INTRASEPTAL PROCAINE ABOLISHES HYPOTHALAMIC STIMULATION-INDUCED WHEEL-RUNNING AND HIPPOCAMPAL THETA FIELD ACTIVITY IN RATS

Rats were implanted chronically with hippocampal recording electrodes, a microinfusion guide cannula aimed at the medial septal nucleus, and an electrode for electrical stimulation of the posterior hypothalamic nucleus (PH). PH stimulation elicited running in rats placed in a whe el and simultaneously occurring hippocampal theta field activity (HPC- theta). In the preprocaine (PRE) testing condition, a positive linear relationship was demonstrated among the intensity of electrical stimul ation of the PH, wheel-running speed, and the peak frequency of HPC-th eta. HPC-theta amplitude reached an asymptote at the lowest levels of electrical stimulation of the PH. Procaine hydrochloride (1.5 mu l, 20 % solution), a local anesthetic, was then infused into the medial sept al nucleus (MS). Five minutes after the infusion, PH stimulation no lo nger induced wheel-running behavior or HPC-theta, and the remaining ir regular field activity was significantly reduced in amplitude. Fifteen minutes after the procaine infusion, PH stimulation still did not eli cit HPC-theta or running behavior in the majority of animals but did e voke large-amplitude sharp-waves. Thirty minutes after the procaine in fusion, PH stimulation again elicited HPC-theta and running behavior, but HPC-theta peak frequency and running speeds were both significantl y reduced compared with PRE values. Forty-five minutes after the infus ion, HPC-theta amplitude had recovered to PRE values, but HPC-theta fr equency and running speeds elicited by PH stimulation were still signi ficantly reduced. By 60 min after procaine administration, the amplitu de and frequency of HPC-theta and the running speeds elicited by PH st imulation recovered to PRE values. Multiple regression analysis reveal ed that the recovery pattern of running behavior reflected the frequen cy rather than the amplitude of HPC-theta. Neither saline control infu sions into the MS nor procaine infusions into the lateral septum and p araventricular thalamic nucleus affected HPC-theta or running behavior . These findings are consistent with the notion that both the locomoto r activity and ''movement-related'' HPC-theta frequency induced by ele ctrically stimulating the PH were attributable to ascending activation of a hypothalamo-septal pathway and not to activation of descending b rainstem or peripheral motor systems.