Sildenafil, the first oral drug approved by the United States Food and Drug Administration for the therapeutic treatment of erectile dysfunction, Sildenafil was firstly synthesized by pharmaceutical scientists working at Pfizer’s research facility in Kent, United Kingdom. The focus of research by the scientists at Pfizer was to treat hypertension and angina pectoris. However, the phase I clinical trials did not show a significant antihypertensive effect and sildenafil had little therapeutic potential for the treatment of angina. Interestingly, sildenafil exhibited a different pharmacological effect in treatment of penile erection. This unexpected pharmacological finding led to patenting sildenafil in 1996 by Pfizer. Later, sildenafil approved by the Food and Drug Administration in 1998 for use in erectile dysfunction.

So, the drug sildenafil became the first oral drug approved for the therapeutic treatment of erectile dysfunction in the United States. Sildenafil citrate is a water soluble aromatic compound and its inhibits the phosphodiesterase-5 (PDE5) in the corpus cavernosum which has the most blood in the penis during erection. PDE5 is from an important  family of proteins that regulates the intracellular level of cyclic guanosine monophosphate (cGMP). Eleven different types of phosphodiesterases are distributed throughout the body. Phosphodiesterases hydrolyse cyclic nucleotides and  are involved in second messenger signaling pathway. Among many types of phosphodiesterases, only three selectively hydrolyze cGMP relative to cAMP. PDE5 hydrolyzes cGMP and is found in several parts of  the body like lungs, platelets, various forms of smooth muscle and several brain regions. Structurally, Sildenafil is similar to the guanosine base of cGMP and the 3-substituent extension fills a space in the enzyme active site occupied by ribose. Interestingly, several recent evidence show that sildenafil offer new strategy in the therapeutic treatment of memory impairment, pain, pulmonary hypertension and multiple sclerosis. In this paper, by studying about 40 differenet articles, we defined various applications of Sildenafil with the focus on its effect on improving Alzheimer’s disease (AD).

Sildenafil (Viagra), a drug used to treat erectile dysfunction and pulmonary arterial hypertension, inhibits phosphodiesterase 5 (PDE5). PDE5 degrades cyclic guanosine monophosphate (cGMP). 

Erectile Dysfunction (E.D.) is the inability to achieve or/and maintain a sufficient penile erection for successful, satisfactory, and pleasant vaginal intercourse(1-3). National Institute of Health's division of NIDDK (National Institute of Diabetes and Digestive and Kidney Diseases) defines erectile dysfunction as a condition in which one person cannot get or keep an erection firm enough for satisfactory sexual intercourse (4, 5). The usual symptoms include problems achieving a solid erection, difficulties maintaining the erected condition of the penis, and decreased sexual desire. Several factors can contribute to these penile disorders, like neurogenic injury, endocrinological disorders, drug-affected pathology, cardiovascular disease, etc(1-5).

The prevalence of E.D. is highly dependent on the age of the male population. Several studies have estimated that less than 10% of men who are younger than 40 years of age are affected by E.D. The prevalence range is between 2 to 9% within the age range of 40-49 years. However, it is increased to 40% if the age range is 60-69%. The highest prevalence of E.D. was 50-100% in men over 70 years (2, 3, 6-8). A similar study in the U.S. showed that the bulk of E.D. affects about 18 million men in America of the age group of 20 years older. The prevalence was more than 70% in men in the U.S. aged 70 years or older. The same study found that the majority was highly significant in men with cardiovascular conditions. Other factors such as diabetes, lack of exercise, and lower sexual education exacerbate the situation (8).

