Roles of the glutamate receptor epsilon 2 and delta 2 subunits in the potentiation and prepulse inhibition of the acoustic startle reflex

We examined the regulation of the acoustic startle response in mutant mice of the N-methyl-d-aspartate (NMDA)- and delta -subtypes of the glutamate receptor (GluR) channel, which play important roles in neural plasticity in the forebrain and the cerebellum, respectively. Heterozygous mutant mice with reduced GluR epsilon 2 subunits of the NMDA receptor showed strongly enhanced startle responses to acoustic stimuli. On the other hand, heterozygous and homozygous mutation of the other NMDA receptor GluR epsilon subunits exerted no, or only small effects on acoustic startle responses. The threshold of the auditory brainstem response of the GluR epsilon 2-mutant mice was comparable to that of the wild-type littermates. The primary circuit of the acoustic startle response is a relatively simple oligosynaptic pathway located in the lower brainstem, whilst the expression of GluR epsilon 2 is restricted to the forebrain. We thus suggest that the NMDA receptor GluR epsilon 2 subunit plays a role in the regulation of the startle reflex. Ablation of the cerebellar Purkinje cell-specific delta 2 subunit of the GluR channel exerted little effect on the acoustic startle response but resulted in the enhancement of prepulse inhibition of the reflex. Because inhibition of the acoustic startle response by a weak prepulse is a measure of sensorimotor gating, the process by which an organism filters sensory information, these observations indicate the involvement of the cerebellum in the modulation of sensorimotor gating.