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G. P. Pearce and P. E. Hughes

Summary. Large White × (Large White × Landrace) prepubertal gilts, 165 days of age, were fitted with indwelling venous catheters and housed in modified metabolism crates. After a period of acclimatization, frequent blood samples were taken at regular intervals before, during and after the 7 gilts were exposed to various degrees of contact with male pigs. The plasma samples were assayed for cortisol concentration using a competitive protein-binding radioassay. Significantly elevated concentrations of plasma cortisol (P < 0·001) occurred only when full physical contact between the boar and the gilts was allowed. Boar exposure without full physical contact induced only minor changes in plasma cortisol concentrations of gilts. Plasma cortisol concentrations have been shown to constitute a reliable indicator of a stress response in pigs, and so the results of this study suggest that tactile stimulation from a male pig induces a stress response in the recipient prepubertal gilt. This stress response in the gilt may be involved in the stimulation of puberty onset by contact with a mature boar (i.e. the 'boar effect').

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G. P. Pearce, A. M. Paterson and P. E. Hughes

Summary. Prepubertal gilts were fitted with jugular vein and carotid artery catheters at 148 days of age. At 160 days of age the 24 gilts were allocated to treatment in a 2 × 2 factorial design involving intra-carotid infusion of cortisol (10 mg in 40 ml saline) or saline alone with or without i.v. injection of 5 μg synthetic GnRH midway through the 1 h infusion. Plasma cortisol concentrations were elevated in gilts infused with cortisol (P < 0·05). The LH response to exogenous GnRH was reduced by cortisol infusion. Treated gilts released less LH (P < 0·001) and had a lower mean LH peak (P < 0·01) than did control gilts but the timing of the induced LH peak was not affected. In the absence of an exogenous GnRH challenge, cortisol infusion increased the endogenous secretion of LH (P < 0·01). These results suggest that acute elevations in plasma cortisol concentration may be involved in mediating changes in pituitary responsiveness and the secretion of LH in the peripubertal gilt.

Keywords: prepubertal gilts; elevated cortisol; LH secretion

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R. N. Kirkwood, J. M. Forbes and P. E. Hughes

Summary. At 6–7 weeks of age the olfactory bulbs were removed from gilts which were then exposed to vasectomized boars for 30 min/day, starting on Day 160. Mean ± s.e.m. age at first oestrus was similar in these gilts (230 ± 15 days) to that in intact gilts not exposed to a boar (234 ± 8 days) but greater (P < 0·05) than for sham- and un-operated gilts exposed to a boar (204 ± 3 and 208 ± 9 days, respectively). There were no differences in growth rate or ovulation rate at first oestrus. It is concluded that the main stimulus involved in the boar effect is olfactory in nature.

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K E Wonnacott, W Y Kwong, J Hughes, A M Salter, R G Lea, P C Garnsworthy and K D Sinclair

The evidence that omega-3 (n-3) and -6 (n-6) polyunsaturated fatty acids (PUFAs) have differential effects on ovarian function, oocytes and embryo quality is inconsistent. We report on the effects of n-3 versus n-6 PUFA-enriched diets fed to 36 ewes over a 6-week period, prior to ovarian stimulation and follicular aspiration, on ovarian steroidogenic parameters and embryo quality. Follicle number and size were unaltered by diet, but follicular-fluid progesterone concentrations were greater in n-3 PUFA-fed ewes than in n-6 PUFA-fed ewes. The percentage of saturated FAs (mostly stearic acid) was greater in oocytes than in either granulosa cells or plasma, indicating selective uptake and/or de novo synthesis of saturated FAs at the expense of PUFAs by oocytes. High-density lipoproteins (HDLs) fractionated from sera of these ewes increased granulosa cell proliferation and steroidogenesis relative to the FA-free BSA control during culture, but there was no differential effect of n-3 and n-6 PUFAs on either oestradiol or progesterone production. HDL was ineffective in delivering FAs to embryos during culture, although n-6 PUFA HDL reduced embryo development. All blastocysts, irrespective of the treatment, contained high levels of unsaturated FAs, in particular linoleic acid. Transcripts for HDL and low-density lipoprotein (LDL) receptors (SCARB1 and LDLR) and stearoyl-CoA desaturase (SCD) are reported in sheep embryos. HDL reduced the expression of transcripts for LDLR and SCD relative to the BSA control. The data support a differential effect of n-3 and n-6 PUFAs on ovarian steroidogenesis and pre-implantation development, the latter in the absence of a net uptake of FAs.