Extended methamphetamine self-administration enhances reinstatement of drug seeking and impairs novel object recognition in rats

RATIONALE

Methamphetamine is a highly addictive psychostimulant, and chronic methamphetamine users show high rates of relapse. Furthermore, prolonged methamphetamine abuse can lead to psychiatric symptoms and has been associated with various cognitive dysfunctions. However, the impact of self-administered methamphetamine on cognitive dysfunction and relapse has not been concurrently examined in an animal model.

OBJECTIVES

The present study determined the effects of short- vs. long-access contingent methamphetamine on self-administration, extinction responding, reinstatement of methamphetamine seeking, and cognitive performance on an object exploration task.

MATERIALS AND METHODS

Long-Evans rats self-administered methamphetamine i.v. (0.02 mg/infusion) or received saline during daily sessions (1 or 2 h) for 10 days, followed by either maintained short- (1 or 2 h) or long-access (6 h) self-administration for 14 days. Lever responding was extinguished prior to reinstatement, which consisted of presentation of drug-paired cues or a priming injection of methamphetamine (1.0 mg/kg). Animals were also tested on an object exploration task prior to self-administration and at 10-12 days after cessation of self-administration, thus providing a comparison of pre-methamphetamine exposure with post-methamphetamine exposure.

RESULTS

Long-access methamphetamine self-administration resulted in escalation of daily intake. Furthermore, animals in both short- and long-access groups showed robust conditioned-cued and drug-primed reinstatement, with long access resulting in enhanced methamphetamine-primed reinstatement. Methamphetamine self-administration also led to access-dependent impairments on novel object recognition but failed to impair recognition of spatial reconfiguration.

CONCLUSIONS

Extended methamphetamine self-administration enhances drug-primed reinstatement and decreases novel object recognition, indicating that prolonged contingent methamphetamine increases motivation for drug seeking following withdrawal while increasing cognitive deficits.

Relapse to cocaine-seeking increases activity-regulated gene expression differentially in the striatum and cerebral cortex of rats following short or long periods of abstinence

One of the most insidious features of cocaine addiction is a high rate of relapse even after extended periods of abstinence. A wide variety of drug-associated stimuli, including the context in which a drug is taken, can gain incentive motivational properties that trigger drug desire and relapse to drug-seeking. Both animal and clinical studies suggest that extensive cocaine exposure may induce a transition from cortical to striatal control over decision-making as compulsive drug-seeking emerges. Using an animal model of relapse to cocaine-seeking, the present study investigated the expression patterns of three different activity-related genes (c-fos, zif/268, and arc) in cortical and striatal brain regions implicated in compulsive drug-seeking in order to determine the neuroadaptations that occur during context-induced relapse following brief or prolonged abstinence from cocaine self-administration. Re-exposure to the environment previously associated with cocaine self-administration following 22 h or 15 days of abstinence produced a significant increase in zif/268 and arc, but not c-fos mRNA, in the caudate-putamen and nucleus accumbens. With the exception of arc mRNA levels following 15 days of abstinence, all three genes were increased in the anterior cingulate cortex of animals with a cocaine history when they were re-exposed to the operant chamber. Additionally, c-fos, zif/268, and arc expression was differentially affected in the motor and sensory cortices at both timepoints. Together, these results support convergent evidence that drug-seeking induced by a cocaine-paired context changes the activity of corticostriatal circuits.

Relapse to cocaine seeking increases activity-regulated gene expression differentially in the prefrontal cortex of abstinent rats

RATIONALE

Alterations in the activity of the prefrontal and orbitofrontal cortices of cocaine addicts have been linked with re-exposure to cocaine-associated stimuli.

OBJECTIVES

Using an animal model of relapse to cocaine seeking, the present study investigated the expression patterns of four different activity-regulated genes within prefrontal cortical brain regions after 22 h or 15 days of abstinence during context-induced relapse.

MATERIALS AND METHODS

Rats self-administered cocaine or received yoked-saline for 2 h/day for 10 days followed by 22 h or 2 weeks of abstinence when they were re-exposed to the self-administration chamber with or without levers available to press for 1 h. Brains were harvested and sections through the prefrontal cortex were processed for in situ hybridization using radioactive oligonucleotide probes encoding c-fos, zif/268, arc, and bdnf.

RESULTS

Re-exposure to the chamber in which rats previously self-administered cocaine but not saline, regardless of lever availability, increased the expression of all genes in the medial prefrontal and orbitofrontal cortices at both time points with one exception: bdnf mRNA was significantly increased in the medial prefrontal cortex at 22 h only if levers previously associated with cocaine delivery were available to press. Furthermore, re-exposure of rats to the chambers in which they received yoked saline enhanced both zif/268 and arc expression selectively in the orbitofrontal cortex after 15 days of abstinence.

CONCLUSIONS

These results support convergent evidence that cocaine-induced changes in the prefrontal cortex are important in regulating drug seeking following abstinence and may provide additional insight into the molecular mechanisms involved in these processes.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

The neural circuitry underlying reinstatement of heroin-seeking behavior in an animal model of relapse

Reinstatement of extinguished drug-seeking has been utilized in the study of the neural substrates of relapse to drugs of abuse, particularly cocaine. However, limited studies have examined the circuitry that drives the reinstatement of heroin-seeking behavior in the presence of conditioned cues, or by heroin itself. In order to test the hypothesis that the circuitry underlying reinstatement in heroin-experienced animals would show overlapping, yet distinct differences from cocaine-experienced animals, we used transient inhibition of several cortical, striatal, and limbic brain regions during reinstatement of heroin-seeking produced by heroin-paired cues, or by a single priming dose of heroin. Rats lever pressed for i.v. heroin discretely paired with a conditioned stimulus (CS) during daily 3-h sessions for a period of 2 weeks, followed by daily extinction of lever responding. Subsequent reinstatement of heroin-seeking was measured as lever responding in the absence of heroin reinforcement. The first set of reinstatement tests involved response-contingent CS presentations following bilateral intracranial infusion of either a combination of GABA receptor agonists (baclofen-muscimol, B/M) or vehicle (saline) into one of 13 different brain regions. The second set of reinstatement tests involved a single heroin injection (0.25 mg/kg, s.c.) following either B/M or vehicle infusions. Our results showed that vehicle-infused animals reinstated to both CS presentations and a priming injection of heroin, while B/M inactivation of several areas known to be important for the reinstatement of cocaine-seeking also attenuated heroin-seeking in response to CS presentations and/or a priming dose of heroin. However, as predicted, inactivation of areas previously shown to not affect cocaine-seeking significantly attenuated heroin-seeking, supporting the hypothesis that the circuitry underlying the reinstatement of heroin-seeking is more diffusely distributed than that for cocaine.

The role of dorsal vs ventral striatal pathways in cocaine-seeking behavior after prolonged abstinence in rats

RATIONALE

Recent studies have implicated an important role for the dorsal striatum during craving for cocaine and in cocaine-seeking after abstinence in rats.

OBJECTIVES

We compared the effects of pharmacological inactivation of mesencephalic dopamine (DA) cell body regions and dorsal vs ventral striatal terminal fields in an animal model of relapse after chronic cocaine self-administration.

MATERIALS AND METHODS

Rats self-administered cocaine for 2 h/day for ten sessions, followed by 2 weeks of abstinence (i.e., no extinction training). Immediately before being returned to the self-administration chamber, we assessed the effects of gamma-aminobutyric acid agonist inhibition of midbrain DA regions (substantia nigra [SN] and ventral tegmental area [VTA]) and striatum (dorsolateral caudate-putamen, nucleus accumbens core, and nucleus accumbens shell) on relapse to cocaine-seeking in the absence of reinforcement. Further testing examined daily extinction responding subsequent to the initial relapse test.

