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.