Distribution of acetylcholinesterase and choline acetyltransferase in the main and accessory olfactory bulbs of the hedgehog (Erinaceus europaeus)

The distribution of cholinergic markers was studied in the main olfactory b ulb (MOB) and accessory olfactory bulb (AOB) of the western European hedgeh og (Erinaceus europaeus) by using choline acetyltransferase (ChAT) immunocy tochemistry and acetylcholinesterase (AChE) histochemistry. A dense network of AChE-containing and ChAT-immunoreactive fibers was observed innervating all layers of the MOB except the olfactory nerve layer, where neither AChE - nor CkAT-labeled elements were found. The highest density of AChE- and Ch AT-positive axons was found in the glomerular layer (GL)/external plexiform layer (EPL) boundary, and in the internal plexiform layer. This general di stribution pattern of ChAT- and AChE-stained axons resembled the distributi on pattern found in rodents. Nevertheless, some interspecies differences, s uch as the lack of atypical glomeruli in the hedgehog, were also found. In addition to fibers, a population of noncholinergic and presumably cholinoce ptive AChE-active neurons was observed in the hedgehog. All mitral and tuft ed cells of the hedgehog MOB showed a dark AChE staining unlike previous ob servations in the mitral and tufted cells of rodents. As in other species p reviously reported, subpopulations of external tufted cells and short-axon cells were also AChE-active. Finally, a population of small AChE-containing cells was observed in the EPL of the hedgehog MOB. The size, shape, and lo cation of these cells coincided with those of satellite and perinidal cells , two neuronal types described previously in the EPL of the hedgehog and no t present in the rodent MOB. The BOB of the hedgehog showed a distribution of AChE- and ChAT-positive fibers similar to the rodent AOB. Nevertheless, a heterogeneous innervation of vomeronasal glomeruli by bundles of AChE- an d ChAT-labeled axons found in the hedgehog has not been previously found in any other species. As in the MOB, all mitral cells in the AOB showed a str ong AChE activity. These results demonstrate some similarities but also imp ortant differences between the distribution of ChAT and AChE in the MOB and AOB of rodents and this primitive mammalian. These variations may indicate a different organization of the cholinergic modulation of the olfactory in formation in the insectivores. J. Comp. Neurol. 403:53-67, 1999. (C) 1999 W iley-Liss, Inc.