Age-dependent improvement in passive avoidance learning of the young chick: cholinergic mediation?

Cholinergic mediation of the age-dependent improvement in response suppression of the young chick was studied by determining the performance of 4-day-old chicks, pretreated with scopolamine, during passive avoidance (PA) and extinction testing. In Experiment 1, chicks were trained briefly to key peck for heat reward (prepunishment training), and then tested for PA learning under immediate, 2-sec-delayed, or no shock condition. Half of the chicks in each wing-shock (5 mA, 5 sec) condition received saline injections before prepunishment training and .5 mg/kg scopolamine injections after prepunishment training. The rest of the chicks received .5 mg/kg scopolamine injections both before and after prepunishment training. For chicks in both scopolamine groups, delaying shock onset resulted in significantly less response suppression than immediate response-contingent shock. In Experiment 2, 4-day-old chicks injected with either saline or scopolamine were trained to key peck for heat reward and then tested for resistance to extinction under either response-contingent shock or nonshock conditions. Punishment decreased the number of extinction responses for both saline and scopolamine groups of chicks. Previous studies have shown that normal 1-day-old chicks do not show a significant delay of punishment effect during PA testing and that response-contingent punishment increases the number of their responses during extinction. Hence, the results of the present experiments indicate that the age-dependent improvement in response suppression of the young chick cannot be explained solely by a significant increase in central cholinergic functioning.