The effects of sarmesin, an Angiotensin II analogue on seizure susceptibility, memory retention and nociception

From Angiotensin II to Cyclic Peptides and Angiotensin Receptor Blockers (ARBs): Perspectives of ARBs in COVID-19 Therapy

The octapeptide hormone angiotensin II is one of the most studied peptides with the aim of designing and synthesizing non-peptide mimetics for oral administration. To achieve this, cyclizations at different positions within the peptide molecule has been a useful strategy to define the active conformation. These studies on angiotensin II led to the discovery of Sarmesin, a type II angiotensin II antagonist, and the breakthrough non-peptide mimetic Losartan, the first in a series of sartans marketed as a new generation of anti-hypertensive drugs in the 1990s. Angiotensin II receptor blockers (ARBS) and angiotensin I converting enzyme inhibitors (ACEI) were recently reported to protect hypertensive patients infected with SARS-CoV-2. The renin–angiotensin system (RAS) inhibitors reduce excess angiotensin II and increase antagonist heptapeptides alamandine and aspamandine which counterbalance angiotensin II and maintain homeostasis and vasodilation.

Effect of captopril, an angiotensin-converting enzyme inhibitor, on morphine analgesia and tolerance in rats, and elucidating the inflammation and endoplasmic reticulum stress pathway in this effect

The purpose of current study was to examine the possible involvement of captopril, an angiotensin-converting enzyme inhibitor, on nociception, morphine analgesia and morphine tolerance development involving inflammation and ER-stress pathways in rats. In this study, thirty-six male Wistar rats were used. Animals were divided into six groups: Saline, 50 mg/kg captopril, 5 mg/kg morphine, morphine + captopril, morphine tolerance and morphine tolerance + captopril. The resulting analgesic effect was measured with hot plate and tail flick analgesia tests. The dorsal root ganglions (DRG) tissues were collected for inflammation parameters, endoplasmic reticulum (ER) stress and apoptosis proteins by using ELISA. Captopril showed anti-nociceptive effect when given alone (p < 0.05 to p < 0.01). In addition, captopril increased the analgesic effect of morphine (p < 0.05 to p < 0.001) and also decreased the tolerance to morphine at a significant level (p < 0.05 to p < 0.001). However, it decreased inflammation and ER-stress when applied with single-dose morphine and tolerance induction (p < 0.001). Moreover, captopril decreased apoptosis proteins after tolerance development (p < 0.001). In conclusion, captopril has antinociceptive properties, increasing analgesic effect of morphine, and preventing tolerance development. These effects may occur by suppressing inflammation and ER-stress pathways.

Experimental Models for the Discovery of Novel Anticonvulsant Drugs: Focus on Pentylenetetrazole-Induced Seizures and Associated Memory Deficits

Epilepsy is a chronic neurological disorder characterized by irregular, excessive neuronal excitability, and recurrent seizures that affect millions of patients worldwide. Currently, accessible antiepileptic drugs (AEDs) do not adequately support all epilepsy patients, with around 30% patients not responding to the existing therapies. As lifelong epilepsy treatment is essential, the search for new and more effective AEDs with an enhanced safety profile is a significant therapeutic goal. Seizures are a combination of electrical and behavioral events that can induce biochemical, molecular, and anatomic changes. Therefore, appropriate animal models are required to evaluate novel potential AEDs. Among the large number of available animal models of seizures, the acute pentylenetetrazole (PTZ)-induced myoclonic seizure model is the most widely used model assessing the anticonvulsant effect of prospective AEDs, whereas chronic PTZ-kindled seizure models represent chronic models in which the repeated administration of PTZ at subconvulsive doses leads to the intensification of seizure activity or enhanced seizure susceptibility similar to that in human epilepsy. In this review, we summarized the memory deficits accompanying acute or chronic PTZ seizure models and how these deficits were evaluated applying several behavioral animal models. Furthermore, major advantages and limitations of the PTZ seizure models in the discovery of new AEDs were highlighted. With a focus on PTZ seizures, the major biochemicals, as well as morphological alterations and the modulated brain neurotransmitter levels associated with memory deficits have been illustrated. Moreover, numerous medicinal compounds with concurrent anticonvulsant, procognitive, antioxidant effects, modulating effects on several brain neurotransmitters in rodents, and several newly developed classes of compounds applying computer-aided drug design (CADD) have been under development as potential AEDs. The article details the in-silico approach following CADD, which can be utilized for generating libraries of novel compounds for AED discovery. Additionally, in vivo studies could be useful in demonstrating efficacy, safety, and novel mode of action of AEDs for further clinical development.

