Effects of Sitagliptin and Celery Seed Extract on Corneal Nerve Morphology and Sensory Dysfunction in Diabetic Rats

Background: Diabetic peripheral neuropathy (DPN) is a common complication of diabetes, characterised by sensory dysfunction and progressive loss of small nerve fibres. Corneal nerves are among the earliest fibres affected and may serve as sensitive markers for early detection and therapeutic intervention. Sitagliptin and celery seed extract have demonstrated anti-hyperglycaemic and neuroprotective properties in experimental diabetes; however, their effects on corneal nerve morphology and function remain unknown. Purpose: The present study aimed to investigate the effects of sitagliptin and celery seed extract on corneal nerve morphology, corneal sensitivity, and sensory behaviour in a high-fat diet, streptozotocin-induced rat model of type 2 diabetes. Methods: Male Sprague Dawley rats (n = 24) were used to induce type 2 diabetes by combining a high-fat diet with streptozotocin. Diabetic rats were treated with sitagliptin (30 mg/kg per day for 4 weeks) or celery seed extract (100 mg/kg per day for 4 weeks). Fasting blood glucose levels were measured throughout the monitoring period. Corneal nerve fibre parameters, including corneal nerve fibre length (CNFL), density (CNFD), and tortuosity, were assessed in βIII-tubulin-stained whole-mount corneas. Corneal sensitivity was measured with the Cochet-Bonnet esthesiometer. Sensory behaviour was evaluated using hot- and cold-water tail immersion and an acetone drop test. Results: Diabetes significantly reduced the CNFL (39.8 ± 4.7 vs. normal: 71.7 ± 7.3 mm/mm²; p < 0.05) and CNFD (7.1 ± 0.9 vs. normal: 10.6 ± 1.2%; p < 0.05) and increased corneal nerve tortuosity (5.8 ± 0.2 vs. normal: 5.1 ± 0.2; p < 0.05). Both sitagliptin and celery seed extract significantly increased the CNFL (68.8 ± 4.5 and 65.8 ± 6.5 mm/mm², respectively) compared to the untreated diabetic group. Tortuosity was significantly decreased in both the sitagliptin (4.3 ± 0.1) and celery seed extract (4.9 ± 0.1) groups compared with the untreated diabetic group. However, the CNFD showed only a modest increase after either treatment. A significant decrease in corneal sensitivity was also observed in rats following diabetes induction (5.8 ± 0.1 in normal vs. 4.5 ± 0.1 cm in diabetic rats; p < 0.05). This was rescued with sitagliptin and celery seed extract treatment (5.5 ± 0.1 and 5.3 ± 0.1 cm, respectively; p < 0.05). Compared with the non-diabetic controls, diabetic rats showed significantly shorter withdrawal latencies in both the hot- (3.9 ± 0.3 vs. 7.6 ± 1.1 s) and cold-water tests (4.3 ± 0.5 vs. 8.1 ± 0.9 s), indicating thermal sensitivity. By contrast, diabetic rats showed a significantly longer response time in the acetone drop test (8.0 ± 0.7 vs. 3.6 ± 0.4 s), indicating altered cold sensitivity (p < 0.05) for all responses. Treatment with sitagliptin and celery seed extract significantly reversed these changes, as evidenced by increased withdrawal latency in the cold-water test and decreased response time in the acetone drop test. However, this improvement was not significant in the hot-water test. Conclusions: Sitagliptin and celery seed extract restored corneal nerve architecture, corneal sensitivity, and sensory dysfunction in diabetic rats, suggesting the therapeutic potential for DPN.