In an attempt to characterize more extensively the distribution and morphologies of acetylcholinesterase- (AChE, EC 22.214.171.124) and norepinephrine-containing neurons in the rat locus ceruleus, we histochemically examined the two neurochemicals either separately or on the same tissue section. Some animals were pretreated with bis(l-methylethyl)phosphorofluoridate, an irreversible cholinesterase inhibitor, in order to reduce background AChE staining and, consequently, to enhance visualization of neurons containing the cholinergic degradative enzyme. Neuronal somata containing AChE were round, oval, fusiform, or pyramidal. On the basis of cell body size, three different populations of cerulear neurons were discerned: small (maximum soma extent: 10-20 μm), medium (maximum soma extent: 20-30 μm), and large (maximum soma extent: > 30 μm). The proportion of large dimension cells increased, whereas the percentage of small neurons decreased, from rostral to caudal levels of locus ceruleus. The proportion of medium-sized cells was roughly constant at all levels of the nucleus. The morphologies and distribution of norepinephrinecontaining neurons appeared identical to those of AChE cells. In tissue sections stained both for AChE and catecholamines, it was found that all cerulear neurons containing norepinephrine also appeared to possess AChE. The significance of AChE's association with norepinephrine somata and proximal processes in locus ceruleus is unknown, but one possibility is that the cholinergic degradative enzyme inactivates acetylcholine released from cholinergic afferents, the origin of which are currently unknown.
- Locus ceruleus
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