ENKEPHALIN NEURONS THAT PROJECT TO THE A7 CATECHOLAMINE CELL GROUP ARE LOCATED IN NUCLEI THAT MODULATE NOCICEPTION - VENTROMEDIAL MEDULLA

The location of methionine enkephalin neurons in the medulla oblongata that project to the dorsolateral pontine tegmentum was investigated u sing anterograde and retrograde tract tracing combined with immunocyto chemical neurotransmitter identification. The results of these experim ents demonstrate that enkephalinergic neurons from areas known to modu late nociception project to the region of the A7 catecholamine cell gr oup in the dorsolateral pontine tegmentum. The medullary nuclei that c ontain these enkephalinergic neurons include the nucleus raphe magnus and the nucleus reticularis gigantocellularis pars alpha in the ventro medial medulla. While some of these enkephalinergic axons appose the s omata and dendrites of A7 neurons, the majority of these axons appear to contact non-catecholamine neurons in the dorsolateral pontine tegme ntum. Unidentified neurons located in the nucleus raphe magnus, the nu cleus reticularis gigantocellularis pars alpha, and the nucleus reticu laris gigantocellularis also project to the A7 area. Many of the neuro ns in the nucleus reticularis gigantocellularis pars alpha appear to c ontact both noradrenergic A7 neurons and non-catecholamine neurons in the dorsolateral pontine tegmentum, whereas most of those in the nucle us raphe magnus appear to contact non-catecholamine neurons. The anato mical findings described in this report and the results of preliminary behavioral studies provide evidence to support a model in which activ ation of the enkephalin-containing neurons in the ventromedial medulla facilitates nociception, while the non-enkephalin neurons mediate par t of the antinociception produced by stimulating sites in the ventrome dial medulla. (C) 1998 IBRO. Published by Elsevier Science Ltd.