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

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Long-term behavioral and neurodegenerative effects of perinatal phencyclidine administration: implications for schizophrenia

Both acute and chronic administration of N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine and dizocilpine have been proposed to mimic some of the symptoms of schizophrenia. The purposes of the present study were first, to characterize the long-term behavioral and neurodegenerative effects of subchronic administration of phencyclidine to perinatal rats and second, to determine whether pretreatment with olanzapine could attenuate these effects. On postnatal days 7, 9 and 11 rat pups were pretreated with either vehicle or olanzapine prior to administration of either saline or phencyclidine (10 mg/kg). Some pups were killed on postnatal day 12 for biochemical determinations and others were tested on postnatal days 24-28 for prepulse inhibition of acoustic startle, on postnatal day 42 for phencyclidine-induced locomotor activity and between postnatal days 33 and 70 for acquisition of a delayed spatial learning task. Phencyclidine treatment resulted in a substantial increase in fragmented DNA in the frontal and olfactory cortices consistent with neurodegeneration by an apoptotic mechanism. An increase in the NMDA receptor NR1 subunit mRNA was also observed in the cortex. Gel shift assays showed that phencyclidine also increased the nuclear translocation of nuclear factor-kappaB proteins in the prefrontal cortex. In tissue from the frontal cortex, western blot analysis revealed that phencyclidine treatment increased Bax and decreased Bcl-X(L) proteins. Later in development, it was observed that perinatal phencyclidine treatment significantly retarded baseline prepulse inhibition of acoustic startle measured shortly after weaning. In 42-day-old rats, it was found that challenge with 2 mg/kg phencyclidine increased locomotor activity to a significantly greater extent in the rats that had been pretreated with phencyclidine. Similarly, perinatal phencyclidine treatment significantly delayed the acquisition of a delayed spatial alternation task. Each of the aforementioned changes (except for the spatial learning task, which was not tested) was significantly inhibited by olanzapine pretreatment, an antipsychotic drug known to be effective against both positive and negative symptoms of schizophrenia. Further, olanzapine treatment for 12 days following the administration of phencyclidine was also able to reverse the phencyclidine-induced deficit in baseline prepulse inhibition. Together these data suggest that perinatal administration of phencyclidine results in long-term behavioral changes that may be mechanistically related to the apoptotic neurodegeneration observed in the frontal cortex. It is postulated that these deficits may model the hypofrontality observed in schizophrenia and that this model may be helpful in designing appropriate pharmacotherapy.

The difference in Mismatch negativity between the acute and post-acute phase of schizophrenia

In order to investigate the trait and state aspects of Mismatch negativity (MMN) amplitude reduction in schizophrenia, auditory MMNs were measured from 13 schizophrenic patients on two occasions, initially when they showed acute exacerbation and later when their symptoms improved. Patients exhibited reduced mean amplitude of the MMN recorded at Fz. There were no significant changes in the amplitude of MMN at Fz between the acute patients and the post-acute patients, despite significant improvement in symptomatology. However, the acute patients showed a significant attenuation of MMN recorded at both mastoids as compared with the post-acute patients. Although the findings of the MMN at Fz support the overall longitudinal stability of MMN deficits in schizophrenia, the acute phase patients showed a modestly altered MMN activity compared with the post-acute phase patients, suggesting that there is some state-dependent modulation of these deficits.

Effects of phencyclidine (PCP) and MK 801 on the EEGq in the prefrontal cortex of conscious rats; antagonism by clozapine, and antagonists of AMPA-, alpha(1)- and 5-HT(2A)-receptors

1. The electroencephalographic (EEG) effects of the propsychotic agent phencyclidine (PCP), were studied in conscious rats using power spectra (0 - 30 Hz), from the prefrontal cortex or sensorimotor cortex. PCP (0.1 - 3 mg kg(-1) s.c.) caused a marked dose-dependent increase in EEG power in the frontal cortex at 1 - 3 Hz with decreases in power at higher frequencies (9 - 30 Hz). At high doses (3 mg kg(-1) s.c.) the entire spectrum shifted to more positive values, indicating an increase in cortical synchronization. MK 801 (0.05 - 0.1 mg kg(-1) i.p.) caused similar effects but with lesser changes in power. 2. In contrast, the non-competitive AMPA antagonists GYKI 52466 and GYKI 53655 increased EEG power over the whole power spectrum (1 - 10 mg kg(-1) i.p.). The atypical antipsychotic clozapine (0.2 mg kg(-1) s.c.) synchronized the EEG (peak 8 Hz). The 5-HT(2A)-antagonist, M100907, specifically increased EEG power at 2 - 3 Hz at low doses (10 and 50 microg kg(-1) s.c.), whereas at higher doses (0.1 mg kg(-1) s.c.) the profile resembled that of clozapine. 3. Clozapine (0.2 mg kg(-1) s.c. ), GYKI 53655 (5 mg kg(-1) i.p.), prazosin (0.05 and 0.1 mg kg(-1) i.p.), and M100907 (0.01 and 0.05 mg kg(-1) s.c.) antagonized the decrease in power between 5 and 30 Hz caused by PCP (1 mg kg(-1) s.c.), but not the increase in power at 1 - 3 Hz in prefrontal cortex.

Dissociable contributions of the orbitofrontal and lateral prefrontal cortex of the marmoset to performance on a detour reaching task

To gain insight into the nature and neural specificity of the relationship between simple problem solving, inhibitory control and prefrontal cortex, comparison of the effects of excitotoxic lesions of the orbitofrontal and lateral prefrontal cortex were examined on the performance of common marmosets on a detour reaching task. Monkeys were required to inhibit reaching directly for food reward in a transparent box and instead make a detour reach around to the side of the box either having had (i) no prior experience on the task (experiment 1) or (ii) previous experience in reaching around the sides of an opaque box (experiment 2). Whilst monkeys with orbitofrontal lesions had difficulty in inhibiting direct reaches to visible food reward (experiment 1), they could resist this prepotent response tendency following extensive prior experience of detour reaching with an opaque box (experiment 2). In marked contrast, monkeys with lateral prefrontal lesions exhibited no difficulty in inhibiting reaching to visible food reward or acquiring detour reaching per se (experiment 1). However, having been given the opportunity to acquire an efficient detour reaching strategy to hidden food reward these lateral prefrontal lesioned monkeys were impaired at transferring this strategy to the new context in which the food reward was made visible (experiment 2). This double dissociation between the effects of orbitofrontal and lateral prefrontal lesions on detour reaching provides evidence for a clear distinction in the level of control over responding exerted by the orbitofrontal and lateral prefrontal cortex, consistent with hierarchical ordering of response control processes within prefrontal cortex.

