Sexual function is a complex multidimensional biological process regulating the central dogma of libido and stimulation . It also controls the primary mechanism for the generation of penile tumescence, sexual arousal, rigidity, orgasm, and ejaculation. In the present study, patients do not differ significantly from the controls regarding the number of (CAG)n or (GGC)n repeats, but those patients with longer trinucleotide repeats have higher testosterone and lower oxytocin levels than other patients and controls. According to the correlation studies, patients with longer trinucleotide repeats have lower IELT and higher PEDT scores than the other patients (i.e. more severe premature ejaculation) and lower IIEF-15 scores i.e. they are more prone to erectile dysfunction.
While, TSH and PRL levels appear to be lower in patients with premature ejaculation and diabetes mellitus than in controls.
Additionally, it is validated that endocrine milieu influences the process of PE and has its effects on the ability to achieve an erection by regulating the pathway of L- arginine-nitric oxide-guanylyl cyclase-cyclic guanosine monophosphate (cGMP) mediated smooth muscle relaxation [27, 28].
This study tends to confirm that long (CAG)n repeats in the AR compromise several androgen -dependent functions, especially erectile function. Testosterone is one of the key players in the sexual function by influencing the sexual desire of men through central and peripheral nervous system. The lower levels of serum testosterone (TT) are directly linked to obesity, metabolic syndrome and type 2 diabetes, whereas high levels are in turn associated with PE .
In our study, the patients with long (CAG)n repeats were found to have lower IIEF- 15 scores than the controls though they had higher testosterone levels. Moreover, an inverse correlation was observed in the patients’ group between (CAG)n repeats length and IIEF-15 score. Thus, in patients with high testosterone levels, long (CAG)n repeats impair the effect of testosterone on erectile function. This confirms the results of the study by Liu CC et al. (who employed the IIEF 5 instead of IIEF-15) who reported that long (CAG)n repeats are an independent risk factor for erectile dysfunction in subjects with testosterone levels above 3.3 ng/mL, but not in those with testosterone levels of 3.3 ng/mL or below . In the PE patients the length of (CAG)n repeats was inversely related to self-estimated IELT and positively related to PEDT, i.e. a greater length was associated with more severe PE. The length of (CAG)n repeats was also positively related to testosterone levels and negatively related to oxytocin levels.
Regarding the PE patients with long (CAG)n repeats, they were found to have higher testosterone levels than the other PE patients and the controls. Testosterone is known to be able to suppress serotonin, a neuromodulator that prevents ejaculations, so that increased testosterone levels could lead to secondary PE. Serotonin is secreted by the brain stem, hypothalamus, and spinal cord and exerts an inhibitory influence on ejaculation. In the patients with long (CAG)n repeats, the high testosterone levels that are found could overtake the lowering effect of (CAG)n repeats length regarding testosterone action on serotonin, and partly explain why the patients with long CAG repeats suffer from PE as well as those patients with normal or short (CAG)n repeats. . Any drug like SSRI, (Selective serotonin reuptake inhibitors) blocks the serotonin reuptake by central neurons successively increased the accumulation of serotonin, which in turn prevents ejaculation and sustained IELT for a bit longer spell .
Regarding the PE patients with short (CAG)n repeats, they were found to have higher oxytocin levels than the other subgroups of patients and controls. Such higher oxytocin levels could account for the occurrence of PE in these patients with short (CAG)n repeats. Oxytocin is a neuromodulator and is involved in the modulation of the symptoms of severe depression. Oxytocin levels may increase during sexual intercourse and facilitate the ejaculation by reducing the ejaculatory latency time . The study performed with mice provided the first empirical evidence that high levels of oxytocin can induce a potent effect on ejaculation by decreasing the IELT and post-copulatory refractory period. Alternatively, rodents with low serum oxytocin levels had prolonged mount and longer refractory spell . Contrarily, in another study, conducted on a small group of active healthy men, the administration of exogenous oxytocin through nasal spray had no substantial outcome on IELT and orgasm .
Thus, this study highlights the fact that androgen receptor polymorphism plays a role in PE and that this role is strongly dependent on hormonal cofactors, especially testosterone and oxytocin: patients with long (CAG)n repeats have higher TT levels that can lead to PE, whereas patients with short (CAG)n repeats may have a higher TT effect (because of short CAG repeats) and higher oxytocin levels, thereby also contributing to occurrence of PE.
The finding of high oxytocin levels in PE + DM patients with short (CAG)n repeats lengths has not yet been reported and remains to be further confirmed in a large cohort study. There is no clear explanation for such a result. However, it is clearly known that testosterone and oxytocin have opposite effects in neuropsychiatric disorders, cognitive and behavioral functions that might be based upon a direct inhibition of AR on oxytocin transcription . A recent study has proposed a clear opposing effect of testosterone and oxytocin in the modulation of psychiatric disorders such as bipolar disorder and major depressive disorder in diabetic patients [36, 37]. Short (CAG)n repeats would have been expected to increase the effects of testosterone and to decrease oxytocin. The inhibiting effect of testosterone on oxytocin seems to be impaired in these patients with short (CAG)n repeats in comparison with controls. It has also been reported that testosterone could exert an indirect role through estradiol, which is a metabolite of testosterone that could stimulate oxytocin release . However, the definite explanation can be given for this finding of high oxytocin levels in patients with short (CAG)n repeats by conducting more robust future studies.
