Organization and morphologies of acetylcholinesterase-containing neurons in the thalamus and hypothalamus of the rat

Abstract

The distribution and morphologies of neurons containing acetylcholinesterase (AChE, EC 3.1.1.7) in the thalamus and hypothalamus of the rat were studied by means of a pharmaco-histochemical technique involving staining for AChE (Karnovsky-Roots' procedure) at various times after administration of bis-(l-methylethyl)phosphorofluoridate. This method enables visualization of individual AChE-containing neuronal somata and their processes to a degree not possible with other protocols for the enzyme. The strongest AChE activity occurring at the level of the thalamus is found within the small, round to oval, somata of nucleus anterior dorsalis. Most of the intralaminar nuclei, as well as nucleus reticularis, are composed of mediumsized multipolar neurons displaying moderate to strong AChE activity. Moderately stained AChE neurons are also found in pars ventralis of nucleus geniculatus lateralis and in pars lateralis of nucleus habenularis. Most of the neurons of the lateral and posterior thalamic territories, however, are nearly devoid of AChE. At the level of the hypothalamus, the neurons of nuclei supraopticus and paraventricularis show strong AChE activity. The AChE neurons of nucleus supraopticus are surrounded by numerous AChE-containing processes of some large lateral preoptic area neurons that stain intensely for the enzyme. Numerous intensely stained AChE perikarya occur in the lateral, dorsal, and supramammillary hypothalamic areas. These neurons often possess several AChE-containing processes. Nuclei arcuatus and ventromedialis consist mainly of neurons displaying a weak to moderate intensity of AChE staining. At the level of the mammillary bodies most neurons show moderate AChE activity except the neuronal somata of nucleus mammillaris lateralis which stain very strongly for the enzyme.