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Keywords:

  • basal ganglia;
  • dendritic dopamine release;
  • microdialysis;
  • nigrostriatal dopamine neuron;
  • serotonin;
  • striatum

Abstract

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Nigrostriatal dopaminergic neurons release dopamine from dendrites in substantia nigra and axon terminals in striatum. The cellular mechanisms for somatodendritic and axonal dopamine release are similar, but somatodendritic and nerve terminal dopamine release may not always occur in parallel. The current studies used in vivo microdialysis to simultaneously measure changes in dendritic and nerve terminal dopamine efflux in substantia nigra and ipsilateral striatum respectively, following intranigral application of various drugs by reverse dialysis through the nigral probe. The serotonin releasers (+/–)-fenfluramine (100 µm) and (+)-fenfluramine (100 µm) significantly increased dendritic dopamine efflux without affecting extracellular dopamine in striatum. The non-selective serotonin receptor agonist 1-(m-chlorophenyl)-piperazine (100 µm) elicited a similar pattern of dopamine release in substantia nigra and striatum. NMDA (33 µm) produced an increase in nigral dopamine of a similar magnitude to mCPP or either fenfluramine drug. However, NMDA also induced a concurrent increase in striatal dopamine. The D2 agonist quinpirole (100 µm) had a parallel inhibitory effect on dopamine release from dendritic and terminal sites as well. Taken together, these data suggest that serotonergic afferents to substantia nigra may evoke dendritic dopamine release through a mechanism that is uncoupled from the impulse-dependent control of nerve terminal dopamine release.

Abbreviations used
DA

dopamine

5-HT

serotonin

mCPP

1-(m-chlorophenyl)-piperazine

Dopamine (DA) neurons of the substantia nigra (SN) possess the ability to release DA not only from axon terminals in striatum, but also from the soma and dendrites within SN (Geffen et al. 1976; Korf et al. 1976; Paden et al. 1976; Nieoullon et al. 1977; Cheramy et al. 1981). A vast body of literature investigating the cellular mechanisms of somatodendritic release indicates that nigral DA efflux is calcium dependent (Geffen et al. 1976; Kalivas and Duffy 1991; Jaffe et al. 1998), inactivated by tetrodotoxin (Santiago et al. 1992; Heeringa and Abercrombie 1995; Elverfors et al. 1997), enhanced by amphetamine (Paden et al. 1976; Heeringa and Abercrombie 1995) and disrupted by reserpine (Bjorklund and Lindvall 1975; Wilson et al. 1977; Hattori et al. 1979; Heeringa and Abercrombie 1995). Taken together, these data suggest that the cellular mechanisms underlying somatodendritic DA efflux are fundamentally similar to those mediating nerve terminal release, such that DA is stored in vesicles within DA neuron dendrites, released via voltage-sensitive calcium-mediated exocytosis, and is inactivated by reuptake. Somatodendritic DA therefore appears to fulfill the classical requirements for neurotransmitter release.

The dependence of nerve terminal neurotransmitter release on impulse activity has been well established (for a review see Cooper et al. 1991). Upon reaching threshold, action potentials generated at the axon hillock propagate down the axon to the terminal region and stimulate neurotransmitter release by activating voltage-sensitive calcium channels. In DA neurons, action potentials can actively propagate back into the dendrites with only modest attenuation owing to the existence of sodium channels within the dendrites (Hausser et al. 1995). Somatodendritic DA release is sensitive to activation of voltage-gated calcium channels (Elverfors et al. 1997; Bergquist et al. 1998), so propagation of sodium-dependent action potentials within the dendrites may provide a mechanism for influx of calcium necessary to evoke somatodendritic DA efflux. In this scenario, somatodendritic and nerve terminal DA release will co-vary in an impulse-dependent manner (Westerink et al. 1992a,b)

Alternatively, several lines of evidence indicate that the unique electrophysiological properties of DA neurons may make possible the uncoupling of local DA release within the dendrites and DA efflux from axon terminals. DA neuron action potentials are comprised of an initial segment spike and a distinctive somatodendritic spike (Grace and Bunney 1983a,b; Kita et al. 1986; Grace and Onn 1989; Grace 1990). This may be caused, in part, by the emergence of the axon from a primary dendrite a significant distance from the soma (Grace and Bunney 1983b; Tepper et al. 1987; Hausser et al. 1995; Richards et al. 1997). Based on this anatomical arrangement, the somatodendritic component potentially could be generated independently from the initial segment spike in an isolated ‘compartment’ within the dendrites (Grace 1990). The somatodendritic spike is pharmacologically distinct from the initial segment spike in that it is mediated by a calcium current (Grace 1990). DA neurons display low-threshold and high-threshold calcium spikes that can be activated independently of fast sodium channels and are expressed within the dendrites (Llinas et al. 1984; Harris et al. 1989; Hounsgaard et al. 1992; Nedergaard and Greenfield 1992). Entry of calcium into the dendrites occuring independently of sodium spikes may therefore provide an additional mechanism for somatodendritic DA release such that action potential generation may not be compulsory for somatodendritic DA release under all conditions. In this way, modulatory afferent inputs may conceivably activate dendritic DA release in the absence of back-propagating sodium-dependent action by eliciting dendritic calcium spikes through activation of dendritically localized conductances. For instance, tetrahydrocannabinol stimulates local dendritic DA release within the ventral tegmental area without affecting terminal release in nucleus accumbens (Chen et al. 1993). Moreover, stimulation of dorsal raphe nucleus attenuates antidromic activation of DA neurons without reducing the probability of neuronal firing (Trent and Tepper 1991). These authors propose that serotonin (5-HT)-evoked dendritic DA release occurs locally in SN and inhibits neighboring dendritic regions through inhibitory autoreceptor mechanisms, without affecting overall neuronal excitability. 5-HT application has been shown to evoke somatodendritic DA release from nigral slices and 5-HT selectively activates a tetrodotoxin-insensitive calcium conductance in DA neurons (Williams and Davies 1983; Nedergaard et al. 1988). Taken together, these data suggest that certain afferent inputs, including 5-HT, may evoke dendritic DA efflux independent of changes in release of DA from axon terminals in striatum.

