These studies examined the hypothalamic site and receptor subtype mediating the serotonergic (5-HT) control of PRL secretion in conscious male rats. Initially, we characterized the pharmacology of the 5-HT releaser and 5-HT agonists that increase PRL release. Subsequently, we performed lesion experiments to locate the 5-HT receptors involved in PRL secretion. p-Chloroamphetamine, a 5-HT releaser, is postulated to enter serotonergic nerve terminals through the 5-HT uptake mechanism, which can be inhibited by fluoxetine. p-Chloroamphetamine (8 mg/kg, ip) increased the plasma PRL concentration approximately 6-fold. The 5-HT uptake inhibitor fluoxetine almost completely prevented this increase, demonstrating that p-chloroamphetamine increases PRL release via a serotonergic mechanism. The 5-HT1C/5-HT2 agonist +(-)-1-(2,5-dimethoxy-4-iodophenyl)2-aminopropane HCl (ip) produced a strong (30-fold) dose-dependent elevation of plasma PRL, which was virtually eliminated by 0.1 mg/kg (sc) ritanserin, a 5-HT1C/5-HT2 antagonist. +(-)-1-(2,5-Dimethoxy-4-iodophenyl)2-aminopropane HCl injected intracerebroventricularly (icv) in doses below those that were peripherally effective also produced a significant (8-fold) increase in PRL secretion that was again attenuated by icv pretreatment with ritanserin (2 micrograms/kg). RU 24969 (5-methoxy-3-[1,2,3,4-tetrahydro-4-pyridinyl]1H-indole) was reported to act as both a 5-HT releaser and a direct postsynaptic 5-HT agonist. To test whether RU 24969 releases 5-HT to increase PRL secretion, we depleted 5-HT stores with the 5-HT synthesis inhibitor p-chlorophenylalanine. The ability of RU 24969 (0.5, 1, 5, and 10 mg/kg, ip) to elevate PRL secretion was not inhibited by pretreatment with p-chlorophenylalanine, suggesting that RU 24969 stimulates PRL secretion only through activation of postsynaptic 5-HT receptors. To test whether RU 24969 acts centrally, it was injected either icv, through chronic icv cannulae, or peripherally (ip). RU 24969 injected icv significantly stimulated PRL secretion (11-fold) at doses 500-fold lower than the peripherally effective doses (10 micrograms/kg vs. 5 mg/kg), suggesting a role for central 5-HT receptors in the regulation of PRL secretion. In addition, rats pretreated with the 5-HT1C/5-HT2 antagonist LY53857 (icv) significantly inhibited the PRL response if RU 24969 was injected ip, but not icv. The results of these experiments suggest that 5-HT1C or 5-HT2 receptors in the brain participate in the serotonergic stimulation of PRL secretion.(ABSTRACT TRUNCATED AT 400 WORDS)