Summary
In superfused rat hypothalamic slices prelabelled with [3H]-noradrenaline, the α2-adrenoceptor agonist UK 14304 inhibited in a concentration-dependent manner the electrically-evoked release of tritium. This inhibition was antagonized by the α2-adrenoceptor blocking agent idazoxan, which by itself increased the electrically-evoked tritium overflow. Exposure to forskolin, an adenylate cyclase activator, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of forskolin (1 μmol/l), both the inhibitory effect of UK 14304 and the increasing effect of idazoxan on the electrically-evoked release of [3H]-noradrenaline were less pronounced than in the absence of the adenylate cyclase activator. Exposure to forskolin and to the phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine shifted to the right the concentration-effect curve for UK 14304 in a similar manner as that observed in the presence of forskolin alone. Exposure to phorbol-12,13-dibutyrate (0.01–10 μmol/l), a drug which activates protein kinase C, increased the electrically-evoked release of [3H]-noradrenaline. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 μmol/l), the concentration effect curve for UK 14304 on tritium overflow was significantly shifted to the right. The increasing effect of idazoxan on tritium overflow was significantly less pronounced in the presence of 1 μmol/l phorbol-12,13-dibutyrate.
In superfused rat hypothalamic slices prelabelled with [3H]-5-hydroxytryptamine, the α2-adrenoceptor agonist UK 14304 significantly inhibited the electrically-evoked release of tritium. Exposure to forskolin increased in a concentration-dependent manner [3H]-5-hydroxytryptamine overflow, but did not modify the UK 14304-mediated inhibition. Exposure to 3-isobutyl-1-methylxanthine enhanced the electrically-evoked release of [3H]-5-hydroxytryptamine. In the presence of both forskolin (1 μmol/l) and 3-isobutyl-l-methylxanthine (1 mmol/l), the concentration-response curve for UK 14304 was significantly shifted to the right. Exposure to phorbol-12,13-dibutyrate (0.01–10 μmol/l) enhanced in a concentration-dependent manner the electrically-evoked overflow of [3H]-5-hydroxytryptamine. In the presence of phorbol-12,13-dibutyrate (0.1 and 1 μmol/l), UK 14304 was significantly less potent to inhibit tritium release than in the absence of the protein kinase C activator.
It is concluded that both cyclic AMP and phosphoinositide turnover are involved in the modulation of noradrenaline and 5-hydroxytryptamine release by presynaptic α2-adrenoceptors in rat hypothalamic slices. However, these interactions do not represent definitive proof for a cause-effect relationship for the second messengers mediating the α2-adrenoceptor induced inhibition of transmitter release either as autoreceptor or as heteroreceptor.
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Ramdine, R., Galzin, AM. & Langer, S.Z. Involvement of adenylate cyclase and protein kinase C in the α2-adrenoceptor-mediated inhibition of noradrenaline and 5-hydroxytryptamine release in rat hypothalamic slices. Naunyn-Schmiedeberg's Arch Pharmacol 340, 386–395 (1989). https://doi.org/10.1007/BF00167039
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DOI: https://doi.org/10.1007/BF00167039