To evaluate the extent to which 5-HT input to nigral DA neurons might selectively evoke local dendritic DA efflux in a manner uncoupled from nerve terminal DA in striatum, microdialysis probes were implanted in the SN and ipsilateral striatum of conscious, freely moving animals. Various pharmacological agents were applied by reverse dialysis through the nigral probe in order to compare the relative changes in extracellular DA efflux simultaneously measured in both SN and striatum.

Animals

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Adult male Sprague–Dawley rats (Zivic-Miller Laboratories, Pittsburgh, PA, USA) were used in all experiments. Before the experiment, animals were housed individually in plastic shoebox cages with free access to food and water. During the microdialysis experiments, animals were housed in plexiglass cylinders (Abercrombie and Finlay 1991), also with food and water ad libitum. Animals were kept under conditions of constant temperature (21°C) and humidity (40%), and maintained on a 12-h light/dark cycle. Rats weighed between 275 and 325 g at the time of each microdialysis experiment. All efforts were made to minimize animal suffering and limit the number of animals utilized for these experiments. Animal procedures were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publications no. 80–23 revised 1996.).

Microdialysis probe implantation

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Microdialysis probes were of the vertical concentric design, as described in detail previously (Abercrombie and Finlay 1991; Cobb and Abercrombie 2002). Probes implanted into SN and striatum had an active area of 1.5 and 2 mm respectively. They were perfused continuously with artificial CSF (147 mm NaCl, 2.5 mm KCl, 1.3 mm CaCl2, 0.9 mm MgCl2, pH ∼ 7.4) at a rate of 1.5 µL/min with a syringe pump (Harvard Apparatus, Holliston, MA, USA). Animals were anesthetized with sodium pentobarbital (60 mg/kg, i.p.) and mounted into a stereotaxic device (David Kopf Instruments, Tujunga, CA, USA). For implantation into SN, probes were set at a 30° lateral angle and lowered into SN at the following coordinates: AP − 5.2 mm, ML ± 5.6 mm relative to bregma, and DV − 8.0 mm below dura (Paxinos and Watson 1986). The probe assembly was fixed to the skull with fast-curing dental cement and two set screws. A second microdialysis probe was implanted into the ipsilateral striatum at the following coordinates: AP + 0.5 mm, ML ± 2.5 mm relative to bregma, and DV − 6.5 mm below dura (Paxinos and Watson 1986). The probe assembly was fixed to the skull with fast-curing dental cement and an additional set screw. Experiments were conducted at least 18 h after probe implantation under conscious conditions.

Analysis of dialysate

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Dialysis samples (20 µL) were collected from both SN and striatum every 15 min throughout the experiment. Subsequently, each sample was analyzed for DA content by HPLC coupled with electrochemical detection. In brief, each system utilized a Velosep RP-18 column (100 × 3.2 mm; Applied Biosystems, Inc., Foster City, CA, USA) and the mobile phase was composed of 0.1 m sodium acetate buffer, pH 4.1, 0.1 mm EDTA, 1.2 mm sodium octyl sulfate and 8.5% (v/v) methanol. An electrochemical detector (Waters Model 460; Millipore Corporation, Bedford, MA, USA) with an amperometric electrode set at an applied potential of + 0.6 V was used. A solvent delivery pump (Model LC-10AD; Shimadzu Corporation, Columbia, MD, USA) delivered the mobile phase at a flow rate of 0.7 mL/min. Each system was calibrated daily with 20 µL of 10 nm standard solution in 0.1 m perchloric acid. Retention time was used to identify DA, which was quantified based on peak height. Samples were analyzed on two separate HPLC systems; the limit of detection for DA in these analyses in SN and striatum was ∼ 0.3 and 2.0 pg respectively.

Experimental manipulations

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

For each experiment, a syringe containing drug solution was prepared by dissolving (+/–)-fenfluramine (100 µm), (+)-fenfluramine (100 µm), 1-(m-chlorophenyl)-piperazine (mCPP), (100 µm), NMDA (33 µm) or quinpirole (100 µm) in artificial CSF to the desired concentration for delivery into SN. The concentrations of (+/–)-fenfluramine, (+)-fenfluramine and mCPP were determined empirically in pilot studies based on the ability to elicit increases of similar magnitude in dendritic DA efflux. After three successive stable baseline samples had been collected from each probe (variability < 10%), the syringe perfusing the probe in SN was switched from one containing artificial CSF only to a syringe containing drug solution. After allowing 15 min for the flow rate to normalize, dialysis sample collection resumed. Extracellular DA content in SN and striatum was analyzed for an additional 3 h while drug was continually delivered through the nigral probe. The time delay before the drug effects were observed can be attributed to the amount of time required for the drug to travel through the tubing of the inlet of the probe to the active area.

