Establishing Diagnostic Criteria for Degenerative Cervical Myelopathy [AO Spine RECODE-DCM Research Priority Number 3]

STUDY DESIGN

Narrative review.

OBJECTIVES

To discuss the importance of establishing diagnostic criteria in Degenerative Cervical Myelopathy (DCM), including factors that must be taken into account and challenges that must be overcome in this process.

METHODS

Literature review summarising current evidence of establishing diagnostic criteria for DCM.

RESULTS

Degenerative Cervical Myelopathy (DCM) is characterised by a degenerative process of the cervical spine resulting in chronic spinal cord dysfunction and subsequent neurological disability. Diagnostic delays lead to progressive neurological decline with associated reduction in quality of life for patients. Surgical decompression may halt neurologic worsening and, in many cases, improves function. Therefore, making a prompt diagnosis of DCM in order to facilitate early surgical intervention is a clinical priority in DCM.

CONCLUSION

There are often extensive delays in the diagnosis of DCM. Presently, no single set of diagnostic criteria exists for DCM, making it challenging for clinicians to make the diagnosis. Earlier diagnosis and subsequent specialist referral could lead to improved patient outcomes using existing treatment modalities.

Predicting population responses to environmental change from individual-level mechanisms: towards a standardized mechanistic approach

Animal populations will mediate the response of global biodiversity to environmental changes. Population models are thus important tools for both understanding and predicting animal responses to uncertain future conditions. Most approaches, however, are correlative and ignore the individual-level mechanisms that give rise to population dynamics. Here, we assess several existing population modelling approaches and find limitations to both 'correlative' and 'mechanistic' models. We advocate the need for a standardized mechanistic approach for linking individual mechanisms (physiology, behaviour, and evolution) to population dynamics in spatially explicit landscapes. Such an approach is potentially more flexible and informative than current population models. Key to realizing this goal, however, is overcoming current data limitations, the development and testing of eco-evolutionary theory to represent interactions between individual mechanisms, and standardized multi-dimensional environmental change scenarios which incorporate multiple stressors. Such progress is essential in supporting environmental decisions in uncertain future conditions.

A single high dose of methamphetamine increases cocaine self-administration by depletion of striatal dopamine in rats

Psychostimulant addicts often take high doses of drugs, and high doses of psychostimulants such as methamphetamine (METH) are neurotoxic to striatal dopamine (DA) terminals. Yet, the effects of high doses of METH on drug-seeking and drug-taking behavior have not been examined. In the present study, we found that single high doses of METH in rats (10-20 mg/kg) dose-dependently increased cocaine self-administration under fixed-ratio 2 (FR2) reinforcement conditions, while higher doses (40 mg/kgx1 or 10 mg/kg/2 hx4) caused high mortality among rats maintained on daily cocaine self-administration. The increased cocaine self-administration appeared to be a compensatory response to reduced cocaine reward after METH, because the same doses of METH caused a dose-dependent reduction both in "break-point" levels for cocaine self-administration under progressive-ratio reinforcement and in nucleus accumbens DA response to acute cocaine. Further, METH (10-20 mg/kg) produced large DA release (4000%-6000% over baseline), followed by a significant reduction in striatal DA and 3,4-dihydroxyphenylacetic acid (DOPAC) contents, but without significant changes in striatal DA transporter levels. These findings suggest that the present high doses of METH caused striatal DA depletion or hypofunction without severe damage in DA terminals, which may contribute to the increased cocaine-taking behavior observed in the present study. Provided that the present doses of METH may mimic METH overdose incidents in humans, the present findings suggest that METH-induced DA depletion or neurotoxicity may lead to an increase in subsequent drug-taking and drug-seeking behavior.

Gamma-vinyl GABA (GVG) blocks expression of the conditioned place preference response to heroin in rats

We examined the effect of gamma-vinyl GABA (GVG) on the expression of the conditioned place preference response to intraperitoneally (i.p.) administered heroin in rats. Heroin, but not vehicle, produced a significant conditioned place preference response. Pretreatment of animals with 300 mg/kg of GVG significantly attenuated the expression of the heroin-induced conditioned place preference response. These results are the first to suggest that systemic GVG may provide an effective alternative to methadone maintenance in the treatment of heroin addiction, since it is without abuse potential and can be used for treatment outside an institutional setting.

The selective sigma(1) receptor agonist, 1-(3,4-dimethoxyphenethyl)-4-(phenylpropyl)piperazine (SA4503), blocks the acquisition of the conditioned place preference response to (-)-nicotine in rats

We examined the effect of the sigma(1) receptor agonist, 1-(3,4-dimethoxyphenethyl)-4-(phenylpropyl)piperazine (SA4503), on the acquisition of the conditioned place preference response to subcutaneously administered (-)-nicotine in rats. (-)-Nicotine, but not SA4503 or vehicle, produced a significant conditioned place preference response. Pretreatment of animals with either 1 or 3 mg/kg of SA4503 significantly attenuated the conditioned place preference response to (-)-nicotine.

Relapse to Cocaine-Seeking After Hippocampal Theta Burst Stimulation

Treatment efforts for cocaine addiction are hampered by high relapse rates. To map brain areas underlying relapse, we used electrical brain stimulation and intracranial injection of pharmacological compounds after extinction of cocaine self-administration behavior in rats. Electrical stimulation of the hippocampus containing glutamatergic fibers, but not the medial forebrain bundle containing dopaminergic fibers, elicited cocaine-seeking behavior dependent on glutamate in the ventral tegmental area. This suggests a role for glutamatergic neurotransmission in relapse to cocaine abuse. The medial forebrain bundle electrodes supported intense electrical self-stimulation. These findings suggest a dissociation of neural systems subserving positive reinforcement (self-stimulation) and incentive motivation (relapse).

What we have learned about addiction from animal models of drug self-administration

Self-administration of addictive drugs in laboratory animals has been widely used for decades as a tool for studying behavioral, neurobiological, and genetic factors in addiction. From such studies has come an enormous amount of information on brain mechanisms involved in addiction, on vulnerability factors in the addictive process, and on possible pharmacotherapeutic treatments for addiction. Modifications of the laboratory animal self-administration paradigm--including progressive ratio break-point models and the "reinstatement" model of relapse to drug-seeking behavior--are currently increasing our knowledge of incentive motivational factors in addiction and of the mechanisms underlying relapse to drug self-administration behavior.

