Abstract
The distribution of serotonin-immunoreactive (5HT-IR) neurons was studied in the brain of a primitive bony fish, Polypterus senegalus. The diencephalon contained a prominent 5HT-IR cell group (G2) consisting mainly of liquor-contacting neurons in the hypothalamic periventricular wall including the paraventricular organ. Their ependymofugal processes arborized profusely and gave rise to dense networks of varicose fibers in the hypothalamus. Other groups of 5HT-IR cells were situated in the preoptic area including the periventricular preoptic nucleus (G1) and in the dorsal thalamus (G3). In the brain stem, 5HT-IR cells were found in the medial part, mainly in the superior raphe nucleus (G4). No 5HT-IR cells were demonstrated in other regions of the brain. The cell groups referred to as G1 and G4 in the present study seem to be new components comprising the serotonergic systems in the Polypterus brain.
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