Transient serotonin-immunoreactive neurons coincide with a critical period of neural development in coho salmon (Oncorhynchus kisutch).

In coho salmon (Oncorhynchus kisutch), smolt transformation has been shown to be associated with sequential surges of neurotransmitters in the brain. In order to determine if the surge of serotonin (5-HT) is correlated with structural changes, we have used immunocytochemistry to observe changes in the serotonin immunoreactivity before, during and after the 5-HT surge. The following stages were studied: 12-month-old freshwater presmolts, 17-month-old freshwater presmolts, 18-month-old saltwater smolts, 19-month-old saltwater postsmolt, 24-month-old postsmolt, and adult spawners. In the 17-month-old samples, but not at any other stage, we found a set of transient (serotonin-immunoreactive) 5-HT-immunoreactive neurons in the lateral preoptic area, as well as a discrete population of 5-HT-immunoreactive neurons in the lateral part of the dorsal right habenular nucleus. In addition, a higher density of serotonergic fibers was found in the telencephalon at this stage compared to the following two stages. Since the transient 5-HT-immunoreactive structures presented here do not appear simultaneously with the 5-HT total brain concentration surge, we conclude that they are unlikely to be the source of the 5-HT surge, but are probably related to other developmental changes in the brain associated with smolt transformation.