||The dose-response effects of clonidine on transmission through somatospinal reflex, viscerospinal reflex, intraspinal, and spinal-bulbospinal reflex pathways were determined in spinal or chloralose-anesthetized cats to assess principal sites of drug action. Clonidine rapidly produced parallel, dose-dependent depression of transmission through each pathway which was antagonized by tolazoline or yohimbine. The order of descending sensitivity was found to be spinal-bulbospinal, intraspinal, and spinal reflex pathway. Analysis of the relative depression of transmission at spinal and at brainstem levels indicates that the spinal site is more sensitive to clonidine that it is generally considered to be. The effect of the serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP) was also assessed on the intraspinal pathway and somatospinal reflex. In contrast to clonidine, 5-HTP was more effective in depressing the spinal reflex than the intraspinal pathway, and both pathways could be depressed completely. Clonidine and 5-HTP appear to depress the excitability of sympathetic neurons by activating alpha(,2)- and 5-HT receptors, respectively. The intraspinal pathway was rapidly and markedly enhanced for 1-2 hours by two methylxanthines. Clonidine depressed intraspinal transmission and prevented enhancement by the xanthines; alpha(,2)-receptor antagonists blocked the effect of clonidine and not only restored but also markedly prolonged the enhancement by the xanthines. The results suggest that the excitability of sympathetic preganglionic neurons may be regulated by cyclic AMP through activation of different subtypes of adrenergic receptors that are either positively or negatively coupled to adenylate cyclase. Methyldopa (MD) produced a moderate enhancement of transmission through three central sympathetic pathways three hours after an i.v. infusion of 150 mg/kg. However, a subsequent dose of 5 mg/kg dose of reserpine, which alone causes no depression, produces prompt, marked depression of transmission through each pathway which is antagonized by yohimbine, suggesting that reserpine releases an active metabolite of MD to depress sympathetic preganglionic neurons by activing alpha(,2)-receptors. Depletion of epinephrine by blockade of phenylethanolamine-N-methyltransferase prevents this depression that occurs with the transmitter release. Propranolol modestly enhances transmission through the pathways tested.