MUTUAL INTERACTIONS AMONG CHOLINERGIC, NORADRENERGIC AND SEROTONERGICNEURONS STUDIED BY IONOPHORESIS OF THESE TRANSMITTERS IN RAT BRAIN-STEM NUCLEI

In urethane-anesthetized rats, single neuronal activity was recorded i n or around the central gray of the caudal mesencephalon to rostral po ns with multibarrel microelectrodes for ionophoretic application of ac etylcholine, noradrenaline and serotonin. Neurons were classified by s pike shape into broad-spike and brief-spike neurons. In the laterodors al tegmental nucleus, locus coeruleus or dorsal raphe, broad-spike neu rons, marked by Pontamine Sky Blue and discriminated in sections proce ssed for histochemistry of reduced nicotinamide adenine dinucleotide p hosphate diaphorase or Nissl staining, were presumed to be cholinergic , noradrenergic or serotonergic, respectively. The majority of these n eurons were inhibited through autoreceptors, except some laterodorsal tegmental neurons which might not be furnished by autoreceptors. Norad renaline and serotonin inhibited more than two-thirds of the laterodor sal tegmental neurons tested, while a few neurons were excited by nora drenaline. Though effects of noradrenaline on dorsal raphe neurons and those of serotonin on locus coeruleus neurons were not clear in many neurons tested, neurons affected in these examinations (30%) were all inhibited clearly and no excitatory effect was observed. Acetylcholine exerted inhibition on about one-half of dorsal raphe neurons, while e ffects of acetylcholine on locus coeruleus neurons were the only case in the present study in which excitation was the major effect, though more than a half of locus coeruleus neurons were not sensitive to this drug. Thus, in this study some new data on the pharmacological proper ties of the cholinergic laterodorsal tegmental neurons were obtained. In addition, mutual interactions between brainstem cholinergic, noradr energic and serotonergic neurons were assayed by comparing the pharmac ological properties of these neurons tested with a uniform procedure. The interactions between these diffuse projection neurons may be invol ved in neural mechanisms controling vigilance, wakefulness and/or slee p.