Activation of α2-adrenoceptors causes inhibition of calcium channels but does not modulate inwardly-rectifying K + channels in caudal raphe neurons

Abstract

Many neurotransmitter receptors that interact with pertussis toxin-sensitive G proteins, including the α 2-adrenergic receptor, can modulate both voltage-dependent calcium channels and G protein-coupled inwardly-rectifying K + channels. Serotonergic neurons of the medulla oblongata (n. raphe obscurus, and n. raphe pallidus), which provide a major projection to sympathetic and motor output systems, receive a catecholaminergic input and express α 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 (∼25%) was less than that induced by 5-hydroxytryptamine (∼50%). The norepinephrine-induced calcium current inhibition was mediated by α 2-adrenergic receptors; it was mimicked by UK 14 304, an α 2-adrenergic receptor agonist and blocked by idazoxan, an α 2-adrenergic receptor antagonist, but not affected by prazosin or propanolol ( α 1 and β adrenergic antagonists, respectively). Calcium current inhibition by norepinephrine was essentially eliminated following application of ω-Conotoxin GVIA and ω-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, α 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 α 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-hydroxytryptamine 1A receptor activation in caudal raphe neurons, and indicates a degree of specificity in the signalling by different pertussistoxin-sensitive G protein-coupled receptors to voltage-dependent calcium channels and G protein-coupled inwardly-rectifying K + channels even within the same cell system.