In target tissues, leptin receptor (Ob-R) gene expression results in an array of alternatively spliced isoforms (Ob-Ra to Ob-Rf) with different functional features. Recent evidence has pointed to a direct role of leptin in the control of testicular function. However, complete elucidation of the pattern of Ob-R gene expression in the male gonad is still pending. The focus of this study was to characterize in detail the developmental pattern of expression and hormonal regulation of Ob-R gene in rat testis. To this end, the overall expression of Ob-R mRNA was compared to that of the fully functional, long Ob-Rb isoform in different experimental settings, using semiquantitative reverse transcription-polymerase chain reaction. Expression of Ob-R mRNA was detected in testes from 15-, 30-, 45-, and 75-day-old rats at rather constant relative levels. In contrast, testicular expression of Ob-Rb mRNA was higher in pubertal testes (15- to 30-day-old rats) and declined in adulthood. In testes from 30-day-old animals, analysis of isoform distribution revealed that, in addition to abundant Ob-Rb mRNA levels, expression of Ob-Ra, Ob-Rf, and, to a lesser extent, Ob-Rc and Ob-Re messages is detected. Testicular Ob-R mRNA expression appeared sensitive to neonatal imprinting as neonatal treatment with estradiol benzoate (500 microg/rat; Day 1 postpartum) resulted in a persistent increase (P: < 0.01) in the relative expression level of Ob-R mRNA, a phenomenon only partially mimicked by neonatal suppression of serum gonadotropins by means of LHRH-antagonist administration. In addition, neonatal estrogenization differentially altered the pattern of expression of Ob-R isoforms in adult rat testis, as expression of Ob-Rb mRNA was decreased to undetectable levels, whereas that of Ob-Rc remained unaltered, and Ob-Ra, Ob-Rf, and, to a lesser extent, Ob-Re mRNA levels were significantly increased (P: < 0.01) by neonatal exposure to estrogen. Finally, down-regulation of testicular Ob-R gene expression by homologous and heterologous signals was demonstrated as relative levels of Ob-R and Ob-Rb mRNAs were significantly decreased (P: < 0.01), in a coordinate manner, in rat testis after exposure to human recombinant leptin in vitro, and after stimulation with hCG and FSH in vivo. In conclusion, our results indicate that testicular Ob-R gene expression is developmentally regulated, imprinted by the neonatal endocrine milieu, and sensitive to regulation by leptin and gonadotropins. The ability of pivotal signals in testicular function to regulate Ob-R gene expression further supports the contention of a direct role of leptin in functional control of the rat testis.