PCP models of schizophrenia
In recent years, increasing evidence supports the idea that dysfunction of the glutamatergic system is a primary pathophysiological change seen in schizophrenia (see Olney and Farber, 1995; Tsai and Coyle, 2002; Coyle et al., 2003; Konradi and Heckers, 2003). Pharmacological evidence for the role of glutamate in schizophrenia centres on findings that blockade of the NMDA receptor by non-competitive antagonists, such as ketamine or PCP, induces delusions and hallucinations in otherwise healthy subjects, symptoms commonly seen in schizophrenia (Cohen et al., 1962; Krystal et al., 1994). Furthermore, in both stabilized chronic and acute schizophrenic patients, PCP rekindles and exacerbates positive symptoms (Javitt and Zukin, 1991), and even at low doses, it produces psychotic symptoms in normal volunteers accompanied by progressive withdrawal and poverty of speech, akin to the negative symptoms of schizophrenia (Luby et al., 1959). Additionally, both acute low-dose and chronic recreational use of PCP impair cognitive performance, which is reversed with cessation of drug administration (Cosgrove and Newell, 1991; Javitt and Zukin, 1991).
As PCP induces several symptoms in humans akin to those seen in schizophrenia, it has been used to attempt to produce a pharmacological rodent model of schizophrenia. Acute PCP administration causes hyperlocomotion (Kalinichev et al., 2007), social withdrawal (Sams-Dodd, 1995), and impairment of both PPI (Mansbach and Geyer, 1989) and cognition (Egerton et al., 2005) in rodents. While it is not possible to perform controlled chronic PCP studies in humans, it has been reported that recreational abuse of PCP produces symptoms that persist beyond the end of treatment (Rainey and Crowder, 1975). Additionally, early PET scans suggested that PCP abuse was accompanied by deficits in the temporal and frontal lobes, which parallels changes seen in schizophrenic patients (Hertzmann et al., 1990). Thus, it has been suggested that chronic PCP use may be used to more accurately mimic the symptoms of schizophrenia (Jentsch and Roth, 1999). This has been the basis for evaluating of the effects of chronic PCP administration in rodents; most commonly using twice-daily administration for 7 days followed by a 7 day washout period before the start of experimentation herein described as subchronic. However, different research groups have developed their own variant of the subchronic PCP treatment, detailed analysis of which is beyond the scope of this review, but some distinguishing features are discussed in Table 2. Notably, variations in the period of administration, dose, gender and strain all affect the peak concentration of PCP in the brain, which could account for many differences reported with these various protocols (Table 2).
Table 2. Comparison of the effects of various subchronic PCP administration protocols on cognitive paradigms in rats and mice as indicated
|Dose||Species/Strain/Sex||Time of test||Behavioural test||Antipsychotic drug effect||Reference|
|Subchronic (2 mg·kg−1)||Rat/Lister hooded/♀||>7 days||Deficit in reversal learning||Reversed by acute ASN, CLZ, OLZ, SRT, ZPD and repeated ASN, RSP and OLZ; no effect of acute HLP or CPM||(Abdul-Monim et al., 2006; Idris et al., 2010; McLean et al., 2010b)|
| ||7 days||Deficit in novel object recognition||Reversed by acute CLZ, MLP, OLZ, RSP, SRT, but not HLP||(Grayson et al., 2007; Idris et al., 2010; Snigdha et al., 2010)|
|Subchronic (2 mg·kg−1)||Rat/Sprague-Dawley/♂||7 days||Deficit in reversal learning|| ||(Jentsch and Taylor, 2001)|
| || ||Deficit in novel object recognition||No effect of concurrent RSP||(McKibben et al., 2010)|
|Subchronic (4.5 mg·kg−1)||Rat/Sprague-Dawley/♂||7 days||Deficit in performance in double Y-maze|| ||(Beninger et al., 2010)|
|Subchronic (5 mg·kg−1)||Rat/Lister hooded/♂||7 days||Deficit in episodic memory||No effect of CLZ||(Le Cozannet et al., 2010)|
| || ||Deficit in attentional set shifting||Reversed by acute SRT, but not RSP or HLP||(Rodefer et al., 2005; Broberg et al., 2009; Goetghebeur and Dias, 2009)|
|Rat/Wistar/♂||72 h||Deficit in delayed alternation task|| ||(Seillier and Giuffrida, 2009)|
