Sex differences in the responses of spinal wide-dynamic range neurons to subcutaneous formalin and in the effects of different frequencies of conditioning electrical stimulation

In conclusion, s.c. administration of different concentrations of formalin shows a distinct sex-related difference in its late tonic nociception of spinal nociceptive sensory neurons. Sex differences in formalin-induced tonic nociception are stimulus intensity dependent and related to the modulation from the supraspinal regions. S.c. formalin-induced late phase nociception in female rats is only sensitive to depression at a frequency of 50Hz, but not 5Hz, of conditioning electrical stimulation. This suggests that the involvement of the central mechanisms in the antinociceptive effects of conditioning electrical stimulation may be different at various frequencies of stimulation.