Of note, enhanced expression levels of the neuropeptides SP (11.7 ± 0.26 vs. 7.26 ± 0.69, P < 0.01) and CGRP (18.51 ± 0.69 vs. 16.45 ± 0.25, P < 0.05) in the cornea were observed in DM-In group mice compared with DM-Con group after 3 days of topical insulin treatment (shown in Fig. 4A). Here, INS is linked to diabetes mellitus.