Stress-induced Antinociception Research Articles

Contribution of the intra-hippocampal orexin system in the regulation of restraint stress response to pain-related behaviors in the formalin test.

Stress-induced antinociception (SIA) is due to the activation of several neural pathways and neurotransmitters that often suppress pain perception. Studies have shown that the orexin neuropeptide system is essential in pain modulation. Therefore, this study aimed to investigate the role of orexinergic receptors in the hippocampal CA1 region in modulating SIA response during the formalin test as an animal model of inflammatory pain. The orexin-1 receptor (OX1r) antagonist, SB334867, at 1, 3, 10, and 30 nmol or TCS OX2 29 as an orexin-2 receptor (OX2r) antagonist at the same doses were microinjected into the CA1 region in rats. Five minutes later, rats were exposed to restraint stress (RS) for 3 h, and pain-related behaviors were monitored in 5-min blocks for the 60-min test period in the formalin test. Results showed that applying RS for 3 h reduced pain responses in the early and late phases of the formalin test. The main findings showed that intra-CA1 injection of orexin receptor antagonists reduced the antinociception caused by stress in both phases of the formalin test. In addition, the contribution of OX2r in mediating the antinociceptive effect of stress was more prominent than that of OX1r in the early phase of the formalin test. However, in the late phase, both receptors worked similarly. Accordingly, the orexin system and its two receptors in the CA1 region of the hippocampus regulate SIA response to this animal model of pain in formalin test.

The integrative role of orexin-1 and orexin-2 receptors within the hippocampal dentate gyrus in the modulation of the stress-induced antinociception in the formalin pain test in the rat.

The stressful experiences, by triggering a cascade of hormonal and neural changes, can produce antinociception commonly referred to as stress-induced antinociception (SIA). Orexin neuropeptides have an essential role in stress responses and pain modulation. The dentate gyrus receives orexinergic projections and has been shown to be involved in pain processing. The current study investigated the possible role of orexin-1 and orexin-2 receptors (OX1r and OX2r, respectively) within the dentate gyrus in SIA in a rat model of formalin-induced pain behavior in one hind paw. Male Wistar rats weighing 230-250 g underwent stereotaxic surgery and a cannula was implanted in their brains, above the dentate gyrus region. Either SB334867 or TCS OX2 29 (OX1r and OX2r antagonists, respectively) was microinjected into the dentate gyrus region at a range of doses at 1, 3, 10, and 30 nmol (control group received DMSO 12% as vehicle), 5 min before the forced swim stress (FSS) exposure. The formalin test was performed to assess pain-related behaviors. The results indicated that FSS exposure relieves pain-related behavior in the early and late phases of the formalin test. Blockade of intra-dentate gyrus OX1 or OX2 receptors reduced the antinociceptive responses induced by FSS in the formalin test, with more impact during the late phase. Our findings support the potential role of intra-dentate gyrus orexin receptors as target sites of orexin neurons in painful and stressful situations. Therefore, understanding the exact mechanisms of SIA and the role of the orexinergic system in this phenomenon can lead to identifying the strategies to guide future research and offer a new approach to discovering new pain therapeutic agents.

Sex effects in the interaction of acute stress and pain perception.

A reciprocity between the stress and the pain system is recognized; however, the manner by which sex affects this reciprocity is unclear. Understanding the interactions of stress, pain, and sex may shed light on the apparent women's vulnerability to chronic pain, which often coexists with increased distress, and to affective disorders, which often coexist with chronic pain. The study's aim was to examine the effect of acute, validated, psychosocial stress on pain perception and modulation of women and men in a controlled manner. Participants were 82 women and 66 men. Heat-pain threshold, heat-pain tolerance, and pain modulation by temporal summation of pain (TSP), and pain adaptation were measured before and after exposure to the Montreal Imaging Stress Task (MIST) or to a sham task. The stress response was verified by perceived ratings of stress and anxiety, autonomic variables, and salivary cortisol. A significant stress response was obtained by the MIST among both sexes; however, women displayed a greater increase in perceived distress, and men displayed a greater increase in cortisol. Among women, TSP decreased and pain adaptation increased following the MIST, responses that were predicted by perceived distress levels. Among men, TSP increased following the MIST but was not predicted by the stress variables. In conclusion, acute stress manipulation seems to differentially affect both stress and pain responses of women and men: women exhibited stress-induced antinociception and men exhibited stress-induced pronociception. Higher perceived stress levels among women may trigger a temporary increase in pain inhibition mechanisms to serve evolutionary purposes.

