Nociceptive sensitivity to a long-term stimulus in the formalin test in female and male rats in postnatal ontogenesis

In Wistar rats, a comparative study of pain sensitivity to the long-term stimulus in the formalin test was carried out at different age periods-the prepubertal (25 days), pubertal (40 days), and sex maturity (90 days) periods. The pain sensitivity was evaluated by standard indexes of the biphasic behavioral response (BBR)-patterns of flexing, shaking, licking of the leg injected by formalin, and by duration of the first and second response phases and of the interphase interval. It is found that with development of the central nervous system the pain sensitivity changed, with differences in the first, acute and the second, tonic BBR phases: in the tonic phase of response the pain sensitivity increased in males by the indexes organized at the spinal, while in females, at the supraspinal level, whereas in the acute phase it decreased essentially at the supraspinal level in individuals of both sexes. The greater number of age differences in the pain sensitivity is revealed in females by the response patterns organized at the supraspinal level. At the same level, essential readjustments in hormonal and neurotransmitter systems are reflected in the high BBR indexes. In adult individuals the sex dimorphism is detected in duration of the interphase intervals. Activity of the bulbospinal descending inhibitory monoaminergic systems is shown to continue increasing for the first three months of life, with predominance of this process in females. The obtained data allow concluding that the BBR characteristics depend on age and sex of individuals and are determined by the organization level in CNS of the response patterns characterizing the pain sensitivity in the formalin test.