PLASMA PROLACTIN CONCENTRATIONS IN THE HORSE - RESPONSE TO OPIOID RECEPTOR BLOCKADE WITH NALOXONE AND COMPARISON OF 2 PROLACTIN ASSAY SYSTEMS

The aim of this study was to investigate an involvement of endogenous opioids in the regulation of prolactin release in the horse. Effects o f the opioid antagonist naloxone on plasma prolactin concentrations we re determined and prolactin was measured with a homologous radio-immun oassay and an immunometric enzyme assay utilizing a combination of two monoclonal antibodies. Both assays were sensitive and specific for pr olactin determination in the horse. However, low prolactin concentrati ons could be better differentiated with the radio-immunoassay. Experim ents in stallions were performed in May, August and December, and in m ares between April and June. Naloxone (approximate to 0.5 mg/kg i.v.) stimulated prolactin release in stallions. Prolactin secretion determi ned by radio-immunoassay in stallions receiving naloxone was significa ntly higher than in controls in May (p < 0.05) and August (p < 0.05), and nearly reached statistical significance in December (p = 0.06). Pr olactin release measured by enzyme immunoassay was significantly diffe rent (p < 0.05) between naloxone and control experiments at all times. Endogenous opioids therefore inhibit prolactin release in stallions. The activation of these opioid systems is affected by season. Opioid-r eceptor blockade with naloxone did not influence prolactin release in mares.