RESPONSES OF FELINE CAUDAL HYPOTHALAMIC CARDIORESPIRATORY NEURONS TO HYPOXIA AND HYPERCAPNIA

Several studies have suggested that the caudal hypothalamus modulates responses to hypercapnia and hypoxia. In addition, this area of the hy pothalamus contains neurons that have a sympathoexcitatory discharge. The purpose of the present study was to determine whether the basal di scharge of caudal hypothalamic neurons that are stimulated by hypercap nia or hypoxia is related to cardiovascular (sympathetic discharge and /or the cardiac cycle) and/or respiratory activity (phrenic nerve disc harge). Hypothalamic single unit activity, phrenic nerve activity, and cervical sympathetic nerve activity were recorded in anesthetized cat s. Computer averaging techniques were used to compare temporally the d ischarge of hypothalamic neurons with cardiovascular and/or respirator y activity. Cardiorespiratory and hypothalamic neuronal responses to v entilation with hypoxic (10% O2/90% N2) and hypercapnic (5% CO2/95% O2 ) gases were determined in intact and in peripherally-chemodenervated, barodenervated cats. Thirty-two percent of hypothalamic neurons were stimulated by a hypercapnic stimulus in intact cats; of those that wer e stimulated by hypercapnia, all had a basal discharge related to card iovascular and/or respiratory activity. Hypoxia significantly increase d the discharge rate of 21% of hypothalamic units in intact animals; 9 0% of those had a cardiovascular and/or respiratory-related rhythm. On ly 13% of the neurons were stimulated by both hypoxia and hypercapnia. Similar results were found in barodenervated, peripherally chemodener vated cats. Neurons excited by these stimuli in both the intact and de nervated cats were found to be concentrated in the posterior hypothala mic area. The results of this study suggest that a group of caudal hyp othalamic neurons contribute to the cardiorespiratory responses to hyp oxia and hypercapnia, but via separate subpopulations of neurons. In a ddition, input from peripheral baroreceptor and chemoreceptor afferent s is not required for this modulation.