Substance P-containing neurons of the avian suprachiasmatic nucleus project directly to the nucleus of Edinger-Westphal.

Retrograde and anterograde pathway tracing techniques were used in the pigeon to study afferent visual input from the suprachiasmatic nucleus (SCN) of the hypothalamus to the nucleus of Edinger-Westphal (EW), the parasympathetic visceral efferent component of the oculomotor complex. Horseradish peroxidase injected into the EW retrogradely labeled numerous neurons in the contralateral SCN, a retinorecipient hypothalamic nucleus, as well as a few neurons in the ipsilateral SCN. Autoradiographic orthograde pathway tracing experiments confirmed that the SCN projects heavily upon the medial subdivision of the contralateral EW and lightly upon the medial subdivision of the ipsilateral EW. Some neurons of the SCN contain substance P and a substance P-positive plexus of fibers was seen in precisely that medial portion of the EW to which the SCN was found to project in the autoradiographic experiments. This substance P-positive fiber plexus in the medial EW was eliminated by bilateral electrolytic lesions of the SCN. These results show that the avian suprachiasmatic nucleus has a heavy contralateral and a much lighter ipsilateral projection to the medial subdivision of the EW and that this projection may be largely substance P positive. Previous studies have suggested that neurons of the medial EW specifically project to the neurons of the ciliary ganglion that control the choriocapillary blood flow of the eye. Since the SCN has been implicated in the control of circadian rhythms in vertebrates, the projection of the SCN to the EW may represent a pathway by which a circadian rhythmicity is imposed on choriocapillary blood flow. Alternatively or in addition, the SCN-EW pathway described in this paper may provide the central neural substrate for a homeostatic regulatory mechanism by which choriocapillary blood flow is controlled by the intensity of retinal illumination.