Cholinergic involvement in inhibitory behavior of the young domestic chick

Four-day-old Vantress × Arbor Acre chicks injected intraperitoneally with scopolamine hydrobromide increased their activity in an open arena, were more resistant to extinction after key-peck conditioning, and were disrupted in passive avoidance learning when compared to saline-injected controls. Also, the young chicks' performance on these three tests of “behavioral inhibition” was differentially sensitive to the effects of scopolamine. Scopolamine, 1 mg/kg, increased the chicks' activity by about 100% and produced greater resistance to extinction, but had no significant effect on passive avoidance learning. Scopolamine, 4 mg/kg, also increased the chicks' activity by about 100%, but disrupted key—peck performance during extinction testing and significantly reduced response suppression in the key-peck passive avoidance test. Scopolamine, therefore, affects the young precocial chicks' performance on three tasks that have been assumed to measure inhibitory behavior, results that are consistent with the effects of cholinergic antagonists on inhibitory behavior of the altricial rat.