The role of presynaptic receptors in the release and synthesis of 3H-dopamine by slices of rat striatum

Effects of thioridazine on apomorphine-elicited stereotypic behavior and motor activity

Bilaterally injected thioridazine (10 micrograms) into the striata of rats augmented the stereotypic behavior elicited by apomorphine. The enhancing effect was attenuated by pretreatment with alpha-methyl-p-tyrosine. At 48 hr postinjection of thioridazine (1.0 mg/kg, IP), motor suppression from a low dose of apomorphine (0.2 mg/kg, IP) was enhanced; however, motor response to a high dose of apomorphine (1 mg/kg, SC) was not affected. Possible mechanisms of action of thioridazine are discussed.

Rotational behavior induced by unilateral electrical stimulations of nigro-striatal dopamine neurons: modification by low doses of apomorphine

Unilateral electrical stimulation of nigro-striatal dopamine axons produced marked turning contralateral to the stimulated side. Iterative stimulation led to a progressive increase of this response, followed by a definitive decrease associated with lesions of dopamine neurons. A low dose of apomorphine (25 micrograms/kg) slightly modified the time course and intensity of this response. On the contrary, in rats with a lesioned contralateral nigro-striatal system, low doses of apomorphine (6.25-50 micrograms/kg) markedly depressed contralateral turning whereas a higher dose (150 micrograms/kg) no longer depressed it. These results are discussed with regard to the dopamine autoreceptors.

Dopamine auto- and postsynaptic receptors: electrophysiological evidence for differential sensitivity to dopamine agonists

The responses of dopamine cells in the substantia nigra to iontophoretically administered dopamine and intravenous apomorphine were compared to the responses of spontaneously active neurons in the caudate nucleus. Dopaminergic cells were six to ten times more sensitive to dopamine and intravenous apomorphine than 86 percent of the caudate cells tested. This differential sensitivity of dopamine auto- and postsynaptic receptors may explain the apparently paradoxical behavioral effects induced by small compared to large doses of some dopamine agonists and may provide a means of developing new types of drugs to antagonize dopaminergic influence in the central nervous system.

Effect of prolactin on dopamine synthesis in medial basal hypothalamus: evidence for a short loop feedback

Several procedures were utilized to study the effects of prolactin on dopamine synthesis in the medial basal hypothalamus of the rat. Elevation of serum prolactin was produced by the administration of trifluoperazine (5 mg/kg, i.p.) and resulted in a significant increase in the conversion of [3',5'-3H]tyrosine to dopamine when measured in slices of medial basal hypothalamus and striatum. Hypophysectomy abolished this effect of trifluoperazine in the medial basal hypothalamus but not in the striatum. In addition, the synthesis of dopamine was significantly elevated in slices of medial basal hypothalamus obtained from rats bearing pituitary tumor implants that secreted microgram quantities of prolactin. In contrast, the in vitro synthesis of dopamine in the striatum of such rats was increased by the secretory products in one tumor line but decreased in another compared to that observed in control animals. It is suggested that the ability of prolactin to accelerate the synthesis of dopamine in the medial basal hypothalamus might constitute a short loop feedback system that finely regulates prolactin secretion.

Effects of neuroleptics on release of 3H-dopamine from slices of rat corpus striatum

The characteristics of 3H-DA release from striatal slices by electrical stimulation were analyzed and the effects of a number of neuroleptics thereon were examined under different experimental conditions. The butyrophenones, haloperidol and spiroperidol, already at low concentrations (0.1 - 1 micronM) increased basal tritium efflux in a dose-dependent manner. The phenothiazines, chlorpromazine and fluphenazine, were much less effective in this respect. The butyrophenones strongly inhibited the electrically stimulated overflow of both 3H-DA and 14C-GABA, while the phenothiazines again had little effect. The action of 1 micronM haloperidol on 3H-DA release could be blocked by 10 micronM cocaine, but not with 1 micronM apomorphine. Apomorphine itself had no significant effect on 3H-DA release. Our data do not support the suggestion that presynaptic DA receptors on dopaminergic nerve terminals may modulate the release of newly taken-up 3H-DA. Some neuroleptics, particularly the butyrophenones may have presynaptic effects not related to interaction with DA receptors. It is suggested that different mechanisms may be involved in the local presynaptic receptor-mediated feedback regulation of transmitter release in noradrenergic and dopaminergic systems in the CNS.

Effects of peripheral and local administration of picrotoxin on the release of newly synthesized 3H-dopamine in the caudate nucleus of the cat

The release of 3H-DA was estimated in superfusates of a superfusion cannula introduced into the caudate nucleus of "encéphale isolé" cats. 3H-DA was continuously formed from L-3,5-3H-tyrosine introduced into the superfusion medium. Injected at the periphery, picrotoxin (2.5 mg/kg, i.p.) markedly enhanced the release of 3H-DA, the effect being particularly pronounced 90 min after the drug injection. The effects of the local introduction of picrotoxin into the substantia nigra and into the caudate nucleus on 3H-DA release were also examined to ensure the specificity of the result. Picrotoxin (10(5) M) added into the superfusing medium of a second superfusion cannula introduced in to the substantia nigra enhanced by 2 to 5 times the release of 3H-DA during the drug application. No effect could be seen when the tip of the superfusion cannula was not exactly localized into the substantia nigra. Although less pronounced, a stimulation of 3H-DA release was also seen when picrotoxin(10(6) M) was added directly into the superfusion medium of the cannula introduced into the caudate nucleus. These results suggest that some gabaergic neurons are involved in the control of DA release by acting on dopaminergic cell bodies or dendrites into the substantia nigra. Other gabaergic neurons may directly or indirectly act on dopaminergic terminals within the caudate nucleus.

Dopaminergic neurons: an in vivo system for measuring drug interactions with presynaptic receptors

An in vivo system has been used to investigate the ability of dopamine agonists and antagonists to alter dopamine synthesis by acting at what appear to be presynaptic dopamine receptors. In order to eliminate postsynaptically induced changes in dopamine synthesis caused by the effects of these drugs on the firing rate of dopamine neurons, gammabutyrolactone was administered to block impulse flow in the nigro-neostriatal pathway. The accumulation of Dopa in the rat striatum after administration of Dopa decarboxylase inhibitor was used as an index of striatal tyrosine hydroxylase activity. It was found that administration of the dopamine agonists, apomorphine or ET-495 [1-(2-pyrimidyl)-piperonyl-piperazine], modified the apparent activity of striatal tyrosine hydroxylase when impulse flow was blocked in dopamine neurons. This presynaptic effect of apomorphine could be prevented by low doses of loxapine haloperidol and spiroperidol. Chlorpromazine, fluphenazine, and thioridizine were much less effective than the butyrophenones in blocking the effects of apomorphine. Molindone and (+) butaclamol, but not (-) butaclamol, reversed the presynaptic agonist effects, pimozide was a weak blocker and clozapine had no effect at all. All these neuroleptics except (-) butaclamol caused a significant increase in Dopa accumulation when impulse flow was intact. Compared with haloperidol the phenothiazines and pimozide appeared less potent in reversing the presynaptic effects of apomorphine than in blocking the behavioral effects of this agonist. Possible functional significance of the presynaptic dopamine receptors are considered.