RESULTS

Inactivation of the dorsal caudate-putamen and midbrain regions attenuated cocaine seeking, while inactivation of the ventral striatum had no such effects. However, subsequent sessions under extinction conditions revealed a rebound in cocaine seeking in animals that had undergone inactivation in all regions except the dorsolateral caudate-putamen.

CONCLUSIONS

The dorsal but not ventral striatum plays a critical role in cocaine seeking immediately after abstinence. These data support the theory that chronic cocaine may shift activity from the ventral to dorsal striatum during drug seeking under certain conditions. While not necessary at the time of relapse, the ventral striatum appears to be involved in processing critical information of the relapse event.

Dopamine D1 or D2 receptor antagonism within the basolateral amygdala differentially alters the acquisition of cocaine-cue associations necessary for cue-induced reinstatement of cocaine-seeking

The basolateral amygdala complex has been implicated in the formation and utilization of cocaine-cue associations in rat models of cue-induced reinstatement to cocaine-seeking behavior. We have previously demonstrated the importance of dopamine inputs to the basolateral amygdala complex in the reinstatement of cocaine-seeking behavior following chronic cocaine self-administration. Here we show that selective blockade of amygdalar dopamine D1 and D2 receptors during acquisition of cocaine-cue associations has distinctive effects on subsequent conditioned-cued cocaine-seeking behavior. Male, Sprague-Dawley rats were first trained to self-administer i.v. cocaine on a fixed ratio 1 schedule for 5 days. Subjects then received bilateral, intra-basolateral amygdala complex infusions of a dopamine D1 receptor antagonist (SCH23390, 0.25-2.0 microg/side; experiment 1), a dopamine D2 receptor antagonist (raclopride, 0.625-5.0 microg/side; experiment 2), or vehicle just prior to a single classical conditioning session, during which a light+tone cue was discretely paired with passive infusions of cocaine in the absence of lever responding. Following five additional days of cocaine self-administration and 7-10 days of extinction training, animals underwent multiple tests for cue-induced reinstatement. SCH23390 (2.0 microg/side), administered at the time of cocaine-cue association only, produced an attenuation of reinstatement to cue-induced cocaine-seeking behavior. In contrast, low doses of raclopride potentiated, while a higher dose of raclopride attenuated cue-induced reinstatement. These results demonstrate unique contributions of D1 vs. D2 receptors in mediating dopamine inputs within the basolateral amygdala complex during the formation of cocaine-stimulus associations that are critical for cue-induced reinstatement.

Muscarinic receptor antagonism in the basolateral amygdala blocks acquisition of cocaine-stimulus association in a model of relapse to cocaine-seeking behavior in rats

Recent evidence has demonstrated a critical role for the basolateral amygdala complex in the reinstatement of extinguished drug-seeking behavior produced by drug-paired cues. In the current study, we utilized a model of the acquisition and expression of cocaine-stimulus associative pairing in order to study the role of cholinergic input to the basolateral amygdala in mediating conditioned-cued reinstatement. Male, Sprague-Dawley rats were first trained daily to self-administer i.v. cocaine on a fixed ratio 1 schedule of reinforcement. The muscarinic acetylcholine receptor antagonist, scopolamine, was directly infused into the basolateral amygdala prior to: a) classically conditioned pairing of a tone+light stimulus with cocaine infusions (acquisition), or b) testing of conditioned-cued reinstatement following a period of withdrawal from cocaine and extinction of cocaine-paired lever responding. Infusion of scopolamine just prior to the classical conditioning trial produced a dose-dependent disruption of cocaine-seeking behavior maintained by cocaine-paired cues during the reinstatement test. In contrast, infusion of scopolamine prior to the reinstatement test had no effect on conditioned-cued reinstatement of cocaine-seeking behavior. These results indicate a crucial role for cholinergic innervation of muscarinic acetylcholine receptors in the basolateral amygdala during the formation, but not the expression, of stimulus-reward associations that mediate cue-induced cocaine-seeking behavior.

Conditioned reinstatement of drug-seeking behavior with a discrete compound stimulus classically conditioned with intravenous cocaine

The present study investigated the ability of a light and tone (LT) compound stimulus paired with cocaine infusions to reinstate cocaine-seeking behavior. Rats were trained to self-administer cocaine in the presence or absence of the LT during daily 3-hr sessions (maintenance). During Maintenance Days 5 and 10, rats underwent classical conditioning, whereby passive cocaine infusions were paired with either short-delayed, random, or no presentations of an LT. After extinction sessions, rats underwent test sessions in which the LT was presented in a noncontingent or response-contingent manner to measure conditioned cocaine-seeking behavior. The results demonstrated that response-contingent LT presentations significantly increased cocaine-seeking behavior and that the LT trained in a classical conditioning format transferred to an operant secondary reinforcer.

Decreased pallidal GABA following reverse microdialysis with clozapine, but not haloperidol

Changes in striatopallidal GABA are believed to play a significant role in the motor side effects produced by antipsychotic drugs (APDs). In the current study, we measured extracellular GABA in the globus pallidus (GP) of rats. GABA release was partially impulse- and Ca2+-dependent, as evidenced by decreased efflux following tetrodotoxin (TTX) or removal of Ca2+. In addition, GABA release was significantly increased by high K+ (100 mM KCl) stimulation. Reverse dialysis of the atypical APD, clozapine (1-100 microM), produced a concentration dependent decrease in extracellular GABA. In contrast, the typical APD, haloperidol (1-100 microM), had no significant effect on GABA levels. These results suggest that clozapine has direct actions within the GP, while the effects of haloperidol are most likely mediated through its effects in the striatum. The clozapine-induced decrease in pallidal GABA may account for its low motor side effect liability.

Conditioned reinstatement of drug-seeking behavior with a discrete compound stimulus classically conditioned with intravenous cocaine

The present study investigated the ability of a light and tone (LT) compound stimulus paired with cocaine infusions to reinstate cocaine-seeking behavior. Rats were trained to self-administer cocaine in the presence or absence of the LT during daily 3-hr sessions (maintenance). During Maintenance Days 5 and 10, rats underwent classical conditioning, whereby passive cocaine infusions were paired with either short-delayed, random, or no presentations of an LT. After extinction sessions, rats underwent test sessions in which the LT was presented in a noncontingent or response-contingent manner to measure conditioned cocaine-seeking behavior. The results demonstrated that response-contingent LT presentations significantly increased cocaine-seeking behavior and that the LT trained in a classical conditioning format transferred to an operant secondary reinforcer.

Differential contributions of the basolateral and central amygdala in the acquisition and expression of conditioned relapse to cocaine-seeking behavior

The amygdala is known to be a critical mediator of emotional learning in aversive and appetitive conditioning. Here we show for the first time that distinct subregions of the amygdala play unique roles in the acquisition and expression of cocaine-seeking behavior maintained by drug-paired cues in a model of relapse. Reversible inactivation of the basolateral amygdala with the sodium channel blocker tetrodotoxin disrupted both the acquisition and expression of the conditioned reinforcing effects maintained by drug-paired stimuli. However, inactivation of the central amygdala disrupted only the expression, but not the acquisition, of the conditioned reinforcing effects of drug-paired stimuli. Our results demonstrate that these nuclei participate as components of an amygdalar circuit to drive cocaine-seeking behavior produced by stimulus-reinforcement associations.

