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 were examined in rats. Acoustic startle was significantly depressed by 12.5 microg TRH, while increasing motor activity. In a fear-potentiated startle paradigm, 12.5 microg TRH reduced the overall startle response amplitude, but did not decrease the amount of fear-potentiated startle. When TRH was administered 15 min before contextual fear conditioning, neither fear-related freezing in acquisition nor in a retention test was affected. In contrast, when TRH was administered 15 min before the retention test, TRH significantly reduced 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 avoidance. It is suggested that the results may be due to TRH's effects on motor activity and arousal, independent of its effects on fear. Copyright 2000 Harcourt Publishers Ltd.