PRINCIPAL CELLS ARE THE POSTSYNAPTIC TARGETS OF SUPRAMAMMILLARY AFFERENTS IN THE HIPPOCAMPUS OF THE RAT

Neurons of the supramammillary nucleus are known to fire phase-locked to hippocampal theta rhythm. Stimulation of this area induces theta ac tivity in the hippocampus via the medial septum and facilitates perfor ant pathway stimulation-evoked population spikes in the dentate gyrus even if the medial septum is inactivated. This latter effect was sugge sted to be due to a direct inhibitory input from the supramammilary nu cleus to hippocampal nonpyramidal cells resulting in disinhibition. In the present study, using anterograde tracing with Phaseolus vulgaris leucoagglutinin, we aimed to identify the types of neurons innervated by the supramammillary projection in the dentate gyrus and Ammons horn , with particular attention to the presumed postsynaptic inhibitory ne urons, which may mediate the proposed disinhibitory action. Double-imm unostaining for the tracer and different neuropeptides (somatostatin, cholecystokinin, neuropeptide Y) or calcium binding proteins (calretin in, parvalbumin, calbindin D-28K) present in different subpopulations of interneurons revealed no multiple contacts between supramammillary afferents and labeled inhibitory cells at the light microscopic level. Furthermore, postembedding immunostaining of electron microscopic sec tions for GABA demonstrated that none of the 68 PHAL-labeled supramamm illary boutons examined and none of their postsynaptic targets were im munoreactive for the inhibitory neurotransmitter. We conclude, therefo re, that most if not all postsynaptic targets of the supramammillary p rojection are principal cells both in the dentate gyrus and in the CA2 -CA3a subfields. This suggests that a mechanism other than disinhibiti on is responsible for the facilitatory effect of this pathway on hippo campal evoked activity. (C) 1994 Wiley-Liss, Inc.