Histology

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Upon completion of each experiment, a lethal dose of sodium pentobarbital was administered (120 mg/kg, i.p.) and animals were perfused transcardially with 10% buffered formalin. The brain was then removed and stored in formalin for subsequent histological processing. Coronal slices of 50 µm thickness were stained with cresyl violet to verify placement of microdialysis probes in SN or striatum. Only data from animals with probe placement within the pars reticulata of SN (Cobb and Abercrombie 2002) and striatum were included in the data analysis.

Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Reverse dialysis application of (+/–)-fenfluramine (100 µm) through the nigral probe significantly increased DA measured in SN. Nigral (+/–)-fenfluramine application elicited a rise in extracellular DA measured in SN from 1.1 ± 0.2 pg/sample immediately before drug application to an overall absolute peak level of 2.4 ± 0.4 pg/sample in the presence of drug perfusion (F14,84 = 8.7; p < 0.01, n = 7; Fig. 1a). The increase was observed in the third post-drug sample and remained for the duration of the experiment.

image

Figure 1. Extracellular DA measured concurrently in SN and striatum following reverse dialysis application of (+/–)-fenfluramine (FEN) through the microdialysis probe in SN (horizontal bar). (a) Local application of (+/–)-fenfluramine (100 µm) elicited a significant rise in nigral DA efflux (n = 7). (b) Application of fenfluramine in SN had no effect on extracellular DA recovered in the ipsilateral striatum (STR) (n = 7). Data are mean ± SEM. *p < 0.05 (one-way anova with repeated measures and Dunn's post hoc test).

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Reverse dialysis application of (+/–)-fenfluramine (100 µm) through the nigral probe had no effect on DA efflux in striatum. No significant increase or decrease in DA was observed while (+/–)-fenfluramine was applied within SN compared with the value immediately before drug application (F14,84 = 0.5; p = 0.94, n = 7; Fig. 1b).

Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Reverse dialysis application of the more active enantiomer (+)-fenfluramine (100 µm) through the nigral probe significantly increased extracellular DA measured in SN from 1.0 ± 0.1 pg/sample immediately before drug application to a peak level of 2.4 ± 0.3 pg/sample during drug administration (F14,56 = 8.7; p < 0.01, n = 5; Fig. 2a). The increase was observed in the third post-drug sample and remained present for the duration of the experiment.

image

Figure 2. Extracellular DA measured simultaneously in SN and striatum following application of (+)-fenfluramine (FEN) through the microdialysis probe in SN (horizontal bar). (a) Local application of fenfluramine elicited a significant increase in nigral DA efflux (n = 5). (b) Application of (+)fenfluramine via the nigral probe did not affect extracellular DA measured in the ipsilateral striatum (STR) (n = 5). Data are mean ± SEM. *p < 0.05 (one-way anova with repeated measures and Dunn's post hoc test).

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Reverse dialysis application of (+)-fenfluramine (100 µm) through the nigral probe had no effect on spontaneous DA release in striatum (F14,48 = 1.0; p = 0.42, n = 5; Fig. 2b).

Effect of mCPP application in SN on extracellular DA in SN and striatum

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Reverse dialysis application of the non-selective 5HT1B and 5-HT2B/2C serotonin receptor agonist mCPP (100 µm) through the nigral probe significantly increased DA measured in SN. Nigral mCPP application increased extracellular DA measured in SN from 1.0 ± 0.1 pg/sample immediately before drug application to a peak level of 2.4 ± 0.3 pg/sample during drug administration (F14,56 = 11.4; p < 0.01, n = 4; Fig. 3a). The increase was observed in the third post-drug sample and remained for the duration of the experiment.

image

Figure 3. Extracellular DA measured simultaneously in SN and striatum following reverse dialysis application of mCPP through the microdialysis probe in SN (horizontal bar). (a) Local application of mCPP caused a significant increase in nigral DA efflux (n = 4). (b) Application of mCPP through the microdialysis probe in SN did not affect extracellular DA measured in the ipsilateral striatum (STR) (n = 4). Data are mean ± SEM. *p < 0.05 (one-way anova with repeated measures and Dunn's post hoc test).

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Reverse dialysis application of mCPP (100 µm) through the nigral probe had no effect on spontaneous DA release in striatum (F14,42 = 1.1; p = 0.41, n = 4; Fig. 3b).

Effect of NMDA application in SN on extracellular DA in SN and striatum

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Local application of NMDA (33 µm) by reverse dialysis through the nigral probe significantly increased DA measured in SN from a pre-drug value of 0.7 ± 0.1 pg/sample to a maximal level of 2.6 ± 0.5 pg/sample during drug administration (F14,84 = 20.3; p < 0.01, n = 7; Fig. 4a). The increase was observed in the third post-drug sample and remained for the duration of the experiment.

image

Figure 4. Extracellular DA measured concurrently in SN and striatum following application of NMDA through the microdialysis probe in SN (horizontal bar). (a) Local application of NMDA (33 µm) significantly increased DA recovered within SN (n = 7). (b) Application of NMDA by reverse dialysis elicited a significant increase in extracellular DA detected in the ipsilateral striatum (STR) (n = 7). Data are mean ± SEM. *p < 0.05 (one-way anova with repeated measures and Dunn's post hoc test).

