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 degrees C ambient temperature and then subjected to acute cold exposure (4 degrees 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 degrees C) conditions. Animal 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.