Brain-derived neurotrophic factor and neurotrophic tyrosine receptor kinase-2 in stallion testes: insights into seasonal changes and potential roles in spermatogenesis.

Brain-derived neurotrophic factor (BDNF) and its receptor neurotrophic tyrosine receptor kinase-2 (NTRK2) have known important roles in the central nervous system for neurite growth, survival, and differentiation. Nevertheless, the significance of BDNF in spermatogenesis remains unclear in stallions. Therefore, the present study was designed 1) to investigate the expression of BDNF and its receptor NTRK2 and 2) the seasonal variation in the expression patterns of BDNF and NTRK2 in stallions' testes. We used testes from eight postpubertal Thoroughbred stallions collected after a field castration during two different seasons of the year (breeding season [BS] and nonbreeding season [NBS]). Reverse transcription-quantitative polymerase chain reaction (RT-qPCR), Western blotting (WB), and immunofluorescence were performed. RT-qPCR results showed upregulation of mRNA levels of BDNF and NTRK2 in the testes collected during the NBS. The quantification of the protein bands obtained after WB displayed significantly higher relative intensity in NBS. The immunofluorescence assay identified the localization of BDNF in the cytoplasm of Sertoli and Leydig cells in BS. The cytoplasm of germs cells and Leydig cells were stained with BDNF in NBS. NTRK2 was observed in the cytoplasm of Leydig cells of BS and NBS. Moreover, different stages of germ cells including undifferentiated spermatogonia and spermatocytes were immune labeled with NTRK2 in the NBS. These findings provided the first evidence of the localization of BDNF and NTRK2 in the testicular cells of stallions, suggesting the potential role of BDNF signaling in testes development and spermatogenesis. Further investigation is necessary to explore the functional implications of BDNF signaling on spermatogenesis, focusing on the regulatory mechanisms that govern the seasonal expression patterns observed. This will help confirm the paracrine/autocrine importance of this neurotrophin in the stallions testes.