The Journal of Neurophysiology Vol. 77 No. 3 March 1997, pp. 1349-1361
Copyright ©1997 The American Physiological Society

Effects of Serotonin on Caudal Raphe Neurons: Activation of an Inwardly Rectifying Potassium Conductance

Douglas A. Bayliss, Yu-Wen Li, and Edmund M. Talley

Department of Pharmacology, University of Virginia, Charlottesville, Virginia 22908

Bayliss, Douglas A., Yu-Wen Li, and Edmund M. Talley. Effects of serotonin on caudal raphe neurons: activation of an inwardly rectifying potassium conductance. J. Neurophysiol. 77: 1349-1361, 1997. We used whole cell current- and voltage-clamp recording in neonatal rat brain stem slices to characterize firing properties and effects of serotonin (5-HT) on neurons(n = 225) in raphe pallidus (RPa) and raphe obscurus (ROb). Of a sample of 51 Lucifer yellow-filled neurons recovered after immunohistochemical processing to detect tryptophan hydroxylase (TPH), 34 were found to be TPH immunoreactive (i.e., serotonergic). Serotonergic neurons had long-duration action potentials and fired spontaneously at low frequency (~1 Hz) in a pattern that was often irregular; at higher firing frequencies the discharge became more regular. These neurons displayed spike frequency adaptation, with maximal 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 were 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 neurons had more hyperpolarized resting membrane potentials, were not spontaneously active, had shorter-duration action potentials, and were depolarized by 5-HT. Caudal raphe neurons responded to 5-HT (1-5 µM) 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 an increase in peak conductance and was highly selective for K⁺. It was completely blocked by 0.2 mM Ba²⁺ but not by glibenclamide, an inhibitor of ATP-sensitive K⁺ channels. Effects of 5-HT were dose dependent, with an EC₅₀ of 0.1-0.3 µM. The 5-HT_1A agonist 8-OH-DPAT mimicked, and the 5-HT_1A antagonists (+)WAY 100135 and NAN 190 blocked, effects of 5-HT. The 5-HT_2A/C antagonist ketanserin did not inhibit the effects of 5-HT. Fewer 5-HT-responsive neurons were encountered in slices exposed acutely to pertussis toxin (~13%) than in adjacent control slices not exposed to pertussis toxin (~85%). In addition, in neurons recorded with pipettes containing GTPS (0.1 mM), 5-HT induced an inwardly rectifying current that did not reverse on washing. In many cells recorded with GTPS, a current developed in the absence of agonist that had properties identical to those of the 5-HT-sensitive current; when followed for extended periods, the agonist-independent GTPS-induced conductance desensitized, returning toward control levels with a time constant of ~18 min. Together these results indicate that serotonergic neurons of ROb and RPa are spontaneously active in a neonatal rat brain stem slice preparation and that hyperpolarization of those neurons by 5-HT_1A receptor stimulation is due to pertussis toxin-sensitive G protein-mediated activation of an inwardly rectifying K⁺ conductance. In addition, we identified a group of nonserotonergic medullary raphe neurons that had distinct electrophysiological properties and that was depolarized by 5-HT.

This article has been cited by other articles:

				A. M. Moruzzi, N. C. Abedini, M. A. Hansen, J. E. Olson, and M. M. Bosma Differential expression of membrane conductances underlies spontaneous event initiation by rostral midline neurons in the embryonic mouse hindbrain J. Physiol., November 1, 2009; 587(21): 5081 - 5093. [Abstract] [Full Text] [PDF]

				D. K. Mulkey, E. M. Talley, R. L. Stornetta, A. R. Siegel, G. H. West, X. Chen, N. Sen, A. M. Mistry, P. G. Guyenet, and D. A. Bayliss TASK Channels Determine pH Sensitivity in Select Respiratory Neurons But Do Not Contribute to Central Respiratory Chemosensitivity J. Neurosci., December 19, 2007; 27(51): 14049 - 14058. [Abstract] [Full Text] [PDF]

				D. S. McGehee GIRK Channels as a Target for SSRIs. Focus on "Reduced 5-HT1A- and GABAB Receptor Function in Dorsal Raphe Neurons Upon Chronic Fluoxetine Treatment of Socially Stressed Rats" J Neurophysiol, July 1, 2007; 98(1): 1 - 2. [Full Text] [PDF]

				L. N. Cornelisse, J. E. Van der Harst, J. C. Lodder, P.J.J. Baarendse, A. J. Timmerman, H. D. Mansvelder, B. M. Spruijt, and A. B. Brussaard Reduced 5-HT1A- and GABAB Receptor Function in Dorsal Raphe Neurons Upon Chronic Fluoxetine Treatment of Socially Stressed Rats J Neurophysiol, July 1, 2007; 98(1): 196 - 204. [Abstract] [Full Text] [PDF]

				M. Czesak, S. Lemonde, E. A. Peterson, A. Rogaeva, and P. R. Albert Cell-Specific Repressor or Enhancer Activities of Deaf-1 at a Serotonin 1A Receptor Gene Polymorphism J. Neurosci., February 8, 2006; 26(6): 1864 - 1871. [Abstract] [Full Text] [PDF]