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A significant increase in DA measured in striatum was also observed during NMDA (33 µm) application into SN, from 8.3 ± 2.6 pg/sample immediately before drug application to a maximal level of 10.5 ± 2.6 pg/sample during reverse dialysis administration of NMDA into SN (F14,84 = 16.8; p < 0.01, n = 7; Fig. 4b).

NMDA also produced a dose-dependent response as greater concentrations of NMDA produced more robust increases in nigral DA release. Intranigral NMDA (50 µm) increased nigral DA efflux from a pre-drug value of 1.0 ± 0.1 pg/sample to a maximal value of 4.8 ± 1.4 pg/sample (n = 4; data not shown) and NMDA at a concentration of 100 µm increased extracellular DA in SN from 1.0 ± 0.3 pg/sample to a peak value of 6.7 ± 0.8 pg/sample (n = 2; data not shown).

Effect of quinpirole application in SN on extracellular DA in SN and striatum

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

Reverse dialysis application of the D2 DA receptor agonist quinpirole (100 µm) through the nigral probe significantly decreased extracellular DA measured in SN from a pre-drug value of 1.0 ± 0.1 pg/sample to a minimum level of 0.5 ± 0.1 pg/sample during drug administration (F14,70 = 10.9; p < 0.01, n = 6; Fig. 5a).

image

Figure 5. Extracellular DA measured concurrently in SN and striatum following application of quinpirole (QUIN) through the microdialysis probe in SN (horizontal bar). (a) Local application of quinpirole (100 µm) caused a significant reduction in DA recovered in SN (n = 7). (b) Application of quinpirole (100 µm) by reverse dialysis in SN elicited a significant decrease in extracellular DA detected in the ipsilateral striatum (STR) (n = 7). Data are mean ± SEM. *p < 0.05 (one-way anova with repeated measures and Dunn's post hoc test).

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A significant decrease in striatal DA was also observed during quinpirole (100 µm) application into SN. Local quinpirole administration decreased striatal DA from 9.2 ± 1.0 pg/sample immediately before drug application to a minimum level of 5.8 ± 0.3 pg/sample during reverse dialysis administration in SN (F14,70 = 7.3; p < 0.01, n = 6; Fig. 5b).

Discussion

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References

In the current set of experiments, we examined the effects of serotonergic, glutamatergic and dopaminergic inputs to SN on DA efflux in pars reticulata of SN and in striatum. To achieve this, animals were implanted with microdialysis probes in SN and the ipsilateral striatum. Drugs were delivered via reverse dialysis through the nigral probe. The release of DA in striatum is fundamentally dependent upon action potential generation in the axon hillock of the midbrain DA neurons (Kuhr et al. 1987; Keefe et al. 1992), so release of DA in striatum provides a relative index for the level of nerve impulse activity. The concurrent comparison of nigral and striatal DA efflux in these experiments provides additional information relevant to consideration of the relationship between nerve impulse activity and somatodendritic DA release.

We observed that nigral application of (+/–)-fenfluramine via the microdialysis probe significantly increased dendritic DA efflux in SN without affecting release from axon terminals in striatum (Fig. 1). Fenfluramine increases extracellular 5-HT by blocking 5-HT uptake and evoking 5-HT release from 5-HT terminals (Gobbi et al. 1992; Crespi et al. 1997). SN is richly innervated by serotonergic terminals arising from the dorsal raphe nucleus (van der Kooy and Hattori 1980; Wirtshafter et al. 1987; Lavoie and Parent 1990; Corvaja et al. 1993), so application of (+/–)-fenfluramine via the nigral probe was used to increase serotonergic tone within SN. The enantiomer (+/–)-fenfluramine may affect DA outflow directly at DA uptake sites (Bettini et al. 1987). Therefore we also administered the more active enantiomer (+)-fenfluramine alone; a similar pattern of DA release in SN and striatum as that induced by the mixed enantiomer (+/–)-fenfluramine was obtained (Fig. 2). In an additional group of animals, application of the non-selective 5-HT receptor agonist mCPP evoked an equivalent increase in nigral DA efflux in the absence of any effect on extracellular DA in striatum (Fig. 3). Intravenous administration of mCPP has been shown to have a minimal inhibitory effect on DA neuron firing rate (Di Giovanni et al. 2000). In this same study, DA release in striatum was unaltered following intraperitoneal application of mCPP as measured by microdialysis. These findings, together with the present data, indicate that stimulation of nigral 5-HT receptors can evoke local somatodendritic DA release selectively without affecting the release of DA from axon terminals in striatum.

