HYPOXIA SENSITIVE NEURONS IN THE CAUDAL HYPOTHALAMUS PROJECT TO THE PERIAQUEDUCTAL GRAY

Previous studies have demonstrated that the caudal hypothalamus modula tes the respiratory responses to hypoxia and hypercapnia. In addition, many of the neurons in this area have a basal discharge related to th e cardiac and/or respiratory cycles and are stimulated by hypoxia or h ypercapnia. The purpose of the present study was to determine if these hypothalamic neurons project to a known cardiorespiratory area, the p eriaqueductal gray in the rat. In a first set of experiments, rhodamin e-tagged microspheres were injected into the periaqueductal gray (PAG) to determine the areas of the caudal hypothalamus that project to the PAG. These studies revealed that the caudal hypothalamus sends strong ipsilateral and weak contralateral projections to the PAG. In a secon d set of experiments, single unit recordings were made from neurons in the caudal hypothalamus; the basal discharge of these neurons were ex amined with signal averaging techniques. Each neuron (n = 79) was test ed for a response to inhalation of a hypoxic (10% O-2) and a hypercapn ic (5% CO2) gas. Antidromic activation techniques were then used to de termine if neurons in the caudal hypothalamus send projections to or t hrough the PAG. Nineteen percent (n = 15) of the hypothalamic neurons studied could be activated from the PAG; approximately 53% (n = 8) of these were excited by hypoxia and 27% (n = 4) by hypercapnia. Most of these neurons tested (42 of 64 neurons) had a basal discharge related temporally to the cardiac and/or respiratory cycles. These findings su ggest that a caudal hypothalamic to periaqueductal gray projection is involved in the integrated response to hypoxia.