Sex differences in formalin-induced pain in prenatally stressed infant rats

The aim of this work was to study the effects of prenatal stress on nociceptive responses in the formalin test in female and male infant (7-day-old) Long-Evans hooded rats. Prenatally stressed infant rats displayed biphasic flinching+ shaking behavior whereas non-stressed animals showed only a weak second phase. Pain sensitivity in prenatally stressed males was significantly greater than that of prenatally non-stressed males during the second phase only; there were no differences in pain sensitivity between prenatally stressed and non-stressed females. Moreover prenatally stressed male rats pups demonstrated that the second phase of the response to formalin was enhanced relative to the second phase in stressed females. The current and previous data [Butkevich IP, Barr GA, Mikhailenko VA, Otellin VA. Increased formalin-induced pain and expression of fos neurons in the lumbar spinal cord of prenatally stressed infants rats. Neurosci Lett 2006a;403:222-226] show increased tonic pain in prenatally stressed infant rats and a large increase in the number of formalin-induced fos-like immunoreactivity in the spinal cord dorsal horn. There is a concomitant decrease in serotonin-like immunoreactivity in the lumbar spinal cord dorsal horn [Butkevich IP, Barr GA, Otellin VA. Effect of prenatal stress on behavioral and neural indices of formalin-induced pain in infant rats. Abstracts, 35th Annual Meeting of Soc. For Neurosci. 2005a. Program No. 512.4 Washington, DC: Society for Neuroscience]. Given the decreased level of perinatal testosterone in prenatally stressed rats to which infant males are more sensitive than females, we suggest that these hormonal, behavioral and neuronal indices are strongly interrelated in prenatally stressed 7-day-old rat pups and that the decreased surge of testosterone may contribute to the increased behavioral response in the second phase in male rat pups. Mechanisms underlying the behavioral pain response induced by inflammation in prenatally stressed rat pups are characterized by sexual dimorphism even prior to the activational effects of sex hormones.