We recently identified a cDNA encoding three novel fish hypothalamic neuropeptides, having LPXRF-NH 2 from the goldfish brain. In this study, to clarify the physiological functions of these three LPXRFamide peptides (gfLPXRFa-1, -2, and -3), we analysed the localisation and hypophysiotrophic activity of these peptides using sockeye salmon, Oncorhynchus nerka, in which immunoassay systems for several anterior pituitary hormones have been developed. gfLPXRFa-immunoreactive cell bodies were detected in the nucleus posterioris periventricularis of the hypothalamus and immunoreactive fibres were distributed in various brain regions and the pituitary.We also detected gfLPXRFa-immunoreactivity in the pituitary by competitive enzyme-linked immunosorbent assay combined with reversed-phase HPLC. These three gfLPXRFamide peptides stimulated the release of FSH, LH and GH, but did not affect the release of prolactin (PRL) and somatolactin (SL) from cultured pituitary cells. These results suggest that novel fish hypothalamic LPXRFamide peptides exist in the brain and pituitary of sockeye salmon and stimulate the release of gonadotrophins and GH from the pituitary.

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To clarify the possible function of gonadotropin-releasing hormone (GnRH) in the brain of a pleuronectiform fish, the barfin flounder Verasper moseri, the distribution of three forms of GnRH in various areas of the brain was examined by radioimmunoassay, and the localization of GnRH-immunoreactive (ir) cell bodies and fibers in the brain and pituitary was determined by immunocytochemistry. The dominant form in the pituitary was seabream GnRH (sbGnRH), levels of which were much higher than those of salmon GnRH (sGnRH) and chicken GnRH-II (cGnRH-II). In contrast, sbGnRH levels were extremely low in all other brain areas examined. Levels of sGnRH and cGnRH-II were high in the anterior and posterior part of the brain, respectively. sbGnRH-ir cell bodies were located in the preoptic area, whereas sbGnRH-ir fibers were localized mainly in the preoptic area-hypothalamus-pituitary and formed a distinctive bundle of axons projecting to the pituitary. sGnRH-ir cell bodies were located in the ventromedial part of the rostral olfactory bulbs and in the terminal nerve ganglion (the transitional area between the olfactory bulb and the telencephalon). cGnRH-II-ir cell bodies were localized to the midbrain tegmentum. sGnRH-ir and cGnRH-II-ir fibers were observed throughout the brain except in the pituitary gland. These results indicate that sbGnRH is responsible for the neural control of the reproductive endocrinology of the barfin flounder (hypothalamo-hypophysial system), and that sGnRH and cGnRH-II function as neurotransmitters or neuromodulators in the brain.

The gustatory responses to tetrodotoxin (TTX) and saxitoxin (STX) recorded from the palatine nerve (VIIth cranial nerve) were studied electrophysiological in rainbow trout (Salmo gairdneri) and Arctic char (Salvelinus alpinus). Both toxins were highly effective gustatory stimuli in both species, in rainbow trout, TTX had a threshold concentration 2 × 10−7 mol/L and at 10−5 mol/L evoked a response four times that of 10−3 mol L-proline/L, the most potent amino acid for this species. The threshold for STX was Sower (10−8 mol/L), but unlike TTX the response magnitude reached a maximum at 10−6 mol/L. The reverse occurred in Arctic char; lower threshold for TTX (10−8 mol/L) than STX (10−7 mol/L) and the response magnitude never exceeded that of 10−3 mol L-proline/L. Cross-adaptation experiments indicated that the receptor(s) for TTX are distinct from those which detect amino acids and bile salts and that TTX and STX do not share the same receptor populations. Furthermore, the integrated response to TTX or STX was a fast-adapting, phasic response and rapidly returned to baseline even with continued stimulation. Perfusion of the gustatory organs with these toxins had little toxic effect. The sensitive, specific gustatory receptor system for the toxins suggests the existence of a mechanism for avoiding poisonous prey organisms that has adaptive advantage to the receiver (predator).