Increased formalin-induced pain and expression of fos neurons in the lumbar spinal cord of prenatally stressed infant rats

Increased nociceptive behaviors and spinal c-Fos expression in the formalin test in a rat repeated cold stress model

Repeated cold stress (RCS) can trigger the development of fibromyalgia (FM)-like symptoms, including persistent deep-tissue pain, although nociceptive changes to the skin have not been fully characterized. Using a rat RCS model, we investigated nociceptive behaviors induced by noxious mechanical, thermal, and chemical stimuli applied to plantar skin. Neuronal activation in the spinal dorsal horn was examined using the formalin pain test. In rats exposed to RCS, nociceptive behavioral hypersensitivity was observed in all modalities of cutaneous noxious stimuli: the mechanical withdrawal threshold was decreased, and the heat withdrawal latency was shortened one day after the cessation of stress. The duration of nocifensive behaviors in the formalin test was prolonged in phase II but not in phase I. The number of c-Fos-positive neurons increased in the entire dorsal horn laminae I-VI, ipsilateral, but not contralateral, to formalin injection at the L3-L5 segments. The duration of nocifensive behavior in phase II was significantly and positively correlated with the number of c-Fos-positive neurons in laminae I-II. These results demonstrate that cutaneous nociception is facilitated in rats exposed to RCS for a short time and that the spinal dorsal horn neurons are hyperactivated by cutaneous formalin in the RCS model.

Local injury and systemic infection in infants alter later nociception and pain affect during early life and adulthood

Newborns in intensive care are regularly exposed to minor painful procedures at developmental time points when noxious stimulation would be normally absent. Pain from these interventions is inconsistently treated and often exists concurrently with systemic infection, a common comorbidity of prematurity. Our understanding of the independent and combined effects of early painful experiences and infection on pain response is incomplete. The main goals of this research therefore were to understand how pain and infection experienced early in life influence future nociceptive and affective responses to painful stimuli. Rat pups were infected with E-coli on postnatal day 2 (PN2) and had left hind paw injury with carrageenan on PN3. Standard thermal tests for acute pain, formalin tests for inflammatory pain, and conditioned place aversion testing were performed at different ages to assess the nociceptive and affective components of the pain response. Early E-coli infection and early inflammatory injury with carrageenan both independently increased pain scores following hind paw reinjury with formalin on PN8, with effects persisting into adulthood in the carrageenan exposed group. When experienced concurrently, early E-coli infection and carrageenan exposure also increased conditioned aversion to pain in adults. Effect of sex was significant only in formalin testing, with males showing higher pain scores in infancy and females showing higher pain scores as adults. These findings demonstrate that infection experienced early in life can alter both the nociceptive and affective components of the pain response and that there is a cumulative effect of local and systemic pro-inflammatory processes on the aversive component of pain.

Orexinergic descending inhibitory pathway mediates linalool odor-induced analgesia in mice

Linalool odor exposure induces an analgesic effect in mice. This effect disappeared in the anosmic model mice, indicating that olfactory input evoked by linalool odor triggered this effect. Furthermore, hypothalamic orexinergic neurons play a pivotal role in this effect. However, the neuronal circuit mechanisms underlying this effect have not been fully addressed. In this study, we focused on the descending orexinergic projection to the spinal cord and examined whether this pathway contributes to the effect. We assessed the effect of intrathecal administration of orexin receptor antagonists on linalool odor-induced analgesia in the tail capsaicin test. We found that the selective orexin type 1 receptor antagonist, but not the selective orexin type 2 receptor antagonist, prevented the odor-induced analgesic effect. Furthermore, immunohistochemical analyses of c-Fos expression induced by the capsaicin test revealed that neuronal activity of spinal cord neurons was suppressed by linalool odor exposure, which was prevented by intrathecal administration of the orexin 1 receptor antagonist. These results indicate that linalool odor exposure drives the orexinergic descending pathway and suppresses nociceptive information flow at the spinal level.

