The number and distribution of sympathetic neurons that innervate the rat pineal gland.

The sympathetic innervation of the rat pineal gland was examined using a variety of anatomical techniques. Following the injection of horseradish peroxidase into the pineal gland, approximately 250 labeled neurons were found in the ipsilateral superior cervical ganglion. No labeled neurons were found in the middle or inferior cervical ganglia. In animals whose left internal carotid nerve was lesioned prior to the injection of peroxidase, an average of only three labeled neurons was found in the ipsilateral superior cervical ganglion. These data suggest that most, if not all, of the sympathetic neurons innervating the pineal gland exit from the superior cervical ganglia via the internal carotid nerves. The distribution of sympathetic neurons innervating the pineal gland was similar, though slightly more rostrally placed, than the distribution of the entire population of superior cervical ganglion neurons which project into the internal carotid nerve. Both the small number of neurons innervating the pineal gland and their wide distribution in the rostral part of the superior cervical ganglion indicate that their study at the level of the ganglion would be difficult. Sympathetic axons reach the pineal gland via the nervi conarii. Electron microscopic studies indicate that in each nervus conarii there are about 440 axons which make contact with the surface of the pineal gland. In certain cases, bundles of axons from the left and right nervi conarii were found to fuse. Additional evidence for the intermingling of axons from the two nervi conarii was seen in orthograde transport studies with horseradish peroxidase.