Streptozotocin-induced diabetes in rodents appears to be the mostsuitable animal model because it reflects the symptoms of diabetes in human (King, 2012) and it ischaracterized by severe loss in body weight (Al-Roujeaie et al., 2016), andthis is also reflected in the present study.
The decreases in body and testisweight in diabetic rats showed the loss or degradation of structural proteinsdue to diabetes and a significant reduction in the serum levels ofthe main androgenic hormone, testosterone (Royet al., 2014). NG treatment enhanced the testicular weight.Recently, Al-Roujeaie and his colleagues (2016) reportedthat, rutin a phenolic compound enhanced the testicular weight of STZ-induceddiabetic rats and enhanced the sexual behavioral activity. This may considerthat phenolic compounds have ability to enhance the testicular weight whichfurther justified with present results.
Epidemiological studies showed the higher prevalence of severe EDin diabetic men compared to normal healthy males (Maiorino et al., 2014; Saenzet al., 2005). Experimentally-induced diabetic models are well-established toelaborate the mechanism of diabetic-induced ED and also a recommended model forestablishing the beneficial effect of any new compound against diabetes ED(Hirata et al., 2009; Li et al., 2016). In present study, sever sexualimpairment was found in STZ-induced diabetic rat by inhibiting the EL, PEI, MLand IL latencies and increasing the MF and IF frequencies.
Our observations are in agreement withearlier reports showed fewer in sexual behaviors of diabetic animals comparedcontrols (Scarano et al., 2006; De et al., 2016). In NG supplemented diabeticrats, we found improvement in all the behavioral parameters that suggests thedrug produces beneficial effect in one of the diabetic-induced complication. Itis well established that the testosterone levels decreases in diabeticconditions. However, this factor alone does not appear responsible for changesin mating behavior as testosterone replacement did not reverse the adverseeffects of diabetes on sexual behavior. While STZ-induced sexual dysfunctionmight be due to reduced testosterone responsiveness, it seems likely that ED inthe diabetic state results from direct or indirect action of insulin and/orglucose on the adrenergic complex (Kniel et al.
, 1986). Our results also showedsignificant decrease in serum testosterone levels compared to controls. It is well known that penile erection depends on decreased penilevascular resistance, which subsequently results in increased penile blood flow.
The action consists of the relaxation of the penile helicine arteries and thecavernous smooth muscle that lines the cavernosal spaces (Andersson and Wagner,1995). It has demonstrated that reduced vasorelaxation or increasedvasoconstriction under diabetic conditions accounts for the development of ED. As previously reported, the rats showed evidence of decliningerectile function following the STZ injection (Choi et al., 2012; Ari et al.,1999; Italiano et al.
, 1993). When Choi and his colleagues (2012) compared tothe age-matched controls, the diabetic rats showed a significant decrease inthe ICP/MAP and AUC at the eight and twelve week assessments after the STZinjection, respectively. Present results also showed similar decline in ICP/MAPratio in diabetic rats compared to normal healthy animals. The NG treatmentmarkedly improved in ICP/MAP ratio compared to untreated diabetic rats.
Furthermore, our results are more consistentwith those from Melman et al, (2009) which have reported significant changes inICP/MAP in as early as 1 month after STZ injection. The necessity for otherindices in addition to the ICP/MAP has been suggested by others. Earlier, Zhangand his colleagues (2011) studied such phenolic compound QT on intracavernouspressure (ICP) of STZ-induced diabetic rats and clearly documented the potentialeffect of QT against ED.
Itis well established that the testosterone levels decreases in diabeticconditions. Hyperglycemia-induced sexual dysfunction might be due to theinhibition of testosterone, in diabetic state results from direct or indirect actionof insulin and/or glucose on the adrenergic complex (Kniel et al., 1986). Present results also showedsignificant decrease in serum testosterone levels compared to controls. The NGtreatment markedly increased the inhibited testosterone levels in our diabeticrats which showed potentials of NG against diabetic-induced ED. Earlier reportsdocumented that, diabetes causes inhibition in sperm count, motility andviability (Scarano etal.
, 2006), similar changes have noted in present study. The decreasedvalues of sperm numbers, motility and viability were significantly enhanced bythe NG treatment. It supports the beneficial effect of NG againstdiabetic-induced ED. The pathophysiologic mechanism of diabetic-induced EDincludes both the functional and structural impairment. Endothelialdysfunction, coordination disorder of the relaxation and contraction ofcorporal smooth muscles via the NO/cGMP pathway and the RhoA/Rho-kinase (ROCK)pathway, autonomic neuropathy and apoptosis in corporal smooth muscles weresimultaneously happened.