The effect of intravenous administration of delta-9-tetrahydrocannabinol on the activity of A10 dopamine neurons recorded in vivo in anesthetized rats

In this pilot study, we examined the effect of the intravenous administration of delta-9-tetrahydrocannabinol (THC) on the activity of spontaneously active A10 dopamine (DA) neurons of anesthetized, male albino rats. This was accomplished using the technique of in vivo extracellular single-unit recording. The administration of THC (0.05-1.6 mg/kg i.v.) did not significantly alter either the basal firing rate or the firing pattern of spontaneously active A10 DA neurons compared to vehicle controls. Our findings suggest that, unlike a number of drugs of abuse, THC does not alter the activity of A10 DA neurons and that the previously reported THC-induced increase in brain DA levels is not due to its action on firing rate or pattern in A10 DA neurons.

GABAergic blockade of cocaine-associated cue-induced increases in nucleus accumbens dopamine

Environments previously associated with drug use can become one of the most common factors triggering relapse to drug-seeking behavior. To better understand the neurochemical mechanisms potentially mediating these cues, we measured nucleus accumbens dopamine levels in animals exposed to environmental cues previously paired with cocaine administration. In animals exposed to a cocaine-paired environment nucleus accumbens dopamine increased by 25%. When administered 2.5 h prior to presentation of the environmental trigger, racemic vigabatrin (an irreversible inhibitor of gamma-aminobutyric acid (GABA)-transaminase) abolished this cue-induced increase. Conversely, R-(-)-vigabatrin, the inactive enantiomer, had no effect. Combined with our earlier findings, these studies support the potential therapeutic benefit of this enzyme-based GABAergic strategy to modulate brain dopamine and the subsequent treatment of drug addiction.

Slow-onset, long-duration 3-(3',4'-dichlorophenyl)-1-indanamine monoamine reuptake blockers as potential medications to treat cocaine abuse

A series of 3-(3',4'-dichlorophenyl)-1-indanamine monoamine reuptake blockers have been synthesized in an effort to develop a compound that could be used as a maintenance therapy to treat cocaine abuse. Since the effects of cocaine on dopamine (DA) and serotonin (5HT) transporters are important components of its pharmacological activity, the focus was on nonselective inhibitors of monoamine transport. To reduce or eliminate the abuse potential of a DA reuptake blocker, the compounds were designed to be slow-onset, long-duration prodrugs whose N-demethylated metabolites would have increased activity over the parent compound with the ideal being a parent compound that has little or no activity. To achieve this, pairs of compounds with different groups on the amine nitrogen and with and without an additional N-methyl group were synthesized. All of the synthesized compounds were screened for binding and reuptake at the cloned human DA, 5HT, and norepinephrine (NE) transporters. As previously found, trans isomers are nonselective blockers of DA, 5HT, and NE reuptake, cis isomers with small N-alkyl groups are selective blockers of 5HT reuptake, and tertiary amines of the trans compounds are less potent than the corresponding N-demethylated secondary amines as blockers of DA reuptake. Larger N-alkyl groups in both the trans and cis series were found to reduce activity for the 5HT and NE transporters with less effect at DA transporters. Selected trans compounds were also screened for locomotor activity in mice and generalization to a cocaine-like profile in rats. With intraperitoneal administration, all of the trans isomers showed a slow onset of at least 20 min and an extremely long duration of action in the locomotor assays. Several of the trans compounds also fully generalized to a cocaine-like pharmacological profile. An initial lead compound, the N,N-dimethyl analogue trans-1b, was resolved into chirally pure enantiomers. Surprisingly, both enantiomers were found to have significant affinity for the DA transporter and to cause locomotor activation. This is in contrast to the N-methyl compound in which only the (+)-enantiomer had significant activity. The absolute configuration of the more active enantiomer was determined by X-ray crystallography to be 3R,1S.

Enhancement of conditioned place preference response to cocaine in rats following subchronic administration of 3, 4-methylenedioxymethamphetamine (MDMA)

In this study, we measured conditioned place preference (CPP) responses to cocaine following subchronic administration of the recreationally abused drug (+/-)-3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") in male Sprague-Dawley rats. Animals were given either vehicle (1 ml/kg of distilled water, s.c.) or MDMA (20 mg/kg, s.c.) twice a day for 4 consecutive days. Two weeks later, CPP responses to cocaine (5, 10, or 20 mg/kg, i.p.) were measured. The MDMA-treated animals showed a significantly greater CPP response to cocaine than the vehicle-treated animals. Since conditioned place preference is believed to be a measure of appetitive behavior, these results suggest that MDMA abuse could lead to an increased vulnerability to the rewarding actions of cocaine and, hence, to increased vulnerability to cocaine addiction and dependence.

Heterogeneity of the mesotelencephalic dopamine fibers: physiology and pharmacology

The mesotelencephalic dopamine (DA) system is heterogeneous with respect to nuclei, terminal loci, DA receptor subtypes, electrophysiological characteristics and response patterns, and neuropharmacological response to a range of agents. The majority of mesocortical and mesolimbic DA neurons originate in the ventral tegmental area. Mesostriatal DA neurons originate in substantia nigra pars compacta. DA neurons originating from the retrorubal field primarily innervate subcortical limbic and neostriatal loci. Mesostriatal terminal loci have relatively low densities of D3 and D4 receptors, compared to mesolimbic and mesocortical loci. The D1 and D2 receptors appear more homogeneously distributed. Electrophysiologically, mesostriatal DA neurons show more regularity in firing pattern (fewer bursting events), and a lower basal firing rate than mesolimbic or mesocortical neurons. Neuropharmacologically, mesocortical DA neurons are less responsive to intravenous d-amphetamine, (+)apomorphine, and chronic antipsychotic drug treatment. Mesocortical DA neurons are also relatively insensitive to iontophoretically applied DA, a finding congruent with their reported relative lack of somatodendritic autoreceptors. Neurochemically, mesoaccumbens DA neurons are more sensitive to systemic administration of drugs with addictive liability.

