Monoaminergic changes associated with socially induced sex reversal in the saddleback wrasse.

The process of sex reversal in fishes is socially mediated and requires a total reorganization of the hypothalamo-pituitary-gonadal axis. When the ratio of males to females in a population of saddleback wrasse (Thalassoma dupperrey) is too low, the largest female becomes male over the course of 6 to 8 weeks. This event requires the conversion of external social cues into internal chemical cues. In an attempt to investigate the role monoamines might play in this process, two females were housed together in floating enclosures in order to induce sex reversal in the larger. Brains were sampled at various time points throughout the process of sex reversal. Monoamines were measured in the amygdala, preoptic area, ventral hypothalamus, locus coeruleus and raphe nucleus. Changes were demonstrated in monoamine metabolism for all brain regions examined. The most important changes in monoamine-system activation were seen during the first week of sex reversal. It is during this time that transitional animals undergo behavioral sex reversal. There is an increase in serotonergic activity in the amygdala which is likely related to territorial acquisition. The absence of male aggression results in a less stressful environment for the female and a reduction in serotonergic activity in the preoptic area allowing for an increase in noradrenergic activity potentially triggering the reorganization of the reproductive axis. In the ventral hypothalamus, there is a decrease in noradrenergic and increase in dopaminergic activity associated with this change from female to male. The locus coeruleus shows an increase in noradrenergic activity later in the process of sex reversal which is probably a response to more circulating androgens. In the raphe nucleus, there is a decrease in serotonergic activity at the time of behavioral sex reversal. This decrease in serotonergic activity is linked to the behavioral component of sex reversal. This study suggests that monoamines play a very important role in both behavioral and gonadal sex reversal in the saddleback wrasse, the former under the control of serotonin in the raphe and the latter mediated via serotonergic effects on norepinephrine in the preoptic area.