Omega-3 fatty acids supplementation prevents learning and memory impairment induced by chronic ethanol consumption in adolescent male rats through restoration of inflammatory and oxidative responses.

OBJECTIVE: Ethanol (Eth) intake is known to cause numerous detrimental effects on the structure and function of the brain, and it is commonly used as a psychostimulant drug by adolescents. Conversely, omega-3 (O3) can reduce the risk of cognitive decline and promote the maintenance of neurophysiological functions. In this study, we investigated the protective effects of O3 on behavioral alterations, oxidative stress, and interleukin-6 (IL-6) levels induced by chronic Eth intake during adolescence in rats. MATERIALS AND METHODS: Adolescent male rats (21 days old) were divided as follows: (1) Vehicle, (2) Eth (Eth in drinking water [20%]), (3-5) Eth + O3 (50/100/150 mg/kg), and (6) O3 (150 mg/kg). After 5 weeks, Morris water maze (MWM) and passive avoidance (PA) tests were performed, and the hippocampal and cortical levels of oxidative stress markers and inflammatory indices were measured. RESULTS: Adolescent Eth intake impairs learning and memory function in MWM and PA tests (groups × day, p < 0.05 and p < 0.001, respectively). It was shown that Eth induced oxidative stress and neuroinflammation. O3 improved learning and impairment induced by Eth by reducing the adverse effects of Eth on the oxidant/antioxidant balance in the hippocampi (for malondialdehyde [MDA]/thiol: p < 0.01, p < 0.001, respectively) and for superoxide dismutase (SOD)/catalase (CAT): p < 0.01 and p < 0.05, respectively). Furthermore, we found that O3 prevented the Eth-induced increase of hippocampal IL-6 (p < 0.001). CONCLUSION: O3 supplementation acts as an effective approach to prevent learning and memory impairments induced by chronic Eth consumption during adolescence. In this respect, the antioxidant and anti-inflammatory properties of O3 seem to be the main underlying mechanisms of neuroprotection.

Omega-3 fatty acids prevent nicotine withdrawal-induced exacerbation of anxiety and depression by affecting oxidative stress balance, inflammatory response, BDNF and serotonin metabolism in rats.

Adolescents are known to be more vulnerable than adults to the adverse effects of nicotine dependence. In the present study, we aimed to investigate whether adolescent nicotine exposure, followed by a period of abstinence, could affect the anxiety- and depressive-like behaviors in rats. For this purpose, behavioral assessments were carried out using open field test, elevated plus maze and forced swimming test in male rats received chronic nicotine intake during adolescence followed by a period of abstinence in adulthood, compared to their control counterparts. In addition, O3 pre-treatment was done at three different doses to reveal whether it could prevent nicotine withdrawal effects. Then, animals were euthanized and the cortical concentrations of oxidative stress markers, inflammatory indices, brain-derived neurotrophic factor, serotonin and the enzymatic activity of monoamine oxidase-A were measured. Results indicated that nicotine withdrawal exacerbates the behavioral signs of anxiety through alteration of the brain oxidative stress balance, inflammatory response and serotonin metabolism. Moreover, we found that omega 3 pre-treatment significantly prevents the nicotine withdrawal-induced complications by restoration of changes in the mentioned biochemical indices. Moreover, the improving effects of O3 fatty acids were found to be dose-dependent in all experiments. Taken together, we would like to suggest the O3 fatty acids supplementation as a safe, inexpensive and effective strategy for prevention or amelioration of detrimental effects induced by nicotine withdrawal at cellular and behavioral levels.

The Role of 5-HT1A Receptors and Neuronal Nitric Oxide Synthase in a Seizur Induced Kindling Model in Rats.

