Dentate gyrus and olfactory bulb responses to olfactory and noxious stimulation in urethane anaesthetized rats

Recent research has shown that olfactory stimuli such as toluene vapor, but not visual, auditory, tactile or gustatory stimuli, elicit a burst of fast waves (15-30 Hz) in the hilus of the dentate gyrus in waking rats. In urethane-anaesthetized rats, toluene odors elicit similar fast waves. The present study shows that noxious stimulation (tail clamp) produces a blockade of spontaneous slow waves (1-12 Hz) in the hilus of the dentate gyrus but does not increase fast wave activity in urethane-anaesthetized rats. This slow wave blockade, which resembles neocortical activation, is independent of olfaction since it is not affected by tracheotomy. In contrast, tracheotomy abolishes the fast wave response 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 hydrobromide (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, evoked potentials elicited in the dentate hilus by single pulse stimulation of the olfactory bulb are not blocked by scopolamine in urethane-anaesthetized rats. The results suggest that several different types of electrical activity in the hippocampal formation are mediated by cholinergic inputs and that the dentate gyrus plays a role in olfaction.