Effects of ethanol on deep pain evoked by formalin injected in TMJ of rat

Preclinical orofacial pain assays and measures and chronic primary orofacial pain research: where we are and where we need to go

Chronic primary orofacial pain (OFP) conditions such as painful temporomandibular disorders (pTMDs; i.e., myofascial pain and arthralgia), idiopathic trigeminal neuralgia (TN), and burning mouth syndrome (BMS) are seemingly idiopathic, but evidence support complex and multifactorial etiology and pathophysiology. Important fragments of this complex array of factors have been identified over the years largely with the help of preclinical studies. However, findings have yet to translate into better pain care for chronic OFP patients. The need to develop preclinical assays that better simulate the etiology, pathophysiology, and clinical symptoms of OFP patients and to assess OFP measures consistent with their clinical symptoms is a challenge that needs to be overcome to support this translation process. In this review, we describe rodent assays and OFP pain measures that can be used in support of chronic primary OFP research, in specific pTMDs, TN, and BMS. We discuss their suitability and limitations considering the current knowledge of the etiology and pathophysiology of these conditions and suggest possible future directions. Our goal is to foster the development of innovative animal models with greater translatability and potential to lead to better care for patients living with chronic primary OFP.

Probiotics attenuate alcohol-induced muscle mechanical hyperalgesia: Preliminary observations

Alcohol use disorder (AUD) is a major health problem that causes millions of deaths annually world-wide. AUD is considered to be a chronic pain disorder, that is exacerbated by alcohol withdrawal, contributing to a high (∼80%) relapse rate. Chronic alcohol consumption has a marked impact on the gut microbiome, recognized to have a significant effect on chronic pain. We tested the hypothesis that modulating gut microbiota through feeding rats with probiotics can attenuate alcohol-induced muscle mechanical hyperalgesia. To test this hypothesis, rats were fed alcohol (6.5%, 4 days on 3 days off) for 3 weeks, which induced skeletal muscle mechanical hyperalgesia. Following alcohol feeding, at which time nociceptive thresholds were ∼37% below pre-alcohol levels, rats received probiotics in their drinking water, either Lactobacillus Rhamnosus GG (Culturelle) or De Simone Formulation (a mixture of 8 bacterial species) for 8 days; control rats received plain water to drink. When muscle mechanical nociceptive threshold was evaluated 1 day after beginning probiotic feeding, nociceptive thresholds were significantly higher than rats not receiving probiotics. Mechanical nociceptive thresholds continued to increase during probiotic feeding, with thresholds approaching pre-alcohol levels 5 days after starting probiotics; nociceptive threshold in rats not receiving probiotics remained low. After probiotics were removed from the drinking water, nociceptive thresholds gradually decreased in these two groups, although they remained higher than the group not treated with probiotic (21 days after ending alcohol feeding). These observations suggest that modification of gut microbiota through probiotic feeding has a marked effect on chronic alcohol-induced muscle mechanical hyperalgesia. Our results suggest that administration of probiotics to individuals with AUD may reduce pain associated with alcohol consumption and withdrawal, and may be a novel therapeutic intervention to reduce the high rate of relapse seen in individuals with AUD attempting to abstain from alcohol.

A Systematic Review of Rat Models With Temporomandibular Osteoarthritis Suitable for the Study of Emerging Prolonged Intra-Articular Drug Delivery Systems

PURPOSE

Development of minimally invasive therapies for temporomandibular joint osteoarthritis (TMJOA) has focused on drug intra-articular injections to avoid the systemic adverse effects experienced when these substances are administered orally. Therefore, we performed a systematic review to answer the question "Which method of induction of a TMJOA-related pain model in rats leads to prolonged painful symptoms, allowing the best assessment of a sustained drug delivery system?"

MATERIALS AND METHODS

Following the PRISMA guidelines, we searched MEDLINE for papers published from 1994 to July 2020 on a TMJ arthritis model using rats. We identified the means of pain induction and of nociception assessment. We assessed protocol bias using an adaptation of the QUADAS-2 tool. Animal selection, the reference standard method of pain assessment, applicability of a statistical assessment, and flow and timing were assessed.