BACKGROUND AND OBJECTIVE: Dentate gyrus (DG) has a high density of 5-HT1A receptors. It has neural nitric oxide synthase (nNOS), which is involved in neural excitability. The purpose of this study was to investigate the role of 5-HT1A receptors and nNOS of DG in perforant path kindling model of epilepsy. MATERIAL AND METHODS: To achieve this purpose, a receptor antagonist (WAY100635, 0.1 mg/kg, intracerebroventricular, i.c.v) and neuronal nitric oxide synthase inhibitor (7-NI, 15 mg/kg, intraperitoneal, i.p.) were injected during kindling aquisition. Adult male Wistar rats (280 ± 20 g) were used in this study Animals were kindled through the daily administration of brief electrical stimulations (10 stimulations per day) to the perforant pathway. Field potential recordings were performed for 20 min in DG beforehand. Additionally, glial fibrillary acidic protein (GFAP) expression rate in the DG was determined using immunohistochemistry as a highly specific marker for glia. RESULTS: WAY100635 (0.1 mg/kg) significantly attenuated the kindling threshold compared to the kindled + vehicle group (P < 0.001). The co-administration of WAY100635 with 7-NI, exerted a significant anticonvulsive effect. Furthermore, the slope of field Excitatory Post Synaptic Potentials (fEPSP) at the end of 10 days in the kindled + 7-NI + WAY100635 group was significantly lower than in the kindled + vehicle group (P < 0.001). Furthermore, immunohistochemistry showed that the density of GAFP+ cells in the kindled + 7-NI + WAY100635 group was significantly higher than in the kindled + vehicle group (P < 0.001). CONCLUSION: Our data demonstrate that antagonists of 5-HT1A receptors have proconvulsive effects and that astrocyte cells are involved in this process, while nNOS has an inhibitory effect on neuronal excitability.

Beneficial effects of vitamin D on anxiety and depression-like behaviors induced by unpredictable chronic mild stress by suppression of brain oxidative stress and neuroinflammation in rats.

The objective of this study was to investigate the protective effects of vitamin D (Vit D) on anxiety and depression-like behaviors induced by unpredictable chronic mild stress and brain tissue oxidative damage criteria and neuroinflammation in rats. The rats were treated as follows: (1) control, (2) UCMS, (3-5) Vit D 100, 1000, and 10,000 iu + UCMS. Rats were subjected to UCMS for a total of 4 weeks. During week 4, they received seven training trials. The brains were then collected to examine inflammation and oxidative stress criteria. Pretreatment with Vit D enhanced performances of the rats in the elevated plus maze (EPM) and open field (OF) and forced swimming test (FST). UCMS also increased MDA and interleukin-6 (IL-6) levels while decreased CAT, SOD, and thiol. Vit D reversed the effects of UCMS. The results of the current research revealed that Vit D improved UCMS-induced anxiety and depression via decreasing brain oxidative stress and inhibiting neuroinflammation.

Effects of aqueous extract of Hyssopus officinalis on seizures induced by pentylenetetrazole and hippocampus mRNA level of iNOS in rats.

OBJECTIVE: We examined the effectiveness of Hyssopus officinalis (hyssop) aqueous extract on pentylenetetrazole (PTZ)-induced acute seizures and the hippocampus iNOS (inducible nitric oxide synthases) gene expression as a potential mediator of the effects. MATERIALS AND METHODS: Adult male Wistar rats were used. Tonic-clonic seizures were induced by intraperitoneal (i.p.) injection of PTZ (80 mg/kg) then behavioral profile during 30 min was characterized by stages defined as seizure scores. Hyssop extract were prepared and injected (i.p.) 15 minutes before the seizure induction at three doses 50, 100 and 200 mg/kg. Experimental groups were as below: (1) saline+PTZ (n=5); (2) Hyssop 50mg/kg+PTZ (n=10); (3) Hyssop 100mg/kg+PTZ (n=10); (4) Hyssop 200 mg/kg+PTZ (n=8). Two hours after the experimental procedure, all animals were decapitated, brain was removed and right hippocampus was quickly dissected. After total RNA extraction and cDNA synthesis quantitative PCR were used for gene expression of iNOS. RESULTS: Our results showed significant increase (p<0.05) in latency to reach stages 5 and 6 of tonic-clonic seizure at dose 100 mg/kg hyssop extract. In addition, this dose caused significant increase in the gene expression of iNOS in the hippocampus. CONCLUSION: It seems a 100 mg/kg dose of hyssop extract might have anticonvulsant effects. However, these anticonvulsant effects might not occur through the iNOS gene expression.