The role of (GGC)n repeats length appears to be a minor one in comparison with that of (CAG)n repeats length. However, there is an inverse significant relationship between the length of (GGC)n repeats and self-estimated IELT and a positive significant relationship with PEDT, which means that longer (GGC)n repeats have an impact on severe PE.
Furthermore, trinucleotide repeats (TNR) polymorphism findings on depression analysis (BDI-II score), demonstrating that all subgroups (PE + DM) had significantly higher state depression than controls, are in line with previous research stated that both PE and ED were associated with severe free-floating anxiety in DM patients [38, 39]. Other published studies suggested that organic and neurobiological factors implicated more than psychological factors in the development of acquired PE [9, 40]. Rastrelli G et al. stated that hypoprolactinemia has been associated with erectile dysfunction and premature ejaculation, findings further confirmed in the general European population and infertile men. They thought that low PRL might be a mirror of an increased dopaminergic or a decreased serotoninergic tone . Corona G et al. showed that PRL (as well as TSH) levels progressively increased from patients with severe PE towards those with an-ejaculation (and that the opposite was observed for testosterone levels). Interestingly, Corona G. et al. included in their discussion the following statements: ‘The speculation that low basal PRL could mirror an impaired serotoninergic pathway is based also on the observation that… hypoprolactinemia was associated with psychobiological features, often considered associated with a low serotoninergic signaling. These include anxiety symptoms and a major propensity towards metabolic syndrome’ . This appears to be in accordance with the finding in the present study of low PRL levels in patients with PE and type 2 diabetes. This low serotoninergic tone is likely to play a role in PE in such patients. Moreover, Corona et al.  give also interesting information regarding TSH and thyroid hormones in PE. Interestingly, the present study also found low TSH levels in PE patients even though we had excluded patients with abnormal TSH levels, unlike Corona’s study (in Corona’s study, 33 /2652 patients suffered from hyperthyroidism).
This lowering effect on TSH levels might be related to the frequent use of metformin that is widely used the drug as the first line of oral hypoglycemic agent for treatment of type 2 diabetes mellitus . Other studies also proposed that metformin has a notable lowering effect on TSH through modification of thyroid hormone receptor binding affinity and by inhibition of AMPK (adenosine 5′ - monophosphate -activated kinase) signaling cascade .
The low levels of TSH lead to boost the frequency of contraction in seminal vesicle as well as bulbospongiosus muscles located in the middle of the perineum and are likely responsible for ejaculation with minimal sexual stimulation . Whereas, recovery from low levels of thyreotropin prolonged the emission and expulsion episodes in PE subjects [46, 47]. Consistent with the same scenario another prospective study showed that 50% hyperthyroid cohort had PE and this condition became reversed after therapy, with twofold increased intravaginal ejaculatory latency time . Contrarily, a study on a small group of Dutch patients did not reflect any association between TSH and PE .
Typically, a multivariate analysis is the crucial stepping stone towards depicting a scenario between the parameters which are often evident in the classic to the parametric system. For instance, Cluster analysis (CA) that separates all the 16 parameters into three clusters and reveals a heterogeneity among observed parameters, which are assorted into varied clusters. Furthermore, CA also shows a close link among nearby, associated variables. In this framework, the first cluster assorted (CAG)n, TT, IELT, PEDT, prolactin, Oxytocin, and TSH from the rest of parameters. This is in agreement with contemporary research evidence, has been described by Humble MB et al., that neurohypophyseal hormones are directly involved in the orgasm function, penile erection, and social bonding. They also facilitate the ejaculation which makes them potential candidates as targets for drug therapy in PE subjects .
The novel finding of this work is reporting an association between longer (CAG)n homopolymeric tracts and IIEF-15 score. This observation was clearer in CA and PCA analysis which also presented a close liaison between (CAG)n, (GGC)n and IIEF-15 score. High linkage distance between the cluster I and III indicates that hormonal status of oxytocin and prolactin is not dependent on the length of polymorphic (GGC)n repeats of the androgen receptor. This lack of correlation between (GGC)n repeats size and hormonal status in PE subjects have also been described previously by Kalliolia E et al. .
PCA analysis projects the biochemical and genetic variables in the first two PCAs space exhibited that concentration of prolactin was positively correlated with orgasm function. Whereas, the second group exposed a close affiliation between self-reported intravaginal ejaculatory latencies, the total serum concentration of testosterone, thyrotropin levels, overall sexual satisfaction, premature ejaculation diagnostic tool (PEDT), and with BMI. Interestingly, PCA analysis has also exhibited the correlation between PE parameters (IELT, PEDT, and OS) and hormonal milieu (TT and TSH). The close relationship between TT and IELT suggests that T might have both exogenous and endogenous role on the ejaculatory mechanism through supraspinal control or spinal nucleus of the bulbocavernosus mediated ejaculatory reflex (SNB) .
One of the main limitations of our study is that most of the participants filled the questionnaires in front of investigators, which might have triggered uncomfortable feelings, resulting in a negative impact on the precision of the results. Secondly, 250 subjects with PE + DM and 150 healthy men were registered in this study; the small sample size might have influenced the outcomes. Third, we do not record the stopwatch measured IELT values in this study, which might affect the exactitude of results. Fourth, certification of the Chinese version of IIEF-15 has not been done before, though it was employed in this study. Fifth, Type 2 DM patients using metformin might have a lowering effect on the thyreotropin levels.