IN-VITRO RESPONSES OF VLM NEURONS TO HYPOXIA AFTER NORMOBARIC HYPOXICACCLIMATIZATION

Hypoxic acclimatization involves an initial rapid ventilatory response followed by a more gradual increase in ventilation over a period of 2 4 to 48 h in both humans and rats. In addition, the acute ventilatory response to hypoxia is accentuated following hypoxic acclimatization. The purpose of the present investigation was to determine if hypoxic a cclimatization augments the acute hypoxic response of neurons in the v entrolateral medulla (VLM). Brain slices (400 mu m) containing the ven trolateral medulla were prepared from Sprague-Dawley rats acclimatized to hypoxia (10% O-2) for 4-5 days (n = 4) and 9-10 days (n = 4) and f rom rats maintained in a normoxic environment (n = 4). Extracellular r ecordings demonstrated that there were no significant differences in t he basal pattern or discharge rate of VLM neurons from animals exposed to short (10.8 +/- 0.9 Hz, n = 51), or long (10.1 +/- 1.1 Hz, n = 59) periods of hypoxia compared to control neurons (10.8 +/- 1.1 Hz, n = 52). The proportion of neurons stimulated (similar to 70%), inhibited (similar to 20%) and unaffected (similar to 10%) by an acute bout of h ypoxia (10% O-2) was also similar among groups. However, acute hypoxia elicited a greater increase in discharge frequency in neurons from ra ts exposed to the short period of hypoxia compared to the responses fr om neurons in the control and longer acclimatization groups. Thus, the responsivity of VLM neurons during the early stages of hypoxic acclim atization is altered in a manner consistent with the respiratory respo nses associated with acclimatization.