In vitro responses of VLM neurons to hypoxia after normobaric hypoxic acclimatization.

Hypoxic acclimatization involves an initial rapid ventilatory response followed by a more gradual increase in ventilation over a period of 24 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 acclimatization augments the acute hypoxic response of neurons in the ventrolateral medulla (VLM). Brain slices (400 microns) containing the ventrolateral medulla were prepared from Sprague-Dawley rats acclimatized to hypoxia (10% O2) for 4-5 days (n = 4) and 9-10 days (n = 4) and from rats maintained in a normoxic environment (n = 4). Extracellular recordings demonstrated that there were no significant differences in the 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 (approximately 70%), inhibited (approximately 20%) and unaffected (approximately 10%) by an acute bout of hypoxia (10% O2) was also similar among groups. However, acute hypoxia elicited a greater increase in discharge frequency in neurons from rats exposed to the short period of hypoxia compared to the responses from neurons in the control and longer acclimatization groups. Thus, the responsivity of VLM neurons during the early stages of hypoxic acclimatization is altered in a manner consistent with the respiratory responses associated with acclimatization.