α2‐Adrenoceptor‐Mediated Inhibition of Electrically Evoked [3H]Noradrenaline Release from Chick Sympathetic Neurons: Role of Cyclic AMP

Abstract: This study explores the role of cyclic AMP in electrically evoked [3H]noradrenaline release and in the α2‐adrenergic modulation of this release in chick sympathetic neurons. Along with an increase in stimulation‐evoked tritium overflow, applications of forskolin enhanced the formation of intracellular cyclic AMP. Both effects of forskolin were potentiated by the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine. The forskolin‐induced increase in overflow was abolished by the Rp‐diastereomer of cyclic AMP‐thioate, an antagonist at cyclic AMP‐dependent protein kinases, and 1,9‐dideoxy‐forskolin, an inactive analogue at adenylyl cyclase, had no effect on the evoked overflow. A 24‐h pretreatment with either cholera toxin or forskolin reduced the subsequent forskolin‐induced accumulation of cyclic AMP and inhibited the stimulation‐evoked release. Basal cyclic AMP production, however, remained unaltered after forskolin treatment and was enhanced after 24 h of cholera toxin exposure. The α2‐adrenergic agonist bromoxidine did not affect the formation of cyclic AMP stimulated by forskolin but reduced electrically evoked release. However, effects of bromoxidine on 3H overflow were attenuated by forskolin as well as by 8‐bromo‐cyclic AMP. Effects of bromoxidine on [3H]noradrenaline release were paralleled by an inhibition of voltage‐activated Ca2+ currents, primarily through a delayed time course of current activation. This effect was abolished when either forskolin or 8‐bromo‐cyclic AMP was included in the pipette solution. Both substances, however, failed to affect Ca2+ currents in the absence of bromoxidine. These results suggest that the signaling cascade of the α2‐adrenergic inhibition of noradrenaline release involves voltage‐activated Ca2+ channels but not cyclic AMP. Elevated levels of cyclic AMP, however, antagonize this α2‐adrenergic reduction, apparently through a disinhibition of Ca2+ channels.

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