Preliminary observations on the physiology and pharmacology of hippocampal theta-off cells.

The discharge patterns of phasic linear and tonic non-linear hippocampal theta-off cells and the simultaneously occurring field activity [theta (theta) and large amplitude irregular activity] were studied in response to sensory input, electrical stimulation of the dorsomedial-posterior hypothalamus and systemic administration of cholinergic agents, in urethane-anesthetized rats. Sensory input, hypothalamic stimulation and the administration of eserine all produced a total reduction of the discharges of phasic linear theta-off cells when the resulting induced theta was of a frequency of 5 Hz and above. The discharges of tonic non-linear theta-off cells were also totally reduced during the same conditions, even at the lowest frequencies of induced theta. Administration of atropine SO4 abolished all theta activity previously elicited by electrical stimulation of the hypothalamus, sensory input and eserine. However, sensory input and hypothalamic stimulation still produced the reduction of discharge rates of both types of theta-off cells in the presence of atropine SO4. The linear relation between the discharge rate of phasic linear theta-off cells and levels of hypothalamic stimulation was also preserved in the presence of atropine. It was suggested that GABA may mediate the discharge properties of hippocampal theta-off cells.