RESULTS

Of the 59 full papers we reviewed, 41 performed no pain assessment after the first 7 days following induction of the TMJ-related pain model. We eventually identified 18 long-term TMJOA-related pain models. Pain was induced by injection of toxic substances, most commonly Freund's complete adjuvant (50 μg per 50 μl), formalin at various concentrations, or monosodium iodoacetate (0,5 mg per 50 μl), into the TMJ, or by physical methods. Few studies reported data on pain after 21 days of follow-up. Heterogeneity of induction methods, pain assessment methods, and flow and timing biases precluded a meta-analysis.

CONCLUSIONS

Given that pain is 1 of the main symptoms of TMJOA, experimental study protocols should include long-term pain assessment.

Factors affecting the incidence of chronic pain following breast cancer surgery: Preoperative history, anesthetic management, and surgical technique

BACKGROUND OR OBJECTIVES

Breast cancer is the most frequent cancer in women. Chronic pain following mastectomy remains a significant problem. This study aimed to evaluate risk factors associated with postoperative chronic pain.

METHODS

Medical records were analyzed retrospectively for 147 consecutive patients with breast cancer who underwent mastectomy between July 2016 and February 2018 with one-year follow-up. All patients received a paravertebral block pre-operatively and then general anesthesia for the surgery.

RESULTS

The median postoperative pain score was 1.3 on the verbal analgesic score. The average total dose of opioids administered postoperatively was 10.4 mg, expressed in morphine milligram equivalents. The incidence of chronic pain after mastectomy was 16.7% at 6 months and 13.3% at 1 year. The satisfaction rate for anesthesia and surgery was 95.7%.

CONCLUSIONS

The data suggested that preoperative paravertebral nerve blocks, appropriate adjuvants, and a balanced general anesthetic may contribute to better postoperative pain control and decreased the incidence of chronic pain. A history of preoperative use of opioids and/or a history of alcohol consumption were both associated with significant increased levels of postoperative pain, and this subgroup warrants attention to diminish the risk of developing chronic pain.

Japanese Rice Wine can reduce psychophysical stress-induced depression-like behaviors and Fos expression in the trigeminal subnucleus caudalis evoked by masseter muscle injury in the rats

We determined if Japanese Rice Wine (Sake) had inhibitory effects on stress-induced enhancement of masseter muscle (MM) nociception in the rats. Male rats were subjected to the repeated forced swim stress (FS) or sham conditionings from Day -3 to -1. Daily administration of Sake or saline was conducted after each stress conditioning. At Day 0 the number of Fos positive cells, a marker for neural activity, was quantified at the trigeminal subnucleus caudalis (Vc) region by MM injury with formalin. FS increased MM-evoked Fos expression in the Vc region, which was inhibited by Sake compared to saline administration. Sake did not alter the number of Fos positive cells under sham conditions, indicating that inhibitory roles of Sake on neural activity in the Vc region were seen under FS conditions. These findings indicated that Sake had inhibitory roles on stress-induced MM nociception at the Vc region in our experimental conditions.

Chronic orofacial pain animal models - progress and challenges

INTRODUCTION

Chronic orofacial pain is one of the most common pain conditions experienced by adults. Animal models are often selected as the most useful scientific methodology to explore the pathophysiology of the disorders that cause this disabling pain to facilitate the development of new treatments. The creation of new models or the improvement of existing ones is essential for finding new ways to approach the complex neurobiology of this type of pain. Areas covered: The authors describe and discuss a variety of animal models used in chronic orofacial pain (COFP). Furthermore, they examine in detail the mechanisms of action involved in orofacial neuropathic pain and orofacial inflammatory pain. Expert opinion: The use of animal models has several advantages in chronic orofacial pain drug discovery. Choosing an animal model that most closely represents the human disease helps to increase the chances of finding effective new therapies and is key to the successful translation of preclinical research to clinical practice. Models using genetically modified animals seem promising but have not yet been fully developed for use in chronic orofacial pain research. Although animal models have provided significant advances in the pharmacological treatment of orofacial pain, several barriers still need to be overcome for better treatment options.

