Melatonin Receptors and Signal Transduction in Photorefractory Siberian Hamsters (Phodopus sungorus)*

Environmental day length drives nocturnal pineal melatonin secretion, which in turn generates or entrains seasonal cycles of physiology, reproduction, and behavior. In mammals, melatonin (MEL) binds to a number of receptor subtypes including high-affinity (MT1 and MT2) and low-affinity (MT3, nuclear orphan receptors) binding sites, which are distributed throughout the central nervous system and periphery. The MEL receptors that mediate photoperiodic reproductive and behavioral responses to MEL have not been identified in a reproductively photoperiodic species. Here I tested the hypothesis that MT1 receptors are necessary and sufficient to engage photoperiodic responses by challenging male Siberian hamsters (Phodopus sungorus), a species that does not express functional MT2 receptors, with ramelteon (RAM), a specific MT1/MT2 receptor agonist. In hamsters housed in a long-day photoperiod, late-afternoon RAM treatment inhibited gonadotropin secretion, induced gonadal regression, and suppressed food intake and body mass, mimicking effects of MEL. In addition, chronic (24 h/d) RAM infusions were sufficient to obscure endogenous MEL signaling, and these treatments attenuated gonadal regression in short days. Together, the outcomes indicate that signaling at the MT1 receptor is sufficient and necessary to mediate the effects of photoperiod-driven changes in MEL on behavior and reproductive function in a reproductively photoperiodic mammal.

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Melatonin and Microglia

International Journal of Molecular Sciences ◽

10.3390/ijms22158296 ◽

2021 ◽

Vol 22 (15) ◽

pp. 8296

Author(s):

Rüdiger Hardeland

Keyword(s):

Oxidative Stress ◽

Nitric Oxide ◽

Signal Transduction ◽

Endothelial Cells ◽

Bacterial Infections ◽

Melatonin Receptors ◽

Sterile Inflammation ◽

High Complexity ◽

Anti Inflammatory ◽

Multiple Signaling

Melatonin interacts in multiple ways with microglia, both directly and, via routes of crosstalk with astrocytes and neurons, indirectly. These effects of melatonin are of relevance in terms of antioxidative protection, not only concerning free-radical detoxification, but also in prevention of processes that cause, promote, or propagate oxidative stress and neurodegeneration, such as overexcitation, toxicological insults, viral and bacterial infections, and sterile inflammation of different grades. The immunological interplay in the CNS, with microglia playing a central role, is of high complexity and includes signaling toward endothelial cells and other leukocytes by cytokines, chemokines, nitric oxide, and eikosanoids. Melatonin interferes with these processes in multiple signaling routes and steps. In addition to canonical signal transduction by MT1 and MT2 melatonin receptors, secondary and tertiary signaling is of relevance and has to be considered, e.g., via the upregulation of sirtuins and the modulation of pro- and anti-inflammatory microRNAs. Many details concerning the modulation of macrophage functionality by melatonin are obviously also applicable to microglial cells. Of particular interest is the polarization toward M2 subtypes instead of M1, i.e., in favor of being anti-inflammatory at the expense of proinflammatory activities, which is well-documented in macrophages but also applies to microglia.

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Social Cues Attenuate Photoresponsiveness of the Male Reproductive System in Siberian Hamsters (Phodopus sungorus)

Journal of Biological Rhythms ◽

10.1177/074873099129000443 ◽

1999 ◽

Vol 14 (1) ◽

pp. 54-61 ◽

Cited By ~ 16

Author(s):

Carol D. Hegstrom ◽

S. Marc Breedlove

Keyword(s):

Reproductive System ◽

Social Cues ◽

Male Reproductive System ◽

Phodopus Sungorus ◽

Siberian Hamsters

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Energy intake and fur in summer- and winter-acclimated Siberian hamsters (Phodopus sungorus)

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.2001.281.2.r519 ◽

2001 ◽

Vol 281 (2) ◽

pp. R519-R527 ◽

Cited By ~ 9

Author(s):

Alexander S. Kauffman ◽

Alessandra Cabrera ◽

Irving Zucker

Keyword(s):

Food Intake ◽

Energy Intake ◽

Ventral Surface ◽

Phodopus Sungorus ◽

Ambient Temperatures ◽

Siberian Hamsters ◽

Daily Food ◽

Brown Adipose ◽

Wide Range ◽

The Impact

Few studies have directly addressed the impact of fur on seasonal changes in energy intake. The daily food intake of Siberian hamsters ( Phodopus sungorus) was measured under simulated summer and winter conditions in intact animals and those with varying amounts of pelage removed. Energy intake increased up to 44% above baseline control values for approximately 2–3 wk after complete shaving. Increases in food intake varied with condition and were greater in hamsters housed in short than long day lengths and at low (5°C) than moderate (23°C) ambient temperatures. Removal of 8 cm2 of dorsal fur, equivalent to 30% of the total dorsal fur surface, increased food intake, but removal of 4 cm2 had no effect. An 8-cm2 fur extirpation from the ventral surface did not increase food consumption. Food intake was not influenced differentially by fur removal from above brown adipose tissue hot spots. Fur plays a greater role in energy balance in winter- than summer-acclimated hamsters and conserves energy under a wide range of environmental conditions.

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