The changing roles and targets for animal models of schizophrenia

Unlike disorders of other fields of medicine (eg., diabetes, heart disease), schizophrenia has been only marginally impacted by the study of animal models. This gap reflects the incomplete understanding of the causes and mechanisms of schizophrenia and the resulting lack of defined targets for model development. However, prior attempts at modeling in animals the complex symptoms of schizophrenia have given way to more promising component models. This review will address the evolving field of animal models of schizophrenia with a focus on models of errors in neurotransmission, and of psychophysiological deficits, with a concluding discussion of the present and future promise of genetic-based models. Evolving models based on the long-held conceptualization of schizophrenia as being based on errors in neurotransmission are discussed as regards the integration of newer findings implicating alterations in dopamine, glutamate and neurotensin function in the pathophysiology and pharmacotherapy of schizophrenia. The case for the more recent conceptualization of schizophrenia as a core deficit in information processing and stimulus filtering is discussed. Animal behavioral paradigms that model psychophysiologic constructs of stimulus processing deficits related to schizophrenia include prepulse inhibition (PPI), a model of sensorimotor gating, or latent inhibition (LI), a model of salience learning. These models represent both better supported associations with schizophrenia and more productive targets and are providing important new information regarding the psychopharmacology of schizophrenia. Genetic models of schizophrenia are based on the demonstrated heritability of the disorder and more recent pharmacogenetic findings for antipsychotic medications. Genetic-based animal models use behavioral or molecular genetic techniques to manipulate behaviors related to schizophrenia by altering the frequencies of related genes. The future development of increasingly informative animal models of schizophrenia will be dependent on a more complete understanding of schizophrenia, an integration of findings across animal models and refinements in the criteria used to assess model "validity" that better reflect the changing nature and roles of animal models of schizophrenia.

Effects of benztropine on ketamine-induced behaviors in Cebus monkeys

Ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, causes a schizophrenic-like psychosis in normal volunteers and exacerbates psychotic symptoms in patients with schizophrenia. Recent work has shown that ketamine and other NMDA antagonists affect a range of behaviors in nonhuman primates, particularly those associated with motor and mental function such as attention and perception. Several lines of study also suggest that NMDA antagonists interact with cholinergic mechanisms. The effects of benztropine, an anticholinergic agent, on ketamine-induced behaviors were evaluated in a double-blind randomized test design in 20 Cebus monkeys. Benztropine (0.05, 0.1 and 0.25 mg/kg, i.m.) was injected 1 hour before ketamine (2.5 and 5.0 mg/kg, i.m.) administration. Behaviors scored for 90 minutes after ketamine administration included salivation, dystonia and reactivity to external stimuli. Benztropine almost completely blocked ketamine-induced hypersalivation, and partially ameliorated the dystonia syndrome by 50%, but did not affect ketamine-induced decreased reactivity to external stimuli. These results suggest that cholinergic mechanisms only moderately influence ketamine-induced central nervous system effects of motor dysfunction, and may not play a substantive role in the ketamine-induced deficit of reactivity to external stimuli, which involves a complex interaction of mental functions such as attention and perception, as well as motor behavior.

Stress-level cortisol treatment impairs inhibitory control of behavior in monkeys

Most studies of cortisol-induced cognitive impairments have focused on hippocampal-dependent memory. This study investigates a different aspect of cognition in a randomized placebo-controlled experiment with monkeys that were treated with cortisol according to a protocol that simulates a prolonged stress response. Young adult and older adult monkeys were assigned randomly to placebo or chronic treatment with cortisol in a 2 x 2 factorial design (n = 8 monkeys per condition). Inhibitory control of behavior was assessed with a test shown previously in primates to reflect prefrontal cortical dysfunction. Failure to inhibit a specific goal-directed response was evident more often in older adults. Treatment with cortisol increased this propensity in both older and young adult monkeys. Age-related differences in response inhibition were consistent across blocks of repeated test trials, but the treatment effects were clearly expressed only after prolonged exposure to cortisol. Aspects of performance that did not require inhibition were not altered by age or treatment with cortisol, which concurs with effects on response inhibition rather than nonspecific changes in behavior. These findings lend support to related reports that cortisol-induced disruptions in prefrontal dopamine neurotransmission may contribute to deficits in response inhibition and play a role in cognitive impairments associated with endogenous hypercortisolism in humans.

Role for dopamine in the behavioral functions of the prefrontal corticostriatal system: implications for mental disorders and psychotropic drug action

We have discussed the role of dopamine in modulating the interactions between cortical and striatal regions that are involved in behavioral regulation. The evidence reviewed seems to suggest that dopamine acts, overall, to promote stimulus-induced responding for conditioned or reward-related stimuli by integrative actions at multiple forebrain sites. It is thus not surprising that dopaminergic dysfunction has been implicated in a number of neuropsychiatric disorders that involve abnormal cognitive and affective function. Future studies aimed at pinpointing the precise anatomical sites of action and molecular mechanisms involved in dopaminergic transmission within the corticolimbic circuit are critical for trying to disentangle the cellular mechanisms by which dopamine exerts its actions. Moreover, the afferent control of dopamine neurons from brainstem and forebrain sites need to be fully explored in order to begin to understand what mechanisms are involved in regulating the dopaminergic response to stimuli with incentive value. Finally, the post-synaptic consequences of prolonged and supranormal dopaminergic activation need to be investigated in order to understand what persistent neuroadaptations result from chronic activation of this neuromodulatory system (e.g. in drug addiction). Answers to these sorts of questions will undoubtedly provide important insights into the nature of dopaminergic function in the animal and human brain.

Subchronic treatment with either clozapine, olanzapine or haloperidol produces a hyposensitive response of the rat cortical cells to N-methyl-D-aspartate

Using the technique of intracellular recording in in vitro brain slice preparations, we examined the effects produced by repeated administration of the antipsychotic drugs clozapine, olanzapine and haloperidol, on N-methyl-D-aspartic acid-induced responses in pyramidal cells of the rat medial prefrontal cortex. Rats were anesthetized and decapitated 24h after the conclusion of daily intraperitoneal injection with either clozapine (25mg/kg), olanzapine (1, 5 or 10mg/kg) or haloperidol (0.5mg/kg) for 21 days, and the slices from medial prefrontal cortex were used for electrophysiological recordings. The concentration-response curves for N-methyl-D-aspartic acid to activate cortical cells shifted markedly to the right in rats which received the subchronic antipsychotic drug treatment, compared with those obtained from rats which received repeated injections of vehicle (1ml/kg/day, i.p. for 21 days). In addition, repeated exposure to antipsychotic drugs caused a significant reduction in the ability of these antipsychotic drugs to augment the N-methyl-D-aspartic acid-induced inward current in pyramidal cells of the rat medial prefrontal cortex. Repeated administration of haloperidol, but not clozapine or olanzapine, significantly hyperpolarized the resting membrane potential and increased membrane resistance in pyramidal cells of the medial prefrontal cortex. Moreover, subchronic treatment with haloperidol, but not clozapine or olanzapine, depressed (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-induced responses. The desensitized response of medial prefrontal cortex cells to N-methyl-D-aspartic acid could be the result of a compensatory response to the facilitating action of antipsychotic drugs on N-methyl-D-aspartic acid receptor-mediated transmission. The inhibitory action of haloperidol on (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid responses may also contribute to the rightward shift of the N-methyl-D-aspartic acid concentration-response curve.Thus, the present study suggests that the atypical antipsychotic drugs, clozapine and olanzapine, as well as the typical antipsychotic drug haloperidol strongly modulate glutamatergic transmission after prolonged treatment. This might be an important factor in the mechanisms by which these drugs alleviate symptoms in schizophrenic patients.