Intra-cerebral and intra-nasal melanocortin-4 receptor antagonist blocks withdrawal hyperalgesia in alcohol-dependent rats

Humans diagnosed with alcohol use disorder are more sensitive to painful stimuli during withdrawal, which suggests that excessive alcohol drinking worsens pain outcomes. Alcohol-dependent rats exhibit increases in nociceptive sensitivity during withdrawal. Data from animal models suggest that brain melanocortin-4 receptors (MC4Rs) mediate alcohol drinking and nociception. Here we tested: (1) the effect of alcohol dependence on thermal nociception in rats, and (2) the ability of acute alcohol and (3) MC4R antagonists to reverse hyperalgesia during withdrawal in alcohol-dependent rats. Rats were trained to self-administer operant alcohol and were tested for baseline thermal nociception. Half of the rats were made dependent on alcohol, then all rats were cannulated in the lateral ventricle. We tested the effects of acute alcohol drinking, acute fixed-dose alcohol, intra-ventricular agouti-related protein (endogenous MC4R antagonist), intra-ventricular HS014 (synthetic MC4R antagonist) and intra-nasal HS014 on hyperalgesia during withdrawal in alcohol-dependent rats, relative to non-dependent drinkers and alcohol-naïve controls. Alcohol-dependent rats exhibit thermal hyperalgesia that is abolished by alcohol drinking, bolus alcohol and intra-ventricular and intra-nasal MC4R antagonists. These manipulations did not affect thermal nociception in non-dependent drinkers and alcohol-naïve controls, suggesting that alcohol dependence produces neuroadaptations in brain MC4R systems. These results suggest that brain MC4R systems may be an effective therapeutic target for reducing nociception in the alcohol-dependent organism.

Electroacupuncture Attenuates Hyperalgesia in Rats Withdrawn from Chronic Alcohol Drinking via Habenular Mu Opioid Receptors

BACKGROUND

Hyperalgesia or increased sensitivity to pain is often found in alcoholics during alcohol withdrawal and may contribute to relapse drinking. Alternative therapies such as acupuncture and electroacupuncture (EA), through mechanisms involving opioid receptors, may reduce pain and substance dependence and withdrawal syndromes. The lateral habenula (LHb), an epithalamic structure rich in mu opioid receptors (MORs), is a critical target for both drugs of abuse and pain. We previously observed hyperalgesia in rats withdrawn from chronic ethanol (EtOH) drinking and found that EA at the acupoint Zusanli (ST36) reduced EtOH intake. This raised question of whether EA can alleviate hyperalgesia during alcohol withdrawal and, if so, whether the mechanism involves MORs in the LHb.

METHODS

We trained male rats to drink EtOH using the intermittent access 20% EtOH 2-bottle free-choice drinking paradigm for 8 weeks, after which the alcohol supply was discontinued. We measured pain sensitivity using radiant heat (a light beam directed at the hind paw of rats) and compared the paw withdrawal latencies (PWLs) with and without EA at ST36.

RESULTS

The PWLs were significantly shorter in rats at 24, 48, and 72 hours and 7 days after the discontinuation of EtOH when compared to EtOH-naïve rats. After a single administration of 2-Hz EA for 20 minutes at ST36, the PWLs at 24 hours after the withdrawal of EtOH were significantly greater than those of the sham group (2-Hz EA at the tail). Furthermore, the effect of EA on PWLs was significantly attenuated by bilateral intrahabenula infusion of the MOR antagonist naltrexone.

CONCLUSIONS

These results suggest that EA can alleviate hyperalgesia during EtOH withdrawal through a mechanism involving MORs in the habenula. Based on this, EA could be of potential value as a therapy for hyperalgesia in alcohol dependence.

