In the retina of the African clawed frog (Xenopus
laevis), melatonin is synthesized by the photoreceptors
at night, and binds to receptors that likely mediate paracrine
responses. Melatonin appears to alter the sensitivity of the
retinal cells to light, and may play a key role in regulating
important circadian events that occur in the eye. A polyclonal
antibody was raised against a 13 amino acid peptide corresponding
to a region of the third cytoplasmic loop of the Xenopus
laevis Mel1c melatonin receptor. Western blot
analysis revealed a major immunoreactive band of approximately
60 kD in neural retina and retinal pigment epithelium (RPE)
membranes. Immunocytochemical labeling of sections of
Xenopus eyes demonstrated intense melatonin receptor-like
immunoreactivity in the inner plexiform layer (IPL). Immunolabeling
with antibodies to glutamate decarboxylase (GAD) or tyrosine
hydroxylase (TOH) appeared to co-localize with the melatonin
receptor immunoreactivity in different sublaminas of the IPL.
This suggests that both GABAergic and dopaminergic amacrine
cells express melatonin receptor protein. There were also some
melatonin receptor immunoreactive varicose fibers in the IPL
that did not co-localize with either TOH or GAD, and may represent
efferent fibers, since they could be followed into the optic
nerve. Melatonin receptor immunoreactivity was also present
on cell soma in the ganglion cell layer. Furthermore, a moderate
level of melatonin receptor immunoreactivity was observed in
the RPE and rod and cone photoreceptor cells. The presence of
melatonin receptor immunoreactivity in these cells supports
previous observations of melatonin receptor RNA expression in
multiple cell types in the Xenopus retina. Expression
of melatonin receptor protein in the photoreceptors suggests
that melatonin may have a direct action on these cells.