Footshock-induced sensitization of electrically elicited startle reflexes.

The acoustic startle reflex can be facilitated by the presentation of a train of footshocks presented in rapid succession (footshock sensitization). Acoustic startle is a short-latency reflex mediated by a neural circuit consisting of the ventral cochlear nucleus (VCN), ventral nucleus of the lateral lemniscus (VLL), the nucleus reticularis pontis caudalis (RPC), and the spinal cord. The present study sought to determine the point along this pathway where footshocks might ultimately alter neural transmission to affect startle response. Rats were implanted bilaterally with stimulating electrodes in either the VCN, VLL, or RPC. Startle could be elicited acoustically with a noise burst or electrically with a single-pulse stimulus to either the VCN, VLL, or RPC before and after a train of ten 0.6-mA, 500-ms shocks presented at a rate of 1 shocks/s. Startle elicited acoustically or electrically in the VCN or VLL was significantly elevated following shocks. In contrast, startle elicited from the RPC showed no sensitization, even though startle elicited acoustically from the same animals during the same test session was facilitated. These data suggest that footshock sensitization ultimately alters transmission in the startle circuit at the RPC.