Temporal dynamics of locus coeruleus modulation of stress-induced antinociception

Pain and stress are largely interdependent. Targeting stress pathways for pain relief is a novel potential therapeutic strategy. However, to take this approach we need to better understand the mechanisms by which stress influences pain. The locus coeruleus (LC) provides the largest noradrenergic (NE) innervation to the central nervous system and is a major intersection of pain and stress neural circuitry. We seek to determine how stress engages the LC-NE system to alter nociception. Using a cell type-selective inhibitory chemogenetic targeting strategy, we show that inhibition of the LC-NE system reverses acute restraint stress-induced antinociception on a hot plate. However, this chemogenetic strategy lacks the temporal specificity necessary to distinguish whether the LC is required for discrete aspects of stress-induced antinociception, namely the experience of the primary stressor versus noxious stimulus detection and processing. To gain temporal precision for these purposes, we use inhibitory optogenetics to restrict cellular inhibition during either the primary stressor or noxious stimulus only. Our results show that LC activity during the experience of the noxious stimulus is not required to elicit stress-induced antinociception. However, somewhat surprisingly, LC activity is required to maintain normal nociceptive processing in the absence of stress. Given this information, we then sought to characterize endogenous LC activity during our stress-induced antinociception behavioral paradigm using in vivo fiber photometry of global LC calcium activity. In acute restraint stress, we demonstrate that LC calcium activity increases during bouts of struggle against the restraint. Additionally, LC calcium activity is increased during discrete nocifensive responses throughout noxious stimulus detection and processing. Together, our combined chemogenetic, optogenetic, and photometry results suggest the LC tone exerts some level of basal antinociception and that the LC may initiate the process of stress-induced antinociception during the primary stressor rather than during the experience of noxious sensation. Grant support from R01 NS117899-01 F31 NS124301-01.

Cannabinoid system of dorsomedial telencephalon modulates behavioral responses to noxious stimulation in the fish Leporinus macrocephalus

Fish dorsomedial telencephalon has been considered a pallial region homologous to mammals amygdala, being considered a possible substrate for nociception modulation in this animal group. The present study aimed to evaluate the participation of the cannabinoid system of Dm telencephalon on nociception modulation in the fish Leporinus macrocephalus. We demonstrated that cannabidiol microinjection in Dm telecephalon inhibits the behavioral nociceptive response to the subcutaneous injection of 3% formaldehyde, and this antinociception is blocked by previous treatment with AM251 microinjection. Furthermore, AM251 microinjection in Dm prior to restraint stress also blockades the stress-induced antinociception. These results reinforce the hypothesis that this pallial telencephalic structure has a pivotal role in nociception modulation in fish.

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

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

GABAA-benzodiazepine receptors in the dorsomedial (Dm) telencephalon modulate restraint-induced antinociception in the fish Leporinus macrocephalus

The possibility that fish experience pain has been denied based on the absence of the neural substrates to support this “experience”. In this context, the identification of brain regions involved in nociception modulation could provide important insights regarding the processing of nociceptive information in fish. Our study evaluated the participation of the GABAA-benzodiazepine receptor in the dorsomedial (Dm) telencephalon in restraint-induced antinociception in the fish Leporinus macrocephalus through the microinjection of the anxiolytic drug midazolam. The microinjection of midazolam in the Dm did not alter the nocifensive response; however, this drug did block the inhibition of the nocifensive response to formaldehyde promoted by restraint stress. The fish that received midazolam (40nmol) microinjection prior to restraint (3 or 5min), followed by subcutaneous injection with formaldehyde presented a higher distance traveled than the fish that received saline microinjection. This effect might reflect the specific action of midazolam on benzodiazepine receptors in the Dm telencephalon, as pre-treatment with flumazenil, a benzodiazepine receptor antagonist, inhibited the effects of this drug. In the present study, we present the first evidence demonstrating a role for the dorsomedial telencephalic region in the modulation of stress-induced antinociception in fish, revealing new perspectives in the understanding of nociceptive information processing in this group.