Coregulation of sleep-pain physiological interplay by orexin system: An unprecedented review.

Accumulating evidence support the critical role of endogenous orexin system in modulation of various physiological functions. Among these, regulation of pain and wakefulness have extensively been investigated, however, by independent series of studies each focusing a distinct side. It is now well established that orexins induce potent analgesic effect via affecting their receptors within several specific brain structures. These mainly include locus coeruleus (LC), lateral paragigantocellularis (LPGi), ventral tegmental area (VTA), dorsal raphe nucleus (DRN), periaquiductal gray (PAG) and tuberomammillary nuclei (TMN). On the other hand, increased activity of orexinergic neurons enhances general wakefulness. Interestingly, a review of literature reveals that brain regions underlying orexin-mediated analgesia are most probably the site of action for orexin wake-promoting effects as well. The present study first pieces together the existing evidence supporting the rationale for the possibility of sleep-pain coregulation by orexin system and then suggests several shared mechanisms through which orexin can control the two mentioned processes. Furthermore, this study explains how imbalanced orexinergic transmission can cause progressive dysregulation of sleep-pain processing.

Involvement of Endogenous Opioid System in Swim Stress-Induced Pain Modulation During the Interphase of the Formalin Test.

INTRODUCTION: Some evidence demonstrates endogenous inhibitory pathways of pain involved in the interphase (phase between early and later phase) of the formalin test. We previously showed that swimming stress modulates the pain-related behaviors during the interphase of the formalin test. In this study, we evaluated the role of the endogenous opioid system in modulating nociceptive responses of the formalin test. METHODS: Swim stress was performed in different heights of water (5, 25, 50 cm) in a swimming tank. The mean nociceptive scores were measured during phase 1 (1-7 min), interphase (8-14 min), and phase 2 (15-90 min) of the formalin test. Opioid receptor antagonist, naloxone (3 mg/kg; IP) was injected immediately before swim stress. RESULTS: Swim stress attenuated nociceptive behaviors in the first phase and increased the duration of interphase in the formalin test in a water-height-dependent manner, compared to the control group. Naloxone significantly increased nociceptive behaviors in the first phase, interphase, and the second phase of the formalin test, compared to the control group. CONCLUSION: Stress could affect the nociceptive response. Swim stress in different heights of water could have different effects on the nociception in different phases of the formalin test. In addition, the involvement of the endogenous opioid system is further demonstrated in the swim stress-induced modulation of pain behaviors in phase 1, phase 2, as well as interphase of formalin test in rats.

Analgesic tolerance induced by repeated morphine injections induces cross-tolerance to the analgesic effect of orexin-A in rats.

