Development changes in pharmacological responsivity of the acoustic startle reflex: effects of picrotoxin.

Using the acoustic startle reflex as the behavioral measure, qualitatively different responses to the GABA antagonist picrotoxin were obtained in developing rats before and after 21 days postnatal (PN) age. Dose-dependent increases in acoustic startle were seen following picrotoxin in PN day 15-16 rat pups. In contrast, dose-dependent decreases in startle following picrotoxin were observed in adult rats. The switch from excitation to inhibition of startle was found to occur abruptly on PN day 21. Excitatory responses to picrotoxin were also found in adult rats following localized infusions of picrotoxin into lumbar spinal cord regions, but not into the forebrain. These results give evidence that picrotoxin-sensitive sites that modulate increases in startle reflex behavior mature first and are analogous to sites in the adult spinal cord, whereas picrotoxin-sensitive sites that modulate decreases in startle reflex behavior mature later (greater than or equal to PN day 21) and are localized in more rostral brain areas.