Protective Effect of Quinine on Chemical Kindling and Passive Avoidance Test in Rats

Background

In humans, convulsive diseases such as temporal lobe epilepsy are usually accompanied by learning and memory impairments. In recent years, the role of gap junction channels as an important target of antiepileptic drugs has been studied and discussed. Quinine, as a gap junction blocker of connexin 36, can abolish ictal epileptiform activity in brain slices.

Objectives

The role of quinine in memory retrieval in pentylenetetrazole (PTZ)-kindled rats was examined using a step-through passive avoidance task.

Methods

Forty rats were used in this experimental study in groups of 10 animals. Quinine (15, 30, and 60 mg/kg, i.p.) and PTZ (35 mg/kg, i.p.) were injected into the rats before the start of the learning test. Then, retention tests were conducted after the treatments ended.

Results

Quinine could attenuate seizure severity at doses of 15, 30 and 60 mg/kg compared with the control at the beginning of the kindling experiment by lowering the mean seizure stages (P < 0.01, P < 0.001, P < 0.001). Quinine at doses of 15 and 30 mg/kg could significantly increase memory retrieval compared with the control in the retention test 24 and 48 hours after training (P < 0.05). Quinine at a dose of 60 mg/kg increased latency to enter the dark chamber 24 and 48 hours after training (P < 0.001). The results of the retention test one and two weeks after training of quinine were not significant (P > 0.05).

Conclusions

Quinine may decrease the severity of seizure and improve the memory retrieval of animals by inhibiting the gap junction channel. However, further studies are needed to evaluate the molecular mechanism underlying the effects of quinine.

Effects of quercetin on oxidative stress and memory retrieval in kindled rats

Flavonoids are a class of polyphenolic compounds present in fruits and vegetables. Several studies have demonstrated a relationship between the consumption of flavonoid-rich diets and the prevention of human diseases including neurodegenerative disorders. Thus, we assessed the effect of quercetin (3,3',4',5,7-pentahydroxyflavone) on oxidative stress and memory retrieval using a step-through passive avoidance task in kindled rats. Quercetin (25, 50, and 100 mg/kg) was administered intraperitoneally (i.p.) before pentylenetetrazole (PTZ) every other day prior to the training. Retention tests were performed to assess memory in rats. Compared to control, pretreatment with 50 mg/kg of quercetin could attenuate seizure severity from the beginning of the kindling experiment by lowering the mean seizure stages. Moreover, quercetin 50 mg/kg significantly increased the step-through latency of the passive avoidance response compared to the control in the retention test. Malondialdehyde (MDA) levels were significantly increased in the quercetin groups compared to the PTZ group in the hippocampus and cerebral cortex following PTZ kindling. In the quercetin groups, higher sulfhydryl (SH) contents were not observed compared to the PTZ group. These results indicate that quercetin at a specific dose results in decreased seizure severity during kindling and performance improvement in a passive avoidance task in kindled rats. All doses of quercetin led to increased oxidative stress in the hippocampi and cerebral cortices of kindled rats.

The effects of rutin on the development of pentylenetetrazole kindling and memory retrieval in rats

Individuals with epilepsy often complain about memory deficits. Various synthetic derivatives of natural flavonoids are known to have neuroactive properties. Rutin is a flavonoid that is an important dietary constituent of foods and plant-based beverages. The aim of the present study was to investigate the effects of rutin on memory retrieval in pentylenetetrazole (PTZ)-kindled rats using a step-through passive avoidance task. We administered rutin and PTZ intraperitoneally every other day prior to the start of training. Two retention tests were subsequently performed to assess memory in these rats. The results suggest that pretreatment with rutin at 50 and 100mg/kg can attenuate seizure severity during the kindling procedure. Furthermore, rutin administration significantly increased the step-through latency in the passive avoidance paradigm. Taken together, these results indicate that rutin has a potential role in enhancing memory retrieval in kindled rats.

