Annual cycle in LH and testosterone release in response to GnRH challenge in male woodchucks (Marmota monax)

Testosterone and LH concentrations were determined in serum samples obtaine d before and 15 min after injections of GnRH (1 mu g kg(-1)) administered a t 4-7 week intervals over 20 months to groups of male woodchucks (n = 6-7) born and maintained in Northern Hemisphere (boreal) versus Southern Hemisph ere (austral) simulated natural photoperiods, beginning at 18-24 months of age. Nadir and peak unstimulated testosterone (0.1 +/- 0.01 and 7.0 +/- 0.1 ng ml(-1), respectively) and LH (0.8 +/- 0.2 and 8.1 +/- 1.1 ng ml(-1), re spectively) concentrations did not differ in boreal versus austral males. I n the five boreal and five austral males that were confirmed to be photoent rained, basal (pre-GnRH) concentrations of LH and testosterone were lowest in summer, increased simultaneously in late autumn or early winter, and dec lined in the spring. GnRH stimulated some LH release throughout the year ex cept for a 1-4 month period in the summer. The initial annual increase in t he LH response to GnRH occurred in early autumn, and in 17 of 20 cycles it occurred 1-2 months before the initial increase in basal LH was detected. I n the three free-running males not entrained to the photoperiod, the endocr ine patterns were similar but were advanced by several months. The results demonstrate that in woodchucks there is a late autumn increase in LH secret ion associated with the onset of testicular recrudescence, and an early aut umn increase in pituitary response to GnRH before a detectable increase in serum testosterone.