Differential effects of alpha-adrenergic antagonists on tonic immobility in domestic fowl

Groups of young chickens were injected with various dosages of several alpha-adrenergic antagonists and tested for the effects of these drugs on tonic immobility. Yohimbine, a very potent alpha2 antagonist, produced a significant decrease in the duration of tonic immobility at doses of 1 mg/kg, while prazosin, a potent alpha1 antagonist, had no apparent effect on immobility duration. Phentolamine, a on-specific alpha antagonist, produced a significant decrease in the duration of immobility at doses of 0.1 mg/kg, but had no effect at higher or lower doses. These results support the involvement of the alpha-adrenergic system with the duration of the tonic immobility reflex, but at the same time they also suggest that alpha2 receptors are more closely involved with the immobility response than alpha1 receptors. However, none of these drugs had any apparent effect on the initial susceptibility of subjects to the immobility reflex, as measured by the number of inductions required to produce tonic immobility.