Ethanol-induced effects on sting extension response and punishment learning in the western honey bee (Apis mellifera)

Acute ethanol administration is associated with sedation and analgesia as well as behavioral disinhibition and memory loss but the mechanisms underlying these effects remain to be elucidated. During the past decade, insects have emerged as important model systems to understand the neural and genetic bases of alcohol effects. However, novel assays to assess ethanol's effects on complex behaviors in social or isolated contexts are necessary. Here we used the honey bee as an especially relevant model system since bees are typically exposed to ethanol in nature when collecting standing nectar crop of flowers, and there is recent evidence for independent biological significance of this exposure for social behavior. Bee's inhibitory control of the sting extension response (SER) and a conditioned-place aversion assay were used to study ethanol effects on analgesia, behavioral disinhibition, and associative learning. Our findings indicate that although ethanol, in a dose-dependent manner, increases SER thresholds (analgesic effects), it disrupts the ability of honey bees to inhibit SER and to associate aversive stimuli with their environment. These results suggest that ethanol's effects on analgesia, behavioral disinhibition and associative learning are common across vertebrates and invertebrates. These results add to the use of honey bees as an ethanol model to understand ethanol's effects on complex, socially relevant behaviors.

The effects of acute restraint stress on nociceptive responses evoked by the injection of formalin into the temporomandibular joint of female rats

The temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute restraint stress on the nociceptive behavioral responses of female rats during proestrus and estrus phases of the estrous cycle. Rats were subjected to one session of restraint stress (15, 30 min or 1 h). They were then either immediately killed to allow the collection of blood for hormonal radioimmunoassay determinations or subjected to TMJ formalin test to evaluate nociception. All stress protocols significantly raised the plasma concentrations of corticosterone. The performance of rats subjected to 15 and 30 min of restraint stress was similar to that of control rats, whereas rats that were stressed for 1 h showed a decrease in nociceptive responses, during both proestrus and estrus phases. The stress-induced analgesia (SIA) was greater in the proestrus phase. To evaluate the role of kappa-opioid receptors, the selective receptor kappa-opioid antagonist nor-binaltorphimine (nor-BNI; 200 microg or saline) was injected into the TMJ 24 h prior to the 1 h stress period and the TMJ formalin test. The local administration of nor-BNI partially reversed the SIA during the proestrus phase. These findings suggest that (1) acute stress for 1 h can produce analgesia both during proestrus and estrus phases; this effect is greater during the proestrus phase and (2) kappa-opioid receptor activation is involved in the SIA observed in the proestrus phase.

Alcohol-induced tolerance and physical dependence in mice with ethanol insensitive alpha1 GABA A receptors

BACKGROUND

Although many people consume alcohol (ethanol), it remains unknown why some become addicted. Elucidating the molecular mechanisms of tolerance and physical dependence (withdrawal) may provide insight into alcohol addiction. While the exact molecular mechanisms of ethanol action are unclear, gamma-aminobutyric acid type A receptors (GABA(A)-Rs) have been extensively implicated in ethanol action. The alpha1 GABA(A)-R subunit is associated with tolerance and physical dependence, but its exact role remains unknown. In this report, we tested the hypothesis that alpha1-GABA(A)-Rs mediate in part these effects of ethanol.

METHODS

Ethanol-induced behavioral responses related to tolerance and physical dependence were investigated in knockin (KI) mice that have ethanol-insensitive alpha1 GABA(A)-Rs and wildtype (WT) controls. Acute functional tolerance (AFT) was assessed using the stationary dowel and loss of righting reflex (LORR) assays. Chronic tolerance was assessed on the LORR, fixed speed rotarod, hypothermia, and radiant tail-flick assays following 10 consecutive days of ethanol exposure. Withdrawal-related hyperexcitability was assessed by handling-induced convulsions following 3 cycles of ethanol vapor exposure/withdrawal. Immunoblots were used to assess alpha1 protein levels.

RESULTS

Compared with controls, KI mice displayed decreased AFT and chronic tolerance to ethanol-induced motor ataxia, and also displayed heightened ethanol-withdrawal hyperexcitability. No differences between WT and KI mice were seen in other ethanol-induced behavioral measures. Following chronic exposure to ethanol, control mice displayed reductions in alpha1 protein levels, but KIs did not.

CONCLUSIONS

We conclude that alpha1-GABA(A)-Rs play a role in tolerance to ethanol-induced motor ataxia and withdrawal-related hyperexcitability. However, other aspects of behavioral tolerance and physical dependence do not rely on alpha1-containing GABA(A)-Rs.

