Influence of season and social environment on basal and thyrotropin releasing hormone-induced prolactin secretion in the adult domestic boar.

Basal and TRH-induced PRL secretions were investigated for adult Landrace boars housed in two social environments. Socially nonrestricted boars (N = 4) were individually penned adjacent to ovariectomized gilts that were hormonally induced into estrus every 3 weeks, while socially restricted boars (N = 4) were individually kept in pens with solid walls. In February, May and August all boars were fitted with jugular catheters for serial blood sampling which took place 2 h prior to and 4 h following in iv injection of TRH (1 microgram/kg). Mean pre-injection serum PRL concentration was not influenced by either month or social environment. The PRL response to TRH injection, however, was influenced by both factors. The magnitude of the PRL response (peak delta value) increased (month, P less than 0.01) progressively in both groups of boars from February through August and was greater (P less than 0.05) in the nonrestricted versus the restricted boars in August. The total amount of PRL secreted was also maximal and greater in the socially nonrestricted boars in August (month x social group, P less than 0.05). These results indicate that TRH-induced and not basal PRL secretion is influenced by both season and social interaction of boars.