Conditioned freezing (generalized motor inhibition) in several rat strains: its usefulness in assessing somato-vegetative responses to nociceptive stress.

Conditioned initial freezing duration was timed in the light-dark box test. Four genetically diverse groups of rats (Wistar, Long Evans, Brattleboro heterozigous for diabetes insipidus, Brattleboro homozygous for diabetes insipidus) were employed. 0.6 mA and 1.8 mA footshocks were administered as nociceptive stressors. The results showed that freezing duration easily and conveniently measures the animal's capacity of responding to stress. This finding is discussed in terms of genetically determined hormonal make-up and of CNS receptorial mechanisms. Initial freezing duration is proposed as a tool to evaluate stress responses.