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.