Dopamine, but not glutamate, receptor blockade in the basolateral amygdala attenuates conditioned reward in a rat model of relapse to cocaine-seeking behavior

RATIONALE

Following chronic cocaine self-administration and extinction, lesions of the basolateral amygdala (BLA) will significantly attenuate responding for secondary reward (tone + light previously paired with cocaine), without disrupting lever responding for primary reward. However, the specific neurotransmitters involved in conditioned reinstatement remain to be determined.

OBJECTIVE

In the present study, we examined possible receptor substrates of amygdalar regulation of conditioned reinstatement after chronic cocaine self-administration.

METHODS

Rats were allowed 2 weeks of 3-h daily sessions of cocaine self-administration along a fixed ratio (FR) 1 schedule. After 1 week of daily 3-h extinction sessions in which no programmed consequences occurred, selective antagonists of glutamate or dopamine (DA) receptors were bilaterally infused at single doses into the BLA prior to testing for a cocaine-conditioned reward (tone + light). Following three more days of extinction trials, receptor antagonist effects on reinstatement of cocaine self-administration in the absence of the conditioned stimulus were determined.

RESULTS

Infusion of an NMDA receptor antagonist (AP-5, 1.97 micrograms/side), a kainate/alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist (CNQX, 0.83 microgram/side), or both drugs together had no significant effects on conditioned reward or reinstatement of cocaine self-administration. In contrast, infusion of a DA D1 receptor antagonist (SCH-23390, 2 micrograms/side) or a combination of SCH-23390 and a DA D2/D3 receptor antagonist (raclopride, 5 micrograms/side) significantly reduced responding for conditioned reward, but did not affect cocaine self-administration. Raclopride alone was without effect on either test day.

CONCLUSIONS

These results suggest that conditioned reinstatement of drug-seeking behavior is dependent on amygdalar D1 receptors.

Chronic haloperidol-induced alterations in pallidal GABA and striatal D(1)-mediated dopamine turnover as measured by dual probe microdialysis in rats

Using dual probe microdialysis, assessment of extracellular neurotransmitter levels in the corpus striatum and globus pallidus was performed in ovariectomized and gonadally intact female, Sprague-Dawley rats following chronic (24 weeks) oral haloperidol administration. Vacuous chewing movements, an animal analog of orofacial dyskinesia, were also recorded at several time points during haloperidol administration and throughout the dialysis sampling session. Basal GABA levels were significantly elevated in the globus pallidus of haloperidol-treated rats compared with vehicle animals. Injection of the dopamine D(1) agonist dihydrexidine (3mg/kg, s.c.) decreased striatal dopamine levels in both vehicle and haloperidol-treated rats, with a larger decrease seen in haloperidol-treated rats. Furthermore, dihydrexidine reduced striatal 3,4-dihydroxyphenylacetic acid and homovanillic acid levels only in haloperidol-treated rats. Gonadal status had no effect on any neurochemical measure. Vacuous chewing movements were significantly elevated in haloperidol-treated groups by the sixth week of treatment, with higher counts seen in gonadally intact rats. Vacuous chewing movements were significantly elevated above baseline in all groups following dihydrexidine, with no differential effect of prior haloperidol treatment or gonadal status. These results indicate a tonic increase in pallidal GABA levels and a hypersensitivity of D(1)-mediated striatal dopamine and dopamine metabolite decreases following chronic haloperidol treatment. While not found to be correlated with neurochemical measures, the heightened vacuous chewing movements in gonadally intact vs ovariectomized rats may serve as a model of hormone-mediated differences in neuroleptic-induced oral dyskinesia.

Decreased choline acetyltransferase immunoreactivity in discrete striatal subregions following chronic haloperidol in rats

Neuronal loss within the basal ganglia has been hypothesized to play a role in movement disorders (e.g., tardive dyskinesia) that often occur following chronic neuroleptic treatment. Previous studies in animal models have provided some support to this possibility, but have not assessed regionally specific changes after chronic neuroleptic administration. The present study examined whether counts of neurons containing acetylcholine, described as large aspiny type II neurons, were altered in subregions of the corpus striatum and nucleus accumbens following chronic haloperidol administration in rats. Rats were administered haloperidol decanoate (21 mg/kg, i.m.) or vehicle every third week for 24 weeks. Following 4 weeks of withdrawal from the drug, predefined regions were examined for choline acetyltransferase (ChAT) immunoreactive (ir) cells. Compared to the vehicle group, the haloperidol group showed significant reductions in ChAT-ir cell counts in the ventrolateral striatum, nucleus accumbens core, and nucleus accumbens lateral shell. No significant differences were found in the other regions examined: dorsolateral striatum, dorsomedial striatum, ventromedial striatum, nucleus accumbens medial shell, and horizontal limb of the diagonal band. These findings indicate that there may be regionally specific alterations in ChAT-ir cells following chronic haloperidol treatment, supporting previous hypotheses of striatal cholinergic cell loss resulting from chronic neuroleptic treatment. More importantly, the regions affected (ventrolateral striatum and nucleus accumbens) are critical in the regulation of oral movements, thus suggesting that alterations in cholinergic cell activity, and perhaps actual loss of cholinergic cells in these regions, may be important in the manifestation of late-onset oral dyskinesia.

An evaluation of the role of 5-HT(2) receptor antagonism during subchronic antipsychotic drug administration in rats

A widely postulated mechanism of action for the atypical profile of many novel antipsychotic drugs (APDs) is their relatively high affinity for 5-HT(2) receptors. The present study investigated motor function and striatal dopamine (DA) efflux and metabolism in rats given 21 daily injections of drugs that differed in 5-HT(2) affinity. These drugs included: risperidone (high 5-HT(2A/2C)/high D(2)), clozapine (high 5-HT(2A/2C)/low D(2)), haloperidol (low 5-HT(2A/2C)/high D(2)), haloperidol+ritanserin (selective 5-HT(2A/2C)), or vehicle. Rats injected with haloperidol (0.5 mg/kg) or haloperidol+ritanserin (0.5 mg/kg and 1.0 mg/kg, respectively) showed extreme catalepsy on day 1, but significantly decreased catalepsy when tested again on days 7 and 21. Acute or subchronic risperidone (0.05 or 0.5 mg/kg), clozapine (20 mg/kg), or vehicle did not induce significant catalepsy. Microdialysis performed 24 h after the last injection demonstrated that rats treated with risperidone, clozapine, or vehicle showed similar increases in DA efflux and metabolism following an acute injection of a selective DA D(2/3) antagonist (raclopride, 0.5 mg/kg). DA efflux showed an attenuated response to raclopride in the haloperidol alone group; this effect was less apparent in the haloperidol+ritanserin group. However, both of these groups showed a similar tolerance effect to the raclopride-induced increase in DA metabolites. These results suggest that the profile seen after subchronic risperidone more closely resembles clozapine than haloperidol. While ritanserin reduced the tolerance-like effects of haloperidol on striatal DA efflux, the overall results demonstrate that potent 5-HT(2) blockade alone may not entirely account for the distinctive profile of novel APDs.