Repeated administration of morphine or orexin-A produces tolerance to their antinociceptive effects. We investigated the possible incidence of cross-tolerance between orexin-A and morphine. Adult male Sprague-Dawley rats (200-250 g) were used. Under deep anesthesia, a stereotaxic apparatus was used to implant a 23 G cannula into the lateral ventricle for an intracerebroventricular (ICV) microinjection. The antinociceptive effect of three different doses of orexin-A (5, 20, and 40 µM; dissolved in 5 µl sterile saline; ICV) was examined using the hot-plate test at 15, 30, 60, and 90 min after infusion. To evaluate tolerance, orexin-A (20 µM; ICV) or morphine (10 mg/kg; intraperitoneal) was administered for 7 consecutive days (twice per day) and the analgesic response was assessed at days 1, 4, and 7. Cross-tolerance was investigated at day 8 with a single injection of morphine (10 mg/kg; intraperitoneal) to the repeated orexin-A group and a single microinjection of orexin-A (20 µM; ICV) to the repeated morphine group. Analgesic responses were then examined. Administration of both orexin-A and morphine produced significant antinociception at day 1 (P<0.001 compared with the saline group). However, a significant reduction in the analgesic effects of both morphine and orexin-A appeared at day 7, following repeated administration (P<0.01). Orexin-A microinjection at day 8 in the repeated morphine group did not result in significant antinociception (P>0.05), whereas morphine injection in the repeated orexin-A group at day 8 showed a significant analgesic effect (P<0.001). These results indicate cross-tolerance to the analgesic effect of orexin-A following morphine tolerance.

Orphan G protein-coupled receptors: The role in CNS disorders.

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

The Orexinergic (Hypocretin) System and Nociception: An Update to Supraspinal Mechanisms.

Chronic pain is a multifaceted and complex condition that is divided into somatic, visceral, and neuropathic pain. Although opioids and nonsteroidal anti-inflammatory drugs cause analgesia and are effective in the treatment of chronic pain, their utility is hampered by side effects, abuse potential, and development of tolerance to their pain-relieving effects. So, finding alternative analgesics with good efficacy and low side effects is of great interest and the orexinergic system is a potential candidate. Orexin-A and -B are exclusively expressed in the lateral hypothalamus and are involved in the feeding, sleep/wake cycle, cardiovascular function, hormone secretion, seizure, and pain modulation. Orexin peptides and their receptors have been proposed as opportunities for developing analgesic drugs. In experimental studies, orexin peptides induce analgesia that is comparable to morphine. Furthermore, there is evidence that orexin receptors 1 and 2 participate in responsiveness to both stressful stimuli and pain. Thus, direct and indirect activation of the orexinergic system is a new therapeutic approach to pain control. This article will review the most recent and important studies describing the role of orexins in pain modulation.

Characterization of Nociceptive Behaviors Induced by Formalin in the Glabrous and Hairy Skin of Rats.

INTRODUCTION: Glabrous skin and hairy skin are innervated by different types of noxious fibers. However, the different nociceptive behaviors induced by formalin, a commonly used model of acute inflammatory pain, have not yet been systematically examined in the glabrous and hairy skin. METHODS: In this study, we compared nociceptive behaviors induced by formalin injections (2%, 4%, and 8%) into either glabrous skin (plantar surface) of the hind paw or hairy skin of the hind limb in adult rats. RESULTS: A typical biphasic nociceptive response was seen after formalin injection into the plantar surface of the hind paw. A brief interphase separates the first and second phases where nociceptive behaviors were barely spotted. However, following subcutaneous injection into the hairy skin nociceptive behaviors were only seen after 10 minutes of formalin injection, which correlates in time with the second phase of the formalin response. First phase nociceptive behaviors were never seen with hairy skin injection, even following multiple injections of formalin. CONCLUSION: These data suggest that nociceptive behaviors and spinal responses induced by formalin injections to glabrous and hairy skin areas are different, and that the first and second phases may be mediated through different noxious afferent fibers.

Administration of orexin receptor 1 antagonist into the rostral ventromedial medulla increased swim stress-induced antinociception in rat.

OBJECTIVES: Intracerebroventricular injection of orexin-A (hypocretin-1) antagonist has been shown to inhibit stress-induced analgesia. However the locations of central sites that may mediate these effects have not been totally demonstrated. This study was performed to investigate the role of rostral ventromedial medulla (RVM) orexin receptor 1 in stress-induced analgesia (SIA). MATERIALS AND METHODS: Forced swim stress in water was employed to adult male rats (200-250 g). Nociceptive responses were measured by formalin test (50 µl injection of formalin 2% subcutaneously into hind paw) and, pain related behaviors were monitored for 90 min following intra-microinjection of SB-334867 (orexin receptor 1 antagonist) into RVM. RESULTS: Exposure to swimming stress test after administration of SB-334867 into RVM significantly reduces the formalin-induced nociceptive behaviors in phase1, interphase, and phase 2 in rats. CONCLUSION: The result demonstrated the involvement of OXR1 in antinociceptive behaviors induced by swim stress in RVM.

