NMDA AND NON-NMDA ANTAGONISTS INFUSED INTO THE NUCLEUS-RETICULARIS PONTIS CAUDALIS DEPRESS THE ACOUSTIC STARTLE REFLEX

The neural pathway that mediates the acoustic startle reflex has been proposed; however, the pharmacology underlying this reflex is less wel l known. The present study examined the role of excitatory amino acid receptors at the level of the nucleus reticularis pontis caudalis, a b rainstem nucleus obligatory for the whole body startle reflex and impl icated as the locus where extrinsic systems such as the amygdala may a ct to modulate acoustic startle. Twenty-nine rats, chronically implant ed with bilateral cannulae aimed at the nucleus reticularis pontis cau dalis, were tested to assess the effects of gamma-D-glutamylglycine (D GG), DL-2-amino-5-phosphonopentanoic acid (AP5), and 6-cyano-7-nitroqu inoxaline-2,3-dione (CNQX) on the amplitude of the acoustic startle re flex. Local infusion of each of the 3 compounds significantly reduced startle amplitude by as much as 70-80%. AP5 and CNQX attenuated startl e over a dose range which indicated that the reticularis pontis caudal is may be much more sensitive to these compounds than other nuclei alo ng the primary startle pathway. These results suggest that, at the lev el of the nucleus reticularis pontis caudalis, an excitatory amino aci d neurotransmitter may mediate acoustic startle, and that both NMDA an d non-NMDA receptor subtypes may be important for the expression of th e acoustic startle reflex.