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.

Presynaptic Inhibition by Concanavalin A: Are α‐Latrotoxin Receptors Involved in Action Potential‐Dependent Transmitter Release?

“…The procedures for dissociating and culturing chick sympathetic neurons have been described in more detail elsewhere (e.g., Boehm et al, , 1994. Cells were plated on poly-D-lysine-coated (Sigma, no.…”

Neural system modeling and simulation using hybrid functional Petri net

“…However, contradictory result was obtained in chick synaptic neuron [29], showing that the activation of adenosine receptor did not alter the release of NE. That makes the problem even more puzzling and complicated.…”

“…Elevated cAMP level was reported to have a negative impact on the effect induced by α-adrenoreceptor activation [29], but the molecular mechanism is not clear yet.…”

Cholera Toxin Induces Cyclic AMP‐Independent Down‐Regulation of Gsα and Sensitization of α2‐Autoreceptors in Chick Sympathetic Neurons

“…The procedures used to dissociate and culture sympathetic neurons have previously been described in detail (e.g., Boehm et al, 1991Boehm et al, , 1994. In brief, lumbosacral paravertebral sympathetic ganglia were derived from 12-day-old chick embryos, trypsinized (0.1% for 30 mm at 36°C),triturated, and resuspended in Dulbecco's modified Eagle's medium (GIBCO no.…”

“…In cultures for superfusion experiments, no precautions were taken to remove these nonneural cells, which neither accumulate nor release [3H]NA (Boehm et al, 1991). To prepare cultures for U-protein assays, ganglia from up to 16 chicks were dissociated simultaneously, and subsequently nonneural cells were reduced to <5% by differential plating, as described (Boehm et al, 1994). The resulting pure neuronal cell suspension was plated onto 35-mm culture dishes at a density of 0.5-1 X 10'~neurons/dish.…”