The Role of the Septohippocampal Pathway in the Regulation of Hippocampal Field Activity and Behavior: Analysis by the Intraseptal Microinfusion of Carbachol, Atropine, and Procaine

Abstract

The role of the septohippocampal pathway in the regulation of hippocampal field activity and behavior was assessed during the technique of intraseptal microinfusion of carbachol, atropine, and procaine. The effects of these manipulations were assessed by comparing them with the effects of intraseptal saline control infusions on spontaneously occurring motor behavior and their correlated hippocampal field activities. In control conditions hippocampal theta field activity, with up to 40% of the total power concentrated in a 1-Hz band around the peak frequency, was recorded only during type 1 voluntary motor behaviors. Hippocampal large-amplitude irregular activity was recorded during type 2 automatic motor behaviors and during immobility. Microinfusion of carbachol into the MS/vDBB (media septum/vertical limb of the diagonal band of Broca) resulted in continuous theta field activity regardless of what type of motor behavior the animal performed or whether it was immobile. However, under these conditions the performance of a voluntary movement consistently resulted in an upward shift of theta frequency, demonstrating that the frequency of carbachol-elicited type 2 theta can be modulated by the behavioral activation of type 1 theta. The subsequent infusion of atropine abolished the carbachol-elicited theta, while that occurring during type 1 voluntary movement was preserved. In contrast to the selective blockade of carbachol-elicited theta by atropine, procaine suppression of the MS/vDBB abolished both spontaneous movement-related (type 1) theta and carbachol-elicited (type 2) theta. During the postprocaine period theta frequency recovered rapidly in contrast to theta amplitude (power). This result applied to the recovery of both the coactivated type 1 and type 2 theta occurring during spontaneous movement and the carbachol-elicited type 2 theta alone. The behavioral results were discussed within the context of Bland's ( Prog. Neurobiol. 26, 1-54, 1986) sensorimotor model which posits that hippocampal theta activity recorded during voluntary movement represents the coactivation of a cholinergically mediated sensory processing component and a feedback (possibly serotonergic) motor component.