Possible involvement of alpha 1-adrenoceptors in the modulation of [3H]noradrenaline release in rat brain cortical and hippocampal synaptosomes.

Nerve terminals obtained from rat brain cortex and hippocampus, were labelled with 0.04 microM of [3H]noradrenaline ([3H]NA). Thereafter the basal release of [3H]NA was measured in a Brandel superfusion apparatus, in the presence of alpha 1-adrenoceptor agonists (phenylephrine or noradrenaline) or these alpha 1-adrenoceptor agonists along with prazosin, an alpha 1-adrenoceptor antagonist. In cortical synaptosomes both alpha 1-adrenoceptor agonists increased the basal release of [3H]NA in a concentration-dependent manner (EC50 = 0.15 microM for phenylephrine and 12.6 microM for noradrenaline). Effects were reversed by 0.01 microM prazosin (EC50 = 2.46 and 130.1 microM, respectively). In synaptosomes from rat brain hippocampus, phenylephrine (EC50 = 1.28 microM) and noradrenaline (EC50 = 33.7 microM) also increased the [3H]NA release and prazosin (0.01 microM) shifted the corresponding concentration-response curves to the right (EC50 = 7.38 and 264.0 microM, respectively). Events produced by noradrenaline acting as alpha 1-adrenoceptor agonist did not show Ca2+ dependence. These results suggest (1) the presence of functional alpha 1-adrenoceptors in nerve terminals from rat brain cortex and hippocampus, (2) that these receptors seem to play a role in the presynaptic modulation of [3H]NA release, and (3) that intraterminal Ca2+ may be involved.