Opioid and nonopioid swim stress-induced analgesia: a parametric analysis in mice

Similar functional roles of the Orexin-1 and Orexin-2 receptors within the dentate gyrus area of the hippocampus in the stress-induced antinociceptive responses in the acute pain model in the rat

Studies establish that the brain's Orexin system is involved in pain modulation. Orexin-1 and orexin-2 receptors (OX1 and OX2r, respectively) are essential in responsiveness to stressful stimuli. Some evidence indicates that the hippocampus's dentate gyrus (DG) potentially modulates pain and stress. The present study examined the involvement of OX1 and OX2 receptors within the DG in response to acute pain after exposure to forced swim stress (FSS). Five to seven days post-stereotaxic surgery, the baseline tail-flick latency (TFL) was taken from the animal, then rats unilaterally received through an implanted cannula either different doses of OX1r antagonist (SB334867; 1, 3, 10, and 30 nmol), OX2r antagonist (TCS OX2 29; 1, 3, 10 and 30 nmol), or vehicle (0.5 μl solution of 12% DMSO). After 5 min, rats were exposed to the FSS for six minutes. Subsequently, the tail-flick test was conducted, and the TFLs were measured at the 60-min time set intervals. Results indicated that FSS produces antinociceptive responses in the tail-flick test. Two-way ANOVA analysis showed that Microinjection of OX1r and OX2r antagonists into the DG region of the brain reduced FSS-induced analgesia in the tail-flick test. The decrement effects of these two antagonists were almost the same. Additionally, results showed that the role of both receptors was the same in modulating stress-induced analgesia (SIA). These findings show that the orexin system in the hippocampal DG region might be partially involved in the SIA in acute pain.

Intra-CA1 injection of orexin receptors antagonism attenuates the stress-induced analgesia in a rat acute pain model

Orexins or hypocretins are excitatory neuropeptides predominantly produced by neuronal clusters in the lateral hypothalamus. The orexinergic system's involvement in pain modulation makes it a candidate for pain control alternative to the opioid system. Moreover, orexin-1 and orexin -2 receptors (OX1r and OX2r, respectively) play a role in responsiveness to stressful stimuli. Some evidence indicates that the Cornu Ammonis 1 (CA1) region of the hippocampus potentially participates in the modulation of both pain and stress. In quest of better understanding the interaction between orexin receptors and stress-induced analgesia (SIA), The present study examined the involvement of OX1r and OX2r within the CA1 in response to acute pain after exposure to forced swim stress (FSS) for a 6-min period. Adult male Wistar rats received different doses of OX1r antagonist (SB334867; 1, 3, 10, and 30 nmol), OX2r antagonist (TCS OX2 29; 3, 10, 30 and 100 nmol), or vehicle (0.5 μl DMSO) through an implanted cannula. After that, animals individually experienced acute pain by performing the tail-flick test. Results indicated that FSS produces antinociceptive responses in the tail-flick test. Blockade of both orexin receptors within the CA1 region attenuated the analgesic effect of FSS. The antinociceptive effect of swim stress was prevented by lower doses of SB334867 than TCS OX2 29. These findings show that the orexinergic system might be partially involved in the SIA via the OX1 and OX2 receptors in the hippocampal CA1 region.

Early Intervention of Cold-Water Swimming on Functional Recovery and Spinal Pain Modulation Following Brachial Plexus Avulsion in Rats

Brachial plexus avulsion (BPA) causes peripheral nerve injury complications with motor and sensory dysfunction of the upper limb. Growing evidence has shown an active role played by cold-water swimming (CWS) in alleviating peripheral neuropathic pain and functional recovery. This study examined whether CWS could promote functional recovery and pain modulation through the reduction of neuroinflammation and microglial overactivation in dorsal horn neurons at the early-stage of BPA. After BPA surgery was performed on rats, they were assigned to CWS or sham training for 5 min twice a day for two weeks. Functional behavioral responses were tested before and after BPA surgery, and each week during training. Results after the two-week training program showed significant improvements in BPA-induced motor and sensory loss (p < 0.05), lower inflammatory cell infiltration, and vacuole formation in injured nerves among the BPA-CWS group. Moreover, BPA significantly increased the expression of SP and IBA1 in dorsal horn neurons (p < 0.05), whereas CWS prevented their overexpression in the BPA-CWS group. The present findings evidenced beneficial rehabilitative effects of CWS on functional recovery and pain modulation at early-stage BPA. The beneficial effects are partially related to inflammatory suppression and spinal modulation. The synergistic role of CWS combined with other management approaches merits further investigation.

The Influence of Cross-Fostering on Alcohol Consumption and Depressive-Like Behaviors in HA and LA Mice: The Role of the Endogenous Opioid System

The development of alcohol dependence and depression is determined by various genetic and environmental factors. In the presented study, we used high analgesia (HA) and low analgesia (LA) mouse lines, characterized by different endogenous opioid system activity and divergent blood–brain barrier permeability, to determine the influence of cross-fostering of these lines raised by surrogate mothers on ethanol consumption and development of depressive-like behaviors. We also investigated ethanol drinking by biological parents or surrogate mothers. Furthermore, we investigated whether these parental changes would alter the effect of naloxone on ethanol intake and depressive-like behaviors in offspring. Our results reveal that cross-fostering of HA and LA raised by surrogate mothers has a greater impact on depressive-like behaviors than ethanol consumption. Ethanol intake by biological parents substantially affected depressive-like behaviors and ethanol consumption in offspring. Moreover, ethanol intake by biological parents or an adoptive mother modified the effect of naloxone on ethanol consumption and preference and depressive-like behaviors in the HA offspring only. Together, these results indicate that cross-fostering differentially affects the effect of naloxone on alcohol consumption and the development of depression.