Gamma-vinyl GABA inhibits methamphetamine, heroin, or ethanol-induced increases in nucleus accumbens dopamine

We examined the acute effect of the irreversible GABA-transaminase inhibitor, gamma-vinyl GABA (GVG, Sabril((R)), Vigabatrin((R))) on increases in nucleus accumbens (NAc) dopamine (DA) following acute administration of methamphetamine, heroin, or ethanol. Methamphetamine (2.5 mg/kg) produced a dose-dependent increase (2, 700%) in NAc DA. GVG preadministration (300 or 600 mg/kg), however, inhibited this response by approximately 39 and 61%, respectively. The lower dose of methamphetamine (1.25 mg/kg), increased DA by 1, 700%. This response was inhibited to a similar extent (44%) regardless of the GVG dose preadministered (300 or 600 mg/kg). In addition, heroin-induced increases in NAc DA (0.5 mg/kg, 170%) were inhibited or completely abolished by GVG (150 or 300 mg/kg, 65 and 100%, respectively). Finally, at half the dose necessary for heroin, GVG (150 mg/kg) also completely abolished ethanol-induced increases in NAc DA following a 0.25 g/kg challenge dose (140%). Taken with our previous findings using nicotine or cocaine as the challenge drug, these results indicate that GVG attenuates increases in NAc DA by a mechanism common to many drugs of abuse. However, it appears unlikely that an acute dose of GVG can completely inhibit increases in NAc DA following challenges with a drug whose mechanism of action is mediated primarily through the DA reuptake site.

A pharmacologic strategy for the treatment of nicotine addiction

Like many psychostimulant drugs, nicotine elevates extracellular and synaptic dopamine (DA) concentrations in the nucleus accumbens (NAc). This elevation has been linked to its reinforcing properties. Dopaminergic transmission within the NAc is modulated by gamma-aminobutyric acid (GABA). Therefore, we examined the utility of gamma vinyl-GABA (GVG, Vigabatrin) for inhibiting nicotine's biochemical effects on NAc DA as well as its effects on behaviors associated with these biochemical changes. Given 2.5 hours prior to nicotine, GVG (75 mg/kg) had no effect on nicotine-induced increases in extracellular NAc DA. However, at 90 mg/kg, GVG significantly inhibited nicotine-induced increases by approximately 50% while at 100 or 150 mg/kg, GVG completely abolished nicotine-induced increases in both naive and chronically nicotine-treated animals. When given 12 or 24 hours prior to nicotine administration at a dose of 100 mg/kg, GVG-induced inhibition was diminished or abolished, respectively. In addition, at a dose of 18.75 mg/kg GVG abolished the expression of nicotine-induced conditioned place preference (CPP) while a dose of 75 mg/kg abolished the acquisition phase of CPP. Finally, using positron emission tomography (PET) and 11C-raclopride in primates, GVG (100 mg/kg) abolished nicotine-induced increases in synaptic DA while having no effect on the rate of metabolism of the radiotracer or its regional distribution. Together, these data suggest that GVG may be useful for the treatment of nicotine addiction and further support the strategy of targeting the GABAergic system with a suicide inhibitor of GABA-transaminase for the treatment of drug addiction.

Cannabinoid transmission and reward-related events

The reward/reinforcement circuitry of the mammalian brain consists of synaptically interconnected neurons associated with the medial forebrain bundle, linking the ventral tegmental area, nucleus accumbens, and ventral pallidum. Electrical stimulation of this circuit supports intense self-stimulation in animals and, in humans, produces intense pleasure or euphoria. This circuit is strongly implicated in the neural substrates of drug addiction and in such addiction-related phenomena as withdrawal dysphoria and craving. This circuit is also implicated in the pleasures produced by natural rewards (e.g., food, sex). Cannabinoids are euphorigenic in humans and have addictive liability in vulnerable persons, but were long considered "anomalous" drugs of abuse, lacking pharmacological interaction with these brain reward substrates. It is now clear, however, that cannabinoids activate these brain substrates and influence reward-related behaviors. From these actions, presumably, derive both the abuse potential of cannabinoids and the possible clinical efficacy in dysphoric states.

A novel strategy for the treatment of cocaine addiction

Cocaine's addictive liability has been linked to its pharmacologic actions on mesotelencephalic dopamine (DA) reinforcement/reward pathways in the central nervous system (CNS). Dopaminergic transmission within these pathways is modulated by gamma-aminobutyric acid (GABA). With this knowledge, we examined the utility of gamma vinylGABA (GVG), a selective and irreversible inhibitor of GABA-transaminase (GABA-T) known to potentiate GABAergic inhibition, to alter cocaine's biochemical effects as well as its effects on behaviors associated with these biochemical changes. GVG significantly attenuated cocaine-induced increases in neostriatal synaptic DA in the non-human primate (baboon) brain as assessed by positron emission tomography (PET) and abolished both the expression and acquisition of cocaine-induced conditioned place preference (CPP). It had no effect on CPP for a food reward, the delivery of cocaine to the brain or locomotor activity. These findings suggest the possible therapeutic utility in cocaine addiction of a pharmacologic strategy targeted at the GABAergic neurotransmitter system, a system distinct from but functionally linked to the DA mesotelencephalic reward/reinforcement system. However, rather than targeting the GABA receptor complex with a direct GABA agonist, this novel approach with GVG takes advantage of the prolonged effects of an irreversible enzyme inhibitor that raises endogenous GABA levels without the addictive liability associated with GABA agonists acting directly at the receptor itself. Human trials with GVG are currently being developed to directly examine the utility of this novel strategy for the treatment of cocaine addiction.

(-)-Nicotine produces conditioned place preference in Lewis, but not Fischer 344 rats

In this study, we sought to determine if Fischer 344 (F344) and Lewis rats showed different conditioned place preference (CPP) responses to subcutaneously administered (-)-nicotine. Lewis rats displayed a CPP response to (-)-nicotine after five pairings, whereas F344 rats showed no preference for nicotine compared to vehicle. After 10 pairings, the F344 rats showed a conditioned place aversion to (-)-nicotine, whereas the Lewis rats still displayed a significant CPP response. These results suggest that the differences in appetitiveness shown between Lewis and F344 rats to other drugs of abuse extend also to (-)-nicotine.

Genetic differences in delta 9-tetrahydrocannabinol-induced facilitation of brain stimulation reward as measured by a rate-frequency curve-shift electrical brain stimulation paradigm in three different rat strains

Lewis, Fischer 344, and Sprague-Dawley rats were implanted with electrodes in the medial forebrain bundle and trained to lever press for brain stimulation reward using a rate-frequency curve-shift electrical brain stimulation paradigm based on a series of 16 pulse frequencies ranging from 25 to 141 Hz in descending order. Once reward thresholds were stable, rats were given 1.0 mg/kg delta 9-tetrahydrocannabinol (delta 9-THC), the psychoactive constituent in marijuana and hashish, or vehicle, by intraperitoneal injection. Lewis rats showed the most pronounced delta 9-THC-induced enhancement of brain reward functions. Sprague-Dawley rats showed an enhancement of brain reward functions that was approximately half that seen in Lewis rats. Brain reward functions in Fischer 344 rats were unaffected by delta 9-THC at the dose tested. These results are consistent with previous work showing Lewis rats to be highly sensitive to the rewarding properties of a variety of drugs of abuse, including opiates, cocaine, and alcohol, while Fischer 344 rats are relatively less sensitive. They extend such previous findings to cannabinoids, and further suggest that genetic variations to other cannabinoid effects may also exist.