About twenty years ago, men with impotence received redemption in the form of a little blue pill. That blue pill that treats impotence named sildenafil or Viagra (brand name, Pfizer inc.) was patented in 1996 and approved by the Food and Drug Administration (FDA) on March 27, 1998, (9). Basically, sildenafil is a potent, selective, reversible inhibitor of phosphodiesterase type 5 (PDE5) used to treat pulmonary arterial hypertension. But, its serendipitous discovery shows the treatment of impotence and male erectile dysfunction, which ultimately opened a new era of clinical application for this class of drug (10). The happenstance of its effects on penile erection provided a quantum leap in the treatment of erectile dysfunction (E.D.) (11). Back In 1974, Zaprinast was synthesized and was later characterized as the first selective PDE 5 inhibitor. Eventually, studies revealed that Zaprinast was not selective only for PDE5. Later, Exploring PDE5 as a target for a range of cardiovascular disorders like hypertension and angina pectoris, Terrett et al. (1996) found that PDE5 was the predominant hydrolyzing enzyme in the cytosolic fraction found in the corpus cavernosum of humans and suggested that one of their synthesized inhibitors which were sildenafil (11). Indisputably, at that time, study stances that it was a potent and highly selective PDE5 inhibitor and could be useful as an orally active treatment for male ED (12). Now, the generic name Sildenafil is an oral tablet that can treat both conditions, but one brand-named drug can treat only one of the conditions. Viagra (brand name) treats E.D. when a man cannot get or maintain a penile erection. On the other hand, Revatio (brand name, Pfizer) is a citrate salt of sildenafil, used to treat PAH when the blood pressure in the lungs is too high. Besides that, due to the vasodilatory activity and high levels of PDE5 expression in the lung tissue, researchers rationally considered that the drug has some possible therapeutic effects against pulmonary fibrosis, a complication of the COVID-19 disease (10).

Alzheimer’s disease (AD) is a progressive age-related neurodegenerative disease that represents nearly 70% of dementia cases worldwide. It is predicted that by 2025; AD patients will increase in numbers several times in comparison to 1980.1 Although the exact pathogenesis of AD is not so clear, a neurotoxic b-Amyloid (Ab) peptide accumulation is considered one of the most important pathogenesis. Such accumulation is either due to Aluminum (AL) exposure or to reduction in cerebral blood flow with a decrease in its clearance from the brain(2).

The sequence of events in AD started from deposition of amyloid filaments and hyperphosphorylated tau along with abundant accumulation of a-synuclein, leading to neuronal degeneration and significant synapses loss(3, 4). Several interacting endothelial cell mediators including nitric oxide (NO), vascular endothelial growth factor (VEGF), and vascular cell adhesion molecule-1 (VCAM-1) were implicated in several neuronal effects including cell proliferation, angiogenesis, inflammation, and neurological impact on glial and neuronal cells(5,6). AD was associated with increased vascular inflammation, as evidenced by elevated expression and localization of inflammatory markers such as VCAM-1 and Tumor necrosis factor- a (TNF-a) surrounding AD-plaques(7). Previous studies reported that decline inVEGF-Awas correlated with the aggravation of AD pathology indicated by the high level of a-synuclein. In turn, overexpression of a-synuclein caused further decline in VEGF-A, indicating that the extent of decline of VEGFA is correlated with the level of expression of a-synuclein. These findings raise the possibility of a vicious cycle in the accumulation of a-synuclein leads to cerebral hypoperfusion, which accelerates the progression of AD(8). Nitric oxide/cyclic guanosine monophosphate/ phosphodiesterase (NO/cGMP/PDE) pathway plays an imporatnt role in AD(9). Interestingly, both NO donors and cGMP analogs counteract the Ab-induced-brain injury(10). One of the most effective ways to up regulate the NO pathway is increasing cGMP levels by PDE 5 inhibition. PDE is expressed in multiple brain regions associated with cognitive functions such as the hippocampus, cortex and cerebellum(11). Sildenafil is a specific PDE 5 inhibitor that activates NO/cGMP pathway and is commonly prescribed in treatment of erectile dysfunction. It has been reported that sildenafil enhanced the learning and memory functions by raising hippocampal cGMP levels without an evident effect on the male reproductive system(12-14). To the best of our knowledge, the effect of sildenafil on VEGA-Aand VCAM-1in AD model was not addressed(14). 

Sildenafil and erectile dysfunction:

Erectile dysfunction is a physiological and psychological process which is influenced by many factors [4]. Mostly,  erection initiated by visual, olfactory, imaginary stimuli and recruitment of penile afferents. Production of nitric oxide (NO) and cGMP reduced by  age and causes erectile dysfunction in many cases (15). With regard to the psychological aspect, erectile dysfunction significantly affects the quality of life, such as objectively measured decreases in physical satisfaction, emotional satisfaction, and general happiness of a patient(16). Erectile dysfunction is connected with feelings of isolation, low selfesteem and depression (17). The factors that significantly affect sexual performances are environmental influence, cultural factors, gender dynamics, availability of partners and physical setting (18). 

