Aytül Önal

Agmatine attenuates neuropathic pain in rats: Possible mediation of nitric oxide and noradrenergic activity in the brainstem and cerebellum

Effect of agmatine (10-400 mg/kg) on neuropathic pain in a rat model produced by loose ligatures around the common sciatic nerve was studied. The involvement of possible alterations in nitric oxide (NO) levels [measured as its stable metabolites nitrate + nitrite] and in noradrenergic activity [measured as norepinephrine and 3-methoxy-4-hydroxyphenylethylene glycol (MHPG) levels] in this effect was also investigated biochemically in the brainstem and cerebellum. Agmatine increased the neuropathic pain threshold at 300 and 400 mg/kg. There was almost a twofold increase in nitrate + nitrite levels in the brainstem and cerebellum of the rats with neuropathic pain and agmatine decreased the high nitrate + nitrite levels only in the brainstem at 300 mg/kg and both in the brainstem and cerebellum at 400 mg/kg. Ligation of sciatic nerve resulted in almost twofold increase in norepinephrine and MHPG levels only in the brainstem of the rats. Agmatine decreased MHPG levels at 300 and 400 mg/kg, however it decreased norepinephrine levels only at the higher dose. These findings indicate that agmatine decreases neuropathic pain, an effect which may involve the reduction of NO levels and noradrenergic activity in the brain.

Agmatine produces antinociception in tonic pain in mice.

Agmatine is an endogenous polyamine metabolite formed by decarboxylation of L-arginine. In this study, the effect of agmatine on tonic pain was compared to its effect on phasic pain by using the formalin and tail-flick (TF) tests in mice. When administered intraperitoneally (ip), agmatine (37.5-300 mg/kg) exhibited a decrease in nociceptive behaviours in the first and second phase of the formalin test, which is a tonic pain model. The alpha(2) adrenoceptor antagonist yohimbine blocked the effect of agmatine in Phase 2 but did not change its effect in Phase 1. In the TF test, there was no significant change in the behaviour of agmatine-administered (75-300 mg/kg) animals. As a result, agmatine appears to have an analgesic effect on tonic rather than phasic pain, and alpha(2) receptors seem partly to have a role in the antinociceptive effect of agmatine on tonic pain.

Milnacipran attenuates hyperalgesia and potentiates antihyperalgesic effect of tramadol in rats with mononeuropathic pain

Milnacipran is a non-tricyclic antidepressant drug which selectively inhibits serotonin and noradrenaline re-uptake and is recommended in the treatment of various chronic pain syndromes. Many studies have shown that compounds known to block monoamine uptake potentiate the antinociceptive effects of opioids. This study investigates the effect of milnacipran alone or in combination with an opiodergic drug, i.e. tramadol, on hyperalgesia in a rat model of neuropathic pain. The contribution of serotonergic, noradrenergic and opioidergic systems in the potential antihyperalgesic effect of milnacipran has also been examined. Chronic constriction injury was induced in rats by loose ligation of the sciatic nerve and neuropathic pain was evaluated 14 days after surgery. Intraperitoneal acute injection of milnacipran 60 mg/kg produced an antihyperalgesic effect which was prevented by pretreating systemically with alpha-methyl-p-tyrosine, an inhibitor of noradrenaline synthesis; parachlorophenylalanine, an inhibitor of serotonin synthesis; and naloxone, an antagonist of opioidergic receptors. Co-administration of milnacipran 40 mg/kg with tramadol (20 and 40 mg/kg) potentiated the antihyperalgesic effect of tramadol. Milnacipran has an antihyperalgesic effect mediated by serotonergic, noradrenergic and opioidergic systems and the combined use of tramadol with milnacipran potentiates the effect of tramadol in the management of neuropathic pain.

Effects of repeated administered ghrelin on chronic constriction injury of the sciatic nerve in rats.

Chronic constriction injury (CCI) is a peripheral mononeuropathic pain model that is caused by an injury to the peripheral nervous system and refractory to available conventional treatment. Mechanisms involved in neuropathic pain are still unclear. Previous studies reveal that proinflammatory cytokines contribute to CCI-induced peripheral nerve pathology. Ghrelin, a novel identified gastric peptide, has been shown to have antinociceptive activity and also anti-inflammatory properties by decreasing proinflammatory cytokines. Therefore, the aim of the present study was to investigate the effects of ghrelin on the CCI and its relationship with proinflammatory cytokines in rats. Wistar rats underwent sciatic nerve ligation to induce CCI fallowed by repeated ghrelin administrations (50 and 100microg/kg i.p., once daily) for a period of 14 days. Mechanical hyperalgesia was assessed before surgery and at day 14 after CCI. TNF-alpha, IL-1beta and IL-6 were measured in blood and spinal cord. The changes of sciatic nerve was assessed histologically by both light and electron microscopy. Ghrelin attenuated mechanical hyperalgesia, reduced spinal TNF-alpha and IL-1beta levels and enhanced sciatic nerve injury with correlated morphometric recovery. These results indicate that the protective effect by ghrelin in the spinal cord is mediated through the suppression of TNF-alpha and IL-1beta. Thus ghrelin may be a promising peptide in the management of neuropathic pain.