Acute administration of a cannabinoid CB1 receptor antagonist impairs stress-induced antinociception in fish

This study evaluated the influence of the pre-treatment with AM251 (a cannabinoid type I receptor (CB1) selective antagonist) on the stress-induced antinociception promoted by restraint in the fish Leporinus macrocephalus. The application of 3 and 5min of restraint stress promoted an inhibition of the behavioural response to the subcutaneous injection of 3% formaldehyde (increase in locomotor activity), suggesting the activation of an antinociceptive system. The acute intraperitoneal administration of AM251 (3mg·kg−1) impaired this antinociceptive response induced by 3 and 5min of restraint stress. The fish treated with AM251 before the application of restraint stress presented an increase in locomotor activity after the subcutaneous injection of formaldehyde, similar to fish not exposed to restraint, suggesting that the stress-induced antinociception promoted by restraint in fish is probably mediated by cannabinoid CB1 receptors. The results presented in this paper suggest the participation of the endocannabinoid system in nociception modulation in fish, supporting the hypothesis that an endogenous antinociceptive system activated by restraint stress is present in fish and that the modulation of antinociception by the CB1 receptor is evolutionary well-conserved across vertebrates.

The roles of neurotensin and its analogues in pain.

Neurotensin (NT) is an endogenous 13 amino acid neuropeptide with profound opioid-independent analgesic effects. This role of NT is thought to be mediated by both neurotensin receptor subtype 1 (NTS1) and neurotensin receptor subtype 2 (NTS2). NT and its receptors are widely distributed in the pain circuits in central nervous system. Thus NT might modulate pain in many structures of pain pathway, such as spinal cord, rostroventral medulla (RVM) and periaqueductal gray (PAG). Actually either intrathecal application of NT or direct injection of NT into RVM or PAG or intracerebroventricular injection of NT showed analgesic effects. NT exerted its antinociceptive effects in both acute pain and chronic pain models. The analgesic effects of NT were originally found in acute pain experiments. In the case of pathological pain, for example, formalin injection induced inflammatory pain and sciatic nerve constriction induced neuropathic pain, NT also shows antinociceptive effects. The effects exist in somatic pain as well as visceral pain induced by noxious colorectal distension (CRD) or writhing test. It should be noted that NT plays an important role in stress-induced antinociception (SIAN), especially in higher intensity stress experiments. However as a neuropeptide, NT is susceptible to degradation by peptidases and cannot cross the blood-brain barrier (BBB). Great efforts have been made to find NT analogues that are more biologically stable and could inhibit pain by systematic administration. The present review focuses on the analgesic role and the underlying mechanisms of NT and its analogues in pain, especially in chronic pain models.

P.1.h.014 The involvement of CCK receptors in stress-induced antinociception in an animal model of neuropathic pain

P.1.h.014 The involvement of CCK receptors in stress-induced antinociception in an animal model of neuropathic pain

Acute stress-induced antinociception is cGMP-dependent but heme oxygenase-independent

Endogenous carbon monoxide (CO), which is produced by the enzyme heme oxygenase (HO), participates as a neuromodulator in physiological processes such as thermoregulation and nociception by stimulating the formation of 3′,5′-cyclic guanosine monophosphate (cGMP). In particular, the acute physical restraint-induced fever of rats can be blocked by inhibiting the enzyme HO. A previous study reported that the HO-CO-cGMP pathway plays a key phasic antinociceptive role in modulating noninflammatory acute pain. Thus, this study evaluated the involvement of the HO-CO-cGMP pathway in antinociception induced by acute stress in male Wistar rats (250-300 g; n=8/group) using the analgesia index (AI) in the tail flick test. The results showed that antinociception induced by acute stress was not dependent on the HO-CO-cGMP pathway, as neither treatment with the HO inhibitor ZnDBPG nor heme-lysinate altered the AI. However, antinociception was dependent on cGMP activity because pretreatment with the guanylate cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxaline-1-one (ODQ) blocked the increase in the AI induced by acute stress.