|Mice/C57BL/6J/♂||7 days||No deficit in operant behaviour or reversal learning|| ||(Brigman et al., 2009)|
|Chronic intermittent (2.6 mg·kg−1, 28 days)||Rat/Long-Evans/♂||72 h||Impaired attentional set shifting|| ||(Egerton et al., 2008)|
| ||24 h||Deficit in novel object recognition|| ||(Spano et al., 2010)|
|3 days per week for 5 weeks (3 mg·kg−1)||Rat/Sprague-Dawley/♂||4 weeks||No effect on attentional set shifting|| ||(Fletcher et al., 2005)|
|Osmotic minipump (15 mg·kg−1·day−1, 14 days)||Rat/Lister hooded/♂||7 days||Impaired attentional set shifting|| ||(Pedersen et al., 2009)|
|12 days (0.5–4 mg·kg−1)||Mice/C57Bl/6J/♂||15 min||Impaired spatial learning at low dose||Reversed by repeated CLZ, but not HLP||(Beraki et al., 2008)|
|14 days (10 mg·kg−1)||Rat/Sprague-Dawley and Long-Evans/♂||48 h||Deficit in spatial delay alternation task (at longer delays)|| ||(Jentsch et al., 1997b) (Marquis et al., 2007)|
|Mice/ICR/♂||5 days||Deficit in novel object recognition||Reversed by acute and repeated ARP, but not HLP||(Nagai et al., 2009)|
|Mice/C57BL/6J and Rat/Sprague-Dawley/♂||7 days||No deficit in spatial performance|| ||(Li et al., 2003)|
|10 days (with 2 day break) (10 mg·kg−1)||Mice/ICR/♂||14 days||Deficit in novel object recognition||Reversed by repeated QTP||(Tanibuchi et al., 2009)|
|6 days (1.3 mg·kg−1)||Rat/Wistar/♂||30 min||Deficit in spatial learning and memory||Reversed by acute CLZ, SRT and RSP; no effect of HLP||(Didriksen et al., 2007)|
|5 days (b.i.d. 5 mg·kg−1)||Rat/Sprague-Dawley/♂||9 days||No deficit in spatial delay alternation task (short delays)|| ||(Stefani and Moghaddam, 2002)|
|5 days (2 mg·kg−1)||Rat/Wistar/♂||30 min||Deficit in attention, cognitive flexibility and speed of processing||Partially attenuated by chronic CLZ, but not QTP||(Amitai et al., 2007; Amitai and Markou, 2009b)|
As with the neurodevelopmental models, hyperlocomotion is frequently used as an index thought to have translational relevance to positive symptoms. Chronic PCP regimes (including 4–10 days, either repeated or intermittent) do not cause spontaneous hyperactivity, but result in locomotor sensitization to a subsequent challenge dose of PCP (Scalzo and Holson, 1992; Xu and Domino, 1994; Johnson et al., 1998; Hanania et al., 1999; Abekawa et al., 2002; Clark et al., 2002; Fletcher et al., 2005; Tenn et al., 2005; McLean et al., 2009). This mirrors the clinic where ‘positive symptoms’ in normal patients are seen while PCP is on-board (Luby et al., 1959). Sensitization to PCP is attenuated by both typical and atypical antipsychotics, such as haloperidol and clozapine, respectively (Phillips et al., 2001), providing predictive validity to the modelling of positive symptoms. Unlike amphetamine, PCP induces changes reminiscent of not only positive, but also negative symptoms seen in patients with schizophrenia (Jentsch and Roth, 1999). Similarly in the rat, chronic PCP (3–21 days) reduces social interaction (thought to reflect social withdrawal; a negative symptom) (Sams-Dodd, 1996), which is also reversed by both acute haloperidol and clozapine injection (Sams-Dodd, 1998). Additionally, 14 day chronic PCP reduces social behaviour in both rats (Lee et al., 2005) and mice (Qiao et al., 2001), the deficit in mice being reversed by clozapine, but not haloperidol. In contrast, Jenkins et al. (2008) reported no overall decrease in social interaction, but an increase in non-contact behaviour in rats following subchronic PCP. These discrepancies may be due to variations in the dosing regimen or the time after administration that social interaction was recorded. Another negative symptom exhibited by schizophrenic patients is dysfunctional reward processing or anhedonia. Interestingly, a patient with schizophrenia typically shows a normal response to an immediate pleasurable stimuli, but they cannot maintain hedonic value, which results in loss of anticipatory pleasure, sometimes referred to as the ‘anhedonia paradox’ (Pizzagalli, 2010). In rodents, subchronic PCP fails to cause any significant difference in sucrose intake, commonly used to evaluate change in reward (Jenkins et al., 2010), and thus thought to relate to anhedonia seen in schizophrenia.
