Distribution of serotonin-immunoreactive neurons in the brain of sockeye salmon fry.

The organization of the serotonergic cell groups in the brainstem of fishes and amphibians has received relatively little attention. It has been generally assumed that they are little differentiated and constitute a median cell column throughout the brainstem, and that laterally migrated serotonergic cell groups are largely lacking. In the present study we present evidence to the contrary. By the use of a sensitive immunocytochemical technique for the visualization of serotonin-immunoreactive (5HTir) neurons, we have been able to make a detailed delineation of the putatively serotonergic neuronal groups throughout the brain. In the epithalamus, 5HTir neurons were located in the left habenular nucleus in its dorsal subdivision. 5HTir neural elements, primarily photoreceptor cells, were present throughout the pineal organ and in some cases also in the parapineal organ. In the periventricular zones of the hypothalamus and posterior tuberculum, 5THir cerebrospinal fluid-contacting neurons were located in the paraventricular organ and in the dorsal, ventral and caudal zones of the periventricular hypothalamus. In the dorsal thalamus/synencephalon, 5THir neurons surround the tractus habenulo-interpeduncularis (fasciculus retroflexus). In the brainstem, several groups of 5HTir neurons could be discerned, that for reasons of topological similarity were named according to Lidov and Molliver a raphe pallidus/obscurus-complex (B1 and B2), raphe magnus (part of B3), median raphe (B8) possibly including raphe pontis (B5), raphe dorsalis (B4, B6 and B7), and B9. 5HTir neurons were observed in the central gray of the IVth ventricle, dorsal to the noradrenergic isthmal neurons and lateral to the brachium conjunctivum, in an area topologically equivalent with the dorsal subdivision of the locus coeruleus in mammals. In addition, small numbers of 5HTir neurons were located in the lobi faciales. Thus, the presence of well-differentiated groups of migrated serotonergic neurons is not an advanced trait of amniote brains, but may be a pattern common to all vertebrates.