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

Abstract

The postnatal development of presynaptic opioid receptors inhibiting the release of acetylcholine (ACh) was studied in rat brain hippocampus, medial septum (MS) and diagonal band of Broca (DB). To this end, the corresponding brain slices (350 μm thick) of rats of various postnatal ages (postnatal day 4 [P4] to P16, and adult) were preincubated with [ 3H]choline and stimulated twice for 2 min ( S 1, S 2: at 3 Hz, 2 ms, 60 mA) during superfusion with physiological buffer containing hemicholinium-3. In parallel, the activity of choline acetyltransferase (ChAT) was determined in crude homogenates of the tissues as a marker for the development of cholinergic neurons. At any postnatal age, the electrically evoked overflow of tritium from slices preincubated with [ 3H]choline was highest in the DB, followed by the MS and the hippocampus. The evoked [ 3H]overflow increased with postnatal age, reached about 50% (MS, DB) or 30% (hippocampus) of the corresponding adult levels at P16 and correlated significantly with the corresponding ChAT activities. Presence of the preferential μ-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 were first observed at P8 only. Moreover, adult levels of inhibition due to DAMGO were reached at P16 in the DB and MS but not in the hippocampus. In septal areas, also the effect of the preferential δ-opioid receptor agonist DPDPE on the evoked [ 3H]overflow was studied: in contrast to DAMGO, however, significant inhibitory effects of DPDPE were first observed at P12 only. In conclusion, the postnatal development of presynaptic μ-opioid receptors on cholinergic neurons in the DB and MS starts earlier than in the hippocampus and precedes that of presynaptic δ-opioid receptors.