Although acute NMDA antagonist injection impairs PPI in rodents (Mansbach and Geyer, 1989), the deficit induced by chronic PCP is not sustained, such that the PPI impairment diminishes within days of PCP cessation (Ehrhardt et al., 1999; Martinez et al., 1999; Schwabe et al., 2005; Tenn et al., 2005; Egerton et al., 2008; Tunstall et al., 2009). This recovery may explain some of the discrepancies in cognition tasks observed following PCP administration in rodents. Chronic PCP usually produces cognitive impairment in both rats and mice, irrespective of strain (Table 2, but note Li et al., 2003; Fletcher et al., 2005; Brigman et al., 2009). Fletcher et al. (2005) failed to see an impairment in attentional set-shifting with a 5 week (3 mg·kg−1) intermittent dosing regimen that induced locomotor sensitization (Tenn et al., 2005). In contrast, Egerton et al. (2008) observed impaired performance in the extra-dimensional shift of the attentional set-shifting task following a similar chronic intermittent PCP regimen. However, Egerton et al. began with 5 consecutive days of PCP followed by 3 weeks of intermittent dosing, and commenced behavioural testing 3 rather than 7 days after the last dose. This is consistent with the suggestion that cognitive impairment may not be permanent, following an intermittent treatment protocol. It is also possible that the initial 5 consecutive days of PCP injection is crucial to establishing a cognitive deficit, because Egerton et al. showed that 5 days of consecutive dosing alone was sufficient to cause cognitive impairment. It is important for a model to induce changes that are stable over time both because this has face validity to the disease and enables predictive evaluation of drug reversal. In addition, with pharmacological models, a suitable ‘drug-free washout’ ensures results are the consequence of the chronic regime and not due to the presence of the pharmacological effects. However, it appears that some cognitive tasks (such as the five-choice serial reaction time task) may only be impaired shortly after the end of the dosing regimen with the drug on-board, but in these cases, chronic administration may produce a more robust deficit than a single dose of PCP (Amitai et al., 2007). Although schizophrenia affects both males and females, there are notable differences in age of onset and response to antipsychotics, which should be replicated in animal models with good face validity. In rats, gender affects both the pharmacokinetics of PCP (Gartlon et al., 2006) and cognitive ability (Sutcliffe et al., 2007), but the same subchronic PCP dosage regimen impairs reversal learning and novel object recognition equally in male (Jentsch and Taylor, 2001; McKibben et al., 2010) and female (Abdul-Monim et al., 2007; Grayson et al., 2007) rats. As discussed previously, animal tests of cognition have been evaluated to mirror most of the seven cognitive domains thought to be affected in schizophrenia (Hagan and Jones, 2005) and PCP appears to cause deficits in at least five of these (see Table 2 and Neill et al., 2010). Chronic PCP impairs working memory (delayed alternation task) (Jentsch et al., 1997b; Marquis et al., 2007; Seillier and Giuffrida, 2009), attention/vigilance and speed of processing (five-choice serial reaction time task) (Amitai et al., 2007; Amitai and Markou, 2009a), visual learning and memory (object recognition) (Grayson et al., 2007; McKibben et al., 2010; Spano et al., 2010) and reasoning and problem solving (attentional set-shifting, operant reversal learning and maze tasks) (Rodefer et al., 2005; Abdul-Monim et al., 2006; Didriksen et al., 2007; Beraki et al., 2008; Egerton et al., 2008; Pedersen et al., 2009; Idris et al., 2010), but to date we are unaware of any studies evaluating social recognition following chronic PCP. Although a few of these tasks are performed shortly after the last dose of PCP, such that the results are the combination of acute PCP after a chronic regimen, the cognitive deficits in others are seen after delays of 7 days and persist for weeks afterwards (Neill et al., 2010). The persistence of cognitive deficits may have face validity with the disease, but would seem to differ from humans where cognitive deficits appear to reduce after cessation of long-term recreational PCP use (Fauman and Fauman, 1978; Cosgrove and Newell, 1991).
