Effects of TRH on acoustic startle, conditioned fear and active avoidance in rats

The effects of intracerebroventricular injection of thyrotropine-releasing hormone (TRH) on acoustic startle, conditioned fear and active avoidance we re examined in rats. Acoustic startle was significantly depressed by 12.5 m u g TRH, while increasing motor activity. In a fear-potentiated startle par adigm, 12.5 mu g TRH reduced the overall startle response amplitude, but di d not decrease the amount of fear-potentiated startle. When TRH was adminis tered 15 min before contextual fear conditioning, neither fear-related free zing in acquisition nor in a retention test was affected. In contrast, when TRH was administered 15 min before the retention test, TRH significantly r educed mean percentage of time spent freezing. TRH had no effect on active avoidance. The results demonstrate that TRH decreased acoustic startle and freezing responses, but had little effect on fear conditioning and active a voidance. It is suggested that the results may be due to TRH's effects on m otor activity and arousal, independent of its effects on fear. (C) 2000 Har court Publishers Ltd.