Developmental aspects and mechanisms of rat caudal hypothalamic neuronal responses to hypoxia.

Previous reports from this laboratory have shown that a high percentage of neurons in the caudal hypothalamus are stimulated by hypoxia both in vivo and in vitro. This stimulation is in the form of an increase in firing frequency and significant membrane depolarization. The goal of the present study was to determine if this hypoxia-induced excitation is influenced by development. In addition, we sought to determine the mechanism by which hypoxia stimulates caudal hypothalamic neurons. Caudal hypothalamic neurons from neonatal (4-16 days) or juvenile (20-40 days) rats were patch-clamped, and the whole cell voltage and current responses to moderate (10% O2) or severe (0% O2) hypoxia were recorded in the brain slice preparation. Analysis of tissue oxygen levels demonstrated no significant difference in the levels of tissue oxygen in brain slices between the different age groups. A significantly larger input resistance, time constant and half-time to spike height was observed for neonatal neurons compared with juvenile neurons. Both moderate and severe hypoxia elicited a net inward current in a significantly larger percentage of caudal hypothalamic neurons from rats aged 20-40 days (juvenile) as compared with rats aged 4-16 days (neonatal). In contrast, there was no difference in the magnitude of the inward current response to moderate or severe hypoxia between the two age groups. Those cells that were stimulated by hypoxia demonstrated a significant decrease in input resistance during hypoxic stimulation that was not observed in those cells unaffected by hypoxia. A subset of neurons were tested independent of age for the ability to maintain the inward current response to hypoxia during synaptic blockade (11.4 mM Mg2+/0. 2 mM Ca2+). Most of the neurons tested (88.9%) maintained a hypoxic excitation during synaptic blockade, and this inward current response was unaffected by addition of 2 mM cobalt chloride to the bathing medium. In contrast, perfusion with the Na+ channel blocker, tetrodotoxin (1-2 microM) or Na+ replacement with N-methyl-D-glucamine (NMDG) significantly reduced the inward current response to hypoxia. Furthermore, the input resistance decrease observed during hypoxia was attenuated significantly during perfusion with NMDG. These results indicate the excitation elicited by hypoxia in hypothalamic neurons is age dependent. In addition, the inward current response of caudal hypothalamic neurons is not dependent on synaptic input but results from a sodium-dependent conductance.