Enduring cognitive deficits and cortical dopamine dysfunction in monkeys after long-term administration of phencyclidine

Subchronic phencyclidine administration alters central vasopressin receptor binding and social interaction in the rat

Arginine vasopressin (AVP) is a peptide involved in social behaviors in rodents. To investigate the mechanism underlying the deficits in social behavior induced by blockade of N-methyl-D-aspartate (NMDA) receptors, this study examined the effect of noncompetitive NMDA antagonists on AVP receptor binding and social interaction in the rat. Subchronic phencyclidine (PCP) administration (2 mg/kg/day, 14 days, i.p.) significantly reduced the density of V1a receptor binding sites, labeled by an [125I]-Linear AVP antagonist, in several brain regions. Subchronic treatment with PCP or MK-801 (0.13 mg/kg/day, 14 days, i.p.) impaired social interactions in rats, as has been previously reported. These results suggest that NMDA antagonists have modulatory effects on the central vasopressinergic system and social interaction.

Molecular mechanisms of tolerance to and withdrawal of GABA(A) receptor modulators

Here, we summarize recent data pertaining to the effects of GABA(A) receptor modulators on the receptor gene expression in order to elucidate the molecular mechanisms behind tolerance and dependence induced by these drugs. Drug selectivity and intrinsic activity seems to be important to evidence at the molecular level the GABA(A) receptor tolerance. On the contrary, we suggested that all drug tested are equally potentially prone to induce dependence. Our results demonstrate that long-lasting exposure of GABA(A) receptors to endogenous steroids, benzodiazepines and ethanol, as well as their withdrawal, induce marked effects on receptor structure and function. These results suggest the possible synergic action between endogenous steroids and these drugs in modulating the functional activity of specific neuronal populations. We report here that endogenous steroids may play a crucial role in the action of ethanol on dopaminergic neurons.

Regulation of NMDA receptors by dopamine D4 signaling in prefrontal cortex

Increasing evidence has suggested that the interaction between dopaminergic and glutamatergic systems in prefrontal cortex (PFC) plays an important role in normal mental functions and neuropsychiatric disorders. In this study, we examined the regulation of NMDA-type glutamate receptors by the PFC dopamine D4 receptor (one of the principal targets of antipsychotic drugs). Application of the D4 receptor agonist PD168077 caused a reversible decrease of the NMDA receptor (NMDAR)-mediated current in acutely isolated and cultured PFC pyramidal neurons, an effect that was blocked by selective D4 receptor antagonists. Furthermore, application of PD168077 produced a potent reduction of the amplitude (but not paired-pulse ratio) of evoked NMDAR EPSCs in PFC slices. The D4 modulation of NMDA receptors in PFC involved the inhibition of protein kinase A, activation of protein phosphatase 1 and the ensuing inhibition of active Ca2+-calmodulin-dependent kinase II (CaMKII). Moreover, PD168077 reduced the surface expression of NMDARs and triggered the internalization of NMDARs in a manner dependent on CaMKII activity. These results identify a mechanistic link between D4 and NMDA receptors in PFC pyramidal neurons, suggesting that D4 receptors may play an important role in modulating synaptic plasticity and thus cognitive and emotional processes in PFC circuits.

Electrophysiological Interactions between Striatal Glutamatergic and Dopaminergic Systems

Glutamatergic and dopaminergic systems play a primary role in frontal-subcortical circuits involved in motor and cognitive functions. Considerable evidence has emerged indicating that the complex interaction between these neurotransmitter systems within the dorsal striatum and nucleus accumbens is critically involved in the gating of information flow in these highly integrative brain regions. As a result, disruptions of the interaction between glutamate and dopamine has been proposed as a pathological basis for a number of disorders, including the pathophysiology of schizophrenia. In this chapter, we discuss recent studies that have significantly advanced our understanding of the reciprocal interactions between glutamatergic and dopaminergic systems within the striatal complex in the normal brain and in pathological states.