Low maternal licking/grooming stimulation increases pain sensitivity in male mouse offspring

Deprivation of maternal care has been associated with higher pain sensitivity in offspring. In the present study, we hypothesized that the maternal licking/grooming behavior was an important factor for the development of the pain regulatory system. To test this hypothesis, we used male F2 offspring of early-weaned (EW) F1 mother mice that exhibit lower frequency of licking/grooming behavior. The formalin test revealed that F2 offspring of EW F1 dams showed significantly higher pain behavior than F2 offspring of normally-weaned (NW) F1 dams. We found that the mRNA levels of transient receptor potential vanilloid 1 (TRPV1), a nociceptor, were higher in the lumbosacral dorsal root ganglion (DRG) of F2 offspring of EW F1 dams than those of F2 offspring of NW F1 dams, suggesting that the higher pain sensitivity may be attributed to low licking/grooming, which may result in developmental changes in nociceptive neurons. In the DRG, mRNA levels of Mas-related G-protein coupled receptor B4 (MrgprB4), a marker of sensory neurons that detect gentle stroking, was also up-regulated in the F2 offspring of EW F1 dams. Considering that gentle touch alleviates pain, Mrgprb4 up-regulation may reflect a compensatory change. The present findings indicate important implications of maternal licking/grooming behavior in the development of the pain regulatory system.

Effects of plant-derived analgesic compounds sinomenine and salvinorin A in infant rats

OBJECTIVE

Premature and ill neonates undergo painful but medically necessary procedures while hospitalized. Although opiate drugs are administered as analgesics, problems associated with their side effects, tolerance, and potential dependence necessitate research into alternative pain-relieving medications. Here we test two plant-derived compounds in infant rats: sinomenine, which targets the Mas-related G-protein-coupled receptor member X2 opioid receptor; and salvinorin A, which is a κ opioid receptor agonist. In adult animals both sinomenine and salvinorin A are analgesic, but neither has been tested in infants.

METHODS

We used the formalin and thermal plantar tests in rats 7 and 21 days of age (PN7 and PN21) for behavioral signs of pain. In addition, brain sections were stained using Fos immunohistochemistry to examine patterns of brain activation in the midbrain periaqueductal gray and the paraventricular nucleus of the hypothalamus.

RESULTS

Sinomenine was analgesic in both the formalin and thermal tests on animals 21 days of age. At PN7 only the highest dose elevated response latencies in the thermal test and there were no effects of sinomenine in the formalin test. Analysis of Fos expression in the sinomenine-treated animals showed no drug effect, in contrast to the behavioral results. Salvinorin A was analgesic in the formalin test only at the highest dose at 21 days of age but not in the thermal test at either age.

CONCLUSION

The increased modest effectiveness of sinomenine in older animals and the minimum salvinorin A drug effect suggest that the compounds act on sites that develop during the preweaning period (sinomenine) or after weaning (salvinorin A).

Effects of Tualang honey in modulating nociceptive responses at the spinal cord in offspring of prenatally stressed rats

OBJECTIVE

This study was done to determine whether Tualang honey could prevent the altered nociceptive behaviour, with its associated changes of oxidative stress markers and morphology of the spinal cord, among the offspring of prenatally stressed rats.

METHODS

Pregnant rats were divided into three groups: control, stress, and stress treated with Tualang honey. The stress and stress treated with Tualang honey groups were subjected to restraint stress from day 11 of pregnancy until delivery. Ten week old male offspring (n = 9 from each group) were given formalin injection and their nociceptive behaviours were recorded. After 2 h, the rats were sacrificed, and their spinal cords were removed to assess oxidative stress activity and morphology. Nociceptive behaviour was analysed using repeated measures analysis of variance (ANOVA), while the levels of oxidative stress parameters and number of Nissl-stained neurons were analysed using a one-way ANOVA.

RESULTS

This study demonstrated that prenatal stress was associated with increased nociceptive behaviour, changes in the oxidative stress parameters and morphology of the spinal cord of offspring exposed to prenatal stress; administration of Tualang honey reduced the alteration of these parameters.

CONCLUSION

This study provides a preliminary understanding of the beneficial effects of Tualang honey against the changes in oxidative stress and neuronal damage in the spinal cord of the offspring of prenatally stressed rats.

