THE SUPRAMAMMILLARY NUCLEUS - IS IT NECESSARY FOR THE MEDIATION OF HIPPOCAMPAL THETA-RHYTHM

Recent evidence suggests that the supramammillary nucleus of the poste rior hypothalamus serves as an important relay in a brainstem to septu m/hippocampus pathway involved in the generation of hippocampal theta rhythm. In order to examine the role of the supramammillary nucleus as a possible relay/mediator of hippocampal theta rhythm, electrolytic l esions and procaine injections were administered to the supramammillar y nucleus of freely moving and urethane-anesthetized rats: respectivel y. In the urethane-anesthetized rat, it was found that procaine inject ions attenuated both the frequency and amplitude of theta rhythm elici ted by stimulation of the pontine reticular formation. These data sugg est that the pontine reticular elicitation of hippocampal theta rhythm is mediated through connections with the supramammillary nucleus. How ever, it was found that lesions of the supramammillary nucleus failed to produce significant changes in the hippocampal electroencephalogram of freely moving animals. Several explanations concerning this appare nt discrepancy are discussed. The most compelling is that multiple bra instem to septum/hippocampus pathways may serve to generate or facilit ate the generation of theta rhythm in the freely moving animal. The pr esent report demonstrates that the supramammillary nucleus plays a que stionable role in the mediation of hippocampal electroencephalogram si gnals which are thought to be important for mnemonic processes.