The effects of acute and chronic haloperidol treatment on dopamine release mediated by the medial forebrain bundle in the striatum and nucleus accumbens.

The delayed therapeutic effects of neuroleptics have been attributed to D2-mediated depolarization inactivation (DI) of mesolimbic dopaminergic neurons and concomitant reduction in dopamine release. Several studies have suggested, however, that DI may not reduce dopamine release and have hypothesized that this is due to increased impulse independent release. To examine the mechanisms that modulate dopamine release during DI, tetrodotoxin (TTX) was infused into the left medial forebrain bundle (MFB) of Sprague Dawley rats. Three-methoxytyramine (3-MT) levels 10 minutes after pargyline (75 mg/kg) were used as a measure of dopamine release. A dose response study showed that infusions of 10(-5) mol/L and 10(-4) mol/L TTX reduced 3-MT levels on the infused side by 70% in the striatum and 50% to 60% in the nucleus accumbens. In a time course study, 10(-5) mol/L TTX reduced striatal 3-MT at 30, 90, and 120 minutes. After intraperitoneal injections of haloperidol (0.4 mg/kg) for 1 or 21 days, TTX infusions again reduced 3-MT levels by approximately 70% in the striatum and 53% to 59% in the nucleus accumbens on the infused side. Acute and chronic haloperidol treatment did not alter the percent of TTX-induced reductions. These data suggest that dopaminergic neuronal impulse flow modulates similar amounts of total transmitter release after both acute and chronic haloperidol treatment. The results do not support the notion that DI mediates the antipsychotic effects of neuroleptics by markedly reducing total basal dopamine release or increasing impulse independent release. Alternatively, DI could reduce psychotic symptoms by changing the responsiveness of the dopamine system to external stimuli or by reducing synaptic dopamine levels that have been hypothesized to be elevated in psychotic patients.