MODULATORY EFFECT OF THYROID HORMONES AND NEUROSTEROIDS OF GABA –EVOKED CURRENTS IN CULTURE DRGs.

Thyroid hormones (THs) and neurosteroids (NSs) are endogenous modulators of GABA-A receptors in the central and peripheral nervous system. Recent studies reported an increase in NSs levels after injury in rats ( Di Michele et al., 2000), and NSs administration reduces inflammatory and neuropathic pain (Charlet et al., 2008) suggesting their involvement in the physiology and pathology of pain transmission. Very little informations are available instead on the participation of THs in pain mechanisms (Bruno et al.2005). The aim of our study was to analyze the effect of THs (3-iodothyronine ,T3, thyroxine, T4) and of the NSs (Pregenolone Sulfate , PS, and allopregnenolone, ALLO) on GABA -evoked current in rat dorsal root ganglia cells in culture. The experiments were performed using the whole cell patch clamp technique. To highlight the nociceptors, at least one sub-population, we used capsaicin, a marker for pain cells expressing TRPV1, a specific nociceptive receptor. Our results show a reduction of GABA current in capsaicin positive cells after applications of THs (both at 10 μM ) and PS (10 uM) of 29,3% ± 8,2 for T3, 23,8%± 5,9 for T4 and 63,8%± 19,6 for PS. ALLO (10 μM) potentiated GABA current of 20 % ±3,5 and applied by itself elicited an inward current. Our results suggest a possible involvement of THs at the level of nociceptive peripheral transmission and open a new view on the complex influences existing among hormones and nociception