Differential role of specific cardiovascular neuropeptides in pain regulation: Relevance to cardiovascular diseases

In many instances, the perception of pain is disproportionate to the strength of the algesic stimulus. Excessive or inadequate pain sensation is frequently observed in cardiovascular diseases, especially in coronary ischemia. The mechanisms responsible for individual differences in the perception of cardiovascular pain are not well recognized. Cardiovascular disorders may provoke pain in multiple ways engaging molecules released locally in the heart due to tissue ischemia, inflammation or cellular stress, and through neurogenic and endocrine mechanisms brought into action by hemodynamic disturbances. Cardiovascular neuropeptides, namely angiotensin II (Ang II), angiotensin-(1-7) [Ang-(1-7)], vasopressin, oxytocin, and orexins belong to this group. Although participation of these peptides in the regulation of circulation and pain has been firmly established, their mutual interaction in the regulation of pain in cardiovascular diseases has not been profoundly analyzed. In the present review we discuss the regulation of the release, and mechanisms of the central and systemic actions of these peptides on the cardiovascular system in the context of their central and peripheral nociceptive (Ang II) and antinociceptive [Ang-(1-7), vasopressin, oxytocin, orexins] properties. We also consider the possibility that they may play a significant role in the modulation of pain in cardiovascular diseases. The rationale for focusing attention on these very compounds was based on the following premises (1) cardiovascular disturbances influence the release of these peptides (2) they regulate vascular tone and cardiac function and can influence the intensity of ischemia - the factor initiating pain signals in the cardiovascular system, (3) they differentially modulate nociception through peripheral and central mechanisms, and their effect strongly depends on specific receptors and site of action. Accordingly, an altered release of these peptides and/or pharmacological blockade of their receptors may have a significant but different impact on individual sensation of pain and comfort of an individual patient.

Endogenous cannabinoid modulation of restraint stress-induced analgesia in thermal nociception

It is thought that endogenous cannabinoids have a role in the analgesia induced by specific forms of stress. We examined if the role of endogenous cannabinoids is also dependent upon the mode of nociception, and whether this could be altered by drugs which block their enzymatic degradation. In C57BL/6 mice, restraint stress produced analgesia in the hot-plate and plantar tests, two thermal pain assays that engage distinct supraspinal and spinal nociceptive pathways. Stress-induced analgesia in the hot-plate test was abolished by pre-treatment with the opioid receptor antagonist naltrexone but was unaffected by the cannabinoid receptor antagonist 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281). By contrast, stress-induced analgesia in the plantar test was abolished by pre-treatment with naltrexone plus AM281, but not by either antagonist individually. Remarkably, inhibiting the breakdown of endocannabinoids, with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195, rescued stress-induced analgesia in the hotplate test when endogenous opioid signalling was blocked by naltrexone. Furthermore, JZL195 recruited analgesia induced by sub-threshold restraint stress in both thermal pain assays. These findings indicate the role of endocannabinoids in stress-induced analgesia differs with the type of thermal pain behaviour. However, by inhibiting their breakdown, endocannabinoids can be recruited to substitute for endogenous opioid signalling when their activity is blocked, indicating a degree of redundancy between opioid and cannabinoid systems. Together these data suggest targeting endocannabinoid breakdown could provide an alternative, or adjuvant to mainstream analgesics such as opioids.

Increased pain sensitivity and decreased opioid analgesia in T-cell-deficient mice and implications for sex differences

The processing of pain in the central nervous system is now known to have an important immune component, including T cells of the adaptive immune system. T cells have been shown to release endogenous opioids, and although it is well known that opioids have effects on T-cell populations, very little attention has been given to the converse: how T cells may affect opioid regulation. We find here that, in addition to displaying significantly increased baseline pain sensitivity across various pain modalities, T-cell-deficient mice (CD-1 nude, Rag1 null mutant, and Cd4 null mutant) exhibit pronounced deficiencies in morphine inhibition of thermal or inflammatory pain. Nude mice are also deficient in endogenous opioid-mediated analgesia, exhibiting no stress-induced analgesia from restraint. The relevant T-cell subpopulation seems to be CD4 T cells because adoptive transfer of them but not CD8 cells into nude mice rescues both the pain and morphine analgesia phenotypes. As previously reported, we also observe a sex difference in CD-1 mice, with females requiring 2- to 3-fold more morphine than males to produce equal analgesia. Nude mice display no sex differences in morphine analgesia, and the sex difference is restored in nude mice of either sex receiving CD4 T cells from CD-1 donor male or female mice. These results suggest that CD4 T cells play an as yet unappreciated role in opioid analgesia and may be a driver of sex differences therein.

Effect of three different forms of handling on the variation of aggression-associated parameters in individually and group-housed male C57BL/6NCrl mice

Mice are social animals hence group-housing of mice is preferred over individual housing. However, aggression in group-housed male mice under laboratory housing conditions is a well-known problem leading to serious health issues, including injury or death. Therefore, group-housed mice are frequently separated for welfare reasons. In this study, we investigated the effect of 3 different handling methods (tail, forceps, tube) in 2 different housing conditions (single vs. group) on the variance of aggression-associated parameters in male C57BL/6NCrl mice over 8 weeks. Blood glucose concentration, body weight, body temperature, plus number and severity of bite wounds and barbering intensity in group-housed mice were recorded. An assessment of nest complexity was also performed weekly. Feces were collected in week 3 and 7 for analysis of corticosterone metabolites. We also monitored the level of aggression by recording the behavior of group-housed animals after weekly cage cleaning. An open field test followed by a social novel object test, a light/dark box test, a hotplate and a resident-intruder test were performed at the end of the 8-week handling period. Post-mortem, we assessed organ weights. We found that forceps-handled mice, independent of the housing condition, had significantly higher levels of stress-induced-hyperthermia and enhanced aggression after cage cleaning, and they performed worse in the nest complexity test. In addition, handling male mice by the tail seems to be most effective to reduce aggressiveness after transferring animals into new cages, thereby representing an appropriate refinement.

Impact of acute stress on itch sensation and scratching behaviour in patients with atopic dermatitis and healthy controls

BACKGROUND

Patients with atopic dermatitis (AD) often report that stress aggravates their itch. However, no study has investigated if and how acute stress influences itch sensation and scratching behaviour in these patients.

OBJECTIVES

We evaluated the impact of acute stress on experimentally induced cowhage itch perception and scratching behaviour in 16 healthy subjects and 15 patients with AD.

METHODS

The Trier Social Stress Test (TSST) was used to induce acute stress. The itch sensation, provoked by applying cowhage to the forearms, and off-site scratching behaviour (not directed at the cowhage application site) were compared before and after performing the TSST or the control condition (watching a video of landscape scenes).