Alpha-adrenoceptor modulation hypothesis of antipsychotic atypicality

Although all currently used antipsychotic drugs act as dopamine (DA) D2 receptor antagonists, clozapine, the prototype for atypical antipsychotics, shows superior efficacy, especially regarding negative and cognitive symptoms, in spite of a significantly reduced central D2 receptor occupancy compared with typical (conventional) antipsychotic drugs. Clozapine, as well as several other atypicals, displays significant affinities also for several other neurotransmitter receptors, including other dopaminergic receptors, alpha-adrenergic receptors and different serotonergic and cholinergic receptors, which in several ways may contribute to the clinical effectiveness of the drugs. Preclinical and clinical results suggest a dysregulated mesocorticolimbic DA system in schizophrenia, with an impaired prefrontal DA projection, which may relate to negative and cognitive symptoms, concomitant with an overactive or overreactive striatal DA projection, with bearing on psychotic (positive) symptomatology. Available data suggest that blockage of alpha1-adrenoceptors by antipsychotics may contribute to suppress positive symptoms, especially in acute schizophrenia, whereas alpha2-adrenoceptor blockage, a prominent effect of clozapine and, to some extent, risperidone but not other antipsychotics, may rather be involved in relief of negative and cognitive symptoms. Whereas alpha1-adrenoceptor blockage may act by suppressing, at the presynaptic level, striatal hyperdopaminergia, alpha2-adrenoceptor blockage may act by augmenting and improving prefrontal dopaminergic functioning. Thus, the prominent alpha1- and alpha2-adrenoceptor blocking effects of clozapine may generally serve to stabilize dysregulated central dopaminergic systems in schizophrenia, allowing for improved efficacy in spite of a reduced central D2 receptor occupancy compared with typical antipsychotic drugs.

Peeling the onion: NMDA dysfunction as a unifying model in schizophrenia

N-methyl-d-aspartate receptor (NMDAR) dysfunction plays a crucial role in schizophrenia, leading to impairments in cognitive coordination. NMDAR agonists (e.g., glycine) ameliorate negative and cognitive symptoms, consistent with NMDAR models. However, not all types of cognitive coordination use NMDAR. Further, not all aspects of cognitive coordination are impaired in schizophrenia, suggesting the need for specificity in applying the cognitive coordination construct.

High-frequency synchronisation in schizophrenia: Too much or too little?

Phillips & Silverstein's focus on schizophrenia as a failure of “cognitive coordination” is welcome. They note that a simple hypothesis of reduced Gamma synchronisation subserving impaired coordination does not fully account for recent observations. We suggest that schizophrenia reflects a dynamic compensation to a core deficit of coordination, expressed either as hyper- or hyposynchronisation, with neurotransmitter systems and arousal as modulatory mechanisms.

No blind schizophrenics: Are NMDA-receptor dynamics involved?

Numerous searches have failed to identify a single co-occurrence of total blindness and schizophrenia. Evidence that blindness causes loss of certain NMDA-receptor functions is balanced by reports of compensatory gains. Connections between visual and anterior cingulate NMDA-receptor systems may help to explain how blindness could protect against schizophrenia.

Setting domain boundaries for convergence of biological and psychological perspectives on cognitive coordination in schizophrenia

The claim that the disorganized subtype of schizophrenia results from glutamate hypofunction is enhanced by consideration of current subtypology of schizophrenia, symptom definition, interdependence of neurotransmitters, and the nature of the data needed to support the hypothesis. Careful specification clarifies the clinical reality of disorganization as a feature of schizophrenia and increases the utility of the subtype.

Schizophrenia: Putting context in context

Although context-processing deficits may be core features of schizophrenia, context remains a poorly defined concept. To test Phillips & Silverstein's model, we need to operationalize context more precisely. We offer several useful ways of framing context and discuss enhancing or facilitating schizophrenic patients' performance under different contextual situations. Furthermore, creativity may be a byproduct of cognitive uncoordination.

Phenomenology, context, and self-experience in schizophrenia

Impairments in cognitive coordination in schizophrenia are supported by phenomenological data that suggest deficits in the processing of visual context. Although the target article is sympathetic to such a phenomenological perspective, we argue that the relevance of phenomenological data for a wider understanding of consciousness in schizophrenia is not sufficiently addressed by the authors.

Guarding against over-inclusive notions of “context”: Psycholinguistic and electrophysiological studies of specific context functions in schizophrenia

Phillips & Silverstein offer an exciting synthesis of ongoing efforts to link the clinical and cognitive manifestations of schizophrenia with cellular accounts of its pathophysiology. We applaud their efforts but wonder whether the highly inclusive notion of “context” adequately captures some important details regarding schizophrenia and NMDA/glutamate function that are suggested by work on language processing and cognitive electrophysiology.

Mechanisms of disrupted language comprehension in schizophrenia

Mechanisms that contribute to perceptual processing dysfunction in schizophrenia were examined by Phillips & Silverstein, and formulated as involving disruptions in both local and higher-level coordination of signals. We agree that dysfunction in the coordination of cognitive functions (disconnection) is also indicated for many of the linguistic processing deficits documented for schizophrenia. We suggest, however, that it may be necessary to add a timing mechanism to the theoretical account.

Theory of mind in schizophrenia: Damaged module or deficit in cognitive coordination?

Schizophrenics exhibit a deficit in theory of mind (ToM), but an intact theory of biology (ToB). One explanation is that ToM relies on an independent module that is selectively damaged. Phillips & Silverstein's analyses suggest an alternative: ToM requires the type of coordination that is impaired in schizophrenia, whereas ToB is spared because this type of coordination is not involved.

