EFFECTS OF SEROTONIN ON CAUDAL RAPHE NEURONS - ACTIVATION OF AN INWARDLY RECTIFYING POTASSIUM CONDUCTANCE

We used whole cell current- and voltage-clamp recording in neonatal ra t brain stem slices to characterize firing properties and effects of s erotonin (5-HT) on neurons (n = 225) in raphe pallidus (RPa) and raphe obscurus (ROb). Of a sample of 51 Lucifer yellow-filled neurons recov ered after immunohistochemical processing to detect tryptophan hydroxy lase (TPH), 34 were found to be TPH immunoreactive (i.e., serotonergic ). Serotonergic neurons had long-duration action potentials and fired spontaneously at low frequency (similar to 1 Hz) in a pattern that was often irregular; at higher firing frequencies the discharge became mo re regular. These neurons displayed spike frequency adaptation, with m aximal steady-state firing rates of < 4 Hz. The overwhelming majority of identified serotonergic neurons was hyperpolarized by bath-applied 5-HT (94%; n = 32 of 34); conversely, most cells in this sample that w ere hyperpolarized by 5-HT were serotonergic (78%; n = 32 of 41). TPH- immunonegative neurons were separated into two populations. One group had properties that were indistinguishable from those of serotonergic caudal raphe neurons. The other group was truly distinct; those neuron s had more hyperpolarized resting membrane potentials, were not sponta neously active, had shorter-duration action potentials, and were depol arized by 5-HT. Caudal raphe neurons responded to 5-HT (1-5 mu M) with membrane hyperpolarization in current clamp (-13.4 +/- 1.1 mV, mean /- SE) or with outward current in voltage clamp (16.0 +/- 1.4 pA). The current induced by 5-HT was inwardly rectifying and associated with a n increase in peak conductance and was highly selective for K+. It was completely blocked by 0.2 mM Ba2+ but not by glibenclamide, an inhibi tor of ATP-sensitive K+ channels. Effects of 5-HT were dose dependent, with an EC(50) of 0.1-0.3 mu M. The 5-HT1A agonist 8-OH-DPAT mimicked , and the 5-HT1A antagonists (+)WAY 100135 and NAN 190 blocked, effect s of 5-HT. The 5-HT2A/C antagonist ketanserin did not inhibit the effe cts of 5-HT. Fewer 5-HT-responsive neurons were encountered in slices exposed acutely to pertussis toxin (similar to 13%) than in adjacent c ontrol slices not exposed to pertussis toxin (similar to 85%). In addi tion, in neurons recorded with pipettes containing GTP gamma S (0.1 mM ), 5-HT induced an inwardly rectifying current that did not reverse on washing. In many cells recorded with GTP gamma S, a current developed in the absence of agonist that had properties identical to those of t he 5-HT-sensitive current; when followed for extended periods, the ago nist-independent GTP gamma S-induced conductance desensitized, returni ng toward control levels with a time constant of similar to 18 min. To gether these results indicate that serotonergic neurons of ROb and RPa are spontaneously active in a neonatal rat brain stem slice preparati on and that hyperpolarization of those neurons by 5-HT1A receptor stim ulation is due to pertussis toxin-sensitive G protein-mediated activat ion of an inwardly rectifying K+ conductance. In addition, we identifi ed a group of nonserotonergic medullary raphe neurons that had distinc t electrophysiological properties and that was depolarized by 5-HT.