Effects of microinjection of angiotensin II and captopril to VTA on morphine self-administration in rats

The dopaminergic mesolimbic system is considered to be crucial in rewarding actions of opiates. Recent studies have suggested probable interaction between the renin-angiotensin and mesolimbic dopaminergic systems. The present study was undertaken to investigate the effects of Ang II and captopril injection into VTA on morphine self-administration. Male Wistar rats were initially trained to receive small pellets of food by pressing the active lever in self-administration apparatus. The animals were divided into 4 groups (saline, morphine, captopril and Ang II) and were placed in self-administration apparatus and allowed to self-administer morphine (0.5 mg per infusion all test groups) or saline (saline group) during consecutive days, for 2 h/sessions. Captopril (30 mug) and Ang II (0.25 nmol) were injected into the VTA in the corresponding groups before each session. The numbers of active and passive levers pressed in each group have been recorded. The number of active lever pressing of morphine group was significantly higher than saline group (p < 0.001). In Ang II group, the number of active lever pressing was significantly lower than morphine group (p < 0.01). This study suggests the probable interaction between Ang II and opioid system in the VTA.

Ang II and Ang IV: unraveling the mechanism of action on synaptic plasticity, memory, and epilepsy

The central angiotensin system plays a crucial role in cardiovascular regulation. More recently, angiotensin peptides have been implicated in stress, anxiety, depression, cognition, and epilepsy. Angiotensin II (Ang II) exerts its actions through AT(1) and AT(2) receptors, while most actions of its metabolite Ang IV were believed to be independent of AT(1) or AT(2) receptor activation. A specific binding site with high affinity for Ang IV was discovered and denominated "AT(4) receptor". The beneficiary effects of AT(4) ligands in animal models for cognitive impairment and epileptic seizures initiated the search for their mechanism of action. This proved to be a challenging task, and after 20 years of research, the nature of the "AT(4) receptor" remains controversial. Insulin-regulated aminopeptidase (IRAP) was first identified as the high-affinity binding site for AT(4) ligands. Recently, the hepatocyte growth factor receptor c-MET was also proposed as a receptor for AT(4) ligands. The present review focuses on the effects of Ang II and Ang IV on synaptic transmission and plasticity, learning, memory, and epileptic seizure activity. Possible interactions of Ang IV with the classical AT(1) and AT(2) receptor subtypes are evaluated, and other potential mechanisms by which AT(4) ligands may exert their effects are discussed. Identification of these mechanisms may provide a valuable target in the development in novel drugs for the treatment of cognitive disorders and epilepsy.

Neural mechanism underlying acupuncture analgesia

Acupuncture has been accepted to effectively treat chronic pain by inserting needles into the specific "acupuncture points" (acupoints) on the patient's body. During the last decades, our understanding of how the brain processes acupuncture analgesia has undergone considerable development. Acupuncture analgesia is manifested only when the intricate feeling (soreness, numbness, heaviness and distension) of acupuncture in patients occurs following acupuncture manipulation. Manual acupuncture (MA) is the insertion of an acupuncture needle into acupoint followed by the twisting of the needle up and down by hand. In MA, all types of afferent fibers (Abeta, Adelta and C) are activated. In electrical acupuncture (EA), a stimulating current via the inserted needle is delivered to acupoints. Electrical current intense enough to excite Abeta- and part of Adelta-fibers can induce an analgesic effect. Acupuncture signals ascend mainly through the spinal ventrolateral funiculus to the brain. Many brain nuclei composing a complicated network are involved in processing acupuncture analgesia, including the nucleus raphe magnus (NRM), periaqueductal grey (PAG), locus coeruleus, arcuate nucleus (Arc), preoptic area, nucleus submedius, habenular nucleus, accumbens nucleus, caudate nucleus, septal area, amygdale, etc. Acupuncture analgesia is essentially a manifestation of integrative processes at different levels in the CNS between afferent impulses from pain regions and impulses from acupoints. In the last decade, profound studies on neural mechanisms underlying acupuncture analgesia predominately focus on cellular and molecular substrate and functional brain imaging and have developed rapidly. Diverse signal molecules contribute to mediating acupuncture analgesia, such as opioid peptides (mu-, delta- and kappa-receptors), glutamate (NMDA and AMPA/KA receptors), 5-hydroxytryptamine, and cholecystokinin octapeptide. Among these, the opioid peptides and their receptors in Arc-PAG-NRM-spinal dorsal horn pathway play a pivotal role in mediating acupuncture analgesia. The release of opioid peptides evoked by electroacupuncture is frequency-dependent. EA at 2 and 100Hz produces release of enkephalin and dynorphin in the spinal cord, respectively. CCK-8 antagonizes acupuncture analgesia. The individual differences of acupuncture analgesia are associated with inherited genetic factors and the density of CCK receptors. The brain regions associated with acupuncture analgesia identified in animal experiments were confirmed and further explored in the human brain by means of functional imaging. EA analgesia is likely associated with its counter-regulation to spinal glial activation. PTX-sesntive Gi/o protein- and MAP kinase-mediated signal pathways as well as the downstream events NF-kappaB, c-fos and c-jun play important roles in EA analgesia.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Angiotensin converting enzyme inhibitor captopril modifies conditioned place preference induced by morphine and morphine withdrawal signs in rats

