dl-Aspartic acid administration improves semen quality in rabbit bucks

https://doi.org/10.1016/j.anireprosci.2009.07.009Get rights and content

Abstract

Recently, d-aspartic acid (d-Asp) has been suggested as being involved in mechanisms regulating reproduction activity in animals and human. In this study we analyzed the effects of dl-Asp oral administration on sperm production in the rabbit. Bucks from 60, bred in a genetic centre and used for semen production, were divided in 2 subgroups of 6 individuals. The treated group was fed with a concentrate containing dl-Asp which assured a daily administration of 1.3 g dl-Asp/head; the control group was fed with the same concentrate without dl-Asp. The treatment was carried out for 2 wk and animals were monitored weekly, from 1 wk before the start of the treatment to 3 wk after the end of the treatment. Through the experimental period there were no significant variations in semen volume between the two groups. A significant increase in both sperm concentration and kinetic parameters, i.e., the overall percentage of motile spermatozoa, the average path velocity, the percentage of progressively motile spermatozoa, etc., was found in the supplemented group. l-Asp values in blood serum and seminal plasma did not vary through the experimental period. d-Asp concentration in blood serum increased more than 4-fold than baseline (P < 0.01) at the end of the treatment and was maintained at higher than baseline values for up to 3 wk after the end of the treatment. d-Asp concentration in seminal plasma was higher than in blood serum before the start of the treatment (13.7 ± 1.6 nM vs 3.5 ± 3.3 nM; P < 0.01) which suggests an elective storage of d-Asp in the male genital tract. Baseline values of d-Asp concentration in seminal plasma significantly increased following treatment and were back to initial values 1 wk after the end of the treatment. In conclusion, dl-Asp administration improved sperm quality in bucks and the high d-Asp content in seminal plasma suggests a primary role for this d-amino acid in regulatory mechanisms of reproductive activity.

Introduction

d-Amino acids represent a well-documented component in the body of living organisms (for review see D’Aniello, 2007) as either free forms (Imai et al., 1997) or included into peptidic chains (Shell et al., 1997, Walosker et al., 2000). Direct effects of d-amino acids on the natural configuration and functionality of proteins were demonstrated in several neural pathologies, such as Alzheimer (Man et al., 1983, Fisher et al., 1991, Fisher et al., 1998) or in ocular diseases, such as cataracts (Garner and Spector, 1978).

d-Aspartic acid (d-Asp) is an endogenous amino acid, which has been found in nervous and endocrine tissues of both invertebrates and vertebrates (D’Aniello et al., 1998a, D’Aniello et al., 1998b). In vertebrates, d-Asp has been isolated in the chicken and rat brain (Dunlop et al., 1986, Hashimoto et al., 1993). In human, d-Asp has been found in both foetal and adult brains as well as in the cerebrospinal fluid of adult individuals (Fisher et al., 1991, Fisher et al., 1994, Fisher et al., 1998, Hamase et al., 1997). In the rat, high d-Asp concentrations have been recorded in testicles at birth as well as following sexual maturity (D’Aniello et al., 1996). Further studies (D’Aniello et al., 1998a, D’Aniello et al., 1998b) have shown highest concentrations of d-Asp in testicular venous blood plasma, whereas a decreasing pattern of d-Asp storage has been revealed in rete testis, epididimus, testicular parenchymal cells (which would comprise mostly spermatids and spermatocytes), luminal fluid from the seminiferous tubules and interstitial extracellular fluid, respectively. These authors suggested that d-Asp was secreted by the testis mostly into the venous blood, passing thence into the rete testis fluid and was incorporated into the spermatozoa at the time or after they left the testis. A specific d-Asp localization was further observed in rat testis either in elongate spermatidits (Sakai et al., 1998) or in Leydig and Sertoli cells (Nagata et al., 1999). A role of d-Asp in the synthesis of sexual hormones was hypothesized on the basis of studies in Rana esculenta where d-Asp affects testosterone production during sexual activity (Di Fiore et al., 1998). Moreover, it has been recently demonstrated that N-methyl-d-aspartic acid (NMDA) is endogenously biosynthesized from d-Asp by an S-adenosylmethionine-dependent enzyme, NMDA synthase (D’Aniello et al., 2000). NMDA is a powerful agonist of neuroexcitatory activity of aspartic and glutamic acids and has neuromodulatory activity eliciting hormone release from the pituitary both in vivo (Arslan et al., 1982, D’Aniello et al., 2000) and in vitro (Barb et al., 1993). In the sheep, d-Asp is endogenously present in tissues and electively stored in endocrine glands, such as the pituitary, and in the brain; NMDA and LH increased following its administration (Boni et al., 2006).

