Latent Sensitization in a Mouse Model of Ocular Neuropathic Pain

Chronic ocular pain is poorly understood and difficult to manage. We developed a murine model of corneal surface injury (CSI)-induced chronic ocular neuropathic pain. The study focuses on changes in corneal nerve morphology and associated short- and long-term pain-like behavior after CSI.

CSI was induced in mice by local application of an alkali solution (0.75 N NaOH). Corneal nerve architecture, morphology, density, and length were studied. Eye-wiping was evaluated before and after CSI in response to hypertonic saline (2 M NaCl). Naltrexone (NTX) or Naloxone-methiodide (NLX-me), opioid receptor antagonists, were given subcutaneously (s.c., 3 mg/kg) or topically (eye drop, 100 μM), and then an eye-wiping test was performed.

CSI caused partial corneal deinnervation followed by gradual reinnervation. Regenerated nerves displayed increased tortuosity, beading, and branching. CSI enhanced hypertonic saline-induced eye-wiping behavior compared to baseline or sham-injury (P < 0.01). This hypersensitivity peaked at 10 days and subsided 14 days after CSI. Administration of NTX, or NLX-me, a selective peripheral opioid antagonist, reinstated eye-wiping behavior in the injury group, but not in the sham groups (P < 0.05).

This study introduces a model of chronic ocular pain and corneal neuropathy following CSI. CSI induces central and peripheral opioid receptor-dependent latent sensitization (LS) that is unmasked by systemic or topical administration of opioid antagonists.

This model of chronic ocular pain establishes LS as a new inhibitory mechanism in the oculotrigeminal system and may be used for potential diagnostic and therapeutic interventions for ocular neuropathy.