Ontogeny of Influence of Clonidine on High Potassium-Induced Release of Noradrenaline and Specific [3H] Clonidine Binding in the Rat Brain Cortex

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Abstract
A high (20 mM) K+-induced, Ca2+-dependent release of L-[3H]-noradrenaline [norepinephrine] (NA) from cerebral cortex slices was detected in rats on day 18 of gestation. Depolarization-evoked release (percent of total radioactivity) increased with age between day 18 of gestation and day 70 of postnatal life. Clonidine (0.1 .mu.M) significantly (P < 0.05) reduced a high K+-induced L-[3H]NA release on days 7 and 70 but not on day 1. Both Kd and Bmax of specific [3H]clonidine binding to cerebral cortex membranes rapidly increased between days 1-7, followed by a gradual increase to the adult level. Presynaptic .alpha.2-adrenoceptors which regulate NA release seems to become sensitive to .alpha.2-agonists and to reach functional maturity between days 1-7 after birth in the cerebral cortex of rats.