An immunohistochemical and ultrastructural analysis of the retina in tadalafil (Cialis) treated rats
Introduction
Tadalafil (Cialis) is a one of the phosphodiesterase type 5 (PDE5) inhibitors that is commonly prescribed for treatment of erectile dysfunction. It has been also used for treatment of pulmonary hypertension and heart diseases (Katz et al., 2000; Sebkhi et al., 2003; Archer and Michelakis, 2015).
Sexual stimulation induces nitric oxide (NO) release from endothelial cells and neurons. Nitric oxide enhances the formation of cyclic guanosine monophosphate (cGMP) in the penile tissue. cGMP activates a series of downstream G proteins leading to smooth muscle relaxation, dilation of the cavernous arteries and penile erection (Lue, 2000).
The enzyme PDE5 is responsible for cGMP hydrolysis. So, its inhibition leads to augmentation of cGMP levels in the cavernous tissues and arteries and subsequent persistent vasodilatation that increases penile blood flow leading to prolongation of the erection process (Corbin, 2004; Kouvelas et al., 2009).
Previous reports indicated that PDE5 is localized in the bipolar cells, ganglion cells and in the endothelial lining and smooth muscle of retinal blood vessels (Foresta et al., 2008). A potential role of PDE5 in the regulation of retinal blood flow has been also reported (Ballard et al., 1998).
Several ocular side effects of PDE5 inhibitors including mydriasis, retinal vascular accidents, conjunctival hyperemia, ocular pain, subconjunctival hemorrhage, and ischemic optic neuropathy were reported (Fraunfelder, 2005). The use of PDE5 inhibitors was also found to be associated with changes in color vision and light perception, blurred vision and transient alterations in electroretinogram (Diederen et al., 2007; Kerr and Danesh-Meyer, 2009).
The use of PDE5 inhibitors is not curable for erectile dysfunction. For that reason, prolonged and chronic use of such drugs is usually indicated (McMurray et al., 2007). This prolonged use increases the possibility of the development of adverse effects including ocular changes. Eltony and Abdelhameed (2017) have investigated the effect of chronic use of PDE5 inhibitors (sildenafil) on the retina and optic nerve of albino rats. However they didn’t correlate between the detected changes and PDE immunoexpression.
Different PDE5 inhibitors vary considerably in their selectivity to PDE isoenzymes (Seftel, 2004). Tadalafil has a weak inhibitory effect on PDE6 and confers a high selectivity for PDE5 versus PDE6 (Huang et al., 2013). The selectivity of tadalafil for PDE5 is 700 times greater than that for PDE6 and its affinity to PDE6 is much lower than other PDE5 inhibitors such as sildenafil or vardenafil (Coward and Carson, 2008; Cordell et al., 2009). Tadalafil rarely causes visual manifestations and was not associated with any significant harmful effects on visual functions compared to other PDE5 inhibitors such as sildenafil and vardenafil (Bischoff, 2004; Cordell et al., 2009; Huang et al., 2013). However, few cases of central serous chorioretinopathy (Gordon-Bennett and Rimmer, 2012; Roy et al., 2014) and damage to the parafoveal photoreceptors (Coscas et al., 2012) were reported following the use of tadalafil. Most of the visual disturbances caused by sildenafil and vardenafil are due to their potent inhibitory effect on PDE6 enzyme; which is localized in rods and cones of the retina and play an important role in phototransduction (Bischoff, 2004; Foresta et al., 2008).
To our knowledge, the effect of chronic use of tadalafil on the retina of experimental animals has not been previously assessed. Therefore, this study was designed in order to analyze the histological and ultrastructural changes that may be provoked by chronic administration of tadalafil in the rat retina, correlate between such changes and PDE5 immunoexpression and to highlight the role of PDE5 in the pathogenesis of these retinal changes. It also aimed to evaluate the possible reversibility of these changes.
Section snippets
Animals and experimental protocol
The present experimental study was conducted at Nile Research Centre in Mansoura city, Egypt. Study protocol was reviewed and approved by the animal ethics committee of Mansoura Faculty of Medicine (Institutional Review Board; IRB, R/17.09.59). The experiment lasted 10 weeks. Thirty Sprague Dawley male rats weighing (200–220 g) were used in this study. Rats were housed in a well-ventilated animal house, kept in separate cages in average temperature (22 ± 2 °C) and average humidity (50–55%);
H and E stain
The retina of the Control group showed the different layers from the sclera to vitreaous side; the retinal pigmented epithelium, the photoreceptor cell layer that was formed of lightly stained outer segments and deeply stained inner segments, the outer limiting membrane. The outer nuclear layer contained packed densely stained rods and cones nuclei. The outer plexiform appeared much thinner than the inner plexiform layer and both had a reticular appearance. The inner nuclear layer was thinner
Discussion
The present study demonstrated profound degenerative and ultrastructural alterations in the retina of Tadalafil group. These findings coincide with the morphometric changes in the total retinal thickness and thickness of the outer and inner nuclear layers.The diminished cell populations that were observed in both nuclear layers might also explain the reduction in thickness of these layers. Similar degenerative changes were described by Eltony and Abdelhameed (2017) in the retina of rats treated
Conclusion
The chronic use of tadalafil could induce apoptotic, degenerative and ultrastructural changes in the retinal neurons due to its direct inhibitory effect on PDE5 expression which might be reflected on their physiological processes. The improvement of the retinal histology and ultrastructure after withdrawal of tadalafil indicated that these changes are reversible and gives a better chance for protection of the retinal structure with intermittent use of the drug.
Recommendations
Chronic abuse of tadalafil should be avoided and a continuous supervision of visual functions is recommended in patients using this drug for long periods.
Conflict of interest
The authors declare that they have no conflict of interest.
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