Control of choroidal blood flow by the nucleus of Edinger-Westphal in pigeons: a laser Doppler study.

Anatomical studies in birds have suggested that choroidal blood flow may be regulated by a circuit involving the following serially-connected components: the retina-the suprachiasmatic nucleus (SCN)-the medial subdivision of the nucleus of Edinger-Westphal (mEW)-the ciliary ganglion-the choroidal blood vessels. In order to better clarify the role of this circuit, we examined the effects of electrical stimulation of EW on choroidal blood flow in the ipsilateral eye, using laser Doppler velocimetry to monitor choroidal blood flow in the superior pole of the eye. Baseline choroidal blood flow values (144-311.3 mg/min per eye) were found to be comparable to those previously reported in rabbits, cats and primates. Stimulation of EW dramatically increased choroidal blood flow. The increases were current-related and the average maximal increases ranged between 300-700% above baseline values. In contrast, EW stimulation had little or no effect on overall bodily blood flow. All EW stimulation sites were later verified histologically. These results indicate that the SCN-mEW circuit in birds may be involved in mediating increases in choroidal blood flow, possibly in response to the levels of retinal illumination. Such adaptive neural regulation of choroidal blood flow may play an important role in mitigating the potentially deleterious effects of light on the retina.