Neuronal plasticity in animal models and the epileptic human hippocampus

Authors


Address correspondence to Gunther Sperk, Department of Pharmacology, Peter-Mayr-Str. 1a, 6020 Innsbruck, Austria. E-mail: guenther.sperk@i-med.ac.at.

Prolonged status epilepticus in humans as in experimental animals can initiate the development of temporal lobe epilepsy (TLE) (Kapur, 1999). Therefore, application of potent convulsant substances such as kainic acid or pilocarpine in rats induces acute status epilepticus that, after a silent period of 1–2 weeks, is followed by spontaneous convulsions. The status epilepticus is characterized by severe limbic seizures and sequelae of neuropathologic signs including opening of the blood–brain barrier, local brain edema, bleeding into the brain, and activation of microglia and astrocytes followed by neurodegeneration in the hippocampus, amygdala, entorhinal cortex, and other brain areas (Sperk et al., 1983; Du et al., 1993; Rizzi et al., 2003). Induced by the seizure activity, neurotransmitters such as γ-aminobutyric acid (GABA), glutamate, or amine transmitters are released from their stores and mechanisms of their resynthesis are strongly activated (Sperk et al., 1983). In addition, pronounced changes in the expression of multiple functionally important proteins have been found in brains of experimental animals and humans (Herdegen et al., 1993; Sperk, 1994; McNamara, 1999; Morimoto et al., 2004).

Some of these dynamic neurochemical changes persist also in the chronically epileptic state or may be altered or substituted by other changes. They are accompanied by progressing rearrangement of neuronal circuitries, characterized by continuing neurodegeneration and by axonal outgrowth. The best-characterized example of such plastic changes is the sprouting of mossy fibers to the inner molecular layer of the dentate gyrus, where they seem to substitute the loss of associational/commissural fibers arising from dentate mossy cells (Houser et al., 1990).

Herein we review some of our findings and the findings of others on neurochemical and morphologic changes related to GABAergic and peptidergic neurotransmission (Table 1; Pirker et al., 2001).

Table 1.    Parameters of GABAergic and peptidergic neurotransmission altered after kainic acid–induced status epilepticus and in temporal lobe epilepsy
 Kainic acid model (rat)TLE (humans)References
Acute (status)Chronic (epilepsy)
  1. ↑, increased ↑↑, markedly increased; (↑), not significantly increased; ↓, decreased; ↓↓, markedly decreased; not altered; ↑/↓, ↑/−, different results, depending on the study cited.

Parameters of GABAergic transmission
 Glutamate decarboxylase (GAD67, GAD65)
  Enzyme activityndSperk et al., 1983
  mRNA Esclapez & Houser, 1999; Sperk et al., 2003
  Histochemistrysproutingsprouting Furtinger et al., 2001
  Ectopic expression in mossy fibers+++Schwarzer & Sperk, 1995
 Vesicular GABA transporter Sperk et al., 2003
 GAT-1 (molecular layer)Sperk et al., 2003; Mathern et al., 1999
 GABAA receptor in dentate gyrus
  Subunit α1Sperk, 2007; Pirker et al., 2003; Loup et al., 2000
  Subunit α2Sperk, 2007; Loup et al., 2000
  Subunit α3Sperk, 2007; Pirker et al., 2003; Loup et al., 2000
  Subunit α4(↑)Sperk, 2007
  Subunit α5Sperk, 2007
  Subunit β2↑↑Sperk, 2007; Pirker et al., 2003
  Subunit β3↑↑Sperk, 2007; Pirker et al., 2003
  Subunit γ2Sperk, 2007; Pirker et al., 2003; Loup et al., 2000
  Subunit δ↓↓↓↓Sperk, 2007
 GABAB receptors in dentate gyrus
  GABABR.1 and GABABR-2(↑)Furtinger et al., 2003a; Furtinger et al., 2003b
Parameters of peptidergic transmission
 Dynorphin
  Dentate gyrus (granule cells)↑↑Douglass et al., 1991; Houser et al., 1990; Pirker et al., 2001
 Neuropeptide Y (mRNA/peptide)
  Dentate gyrus
   Interneurons↑/↓↑/↑↑/↑ (cell loss)Sperk, 1994; Furtinger et al., 2001
   Mossy fiber↑/−↑/↑−/−Furtinger et al., 2001; Sperk et al., 1992; Vezzani et al., 1999
   CA1/Subiculum↑/−↑/↑ (pyramidal neurons)↑/↑ (interneurons)Furtinger et al., 2001; Vezzani & Sperk, 2004
 Y2 receptors (mossy fibers)↑/↑Furtinger et al., 2001; Sperk et al., 1992
 Somatostatin
  Dentate gyrus↑/↓↑/↑↑/↑ (cell loss)Furtinger et al., 2001; Marksteiner et al., 1992
  CA1/Subiculum↑/−↑/↑ (interneurons and pyr. neurons)↑ (interneurons, sproutingDrexel, in preparation
  SSR-2 receptorsMoneta et al., 2002; Csaba et al., 2005
  Neurokinin B (dentate granule cells)ndMarksteiner et al., 1992
  Galanin (dentate granule cells) ndMazarati et al., 1998; Mazarati et al., 2004