Orexin-a modulates firing of rat rostral ventromedial medulla neurons: an in vitro study.

The rostral ventromedial medulla (RVM) acts a key role in the descending inhibitory pain modulation. Neuropeptide orexin-A (ORXA) is confined to thousands of neurons in the lateral hypothalamus (LH). While RVM gets the orexinergic projections, the orexin receptors are also expressed in this structure. The aim of this study was to specify the cellular effects of ORXA on RVM neurons in vitro by using the whole cell patch-clamp recording. RVM neurons were classified into three types based on their electrophysiological characteristics. Type 1 neurons exhibited an irregular spontaneous activity which was interrupted by periods of pause in 25% of recorded neurons. Type 2 neurons did not show any spontaneous baseline activity (53.8% of recorded neurons). Type 3 neurons fired repetitively without interruption (51.2% of recorded neurons). ORXA had either inhibitory or excitatory effects on 53.8% (7/13) of type 1 neurons. ORXA excited 46.4% (13/28) of type 2 neurons and 27.3% (3/11) of type 3 neurons. The excitatory effect of ORXA observed in type 2 neurons was suppressed by an orexin 1 receptor (OXR1) antagonist, SB-334867. Briefly, we hypothesized that the ORXA mediated excitation and/or inhibition in RVM neurons might work as a mechanism to modulate pain processing by orexinergic neurons.

Repeated injections of orexin-A developed behavioral tolerance to its analgesic effects in rats.

OBJECTIVES: Reduction of pharmacological effectiveness or tolerance appears following repeated administration of many analgesic drugs. We investigated tolerance to anti-nociceptive effects of orexin-A, an endogenous potent analgesic peptide using the hot-plate test. MATERIALS AND METHODS: Orexin-A was microinjected ICV (intracerebroventricular) with an interval of 12 hr for 7 continuous days and its anti-nociceptive responses were measured on days 1, 4 and 7 using the hot-plate test following the first day of administration. Orexin-A was used at a dose of 100 pmol to induce analgesic effects. RESULTS: ICV administration of orexin-A produced an effective anti-nociception on the first day of experiment as measured by hot-plate 5, 15, and 30 min after the injection, in comparison with both baselines (hot-plate test one day before the beginning of orexin-A administration and control, saline-administrated group). However, repeated administration of orexin-A on the following days revealed a significant reduction in this analgesic effect during day 4 to day 7. However, to rule out any associative tolerance resulting from learning related to experimental procedures and/or environmental cues, a single injection of orexin-A was administrated to animals of control group (which were receiving saline during 7 days of experiments) and the analgesic effect was observed. CONCLUSION: These results, for the first time, indicated the appearance of tolerance to anti-nociceptive effects of orexin-A, following repeated administrations of this agent.

Termination of Nociceptive Bahaviour at the End of Phase 2 of Formalin Test is Attributable to Endogenous Inhibitory Mechanisms, but not by Opioid Receptors Activation.

