Nitrergic innervation of the cardiac small intensely fluorescent cells in rats

Background: This study aims to analyse the innervation of the small intensely fluorescent (SIF) cells within the rat heart, seeking to examine the supposable neuroanatomical basis for the paradoxical increase of the sympathetic activity of the heart following electrical stimulation of the vagal nerves. Methods: Neuronal markers and confocal laser scanning microscopy were used to analyse cardiac SIF cells and their innervation in 12 rats. Results: The findings of the present study demonstrate the limited number of nerve fibres (NFs) adjacent to SIF cells. These NFs were of nitrergic phenotype and negative to adrenergic and cholinergic neuronal markers. We revealed two morphologic patterns of these NFs networking SIF cells. The first type of nitrergic NFs surrounded the SIF cells tightly, and their axons contained numerous varicosities. The second pattern had nitrergic axons with fewer or no varicosities on their terminals. Since most SIF cells identified in the rat hearts (∼80 %) had no nerve fibres targeting them, the innervated SIF cells may be considered as particular catecholaminergic cells mediating between the heart neural regulation and the endocrine mechanism of SIF cells. Conclusion: Since our earlier data demonstrate the absolute prevalence of nitric oxide synthesising neurons in the rat vagal sensory ganglia, we conclude that nitrergic nerve axons basketed SIF cells are the vagal sensory axons presumably controlling the cardiac SIF cells in rats.