Analgesic Effects of Duloxetine on Formalin-Induced Hyperalgesia and Its Underlying Mechanisms in the CeA

In rodents, the amygdala has been proposed to serve as a key center for the nociceptive perception. Previous studies have shown that extracellular signal-regulated kinase (ERK) signaling cascade in the central nucleus of amygdala (CeA) played a functional role in inflammation-induced peripheral hypersensitivity. Duloxetine (DUL), a serotonin and noradrenaline reuptake inhibitor, produced analgesia on formalin-induced spontaneous pain behaviors. However, it is still unclear whether single DUL pretreatment influences formalin-induced hypersensitivity and what is the underlying mechanism. In the current study, we revealed that systemic pretreatment with DUL not only dose-dependently suppressed the spontaneous pain behaviors, but also relieved mechanical and thermal hypersensitivity induced by formalin hindpaw injection. Consistent with the analgesic effects of DUL on the pain behaviors, the expressions of Fos and pERK that were used to check the neuronal activities in the spinal cord and CeA were also dose-dependently reduced following DUL pretreatment. Meanwhile, no emotional aversive behaviors were observed at 24 h after formalin injection. The concentration of 5-HT in the CeA was correlated with the dose of DUL in a positive manner at 24 h after formalin injection. Direct injecting 5-HT into the CeA suppressed both the spontaneous pain behaviors and hyperalgesia induced by formalin injection. However, DUL did not have protective effects on the formalin-induced edema of hindpaw. In sum, the activation of CeA neurons may account for the transition from acute pain to long-term hyperalgesia after formalin injection. DUL may produce potent analgesic effects on the hyperalgesia and decrease the expressions of p-ERK through increasing the concentration of serotonin in the CeA.

Dissociation of morphine analgesic effects in the sensory and affective components of formalin-induced spontaneous pain in male and female rats

Sex differences in the analgesic effects of morphine have been previously reported in various models that represent the sensory component of pain. However, pain sensation is a complex process that consists of both sensory and affective components. It is presently unclear whether the analgesic effects of morphine between the sensory and affective components of pain are sexually dimorphic. Moreover, differences in morphine dose-response in the two components of pain have not been examined in male and female rats. Therefore, we examined the analgesic effects of morphine on the sensory and affective components of formalin-induced pain behaviors in male and female rats. To discern the sensory component, rats were pretreated with varying doses of morphine and then intraplantar formalin-induced paw flinches were measured. Morphine reduced the number of formalin-induced paw flinches at a treatment dose of 4.0mg/kg. Morphine analgesia was similar across the sexes in the early (phase 1) and late phase (phase 2) of the formalin test. To examine the affective component, rats were pretreated with varying doses of morphine, and then intraplantar formalin-induced conditioned place aversion (CPA) was examined. Formalin produced CPA, which was blocked by morphine at doses of 1.0mg/kg and higher in male and female rats. Lastly, formalin-induced cFos expression and the effects of systemic morphine were examined in the superficial dorsal horn of the spinal cord. Intraplantar formalin produced robust expression of cFos; however, morphine did not attenuate the cFos expression. These results demonstrate a notable dissociation of the analgesic effects of morphine by detecting a fourfold shift in the minimum effective dose between the sensory and affective components of formalin-induced spontaneous pain, that were similar between male and female rats. The findings further suggest disparate mechanisms involved in systemic morphine-induced analgesia in the two components of formalin-induced pain.

Allopregnanolone in the brain: protecting pregnancy and birth outcomes

A successful pregnancy requires multiple adaptations in the mother's brain that serve to optimise foetal growth and development, protect the foetus from adverse prenatal programming and prevent premature delivery of the young. Pregnancy hormones induce, organise and maintain many of these adaptations. Steroid hormones play a critical role and of particular importance is the progesterone metabolite and neurosteroid, allopregnanolone. Allopregnanolone is produced in increasing amounts during pregnancy both in the periphery and in the maternal and foetal brain. This review critically examines a role for allopregnanolone in both the maternal and foetal brain during pregnancy and development in protecting pregnancy and birth outcomes, with particular emphasis on its role in relation to stress exposure at this time. Late pregnancy is associated with suppressed stress responses. Thus, we begin by considering what is known about the central mechanisms in the maternal brain, induced by allopregnanolone, that protect the foetus(es) from exposure to harmful levels of maternal glucocorticoids as a result of stress during pregnancy. Next we discuss the central mechanisms that prevent premature secretion of oxytocin and consider a role for allopregnanolone in minimising the risk of preterm birth. Allopregnanolone also plays a key role in the foetal brain, where it promotes development and is neuroprotective. Hence we review the evidence about disruption to neurosteroid production in pregnancy, through prenatal stress or other insults, and the immediate and long-term adverse consequences for the offspring. Finally we address whether progesterone or allopregnanolone treatment can rescue some of these deficits in the offspring.