20 to 30 million American men suffer from erectile dysfunction. Erectile dysfunction entangle nearly 150 million men worldwide and this number will doubled over the next 25 years. Patients with this dysfunction usually try various natural products and other local treatments for better performance. Practitioners of Ayurvedic (ancient Indian medicine) and Chinese medicine, African herbalists and traditional healers from different cultures around the world have many centuries of knowledge regarding the use of medicines of plant origin to treat a wide variety of sexual dysfunction. Certain botanicals have been identified as having significant aphrodisiac properties and the ability to improve sexual functioning and overall systemic health. Recent studies have shown that certain botanicals have the ability to improve peripheral and systemic blood flow and to act as vasodilators, leading to increased blood flow to the penis and erectile tissue and, therefore, augment the sexual pleasure and performance(19). There are many problems associated with the use of botanicals with regard to its quality, dose and dosage forms. Adverse effects and long-term toxic response of these botanicals have not been well characterized in animals and humans. Patients with erectile dysfunction positively responded to the current pharmacological drug (synthetic) therapy. Drug therapies are based on their ability to substitute, partially or completely, the degenerated or faulty endogenous mechanisms that control penile erection. Most drugs have a direct action on penile tissue facilitating penile smooth muscle relaxation, including prostaglandin E1, NO donors, phosphodiesterase inhibitors, and alpha-adrenoceptor antagonists. Dopamine receptors in central nervous centers that participate in the initiation of erection have also been targeted for the treatment of erectile dysfunction.

Sildenafil has become a first-line treatment option for erectile dysfunction. Several recent reviews focused on the beneficial effect of regular PDE5 inhibitor administration on the improvement of erectile function and the mechanism of drug action(20-25). Three PDE5 inhibitors (sildenafil, tadalafil, and vardenafil) in a range of doses are available. Sildenafil is a potent inhibitor of cGMP specific PDE5 which is responsible for degradation of cGMP in the corpus cavernosum . Inhibition of PDE5 reduces the degradation of cGMP which allows erectile function to occur by relaxation of penile smooth muscle. Sildenafil during the last 8–9 years has been prescribed by more than million physicians around the globe. Sildenafil fights sexual dysfunction caused by diverse etiology or by such different factors as drugs, psychological barriers, aging or induced by diseases like diabetes mellitus, Parkinson’s disease, depression and renal failure.

Sildenafil and pulmonary hypertension:

Pulmonary hypertension occurs in the major pulmonary artery that carries blood from the right ventricle of the heart to the lungs. The high blood pressure in the lungs is referred as pulmonary hypertension. In the lungs, when the smaller blood vessels become more resistant to blood flow, the right ventricle is under stress to pump adequate blood to the lungs. Pulmonary hypertension is an uncommon blood vessel disorder of the lung which may be defined as a pulmonary artery systolic pressure greater than 30 mmHg or a pulmonary artery mean pressure greater than 20 mmHg. Pulmonary hypertension can  classified as primary and secondary hypertension. The cause of primary pulmonary hypertension is not obviously understood. However, the secondary pulmonary hypertension may be due to pulmonary, cardiac and extrathoracic disorder. Cor pulmonale may lead to a variety of disorders of the respiratory system. Pulmonary hypertension usually paves the way to cor pulmonale. The estimated prevalence of primary pulmonary hypertension is 1–2 cases/million. The prevalence of pulmonary hypertension is about 1.7 times higher in women than in men. However, in children pulmonary hypertension is seen equally often in both sexes but this is changed after puberty. Primary pulmonary hypertension mostly develops between 20 to 40 years of age and has no racial preference [100]. Pain in the chest, shortness of breath with minimal exertion, fatigue, dizzy spells and fainting are the main symptoms of pulmonary hypertension. Pulmonary hypertension can also be induced mostly by breathing disorders like emphysema and bronchitis. The other fewer causes may include scleroderma, systemic lupus erythematosus, congenital heart disease, pulmonary thromboembolism, HIV infection, liver disease, diet and drugs, like fenfluramine and dexfenfluramine.

