Purpose: Aging impairs corneal nerve density and sensitivity. Substance P (SP), a neuropeptide secreted by sensory nerves, regulates nerve morphology and nociception. Here, we investigate the relationship between aging, nerve morphology, and SP expression in mouse and human corneas.
Methods: SP levels in mouse corneas (wild type and substance P-knockout) and human corneas and tears were quantified with an ELISA assay. Corneal total nerve length (TNL) was measured with whole-mount β3-tubulin immunofluorescence in mouse and in vivo laser corneal confocal microscopy in humans. SP and β3-tubulin stained cross-sections were used to assess the colocalization of SP and nerves in human and mouse corneas. Ocular surface nociception was assessed with a wiping test in mice.
Results: SP colocalizes with sub-basal neurons in mice and humans. In WT mice, SP levels decrease with age (P = 0.0045, 8 vs. 52 weeks; P = 0.004, 26 vs. 52 weeks) as well as TNL (P = 0.018, 8 vs. 26 weeks; P = 0.0001, 8 vs. 52 weeks). Knockout mice show a greater TNL reduction (8 vs. 26 weeks, P = 0.0016) than WT mice. In the oldest WT and age-matched KO mice, nociception is impaired (P = 0.007 and P < 0.0001, respectively), and KO mice sensitivity is restored by topical SP treatment. In humans, SP levels are reduced in old subject corneas and correlate, in tears, with age (P = 0.0368); TNL also decreases in older patients (P = 0.0002).
Conclusions: Age-associated corneal nerve loss is paralleled by reduction of SP expression in mice and humans. SP promotes the maintenance of normal nerve morphology in the long term and modulates nociception in the cornea.