Cellular actions of melatonin

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Photoperiodic variation in CD45-positive cells and cell proliferation in the mediobasal hypothalamus of the soay sheep

The Earth's solar orbit induces annual climatic changes challenging to survival. Many animals have evolved to cope with seasonal variability through compensatory annual changes in their physiology and behavior, which involve innate long-term timing and photoperiodic synchronization to anticipate the environmental seasonal cycles. Here we considered the potential involvement of cyclical histogenesis in seasonal timing mechanisms in the sheep. Adult Soay rams were established in three distinctive seasonal states by controlled photoperiod exposure. A first group, representing the condition in late spring (long-photoperiod [LP] group), was taken indoors in May and exposed to 4 wks of 16 h light/day (LP). A second group was exposed to 20 wks of LP to establish a late-summer/long-day refractory condition (LPR group). A third group of animals was brought indoors in August and exposed to 4 wks of LP followed by 4 wks of 8 h light/day (short photoperiod [SP]) to establish an autumn-like condition (SP group). At the end of these regimes, we injected 5-bromo-2-deoxyuridine (BrdU), and animals were killed 24 h or 4 wks later. When BrdU was administered 24 h before death, more BrdU-immunopositive cells were detected in the hilus of the hippocampus in LP compared with SP animals, indicative of a higher proliferation rate. When BrdU was administered 4 wks before death, more BrdU-positive cells were detected in the hippocampus under LP, compared with SP, indicating increased cell survival. These mitotic cells were occasionally seen to adopt a neuronal phenotype in the hippocampus, but not in the hypothalamus. Approximately 10% of BrdU-positive cells in the basal hypothalamus coexpressed the pan-leukocytic marker CD45, and showed morphological features and regional distribution consistent with ameboid microglia. Increased numbers of these cells were detected in the region of the median eminence and tuberoinfundibular sulcus of animals kept in SP compared with LP or LPR. These data suggest that neuroimmune mechanisms may be involved in photoperiod-dependent seasonal remodeling of the adult brain

A transgenic test of the role of Cryptochrome1 in the seasonal neuroendocrine response in mammals

International audienc

Neuroendocrine control of parr-smolt transformation in Atlantic salmon

International audienc

Modulation of MT1 function in the PT: a role for RGS4 in photoperiodic signaling

International audienc

Evidence for RGS4 modulation of melatonin and thyrotrophin signalling pathways in the pars tuberalis

 absen

Circadian pacemaking within the suprachiasmatic nucleus analysed with a novel mCry1::luciferase transgenic mouse

International audienc

Deiodinases in the sheep hypothalamus: differential photoperiodic and endogenous spatiotemporal gene expression

International audienc

Circannual variation in thyroid hormone deiodinases in a short-day breeder

At temperate latitudes, many mammals and birds show internally timed, long-term changes in seasonal physiology, synchronised to the seasons by changing day length (photoperiod). Photoperiodic control of thyroid hormone levels in the hypothalamus dictates the timing. This is effected through reciprocal regulation of thyroid hormone deiodinase gene expression. The local synthesis of type 2 deiodinase (Dio2) promotes triiodothyronine (T3) production and summer biology, whereas type 3 deiodinase (Dio3) promotes T3 degradation and winter biology. In the present study, we investigated the extent to which the hypothalamic expression of Dio2 and Dio3 is circannually regulated in the Soay sheep, a short-day breeding mammal. Male sheep were exposed to a long photoperiod (LP; 16:24h light/dark cycle) or a short photoperiod (SP; 8:24h light/dark cycle), for up to 28weeks to establish four different endocrine states: (i) LP animals in a spring/summer-like state of reproductive arrest; (ii) LP refractory (LPR) animals showing spontaneous reproductive reactivation; (iii) SP animals showing autumn/winter-like reproductive activation; and (iv) SP refractory (SPR) animals showing spontaneous reproductive arrest. A complex pattern of hypothalamic Dio2 and Dio3 expression was observed, revealing distinctive photoperiod-driven and internally timed effects for both genes. The patterns of expression differed both spatially and temporally, with phases of peak Dio2 expression in the median eminence and tuberoinfundibular sulcus, as well as in the paraventricular zone (PVZ) (maximal under LP), whereas Dio3 expression was always confined to the PVZ (maximal under SP). These effects likely reflect the distinct roles of these enzymes in the localised control of hypothalamic T3 levels. The spontaneous decline in Dio2 and spontaneous increase in Dio3 in LPR animals occurred with a corresponding decline in thyroid-stimulating hormone beta expression in the neighbouring pars tuberalis (PT), although this relationship did not hold for the corresponding Dio2 increase/Dio3 decrease seen in SPR animals. We conclude that internally timed and spatially regulated changes in Dio2 and Dio3 expression may drive the cycling between breeding and nonbreeding states in long-lived seasonal species, and may be either PT-dependent or PT-independent at different phases of the circannual cycle