Characterization of extracellular dopamine clearance in the medial prefrontal cortex: role of monoamine uptake and monoamine oxidase inhibition

In vitro rotating disk electrode (RDE) voltammetry and in vivo microdialysis were used to characterize dopamine clearance in the rat medial prefrontal cortex (mPFC). RDE studies indicate that inhibition by cocaine, specific inhibitors of the dopamine transporter (DAT) and norepinephrine transporter (NET), and low Na(+) produced a 50-70% decrease in the velocity of dopamine clearance. Addition of the monoamine (MAO) inhibitors, l-deprenyl, clorgyline, pargyline, or in vivo nialamide produced 30-50% inhibition. Combined effects of uptake inhibitors with l-deprenyl on dopamine clearance were additive (up to 99% inhibition), suggesting that at least two mechanisms may contribute to dopamine clearance. Dopamine measured extracellularly 5 min after exogenous dopamine addition to incubation mixtures revealed that most conditions of DAT/NET inhibition did not produce elevated dopamine levels above controls. Inhibition of MAO produced elevated dopamine levels only after long-term, but not short-term, incubation in vitro. Short-term incubation of l-deprenyl combined with DAT and NET uptake inhibitors increased dopamine above control levels, consistent with more than one mechanism of dopamine clearance. Local infusion of pargyline (100 or 300 microm) into the mPFC or striatum via microdialysis produced more pronounced and immediate increases in mPFC dopamine levels compared with striatum. Furthermore, dopamine elevation in the mPFC was not accompanied by a decrease in the dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, as found in the striatum. These findings may have revealed a unique mechanism of mPFC dopamine clearance and therefore contribute to the understanding of multiple behaviors that involve mPFC dopamine transmission, such as schizophrenia, drug abuse, and working memory function.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Inhibition of stress- or anxiogenic-drug-induced increases in dopamine release in the rat prefrontal cortex by long-term treatment with antidepressant drugs

The effects of long-term treatment with imipramine or mirtazapine, two antidepressant drugs with different mechanisms of action, on the response of cortical dopaminergic neurons to foot-shock stress or to the anxiogenic drug FG7142 were evaluated in freely moving rats. As expected, foot shock induced a marked increase (+ 90%) in the extracellular concentration of dopamine in the prefrontal cortex of control rats. Chronic treatment with imipramine or mirtazapine inhibited or prevented, respectively, the effect of foot-shock stress on cortical dopamine output. Whereas acute administration of the anxiogenic drug FG7142 induced a significant increase (+ 60%) in cortical dopamine output in control rats, chronic treatment with imipramine or mirtazapine completely inhibited this effect. In contrast, the administration of a single dose of either antidepressant 40 min before foot shock, had no effect on the response of the cortical dopaminergic innervation to stress. These results show that long-term treatment with imipramine or mirtazapine inhibits the neurochemical changes elicited by stress or an anxiogenic drug with an efficacy similar to that of acute treatment with benzodiazepines. Given that episodes of anxiety or depression are often preceded by stressful events, modulation by antidepressants of the dopaminergic response to stress might be related to the anxiolytic and antidepressant effects of these drugs.

The effects of phencyclidine pretreatment on amphetamine-induced behavior and c-Fos expression in the rat

Previous data demonstrate that a single injection of phencyclidine enhances amphetamine-induced behaviors 24 h later, suggesting that the delayed effects of a single dose of phencyclidine may produce a schizophrenia-like state in animals. These behavioral changes were accompanied by altered patterns of c-Fos induction, suggesting possible neurochemical correlates to the observed behaviors. Because investigations into PCP's ability to model schizophrenia have found that the effects of repeated, or subchronic, PCP administration differ according to the dose and administration paradigm, this study sought to determine whether single and subchronic PCP exposure produce different effects on amphetamine-induced behaviors and c-Fos induction. No differences were observed between these administration paradigms; both single and subchronic PCP exposure enhanced amphetamine-induced c-Fos in the striatum, decreased c-Fos in the prefrontal cortex, and decreased the number of cage-crossings. However, the observation that PCP pretreatment affected c-Fos induction in the same manner observed previously while having an opposite effect on amphetamine-induced behavior suggests that these behavioral and neurochemical effects are dissociated.

The effects of DCG-IV and L-CCG-1 upon phencyclidine (PCP)-induced locomotion and behavioral changes in mice

The behavioral changes of mice induced by acute and repeated i.p. injection of phencyclidine (PCP) were observed by measuring locomotor activity and stereotyped behavior. Then, the effects of metabotropic glutamate receptor (mGluR) agonists, DCG-IV and L-CCG-1, on the above behavioral changes induced by PCP were found. The effects of DCG-IV were very strong and completely depressed the PCP-induced hyperlocomotion. The effects of L-CCG-1 were not so strong. Repeated injection of PCP for 20 days into mice induced lower locomotor activity than that in acutely injected mice. These behavioral changes may be related with the negative symptoms of schizophrenia. In order to examine some molecular mechanisms of PCP-induced behavioral changes, Northern blot analysis of total RNA from prefrontal cortical tissues of mice treated with PCP, DCG-IV, and L-CCG-1 was carried out.

[(3)H]PNU-101958, a D(4) dopamine receptor probe, accumulates in prefrontal cortex and hippocampus of non-human primate brain

The D(4) dopamine receptor has been investigated for its potential role in neuropsychiatric disorders, "novelty-seeking" behaviors, and effects produced by some psychostimulants. An accurate map of D(4) distribution and density in brain is essential to clarify the role of this receptor subtype in normal brain function and in neuropsychiatric disorders. We investigated the autoradiographic distribution of D(4) receptors in non-human primate (Macaca mulatta) brain (N = 3) with the novel D(4) receptor probe [(3)H]PNU-101958. Quantification of [(3)H]PNU-101958 binding sites in 77 brain regions revealed dense levels of D(4) receptors in several cortical areas, especially in prefrontal cortex, uncus, hypothalamic median eminence, hippocampal formation, and distinct thalamic nuclei, but were significantly lower in striatum. The results correspond well with previous reports of brain distribution of D(4) receptors using other radiolabeled probes, and of D(4) mRNA localization (with some exceptions). Overall, this study reveals that [(3)H]PNU-101958 binding sites in non-human primate brain appear to reflect D(4) dopamine receptor distribution. The significance of a dense localization of D(4) receptors in prefrontal cortex and hippocampus, and broad distribution in other brain areas, allows for investigation of the relationship of these receptors to specific neuropsychiatric disorders and effects produced by psychostimulants.