D-Asp seems to play a crucial role in reproduction either due to its suggested role as neuromodulator with a special store in the pituitary gland or because it is involved in the biosynthesis and release of sexual hormones. Recent studies analyzed the role of d-Asp in human reproduction in both female and male genders and in men, a lower d-Asp content was found in oligo-astheno-teratospermic donors (D’Aniello et al., 2005). In women, d-Asp content in follicular fluid decreased with age and the event is related to decrease of oocyte quality and fertilization competence (D’Aniello et al., 2007).

Until now, the effect of d-Asp on reproductive biology has been analyzed in mammalian models of mouse and rat where it is not feasible to monitor biological patterns as sperm production within each individual through time. Rabbit represents a good solution for this problem since (i) it is an efficient experimental model for repeated semen collections, (ii) genetic centres are established and organized platforms to perform such experiment with consistent results, (iii) allows low experimental costs, and (iv) does not merely represent an experimental model but has an economic significance to livestock species.

This study aims to evaluate the effect of dl-Asp administration on semen production and quality in rabbit bucks. In addition, the concentration of this amino acid was analyzed in blood serum and seminal plasma in order to monitor its dynamics following oral administration of sodium dl-aspartate.

Section snippets

Animals

Twelve sexually mature New Zealand rabbit males (1.8 ± 0.2 yr) weighing 2.15 ± 0.32 kg were used. They were individually housed indoors in the Genetic Centre of Potenza Breeders’ Association in Laurenzana (Potenza, Italy) under natural photoperiod (temperature range, 15–25 °C) with free access to food and water. Animals were fed ad libitum with commercial rabbit pellets (Petrini, Bastia Umbra, PG, Italy) (14.5% crude protein, 2.6% fat, 15% crude fiber, as-fed). Food consumption was measured weekly

Statistical analysis

Values are presented as means ± standard deviation (S.D.). Data were analyzed by analysis of variance (ANOVA, Systat 11.0) for repeated measurements after arcsine transformation in the case of percentages. Pairwise comparison of the means was made with Fisher's LSD test.

Results

Fig. 1 shows mean (±S.D.) variations of ejaculate volume, sperm concentration and sperm kinematic activity through the experimental period. The volume of ejaculates did not show significant differences among weeks and varied from 0.80 ± 0.12 to 0.97 ± 0.17 and from 0.86 ± 0.17 to 0.91 ± 0.17 mL in treated and control groups, respectively. Overall, sperm concentration was significantly (P < 0.05) affected by the treatment. This effect was, however, not clear since the treated group showed a higher

Discussion

This study clearly demonstrates the effect of dl-aspartic acid treatment on sperm quality in rabbit. The volume of semen did not seem to be affected by this amino acid; sperm concentration showed a large variability suggesting its low relevance for the evaluation of the experimental results. In humans, the concentration of d-Asp dramatically decreases either in the seminal plasma or in the spermatozoa of infertile donors (D’Aniello et al., 2005). These authors, however, did not demonstrate the

Acknowledgments

We thank Dr. Antimo D’Aniello for his helpful comments and Dr. Adrianna Ianora for her critical revision of the manuscript.

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