An extrahippocampal projection from the dentate gyrus to the olfactory tubercle

Background:

The dentate gyrus is well known for its mossy fiber projection to the hippocampal field 3 (CA3) and its extensive associational and commissural connections. The dentate gyrus, on the other hand, has only few projections to the CA1 and the subiculum, and none have clearly been shown to extrahippocampal target regions.

Results:

Using anterograde and retrograde tracer techniques in the Madagascan lesser hedgehog tenrec (Afrosoricidae, Afrotheria) it was shown in this study that the dentate hilar region gave rise to a faint, but distinct, bilateral projection to the most rostromedial portion of the olfactory tubercle, particularly its molecular layer. Unlike the CA1 and the subiculum the dentate gyrus did not project to the accumbens nucleus. A control injection into the medial septum-diagonal band complex also retrogradely labeled cells in the dentate hilus, but these neurons were found immediately adjacent to the heavily labeled CA3, while the tracer injections into the rostromedial tubercle did not reveal any labeling in CA3.

Conclusion:

The dentate hilar neurons projecting to the olfactory tubercle cannot be considered displaced cells of CA3 but represent true dentato-tubercular projection neurons. This projection supplements the subiculo-tubercular projection. Both terminal fields overlap among one another as well as with the fiber terminations arising in the anteromedial frontal cortex. The rostromedial olfactory tubercle might represent a distinct ventral striatal target area worth investigating in studies of the parallel processing of cortico-limbic information in tenrec as well as in cat and monkey.

Figure 3

Retrograde labeling in DtHi and HCt following tracer injections into the olfactory tubercle. Tracer injections into the rostromedial Tu (A) consistently labeled a few dentate hilar neurons (D-H; arrows point to labeled cells), but failed to label the CA3 (D-F). A-E are from Et03-58W, F-H from Et01-47W. Some labeled neurons are also noted in the ipsilateral, anterior HCt (B, C) confirming previous anterograde data [28]. Remarkably, both the HCt and the DtHi project to the rostromedial Tu, but not to the Acb. Arrow heads point to similar location. OfB, olfactory bulb; PCx, paleocortex. Scale bars = 0.8 mm in A, 0.4 mm in B and D, 0.3 mm in F, 0.2 mm in C, E and H (as G).

Heinz Künzle.
An extrahippocampal projection from the dentate gyrus to the olfactory tubercle.
BMC Neurosci. 2005; 6: 38.


http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1180450&rendertype=figure&id=F3