Dopaminergic and GABAergic amacrine cells are direct targets
of melatonin: Immunocytochemical study of mt1
melatonin receptor in guinea pig retina
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abstract
Distribution of the mt1 melatonin receptor
in the guinea pig retina was immunocytochemically investigated
using peptide-specific anti-mt1 receptor antibody.
Western blots of the guinea pig retina showed a single
band at approximately 37 kilodalton (kD) immunoreactive
to the anti-mt1 antibody. The most intense immunoreactivity
for the mt1 receptor was detected in the cell
bodies of ganglion cells. Their dendrites and axons were
also immunolabeled. Subpopulations of amacrine cells, the
inner plexiform layer, and the outer plexiform layer also
exhibited moderate to weak immunolabeling. The mt1-positive
amacrine cells were located either at the vitreal border
of the inner nuclear layer or displaced in the ganglion
cell layer. Double immunolabeling using antibodies to the
mt1 receptor and tyrosine hydroxylase revealed
that the majority of dopaminergic amacrine cells showed
mt1 immunoreactivity. Almost all the 1CA type
dopaminergic cells were mt1 positive while the
2CA type cells less frequently exhibited mt1
immunoreaction. By double immunolabeling for the mt1
receptor and GABA, more than 50% of the mt1-immunoreactive
amacrine cells were shown to be GABAergic neurons. Approximately
one-third of the GABAergic amacrine cells were immunolabeled
for the mt1 receptor. The present results demonstrate
expression of the mt1 receptor in diverse neuronal
cell types in the guinea pig retina and provide the first
evidence for the direct effect of melatonin on dopaminergic
and GABAergic amacrine cells via the mt1
receptor.
