RELEASE OF ACETYLCHOLINE AND NORADRENALINE FROM THE CHOLINERGIC AND ADRENERGIC AFFERENTS IN RAT HIPPOCAMPAL CA1, CA3 AND DENTATE GYRUS REGIONS

An attempt was made to study the release of acetylcholine (ACh) and no radrenaline and their presynaptic modulation in isolated slice prepara tions dissected from different subfields of the hippocampus: CA1, CA3 and the dentate gyrus. The slices were perfused and loaded with [H-3]c holine or with [H-3]noradrenaline. The release in response to field st imulation was determined radiochemically and the content of transmitte rs was assayed by a chemiluminescent method or by HPLC combined with e lectrochemical detection. After 30 min of loading with [H-3]choline th ere were marked subregional differences in the specific activity of [H -3]ACh content. The highest concentration was measured in the dentate gyrus and the lowest in CA3. Evidence was obtained that in all three s ubfields the cholinergic axon terminals are equipped with inhibitory m uscarinic autoreceptors and the noradrenergic terminals with alpha(2)- autoreceptors, as indicated by an increase in transmitter release when the tissue was exposed to selective muscarinic or or alpha(2)-adrenoc eptor antagonists. In contrast, the cholinergic boutons are not equipp ed with alpha(2)-adrenoceptors, and noradrenergic terminals do not pos sess inhibitory muscarinic receptors. It is therefore concluded that w hile the release of both ACh and noradrenaline is controlled by negati ve feedback modulation, there is no possibility of establishing a pres ynaptic inhibitory interaction between the two.