Septohippocampal properties of N-methyl-D-aspartate-induced theta-band oscillation and synchrony

Microinfusion of N-methyl-D-aspartate (NMDA) into apical dendrites of hippocampal CA1 pyramidal cells of urethane-anesthetized rats resulted in long lasting (20-30 min) induction of hippocampal synchrony at the field and cellular level. Power but not frequency of NMDA-induced theta was significantly greater than tail pinch-induced theta activity. This effect was antagonized by intrahippocampal infusion of AP5, but unaffected by i.v. atropine sulfate. During AP5 blockade tail pinch theta frequency and power were significantly reduced. Microinfusion of NMDA into the medial septum also resulted in long lasting induction of hippocampal theta field activity. Contrary to the results of hippocampal NMDA microinfusions, frequency but not power of NMDA-induced theta was significantly greater than tail pinch-induced theta activity. Microinfusion of AP5 into the medial septum significantly lowered power of tail pinch-induced theta but did not affect frequency. Wheel running behavior of rats induced by low levels of electrical stimulation of the posterior hypothalamic nucleus (PH) was completely abolished by microinfusion of AP5 into the medial septum, accompanied by a significant reduction in theta power and frequency. Wheel running and theta were maintained at control levels with high intensity PH stimulation. We propose that: (1) the glutamatergic septohippocampal projection represents a third pathway capable of generating hippocampal field and cellular synchrony, independent of that generated by the septohippocampal cholinergic and GABAergic projections, and (2) the septohippocampal glutamatergic projection serves to function as an interface between cholinergic and GABAergic modulated sensory processing Type 2 theta and movement related Type 1 theta.