DEVELOPMENT OF A FUNCTIONAL INNERVATION OF THE IRIS DILATOR MUSCLE BY SYMPATHETIC-NERVE FIBERS IN THE RAT

These studies using both in situ and in vitro preparations were designed to determine the time of onset of a functional sympathetic innervation to the dilator muscle of the rat iris during postnatal development. The preganglionic nerve trunk to the superior cervical ganglion was stimulated and measurements made of pupil diameter. Alternatively, intracellular recordings were made from the dilator muscle cells and the response to stimulation of the intramural nerves was measured. During the second postnatal week, a decrease in the resting pupil diameter under low lighting conditions was observed, reflecting the normal developmental increase in size of the iris. On account of the small size of the iris in rats up to 10 days postnatal, the in vivo experiments were a reliable test of a functional innervation only in older animals. In all cases, stimulation of the preganglionic nerve trunk in situ caused dilatation of the pupil. Stimulation of the sympathetic nerves in vitro, in preparations of iris taken from rats of various ages, evoked a slow depolarisation mediated via alpha adrenoceptors. Such responses could be recorded in irides from rats as young as 3 days postnatal. Spontaneous activity was recorded in irides towards the end of the second postnatal week just prior to the time of normal eye opening, and may reflect the onset of photomechanical coupling. We conclude that the sympathetic nerves within the iris are capable of evoking membrane potential changes in the dilator muscle cells in the early postnatal period, well before the nerve plexus in the iris is fully differentiated.