Cognitive coordination and its neurobiological bases: A new continent to explore

The additional arguments and evidence supplied by the commentaries strengthen the hypothesis that underactivity of NMDA receptors produces impaired cognitive coordination in schizophrenia. This encourages the hope that though the distance from molecules to mind is great, it can nevertheless be traversed. We therefore predict that in this decade or the next molecular psychology will be seen to be as fundamental to our understanding of mind as molecular biology is to our understanding of life.

Context rules

It is proposed that cortical activity is normally coordinated across synaptically connected areas and that this coordination supports cognitive coherence relations. This view is consistent with the NMDA- hypoactivity hypothesis of the target article in regarding disorganization symptoms in schizophrenia as arising from disruption of normal interareal coordination. This disruption may produce abnormal contextual effects in the cortex that lead to anomalous cognitive coherence relations.

The ketamine model for schizophrenia

This commentary compares clinical aspects of ketamine with the amphetamine model of schizophrenia. Hallucinations and loss of insight, associated with amphetamine, seem more schizophrenia-like. Flat affect encountered with ketamine is closer to the clinical presentation in schizophrenia. We argue that flat affect is not a sign of schizophrenia, but rather, arisk factorfor chronic schizophrenia.

Cortical connectivity in high-frequency beta-rhythm in schizophrenics with positive and negative symptoms

In chronic schizophrenic patients with both positive and negative symptoms (see Table 1), interhemispheric connections at the high frequency beta2-rhythm are absent during cognitive tasks, in contrast to normal controls, who have many interhemispheric connections at this frequency in the same situation. Connectivity is a fundamental brain feature, evidently greatly promoted by the NMDA system. It is a more reliable measure of brain function than the spectral power of this rhythm.

Schizophrenia-like syndrome inducing agent phencyclidine failed to impair memory for temporal order in rats

Although subchronic phencyclidine (PCP) administration is recognized as a probative method to model schizophrenia-like symptoms in animals, only a few sets of data support the hypothesis of a cognitive prefrontal cortex (PFc) dysfunction in PCP-treated monkeys and rodents. Two experiments were here conducted to further test the integrity of prefrontal function in two versions of a memory for temporal order (MTO) task administered to rats. Original versions of this task elaborated by Kesner repeatedly yielded moderate to severe performance deficits in PFc lesioned rats. MTO assessment in an eight-arm radial maze consisted in a recency discrimination between two arms previously explored in the context of sequential forced choices. In Experiment 1, 16 naive Long-Evans rats were pre-trained on a variable version of the MTO task involving randomly re-mixed sequences until they reached a group criterion. Then, rats were treated daily for 21 days with PCP (10mg/kg) or saline vehicle and were tested on the same task approximately 20 h after an injection. The performance of the groups did not differ. In Experiment 2, 16 naive Long-Evans rats untrained prior to treatment received 27 daily injections of either PCP (10mg/kg) or saline vehicle and were tested, 20 h after each injection, on a constant version of the MTO task. This time, a fixed set of four sequences of successive arm entries was repeated within each daily session as well as across days. Again, prolonged PCP exposure failed to impair discrimination of temporal order despite the stability of sequential information over time. These negative results are not consistent with long-lasting hypofrontality, a major landmark of human schizophrenia, in the PCP rat model.

Persistent and anatomically selective reduction in prefrontal cortical dopamine metabolism after repeated, intermittent cannabinoid administration to rats

Long-term abuse of delta-9-tetrahydrocannabinol (THC), the major psychoactive constituent of marijuana, produces behavioral and metabolic signs of frontal cortical dysfunction in humans; these effects persist even after short-term abstinence. Based on a preliminary finding that repeated administration of THC to rats reduces basal frontal cortical dopamine turnover (Jentsch et al. [1998] Neurosci Lett 246:169-172), we further investigated the effects of repeated administrations of THC or WIN 55,212-2 (WIN), a synthetic cannabinoid receptor agonist, on dopamine turnover in the prefrontal cortex, striatum, and nucleus accumbens. THC or WIN (twice daily for 7 or 14 days) caused a persistent and selective reduction in medial prefrontal cortical dopamine turnover; no significant alterations of dopamine metabolism were observed in the nucleus accumbens or striatum. Importantly, these dopaminergic deficits in the prefrontal cortex were observed after a drug-free period of up to 14 days. Thus, the cognitive dysfunction produced by heavy, long-term cannabis use may be subserved, in part, by drug-induced alterations in frontal cortical dopamine turnover.

