Effects of d- and l-amphetamine on habituation and sensitization of the acoustic startle response in rats

In a series of 3 experiments the effects of 2, 4, 8, or 16 mg/kg d-amphetamine and 4, 8, 16, or 32 mg/kg l-amphetamine on acoustic startle amplitude in the rat were investigated. d-Amphetamine was 4–5 times as potent as l-amphetamine in augmenting startle amplitude. Startle potentiation was associated with vigorous stereotypies but the resultant cage movement could not account for the change in startle. Pretreatment with -methyl-p-tyrosine (100 mg/kg, 1 hr before) had only a slight depressant effect on startle but essentially eliminated augmentation of startle by either d-amphetamine (8 mg/kg) or l-amphetamine (32 mg/kg). d-Amphetamine did not have a direct effect on startle but instead enhanced sensitization produced by the startle stimuli without altering sensitization produced by background white noise or habituation. The results suggest that startle sensitization is enhanced by increased availability of catecholamines and, by virtue of the different potencies of the d- and l-isomers, that dopamine and norepinephrine may affect startle differently.