Basal telencephalic regions connected with the olfactory bulb in a Madagascan hedgehog tenrec

In an attempt to gain insight into the organization and evolution of the ba sal forebrain, the region was analysed cytoarchitecturally, chemoarchitectu rally, and hodologically in a lower placental mammal, the lesser hedgehog t enrec. Particular emphasis was laid on the subdivision of the olfactory tub ercle, the nuclear complex of the diagonal band, and the cortical amygdala. The proper tubercule and the rostrolateral tubercular seam differed from e ach other with regard to their immunoreactivity to calbindin and calretinin , as well as their afferents from the piriform cortex. Interestingly, the t ubercular seam showed similar properties to the dwarf cell compartment, loc ated immediately adjacent to the islands of Calleja. The most prominent inp ut to the olfactory bulb (OfB) originated from the diagonal nuclear complex . This projection was ipsilateral, whereas the bulbar afferents from the hy pothalamus and the mesopontine tegmentum were bilateral. The amygdala proje cted only sparsely to the OfB, but received a prominent bulbar projection. An exception was the nucleus of the lateral olfactory tract, which was poor ly connected with the OfB. Unlike other species with an accessory OfB, the projections from the tenrec's main OfB did not: show a topographic organiza tion upon the lateral and medial olfactory amygdala. However, there was an accessory amygdala, which could be differentiated from the lateral nuclei b y its intense reaction to NADPh-diaphorase. This reaction was poor in the d iagonal nuclear complex as in monkey but unlike in rat. The variability of cell populations and olfactory bulb connections shown here may help to clar ify both phylogenetic relationships and the significance of individual basa l telencephalic subdivisions. J. Comp. Neurol. 423:706-726, 2000. (C) 2000 Wiley-Liss, Inc.