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

Abstract

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 alpha 2 antagonist, produced a significant decrease in the duration of tonic immobility at doses of 1 mg/kg, while prazosin, a potent alpha 1 antagonist, had no apparent effect on immobility duration. Phentolamine, a non-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 alpha 2 receptors are more closely involved with the immobility response than alpha 1 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.