Changes in cardiac innervation during maturation in long-term diabetes

Abstract Diabetic autonomic neuropathy being a common complication of diabetes mellitus (DM) is related to an increased risk of cardiovascular mortality. However, mechanisms underlying changes of innervation density in affected hearts remain insufficiently understood. Hence, the aim of this study was to describe quantitative changes of intra-myocardial nerve terminals in hearts of diabetic rats of various ages. Male Sprague–Dawley rats were injected with 55 mg/kg streptozotocin (STZ) (DM group) or with citrate buffer (control). After 2 weeks, 2 months, 6 months and 12 months, sections of their hearts were analyzed in five areas—left ventricle, interventricular septum, right ventricle, anterior and posterior wall. Nerve fibers were visualized immunohistochemically, using antibody against a general neuronal marker, protein gene product 9.5 (PGP 9.5). Significant increase in total nerve fibers from all heart areas was observed 2 weeks and 2 months after diabetes induction, followed by a decrease at 6 months and again increase at 12 months was observed in both control and diabetic rats. Significant difference between control and diabetic rats was visible after 2 weeks and 2 months, with diabetic rats exhibiting significantly more nerve fibers. There were no consistent differences in quantity of nerve fibers in different areas of the heart within a particular age-related group of animals. In conclusion, cardiac innervation undergoes dynamic changes both in control and in diabetic rats, with a time-dependent significant increase in neuronal fiber density in diabetic animals. This novel information may contribute to our understanding of pathophysiological changes associated with diabetic cardiac neuropathy.