Zélia Velez

Evidence for functional asymmetry in the olfactory system of the senegalese sole ( solea senegalensis )

The two olfactory epithelia of flatfish of the family Soleidae are essentially in contact with two distinct environments; the upper (right) side samples open water while the lower (left) side samples interstitial water. This study assessed whether there are differences in the responsiveness of the two epithelia by use of the electro‐olfactogram in the Senegalese sole (Solea senegalensis). The upper epithelium was significantly more responsive to the basic amino acids (l‐lysine and l‐arginine), glycine, and l‐threonine than the lower epithelium. The lower epithelium was significantly more responsive to aromatic amino acids (l‐tryptophan, l‐tyrosine, l‐DOPA, and l‐phenylalanine), l‐leucine, and l‐asparagine than the upper. Both epithelia had similar responsiveness to the sulphur‐containing amino acids (l‐cysteine and l‐methionine), l‐alanine, l‐serine, and l‐glutamine. Neither side was responsive to the acidic amino acids (l‐aspartate and l‐glutamate) or the D‐isomers of any amino acid tested. The upper olfactory organ was much more responsive to conspecific‐derived stimuli (bile and intestinal fluid) than the lower organ. We suggest that these differences in responsiveness may be related to different functional roles of the upper and lower epithelia in feeding and chemical communication.

Olfactory transduction pathways in the Senegalese sole Solea senegalensis

This study tested whether differences in sensitivity between the upper and lower olfactory epithelia of Solea senegalensis are associated with different odorant receptors and transduction pathways, using the electro-olfactogram. Receptor mechanisms were assessed by cross-adaptation with amino acids (L-cysteine, L-phenylalanine and 1-methyl-L-tryptophan) and bile acids (taurocholic acid and cholic acid). This suggested that relatively specific receptors exist for 1-methyl-L-tryptophan and L-phenylalanine (food-related odorants) in the lower epithelium, and for taurocholic acid (conspecific-derived odorant) in the upper. Inhibition by U73122 [a phospholipase C (PLC) inhibitor] suggested that olfactory responses to amino acids were mediated mostly, but not entirely, by PLC-mediated transduction (IC50 ; 15-55 nM), whereas bile acid responses were mediated by both PLC and adenylate cyclase-cyclic adenosine monophosphate (AC-cAMP) (using SQ-22536; an AC inhibitor). Simultaneous application of both drugs rarely inhibited responses completely, suggesting possible involvement of non-PLC and non-AC mediated mechanisms. For aromatic amino acids and bile acids, there were differences in the contribution of each transduction pathway (PLC, AC and non-PLC and non-AC) between the two epithelia. These results suggest that differences in sensitivity of the two epithelia are associated with differences in odorant receptors and transduction mechanisms.

Variation in Urinary Amino Acids in the Mozambique Tilapia: A Potential Signal of Dominance or Individuality?

The urine of male tilapia (Oreochromis mossambicus) contains a pheromone which is—at least in part—composed of two steroid glucuronates. However, the polar fraction of urine (not containing steroids) also contains potent odorants, the identity and relevance of which are unknown. Amino acids are polar, potent odorants for fish, and are often found at high concentrations in fish urine. We therefore measured urinary amino acid concentrations (by gas-chromatography/mass-spectrometry) and assessed their contribution to the odor of male urine (by electro-olfactogram)—do amino acids form part of the “dominance pheromone” in tilapia? Urine was taken from males of different social status (dominant and subordinate) and passed through C18 solid-phase extraction cartridges. The polar fraction (aqueous flow-through) contained highly variable levels (up to 17 mM) of l-arginine, l-glutamate, and l-phenylalanine. Urinary [l-arginine] was higher, and [l-phenylalanine] lower, in urine from dominant males than that from subordinates. Although no statistical differences were seen for urinary [l-glutamate], a weak positive correlation with the donor’s social rank was found. In addition, comparison of olfactory responses to an artificial mixture of amino acids, based on the concentrations in a pool of urine from dominant males, showed that these three constitute most—but not all—of the olfactory potency of the polar fraction. These results show that (1) urinary amino acids constitute part of the olfactory potency of male urine, (2) levels of l-arginine, l-phenylalanine and, to a lesser extent l-glutamate, depend on social status, and (3) odorants other than amino acids are present in the aqueous polar fraction. We suggest that amino acids contribute to a urinary “signature mixture”; this could be important in individual recognition during aggressive male–male interactions, wherein urine has previously been shown to play a modulatory role.

Adaptation to reduced salinity affects the olfactory sensitivity of Senegalese sole (Solea senegalensis Kaup 1858) to Ca2+ and Na+ but not amino acids

SUMMARY The Senegalese sole is a marine flatfish, which often penetrates into estuarine waters to feed. It cannot, however, survive in full freshwater. The current study investigated the effect of adaptation to low salinity (10‰) on olfactory responses to changes in environmental [Ca2+] and [Na+] and amino acids by the electro-encephalogram (EEG) recorded from the olfactory bulb. The sole showed olfactory responses to increases in environmental [Na+] and decreases in environmental [Ca2+]; sensitivity to Na+ was greater at 10‰ whereas sensitivity to Ca2+ was greater at 35‰. Decreased environmental [Na+] increased sensitivity to changes in [Ca2+] whereas increased environmental [Ca2+] decreased bulbar responses to changes in [Na+]. Sensitivity to amino acids was unaffected by external salinity. However, the absence of external Na+ strongly decreased bulbar responses to amino acids in fish adapted to 35‰ seawater but not in those at 10‰. The absence of external Ca2+ had no such effect at either salinity. This suggests that odorant-receptor binding and/or olfactory transduction is reliant on external Na+ (but not Ca2+) at higher salinities but the olfactory system is able to adapt to lower environmental [Na+]. Taken together, these results suggest that reductions of external salinity modulate olfactory sensitivity to environmental Ca2+ and Na+ but not amino acids. However, at low salinities, olfactory sensitivity to amino acids is maintained by decreasing reliance on external Na+.

Responsiveness of pituitary to galanin throughout the reproductive cycle of male European sea bass ( Dicentrarchus labrax )

The neuropeptide galanin (Gal) is a putative factor regulating puberty onset and reproduction through its actions on the pituitary. The present study investigated the pituitary responsiveness to galanin and the patterns of galanin receptors (Galrs) expression throughout the reproductive cycle of two years old male European sea bass (Dicentrarchus labrax), an important aquaculture species. Quantitative analysis of pituitary and hypothalamus transcript expression of four galr subtypes revealed differential regulation according to the testicular developmental stage, with an overall decrease in expression from the immature stage to the mid-recrudescence stage. Incubation of pituitary cells with mammalian 1-29Gal peptide induced significant changes in cAMP concentration, with sensitivities that varied according to the testicular development stages. Furthermore 1-29Gal was able to stimulate both follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) release from pituitary cell suspensions. The magnitude of the effects and effective concentrations varied according to reproductive stage, with generalized induction of Fsh and Lh release in animals sampled in January (full spermiation). The differential expression of galrs in pituitary and hypothalamus across the reproductive season, together with the differential effects of Gal on gonadotropins release in vitro strongly suggests the involvement of the galaninergic system in the regulation the hypothalamus-pituitary-gonad axis of male sea bass. This is to our knowledge the first clear evidence for the involvement of galanin in the regulation of reproduction in non-mammalian vertebrates.