Pulmonary hypertension is often not diagnosed properly and is thus advanced to a critical state by the time it is correctly diagnosed. Because of improper disease management, pulmonary hypertension has been historically chronic and incurable resulting in a poor survival rate. Interestingly, new therapeutic treatments are available which have significantly improved prognosis and augmented the survival time, and patients are able to manage the disease for 15 to 20 years or longer. The therapeutic treatment of primary pulmonary hypertension mainly involves calcium channel blockers, anticoagulants, short-acting vasodilators, inhibitors of platelet aggregation, ionotropic agents, corticosteroids or other immunosuppressive agents. Thus, the treatment of pulmonary hypertension is complex and it does not lead to a complete therapeutic cure. Recent studies have shown that sildenafil attenuated pulmonary hypertension by increasing the supply of  blood to the lungs (26-28). Sildenafil can act by relaxing the arterial wall and by decreasing the pulmonary arterial resistance. Due to the presence of PDE5 in the arterial wall, smooth muscle and lungs, sildenafil acts in these areas and induces vasodilatation. Pfizer submitted an additional registration for sildenafil as an oral therapy for pulmonary arterial hypertension with the FDA, and the drug was approved for this indication in June 2005. The formulation for the pulmonary hypertension therapy was named Revatio, avoiding confusion with Viagra. 

Sildenafil and pain:

Pain is a complex process which involves both the peripheral and central nervous system(29). Pain is a self-protective mechanism which forces the body to move away from danger, and then, to rest the injured part, giving the body the chance to heal itself. According to its origins, pain may be classified to neuropathic, psychogenic, referred, somatic and visceral. In acute pain (predominantly nociceptive), visceral, somatic and referred mechanisms play important roles in the  perception. There have been sufficient studies in recent years indicating that pain perception is no longer a straightforward afferent transmission of pain signal. Pain is perceived as a consequence of the response to electrical (neural) and chemical (hormonal) changes in the body as a result of damage, disease or injury. The signals resulting from any insult, damage or injury are picked up by sensory receptors in nerve endings(30). Ultimately the neurons transmit the signal from the site of injury to the spinal cord, then into the brain where that signal is perceived as pain. Anatomically specific ascending excitatory and descending inhibitory pathways play a crucial role in pain signal transmission. Centralization of the pain signal generators (cephalad relocation in the central nervous system) occurs spontaneously or neural pathways are interrupted, leading to totally unexpected pain syndromes. Scientific evidence shows that acute persistent pain eventually sensitizes wide dynamic neurons in the dorsal horn of the spinal cord, called the wind-up phenomenon. This phenomenon may constitute the basic etiology for chronic pain syndromes. Persistent and excessive pain is harmful to the well being and, therefore, pain needs to be treated as early and as completely as possible. Frequently in non-nociceptive, chronic pain, neuropathic and psychogenic mechanisms prevail, resulting in protracted suffering and disability both physically and mentally(31).

Chronic neuropathic pain, often associated with injury of peripheral or central nerves, has been confirmed to be very difficult to treat therapeutically (31). Opioids are a major class of analgesics which have been used in management of moderate to severe pain. Nevertheless, treatment with opioids has been found ineffective in alleviating neuropathic pain. Recent studies have shown that NMDA receptor seems to play a major role in neuropathic pain and in the development of opioid tolerance (32). Many experiments in both animals and humans have established that NMDA antagonists such as ketamine and dextromethorphan can reduce neuropathic pain and reverse opioid tolerance.

Chronic pain in patients due to spinal cord injury could be reduced by a very low dose of ketamine(33). 

Recently sildenafil has been shown to have immense potential for the treatment of pain in animals and humans. Sildenafil produced antinociceptive effect in animal models of pain after local peripheral and systemic administration (29, 30, 34, 35). Acetylcholine and cholinomimetic agents with predominant muscarinic action are known to increase the concentration of cGMP by activation of the NO signaling pathway in the nociceptive conditions. Patil et al. (36) investigated NO-cGMP-PDE5 pathway in nociceptive conditions in the experimental animals. Acetylcholine or neostigmine (cholinomimetic agent) augmented the peripheral antinociceptive effect of sildenafil. Peripheral accumulation of cGMP may be responsible for antinociceptive effect of sildenafil (37). There may be a possible interaction between cholinergic agents and PDE5 system in models of nociception(35) . Nearly 50% of diabetes mellitus patients may develop diabetic neuropathy (38, 39). The treatment of pain in diabetic patients in many ways is unacceptable. Anticonvulsants, antidepressants and opioids have become the mainstay in the treatment of chronic neuropathic pain(40). Sildenafil is a new candidate for a pathogenetically valid treatment in diabetic patients with chronic neuropathic pain(41). Sildenafil inhibits spinal PDE5 and leads to the accumulation of cGMP that produces intrathecal antinociception. Results of the recent study suggest that cGMP accumulates as a result of PDE5 inhibition and interacts with the cholinergic system to mediate this pain-reducing effect.