5-HT has a weak inhibitory influence over DA neuron firing rate (Dray et al. 1976; Kelland et al. 1990, 1993), yet our data reveal that 5-HT has a facilitory effect on dendritic DA. Trent and Tepper (1991) hypothesized that the apparent inhibitory effect of dorsal raphe stimulation on DA neuron dendritic excitability that they observed in the absence of an effect on overall neuronal excitability was the result of 5-HT-evoked somatodendritic DA release and a subsequent increase in the binding of D2 inhibitory autoreceptors localized to the soma and dendrites (Groves et al. 1975; Paden et al. 1976; Trent and Tepper 1991; Pucak and Grace 1994). 5-HT has been shown to have excitatory effects on DA neurons in vitro, including activation of a high-threshold calcium spike in the dendritic region of DA neurons (Nedergaard et al. 1988, 1991). Moreover, exogenous 5-HT application stimulates DA release from nigral slices (Williams and Davies 1983). Whereas DA release from axon terminals relies exclusively upon action potential generation for initiation, the results from the current study indicate that additional mechanisms may have the ability to evoke somatodendritic DA release such that dendritic DA efflux may not be coupled to neuronal impulse activity under all conditions. As a result, afferent input may excite local dendritic regions and stimulate somatodendritic DA release without changing the overall firing rate or firing pattern of the neuron. In this scenario, select excitatory inputs that are incapable of generating action potentials may powerfully regulate pars reticulata output neurons of SN and the output of the basal ganglia via stimulation of local somatodendritic DA release in SN (Cameron and Williams 1993; Timmerman and Abercrombie 1996; Abercrombie and DeBoer 1997; Radnikow and Misgeld 1998).

Our data reveal that stimulation of nigral 5-HT receptors evokes somatodendritic DA release, but it is unclear if this is a direct effect through activation of 5-HT receptors localized to DA neurons or whether this action is mediated through stimulation of 5-HT receptors localized to afferent terminals or non-dopaminergic neurons in SN. SN contains among the densest serotonergic innervation in the brain and serotonergic terminals directly contact TH-positive dendrites predominantly in SN pars reticulata but also within SN pars compacta (Nedergaard et al. 1988; Corvaja et al. 1993; Moukhles et al. 1997). Both 5-HT1 and 5-HT2 serotonin receptor classes are expressed within SN (Pazos and Palacios 1985; Waeber et al. 1989; Pompeiano et al. 1994; Wright et al. 1995; Eberle-Wang et al. 1997; Di Matteo et al. 2001; Varnas et al. 2001), but the specific profile of 5-HT receptor expression by nigrostriatal DA neurons compared with afferent terminals or other neurons in SN has yet to be defined fully. Therefore, serotonergic interactions within SN will require further investigation to identify the exact mechanisms of serotonergic action in SN. An indirect effect of 5-HT on NMDA-mediated stimulation of dendritic DA efflux in SN appears unlikely based on the present data because nigral application of NMDA was associated with increased DA output in striatum, whereas the pharmacological 5-HT manipulations were devoid of this action.

It is possible that reverse dialysis application of either mCPP or fenfluramine increases impulse activity in nigrostriatal DA neurons, but the net effect is too small for the temporal and spatial resolution of the microdialysis probe within the striatum to allow for detectable changes. To investigate this possibility further we applied NMDA or quinpirole via reverse dialysis and measured extracellular DA in SN and striatum. NMDA application elicits excitatory post-synaptic currents in DA neurons, increases DA neuron impulse activity and induces burst firing (Mereu et al. 1991; Overton and Clark 1992; Zhang et al. 1994; Christoffersen and Meltzer 1995; Meltzer et al. 1997). Moreover, NMDA applied directly into SN has also been shown to evoke somatodendritic and terminal DA release in the presence of nomifensine (Westerink et al. 1992b). Conversely, nigral application of quinpirole suppresses DA neuron discharge rate via activation of inhibitory autoreceptors in SN and decreases DA efflux in SN and striatum (Santiago and Westerink 1991; Westerink et al. 1992a; Shi et al. 1997). In the current studies, we replicated these findings in the absence of DA uptake inhibition. Importantly, we found that the increase in extracellular DA in SN produced by nigral application of NMDA was of similar amplitude to the increase observed following administration of mCPP or fenfluramine (Fig. 4). However, the increase in nigral DA efflux evoked by intranigral NMDA administration induced a simultaneous increase in extracellular DA measured within striatum, which was absent during nigral application of mCPP or fenfluramine despite the equivalent effects of these compounds on nigral DA efflux. Consequently, our data suggest that 5-HT or glutamate inputs to SN may differentially regulate DA neuron physiology, such that an increase of similar magnitude of somatodendritic DA efflux may or may not be coupled with a concurrent increase in DA release from nerve terminals in striatum. Higher concentrations of NMDA (50 µm, n = 4; 100 µm, n = 2) evoked larger increases in extracellular DA in SN and striatum (data not shown) so that this disparity cannot be explained as a ceiling effect. Furthermore, quinpirole mediated a parallel inhibitory effect on DA efflux in SN and striatum (Fig. 5).

Taken together, the present data have demonstrated that, under conditions whereby stimulation of nigral NMDA receptors and stimulation of nigral 5-HT receptors have equivalent effects in stimulating dendritic DA efflux, differential effects on nerve terminal DA release in striatum are observed. Based on these observations, it is suggested that action potential generation at the axon hillock of the DA neuron is not compulsory for initiation of somatodendritic DA release under all conditions, and that somatodendritic and nerve terminal DA release therefore does not necessarily parallel under all conditions. Interestingly, a TTX-insensitive mechanism for the initiation of dendritic DA release by 5-HT is predicted by this model (Cheramy et al. 1981). The physiological condition(s) under which such a mechanism might contribute to the regulation of basal ganglia function remain to be determined.

