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R. D. MacDonald, J. L. Peters, and D. R. Deaver

Summary. Administration of naloxone (100 mg i.v.; approximately 1·21 mg/kg body weight0·75) to 10 intact calves (24 weeks of age) caused an acute release of LH that was similar in amplitude and duration to spontaneous discharges of LH that occur at the same age. The naloxone-induced release of LH was abolished in 9/10 calves (intact and castrated) treated with oestradiol-17β. To determine the ontogeny of opioid control of secretion of LH, 12 calves were randomly assigned to receive saline or naloxone (1·21 mg/kg body weight0·75, i.v.) at 3, 5, 7, 9, 11, 13, 17 and 21 weeks of age. At each age, blood was collected at 10-min intervals for 4 h and saline or naloxone was administered (i.v.) after collection of the 120-min sample. Before administration of naloxone, plasma LH values increased with age (P < 0·01) but did not differ between the control and naloxone groups (age × treatment, P > 0·05). Administration of naloxone caused concentrations of plasma LH to increase at 3, 11, 13, 17 and 21 weeks of age (treatment × time, P < 0·001). Concentrations of LH (saline vs naloxone, ng/ml) reached a maximum within 20 min after treatment at Weeks 3 (0·3 vs 1·2), 11 (0·6 vs 2·6), 13 (0·6 vs 3·7), 17 (1·1 vs 2·6), and within 40 min after treatment at Week 21 (1·0 vs 3·5). Based on these results, it is concluded that endogenous opioids inhibit the secretion of LH during infancy and the later prepubertal periods of life in bulls but not around the time of the spontaneous increase in secretion of LH that marks the end of the infantile period.

Keywords: naloxone; LH; puberty; bulls

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J. M. McAndrews, J. L. Peters, and D. R. Deaver

The objectives of this study were (i) to determine whether age-related changes in the secretion of LH are associated with alterations in secretory activity or numbers of gonadotrophs, and (ii) to determine whether gonadotrophs obtained from the pars distalis and pars tuberalis undergo similar age-related changes in function. Blood samples were collected from Holstein bull calves every 15 min for 12 h at <1, 12, and 42 weeks of age (n = 5 per age group) to characterize the secretion of LH. Calves were killed 3–5 days later. The pars distalis and pars tuberalis were enzymatically dispersed into suspensions of single cells. Cells from the pars distalis were (i) extracted with 0.01 mol NaHCO3 l−1, (ii) fixed for immunocytochemical analysis, and (iii) cultured in six-well plastic plates at a density of 500 000 cells per well in media containing 2.5% homologous calf serum for 18 and 72 h. Cells from the pars tuberalis were cultured as for pars distalis cells. As expected, LH pulse frequency increased (P < 0.01) between one and 12 weeks of age and then declined. The percentage of cells from the pars distalis containing immunoreactive LH averaged 8.4%, and did not change with age. The mass of the pars distalis and the total number of cells recovered increased with age (P < 0.05); consequently, the number of gonadotrophs recovered also increased. The initial content of LH of pars distalis cells changed with age and was greatest at 12 weeks. At <1 and 12 weeks of age, the difference in initial content was reflected in the amount of LH released in vitro after both 18 and 72 h in culture. In addition, at these ages the total amount of LH present (media plus cell extract) at the end of the culture period was similar to the initial cell content. In contrast, the total LH content of cells obtained from the pars distalis of calves at 42 weeks of age increased by approximately 3.5 times during the first 18 h of culture. The total content of LH from pars tuberalis cells after 18 h in culture increased slightly with age but this increase was not significant. In summary, age-related differences in the numbers and secretory activity of gonadotrophs located within the pars distalis contribute to age-related increases in concentrations of LH in plasma. Changes in the synthesis and release of LH from pars distalis and pars tuberalis cells appear to differ in culture.

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M. Shahab, K. D. Nusser, J. L. Peters, and D. R. Deaver