Withdrawal from chronic phencyclidine treatment induces long-lasting depression in brain reward function

Phencyclidine (PCP) is a drug of abuse that has rewarding and dysphoric effects in humans. The complex actions of PCP, and PCP withdrawal in particular, on brain reward function remain unclear. The purpose of the present study was to characterize the effects of withdrawal from acute and chronic PCP treatment on brain reward function in rats. A brain stimulation reward procedure was used to evaluate the effects of acute PCP injection (0, 5, or 10 mg/kg) or chronic PCP treatment (0, 10, 15, or 20 mg/kg/day for 14 days delivered via subcutaneous osmotic minipumps) on brain reward function. Withdrawal from acute administration of 5 and 10 mg/kg PCP produced a decrease in brain reward function as indicated by a sustained elevation in brain reward thresholds. When administered chronically, 10, 15, or 20 mg/kg/day PCP induced a progressive dose-dependent potentiation of brain stimulation reward, while cessation of the treatment resulted in significant elevations in reward thresholds reflecting diminished reward. Specifically, withdrawal from 15 or 20 mg/kg/day PCP induced a depression in brain reward function that lasted for the entire month of observation. These results indicate that prolonged continuous administration of high PCP doses facilitates brain stimulation reward, while withdrawal from acute high PCP doses or chronic PCP treatment results in a protracted depression of brain reward function that may be analogous to the dysphoric and anhedonic symptoms observed in PCP dependence, depression, and schizophrenia.

Effect of repeated administration of phencyclidine on spatial performance in an eight-arm radial maze with delay in rats and mice

Phencyclidine (PCP) is an N-methyl-D-aspartate (NMDA) glutamate receptor channel noncompetitive antagonist that produces some of the symptoms of schizophrenia, including delusions, hallucinations, and negative symptoms as well as cognitive impairment. Thus, administration of PCP to rodents and nonhuman primates has been suggested to provide a potential animal model for schizophrenia. There have been some reports that 7-14 days of PCP administration can bring about enduring impairments in working memory in rodents but not all studies have been consistent in this regard. The present study determined whether repeated PCP administration impaired spatial performance in rats or mice trained to make minimal errors in an eight-arm radial maze task with a delay. Male Sprague-Dawley rats and C57BL/6J mice received 14 daily injection of vehicle or PCP (10 mg/kg, s.c.) followed by a withdrawal period of 1 week. The number of arm reentry errors and the distance traveled to complete the task were not significantly different between PCP-treated and vehicle-treated rats on 2, 8, and 14 days of PCP administration or 8 days following withdrawal of PCP. Mice treated with PCP for up to 2 weeks also had no significant differences in the number of arm reentry errors, travel distances, the numbers of visits to different arms during the first eight choices, or latencies to take all eight pellets compared to the vehicle-treated group. Thus, the present study failed to demonstrate that repeated administration of PCP to rats or mice produces enduring memory impairment. Factors potentially contributing to the discrepancies between various studies are discussed.

Schizophrenia: from phenomenology to neurobiology

Schizophrenia is a common and debilitating illness, characterized by chronic psychotic symptoms and psychosocial impairment that exact considerable human and economic costs. The literature in electronic databases as well as citations and major articles are reviewed with respect to the phenomenology, pathology, treatment, genetics and neurobiology of schizophrenia. Although studied extensively from a clinical, psychological, biological and genetic perspective, our expanding knowledge of schizophrenia provides only an incomplete understanding of this complex disorder. Recent advances in neuroscience have allowed the confirmation or refutation of earlier findings in schizophrenia, and permit useful comparisons between the different levels of organization from which the illness has been studied. Schizophrenia is defined as a clinical syndrome that may include a collection of diseases that share a common presentation. Genetic factors are the most important in the etiology of the disease, with unknown environmental factors potentially modulating the expression of symptoms. Schizophrenia is a complex genetic disorder in which many genes may be implicated, with the possibility of gene-gene interactions and a diversity of genetic causes in different families or populations. A neurodevelopmental rather than degenerative process has received more empirical support as a general explanation of the pathophysiology, although simple dichotomies are not particularly helpful in such a complicated disease. Structural brain changes are present in vivo and post-mortem, with both histopathological and imaging studies in overall agreement that the temporal and frontal lobes of the cerebral cortex are the most affected. Functional imaging, neuropsychological testing and clinical observation are also generally consistent in demonstrating deficits in cognitive ability that correlate with abnormalities in the areas of the brain with structural abnormalities. The dopamine and other neurotransmitter systems are certainly involved in the treatment or modulation of psychotic symptoms. These broad findings represent the distillation of a large body of disparate data, but firm and specific findings are sparse, and much about schizophrenia remains unknown.