Involvement of peripheral TRPV1 in TMJ hyperalgesia induced by ethanol withdrawal

Ethanol withdrawal increases nociception after the injection of formalin into the rat's temporomandibular joint (TMJ). Little is known about the neurological basis for hyperalgesia induced by ethanol withdrawal, but it has been reported that ethanol can potentiate the response of transient receptor potential vanilloid receptor-1 (TRPV1) in superficial tissues. The present study was designed to test the hypothesis that peripheral TRPV1 could be involved on nociceptive behavioral responses induced by the injection of formalin into the TMJ region of rats exposed to chronic ethanol administration and ethanol withdrawal. Behavioral hyperalgesia was verified 12 h after ethanol withdrawal in rats that drank an ethanol solution (6.5%) for 10 days. In another group submitted to the same ethanol regimen, the selective vanilloid receptor antagonist capsazepine (300, 600 or 1200 microg/25 microl) or an equal volume of vehicle were injected into the TMJ regions 30 min before the TMJ formalin test. The local injections of capsazepine reduced the increased nociceptive responses induced by ethanol withdrawal. The effect of capsazepine on rats that did not drink ethanol was not significant. These results indicate that the peripheral TRPV1 can contribute to the hyperalgesia induced by ethanol withdrawal on deep pain conditions.

How may stressful experiences contribute to the development of temporomandibular disorders?

Temporomandibular disorders (TMD) comprise the most common cause of chronic facial pain conditions, and they are often associated with somatic and psychological complaints including fatigue, sleep disturbances, anxiety, and depression. For many health professionals, the subjectivity of pain experience is frequently neglected even when the clinic does not find any plausible biologic explanation for the pain. This strictly biomedical vision of pain cannot be justified scientifically. The purpose of this study is to demonstrate, by original articles from the literature and recent studies conducted in our own laboratory, the biological processes by which psychological stress can be translated into the sensation of pain and contribute to the development of TMD. The role of the hypothalamic-pituitary-adrenal axis, the serotoninergic and opioid systems in the pathogenesis of facial pain is exposed, including possible future therapeutic approaches. It is hoped that knowledge from apparently disparate fields of dentistry, integrated into a multidisciplinary clinical approach to TMD, will improve diagnosis and treatment for this condition through a clinical practice supported by scientific knowledge.

Tolerance to the antinociceptive effects of ethanol during ethanol withdrawal

Prior research has indicated that tolerance develops to the antinociceptive effects of ethanol and continues even during withdrawal. Three potential pharmacological mechanisms for this tolerance are examined, using nitrendipine (L-type calcium channel blocker), theophylline (adenosine A1/A2 antagonist) and flumazenil (benzodiazepine antagonist). Rats received 10 days of exposure to an ethanol-containing liquid diet (6.5% w/v). A radiant heat tail-flick assay was used to assess hyperalgesia at 12 h after removal of the liquid diet, as well as tolerance to the effects of cumulative doses of ethanol (0.5-2 g/kg). Co-administration of flumazenil (10 mg/kg, i.p., b.i.d.), nitrendipine (5 mg/kg, i.p., b.i.d.) or theophylline (1 mg/kg, i.p., b.i.d.) with chronic ethanol prevented development of the hyperalgesia produced by ethanol withdrawal, but only theophylline reduced tolerance to the antinociceptive effects of ethanol administered during ethanol withdrawal. In contrast, when administered during ethanol withdrawal, theophylline (1-10 mg/kg) blocked the anti-hyperalgesic effects of ethanol during ethanol withdrawal, whereas nitrendipine (5-25 mg/kg) enabled ethanol to produce levels of antinociception comparable to non-dependent rats. These findings indicate that L-type calcium channels and adenosine receptors play important, but differing roles in the development of hyperalgesia during withdrawal, and to tolerance to the antinociceptive effects of ethanol.

