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The effect of exogenous prolactin (PRL) on the weight and lipid content of the liver and abdominal adipose tissue as well as on the concentrations of total lipids and lipoprotein fractions (portomicrons, VLDL, LDL, HDL) in plasma was investigated in chickens. PRL or its solvent were administered at different time points (0, 4, 8, and 12 hours after light onset = HALO) during 5 consecutive days. The birds were exsanguinated at the same time of the day when they received the injections, 24 hours after the last one. At the same time plasma corticosterone (CS) concentration was determined in these chickens and also during 24 hours in intact chickens matched for age. PRL affected fat accumulation in the experimental birds and the intensity of this effect depends on the time of the day at which it was administered. PRL caused rise of the weight of the fat tissue and its lipid content when it was injected at 8 or 12 HALO. This was associated with a significant increase of total plasma lipids and with an insignificant increase in the level of lipoprotein fractions excluding portomicrons. PRL increased also (at 4 HALO) or decreased (at 8 HALO) the liver weight. Handling of the birds during vehicle injections decreased the corticosterone level in the control chickens as compared with the intact ones; PRL raised the level of CS at 0 HALO significantly. The role of PRL in the intensification of fat transport is discussed.

The purpose of this experiment was to elucidate whether the effect of exogenous prolactin (PRL) on immunity parameters of White Leghorn chickens varies during the day. The experiment was carried out on cockerels kept after hatching during 6 weeks under L:D = 12:12 conditions. During 5 consecutive days cockerels were injected with bovine PRL (150 micrograms per bird daily) or its solvent at different time points, i.e., at 0, 4, 8 or 12 HALO (Hours After Light Onset). The birds were sacrificed 24 hours after the last injection at the same time point when the injections were given. It was found that given at 4 HALO prolactin raised, whereas at 8 HALO it decreased the peripheral lymphocyte number. PRL had no effect on granulocyte number, natural anti-rabbit red blood cells (anti-RRBC) agglutinin titre and serum lysozyme activity. Administered at 0 and 12 HALO PRL tended to decrease the thymus and bursa of Fabricius weight. This different PRL effect on peripheral lymphocytes in chickens during the circadian period may be one of the causes of varying effect of this hormone on immunity (stimulatory or suppressive) described in literature. The role of PRL in regulation of immunity is discussed in relation to the possibility of PRL receptors occurrence on chickens lymphocytes.