Increase in testosterone metabolism in the rat central nervous system by formalin-induced tonic pain

The effects of formalin-induced tonic pain (FITP) on testosterone (T) concentrations in the central nervous system (CNS) and serum were investigated in rats. T was nearly eliminated from the brain and spinal cord 1.5 and 24 h after a single subcutaneous injection (100 μl/rat, sc) of 5% formalin and its levels were similar to that seen following castration. In serum, T concentrations were decreased significantly 1.5 h following formalin injection, but after 24 h, the serum level of T was within normal range. T concentrations in the brain, spinal cord, and serum were not modified 20 min after formalin injection. Pretreatment of rats with finasteride, a 5α-reductase (5α-R) inhibitor (5 mg/kg, sc) blocked T elimination from the brain and spinal cord by FITP, but it failed to prevent decrease in serum T. However, 3 h after administration of exogenous T (5 mg/kg, sc), FITP did not cause a significant decrease in T levels in the CNS and serum. These results suggest that FITP eliminates endogenous T in the brain and spinal cord by increasing 5α-R activity in the CNS.