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Juana Gallar, Gerard Callejo, Carolina L. Luna, M.Carmen Acosta, Carlos Belmonte, Xavier Gasull; Effects of Long-Term Eye Dryness on Voltage-Gated Na+ Currents of Guinea-Pig Cold-Sensitive Ocular Trigeminal Ganglion Neurons. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1792.
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© ARVO (1962-2015); The Authors (2016-present)
To determine whether the enhanced firing of corneal cold thermoreceptors caused by chronic ocular dryness is associated to alterations in Na+ currents (INa), in cold-sensitive corneal trigeminal ganglion (TG) neurons.
The main lacrimal gland was surgically removed in guinea pigs to produce and experimental dry eye (DE). Twenty-two days later, 5mM FM1-43 was applied to the central cornea during 40min to retrogradely label TG neurons innervating the cornea. Six days later (4 weeks after surgery), animals were sacrificed and TG neurons were dissociated and cultured. 24 h later, FM1-43 labelled corneal neurons (1-2/coverslip) perfused at room temperature were selected for whole-cell patch-clamp recordings of INa. A voltage protocol to separate TTX-R and TTX-S INa was used. Inward currents evoked by 100μM menthol, 100μM AITC and 500nM capsaicin were used to classify neurons as cold sensory neurons (menthol +, AITC -, capsaicin -).
In control animals, 7 out of 14 (50%) recorded neurons were classified as cold neurons while only 7/26 (27%) were found in the DE group. TTX-S INa started activating around -40mV in both groups with maximal currents at -30mV in DE and -20mV in control. Current amplitude was significantly larger in DE at -40mV (-63.8±18.2 vs. -6.0±5.0 pA/pF; p<0.01) and at -30mV (-86.9±9.6 vs. -46.0±18.8 pA/pF; p<0.05). TTX-S INa were activated at more negative membrane potentials in DE compared with control neurons (mid-point of activation, V0.5= -42.8±1.9 vs. -29.7±2.8 mV, respectively; p<0.01). TTX-R INa activated at -30mV in both groups and reached maximum activation at -20 (DE) and -10 mV (control); current amplitudes at -20 mV were slightly larger in DE (-69.5±13.8 vs. -50.9±17.7 pA/pF; p=0.16). G/Gmax TTX-R INa activation was shifted towards more negative voltages in DE (V0.5= -29.0±2.4 vs. -21.9±3.0 mV in controls; p<0.05) although the activation curve slopes showed only small differences (k= 1.3±0.5 vs. 1.9±0.8 mV).
The leftward shift in the activation of TTX-R and especially of TTX-S INa suggest that Na+ currents may contribute, at least in part, to the increased excitability developed by corneal cold sensory fibers after long-term ocular dryness.
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