INTRODUCTION: Formalin injection induces nociceptive bahaviour in phase I and II, with a quiescent phase between them. While active inhibitory mechanisms are proposed to be responsible for initiation of interphase, the exact mechanisms which lead to termination of nociceptive response in phase II are not clear yet. Phase II is a consequence of peripheral and central sensitization processes, which can lead to termination of the noxious stimuli responses; 45-60 minutes after formalin injection via possible recruitment of active inhibitory mechanisms which we have investigated in this study. METHODS: To test our hypothesis, in the first set of experiments, we evaluated nociceptive response after two consecutive injection of formalin (50µL, 2%), with intervals of 5 or 60 minutes. In the next set, formalin tests were carried out in companion with injection of Naloxone Hydrochloride, a non-selective antagonist of opioid receptors, pre-formalin injection and 30 and 45 minutes post formalin injection. RESULTS: While normal nociceptive behaviour was observed in the group receiving one injection of formalin, a diminished response was observed in phases I and II of those receiving consequent injection of formalin, 60 minute after first injection. While second injection of formalin, 5 minute after first injection, had no effect. Administration of naloxone (1mg/kg) decreased nociception in phase 2A; but had no effect on delayed termination of formalin test. DISCUSSION: The results of this study suggest the existence of an active inhibitory mechanism, other than the endogenous opioids, that is responsible for termination of nociceptive behaviour at the end of formalin test.

Effects of Hydroalcoholic Extract of Tanacetum Sonbolii (Asteraceae) on Pain-related Behaviors during Formalin Test in Mice.

INTRODUCTION: Tanacetum sonbolii (Asteraceae) is an endemic species in Iran. In the present study, we examined the effects of Tanacetum sonbolii hydroalcoholic extract on the formalin test in mice. METHODS: 126 Swiss albino mice weighing 230-280g were used as subjects. The formalin test was performed on two control groups (marked as intact and saline groups; n = 6 in each group) and an experimental group. In all groups, the formalin test was recorded for 60 min after administration of extract and drugs in mice. RESULTS: The results showed that Tanacetum sonbolii (150 and 300 mg/kg) produced significant antinociception in phase 2. In addition, different doses of Tanacetum sonbolii extract (600, 900 and 1200 mg/kg) also induced antinociceptive effects in phase1 and phase 2. On the other hand, morphine could induce antinociception in a dose-dependent manner. Diclofenac (10 mg/kg) failed to affect the pain scores compared to Tanacetum sonbolii (300 mg/kg) group. DISCUSSION: It seems that administration of hydroalcoholic extract of Tanacetum sonbolii has the potential to relieve pain through both central and peripheral mechanisms in persistent inflammatory nociception.

Evaluation of the association between blood homocysteine concentration and the degree of behavioral symptoms in the 6-hydroxydopamine-induced Parkinsonism in rat.

Growing evidence indicates that homocysteine (Hcy) may be involved in the pathophysiology of several neurological disorders including Parkinson's disease. In the present study, the association between blood Hcy concentration and the degree of behavioral symptoms in the 6-hydroxydopamine (6-OHDA)-induced Parkinsonism in rat was evaluated. Total serum Hcy (tHcy) was measured before and 6 weeks after the intracerebral injection of 6-OHDA. Apomorphine-induced rotational test was performed at second, third and sixth weeks after 6-OHDA injection. Subsequently, cell replacement therapy was performed on rats with good rotation score. No correlation between tHcy in before 6-OHDA injection and severity of the rotations after 6-OHDA injection was observed. On the other hand, 6-OHDA treatment significantly decreased tHcy level. However, this reduction was only observed in animals with low degree of rotations and in rats with high number of rotations; tHcy did not change significantly. Furthermore, 10 weeks after cell transplantation, tHcy was significantly lower than that found before therapy if the rats showed good improvement in the degree of rotations. We also examined the effect of different supplements of B vitamins on tHcy before and after 6-OHDA injection. In healthy rats, all kinds of B vitamins and also supplement B6 or B12 alone reduced tHcy. Following 6-OHDA injection, B vitamin supplementation failed to cause remarkable effect. Considering the direct correlation between the severity of rotational behavior and the degree of lesion in the substantia nigra (SN), our data indicate that higher tHcy values can predict higher SN dopaminergic neurodegeneration.

Permanent lesion in rostral ventromedial medulla potentiates swim stress-induced analgesia in formalin test.

