Effects of serotonin on caudal raphe neurons: inhibition of N- and P/Q-type calcium channels and the afterhyperpolarization

J Neurophysiol. 1997 Mar;77(3):1362-74. doi: 10.1152/jn.1997.77.3.1362.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Barium / metabolism
  • Calcium Channel Blockers / pharmacology*
  • Calcium Channels / drug effects
  • Calcium Channels / metabolism*
  • Electrophysiology
  • GTP-Binding Proteins / metabolism
  • In Vitro Techniques
  • Membrane Potentials / drug effects
  • Membrane Potentials / physiology
  • Neurons / drug effects
  • Neurons / metabolism
  • Patch-Clamp Techniques
  • Pertussis Toxin
  • Raphe Nuclei / cytology
  • Raphe Nuclei / drug effects*
  • Rats
  • Receptors, Serotonin / drug effects
  • Serotonin / metabolism
  • Serotonin / pharmacology*
  • Virulence Factors, Bordetella / pharmacology


  • Calcium Channel Blockers
  • Calcium Channels
  • Receptors, Serotonin
  • Virulence Factors, Bordetella
  • Barium
  • Serotonin
  • Pertussis Toxin
  • GTP-Binding Proteins