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.