Antihyperalgesic and antiallodynic effect of sirolimus in neuropathic pain and the role of cytokines in this effect.

Recent studies have revealed that T lymphocytes play a role in neuropathic pain following nerve injury in rats through releasing several cytokines. Sirolimus is an immunosuppressive antibiotic inhibiting T cell activation. This study aimed to determine the effect of sirolimus on hyperalgesia and allodynia and on serum and spinal cord TNF-alpha, IL-1beta and IL-6 levels in rat neuropathic pain. Neuropathic pain was induced by loose ligation of the sciatic nerve and evaluated by tests measuring the mechanical hyperalgesia and allodynia. Sirolimus (0.75 and 1.5 mg/kg) was administered intraperitoneally once every 3 days for 2 weeks (7 doses totally). This dosing regimen revealed acceptable blood concentrations in neuropathic rats. Chronic constriction injury of the sciatic nerve resulted in hyperalgesia and allodynia. Serum levels of cytokines remained unchanged in neuropathic rats. However, TNF-alpha, but not IL-1beta or IL-6, protein level was increased in the spinal cord tissue as evaluated by Western blotting analysis. Treatment with sirolimus resulted in antihyperalgesic and antiallodynic effects and prevented the increased spinal cord TNF-alpha level. It seems that sirolimus could be a promising immunosuppressive agent in the treatment of neuropathic pain.

Effect of Nabumetone Treatment on Vascular Responses of the Thoracic Aorta in Rat Experimental Arthritis

Nabumetone is a nonsteroidal anti-inflammatory (NSAI) drug which is known to cause less gastrointestinal damage than other NSAI drugs. This study was performed to evaluate whether nabumetone treatment might alter the vascular aberrations related to inflammation in a rat model of adjuvant-induced arthritis. Nabumetone treatment (120 or 240 mg·kg–1·day–1, orally) was initiated on the 15th day of adjuvant inoculation and continued for 14 days. Arthritic lesions, vascular contractile and relaxant responses and gastroduodenal histopathological preparations were evaluated 29 days after adjuvant inoculation. The contractile responses of aortic rings to phenylephrine and KCl were increased in grade 2 arthritic rats. In grade 3 arthritis only the phenylephrine contractility was decreased. The relaxant responses to acetylcholine and sodium nitroprusside were decreased in grades 2 and 3. In healthy rats, nabumetone did not change the vascular responses. After treatment of arthritic rats with nabumetone, both the contractile and relaxant response of the aortic rings returned to normal, and arthritic score and paw swelling were reduced. Gastroduodenal histopathology did not show erosions or ulcers in any of the groups. In conclusion, nabumetone improved the systemic signs and vascular alterations in experimental arthritis without showing any gastrointestinal side effects.

The antinociceptive effect of leukotriene D4 receptor antagonist, MK-571, in mice : Possible involvement of opioidergic mechanism

The effect of a leukotriene D(4) receptor antagonist, (3-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl(3-dimethyl amino-3-oxo propyl)thio)methyl)thio) propanoic acid (L-660,711; MK-571), was investigated on nociceptive responses in mice using three different assays: acetic-acid-induced abdominal constrictions, formalin response and tail-flick test. MK-571 (8-32 mg/kg, i.v.) produced dose-dependent protection against acetic-acid-induced abdominal constriction (ED(50)=30 mg/kg). The compound (10-80 mg/kg, i.p.) was also effective, in a dose-dependent manner, on the second phase of the formalin response (ED(50)=26 mg/kg). However, it had no effect on the first phase of the formalin response and in the tail-flick test. Naloxone (1 mg/kg, i.v.), an opioid antagonist, almost completely blocked the antinociceptive effect of MK-571 in both acetic-acid-induced abdominal constriction and the second phase of the formalin test. These results provide evidence for an antinociceptive action of MK-571 at peripheral sites and suggest that opioid mechanisms are involved.

Antihyperalgesic and antiallodynic effect of sirolimus in rat model of adjuvant arthritis.

