Repeated cocaine intoxication can result in the development of behavioral sensitization in animals and psychosis in humans, phenomena that have been associated with alterations in dopamine (DA) function. Using electrophysiologic and autoradiographic techniques, modifications of central serotonin (5-hydroxytryptamine; 5-HT) systems were investigated in rats treated with a regimen of cocaine administration that produced behavioral sensitization. The inhibitory response of single 5-HT neurons in the dorsal raphe (DR) to (-)-cocaine, the 5-HT uptake inhibitor fluoxetine or the 5-HT1A agonist 8-hydroxy-2-[di-N-propylamino]tetralin (8-OHDPAT) was significantly enhanced in cocaine-treated rats. Furthermore, several brain areas that contain either cell bodies (DR) or terminals for 5-HT (medial and sulcal prefrontal cortex, frontal cortex) showed cocaine-induced elevations in [3H]imipramine-labeled 5-HT uptake sites, while [3H]-8-OHDPAT-labeled 5-HT1A receptors were decreased only in the central medial amygdala. These results suggest that modifications of autoregulatory mechanisms secondary to alterations of 5-HT uptake processes may contribute to the development of cocaine sensitization.

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The central nervous system (CNS) of the rat is known to contain serotonin (5-HT) type -3 receptors (5-HT3). Behavioral evidence suggests that 5-HT3 receptors interact with mesolimbic dopamine (DA) systems and that 5-HT3 antagonists can interfere with the hyperlocomotive effects of amphetamine and cocaine and the rewarding and stimulus effects of morphine, nicotine and ethanol. Cocaine, which blocks the reuptake of DA, norepinephrine (NE), and 5-HT in the CNS, also may be an antagonist at 5-HT3 receptors. The purpose of the present study was to determine whether systemic administration of the 5-HT3 antagonists ICS 205930 or MDL 72222 could mimic or block the discriminative stimulus properties of cocaine. Once rats (N = 16) were trained to discriminate cocaine (10 mg/kg) from saline, substitution tests with various doses of cocaine (0.313-10 mg/kg), ICS 205930 (2-24 mg/kg), and MDL 72222 (2-16 mg/kg) were conducted. Cocaine produced a dose-related increase in cocaine-appropriate responding while the 5-HT3 antagonists engendered primarily saline-lever responding. Neither ICS 205930 nor MDL 72222 were able to antagonize the stimulus effects of cocaine (5 mg/kg). Response rates were not significantly reduced when the 5-HT3 antagonists were given in combination with cocaine. The results indicate that although 5-HT3 antagonists can inhibit some of the unconditioned behavioral effects of psychomotor stimulants, the discriminative stimulus effects of cocaine remain intact.