Dissociation of primary and secondary reward-relevant limbic nuclei in an animal model of relapse

The neural substrates underlying relapse to drug-seeking behavior after chronic drug abuse may differ from those underlying immediate drug-taking behavior. In a model of relapse to drug-seeking behavior following chronic cocaine self-administration and prolonged extinction, we have previously shown that rats will significantly reinstate lever responding for either primary reward (cocaine) or secondary reward (tone + light stimulus previously paired with cocaine). In the present study, we utilized reversible inactivation of discrete brain nuclei with tetrodotoxin (TTX) in order to examine the neural substrates mediating primary and secondary cocaine reward in rats allowed two weeks of cocaine self-administration. After one week of daily extinction sessions, bilateral inactivation of the basolateral amygdala resulted in significant attenuation of lever pressing for a cocaine-conditioned reward (tone + light). Following three more days of extinction, bilateral TTX inactivation of the basolateral amygdala had no effect on the reinstatement of cocaine self-administration. In contrast, TTX inactivation of the nucleus accumbens produced the exact opposite effects, with significant blockade of primary reward (cocaine alone), but not secondary reward (tone + light). Thus, cocaine-conditioned reward is neuroanatomically dissociated from primary cocaine reward.

The importance of a compound stimulus in conditioned drug-seeking behavior following one week of extinction from self-administered cocaine in rats

Previous studies have demonstrated that conditioned stimuli can increase responding on a drug-associated lever after extinction from drug self-administration. The present study investigated singular stimuli (tone or light) or a compound stimulus (tone + light) for their ability to increase extinguished responding following chronic cocaine self-administration. Rats self-administered cocaine for 2 weeks on a fixed ratio (FR1) schedule of reinforcement, in which lever responding resulted in varied presentation of a tone, light, or tone + light combination. The rats were then exposed to 1 week of daily extinction sessions. Presentation of the tone + light on day 8 of extinction in the absence of cocaine reinforcement resulted in a significant increase in responding, while either stimulus component alone was much weaker or failed to produce any changes from extinction rates of responding. In addition, changing the duration of the single elements of the compound did not affect the magnitude of increased responding to the compound. Following three final extinction sessions, robust lever responding for cocaine infusions on day 12 of extinction was seen across all groups. These findings suggest that compound stimuli may be critical to fully activate drug-seeking behavior in conditions of craving and relapse following prolonged extinction.

Predicting relapse to cocaine-seeking behavior: a multiple regression approach

Recovery of previously extinguished responding to stimuli paired with the administration of drugs of abuse is becoming a widely utilized animal model of relapse to drug-seeking behaviors. While this approach is useful for identifying factors such as conditioned stimuli that are associated with drug-seeking, it has not directly identified behaviors that might predict susceptibility to relapse. In this study, rats were initially screened for locomotor activity in response to a novel environment. Rats were then trained to self-administer cocaine. A stimulus light and tone were paired with each infusion of cocaine. After 14 days of self-administration (maintenance) rats underwent 7 days of extinction trials (extinction phase 1), in which responding yielded neither cocaine nor the presentation of the conditioned stimulus. After extinction phase 1, rats responded for presentations of the compound stimulus in the absence of cocaine (test day 1). Rats then underwent 3 more days of extinction (extinction phase 2). After extinction phase 2, rats were once again allowed to self-administer cocaine (test day 2) and received presentations of the compound stimulus. Hierarchical regression equations, utilizing data from locomotor screening and the average responding during maintenance and extinction phases 1 and 2, were then constructed in order to predict the magnitude of responding on test days 1 and 2. A model utilizing locomotor activity, maintenance responding and extinction phase 1 responding accounted for over 76% of the variance associated with responding on test day 1, with extinction phase 1 as best predictor. A further model indicated that locomotor activity was the best predictor of propensity to self-administer cocaine on test day 2. These regression models provide a novel approach to determining factors that may predict relapse to drug-seeking behaviors.

Risperidone-induced increase of plasma norepinephrine is not correlated with symptom improvement in chronic schizophrenia

BACKGROUND

Previous studies have shown an increase in plasma levels of norepinephrine (NE) after clozapine treatment of schizophrenia. This effect has been suggested to relate to improvement in symptoms.

METHODS

To test whether other novel antipsychotic drugs have such an effect, the present experiment examined schizophrenic symptoms and plasma levels of NE before and after 5 weeks of treatment with risperidone or haloperidol.

RESULTS

Risperidone, but not haloperidol, significantly increased plasma NE; however, there was no correlation of this effect with clinical improvement on any symptom scale.

CONCLUSIONS

This finding suggests that risperidone shares similar properties with clozapine in enhancing peripheral NE, but that these changes in plasma NE may not be a consistent indicator of atypical antipsychotic drug efficacy.

Unique activation of extracellular striato-pallidal neurotransmitters in rats following acute risperidone

The present study investigated the effects of the putative atypical antipsychotic drug (APD), risperidone, on striatal monoamine and pallidal gamma-aminobutyric acid (GABA) function using dual probe in vivo microdialysis. Risperidone (0.03, 0.3, 3 mg/kg) or vehicle was injected (s.c.) into female, Sprague-Dawley rats fitted with dual microdialysis probes in the striatum and the globus pallidus (GP). In the striatum, risperidone increased extracellular levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) at all doses and the serotonin (5-HT) metabolite, 5-hydroxyindoleacetic acid (5-HIAA), at the highest dose. The increase in striatal DA was most pronounced at the lowest dose of risperidone; however, DOPAC showed a dose dependent increase. Risperidone at the medium and high doses significantly reduced extracellular GABA levels in the GP. Simultaneous measurement of limb rigidity during microdialysis showed that risperidone dose-dependently produced significant increases in horizontal bar test catalepsy and fore- and hindlimb paw retraction latencies. The current results suggest novel effects of risperidone on striatal DA release, while the pallidal GABA changes are similar to previous results obtained with the atypical antipsychotic drug, clozapine. Additionally, the behavioral results predict the clinical expression of extrapyramidal motor side effects at high doses. Overall, these results support an atypical profile of risperidone when compared with typical APDs, yet one with unique neurochemical and behavioral properties.

Emergence of oral and locomotor activity in chronic haloperidol-treated rats following cortical N-methyl-D-aspartate stimulation

Neuroleptic-induced orofacial movements in rats have been widely utilized as an animal model of tardive dyskinesia (TD). The present study investigated the role of the oral motor cortex in these movements by applying direct cortical stimulation in rats exposed to chronic haloperidol. Rats received depot i.m. injections of haloperidol decanoate or sesame oil vehicle every 3 weeks (10 rats per group). After 24 weeks of injections and a 3-week withdrawal period, bilateral guide cannulae were implanted into the primary oral motor cortex. After a 1-week recovery, bilateral microinfusions of saline vehicle followed by 1, 3, and 10 mM N-methyl-D-aspartate (NMDA) were given and observations of oral activity, locomotion, rearing, and grooming were recorded. Haloperidol-treated rats displayed a significant emergence of NMDA stimulated oral activity (nondirected oral movements, oral tremor, audible teeth grinding, and directed oral movements). In addition, rearing and locomotion were significantly elevated in these animals. In contrast to haloperidol-treated rats, sesame oil-treated rats showed no significant emergence of any motor activity. These results suggest that chronic haloperidol administration alters primary motor cortex efferents, and that this effect may be a factor in the manifestation of chronic neuroleptic induced motor side effects, such as TD.

