TESTICULAR GROWTH AND LOCOMOTOR-ACTIVITY OF SIBERIAN HAMSTERS FROM SHORT-DAY-RESPONSIVE AND SHORT-DAY-NONRESPONSIVE LINEAGES

Young of short-day-responsive (SDR/Y) and short-day-nonresponsive (SDN R/Y) Siberian hamsters were reared in a long (16L:8D) photoperiod. At weaning, males from both lineages were maintained in 16L:8D or transfe rred to a short (12L:12D) photoperiod; body weight and testis size wer e measured every 7-10 days until 45 or 140 days of age. Contrary to ex pected results, juvenile SDNR/Ys responded to the inhibitory effects o f the shorter photoperiods with decreased body weight and inhibited te sticular growth. Under the long photoperiods, SDR/Y and SDNR/Y animals exhibited accelerated body weight gains and testicular growth from 18 to 69 days of age. Beginning at 69 days of age, however, SDNR/Ys exhi bited a transient decrease in testis size that troughed at 86 days of age; testis growth subsequently resumed. Consistent with their ability to respond to short photoperiods, SDNR/Y young, in a second experimen t, showed locomotor activity patterns similar to those of SDR/Ys in sh ort days (9L:15D). It was only in adult SDNR/Ys that the delayed onset of activity characteristic of nonresponders was apparent. These resul ts indicate that the circadian system of nonresponders changes with ag e, rendering animals nonresponsive to short photoperiods. The period o f transient testicular decline observed in SDNR/Ys housed in 16L:8D ma y mark a critical period in the onset of these changes; additional stu dies are required to test this hypothesis.