Aromatase activity in quail brain: correlation with aggressiveness.

Testosterone (T) triggers aggressive behavior in males of many vertebrate species; however, the neural and hormonal basis of individual differences in the frequency or intensity of aggressive behavior is still debated. Using the Japanese quail (Coturnix coturnix japonica), a species in which individuals exhibit a wide range of aggressiveness in nature and the laboratory, together with a newly devised test procedure for quantifying aggressiveness, we recently demonstrated that aggression is estrogen dependent. Here we extend these studies by testing the hypothesis that aromatization in brain is a rate-limiting step in the expression of individual differences in aggressiveness. Using procedures previously validated for this species, aromatase and 5 alpha- and 5 beta-reductase activities were estimated in selected brain regions of reproductively active male quail by measuring conversion of [3H]androstenedione to [3H]estrone, [3H]5 alpha-androstanedione, and [3H]5 beta-androstanedione, respectively. In Exp 1, behaviorally inexperienced test birds were killed 90 sec after a single behavioral test. Aggressiveness of individuals in this group, as determined by pecking and locomotor activity in response to visualization of a conspecific, ranged 3- to 4-fold from high to low. Aromatase activity in the posterior hypothalamus (PHYP) was significantly higher in males rated high for aggressiveness than in animals rated low (1.04 vs. 0.59 pmol/h.mg protein; P less than 0.02). Similar differences were observed in the anterior hypothalamus/preoptic area (AHPOA) but were not significant. In Exp 2, sexually mature males were behaviorally tested eight times over 22 days and killed 24 h after the final test. Aggressiveness varied 5-fold from high to low, although the rating in a given bird remained constant with time and repeat testing. Aromatase activity in the AHPOA was significantly greater in birds rated high for aggressiveness than in low aggressiveness birds (3.77 vs. 2.80 pmol/h.mg protein; P less than 0.02). In addition, when AHPOA aromatase in all birds was plotted against behavioral intensity, there was a 2-fold variation and a significant positive correlation (r = 0.556; P less than 0.02). Similar differences were observed in PHYP, but these were of borderline significance. By contrast, aromatase levels outside the AHPOA and PHYP were unrelated to behavior. Moreover, in both Exp 1 and 2, 5 alpha- and 5 beta-reductase activities in AHPOA, PHYP, and other brain regions; plasma T, 5 alpha-dihydrotestosterone, and total estrogens; and relative testicular weights were not consistently related to aggression.(ABSTRACT TRUNCATED AT 400 WORDS)