Differential effects of centrally‐active antihypertensives on 5‐HT1A receptors in rat dorso‐lateral septum, rat hippocampus and guinea‐pig hippocampus

Abstract

The electrophysiological responses elicited by 5‐hydroxytryptamine1A‐(5‐HT1A) receptor agonists in rat and guinea‐pig CA1 pyramidal neurones and rat dorso‐lateral septal neurones were compared in vitro by use of conventional intracellular recording techniques. In the presence of 1 μm tetrodotoxin (TTX), to prevent indirect effects, 5‐HT, N,N‐dipropyl‐5‐carboxamidotryptamine (DP‐5‐CT) and 8‐hydroxy‐2(di‐n‐propylamino) tetralin (8‐OH‐DPAT) hyperpolarized the neurones from rat and guinea‐pig brain. The hypotensive drug flesinoxan, a selective 5‐HT1A receptor agonist, hyperpolarized neurones in all three areas tested; however, another hypotensive agent with high affinity at 5‐HT1A‐receptors, 5‐methyl‐urapidil, hyperpolarized only the neurones in rat hippocampus and septum. In guinea‐pig hippocampal neurones, 5‐methyl‐urapidil behaved as a 5‐HT1A‐receptor antagonist. The relative efficacies (5‐HT = 1) of DP‐5‐CT, 8‐OH‐DPAT, flesinoxan and 5‐methyl‐urapidil at the three sites were: rat hippocampus, 1.09, 0.7, 0.5 and 0.24; rat septum, 0.88, 0.69, 0.82 and 0.7; guinea‐pig hippocampus, 1.0, 0.69, 0.89 and 0, respectively. It is concluded that the hypotensive agents flesinoxan and 5‐methyl‐urapidil appear to have different efficacies at 5‐HT1A receptors located in different regions of the rodent brain. Whether these regional and species differences arise from receptor plurality or variability in intracellular transduction mechanisms remains to be elucidated. 1994 British Pharmacological Society