Biochemical and behavioural effects of isamoltane, a beta-adrenoceptor antagonist with affinity for the 5-HT1B receptor of rat brain

The biochemical and behavioural effects of isamoltane, a beta-adrenoceptor and 5-HT1B receptor antagonist that has higher affinity for 5-HT1B receptors than for 5-HT1A receptors, on 5-HT neurotransmission in the rat brain were examined. In binding experiments isamoltane was found to be about five times more potent as a ligand for the 5-HT1B receptor than for the 5-HT1A receptor (Ki values 21 and 112 nmol/l, respectively). Isamoltane increased the K(+)-evoked overflow of 3H from 3H-5-HT loaded slices of rat occipital cortex at 0.1 mumol/l, consistent with inhibition of the terminal 5-HT autoreceptor. In vivo, isamoltane significantly increased the concentration of 5-hydroxyindoleacetic acid in hypothalamus and hippocampus indicating an increased 5-HT turnover with a maximal effect at 3 mg/kg s.c. A higher dose produced a less pronounced effect. This effect did not seem to be due to the beta-adrenoceptor blocking action of isamoltane since the beta-adrenoceptor antagonists. (-)-alprenolol, betaxolol or ICI 118.551 had no significant effects on 5-HT turnover at 5 mg/kg s.c. Isamoltane at 3 mg/kg s.c. induced the wet-dog shake response which was blocked by the Tryptophan Hydroxylase Inhibitor p-chlorophenylalanine. In contrast, the same response induced by the 5-HT2 receptor agonist quipazine was not blocked by pretreatment with p-chlorophenylalanine. The wet-dog shakes evoked by isamoltane and quipazine were blocked by ritanserin, which indicates that 5-HT2 receptors are involved in their expression. These observations indicate that isamoltane, by inhibiting the terminal 5-HT autoreceptors, increased the synaptic concentration of 5-HT to a level that induced a behavioural response.