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

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

Stress-induced antinociception in fish reversed by naloxone.

Pain perception in non-mammalian vertebrates such as fish is a controversial issue. We demonstrate that, in the fish Leporinus macrocephalus, an imposed restraint can modulate the behavioral response to a noxious stimulus, specifically the subcutaneous injection of 3% formaldehyde. In the first experiment, formaldehyde was applied immediately after 3 or 5 min of the restraint. Inhibition of the increase in locomotor activity in response to formaldehyde was observed, which suggests a possible restraint-induced antinociception. In the second experiment, the noxious stimulus was applied 0, 5, 10 and 15 min after the restraint, and both 3 and 5 min of restraint promoted short-term antinociception of approximately 5 min. In experiments 3 and 4, an intraperitoneal injection of naloxone (30 mg.kg−1) was administered 30 min prior to the restraint. The 3- minute restraint-induced antinociception was blocked by pretreatment with naloxone, but the corresponding 5-minute response was not. One possible explanation for this result is that an opioid and a non-preferential μ–opioid and/or non-opioid mechanism participate in this response modulation. Furthermore, we observed that both the 3- and 5- minutes restraint were severely stressful events for the organism, promoting marked increases in serum cortisol levels. These data indicate that the response to a noxious stimulus can be modulated by an environmental stressor in fish, as is the case in mammals. To our knowledge, this study is the first evidence for the existence of an endogenous antinociceptive system that is activated by an acute standardized stress in fish. Additionally, it characterizes the antinociceptive response induced by stress in terms of its time course and the opioid mediation, providing information for understanding the evolution of nociception modulation.

Activation of type 5 metabotropic glutamate receptors and diacylglycerol lipase-α initiates 2-arachidonoylglycerol formation and endocannabinoid-mediated analgesia.

Acute stress reduces pain sensitivity by engaging an endocannabinoid signaling circuit in the midbrain. The neural mechanisms governing this process and molecular identity of the endocannabinoid substance(s) involved are unknown. We combined behavior, pharmacology, immunohistochemistry, RNA interference, quantitative RT-PCR, enzyme assays, and lipidomic analyses of endocannabinoid content to uncover the role of the endocannabinoid 2-arachidonoyl-sn-glycerol (2-AG) in controlling pain sensitivity in vivo. Here, we show that footshock stress produces antinociception in rats by activating type 5 metabotropic glutamate receptors (mGlu(5)) in the dorsolateral periaqueductal gray (dlPAG) and mobilizing 2-AG. Stimulation of mGlu(5) in the dlPAG with DHPG [(S)-3,5-dihydroxyphenylglycine] triggered 2-AG formation and enhanced stress-dependent antinociception through a mechanism dependent upon both postsynaptic diacylglycerol lipase (DGL) activity, which releases 2-AG, and presynaptic CB(1) cannabinoid receptors. Pharmacological blockade of DGL activity in the dlPAG with RHC80267 [1,6-bis(cyclohexyloximinocarbonylamino)hexane] and (-)-tetrahydrolipstatin (THL), which inhibit activity of DGL-α and DGL-β isoforms, suppressed stress-induced antinociception. Inhibition of DGL activity in the dlPAG with THL selectively decreased accumulation of 2-AG without altering levels of anandamide. The putative 2-AG-synthesizing enzyme DGL-α colocalized with mGlu(5) at postsynaptic sites of the dlPAG, whereas CB(1) was confined to presynaptic terminals, consistent with a role for 2-AG as a retrograde signaling messenger. Finally, virally mediated silencing of DGL-α, but not DGL-β, transcription in the dlPAG mimicked effects of DGL inhibition in suppressing both endocannabinoid-mediated stress antinociception and 2-AG formation. The results indicate that activation of the postsynaptic mGlu(5)-DGL-α cascade triggers retrograde 2-AG signaling in vivo. This pathway is required for endocannabinoid-mediated stress-induced analgesia.

