Naltrexone and insulin are independently effective but not additive in accelerating corneal epithelial healing in type I diabetic rats

Abstract

Patients with diabetes are at increased risk for developing corneal disorders, termed diabetic keratopathy. Treatments for diabetic keratopathy are limited. Preclinical studies have demonstrated that topical administration of either naltrexone (NTX) or insulin (INS) accelerates corneal reepithelialization in Type I diabetic rats. This study determined whether the combination of NTX and INS would have additive effect(s) on the reepithelialization of corneal abrasions in diabetic male Sprague-Dawley rats beyond either agent alone. Type 1 diabetes (DB) (glucose levels >400 mg/dl) was induced with streptozotocin; glycemic levels were not controlled with INS. Eight weeks after induction of diabetes, a 5 mm diameter circular abrasion was created in the center of the cornea in one eye of each rat. Eye drops (0.05 ml) of INS [1U (~ 6 nM)] and NTX (10 −5 M) in Vigamox were administered separately 4 times daily for 7 days (NTX/INS); DB control rats received drops of sterile vehicle (DB SV) 4 times daily. Two other groups of rats were given only NTX (DB NTX) or only INS (DB INS). Reepithelialization was monitored by fluorescein staining, and images were recorded with a CCD camera. Areal measurements were made using Optimas software, and the percentage of epithelial defect over a 40 hr period was calculated. Twenty-four hr after formation of an abrasion (~21.7±0.4 mm 2 area), corneal wounds in DB rats treated with NTX, INS, or NTX/INS were significantly smaller (p<0.001) than those in DB SV rats, with reductions in the size of the defect ranging from 24-84%. DB rats treated with NTX or INS alone also were observed to have reductions in wound size of 22 and 29%, respectively, from subjects in the DB SV group at 16 h. At 16 h both the DB NTX and DB INS groups had defects that were 13 and 27%, respectively, smaller than those for the DB NTX/INS group, and at 40 h the DB INS animals had 78% smaller corneal wounds than in the DB NTX/INS group. Therefore, the DB NTX/INS group exhibited some slight delays in wound repair compared to the DB NTX and DB INS groups. Topical application of NTX and/or INS to the cornea had no effect on non-invasive measures that included ocular morphology, intraocular pressure, or corneal thickness. These data demonstrate that although NTX or INS accelerate wound healing, concomitant application of NTX and INS to corneal abrasions in diabetic animals does not have an additive effect on reepithelialization.

Wounding the cornea to learn how it heals

“…Although now there are several strains of rats that develop diabetes spontaneously due to genetic mutations (Li et al, 2007), the injection of Sprague-Dawley or Wistar rats with streptozotocin remains the most well-studied diabetic model in use (Inoguchi et al, 1992; Shimomura et al, 1999). There has also been significant progress made in understanding the role of opioid signaling in corneal wound healing using diabetic rats (Klocek et al, 2009; McLaughlin et al, 2010; Zagon et al, 2009). With streptozotocin treated rats, Kloecek and colleagues (Klocek et al, 2009) showed that the opioid antagonist naltrexone accelerated corneal reepithelialization after debridement wounding.…”

“…There has also been significant progress made in understanding the role of opioid signaling in corneal wound healing using diabetic rats (Klocek et al, 2009; McLaughlin et al, 2010; Zagon et al, 2009). With streptozotocin treated rats, Kloecek and colleagues (Klocek et al, 2009) showed that the opioid antagonist naltrexone accelerated corneal reepithelialization after debridement wounding. Researchers are also studying Sprague-Dawley or Wistar rats placed on high-fat diets to better understand and improve treatments for obesity-induced pathologies (Qin et al, 2004; Srinivasan et al, 2005; Thaler et al, 2012).…”

Diabetic keratopathy and treatment by modulation of the opioid growth factor (OGF)–OGF receptor (OGFr) axis with naltrexone: A review

“…Moreover, topical insulin did not alter corneal thickness, intraocular pressure or serum glucose. Concomitant administration of topical NTX (10 −5 M) and insulin (1 U/0.05 ml) provided no additive effects at corneal restoration [63]. …”

Topical application of naltrexone facilitates reepithelialization of the cornea in diabetic rabbits

“…NTX (Sigma-Aldrich; Indianapolis, IN) was prepared at a concentration of 10 −4 M in Vigamox (moxifloxacin hydrochloride ophthalmic solution, Alcon, Inc., Ft. Worth, TX); this concentration was selected based on data obtained earlier for rats [21,22,41,50]. Vigamox alone was administered as the control vehicle.…”

“…Naltrexone hydrochloride (NTX), an opioid antagonist, when applied topically in rats [21,22] and rabbits [46], systemically in normal and diabetic rats [40,45], or included in organ cultures of human [47] or rabbit [46] corneas, markedly accelerates DNA synthesis and corneal reepithelialization. The mechanism of action for NTX is the blockade of the opioid growth factor (OGF) interaction with the OGF receptor (OGFr) [41,44,46–49].…”