Postnatal phencyclidine-induced deficit in adult water maze performance is associated with N-methyl-D-aspartate receptor upregulation

The N-methyl-D-aspartate (NMDA) receptor plays an important role in developmental plasticity. Earlier, we have shown that blocking the NMDA receptor with the non-competitive antagonist phencyclidine (PCP), during a brief postnatal period, disrupts the water maze performance in young juvenile rats (starting at 25 days of age). We now show the long-term effects of postnatal phencyclidine exposure on spatial learning and memory. Male and female rats were exposed to PCP (1 and 5mg/kg) or saline, from postnatal days 5-15, and their performance in the Morris water maze (MWM) was tested both as adolescents (starting on postnatal day (PD) 35) and as adults (starting on postnatal day 60). Separate groups of adult male and female postnatal PCP-treated and saline-treated rats were sacrificed and saturation [3H]MK-801 binding experiments were carried out in their hippocampi and frontal cortices; hippocampus and frontal cortex have high densities of NMDA receptors and both regions are important in spatial learning and memory. Postnatal PCP administration disrupted the water maze performance both in adolescent and adult rats of both sexes. Adult male and female rats treated postnatally with PCP had increased maximal [3H]MK-801 binding in the hippocampus and frontal cortex compared to same-sex saline-treated controls. Taken together, repeated postnatal PCP (RPP) administration impaired the acquisition of spatial learning in adolescent and adult male and female rats, and this cognitive deficit was associated with increased [3H]MK-801 labeled NMDA receptor in the hippocampus and frontal cortex. These findings are consistent with the hypothesis that PCP treatment during the postnatal period produces deficits in the water maze performance by disrupting the developing glutamatergic system.

Where the rubber meets the road: The importance of implementation

Phillips & Silverstein argue that a range of cognitive disturbances in schizophrenia result from a deficit in cognitive coordination attributable to NMDA receptor dysfunction. We suggest that the viability of this hypothesis would be further supported by explicit implementation in a computational framework that can produce quantitative estimates of the behavior of both healthy individuals and individuals with schizophrenia.

Context, connection, and coordination: The need to switch

Context, connection, and coordination (CCC) describe well where the problems that apply to thought-disordered patients with schizophrenia lie. But they may be part of the experience of those with other symptom constellations. Switching is an important mechanism to allow context to be applied appropriately to changing circumstances. In some cases, NMDA-voltage modulations may be central, but gain and shift are also functions that monoaminergic systems express in CCC.

Synchronous dynamics for cognitive coordination: But how?

Although interesting, the hypotheses proposed by Phillips & Silverstein lack unifying structure both in specific mechanisms and in cited evidence. They provide little to support the notion that low-level sensory processing and high-level cognitive coordination share dynamic grouping by synchrony as a common processing mechanism. We suggest that more realistic large-scale modeling at multiple levels is needed to address these issues.

A wide-spectrum coordination model of schizophrenia

The target article presents a model for schizophrenia extending four levels of abstraction: molecules, cells, cognition, and syndrome. An important notion in the model is that of coordination, applicable to both the level of cells and of cognition. The molecular level provides an “implementation” of the coordination at the cellular level, which in turn underlies the coordination at the cognitive level, giving rise to the clinical symptoms.

Linking brain to mind in normal behavior and schizophrenia

To understand schizophrenia, a linking hypothesis is needed that shows how brain mechanisms lead to behavioral functions in normals, and also how breakdowns in these mechanisms lead to behavioral symptoms of schizophrenia. Such a linking hypothesis is now available that complements the discussion offered by Phillips & Silverstein (P&S).

Spatial integration in perception and cognition: An empirical approach to the pathophysiology of schizophrenia

Evidence for a dysfunction in cognitive coordination in schizophrenia is emerging, but it is not specific enough to prove (or disprove) this long-standing hypothesis. Many aspects of the external world are spatially mapped in the brain. A comprehensive internal representation relies on integration of information across space. Focus on spatial integration in the perceptual and cognitive processes will generate empirical data that shed light on the pathophysiology of schizophrenia.

Inferring contextual field interactions from scalp EEG

This commentary highlights methods for using scalp EEG to make inferences about contextual field interactions, which, in view of the target article, may be specially relevant to the study of schizophrenia. Although scalp EEG has limited spatial resolution, prior knowledge combined with experimental manipulations may be used to strengthen inferences about underlying brain processes. Both spatial and temporal context are discussed within the framework of nonlinear interactions. Finally, results from a visual contour integration EEG pilot study are summarized in view of a hypothesis that relates receptive field and contextual field processing to evoked and induced activity, respectively.

Reconciling schizophrenic deficits in top-down and bottom-up processes: Not yet

This commentary challenges the authors to use their computational modeling techniques to support one of their central claims: that schizophrenic deficits in bottom-up (Gestalt-type tasks) and top-down (cognitive control tasks) context processing tasks arise from the same dysfunction. Further clarification about the limits of cognitive coordination would also strengthen the hypothesis.

Cognitive coordination deficits: A necessary but not sufficient factor in the development of schizophrenia

The Phillips & Silverstein model of NMDA-mediated coordination deficits provides a useful heuristic for the study of schizophrenic cognition. However, the model does not specifically account for the development of schizophrenia-spectrum disorders. The P&S model is compared to Meehl's seminal model of schizotaxia, schizotypy, and schizophrenia, as well as the model of schizophrenic cognitive dysfunction posited by McCarley and colleagues.

