PHARMACOLOGICAL CHARACTERIZATION OF 5-HYDROXYTRYPTAMINE-INDUCED EXCITATION OF AFFERENT CERVICAL VAGUS NERVE IN ANESTHETIZED RATS

1 An excitatory response to 5-hydroxytryptamine (5-HT) was measured from the afferent vagus nerve of anaesthetized rats. Measurements were determined by an extracellular recording from the whole nerve. 2 Intravenous bolus injection of 5-HT (1.56-100-mu-g kg-1) evoked a dose-dependent excitation of afferent vagus nerve activity. This response was blocked not only by a selective 5-HT3 receptor antagonist, GR38032F (10 and 100-mu-g kg-1), but also by a 5-HT2 receptor antagonist, ketanserin (10 and 100-mu-g kg-1). 3 Both a 5-HT3 receptor agonist, 2-methyl-5-HT (3.12-100-mu-g kg-1), and a 5-HT2 receptor agonist, alpha-methyl-5-HT (3.12-50-mu-g kg-1), produced a dose-dependent excitation of afferent vagus nerve activity. These excitatory effects were antagonized by GR38032F (10-mu-g kg-1) and ketanserin (10-mu-g kg-1), respectively. 4 A 5-HT1 like receptor agonist, 5-carboxamidotryptamine (50-mu-g kg-1), and a putative 5-HT4 receptor agonist, 5-methoxytryptamine (100 Ag kg-1), failed to produce excitatory effects on the afferent vagus nerve. 5 These results suggest that the 5-HT-induced excitatory response of the afferent vagus nerve might be mediated not only via 5-HT3 receptors but also via 5-HT2 receptors in anaesthetized rats. It is unlikely, however, that either 5-HT1-like or putative 5-HT4 receptors are involved in the excitatory response of the afferent vagus nerve to 5-HT.