THE EFFECTS OF TROPICAMIDE ON MYDRIASIS IN YOUNG-RATS EXHIBITING A NATURAL DEFICIT IN PASSIVE-AVOIDANCE RESPONDING

The young rat at post-natal day 18-22 exhibits a natural deficit in pa ssive-avoidance responding that can be corrected with the acute system ic administration of different cholinomimetic drugs, such as tacrine. In order to evaluate the generality of this apparent cholinergic hypof unction, different doses of the anticholinergic agent tropicamide, wer e administered either systemically or dropped directly into the eye of young or adult rats. Tropicamide produced mydriasis in a dose-depende nt manner. The ED50 for tropicamide dropped into the eye was 0.025% fo r adult rats and 0.12% for young rats. When doses between 0.3 and 100 mg/kg were delivered systemically, the mean time course for recovery t o baseline pupil size was accelerated in young rats. The average time to recovery across all doses was 112 +/- 27 min (mean +/- SE) for youn g rats and 274 +/- 70 min for adults. When subcutaneous tacrine was gi ven immediately to young rats after training in a passive-avoidance re sponse (PAR) task, retention was enhanced at testing 24 hours later in a dose-dependent manner. The response latencies were statistically di fferent from saline-treated controls at doses of 0.003 and 0.01 mg/kg. This was not observed in adult rats. Taken together these results sug gest that the PAR, along with the mydriacyl response of the young rat to tropicamide, may be regulated by a system of subsensitive cholinerg ic receptors.