The other side of the medal: how chemokines promote analgesia

Chemokines are chemotactic mediators controlling cell trafficking under physiological and pathological conditions. Chemokines are not only important under various inflammatory conditions but also play a role in pain and analgesia. While many studies examined the hyperalgesic action of chemokines, recent evidence also points towards antinociceptive effects of chemokines. Such effects are indirect by recruitment of opioid containing leukocytes and stimulation of release of opioid peptides. Opioid peptides then bind to opioid receptors on peripheral sensory neurons eliciting potent analgesia. This review focuses on the analgesic role of chemokines in the periphery under inflammatory and non-inflammatory conditions.     

Empirical contributions

To evaluate the possible negative association between risk for hypertension and pain sensitivity in women, 24 healthy young adult women with a parental history of hypertension and 24 without a parental history of hypertension participated in 2 laboratory sessions in which electric shock and the cold-pressor test were administered. To assess the possible role of stress-induced analgesia in blood pressure-related hypoalgesia, the sessions were identical with the exception of the fact that participants were exposed to a stressful 20-min videogame before the pain stimuli on one occasion and a nonstressful control task on the other. Women with a parental history of hypertension and high blood pressure reactivity to the videogame displayed a reduced sensitivity to electric shock on both days, suggesting that risk for hypertension is associated with reduced sensitivity to at least some pain stimuli in women. Blood pressure reactivity to stress-associated baroreceptor stimulation was not implicated as a mediator of decreased pain perception. However, other analyses revealed an effect of family history on shock pain only among women who reported relatively high anxiety, suggesting that other aspects of the stress response may be involved in this phenomenon. This research was supported by the Heart and Stroke Foundation of Quebec and an Academic Challenge grant from the state of Ohio     

Hypothesis on Reciprocal Interactions between the Central and Peripheral Components of the Endogenous Opioid System

A hypothesis on reciprocal interactions between the central and peripheral components of the endogenous opioid system was formulated on the basis of results of our experimental studies and published data. In order to verify this hypothesis, we studied the effects of peripheral administration of loperamide (μ-opioid receptor agonist) and methyl-naloxone (opioid receptor antagonist) not penetrating through the blood-brain barrier on the pain sensitivity of rats, morphine-induced analgesia, and formation of morphine analgesia tolerance. Peripheral loperamide and methylnaloxone modulated the central mechanisms of perception of painful stimuli. This fact confirmed the hypothesis on the reciprocal interactions between the central and peripheral compartments of the endogenous opioid system. Methylnaloxone exhibits an antagonistic effect on peripheral μopioid receptors, which probably leads to activation of the central μ-opioid receptors and to the development of analgesia. Loperamide activates peripheral, but suppresses the central μ-opioid receptors, which leads to hyperalgesia. Methylnaloxone suppresses morphine-induced analgesia under conditions of morphine activation of the central antinociceptive mechanisms. Peripheral injection of μ-opioid agonist loperamide virtually did not modify the central compartments of the opioid system under conditions of morphine treatment. Methylnaloxone and loperamide partially prevented the development of morphine analgesia tolerance. Hence, the results confirm the hypothesis about the reciprocal interactions between the central and peripheral compartments of the endogenous opioid system. The relationships between the central and peripheral compartments of the opioid system can be more intricate when its function is modulated. Translated from Byulleten’ Eksperimental’noi Biologii i Meditsiny, Vol. 146, No. 12, pp. 604–607, December, 2008 Original article submitted September 8, 2008     

Stress-induced analgesia evoked by intraperitoneal injection of hypertonic saline: evidence for its occurrence in vasopressin deficient rats

Experiments were carried out to investigate whether vasopressin is involved in stress-induced analgesia. Intraperitoneal injection of hypertonic saline caused a significant and dose-related increase in the latency to the tail-flick response of the rat to noxious heat and was used as a stimulus for stress-induced analgesia. Neither the pituitary nor opioid peptides appeared to be involved, since the response occurred in hypophysectomized rats and was not reduced by the opiate antagonist naloxone. Furthermore hypertonic-saline analgesia was clearly potentiated in hypophysectomized rats in comparison to sham-operated controls. Hypertonic-saline analgesia was also observed in vasopressin-deficient (homozygous Brattleboro) rats similar in both magnitude and duration to that in normal rats of the same strain (Long Evans). It was concluded that vasopressin was not involved in stress-induced analgesia evoked by hypertonic saline.     

