Sensitization of the acoustic startle reflex by footshock.

Administration of footshock (500-ms duration, 0.2-1.4 mA) increased the amplitude of the startle reflex for a long time after its presentation. The effect occurred with a single footshock, although its magnitude and consistency across animals was greater with 5 or 10 footshocks presented 1/s. The facilitatory effect came on within 2-4 min with a 0.6-mA shock, peaking in about 10 min and then dissipating over the next 40 min. Stronger shocks also increased startle, but with a more delayed onset of facilitation (8-10 min). Footshocks increased startle in rats not previously given startle-eliciting stimuli, indicating sensitization rather than dishabituation. The facilitatory effect may not be attributable to a rapid conditioning to the experimental context, because a change in lighting conditions from shock presentation to testing did not attenuate shock sensitization. This excitatory effect of shock on startle may represent the unconditioned effect of shock that can become associated with a neutral stimulus to support classical fear conditioning.