OBJECTIVE(S): There are many reports about the role of rostral ventromedial medulla (RVM) in modulating stress-induced analgesia (SIA). In the previous study we demonstrated that temporal inactivation of RVM by lidocaine potentiated stress-induced analgesia. In this study, we investigated the effect of permanent lesion of the RVM on SIA by using formalin test as a model of acute inflammatory pain. MATERIALS AND METHODS: Three sets of experiments were conducted: (1) Application of stress protocol (2) Formalin injection after exposing the animals to the swim stress (3) Either the relevant vehicle or dopamine receptor 1 (D1) agonist R-SKF38393 was injected into the RVM to cause a lesion. For permanent lesion of RVM, R-SKF38393 was injected into the RVM. Forced swim stress in water was employed in adult male rats. Nociceptive responses were measured by formalin test (50µl injection of formalin 2% subcutaneously into hind paw) and pain related behaviors were monitored for 90 min. RESULTS: In the unstressed rats, permanent lesion of the RVM by R-SKF38393 decreased formalin-induced nociceptive behaviors in phase 1, while in stressed rats, injection of R-SKF38393 into the RVM potentiated swim stress-induced antinociception in phase 1 and interphase, phase 2A of formalin test. Furthermore, R-SKF38393 had pronociceptive effects in phase2B whereas injections of R-SKF38393 resulted in significant difference in nociceptive bahaviours in all phases of formalin test (P<0.05). CONCLUSION: The result of the present study demonstrated that permanent inactivation of RVM can potentiate stress-induced analgesia in formalin test.

Orexin-A microinjection into the rostral ventromedial medulla causes antinociception on formalin test.

Orexins are produced from neurons which are restricted to a few regions of the lateral hypothalamus (LH), where they are important in pain modulation. The orexin receptors and orexinergic projections are localized in regions previously shown to play a role in pain modulation such as rostral ventromedial medulla (RVM). The effect of orexin-A (ORXA) microinjection into the RVM on nociceptive behaviors was examined using the formalin test. Microinjection of ORXA into the RVM, but not adjacent reticularis gigantocellularis (Gi) nucleus, decreased formalin induced nociceptive behaviors. Pretreatment with a selective OX1R antagonist, SB-334867 inhibited the antinociception produced by ORXA, while the administration of SB-334867 alone had no effect. These data demonstrate that ORXA-induced antinociception in the formalin test is mediated in part through orexin1 receptors in the RVM.

Tail flick modification of orexin-a induced changes of electrophysiological parameters in the rostral ventromedial medulla.

OBJECTIVE: It is well known that intracerebroventricular (ICV) and supraspinal injections of orexin-A elicit analgesia, but the mechanism(s) of action remains unidentified. This study aims to characterize the effect of orexin-A on rostral ventromedial medulla (RVM) neurons which are involved in the descending nociception modulating pathway. MATERIALS AND METHODS: In this experimental study, we injected orexin-A or vehicle di- rectly into rats' ICV while the tail flick (TF) latencies were measured and the on- and off-cell firing activities were monitored for more than 60 minutes. RESULTS: In response to noxious stimuli on the tail, we observed increased firing rate of on-cells and a decreased association with the firing rate of off-cells and in neutral cells the firing rate was unchanged just prior to tail flicking. ICV injection of orexin-A decreased the spontaneous firing rate of on-cells (the type of RVM neurons believed to have facilitatory action on nociception). Furthermore, orexin-A increased firing rate of off-cells (the type of RVM neurons believed to have an inhibitory action on nocicep- tion). Orexin-A reduced the TF-related responses of on-cells and TF-related pause duration of off-cells. CONCLUSION: These results have shown that the analgesic effect produced by orexin-A may be induced by brainstem neurons. It is suggested that the orexinergic system from the hypothalamus to the RVM may have a potential role in modulation of nociceptive transmission.