Leukocytes in the regulation of pain and analgesia

When tissue is destroyed or invaded by leukocytes in inflammation, numerous mediators are delivered by the circulation and/or liberated from resident and immigrated cells at the site. Proalgesic mediators include proinflammatory cytokines, chemokines, protons, nerve growth factor, and prostaglandins, which are produced by invading leukocytes or by resident cells. Less well known is that analgesic mediators, which counteract pain, are also produced in inflamed tissues. These include anti-inflammatory cytokines and opioid peptides. Interactions between leukocyte-derived opioid peptides and opioid receptors can lead to potent, clinically relevant inhibition of pain (analgesia). Opioid receptors are present on peripheral endings of sensory neurons. Opioid peptides are synthesized in circulating leukocytes, which migrate to inflamed tissues directed by chemokines and adhesion molecules. Under stressful conditions or in response to releasing agents (e.g., corticotropin-releasing factor, cytokines, noradrenaline), leukocytes can secrete opioids. They activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of excitatory neuropeptides. This review presents discoveries that led to the concepts of pain generation by mediators secreted from leukocytes and of analgesia by immune-derived opioids.     

Increased sensitivity to acute and persistent pain in neuron-specific endothelin-1 knockout mice

Endothelin-1 (ET-1) exists in endothelial cells as well as a variety of other cell types. The presence of ET-1 and its receptors in neurons suggests its possible role as a neurotransmitter and/or neuromodulator. Studies utilizing exogenous ET-1 have suggested that ET-1 affects pain transmission. This study was designed to examine the possible role(s) of neuronal ET-1 in pain processing. We produced neuron-specific ET-1 knockout mice using the Cre/loxP system with a synapsin I promoter and examined the effects produced by the lack of neuronal ET-1 on pain behavior using common pain models and a model of stress-induced analgesia. In acute nociceptive pain models, paw withdrawal thresholds to radiant heat and mechanical stimuli applied with von Frey hairs were significantly lower in the knockout mice compared with control. This indicated that the absence of neuronal ET-1 leads to greater sensitivity to acute nociceptive stimuli. After inflammation was produced by intraplantar injection of carrageenan, there was a significantly greater degree of thermal hyperalgesia and mechanical allodynia in the knockout mice even after the difference in baseline values was compensated. Furthermore, in a neuropathic pain model produced by spinal nerve ligation, there was also a greater degree of mechanical allodynia in the knockout mice. Finally, in a swim-stress model, the magnitude of stress-induced analgesia was less in the knockout mice, indicating the involvement of neuronal ET-1 in stress-induced analgesia. The results suggest that there is a basal release of ET-1 from neurons that affect baseline pain thresholds as well as an additional release during persistent pain states that acts to suppress the pain. The involvement of neuronal ET-1 in stress-induced analgesia further suggests its role in endogenous pain inhibitory systems. To confirm that ET-1 is released in persistent pain states and to determine which part of the CNS is involved, we measured the concentrations of ET-1 before and after inducing peripheral inflammation in different parts of the CNS involved in endogenous pain inhibitory systems in normal mice. We found that ET-1 was increased in the hypothalamus while no significant increase was observed in the midbrain, medulla and spinal cord. The results of the present study suggest that neuronal ET-1 is involved in endogenous pain inhibitory control likely via pathways through the hypothalamus.     

Cimetidine-sensitive analgesia: investigation of the importance of stress-induced changes in arterial pressure

Because previous studies have suggested that activation of baroreceptors could mediate stress-induced analgesia, the effect of acute exposure to footshock on mean arterial pressure (MAP) and pain sensitivity was simultaneously determined in conscious rats receiving the histamine H2 receptor antagonist cimetidine or vehicle. Continuous exposure to 3 min of inescapable footshock (3.5 mA) dramatically decreased pain sensitivity, with no increase in post-stress MAP, when compared to no shock controls. The histamine H2-antagonist cimetidine (100 mg/kg, IP) had no significant effect on MAP in resting or stressed animals, but inhibited the stress-induced analgesia, showing that the antagonism of the analgesia is not mediated by modulation of post-stress MAP. Although footshock failed to elicit a significant increase in MAP, a highly significant correlation was found between individual analgesic scores and shock-induced pressure changes in animals treated with cimetidine; in animals receiving vehicle, no such correlation was observed, although the use of a cutoff in analgesic testing may explain this. These results suggest the existence of a stress-induced analgesic mechanism resistant to cimetidine, but associated with elevated MAP.     

