Opiate-dopamine interactions in the neural substrates of acoustic startle gating in the rat.

1. The acoustic startle reflex (ASR) was measured in adult male Dawley rats using startling acoustic stimuli presented either alone or 60-500 msec after a weak acoustic prepulse. 2. The inhibition of the ASR by the prepulse, termed "prepulse inhibition" (PPI), was blocked in animals treated either with the indirect dopamine (DA) agonist d-amphetamine (AMPH) or with the direct DA receptor agonist apomorphine (APO). 3. Pretreatment with the opiate receptor antagonist naloxone (NAL) prevented the AMPH-induced loss of PPI, but did not diminish the APO-induced loss of PPI. 4. The opiate heroin had no significant effect on PPI. 5. Dopaminergic mechanisms that modulate PPI in the rat may be regulated by opiate systems that act presynaptic to the DA receptor; brain opiate receptors may not have direct effects on startle gating independent of this opiate-DA interaction.