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a Center for Reproductive Science, Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208
FSH levels begin to rise 3–5 days after male Siberian hamsters are transferred from inhibitory short photoperiods to stimulatory long photoperiods. In contrast, LH levels do not increase for several weeks. This differential pattern of FSH and LH secretion represents one of the most profound in vivo examples of differential regulation of the gonadotropins. The present study was undertaken to characterize the molecular mechanisms controlling differential FSH and LH synthesis and secretion in photostimulated Siberian hamsters. First, we cloned species-specific cDNAs for the three gonadotropin subunits: the common 
subunit and the unique FSHß and LHß subunits. All three subunits share high nucleotide and predicted amino acid sequence identity with the orthologous cDNAs from rats. We then used these new molecular probes to examine the gonadotropin subunit mRNA levels from pituitaries of short-day male hamsters transferred to long days for 2, 5, 7, 10, 15, or 20 days. Short-day (SD) and long-day (LD) controls remained in short and long days, respectively, from the time of weaning. We measured serum FSH and LH levels by RIA. FSHß, LHß, and subunit mRNA levels were measured from individual pituitaries using a microlysate ribonuclease protection assay. Serum FSH and pituitary FSHß mRNA levels changed similarly following long-day transfer. Both were significantly elevated after five long days (2.3- and 3.6-fold, respectively; P < 0.02) and declined thereafter, but they remained above SD control values through 20 long days. Alpha subunit mRNA levels also increased significantly relative to SD control values (maximum 2-fold increase after seven long days; P < 0.03), although to a lesser extent than FSHß. Neither serum LH nor pituitary LHß mRNA levels changed significantly following long-day transfer. The results indicate that long-day-associated increases in serum FSH levels in Siberian hamsters reflect an underlying increase in pituitary FSHß and subunit mRNA accumulation.
1 The research was supported by NIH grants F32-MH11493 (D.J.B.), R01-HD09885, P01-HD2192, and P30-HD28048 (F.W.T.).
2 Correspondence: Daniel J. Bernard, Department of Neurobiology and Physiology, Northwestern University, 2153 N. Campus Drive, Evanston, IL 60208. FAX: 847 467 4065; dbernard{at}nwu.edu
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