Object retrieval/detour deficits in monkeys produced by prior subchronic phencyclidine administration: evidence for cognitive impulsivity

BACKGROUND

Impulsivity associated with frontal cortical dysfunction appears to be a direct consequence of chronic consumption of drugs of abuse, though few investigations in animals have attempted to directly address this issue. In this study the effects of repeated, intermittent administration of a psychotomimetic drug of abuse, phencyclidine, on the acquisition and performance of a task sensitive to corticostriatal function was examined in nonhuman primates.

METHODS

Monkeys were repeatedly exposed to phencyclidine (0.3 mg/kg) twice daily for 14 days. Acquisition and performance on an object-retrieval detour task was subsequently examined for up to 28 days after drug withdrawal.

RESULTS

Animals treated with phencyclidine exhibited impaired acquisition of the task. The performance of trials requiring inhibitory control (as opposed to solely sensory-guided responding) was specifically impaired by prior phencyclidine administration. Impairments were found to be due to increased perseveration and barrier reaching. As is the case after frontal cortex ablation, the behavioral deficits were particularly evident during acquisition and appeared to be alleviated by prolonged training.

CONCLUSIONS

The current data demonstrate that subchronic administration of phencyclidine can produce deficits in inhibitory response control that are manifest as impulsivity (increased control of behavior by unconditioned, appetitive stimuli). These data suggest that long-term phencyclidine exposure induces frontostriatal-like cognitive impairments and may represent a potential (drug induced) model for the study of prefrontal cortical cognitive and dopaminergic dysfunction.

The delayed effects of DTG and MK-801 on latent inhibition in a conditioned taste-aversion paradigm

The delayed effects of phencyclidine (PCP) have been shown to disrupt latent inhibition (LI) in a conditioned taste-aversion paradigm. In an attempt to understand the mechanism of this disruption, the delayed effects of the selective sigma receptor agonist 1,3-Di(2-tolyl)guanidine (DTG) and the selective NMDA receptor antagonist MK-801 on latent inhibition were assessed in the same paradigm. Water-deprived male rats were allowed access to either water (nonpreexposed; NPE) or 5% sucrose (preexposed; PE) for 30 min on 2 consecutive days. On the third day, animals were allowed access to sucrose and subsequently injected with lithium chloride. On the forth day, animals were allowed access to both sucrose and water. LI was assessed by comparing the percent sucrose consumed in PE and NPE groups on the fourth day. DTG (1.0, 5.0, or 10.0 mg/kg), MK-801 (0.5, 1.0, or 2.0 mg/kg), or vehicle was administered IP 20 h before preexposure (days 1 and 2) and conditioning (day 3). In vehicle-treated groups, PE animals consumed a significantly higher percent sucrose on the test day than NPE animals, indicating the presence of LI. DTG (10.0 mg/kg) and MK-801 (2.0 mg/kg) decreased the percent sucrose consumed by animals in the PE group to the level observed in the NPE group, indicating disrupted LI. However, this dose of MK-801 was found to produce a decrease in percent sucrose consumed in PE animals not treated with lithium chloride, indicating that the decrease observed in the LI paradigm could be due to MK-801-induced decrease in taste preference for sucrose rather than a disruption of LI. Lower doses of MK-801 that did not produce a decrease in taste preference for sucrose did not significantly disrupt LI. None of the doses of DTG tested altered taste preference for sucrose. These data suggest a role for sigma receptors in the previously observed PCP-induced disruption of LI. Published by Elsevier Science Inc., 2000

Blockade of phencyclidine-induced effects by a nitric oxide donor

1. Phencyclidine (PCP) is widely used as an animal model of schizophrenia. The aim of this study was to better understand the role of nitric oxide (NO) in the mechanism of action of PCP and to determine whether positive NO modulators may provide a new approach to the treatment of schizophrenia. 2. The effects of the NO donor, sodium nitroprusside (SNP), were studied in PCP-treated rats. Following drug administration, behavioural changes and the expression of c-fos, a metabolic marker of functional pathways in the brain, were simultaneously monitored. 3. Acute PCP (5 mg kg(-1), i. p.) treatment induced a complex behavioural syndrome, consisting of hyperlocomotion, stereotyped behaviours and ataxia. Treatment with SNP (2 - 6 mg kg(-1), i.p.) by itself produced no effect on any behaviour studied but completely abolished PCP-induced behaviour in a dose- and time-dependent manner. 4. PCP had differential regional effects on c-fos expression in rat brain, suggesting regionally different patterns of neuronal activity. The most prominent immunostaining was observed in the cortical regions. Pre-treatment with SNP blocked PCP-induced c-fos expression at doses similar to those that suppress PCP-induced behavioural effects. 5. These results implicate the NO system in the mechanism of action of PCP. The fact that SNP abolished effects of PCP suggests that drugs targeting the glutamate-NO system may represent a novel approach to the treatment of PCP-induced psychosis and schizophrenia.

Enhancement of NMDA-induced current by the putative NR2B selective antagonist ifenprodil

Ifenprodil has been widely used as an antagonist selective for NMDA receptors containing the NR2B subunit. Evidence suggests, however, that ifenprodil also increases NMDA receptor affinity. Using rat brain slices, we found that ifenprodil enhanced NMDA-induced current in both cortical and subcortical areas examined. To test whether the effect is due to an increase in NMDA receptor affinity, we compared the effect of ifenprodil on currents induced by different concentrations of NMDA. Consistent with the hypothesis, the enhancing effect (percent increase) was relatively constant at low NMDA concentrations. As NMDA concentration increased, however, the effect decreased. To test whether the effect is blocked when NMDA binding sites are saturated with NMDA, high concentrations of NMDA were applied. To partially block Ca(2+) influx and prevent cells from deteriorating, the experiments were performed in the presence of either MK801 or kynurenate, two noncompetitive antagonists. Under such conditions, ifenprodil not only failed to potentiate NMDA currents, but consistently suppressed the current. When the same concentration of NMDA was applied in the presence of the competitive antagonist CGP37849, ifenprodil regained its ability to potentiate NMDA currents. Furthermore, the higher the concentration of CGP37849 the more the NMDA current was potentiated by ifenprodil. These results, combined with previous studies, suggest that the enhancing effect is due to an increase in NMDA receptor affinity and is specific for responses induced by low NMDA concentrations. As NMDA concentration increases, the affinity-enhancing effect decreases. Consequently, the channel-suppressing effect becomes more prominent.

