**Background:** Dry eye disease (DED) involves tear film instability, inflammation, and abnormal corneal neurosensitivity. Corneal cold thermoreceptors detect cooling and osmolarity changes, regulating basal tearing and blinking. In DED, these receptors become hyperexcitable, contributing to dryness sensation and discomfort. Cyclosporine A (CsA) is an immunomodulator used topically for DED, but its direct effects on sensory nerve activity were unknown. This study aimed to evaluate whether CsA acutely modulates the enhanced activity of corneal cold thermoreceptors in a guinea pig model of tear-deficient dry eye.
**Methods:** Tear-deficient dry eye was induced in 18 Dunkin Hartley guinea pigs by unilateral excision of the main lacrimal gland; 13 non-operated animals served as controls. Four weeks post-surgery, ex vivo extracellular recordings of corneal cold thermoreceptor nerve terminal impulses (NTIs) were performed. Corneas were superfused with physiological solution at 34°C, and cooling ramps (34°C to 15°C at −0.5°C/s) were applied. Spontaneous activity at basal temperature, cooling threshold (temperature at which firing increased 25%), and peak response to cold (maximal impulses/s during cooling) were measured before and during perfusion with 50 µM CsA. NTI shape parameters (positive/negative peak amplitudes, maximum rates of voltage change, duration) were also analyzed. In separate awake animals, blinking frequency (5-min observation) and tearing rate (phenol red thread test, 30 s) were measured before, immediately after, and 10 min after topical CsA (10 µL of 50 µM). Data are presented as mean ± SEM; statistical significance set at p<0.05.
**Key Results:** In tear-deficient corneas, cold thermoreceptors showed increased spontaneous activity (e.g., 8.2 ± 1.1 imp/s vs. 6.8 ± 0.8 imp/s in controls) and a larger peak response to cold (e.g., 38.5 ± 5.2 imp/s vs. 26.2 ± 5.4 imp/s in controls). Perfusion with 50 µM CsA significantly decreased the peak response to cold in tear-deficient terminals (from 38.5 ± 5.2 to 28.1 ± 4.3 imp/s, p<0.05), an effect reversed after 15–30 min washout. The decrease correlated strongly with the pre-CsA peak response (r=0.85, p<0.01). Spontaneous activity and cooling threshold were not significantly altered by CsA in either group. In control corneas, CsA had no significant effect on any electrophysiological parameter. NTI shape analysis revealed that tear-deficient terminals had significantly shorter NTI duration (e.g., 2.1 ± 0.2 ms vs. 2.8 ± 0.3 ms in controls, p<0.05) and reduced positive peak amplitude (e.g., 0.45 ± 0.05 mV vs. 0.62 ± 0.06 mV in controls, p<0.05). CsA did not significantly change NTI shape parameters in either group, though a trend toward reversing the duration decrease was noted in tear-deficient terminals. In vivo, topical CsA increased blinking frequency in control animals (from 0.2 ± 0.1 to 2.1 ± 0.5 blinks/min, p<0.01) and tear-deficient animals (from 8.5 ± 1.2 to 11.3 ± 1.5 blinks/min, p<0.05). Tearing rate increased in controls (from 12.5 ± 2.1 mm to 18.3 ± 2.5 mm, p<0.01) and tear-deficient animals (from 4.8 ± 0.6 mm to 7.2 ± 0.9 mm, p<0.05). These effects reversed within 10 min.
**Clinical Implications:** This study provides the first evidence that CsA acutely reduces the hyperexcitability of corneal cold thermoreceptors in a dry eye model, specifically dampening their exaggerated response to cooling without affecting baseline firing or threshold. The effect was selective for sensitized terminals, as normal receptors were unaffected. This suggests CsA may directly modulate abnormal sensory nerve activity, contributing to symptom relief beyond its anti-inflammatory actions. The rapid onset (minutes) and reversibility point to a non-genomic mechanism, possibly involving ion channel modulation (e.g., voltage-gated sodium or potassium channels) rather than calcineurin-dependent gene transcription. The increase in blinking and tearing after topical CsA, despite no activation of cold receptors, implies additional effects on polymodal nociceptors or lacrimal gland function. These findings support the use of CsA to normalize aberrant corneal nerve activity in DED, potentially reducing dryness and discomfort. Future clinical studies should explore whether combining CsA with artificial tears yields greater symptom improvement by targeting both tear film stability and neural hyperexcitability. Limitations include the animal model (guinea pig), single CsA concentration, and small sample size for NTI shape analysis. Further research is needed to elucidate the precise molecular targets of CsA on sensory nerves.