5-HT3 agonist-induced dopamine overflow during withdrawal from continuous or intermittent cocaine administration.

This experiment examined alterations in the ability of the highly selective 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide (mCPBG), to induce dopamine (DA) overflow in caudate brain slices obtained from rats withdrawn from continuous or intermittent cocaine administration. Rats were pretreated with 40 mg/kg per day cocaine for 14 days by either subcutaneous injections or osmotic minipumps, and then withdrawn from this regimen for 7 days. Caudate brain slices were obtained, and perfused with artificial cerebrospinal fluid. Following an equilibration period, the slices were then perfused with 25, 50, or 100 microM mCPBG. The samples were assayed for DA content by HPLC with electrochemical detection. The results indicated that the pretreatment with intermittent cocaine did not consistently alter the ability of mCPBG to induce DA overflow, although there was a reduction in the amount of DA released by the highest concentration of mCPBG. In contrast, pretreatment with continuous cocaine administration consistently and significantly attenuated the ability of mCPBG to induce DA overflow. The DA overflow induced by mCPBG was partially dependent on extracellular Ca2+ in the perfusion medium for the saline control and intermittent administration subjects: elimination of Ca2+ from the medium significantly reduced, but did not eliminate, DA overflow for these two groups. In contrast, elimination of Ca2+ from the perfusion medium had a significant enhancing effect on mCPBG-induced DA overflow in the continuous administration rats. These results suggest that distinct temporal patterns of cocaine administration differentially alter the ability of a 5-HT3 agonist to increase extracellular DA levels, and that this effect may be related to an impairment of Ca(2+)-dependent release.(ABSTRACT TRUNCATED AT 250 WORDS)