Responses of septal theta-on and theta-off cells to activation of the dorsomedial-posterior hypothalamic region.

The effect of different levels of electrical stimulation of the dorsomedial-posterior hypothalamic region, on the discharge properties of septal theta-on and theta-off cells, was investigated in urethane-anesthetized rats. In the present sample of 45 cells recorded in the medial septum/ventral limb of the diagonal band of Broca, 43 of the 45 (96%) were affected by the stimulation. The majority of theta-on cells were intensely activated by this stimulation (two were inhibited) and all theta-off cells were inhibited (ranging from partial to complete inhibition). Stimulation did not change a cell's classification as "on" or "off," nor did it alter its discharge pattern (phasic or tonic). Stimulation did have a profound effect on phasic nonlinear theta-on cells. Phasic nonlinear theta-on cells, by definition, did not alter their discharge rate in relation to the increasing frequency of spontaneously occurring or tail-pinch-induced theta. However, stimulation of the dorsomedial-posterior hypothalamus resulted in 11 out of 13 (85%) of the phasic nonlinear cells increasing their discharge rates in relation to the increasing frequency of the hypothalamically induced theta (i.e., became linear). Stimulation had no effect on two cells classified as unrelated to theta.