RESULTS

In patients with AD, stress induced by TSST caused a significant reduction of cowhage-evoked itch but significantly increased off-site scratching behaviour. Such changes in itch perception and scratching behaviour were not observed in healthy controls. In addition, a significant positive correlation was noted between stress induced by TSST and clinical severity of eczema.

CONCLUSIONS

We speculate that psychological stress increases spontaneous scratching in patients with AD, which may enhance the vicious cycle of itching and scratching, resulting in aggravation of the skin eczema. These results provide new insights on the mechanism of acute stress-related exacerbation of itch in patients with AD.

Sex Differences in Nociceptin/Orphanin FQ Peptide Receptor-Mediated Pain and Anxiety Symptoms in a Preclinical Model of Post-traumatic Stress Disorder

Nociceptin/Orphanin FQ (N/OFQ) is a neuropeptide that modulates pain transmission, learning/memory, stress, anxiety, and fear responses via activation of the N/OFQ peptide (NOP or ORL1) receptor. Post-traumatic stress disorder (PTSD) is an anxiety disorder that may arise after exposure to a traumatic or fearful event, and often is co-morbid with chronic pain. Using an established animal model of PTSD, single-prolonged stress (SPS), we were the first to report that NOP receptor antagonist treatment reversed traumatic stress-induced allodynia, thermal hyperalgesia, and anxiety-like behaviors in male Sprague-Dawley rats. NOP antagonist treatment also reversed SPS-induced serum and CSF N/OFQ increase and circulating corticosterone decrease. The objective of this study was to examine the role of the NOP receptor in male and female rats subjected to traumatic stress using Wistar wild type (WT) and NOP receptor knockout (KO) rats. The severity of co-morbid allodynia was assessed as change in paw withdrawal threshold (PWT) to von Frey and paw withdrawal latency (PWL) to radiant heat stimuli, respectively. PWT and PWL decreased in male and female WT rats within 7 days after SPS, and remained decreased through day 28. Baseline sensitivity did not differ between genotypes. However, while male NOP receptor KO rats were protected from SPS-induced allodynia and thermal hypersensitivity, female NOP receptor KO rats exhibited tactile allodynia and thermal hypersensitivity to the same extent as WT rats. Male NOP receptor KO rats had a lower anxiety index (AI) than WT, but SPS did not increase AI in WT males. In contrast, SPS significantly increased AI in WT and NOP receptor KO female rats. SPS increased circulating N/OFQ levels in male WT, but not in male NOP receptor KO, or WT or KO female rats. These results indicate that the absence of the NOP receptor protects males from traumatic-stress-induced allodynia and hyperalgesia, consistent with our previous findings utilizing a NOP receptor antagonist. However, female NOP receptor KO rats experience allodynia, hyperalgesia and anxiety-like symptoms to the same extent as WT females following SPS. This suggests that endogenous N/OFQ-NOP receptor signaling plays an important, but distinct, role in males and females following exposure to traumatic stress.

Modulation of the anticonvulsant effect of swim stress by agmatine

Agmatine is an endogenous l-arginine metabolite with neuroprotective effects in the stress-response system. It exerts anticonvulsant effects against several seizure paradigms. Swim stress induces an anticonvulsant effect by activation of endogenous antiseizure mechanisms. In this study, we investigated the interaction of agmatine with the anticonvulsant effect of swim stress in mice on pentylenetetrazole (PTZ)-induced seizure threshold. Then we studied the involvement of nitric oxide (NO) pathway and endogenous opioid system in that interaction. Swim stress induced an anticonvulsant effect on PTZ seizures which was opioid-independent in shorter than 1-min swim durations and opioid-dependent with longer swims, as it was completely reversed by pretreatment with naltrexone (NTX) (10mg/kg), an opioid receptor antagonist. Agmatine significantly enhanced the anticonvulsant effect of opioid-independent shorter swim stress, in which a combination of subthreshold swim stress duration (45s) and subeffective dose of agmatine (1mg/kg) revealed a significantly higher seizure threshold compared with either one. This effect was significantly reversed by NO synthase inhibitor N^G-nitro-l-arginine (L-NAME (Nω-Nitro-L-arginine methyl ester), 5mg/kg), suggesting an NO-dependent mechanism, and was unaffected by NTX (10mg/kg), proving little role for endogenous opioids in the interaction. Our data suggest that pretreatment of animals with agmatine acts additively with short swim stress to exert anticonvulsant responses, possibly by mediating NO pathway.

Naloxone exacerbates memory impairments and depressive-like behavior after mild traumatic brain injury (mTBI) in mice with upregulated opioid system activity

The neuroprotective role of the endogenous opioid system in the pathophysiological sequelae of brain injury remains largely ambiguous. Noteworthy, almost no data is available on how its genetically determined activity influences the outcome of mild traumatic brain injury. Thus, the aim of our study was to examine the effect of opioid receptor blockage on cognitive impairments produced by mild traumatic brain injury in mice selectively bred for high (HA) and low (LA) swim-stress induced analgesia that show innate divergence in opioid system activity. Mild traumatic brain injury was induced with a weight-drop device on anaesthetized mice. Naloxone (5mg/kg) was intraperitoneally delivered twice a day for 7days to non-selectively block opioid receptors. Spatial memory performance and manifestations of depressive-like behavior were assessed using the Morris Water Maze and tail suspension tests, respectively. Mild traumatic brain injury resulted in a significant deterioration of spatial memory performance and severity of depressive-like behavior in the LA mouse line as opposed to HA mice. Opioid receptor blockage with naloxone unmasked cognitive deficits in HA mice but was without effect in the LA line. The results suggest a protective role of genetically predetermined enhanced opioid system activity in suppression of mild brain trauma-induced cognitive impairments. Mice selected for high and low swim stress-induced analgesia might therefore be a useful model to study the involvement of the opioid system in the pathophysiology and neurological outcome of traumatic brain injury.

