Brain stem circuits mediating prepulse inhibition of the startle reflex

Rationale: Prepulse inhibition (PPI) of the startle reflex occurs when brie f. non-startling tactile, acoustic or visual stimuli are presented 20-500 m s before the startling stimulus. Objective: To review information about PPI -mediating brain stem circuits and transmitters, and their functions. Resul ts: Midbrain systems are most critical for the fast relay of these PPI stim uli. Acoustic prepulses for PPI are relayed through the inferior colliculus (IC). The superior colliculus (SC) is important for acoustic PPI, and may be important for the mediation of tactile and visual prepulses. This collic ular activation for PPI is quickly relayed through the pedunculopontine teg mental nucleus (PPTg), with lesser contributions to PPI from the laterodors al tegmental nucleus (LDTg) and substantia nigra, pars reticulata (SNR). Th e transient activation of midbrain nuclei by PPI stimuli is converted into long-lasting inhibition of the giant neurons of the caudal pontine reticula r nucleus (PnC). We propose that muscarinic and GABAB inhibitory receptors (both metabotropic receptors) on PnC giant neurons combine to produce the l ong-lasting inhibition of startle. Activation of mesopontine cholinergic ne urons leads to cortical arousal, turning and exploratory approach responses . Conclusion: PPI is mediated by a circuit involving the IC, SC, PPTg, LDTg , SNR and NC. By reducing startle, PPI allows the execution of approach res ponses and perceptual processing following salient stimuli.