Increased melatonin levels in aqueous humor of patients with proliferative retinopathy in type 2 diabetes mellitus

Effects of Melatonin on Diabetic Neuropathy and Retinopathy

Diabetes mellitus (DM) leads to complications, the majority of which are nephropathy, retinopathy, and neuropathy. Redox imbalance and inflammation are important components of the pathophysiology of these complications. Many studies have been conducted to find a specific treatment for these neural complications, and some of them have investigated the therapeutic potential of melatonin (MEL), an anti-inflammatory agent and powerful antioxidant. In the present article, we review studies published over the past 21 years on the therapeutic efficacy of MEL in the treatment of DM-induced neural complications. Reports suggest that there is a real prospect of using MEL as an adjuvant treatment for hypoglycemic agents. However, analysis shows that there is a wide range of approaches regarding the doses used, duration of treatment, and treatment times in relation to the temporal course of DM. This wide range hinders an objective analysis of advances and prospective vision of the paths to be followed for the unequivocal establishment of parameters to be used in an eventual therapeutic validation of MEL in neural complications of DM.

Circadian rhythms in diabetic retinopathy: an overview of pathogenesis and investigational drugs

INTRODUCTION

Circadian rhythm is a natural endogenous process occurring roughly every 24 hours. Circadian rhythm dysfunction is involved in diabetic retinopathy (DR) pathogenesis. Interestingly, there are investigational drugs that exhibit potential in the treatment of DR by targeting circadian rhythm dysfunction.

AREAS COVERED

We performed a literature search in June 2020 using PubMed's Medical Subject Heading (MeSH) terms 'circadian clock,' 'circadian rhythms,' and 'diabetic retinopathy.' This article offers an overview of the physiology of the biological clock and clock regulatory genes and presents an examination of the retinal clock. It discusses the pathogenic mechanisms of DR and emphasizes how circadian rhythm dysfunction at structural, physiological, metabolic and cellular levels, plays a critical role in the development of DR. The latter part of the paper sheds light on those investigational drugs (such as melatonin, tasimelteon and metformin) which exhibit potential in the treatment of DR by the targeting of circadian rhythm dysfunction.

EXPERT OPINION

An enhanced understanding of circadian rhythm and its role in DR could offer therapeutic potential by targeting of circadian rhythm dysfunction.

Melatonin Affects Mitochondrial Fission/Fusion Dynamics in the Diabetic Retina

Mitochondrial fission and fusion are dependent on cellular nutritional states, and maintaining this dynamics is critical for the health of cells. Starvation triggers mitochondrial fusion to maintain bioenergetic efficiency, but during nutrient overloads (as with hyperglycemic conditions), fragmenting mitochondria is a way to store nutrients to avoid waste of energy. In addition to ATP production, mitochondria play an important role in buffering intracellular calcium (Ca2+). We found that in cultured 661W cells, a photoreceptor-derived cell line, hyperglycemic conditions triggered an increase of the expression of dynamin-related protein 1 (DRP1), a protein marker of mitochondrial fission, and a decrease of mitofusin 2 (MFN2), a protein for mitochondrial fusion. Further, these hyperglycemic cells also had decreased mitochondrial Ca2+ but increased cytosolic Ca2+. Treating these hyperglycemic cells with melatonin, a multifaceted antioxidant, averted hyperglycemia-altered mitochondrial fission-and-fusion dynamics and mitochondrial Ca2+ levels. To mimic how people most commonly take melatonin supplements, we gave melatonin to streptozotocin- (STZ-) induced type 1 diabetic mice by daily oral gavage and determined the effects of melatonin on diabetic eyes. We found that melatonin was not able to reverse the STZ-induced systemic hyperglycemic condition, but it prevented STZ-induced damage to the neural retina and retinal microvasculature. The beneficial effects of melatonin in the neural retina in part were through alleviating STZ-caused changes in mitochondrial dynamics and Ca2+ buffering.