The isolated eye of the mollusc, Aplysia califomica, contains a circadian pacemaker whose phase can be regulated by serotonin. The results of previous biochemical and physiological studies indicate that serotonin is used as a transmitter of circadian information in the eye. Although the effects of serotonin on various physiological processes in the Aplysia eye have been studied, very little is known about the anatomy of the serotonergic innervation. We have examined the innervation of the eye using immuno-cytochemical methods. Serotonin-immunoreactive processes were observed in the optic nerve, in the accessory optic nerves, in the connective tissue capsule surrounding the eye, and within the eye itself. There appeared to be two sources of serotonergic input to the eye of Aplysia. One set of immunoreactive fibers was contained in the optic nerve and entered the eye in the neuropil region before radiating outward towards the peripheral retina in the layer below the photoreceptor cell bodies. A second serotonin-immunoreactive input to the eye entered from the accessory optic nerves and these fibers formed a dense plexus of fibers in the connective tissue capsule surrounding the eye. Serotonin-immunoreactive fibers from the plexus penetrated the eye and appeared to terminate in the peripheral portion of the retina. No serotonin-immunoreactive cell bodies were observed in the eye, nerves, or connective tissue capsule. These results support the hypothesis that serotonergic fibers innervate the retina of Aplysia and that these fibers travel through two distinct anatomical pathways: the optic nerve and the accessory optic nerves.
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