Intermittent hypoxia, such as that associated with obstructive sleep apnea, can cause neuronal death and neurobehavioral dysfunction. The cellular and molecular mechanisms through which hypoxia alter hippocampal function are incompletely understood. This study used in vitro [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) autoradiography to test the hypothesis that carbachol and DAMGO activate hippocampal G proteins. In addition, this study tested the hypothesis that in vivo exposure to different oxygen (O(2)) concentrations causes a differential activation of G proteins in the CA1, CA3, and dentate gyrus (DG) regions of the hippocampus. G protein activation was quantified as nCi/g tissue in CA1, CA3, and DG from rats housed for 14 days under one of three different oxygen conditions: normoxic (21% O(2)) room air, or hypoxia (10% O(2)) that was intermittent or sustained. Across all regions of the hippocampus, activation of G proteins by the cholinergic agonist carbachol and the mu opioid agonist [D-Ala(2), N-Met-Phe(4), Gly(5)] enkephalin (DAMGO) was ordered by the degree of hypoxia such that sustained hypoxia > intermittent hypoxia > room air. Carbachol increased G protein activation during sustained hypoxia (38%), intermittent hypoxia (29%), and room air (27%). DAMGO also activated G proteins during sustained hypoxia (52%), intermittent hypoxia (48%), and room air (43%). Region-specific comparisons of G protein activation revealed that the DG showed significantly less activation by carbachol following intermittent hypoxia and sustained hypoxia than the CA1. Considered together, the results suggest the potential for hypoxia to alter hippocampal function by blunting the cholinergic activation of G proteins within the DG.