Neuroendocrine correlates of stress and tonic immobility.

Threatening stimuli challenging animal homeostasis are the primary events triggering defensive responses, including TI. The stress-response system (allostasis) is signaled by increased corticosteroid basal levels. In bird animal lines genetically selected for stress-induced corticosterone, there is a covariation between stress physiology and coping styles. Rabbit studies, in which the effects of TI are dissociated from those of induction per se, support the view that TI takes part in the homeostatic stress-response system. An increase of corticosterone is recorded just after the end of the induction procedure but not in the corresponding groups in which induction is followed by TI, suggesting a recovery process during TI. Similarly to corticosterone but in opposite direction, testosterone plasma levels decrease following induction and recover during TI. Recovery mechanisms are also suggested in two bird genotypes selected for long and short TI duration. The positive relation between corticosterone levels and TI duration has been confirmed after exogenous corticosterone administration.