NMDA-receptor hypofunction versus excessive synaptic elimination as models of schizophrenia

We propose that the primary cause of schizophrenia is a pathological extension of synaptic pruning involving local connectivity that unfolds ordinarily during adolescence. Computer simulations suggest that this pathology provides reasonable accounts of a range of symptoms in schizophrenia, and is consistent with recent postmortem and genetic studies. NMDA-receptors play a regulatory role in maintaining and/or eliminating cortical synapses, and therefore may play a pathophysiological role.

Is sensory gating a form of cognitive coordination?

Neurophysiological investigations of the past two decades have consistently demonstrated a deficit in sensory gating associated with schizophrenia. Phillips & Silverstein interpret this impairment as being consistent with cognitive coordination dysfunction. However, the physiological mechanisms that underlie sensory gating have not been shown to involve gamma-band oscillations or NMDA-receptors, both of which are critical neural elements in the cognitive coordination model.

Why do schizophrenic patients hallucinate?

Phillips & Silverstein argue that schizophrenia is a result of a deficit of the contextual coordination of neuronal responses. The authors propose that NMDA-receptors control these modulatory effects. However, hallucinations, which are among the principle symptoms of schizophrenia, imply a flaw in the interactions between neurons that is more fundamental than just a general weakness of contextual modulation.

Schizophrenic cognition: Taken out of context?

This commentary addresses: (a) the problems of definition which have been prominent in the use of the term context in schizophrenia research; (b) potentially useful distinctions and links with other theories of schizophrenic cognition; and (c) possible pathways to schizophrenic symptoms. It is suggested that at least two major aspects of the operation of context may be distinguished and that both may be impaired in schizophrenia.

Induction of metabolic hypofunction and neurochemical deficits after chronic intermittent exposure to phencyclidine: differential modulation by antipsychotic drugs

Numerous human imaging studies have revealed an absolute or relative metabolic hypofunction within the prefrontal cortex, thalamus and temporal lobes of schizophrenic patients. The former deficit correlates with cognitive deficits and negative symptoms, whereas the latter correlates with positive symptomologies. There is also general consensus that schizophrenia is associated with decreased parvalbumin expression in the prefrontal cortex. Since the drug phencyclidine can induce a psychosis resembling schizophrenia in humans, we have examined whether repeated phencyclidine (PCP) treatment to rats could produce similar metabolic and neurochemical deficits to those occurring in schizophrenia and whether these deficits could be modulated by antipsychotic drugs. We demonstrate here that chronic intermittent exposure to PCP (2.58 mg kg(-1) i.p.) elicits a metabolic hypofunction, as demonstrated by reductions in the rates of glucose utilization, within the prefrontal cortex, reticular nucleus of thalamus and auditory system, key structures displaying similar changes in schizophrenia. Moreover, chronic PCP treatment according to this regime also decreases parvalbumin mRNA expression in the rat prefrontal cortex and reticular nucleus of the thalamus. Chronic coadministration of haloperidol (1 mg kg(-1) day(-1)) or clozapine (20 mg kg(-1) day(-1)) with PCP did not modulate PCP-induced reductions in metabolic activity in the rat prefrontal cortex, but reversed deficits in the structures of the auditory system. Clozapine, but not haloperidol, reversed PCP-induced decreases in parvalbumin expression in prefrontal cortex GABAergic interneurons, whereas both drugs reversed the deficits in the reticular nucleus of the thalamus. These data provide important new information, which strengthen the validity of chronic PCP as a useful animal model of schizophrenia, when administered according to this protocol. Furthermore, we propose that reversal of PCP-induced reductions in parvalbumin expression in the prefrontal cortex may be a potential marker of atypical antipsychotic activity in relation to amelioration of cognitive deficits and negative symptoms of schizophrenia.

NMDA synapses can bias competition between object representations and mediate attentional selection

Phillips & Silverstein emphasize the gain-control properties of NMDA synapses in cognitive coordination. We endorse their view and suggest that NMDA synapses play a crucial role in biased attentional competition and (visual) working memory. Our simulations show that NMDA synapses can control the storage rate of visual objects. We discuss specific predictions of our model about cognitive effects of NMDA-antagonists and schizophrenia.

Combating fuzziness with computational modeling

Phillips & Silverstein's ambitious link between receptor abnormalities and the symptoms of schizophrenia involves a certain amount of fuzziness: No detailed mechanism is suggested through which the proposed abnormality would lead to psychological traits. We propose that detailed simulation of brain regions, using model neural networks, can aid in understanding the relation between biological abnormality and psychological dysfunction in schizophrenia.

