The master circadian oscillator in the hypothalamic suprachiasmatic nucleus is entrained to the day/night cycle by retinal photoreceptors. Melanopsin (Opn4), an opsin-based photopigment, is a primary candidate for photoreceptor-mediated entrainment. To investigate the functional role of melanopsin in light resetting of the oscillator, we generated melanopsin-null mice (Opn4-/-). These mice entrain to a light/dark cycle and do not exhibit any overt defect in circadian activity rhythms under constant darkness. However, they display severely attenuated phase resetting in response to brief pulses of monochromatic light, highlighting the critical role of melanopsin in circadian photoentrainment in mammals.

Although mice lacking rod and cone photoreceptors are blind, they retain many eye-mediated responses to light, possibly through photosensitive retinal ganglion cells. These cells express melanopsin, a photopigment that confers this photosensitivity. Mice lacking melanopsin still retain nonvisual photoreception, suggesting that rods and cones could operate in this capacity. We observed that mice with both outer-retinal degeneration and a deficiency in melanopsin exhibited complete loss of photoentrainment of the circadian oscillator, pupillary light responses, photic suppression of arylalkylamine-N-acetyltransferase transcript, and acute suppression of locomotor activity by light. This indicates the importance of both nonvisual and classical visual photoreceptor systems for nonvisual photic responses in mammals.

The cloning of the melanocyte-stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH) receptors (MC1-R and MC2-R, respectively) recently has led to the identification of three additional melanocortin receptors, MC3-R, MC4-R, and MC5-R. The MC2 receptor primarily recognizes only ACTH peptides, but the other four receptors all recognize alpha-melanocyte-stimulating hormone (alpha-MSH) and potent alpha-MSH agonists such as [Nle4,D-Phe7]alpha-MSH-NH2 and Ac-Nle4-c[Asp5,D-Phe7,Lys10]alpha-MSH-(4-10)-NH2 as well as ACTH. The absence of any known physiological role for these new receptors, expressed both in the brain (MC3-R and MC4-R) and throughout a number of peripheral tissues (MC5-R), has necessitated as search for potent and receptor selective agonists and antagonists. We report here that analogues of the superpotent cyclic agonist analogue Ac-Nle4-c[Asp5,D-Phe7, Lys10]alpha-MSH-(4-10)-NH2, in which a bulky aromatic amino acid is substituted in the 7-position, can produce potent and selective antagonists for melanocortin receptors. Thus, the D-p-iodophenylalanine7-containing analogue Ac-Nle4-c[Asp5,D-Phe(pI)7,Lys10]alpha-MSH-(4-10)-NH2 is a potent antagonist (pA2 = 10.3) in the classical frog skin (Rana pipiens) assay (MC1-R), as is the D-2'-naphthylalanine7 (D-Nal(2)7)-containing analogue Ac-Nle4-c[Asp5,D-Nal(2)7,Lys10]alpha-MSH-(4-10)-NH2 (pA2 > 10.3). Interestingly, the D-p-chloro- and D-p-fluorophenylalanine7-containing analogues lacked antagonist activities at all melanotropin receptors, and both exhibited full agonist potency in the frog skin assay. The activity of these analogues also was examined at four mammalian melanocortin receptors. Interestingly, Ac-Nle4-c[Asp5,(D-Nal(2)7,Lys10] alpha-MSH-(4-10)-NH2 was found to be a potent antagonist of the MC4-R (pA2 = 9.3) with minimal agonist activity, a less potent antagonist of the MC3-R (pA2 = 8.3) with minimal agonist activity, and a full agonist of the MC1 and MC5 receptors. Surprisingly, Nle4-c[Asp5,D-Phe(pI)7,Lys10]alpha-MSH was found to be a potent agonist at the cloned human MC1-R (EC50 = 0.055 nM) and mouse MC1-R (EC50 = 0.19 nM) but had potent antagonist activities at the human MC4-R (pA2 = 9.7) and human MC3-R (pA2 = 8.3) with significant partial agonist activities (EC50 = 0.57 and 0.68 nM, respectively) as well. Thus, highly potent and receptor selective antagonist analogues can arise from substitution of the D-Phe7 residue with a bulky aromatic amino acid. These analogues can be used to help determine the functional roles of these receptors.