Sildenafil and multiple sclerosis:

Multiple sclerosis is an inflammatory disease of the central nervous system characterized by pathologic changes including demyelination and axonal injury in the brain, spinal cord, and optic nerves. Myelin sheathes surround nerves in the brain and spinal cord. In multiple areas,  Myelin is lost, leaving plaques or scars called scleroses so named multiple sclerosis. Myelin is the fatty substance that coats and protects these fibers, similar to the way insulation shields electrical wires. Multiple sclerosis is a chronic, potentially debilitating disorder that affects the central nervous system. The central nervous system contains millions of nerve cells joined together to form nerve fibers. Electrical impulses begin in nerve cells and travel along the nerve fibers to and from the brain. Inflammation and injury to the myelin sheath leads to neuronal injury. The result may be multiple areas of scarring (sclerosis) in the central nervous system. Ultimately, this scaring or sclerosis can slow or block the nerve signals that control muscle coordination, strength, sensation and vision. Multiple sclerosis affects approximately 400,000 individuals in the United States and 2.5 million worldwide, with a typical onset during the productive years between the ages of 20 and 50. Multiple sclerosis spread more in women. Multiple sclerosis patients experience their first symptoms between the ages of 20 and 40. Symptoms of multiple sclerosis vary widely, depending on the location of affected nerve fibers. Signs and symptoms of multiple sclerosis may include fatigue, numbness or weakness of limbs, partial or complete loss of vision, double vision or blurring of vision, tingling or pain in numb areas of the body, electric-shock sensations that occur with certain head movements, tremor, lack of coordination or unsteady gait and dizziness. Some patients may also develop slurred speech, muscle stiffness or spasticity, paralysis, or problems with bladder, bowel or sexual function. Mental changes such as forgetfulness or difficulties with concentration also can occur. Beta interferons, glatiramer (Copax one), mitoxantrone (Novantrone), corticosteroids, muscle relaxants and medications to reduce fatigue are the current therapeutic strategies used in multiple sclerosis. Multiple sclerosis leads to memory impairment through disturbances in the cortical and subcortical pathways as a result of demyelination and axonal transection. Neurologists employ magnetic resonance imaging (MRI) to track the effect of drug therapy on the development or lack of new lesions (42). Sildenafil has been shown to protect multiple sclerosis patients from neurodegeneration through increased gray matter perfusion in the brain. Functional MRI taken after oral administration of sildenafil has shown significant increase in gray matter perfusion in multiple sclerosis patients. Sildenafil has been shown to enhance neurogenesis suggesting its role in neuroprotection in multiple sclerosis.

Sildenafil and neurogenesis:

It has been said that neurogenesis, or the birth of new neuronal cells, could occur only in developing organisms. However, recent scientific studies have demonstrated that neurogenesis occurs continuously into and throughout adult life in both vertebrate and invertebrate organisms (43, 44). Neurogenesis is significant in the hippocampus of mammals (45, 46), song control nuclei of birds(47) and the olfactory pathway of rodents (48), insects (49) and crustaceans (50). Neurogenesis occurs in adult forebrain regions of the subventricular zone and the dentate gyrus in various species (44). Ongoing neurogenesis is thought to be an important mechanism underlying neuronal plasticity, enabling organisms to adapt to environmental changes and influencing learning and memory throughout life. Neurogenesis, essential for synaptic plasticity and formation of memory, generally reduces with age and is associated with neurodegenerative diseases (51, 52). Recently, many factors that regulate neurogenesis have been identified. Physical activity and environmental conditions are significant aspects that have been known to affect proliferation and survival of neurons in vertebrates as well as invertebrates. Crayfish in an enriched environment had improved neurogenesis and neuronal survival compared to siblings in an “impoverished” environment (53). Hormones, such as testosterone and adrenal steroids, have also been found to influence the rate of neurogenesis in vertebrates and invertebrates (54). Serotonin is known to play a key role in neurogenesis in a variety of organisms. In lobsters, serotonin depletion significantly decreased the proliferation and survival of olfactory projection neurons and local interneurons. Interestingly, neurogenesis follows a circadian rhythm in the juvenile lobster. Even though new neurons are generated regularly throughout the day, extensively more neurons were generated in the evening or night, the most active time for lobsters. Studies in animals showed that both serotonin reuptake inhibitors and the antiepileptic drug phenytoin (dilantin) blocked the effects of stress on the hippocampus.