Different roles of group I and group II metabotropic glutamate receptors on phencyclidine-induced dopamine release in the rat prefrontal cortex

The dopamine system in the limbic-prefrontal cortex has been assumed to play an important role in the cognitive dysfunction of schizophrenia and phencyclidine (PCP)-induced psychosis. In the present study, the role of metabotropic glutamate (mGlu) receptor subtypes on PCP-induced cortical dopamine release was investigated using the microdialysis technique. Infusion of 50 and 100 microM of non-selective mGlu receptor agonist trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid inhibited PCP-induced dopamine release, while the basal dopamine level was not significantly affected. A similar inhibition of PCP-induced dopamine release was observed with 100 and 500 microM of selective group I mGlu receptor agonist, (+)-3-hydroxy-phenylglycine. On the other hand, infusion of 10 microM of selective group II mGlu receptor agonist, 2-(2, 3-dicarboxycyclopropyl)-glycine, enhanced the PCP-induced dopamine increase. These results suggest that group I and II mGlu receptors exert opposite modulations on the PCP-induced dopamine release.

Blockade of phencyclidine-induced cortical apoptosis and deficits in prepulse inhibition by M40403, a superoxide dismutase mimetic

Repetitive administration of phencyclidine (PCP) in the perinatal period results in cortical apoptosis and a long-lasting deficit in sensorimotor gating. Because these changes are olanzapine-sensitive, we have suggested that the effects of perinatal PCP could be used to model certain aspects of schizophrenia. Studies of PCP and N-methyl-D-aspartate-induced cell death suggested that superoxide could play a role in the pathway leading to death after PCP administration. The purpose of the current study was to determine whether the in vivo administration of M40403, a superoxide dismutase mimetic, could prevent PCP-induced cortical apoptosis and/or deficits in prepulse inhibition. Perinatal rat pups were administered 10 mg/kg PCP on postnatal (PN) days 7, 9, and 11 with or without treatment with 10 mg/kg M40403. Pups were either killed on PN 12 for analysis of various apoptotic markers or they were assessed for prepulse inhibition on PN 24 to 26. Treatment with M40403 2 and 24 h after each PCP treatment prevented PCP-induced increases in two measures of apoptosis in the dorsolateral frontal cortex and in the olfactory cortex. PCP-induced proapoptotic changes in Bax and Bcl-X(L) were also prevented by M40403 treatment. This regimen did not prevent the deficit in prepulse inhibition caused by PCP treatment, but when the treatment regimen was extended through PN 23, M40403 completely prevented the PCP-induced deficit in prepulse inhibition. These data suggest that perinatal PCP treatment leads to long-lasting changes in the pathway(s), leading to cell death and behavioral deficits, and that the superoxide radical plays a critical role in the underlying mechanism.

Progesterone enhances ethanol-induced modulation of mesocortical dopamine neurons: antagonism by finasteride

The effect of endogenous 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-TH PROG) on the modulation of mesocortical dopamine extracellular concentration by ethanol was investigated by microdialysis in rats. Intraperitoneal injection of progesterone (5 mg/kg, once a day for 5 days) increased the cortical content of 3alpha,5alpha-TH PROG and potentiated the biphasic effect of acute intraperitoneal administration of ethanol on dopamine content. A dose of ethanol (0.25 g/kg) that was ineffective in naïve rats induced a 55% increase in dopamine extracellular concentration in rats pretreated with progesterone. This increase was similar to that induced by a higher dose (0.5 g/kg) of ethanol in naïve rats. Administration of ethanol at 0.5 g/kg to progesterone-pretreated rats inhibited dopamine content by an extent similar to that observed with an even higher dose (1 g/kg) in naïve rats. The administration of the 5alpha-reductase inhibitor finasteride (25 mg/kg, subcutaneous), together with progesterone, prevented the effects of the latter, both on the cortical concentration of 3alpha,5alpha-TH PROG and on the modulation by ethanol of dopamine content. These data suggest that 3alpha,5alpha-TH PROG contributes to the action of ethanol on the mesocortical dopaminergic system. They also suggest that physiological fluctuations in the brain concentrations of neuroactive steroids associated with the oestrous cycle, menopause, pregnancy and stress may alter the response of mesocortical dopaminergic neurons to ethanol.

Dopamine D4 receptors modulate GABAergic signaling in pyramidal neurons of prefrontal cortex