Fetal tissue transplants in animal models of Huntington's disease: the effects on damaged neuronal circuitry and behavioral deficits

Accumulating evidence indicates that grafts of embryonic neurons achieve the anatomical and functional reconstruction of damaged neuronal circuitry. The restorative capacity of grafted embryonic neural tissue is most illustrated by studies with striatal tissue transplantation in animals with striatal lesions. Striatal neurons implanted into the lesioned striatum receive some of the major striatal afferents such as the nigrostriatal dopaminergic inputs and the gluatmatergic afferents from the neocortex and thalamus. The grafted neurons also send efferents to the primary striatal targets, including the globus pallidus (GP, the rodent homologue of the external segment of the globus pallidus) and the entopeduncular nucleus (EP, the rodent homologue of the internal segment of the globus pallidus). These anatomical connections provide the reversal of the lesion-induced alterations in neuronal activities of primary and secondary striatal targets. Furthermore, intrastriatal striatal grafts improve motor and cognitive deficits seen in animals with striatal lesions. Since the grafts affect motor and cognitive behaviors that are critically dependent on the integrity of neuronal circuits of the basal ganglia, the graft-mediated recovery in these behavioral deficits is most likely attributable to the functional reconstruction of the damaged neuronal circuits. The fact that the extent of the behavioral recovery is positively correlated to the amount of grafted neurons surviving in the striatum encourages this view. Based on the animal studies, embryonic striatal tissue grafting could be a viable strategy to alleviate motor and cognitive disorders seen in patients with Huntington's disease where massive degeneration of striatal neurons occurs.

Pharmacological and molecular targets in the search for novel antipsychotics

The recent enthusiasm among clinicians for the so-called 'atypical antipsychotics' has both improved treatment for schizophrenic patients and provided a welcome stimulus for basic research on antipsychotic mechanisms. Even the newer drugs have shortcomings, and research is underway aimed at identifying novel agents with greater efficacy and safety. Much of this effort is directed towards compounds which, in addition to blocking dopamine receptors, also act on other neurotransmitter receptors such as 5-HT2, 5-HT1A and alpha2-adrenergic receptors. However, there is also a large amount of scientific activity seeking to discover and develop selective dopamine receptor subtype antagonists (including compounds which specifically block D3 or D4 receptors) or drugs that specifically target the dopamine autoreceptor. Finally, a number of drug development programmes are searching for non-dopaminergic antipsychotics. Drugs that do not have affinity for dopamine receptors but act through neurotensin, sigma or cannabinoid CB1 receptors or glutamatergic mechanisms are currently being evaluated. If any of these agents prove to have clinical efficacy this may lead to a third generation of antipsychotics. It is likely, however, that the mechanisms of action of such drugs will nevertheless imply the intimate involvement of dopaminergic pathways.

Neuroleptics ameliorate phencyclidine-induced impairments of short-term memory

1. Phencyclidine (PCP), a non-competitive NMDA-receptor antagonist, is able to induce schizophrenia-like symptoms in animals and in humans. It is known that schizophrenic patients have deficits in memory processes. 2. Therefore, it was investigated whether subchronic pulsatile or continuous application of 5.0 mg kg(-1) PCP over 5 days induce short-term memory deficits in holeboard learning and the action of two different neuroleptics on this behavioural test. 3. First, an impairment in the holeboard task was described when the animals were tested 24 h after the last application but not after 15 min or 1 h after the last injection. Secondly, the influence of haloperidol and risperidone on the PCP-induced short-term memory changes was tested. 4. The combined application of PCP and risperidone led to a complete antagonism of the short-term deficits, but the combined treatment with haloperidol was accompanied by a partial abolishment of the PCP-induced deficits. 5. PCP led to an upregulation of the glutamate binding sites in striatum and nucleus accumbens whereas the D(2) binding sites were reduced in striatum. The D(1) binding sites seem to be unchanged. The receptor protein expression of glutamate receptors mGluR1, GluR2, GluR5/7 and NMDAR1 were not modified in response to PCP treatment. 6. The determination of a subpopulation of GABAergic interneurons shows a decrease of the cells within the CA3 of the hippocampal formation. 7. These findings indicate that PCP induced impairments in short term memory can be detected by holeboard learning and may provide an interesting tool for the search of new neuroleptics.

Effects of age, medication, and illness duration on the N-acetyl aspartate signal of the anterior cingulate region in schizophrenia

The authors performed a MRSI study of the anterior cingulate gyrus in 19 schizophrenic patients under stable medication and 16 controls in order to corroborate previous findings of reduced NAA in the anterior cingulate region in schizophrenia. Furthermore, correlations between NAA in the anterior cingulate gyrus and age or illness duration have been determined. A decreased NAA signal was found in the anterior cingulate gyrus of patients compared to controls. Subdividing the patient group into two groups depending on medication revealed that the group of patients receiving a typical neuroleptic medication showed a lower mean NAA in comparison to the group of patients receiving atypical antipsychotic drugs. No significant group differences in the creatine and phosphocreatine signal or the signal from choline-containing compounds were found. The NAA signal significantly correlated with age, and therefore, individual NAA values were corrected for the age effect found in the control group. The age-corrected NAA signal in schizophrenia correlated significantly with the duration of illness. The detected correlations of NAA decrease with age and illness duration are consistent with recent imaging studies where progressing cortical atrophy in schizophrenia was found. Further studies will be needed to corroborate a possible favorable effect of atypical antipsychotics on the NAA signal.

Does MK-801 discrimination constitute an animal model of schizophrenia useful for detecting atypical antipsychotics?

Two groups of female Wistar rats were trained to discriminate two doses (0.075 and 0.0375 mg/kg) of the noncompetitive NMDA antagonist MK-801 (dizocilpine) in a food-rewarded operant FR30 drug discrimination task. The atypical neuroleptic clozapine (2-6 mg/kg) produced only minimal antagonism (max. 32%) of the MK-801 cue at either training dose, and the "antagonist" effects were not clearly dose related. Furthermore, in the 0.075 mg/kg trained animals clozapine at 3 mg/kg failed to shift the MK-801 dose-response curve to the right. The alpha1-adrenoceptor antagonist prazosin (1-8 mg/kg) was also tested for antagonism of the 0.0375 mg/kg MK-801 cue, and again, only partial antagonism was seen (maximum 36%). Recently, it was suggested [4] that as the discriminative stimulus produced by MK-801 (0.075 mg/kg) was fully antagonized by clozapine at 3 mg/kg, but not by the typical neuroleptic haloperidol, this assay may be a useful screen for detecting atypical neuroleptics. It would seem, however, that this is not necessarily the case, and that the MK-801 discriminative cue may not be psychotomimetic. However, as this was a food rewarded rather than an avoidance paradigm that was used in the prior study [4], it may be that the drug discrimination procedure itself is a critical factor, although this hypothesis requires empirical testing.

Enhanced cortical dopamine output and antipsychotic-like effects of raclopride by alpha2 adrenoceptor blockade

Clozapine exerts superior clinical efficacy and markedly enhances cortical dopamine output compared with classical antipsychotic drugs. Here the alpha2 adrenoceptor antagonist idazoxan was administered to rats alone or in combination with the D2/3 dopamine receptor antagonist raclopride. Dopamine efflux in the medial prefrontal cortex and conditioned avoidance responding were analyzed. Idazoxan selectively potentiated the cortical output of dopamine and augmented the suppression of conditioned avoidance responding induced by raclopride. These results challenge basic assumptions underlying the dopamine hypothesis of schizophrenia and provide insight into clozapine's mode of action.

