Seasonal patterns of basal and GnRH-induced LH, FSH and testosterone secretion in Eld's deer stags ( <i>Cervus eldi thamin</i> )

Abstract

Plasma LH, FSH and testosterone were measured in blood samples collected via remote catheterization from six adult Eld's deer stags every 10 min for 8 h before and 2 h after GnRH (1 μg kg−1, i.v.) administration. Blood samples were collected within 2 weeks of the summer solstice (21 June), autumn equinox (22 September), winter solstice (21 December) and spring equinox (20 March). Marked seasonal variations in basal LH, FSH and testosterone concentrations were observed. From autumn, well-defined LH pulses were temporally associated with small, but detectable pulses in testosterone. During the winter transition into the breeding season, episodic LH pulses were also temporally associated with corresponding testosterone surges that lasted 2–3 h. High amplitude, low frequency testosterone surges were also observed during the spring, but often in the absence of detectable LH pulses. Basal LH and testosterone concentrations decreased during the summer and, although LH pulses were detected, associated testosterone pulses were absent. Only 37% of LH pulses occurred coincidentally with FSH pulses, and FSH pulses were generally less prominent. The increases in LH and FSH above basal concentrations after GnRH treatment were significant (P &amp;lt; 0.05) for all seasons. Increases in testosterone after GnRH treatment were greatest during the winter and spring, but testosterone also increased to a lesser extent during the autumn (P &amp;lt; 0.05). In contrast, testosterone concentrations were not different before and after GnRH treatment during summer. The net LH increase after GnRH treatment was similar (P &amp;gt; 0.05) for all seasons, whereas the proportional increase in LH was greatest (P &amp;lt; 0.05) during the summer and autumn when basal concentrations were lowest. Although the net FSH increase after GnRH treatment ranged only from 20 to 40 ng ml−1 during all seasons, the proportional rise in FSH after GnRH treatment tended to be highest during the summer and autumn when basal concentrations were lowest. Basal testosterone secretion and the net increase in testosterone after GnRH treatments were greatest during the spring rut (P &amp;lt; 0.05). Thus, in Eld's deer, it appears that pituitary gonadotrophs seasonally adjust their basal secretory 'set point', but the capacity to respond above basal concentrations is constrained to a maximum net increase in both LH and FSH, regardless of season. Because both basal LH and the net releasable pool of LH remain relatively constant, increased testosterone secretion after GnRH treatment probably represents increased testicular sensitivity to LH.