Sympathetic innervation of the rat choroid: an autoradiographic tracing and immunohistochemical study.

The sympathetic innervation of the choroid was investigated by means of the anterograde tracer 3H-leucine, injected into the rostral part of the superior cervical ganglion. The tracer was autoradiographically visualized at the light- and electron-microscopic levels. Labelled unmyelinated fibres were found in the choroid and labelled terminals were observed in close proximity to the smooth muscle cells of arterioles. The labelled terminals contain granular vesicles of different size and clear vesicles; the mitochondria were of the electron-dense type and no cell-to-cell synaptic contacts were observed. In addition, tyrosine hydroxylase immunocytochemistry was performed. Immunostained terminals found in the choroid show ultrastructural features similar to those found in the tracing experiments: granular vesicles of different size, clear vesicles and electron-dense mitochondria. On the basis of previous studies and the present observations, it is concluded that the choroid has a dual innervation, a parasympathetic innervation from the pterygopalatine ganglion and a sympathetic innervation from the superior cervical ganglion. The dual innervation is most likely involved in maintaining the homeostasis of vasodilatation and vasoconstriction in the choroid.