There are clear indications for a loss of excitatory as well as of inhibitory GABAergic neurons early after induction of the status epilepticus. At the same time, expression of immediate early genes and of many proteins becomes severely altered, mostly activated presumably leading to an altered functioning of neuronal circuitries (Herdegen et al., 1993; Sperk, 1994; Morimoto et al., 2004). Expression of the GABA-synthesizing enzymes glutamate decarboxylases GAD65 and GAD67 and of an embryonic form of GAD67 becomes enhanced (Sperk et al., 1983, 2003; Esclapez & Houser, 1999; Szabo et al., 2000), indicating enhanced GABA synthesis in the surviving neurons. Also at the receptor level, GABAergic transmission appears to be markedly altered. In human TLE, as in animal models, GABAA and GABAB receptors undergo dynamic changes in their expression. Whereas expression of GABAB receptors is decreased initially after status epilepticus (perhaps resulting in enhanced release of glutamate), it is increased in patients with chronic TLE (Furtinger et al., 2003a,b). Changes in the expression of GABAA receptor subunits are complex. In animal models, typically expression of the β-subunits (β2 and β3) containing the binding site for GABA, and of α2 and γ2, contributing to the binding of the anticonvulsant benzodiazepines is increased. On the other hand, levels of subunits presumably comprising extrasynaptic receptors involved in tonic GABA-mediated inhibition, such as δ and α5 (in mice), become decreased in the dentate gyrus after status epilepticus. Interestingly in human TLE most subunits expressed in the hippocampus seem to be upregulated (notably subunits α2, α3, α5, β1-3, γ2, and δ), indicating little functional changes but consistent upregulation of the receptors presumably leading to generally enhanced GABAergic transmission. (Table 1; Loup et al., 2000; Pirker et al., 2001).

Neuropeptides are cotransmitters of classical neurotransmitters. They are rapidly released during status epilepticus but are considerably slower resynthesized than classical neurotransmitters (Vezzani et al., 1996). It has been well documented that synthesis of neuropeptides is dynamically regulated by seizures and that neuropeptides may potently influence later epileptic events in different ways. Therefore, the peptides thyrotropin-releasing hormone (TRH) and neurokinin B exert proconvulsive actions, and neuropeptide Y (NPY), galanin, and dynorphin exert potent anticonvulsive actions (Vezzani et al., 1999; Mazarati & Wasterlain, 2002). Expression of all of these peptides is altered by the status epilepticus. NPY exerts its anticonvulsive effects through presynaptic Y2 receptors located presynaptically on glutamate neurons and by mediating inhibition of the release of the excitatory transmitter (Vezzani et al., 1999; Furtinger et al., 2001). Seizures not only cause marked upregulation of NPY but also of Y2 receptors in mossy fibers of rats and patients with TLE (Furtinger et al., 2001). Interestingly, whereas NPY is expressed ectopically in principal neurons of epileptic rats and may act there on presynaptic receptors, it becomes overexpressed in GABA/NPY neurons that prominently sprout in human TLE. In contrast to the rat, in human TLE, the peptide may be released from interneurons upon nerve endings of excitatory neurons and may result in impaired glutamate release (Furtinger et al., 2001).

Other than for NPY, expression of dynorphin becomes decreased in the hippocampus of epileptic rats (Douglass et al., 1991). Consequently, its endogenous action may be limited in epileptic rats. In contrast, in patients with TLE, expression of dynorphin is markedly upregulated in mossy fibers. mRNA levels are especially high in patients that experienced seizures within 48 h prior to epilepsy surgery, indicating a confounding effect of seizures on dynorphin expression (Pirker et al., 2009). Because dynorphin exerts anticonvulsive actions (mediated by κ-opioid receptors) in experimental animals, it may act as an endogenous anticonvulsant peptide in human TLE, upregulated by a previous seizure episode. The anticonvulsant potency of various neuropeptides, notably of NPY and galanin, has recently led to the concept of using viral vectors overexpressing the neuropeptides, which then may be selectively released during epileptic seizures and may exert anticonvulsive action.

Acknowledgment

The work was supported by the Austrian Science Foundation (P 19 464 and SFB 35-12) and European Union Grant FP6 EPICURE (LSH-CT-2006-037315).

The authors confirm that they have read the Journal’s position on issues involved in ethical publication and affirm that this article is consistent with those guidelines.

Disclosure: None of the authors has any conflict of interest to disclose.

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