Activation of alpha 2-adrenoceptors causes inhibition of calcium channels but does not modulate inwardly-rectifying K+ channels in caudal raphe neurons.
Many neurotransmitter receptors that interact with pertussis toxin-sensitive G proteins, including the alpha 2-adrenergic receptor, can modulate both voltage-dependent calcium channels and G protein-coupled inwardly-rectifying K+ channels. Serotonergic neurons of the medulla oblongata (nucleus raphe obscurus and nucleus raphe pallidus), which provide a major projection to sympathetic and motor output systems, receive a catecholaminergic input and express alpha 2-adrenergic receptors. Therefore, we tested the effects of norepinephrine on voltage-dependent calcium channels and G protein-coupled inwardly-rectifying K+ channels in neonatal raphe neurons using whole-cell recording in a brainstem slice preparation. Calcium channel currents were inhibited by norepinephrine in the majority of raphe neurons tested (88%) and in all identified tryptophan hydroxylase-immunoreactive (i.e. serotonergic) neurons. When tested in the same neurons, the magnitude of calcium current inhibition by norepinephrine (approximately 25%) was less than that induced by 5-hydroxytryptamine (approximately 50%). The norepinephrine-induced calcium current inhibition was mediated by alpha 2-adrenergic receptors; it was mimicked by UK 14304, an alpha 2-adrenergic receptor agonist and blocked by idazoxan, an alpha 2-adrenergic receptor antagonist, but not affected by prazosin or propanolol (alpha 1 and beta adrenergic antagonists, respectively). Calcium current inhibition by norepinephrine was essentially eliminated following application of omega-Conotoxin GVIA and omega-Agatoxin IVA, indicating that norepinephrine modulated N- and P/Q-type calcium channels predominantly. Calcium current inhibition by norepinephrine was voltage-dependent and mediated by pertussis toxin-sensitive G proteins. Thus, as expected, alpha 2-adrenergic receptor activation inhibited N- and P/Q-type calcium currents in medullary raphe neurons via pertussis toxin-sensitive G proteins. In parallel experiments, however, we found that norepinephrine had no effect on G protein-coupled inwardly-rectifying K+ channels in any raphe neurons tested, despite the robust activation of those channels in the same neurons by 5-hydroxytryptamine. Together, these data indicate that alpha 2-adrenergic receptors can modulate N- and P/Q-type calcium channels in caudal medullary raphe neurons but do not couple to the G protein-coupled inwardly-rectifying K+ channels which are also present in those cells. This is in contrast to the effect of 5-hydroxytryptamine1A receptor activation in caudal raphe neurons, and indicates a degree of specificity in the signalling by different pertussis toxin-sensitive G protein-coupled receptors to voltage-dependent calcium channels and G protein-coupled inwardly-rectifying K+ channels even within the same cell system.
Li
Y W
YW
Department of Pharmacology, University of Virginia, Charlottesville 22908, USA.
Bayliss
D A
DA
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism*; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000378:metabolism; D007150:Immunohistochemistry; D008564:Membrane Potentials / Q000502:physiology; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000819:agonists / Q000378:metabolism*; D011903:Raphe Nuclei / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000494:pharmacology; D014365:Tryptophan Hydroxylase / Q000378:metabolism
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012701:Serotonin; D014365:Tryptophan Hydroxylase; D019204:GTP-Binding Proteins; D009638:Norepinephrine