Differential distribution of melatonin receptors in the pituitary gland of Xenopus laevis

Abstract

A major target site for melatonin action is thought to be the pituitary gland. We have detected differential expression and co-localization of the Mel(1a) and Mel(1c) receptors in cells of the Xenopus laevis pituitary gland. Sections of Xenopus pituitary glands were labeled with Mel(1a) and/or Mel(1c) antibodies, in combination with antibodies to arginine vasotocin (AVT), alpha-melanocyte stimulating hormone (alpha-MSH), prolactin (PRL), and luteinizing hormone (LH). Mel(1a) immunoreactivity was localized to cells of the pars intermedia and to elements within the pars nervosa. Mel(1c) immunoreactivity was also localized to the pars nervosa, and significant labeling was also observed in discrete clusters of cells in the pars distalis. Mel(1a) was absent from the pars distalis, while Mel(1c) was absent from the pars intermedia. Mel(1a) and Mel(1c) were co-localized in the pars nervosa. AVT was present in the pars nervosa, and appeared to be localized to the cell clusters of the pars distalis in which the Mel(1c) receptor was localized. alpha-MSH co-localized with the Mel(1a) receptor in the pars intermedia. LH appeared to localize to many of the cells in the pars distalis, with the notable exception of the Mel(1c) receptor-positive clusters of cells. PRL did not appear to co-localize with either melatonin receptor. The pattern of differential expression of the Mel(1a) and Mel(1c) receptors suggests that the receptors specifically mediate the cellular response to melatonin binding in the specific cell populations.

Two light‐activated neuroendocrine circuits arising in the eye trigger physiological and morphological pigmentation

“…Interestingly, the physiological response is always induced concomitantly with morphological pigmentation, suggesting the existence of a neuronal and/or endocrine link between circuits. An interesting possibility is a neuronal circuit originating with the classical RGC pathway negatively regulates the secretion of α ‐MSH from melanotropes in a manner similar to that of mRGCs, in that Mel1a is expressed by α ‐MSH+ cells of the adult Xenopus pars intermedia pituitary (Wiechmann et al., ). Given that there is no direct evidence for such a role, the regulation of α ‐MSH secretion through the classical RGC pathway or melatonin needs to be explored.…”

The Effects of Melatonin on Brain Arginine Vasotocin: Relationship with Sex and Seasonal Differences in Melatonin Receptor Type 1 in Green Treefrogs (Hyla cinerea)

“…Similar to most vertebrates, anuran amphibians possess two subtypes of high affinity, membrane‐bound G protein‐coupled melatonin receptors: MT1 and MT2 (also known as Mel1a and Mel1b, respectively) . A third high affinity receptor subtype, Mel1c, is found in fish, amphibians and birds.…”

“…Vasotocin‐IR cell and fibre populations have been described previously in several amphibians, including H. cinerea . By contrast, this is the first report of MT1 immunohistochemistry in H. cinerea , although MT1 immunohistochemistry has been performed in Xenopus laevis , another anuran amphibian . The MT1 antiserum that we used (item sc13179; Santa Cruz Biotechnology, Inc.) is a polyclonal antibody raised against the N‐terminus of human MT1, a 37‐kDa peptide containing 350 amino acids.…”

Sexually Dimorphic Effects of Melatonin on Brain Arginine Vasotocin Immunoreactivity in Green Treefrogs <b><i>(Hyla cinerea)</i></b>

“…Information regarding the neural distribution of melatonin receptors in other amphibians, however, is limited. Like most ectotherms, anuran amphibians possess three different melatonin receptor subtypes: melatonin 1a, 1b, and 1c receptors ([Wiechmann et al 2003; Wiechmann et al 2009], but see also [Wiechmann et al 1999; Serino et al 2011]). No studies have addressed the presence and/or distribution of melatonin receptors in H. cinerea .…”