Repeated low-level formaldehyde exposure produces cross-sensitization to cocaine: possible relevance to chemical sensitivity in humans

Sensitivity to chemicals in humans has been proposed to be an acquired disorder in which individuals become increasingly sensitive to chemicals in the environment. A possible link between the manifestation of psychiatric symptoms in individuals claiming sensitivity to chemicals was investigated based on a leading hypothesis put forth by Bell and co-workers (1992) to explain the amplification of symptoms after chemical exposure. The hypothesis is that chemical sensitivities may be akin to sensitization observed in rodents after repeated psychostimulants. Repeated exposure to psychostimulants enhances behavioral activity and the underlying neurochemical responses in specific limbic pathways; a similar sensitization of limbic pathways has been proposed to occur in individuals who become sensitive to chemicals. To test this hypothesis, female Sprague-Dawley rats were exposed to either air or formaldehyde (Form) for 1 h/day for 7 days or 20 days (5 days/week x 4 weeks). Two to 4 days after the last exposure, rats were given a cocaine challenge (= early withdrawal) followed by an additional cocaine challenge 4-6 weeks later (= late withdrawal). No differences in cocaine-induced locomotor activity were noted between groups after 7 days of exposure. However, after 20 days of exposure to Form, vertical activity was significantly elevated at both early and late withdrawal times. These studies demonstrate that behavioral sensitization occurs after long-term, but not short-term, low-level exposure to Form, and lends support to the limbic system sensitization hypothesis of sensitivity to chemicals in humans.

Lesions of the basolateral amygdala abolish the ability of drug associated cues to reinstate responding during withdrawal from self-administered cocaine

This study investigated the ability of bilateral excitotoxic lesions of the basolateral amygdala (BLA) to disrupt cocaine self-administration, responding during extinction sessions, and stimulus cued recovery of extinguished responding in rats. BLA and sham lesions following 7 days of 3 h limited access cocaine self-administration sessions (0.33 mg/infusion) on a fixed ratio 1 (FR1) schedule of reinforcement failed to alter cocaine intake and responding on 7 subsequent days of self-administration. This lack of effect suggests that the BLA is not paramount for the maintenance of cocaine's reinforcing effects. In contrast, BLA lesions, but not sham lesions, following 7 to 14 days of cocaine self-administration attenuated responding on a lever associated with cocaine infusions on the first day of extinction sessions and blocked the ability of drug associated stimuli to reinstate extinguished responding following 20 daily extinction sessions. However, lesions of the BLA did not attenuate stimulus cued recovery of responding following 43 days of withdrawal. These results are consistent with the hypothesis that the BLA is important for the conditioned incentive properties of reinforcers, but not primary reinforcement itself.

Cocaine self-administration in ovariectomized rats is predicted by response to novelty, attenuated by 17-beta estradiol, and associated with abnormal vaginal cytology

Parameters of cocaine self-administration behavior and vaginal cytological changes were assessed in ovariectomized rats during and after chronic self-administration. Response to novelty as measured by locomotor activity was found to correlate with cocaine self-administration on both fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement. Chronic 17-beta estradiol (E2) replacement did not affect cocaine self-administration on FR or PR schedules of reinforcement. Acute E2 administration decreased cocaine self-administration on a PR schedule of reinforcement on the day of and on the two days following hormone treatment. Finally, the proliferation of vaginal epithelia following acute E2 was potentiated in rats both during cocaine self-administration and 30 days into withdrawal when compared with non-drug-exposed animals. These results identify response to novelty as a behavioral index predictive of cocaine self-administration liability in female rats and additionally provide evidence of psychostimulant-hormonal interactions highly relevant to female behavior and physiology.

The neurotensin receptor antagonist SR 48692 decreases extracellular striatal GABA in rats

Intracranial microdialysis was utilized to assess the effects of the novel neurotensin antagonist SR 48692 on extracellular gamma-amino-butyric acid (GABA) and glycine in the striatum. Subcutaneous injection of SR 48692 (0.2 mg/kg) significantly decreased extracellular striatal GABA levels, with peak decreases occurring 2-3 h post-injection. Injection of SR 48692 had no significant effect on glycine levels. These data suggest that endogenous neurotensin may modulate striatal GABA levels.

Duration-dependent increase in striatal glutamate following prolonged fluphenazine administration in rats

Chronic neuroleptic administration has been shown to selectively increase striatal extracellular glutamate concentration. In the current study, age-matched female rats were administered chronic oral fluphenazime or no drug via their drinking water for 3 or 32 weeks. Microdialysis probes were inserted into the ventrolateral caudate putamen and the medial nucleus accumbens and dialysis samples were analyzed for glutamate and gamma-aminobutyric acid (GABA) concentrations Glutamate levels were significantly increased only in the ventrolateral caudate putamen after 32 weeks. No significant effects were seen for GABA levels. Neuroleptic-induced enhancement of striatal glutamate levels thus appears to increase with chronic exposure and this increase may relate to late onset motor side effects.

Proposed animal neurosensitization model for multiple chemical sensitivity in studies with formalin

A potentially promising line of animal research relevant to multiple chemical sensitivity (MCS) is that of sensitization in the central nervous system (CNS), particularly limbic pathways in the brain. Sensitization is the progressive and enduring enhancement in behavioral and neurochemical responses that occurs after repeated exposure to psychostimulants or environmental stressors. Since the onset and progression of sensitization has many parallels with that of MCS, it has been proposed that MCS may be initiated through a mechanism similar to the sensitization of CNS components occurring in the rodent. To test this hypothesis, female Sprague-Dawley rats were exposed to formalin vapors (FORM, 11 ppm) or water vapor (control) 1 h/day for 7 days. The next day, a saline injection was given followed by a cocaine injection (15 mg/kg, i.p.) 24 h later, and locomotor activity was monitored. Animals pretreated with repeated FORM inhalation demonstrated a significantly enhanced locomotor response to cocaine compared to controls, an indicator that specific limbic pathways may have been sensitized. At 4 weeks of withdrawal from FORM exposure, a subset of animals remained sensitized to a cocaine challenge. No differences were found between groups after a saline injection. In a second experiment, animals were screened prior to FORM or water exposure for their response to a novel situation, a measure believed to reflect an animal's general responsiveness to stimuli. Rats were divided into high responders (HR) or low responders (LR), based on their locomotion in a novel cage. Results from three behavioral tests demonstrated that HR and LR were differentially affected by exposure to FORM. In a passive avoidance test, HR and LR appeared to be different in their distribution of responses, while HR and LR responses in the FORM group were nearly identical. On the elevated plus maze test of anxiety, HR spent more time on the open arms than LR in both treatment groups, with significant differences between HR and LR in the FORM, but not water, treated group. On a hot plate test to measure nociceptive levels, no differences occurred between HR and LR in the control group, whereas nociception of LR tended toward an increase compared to HR in the FORM-exposed group. Results from the second experiment suggest that the effects of FORM exposure may be obscured by examining behavior in a heterogeneous population (HR and LR). This approach using animal models may help define neural substrates that mediate the amplification of responses of a subpopulation of individuals to chemicals in the environment.

