DENTATE GYRUS AND OLFACTORY-BULB RESPONSES TO OLFACTORY AND NOXIOUS-STIMULATION IN URETHANE-ANESTHETIZED RATS

Recent research has shown that olfactory stimuli such as toluene vapor , but not visual, auditory, tactile or gustatory stimuli, elicit a bur st of fast waves (15-30 Hz) in the hilus of the dentate gyrus in wakin g rats. In urethane-anaesthetized rats, toluene odors elicit similar f ast waves. The present study shows that noxious stimulation (tail clam p) produces a blockade of spontaneous slow waves (1-12 Hz) in the hilu s of the dentate gyrus but does not increase fast wave activity in ure thane-anaesthetized rats. This slow wave blockade, which resembles neo cortical activation, is independent of olfaction since it is not affec ted by tracheotomy. In contrast, tracheotomy abolishes the fast wave r esponse to toluene presentation to the snout unless the toluene vapor is drawn into the nasal passages by suction at the rostral end of the severed trachea. Both the toluene odor-induced fast wave and the tail clamp-induced activation responses are abolished by scopolamine hydrob romide (5.0 mg/kg, i.p.) but not by scopolamine methyl bromide (5.0 mg /kg, i.p.) which does not cross the blood-brain barrier. However, evok ed potentials elicited in the dentate hilus by single pulse stimulatio n of the olfactory bulb are not blocked by scopolamine in urethane-ana esthetized rats. The results suggest that several different types of e lectrical activity in the hippocampal formation are mediated by cholin ergic inputs and that the dentate gyrus prays a role in olfaction.