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.

This report describes the connections of the olfactory bulb in juvenile chinook salmon (Oncorhynchus tshawytscha) as revealed by the retrograde and anterograde transport of biotinylated dextran amine (BDA). Pressure injections of BDA to discrete locations within the bulb reveal that the olfactory bulb projects via medial and lateral tracts to various regions of the dorsal and ventral telencephalon as well as to the diencephalon. Terminal-like boutons are present in the ventral, dorsal, lateral and supracommissural nuclei of the ventral telencephalon and in the lateral-ventral and posterior zones of the dorsal telencephalon. There is also a projection to the nucleus taenia and to the contralateral olfactory bulb. A diencephalic terminal field occurs in the posterior tuberal region of the ventral diencephalon. Olfactory bulb fibers in the preoptic region have axonal varicosities near neurons containing releasing hormones. A synaptic connection is suggested allowing for the possibility that output from the olfactory bulb may directly modulate the activity of these neurons. Telencephalic injections of BDA are used to determine the origins of olfactory bulb efferents. Neurons are retrogradely labeled by BDA in a Golgi-like manner, allowing one to visualize the morphology of olfactory bulb efferents. Three types of neurons in the bulb are filled by injections into the posterior zone of the dorsal telencephalon: mitral cells and ruffed cells in the external cellular layer, and neurons in the internal cellular layer. Although it had been known that both mitral cells and neurons in the internal cellular layer project out of the bulb in teleosts, the use of BDA allowed for an accurate description of the morphology of these extrinsically projecting neurons. This is the first evidence that the ruffed cells in salmon project out of the bulb. The results of this study suggest that the secondary olfactory projections in chinook salmon are consistent with those in other teleosts with minor variations. The data also show that olfactory bulb efferents originate not solely from mitral cells in the external cellular layer but also from ruffed cells in the external cellular layer and neurons in the internal cellular layer. It is proposed that the extrinsically projecting neurons in the internal cellular layer constitute an anterior olfactory nucleus in salmon.

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