Mechanisms of exercise-induced hypoalgesia

The purpose of this study was to examine opioid and endocannabinoid mechanisms of exercise-induced hypoalgesia (EIH). Fifty-eight men and women (mean age = 21 years) completed 3 sessions. During the first session, participants were familiarized with the temporal summation of heat pain and pressure pain protocols. In the exercise sessions, following double-blind administration of either an opioid antagonist (50 mg naltrexone) or placebo, participants rated the intensity of heat pulses and indicated their pressure pain thresholds and pressure pain ratings before and after 3 minutes of submaximal isometric exercise. Blood was drawn before and after exercise. Results indicated that circulating concentrations of 2 endocannabinoids, N-arachidonylethanolamine and 2-arachidonoylglycerol, as well as related lipids oleoylethanolamide, palmitoylethanolamide, N-docosahexaenoylethanolamine, and 2-oleoylglycerol, increased significantly (P < .05) following exercise. Pressure pain thresholds increased significantly (P < .05), whereas pressure pain ratings decreased significantly (P < .05) following exercise. Also, temporal summation ratings were significantly lower (P < .05) following exercise. These changes in pain responses did not differ between the placebo and naltrexone conditions (P > .05). A significant association was found between EIH and docosahexaenoylethanolamine. These results suggest involvement of a nonopioid mechanism in EIH following isometric exercise.

PERSPECTIVE

Currently, the mechanisms responsible for EIH are unknown. This study provides support for a potential endocannabinoid mechanism of EIH following isometric exercise.

The critical role of spinal 5-HT7 receptors in opioid and non-opioid type stress-induced analgesia

The opioid and non-opioid types of stress-induced analgesia have been well defined. One of the non-opioid type involve the endocannabinoid system. We previously reported that the spinal serotonin 7 receptor (5-HT7) blockers inhibit both morphine and cannabinoid-induced analgesia, thus we hypothesized that descending serotonergic pathways-spinal 5-HT7 receptor loop might contribute to stress-induced analgesia. Stress-induced analgesia was induced with warm (32°C) or cold (20°C) water swim stress in male Balb-C mice. The effects of intrathecal injection of a selective 5-HT7 receptor antagonist, SB 269970, of the denervation of serotonergic neurons by intrathecal administration of 5,7-dihydroxytryptamine (5,7-DHT) and of lesions of the dorsolateral funiculus on opioid and non-opioid type stress-induced analgesia were evaluated with the tail-flick and hot plate tests. The expression of 5-HT7 receptors mRNA in the dorsal lumbar region of spinal cord were analyzed by RT-PCR following spinal serotonin depletion or dorsolateral funiculus lesion. The effects of the selective 5-HT7 receptor agonists LP 44 and AS 19 were tested on nociception. Intrathecal SB 269970 blocked both opioid and non-opioid type stress-induced analgesia. Dorsolateral funiculus lesion or denervation of the spinal serotonergic neurons resulted in a marked decrease in 5-HT7 receptor expression in the dorsal lumbar spinal cord, accompanied by inhibition of opioid and non-opioid type stress-induced analgesia. However, the systemic or intrathecal LP 44 and AS 19 alone did not produce analgesia in unstressed mice. These results indicate that descending serotonergic pathways and the spinal 5-HT7 receptor loop play a crucial role in mediating both opioid and non-opioid type stress-induced analgesia.

Significance of neuronal cytochrome P450 activity in opioid-mediated stress-induced analgesia

Stressful environmental changes can suppress nociceptive transmission, a phenomenon known as "stress-induced analgesia". Depending on the stressor and the subject, opioid or non-opioid mechanisms are activated. Brain μ opioid receptors mediate analgesia evoked either by exogenous agents (e.g. morphine), or by the release of endogenous opioids following stressful procedures. Recent work with morphine and neuronal cytochrome P450 (P450)-deficient mice proposed a signal transduction role for P450 enzymes in µ analgesia. Since µ opioid receptors also mediate some forms of stress-induced analgesia, the present studies assessed the significance of brain P450 activity in opioid-mediated stress-induced analgesia. Two widely-used models of opioid stress-induced analgesia (restraint and warm water swim) were studied in both sexes of wild-type control and P450-deficient (Null) mice. In control mice, both stressors evoked moderate analgesic responses which were blocked by pretreatment with the opioid antagonist naltrexone, confirming the opioid nature of these responses. Consistent with literature, sex differences (control female>control male) were seen in swim-induced, but not restraint-induced, analgesia. Null mice showed differential responses to the two stress paradigms. As compared with control subjects, Null mice showed highly attenuated restraint-induced analgesia, showing a critical role for neuronal P450s in this response. However, warm water swim-induced analgesia was unchanged in Null vs. control mice. Additional control experiments confirmed the absence of morphine analgesia in Null mice. These results are the first to show that some forms of opioid-mediated stress-induced analgesia require brain neuronal P450 activity.

Association between the A107V substitution in the δ-opioid receptors and ethanol drinking in mice selected for high and low analgesia

Experimental evidence suggests that endogenous opioids play an important role in the development of ethanol addiction. In this study, we employed two mouse lines divergently bred for opioid-mediated stress-induced analgesia. In comparison with HA (high analgesia line) mice, LA (low analgesia line) mice, having lower opioid receptor system activity, manifest enhanced basal as well as stress-induced ethanol drinking. Here, we found that recently discovered C320T transition in exon 2 of the δ-opioid receptor gene (EU446125.1), which results in an A107V substitution (ACA23171.1), leads to higher ethanol preference in CT mice compared with CC homozygotes. This genetic association is particularly evident under chronic mild stress (CMS) conditions. The interaction between stress and ethanol intake was significantly stronger in HA than in LA mice. Ethanol almost completely attenuated the pro-depressive effect of CMS (assessed with the tail suspension test) in both the CC and CT genotypes in the HA line. In the LA mice, a lack of response to ethanol was observed in the CC genotype, whereas ethanol consumption strengthened depressive-like behaviours in CT individuals. Our results suggest that constitutively active A107V substitution in δ-opioid receptors may be involved in stress-enhanced vulnerability to ethanol abuse and in the risk of ethanol dependence.

Olfactory exposure to males, including men, causes stress and related analgesia in rodents

We found that exposure of mice and rats to male but not female experimenters produces pain inhibition. Male-related stimuli induced a robust physiological stress response that results in stress-induced analgesia. This effect could be replicated with T-shirts worn by men, bedding material from gonadally intact and unfamiliar male mammals, and presentation of compounds secreted from the human axilla. Experimenter sex can thus affect apparent baseline responses in behavioral testing.

