Is the noradrenergic “feeding circuit” in hypothalamus really an olfactory system?

Since the act of feeding releases catecholamines from certain areas of the rat's hypothalamus, we have examined the possibility of a sensory component underlying this release. An individual diencephalic site in the unanesthetized rat was radiolabeled by microinjection of 1.0–2.0 μCi of ¹⁴C-NE, in a volume of 0.5–1.0 μ1, through a permanently implanted guide cannula. Then 30 min later, the labeled site was perfused by means of push-pull cannulae, with an osmotically balanced CSF solution at a rate of 25 μl/min. The interval of perfusion was 5 min with 10 min intervening. In the midpoint of a sequence of 7 perfusions, either the rat was allowed to consume food, or one of two odoriferous substances was placed beneath its cage floor, i.e., peanut butter or pyridine. During the presentation of either of the two olfactory stimuli to the animal, the efflux of ¹⁴C-NE was enhanced from the same circumscribed site of perfusion in which feeding augmented similarly the release of the ¹⁴C-NE isotope. An analysis of selected hypothalamic perfusates by TLC verified quantitatively the alteration in the profile of ¹⁴C-NE and its labeled metabolites. Our results thus support the view that the changes in the catecholamine release within the hypothalamus reflect synaptic activity of the sensory pathways which mediate the olfactory input to this diencephalic structure.