VENTROLATERAL MEDULLARY NEURONS SHOW AGE-DEPENDENT DEPOLARIZATIONS TOHYPOXIA IN-VITRO

Central mechanisms are likely responsible for the larger respiratory a ctivation in response to hypoxia in the adult compared to the neonatal animal. One possible site for this effect is in the ventrolateral med ulla, an area known to be involved in the cardiorespiratory responses to hypoxia. Neurons in this area are stimulated by hypoxia both in viv o and in vitro. The purpose of the present study; was to determine if changes in the magnitude of this excitatory response occur during earl y postnatal development. Whole-cell patch recordings were made from ne urons in the ventrolateral medulla (VLM) in a 400-mu m brain slice pre paration. The basal properties and responses to a brief (90 s) hypoxic stimulus (5% CO2/95% N-2) were compared between neurons from neonatal (P < 16) and juvenile (P16-38) rats. An excitation consisting of a de polarization, increase in spike frequency and decrease in input resist ance was observed during hypoxia in eighty-three percent of juvenile b ut in only 58% of the neonatal VLM neurons. Moreover, the magnitude of this response was greater in the juvenile (8.2 +/- 1.3 mV) than in th e neonatal (4.8 +/- 0.5 mV) neurons. A second type of depolarizing res ponse, consisting bf a more pronounced depolarization interrupted by a brief hyperpolarization that returned to a depolarized state and not associated with an increased discharge frequency, occurred in only 3% of the neurons from the juvenile animals compared to 18% of those from neonatal rats. The remaining proportion of the VLM neurons studied we re hyperpolarized or were unaffected by hypoxia. Measurements of tissu e pO(2) indicate that none of the above differences are due to variati ons in the hypoxic stimulus between neonatal and adult slices. The res ults of this study suggest that the hypoxic-induced depolarizations ob served in VLM neurons change during development. These developmental c hanges may contribute two the changes that occur in cardiorespiratory responses to acute systemic hypoxia during early development.