LOCALIZATION OF LATERAL HYPOTHALAMIC NEURONS PROJECTING TO THE MEDIAL PART OF THE PARABRACHIAL AREA OF THE RAT

We recently showed that electrical self-stimulation registered in the medial part of the parabrachial area, as well as the preference-aversion threshold to saccharin solutions were both significantly altered following ibotenic acid lesion of the lateral hypothalamic neurons. In order to identify the location of the neurons in the lateral hypothalamus directly projecting to the parabrachial area, we injected in the medial part of this area the retrograde tracer wheat germ agglutinin-inactive horseradish peroxidase coupled to colloidal gold. In the lateral hypothalamus a large number of labeled cells was constantly observed. This cluster of cells was located in the middle and posterior parts of the lateral hypothalamic area between the frontal plane corresponding to the posterior third of the ventromedial hypothalamic nucleus and the plane corresponding to the premammillary nuclei. In contrast, the anterior part of the lateral hypothalamus was unlabeled. Since the labeled neurons are located in the same region as those destroyed by our ibotenic acid lesions, the present results strongly suggest that these descending projections, originating in the posterior lateral hypothalamus, are implicated in reward mechanisms elicited from the parabrachial area. © 1990.