Sexually dimorphic effects of melatonin on prolactin cell function in male and female syrian hamsters

Initial expression of the common alpha-chain in hypophyseal pars tuberalis-specific cells in spontaneous recrudescent hamsters

When exposed to short-day conditions, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure to short days, however, the animals become photorefractory and gonadal recrudescence occurs. The underlying mechanism for this insensitivity is still unknown. There is growing evidence, however, that specific cells of the pituitary pars tuberalis (PT) mediate these photoperiod/nonphotoperiod-dependent changes as a direct or indirect "Zeitgeber" for the endocrine system. We investigated messenger RNA (mRNA)/protein formation for several hypophyseal hormones (beta-TSH, beta-LH, PRL, common alpha-chain) in the pars distalis (PD) and PT of female Djungarian hamsters in long photoperiod (LP) and after 18, 28, and 38 weeks of short photoperiod (SP). As indicated by gonadal and body weight, the hamsters displayed gonadal regression after 18 and 28 weeks of SP; after 38 weeks of SP, all animals showed recrudescence. At 18 and 28 weeks of SP, only PRL mRNA and protein levels were significantly reduced in the PD and returned to LP values after 38 weeks of SP. The expression of hypothalamic tyrosine hydroxylase in the arcuate nucleus that was determined by immunocytochemistry and by in situ hybridization was also down-regulated in SP18 and SP28 with increasing levels at SP38. Urinary 6-sulfatoxymelatonin levels were elevated in SP with highest levels in the SP18 group. In the PT, beta-TSH mRNA and protein were not detectable in all SP groups compared with the moderate signal intensity in LP. The common alpha-chain mRNA and protein, however, which were also reduced in the animals of the SP18 group, were already elevated after 28 weeks of SP and nearly reached LP-levels after 38 weeks of SP. These results show that, in contrast to LH and TSH, PRL expression in the PD is a sensitive indicator for photoperiod dependent changes of the endocrine system and seems to be tyrosine hydroxylase independent. The increase of common alpha-chain expression in PT-specific cells depending upon duration of SP that precedes the hormonal changes in the PD leads us to speculate that PT-specific cells initiate spontaneous recrudescence via a PT-PD pathway.

Ultrastructural morphology of the aged pineal

Thirty-four pineals of 18-month-old rats were studied, of which 12 belonged to animals exposed to 7% and 13 to a group exposed to 10% oxygen in breathing gas. Nine pineals were from controls kept under normoxic conditions. For comparison 2 pineals of 6-week-old rats were also examined. No influence of hypoxia on histology of the pineal gland can be stated. In this the pineal behaves like other parts of the rat brain, which in previous experiments has been shown to adapt well to low-oxygen concentrations. Also no important difference between pineals of young and advanced aged rats were found by us. In 8 of 34 pineals striated muscle cells were present. We consider this an important finding, as pineal cells have a unique differentiation potential in cell culture systems and in tumorous lesions of the pineal. As rats exposed to severe hypoxia exhibit body organ changes indicating stress, it would be interesting to measure melatonin secretion and pineal melatonin content under prolonged and increasing hypoxic conditions.