Morphine tolerance and dependence in mice with history of repeated exposures to NMDA receptor channel blockers

Mice were subjected to two successive treatment protocols: first with NMDA receptor channel blockers (14 days, once a day) and second with morphine (5 mg/kg, 8 days, once a day). Treatment with the higher doses of dizocilpine (1 mg/kg), memantine (30 mg/kg), and MRZ 2/576 (30 mg/kg) upon discontinuation revealed only minor behavioral abnormalities attributable to the state of withdrawal. Following repeated administration of low-dose morphine, tolerance to morphine analgesia developed in mice preexposed to dizocilpine (1 mg/kg but not 0.3 mg/kg) but not memantine (10 and 30 mg/kg), MRZ 2/579 (10 and 30 mg/kg), or saline. There were no signs of morphine dependence in any treatment group. Overall, the present study found only minor effects of the subchronic administration of high doses of NMDA receptor channel blockers, suggesting that clinical use of NMDA receptor channel blockers such as memantine will not be accompanied by increased propensity to induction of morphine tolerance and dependence.

The delayed effects of phencyclidine enhance amphetamine-induced behavior and striatal C-Fos expression in the rat

The ability for the delayed effects of phencyclidine to model schizophrenia-like symptomatology was investigated by assessing the effects of phencyclidine pretreatment on amphetamine-induced behavior. Corresponding changes in striatal, nucleus accumbens and anterior cingulate cortex c-Fos induction were also assessed in order to test the hypothesis that alterations in the neurochemistry of these regions accompany phencyclidine-induced changes in amphetamine-induced behaviors. Rats were treated with 15.0 mg/kg phencyclidine or vehicle 24 h prior to behavioral testing following vehicle, 0.5, 2.5 or 5.0 mg/kg amphetamine. Phencyclidine pretreatment significantly increased amphetamine-induced locomotion and rearing in response to 0.5 mg/kg amphetamine. Likewise, phencyclidine pretreatment produced an increase in the number of striatal cells expressing c-Fos following treatment with 0.5 mg/kg amphetamine. Phencyclidine pretreatment did not alter c-Fos induction in the nucleus accumbens, but did decrease the basal number of c-Fos-containing cells in the anterior cingulate cortex. While stereotypy rating revealed that phencyclidine pretreatment enhanced the behavioral response to 5.0 mg/kg amphetamine over time, no other alterations in behavior or c-Fos expression in response to the higher doses of amphetamine were induced by phencyclidine pretreatment. These data demonstrate that the delayed effects of a single dose of phencyclidine alter anterior cingulate cortex neurochemistry, and enhance the behavioral and striatal c-Fos response to a low dose of amphetamine. These findings suggest that the delayed effects of a single dose of phencyclidine may produce a reasonable animal model for schizophrenia.

Phencyclidine in the social interaction test: an animal model of schizophrenia with face and predictive validity

Phencyclidine (PCP) is a hallucinogenic drug that can mimic several aspects of the schizophrenic symptomatology in healthy volunteers. In a series of studies PCP was administered to rats to determine whether it was possible to develop an animal model of the positive and negative symptoms of schizophrenia. The rats were tested in the social interaction test and it was found that PCP dose-dependently induces stereotyped behaviour and social withdrawal, which may correspond to certain aspects of the positive and negative symptoms, respectively. The effects of PCP could be reduced selectively by antipsychotic drug treatment, whereas drugs lacking antipsychotic effects did not alleviate the PCP-induced behaviours. Together these findings indicate that PCP effects in the rat social interaction test may be a model of the positive and negative symptoms of schizophrenia with face and predictive validity and that it may be useful for the evaluation of novel antipsychotic compounds.

Engagement in a non-escape (displacement) behavior elicits a selective and lateralized suppression of frontal cortical dopaminergic utilization in stress

Although the preferential activation of the prefrontal cortical (PFC) dopaminergic system is generally observed in stress, limited exceptions to this have been observed. Certain non-escape behaviors have been demonstrated to attenuate physiological indices of stress (e.g., coping or displacement responses). One well-characterized non-escape behavior observed in stress is chewing, or gnawing, of inedible objects. Engagement in this behavior attenuates stress-related activation of the hypothalamopituitary-adrenal axis, in a variety of species. We examined the degree to which engagement in this non-escape behavior modulates stressor-induced activation of the PFC dopamine (DA) system. Rats and mice were exposed to a brightly lit novel environment (novelty stress) in the presence or absence of inedible objects. Following novelty exposure, various dopaminergic terminal fields were collected and dopamine and its major catabolite, DOPAC, were measured using HPLC with electrochemical detection. DOPAC/DA ratios were calculated as an index of DA utilization. In some cases serotonin (5-HT) and its major catabolite, 5-HIAA, were also measured. In animals that did not chew, novelty exposure elicited significant increases in DOPAC/DA levels within PFC, nucleus accumbens (shell and core subdivisions), and striatum (relative to quiet-controls). DOPAC/DA responses were greater in the right PFC than in the left PFC. Animals that chewed displayed significantly lower DOPAC/DA responses in PFC, but not other dopaminergic terminal fields. This effect of chewing was always observed in the right PFC and less consistently in the left PFC. Chewing did not alter novelty-induced increases in PFC 5-HIAA/5-HT responses. Thus, engagement in this non-escape behavior elicits a neuroanatomically and neurochemically specific attenuation of the PFC DA response in stress. Given the pivotal role of the PFC in certain cognitive and affective processes, behavioral regulation of PFC DA utilization may modulate cognitive and/or affective function in stress.

An integrated view of pathophysiological models of schizophrenia

Pathophysiological processes that underlie the profound neuropsychiatric disturbances in schizophrenia are poorly understood. However, the clinical course of the disease, and a number of clinical and basic science observations, provide direction for formulating pathophysiological models that could be empirically tested. For example, repeated psychostimulant administration to healthy subjects can induce psychotic symptoms, and acute stimulant challenge in schizophrenia patients can precipitate psychosis. Also, NMDA antagonists induce positive, negative, and cognitive schizophrenic-like symptoms in healthy volunteers and precipitate thought disorder and delusions in schizophrenia patients. These human studies provide support for the dopamine and NMDA receptor hypofunction hypotheses of schizophrenia. Well-documented effects of NMDA antagonists on dopamine systems provide a basis to integrate the dopamine and NMDA receptor hypofunction hypotheses. Furthermore, it has become apparent that prominent actions of antipsychotic drugs, especially those with 'atypical' properties, involve antagonism of behavioral, electrophysiological and brain metabolic effects produced by administration of NMDA receptor antagonists. A confluence of clinical and basic science data suggests that an early developmental insult, potentially involving reduced NMDA receptor function, could facilitate sensitization of dopamine systems, leading to the formal onset of schizophrenia in late adolescence and early adulthood. Although clearly speculative, this conceptual model is consistent with existing evidence and suggests lines of future experimental investigation.

