Abstract
Increased acoustic startle in the presence of bright ambient light, a phenomenon called light-enhanced startle (LES), is dependent on the bed nucleus of the stria terminalis. In contrast to gonadally intact male rats, LES was seen reliably in castrated male rats and in female rats, although it fluctuated significantly with reproductive state. Replacement with testosterone (T) or combined estradiol (E2) and dihydrotestosterone (DHT), but not with either E2 or DHT alone, attenuated LES in castrated rats. However, replacement with T or E2 in ovariectomized rats did not decrease LES. In contrast, no sex difference was seen in the central amygdala-dependent acquisition or expression of fear-potentiated startle. In addition, T did not reduce expression of fear-potentiated startle in castrated rats. T-replaced castrated males injected centrally with mixed arginine vasopressin (AVP) V1a/b receptor antagonists daily throughout the replacement period failed to show a reduction in the expression of LES. These data suggest that T attenuates LES, but not fear-potentiated startle, through a mechanism that may involve AVP.
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