Melanopsin is the photopigment of intrinsically photosensitive retinal ganglion cells (ipRGCs). Melanopsin immunoreactivity reveals two dendritic plexuses within the inner plexiform layer (IPL), and morphologically heterogeneous retinal ganglion cells. Using enhanced immunohistochemistry, we provide a fuller description of murine cell types expressing melanopsin, their contribution to the plexuses of melanopsin dendrites, and mosaics formed by each type. M1 cells, corresponding to the originally described ganglion-cell photoreceptors, occupy the ganglion cell or inner nuclear layers. Their large, sparsely branched arbors (mean diameter 275 µm) monostratify at the outer limit of the OFF sublayer. M2 cells also have large, monostratified dendritic arbors (mean diameter 310 µm), but ramify in the inner third of the IPL, within the ON sublayer. There are approximately 900 M1 cells and 800 M2 cells per retina; each type comprises roughly 1–2% of all ganglion cells. The cell bodies of M1 cells are slightly smaller than those of M2 cells (mean diameters: 13 µm for M1, 15 µm for M2). Dendritic field overlap is extensive within each type (coverage factors ~3.8 for M1 and 2.5 for M2 cells). Rare bistratified cells deploy terminal dendrites within both melanopsin-immunoreactive plexuses. Because these are too sparsely distributed to permit complete retinal tiling, they lack a key feature of true ganglion cell types and may be anomalous hybrids of the M1 and M2 types. Finally, we observed weak melanopsin immunoreactivity in other ganglion cells, mostly with large somata, that may constitute one or more additional types of melanopsin-expressing cells.

Utilizing results from previous structure-activity relationships and theoretical studies of alpha-melanotropin (alpha-MSH, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2) and its related superpotent analogues, Ac-[Nle4,D-Phe7]-alpha-MSH and Ac-[Cys4,Cys10]-alpha-MSH, we have designed a new class of alpha-MSH4-13 and alpha-MSH4-10 cyclic lactam fragment analogues of alpha-melanotropin. The cyclic peptides have the following general structures: Ac-[Nle4,Xxx5,D-Phe7,Yyy10,Gly11]-alpha-MSH4-13- NH2 and Ac-[Nle4,Xxx5,D-Phe7,Yyy10]-alpha-MSH4-10-NH2, where Xxx = Glu or Asp and Yyy = Lys, Orn, Dab, or Dpr. Formation of the lactam bridge between the side-chain groups Xxx and Yyy was performed either in solution or on a solid-phase support. Seven cyclic peptides were prepared and bioassayed for their melanotropic potency by using standard frog (Rana pipiens) and lizard (Anolis carolinensis) skin bioassays. Relative to alpha-MSH (relative potency = 1), the potencies of the cyclic peptides in the lizard skin bioassay were as follows: alpha-MSH (1); Ac-[Nle4,Glu5,D-Phe7,Lys10,Gly11]-alpha-MSH4-13- NH2 (6); Ac-[Nle4,Asp5,D-Phe7,Lys10,Gly11]-alpha-MSH4-13- NH2 (100); Ac-[Nle4,Glu5,D-Phe7,Lys10]-alpha-MSH4-10-NH2 (9); Ac-[Nle4,Asp5,D-Phe7,Lys10]-alpha-MSH4-10-NH2 (90); Ac-[Nle4,Asp5,D-Phe7,Orn10]-alpha-MSH4-10-NH2 (20); Ac-[Nle4,Asp5,D-Phe7,Dab10]-alpha-MSH4-10-NH2 (5); Ac-[Nle4,Asp5,D-Phe7,Dpr10]-alpha-MSH4-10-NH2 (5). Similar results were obtained in the frog skin bioassay, but the analogues were much less potent. Cyclic melanotropins with 23-membered rings exhibited 100-fold higher melanotropic potency than alpha-MSH with selectivity for the lizard melanocyte receptors over the frog melanocyte receptors. Increasing or decreasing the ring size of these cyclic melanotropins from 23 diminishes the biological potency of the resulting cyclic peptide. The 23- and 24-membered ring analogues showed prolonged (residual) biological activities in both biological assays, but the smaller ring systems (20, 21, 22) did not. These results provide new insights into the structural and conformational requirements of alpha-MSH and its analogues at two different types of pigment cell (melanocyte) receptors.