Chronic mild stress facilitates melanoma tumor growth in mouse lines selected for high and low stress-induced analgesia

Both chronic stress conditions and hyperergic reaction to environmental stress are known to enhance cancer susceptibility. We described two mouse lines that displayed high (HA) and low (LA) swim stress-induced analgesia (SSIA) to investigate the relationship between inherited differences in sensitivity to stress and proneness to an increased growth rate of subcutaneously inoculated melanoma. These lines display several genetic and physiological differences, among which distinct sensitivity to mutagens and susceptibility to cancer are especially noticeable. High analgesic mice display high proneness both to stress and a rapid local spread of B16F0 melanoma. However, stress-resistant LA mice do not develop melanoma tumors after inoculation, or if so, tumors regress spontaneously. We found that the chronic mild stress (CMS) procedure leads to enhanced interlinear differences in melanoma susceptibility. Tumors developed faster in stress conditions in both lines. However, LA mice still displayed a tendency for spontaneous regression, and 50% of LA mice did not develop a tumor, even under stressed conditions. Moreover, we showed that chronic stress, but not tumor progression, induces depressive behavior, which may be an important clue in cancer therapy. Our results clearly indicate how the interaction between genetic susceptibility to stress and environmental stress determine the risk and progression of melanoma. To our knowledge, HA/LA mouse lines are the first animal models of distinct melanoma progression mediated by inherited differences in stress reactivity.

Cellular mechanisms of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor regulation and heterologous regulation by N/OFQ

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is the fourth and most recently discovered member of the opioid receptor superfamily that also includes μ, δ, and κ opioid receptor subtypes (MOR, DOR, and KOR, respectively). The widespread anatomic distribution of the NOP receptor enables the modulation of several physiologic processes by its endogenous agonist, N/OFQ. Accordingly, the NOP receptor has gained a lot of attention as a potential target for the development of ligands with therapeutic use in several pathophysiological states. NOP receptor activation frequently results in effects opposing classic opioid receptor action; therefore, regulation of the NOP receptor and conditions affecting its modulatory tone are important to understand. Mounting evidence reveals a heterologous interaction of the NOP receptor with other G protein-coupled receptors, including MOR, DOR, and KOR, which may subsequently influence their function. Our focus in this review is to summarize and discuss the findings that delineate the cellular mechanisms of NOP receptor signaling and regulation and the regulation of other receptors by N/OFQ and the NOP receptor.

Forced swim-induced musculoskeletal hyperalgesia is mediated by CRF2 receptors but not by TRPV1 receptors

The exacerbation of musculoskeletal pain by stress in humans is modeled by the musculoskeletal hyperalgesia in rodents following a forced swim. We hypothesized that stress-sensitive corticotropin releasing factor (CRF) receptors and transient receptor vanilloid 1 (TRPV1) receptors are responsible for the swim stress-induced musculoskeletal hyperalgesia. We confirmed that a cold swim (26 °C) caused a transient, morphine-sensitive decrease in grip force responses reflecting musculoskeletal hyperalgesia in mice. Pretreatment with the CRF2 receptor antagonist astressin 2B, but not the CRF1 receptor antagonist NBI-35965, attenuated this hyperalgesia. Desensitizing the TRPV1 receptor centrally or peripherally using desensitizing doses of resiniferatoxin (RTX) failed to prevent the musculoskeletal hyperalgesia produced by cold swim. SB-366791, a TRPV1 antagonist, also failed to influence swim-induced hyperalgesia. Together these data indicate that swim stress-induced musculoskeletal hyperalgesia is mediated, in part, by CRF2 receptors but is independent of the TRPV1 receptor.

μ-Opioid blockade reduces ethanol effects on intake and behavior of the infant rat during short-term but not long-term social isolation

Numerous findings in adult and infant rats have shown that the endogenous opioid system is involved in control of ethanol consumption and its reinforcing effects. Opioid systems are also involved in reactivity to social isolation with several factors (age, duration, and type of isolation) affecting this modulation. The present study investigated the effects of a selective mu-opioid antagonist CTOP (0, 0.1, 0.5mg/kg), ethanol (0, 0.5 g/kg), and the interaction of the two drugs on the behavioral consequences of two types of social isolation given to preweanling rats: 1) short-term social isolation from littermates (STSI, duration 8 min) and 2) relatively long-term (5h) isolation (LTSI) from the dam and littermates. Voluntary intake of saccharin, locomotion, rearing activity, paw licking, and grooming were assessed during an 8-min. intake test. Thermal nociceptive reactivity was also measured before and after the testing session with normalized differences in pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). The results indicate that pharmacological blockade of mu-opioid receptors by CTOP substantially attenuated ethanol's anxiolytic effects on the developing rat's reactions to social isolation. Some of these stress-attenuating effects of CTOP were observed only in animals exposed to short-term isolation (STSI) but not in pups isolated for 5h (LTSI). Ethanol selectively increased saccharin intake during STSI in females and CTOP blocked this effect. Ethanol decreased the magnitude of analgesia associated with STSI but had no effect on pain reactivity during LTSI. CTOP by itself did not affect IIA or saccharin intake in sober animals. The findings of the present experiments indicate that the anxiolytic effects of 0.5 g/kg ethanol on pups exposed to STSI are modulated by endogenous opioid activity.