References

  1. Top of page
  2. Abstract
  3. Experimental procedures
  4. Materials
  5. Animals
  6. Microdialysis probe implantation
  7. Analysis of dialysate
  8. Experimental manipulations
  9. Histology
  10. Data analysis
  11. Results
  12. Effect of (+/–)-fenfluramine application in SN on extracellular DA in SN and striatum
  13. Effect of application of (+)-fenfluramine in SN on extracellular DA in SN and striatum
  14. Effect of mCPP application in SN on extracellular DA in SN and striatum
  15. Effect of NMDA application in SN on extracellular DA in SN and striatum
  16. Effect of quinpirole application in SN on extracellular DA in SN and striatum
  17. Discussion
  18. Acknowledgements
  19. References
  • Abercrombie E. D. and DeBoer P. (1997) Substantia nigra D1 receptors and stimulation of striatal cholinergic interneurons by dopamine: a proposed circuit mechanism. J. Neurosci. 17, 84988505.
  • Abercrombie E. D. and Finlay J. M. (1991) Monitoring extracellular norepinephrine in brain using in vivo microdialysis and HPLC-EC, in Microdialysis in the Neurosciences, Vol. 7 (Robinson T. E. and Justice J. B., eds), pp. 253274. Elsevier Science Publishers, New York.
  • Bergquist F., Jonason J., Pileblad E. and Nissbrandt H. (1998) Effects of local administration of L-, N-, and P/Q-type calcium channel blockers on spontaneous dopamine release in the striatum and the substantia nigra: a microdialysis study in rat. J. Neurochem. 70, 15321540.
  • Bettini E., Ceci A., Spinelli R. and Samanin R. (1987) Neuroleptic-like effects of the l isomer of fenfluramine on striatal dopamine release in freely moving rats. Biochem. Pharmacol. 36, 23872391.
  • Bjorklund A. and Lindvall O. (1975) Dopamine in dendrites of substantia nigra neurons: suggestions for a role in dendritic terminals. Brain Res. 83, 531537.
  • Cameron D. L. and Williams J. T. (1993) Dopamine D1 receptors facilitate transmitter release. Nature 366, 344347.
  • Chen J., Marmur R., Pulles A., Paredes W. and Gardner E. L. (1993) Ventral tegmental microinjection of delta 9-tetrahydrocannabinol enhances ventral tegmental somatodendritic dopamine levels but not forebrain dopamine levels: evidence for local neural action by marijuana's psychoactive ingredient. Brain Res. 621, 6570.
  • Cheramy A., Leviel V. and Glowinski J. (1981) Dendritic release of dopamine in the substantia nigra. Nature 289, 537542.
  • Christoffersen C. L. and Meltzer L. T. (1995) Evidence for N-methyl-d-aspartate and AMPA subtypes of the glutamate receptor on substantia nigra dopamine neurons: possible preferential role for N-methyl-d-aspartate receptors. Neuroscience 67, 373381.
  • Cobb W. S. and Abercrombie E. D. (2002) Distinct roles for nigral GABA and glutamate receptors in the regulation of dendritic dopamine release under normal conditions and in response to systemic haloperidol. J. Neurosci. 22, 14071413.
  • Cooper J. R., Bloom F. E. and Roth R. H. (1991) The steps of synaptic transmission. The Biochemical Basis of Neuropharmacology, VIth edn, pp. 4446. Oxford Press, New York.
  • Corvaja N., Doucet G. and Bolam J. P. (1993) Ultrastructure and synaptic targets of the raphe-nigral projection in the rat. Neuroscience 55, 417427.
  • Crespi D., Mennini T. and Gobbi M. (1997) Carrier-dependent and Ca2+-dependent 5-HT and dopamine release induced by (+)-amphetamine, 3,4-methylendioxymethamphetamine, p-chloroamphetamine and (+)-fenfluramine. Br. J. Pharmacol. 121, 17351743.
  • Di Giovanni G., Di Matteo V., Di Mascio M. and Esposito E. (2000) Preferential modulation of mesolimbic vs. nigrostriatal dopaminergic function by serotonin (2C/2B) receptor agonists: a combined in vivo electrophysiological and microdialysis study. Synapse 35, 5361.
  • Di Matteo V., De Blasi A., Di Giulio C. and Esposito E. (2001) Role of 5-HT (2C) receptors in the control of central dopamine function. Trends Pharmacol. Sci. 22, 229232.
  • Dray A., Gonye T. J., Oakley N. R. and Tanner T. (1976) Evidence for the existence of a raphe projection to the substantia nigra in rat. Brain Res. 113, 4557.
  • Eberle-Wang K., Mikeladze Z., Uryu K. and Chesselet M. F. (1997) Pattern of expression of the serotonin2C receptor messenger RNA in the basal ganglia of adult rats. J. Comp. Neurol. 384, 233247.
  • Elverfors A., Jonason J., Jonason G. and Nissbrandt H. (1997) Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices. Synapse 26, 359369.
  • Geffen L. B., Jessell T. M., Cuello A. C. and Iversen L. L. (1976) Release of dopamine from dendrites in rat substantia nigra. Nature 260, 258260.
  • Gobbi M., Frittoli E., Mennini T. and Garattini S. (1992) Releasing activities of d-fenfluramine and fluoxetine on rat hippocampal synaptosomes preloaded with [3H]serotonin. Naunyn Schmiedebergs Arch. Pharmacol. 345, 16.
