Activation of cerebrocortical type II 5′-deiodinase activity in Syrian hamsters kept under short photoperiod and reduced ambient temperature

Cerebrocortical type II thyroxine 5′-deiodinase (5′-D II) was studied in the Syrian hamster. Animals maintained in 14 hr light/day and 20°C ambient temperature and then subjected to acute cold exposure (4°C) for 4 hours did not exhibit changes in cortical 5′-D II activity. Prolonged exposure of hamsters to natural autumnal short photoperiods and reduced ambient temperatures for 8 weeks increased cortical 5′-D II activity compared to hamsters maintained inside under controlled long photoperiod (14:10 LD) and temperature (20 ± 2°C) conditions. Animals given subcutaneous implants of melatonin and exposed to the natural autumnal condition for 8 weeks exhibited a greater increase in the cortical 5′-D II activity than that in hamsters with blank pellets kept under the same conditions of reduced photoperiod and temperature. Type II 5′-deiodinase activity was not affected by treatment with 6-methoxy-2-benzoxazolinone (6-MBOA), a compound with a chemical structure related to melatonin. All animals maintained outdoors in the natural photoperiod and temperature conditions had depressed circulating thyroxine (T₄) and elevated triiodothyronine (T₃) concentrations; neither implants of melatonin nor 6-MBOA affected these thyroid hormone levels. It is concluded that serum T₄ is depressed and cerebrocortical 5′-D II activity is stimulated despite the elevation of T₃ levels in this autumnal condition. These responses, unlike classic pineal-mediated responses to environmental changes, are not blocked by subcutaneous implants of melatonin. However, chronic administration of melatonin might augment the cortical 5′-D II response to low serum T₄ or some other component of the autumnal condition.