Nociception- and anxiety-like behavior in rats submitted to different periods of restraint stress

The aim of this study was to evaluate the effect of acute, sub-chronic and chronic stress on nociception induced by formalin injection in rats' temporomandibular joint (TMJ). It was evaluated the relation between blood levels of adrenocorticotropin, corticosterone, the levels of anxiety and nociceptive responses recorded after different stress protocols. Animals were initially submitted to acute restraint stress (15; 30 min and 1 h), or exposed to sub-chronic (3 days-1 h/day) or chronic stress (40 days-1 h/day). Then, animals were (1) killed immediately to collect blood for hormonal determinations; or (2) submitted to the elevated plus-maze to evaluate anxiety; or (3) submitted to the TMJ formalin test to evaluate nociception. It was also evaluated the role of serotoninergic and opioid systems in nociceptive changes induced by stress. For this, the serotonin-selective reuptake inhibitor (fluoxetine 10 mg/kg) and the opioid agonist (morphine 1-5 mg/kg) were administered before the nociception test. All stress protocols significantly raised the levels of ACTH or corticosterone, as well as the anxiety behavior. In relation to nociception, the chronic stressed animals showed an increase in nociceptive responses (hyperalgesia). In this group, there was a reduction in the morphine analgesic effects, suggesting dysfunction in the endogenous opioid system. Fluoxetine had an analgesic effect in both stressed and control groups, although this effect was more evident in the stressed group. It was concluded that stress-induced hyperalgesia may result from changes in the serotoninergic and opioid systems, which can explain, at least in part, the important link between stress and orofacial pain.

The effects of restraint stress on nociceptive responses induced by formalin injected in rat's TMJ

It has been reported that stress can alter nociception from superficial tissues, such as skin and subcutaneous region. However, the influence of stress on an experimental deep nociception model is not understood. In this study, the temporomandibular joint (TMJ) formalin test was used to evaluate the effects of acute and chronic restraint stress on nociceptive responses in rats. Animals were initially submitted to one session of acute restraint stress (1 h) or exposed to chronic stress (40 days-1 h/day). Then, animals were killed immediately to collect blood for hormonal determinations by radioimmunoassay, or submitted to the TMJ formalin test to evaluate nociception. Rats submitted to acute restraint presented a performance similar to unstressed controls in the TMJ formalin test, whereas chronically stressed rats showed an increase in nociceptive responses. After 40 days of restraint, morphine was injected i.p. (1, 5 mg/kg or saline). The stressed rats displayed decreased morphine effects on nociception compared to unstressed controls. These findings suggest that repeated stress can produce hyperalgesia, which is, at least in part, due to alterations in the activity of opioid systems. This model may help elucidate the underlying neural mechanisms that mediate the effects of repeated stress on orofacial pain.

A novel method for subarachnoid drug delivery in the medullary region of rats

This study describes a novel method for direct subarachnoid drug delivery to the medullary dorsal horn region of rats, without introducing a catheter. The reliability of the method was demonstrated by a pharmacological validation; that is, morphine administration to the medullary region blocked the nociceptive response to formalin injected in the temporomandibular joint (TMJ) region, an effect that was prevented by co-administration of naloxone. The method proposed offers many advantages over the existing methods for medullary drug delivery with catheter implantation. It is easy to be employed, it does not induce any sign of motor impairment, and it does not require the neck surgery performed to implant a catheter in the medullary dorsal horn region. Therefore, it is a useful method for subarachnoid drug delivery in behavioral trigeminal pain studies, particularly when nociceptive behavioral measures that require normal neck muscle activity to occur, such as head withdraw or head flinch, are evaluated.

Evidence for the involvement of endogenous ATP and P2X receptors in TMJ pain

Evidence is accumulating which supports a role for ATP in the initiation of pain by acting on P2X receptors expressed on nociceptive afferent nerve terminals. To investigate whether these receptors play a role in temporomandibular (TMJ) pain, we studied the presence of functional P2X receptors in rat TMJ by examining the nociceptive behavioral response to the application of the selective P2X receptor agonist alpha,beta-methylene ATP (alpha,beta-meATP) into the TMJ region of rat. The involvement of endogenous ATP in the development of TMJ inflammatory hyperalgesia was also determined by evaluating the effect of the general P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) on carrageenan-induced TMJ inflammatory hyperalgesia. Application of alpha,beta-meATP into the TMJ region of rats produced significant nociceptive responses that were significantly reduced by the co-application of lidocaine N-ethyl bromide quaternary salt, QX-314, (2%) or of the P2 receptor antagonist PPADS. Co-application of PPADS with carrageenan into the TMJ significantly reduced inflammatory hyperalgesia. The results indicate that functional P2X receptors are present in the TMJ and suggest that endogenous ATP may play a role in TMJ inflammatory pain mechanisms possibly by acting primarily in these receptors.

