K⁺ channel and 5-hydroxytryptamine_1A autoreceptor interactions in the rat dorsal raphe nucleus: An in vitro electrophysiological study

Extracellular recordings were made from serotonergic neurons of the rat dorsal raphe nucleus in a slice preparation. In the presence of phenylephrine (3 μM) to restore the pacemaker activity of otherwise silent serotonergic neurons, superfusion with the 5-hydroxytryptamine_1A agonist ipsapirone depressed the firing of these neurons with an ic₅₀ of ~50 nM. Complete inhibition was achieved with 100-300 nM of the drug. Concomitant superfusion with the 5-hydroxytryptamine_1A antagonists spiperone (100 nM) or propranolol (10 μM) markedly reduced the inhibitory effect of ipsapirone (100 nM). Superfusion with K⁺ channel blockers such as apamin (50-100 nM), charybdotoxin (100 nM) or Ba²⁺ (1 mM) did not induce any changes in the electrical activity of serotonergic neurons. However, 4-aminopyridine (0.1-1 mM) disrupted the regularity of their discharge without affecting the mean firing rate. The ipsapirone-induced inhibition was unchanged by apamin and charybdotoxin, but was markedly reduced by Ba²⁺ and 4-aminopyridine. Thus the ic₅₀ of ipsapirone was shifted to ~150 nM in the presence of 1 mM of 4-aminopyridine. These results indicate that, in serotonergic neurons within the dorsal raphe nucleus, the K⁺ channel opened through the stimulation of 5-hydroxytryptamine_1A autoreceptors is 4-aminopyridine-sensitive.