Novelty-induced opioid analgesia in the terrestrial snail, Cepaea nemoralis

Exposure for 1-15 min to the surface of either a non-functional (22 degrees C) hot-plate or a polypropylene tube increased the thermal (38.5 degrees C) nociceptive thresholds of the terrestrial snail, Cepaea nemoralis. This "analgesic response," which was evident 10-15 sec after exposure to the new stimuli and lasted for 15-30 min, was blocked by the opiate antagonist naloxone. The analgesic response was not observed in snails previously made familiar with the test situation. Snails exposed to a functional (36.5 degrees C) stressful hot-plate surface to which they were aversive also displayed a naloxone-reversible analgesia. However, the level and duration of analgesia was markedly greater than that observed following exposure to the novel non-physically stressful stimuli and was not inhibited by prior familiarization with the stimuli. These observations demonstrate that novelty per se can activate endogenous opioid system(s) and induce an analgesic response in a snail. They also suggest a phylogenetic continuity in responses to novel stimuli and environmental conditions.     

A CB2 receptor agonist,A-836339, modulates wide dynamic range neuronal activity in neuropathic rats: Contributions of spinal and peripheral CB2 receptors

We investigated the systemic and site-specific actions of a selective CB₂ receptor agonist, A-836339 on mechanically evoked (10 g von Frey hair) and spontaneous firing of spinal wide dynamic range (WDR) neurons in neuropathic (L5 and L6 ligations) and sham rats. Systemic administration of A-836339 (0.3–3 μmol/kg, i.v.) reduced both evoked and spontaneous WDR neuronal activity in neuropathic, but not sham rats. The effects in neuropathic rats were blocked by pre-administration of a CB₂, but not a CB₁, receptor antagonist. Similar to systemic delivery, intra-spinal injection of A-836339 (0.3 and 1 nmol) also attenuated both von Frey–evoked and spontaneous firing of WDR neurons in neuropathic rats. Intra-spinal injections of A-836339 were ineffective in sham rats. Application of A-836339 (3–30 nmol) onto the ipsilateral L5 dorsal root ganglion (DRG) of neuropathic rats reduced the von Frey–evoked activity of WDR neurons, but spontaneous firing was unaltered. All effects of A-836339 on WDR neuronal activity following either intra-spinal or intra-DRG administration were blocked by pre-administration of a CB₂ receptor antagonist. Pre-administration of a CB₁ receptor antagonist did not alter the site-specific effects of A-836339. Injection of A-836339 (300 nmol) into the neuronal receptive field on the ipsilateral hind paw did not affect evoked or spontaneous firing of WDR neurons. Thus, the current data demonstrate that modulation of spinal neuronal activity by a CB₂ receptor agonist is enhanced following peripheral nerve injury, and further delineate the contribution of spinal and peripheral CB₂ receptors to this modulation.     

The role of hypothalamo-hypophyseal-adrenocortical system hormones in controlling pain sensitivity

The present review addresses analysis of data demonstrating the role of the hypothalamo-hypophyseal-adrenocortical axis (HHACA) in controlling pain sensitivity. Experiments on rats have demonstrated the analgesic effects of exogenous hormones of all components of the HHACA — corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and glucocorticoids — in the same models, and have also shown that the opioid and non-opioid mechanisms contribute to the development of the analgesia induced by these hormones. Endogenous glucocorticoids are involved in the development of analgesia mediated by non-opioid mechanisms. Along with the non-opioid mechanisms associated with endogenous glucocorticoids, the analgesic effect of ACTH can be mediated by the opioid mechanism. Unlike the situation with ACTH, the analgesic effect of CRH is mediated exclusively by non-opioid mechanisms, one of which is associated with HHACA hormones, while the other, appearing only on systemic administration, is not associated with these hormones. The actions of glucocorticoids on pain are mediated by neurons in the central gray matter of the midbrain. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 93, No. 11, pp. 1252–1262, November, 2007.     

