Effects of serotonin on caudal raphe neurons: inhibition of N- and P/Q-type calcium channels and the afterhyperpolarization.
We characterized whole cell barium currents through calcium channels and investigated the effects of serotonin (5-HT) on calcium channel currents and firing behavior in visualized caudal raphe neurons of the neonatal rat in brain stem slices (n = 201). A subpopulation of recorded neurons was recovered after staining for tryptophan hydroxylase (TPH), the 5-HT synthesizing enzyme (n = 21); of those cells, 86% were TPH immunoreactive, suggesting that the majority of recorded neurons was serotonergic. Calcium channel currents began to activate at about -40 mV in caudal raphe neurons and showed a peak amplitude of 952.2 +/- 144.2 (SE) pA at -10 mV. A small low-voltage activated current was also observed (approximately 22 pA). Calcium channel currents were potently inhibited by bath-applied 5-HT in most cells tested (approximately 90%). The EC50 for inhibition of calcium current by 5-HT was 0.1 microM; a saturating concentration (1.0 microM) blocked approximately 40% of the current evoked at 0 mV from a holding potential of -70 mV (n = 101). Current inhibition was associated with a slowing of activation kinetics and a shift in the peak of the current-voltage relationship, and was partially relieved by strong depolarizations. Current inhibition by 5-HT was mimicked by 8-OH-DPAT, a specific 5-HT1A agonist, and blocked by the 5-HT1a antagonists NAN 190 and (+) WAY 100135, but was unaffected by ketanserin, a 5-HT2A/C antagonist. omega-Conotoxin GVIA (omega-CgTx)-sensitive N-type channels and omega-agatoxin IVA (omega-AgaIVA)-sensitive P/Q-type channels together accounted for most of the calcium current (36 and 37%, respectively). Nimodipine had no effect on the calcium current, indicating that caudal raphe neurons do not express dihydropyridine-sensitive L-type currents. A substantial residual current (27%) remained after application of omega-CgTx, omega-AgaIVA, and nimodipine. Most of the 5-HT-sensitive calcium current was blocked by omega-CgTx and omega-AgaIVA; 5-HT had little effect on the residual current. Inhibition of calcium current by 5-HT was irreversible when GTP gamma S, a nonhydrolyzable guanosine 5'-triphosphate (GTP) analogue, was substituted for GTP in the pipette. In addition, the effects of 5-HT were blocked by pretreating slices with pertussis toxin (PTX). Together these data indicate that inhibition of N- and P/Q-type calcium current in serotonergic caudal raphe neurons is mediated by a 5-HT1A receptor via PTX-sensitive G proteins. Under current clamp, calcium channel toxins (omega-CgTx and omega-AgaIVA) and 5-HT each caused a decrease in the spike afterhyperpolarization and enhanced the repetitive firing response to injected current. The similar effects of 5-HT and the calcium channel toxins on firing behavior suggest that those effects of 5-HT were secondary to inhibition of N- and P/Q-type calcium channels.
Bayliss
D A
DA
Department of Pharmacology, University of Virginia, Charlottesville 22908, USA.
Li
Y W
YW
Talley
E M
EM
D000818:Animals; D001464:Barium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000378:metabolism; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin; D011903:Raphe Nuclei / Q000166:cytology / Q000187:drug effects*; D051381:Rats; D011985:Receptors, Serotonin / Q000187:drug effects; D012701:Serotonin / Q000378:metabolism / Q000494:pharmacology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D011985:Receptors, Serotonin; D010566:Virulence Factors, Bordetella; D001464:Barium; D012701:Serotonin; D037342:Pertussis Toxin; D019204:GTP-Binding Proteins