The effect of d-amphetamine on optokinetic nystagmus in the guinea pig.

The effect of d-amphetamine oral administration in doses of 1-2.5 mg/kg on horizontal optokinetic nystagmus (OKN) and after nystagmus (OKAN) was investigated in the guinea pig. Eye movements were recorded by means of the electromagnetic search-coil technique. After amphetamine administration the range of stimulus velocities effective for eliciting OKN was 10-20 deg/s higher than before treatment. The mean values and the fluctuations of the eye velocity during slow nystagmus phases before and after treatment did not differ. Administration of amphetamine led to 2-8 s increase in OKAN duration. The OKAN prolongation did not depend on stimulation velocity. The dependency of OKAN duration on stimulation velocity was well approximated by a linear regression. The slope of the regression line was 0.160 +/- 0.022 before and 0.177 +/- 0.028 after treatment. Similarity in the coefficients indicates that amphetamine did not alter the relationship between the velocity of optokinetic stimulus and the duration of after nystagmus. Constant prolongation of OKAN over the whole range of stimulation velocities could reflect a constant shift in activity of neurons representing the velocity storage. The effects observed on OKN gain curves and the increase in OKAN duration did not display a clear dependency on the dosages of d-amphetamine used in the experiments. We assume that the effects of treatment reflected a general increase in attentiveness and motility of animals resulting from the arousal action of amphetamine.