Systemic cocaine challenge after chronic cocaine treatment reveals sensitization of extracellular dopamine levels in nucleus accumbens but direct cocaine perfusion into nucleus accumbens does not: implications for the neural locus of cocaine sensitization

Rats were treated chronically with 20 mg/kg/day cocaine (by intraperitoneal injection) for 16 days, followed by 7 days of cocaine wash-out. On the next day, rats were challenged with an acute dose of cocaine administered by one of two routes (systemic or intracranial), and extra-cellular dopamine (DA) in the nucleus accumbens (Acb) was measured by in vivo microdialysis. Rats acutely challenged systemically with 20 mg/kg cocaine showed enhanced Acb extracellular DA levels (compared to control rats that had not previously received chronic cocaine). However, rats acutely challenged with intracranial cocaine by perfusion of 10(-5) M cocaine directly into the Acb did not. It is suggested that both the development and triggering of cocaine sensitization, as manifested by enhanced Acb DA content to subsequent acute cocaine challenge, may involve more than just neural mechanisms occurring locally within the Acb.

Conditioned place preference induced by delta 9-tetrahydrocannabinol: comparison with cocaine, morphine, and food reward

The rewarding property of delta 9-tetrahydrocannabinol (THC), the psychoactive constituent of marijuana and hashish, was studied using the conditioned place preference paradigm, and compared to that of cocaine, morphine, and food reward. The results of Experiment 1 demonstrated that 2.0 and 4.0 mg/kg doses produced a reliable shift in preference for the THC-paired compartment. The THC place preference observed at 2.0 and 4.0 mg/kg was nearly equivalent to that produced by low doses of cocaine (5.0 mg/kg), morphine (4.0 mg/kg), and food in non food-deprived animals. The second experiment used a different conditioning procedure that included a washout period for THC. The results of Experiment 2 demonstrated that a THC place preference could be obtained using a lower dose of THC (1.0 mg/kg), and that this THC place preference was equivalent to that produced by 10 mg/kg cocaine. At higher doses (2.0 and 4.0 mg/kg), THC produced a dose-dependent place aversion. These results suggest that THC's action on brain reward substrates, previously demonstrated by electrical brain stimulation reward, in vivo brain microdialysis, and in vivo brain electrochemistry studies, reflects itself behaviorally in increased appetitive motivational value for environmental stimuli associated with ingestion of marijuana and hashish.

Clozapine produces potent antidopaminergic effects anatomically specific to the mesolimbic system

Although weak as an antagonist at brain dopamine receptors, clozapine is very strongly antidopaminergic when measured by in vivo single-neuron electrophysiologic recording, in vivo brain microdialysis, in vivo brain voltammetric electrochemistry, in vivo electrical brain stimulation, or in vivo neurobehavioral techniques. Three aspects of clozapine's strong antidopaminergic actions are remarkable. First, clozapine's antidopaminergic potency is stronger than can be explained by its comparatively weak interaction with dopamine receptors, implying that a significant portion of its antidopaminergic action may be transsynaptic and secondary to actions on nondopamine neurotransmitter systems. Second, clozapine's strong antidopaminergic action is seen more reliably with chronic rather than acute administration, implying that some cumulative neurochemical process underlies the gradual recruitment of dopamine blockade. Third, and most remarkably, clozapine's strong antidopaminergic action is anatomically specific--present in the mesolimbic dopamine system but absent in the nigrostriatal dopamine system. This mesolimbic-specific dopamine blockade may underlie at least a portion of clozapine's atypical clinical profile.

Ventral tegmental microinjection of delta 9-tetrahydrocannabinol enhances ventral tegmental somatodendritic dopamine levels but not forebrain dopamine levels: evidence for local neural action by marijuana's psychoactive ingredient

Basal extracellular levels of dopamine (DA) and its metabolites in both ventral tegmental area (VTA) and nucleus accumbens (Acb) were simultaneously monitored by in vivo brain microdialysis in laboratory rats. Microinjection of 12 micrograms or 24 micrograms delta 9-tetrahydrocannabinol (delta 9-THC), the psychoactive ingredient in marijuana and hashish, into the VTA produced a dose-dependent increase in somatodendritic DA levels in VTA but failed to produce any simultaneous change in extracellular DA in Acb. Direct delta 9-THC perfusion (5 x 10(-5) and 2 x 10(-4) M) into Acb via the microdialysis probes caused a significant increase in extracellular DA levels in Acb. These findings suggest that (1) delta 9-THC has a facilitating effect on somatodendritic DA efflux, and (2) the elevation of Acb DA levels seen in our previous studies with systemic administration of delta 9-THC may result from local actions of delta 9-THC at or near the DA terminal projections in Acb.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Effects of haloperidol on human plasma magnesium

The effect of haloperidol treatment on human plasma levels of magnesium (Mg), calcium (Ca) and phosphorus (P) was assessed. Haloperidol treatment significantly reduced plasma Mg but not plasma Ca or P levels. This finding contrasts with studies using other neuroleptics where reductions in both Mg and Ca concentrations were observed. The addition of lithium to haloperidol treatment resulted in an elevation of plasma Mg, but not Ca or P, to levels significantly greater than those at baseline. The reduction of Mg levels by haloperidol may reflect the ion's involvement in the pathology of psychosis and/or in the mechanism of generating extrapyramidal side effects.

