Duration of tonic immobility in chickens as a function of alpha-adrenergic receptor stimulation and blockade.

Tonic immobility in chickens was influenced by a variety of drugs that act on the adrenergic neurochemical system of the body. Alpha 1 agonists such as methoxamine and phenylephrine produced decreases in the duration of immobility, although the former compound also caused a significant increase in the immobility response at high dosages. Alpha 2 agonists such as clonidine, naphazoline, and guanfacine enhanced the duration of immobility, but clonidine also produced an apparent reversal of this effect at high dosages. Subsequent experiments examined more fully the biphasic effects by methoxamine and clonidine on tonic immobility through interactions with alpha antagonists. Yohimbine, an alpha 2 blocker, attenuated the duration of immobility, either alone or in conjunction with various dosages of methoxamine. Prazosin, an alpha 1 blocker, had no direct effect on tonic immobility, but potentiated the duration of the response when given in conjunction with various dosages of methoxamine. When these antagonists were given in conjunction with clonidine, yohimbine reduced immobility durations, while prazosin had no apparent effect on this response. These results are discussed in terms of the relative contributions of the alpha 1 and alpha 2 adrenoceptors to the duration of the immobility response.