Effect of cold acclimation and repeated swimming on opioid and nonopioid swim stress-induced analgesia in selectively bred mice

Swiss-Webster mice selectively bred for high swim stress-induced analgesia (SSIA) were exposed to continuous ambient cold (5 degrees C) for 6 weeks or to daily 3-min swims for 14 consecutive days either in 20 or 32 degrees C water. Thereafter, mice subjected to the particular procedure were injected intraperitoneally with 10 mg/kg of naltrexone HCl and were tested for modification of the opioid and nonopioid component of SSIA. SSIA was reduced following swims at either water temperature and was antagonized by naltrexone to greater extent than in nonswimming mice. Thus, the nonopioid (i.e. naltrexone-resistant) portion of the overall SSIA was significantly reduced, whereas the opioid (naltrexone-sensitive) portion became relatively augmented. In contrast, SSIA differed neither in magnitude nor in sensitivity to naltrexone between cold-acclimated and unacclimated mice. Swim hypothermia as well as the nonopioid component of SSIA were decreased after repeated swimming at 32 and 20 degrees C, but remained unchanged after cold acclimation. This argues for the essential role of an extrathermal, probably emotional in nature, factor not only in the elicitation of nonopioid SSIA, but also in the modulation of thermoregulatory processes during swimming. We suggest that the emergency component of swim stress, together with initial moderate hypothermic challenge, first produces the opioid form of SSIA, and subsequently, as the swim continues, also affects the thermoregulatory processes maintaining thermal homeostasis. This causes further increase in swim hypothermia and raises its stressing property to induce the nonopioid form of SSIA.     

The effects of μ, δ- and ±-opioid receptor antagonists on the pain threshold increase following muscle stimulation in the rat

In a previous study, prolonged low-frequency muscle stimulation, inducing dynamic contractions in the hind leg of unanaesthetized rats, was shown to give rise to a hypoalgesia. The increase in pain threshold, measured as squeak threshold to noxious electric pulses, lasted 3 h. In the present study, the involvement of the endogenous opioid system in the post-stimulatory analgesia was investigated using selective opioid receptor antagonists. The post-stimulatory analgesia was completely reversed back to prestimulatory control levels by naloxone, 1 mg kg^-1. ICI 154, 129 and MR 2266 BS, selective δ- and ±-receptor antagonists respectively, did not significantly influence the post-stimulatory analgesia, although ICI 154, 129 had a minor pain threshold-lowering effect. Rats pretreated with β-funaltrexamine, a μ-preceptor antagonist, did not exhibit any post-stimulatory analgesia. These results suggest that opioid systems are involved in the increase in pain threshold after muscle stimulation and that the analgesic response is both elicited and maintained by the μ-preceptor.     

Activation of alpha2-adrenoceptors in the trigeminal region produces sex-specific modulation of nociception in the rat

Sex-related differences in the sensitivity to pain and in the response to analgesics have been reported including higher perceptual responses to experimentally induced pain and the higher prevalence of many pain syndromes in women compared with men. This study examines whether alpha2-adrenoceptor-mediated antinociceptive effects are reduced by estrogen which could account for the sex-related differences in pain perception and modulation. Clonidine, an alpha2-adrenoceptor agonist, has been shown to inhibit noxious stimulus-evoked nociceptive behavior as well as the responses of nociceptive neurons in the medullary dorsal horn. Intracisternal microinjection of clonidine (7 microg/5 microl) through the implanted PE-10 cannulae dorsal to the trigeminal region in male, ovariectomized (OVX), and diestrous (DiE) Sprague-Dawley rats produced a strong antinociceptive effect on N-methyl-D-aspartic acid (NMDA)-induced nociceptive scratching behavior and heat-induced face withdrawal nociceptive tests. However, it failed to produce any inhibition in the estradiol-treated ovariectomized (OVX+E) group regardless of the dose of estradiol (1, 10 or 100 microg/100 microl sesame oil) or in the proestrous (ProE) group. Further, clonidine produced dose-dependent effects in male and OVX groups but not in the OVX+E group on the NMDA-induced nociceptive behavior. Finally, the effect of clonidine was reversed by yohimbine, an alpha2-adrenoceptor antagonist, in male and OVX groups on thermal nociceptive test. These results lead us to conclude that activation of alpha2-adrenoceptors produces sex-specific, estrogen dependent modulation of nociception in the trigeminal region of the rat. A decreased alpha2-adrenoceptor-mediated inhibition could be one of the factors responsible for the higher prevalence of pain syndromes in females.     