The minimal sequence required for biological activity of alpha-MSH (alpha-melanotropin, alpha-melanocyte stimulating hormone) was determined in the frog (Rana pipiens) skin bioassay. The sequence required to elicit measurable biological activity was the central tetrapeptide sequence, Ac-His-Phe-Arg-Trp-NH2 (Ac-alpha-MSH6-9-NH2), which was about 6 orders of magnitude less potent than the native tridecapeptide. Smaller fragments of this sequence (Ac-His-Phe-NH2, Ac-Phe-Arg-NH2, Ac-His-Phe-Arg-NH2) were devoid of melanotropic activity at concentrations as high as 10(-4) M. We were unable to demonstrate biological activity for the tetrapeptide, Ac-Phe-Arg-Trp-Gly-NH2 (Ac-alpha-MSH7-10-NH2), and for several carboxy terminal analogues including Ac-Lys-Pro-Val-NH2 (Ac-alpha-MSH11-13-NH2). We prepared a series of fragment analogues of alpha-MSH in an attempt to determine the contribution of each individual amino acid to the biological activity of the native hormone. The minimal potency of Ac-alpha-MSH6-9-NH2 could be enhanced about a factor of 16 by the addition of glycine to the C-terminus, yielding Ac-alpha-MSH6-10-NH2 (Ac-His-Phe-Arg-Trp-Gly-NH2). Addition of glutamic acid to the N-terminus provided the peptide, Ac-alpha-MSH5-10-NH2, which was only slightly more potent than Ac-alpha-MSH6-10-NH2, indicating that position 5 contributes little to the biological potency of alpha-MSH in this assay. Addition of methionine to the N-terminus of Ac-alpha-MSH5-10-NH2 resulted in the heptapeptide, Ac-alpha-MSH4-10-NH2, which was only about 4-fold more potent than Ac-alpha-MSH5-10-NH2. Addition of lysine and proline to the C-terminal of the Ac-alpha-MSH4-10-NH2 sequence yielded the peptide, Ac-alpha-MSH4-12-NH2 with a 360-fold increase in potency relative to Ac-alpha-MSH4-10-NH2. This peptide was only about 6-fold less potent than alpha-MSH. A series of Nle-4-substituted analogues also were prepared. Ac-[Nle4]-alpha-MSH4-10-NH2 was about 4 times more potent than Ac-alpha-MSH4-10-NH2. Ac-[Nle4]-alpha-MSH4-11-NH2 also was about 4 times more potent than Ac-alpha-MSH4-10-NH2, demonstrating that lysine-11 contributes somewhat to the biological activity of alpha-MSH on the frog skin melanocyte receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Melanopsin is the photopigment that confers light sensitivity on intrinsically photosensitive retinal ganglion cells. Mammalian intrinsically photosensitive retinal ganglion cells are involved in the photic synchronization of circadian rhythms to the day-night cycle. Here, we report molecular components of melanopsin signaling using the cultured Xenopus dermal melanophore system. Photoactivated melanopsin is shown to initiate a phosphoinositide signaling pathway similar to that found in invertebrate phototransduction. In melanophores, light increases the intracellular level of inositol trisphosphate and causes the dispersion of melanosomes. Inhibition of phospholipase C and protein kinase C and chelation of intracellular calcium block the effect of light on melanophores. At least four proteins, 43, 74, 90, and 134 kDa, are phosphorylated by protein kinase C upon light stimulation. This provides evidence of an invertebrate-like light-activated signaling cascade within vertebrate cells.melanophore ͉ phosphoinositide ͉ phospholipase C ͉ photoreception T he canonical visual photoreceptors (rods and cones) are not the only photosensitive cells in the mammalian retina. A subpopulation of retinal ganglion cells are intrinsically photosensitive, depolarizing in response to light (1). These cells project to the master circadian clock, the hypothalamic suprachiasmatic nucleus, and other brain sites known to participate in nonvisual responses to light (2-4). It has been demonstrated that these intrinsically photosensitive retinal ganglion cells (ipRGC) are important for photic regulation of the circadian oscillator, acute suppression of pineal melatonin, and acute suppression of activity (masking) in rodents (5, 6). Melanopsin (gene symbol Opn4), a recently identified opsin-based photopigment (7,8), is expressed in ipRGC and is required for their photosensitivity (9). Melanopsin was initially cloned from cultured photosensitive dermal melanophores derived from Xenopus laevis embryos (7). Its peptide sequence is consistent with melanopsin being a member of the superfamily of heptahelical G protein-coupled receptors; specifically, the family of photopigment proteins known as opsins. Despite its vertebrate source, melanopsin's predicted peptide sequence bears greater homology to invertebrate than to vertebrate opsins (7). Although phototransduction is well understood in vertebrate visual photoreceptors and the photoreceptors of some invertebrates, virtually nothing is known currently about intracellular melanopsin-initiated signaling pathways (1).Photosensitive amphibian melanophores are an ideal model in which to investigate melanopsin signal transduction. These cells grow at room temperature by using atmospheric air and show a robust melanosome dispersion in response to illumination. Scoring of this photoresponse can be automated through absorbance monitoring in a microtiter plate reader (10-12). Although ipRGC and Xenopus melanophores naturally express melanopsin, the ultimate cellular responses to melanopsin activation ar...

We have discovered an expansive photoreceptive 'net' in the mouse inner retina, visualized by using an antiserum against melanopsin, a likely photopigment. This immunoreactivity is evident in a subset of retinal ganglion cells that morphologically resemble those that project to the suprachiasmatic nucleus (SCN), the site of the primary circadian pacemaker. Our results indicate that this bilayered photoreceptive net is anatomically distinct from the rod and cone photoreceptors of the outer retina, and suggest that it may mediate non-visual photoreceptive tasks such as the regulation of circadian rhythms.