Sensitivity to ethanol across development in rats: comparison to [3H]zolpidem binding.
Previous research has suggested that rats tested at 28 to 30 days of age show a marked subsensitivity to the sedative effects of ethanol. In the present study, rats of different ages were tested for aerial righting following acute ethanol (3 g/kg) treatment. These results were compared with the effects of the atypical benzodiazepine zolpidem (3 and 5 mg/kg) and pentobarbital (10 and 15 mg/kg). Animals tested at 25, 28, or 35 days of age were significantly less impaired by ethanol than preweanling rats (age 20 days) or older rats (age 65 to 75 days), whereas animals tested at 25 or 28 days of age were less impaired by the higher dose of zolpidem. With pentobarbital, the most distinct age-related trend was greater impairment in 20-day-old rats. Because ethanol may be active at the same type I GABA(A) receptor site selectively labeled by [3H]zolpidem, levels of [3H]zolpidem binding were determined for rats of different ages. Although some brain regions showed progressive increases in binding of [3H]zolpidem across development, other regions demonstrated increased binding from day 12 or 17 to day 20, then a plateau of binding levels across days 20, 25, and 28, with further increases occurring by day 36 or day 60. This pattern was observed in the cingulate cortex, medial septal nucleus, globus pallidus, inferior colliculus, red nucleus, and cerebellum. Overall, the results indicate that the period of subsensitivity to the sedative effects of ethanol is coincident with a change in the developmental pattern of GABA(A) receptor sites targeted by [3H]zolpidem.