The estrous cycle and estrogen modulate stress-induced analgesia

In this article we investigate the impact of estrous cycle, ovariectomy, and estrogen replacement on both opioid and nonopioid stress-induced analgesia. Stage of estrous strongly influenced analgesia. Diestrus females exhibited the typical male pattern produced by the analgesia inducing procedures used--strong nonopioid analgesia following 10-20 tailshocks, and strong opioid analgesia following 80-100 tailshocks. In these experiments the nonopioid analgesia was slightly attenuated during estrus, but the opioid analgesia was markedly reduced. The role of estrogen in producing these changes was studied with estrogen replacement in ovariectomized subjects. Ovariectomy only slightly altered nonopioid analgesia but eliminated opioid analgesia, which suggests that some estrogen might be necessary to maintain the integrity of the system(s) underlying opioid analgesia. Estrogen administration restored opioid analgesia, but further estrogen suppressed opioid analgesia, duplicating the estrus pattern. It did not suppress nonopioid analgesia. Opioid analgesia was enhanced 102 hr after estrogen replacement, thus duplicating the diestrus pattern. Estrogen thus appears to be responsible for the impact of estrous cycle on opioid but not on nonopioid analgesia. These results suggest that ovarian hormones may modulate the impact of stressors on endogenous pain inhibition and other stress-responsive systems.     

Stress response dysregulation and stress-induced analgesia in nicotine dependent men and women

Alterations in the stress response and endogenous pain regulation mechanisms may contribute directly and indirectly to maintenance of nicotine dependence and relapse. We examined the extent to which nicotine dependence alters endogenous pain regulatory systems, including the hypothalamic-pituitary-adrenocortical axis, cardiovascular activity, and stress-induced analgesia. Smokers and nonsmokers attended a laboratory session that included assessment of hormonal and cardiovascular responses to stress. Smokers smoked at their regular rate prior to the session. The hand cold pressor and heat thermal pain tests were completed twice, once after acute stress (public speaking and math tasks) and the other after rest. While smokers and nonsmokers exhibited significant hormonal and cardiovascular responses to stress, smokers exhibited blunted stress responses relative to nonsmokers. They also exhibited diminished stress-induced analgesia. Results demonstrate altered stress response and diminished stress-induced analgesia among chronic smokers, and suggest that these dysregulated physiological responding may contribute to altered endogenous pain regulation.     

全膝关节置换后的多模式镇痛

背景：围全膝关节置换期疼痛处理一直是临床所关注的重点问题,寻找安全有效的镇痛方式,成为关节外科医生的重要任务之一。 目的：比较硬膜外镇痛和股神经阻滞镇痛在患者全膝关节置换后镇痛、康复的效果,探索相关的多模式联合镇痛方案。 方法：随机选取行单侧膝关节置换的患者40例,按照镇痛方案的不同分为硬膜外镇痛组和股神经阻滞镇痛组,每组20例。患者在连续硬膜外麻醉下进行单侧膝关节置换并进行术前宣教和塞来昔布给药。置换后硬膜外镇痛组通过留置导管连接0.2%罗哌卡因、2 mg/L芬太尼止痛泵镇痛;股神经阻滞镇痛组通过股神经阻滞导管间断注射0.2%罗哌卡因镇痛。观察全膝关节置换后患者6,12,24 h和2-7 d每天的疼痛程度,以及2-7 d每天的膝关节活动度。 结果与结论：全膝关节置换后2-7 d,2组患者每天的静息痛和活动痛的目测类比评分均呈下降的趋势,股神经阻滞镇痛组患者的疼痛程度小于硬膜外镇痛组患者。置换后2-7 d,股神经阻滞镇痛组和硬膜外镇痛组患者膝关节活动度都逐渐升高,股神经阻滞镇痛组患者膝关节活动度大于硬膜外镇痛组。结果说明虽然硬膜外镇痛和股神经阻滞镇痛都能缓解全膝关节置换术后疼痛,但股神经阻滞镇痛在全膝关节置换后近期的运动镇痛效果优于硬膜外镇痛,能加快患者关节功能的康复,且多模式联合镇痛方案能有效控制疼痛。     

