Modulatory effects of neurosteroids and thyroid hormones on GABA-evoked currents in cultured dorsal root ganglion cells

Neurosteroids (NSs) and Thyroid hormones (THs, T3, triiodothyronine and T4, thyroxine) are important endogenous modulators of GABAA receptor (GABAAR) function (Puia and Losi, 2011). The involvement of NSs in several physiological and pathological processes has been largely acknowledged, among them pain transmission. Several studies revealed the antinociceptive properties of some NSs and demonstrated that they can induce a potent peripheral analgesia via a direct GABAAR allosteric modulation (Poisbeau et al., 2014). Very little is known instead of how THs affect synaptic transmission in structures devoted to pain transmission. Pain sensitivity is related to the thyroid status, indeed hyperthyroidism confers greater sensitivity to thermal noxious stimuli (Edmondson et al., 1990) and alters the nociceptive responses in rats (Bruno et al., 2005). Dorsal root ganglion (DRG) cells are primary sensory neurons playing important roles in pain transmission between periphery and CNS. By using the patch clamp technique in the whole cell configuration we analyzed the effect of THs and of some NSs (Pregnenolone Sulfate, PS, and Allopregnenolone, ALLO) on GABA-evoked currents in rat DRG cells grown in primary cultures T3, T4 and PS (from 500 nM to 50 µM) reduce GABA-evoked currents with an IC50 of 0.8±0.3μM for T3 (eff max =-42±9%), of 1.4±0.7 μM for T4 (eff max =-41±6%) and of 4.3±1.2μM (eff max =-60±8%) for PS. ALLO potentiates GABA-evoked current in DRG neurons with an IC50 of 1.3±0.8 μM and a maximal effect of 110±20%. To investigate the mechanism of action of THs and PS we applied increasing concentrations of GABA (5, 10 and 50 µM) to the same concentration of modulator (10 µM). The effect of T3, T4 and PS was not dependent on the GABA concentration used suggesting that they act in a non-competitive way. The modulatory activity of THs and PS on sIPSCs amplitude and frequency measured in lamina II neurons of acutely dissociated spinal cord slices was also investigated. In conclusion, since DRG neurosteroidogenesis is a physiologically relevant process (Schaeffer et al., 2010), our findings suggest that NSs modulation of GABAAR in this cells could play an important role in pain transmission from periphery to spinal cord. Furthermore the decreased GABAAR activity induced by T3 and T4 result in a reduced inhibitory neurotransmission that could contribute to the increased pain sensitivity detected in hyperthyroid animals.