The Djungarian or Siberian hamster (Phodopus sungorus) is a seasonally breeding rodent in which maturation of gonadal function depends upon the photoperiod during rearing. It was hypothesized that the ability of short days to block testicular growth resulted from insufficient gonadotropin secretion during critical stages of development. This question was studied by measurement of serum concentrations of FSH, LH, PRL, and androgens by RIA systems validated for use in this species. Males reared from birth in long (16 h of light, 8 h of darkness) or short (10 h of light, 14 h of darkness) photoperiods were killed at 5- to 10-day intervals between 5 and 60 days of age. Regardless of photoperiod before 15 days of age, body and testes weights similarly increased. Serum concentrations of FSH and PRL gradually increased during this age period, although PRL concentrations were statistically higher in males under long days compared to those under short days. Circulating serum LH and androgen levels were high before 10 days of age, but decreased by 15-20 days of age in both photoperiods. Under long days, the period between 15 and 30 days of age was characterized by rapid body (1 g/day) and testicular (10-38 mg/day) growth, peak serum FSH concentrations (20-25 days), sustained elevation in serum LH and androgen concentrations, and further increases in serum PRL values. After 30 days of age, a reduced growth rate for body and testes occurred; serum FSH levels declined, while adult serum concentrations of LH, PRL, and androgens were attained. In contrast, hamsters exposed to short days from birth exhibited a slower rate of body and testicular growth by 20 days of age. Short days blocked peak FSH secretion and suppressed serum concentrations of LH and androgens after 20 days of age. PRL titers were significantly lower in short day compared to those in long day housed hamsters at all ages. These results support the hypothesis that the short day-induced suppression of gonadotropins and PRL secretion during development blocked gonadal maturation.
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