Abstract

Recently, we found that the beta 1/beta 2 adrenoceptor blocking agent (-)penbutolol prevents behavioral and biochemical actions of the specific serotonin (5-HT)1A agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin. The putative 5-HT1 receptor antagonist profile of (-)penbutolol was further explored in the present study, using in vivo microdialysis methods to assess its effects on central 5-HT release. (+)Penbutolol and (-)pindolol were included for comparison purposes. In contrast to (-)pindolol (8.0 mg/kg s.c.), administration of (-)penbutolol (2.0 or 8.0 mg/kg s.c.) increased hippocampal 5-HT output. The (-)penbutolol-induced 5-HT response was dose-related, stereoselective and Ca(++)-dependent. In addition, the 5-HT response to (-)penbutolol was abolished by omitting the 5-HT reuptake blocker citalopram from the perfusion medium, suggesting the need for endogenous 5-HT tone. Local (-)penbutolol (1 microM) perfusion increased the 5-HT output per se, and also blocked 5-HT release suppression caused by the 5-HT1B receptor agonist CP-93,129. Furthermore, (-)penbutolol, but not its (+)antipode, prevented the decrease of 5-HT release induced by the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin. By comparison, the 5-HT1 receptor inactive beta adrenoceptor blockers metoprolol (beta 1) and ICI 118,551 (beta 2), given alone or in combination, did not increase 5-HT output and were ineffective in antagonizing the (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin response. The data indicate that (-)penbutolol possesses 5-HT1A and 5-HT1B autoreceptor antagonist properties, and may be a useful tool in studies of central 5-HT receptor-mediated function.