POSTNATAL-DEVELOPMENT OF MUSCARINIC AUTORECEPTORS MODULATING ACETYLCHOLINE-RELEASE IN THE SEPTOHIPPOCAMPAL CHOLINERGIC SYSTEM - I - AXON TERMINAL REGION - HIPPOCAMPUS

We studied the postnatal development of the release of acetylcholine ( ACh) and of presynaptic, release-inhibiting muscarinic autoreceptors i n the rat hippocampus. To this end, hippocampal slices (350 mu m thick ) from rats of various postnatal ages (postnatal day 3 [P3] to P16) we re preincubated with [H-3]choline and stimulated twice (S-1, S-2: 360 pulses, 2 ms, 3 Hz, 60 mA) during superfusion with physiological buffe r containing hemicholinium-3 (10 mu M). In parallel, the activities of hemicholinium-sensitive high-affinity choline uptake (HACU, in synapt osomes) and of choline acetyltransferase (ChAT, in crude homogenates) were determined as markers for the cholinergic ingrowth. In hippocampa l slices preincubated with [H-3]choline, the electrically evoked overf low of H-3 at S-1 increased from 0.11 (P3) to 0.81% of tissue H-3 (P16 ), the latter value being still much lower than that of hippocampal sl ices from adult rats (2.89% of tissue 3H). Already at P3 the evoked ov erflow of H-3 was Ca2+-dependent and sensitive to tetrodotoxin, indica ting an action potential-evoked exocytotic mechanism of ACh release. T he muscarinic agonist oxotremorine (1 mu M) significantly inhibited th e evoked ACh release in hippocampal slices with increasing effectivity from P4 to P16; no significant effect was detectable at P3. The ACh e sterase inhibitor physostigmine and the muscarinic antagonist atropine (1 mu M, each) exhibited significant inhibitory and facilitatory effe cts, respectively, only at P15-16. The specific activities of both hip pocampal HACU (pmoles/mg protein/min) and ChAT (nmoles/mg protein/min) continuously increased from P3 to P16. It is concluded (1) that choli nergic nerve terminals arriving at the hippocampal formation during po stnatal ingrowth are already endowed with the apparatus for action pot ential-induced, Ca2+-sensitive (exocytotic) ACh release; (2) that, in contrast, the expression of presynaptic muscarinic autoreceptors on th ese cholinergic axon terminals is delayed; and (3) that autoinhibition due to endogenous ACh develops even later, probably when the density of presynaptic terminals in the hippocampus and hence, the concentrati on of released ACh has reached a suprathreshold value. (C) 1998 Elsevi er Science B.V. All rights reserved.