Reproduction is a complicated affair, requiring the well-timed coordination of hormone levels, brain signals and behavior. The humble zebrafish makes it look easy – their frequent reproduction and abundant offspring contribute to the popularity of this species in research. Estrogens are among the most important steroid hormones involved in reproduction. Fish have a unique quirk in their estrogen production system due to a gene duplication: they have two specialized versions of the estrogen-producing enzyme whereas most mammals and birds have just one. One version is primarily found in the gonads of fish, including the ovaries, where estrogen production is critical for proper development and function. The other version is primarily found in the brain, but its role there is not so clear. Could this tissue-specific production of estrogen in the brain of fish impact their reproduction? Katherine Shaw (University of Ottawa, Canada) and colleagues from the same institution and Sun Yat-Sen University, China, investigated whether disruption of estrogen production in the fish brain can really throw a wrench into the gears of reproduction.
To tackle the question, Shaw and colleagues genetically modified zebrafish to disrupt the gene that codes for this estrogen-producing enzyme in the brain. This approach allowed them to specifically inhibit estrogen production in the brain, while leaving ovarian estrogen production intact. Accordingly, they measured lower levels of estradiol (the primary estrogen in fish) in the brain of these mutated female fish. They also observed that these female fish were about four times slower to start releasing eggs when paired with a normal male during their morning mating session. Without this enzyme supplying estrogen in their brain, the normal mating behavior of the female zebrafish was thrown off. But this result led to another question – how does a drop in brain estradiol levels bring about the delay in egg release?
This is where an important duo of signaling molecules – arginine vasopressin and oxytocin – join the story. These two molecules are produced in the brain of fish and can contribute to signaling pathways that control reproductive behavior and gamete release. Since egg release was delayed in the mutated females, the researchers suspected that arginine vasopressin and/or oxytocin might link the estrogen hormone in the brain to the timing of egg release. They measured these signaling molecules in the brain of the mutated female fish and found that both were reduced. Was this the link between reduced brain estradiol and the delay in releasing eggs? Could they perhaps restore the normal timing of egg release during mating by increasing levels of arginine vasopressin or oxytocin in the fish's body?
To find out, the team injected mutated female fish (with reduced brain estradiol) with either arginine vasopressin or oxytocin and observed them during mating. They found that the arginine vasopressin-injected fish now released their eggs within a more typical timeframe. The restoration of this behavior showed that arginine vasopressin is a key signal linking estrogen production in the brain to egg release during mating. When the researchers fluorescently labeled arginine vasopressin and the enzyme for estrogen production in the zebrafish brain, they observed that the two molecules were produced in different cell types; however, the cells were in close proximity, potentially allowing brain-derived estrogens to stimulate nearby production of arginine vasopressin.
What have we learned from all this? Fish have a gene that encodes brain-specific estrogen production, which is, with the help of arginine vasopressin, critical in the prompt timing of egg release during zebrafish mating. This has laid a foundation to further explore the roles of brain-specific estrogen production in zebrafish as well as in other fishes, so researchers can spawn new ideas about how sex steroids influence the brain and behavior.