Acute administration of the typical antipsychotic, haloperidol, is unable to reverse deficits in novel object recognition, reversal learning, attentional set shifting and spatial learning induced by chronic PCP (see Table 2 for references). In contrast, many of the PCP-induced cognitive deficits appear to be reversed by several atypical antipsychotics. However, acute clozapine failed to reverse a subchronic PCP-induced impairment in episodic memory (Le Cozannet et al., 2010), and only sertindole, but not risperidone, restored performance in the extra-dimensional shift of the attentional set-shifting task (Goetghebeur and Dias, 2009). Few studies have evaluated the effect of repeated or chronic antipsychotic drug treatment on cognitive impairment in PCP models. Repeated risperidone, commenced after subchronic PCP, reversed the impairment in reversal learning (McLean et al., 2010b), but when given concurrently, it failed to attenuate a deficit in novel object recognition (McKibben et al., 2010). Chronic quetiapine did not improve performance in the five-choice serial reaction time task (Amitai and Markou, 2009b), whereas chronic clozapine partially attenuated the impairment (Amitai et al., 2007). In mice, both repeated quetiapine and aripiprazole restored performance in the novel object recognition task (Nagai et al., 2009; Tanibuchi et al., 2009).
The reversal of PCP-induced cognitive impairments produced by atypical antipsychotics is in marked contrast with clinical evidence, suggesting that these drugs have a relatively small, if any, beneficial cognitive effect and little difference in effectiveness compared with typical antipsychotic drugs (Keefe et al., 2007). This raises questions about the predictive validity of the PCP models and the ability to screen out false positives. A recent study has shown that co-administration of the selective 5-HT2A receptor inverse agonists, primavanserin and volinanserin, with ineffective doses of atypical antipsychotics, reversed the subchronic PCP-induced deficit in novel object recognition (Snigdha et al., 2010). While the move to testing adjunct therapy is encouraging, use of atypical antipsychotics may confound these experiments, providing false positives. In this case, their effect alongside typical antipsychotics, such as haloperidol, may have more relevance to the clinical treatment of cognitive deficits.