  • Grace A. A. (1990) Evidence for the functional compartmentalization of spike generating regions of rat midbrain dopamine neurons recorded in vitro. Brain Res. 524, 3141.
  • Grace A. A. and Bunney B. S. (1983a) Intracellular and extracellular electrophysiology of nigral dopaminergic neurons – 1. Identification and characterization. Neuroscience 10, 301315.
  • Grace A. A. and Bunney B. S. (1983b) Intracellular and extracellular electrophysiology of nigral dopaminergic neurons – 2. Action potential generating mechanisms and morphological correlates. Neuroscience 10, 317331.
  • Grace A. A. and Onn S. P. (1989) Morphology and electrophysiological properties of immunocytochemically identified rat dopamine neurons recorded in vitro. J. Neurosci. 9, 34633481.
  • Groves P. M., Wilson C. J., Young S. J. and Rebec G. V. (1975) Self-inhibition by dopaminergic neurons. Science 190, 522528.
  • Harris N. C., Ramsay S., Kelion A. and Greenfield S. A. (1989) Electrophysiological evidence for the dendritic localization of a calcium conductance in guinea-pig substantia nigra neurones in vitro. Exp. Brain Res. 74, 411416.
  • Hattori T., McGeer P. L. and McGeer E. G. (1979) Dendro axonic neurotransmission. II. Morphological sites for the synthesis, binding and release of neurotransmitters in dopaminergic dendrites in the substantia nigra and cholinergic dendrites in the neostriatum. Brain Res. 170, 7183.
  • Hausser M., Stuart G., Racca C. and Sakmann B. (1995) Axonal initiation and active dendritic propagation of action potentials in substantia nigra neurons. Neuron 15, 637647.
  • Heeringa M. J. and Abercrombie E. D. (1995) Biochemistry of somatodendritic dopamine release in substantia nigra: an in vivo comparison with striatal dopamine release. J. Neurochem. 65, 192200.
  • Hounsgaard J., Nedergaard S. and Greenfield S. A. (1992) Electrophysiological localization of distinct calcium potentials at selective somatodendritic sites in the substantia nigra [letter]. Neuroscience 50, 513518.
  • Jaffe E. H., Marty A., Schulte A. and Chow R. H. (1998) Extrasynaptic vesicular transmitter release from the somata of substantia nigra neurons in rat midbrain slices. J. Neurosci. 18, 35483553.
  • Kalivas P. W. and Duffy P. (1991) A comparison of axonal and somatodendritic dopamine release using in vivo dialysis. J. Neurochem. 56, 961967.
  • Keefe K. A., Zigmond M. J. and Abercrombie E. D. (1992) Extracellular dopamine in striatum: influence of nerve impulse activity in medial forebrain bundle and local glutamatergic input. Neuroscience 47, 325332.
  • Kelland M. D., Freeman A. S. and Chiodo L. A. (1990) Serotonergic afferent regulation of the basic physiology and pharmacological responsiveness of nigrostriatal dopamine neurons. J. Pharmacol. Exp. Ther. 253, 803811.
  • Kelland M. D., Freeman A. S., Rubin J. and Chiodo L. A. (1993) Ascending afferent regulation of rat midbrain dopamine neurons. Brain Res. Bull. 31, 539546.
  • Kita T., Kita H. and Kitai S. T. (1986) Electrical membrane properties of rat substantia nigra compacta neurons in an in vitro slice preparation. Brain Res. 372, 2130.
  • van der Kooy D. and Hattori T. (1980) Dorsal raphe cells with collateral projections to the caudate–putamen and substantia nigra: a fluorescent retrograde double labeling study in the rat. Brain Res. 186, 17.
  • Korf J., Zieleman M. and Westerink B. H. C. (1976) Dopamine release in the substantia nigra? Nature 260, 257258.
  • Kuhr W. G., Wightman R. M. and Rebec G. V. (1987) Dopaminergic neurons: simultaneous measurements of dopamine release and single-unit activity during stimulation of the medial forebrain bundle. Brain Res. 418, 122128.
  • Lavoie B. and Parent A. (1990) Immunohistochemical study of the serotoninergic innervation of the basal ganglia in the squirrel monkey. J. Comp. Neurol. 299, 116.
  • Llinas R., Greenfield S. A. and Jahnsen H. (1984) Electrophysiology of pars compacta cells in the in vitro substantia nigra – a possible mechanism for dendritic release. Brain Res. 294, 127132.
  • Meltzer L. T., Christoffersen C. L. and Serpa K. A. (1997) Modulation of dopamine neuronal activity by glutamate receptor subtypes. Neurosci. Biobehav. Rev. 21, 511518.
  • Mereu G., Costa E., Armstrong D. M. and Vicini S. (1991) Glutamate receptor subtypes mediate excitatory synaptic currents of dopamine neurons in midbrain slices. J. Neurosci. 11, 13591366.
  • Moukhles H., Bosler O., Bolam J. P., Vallee A., Umbriaco D., Geffard M. and Doucet G. (1997) Quantitative and morphometric data indicate precise cellular interactions between serotonin terminals and postsynaptic targets in rat substantia nigra. Neuroscience 76, 11591171.
  • Nedergaard S. and Greenfield S. A. (1992) Sub-populations of pars compacta neurons in the substantia nigra: the significance of qualitatively and quantitatively distinct conductances. Neuroscience 48, 423437.