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.

Increased Nociceptive Sensitivity and Nociceptin/Orphanin FQ Levels in a Rat Model of PTSD

BackgroundClinical studies indicate that post-traumatic stress disorder (PTSD) frequently shares co-morbidity with chronic pain. Although in animals acute stress-induced antinociception is well documented, the effect of PTSD-like stress on nociceptive sensitivity is unclear. Though a few studies measured nociceptive responses at a single time point, no studies have examined changes in nociceptive sensitivity over time following exposure to PTSD-like stress. Nociceptin/orphanin FQ (N/OFQ), an endogenous ligand for the N/OFQ peptide (NOP) receptor, modulates various biological functions in the central nervous system that are affected by PTSD, including nociceptive sensitivity, stress and anxiety, learning and memory.ResultsThe present study examined thermal and mechanical nociceptive sensitivity in male Sprague Dawley rats between 7 and 28 days after single-prolonged stress (SPS), an established animal model for PTSD. Rat paw withdrawal thresholds (PWT) to von Frey and paw withdrawal latencies (PWL) to radiant heat stimuli, respectively, dramatically decreased as early as 7 days after initiation of SPS and lasted the length of the study, 28 days. In addition, N/OFQ levels increased in cerebrospinal fluid (CSF; on days 9, 14 and 28) and serum (day 28), while levels of circulating corticosterone (CORT) decreased 28 days after initiation of SPS. SPS exposure induced anxiety-like behavior and enhanced inhibition of the hypothalamo-pituitary-adrenal (HPA) axis, as previously reported for this model.ConclusionsOur results demonstrate that SPS induces the development of persistent mechanical allodynia and thermal hyperalgesia that is accompanied by increased N/OFQ content in the CSF, and eventually, in serum. These findings suggest a link between N/OFQ and the development of hyperalgesia and allodynia in a rat model of PTSD.

Environmentally induced antinociception and hyperalgesia in rats and mice

Stress can enhance and inhibit nociception depending on the situation. Thus, simply shifting the context from the elevated plus maze (EPM) which has been shown to produce stress-induced antinociception to a different environment could produce drastic and rapid changes in nociception. The present experiment tested this hypothesis by assessing nociception in rats and mice during and immediately after removal from the maze. Experiment 1 found hyperalgesia in female and male rats tested on the hot plate immediately after exposure to the elevated plus maze. This hyperalgesia occurred with or without the added stress of a hind paw formalin injection and regardless of whether rats were exposed to an EPM with open (oEPM) or enclosed (eEPM) arms despite a clear antinociceptive effect while on the oEPM. Experiment 2 showed a similar shift from antinociception to nociception on the formalin test in mice immediately after removing them from the EPM. These data demonstrate that a mild stressor such as the EPM can produce both antinociception and hyperalgesia depending on the context. This shift from antinociception to hyperalgesia occurs rapidly and is evident in mice, male and female rats, and with the hot plate and formalin tests.

α-Adrenoceptor Involvement in Swim Stress-induced Antinociception in the Mouse

Three different intensities of swim stress produced stress-induced antinociception (SIA) in mice which was assessed either by the reduction in the number of abdominal constrictions produced by acetic acid or by an increase in reaction time on a hot-plate. The involvement of alpha-adrenoceptors in the three models of SIA was investigated using selective antagonists. SIA produced by the mild stress of a 30 s warm water swim was attenuated by idazoxan (0.5-1 mg kg-1), and by yohimbine at a dose (1 mg kg-1) which reduced antinociception produced by clonidine (12.5-50 micrograms kg-1). Indoramin (1-2 mg kg-1) did not affect this model of SIA, but reversed phenylephrine induced inhibition of the constrictions. A 3 min room temperature swim increased reaction times on the hot-plate and this naloxone-sensitive SIA was reduced significantly by prazosin (1-2 mg kg-1), idazoxan (0.5-1 mg kg-1) and yohimbine (0.5-1 mg kg-1) but enhanced by clonidine (0.5 mg kg-1) and noradrenaline (NA) (10 micrograms i.c.v.). Mice treated with 6-hydroxydopamine (60 + 60 micrograms i.c.v.) were hypersensitive to the hot-plate and did not develop SIA. Levels of noradrenaline in the brain (minus the cerebellum) were decreased after the room temperature swim SIA. The most severe stress of a cold water swim produced SIA on the hot-plate which was initially naloxone-insensitive.(ABSTRACT TRUNCATED AT 250 WORDS)