The neuroendocrine mechanisms underlying the development of pulsatile release of LH in bull calves are poorly understood. The hypothesis that endogenous excitatory amino acids, working through N-methyl-d-aspartate (NMDA) receptors, are involved in the generation of pulsatile LH release during sexual maturation of bull calves was tested. Holstein bull calves were administered i.v. 0.001, 0.01 and 0.1 mg kg−1 body mass of MK-801, a specific noncompetitive NMDA receptor antagonist, on alternate days at 1, 12 and 24 weeks of age (n = 6 per dose), using a replicated 3×3 latin square design. Blood samples were obtained from jugular catheters at intervals of 10 min for 2 h before and 2 h after MK-801 treatment at 1 week and for periods of 4 h before and after MK-801 administration at 12 and 24 weeks of age. Plasma concentrations of LH were measured by a specific radioimmunoassay and pulsatile LH secretion was analysed using the Pulsar algorithm. Basal LH secretion at 1 week of age was low but was increased by 12 weeks (0.38 ± 0.01 versus 1.38 ± 0.11 ng ml−1; P < 0.001) with the establishment of frequent high amplitude pulses. Mean LH concentrations and pulse amplitude, but not frequency, were lower at 24 than at 12 weeks of age (0.88 ±0.08 versus 1.38 ± 0.11 ng ml−1: P < 0.001, 1.74 ± 0.14 versus 2.82 ± 0.36 ng ml−1: P < 0.01, and 1.70 ± 0.30 versus 1.70 ± 0.41 in 4 h: P > 0.05, respectively). Administration of MK-801 did not affect LH secretion in 1- and 12-week-old calves. In contrast, the antagonist at the highest dose tested caused a significant decrease in mean LH concentrations (0.30 ± 0.01 versus 0.70 ±0.10 ng ml−1; P < 0.05), which was associated with a reduction in the number of LH pulses (0.70 ± 0.30 versus 1.50 ± 0.30 in 4 h; P < 0.01) and pulse amplitude (0.54 ± 0.04 versus 1.72 ± 0.20 ng ml−1; P < 0.001), in 24-week-old calves. These data suggest that the initiation of pulsatile secretion of LH during the juvenile phase in bull calves is not mediated by the endogenous excitatory amino acids operating through NMDA receptors. However, maintenance of the pulsatile secretory pattern of LH in prepubertal bull calves, which is required for continued testicular maturation, does involve an NMDA-receptor-mediated neuroexcitatory drive to GnRH-releasing neurones.

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G. J. Killian, D. A. Chapman, J. F. Kavanaugh, D. R. Deaver, and H. B. Wiggin

Summary. The oviducts of 4 cows were cannulated and oviduct fluid was collected daily from the exteriorized cannulas for a total of 5 oestrous cycles. Daily serum samples were assayed for oestradiol-17β and progesterone to monitor the oestrous cycle. Data for each cycle were compared for oviduct fluid collected during the non-luteal phase (serum progesterone ⩽ 1·5 ng/ml) and the luteal phase (serum progesterone > 1·5 ng/ml). During the non-luteal phase oviduct fluid volume was higher and the osmolality was lower than during the luteal phase. Total protein, cholesterol and phospholipid secreted daily was greater during the non-luteal phase. Cholesterol and protein concentrations were generally lower during the non-luteal phase, but phospholipid concentrations were generally higher. About 40% of the phospholipid in oviduct fluid was phosphatidylcholine and lysophosphatidylcholine, while phosphatidylinositol and lysophosphatidylinositol accounted for 20%. The ratio of 1-acyl-phospholipid to diacylphospholipid increased during the non-luteal phase. An increased cholesterol to phospholipid ratio, and a decreased cholesterol to protein ratio in oviduct fluid also were associated with the non-luteal phase. Changes in the lipid composition of oviduct fluid during the oestrous cycle may play a role in the preparation of gametes for fertilization.

Keywords: oviduct fluid; cow; phospholipid; cholesterol; protein

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P. J. Battista, J. P. Poff, D. R. Deaver, and W. A. Condon

Summary. Biogenic amines were administered using osmotic pumps placed subcutaneously in the neck region of regularly cycling, non-lactating dairy cows on Days 9–11 (oestrus = Day 0) of the oestrous cycle. Blood samples were collected using indwelling jugular catheters and the plasma progesterone concentrations were measured. Samples were collected at 4-h intervals for the first 12 h of treatment and thereafter at 12-h intervals for the remainder of the 72-h treatment period. After administration of various doses of noradrenalne, adrenaline and serotonin (0·5–2·0 μg/kg/h) significant elevation of plasma progesterone was achieved at a dosage of 2·0 μg/kg/h (P < 0·01). The response to adrenaline was greater than that observed for noradrenaline and serotonin (P < 0·05). Within-treatment comparison to pretreatment samples showed plasma progesterone concentrations to increase within 4 h after the administration of noradrenaline, adrenaline and serotonin (P < 0·05) and this enhancement was maintained throughout the treatment period (P < 0·05). The elevation in plasma progesterone concentrations induced by noradrenaline, adrenaline and serotonin was independent of changes in circulating concentrations of luteinizing hormone. These results support a physiological role for endogenous biogenic amines in the control of bovine luteal progesterone production.