Effects of acute stressors on itch- and pain-related behaviors in rats

Many acute stressors reduce pain, a phenomenon called stress-induced antinociception (SIA). Stress also is associated with increased scratching in chronic itch conditions. We investigated effects of acute stressors on facial itch and pain using a recently introduced rat model. Under baseline (no-swim) conditions, intradermal (id) cheek microinjection of the pruritogen serotonin (5-HT) selectively elicited hindlimb scratch bouts, whereas the algogen mustard oil (allyl isothiocyanate [AITC]) selectively elicited ipsilateral forepaw swipes, directed to the cheek injection site. To test effects of swim stress, rats received id cheek microinjection of 5-HT (1%), AITC (10%), or vehicle, and were then subjected to one of the following swim conditions: (1) weak SIA (W-SIA), (2) naltrexone-sensitive SIA (intermediate or I-SIA), or (3) naltrexone-insensitive SIA (strong or S-SIA). After the swim, we recorded the number of hindlimb scratch bouts and forelimb swipes directed to the cheek injection site, as well as facial grooming by both forepaws. Under S-SIA, AITC-evoked swiping and 5-HT-evoked scratching were both reduced. I-SIA reduced AITC-evoked swiping with no effect on 5-HT-evoked scratching. Facial grooming immediately post-swim was suppressed by S-SIA, but not I- or W-SIA. W-SIA tended to equalize scratching and swiping elicited by 5-HT and AITC compared with no-swim controls, suggesting altered itch and pain processing. Exercise (wheel-running), novelty, cold exposure, and fear (shaker table), key components of swim stress, differentially affected tail-flick latencies and 5-HT-evoked swiping and scratching behavior. Thus, itch and pain can be simultaneously suppressed by a combination of acute stress-related factors via an opioid-independent mechanism.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

Pain sensitivity and vasopressin analgesia are mediated by a gene-sex-environment interaction

Quantitative trait locus mapping of chemical/inflammatory pain in the mouse identified the Avpr1a gene, encoding the vasopressin-1A receptor (V1AR), as responsible for strain-dependent pain sensitivity to formalin and capsaicin. A genetic association study in humans revealed the influence of a single nucleotide polymorphism (rs10877969) within AVPR1A on capsaicin pain levels, but only in male subjects reporting stress at the time of testing. The analgesic efficacy of the vasopressin analog, desmopressin, revealed a similar interaction between the drug and acute stress, as desmopressin inhibition of capsaicin pain was seen only in non-stressed subjects. Additional experiments in mice confirmed the male-specific interaction of V1AR and stress, leading to the conclusion that vasopressin activates endogenous analgesia mechanisms unless they have already been activated by stress. These findings represent the first explicit demonstration of analgesic efficacy depending on the emotional state of the recipient, and illustrate the heuristic power of a bench-to-bedside-to-bench translational strategy.

Influence of preferred versus prescribed exercise on pain in fibromyalgia

PURPOSE

The purpose of this study was to examine the influence of a preferred- versus a prescribed-intensity exercise session on pain in women with fibromyalgia (FM).

METHODS

Twenty-one women with FM (mean age = 44 yr) completed two randomly assigned exercise sessions consisting of 20 min of cycle ergometry at a self-selected intensity and a prescribed intensity. Experimental pain perception was assessed before and after aerobic exercise. During exercise, HR, watts, RPE, and muscle pain were assessed every 5 min. Clinical pain was assessed with the Short-Form McGill Pain Questionnaire (SF-MPQ) immediately and 24, 48, 72, and 96 h after exercise. Data were analyzed with repeated-measures ANOVA.

RESULTS

Women with FM preferred a lower intensity of exercise than what was prescribed as indicated by significantly lower HR, watts, and RPE responses (P < 0.05). Muscle pain in the legs, however, was similar in the two conditions and significantly increased during exercise (P < 0.05). Pain thresholds and pain tolerances increased significantly after exercise, whereas peak pain ratings decreased after exercise (P < 0.05). Furthermore, pain (SF-MPQ) in the follow-up period was found to be lower than baseline (P < 0.05).

CONCLUSIONS

It is concluded that the women with FM who participated in this study experienced significant improvements in pain after exercise. The results from this study are novel and indicate that recommendations for exercise prescription for individuals with FM should consider the preferred-intensity exercise model as a strategy to reduce pain.

Newborn Analgesia Mediated by Oxytocin during Delivery

The mechanisms controlling pain in newborns during delivery are poorly understood. We explored the hypothesis that oxytocin, an essential hormone for labor and a powerful neuromodulator, exerts analgesic actions on newborns during delivery. Using a thermal tail-flick assay, we report that pain sensitivity is two-fold lower in rat pups immediately after birth than 2 days later. Oxytocin receptor antagonists strongly enhanced pain sensitivity in newborn, but not in 2-day-old rats, whereas oxytocin reduced pain at both ages suggesting an endogenous analgesia by oxytocin during delivery. Similar analgesic effects of oxytocin, measured as attenuation of pain-vocalization induced by electrical whisker pad stimulation, were also observed in decerebrated newborns. Oxytocin reduced GABA-evoked calcium responses and depolarizing GABA driving force in isolated neonatal trigeminal neurons suggesting that oxytocin effects are mediated by alterations of intracellular chloride. Unlike GABA signaling, oxytocin did not affect responses mediated by P2X3 and TRPV1 receptors. In keeping with a GABAergic mechanism, reduction of intracellular chloride by the diuretic NKCC1 chloride co-transporter antagonist bumetanide mimicked the analgesic actions of oxytocin and its effects on GABA responses in nociceptive neurons. Therefore, endogenous oxytocin exerts an analgesic action in newborn pups that involves a reduction of the depolarizing action of GABA on nociceptive neurons. Therefore, the same hormone that triggers delivery also acts as a natural pain killer revealing a novel facet of the protective actions of oxytocin in the fetus at birth.

Avaliação funcional da nocicepção do joelho de ratos tratada com laser de baixa potência e natação

O objetivo deste estudo foi avaliar e comparar os efeitos do laser de baixa potência e da natação forçada em modelo de nocicepção articular, de ratos Wistar, avaliando a dor de forma funcional, pelo tempo de elevação da pata (TEP) durante marcha em cilindro metálico. Foram utilizados 32 ratos Wistar, divididos em quatro grupos: GC - animais submetidos à indução de nocicepção no joelho direito e não tratados; GL - nocicepção e tratados com laser de baixa potência 670nm, 8J/cm²; GN - nocicepção e natação por 10 minutos em água a 30-32ºC; GNL - nocicepção e tratados com natação e laser. Para realizar a nocicepção foi injetado, no espaço tibiofemoral medial direito, 50µL de formalina 5%. A avaliação funcional da dor foi realizada com o teste de incapacidade funcional, que avalia o tempo de pata no ar (TEP) da marcha durante um minuto sobre um cilindro metálico, as avaliações ocorreram antes da indução da nocicepção (AV1), após 15 (AV2) e 30 minutos (AV3) da mesma, sendo que após a AV2, ocorreram os protocolos de tratamento. Os resultados mostraram que o grupo laser foi o único a apresentar restauração dos valores na AV3, comparando com AV1. GN foi o único a não apresentar redução ao comparar AV3 com AV2. Conclui-se que, pela avaliação funcional, o laser de baixa potência apresentou efeitos analgésicos, enquanto a natação produziu aumento do quadro de dor, o qual foi parcialmente revertido com o uso do laser associado