				W. Hirdes, M. Schweizer, K. S Schuricht, S. S Guddat, I. Wulfsen, C. K Bauer, and J. R Schwarz Fast erg K+ currents in rat embryonic serotonergic neurones J. Physiol., April 1, 2005; 564(1): 33 - 49. [Abstract] [Full Text] [PDF]

				S. F. Morrison Activation of 5-HT1A receptors in raphe pallidus inhibits leptin-evoked increases in brown adipose tissue thermogenesis Am J Physiol Regulatory Integrative Comp Physiol, May 1, 2004; 286(5): R832 - R837. [Abstract] [Full Text] [PDF]

				T. Sickmann and C. Alzheimer Short-Term Desensitization of G-Protein-Activated, Inwardly Rectifying K+ (GIRK) Currents in Pyramidal Neurons of Rat Neocortex J Neurophysiol, October 1, 2003; 90(4): 2494 - 2503. [Abstract] [Full Text] [PDF]

				Y Ootsuka and W W Blessing 5-Hydroxytryptamine 1A Receptors Inhibit Cold-Induced Sympathetically Mediated Cutaneous Vasoconstriction in Rabbits J. Physiol., October 1, 2003; 552(1): 303 - 314. [Abstract] [Full Text] [PDF]

				X.-M. Ou, S. Lemonde, H. Jafar-Nejad, C. D. Bown, A. Goto, A. Rogaeva, and P. R. Albert Freud-1: A Neuronal Calcium-Regulated Repressor of the 5-HT1A Receptor Gene J. Neurosci., August 13, 2003; 23(19): 7415 - 7425. [Abstract] [Full Text] [PDF]

				T. S. Brink and P. Mason Raphe Magnus Neurons Respond to Noxious Colorectal Distension J Neurophysiol, May 1, 2003; 89(5): 2506 - 2515. [Abstract] [Full Text] [PDF]

				Z. Xiang and D. A. Prince Heterogeneous Actions of Serotonin on Interneurons in Rat Visual Cortex J Neurophysiol, March 1, 2003; 89(3): 1278 - 1287. [Abstract] [Full Text] [PDF]

				C. P. Washburn, J. E. Sirois, E. M. Talley, P. G. Guyenet, and D. A. Bayliss Serotonergic Raphe Neurons Express TASK Channel Transcripts and a TASK-Like pH- and Halothane-Sensitive K+ Conductance J. Neurosci., February 15, 2002; 22(4): 1256 - 1265. [Abstract] [Full Text] [PDF]

				W. Wang, J. K. Tiwari, S. R. Bradley, R. V. Zaykin, and G. B. Richerson Acidosis-Stimulated Neurons of the Medullary Raphe Are Serotonergic J Neurophysiol, May 1, 2001; 85(5): 2224 - 2235. [Abstract] [Full Text] [PDF]

				K. Yamada, H. Hasuo, M. Ishimatsu, and T. Akasu Characterization of Outward Currents Induced by 5-HT in Neurons of Rat Dorsolateral Septal Nucleus J Neurophysiol, April 1, 2001; 85(4): 1453 - 1460. [Abstract] [Full Text] [PDF]

				Q.-Q. Sun, J. R Huguenard, and D. A Prince Neuropeptide Y receptors differentially modulate G-protein-activated inwardly rectifying K+ channels and high-voltage-activated Ca2+ channels in rat thalamic neurons J. Physiol., February 15, 2001; 531(1): 67 - 79. [Abstract] [Full Text] [PDF]

				Q.-Q. Sun, G. Akk, J. R Huguenard, and D. A Prince Differential regulation of GABA release and neuronal excitability mediated by neuropeptide Y1 and Y2 receptors in rat thalamic neurons J. Physiol., February 15, 2001; 531(1): 81 - 94. [Abstract] [Full Text] [PDF]

				I. M. Raman, A. E. Gustafson, and D. Padgett Ionic Currents and Spontaneous Firing in Neurons Isolated from the Cerebellar Nuclei J. Neurosci., December 15, 2000; 20(24): 9004 - 9016. [Abstract] [Full Text] [PDF]

				I. M. Raman and B. P. Bean Ionic Currents Underlying Spontaneous Action Potentials in Isolated Cerebellar Purkinje Neurons J. Neurosci., March 1, 1999; 19(5): 1663 - 1674. [Abstract] [Full Text] [PDF]

				Y.-W. Li and D. A Bayliss Presynaptic inhibition by 5-HT1B receptors of glutamatergic synaptic inputs onto serotonergic caudal raphe neurones in rat J. Physiol., July 1, 1998; 510(1): 121 - 134. [Abstract] [Full Text] [PDF]

				Y.-W. Li, P. G. Guyenet, and D. A. Bayliss Voltage-Dependent Calcium Currents in Bulbospinal Neurons of Neonatal Rat Rostral Ventrolateral Medulla: Modulation by alpha 2-Adrenergic Receptors J Neurophysiol, February 1, 1998; 79(2): 583 - 594. [Abstract] [Full Text] [PDF]

				D. A. Bayliss, Y.-W. Li, and E. M. Talley Effects of Serotonin on Caudal Raphe Neurons: Inhibition of N- and P/Q-Type Calcium Channels and the Afterhyperpolarization J Neurophysiol, March 1, 1997; 77(3): 1362 - 1374. [Abstract] [Full Text] [PDF]