Immunohistochemical localization of thyrotropin‐releasing hormone in the brain of chinook salmon (oncorhynchus tshawytscha)

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Abstract
This report describes the distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of juvenile chinook salmon. TRH-positive cell bodies are observed in the preoptic region of the diencephalon, in the supracommissural nucleus of the ventral telencephalon, and in the internal cellular layer of the olfactory bulb. Immunoreactive fibers occur in the olfactory bulb, the dorsal and ventral telencephalon and were particularly extensive in hypothalamic regions. TRH-positive fibers also are observed in the optic tectum, posterior pituitary and the brainstem. The cell bodies in the preoptic area reside in the magnocellular preoptic nucleus. The position of these cell bodies along with the location of fibers in the hypothalamus and pituitary is consistent with the role of TRH as a hypothalamic releasing hormone. TRH-positive cell bodies also occur in the supracommissural nucleus of the ventral telencephalon and in the internal cellular layer of the olfactory bulb. The cell bodies in the olfactory bulb may account for some of the fibers in the telencephalon, as there are TRH fibers in the olfactory tracts. The presence of TRH-positive, fibers with bouton-like swellings raise the possibility that the TRH peptide may act as a central neurotransmitter of neuromodulator. The results of this study suggest that TRH functions as a modulator of the pituitary activity in salmonids and that TRH is used as a transmitter or modulator in the olfactory system. The presence of TRH-positive somata in the olfactory bulb and ventral telencephalon provides new insights into the comparative anatomy of the salmon telencephalon.