Testosterone, LH and FSH episodic secretory patterns in GnRH-immunized bulls

The objective was to determine: (i) the secretory patterns of LH, FSH and testosterone in bulls, and the temporal relationships between the pulses of these hormones; and (ii) the effect of GnRH immunization on these parameters. Friesian bulls (n = 72) were given a primary immunization on day 0 (10-week-old) and a booster immunization on either day 28 (n = 36) or day 56 (n = 36) against either a GnRH-human serum albumin (HSA) conjugate (n = 48) or HSA (n = 24; control). On the basis of GnRH antibody titres at a plasma dilution of 1:160, 1 week after booster immunization, 12 GnRH-immunized and six control bulls from each booster-immunized group were selected and allocated to one of three groups: control bulls and bulls with medium and high antibody titres (0.3, 32 and 51% binding, respectively; pooled SED 4.3%). Blood samples were taken from these animals (n = 36) every 15 min for 8 h on three occasions: (i) 2 weeks after booster immunization when bulls were 4-5 months of age (prepubertal); (ii) at 7 months of age (peripubertal); and (iii) at 11 months of age (post-pubertal). Data were analysed by PULSAR, ANOVA and chi-squared test. The mean antibody titre of the high antibody titre group was greater (P < 0.05) than that of the medium antibody titre group in prepubertal bulls only, but the mean antibody titres of both antibody titre groups were greater (P < 0.05) than that of the control bulls at all times. The frequency and amplitude of LH and FSH pulses in the control bulls were greater (P < 0.05) during prepuberty than after puberty. The frequency, amplitude and duration of LH pulses were greater (P < 0.05) in control bulls than those in medium and high antibody titre bulls at prepuberty. The mean and basal concentrations of FSH and the amplitude and duration of FSH pulses were greater (P < 0.05) in the control bulls than in the high antibody titre bulls at prepuberty. The frequency of testosterone pulses was greater (P < 0.05) in the control bulls than in the medium and high antibody titre bulls at peripuberty. The mean and basal concentrations and pulse amplitude of testosterone were greater (P < 0.05) in high antibody titre bulls than in control bulls after puberty. There was a close temporal relationship between LH and FSH (62.5% of LH pulses were followed by FSH pulses within 75 min) at prepuberty in the control bulls but there was no relationship after puberty. The opposite trend occurred in the high antibody titre bulls, that is, there was no relationship between LH and FSH at prepuberty but there was a close temporal relationship after puberty. The temporal relationship between LH and testosterone was closest at peripuberty (86.7%) in the control bulls, but increased in the high antibody titre bulls from 0% at prepuberty to 57.1% after puberty. In summary, there was an age-related decrease in LH and FSH pulse frequency and amplitude and also in the temporal relationships between these hormones in control bulls. Prepubertal GnRH immunization had a suppressing effect on the pulsatile release of LH, FSH and testosterone before but not after puberty. The presence of high amplitude testosterone pulses in the GnRH-immunized bulls after puberty indicates that the long-term tonic release of LH may be sufficient to initiate a late pubertal-type increase in testosterone concentrations.