Pain control in tonic immobility (TI) and other immobility models.

This chapter deals with the mechanisms modulating pain during TI and other immobility responses in different animal species. In mammals the presence of high voltage slow waves in the electroencephalogram during TI suggests the activation of the thalamic gate, a mechanism blocking all sensory information, including pain. In rabbits TI transiently suppresses all the behavioral responses to persistent nociceptive stimulation by the activation of an opioid mechanism outlasting TI offset by 1h. On the other hand, in rodents, also not injuring nociceptive stimuli applied during TI elicit a delayed opioid analgesia that develops within 45min. Moreover, both opioid and non-opioid mechanisms of analgesia have been observed. TI strongly reduces inflammatory responses by activating the vagal-neocortical-sympathetic axis, a feedback control of neuro-immune mechanisms. Several models of noxious and non-noxious restraint and of post-restraint immobility resembling TI have been proposed. Moreover in lizards, hyperalgesia occurs during and after TI.