Dopaminergic neurotransmission in the prefrontal cortex (PFC) plays an important role in regulating cognitive processes and emotional status. The dopamine D4 receptor, which is highly enriched in the PFC, is one of the principal targets of antipsychotic drugs. To understand the cellular mechanisms and functional implications of D4 receptors, we examined the impact of D4 receptors in PFC pyramidal neurons on GABAergic inhibition, a key element in the regulation of "working memory." Application of the D4 agonist N-(methyl)-4-(2-cyanophenyl)piperazinyl-3-methylbenzamide maleate caused a reversible decrease in postsynaptic GABA(A) receptor currents; this effect was blocked by the D4 antagonist 3-[(4-[4-chlorophenyl]piperazine-1-yl)methyl]-[1H]-pyrrolo[2,3-b]pyridine but not by the D2 antagonist sulpiride, suggesting mediation by D4 receptors. Application of PD168077 also reduced the GABA(A) receptor-mediated miniature IPSC amplitude in PFC pyramidal neurons recorded from slices. The D4 modulation of GABA(A) receptor currents was blocked by protein kinase A (PKA) activation and occluded by PKA inhibition. Inhibiting the catalytic activity of protein phosphatase 1 (PP1) also eliminated the effect of PD168077 on GABA(A) currents. Furthermore, disrupting the association of the PKA/PP1 complex with its scaffold protein Yotiao significantly attenuated the D4 modulation of GABA(A) currents, suggesting that Yotiao-mediated targeting of PKA/PP1 to the vicinity of GABA(A) receptors is required for the dopaminergic signaling. Together, our results show that activation of D4 receptors in PFC pyramidal neurons inhibits GABA(A) channel functions by regulating the PKA/PP1 signaling complex, which could underlie the D4 modulation of PFC neuronal activity and the actions of antipsychotic drugs.

Adaptation to chronic PCP results in hyperfunctional NMDA and hypofunctional GABA(A) synaptic receptors

Schizophrenia is currently thought to be associated with a hypoglutamatergic state that is mimicked by acute phencyclidine (PCP), an antagonist of the N-methyl-D-aspartate (NMDA) receptor subtype. In this study we tested the hypothesis that chronic treatment of rats with this antagonist may be a more appropriate animal model than acute exposure since it could result in adaptive synaptic responses that would model certain aspects of the schizophrenic state in humans. In vitro intracellular electrophysiological recordings employing brain slices from rats treated chronically in vivo with PCP demonstrated that chronic PCP caused a substantial increase in synaptic responses mediated by NMDA receptors without any significant changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-mediated synaptic responses. At the same time, GABA(A) receptor-mediated inhibitory responses were depressed significantly. Pharmacological and paired-pulse facilitation experiments demonstrated that these adaptive responses following chronic PCP administration were not the result of altered glutamate or GABA release. Immunoblot analyses suggest that the hyperfunctional NMDA response is at least partially mediated by an increased synthesis of NR1 and NR2A subunits as well as a change in the subunit stoichiometry of the NMDA receptor. This change in receptor composition was also supported by pharmacological experiments with a subunit selective NMDA antagonist. Our data support a reconsideration of NMDA and GABA(A) receptor responsiveness following a chronic, not acute, exposure to PCP and the adaptations that persist after such a regimen.

Effects of repeated treatment with amphetamine or phencyclidine on working memory in the rat

Repeated exposure to psychomotor stimulants produces long-lasting molecular, cellular and locomotor behavioral changes. Such changes are likely to contribute to the development of drug addiction and psychosis. It is not clear whether these durable changes are accompanied by lasting changes in cognition. We examined the long-term effects of repeated treatment with phencyclidine (PCP) or amphetamine on working memory, using a discrete, paired-trials, delayed-alternation task sensitive to the acute effects of PCP and amphetamine, and to the integrity of the prefrontal cortex. Twice daily treatment with PCP (5.0 mg/kg) or amphetamine (2.5 mg/kg) for 5 days did not produce lasting, significant impairments in alternation performance in comparison to either pre-treatment baseline performance or to the vehicle-treated group. Subsequent challenge doses of PCP (1, 3 and 5 mg/kg) produced alternation deficits in vehicle, PCP, and amphetamine pre-treated groups that were dependent on dose, but not on pre-treatment regimen. However, rats pre-treated with PCP showed a trend towards sensitization in response to PCP challenge. The present data suggest that psychostimulant treatment regimens that are reported to produce long-lasting changes in neural morphology and locomotor behavior may not produce equally durable changes in working memory.

Prazosin inhibits spontaneous locomotor activity in diabetic mice

We examined the effect of prazosin, a selective alpha1-adrenergic receptor antagonist, on the enhanced spontaneous locomotor activity in streptozotocin-induced diabetic mice. Spontaneous locomotor activity in diabetic mice was significantly greater than that in nondiabetic mice. Pretreatment with either intracerebroventricular (5, 10 nmol) or intraperitoneal (0.5, 1.0 mg/kg) injection of prazosin dose-dependently reduced the spontaneous locomotor activity in diabetic mice, but not in nondiabetic mice. Furthermore, the enhanced dopamine turnover ratio in the limbic forebrain in diabetic mice was reduced to the same level as that in nondiabetic mice after the administration of prazosin. Thus, these results suggest that alpha1-adrenergic receptors might play an important role in the enhanced spontaneous locomotor activity in diabetic mice. Furthermore, alpha1-adrenoceptor antagonism might have an inhibitory effect on presynaptic dopaminergic neurotransmission in the limbic forebrain in diabetic mice.