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

Abstract

In an attempt to gain insight into the organization and evolution of the basal forebrain, the region was analysed cytoarchitecturally, chemoarchitecturally, and hodologically in a lower placental mammal, the lesser hedgehog tenrec. Particular emphasis was laid on the subdivision of the olfactory tubercle, the nuclear complex of the diagonal band, and the cortical amygdala. The proper tubercule and the rostrolateral tubercular seam differed from each other with regard to their immunoreactivity to calbindin and calretinin, as well as their afferents from the piriform cortex. Interestingly, the tubercular seam showed similar properties to the dwarf cell compartment, located immediately adjacent to the islands of Calleja. The most prominent input to the olfactory bulb (OfB) originated from the diagonal nuclear complex. This projection was ipsilateral, whereas the bulbar afferents from the hypothalamus and the mesopontine tegmentum were bilateral. The amygdala projected only sparsely to the OfB, but received a prominent bulbar projection. An exception was the nucleus of the lateral olfactory tract, which was poorly connected with the OfB. Unlike other species with an accessory OfB, the projections from the tenrec's main OfB did not show a topographic organization upon the lateral and medial olfactory amygdala. However, there was an accessory amygdala, which could be differentiated from the lateral nuclei by its intense reaction to NADPh-diaphorase. This reaction was poor in the diagonal nuclear complex as in monkey but unlike in rat. The variability of cell populations and olfactory bulb connections shown here may help to clarify both phylogenetic relationships and the significance of individual basal telencephalic subdivisions.

Amygdalar connections in the lesser hedgehog tenrec

“…The present study on the extrinsic connections of the amygdala (Ay) in the lesser hedgehog tenrec supplements a previous investigation on the bulbo-amygdalar connections (Künzle and Radtke-Schuller 2000) and helps to reduce the knowledge gap with regard to the amygdala's connectivity in lower mammals. Unlike numerous investigations in nonmammals and higher mammals (for review : Petrovich et al 2001;Pitkänen 2001;Sah et al 2003;DeOlmos et al 2004;Martínez-García et al 2007), there is only one study with tracer injections aimed into the Ay in mammals with poorly differentiated brain.…”

“…Nevertheless, the distribution of cells giving rise to Ay-hypothalamic projections in the tenrec is similar to that in the rat as far as the amygdalar cells are best represented medially (AyM) and are largely absent laterally, and in both species there are hypothalamic projections arising from the AyCo and none from the OfTL (Pitkänen 2001;Santiago and Shammah-Lagnado 2004). Unusual in the tenrec is the presence of hypothalamus projecting cells medially (an area considered as a part of AyA; see Künzle and Radtke-Schuller 2000) and laterally (proper CoA) adjacent to the OfTL (the so-called POfT), even in the case with no labeling in the main portion of the anterior AyCo (further discussion see below). As in the opossum (McDonald and Culberson 1986) there is only one circumscribed amygdalo-hypothalamic target area, presumably corresponding to the nucleus ventralis medialis in the rat (Canteras et al 1994).…”

“…6e). The retrograde labeling of the layer 3 is noteworthy because we failed previously to identify this layer (Künzle and Radtke-Schuller 2000). The AyL, AyB and AyP also gave rise to commissural connections (Fig.…”

An extrahippocampal projection from the dentate gyrus to the olfactory tubercle

“…It receives few if any direct projections from the olfactory bulb (personal reinvestigation of the material published previously [63]) but gets a distinct input from the anteromedial frontal cortex [29]. The dentato-tubercular target area is likely to be part of a cortico-basal ganglia-thalamo-cortical loop involving the mediodorsal nucleus [64-67] and the prefrontal cortex, at least in subprimate species (rat: [68-70]; cat: [71]; monkey: [72-74]).…”

Neocortical connections with perihippocampal and periamygdalar regions in the hedgehog tenrec

“…also receives fibers from the cortex and projects to the hippocampus-data we have been unable so far to show for the tenrec's PCxV; see Künzle 2002). It may be mentioned that we have tried previously to characterize the tenrec's cortical amygdala and did not include the PCxV (Künzle and Radtke-Schuller 2000a). Obviously one has to reconsider these interpretations.…”