Tuberoinfundibular peptide of 39 residues (TIP39) signaling modulates acute and tonic nociception

Tuberoinfundibular peptide of 39 residues (TIP39) synthesizing neurons at the caudal border of the thalamus and in the lateral pons project to areas rich in its receptor, the parathyroid hormone 2 receptor (PTH2R). These areas include many involved in processing nociceptive information. Here we examined the potential role of TIP39 signaling in nociception using a PTH2R antagonist (HYWH) and mice with deletion of TIP39's coding sequence or PTH2R null mutation. Intracerebroventricular (icv) infusion of HYWH significantly inhibited nociceptive responses in tail-flick and hot-plate tests and attenuated the nociceptive response to hindpaw formalin injection. TIP39-KO and PTH2R-KO had increased response latency in the 55°C hot-plate test and reduced responses in the hindpaw formalin test. The tail-flick test was not affected in either KO line. Thermal hypoalgesia in KO mice was dose-dependently reversed by systemic administration of the cannabinoid receptor 1 (CB1) antagonist rimonabant, which did not affect nociception in wild-type (WT). Systemic administration of the cannabinoid agonist CP 55,940 did not affect nociception in KO mice at a dose effective in WT. WT mice administered HYWH icv, and both KOs, had significantly increased stress-induced analgesia (SIA). Rimonabant blocked the increased SIA in TIP39-KO, PTH2R-KO or after HYWH infusion. CB1 and FAAH mRNA were decreased and increased, respectively, in the basolateral amygdala of TIP39-KO mice. These data suggest that TIP39 signaling modulates nociception, very likely by inhibiting endocannabinoid circuitry at a supraspinal level. We infer a new central mechanism for endocannabinoid regulation, via TIP39 acting on the PTH2R in discrete brain regions.

Examination of the dose-response relationship between pain perception and blood pressure elevations induced by isometric exercise in men and women

The purpose of this study was to examine exercise-induced hypoalgesia (EIH) in men and women, and whether different magnitudes of BP elevations induced by isometric exercise systematically influenced pain perception. Twenty-five men and 25 women performed isometric exercise at 25% MVC for 1-min, 3-min, and 5-min while BP and pain perception were assessed. Results indicated that BP was significantly elevated (p<0.05) by isometric exercise in a dose-response manner. Pain thresholds were found to be elevated while pain ratings were lower (p<0.05) immediately following isometric exercise but not in a dose-response manner. It was concluded that isometric exercise produced EIH in men and women, and there was not a dose-response relationship between BP and EIH.

Involvement of NTS2 receptors in stress-induced analgesia

Stress activates multiple neural systems that suppress pain sensation. This adaptive phenomenon referred as stress-induced analgesia (SIA) is mediated by the activation of endogenous pain inhibitory systems. Both opioid and non-opioid forms of SIA have been elicited in rodents according to stressor parameters and duration. There is accumulating evidence that the endogenous neurotensin (NT) system plays an important role in SIA. Especially, NT-deficient mice were shown to exhibit reduced SIA following water avoidance or restraint stress. Since central NT produces naloxone-insensitive analgesic effects by acting on spinal and supraspinal NTS2 receptors, we hypothesized that NT might mediate non-opioid SIA through NTS2 activation. Here, we evaluated the influence of an opioid-independent severe stress produced by a cold-water swim for 3 min at 15 degrees C on rodent offspring's pain perception. Our results demonstrated that mice lacking NTS2 exhibit significantly reduced SIA following cold-water swim stress. Indeed, NTS2 knockout mice submitted to both acute (plantar test) and tonic (formalin test) pain stimuli show a greater sensitivity to pain in comparison to wild-type littermates. Accordingly, pretreatment with the NT receptor antagonist SR142948A results in a hyperalgesic response to stress induced by cold-water swim. Endogenous NT regulates hypothalamic-pituitary-adrenal axis activity in stress condition by increasing corticosterone plasma levels. Accordingly, the plasma levels of corticosterone measured by radioimmunoassay are significantly reduced in non-stressed and stressed NTS2-deficient mice in comparison with wild-type mice. To further investigate the site of action of NT in mediating SIA, we microinjected NTS2 agonists in lumbar spinal cord and quantified post-stress sensitivity to pain in rats using the plantar test. Exogenously administered NTS2 analogs, JMV-431, beta-lactotensin and NT69L markedly enhance the magnitude and duration of stress antinociception in both 25- and 60-day-old rats. In sum, by using genetic and pharmacological approaches, we demonstrated here that NTS2 receptors mediate non-opioid SIA. Our results also revealed that the release of endogenous NT in response to stress requires the presence of NTS2 to stimulate corticotropin-releasing factor (CRF)-induced elevation of plasma corticosterone, and that NTS2 receptors localized at the lumbar spinal cord participate to the disinhibition of descending pain control pathways. Therefore, these data highlight the significance of NTS2 as a novel target for the treatment of pain and stress-related disorders.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.

Mode of delivery modulates physiological and behavioral responses to neonatal pain

OBJECTIVE

To study whether the mode of delivery alters pain expression.

STUDY DESIGN

Full-term infants born by vaginal delivery or elective caesarean section were observed following high- and low-intensity pain stimuli, with recording of electrocardiogram, facial expression and vocalization.

RESULT

Graded physiological and behavioral responses occurred, with greater responses to higher than lower intensity pain stimuli. Elevation in heart rate following both stimuli increased with time after vaginal delivery. Infants delivered by elective caesarean section showed stronger facial expressions and briefer time in vocalizations response to both interventions.

CONCLUSION

Diminished responses following vaginal delivery suggest that physiological events associated with a normal delivery reduce the physiologic and sympathoadrenal activation by nociceptive mechanisms. Pain and stress reactivity appear to be inhibited during fetal life and sensory inputs during vaginal delivery may reverse this inhibition. To minimize neonatal pain, we recommend that postnatal invasive procedures to be performed shortly after vaginal birth.