Neurobiology of stress-induced hyperalgesia

The intensity and severity of perceived pain does not correlate consistently with the degree of peripheral or central nervous system tissue damage or with the intensity of primary afferent or spinal nociceptive neurone activity. In this respect, the modulation of pain by emotion and context is now widely recognized. In particular, stress, fear and anxiety exert potent, but complex, modulatory influences on pain. Stress can either suppress pain (stress-induced analgesia) or exacerbate it (stress-induced hyperalgesia; SIH) depending on the nature, duration and intensity of the stressor. Herein, we review the methods and models used to study the phenomenon of SIH in rodents and humans and then present a detailed discussion of our current understanding of neural substrates and neurobiological mechanisms. The review provides perspectives and challenges for the current and future treatment of pain and the co-morbidity of pain with stress-related psychiatric disorders including anxiety and depression.

Phospholipase D-mediated hypersensitivity at central synapses is associated with abnormal behaviours and pain sensitivity in rats exposed to prenatal stress

Adverse events at critical stages of development can lead to lasting dysfunction in the central nervous system (CNS). To seek potential underlying changes in synaptic function, we used a newly developed protocol to measure alterations in receptor-mediated Ca(2+) fluorescence responses of synaptoneurosomes, freshly isolated from selected regions of the CNS concerned with emotionality and pain processing. We compared adult male controls and offspring of rats exposed to social stress in late pregnancy (prenatal stress, PS), which showed programmed behavioural changes indicating anxiety, anhedonia and pain hypersensitivity. We found corresponding increases, in PS rats compared with normal controls, in responsiveness of synaptoneurosomes from frontal cortex to a glutamate receptor (GluR) agonist, and from spinal cord to activators of nociceptive afferents. Through a combined pharmacological and biochemical strategy, we found evidence for a role of phospholipase D1 (PLD1)-mediated signalling, that may involve 5-HT2A receptor (5-HT2AR) activation, at both levels of the nervous system. These changes might participate in underpinning the enduring alterations in behaviour induced by PS.

Maternal prenatal stress in rats influences c-fos expression in the spinal cord of the offspring

Previous studies in humans have reported a link between maternal stress and disturbed infant physiological behavior. The objective of our study was to examine in experimental rats how maternal prenatal stress induced by a forced swim test affects offspring afferent spinal responses mediated by stimulation of vaginocervical receptors. The activation of spinal cord neurons showing c-fos expression was examined following vaginocervical mechanical stimulation in adult rats, which were the offspring of dams exposed to gestational stress from E10 until delivery. Vaginocervical stimulation of both prenatal-stressed and non-prenatal-stressed rats induced an increase in immunoreactive protein in the spinal cord ranging from T12 to S1 segmental levels. However, a significantly higher (40%) increase in the expression of Fos-immunoreactive neurons was observed in vaginocervical stimulated prenatally stressed rats than in non-stimulated prenatally stressed ones. This increase was higher in L5-S1 levels than in T12-L4. When the regional distribution was examined, results showed that up to 80% of activated neurons were located in the dorsal horn in both non-stimulated prenatally stressed and stimulated prenatally stressed groups, with a significantly higher density in the latter. Our results demonstrate that maternal prenatal stress can have consequences on vaginocervical responses conveyed to the spinal cord. The increase in Fos labeled neurons in T12-S1 in prenatally stressed rats induced by vaginocervical stimulation suggests the hypersensitivity of the genital tract associated with activation of spinal circuits spanning multiple segments.