Differential effects of unique profile antipsychotic drugs on extracellular amino acids in the ventral pallidum and globus pallidus of rats

The effects of antipsychotic drugs (APDs) on brain dopamine receptors in the striatum are ultimately expressed through efferent projections which primarily use amino acid transmitters, including gamma-aminobutyric acid and glutamate. The present study examined the effects of APDs on extracellular amino acid levels in the rat ventral pallidum (VP) and globus pallidus (GP), areas that receive projections from distinct striatal subregions. Clozapine, an APD with low motor side effect liability, and metoclopramide, a low-potency APD with high motor side effect liability, were compared with haloperidol, a widely used APD with high motor side effect liability. Drugs were administered subcutaneously and amino acid levels were monitored concurrently in the VP and GP by intracranial microdialysis. High doses of haloperidol and metoclopramide increased and clozapine decreased extracellular gamma-aminobutyric acid levels in the GP but not the VP. Low, but not high, doses of the three drugs tended to increase extracellular glutamate levels in both pallidal regions. Clozapine, but not the other two drugs, decreased extracellular threonine in the GP and glycine and threonine in the VP. Results indicate a correlation between increased gamma-aminobutyric acid levels in the GP and the propensity of the APDs tested to induce motor side effects. The novel effects of clozapine on extracellular glycine and threonine further distinguish this drug as a unique antipsychotic compound.

Subchronic administration of clozapine, but not haloperidol or metoclopramide, decreases dopamine D2 receptor messenger RNA levels in the nucleus accumbens and caudate-putamen in rats

The effects of unique profile antipsychotic drugs on dopamine D2 receptors and D2 receptor messenger RNA were assessed following subchronic administration in rats. Male, Sprague-Dawley rats were administered oral haloperidol, clozapine, metoclopramide or no drug for three weeks via their drinking water. Tissue from the medial nucleus accumbens and dorsolateral caudate-putamen was dissected and analyzed by Northern blot analysis for levels of dopamine D2 receptor messenger RNA and binding assays conducted with [3H]spiperone for dopamine D2 receptors. Haloperidol and metoclopramide, but not clozapine, significantly increased [3H]spiperone in the caudate-putamen, but not the nucleus accumbens. Clozapine significantly decreased D2 messenger RNA levels in the caudate-putamen and the nucleus accumbens, while metoclopramide and haloperidol had no significant effect in either brain region. The finding of decreased D2 receptor messenger RNA levels produced by subchronic clozapine may account for the lack of striatal D2 receptor up-regulation, which was robustly observed after subchronic haloperidol and metoclopramide. Furthermore, since haloperidol and metoclopramide have a high liability for motor side effects, the current results relate favorably to the low motor side effect profile of clozapine.

Chronic haloperidol-induced changes in regional dopamine release and metabolism and neurotensin content in rats

Chronic neuroleptic administration has previously been shown to alter in vivo measures of dopaminergic function and lead to regionally selective increases in neurotensin levels. In the current study, female rats were administered chronic haloperidol for 6 months via subcutaneous silastic implants. After 24 weeks of administration, microdialysis probes were inserted into the lateral caudate putamen and the medial prefrontal cortex. Basal samples were collected prior to infusion of a high K+ concentration (100 mM KCl). Extracellular concentrations of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were assessed using HPLC. Chronic haloperidol-treated rats showed increased basal dopamine metabolite levels in the caudate putamen and an altered response to the effects of high K+ on 3,4-dihydroxyphenylacetic acid; no significant differences were seen with other analytes in the caudate putamen. Although basal concentrations were not different between groups in the prefrontal cortex, haloperidol-treated rats showed a significant attenuation of response to the effects of high K+ infusion on dopamine metabolite concentrations. Radioimmunoassay measurement of tissue neurotensin content showed highly significant elevations of neurotensin concentrations in the caudate putamen and nucleus accumbens, but not in other brain regions analyzed. These results suggest a confluence of altered dopamine and neurotensin function in the caudate putamen which may be related to motor side effects of haloperidol, whereas changes in prefrontal dopamine function are not associated with altered neurotensin levels.

Chronic haloperidol potentiates stimulated glutamate release in caudate putamen, but not prefrontal cortex

Rats were given haloperidol continuously for 6 months via subcutaneous implants. Extracellular concentrations of basal and high K(+)-stimulated GABA and glutamate in the lateral caudate putamen and the medial prefrontal cortex were then assessed using microdialysis. While there were no significant differences in basal extracellular concentrations in either brain region, chronic haloperidol-treated rats showed significantly greater increases in glutamate following stimulation with high K+ in the caudate putamen, but not the prefrontal cortex. This effect was accompanied by an attenuation of K(+)-stimulated GABA overflow in the caudate putamen. These results suggest regionally selective alterations in amino acid transmitter function which may be related to chronic neuroleptic-induced motor side effects.

Tolerance-like attenuation to contingent and noncontingent cocaine-induced elevation of extracellular dopamine in the ventral striatum following 7 days of withdrawal from chronic treatment

Time-dependent changes in mesolimbic dopamine (DA) function are believed to play a role in behavioral sensitization and drug craving experienced during withdrawal from chronic cocaine administration. The present study utilized intravenous (IV) cocaine self-administration coupled with intracranial microdialysis in rats to investigate time dependent changes during withdrawal from chronic cocaine exposure. Following 2 weeks of IV cocaine self-administration, rats were allowed contingent access to cocaine at 1 and 7 days of withdrawal while extracellular levels of DA were measured from the ventral striatum. A second group of animals received yoked, noncontingent cocaine for 2 weeks and were then administered noncontingent cocaine on days 1 and 7 of withdrawal. In addition, a third group of animals received 2 weeks of yoked saline followed by noncontingent cocaine 1 day after withdrawal. There were no significant differences between groups for the overall cocaine dosage or temporal pattern of infusions on days 1 and 7 of withdrawal. Basal extracellular DA concentrations did not differ between any treatment groups at either withdrawal time. Extracellular DA levels were increased throughout the session on both days; however, the increases at day 7 were significantly less than day 1 for both contingent and noncontingent conditions. DA overflow on day 1 did not differ between animals receiving chronic yoked cocaine or saline. These results suggest that tolerance-like attenuation to the DA-elevating effects of cocaine is not apparent early in withdrawal, but does develop by later time points.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of 3-PPP enantiomers on extracellular dopamine concentration in the caudate-putamen and nucleus accumbens of rats

The ability of the enantiomers of 3-PPP (3-(3-hydroxyphenyl)-N-n-propylpiperidine) to modify the extracellular concentration of dopamine (DA) in the caudate-putamen and the nucleus accumbens of awake rats was assessed using intracranial microdialysis. The enantiomers and the racemate were directly infused at 3 incremental concentrations (1-100 microM) or systematically administered by subcutaneous injection (10 mg/kg). Both systemic administration and direct infusion of (-)3-PPP at the highest concentration (100 microM) significantly increased extracellular DA in both brain regions. This increase was also seen in the presence of higher extracellular DA levels following reuptake inhibition by nomifensine (10 microM). In contrast to the effects of (-)3-PPP, systemic administration of both (+)3-PPP and (+/-)3-PPP decreased DA levels. Infusion of (+/-)3-PPP led to slight increases in DA levels in both brain regions at the highest concentration (100 microM), while (+)3-PPP at 100 microM also produced a significant increase in the caudate-putamen, but not in the nucleus accumbens. However, in the presence of nomifensine-induced elevations in extracellular DA, (+)3-PPP produced a significant decrease in DA concentrations in both brain regions. These results support previous findings that the enantiomers of 3-PPP show unique profiles in their in vivo effects on DA terminal functions. Furthermore, these effects are dependent on the mode of 3-PPP administration, the brain region in which DA overflow is measured, and the level of basal extracellular DA.

