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

Effects of the selective adenosine A2A receptor antagonist, SCH 412348, on the parkinsonian phenotype of MitoPark mice

Adenosine A2A receptors are predominantly localized on striatopallidal gamma-aminobutyric acid (GABA) neurons, where they are colocalized with dopamine D2 receptors and are involved in the regulation of movement. Adenosine A2A receptor antagonists have been evaluated as a novel treatment for Parkinson's disease and have demonstrated efficacy in a broad spectrum of pharmacological and toxicological rodent and primate models. Fewer studies have been performed to evaluate the efficacy of adenosine A2A receptor antagonists in genetic models of hypodopaminergic states. SCH 412348 is a potent and selective adenosine A2A receptor antagonist that shows efficacy in rodent and primate models of movement disorders. Here we evaluated the effects of SCH 412348 in the MitoPark mouse, a genetic model that displays a progressive loss of dopamine neurons. The dopamine cell loss is associated with a profound akinetic phenotype that is sensitive to levodopa (l-dopa). SCH 412348 (0.3-10mg/kg administered orally) dose dependently increased locomotor activity in the mice. Moreover, SCH 412348 retained its efficacy in the mice as motor impairment progressed (12-22 weeks of age), demonstrating that the compound was efficacious in mild to severe Parkinson's disease-like impairment in the mice. Additionally, SCH 412348 fully restored lost functionality in a measure of hind limb bradykinesia and partially restored functionality in a rotarod test. These findings provide further evidence of the anti-Parkinsonian effects of selective adenosine A2A receptor antagonists and predict that they will retain their efficacy in both mild and severe forms of motor impairment.

Applicability of reverse microdialysis in pharmacological and toxicological studies

A recent application of microdialysis is the introduction of a substance into the extracellular space via the microdialysis probe. The inclusion of a higher amount of a drug in the perfusate allows the drug to diffuse through the microdialysis membrane to the tissue. This technique, actually called as reverse microdialysis, not only allows the local administration of a substance but also permits the simultaneous sampling of the extracellular levels of endogenous compounds. Local effects of exogenous compounds have been studied in the central nervous system, hepatic tissue, dermis, heart and corpora luteae of experimental animals by means of reverse microdialysis. In central nervous studies, reverse microdialysis has been extensively used for the study of the effects on neurotransmission at different central nuclei of diverse pharmacological and toxicological agents, such as antidepressants, antipsychotics, antiparkinsonians, hallucinogens, drugs of abuse and experimental drugs. In the clinical setting, reverse microdialysis has been used for the study of local effects of drugs in the adipose tissue, skeletal muscle and dermis. The aim of this review is to describe the principles of the reverse microdialysis, to compare the technique with other available methods and finally to describe the applicability of reverse microdialysis in the study of drugs properties both in basic and clinical research.

Changes in extracellular dopamine in the rat globus pallidus induced by typical and atypical antipsychotic drugs

Typical antipsychotic drugs with a high extrapyramidal motor side-effects liability markedly increase extracellular dopamine in the caudate-putamen, while atypical antipsychotic drugs with a low incidence of extrapyramidal motor side-effects have less pronounced stimulating actions on striatal dopamine. Therefore, it has been suggested that the extrapyramidal motor side-effects liability of antipsychotic drugs (APD) is correlated with their ability to increase extracellular dopamine in the caudate-putamen. The globus pallidus (GP) is another basal ganglia structure probably mediating extrapyramidal motor side-effects of typical antipsychotic drugs. Therefore, the present study sought to determine whether extracellular dopamine in the globus pallidus might be a further indicator to differentiate neurochemical actions of typical and atypical antipsychotic drugs. Using in vivo microdialysis we compared effects on pallidal dopamine induced by typical and atypical antipsychotic drugs in rats. Experiment I demonstrated that systemic administration of haloperidol (1 mg/kg; i.p.) and clozapine (20 mg/kg; i.p.) induced a significant pallidal dopamine release to about 160 and 180% of baseline, respectively. Experiment II revealed that reverse microdialysis of raclopride and clozapine using a cumulative dosing regimen did not stimulate extracellular dopamine in the globus pallidus if low (1microM) or intermediate (10 and 100 microM) concentrations were used. Only at a high concentration (1,000 microM), raclopride and clozapine induced a significant pallidal dopamine release to about 130 and 300% of baseline values, respectively. Thus, effects of typical and atypical antipsychotic drugs on pallidal dopamine were similar and thus, may not be related to their differential extrapyramidal motor side-effects liability. Furthermore, the finding that reverse microdialysis of raclopride over a wide range of concentrations did not stimulate pallidal dopamine concentrations tentatively suggests that pallidal dopamine release under basal conditions is not regulated by D2 autoreceptors.