Preclinical studies indicate that stress is associated with changes in structure of the hippocampus. The hippocampus is the area of the brain which plays a key role in memory and neurogenesis. Imaging studies measuring magnetic resonance have found a smaller volume of the hippocampus in patients with post-traumatic stress disorder related to both combat and childhood abuse. These patients were also found to have deficits in memory by neuropsychological testing. Functional imaging studies using positron emission tomography found decreased hippocampal activation with memory tasks. However, paroxetine significantly increased the hippocampal volume leading to enhanced memory function. Like paroxetine, phenytoin was also effective in post-traumatic stress disorder in increasing hippocampal volume but without significant change in memory(55).

Sildenafil also enhances functional recovery and neurogenesis after stroke in rats. Sildenafil has been shown to increase cGMP levels in the brain, induce neurogenesis and reduce neurological deficits in rats after stroke (56). Neuronal growth is decreased with age mainly due to lowered production of cGMP and stroke reduces the number of functional neurons in the brain. Prenatal brain development is influenced by neuronal nitric oxide synthase and suggests a role for cGMP in both neurogenesis and synaptogenesis (57). Production of NO and cGMP is reduced with age suggesting that decreased cGMP levels could contribute to age-related decrements in neurogenesis. Thus, studies on aged animals have important clinical implications for stroke treatment. Stroke remains as a major cause of death and disability in aged population [82]. Sildenafil promotes cell proliferation in neurospheres isolated from the subventricular zone of adult rat . PDE5 is expressed in neurospheres. Sildenafil significantly increased cGMP levels and neurogenesis in neurospheres. Sildenafil also significantly phosphorylated protein  kinase B (Akt) in neurospheres. Phosphorylated Akt has been associated with an increase in phosphorylation of glycogen synthase kinase 3 (GSK-3), a downstream target of Akt. This was confirmed in a study with PI3-K/Akt inhibitor, LY 294002. Co-incubation of LY 294002 with neurospheres abolished the phosphorylation. This study suggests that Sildenafil-enhanced neurogenesis likely occurs through activation of the PI3-K/Akt/GSK-3 pathway. Sildenafil increases the cGMP levels and enhances neurogenesis in cell culture studies. Suzuki et al. (58), have shown that glutamate can significantly increase the proliferation rates of human neural progenitor cells. Therefore, it is clear that glutamate-NO-cGMP pathway plays a role in sildenafil-mediated neurogenesis. The use of specific PDE5 inhibitors such as sildenafil may offer an innovative approach for improvement of brain function in the aged population.

Sildenafil and memory enhancement:

Alzheimer’s disease is a progressive neurodegenerative disorder that is mainly characterized by cognitive impairment. It has been estimated  that 4 million people have Alzheimer’s disease in the United States, affecting 30–50% of individuals aged 85 and older(59). The specific cause of Alzheimer’s disease is unknown, but genetic abnormalities appear to play a role and neuroinflammation is now recognized as a major feature in Alzheimer’s pathology (60). Therapeutic treatment consists of alleviating symptoms, providing long term care at a minimal cost with fewer adverse effects. Progressive neurodegeneration results in chronic cognitive decline culminating in memory loss and motoneuronal dysfunctions. Alzheimer’s disease patients find difficulty reasoning, making judgments, communicating and carrying out daily activities. With the progression of Alzheimer’s, patients may also experience changes in personality, behavior and life style, such as anxiety, suspiciousness or agitation, as well as delusions or hallucinations (61).

