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M C. Acosta, Victor Meseguer, Illés Kovacs, Carolina L. Luna, Juana Gallar, Carlos Belmonte; Intracellular Calcium Responses To Natural Stimuli Of Ocular Trigeminal Ganglion Neurons Of Guinea Pigs After Long-term Eye Dryness. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3775.
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Decreased tear secretion induces changes in nerve impulse activity of corneal sensory receptors in a guinea pig model of dry eye (Kovacs et al, 2010; ARVO E-Abstract 3403). We tried to elucidate if this is associated to changes in the responsiveness of the soma of trigeminal ganglion (TG) neurons innervating the cornea.
The main lacrimal gland was surgically removed in guinea pigs. 22 days later, 5mM FM1-43 was applied to the center of the cornea for 40min to retrogradely label TG neurons innervating the ocular surface. 6 days later (4 weeks after surgery), animals were sacrificed. TG neurons were cultured and incubated 24h later with 5µM Fura-2 AM (45min, 37ºC). Changes in intracellular calcium concentration were measured using ratiometric digital fluorescence calcium imaging. Responses to cooling ramps to 20ºC, 100µM menthol, 100µM cinnamaldehyde and 1µM capsaicin were recorded. Calcium responses in TG neurons from non-operated animals served as control.
Each culture coverslip contained 1-2 labeled neurons originated at the corneal surface, identified by their fluorescence when excited at 470 nm. In these neurons, the peak value of the intracellular calcium responses evoked by temperature drops and menthol in cold-thermosensitive TG neurons, and by capsaicin (TRPV1-agonist) and cinnamaldehyde (TRPA1-agonist) on putative cold-thermosensory and polymodal nociceptive TG neurons were not significantly different from controls.
Changes in excitability and sensitivity to natural stimuli seen in cold and polymodal corneal nerve endings following chronic lacrimal gland removal are not evident in the soma of TG neurons. An altered expression, location and/or function of ion channels involved in transduction and nerve impulse generation in response to cold and chemical stimuli appear to be behind such membrane excitability changes, although they seem to be more prominent in corneal nerve terminals than in the cell body.
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