The role of acetaldehyde in the neurobehavioral effects of ethanol: A comprehensive review of animal studies

Acetaldehyde has long been suggested to be involved in a number of ethanol's pharmacological and behavioral effects, such as its reinforcing, aversive, sedative, amnesic and stimulant properties. However, the role of acetaldehyde in ethanol's effects has been an extremely controversial topic during the past two decades. Opinions ranged from those virtually denying any role for acetaldehyde in ethanol's effects to those who claimed that alcoholism is in fact "acetaldehydism". Considering the possible key role of acetaldehyde in alcohol addiction, it is critical to clarify the respective functions of acetaldehyde and ethanol molecules in the pharmacological and behavioral effects of alcohol consumption. In the present paper, we review the animal studies reporting evidence that acetaldehyde is involved in the pharmacological and behavioral effects of ethanol. A number of studies demonstrated that acetaldehyde administration induces a range of behavioral effects. Other pharmacological studies indicated that acetaldehyde might be critically involved in several effects of ethanol consumption, including its reinforcing consequences. However, conflicting evidence has also been published. Furthermore, it remains to be shown whether pharmacologically relevant concentrations of acetaldehyde are achieved in the brain after alcohol consumption in order to induce significant effects. Finally, we review current evidence about the central mechanisms of action of acetaldehyde.

Peripheral effect of a kappa opioid receptor antagonist on nociception evoked by formalin injected in TMJ of pregnant rats

The effect of sex hormones on orofacial pain modulation is poorly understood. Therefore, this study aimed to investigate the effect of hormonal changes as a result of pregnancy, as well as that of the kappa (kappa) opioid receptor antagonist on female rats' sensitivity to the temporomandibular joint (TMJ) formalin test. Initially, female rats at estrus and pregnant females on day 19 of pregnancy received a 50 microl formalin (1.5%) injection in the right TMJ. The pregnant females showed a reduction in nociceptive responses to the TMJ formalin test when compared with those at estrus. Then, the selective kappa-opioid receptor antagonist nor-Binaltorphimine (nor-BNI), was co-administered with the formalin. Next, additional groups received the kappa (200 microg) receptor antagonist or 0.9% NaCl 24 hours prior to the periarticular injection of formalin. Co-administration of nor-BNI with formalin into the TMJ region had no significant effect. The pre-injection of selective kappa-opioid receptor antagonist, nor-BNI, significantly enhanced the nociceptive behavioral responses in pregnant females. When applied in the contralateral TMJ, nor-BNI did not affect the magnitude of the nociceptive response induced by formalin. It can be concluded that: 1) The increase of the sex hormone levels, as result of pregnancy, induces a reduction of nociceptive behavioral responses to the TMJ formalin test; 2) the peripheral kappa opioid receptor activation, by endogenous opioid agonists release, is involved in the antinociception to TMJ formalin test, induced by pregnancy.

An analgesic evaluation of isoxsuprine in horses

Isoxsuprine is used clinically to treat navicular disease and laminitis in horses. Although it is thought to increase digital and laminar blood flow, isoxsuprine's mechanism of action remains controversial, and analgesia has been suggested recently as such possible mechanism. This research investigated the analgesic potential of isoxsuprine in healthy horses submitted to a mechanical nociceptive test. Isoxsuprine (1.2 mg/kg), xylazine (1.1 mg/kg), distilled water : ethanol 95% (2 : 1, v/v, 20 ml) and saline (0.9%, 20 ml) were injected intravenously, and nociceptive thresholds were measured over 90 min. Only xylazine significantly increased nociceptive thresholds, confirming that alpha(2)-adrenoceptor agonists produce analgesia in horses. Our results do not support an analgesic mechanism of action for isoxsuprine in horses, suggesting that other mechanisms might account for the clinical efficacy of this drug or that mechanical nociceptive testing may not be sufficiently sensitive to demonstrate an analgesic effect for this drug.