Impaired Corneal Sensation and Nerve Loss in a Type 2 Rat Model of Chronic Diabetes Is Reversible With Combination Therapy of Menhaden Oil, α-Lipoic Acid, and Enalapril

Abstract

Purpose: This study investigated the efficacy of monotherapy versus combination of menhaden oil, α-lipoic acid, and enalapril on corneal sensation and morphometry and other neuropathy-related endpoints in a rat model of type 2 diabetes.

Methods: Male Sprague-Dawley rats (aged 12 weeks) were fed a high-fat diet for 8 weeks followed by 30 mg/kg streptozotocin. After 16 weeks of hyperglycemia, 12-week treatments consisting of menhaden oil, α-lipoic acid, enalapril, or their combination were initiated. Before and after treatments, we performed analyses of multiple neural and vascular endpoints including corneal sensitivity, corneal nerve density, vascular reactivity of epineurial arterioles, motor and sensory nerve conduction velocity, intraepidermal nerve fiber density, and thermal nociception.

Results: Before treatment, all the neural and vascular endpoints in diabetic rats were impaired. Treating diabetic rats with monotherapy was effective in improving neural and vascular deficits with menhaden oil being most efficacious. However, the combination therapy provided the greatest benefit and improved/reversed all nerve and vascular deficits. The effect of combination therapy on corneal relative sensitivity and structure (in mm/mm), primary endpoints for this study, for control, diabetic, and diabetic treated rats was 4.2 ± 1.4 and 7.5 ± 0.5, 12.1 ± 1.3* and 3.8 ± 0.2*, and 6.6 ± 2.3 and 7.3 ± 0.5, respectively (*P < 0.05 compared with control rats; P < 0.05 compared with diabetic rats).

Conclusions: These studies suggest that a combination therapeutic approach may be most effective for treating vascular and neural complications of type 2 diabetes.

Figure 1. Glucose utilization curve for control rats, untreated type 2 diabetic rats and type 2 diabetic rats treated with menhaden oil, α-lipoic acid, enalapril or their combination

Fasting blood glucose at time 0 for control, untreated type 2 diabetic rats and type 2 diabetic rats treated with menhaden oil, α-lipoic acid, enalapril or their combination for early intervention was 92 ± 3, 339 ± 35^*, 373 ± 60^*,+, 206 ± 19^*, 362 ± 32^* and 260 ± 43^* mg/dl, respectively and for late intervention was 94 ± 3, 325 ± 24^*, 355 ± 51^*, 250 ± 27^*, 324 ± 36^* and 280 ± 43^* mg/dl, respectively (* p < 0.05, compared to control rats). Data are the mean ± S.E.M. The area under the curve (AUC) was significantly different, p < 0.05 (impaired), for untreated type 2 diabetic rats and type 2 diabetic rats treated with menhaden oil, α-lipoic acid, enalapril or the combination vs. control rats. The number of rats in each group was the same as shown in Table 1.

Figure 2. Effect of treating diabetic rats with menhaden oil, α-lipoic acid, enalapril or their combination on vascular relaxation by acetylcholine in epineurial arterioles

Vascular relaxation to acetylcholine was determined as described in the legend of Figure 3. The number of rats in each group was the same as shown in Table 1. Data are presented as the mean of % relaxation ± S.E.M. * p < 0.05 compared to control rats; + p < 0.05 compared to diabetic rats.

Figure 3. Effect of treating diabetic rats with menhaden oil, α-lipoic acid, enalapril or their combination on corneal reactivity to a hypertonic solution

Corneal reactivity was determined and quantified as described in the Methods section. The data set labeled as basal was obtained from control and diabetic rats examined prior to initiation of treatments (n = 6). The number of rats for the treatment groups was the same as shown in Table 1. Data are presented as area under the curve. A larger area under the curve occurs with little squinting and lack of narrowing of the inter-palpebral area due to loss of corneal sensation in response to a hypertonic saline drop. * p < 0.05 compared to control rats; + p < 0.05 compared to diabetic rats.