Does prolactin mediate induced nest-building behaviour in pseudopregnant gilts treated with PGF2α?

Nest-building behaviour occurs 6-24 h before parturition in pigs (gestation = 116 days). Pseudopregnancy in pigs (induced with oestradiol valerate injections) lasts 50-80 days. We have shown that prostaglandin F2 alpha (PG) administration on day 47 of pseudopregnancy induces nest-building and changes to plasma prolactin, oxytocin, cortisol and progesterone similar to those seen before normal parturition, Peripheral prolactin has been proposed as a modulator of nest-building. This study assessed nest-building behaviour in prolactin-deprived gilts. Jugular vein catheters were inserted on day 39 of pseudopregnancy and blood samples collected daily from days 40-48. Animals were injected im with either 40 mg bromocriptine in 2 ml 70% ethanol (n = 8) or vehicle (n = 7) at 17.00 h on day 46 and 09.00 h on day 47 of pseudopregnancy. PG (15 mg Lutalyse: Upjohn) was injected im at 11.00 h on day 47. Blood and behavioural samples were taken from 90 min before PG to 6 h post-PG. Plasma prolactin increased in control but not bromocriptine treated animals following PG (P < 0.05). Elevations in oxytocin, cortisol and progesterone (P < 0.05) above pre-PG concentrations were also seen, but of these only progesterone showed between group differences [greater (P < 0.05) in control gilts on both days 47 and 48]. PG significantly (P < 0.05) increased both the rate and proportion of total time spent performing straw/floor-directed behaviours not including foraging (an index of nesting behaviour) in both treatment groups with no significant differences between groups. There were also no significant differences between groups in time spent performing pen fixture directed activities before or after PG. Bromocriptine suppressed the rise in prolactin concentrations after PG without suppressing nest-building behaviour. We conclude that peripheral prolactin is not an essential component of the nest-building complex in pigs.