Regulation of Noradrenaline Release from Rat Occipital Cortex Tissue Chops by α2‐Adrenergic Agonists

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Abstract
Noradrenaline (NA) and the α2-adrenergic agonists clonidine, BHT-920, and UK 14304-18 inhibit potassiumevoked release of [3H]NA from rat occipital cortex tissue chops with similar potencies. NA (10−5M) was most effective as up to 85% inhibition could be observed compared with 75%, 55%, and 35% for UK 14304-18, clonidine, and BHT-920, respectively, all at 10−5M. Potassium-evoked release was enhanced by both forskolin (10−5M) and 1 mM dibutyryl cyclic AMP. Pretreatment of tissue chops with 1 mM dibutyryl cyclic AMP in the presence of 3-isobutyl-1-methylxanthine partially reversed the α2-adrenergic agonist inhibition of NA release. No reversal of inhibition was observed following pretreatment with 10−5M forskolin. The effects of clonidine, BHT-920, UK-14308–18, and NA on cyclic AMP formation stimulated by (a) forskolin, (b) isoprenaline, (c) adenosine, (d) potassium, and (e) NA were examined. Only cAMP formation stimulated by NA was inhibited by these α2-adrenergic agonists. These results suggest that only a small fraction of adenylate cyclase in rat occipital cortex is coupled to α2-adrenergic receptors. These results are discussed in relation to recent findings that several α2-adrenergic receptor subtypes occur, not all of which are coupled to the inhibition of adenylate cyclase, and that α2-adrenergic receptors inhibit NA release in rat occipital cortex by a mechanism that does not involve decreasing cyclic AMP levels.

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