A morphometric study of age-related changes in serotonin-immunoreactive cell groups in the brain of the coho salmon, Oncorhynchus kisutch walbaum

In the coho salmon there is a transient increase in total brain concentrations of serotonin during smolt transformation which occurs midlife, just before down-stream migration to the ocean. There is also a gradual age-related increase in total brain serotonin concentrations. These increases may be due to reorganization of the central serotonergic system, changes in serotonin turnover, or both. They may be related to the specific physiological conditions during different life stages of salmon, or to ongoing growth and plastic changes of the brain. In the present study we have compared serotonin-immunoreactive (5-HTir) cell groups in 1-year-old freshwater presmolt and 2-year-old seawater postsmolt salmon. Our data indicate a continuous growth of the 5-HTir cell groups in terms of an increase in numbers of 5-HTir neurons in the cell groups of the pretectum and the brain stem, and an increase in the volumes of such neurons and cell groups. However, when related to the increase in total brain volume, i.e., the volume that may be innervated by the 5-HTir neurons, the ratio of 5-HTir neurons per mm³ decreased. The largest decreases were observed in the median raphe nucleus (P < 0.005) and the B9 group (P < 0.05). The ratio of volumes of the brain nuclei containing 5-HTir neurons relative to total brain volume was remarkably constant when comparing pre- and postsmolt brains: only the pretectal nucleus showed a significant decrease (P < 0.01) in relative volume. The total volume of 5-HTir neurons increased in postsmolts (P < 0.005). Since there was no increase in size of 5-HTir somata in any nucleus, changes in total volume reflect the changes in numbers of 5-HTir neurons. Thus, to account for the observed age-related increase in total brain concentrations of serotonin, an increase in the net production of serotonin, possibly accompanied by an increase in the density of serotonergic innervation in certain brain areas, may be postulated. The transient surge of whole brain content of serotonin observed at smolt transformation probably reflects a transient change in serotonin metabolism rather than an increase in the number of neurons.