title: Excitatory projections from the anterior hypothalamus to periaqueductal gray neurons that project to the medulla: a functional anatomical study.
abstract: The present study was designed to investigate the organization of excitatory projections from regions of the anterior hypothalamus that are known to co-ordinate autonomic and sensory functions to medullo-output neurons in the periaqueductal gray. The induction of Fos protein was used to identify neurons in the periaqueductal gray that were activated synaptically by chemical stimulation at sites in the anterior hypothalamus from which either increases or decreases in arterial blood pressure were evoked (pressor sites and depressor sites, respectively). This was combined with retrograde tracing using fluorescent latex microspheres from sites in the medulla. When compared to control animals, neuronal activation at pressor sites in the anterior hypothalamus evoked Fos-like immunoreactivity in significantly more neurons in all but one sub-division of the periaqueductal gray (P at least < 0.05). The majority of Fos-positive neurons following a pressor response were located in the caudal half of the periaqueductal gray where significantly more neurons contained Fos-like immunoreactivity in lateral than in any other sub-division (P < 0.01). In all but two of 14 subdivisions of the periaqueductal gray, the numbers of neurons that expressed Fos-like immunoreactivity following stimulation at depressor sites in the anterior hypothalamus were not significantly different from controls. When neuronal activation at pressor or depressor sites in the anterior hypothalamus was combined with retrograde tracing from the rostral ventrolateral medulla, nucleus raphe magnus and/or nucleus raphe obscurus the majority of double-labelled neurons were located in the caudal half of the periaqueductal gray. Comparisons between the numbers of double-labelled neurons that resulted from different combinations of hypothalamic and medullary injection sites revealed that neuronal activation at pressor sites in the anterior hypothalamus combined with retrograde tracing from the rostral ventrolateral medulla resulted in the greatest numbers of double-labelled neurons. The identification of double-labelled neurons indicates that medullo-output neurons in the periaqueductal gray receive excitatory inputs predominantly from pressor compared to depressor sites in the anterior hypothalamus. These results are discussed in relation to the roles of the different longitudinal columns of the periaqueductal gray, and the organisation of their projections to the medulla, in the co-ordination of autonomic and sensory functions.
authors.affiliation: Department of Physiology, School of Medical Sciences, University of Bristol, UK.
authors.forename: F M
authors.initials: FM
authors.lastname: Semenenko
authors.forename: B M
authors.initials: BM
authors.lastname: Lumb
mesh_terms: D000818:Animals; D000906:Antibodies; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D005456:Fluorescent Dyes; D006710:Homocysteine / Q000031:analogs & derivatives / Q000494:pharmacology; D007032:Hypothalamus, Anterior / Q000166:cytology* / Q000502:physiology; D008297:Male; D008526:Medulla Oblongata / Q000166:cytology* / Q000502:physiology; D008863:Microspheres; D009434:Neural Pathways; D009474:Neurons / Q000737:chemistry / Q000378:metabolism; D010487:Periaqueductal Gray / Q000737:chemistry / Q000166:cytology* / Q000502:physiology; D016760:Proto-Oncogene Proteins c-fos / Q000032:analysis / Q000096:biosynthesis / Q000276:immunology; D051381:Rats; D017208:Rats, Wistar; D012235:Rhodamines
chemical_list: D000906:Antibodies; D005456:Fluorescent Dyes; D016760:Proto-Oncogene Proteins c-fos; D012235:Rhodamines; D006710:Homocysteine; C007956:homocysteic acid