In vertebrates, the pituitary gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are the main regulators of gametogenesis and gonadal hormone production. To exert their biological actions, gonadotropins must bind to, and activate specific cell-surface receptors. The primary aim of the studies underlying this thesis was to investigate the role of gonadotropins in the sea bass (Dicentrarchus labrax) reproductive function by studying their cognate receptors. Two cDNAs encoding a FSH receptor (sbsFSHR) and a LH receptor (sbsLHR) were cloned and characterized. Their gene structure, expression, and functional activity were evaluated. Sea bass FSHR and LHR mature proteins display typical features of the glycoprotein hormone receptor family members, but the sbsFSHR also contains some remarkable differences when compared with other fish or mammalian FSHRs. Among them, a distinct extracellular N-terminal cysteine domain as regards to its length and cysteine number, and the presence of an extra leucine-rich repeat. sbsLHR is more conserved than sbsFSHR when comparing their genomic primary structures with other vertebrate gonadotropin receptors, particularly in the extracellular domain. Furthermore, two alternately spliced variants of the sbsFSHR were amplified. Both correspond to transcripts originated by removal of exons from the extracellular domain. Expression analysis revealed that the sbsFSHR is exclusively expressed in gonads, specifically in the wall of previtelogenic and early-vitelogenic follicles. On the contrary, sbsLHR mRNA was found to be widely distributed in somatic tissues. Recombinant sbsFSHR was specifically stimulated by bovine FSH, while sbsLHR was activated by both bovine LH and FSH. Nevertheless, specific stimulation of both sea bass FSHR and LHR was observed when recombinant sea bass gonadotropins were used advancing specific interactions. This behaviour differs from studies in other teleost where promiscuous activation was reported. The expression profiles of sea bass gonadotropin receptors were investigated in the gonads of both sexes during a complete reproductive cycle, in parallel with the expression of steroidogenic acute regulatory protein (StAR) gene and reproductive hormones. While in male sea bass, both gonadotropin receptors show different expression patterns, in females their expression patterns were somehow similar. Expression of the sbsFSHR was connected with early stages of gonadal development, but also with the spermiation period in males and the maturation-ovulation period in female. The expression profile of the sbsLHR, in both sexes, supports the already suggested involvement of LH in the regulation of the final stages of fish gamete maturation and ovulation/spermiation. sbsStAR expression was strongly correlated with sbsLHR expression. In addition, the expression profile obtained for sbsStAR suggests that this protein mediates the rapid delivery of sterol substrate for the synthesis of gonadal maturation inducing steroids. No strong relationship between gonadotropin receptor expression and plasma levels of the analysed sex steroids was seen. Further studies will be needed to understand how gonadotropins and sex steroids interact to regulate sea bass reproduction. Recent studies reported abundant expression of TSHR transcripts in both the ovary and testis of some fish species. To help elucidate the physiological role that TSHR may have in the gonadal function of sea bass, a cDNA encoding a TSHR was isolated from the gonads of this fish species. The mature protein displays typical features of the members of the glycoprotein hormone receptor family. By RT-PCR analysis we demonstrate the extra-thyroidal expression of sbsTSHR in numerous tissues of the sea bass. Seasonal changes in sbsTSHR mRNA levels in female and male sea bass during puberty suggest that in females the TSHR could participate in active vitellogenesis and in the regulation of gamete maturation and ovulation, whereas in males, the TSHR would be involved in the regulation of early stages of the gonadal development and also of gamete maturation and spermiation.RESUMEN En los vertebrados, las gonadotrofinas (FSH y LH) son hormonas esenciales en el control de la gametogénesis y esteroidogénesis gonadal. Sus acciones están mediadas por la activación de sus respectivos receptores. A pesar de su importancia en la función reproductora, en teleósteos se dispone de muy poca información relativa a estos receptores. En este trabajo se investigaron las características moleculares, estructurales y funcionales de los receptores de gonadotrofinas (FSHR y LHR) de un teleósteo marino, la lubina (Dicentrarchus labrax). Además, se evaluó la especificidad de unión al ligando de estos receptores. Usando un método de RT-PCR cuantitativo, se midieron los perfiles de expresión de los receptores de gonadotrofinas durante la primera maduración gonadal de machos y hembras de lubina. Asimismo, se investigaron posibles relaciones entre estos perfiles, los de la proteína responsable de la respuesta aguda de la esteroidogénesis (StAR) y los perfiles plasmáticos de esteroides sexuales. Estudios recientes realizados en diversas especies de peces sugieren que el receptor de TSH (TSHR), que comparte características estructurales con los receptores de gonadotrofinas, podría estar directamente implicado en la fisiología gonadal. Por ese motivo, en este trabajo se investigó una posible participación del TSHR en la función reproductora de la lubina. Los resultados obtenidos contribuyen a entender en que momento del ciclo reproductor las células germinales en crecimiento son sensibles a las señales procedentes de la hipófisis (gonadotrofinas). Además, se ha demostrado que, a diferencia de otros peces, las interacciones entre los receptores de gonadotrofinas de lubina y sus respectivos ligandos son específicas. Asimismo, podemos decir que en la lubina, los receptores de gonadotrofinas se expresan en momentos diferentes del ciclo reproductor. Esto apoya la idea de que la FSH y la LH poseen diferentes acciones en el control de la función gonadal. Desde de un punto de vista aplicado, estos conocimientos podrían ser de gran importancia en la manipulación del ciclo reproductor de esta especie de gran valor económico. Por lo tanto, estos estudios son relevantes tanto por su componente de investigación básica como por las posibles aplicaciones que pueden derivarse. __________________________________________________________________________________________________