Possible involvement of light regulated gonadotropin-releasing hormone neurons in biological clock for reproduction in the cerebral ganglion of the ascidian, Halocynthia roretzi.

Since ascidians, a primitive chordate, spawn at a fixed latency after sunrise, light must regulate a biological clock for reproduction in the ascidians. A retinal protein found in the cerebral ganglion of the ascidian is a candidate for the photoreceptor that might drives the change in gonadal activity via the gonadotropin-releasing hormone (GnRH) system. Photoresponses of the cerebral ganglion of ascidian, Halocynthia roretzi, were examined and two light-evoked responses recorded extracellularly, a light-evoked slow potential and light inhibition of high frequency spontaneous discharges. These results suggest that pacemaker signals of GnRH neurons might be regulated by photoreceptor activation. Immunohistochemical studies showed photoreceptor cells located close to the GnRH neurons and thus the photosignal might proceed from photoreceptor cell to GnRH neuron intercellularly.