Ultramicroscopic organization of the exterior olfactory organ in Anguilla vulgaris in relation to its spawning migration

Background

Catadromous fishes have well-developed elongated olfactory organs with numerous lamellae and different types of receptor neurons related to their breeding migration.

Aim

The current study showed how the olfactory system adapted to the catadromous life. Our work declared the need of the migratory fishes for the sense of smell that is exhibited by a higher number of the olfactory lamellae and the receptor neuron verification in the olfactory epithelium.

Methods

Ten specimens of fully grown, but pre-matured, silver eels of Anguilla vulgaris were captured at the outlet of Edco Lake, overlooking the Mediterranean Sea, east of Alexandria. Olfactory rosettes were dissected and fixed for scanning electron microscope (SEM) and transmission electron microscope (TEM).

Results

Our study gave a morphological description of the olfactory system of A. vulgaris. At the ultrastructural level using SEM and TEM, one olfactory rosette was provided with 90-100 flat radial olfactory lamellae. The nasal configuration allowed water to enter and exit, transferring odorant molecules to olfactory receptor cells which comprise long cylindrical ciliated and microvillous receptors as well as rod-tipped cells. These cells are bipolar neurons with upward dendritic knobs. The olfactory epithelia also include crypt receptor cells. Interestingly, the olfactory neurons are delimited by nonsensory supporting cells, including long motile kinocilia and sustentacular supporting cells beside mucus secretory goblet cells and ionocytes or labyrinth cells that contribute to the olfaction process.

Conclusion

Olfaction is crucial in all vertebrates, including fishes as it involves reproduction, parental, feeding, defensive, schooling, and migration behaviors. Here, A. vulgaris is an excellent model for catadromous fishes. It has a well-developed olfactory organ to cope with the dramatic climate change, habitat loss, water pollution, and altered ocean currents effect during their catadromous life for reproduction.

Androgen induces olfactory expression of prostaglandin E2 receptor Ep1 in the burrow-living fish Bostrychus sinensis

It is well documented that androgens modify olfactory processing in vertebrates. In fish, several lines of evidence indicate that androgens increase olfactory sensitivity to prostaglandin pheromone, but the molecular mechanism is still unclear. Our previous studies showed that prostaglandin E₂ (PGE₂) is a sex pheromone in the burrowing-living fish Chinese black sleeper (Bostrychus sinensis) and that the PGE₂ receptor 1 (Ep1) in the olfactory rosette is a candidate receptor for sensing sex pheromone PGE₂. In the present study, we found that testosterone (T) and 11-ketotestosterone (11-KT) exhibited stimulatory effects on the expression of ep1 in the olfactory rosette in vivo and ex vivo. Moreover, the androgen receptor (Ar) agonist R1881 had similar effects to 11-KT on the expression of ep1 ex vivo, suggesting the up-regulatory effect is mediated by Ar. The amount of arα transcripts (˜1500 copies/100 ng total RNA) was greater than that of arβ (˜300 copies/100 ng total RNA) in the olfactory rosette, and the expression levels of arα increased with spermatogenesis and peaked at late meiosis stage. Moreover, activated Arα but not Arβ transactivated a 2k bp ep1 promoter in HEK293T cell, and some OSNs exhibited co-localization of arα mRNA and Ep1 protein signals. Taken together, our results suggest that Arα, but not Arβ, plays a crucial role in mediating the androgen-induced up-regulation of ep1 expression in B. sinensis. The present study is the first to shed light on the molecular mechanisms whereby androgens enhance responsiveness to prostaglandin sex pheromones in teleosts.

Cloning and olfactory expression of progestin receptors in the Chinese black sleeper Bostrichthys sinensis

Our previous studies suggested that 17α,20β-dihydroxy-4-pregnen-3-one (DHP), an oocyte maturation inducing progestin, also acts as a sex pheromone in Chinese black sleeper Bostrichthys sinensis, a fish species that inhabits intertidal zones and mates and spawns inside a muddy burrow. The electro-olfactogram response to DHP increased during the breeding season. In the present study, we cloned the cDNAs of the nine progestin receptors (pgr, paqr5, 6, 7(a, b), 8, 9, pgrmc1, 2) from B. sinensis, analyzed their tissue distribution, and determined the expression in the olfactory rosette during the reproductive cycle in female and male fish. The deduced amino acid sequences of the nine progestin receptors share high sequence identities with those of other fish species and relatively lower homology with their mammalian counterparts, and phylogenetic analyses classified the nine B. sinensis progestin receptors into their respective progestin receptor groups. Tissue distribution of B. sinensis progestin receptors showed differential expression patterns, but all these nine genes were expressed in the olfactory rosette. Interestingly, paqr5 mRNA was found in the intermediate and basal parts of the olfactory epithelium but not in the central core using in situ hybridization, and its expression level was the highest in the olfactory rosette among the tissues examined. These results suggested Paqr5 may have an important role for transmitting progestin signaling in the olfactory system. The expression levels of paqr7a and paqr7b, pgr and pgrmc2 mRNA peaked around the mid meiotic stage, and that of paqr8 peaked at late meiotic stage in the olfactory rosette in males, while the olfactory expression of paqr5 decreased gradually as spermatogenesis progressed. In contrast, the expression of the progestin receptors did not change significantly during the development of the ovary in the olfactory rosette in females, except that of pgr. Interestingly, the changes of paqr8 expression in the olfactory rosette in males mirrored the changes of plasma DHP levels in females during the reproductive cycle, suggesting the Paqr8 may also be important for deciphering progestin signaling released by female. To our knowledge, this is the first time to demonstrate the presence of all known progestin receptors in a teleost olfactory rosette, and to show different expressions between the males and females during the reproductive cycle. This study provides the first evidence on changes of all purported progestin receptors during a reproductive cycle in teleost olfactory rosette, and suggests that distinct olfactory sensitivities to DHP may be due to the changes and compositions of each progestin receptor in B. sinensis.