Clozapine's functional mesolimbic selectivity is not duplicated by the addition of anticholinergic action to haloperidol: a brain stimulation study in the rat

This study examined whether the anticholinergic potency of the clinically superior antipsychotic drug clozapine contributes to clozapine's anatomically-selective functional inhibition of the mesolimbic dopamine (DA) system, using an electrical brain-stimulation reward (BSR) paradigm in rats that has been previously shown to be highly sensitive to clozapine's mesolimbic functional selectivity. Rats were chronically administered saline, clozapine, haloperidol, or haloperidol plus the anticholinergic compound trihexyphenidyl, and threshold sensitivity of the mesolimbic and nigrostriatal DA systems was assessed using the BSR paradigm, to infer degree of functional DA blockade produced by the chronic drug regimens. Chronic saline produced no change in either DA system. Congruent with previous findings, chronic clozapine powerfully inhibited the mesolimbic DA system but spared the nigrostriatal DA system. Also congruent with previous findings, chronic haloperidol powerfully inhibited both DA systems. Compared to chronic haloperidol alone, chronic haloperidol plus chronic trihexyphenidyl exerted diminished anti-DA action in both the mesolimbic and nigrostriatal DA systems. These results suggest that clozapine's anticholinergic potency is not an adequate explanation for its functional mesolimbic selectivity.

Serotonin denervation enhances responsiveness of presynaptic dopamine efflux to acute clozapine in nucleus accumbens but not in caudate-putamen

Clozapine alters mesolimbic dopamine (DA) function but spares nigrostriatal DA function in laboratory animals, but the underlying mechanism is unknown. In the present study, acute intraperitoneal injection of clozapine (5-40 mg/kg) increased extracellular DA levels in nucleus accumbens (Acb) and caudate-putamen (CPu) of awake, freely moving rats as measured by in vivo brain microdialysis, without anatomic selectivity. However, in serotonin (5HT)-denervated rats acute clozapine preferentially enhanced DA levels in Acb as compared to CPu. Since (i) up-regulation of 5HT receptors on DA neurons may result from 5HT denervation, (ii) clozapine has potent anti-5HT action, and (iii) 5HT receptors are more dense in Acb than CPu, these data appear to add additional weight to previous suggestions that a serotonergic mechanism may partly underlie clozapine's mesolimbic selectivity.

Effects of acute and chronic clozapine on dopaminergic function in medial prefrontal cortex of awake, freely moving rats

We previously showed that chronic administration of the clinically atypical and clinically superior antipsychotic drug clozapine selectively reduces dopamine (DA) release in the nucleus accumbens but not neostriatum, and that this effect appears mediated by anatomically selective mesolimbic DA depolarization blockade. The present study extends that research to another mesocorticolimbic DA locus, the medial prefrontal cortex. Acute clozapine challenge (5-40 mg/kg i.p.) produced dose-dependent increased extracellular levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the medial prefrontal cortex of awake, free-moving rats as measured by in vivo brain microdialysis. Chronic clozapine treatment (20 mg/kg/day for 21 days) did not significantly change basal extracellular levels of DA, DOPAC or HVA. Acute clozapine challenge on day 22 in the chronic clozapine-treated animals produced no significant differences in medial prefrontal cortex DA, DOPAC or HVA as compared to chronic vehicle-treated animals, indicating that tolerance to clozapine does not develop in the mesocortical DA system, in contrast to the mesolimbic system. The DA agonist apomorphine (100 micrograms/kg) produced decreased basal extracellular levels of DA, DOPAC and HVA in medial prefrontal cortex of both chronic clozapine-treated and chronic vehicle-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Marijuana's interaction with brain reward systems: update 1991

The most pervasive commonality amongst noncannabinoid drugs of abuse is that they enhance electrical brain stimulation reward and act as direct or indirect dopamine agonists in the reward relevant dopaminergic projections of the medial forebrain bundle (MFB). These dopaminergic projections constitute a crucial drug sensitive link in the brain's reward circuitry, and abused drugs derive significant abuse liability from enhancing these circuits. Marijuana and other cannabinoids were long considered "anomalous" drugs of abuse, lacking pharmacological interaction with these brain reward substrates. It is now clear, however, that delta 9-tetrahydrocannabinol (delta 9-THC), marijuana's principal psychoactive constituent, acts on these brain reward substrates in strikingly similar fashion to noncannabinoid drugs of abuse. Specifically, delta 9-THC enhances MFB electrical brain stimulation reward, and enhances both basal and stimulated dopamine release in reward relevant MFB projection loci. Furthermore, delta 9-THC's actions on these mechanisms is naloxone blockable, and delta 9-THC modulates brain mu and delta opioid receptors. This paper reviews these data, suggests that marijuana's interaction with brain reward systems is fundamentally similar to that of other abused drugs, and proposes a specific neural model of that interaction.

Strain-specific facilitation of dopamine efflux by delta 9-tetrahydrocannabinol in the nucleus accumbens of rat: an in vivo microdialysis study

This study tests the hypothesis that delta 9-tetrahydrocannabinol (delta 9-THC) has a strain-specific facilitatory effect on dopamine (DA) efflux in rat nucleus accumbens, a crucial forebrain convergence of reward-relevant DA neural fibers that has been implicated as a focal brain locus mediating the euphorigenic properties of drugs of abuse. The dependent variable is presynaptic DA efflux measured by in vivo microdialysis in the nucleus accumbens. The independent variables are: (1) intraperitoneal injections of delta 9-THC at 0.0 (vehicle), 0.5 and 1.0 mg/kg; (2) Sprague-Dawley vs Lewis strain rat. Results show that delta 9-THC produces a dose-dependent, strain-specific enhancement of basal DA efflux in Lewis strain rats. These results suggest that genetic variation influences drug abuse vulnerability.

Activation of 5-HT3 receptor by 1-phenylbiguanide increases dopamine release in the rat nucleus accumbens

The serotonin-3 (5-HT3) agonist 1-phenylbiguanide (0.1-1.0 mM in perfusate) caused a robust, dose-dependent enhancement of extracellular dopamine content in nucleus accumbens as measured by in vivo microdialysis. This action was antagonized by co-perfusion of the 5-HT3 antagonists zacopride and GR38032F (1 mM in perfusate). Similar effects were observed in 5-HT-denervated rats. These findings suggest that there is a potent modulation of dopamine (DA) release in the nucleus accumbens mediated via 5-HT3 receptors, which appear to be located presynaptically on DA terminals of the mesolimbic DA pathway.

Chronic treatment with clozapine selectively decreases basal dopamine release in nucleus accumbens but not in caudate-putamen as measured by in vivo brain microdialysis: further evidence for depolarization block

As measured using in vivo brain microdialysis in conscious freely-moving rats, chronic treatment (20 mg/kg/day i.p. for 21 days) with the clinically atypical neuroleptic clozapine selectively reduced basal dopamine (DA) release in the nucleus accumbens (Acb) but not in caudate-putamen (CPu). Apomorphine (100 micrograms/kg s.c.) enhanced presynaptic Acb DA release in clozapine-treated rats, but reduced Acb DA release in vehicle-treated rats. These findings provide further evidence that depolarization block of mesolimbic DA neurons projecting to Acb but not of nigrostriatal DA neurons projecting to CPu may underlie clozapine's unusual clinical efficacy and its lack of production of extrapyramidal motoric effects.