Prevalence of Alzheimer’s disease is increasing in trend, particularly in economically developed countries due to many reasons including increase in life expectancy and change in food habits. A lot of investments are currently made for the therapeutic intervention and cure of Alzheimer’s disease. The cholinergic and glutamatergic neurotransmitter systems, which share a close functional relationship play a role in the pathogenesis of Alzheimer’s disease. Acetylcholinesterase inhibitors (AChEI) are effective for the treatment of mild to moderate Alzheimer’s disease. Currently the three cholinesterase inhibitors donepezil, rivastigmine, and galantamine are widely recommended for clinical use (62, 63). Galantamine is of particular interest for it has a dual mechanism of action: it is postulated to be both an AChEI and an allosteric modulator of nicotinic receptors. Memantine, an N-methyl-D-aspartate (NMDA) inhibitor, has been approved for the therapeutic use in moderate to severe Alzheimer’s disease(64, 65). Modulation of NMDA and nicotinic receptors by memantine and galantamine may provide the best combination therapy for Alzheimer’s disease. However, there is no perfect drug currently available for the treatment. Recent studies have shown that PDE5 inhibitors can counteract deficits in long-term memory caused by pharmacological agents or aging. Therefore, targeting PDE5 with a selective inhibitor like sildenafil may offer a novel therapy aimed at slowing progression, prevention and, eventually, therapy of Alzheimer’s disease. Event-related brain potentials recorded following sildenafil administration suggest an enhanced ability in young men to focus attention on auditory stimuli. This finding is significant as the first attempt to study the cognitive effects of sildenafil in humans using electrophysiological techniques. Animal studies have shown sildenafil to enhance memory. Administration of sildenafil directly into the hippocampus after the first trail in object recognition task, improved memory in mice (66, 67) and enhanced the processes of consolidation of object information(68). Similarly, sildenafil has been shown to weaken memory impairment induced by nitric oxide synthase inhibition, hyperammonemia  and blockage of muscarinic cholinergic receptors. Sildenafil administration improved the cognitive performance in diabetic conditions and electroconvulsive shock-induced animal models(69).

There are several theories proposed to explain the memory enhancement by phosphodiesterase inhibition. PDE5 inhibition causes vasodilatation, probably through cGMP, in rats. Thus, one of the suggested mechanisms is memory improvement through increased blood flow and consequent glucose metabolism in the brain. The NO-cGMP-cGK pathway is involved in learning-related forms of synaptic plasticity. Studies have revealed a variety of molecular mechanisms, including retrograde signaling and activation of presynaptic PKG and calmodulin-dependent protein kinase II (CaM KII) for the expression of  long-term potentiation (LTP). LTP in the associational/ commissural pathway requires NMDA receptor activation, postsynaptic depolarization, a rise in postsynaptic Ca2+, and insertion of postsynaptic AMPA type glutamate receptors. Induction of LTP is blocked by inhibition of nitric oxide synthase. Evidence from this study and other reviews implicates NO as a retrograde messenger in memory mechanism [28]. Though LTP is considered as a postsynaptic event, retrograde signaling of NO followed by cGMP-stimulated release of glutamate is suggested as a presynaptic mechanism(70, 71). A unifying hypothesis intended to explain the sildenafil-mediated memory enhancement proposes that accumulation of cGMP initiates a complex cascade. Presynaptic PDE5 inhibition increases the cGMP level and triggers the release of glutamate and subsequent NMDA receptor activation. Postsynaptic inhibition of PDE5 increases protein synthesis and synaptogenesis. Increased activity of cGMP-coupled ion channels may lead to early consolidation of information into memory. Electrophysiological experiments with long-term potentiation revealed that cGMP had to be kept high but below a certain threshold to reach the peak capacity to learn (72). Bernabeu et al., found that administration of cAMP into the hippocampus enhanced passive avoidance learning, suggesting cAMP involvement in later stages of memory consolidation processes. Direct administration of cGMP into the hippocampus improved object memory in rats whereas there was no improvement by cAMP. Cyclic GMP-regulated processes in the hippocampus play a significant role in the early stages of memory consolidation and cAMP signaling pathways are occupied in the late posttraining memory processing of inhibitory avoidance learning [10, 13]. LTP is mainly a postsynaptic event. However, LTP is expressed by increasing the presynaptic release of glutamate through the GC/ cGMP/PKG pathway. Glutamate- NO-cGMP pathway modulates important cerebral processes such as intercellular communication, the circadian rhythms and LTP (73). Sildenafil may also reduce the cognitive deficits associated with aging and be used for treating age-related neurodegeneration.

