Hippocampal formation neurons code the level of activation of the cholinergic septohippocampal pathway.

In these experiments, differing levels of activation of the cholinergic septohippocampal pathway, as reflected by shifts in type 2 slow wave theta frequency recorded in the hippocampal formation, were studied in urethane-anesthetized rats. The purpose was to determine whether the discharge rates of cells in the hippocampal formation varied in any systematic way with the frequency shifts produced during spontaneous theta and by stimulation of the dorsomedial-posterior hypothalamic region. Linear regression analysis revealed that a class of neurons called theta-on cells precisely coded the level of activation of the septohippocampal pathway through their discharge rates. Administration of atropine sulfate abolished this coding. A second class of neurons called theta-off cells were shown to have reciprocal relationships with theta-on cells. Implications for the sensorimotor model of hippocampal formation theta subsystems were discussed.