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 a nd in vitro. This stimulation is in the form of an increase in firing frequ ency and significant membrane depolarization. The goal of the present study was to determine if this hypoxia-induced excitation is influenced by devel opment. In addition, we sought to determine the mechanism by which hypoxia stimulates caudal hypothalamic neurons. Caudal hypothalamic neurons from ne onatal (4-16 days) or juvenile (20-40 days) rats were patch-clamped, and th e whole cell voltage and current responses to moderate (10% O-2) or severe (0% O-2) hypoxia were recorded in the brain slice preparation. Analysis of tissue oxygen levels demonstrated no significant difference in the levels o f tissue oxygen in brain slices between the different age groups. A signifi cantly larger input resistance, time constant and half-time to spike height was observed for neonatal neurons compared with juvenile neurons. Both mod erate and severe hypoxia elicited a net inward current in a significantly l arger percentage of caudal hypothalamic neurons from rats aged 20-40 days ( juvenile) as compared with rats aged 4-16 days (neonatal). In contrast, the re was no difference in the magnitude of the inward current response to mod erate or severe hypoxia between the two age groups. Those cells that were s timulated by hypoxia demonstrated a significant decrease in input resistanc e during hypoxic stimulation that was not observed in those cells unaffecte d by hypoxia A subset of neurons were tested independent of age for the abi lity to maintain the inward current response to hypoxia during synaptic blo ckade (11.4 mM Mg2+/0.2 mM Ca2+). Most of the neurons tested (88.9%) mainta ined 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 mu M) or Na+ replacement with N-methyl-D-glucamine (NMDG) significantl y reduced the inward current response to hypoxia. Furthermore, the input re sistance decrease observed during hypoxia was attenuated significantly duri ng perfusion with NMDG. These results indicate the excitation elicited by h ypoxia in hypothalamic neurons is age dependent. In addition, the inward cu rrent response of caudal hypothalamic neurons is not dependent on synaptic input but results from a sodium-dependent conductance.