Deficiency of Antinociception and Excessive Grooming Induced by Acute Immobilization Stress in Per1 Mutant Mice

Acute stressors induce changes in numerous behavioral parameters through activation of the hypothalamic-pituitary-adrenal (HPA) axis. Several important hormones in paraventricular nucleus of the hypothalamus (PVN) play the roles in these stress-induced reactions. Corticotropin-releasing hormone (CRH), arginine-vasopressin (AVP) and corticosterone are considered as molecular markers for stress-induced grooming behavior. Oxytocin in PVN is an essential modulator for stress-induced antinociception. The clock gene, Per1, has been identified as an effecter response to the acute stresses, but its function in neuroendocrine stress systems remains unclear. In the present study we observed the alterations in grooming and nociceptive behaviors induced by acute immobilization stress in Per1 mutant mice and other genotypes (wild types and Per2 mutant). The results displayed that stress elicited a more robust effect on grooming behavior in Per1 mutant mice than in other genotypes. Subsequently, the obvious stress-induced antinociception was observed in the wild-type and Per2 mutant mice, however, in Per1 mutant, this antinociceptive effects were partially-reversed (mechanical sensitivity), or over-reversed to hyperalgesia (thermal sensitivity). The real-time qPCR results showed that in PVN, there were stress-induced up-regulations of Crh, Avp and c-fos in all of genotypes; moreover, the expression change of Crh in Per1 mutant mice was much larger than in others. Another hormonal gene, Oxt, was up-regulated induced by stress in wild-type and Per2 mutant but not in Per1 mutant. In addition, the stress significantly elevated the serum corticosterone levels without genotype-dependent differences, and accordingly the glucocorticoid receptor gene, Nr3c1, expressed with a similar pattern in PVN of all strains. Taken together, the present study indicated that in acute stress treated Per1 mutant mice, there are abnormal hormonal responses in PVN, correlating with the aberrant performance of stress-induced behaviors. Therefore, our findings suggest a novel functional role of Per1 in neuroendocrine stress system, which further participates in analgesic regulation.

The effects of chronic losartan pretreatment on restraint stress-induced changes in motor activity, nociception and pentylenetetrazol generalized seizures in rats

Accumulated evidence has shown that renin-angiotensin system has a pivotal role in stress responses. to assess the participation of AT1 receptor in stress-induced modulation of motor activity, nociception and seizure susceptibility in male Wistar rats. AT1 receptor antagonist losartan was administered subcutaneously to rats for 10 days at a dose of 10 mg/kg either alone or as a pretreatment before chronic restraint stress applied for 10 days. Locomotor and exploratory activity (open field test), the nociception (paw-pressure test) and the seizure susceptibility (pentylenetetrazol seizure test) were analysed. Chronic restraint stress decreased motor activity and increased anxiety-like behaviour (grooming) while losartan pretreatment alleviated anxiety-like behaviour. Chronic restraint stress had an antinociceptive effect in paw-pressure test and losartan pretreatment abolished stress-induced antinociception. Both chronic restraint stress and losartan showed anticonvulsant activity in pentylenetetrazol seizure test. However, drug pretreatment attenuated this effect in chronically-stressed rats. Our findings suggest that the AT1 receptor is involved in the mechanism of stress-induced changes in anxiety-like behaviour, nociception and seizure susceptibility in rats.