Effects of acute or 3-day treatments of Hypericum caprifoliatum Cham. & Schltdt. (Guttiferae) extract or of two established antidepressants on basal and stress-induced increase in serum and brain corticosterone levels

Since depressive patients present alterations in the hypothalamo-pituitary-adrenal (HPA) axis that are normalised by antidepressants, this HPA axis has been considered as a target of their actions. We have investigated the mechanism of action of a cyclohexane extract of Hypericum caprifoliatum (HCP), which displays antidepressant like activity, by studying, in mice, the influence of HCP and of two established antidepressant drugs, imipramine and bupropion, administered either acutely or semi-chronically (once a day, three consecutive days), on serum and brain cortex corticosterone levels, either in basal conditions or shortly after a forced-swimming session (FSS). Administered acutely, imipramine (20 mg/kg, per os (p.o.)), bupropion (30 mg/kg, p.o.) and HCP (360 mg/kg, p.o.) significantly reduced the immobility time and had no effects on FSS-induced increase of serum and cortical corticosterone levels. Conversely, 3 days repeated treatment with imipramine or bupropion resulted in a significant reduction of immobility time and FSS-induced increase of serum and cortical corticosterone levels. In a different way, repeated treatment with HCP significantly reduced the immobility time and only cortical corticosterone levels in stressed mice. These results indicate that short-term treatments with antidepressants are sufficient to induce modifications in the HPA axis reactivity to stress; and that apparently HCP has an influence on corticosterone levels by a mechanism diverse from the other tested antidepressants.

Investigating the effects of distracting stimuli on nociceptive behaviour and associated alterations in brain monoamines in rats

Distraction interventions are used clinically to relieve pain. Exposure to distracting stimuli causes withdrawal of attention from the painful stimulus and reduces perceived pain. However, the neurobiological mechanisms mediating distraction-induced analgesia are poorly understood due, in part, to a paucity of animal studies modelling this phenomenon. The present study investigated the effects of three distracting stimuli on formalin-evoked nociceptive behaviour and brain tissue monoamine levels in rats. The three distractors were: exposure to a novel environment, exposure to a novel object, and exposure, without contact, to another rat (conspecific). A control group, habituated to the test arena, was also included. Formalin-evoked nociceptive behaviour was significantly reduced in rats exposed to the novel object or novel arena, but not those exposed to the conspecific, compared with controls. Antinociception resulting from exposure to the novel object was of longer duration than that resulting from exposure to the novel arena. Failure to detect any distractor-induced effects on plasma corticosterone levels or aversive behaviours suggests that the stimuli used were non-stressful. HPLC analysis revealed that there was a significant reduction in serotonin and dopamine metabolites in the medial prefrontal cortex in animals exposed to the novel object. These results indicate that exposure to a novel object or arena reduces nociceptive behaviour in rats, effects accompanied by discrete alterations in serotonin and dopamine metabolites in the medial prefrontal cortex.

Chronic forced swim stress inhibits ultra-low dose morphine-induced hyperalgesia in rats

Ultra-low doses of morphine (UL-morphine) induce hyperalgesia, which is assumed to be mediated by stimulatory G proteins (G(alphas)) signaling pathway. G(alphas) pathway inhibition and chronic stress both attenuate development of tolerance to analgesic effect of morphine. This study evaluated the effect of chronic stress on UL-morphine-induced hyperalgesia to find out if chronic stress interacts with the G(alphas) signaling pathway. Repeated daily forced swim stress was applied to induce chronic stress. UL-morphine (1 microg/kg, intraperitoneal)-induced hyperalgesia was assessed using the tail-flick test on day 6, in male rats that during days 1-5 received different treatments of swim stress, dexamethasone, swim stress following adrenalectomy (ADX) or swim stress after sham operation. Chronic stress by itself induced hyperalgesia in control and sham-operated rats but inhibited UL-morphine-induced hyperalgesia. In ADX animals, chronic stress did not produce hyperalgesia and could not inhibit UL-morphine-induced hyperalgesia. Chronic dexamethasone produced hyperalgesia but did not change the UL-morphine-induced hyperalgesia. Inhibition of UL-morphine hyperalgesia by chronic stress suggests that chronic stress interacts with the G(alphas) signaling pathway, which is responsible for UL-morphine-induced hyperalgesia. The absence of this effect in the ADX-rats or after repetitive dexamethasone administration demonstrates that hypothalamic-pituitary-adrenal (HPA) axis activation is necessary for controlling UL-morphine-induced hyperalgesia. Finally, the interaction of stress with the G(alphas) signaling pathway may provide an explanation for the inhibitory effect of stress on development of tolerance to the analgesic effect of morphine.

Neonatal morphine enhances nociception and decreases analgesia in young rats

The recognition of the impact of neonatal pain experience on subsequent sensory processing has led to the increased advocacy for the use of opioids for pain relief in infants. However, following long-term opioid exposure in intensive care units more than 48% of infants exhibited behaviors indicative of opioid abstinence syndrome, a developmentally equivalent set of behaviors to opioid withdrawal as seen in adults. Little is known about the long-term influence of repeated neonatal morphine exposure on nociception and analgesia. To investigate this, we examined mechanical and thermal nociception on postnatal days 11, 13, 15, 19, 24, 29, 39 and 48 following subcutaneous administration of morphine (3 mg/kg) once daily on postnatal days 1-9. The cumulative morphine dose-response was assessed on postnatal days 20 and 49, and stress-induced analgesia was assessed on postnatal days 29 and 49. Both basal mechanical and thermal nociception in neonatal, morphine-exposed rats were significantly lower than those in saline-exposed, handled-control rats and naive rats until P29. A rightward-shift of cumulative dose-response curves for morphine analgesia upon chronic neonatal morphine was observed both on P20 and P49. The swim stress-induced analgesia was significantly decreased in neonatal morphine-exposed rats on P29, but not on P49. These data indicate that morphine exposure equivalent to the third trimester of gestation produced prolonged pain hypersensitivity, decreased morphine antinociception, and decreased stress-induced analgesia. The present study illustrates the need to examine the long-term influence of prenatal morphine exposure on pain and analgesia in the human pediatric population.