Delta 9-tetrahydrocannabinol enhances presynaptic dopamine efflux in medial prefrontal cortex

Acute administration of 1.0-2.0 mg/kg delta 9-tetrahydrocannabinol (delta 9-THC) increased presynaptic dopamine (DA) efflux in the medial prefrontal cortex of rats, as measured by intracerebral microdialysis in awake, behaving rats. These data are congruent with suggestions that (1) marijuana's euphorigenic effects and abuse potential may be related to augmentation of presynaptic DA mechanisms, and (2) the medial prefrontal cortex may be an important site of action for drugs of abuse in general and for delta 9-THC in particular.

Delta 9-tetrahydrocannabinol produces naloxone-blockable enhancement of presynaptic basal dopamine efflux in nucleus accumbens of conscious, freely-moving rats as measured by intracerebral microdialysis

This study examined the effects of acute administration of delta-9-tetrahydrocannabinol (delta 9-THC), the psychoactive ingredient in marijuana, on extracellular efflux of dopamine (DA) and its metabolites as measured by in vivo microdialysis in nucleus accumbens of conscious, freely-moving rats. delta 9-THC, at low doses (0.5-1.0 mg/kg), which significantly enhance brain stimulation reward (intracranial self-stimulation), significantly increased DA efflux in nucleus accumbens. Augmentation of DA efflux by delta 9-THC was abolished by removal of calcium (Ca++) ions from the perfusion fluid, indicating a Ca(++)-dependence of delta 9-THC's action. Augmentation of DA efflux by delta 9-THC was either totally blocked or significantly attenuated by doses of naloxone as low as 0.1 mg/kg. Given the postulated role of mesocorticolimbic DA circuits in mediating and/or modulating brain stimulation reward, the present data raise the possibility that marijuana's rewarding effects, and hence its euphorigenic effects and abuse potential, may be related to pharmacological augmentation of presynaptic DA mechanisms. Additionally, the DA mechanisms enhanced by marijuana appear to be modulated by an endogenous opioid peptide system.

The effects of delta 9-tetrahydrocannabinol on potassium-evoked release of dopamine in the rat caudate nucleus: an in vivo electrochemical and in vivo microdialysis study

The effect of systemically administered delta 9-tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, on the potassium-evoked release of dopamine (DA) was examined in the neostriatum of the chloral hydrate anesthetized rat. Both in vivo electrochemical and in vivo microdialysis techniques were employed. A low dose of THC (0.5 mg/kg, i.p.) increased the time course of potassium-evoked in vivo electrochemical signals corresponding to released extracellular DA. In vivo microdialysis showed an increase in potassium-evoked DA release following 0.5 and 2.0 mg/kg doses of THC. Potassium-evoked electrochemical signals corresponding to released extracellular DA were augmented in time course following i.p. administration (5.0 mg/kg) of nomifensine, a recognized and potent catecholaminergic reuptake blocker. In addition, in vivo brain microdialysis studies of nomifensine (5.0 mg/kg i.p.) on neostriatal potassium-evoked DA release showed that DA levels were augmented in magnitude over the time course of the microdialysis. Taken together, these studies indicate that THC has a potent presynaptic augmenting effect on at least the neostriatal portions of the mesotelencephalic DA system in the rat, although the possibility that this effect could be mediated transsynaptically cannot be ruled out. Given the previous extensive evidence for an involvement of portions of the mesotelencephalic DA system in mediating the reinforcing and euphorigenic properties of many classes of abused drugs, and in mediating direct electrical brain stimulation reward, we suggest that the presently demonstrated effects of THC on forebrain dopamine function may be related to marijuana's euphorigenic properties and, thus, to its abuse potential.

Facilitation of brain stimulation reward by delta 9-tetrahydrocannabinol

The present experiment explored whether delta 9-tetrahydrocannabinol (delta 9-THC), the psychoactive ingredient in marijuana, shares with other drugs of abuse the ability to facilitate brain stimulation reward acutely, as measured by electrical intracranial self-stimulation (ICSS). Laboratory rats were implanted with stimulation electrodes in the medial forebrain bundle, and trained to stable performance on a self-titrating threshold ICSS paradigm. delta 9-THC, at a dose believed pharmacologically relevant to moderate human use of marijuana, acutely lowered ICSS thresholds, suggesting that marijuana acts on similar CNS hedonic systems to most other drugs of abuse.

Visualization of multiple opioid-receptor types in rat striatum after specific mesencephalic lesions

In order to gain insight into a possible modulatory role for mu, delta, and kappa opioid receptors of the nigrostriatal dopaminergic pathway, we investigated the topographical organization of the receptors with respect to pre- and postsynaptic membranes. Dopaminergic terminals projecting from the substantia nigra to the corpus striatum were destroyed by unilateral injection of 6-hydroxydopamine into the substantia nigra. Quantitative receptor assays using highly specific radioligands were used to measure the density of striatal mu, delta, and kappa receptors before and after denervation. Denervation caused a 34 +/- 2% loss of striatal mu receptors and a 32 +/- 1% loss of striatal delta receptors on the lesioned side of the brain; in contrast, kappa receptors did not change significantly in density. Quantitative in vitro autoradiography was used to visualize the neuroanatomical pattern of receptors on lesioned and nonlesioned sides of the brain under the light microscope. Loss of mu receptors in striatal patches was striking in the ventrolateral areas of the striatum, whereas the most notable loss of delta receptors was found in the central striatum. Other brain areas did not differ significantly in mu receptor density between the lesioned and nonlesioned sides, as determined by autoradiography. These findings suggest that a high percentage of mu and delta receptors in the striatum are located on the nigrostriatal dopaminergic terminals and support the concept of a modulatory role for mu and delta opioid peptides in the nigrostriatal dopaminergic pathway.