Chronic haloperidol, but not clozapine, produces altered oral movements and increased extracellular glutamate in rats

Rats administered chronic haloperidol or clozapine in their drinking water for 6 months were monitored for changes in oral movements using a computerized video analysis system. Haloperidol-treated animals exhibited late onset increases in small amplitude oral movements and an increase in the percentage of oral movements in the 1-2 Hz range, accompanied by a decrease in oral movements in the higher frequency range (> 6 Hz) as determined by fast fourier analysis. In contrast, clozapine-treated rats showed a decrease in medium-sized amplitude oral movements, but did not demonstrate significant changes in the distribution of oral movements across frequencies. Extracellular concentrations of gamma-aminobutyric acid (GABA) and glutamate in the ventrolateral striatum were then assessed by intracranial microdialysis during oral drug administration and 3 days after drug withdrawal. Extracellular GABA and glutamate levels were not significantly different between groups during drug administration. However, 3 days after drug withdrawal, there was a significant increase in glutamate in the haloperidol-treated rats. No changes were noted for glutamate levels in clozapine-treated rats or for GABA levels in either group following withdrawal. These results confirm the atypical profile of clozapine in an animal model of tardive dyskinesia and suggest that alterations in striatal glutamatergic function follow typical, but not atypical, antipsychotic drug administration.

Single preexposure to fluphenazine produces persisting behavioral sensitization accompanied by tolerance to fluphenazine-induced striatal dopamine overflow in rats

Single, previous exposure to a neuroleptic has been shown to produce long-lasting changes in various measures of behavior and neurochemistry upon subsequent drug exposure. The present study examined the effects of a single preexposure to fluphenazine (0.3 or 1.0 mg/kg) or vehicle on the effects of subsequent fluphenazine administration 15 or 30 days later. Intracranial microdialysis was used to assess changes in striatal extracellular dopamine concentrations. Animals were tested for catalepsy response on a horizontal bar test while concurrently collecting dialysis samples. Previous fluphenazine exposure produced a profound tolerance to the effects of subsequent fluphenazine at day 15 or day 30 on increasing extracellular dopamine levels. In addition, animals that had received fluphenazine on the first trial showed significant sensitization to the cataleptic effects of fluphenazine at both time points. Pretreatment with vehicle did not result in tolerance to dopamine overflow and there was only minimal evidence of cataleptic sensitization to a subsequent fluphenazine challenge. Although the tolerance to dopamine overflow may only indirectly relate to behavioral sensitization, these results support the hypothesis that significant behavioral and neurochemical alterations persist for prolonged time periods following single neuroleptic exposure.

Assessment of striatal extracellular dopamine and dopamine metabolites by microdialysis in haloperidol-treated rats exhibiting oral dyskinesia

Rats that had received continuous, chronic haloperidol (HAL) for 32 weeks were mentioned for changes in oral movements as determined by Fast Fourier analysis of jaw movements recorded with a computerized video analysis system. Beginning at 24 weeks of administration, HAL-treated animals exhibited a pattern of significant increases in oral movement activity in the 1 to 3 Hz range and decreases in the 5 to 8 Hz range when compared to control animals. The release and metabolism of dopamine (DA) in the striatum of these animals was then assessed using intracranial microdialysis during week 32 of HAL administration and 3 days after withdrawal of HAL. Basal extracellular concentrations of the DA metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid were significantly increased in the HAL-treated rats during continuous HAL administration. Three days after withdrawal from HAL, no difference was seen in basal extracellular concentrations of any of the analytes. No difference in the magnitude of DA release was seen between groups following local application of amphetamine (10 microM) through the dialysis probe during or after chronic HAL administration. These results confirm previous findings that long-term HAL administration produces increased DA turnover during HAL administration, but that this increase does not persist following HAL withdrawal. The increased striatal DA turnover seen during chronic HAL administration may have relevance to the development of late-onset neuroleptic-induced dyskinesia in rats.

Chronic haloperidol does not alter G protein alpha-subunit levels in rats

Female Sprague-Dawley rats were administered continuous haloperidol or no drug for 32 weeks via subcutaneous silastic implants. Three days after drug withdrawal, animals were rapidly decapitated and tissue samples were analyzed for levels of G alpha subunits for Gi and Gs using immunoblotting procedures. No significant differences were seen between groups in the dopaminergic terminal regions of the medial prefrontal cortex, nucleus accumbens, or dorsolateral caudate-putamen. These results suggest that haloperidol administered in a regimen known to produce alterations in several parameters of dopamine function fails to alter the amount of receptor-linked G protein subunits in rat brain.

Comparison of chronic intermittent haloperidol and raclopride effects on striatal dopamine release and synaptic ultrastructure in rats

The effects of chronic intermittent administration (7 months) of two neuroleptics, haloperidol (HAL) and raclopride (RAC), were compared using several different measures. Both drugs were administered weekly by subcutaneous injection at 7.0 mg/kg. Both neuroleptics consistently produced catalepsy throughout the treatment period, although HAL was generally more cataleptogenic than RAC. Assessment of dopamine (DA) release in the caudate putamen (CPu), through the use of in vivo microdialysis, showed that chronic HAL or RAC administration caused a prolonged decrease of DA release in response to a low dose of the DA D2 agonist quinpirole (0.03 mg/kg, sc). Injection of the muscarinic agonist pilocarpine (1.0 mg/kg, IP) did not have any significant within-group effects, although both neuroleptic treatment groups showed decreased DA release when compared to controls. Ultrastructural analysis of the dorsolateral CPu showed that both HAL and RAC treatment resulted in a significant increase in the number of perforated synapses, which contain a discontinuous density along the postsynaptic membrane. These results demonstrate that two different DA D2 receptor antagonists produce a similar effect on DA function and ultrastructural changes within the CPu following chronic, intermittent treatment.

Regional differences in chronic neuroleptic effects on extracellular dopamine activity

The extracellular levels of dopamine (DA) and DA metabolites in the caudate-putamen (CPu) and the nucleus accumbens (NA) of rats following administration of haloperidol (HAL) decanoate and fluphenazine (FLU) decanoate for 8 months were assessed using intracranial microdialysis 1 month after final injection. Both HAL- and FLU-treated animals showed persisting plasma neuroleptic levels at time of sacrifice. Extracellular basal levels of homovanillic acid (HVA) in the CPu were significantly elevated in the FLU-treated animals, while basal levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the CPu were significantly elevated in the HAL-treated animals. Basal levels of DA and the serotonin metabolite, 5-hydroxyindoleacetic acid (5HIAA) in the CPu were not significantly different between groups. No significant between-group differences were found for basal levels of any of the analytes in the NA. Neuroleptic-treated animals showed an enhanced response to direct infusion through the dialysis probe of amphetamine (1 microM) and nomifensine (10 microM) in the CPu but not the NA. These results suggest that chronic neuroleptic treatment produces enhanced extracellular DA activity in nigrostriatal, but not mesolimbic DA pathways.

Changes in striatal dopamine release and metabolism during and after subchronic haloperidol administration in rats

The release and metabolism of dopamine (DA) in the striatum of rats during and after subchronic haloperidol (HAL) administration (3 weeks) was assessed using in vivo microdialysis. Basal extracellular levels of DA, DA metabolites (homovanillic acid and 3,4-dihydroxyphenylacetic acid) and the serotonin metabolite (5-hydroxyindoleacetic acid) were not significantly different from control values at 3 weeks of HAL administration and 3 days after drug withdrawal. The specific DA D2 receptor antagonist, raclopride (0.5 mg/kg, i.p.), significantly increased DA release and metabolism in control animals, but decreased DA release in the HAL-treated groups at 3 weeks of drug treatment. This effect was not significant following drug withdrawal. These results contrast with our previous finding that chronic HAL treatment (32 weeks) increases basal DA metabolism and further support the possibility that changes in DA function differ following short term vs. long term neuroleptic exposure.

