Inhibition of acoustic startle in the rat by a footshock prestimulus: effects of morphine and naloxone

In a series of three experiments, we investigated the ability of footshock to inhibit the rat's acoustic startle response. Based on the mean thresholds obtained from a flinch-jump test, 8 rats were tested in a startle inhibition procedure with prestimulus intensities of 0.1, 0.2, 0.4, and 0.8 mA. Subjects were presented with a series of startle-eliciting noise bursts, preceded by a 300-ms footshock of one of the four intensities. Control trials consisting of the noise alone were also included. Startle amplitude was measured during the 100 ms immediately following the noise onset. Footshock reliably inhibited startle at all intensities; inhibition was near maximum at 0.2 mA. Morphine administration reliably interfered with the inhibition at all intensities, and this was reversed by naloxone administration. These data suggest that morphine may have a more general effect than is currently believed.