Depending on their native environment, different species of fish respond to osmotic changes in different ways. Some fish are able to live in multiple salinity concentrations and are known as euryhaline, while others are only able to survive in one and are known as stenohaline. Both stenohaline and euryhaline fish tightly regulate internal concentrations of salt and water by tending to excrete more of the substance present in their environment: marine fish excrete large amounts of salt and little amounts of water, whereas freshwater fish excrete large amounts of water and little amounts of salt (Karlstrom 2019). This ionocytic regulation itself is regulated by the hypothalamic-pituitary axis in the brain, which can be further investigated through the zebrafish, a freshwater model organism. It is hypothesized that as a response to changing osmotic demands, the zebrafish hypothalamus alters its dopamine hormone output in the short-term to the pituitary (which in turn regulates prolactin and ionocytes) and in the long-term alters cell proliferation rates to establish new regulatory populations of dopaminergic neurons. If zebrafish are exposed to altered salt concentrations in their environment, cell proliferation rates of dopaminergic neurons in the hypothalamus will increase as a long-term response to this new, stressful osmotic demand.
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