Modulation of dorsal hippocampus field CA1 pyramidal cell excitability by an ascending relay from hypothalamic supramammillary nucleus

Abstract

Experiments were performed on the urethane anaesthetized rat to test the hypothesis that theta generating reticular stimulation affected the excitability of CA1 pyramidal cells. In this context, it was observed that reticular stimulation induced a suppression of the CA1 pyramidal cell population spike and the corresponding dendritic field excitatory postsynaptic potential evoked by field CA3 stimulation. However, this suppression was observed at stimulation intensity below the threshold for generation of CA1 theta and was maximal at the threshold for theta. The frequency and amplitude of theta waves, by contrast, increased further with increasing reticular stimulation voltage suggesting dissociation between mechanisms underlying suppression and theta generation. Further studies involving reversible neural inactivation with local microinjection of drugs indicated that the reticularlya??elicited suppression was mediated by neural substrate(s) in the medial supramammillary region and the medial septum. Interestingly, cholinergic mechanisms in the lateral supramammillary region evoked CA1 suppression also via the medial septum.