AN ANALYSIS OF SEX AND BREEDING STAGE DIFFERENCES IN PROLACTIN BINDING-ACTIVITY IN BRAIN AND HYPOTHALAMIC GNRH CONCENTRATION IN WILSONS PHALAROPE, A SEX ROLE-REVERSED SPECIES

Wilson's Phalarope (Phalaropus tricolor) is a sex role-reversed specie s in which incubation of eggs and care of young is performed exclusive ly by the male. Plasma levels of prolactin (PRL), the hormone most ass ociated with parental care in birds, are higher in incubating males th an in nonincubating males or females. Conversely, plasma testosterone levels are reduced in males during incubation. In an attempt to charac terize the Physiological basis of this unusual parental care system, e ve used quantitative film autoradiography and densitometry to measure the specific binding in vitro of I-125-ovine PRL to 12 brain regions i n females, nonincubating males, and incubating males during the normal breeding season. We also measured hypothalamic chicken gonadotropin-r eleasing hormone I (cGnRH-I) in three brain areas in these same birds, as well as plasma levels of PRL and testosterone. Analysis revealed t hat cGnRH-I concentrations in the preoptic area and plasma testosteron e levels were significantly lower in incubating males than in nonincub ating males. Specific binding of I-125-ovine PRL was detected in choro id plexus and in several diencephalic brain regions of both sexes, wit h highest binding activity recorded in the dorsolateral thalamus, medi al habenula, nucleus subrotundus, and preoptic area. When adjustments were made for the large number of comparisons performed, specific bind ing did not vary significantly by sex or breeding stage in any single brain region. However, average specific binding values in nonincubatin g males exceeded those of incubating males in 9 of the 11 PRL-sensitiv e regions examined. Increased occupancy of the receptor by endogenous PRL during incubation could have contributed to this result, since pla sma PRL levels were elevated in incubating males. In addition, PRL bin ding activity in several of these brain regions tended to correlate ne gatively with plasma PRL. The two exceptions to this general pattern w ere the preoptic area and the lateral septum, where mean specific bind ing was 14-15% higher in incubating males than in nonincubating males. This raises the interesting possibility that PRL sensitivity is up-re gulated during incubation in some regions of the male phalarope brain, such as the preoptic area and lateral septum, that have been implicat ed in PRL-modulated changes in behavior and reproductive activity duri ng this breeding stage. (C) 1998 Academic Press.