ACETYLCHOLINESTERASE-CONTAINING INTRINSIC NEURONS IN THE RAT MAIN OLFACTORY-BULB - CYTOLOGICAL AND NEUROCHEMICAL FEATURES

Acetylcholinesterase (AChE) histochemistry in light and electron micro scopy was used to identity cholinoceptive neurons in the olfactory bul b of adult and 16-day-old rats. Double-labelling experiments using ACh E histochemistry and either tyrosine hydroxylase or GABA immunocytoche mistry with light microscopy were also performed in order to specify t he chemical nature of cholinoceptive neurons. Superficial short-axon c ells and several morphological subtypes of deep short-axon cells (seco nd-order interneurons) are the most numerous AChE-containing intrinsic neurons in the olfactory bulb. Short-axon interneurons seem to be the only neurons expressing AChE in the deep olfactory bulb since the num erous granule cells (first-order interneurons) were never found to be AChE-positive, even in electron microscopy. In the superficial olfacto ry bulb, cholinoceptive cells belong to several neuronal categories. I n addition to the intensely labelled superficial short-axon cells, a f ew periglomerular cells (first-order interneurons) display weak but si gnificant AChE expression, clearly visible in electron microscopy. Bot h ultrastructural and double-labelling observations support the hypoth esis that a subset of superficial tufted cells is also cholinoceptive. The coexistence of AChE and tyrosine hydroxylase in large neurons loc ated in the glomerular and superficial external plexiform layers indic ates that some, if not all, cholinoceptive tufted cells belong to the dopaminergic population previously observed in this area. These observ ations indicate that several types of intrinsic neurons express AChE a nd can be tentatively considered as cholinoceptive. Our results provid e an anatomical substrate for hypotheses concerning the complex effect s of acetylcholine in the processing of sensory information in the olf actory bulb.