Characterization of prolactin secretion in gray wolves (Canis lupus)

Abstract

From 1980 to 1989, experiments were conducted on 28 intact (13 females, 15 males) and 10 neutered wolves (5 females, 5 males) to characterize prolactin (PRL) release. From these studies, we have (i) adapted and validated the canine PRL radioimmunoassay of Parlow for wolves; (ii) determined that the plasma half-life of PRL is 44 min; (iii) demonstrated that anesthesia with 400 mg ketamine plus 50 mg promazine, 400 mg ketamine plus 30 mg xylazine, or 7.5 μg∙kg−1 etorphine plus 0.5 mg∙kg−1 xylazine, administered intramuscularly, does not alter PRL rhythms or control mechanisms; (iv) indicated that PRL is not secreted in response to handling stress (P ≥ 0.78) or by activation of the hypothalamic–pituitary–adrenocortical axis simulated by intravenous injection of 50 μg ovine corticotropin-releasing factor (P = 0.28); (v) demonstrated a circannual rhythm in intact and neutered wolves characterized by elevated PRL levels just prior to summer solstice; (vi) detected a circadian PRL rhythm in females; (vii) provided evidence for dopaminergic control of PRL secretion by injecting 2.0 mg∙kg−1 promazine, a dopaminergic antagonist, which increased PRL levels significantly (P = 0.0001); (viii) shown that intravenous injection of 100 μg thyrotropin-releasing hormone results in PRL release (P = 0.0001), but that this release could not be attenuated by prior administration of 1.0 mg∙kg−1 of the opioid antagonist naloxone; (ix) determined that feeding 2.0 mg of melatonin daily reduced basal PRL levels in May and June (P = 0.03), but not in October–December (P = 0.42), and that PRL stimulation with 100 μg thyrotropin-releasing hormone was consistently lower in melatonin-fed wolves; and (x) shown that pinealectomy does not abolish the circadian PRL rhythm nor alter absolute PRL levels relative to sham-operated control wolves (P = 0.33).