A.E. Bennett Research Award. Reversal of phencyclidine-induced effects by glycine and glycine transport inhibitors

BACKGROUND

Phencycline (PCP, "angel dust") and other noncompetitive antagonists of N-methyl-D-aspartate (NMDA)-type glutamatergic neurotransmission induce psychotic effects in humans that closely resemble positive, negative, and cognitive symptoms of schizophrenia. Behavioral effects of PCP in rodents are reversed by glycine (GLY) and other NMDA augmenting agents. In rodents, behavioral effects of PCP are mediated, in part, by secondary dysregulation of subcortical dopaminergic neurotransmission. This study evaluates effects of GLY and GLY transport antagonists on behavioral and neurochemical consequences of PCP administration in rodents.

METHODS

Two separate experiments were performed. In the first, effects of GLY on PCP-induced stimulation of dopaminergic neurotransmission in nucleus accumbens were evaluated using in vivo microdialysis in awake animals. In the second, effects of a series of GLY transport antagonists were evaluated for potency in inhibiting PCP-induced hyperactivity.

RESULTS

In microdialysis studies, GLY significantly inhibited PCP-induced stimulation of subcortical DA release in a dose-dependent fashion. In behavioral studies, the potency of a series of GLY transport antagonists for inhibiting PCP-induced hyperactivity in vivo correlated significantly with their potency in antagonizing GLY transport in vitro.

CONCLUSIONS

These findings suggest, first, that GLY reverses not only the behavioral, but also the neurochemical, effects of PCP in rodents. Second, the findings suggest that GLY transport antagonists may induce similar effects to GLY, and may therefore represent an appropriate site for targeted drug development.

Altered frontal cortical dopaminergic transmission in monkeys after subchronic phencyclidine exposure: involvement in frontostriatal cognitive deficits

Long-term exposure to the psychotomimetic drug phencyclidine produces prefrontal cortical cognitive and dopaminergic dysfunction in rats and monkeys, effects possibly relevant to the frontal cortical impairments of schizophrenia. In the present study, the effects of subchronic phencyclidine administration (0.3 mg/kg twice-daily for 14 days) on monoamine systems in the monkey brain were examined and related to cognitive performance on an object retrieval/detour task, which has been linked with frontostriatal function. Long-term (14 days) administration of phencyclidine resulted in a marked and persistent reduction in dopamine utilization in the frontal cortex. Moreover, the degree of cognitive impairment in phencyclidine-treated monkeys correlated significantly with the magnitude of dopaminergic inhibition within the dorsolateral prefrontal cortex and prelimbic cortex. No specific correlation was measured for dopamine utilization in other cortical regions or for indices of serotonin transmission in any brain region. These data show that repeated exposure to phencyclidine reduces prefrontal cortical dopamine transmission, and this inhibition of dopaminergic function is associated with performance impairments on a task sensitive to frontostriatal cognitive dysfunction. Thus, the cognitive deficits of phencyclidine-treated monkeys, as in schizophrenia, appear to be mediated, in part, by reduced dopaminergic function in specific subregions of the frontal cortex.

Pharmacologic strategies for augmenting cognitive performance in schizophrenia

There is recognition that the cognitive symptoms of schizophrenia have the most substantial impact on illness outcome. Domains of cognition reported to be significantly affected include serial learning, executive function, vigilance, and distractibility, to name a few. Dopamine activity at D1 receptors mediates many cognitive processes subserved by the prefrontal cortex (PFC), particularly working memory. The number of D1 receptors in the PFC is decreased in schizophrenics and is unaffected by chronic administration of typical neuroleptics. Therefore, medications that increase dopamine in the PFC, such as atypical neuroleptics, or that directly activate the D1 receptor may prove useful in the remediation of prefrontal-dependent cognitive deficits in schizophrenia. Decreased levels of cortical norepinephrine (NE) are associated with impaired learning and working memory in animal models, and can be reversed by drugs that restore NE activity. More specifically, alpha-2 adrenergic receptor agonists have been particularly effective in improving delayed response performance in young monkeys with localized 6-hydroxydopamine lesions in the PFC. Furthermore, human postmortem studies have demonstrated decreased NE in the frontal cortex of demented schizophrenic patients. Therefore, alpha-2 receptor agonists hold promise as drugs to improve cognitive performance on tasks dependent upon PFC function in schizophrenics. Finally, the finding that cortical choline acetyl transferase activity correlates with Clinical Dementia Rating scores in schizophrenic patients and that cholinomimetic drugs enhance cognition in healthy subjects suggests that cholinergic drugs may also treat cognitive symptoms in schizophrenia. Two potential types of cholinomimetics for use in schizophrenics are the acetylcholinesterase inhibitors and M1/M4 muscarinic agonists, both of which increase cortical cholinergic activity.

Reduced prefrontal cortical dopamine, but not acetylcholine, release in vivo after repeated, intermittent phencyclidine administration to rats

Subchronic administration of phencyclidine to rats or monkeys produces prefrontal cortical cognitive dysfunction, as well as reduced frontal cortical dopamine utilization. In the current study, the effects of subchronic exposure to phencyclidine on dopamine and acetylcholine release in the prefrontal cortex were assessed, using in vivo microdialysis in conscious rats. Subchronic exposure to phencyclidine (5 mg/kg twice daily for 7 days) reduced both basal extracellular concentrations of dopamine as well as the increase in dopamine release produced by an acute phencyclidine challenge. The increase in dopamine release induced by a high potassium concentration in the perfusate tended to be reduced after subchronic phencyclidine treatment, while basal and evoked acetylcholine release was unaffected. These data demonstrate that altered dopamine turnover in subjects after subchronic exposure to phencyclidine is directly reflective of reduced release, and as such, represents a functionally relevant phenomenon.

Potentiation of D2-dopamine receptor-mediated suppression of zif 268 by non-competitive NMDA receptor antagonists in reserpinized rats

Striatopallidal output neurons, which coexpress D2-dopamine receptors and NMDA receptors, are logically a potential site of interaction between corticostriatal glutamatergic input and dopaminergic systems. Recent hypotheses about the etiology of schizophrenia have implicated both excitatory amino acid and dopamine systems. The present study was designed to examine, in vivo, the interaction between D2-dopamine receptors and NMDA receptors in the regulation of the expression of the early immediate genes (IEGs), zif 268 and jun B, in striatopallidal neurons. We tested whether coadministration of NMDA antagonists interacted with the actions of the D2 agonist, quinpirole, on IEG expression following dopamine depletion with reserpine. When rats were pretreated with the non-competitive NMDA receptor antagonists, MK 801 (1 mg/kg) or PCP (20 mg/kg), together with quinpirole, the quinpirole reversal of reserpine induction of zif 268 mRNA was potentiated in all regions examined. MK 801 alone had no significant effect on reserpine induction of zif 268 mRNA. Pretreatment with the competitive NMDA receptor antagonist, CPP (5 mg/kg), did not significantly alter the dose response of zif 268 mRNA expression to quinpirole in any region. There was no significant effect of MK 801 on jun B mRNA expression, either on the response to quinpirole or when administered alone with reserpine. Our findings provide evidence of an interaction between the NMDA receptor channel system and the D2-dopamine system on a molecular level in striatopallidal neurons carrying output from the basal ganglia.