Heterogeneity of the infant stage of rat development: inflammatory pain response, depression-related behavior, and effects of prenatal stress

The infant stage of rat development is a very important period for potential correction of adverse consequences produced by negative prenatal events. However the age limit for this correction needs to be investigated. The last prenatal and first two weeks after birth are "critical" for maturation of the nociceptive and emotional systems. Clinical observations suggest a correlation between persistent pain response and emotional behavior. In infant male rats of different ages, we studied indices of the inflammatory pain response (the number of flexes+shakes in the formalin test), depression-related behavior (immobility in the forced swim test) and the relations between them, as well as the effects of prenatal stress on these indices. Furthermore, we assessed the trend of body weight and the relations between body weight and the depression- and pain-related behaviors. We demonstrate heterogeneity of the infant stage: control prenatally non-stressed rat pups showed significantly lower immobility at 7 days of age than at 10 days; prenatal stress caused an increase of immobility and the number of flexes+shakes in 7-8-day-old pups but not in 10-11-day-olds. These findings should be taken into account in the treatment of abnormalities of emotional and inflammatory pain-related behaviors produced by prenatal stressful events. The present data and our previous findings indicate that the deficiency of body weight in prenatally stressed newborns may predict the development of abnormalities in inflammatory pain-related responses during postnatal ontogeny.

Lactobacillus farciminis treatment attenuates stress-induced overexpression of Fos protein in spinal and supraspinal sites after colorectal distension in rats

Abstract Irritable bowel syndrome (IBS), frequently associated with psychological distress, is characterized by hypersensitivity to gut wall distension. Some probiotics are able to alleviate IBS symptoms and reduce visceromotor response to mechanical stimuli in animals. Moreover, we have previously shown that Lactobacillus farciminis treatment abolished the hyperalgesia to colorectal distension (CRD) induced by acute stress. The aims of the present study were to determine whether (i) stress-induced visceral hyperalgesia modifies the expression of Fos, a marker of general neuronal activation, induced by CRD, (ii) this activation can be modulated by L. farciminis treatment. Female rats were treated by L. farciminis and CRD was performed after partial restraint stress (PRS) or sham-PRS. The expression of Fos protein was measured by immunohistochemistry. After CRD or PRS, Fos expression was increased in spinal cord section (S1), nucleus tractus solitarius (NTS), paraventricular nucleus (PVN) of the hypothalamus, and in the medial nucleus of the amygdala (MeA). The combination of both stimuli, PRS and CRD, markedly increased this Fos overexpression in the sacral spinal cord section, PVN and MeA, but not in NTS. By contrast, a pretreatment with L. farciminis significantly reduced the number of Fos positive cells in these area. This study shows that PRS enhances Fos protein expression induced by CRD at the spinal and supraspinal levels in rats. Lactobacillus farciminis treatment inhibited this enhancing effect, suggesting that the antinociceptive effect of this probiotic strain results from a decrease of the stress-induced activation/sensitization of sensory neurons at the spinal and supraspinal level.

Sex differences in formalin-induced pain in prenatally stressed infant rats

The aim of this work was to study the effects of prenatal stress on nociceptive responses in the formalin test in female and male infant (7-day-old) Long-Evans hooded rats. Prenatally stressed infant rats displayed biphasic flinching+ shaking behavior whereas non-stressed animals showed only a weak second phase. Pain sensitivity in prenatally stressed males was significantly greater than that of prenatally non-stressed males during the second phase only; there were no differences in pain sensitivity between prenatally stressed and non-stressed females. Moreover prenatally stressed male rats pups demonstrated that the second phase of the response to formalin was enhanced relative to the second phase in stressed females. The current and previous data [Butkevich IP, Barr GA, Mikhailenko VA, Otellin VA. Increased formalin-induced pain and expression of fos neurons in the lumbar spinal cord of prenatally stressed infants rats. Neurosci Lett 2006a;403:222-226] show increased tonic pain in prenatally stressed infant rats and a large increase in the number of formalin-induced fos-like immunoreactivity in the spinal cord dorsal horn. There is a concomitant decrease in serotonin-like immunoreactivity in the lumbar spinal cord dorsal horn [Butkevich IP, Barr GA, Otellin VA. Effect of prenatal stress on behavioral and neural indices of formalin-induced pain in infant rats. Abstracts, 35th Annual Meeting of Soc. For Neurosci. 2005a. Program No. 512.4 Washington, DC: Society for Neuroscience]. Given the decreased level of perinatal testosterone in prenatally stressed rats to which infant males are more sensitive than females, we suggest that these hormonal, behavioral and neuronal indices are strongly interrelated in prenatally stressed 7-day-old rat pups and that the decreased surge of testosterone may contribute to the increased behavioral response in the second phase in male rat pups. Mechanisms underlying the behavioral pain response induced by inflammation in prenatally stressed rat pups are characterized by sexual dimorphism even prior to the activational effects of sex hormones.