Characteristics of prolactin binding sites in the brain of the ring dove ( Streptopelia risoria)

Abstract

Radiolabeled ovine prolactin ([ 125I]oPRL) was found to bind specifically to crude membrane fractions prepared from ring dove brain homogenates. Specific binding changed with incubation time and temperature but did not fluctuate systematically with pH. Specific binding levels were highest in hypothalamus and forebrain and were lowest in cerebellum and brain stem. A linear increase in specific binding was observed in pooled midbrain and forebrain membrane fractions as membrane protein concentrations increased from 0.2 to 6.0 mg/ml. Scatchard analysis of binding saturation and competitive binding data revealed that dove midbrain/forebrain membranes bound oPRL with high affinity ( K d = 2.1–2.6 × 10 −10 M) but had low binding capacity ( B max = 2.9–4.8 fmol/mg protein). Unlabeled oPRL and human growth hormone competed most effectively with radiolabeled oPRL for occupation of midbrain/forebrain binding sites. Turkey PRL and human placental lactogen were less effective as competitors but were 8–15 times more effective than ovine or turkey growth hormone, porcine insulin, or ovine luteinizing hormone. Subcutaneous injection of unlabeled oPRL lowered specific binding in liver and midbrain/forebrain by 76 and 39%, respectively, over that observed after saline injection. Prior MgCl 2-induced desaturation of membrane binding sites greatly reduced or eliminated these treatment differences. These results suggest the existence of specific, saturable binding sites for PRL in dove brain which conceivably could mediate the reported effects of PRL on behavior and gonadal function in this species.