Modulation of rat brain opioid receptors by cannabinoids

The interaction of delta 9-tetrahydrocannabinol (delta 9-THC) and related cannabinoids with opioid receptors of neuronal membranes has been investigated. Treatment of membranes with delta 9-THC consistently decreased specific in vitro binding of [3H]dihydromorphine (mu opioid) in a dose-dependent fashion. Similar dose-dependent changes were elicited by cannabidiol and (+/-)-hexahydrocannabinol. Equilibrium binding studies in which brain membranes were titrated with [3H]dihydromorphine in the presence of delta 9-THC demonstrated that the decrease in [3H]dihydromorphine binding is due to a reduction in the number of binding sites, with no significant alteration in receptor affinity. This result suggests that the interaction of delta 9-THC with opioid receptors is a noncompetitive one. Delta 9-THC also inhibited the binding of the delta opioid [3H]D-Pen2, D-Pen5-enkephalin and the opioid antagonist [3H]naloxone (Ki = 16 and 19 microM, respectively) but failed to inhibit the binding of the kappa opioid [3H]ethylketocyclazocine (after suppression of mu and delta receptor binding), the phencyclidine analog [3H]N-(1-[2-theinyl]cyclohexyl)piperidine, the dopamine antagonist [3H]spiroperidol or the muscarinic antagonist [3H]quinuclidinyl benzilate. Moreover, delta 9-THC inhibited the binding of [3H]etorphine (potent opioid agonist) to solubilized, partially purified opioid receptors with a Ki value similar to that observed for the membrane-bound receptors. This finding indicates that the allosteric modulation of the opioid receptor by delta 9-THC is the result of a direct interaction with the receptor protein or with a specific protein-lipid complex and not merely the result of a perturbation of the lipid bilayer of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of [3H](-)-SKF-10,047 with brain sigma receptors: characterization and autoradiographic visualization

The sigma opiates differ from other opiates in their stimulatory and psychotomimetic actions. The sigma opiate [3H](-)-SKF-10,047 has been used to characterize sigma receptors in rat nervous tissue. Binding of [3H](-)-SKF-10,047 to rat brain membranes was of high affinity, saturable, and reversible. Scatchard analysis revealed the apparent interaction of this drug with two distinct binding sites characterized by affinities of 0.03 and 75 nM (5 mM Tris-HCl buffer, pH 7.4, at 4 degrees C). Competition analyses involving rank order determinations for a series of opiates and other drugs indicate that the high-affinity binding site is the mu opiate receptor. The lower-affinity site (revealed after suppression of mu and delta receptor binding) has been identified as the sigma opiate/phencyclidine receptor. In vitro autoradiography has been used to visualize neuroanatomical patterns of receptors labeled using [3H](-)-SKF-10,047 in the presence of normorphine and [D-Ala2,D-Leu5]enkephalin to block mu and delta interactions, respectively. Labeling patterns differ markedly from those for mu, delta, or kappa receptors. The highest densities (determined by quantitative autoradiography) are found in the medial portion of the nucleus accumbens, amygdaloid nucleus, hippocampal formation, central gray, locus coeruleus, and the parabrachial nuclei. Receptors in these structures could account for the stimulatory, mood-altering, and analgesic properties of the sigma opiates. Although not the most selective sigma opiate ligand, [3H](-)-SKF-10,047 binds to sigma opiate receptors in brain, and this interaction can be readily distinguished from its interactions with other classes of brain opiate receptors.

Neurochemical and functional correlates of naltrexone-induced opiate receptor up-regulation

The neurochemical and functional correlates of opioid receptor up-regulation after chronic antagonist administration in vivo and of down-regulation after withdrawal of antagonist were examined. Total brain opioid receptors increased 1.9-fold by day 8 of naltrexone administration, after which no further increase was observed; the newly synthesized or unmasked receptors exhibited an enhanced sensitivity to guanyl nucleotide modulation. Withdrawal from chronic naltrexone treatment resulted in a return to nearly control levels of receptor density and guanyl nucleotide sensitivity in a period of 6 days. These results suggest that up-regulation is accompanied by an increased coupling of the receptors to the inhibitory guanyl nucleotide binding protein (Ni) and that down-regulation involves the dissociation of the receptor/Ni complex. In experiments designed to target opiate receptor subtypes, long-term treatment with naltrexone was found to produce a coordinated up-regulation of brain mu and delta receptors, but did not cause a significant change in the density or affinity of kappa or sigma receptors. These findings indicate that the kappa and sigma opiate receptor classes may be subject to independent control mechanisms. Chronic naltrexone treatment also resulted in an enhanced morphine-induced analgesia. This result indicates that a functional supersensitivity occurs as a result of the selective up-regulation of mu and delta receptors. After withdrawal from naltrexone, supersensitivity to morphine-induced analgesia decreased monotonically and, in parallel to opioid receptor density, to prenaltrexone treatment levels within 6 days. Together, these results suggest a functional significance for antagonist-induced mu and delta opiate receptor up-regulation.

Visualization of opiate receptor upregulation by light microscopy autoradiography

Light microscopy autoradiography has been used to visualize neuroanatomical patterns of brain opiate receptor upregulation in response to chronic naltrexone administration. Slide-mounted brain sections of frozen rat brain were labeled in vitro with dihydro[3H]morphine, a relatively selective mu opioid ligand. The greatest relative increases in opiate receptor density were observed in the nucleus accumbens, the amygdala, striatal patches, nuclei of the thalamus and hypothalamus, layers I and III of neocortex, substantia nigra compacta, midbrain periaqueductal gray regions, and the parabrachial nuclei of the brainstem. The substantia nigra reticulata, surrounding areas of striatal patches, and the locus ceruleus, were not affected by this drug treatment. These findings demonstrate that chronically administered naltrexone differentially regulates opiate receptors throughout the brain. In particular, three brain systems appear to be target areas of receptor upregulation : (i) the dopamine A9/A10 systems, (ii) the limbic system, and (iii) structures that receive input from afferent sensory pathways. Two possible mechanisms to account for this finding are (i) that the drug does not have uniform effects throughout the brain or (ii) that the receptors themselves may be associated with different functional systems. Receptor density changes are paralleled by increases in methionine-enkephalin content in the striatum, nucleus accumbens, periaqueductal gray, and hypothalamic areas of chronic naltrexone-treated rats relative to control rats. Thus opiate receptors and opioid peptides appear to be subject to regulatory mechanisms similar to those that modulate other neurotransmitters and their receptors. These results document in a visual manner brain patterns of opiate receptor upregulation .