Chronic neuroleptic administration decreases extracellular GABA in the nucleus accumbens but not in the caudate-putamen of rats

The extracellular levels of gamma-aminobutyric acid (GABA) in the caudate-putamen and the nucleus accumbens of rats following administration of haloperidol decanoate, fluphenazine decanoate, or vehicle for 8 months were assessed using intracranial microdialysis. Basal levels of extracellular GABA were significantly decreased in the nucleus accumbens of both neuroleptic-treated groups while levels of GABA in the caudate-putamen were not significantly different between groups. These results provide evidence for selective chronic neuroleptic-induced effects on in vivo GABA function in different terminal regions containing dopamine receptors.

Neurotensin increases extracellular striatal dopamine levels in vivo

This study employed intracranial microdialysis to assess the effects of neurotensin (NT) infusion on extracellular dopamine (DA) and DA metabolite concentrations in the rat striatum and nucleus accumbens, and the effects of NT on alterations in extracellular DA levels induced by cocaine and the DA D-2 receptor agonist, quinpirole. Direct NT infusion (.10, 1.0, 10.0 microM) did not significantly affect extracellular DA in the nucleus accumbens, but did produce a significant increase in the DA metabolite homovanillic acid (HVA). In contrast, direct NT infusion produced an increase in striatal DA levels, without altering DA metabolites. Neurotensin infusion (.10 microM) into the striatum significantly attenuated the peak DA increase induced by an intraperitoneal (IP) injection of a low dose (10.0 mg/kg) but not a high dose (30.0 mg/kg) of cocaine. Neurotensin infusion (.10 microM) did not affect the decrease in DA and its metabolites induced by an IP injection of a low dose of quinpirole (.03 mg/kg), but did alter the decrease in HVA induced by a high dose of quinpirole (.10 mg/kg). These results suggest that NT differentially affects in vivo DA release in the striatum and nucleus accumbens, and further strengthens the assertion that NT is an important modulator of dopaminergic function.

In vivo assessment of release and metabolism of dopamine in the ventrolateral striatum of awake rats following administration of dopamine D1 and D2 receptor agonists and antagonists

The ability of specific dopamine (DA) receptor agonists and antagonists to modify the release and metabolism of DA in the ventrolateral striatum of awake rats was assessed using in vivo microdialysis. The specific DA D2 receptor antagonist, raclopride (0.1, 0.5 and 2.0 mg/kg, i.p.), dose-dependently increased release of DA and levels of the metabolites DOPAC and HVA, while the D2 receptor agonist, quinpirole (0.03, 0.1 and 0.3 mg/kg), decreased levels of DA, DOPAC and HVA. The DA D1 receptor antagonist, SCH23390 [(R + (+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl- 1H-3-benzazepin-7-ol) (0.01, 0.05 and 0.25 mg/kg), produced an increase in DA, DOPAC and HVA but of a lesser magnitude than raclopride. The D1 agonist SKF38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (1.0, 3.0 and 10.0 mg/kg) failed to affect the release of metabolism of DA at any dose. These results support previous findings that activation of D2 receptors has greater control over in vivo DA function, than drugs specifically affecting D1 receptors.

Striatal dopamine metabolism increases during long-term haloperidol administration in rats but shows tolerance in response to acute challenge with raclopride

The release and metabolism of dopamine (DA) in the striatum of rats during long-term haloperidol administration (32 weeks) was assessed using in vivo microdialysis. Basal levels of homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) were significantly elevated over control values, while basal DA release was not significantly increased. The specific DA D2 receptor antagonist, raclopride (0.5 mg/kg, i.p.), increased DA release and metabolism in control animals, but this effect was profoundly blocked in the haloperidol treated group. These results suggest that chronic haloperidol treatment may induce compensatory increases in basal DA activity even though response to an acute D2 antagonist shows significant tolerance.

Cholinergic modulation of oral activity in drug-naive and chronic haloperidol-treated rats

The cholinergic agonists pilocarpine, physostigmine, oxotremorine, and arecoline were administered IP at various doses to rats. Oral activity was assessed in these animals with a computerized video analysis system that determined the number and form of jaw openings and closings (computer scored movelets "CSMs"). The different cholinergic drugs produced distinctive changes in the number of CSMs at various amplitudes and in the frequency distribution of CSMs as determined by fast fourier analysis. Rats treated for 28 weeks with haloperidol showed a previously described, late onset oral dyskinesia characterized by increases in small amplitude CSMs, decreases in CSM slope, increased energy at the 1-3 Hz range and decreased energy at the 5-7 Hz range. Administration of pilocarpine (1.0 mg/kg) reversed all of these effects, while the anticholinergic drug, scopolamine (0.05 mg/kg), had no effect. These results indicate that different cholinomimetics can uniquely alter oral activity in rats and that symptoms of late onset, neuroleptic-induced oral dyskinesia are modified by a cholinergic agonist.

Comparison of chronic administration of haloperidol and the atypical neuroleptics, clozapine and raclopride, in an animal model of tardive dyskinesia

Rats were administered haloperidol, clozapine, raclopride, or no drug for either 28 days or 8 months and then withdrawn from drug treatment for 3 weeks. Oral movements were repeatedly recorded, both by a human observer and by a computerized video analysis system which determined mouth openings and closings, or computer-scored movelets (CSMs). Four weeks of neuroleptic administration produced no changes in CSMs in any drug-treated group. Long-term administration induced distinctively different patterns of oral activity in the three drug groups, both in number of CSMs and the form of these movements. The oral movements which developed in the haloperidol-treated rats fit a previously described syndrome of late-onset oral dyskinesias which increased upon drug withdrawal. The clozapine- and raclopride-treated rats did not show the increased oral movements seen in the haloperidol animals, but each exhibited uniquely different CSM characteristics compared to controls. The results from this rodent model imply that haloperidol, but not clozapine or raclopride, produces late-onset oral dyskinesias in rats that fit the pattern expected for tardive dyskinesia.

Autoradiographic analysis of regional alterations in brain receptors following chronic administration and withdrawal of typical and atypical neuroleptics in rats

Rats were administered haloperidol, clozapine, raclopride, or no drug for 28 days or 8 months. Following a 3 week withdrawal period, in vitro autoradiography was utilized to examine receptor binding for dopamine D2 ([3H]spiperone and [3H]raclopride), dopamine D 1 ([3H]SCH23390), GABA(A) ([3H]muscimol), benzodiazepine ([3H]RO15-1788), and muscarinic ACh receptors ([3H]QNB). [3H]spiperone was elevated in striatal subregions only in haloperidol-treated rats, with the largest increases seen in the 8 month duration animals. Striatal [3H]raclopride binding was increased after both short- and long-term treatment in both haloperidol and raclopride, but not clozapine-treated animals. Clozapine-treated rats showed significant increases in [3H]SCH23390 in the nucleus accumbens after 28-day administration; otherwise no changes were seen for this ligand in any other groups. Increases in [3H]muscimol binding in the substantia nigra reticulata were seen in haloperidol-treated rats after 8 month treatment. Binding of [3H]QNB and [3H]RO151788 were not significantly different from control for any of the drug-treated groups. These data suggest that persisting alterations in receptor binding are primarily seen in dopamine D2 and GABA receptors after withdrawal from chronic administration of haloperidol but not the atypical neuroleptics, clozapine and raclopride.