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Diabetes mellitus impairs testicular activity and leads to infertility. Leptin is one of the endogenous regulators of the male reproductive functions, but the role of leptin and its receptor (LEPR/Ob-R) in the control of testosterone production and testicular proliferation has not been investigated so far, especially in the Type 1 diabetes mellitus (DM1). Metformin is an anti-hyperglycemic drug which is beneficial for treating the both DM2 and DM1. The aim of this work was to study the possible role of leptin and Ob-R in the regulation of steroidogenesis and proliferation in the testes of mice with streptozotocin-induced DM1 (75 mg/kg/day, 4 days) and to estimate the restoring effect of metformin treatment (500 mg/kg, 2 weeks) on the diabetic testes. In the diabetic testes, the plasma and intratesticular leptin levels and plasma testosterone levels were reduced and completely restored by metformin treatment. Metformin also restored the expression of the steroidogenic transport protein steroidogenic acute regulatory protein reduced in DM1. In the diabetic testes, the expression of Ob-R was downregulated and the immunolocalization of Ob-R showed weak staining in the Leydig cells, the primary spermatocytes and the round spermatids. The germ cell proliferation was also reduced in DM1, as noticed with proliferating cell nuclear antigen (PCNA) expression. Metformin increased the Ob-R expression and immunostaining in the different cell types and improved the PCNA expression. Thus, DM1 impairs the testicular steroidogenesis and proliferation by inhibiting the leptin signaling, causing a decrease in leptin levels and Ob-R expression in the testes of diabetic mice, while metformin improves the leptin signaling and restores testosterone production and testicular proliferation. K E Y W O R D S diabetes mellitus, leptin, metformin, Ob-R, testes, testosterone 1 | INTRODUCTION The Types 1 and 2 diabetes mellitus (DM1 and DM2) are widespread metabolic diseases. The DM1 is characterized by insulin deficiency and severe hyperglycemia, while the DM2 is characterized by insulin resistance and impaired glucose tolerance (American Diabetes Association, 2013). In DM, the prolonged hyperglycemia, the pathological changes in energy metabolism and

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