The relationship between pain sensitivity and conditioned fear response in rats.

It might seem obvious that pain sensitivity would predict individual, inborn susceptibilities to aversive stimuli and the strength of fear-conditioned responses. Such relationships are based on the assumption that there is a close association between fear-evoked behavioral reactions and the responses to painful, aversive stimuli. However, this problem has not been systematically studied. To this end, we investigated the relationship between pain sensitivity in two pain tests (the 'tail-flick' and 'flinch-jump' tests) and a conditioned, fear-evoked, freezing response in rats. The results show that there was no correlation between: (1) the conditioned (associative) and the novelty-evoked (non-specific stress-related) fear response and (2) individual differences in pain threshold and fear responses. Furthermore, factor analysis did not group freezing in the conditioned fear test, individual footshock sensibility, or 'tail-flick' reaction to painful stimuli together. These results indicate that pain sensitivity and conditioned emotional responses to pain are not directly correlated.