sildenafil-citrate and Retinitis-Pigmentosa

Sildenafil, the active ingredient in Viagra, has been reported to cause transient visual disturbance from inhibition of phosphodiesterase 6 (PDE6), a key enzyme in the visual phototransduction pathway. This study investigated the effects of sildenafil on the rd1(+/-) mouse, a model for carriers of Retinitis Pigmentosa which exhibit normal vision but may have a lower threshold for cellular stress caused by sildenafil due to a heterozygous mutation in PDE6. Sildenafil caused a dose-dependent decrease in electroretinogram (ERG) responses of normal mice which mostly recovered two days post administration. In contrast, rd1(+/-) mice exhibited a significantly reduced photoreceptor and a supernormal bipolar cell response to sildenafil within 1 h of treatment. Carrier mice retinae took two weeks to return to baseline levels suggesting sildenafil has direct effects on both the inner and outer retina and these effects differ significantly between normal and carrier mice. Anatomically, an increase in expression of the early apoptotic marker, cytochrome C in rd1(+/-) mice indicated that the effects of sildenafil on visual function may lead to degeneration. The results of this study are significant considering approximately 1 in 50 people are likely to be carriers of recessive traits leading to retinal degeneration.

Topics: Animals; Cytochromes c; Disease Models, Animal; Dose-Response Relationship, Drug; Electroretinography; Female; Fluorescent Antibody Technique, Indirect; Glial Fibrillary Acidic Protein; Heterozygote; In Situ Nick-End Labeling; Male; Mice; Mice, Inbred C57BL; Phosphodiesterase 5 Inhibitors; Photoreceptor Cells, Vertebrate; Piperazines; Purines; Retina; Retinal Bipolar Cells; Retinitis Pigmentosa; Sildenafil Citrate; Sulfones

We determined the metabolic changes that precede cell death in the dystrophic proline-23-histidine (P23H) line 3 (P23H-3) rat retina compared with the normal Sprague-Dawley (SD) rat retina. Metabolite levels and metabolic enzymes were analyzed early in development and during the early stages of degeneration in the P23H-3 retina. Control and degenerating retinas showed an age-dependent change in metabolite levels and enzymatic activity, particularly around the time when phototransduction was activated. However, lactate dehydrogenase (LDH) activity was significantly higher in P23H-3 than SD retina before the onset of photoreceptor death. The creatine/phosphocreatine system did not contribute to the increase in ATP, because phosphocreatine levels, creatine kinase, and expression of the creatine transporter remained constant. However, Na(+)-K(+)-ATPase and Mg(2+)-Ca(2+)-ATPase activities were increased in the developing P23H-3 retina. Therefore, photoreceptor apoptosis in the P23H-3 retina occurs in an environment of increased LDH, ATPase activity, and higher-than-normal ATP levels. We tested the effect of metabolic challenge to the retina by inhibiting monocarboxylate transport with alpha-cyano-4-hydroxycinnamic acid or systemically administering the phosphodiesterase inhibitor sildenafil. Secondary to monocarboxylate transport inhibition, the P23H-3 retina did not demonstrate alterations in metabolic activity. However, administration of sildenafil significantly reduced LDH activity in the P23H-3 retina and increased the number of terminal deoxynucleotidyl transferase biotin-dUPT nick end-labeled photoreceptor cells. Photoreceptor cells with a rhodopsin mutation display an increase in apoptotic markers secondary to inhibition of a phototransduction enzyme (phosphodiesterase), suggesting increased susceptibility to altered cation entry.

Topics: Adenosine Triphosphate; Age Factors; Animals; Apoptosis; Ca(2+) Mg(2+)-ATPase; Coumaric Acids; Creatine; Creatine Kinase; Disease Models, Animal; Disease Progression; Energy Metabolism; Histidine; L-Lactate Dehydrogenase; Membrane Transport Proteins; Monocarboxylic Acid Transporters; Mutation; Phosphocreatine; Phosphodiesterase Inhibitors; Piperazines; Proline; Purines; Rats; Rats, Sprague-Dawley; Rats, Transgenic; Retina; Retinitis Pigmentosa; Sensory Rhodopsins; Sildenafil Citrate; Sodium-Potassium-Exchanging ATPase; Sulfones

Retinitis pigmentosa (RP) is a common inherited degenerative retinal disease that has many causes including mutations in the genes coding for cyclic guanosine monophosphate (cGMP) phosphodiesterase 6 (PDE6). Sildenafil (Viagra; Pfizer Pharmaceuticals, New York, NY), a widely used medication for erectile dysfunction, is a specific inhibitor of PDE, with the potential to affect PDE6 in the retina. The purpose of this study was to investigate the retinal effects of sildenafil on knockout mice heterozygous for a mutation causing absence of the gamma subunit of rod PDE6 (PDEG:(tm1)/+).. Wild-type mice and PDEG:(tm1)/+ mice were subjected to electroretinography (ERG) 1 hour after exposure to one of three treatments: 1) no drug, 2) an intraperitoneal injection of sildenafil at 2 times the equivalent maximal daily recommended dosage for humans, or 3) 10 times this dosage. Control ERGs were also obtained to evaluate the reversibility of changes in retinal function after sildenafil treatment. A minimum of 48 hours elapsed between electroretinogram (ERG) recordings for drug washout and animal recovery.. ERGs of the PDEG:(tm1)/+ mice treated with sildenafil showed a reversible, dose-dependent decrease in a- and b-wave amplitudes. Wild-type mice treated with sildenafil did not show significant differences in either a- or b-wave amplitudes compared with untreated control animals.. These findings suggest that sildenafil has a significant impact on retinal function in PDEG:(tm1)/+ mice and may have implications in human carriers of RP. In addition, extension of these results in other model systems could be useful in understanding the mechanisms of RP and other forms of retinal degeneration.

Topics: 3',5'-Cyclic-GMP Phosphodiesterases; Animals; Cyclic Nucleotide Phosphodiesterases, Type 6; Dose-Response Relationship, Drug; Electroretinography; Female; Male; Mice; Mice, Inbred C57BL; Mice, Knockout; Phosphodiesterase Inhibitors; Piperazines; Purines; Retina; Retinitis Pigmentosa; Sildenafil Citrate; Sulfones

Topics: Contraindications; Enzyme Inhibitors; Humans; Male; Piperazines; Purines; Retinitis Pigmentosa; Sildenafil Citrate; Sulfones