Effects of immobilization stress on shock discrimination performance in rats

It has been suggested that acute stress induces analgesia in rats and mice. However, stress can have a myriad of debilitating effects on organisms, and it is not clear whether stress decreases the perceived intensity of the painful stimulus (i.e., a change in sensitivity) or decreases the willingness or ability of the organism to carry out the appropriate bodily movement (i.e., a change in responsivity). Since traditional animal analgesia tests confound sensitivity with responsivity, we assessed the effects of immobilization stress on rats trained on a two-choice, two-shock discrimination task, in which the dependent measures are presumably less reflective of alterations in pain responsivity. Immobilization stress significantly disrupted shock discrimination performance, with the disruption occurring primarily in choice performance signalled by the higher intensity shock discriminative stimulus. These effects completely dissipated with successive daily exposures to stress. The data suggest that immobilization stress has a specific effect on pain sensitivity which is separate from its possible effects on pain responsivity.