Distribution and quantification of corticotropin‐releasing hormone (CRH) in the brain of the teleost fish Oreochromis mossambicus (tilapia)

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Abstract
The recent characterization of the corticotropin‐releasing hormone (CRH) prehormone of the fish tilapia (Oreochromis mossambicus) showed that more variation exists between vertebrate CRH amino acid sequences than recognized before. The present study investigates whether the deviating composition of tilapia CRH coincides with an atypical distribution of CRH in the brain. For this purpose we applied immunohistochemistry, as well as radioimmunoassay (RIA) quantification in brain slices. The results are plotted in a new atlas and reconstruction of the tilapia brain. The largest population of CRH‐immunoreactive (ir) neurons is present in the lateral part of the ventral telencephalon (Vl). Approximately tenfold less CRH‐ir neurons are observed in the preoptic and tuberal region. The CRH‐ir neurons observed in the preoptic region are parvocellular and do not, or hardly, display arginine‐vasotocin (AVT) immunoreactivity. CRH‐ir neurons are also present in the glomerular layer of the olfactory bulb, in the periventricular layer of the optic tectum, and caudal to the glomerular nucleus. A very dense plexus of CRH‐ir terminals is located in the most rostral part of the dorsal telencephalon. This region has not been described in other teleosts and is in the present study subdivided into the anterior part of the dorsal telencephalon (Da) and the anterior part of the laterodorsal telencephalon (Dla). High densities of CRH‐ir terminals were observed in and around Vl, in the tuberal region, around the rostral part of the lateral recess, and in the caudal part of the vagal lobe. In the pituitary, CRH‐ir terminals are concentrated in the neuro‐intermediate lobe. Overall, the immunohistochemical and quantitative data correlated well, as the RIA CRH profile in serial 160‐μm slices revealed four peaks, which corresponded with major ir‐cell groups and terminal fields. Our results strongly suggest that the CRH‐ir cells of Vl project to the rostro‐dorsal telencephalon. Consequently, they may not be primarily involved in regulation of pituitary cell types but may subserve other functions. The presence of a CRH‐containing Vl‐Da/Dla projection seems to be restricted to the most modern group of teleosts, i.e., the Acanthopterygians. Further anatomic indications for non‐pituitary‐related functions of CRH are found in the vagal lobe and the optic tectum of tilapia. Although the low CRH content of the preoptic region reported here for tilapia may be typical for unstressed fish, the fact remains that remarkably few CRH‐ir neurons are involved in regulating the pituitary. Overall, the CRH distribution in the brain of tilapia is more widespread than previously reported for other teleosts. J. Comp. Neurol. 453:247–268, 2002.

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