  • Nedergaard S., Bolam J. P. and Greenfield S. A. (1988) Facilitation of a dendritic calcium conductance by 5-hydroxytryptamine in the substantia nigra. Nature 333, 174177.
  • Nedergaard S., Flatman J. A. and Engberg I. (1991) Excitation of substantia nigra pars compacta neurones by 5-hydroxy-tryptamine in-vitro. Neuroreport 2, 329332.
  • Nieoullon A., Cheramy A. and Glowinski J. (1977) Release of dopamine in vivo from cat substantia nigra. Nature 266, 375377.
  • Overton P. and Clark D. (1992) Iontophoretically administered drugs acting at the N-methyl-d-aspartate receptor modulate burst firing in A9 dopamine neurons in the rat. Synapse 10, 131140.
  • Paden C., Wilson C. J. and Groves P. M. (1976) Amphetamine-induced release of dopamine from the substantia nigra in vitro. Life Sci. 19, 14991506.
  • Paxinos G. and Watson C. (1986) The Rat Brain in Stereotaxic Coordinates, 2nd edn. Academic Press, New York.
  • Pazos A. and Palacios J. M. (1985) Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors. Brain Res. 346, 205230.
  • Pompeiano M., Palacios J. M. and Mengod G. (1994) Distribution of the serotonin 5-HT2 receptor family mRNAs: comparison between 5-HT2A and 5-HT2C receptors. Brain Res. Mol. Brain Res. 23, 163178.
  • Pucak M. L. and Grace A. A. (1994) Evidence that systemically administered dopamine antagonists activate dopamine neuron firing primarily by blockade of somatodendritic autoreceptors. J. Pharmacol. Exp. Ther. 271, 11811192.
  • Radnikow G. and Misgeld U. (1998) Dopamine D1 receptors facilitate GABAA synaptic currents in the rat substantia nigra pars reticulata. J. Neurosci. 18, 20092016.
  • Richards C. D., Shiroyama T. and Kitai S. T. (1997) Electrophysiological and immunocytochemical characterization of GABA and dopamine neurons in the substantia nigra of the rat. Neuroscience 80, 545557.
  • Santiago M. and Westerink B. H. (1991) The regulation of dopamine release from nigrostriatal neurons in conscious rats: the role of somatodendritic autoreceptors. Eur. J. Pharmacol. 204, 7985.
  • Santiago M., Machado A. and Cano J. (1992) Fast sodium channel dependency of the somatodendritic release of dopamine in the rat's brain. Neurosci. Lett. 148, 145147.
  • Shi W. X., Smith P. L., Pun C. L., Millet B. and Bunney B. S. (1997) D1–D2 interaction in feedback control of midbrain dopamine neurons. J. Neurosci. 17, 79887994.
  • Tepper J. M., Sawyer S. F. and Groves P. M. (1987) Electrophysiologically identified nigral dopaminergic neurons intracellularly labeled with HRP: light-microscopic analysis. J. Neurosci. 7, 27942806.
  • Timmerman W. and Abercrombie E. D. (1996) Amphetamine-induced release of dendritic dopamine in substantia nigra pars reticulata: D1-mediated behavioral and electrophysiological effects. Synapse 23, 280291.
  • Trent F. and Tepper J. M. (1991) Dorsal raphe stimulation modifies striatal-evoked antidromic invasion of nigral dopaminergic neurons in vivo. Exp. Brain Res. 84, 620630.
  • Varnas K., Hall H., Bonaventure P. and Sedvall G. (2001) Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [3H]GR 125743. Brain Res. 915, 4757.
  • Waeber C., Dietl M. M., Hoyer D. and Palacios J. M. (1989) 5HT1 receptors in the vertebrate brain. Regional distribution examined by autoradiography. Naunyn. Schmiedebergs Arch. Pharmacol. 340, 486494.
  • Westerink B. H., Santiago M. and De Vries J. B. (1992a) In vivo evidence for a concordant response of terminal and dendritic dopamine release during intranigral infusion of drugs. Naunyn Schmiedebergs Arch. Pharmacol. 346, 637643.
  • Westerink B. H., Santiago M. and De Vries J. B. (1992b) The release of dopamine from nerve terminals and dendrites of nigrostriatal neurons induced by excitatory amino acids in the conscious rat. Naunyn Schmiedebergs Arch. Pharmacol. 345, 523529.
  • Williams J. and Davies J. A. (1983) The involvement of 5-hydroxytryptamine in the release of dendritic dopamine from slices of rat substantia nigra. J. Pharm. Pharmacol. 35, 734737.
  • Wilson C. J., Groves P. M. and Fifkova E. (1977) Monoaminergic synapses, including dendro-dendritic synapses in the rat substantia nigra. Exp. Brain Res. 30, 161174.
  • Wirtshafter D., Stratford T. R. and Asin K. E. (1987) Evidence that serotonergic projections to the substantia nigra in the rat arise in the dorsal, but not the median, raphe nucleus. Neurosci. Lett. 77, 261266.
  • Wright D. E., Seroogy K. B., Lundgren K. H., Davis B. M. and Jennes L. (1995) Comparative localization of serotonin 1A, 1C, and 2 receptor subtype mRNAs in rat brain. J. Comp. Neurol. 351, 357373.
  • Zhang J., Chiodo L. A. and Freeman A. S. (1994) Influence of excitatory amino acid receptor subtypes on the electrophysiological activity of dopaminergic and nondopaminergic neurons in rat substantia nigra. J. Pharmacol. Exp. Ther. 269, 313321.