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

Abstract

Arginine vasotocin (AVT) and its mammalian homologue, arginine vasopressin (AVP), regulate a variety of social and reproductive behaviors, often with complex species-, sex- and context-dependent effects. Despite extensive evidence documenting seasonal variation in brain AVT/AVP, relatively few studies have investigated the environmental and/or hormonal factors mediating these seasonal changes. In the present study, we investigated whether the pineal hormone melatonin alters brain AVT immunoreactivity in green treefrogs (Hyla cinerea). Reproductively active male and female frogs were collected during the summer breeding season and a melatonin-filled or blank silastic capsule was surgically implanted subcutaneously. The duration of hormone treatment was 4 weeks, at which time frogs were eutha-nized and the brains and blood collected and processed for AVT immunohistochemistry and steroid hormone assay. We quantified AVT-immunoreactive (AVT-ir) cell bodies in the nucleus accumbens (NAcc), caudal striatum and amygda- la (AMG), anterior preoptic area, suprachiasmatic nucleus (SCN) and infundibular region of the ventral hypothalamus. Sex differences in AVT-ir cell number were observed in all brain regions except in the anterior preoptic area and ventral hypothalamus, with males having more AVT-ir cells than females in the NAcc, amygdala and SCN. Brain AVT was sensitive to melatonin signaling during the breeding season, and the effects of melatonin varied significantly with both region and sex. Treatment with melatonin decreased AVT immunoreactivity in both the NAcc and SCN in male H. cinerea. In contrast, brain AVT was relatively insensitive to melatonin signaling in females, indicating that the regulation of the AVT/AVP neuropeptide system by melatonin may be sexually dimorphic. Finally, melatonin did not significantly influence testosterone or estradiol concentrations of male or female frogs, respectively, suggesting that the effects of melatonin on AVT immunoreactivity are independent of changes in gonadal sex steroid hormones. Collectively, our results indicate that the AVT/AVP neuronal system may be an important target for melatonin in facilitating seasonal changes in reproductive physiology and social behavior.

Melatonin action in a midbrain vocal-acoustic network

“…Melatonin has been shown to interact with neuropeptide and steroid pathways in many vertebrates, including teleost fishes (Falcón et al, 2010;Maitra and Chattoraj, 2006), frogs (Lutterschmidt and Wilczynski, 2012) and birds (Chowdhury et al, 2010;Ubuka et al, 2005). Unlike nonapeptides and steroid hormones (Goodson and Bass, 2000a;Remage-Healey and Bass, 2004), pilot studies showed that acute melatonin injections did not rapidly (5-120 min) change the output of the midshipman vocal system under our testing conditions (N.Y.F., personal observations).…”

Compared to what: what can we say about nonapeptide function and social behavior without a frame of reference?

“…Indeed, advances in understanding nonapeptide function and social behavior in the lab have been made in several examples of tractable, non-traditional species. These examples include meadow voles [38], mandarin voles [39], tuco tucos [40], deer mice [41, 42], African striped mice [43, 44], guinea pigs [45], tree shrews [46•], titi monkeys [47•], rhesus macaques [48, 49], zebra finches [50, 51], anole lizards [52], tree frogs [53], butterfly fish [54], and cichlid fishes [55, 56]. …”

Seasonal Variation in Cell Proliferation and Cell Migration in the Brain of Adult Red-Sided Garter Snakes <b><i>(Thamnophis sirtalis parietalis)</i></b>

“…These counting methods allowed us to retain more animals in the statistical analyses, in contrast to eliminating an animal if 2 or more consecutive sections were missing or damaged [e.g. Lutterschmidt and Wilczynski, 2012]. The mean number of missing or damaged sections across all animals and regions was 1.42 (±0.03 SE).…”