Increase in testosterone metabolism in the rat central nervous system by formalin-induced tonic pain Hossein Amini, Abolhassan Ahmadiani * Department of Pharmacology, Neuroscience Research Center, Shaheed Beheshti University of Medical Sciences, Tehran, Iran Received 19 February 2002; received in revised form 5 June 2002; accepted 9 August 2002 Abstract 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 ml/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 5a-reductase (5a-R) inhibitor (5 mg/kg, sc) blocked Telimination 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 5a-R activity in the CNS. D 2002 Published by Elsevier Science Inc. Keywords: Formalin-induced tonic pain; Testosterone; 5a-Reductase; Brain; Spinal cord; Serum; Rat 1. Introduction The formalin test is used as a model of acute tissue injury- induced pain in rodents (Abbott et al., 1995). Immediately after formalin injection into an animal’s paw, there is a vigorous behavioral response, presumably due to direct stimulation of C-fibers by formalin. The initial response subsides after a few minutes, and 15–20 min later, shaking, mouthing, and protection of the limb reappear and continue for 30 – 60 min (Matthies and Franklin, 1992; Yamamoto and Yaksh, 1992). The second phase is thought to reflect tonic pain associated with inflammation and the brainstem plays a critical role in its generation (Matthies and Franklin, 1992). Evidence from numerous studies suggests that sex steroids, such as testosterone (T), influence pain process- ing (Fillingim and Ness, 2000). In animal models of transient (phasic) pain, castration of males has been reported to slightly decrease analgesia (Forman et al., 1989; Kepler et al., 1989), increase it (Rao and Saifi, 1981), or to not alter nociceptive responses (Ali et al., 1995; Kepler et al., 1991). In the second phase of the formalin test, castrated rats experience analgesia (Nayebi and Ahmadiani, 1999). It is postulated that the long- lasting painful stimuli in the second phase of the formalin test may be better analogous to chronic pain in people (Aloisi et al., 1998). Although the studies on the role of sex steroids in pain perception and pain inhibition have received great atten- tion, the effects of pain on endogenous levels of various steroids have not been extensively studied. Acute stress has been shown to elicit a marked rapid increase in brain pro- gesterone and its metabolites, allopregnanolone and allote- trahydrodeoxycorticosterone (Barbaccia et al., 1996a,b; Purdy et al., 1991). These two metabolites that are pro- duced de novo in the brain (Pinna et al., 2000) are among the most potent known ligands of gamma aminobutyric acid type A (GABA A ) receptors in the central nervous system (CNS) (Concas et al., 1999). Changes in the brain concentrations of neuroactive steroids may play an import- ant role in the homeostatic mechanism that counteract the neuronal overexcitation elicited by acute stress (Barbaccia et al., 2001). It has been reported by numerous investigators that stress inhibits T production in male rats (Srivastava et al., 1993). 0091-3057/02/$ – see front matter D 2002 Published by Elsevier Science Inc. PII:S0091-3057(02)00986-3 * Corresponding author: Fax: +98-21-240-3154. E-mail address: aaz@farabi.hbi.dmr.or.ir (A. Ahmadiani). www.elsevier.com/locate/pharmbiochembeh Pharmacology, Biochemistry and Behavior 74 (2002) 199–204