These processes do not work independently; diversecombinations of each pathologic mechanism may manifest as DM progresses (Maiorino et al., 2014; Hadi and Suwaidi, 2007; Park et al., 2011;Cho et al.
, 2011). Therefore, to prevent andreinstate the endothelial function of the corpus cavernosum before progressionof the irreversible changes is the best treatment option to overcome diabetic-inducederectile dysfunction. Indeed, the role of oxidative impairments inthe male reproductive system has been suggested in several experimental studies(Agbaje et al.
, 2007; Shrilatha, and Muralidhara, 2007; Amaralet al., 2006). Induction of lipid peroxidationprocess and elevation of its biomarker like TBARS in the testicular tissues wasshown to have fundamental implications on testicular physiology and spermfunction (Agarwal and Said, 2005). Diabetesmellitus is well known to be strongly correlated with oxidative stress leadingto production of free radicals and act as intercellular second messengers thatcan induce activation downstream signaling of many molecules, includingtranscription factors like nuclear factor kappa B (NF-?B). These NF-?B andother transcription factors mediate vascular smooth muscle cell growth andmigration as well as the expression of pro-in?ammatory cytokines such as TNF-?,IL-1?, and IL-6 (Touyz, 2004).
These elevated pro-in?ammatory mediatorsantagonize insulin action because of their ability to augment insulin receptorsubstrate phosphorylation, leading to insulin resistance (Emanuelli et al., 2000; Senn et al., 2003). Therefore, attenuation of free radicalinduced NF-?B translocation and ameliorating oxidative stress in diabetic ratsexplains an associative relationship between the in?ammatory cytokines and DM.In the present study, also found an elevation in the levels of pro-inflammatoryand oxidative stress biomarkers. Such as TNF-?, IL-1? and IL-6 were markedlyelevated in the diabetic group of animals. NG treatment to diabetic rats foundmarkedly inhibited the pro-inflammatory biomarkers. These findings show theanti-inflammatory potential of NG which earlier, Linand Lin, (2011), reported the anti-inflammatory effect NG wasin primary mouse splenocytes in the absence or presence of lipopolysaccharide(LPS) and it showed strong anti-inflammatory activity.
Inaddition, a beneficial systemic effect of NG on diabetic rats observed bylowering fasting glucose and enhanced insulin levels in the serum of diabeticrats. Similar systemic effects of NG on experimentally induced diabetes werereported in earlier studies also (Hasanein and Fazeli, 2014; Annadurai et al.,2012; Al-rejaie et al.
, 2015). Inpresent study, cGMP levels in penile tissues of diabetic rats weresignificantly inhibited. Similar cGMP inhibition was seen earlier in experimentally-induceddiabetic animal’s penile tissue (Yang et al., 2013; Yang et al., 2008).
Indeed,the role of oxidative impairments in the male reproductivesystem has been suggested in several experimental studies (Agbaje et al., 2007; Amaral et al., 2006; Shrilathaand Muralidhara, 2007). It is known that hyperglycemia induced increases in the productionof glycation end-products, reactive oxygen and nitrogen species impair nitricoxide bioavailability and affect penile tissue, leading to changes inendothelium-dependent vasorelaxation mechanisms (Newsholme et al., 2007;Agarwal et al., 2006).
Adenoviralgene transfer of EC-SOD in vivo can reduce corporal superoxide anion levels andraise cavernosal cGMP levels by increasing NO bioavailability thus restoringerectile function in the STZ-diabetic rat (Bivalacqua et al., 2005b). Thepresent study determined TBARS and GSH levels and SOD, CAT, GST and GPxactivities as a marker for oxidative stress in testicular cells. Decreased GSHlevels, SOD, CAT, GST and GPx activities and elevated levels of TBARS werefound in STZ-diabetic rats compared with that in normal controls. In addition,this abnormality in diabetic rats was significantly restored by NG treatment indose dependent manner. Earlier reports revealed that NG hasantioxidant potentials (Jagetia andReddy, 2002; Al-Rejaie et al.
, 2015). This suggests that NG treatment couldimprove ED in diabetic rats partly by restoring the enzymatic activities andinhibiting oxidative stress in testicular cells. Insummary, we found that diabetic rats exhibited decreased sexual performance,ICP levels and ICP/MAP ratio, sperm count inhibition with low motility,viability, reduced levels of enzymatic activities including SOD, CAT, GST andGPx, as well as elevated levels of TBARS and inhibited GSH levels in testicularcells.
Treatment with NG markedly corrected these diabetic-induced changes inmostly dose dependent manner. Finally, could conclude that NG supplementationmay improve the diabetic-induced testicular oxidative stress, inflammation andthat can make improvement in sexual performance. Further preclinical researchinto the utility of NG treatment may indicate its usefulness as a valuabletherapeutic approach in ED.