Several studies have shown that dopamine and other catecholamines are present in oviduct luminal fluid. We recently reported that dopamine type 2 receptors (DRD2) are present in a wide range of mammalian sperm, suggesting a role for dopaminergic signaling in events such as fertilization, capacitation, and sperm motility. In the present study, we used Western blot analysis to show that boar sperm express DRD2 and that their activation with dopamine (100 nM) has a positive effect on cell viability that can be correlated with AKT/PKB phosphorylation. Bromocriptine (100 nM) and dopamine (100 nM and 10 μM) increased tyrosine phosphorylation during the capacitation period. Immunofluorescence analysis indicated that DRD2 localization is dynamic and depends on the capacitation stage, colocalizing with tyrosine phosphorylated proteins in the acrosome and midpiece region of capacitated boar sperm. This association was confirmed by coimmunoprecipitation analysis. We also showed that bromocriptine (100 nM) and low-concentration dopamine (100 nM and 10 μM) increased total and progressive motility of sperm. However, high concentrations of dopamine (1 mM) decreased tyrosine phosphorylation and motility in in vitro sperm capacitation assays. This can be explained by the presence of the dopamine transporters (DAT, official symbol SLC6A3) in sperm, as demonstrated by Western blot analysis and immunocytochemistry. Taken together, our results support the idea that dopamine may have a fundamental role during sperm capacitation and motility in situ in the female upper reproductive tract.

Dopamine is a recognized modulator in the central nervous system (CNS) and peripheral organ functions. The presence of peripheral dopamine receptors outside the CNS has suggested an intriguing interaction between the nervous system and other functional systems, such as the reproductive system. In the present study we analyzed the expression of D2R receptors in rat testis, rat spermatogenic cells and spermatozoa, in different mammals. The RT‐PCR analysis of rat testis mRNA showed specific bands corresponding to the two dopamine receptor D2R (L and S) isoforms previously described in the brain. Using Western blot analysis, we confirmed that the protein is present in rat testis, isolated spermatogenic cells and also in spermatozoa of a range of different mammals, such as rat, mouse, bull, and human. The immunohistochemistry analysis of rat adult testis showed that the receptor was expressed in all germ cells (pre‐ and post‐meiotic phase) of the tubule with staining predominant in spermatogonia. Confocal analysis by indirect immunofluorescence revealed that in non‐capacitated spermatozoa of rat, mouse, bull, and human, D2R is mainly localized in the flagellum, and is also observed in the acrosomal region of the sperm head (except in human spermatozoa). Our findings demonstrate that the two D2 receptor isoforms are expressed in rat testis and that the receptor protein is present in different mammalian spermatozoa. The presence of D2R receptors in male germ cells implies new and unsuspected roles for dopamine signaling in testicular and sperm physiology. J. Cell. Biochem. 100: 141–150, 2007.

It has been demonstrated that glutamatergic activity induces ascorbic acid (AA) depletion in astrocytes. Additionally, different data indicate that AA may inhibit glucose accumulation in primary cultures of rat hippocampal neurons. Thus, our hypothesis postulates that AA released from the astrocytes during glutamatergic synaptic activity may inhibit glucose uptake by neurons. We observed that cultured neurons express the sodium‐vitamin C cotransporter 2 and the facilitative glucose transporters (GLUT) 1 and 3, however, in hippocampal brain slices GLUT3 was the main transporter detected. Functional activity of GLUTs was confirmed by means of kinetic analysis using 2‐deoxy‐d‐glucose. Therefore, we showed that AA, once accumulated inside the cell, inhibits glucose transport in both cortical and hippocampal neurons in culture. Additionally, we showed that astrocytes are not affected by AA. Using hippocampal slices, we observed that upon blockade of monocarboxylate utilization by α‐cyano‐4‐hydroxycinnamate and after glucose deprivation, glucose could rescue neuronal response to electrical stimulation only if AA uptake is prevented. Finally, using a transwell system of separated neuronal and astrocytic cultures, we observed that glutamate can reduce glucose transport in neurons only in presence of AA‐loaded astrocytes, suggesting the essential role of astrocyte‐released AA in this effect.