Stress-induced analgesia: adaptive pain suppression

In this paper we have examined the phenomenon of stress-induced analgesia. We have described the procedures used to measure analgesia and have suggested that the tests can be designed not only to indicate changes in pain threshold but also to allow for the determination of the capacity to execute adaptive behavior. Aside from enabling the analysis of responses, tests that induce reflexive as well as nonreflexive behavior may have the capacity to separate the more complex aspects of pain such as the possible presence of two components of pain, sensory/discriminative and motivational/affective. These components may be of fundamental importance for any attempt to understand the biological significance of SIA. Our examination of the neurotransmitter and neuropeptide systems has revealed that they are affected by the same manipulations that induce SIA. These amines and perhaps peptides play an integral role in learning, motivation, and performance. We conclude that the functional advantage of a reduction of pain during stressful situations is significant because it allows the animal to react in threatening and perhaps critical situations as if there were no pain. Once the pain system is inhibited, other systems modulate and mediate adaptive responses that expedite the survival of the animal.     

Effects of μ-, δ-, and κ-opioid agonists and enkephalinase inhibitor RB101 in two inbred rat strains

Analgesic and suppressive effects of selective μ- (DAGO), δ- (DME), and κ- (DAKLI) opioid agonists are compared with those of aminopeptidase N and neutral endopeptidase RB101 in rats of WAG/G and Fischer-344 rats. Fischer-344 rats were more susceptible to suppressive effects of DAGO and analgesic effect of DME. It is concluded that in these rats peculiarities of the μ- and δ-opioid systems determine susceptibility to locomotor depression and analgesia, respectively. There is no correlation between effects of DAGO and RB101 in these strains. This implies that depressive effect of RB101 is not mediated though μ-opioid systems. In contrast, the effects of DMA on pain sensitivity in WAG/G and F-344 rats are opposite to those of RB101. This suggests that specific features in the activity of cerebral δ-OS can determine the sensitivity of RB101-induced analgesia. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 125, No. 5, pp. 551–554, May, 1998     

Effects of time of testing, stress level, and number of conditioning days on naloxone sensitivity of conditioned stress-induced analgesia in rats

Controversy exists as to whether conditioned stress-induced analgesia (CSIA) utilizes opioid or nonopioid mechanisms. In two experiments, both an anticipatory conditioned stimulus (CS) and a shock-associated CS were used. The duration of exposure to the anticipatory CS was long, as in studies reporting opioid CSIA, whereas the duration of the shock-associated CS was short, as in studies reporting nonopioid CSIA. In addition, the effects of unconditioned stimulus (UCS) strength were investigated by using three levels of footshock (no, moderate, and high), and the development of CSIA was monitored by using different levels of training (1 to 6 days). CSIA, measured in both anticipatory and postexposure test periods, was found to be relatively stable across tail-flick trials within days and insensitive to strength of shock. As training progressed, CSIA increased with repeated CS-UCS pairings. We tested for opioid involvement using naloxone and found opioid and nonopioid mechanisms underlying CSIA; these mechanisms combined to form a stable level of analgesia. Our data suggest that stress level and amount of training interact to activate opioid and nonopioid mechanisms of CSIA. Apparent discrepancies in previous studies regarding naloxone sensitivity of CSIA may therefore be attributable to differences in stress levels, test periods and durations of exposure to shock-related cues.     

The CB1 receptor agonist, WIN 55,212-2, dose-dependently disrupts object recognition memory in adult rats

Drugs that act as agonists at the cannabinoid CB₁ receptor have been reported to interfere with a diverse range of cognitive functions, including object recognition memory. However, to date, most of the studies conducted on this aspect of memory have suggested that these effects occur mainly in pubertal or pre-pubertal, rather than adult, rats. In this study we revisited this issue and evaluated the effects of a single s.c. injection of the CB₁ receptor agonist, WIN 55,212-2 (‘WIN’), at 1, 3 or 5 mg/kg, on object recognition memory. We found that WIN significantly reduced the total exploration time for objects at the 5 mg/kg dose only (P < 0.05). This was presumably due to its sedative effects at this dose. However, the discrimination index, which controlled for the general effects of WIN on object exploration, was significantly reduced only for the 1 mg/kg WIN group (P < 0.05), suggesting that only at this low dose did WIN specifically interfere with object discrimination. These results suggest that WIN can interfere with object recognition memory even in adult rats following a single injection of a low dose.