Acetylcholinesterase-containing intrinsic neurons in the rat main olfactory bulb: cytological and neurochemical features.

Acetylcholinesterase (AChE) histochemistry in light and electron microscopy was used to identify cholinoceptive neurons in the olfactory bulb of adult and 15-day-old rats. Double-labelling experiments using AChE histochemistry and either tyrosine hydroxylase or GABA immunocytochemistry with light microscopy were also performed in order to specify the chemical nature of cholinoceptive neurons. Superficial short-axon cells and several morphological subtypes of deep short-axon cells (second-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 numerous granule cells (first-order interneurons) were never found to be AChE-positive, even in electron microscopy. In the superficial olfactory bulb, cholinoceptive cells belong to several neuronal categories. In addition to the intensely labelled superficial short-axon cells, a few periglomerular cells (first-order interneurons) display weak but significant AChE expression, clearly visible in electron microscopy. Both ultrastructural and double-labelling observations support the hypothesis that a subset of superficial tufted cells is also cholinoceptive. The coexistence of AChE and tyrosine hydroxylase in large neurons located in the glomerular and superficial external plexiform layers indicates that some, if not all, cholinoceptive tufted cells belong to the dopaminergic population previously observed in this area. These observations indicate that several types of intrinsic neurons express AChE and can be tentatively considered as cholinoceptive. Our results provide an anatomical substrate for hypotheses concerning the complex effects of acetylcholine in the processing of sensory information in the olfactory bulb.