NMDA RECEPTORS IN THE PONTINE BRAIN-STEM ARE NECESSARY FOR FEAR POTENTIATION OF THE STARTLE RESPONSE

The fear-potentiated startle model in rats is a valuable animal test f or the investigation of the neural and neurochemical basis of fear. In this model, rats are trained to associate a neutral stimulus with an aversive stimulus, so that after conditioning the conditioned stimulus alone elicits a state of fear leading to an exaggerated acoustic star tle response. The fear-potentiated startle model does not require inst rumental responding for the indication of states of fear. The acoustic startle response is mediated by a simple brainstem circuit, with the caudal pontine reticular nucleus as an interface that receives input f rom startle-enhancing circuits. In the present study, we tested the hy pothesis that N-methyl-D-aspartate (NMDA) receptors on neurones of the caudal pontine reticular nucleus are involved in the mediation of fea r-potentiated startle. After fear-conditioning, we injected the NMDA r eceptor antagonist DL-2-amino-5-phosphonopentanoic acid (AP-5), into t he caudal pontine reticular nucleus of awake rats and tested the effec t on the expression of fear-potentiated startle. Injections of AP-5 (0 .125-0.5 nmol) into the caudal pontine reticular nucleus dose dependen tly attenuated fear-potentiated startle without affecting the baseline amplitude of the acoustic startle response. The results suggests that , in the caudal pontine reticular nucleus, glutamate may mediate fear- potentiated startle via NMDA receptors.