Hormonal reactions to fighting in rat colonies: Prolactin rises during defense, not during offence

Abstract

Male Wistar rats living in colonies of 4 males plus 4 females were compared to noncolony males, cohabitating with a female. Irreversible dominance relationships developed between one dominant male (D) and three subordinates (S). Dominants developed high basal testosterone levels and large preputial glands. Subordinates had reduced preputial glands despite normal testicle weights and normal basal testosterone levels. Basal corticosterone was elevated in both ranks, in S more so than in D, while in acute encounters both ranks showed a similar increase in the corticosterone-to-ACTH ratio. They also underwent a similar reduction in thymus weight, while an increase in adrenal weight was more pronounced in D. In D-S-I encounters, during which D simultaneously attacked S and an intruder (I) for 20 minutes, both defenders showed a 3–4 fold increase in plasma prolactin, while in the offensive dominant the level remained low. Similar, but weaker hormonal contrasts between offence and defence were found for β-endorphin, ACTH, and corticosterone, while α-MSH and testosterone did not discriminate. In our view, the marked hyporesponsiveness of prolactin to acute offence may be associated with a specific offensive setting of dopaminergic inhibitory and β-adrenergic stimulatory influences.