Postnatal development of opioid receptors modulating acetylcholine releasein hippocampus and septum of the rat

The postnatal development of presynaptic opioid receptors inhibiting the re lease of acetylcholine (ACh) was studied in rat brain hippocampus. medial s eptum (MS) and diagonal band of Broca (DB). To this end, the corresponding brain slices (350 mum thick) of rats of various postnatal ages (postnatal d ay 4 [P4] to P16, and adult) were preincubated with [H-3]choline and stimul ated twice for 2 min (S-1, S-2: at 3 Hz, 2 ms, 60 mA) during superfusion wi th physiological buffer containing hemicholinium-3. In parallel, the activi ty of choline acetyltransferase (ChAT) was determined in crude homogenates of the tissues as a marker for the development of cholinergic neurons. At a ny postnatal age, the electrically evoked overflow of tritium from slices p reincubated with [H-3]choline was highest in the DB, followed by the MS and the hippocampus. The evoked [H-3]overflow increased with postnatal age, re ached about 50% (MS, DB) or 30% (hippocampus) of the corresponding adult le vels at P16 and correlated significantly with the corresponding ChAT activi ties. Presence of the preferential mu -opioid receptor agonist DAMGO during S-2 significantly inhibited the evoked overflow of tritium already at P4, in DB and MS, whereas in the hippocampus significant inhibitory effects wer e first observed at P8 only, Moreover, adult levels of inhibition due to DA MGO were reached at P16 in the DB and MS but not in the hippocampus. In sep tal areas, also the effect of the preferential delta -opioid receptor agoni st DPDPE on the evoked [H-3]overflow was studied: in contrast to DAMGO, how ever, significant inhibitory effects of DPDPE were first observed at P12 on ly. In conclusion, the postnatal development of presynaptic mu -opioid rece ptors on cholinergic neurons in the DB and MS starts: earlier than in the h ippocampus and precedes that of presynaptic delta -opioid receptors. (C) 20 00 Elsevier Science B.V. All rights reserved.