Production of  NO and cGMP is reduced with age and to some extent contributes to age-related

memory decline. Sildenafil, a specific inhibitor of cGMP degrading enzyme, therefore, offers a new strategy for memory improvement and a novel therapy for Alzheimer’s disease in the future.

Mechanism of action of sildenafil: 

From ancient times, penile erection has been considered a remarkably interesting topic. So many antique hypotheses and experimental findings have been given by several types of scientists, philosophers, and investigators. Hippocrates (460-370 BC) thought that the erection of the penis is caused by a critical spirit flowing through the penis, which is based on the perfect balance of four humors and four body elements. The four humors were blood, phlegm, yellow bile, and black bile. The four elements were earth, air, fire, and water. He considered the testes as the pulleys connecting the penis. On the other hand, another great philosopher Aristotle (384-322BC), hypothesized that the penile erection is an imagination-driven movement of the penis. He considered the testes as the fulcrums helping to lift the penis. Later Galen (129-200) considered that the penile erection is caused by the accumulation of air in the corpus cavernosum. But Da Vinci was the first person who correctly theorized that blood causes the erection of the penis. And ultimately, Von Haller from Switzerland theorized that this penile erection is controlled by the nervous system(74). These hypotheses demanded a deeper understanding of the penile erection, and animal studies were performed to understand the underlying mechanism behind this physiologic activity. It was found that the nervous system controlled smooth muscle relaxation in the corpora cavernosa and was responsible for the erection and hardening of the penis(74).

The contraction and relaxation of penile smooth muscles determine the flaccidity and rigidity of the penis, respectively. Intracellular Ca2+ ion concentration is the determinant of cell contraction or muscle contraction. Therefore, if the Ca2+ concentration decreases, the contraction will be decreased, and the relaxation will be triggered, resulting in a rigid and erect penis. Due to various kinds of sexual arousal, non-adrenergic, non-cholinergic neurotransmitter NO is synthesized due to the action of NO synthases. NO is produced from the nerve ends as well as from the endothelial cells. Diffused into the smooth muscles, they bind with the guanylyl cyclase to activate cyclic guanosine monophosphate (cGMP) synthesis. Upon synthesis, cGMP triggers the cGMP-dependent ion channels and/or cGMP-dependent protein kinases. This result in the reduction of the intracellular Ca2+ ion concentration. So, contraction is reduced, and the relaxation is induced to result in penile erection.

Sildenafil citrate is used to treat erectile dysfunction (E.D.) in men. It inhibits the phosphodiesterase 5 (PDE5) enzyme selectively. PDE5 is the main enzyme to degrade cGMP in the penile smooth muscle. Sildenafil inhibits the degradation of cGMP by selectively blocking the enzyme PDE5. Nitric oxide (NO), when released from nerve endings or endothelial cells, it binds with guanylyl cyclase. Activated guanylyl cyclase increases the concentration of cGMP intracellularly. Intracellular cGMP causes a reduction in Ca2+ concentration to provide muscle relaxation and erection of the penis.

PDE5 is a negative feedback mechanism to degrade the cGMP and helps return the penis to its original flaccid condition. Sildenafil citrate blocks the catalytic site of PDE5. Because the molecular structures of cGMP and sildenafil citrate are similar. Sildenafil competes with the cGMP for the binding site of the PDE5. Consequently, it extends the time needed to degrade the cGMP in the penile smooth muscle and provides erection for a longer time.

Beside that, there have been numerous reports of the use of sildenafil in cardiovascular diseases (80, 81), the Raynaud phenomenon (82), cystic fibrosis, cancer, diabetes, and neurological disorders such as Alzheimer's. Sildenafil also has 10

Sildenafil (Viagra) is a drug for treating erectile dysfunction and pulmonary arterial hypertension. It inhibits phosphodiesterase 5 (PDE5) which degrades cyclic guanosine monophosphate (cGMP). 

It is used for Erectile Dysfunction (E.D.). Although it has different kinds of applications, its function in improving Alzheimer’s disease remain unknown. By this study, we attempt to reveal this using aging model in rats.