In vivo electrochemical and behavioral evidence for specific neural substrates modulated differentially by enkephalin in rat stimulant stereotypy and locomotion

The enkephalinamide, D-Ala2-D-Pro5-enkephalinamide monoacetate (WY 42, 186), when systemically administered to male Sprague-Dawley rats, significantly inhibited sniffing, repetitive head movements, and frequency of rearing, stereotyped behaviors which are often associated with nigrostriatal dopamine activation. On the other hand, the locomotor component of amphetamine-induced stereotyped behavior, which is associated with mesolimbic dopaminergic activation, was not inhibited. In vivo electrochemical analysis showed a significant decrease in striatal dopamine release from striatum after systemic administration of D-Ala2-D-Pro5-enkephalinamide monoacetate in chloral hydrate anesthetized rats, whereas the dopamine signal from the nucleus accumbens, a mesolimbic neuroanatomigic modulation of dopamine both behaviorally and biochemically. Also, the concept of separate neural systems for the stereotypic and locomotor components of amphetamine-induced stereotypy is reinforced.

Neurobehavioral evidence for mesolimbic specificity of action by clozapine: studies using electrical intracranial self-stimulation

The ability of chronic treatment with the atypical neuroleptic clozapine to induce functional dopaminergic hypersensitivity in laboratory rats was assessed. The intracranial electrical self-stimulation paradigm, known to be sensitive to changes in functional dopaminergic sensitivity, was used. Animals with electrodes in the ventral tegmental nucleus (mesolimbic dopamine cell body area) showed a marked increase in self-stimulation rate following 3 weeks of chronic clozapine. This increase was similar in magnitude and duration to that shown by animals given 3 weeks of chronic haloperidol. In contrast, animals with electrodes in the substantia nigra (nigrostriatal dopamine cell body area) showed no change in self-stimulation rate following 3 weeks of chronic clozapine. These data are interpreted in the light of previous suggestions that clozapine and other atypical neuroleptics may possess functional selectivity for the mesolimbic dopamine system.

Evidence for a role of endogenous opioids in the nigrostriatal system: influence of naloxone and morphine on nigrostriatal dopaminergic supersensitivity

The effects of morphine and naloxone on nigrostriatal function were evaluated by their influence on rotational behavior in rats with unilateral lesions of the substantial nigra. Two different rotational syndromes which result from different lesion placements, were examined. Rats with the contraversive syndrome, when given apomorphine, rotate away from the lesioned side, while rats with the ipsiversive syndrome rotate toward the lesioned side. In both syndromes, rats rotate toward the lesioned side when given amphetamine. Morphine or naloxone, alone, was without effect in either syndrome. Morphine antagonized rotation by either apomorphine or amphetamine in both syndromes. Naloxone stimulated apomorphine-induced rotation in contraversive rats and antagonized amphetamine-induced rotation in ipsiversive rats. These findings support a functional role of endogenous opioids in this dopaminergic system. The effects of morphine and naloxone on apomorphine-induced rotation indicate that opiates act at a postsynaptic site in this system. Finally, the different responses to naloxone and morphine in the two rotational syndromes suggest that an enkephalinergic asymmetry may underlie the differences in behavioral responses between these two syndromes.

Similar effects of an enkephalin analog on mesolimbic dopamine release and hyperactivity in rats

The effect of the metabolically stable enkephalin pentapeptide analog, D-Ala2-D-Pro5-enkephalinamide monoacetate (DAP) (WY 42, 186) was studied on amphetamine-induced hyperactive behavior and on dopamine release from tuberculum olfactorium in male, Sprague-Dawley rats. The behavioral results showed that D-Ala2-D-Pro5-enkephalinamide monoacetate did not significantly alter hyperactivity, the mesolimbic component of amphetamine-induced stereotypy. In vivo electrochemical evidence, derived from catecholamine sensitive electrodes, showed that the D-Ala2-D-Pro5-enkephalinamide monoacetate did not significantly alter dopamine release from the tuberculum olfactorium, a mesolimbic terminal brain region. The similarity in the behavioral and biochemical responses of dopamine to the enkephalinamide analog suggests that the behavior and biochemistry may be subserved by similar underlying neural mechanisms.

Naltrexone-induced opiate receptor supersensitivity

Chronic administration of the long-lived narcotic antagonist naltrexone resulted in a marked increase in brain opiate receptors. Similar changes in receptor density were observed for binding of the putative mu agonist [3H]dihydromorphine, the mu antagonist [3H]naloxone, the putative delta ligand [3H]D-Ala2,D-Leu5-enkephalin and [3H]etorphine. In addition, the sensitivity of agonist binding to guanyl nucleotide inhibition increased significantly. In contrast, no such changes in opiate binding were observed following acute administration of naltrexone. The increase in opiate receptor number following chronic naltrexone was highest in the mesolimbic and frontal cortex areas, and lowest in the dorsal hippocampus and periaqueductal gray. These results indicate a degree of plasticity in the opiate receptor system that may correlate with specific functional pathways.

Behavioral and biochemical aspects of neuroleptic-induced dopaminergic supersensitivity: studies with chronic clozapine and haloperidol

Rats were chronically injected with saline, clozapine, or haloperidol and tested for alterations in dopamine (DA)-mediated behavior, DA receptor binding, and both acetylcholine (ACH) concentration and choline acetylase activity. Behaviorally, chronic haloperidol significantly enhanced apomorphine-induced chewing and sniffing stereotypies, associated with DA nigrostriatal activation, while clozapine selectively enhanced apomorphine locomotor activity and cage-floor crossing, behavior associated with DA mesolimbic activation. Biochemically, chronic haloperidol significantly enhanced 3H-spiroperidol binding in striatum and in mesolimbic loci (nucleus accumbens/olfactory tubercle) while chronic clozapine failed to produce such enhancement. Acute haloperidol induced an initial decrease in striatal ACH concentration followed by a return of ACH to normal levels within 1 week. There was no change in choline acetylase activity during the same time interval. These findings suggest that haloperidol may inhibit DA mechanisms in both the nigrostriatal may inhibit DA mechanisms in both the nigrostriatal and mesolimbic systems, but that the effect of clozapine on DA mechanisms may be specific to mesolimbic rather than striatal structures. At the same time, the lack of effect of clozapine on 3H-spiroperidol binding may indicate that behaviorally important changes in DA sensitivity can develop independent of changes in post-synaptic DA receptors. The pattern of cholinergic changes with chronic haloperidol suggests that the increase in striatal DA receptor number seen with chronic treatment re-establishes DA inhibition of cholinergic firing within the striatum.