Anticholinergic drugs, blood-brain-barrier and tonic immobility in chickens.

A series of three experiments examined the effects of atropine, scopolamine, and their methyl analogs on tonic immobility in chickens. The first experiment demonstrated significant attenuation of immobility duration by atropine and scopolamine. Methyl atropine and methyl scopolamine attenuated immobility duration but did so significantly less than equivalent amounts of atropine and scopolamine. Experiments 2 and 3 examined effects of atropine, methyl atropine, scopolamine, and methyl scopolamine on immobility duration as a function of posthatch age. In these experiments atropine and scopolamine attenuated TI duration at each test age examined. The methyl analogs attenuated TI duration only at the youngest age (7-days posthatch) and failed to do so at 14-days posthatch or older. These age-dependent differences between the tertiary and quaternary compounds were attributed to blood-brain-barrier development. In Experiments 2 and 3 immobility duration increased with increased posthatch age. Scopolamine prevented the age-related increase but atropine did not. This difference is discussed in terms of fear arousal.