Sirolimus is an immunosupressive drug that specifically inhibit the activation of T-lymphocytes. This study was undertaken to investigate whether treatment with sirolimus exert analgesic effect in rat adjuvant-induced arthritis, an animal model of rheumatoid arthritis. Arthritis was induced by a single subcutaneous injection of Freunds complete adjuvant to male Wistar rats that were divided into four groups; control (saline), vehicle (ethanol), sirolimus 0.75 and sirolimus 1.5. Sirolimus (0.75 and 1.5mg/kg/day) was administered intraperitoneally using Monday-Wednesday-Friday dosing schedule for 29 days, this dosing regimen revealed acceptable trough blood concentrations in arthritic rats. Adjuvant inoculation resulted in paw inflammation, hyperalgesia and allodynia as assessed by pletismometer, analgesymeter and dynamic plantar aesthesiometer respectively. Light microscopic evaluation of the arthritic metacarpophalangeal joints revealed synovial hypertrophy with inflammatory cellular infiltration, cartilage destruction and partial subchondral bone resorption. ELISA tests of serum TNF-α, IL-1β or IL-6 did not show any change in arthritic rats, while Western blotting analysis revealed a significant increase in TNF-α (P<0.001), but not IL-1β or IL-6, protein expression in the lumbar spinal cord of arthritic rats. Treatment with sirolimus significantly decreased the arthritic lesions (P<0.001) and paw swelling (P<0.05), alleviated the histological features in the metacarpophalangeal joint, resulted in antihyperalgesic and antiallodynic effects without affecting the locomotor activity and prevented the increased spinal cord TNF-α level (P<0.05). It seems that prevention of the increased TNF-α expression in the spinal cord may partially contribute to the antihyperalgesic effect of sirolimus in adjuvant arthritic rats and sirolimus could be a promising immunosupressive agent in the treatment of arthritic pain.

The relationship between pupil diameter and pain by the administration of morphine and antidepressant drugs in mice.

Because the pain sensation is subjective, it is difficult to evaluate the responses to analgesic drugs. Some analgesics that affect the central nervous system are known to change the pupil diameter. The pupil diameter is a more objective criterion that shows the drug effect. We studied the relation between the pupil diameter and analgesia responses to morphine and antidepressants by using the selective micro-receptor agonist morphine (2 and 4 mg/kg), the noradrenaline reuptake inhibitor desipramine (7.5 and 10 mg/kg), the mixed serotonergic and noradrenergic uptake inhibitor and cholinergic receptor antagonist amitriptyline (2.5 and 5 mg/kg), and the selective serotonin reuptake inhibitor sertraline (2.5 and 5 mg/kg) in mice. Both monocular microscopy to assess pupil measurement and the hot-plate test to assess nociceptive thresholds were used in the same animals. We found that morphine played an important role in both mydriasis and analgesia, whereas amitriptyline and desipramine had a greater effect on pupil response than on nociception. Sertraline produced antinociception without causing a change in pupil diameter. As a result, although the pupil response is an important criterion in evaluating the analgesic effect of morphine, it is not possible to put forward the same criterion for the antidepressant drugs. Because different neurotransmitters are involved in pupil and pain mechanisms of antidepressant drugs, it is difficult to evaluate the analgesic response with the pupil diameter.

NADPH-d and Fos reactivity in the rat spinal cord following experimental spinal cord injury and embryonic neural stem cell transplantation

AIMS The aim of this study is to determine the role of nitric oxide (NO) in neuropathic pain and the effect of embryonic neural stem cell (ENSC) transplantation on NO content in rat spinal cord neurons following spinal cord injury (SCI). MAIN METHODS Ninety adult male Sprague-Dawley rats were divided into 3 groups (n=30, each): control (laminectomy), SCI (hemisection at T12-T13 segments) and SCI+ENSC. Each group was further divided into sub-groups (n=5 each) based on the treatment substance (L-NAME, 75 mg/kg/i.p.; L-arginine, 225 mg/kg/i.p.; physiological saline, SF) and duration (2h for acute and 28 days for chronic groups). Pain was assessed by tail flick and Randall-Selitto tests. Fos immunohistochemistry and NADPH-d histochemistry were performed in segments 2 cm rostral and caudal to SCI. KEY FINDINGS Tail-flick latency time increased in both acute and chronic L-NAME groups and increased in acute and decreased in chronic L-arginine groups. The number of Fos (+) neurons decreased in acute and chronic L-NAME and decreased in acute L-arginine groups. Following ENSC, Fos (+) neurons did not change in acute L-NAME but decreased in the chronic L-NAME groups, and decreased in both acute and chronic L-arginine groups. NADPH-d (+) neurons decreased in acute L-NAME and increased in L-arginine groups with and without ENSC transplantation. SIGNIFICANCE This study confirms the role of NO in neuropathic pain and shows an improvement following ENSC transplantation in the acute phase, observed as a decrease in Fos(+) and NADPH-d (+) neurons in spinal cord segments rostral and caudal to injury.