Mapping the differential effects of procaine on frequency and amplitude of reticularly elicited hippocampal rhythmical slow activity

Hippocampal rhythmical slow activity (RSA, theta) was elicited in urethanized rats by high-frequency stimulation in the reticular formation. The effects of procaine infusion (0.5 microliters, 20% wt/vol) at various loci in the ascending system from pontine reticular formation to the medial septum/diagonal band area were investigated. It was found that procaine injected at points in the ascending system anterior to the supramammillary nucleus, in the region of the medial forebrain bundle or in the medial septum, reduced the amplitude of reticularly elicited RSA but had no effect on its frequency. Procaine injected at points in the ascending system from just anterior to the reticular formation stimulation site, up to, and including the supramammillary nucleus, reduced both the frequency and amplitude of reticularly elicited RSA. These results indicate that the frequency of reticularly elicited RSA is encoded in the supramammillary area, rather than in the medial septum/diagonal band as have previously been suggested.