Selective regulation of 3α-hydroxysteroid oxido-reductase expression in dorsal root ganglion neurons: A possible mechanism to cope with peripheral nerve injury-induced chronic pain

The enzyme 3 alpha-hydroxysteroid oxido-reductase (3 alpha-HSOR) catalyzes the synthesis and bioavailability of 3 alpha,5 alpha-neurosteroids as allopregnanolone (3 alpha,5 alpha-THP) which activates GABA(A) receptors and blocks T-type calcium channels involved in pain mechanisms. Here, we used a multidisciplinary approach to demonstrate that 3 alpha-HSOR is a cellular target the modulation of which in dorsal root ganglia (DRG) may contribute to suppress pain resulting from peripheral nerve injury. Immunohistochemistry and confocal microscope analyses showed 3 alpha-HSOR-immunostaining in naive rat DRG sensory neurons and glial cells. Pulse-chase, high performance liquid chromatography and Flo/One characterization of neurosteroids demonstrated 3 alpha,5 alpha-THP production in DRG. Behavioral methods allowed identification of pain symptoms (thermal and mechanical hyperalgesia and/or allodynia) in rats subjected to sciatic nerve chronic constriction injury (CCI). Reverse transcription and real-time polymerase chain reaction revealed that 3 alpha-HSOR mRNA concentration in CCI-rat ipsilateral DRG, 5-fold higher than in contralateral DRG, was also 4- to 6-fold elevated than that in sham-operated or naive rat DRG. Consistently, Western blotting confirmed increased 3 alpha-HSOR protein levels in CCI-rat ipsilateral DRG and double immunolabeling showed that 3 alpha-HSOR overexpression occurred in DRG neurons but not in glia. Functional plasticity of 3 alpha-HSOR leading to increased 3 alpha,5 alpha-THP production was evidenced in CO-rat DRG. Interestingly, behavioral and molecular time-course investigations revealed that 3 alpha-HSOR gene upregulation was correlated to pain symptom development. Most importantly, in vivo knockdown of 3 alpha-HSOR expression in healthy rat DRG using 6-carboxyfluorescein-3 alpha-HSOR-siRNA exacerbated thermal and mechanical pain perceptions. This paper is the first to show that siRNA-induced knockdown of a key neurosteroid-synthesizing enzyme directly affects an important function as nociception. Hopefully, these results may be useful for the development of novel analgesics. (C) 2010 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.