Chronic PCP induces several neurochemical changes that correlate well with those thought to occur in schizophrenia. For instance, the mesolimbic dopamine system in the rat is hyper-responsive to amphetamine and mild stress following chronic PCP (Jentsch et al., 1998). Microdialysis data show that both basal and stress-induced PFc dopamine levels are reduced in rats chronically treated with PCP (Jentsch et al., 1997b; 1998), consistent with the suggestion of decreased PFc dopamine in schizophrenia patients (Akil et al., 1999). Similarly, chronic PCP (10 mg·kg−1·day−1 for 14 days) reduces basal PFc glutamate release in freely moving rats (Fattorini et al., 2008) and mice, and increases PFc glutamate–aspartate transporter (GLAST) levels in the latter, consistent with cortical glutamatergic hypofunction (Murai et al., 2007). Decreased synaptic spines on frontal cortex neurones (Flores et al., 2007; although see Hajszan et al., 2006) and a reduced number of cortical and hippocampal parvalbumin-immunoreactive neurones are observed following subchronic PCP (Reynolds et al., 2004; Abdul-Monim et al., 2007; Jenkins et al., 2008; 2010; McKibben et al., 2010), mirroring deficits seen in schizophrenia. Interestingly, rats treated chronically with MK-801 show a similar reduction in the number of parvalbumin-containing neurones in the dentate gyrus and CA1 region of the hippocampus, but no change in the PFc (Braun et al., 2007), which supports the preferential use of PCP in pharmacological models. In the chronic intermittent PCP model, reduced basal glucose utilization indicative of hypometabolism occurs in the PFc, reticular nucleus of the thalamus and auditory cortex (Cochran et al., 2003). Furthermore, there is decreased thalamic and PFc parvalbumin mRNA, which is reversed by chronic clozapine, while haloperidol only reversed the effect in the thalamus (Cochran et al., 2003). Interestingly, clozapine reversed the hypometabolism observed in the auditory cortex, but neither drug reversed it in the PFc. The inability of either clozapine or haloperidol to reverse the PCP-induced prefrontal hypometabolism may reflect the inability to restore cognitive deficits in patients (Cochran et al., 2003). Chronic intermittent PCP reduces N-acetylaspartate (NAA) and N-acetylaspartylglutamate (NAAG) levels in the temporal cortex, and increases NAAG in the hippocampus, which is thought to reflect neuronal dysfunction and closely resembles post-mortem changes seen in schizophrenia (Reynolds et al., 2005). Reductions in mRNA of the GABA-synthesizing enzymes GAD67 and GAD65, and the pre-synaptic transporter GAT-1 also occur along with increases in GABAA subunits in the cerebellum, following chronic intermittent PCP, findings also akin to those seen in schizophrenia patients (Bullock et al., 2009). Multiple changes in receptor expression have been reported following chronic PCP including: decreased dopamine D1 expression in the medial and lateral striatum (without changes in D2 or D4 receptors), increased 5-HT1A expression in medial–prefrontal and dorsolateral frontal cortices, and altered GABA expression in the Fc, hippocampus and striatum (Choi et al., 2009; Beninger et al., 2010). Long-term decreases in NMDA receptor binding occur in many areas, including the hippocampus, nAcc, caudate putamen, thalamus and cortex, although this pattern of binding was considerably different from that seen immediately after the cessation of dosing, emphasizing the importance of the washout period (Newell et al., 2007).
One of the advantages of chronic PCP models over others is the ability to translate findings to primates. Again, chronic PCP is thought to be a better model than acute PCP in part due to the absence of impaired motor function and motivation (Jentsch and Roth, 1999). PCP twice a day for 14 days in monkeys induced a deficit in a PFc-dependent object retrieval task, which was reversed by acute clozapine (Jentsch et al., 1997a). Additionally, a reduction in PFc parvalbumin-containing neurones also occurs in primates (Morrow et al., 2007). Over a 7 month period, PCP significantly reduced the frequency and duration of primate social behaviour, mirroring the negative symptoms seen in schizophrenia patients (Mao et al., 2008). Interestingly, the ‘negative symptoms’ produced following 56 days of osmotic minipump infusion of PCP seen by Linn et al. (2007) were attenuated by concurrent glycine administration, a class of treatment that has had some benefits as an adjunctive therapy on cognitive and negative symptoms in clinical trials. Thus, chronic PCP models do appear to have some translational relevance across rodents, non-human primates and humans.
