The postnatal development of tyrosine hydroxylase immunoreactive nerves in rat atrium, studied with immunofluorescence and confocal laser scanning microscopy.

The developmental pattern and distribution of tyrosine hydroxylase (TH) immunoreactive nerves in the rat heart has been studied by indirect immunofluorescence. Anti-TH and monoclonal antibodies against the synaptic vesicle antigen SV2 were applied to whole-mount stretch preparations of the right atria from hearts of newborn to 40-day-old animals. Immunofluorescence was studied with confocal laser scanning microscopy (CFLM). Nerve fibres with TH-like immunoreactivity (TH-LI) were present throughout the atria already at birth, with the highest density around the sino-atrial (SA) node, colocalized in most nerve terminal arborizations with the synaptic vesicle marker SV2. Also, TH-positive ganglion cells were present around the openings of the caval veins. In comparison with TH-positive fibres, SV2-positive nerve terminal fibres were more delicate, and the number of fibres larger in all areas studied. In axon bundles, TH-LI was very prominent, while the SV2-LI was weak. A gradual increase in the density of innervation was observed up to the age of 40 days. The density of innervation decreased from the epicardium to the endocardium, confirmed in cross-sections of the cardiac wall. The relative density of TH-positive nerve terminals was estimated in the CFLM from the area around the SA node. At birth, about 600 white (strongly fluorescent) pixels were registered per scanned field, with the pixel number increasing gradually up to 20,000 pixels per frame in 18- and 40-day-old animals. The results show that TH-positive, possibly adrenergic, nerve fibres are present at birth, and that the pattern is qualitatively similar to that observed in adults, with the expression of synaptic vesicle antigen being present some days before the development of noradrenaline neuroeffector maturation, as shown in a number of physiological and pharmacological studies.