The Mechanisms of Action of Antipsychotics and Antidepressant Drugs

Chronic treatment with a classical neuroleptic alters excitatory amino acid and GABAergic neurotransmission in specific regions of the rat brain

The purpose of the following experiments was to describe some of the neurochemical changes that occur in the basal ganglia of rats exposed chronically to a classical neuroleptic, fluphenazine, and to relate these changes to extrapyramidal motor dysfunction. For these studies a combination of behavioural, receptor autoradiographic and in situ hybridization methods were employed. Preliminary pharmacological studies on GABA receptors showed that incubation in Tris-acetate rather than Tris-citrate buffer increased the number of binding sites labelled by [3H]muscimol by over 120% without affecting binding affinity or selectivity. The results of experiments with fluphenazine showed that treatment for six months increased the frequency of vacuous chewing movements compared to controls. In the striatum, changes in GABA transmission were observed in fluphenazine-treated rats with increases in glutamate decarboxylase mRNA levels in the caudate nucleus, dorsal shell and core of the accumbens and decreases in [3H]muscimol binding in the caudate and dorsal shell regions. These data suggest that fluphenazine treatment increased GABA transmission in specific subregions of the caudate and accumbens nuclei. In addition, glutamate decarboxylase mRNA levels were elevated in the entopeduncular nucleus of fluphenazine-treated animals. Autoradiographic analysis of excitatory amino acid binding showed that fluphenazine exposure decreased [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid binding in entopeduncular nucleus and in the ventrolateral thalamic nucleus and decreased [3H]dizocilpine maleate binding in the medial geniculate nucleus. These experiments show that in addition to altering GABA transmission, chronic neuroleptic exposure alters excitatory amino acid transmission in specific regions of the basal ganglia-thalamocortical motor system. The neuroleptic dependent increases in glutamate decarboxylase mRNA levels in the entopeduncular nucleus may reflect changes in neurotransmission in the indirect pathway connecting the major input and output nuclei of the basal ganglia. Changes in some of these brain regions may be related to the occurrence of extrapyramidal motor disturbances.