Substantia nigra pars reticulata GABA is involved in the regulation of operant lever pressing: pharmacological and microdialysis studies

Substantia nigra pars reticulata (SNr) is an important mesencephalic nucleus that functions as a relay area for basal ganglia output. SNr receives GABAergic inputs from the neostriatum and globus pallidus, and in turn sends projections to a variety of motor areas. Although a large number of studies have focused upon the behavioral functions of basal ganglia dopamine, much less is known about the behavioral functions of SNr GABA. The present studies were undertaken to investigate the role of SNr GABA in lever pressing behavior. In the first experiment, the GABA(A) antagonist bicuculline was infused locally into SNr to determine if blockade of GABA receptors interfered with the performance of lever pressing on a fixed ratio 5 schedule. SNr injections of bicuculline produced a dose-related suppression of operant responding. Analysis of interresponse time bins showed that SNr bicuculline produced a response slowing characterized by a relative reduction in the number of fast interresponse times, and an increase in the relative number of pauses. In an additional experiment, microdialysis methods were used to determine if extracellular GABA is elevated during the performance of fixed ratio five lever pressing. During the 30 min lever pressing session, extracellular GABA showed a significant and substantial increase relative to baseline levels. These data support the hypothesis that SNr GABA is involved in the regulation of motor output, and indicate that GABA release in this structure is increased during behavioral stimulation.

Differential effects of antipsychotics on haloperidol-induced vacuous chewing movements and subcortical gene expression in the rat

The behavioral and neurochemical effects of switching from typical to atypical medications have not been evaluated in the rodent models of tardive dyskinesia. Thus, we treated rats with haloperidol-decanoate for 12 weeks, and assessed the effects of additional treatment with olanzapine, haloperidol, clozapine, or vehicle on vacuous chewing movements and expression of transcripts for dopamine receptors, tyrosine hydroxylase, delta-opioid receptor, prodynorphin, preproenkephalin, glutamic acid decarboxylase-65 (glutamic acid decarboxylase (GAD)-65) and GAD-67 and N-methyl-D-aspartate (NMDA) receptor subunits in the striatum and its efferent pathways. Haloperidol-decanoate induced vacuous chewing movements extinguished following an additional 4 weeks of treatment with vehicle, olanzapine or haloperidol, but not clozapine. Post-treatment, vacuous chewing movements in the clozapine group were significantly higher than the vehicle, olanzapine and haloperidol groups. GAD-67 mRNA expression in the globus pallidus was decreased following additional treatment with olanzapine or haloperidol, but not clozapine. Changes in expression of other transcripts were not detected. These findings demonstrate important differences in the effects of typical and atypical antipsychotics on chronic vacuous chewing movements.

Convergent evidence from microdialysis and presynaptic immunolabeling for the regulation of gamma-aminobutyric acid release in the globus pallidus following acute clozapine or haloperidol administration in rats

Antipsychotic drugs (APDs) have been primarily characterized for their effects on dopaminergic terminal regions in the brain, especially within the corpus striatum. Efferent GABA pathways are the primary outflow of striatal processing via their projections to the substantia nigra and the globus pallidus (GP). In the current study, we analyzed changes in pallidal GABA function following acute APD administration by means of in vivo microdialysis, followed by immunolabeling of presynaptic GABA terminal density in the contralateral hemisphere of the same animals. Acute administration of the atypical APD, clozapine (10 or 30 mg/kg, s.c.), produced a dose-dependent decrease in extracellular GABA. A corresponding dose-dependent increase in the density of presynaptic terminal GABA immunolabeling in the GP was found. In contrast, the typical APD, haloperidol (1 or 3 mg/kg, s.c.), had no significant effects on either measure, although a non-significant increase in extracellular GABA and decrease in the density of GABA terminal immunolabeling was noted. Paw retraction tests conducted during the time of microdialysis showed that haloperidol produced a typical pattern of highly pronounced motor impairment, while clozapine showed an atypical profile of minimal catalepsy. These complementary results obtained from in vivo neurochemistry and presynaptic neurotransmitter labeling suggest that systemic clozapine suppresses neuronal GABA release within the GP. This decrease in released pallidal GABA may play a role in the low motor side-effect liability of atypical APDs.