Hypoxic responses of sympathetic preganglionic neurons in the acute spinal cat

The relation between arterial O2 tension (PaO2) and the firing rate of sympathetic preganglionic neurons (SPN) of the cervical sympathetic trunk was studied during graded isocapnic hypoxia and hyperoxia in unanesthetized acute C-1 spinal cats. In the PaO2 range between 40 and 400 Torr there was no relation between the two variables. Below 40 Torr firing rate increased as PaO2 decreased, reaching an average peak value of ten times control at a PaO2 of 20 Torr. Mean arterial blood pressure (MABP) was also independent of PaO2 between 40 and 400 Torr and increased by an average of 25% at PaO2 values below 40 Torr. Intravenous administration of hexamethonium or phentolamine abolished the MABP response but not the SPN response to hypoxia. Pentobarbital (5-60 mg/kg iv) did not modify the SPN response to hypoxia, although normoxic SPN background firing was considerably depressed. The excitatory effect of hypoxia seems independent of excitatory afferent input and appears to be a general property of SPN.