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 bree ders 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 unkno wn. There is growing evidence, however, that specific cells of the pit uitary pars tuberalis (PT) mediate these photoperiod/nonphotoperiod-de pendent changes as a direct or indirect ''Zeitgeber'' for the endocrin e system. We investigated messenger RNA (mRNA)/protein formation for s everal hypophyseal hormones (beta-TSH, beta-LH, PRL, common alpha-chai n) in the pars distalis (PD) and PT of female Djungarian hamsters in l ong photoperiod (LP) and after 18, 28, and 38 weeks of short photoperi od (SP). As indicated by gonadal and body weight, the hamsters display ed gonadal regression after 18 and 28 weeks of SP; after 38 weeks of S P, all animals showed recrudescence. At Is and 28 weeks of SP, only PR L mRNA and protein levels were significantly reduced in the PD and ret urned to LP values after 38 weeks of SP. The expression of hypothalami c tyrosine hydroxylase in the arcuate nucleus that was determined by i mmunocytochemistry and by in situ hybridization was also down-regulate d in SP18 and SP28 with increasing levels at SP38. Urinary 6-sulfatoxy melatonin levels were elevated in SP with highest levels in the SP18 g roup. In the PT, beta-TSH mRNA and protein were not detectable in all SP groups compared with the moderate signal intensity in LP. The commo n alpha-chain mRNA and protein, however, which were also reduced in th e animals of the SP18 group, were already elevated after 28 weeks of S P and nearly reached LP-levels after 38 weeks of SP. These results sho w that, in contrast to LH and TSH, PRL expression in the PD is a sensi tive indicator for photoperiod dependent changes of the endocrine syst em and seems to be tyrosine hydroxylase independent. The increase of c ommon alpha-chain expression in PT-specific cells depending upon durat ion of SP that precedes the hormonal changes in the PD leads us to spe culate that PT-specific cells initiate spontaneous recrudescence via a PT-PD pathway.