Angiotensin II and angiotensin converting enzyme (ACE) inhibitors have analgesic, anticonvulsant and antidepressive effects and in some cases they can antagonize morphine. In the present study effects of angiotensin II and ACE inhibitor captopril administered intracerobroventricularily (icv) on conditioned place preference induced by morphine as well as on morphine withdrawal signs has been evaluated in rats. Icv canullas were implanted in anesthetized male rats. Rats were allowed to recover from the surgery and conditioned place preference was induced by morphine, and the time spent in morphine compartment was compared in saline, morphine, captopril and Ang II groups. Morphine withdrawal signs were compared in three other groups of rats: morphine alone, captopril+morphine and Ang II+morphine 4 days after morphine injections (three times in each day) with naloxone injection on 4th day. Results with rats conditioned place preference induced by morphine showed that icv captopril decreased significantly the time in morphine compartment (P<0.01) while Ang II had no effect. In morphine dependent rats captopril decreased some withdrawal signs after naloxone precipitation (P<0.05 and P<0.01). Ang II administration augmented some of withdrawal signs than in the morphine group (P<0.01 and P<0.001). In conclusion captopril reduced conditioned place preference induced by morphine and some withdrawal signs in morphine dependent rats.

Nociception and locomotor activity are increased in ketogenic diet fed rats

Ketogenic diets have been used to treat epilepsy in children for almost 80 years. However, there are only few studies concerning behavioral effects of these diets, besides their efficacy in treating seizure disorders induced by kainic acid or pentylenetetrazol in rats. Here, rats were fed with a ketogenic diet and locomotion, anxiety and nociception were investigated after 10 weeks. Male Wistar rats were weight matched and divided into two groups: control rats, that received regular laboratory ration, and KD rats, that received ketogenic diet (70% fat, 24% protein and no carbohydrate). Behavioral tests were applied after 10-12 weeks of treatment, and included tests to evaluate exploration (habituation to the open field), anxiety (plus-maze), and nociception (tail-flick measurement). Performance of the animals in the open field revealed a significant difference in the number of crossings, suggesting a higher locomotor activity in animals fed with a ketogenic diet. No differences in anxiety were observed, as evaluated by the plus-maze test. Nociception was measured by the latency in the tail-flick test, and ketogenic rats presented a hypernociceptive response. Yet, these animals responded to a stressor with the classic analgesia, similarly to the controls. The response of ketogenic diet fed rats to the stressor, however, was more prolonged. Exposure to a ketogenic diet may induce higher locomotor activity, together with a hypernociceptive state in the animals, possibly as a result of some alteration in the neural systems involved in the modulation of these behaviors.

Angiotensin peptides modulatory system: how is it implicated in the control of seizure susceptibility?

Accumulated studies support the concept that angiotensin peptides, ANG II, ANG III, and ANG IV act as neurotransmitters or neuromodulators in specific neuronal pathways in the brain stem, the hypothalamus, and the forebrain. They have been implicated in the regulation of several physiological processes, particularly in excitable brain structures that express high concentration of their receptors. With the help of pharmacological approaches it was shown that angiotensin peptides appear to be anticonvulsant in a variety of experimental seizure models. Thus, ANG II increases the threshold for pentylenetetrazol (PTZ)-, bicuculline-and picrotoxin-induced seizures in mice. It also attenuates the intensity of clonic seizures evoked by PTZ and 3-mercaptopropionic acid and is effective in the maximal electroshock test. Furthermore, ANG II, ANG III, and ANG IV protect against the clonic convulsions in the PTZ kindling model of epilepsy in mice. From the accumulated results it could be assumed that the angiotensin peptides appear to realize their effects acting directly on their receptors (AT(1), AT(2) and AT(4)) and through close interaction with different neurotransmitter/neuromodulator systems as dopamine (DA)-, gamma-aminobutyric acid (GABA)-and adenosine. This may contribute to a new potential use of angiotensin drugs either alone or in combination with other neuroprotective agents acting through the above mentioned systems, thus providing a more rational strategy for the treatment of neurodegenerative disorders such as epilepsy.