One criticism of chronic PCP models is that the intervention is given to adult rats, which does not have construct validity to the proposed neurodevelopmental origin of schizophrenia. The neonatal PCP model of schizophrenia attempts to address this issue. Typically, rat pups receive 10 mg·kg−1 PCP on PND 7, 9 and 11. There is mounting evidence that neonatal PCP administration produces enduring behavioural changes in adulthood (Mouri et al., 2007). Corresponding to chronic PCP, locomotor sensitization to PCP challenge has been reported following neonatal PCP, which is reversed by acute administration of the atypical antipsychotics, olanzapine and risperidone (Wang et al., 2001; Anastasio and Johnson, 2008a; Boctor and Ferguson, 2010). Deficits in PPI have been reported by some groups to be attenuated by acute olanzapine and risperidone (Wang et al., 2001; Takahashi et al., 2006; Anastasio and Johnson, 2008a), but not seen following neonatal PCP in other studies (Rasmussen et al., 2007; Boctor and Ferguson, 2009). It should be noted that when PCP was given only once on PND 7, neither deficits in PPI nor locomotor sensitization were reported, emphasizing the importance of the repeated dosing regimen (Anastasio and Johnson, 2008a).
Enduring cognitive deficits have also been reported in the social recognition task (Depoortere et al., 2005; Harich et al., 2007), performance in the Morris water maze (Sircar, 2003; Andersen and Pouzet, 2004), acquisition of a delayed spatial alternation task (Wiley et al., 2003), disrupted performance in a continuous spatial alternation task (Boctor and Ferguson, 2010) and impaired flexibility in a set-shifting task (Stefani and Moghaddam, 2005) with the latter being reversed by acute sertindole (Broberg et al., 2009). That performance in the Morris water maze was improved by chronic d-serine treatment suggests a hypoglutamatergic state occurs in the neonatal PCP model (Andersen and Pouzet, 2004). In contrast, other groups giving repeated neonatal PCP have found no enduring effect on cognition nor any sensitization to the lomotor activity response. Several independent research groups have also shown long-lasting behavioural changes, including increased spontaneous (Harris et al., 2003) (but see Stefani and Moghaddam, 2005; Kawabe et al., 2007) and methamphetamine-induced (Uehara et al., 2010) locomotion, attentuated PPI (provided sufficient treatment length is used) (Uehara et al., 2009; 2010) and deficits in cognition, such as non-matching to position (Kawabe and Miyamoto, 2008) and radial arm maze (spatial working memory) learning and attentional set shifting (Stefani and Moghaddam, 2005) following neonatal administration of MK-801 (typically 0.13–0.4 mg·kg−1 s.c. or i.p., PND 7–10 or 20), which are not seen when the drug is given to adult rats (Kawabe et al., 2007). However, neonatal MK-801 (Kawabe et al., 2007; Uehara et al., 2009; 2010) and PCP (Boctor and Ferguson, 2010) cause a significant decrease in body weight across development, a feature not seen in schizophrenia.
The finding that pro-apoptotic genes are up-regulated and anti-apoptotic genes are down-regulated on PND 12 (Wang et al., 2001; Liu et al., 2010) supports the suggestion that the changes following neonatal NMDA receptor antagonists are the result of neurotoxicity preferentially in the frontal cortex (Wang and Johnson, 2005). This neurotoxicity can be prevented by enhancing NMDA receptor function (Lei et al., 2009). Alterations in glutamate function have been reported following neonatal PCP with increased levels of NMDA NR1, NR2A and NR2B subunits in the Fc (Wang et al., 2001; Anastasio and Johnson, 2008a,b), and increased NMDA receptors in the Fc and hippocampus (Sircar, 2000). Morphological changes in PCP rats including decreased hippocampal volume and neuronal number, and decreased synaptophysin mRNA all support the suggestion of synaptic dysfunction (Wiseman Harris et al., 2003). Furthermore, neonatal PCP also produces a sustained elevation in hippocampal and entorhinal BDNF in 8-week-old rats (Takahashi et al., 2006) similar to clinical observations in the corticolimbic system of patients with chronic schizophrenia (Takahashi et al., 2000). While most studies have occurred in rats, one notable study in mice observed hyperlocomotion, a deficit in spatial working memory and decreased social interaction, the latter being reversed by clozapine (Nakatani-Pawlak et al., 2009). Additionally, the behaviour was associated with decreases in parvalbumin-immunoreactive neurones in the Fc and hippocampus, similar to those seen in chronic PCP-treated rats and schizophrenic patients.