Effects of repeated nicotine pre-treatment on mesoprefrontal dopaminergic and behavioral responses to acute footshock stress

The effects of acute and repeated nicotine administration on the stress response of rat mesoprefrontal dopaminergic pathways were examined. Rats were given daily injections of nicotine (0.15 or 0.60 mg/kg, s.c., freebase) or saline for 4 days, then challenged with either nicotine or saline. A regimen of inescapable electrical footshocks or no footshocks was then administered. Thirty minutes after final injection, rats were sacrificed, brains removed and dopamine (DA) and its metabolite dihydroxy-O-phenylacetic acid (DOPAC) were extracted from medial prefrontal cortex (mPFC), nucleus accumbens septi (NAS) and dorsolateral striatum and quantified by high performance liquid chromatography with electrochemical detection. Acute administration of low dose nicotine (0.15 mg/kg) produced an increase in DA utilization (increased DOPAC/DA ratio) in mPFC and NAS, but not striatum. High dose nicotine (0.60 mg/kg) produced activation in NAS, but not mPFC or striatum. Repeated low dose nicotine pre-treatment produced tolerance to the effects of nicotine challenge in the mPFC, and reduced its effects in NAS. Footshock stress preferentially increased DA utilization in mPFC and associated footshock stress-induced immobility responses, and these were reduced by low, but not high, dose repeated nicotine pre-treatment. Further, a single dose of the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (MCA) 30 min prior to nicotine challenge dose-dependently blocked the reduction of mesoprefrontal DA stress responsivity and immobility responses produced by repeated nicotine pre-treatment. These results indicate that: (1) there are dose-dependent differential effects of acute and repeated nicotine pre-exposure on regional DA utilization; (2) low, but not high, dose repeated nicotine reduces both the mesoprefrontal DA and behavioral effects of acute footshock stress; and (3) these effects of repeated nicotine may depend on mecamylamine-sensitive nAChR stimulation. These results may have relevance to acute stress and nicotine dependence, particularly in schizophrenic disorders, which have high prevalence rates of co-morbid nicotine dependence, stress-induced symptom exacerbation and prefrontal cortical dysfunction.

Fetal striatal allografts reverse cognitive deficits in a primate model of Huntington disease

Substitutive therapy using fetal striatal grafts in animal models of Huntington disease (HD) have already demonstrated obvious beneficial effects on motor indices. Using a new phenotypic model of HD recently designed in primates, we demonstrate here complete and persistent recovery in a frontal-type cognitive task two to five months after intrastriatal allografting. The striatal allografts also reduce the occurrence of dystonia, a major abnormal movement associated with HD. These results show the capacity of fetal neurons to provide a renewed substrate for both cognitive and motor systems in the lesioned adult brain. They also support the use of neural transplantation as a potential therapy for HD.

Corticolimbic dopamine neurotransmission is temporally dissociated from the cognitive and locomotor effects of phencyclidine

The behavioral syndrome produced by phencyclidine (PCP) and its analog ketamine represents a pharmacological model for some aspects of schizophrenia. Despite the multifaceted properties of these drugs, the main mechanism for their psychotomimetic and cognitive-impairing effects has been thought heretofore to involve the corticolimbic dopamine system. The present study examined the temporal relationship between alterations in corticolimbic dopamine and glutamate neurotransmission and two dopamine-dependent behavioral effects of PCP in the rodent that have relevance to the clinical phenomenology, namely, impairment of working memory, which is used to model the frontal lobe deficits associated with schizophrenia, and hyperlocomotion, which is used as a predictor of the propensity of a drug to elicit or exacerbate psychosis. PCP increased dopamine and glutamate efflux in the prefrontal cortex and nucleus accumbens, as measured by microdialysis. The increase in dopamine in both regions remained elevated well above baseline 2.5 hr after the injection, at which time the experiment was terminated. However, locomotor activity returned to baseline in <2 hr after injection. Furthermore, impaired performance in a discrete trial delayed alternation task, a rodent working memory task, was only evident up to 60 min after PCP injection; animals tested 80 min after injection, when cortical dopamine release was elevated at 300% of baseline, did not exhibit impaired performance. These findings indicate that activation of dopamine neurotransmission is not sufficient to sustain PCP-induced locomotion and impairment of working memory. Thus, effects of PCP, including a glutamatergic hyperstimulation, may be necessary to account for the psychotomimetic and cognitive-impairing effects of this drug.

Repeated exposure to delta 9-tetrahydrocannabinol reduces prefrontal cortical dopamine metabolism in the rat

Long-term abuse of marijuana by humans can induce profound behavioral deficits characterized by cognitive and memory impairments. In particular, deficits on tasks dependent on frontal lobe function have been reported in cannabis abusers. In the current study, we examined whether long-term exposure to delta9-tetrahydrocannabinol, the active ingredient in marijuana, altered the neurochemistry of the frontal cortex in rats. Two weeks administration of delta9-tetrahydrocannabinol reduced dopamine transmission in the medial prefrontal cortex, while dopamine metabolism in striatal regions was unaffected. These data are consistent with earlier findings of dopaminergic regulation of frontal cortical cognition. Thus, cognitive deficits in heavy abusers of cannabis may be subserved by drug-induced alterations in frontal cortical dopamine transmission.

Effects of the experimental diabetes on dopamine D1 receptor-mediated locomotor-enhancing activity in mice

The effects of diabetes on the dopamine-related locomotor-enhancing activities were studied in mice. Although spontaneous locomotor activity in diabetic mice was significantly greater than that in nondiabetic mice, the locomotor-enhancing effects of methamphetamine (4 mg/kg, s.c.), cocaine (20 mg/kg, s.c.) and SKF82958 (1 mg/kg, s.c.), a selective dopamine D1-receptor agonist, in diabetic mice were significantly lower than those in nondiabetic mice. When dopamine level in the whole brain was reduced by pretreatment with 6-hydroxydopamine (6-OHDA), spontaneous locomotor activity was significantly reduced in both nondiabetic and diabetic mice. There was no significant difference in the total spontaneous locomotor activity counts within 3 h between 6-OHDA-treated nondiabetic and 6-OHDA-treated diabetic mice. Furthermore, the locomotor-enhancing effect of SKF82958 in 6-OHDA-treated diabetic mice was also significantly lower than that in 6-OHDA-treated nondiabetic mice. In a binding assay, the Bmax values of [3H]SCH23390 binding to whole-brain membranes of diabetic mice were significantly lower than those in nondiabetic mice. However, there was no significant difference in the Kd values between nondiabetic and diabetic mice. These results suggest that the decreased density of dopamine D1 receptors in diabetic mice may result in hyporesponsiveness to dopamine-related locomotor enhancement.