PREPULSES INHIBIT STARTLE-INDUCED REDUCTIONS OF EXTRACELLULAR DOPAMINE IN THE NUCLEUS-ACCUMBENS OF RAT

In vivo brain microdialysis was used to monitor extracellular levels o f dopamine (DA) in the nucleus accumbens (NAc) of rats during exposure to startling acoustic stimuli. ten rats were prepared with guide cann ulae into which dialysis probes were inserted 1 d before testing. Two to three hours after the start of perfusion, rats were placed into the startle chamber and exposed to a continuous 70 dB(A) background noise . Dialysis samples (2.0 mu l/min) were collected at 6 min intervals. S tartle pulses (120 dB[A] noise) were presented in 20-trial blocks last ing 5 min. In some blocks, an 86 dB[A] prepulse preceded each of the 2 0 pulses by 100 msec, with the order of presentation of pulse-alone or prepulse + pulse blocks being counterbalanced between animals, Three to six sample periods intervened between stimulus-containing blocks. M onoamine and metabolite levels were measured using HPLC with electroch emical detection. During the presentations of startling stimuli, DA le vels in the NAc decreased relative to the immediately preceding 12 min baseline, This decrease in DA was maintained for only one additional sample period. By contrast, the presentation of prepulse + pulse trial s failed to affect dialysate levels of NAc DA during or immediately af ter the stimulation. Thus, startling acoustic stimuli produce signific ant and transient decreases in dialysate levels of DA in the NAc. Furt hermore, prepulse stimuli effectively inhibit these neurochemical effe cts of startling stimuli, in parallel with their established ability t o inhibit the amplitude of the behavioral startle response.