Inhibition of N-type Calcium Channels: The Only Mechanism by which Presynaptic α2-Autoreceptors Control Sympathetic Transmitter Release

Authors

  • Stefan Boehm,

    Corresponding author
    1. Department of Neuropharmacology, University of Vienna, A-1090 Vienna, Austria
      Stefan Boehm, Max-Planck-Institut für Hirnforschung, Abteilung Neurochemie, Deutschordenstrasse 46, D-60528 Frankfurt/Main, Germany
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      Max-Planck-Institut für Hirnforschung, Abteilung Neurochemie, Deutschordenstrasse 46, D-60528 Frankfurt/Main, Germany

  • Sigismund Huck

    1. Department of Neuropharmacology, University of Vienna, A-1090 Vienna, Austria
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Stefan Boehm, Max-Planck-Institut für Hirnforschung, Abteilung Neurochemie, Deutschordenstrasse 46, D-60528 Frankfurt/Main, Germany

Abstract

α2-Adrenoceptors are known to inhibit voltage-dependent Ca2+ channels located at neuronal cell bodies; the present study investigated whether this or alternative mechanisms, possibly downstream of Ca2+ entry, underlie the presynaptic α2-adrenergic modulation of transmitter release from chick sympathetic neurons. Using chick sympathetic neurons, overflow of previously incorporated [3H]noradrenaline was elicited in the presence of extracellular Ca2+ by electrical pulses, 25 mM K+ or 10μM nicotine, or by adding Ca2+ to otherwise Ca2+-free medium when cells had been made permeable by the calcium ionophore A23187 or by α-latrotoxin. Pretreatment of neurons with the N-type Ca2+ channel blocker ω-conotoxin GVIA and application of the α2-adrenergic agonist UK 14304 reduced the overflow elicited by electrical pulses, K+ or nicotine, but not the overflow caused by Ca2+ after permeabilization with α-latrotoxin or A23187. In contrast, the L-type Ca2+ channel blocker nitrendipine reduced the overflow due to K+ and nicotine, but not the overflow following electrical stimulation or α-latrotoxin- and A23187-permeabilization. The inhibition of electrically evoked overflow by UK 14304 persisted in the presence of nitrendipine and the L-type Ca2+ channel agonist BayK 8644, which per se enhanced overflow. In ω-conotoxin GVIA-treated cultures, electrically evoked overflow was also enhanced by BayK 8644 and almost reached the value obtained in untreated neurons. However, UK 14304 lost its effect under these conditions. Whole-cell recordings of voltage-activated Ca2+ currents corroborated these results: UK 14304 inhibited Ca2+ currents by 33%, nitrendipine caused a 7% reduction, and BayK 8644 increased the currents by 30%. Moreover, the dihydropyridines failed to abolish the inhibition by UK 14304, but pretreatment with ω-conotoxin GVIA, which reduced mean amplitude from 0.95 to 0.23 nA, entirely prevented α2-adrenergic effects. Our results indicate that the α2-autoreceptor-mediated modulation of noradrenaline release from chick sympathetic neurons relies exclusively on the inhibition of ω-conotoxin GVIA-sensitive N-type Ca2+ channels. Mechanisms downstream of these channels and voltage-sensitive Ca2+ channels other than N-type appear not to be important.

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