ULTRASTRUCTURAL IDENTIFICATION OF CHOLINERGIC NEURONS IN THE EXTERNALCUNEATE NUCLEUS OF THE GERBIL - ACETYLCHOLINESTERASE HISTOCHEMISTRY AND CHOLINE-ACETYLTRANSFERASE IMMUNOCYTOCHEMISTRY

Using acetylcholinesterase histochemical and choline acetyltransferase immunocytochemical localization methods, this study has provided conc lusive evidence for the existence of cholinergic neurons in the extern al cuneate nucleus of gerbils. By light microscopy, both acetylcholine sterase and choline acetyltransferase labelling was confined to the ro stral portion of the external cuneate nucleus. Ultrastructurally, acet ylcholinesterase reaction products were found in the nuclear envelope, cisternae of rough endoplasmic reticulum and Golgi saccules of some s omata and large dendrites as well as in the membranes of small dendrit es, myelinated axons and axon terminals. These neuronal elements were also stained for choline acetyltransferase; immunoreactivity was assoc iated with nuclear pores, nuclear envelope, perikaryal membrane and al l the membranous structures within the cytoplasm. Of the total choline acetyltransferase-labelled neuronal profiles analysed, 79% were myeli nated axons, 15% dendrites, 4% somata and 2% axon terminals. The immun ostained axon terminals consisted of two types containing either round (Rd type; 62.5%) or pleomorphic (Pd type; 37.5%) vesicles. Both were associated directly with choline acetyltransferase-positive dendrites. In contrast to the paucity of choline acetyltransferase-labelled axon terminals, numerous choline acetyltransferase-positive myelinated axo ns were present. It may thus be hypothesized that most, if not all, of the external cuneate nucleus cholinergic neurons are projection cells ; such cells may give rise to axonal collaterals which synapse onto th eir own dendrites for possible feedback control. Choline acetyltransfe rase-positive dendrites were contacted by numerous unlabelled presynap tic boutons, 60% of which contained round or spherical synaptic vesicl es (Rd boutons) and 40% flattened vesicles (Fd boutons), suggesting th at these neurons are under strong inhibitory control. The preferential concentration of cholinergic components in the rostral external cunea te nucleus may be significant in the light of the highly organized som atotopy in the external cuneate nucleus and its extensive efferent pro jections to medullary autonomic-related nuclei. Our results suggest th at the cholinergic neurons may be involved in somatoautonomic integrat ion.