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Abstract
Acute physical restraint represents a potent stressor in several animal species and
is accompanied by a complex pattern of hormonal responses and functional changes in
the central nervous system. Repeated immobilization leads to partial blunting of the
behavioral and hormonal responses, with transient modifications of neurotransmitter
systems in the brain. Pain reactions, as investigated by different kinds of nociceptive
tests, are usually attenuated both during and immediately following acute immobilization
and the analgesic effect of opiate compounds potentiated; these behavioral alterations
may be attributed at least in part to activation of an endogenous opioid system. In
some species, restraint may induce a reflex immobility (animal hypnosis or tonic immobility)
which is also characterized by suppression of pain reactions in rabbits, probably
subserved by different mechanisms. Analysis of available data suggests that pain testing
in unanesthetized, restrained animals may involve alterations of the animal's reactivity
to noxious stimuli.