Functional near-infrared spectroscopy to assess pain in neonatal circumcisions.

INTRODUCTION: Pain assessment is challenging in neonates. Behavioral and physiological pain scales do not assess neocortical nociception, essential to pain encoding and central pain pathway development. Functional near-infrared spectroscopy (fNIRS) can assess neocortical activation to noxious stimuli from changes in oxy-(HbO) and total-hemoglobin concentrations (HbT). This study aims to assess fNIRS nociceptive functional activation in the prefrontal cortex of neonates undergoing circumcision through changes in HbO and HbT, and the correlation between changes in fNIRS and Neonatal Infant Pain Scale (NIPS), a behavioral pain assessment scale. METHODS: In healthy term neonates, HbO, HbT, and NIPS were recorded during sequential circumcision events 1-Prep before local anesthetic injection; 2-Local anesthetic injection; 3-Prep before incision; 4-Oral sucrose; 5-Incision; 6-Gomco (hemostatic device) attached; 7-Gomco twisted on; and 8-Gomco removed. fNIRS and NIPS changes after each event were assessed with Wilcoxon signed-rank test and summarized as median and interquartile range (IQR). Changes in fNIRS vs. NIPS were correlated with Spearman coefficient. RESULTS: In 31 neonates fNIRS increased (median [IQR] µmol/L) with noxious events: Local injection (HbO: 1.1 [0.5, 3.1], p < .001; HbT: 2.3 [0.2, 7.6], p < .001), Gomco attached (HbO: 0.7 [0.1, 1.7], p = .002; HbT: 0.7 [-0.2, 2.9], p = .02), and Gomco twisted on (HbO: 0.5 [-0.2, 1.7], p = .03; HbT: 0.8 [-0.1, 3.3], p = .02). fNIRS decreased with non-noxious event: Prep before incision (HbO: -0.6 [-1.2, -0.2] p < .001; HbT: -1 [-1.8, -0.4], p < .001). Local anesthetic attenuated fNIRS increases to subsequent sharp stimuli. NIPS increased with subsequent sharp stimuli despite local anesthetic. Although fNIRS and NIPS changed in the same direction, there was not a strong correlation between them. CONCLUSIONS: During neonatal circumcision, changes in fNIRS differed between different types of painful stimuli, which was not the case for NIPS, suggesting that fNIRS may complement NIPS to assess the quality of pain.

Enduring good memories of infant trauma: rescue of adult neurobehavioral deficits via amygdala serotonin and corticosterone interaction.

Children form a strong attachment to their caregiver--even when that caretaker is abusive. Paradoxically, despite the trauma experienced within this relationship, the child develops a preference for trauma-linked cues--a phenomenon known as trauma bonding. Although infant trauma compromises neurobehavioral development, the mechanisms underlying the interaction between infant trauma bonding (i.e., learned preference for trauma cues) and the long-term effects of trauma (i.e., depressive-like behavior, amygdala dysfunction) are unknown. We modeled infant trauma bonding by using odor-shock conditioning in rat pups, which engages the attachment system and produces a life-long preference for the odor that was paired with shock. In adulthood, this trauma-linked odor rescues depressive-like behavior and amygdala dysfunction, reduces corticosterone (CORT) levels, and exerts repair-related changes at the molecular level. Amygdala microarray after rescue implicates serotonin (5-HT) and glucocorticoids (GCs), and a causal role was verified through microinfusions. Blocking amygdala 5-HT eliminates the rescue effect; increasing amygdala 5-HT and blocking systemic CORT mimics it. Our findings suggest that infant trauma cues share properties with antidepressants and safety signals and provide insight into mechanisms by which infant trauma memories remain powerful throughout life.

Repeated recall and PKMζ maintain fear memories in juvenile rats.

We examined the neural substrates of fear memory formation and maintenance when repeated recall was used to prevent forgetting in young animals. In contrast to adult rats, juveniles failed to show contextual fear responses at 4 d post-fear conditioning. Reconsolidation sessions 3 and 6 d after conditioning restored contextual fear responses in juveniles 7 d after initial training. In juveniles that received reconsolidation sessions, protein kinase M zeta (PKMζ) increased in the amygdala, but not in the hippocampus. These data suggest that repeated reminders and increased PKMζ maintain fear responses in juvenile animals that otherwise would not exhibit this behavior.

Interactions between glia, the immune system and pain processes during early development.

Pain is a serious problem for infants and children and treatment options are limited. Moreover, infants born prematurely or hospitalized for illness likely have concurrent infection that activates the immune system. It is now recognized that the immune system in general and glia in particular influence neurotransmission and that the neural bases of pain are intimately connected to immune function. We know that injuries that induce pain activate immune function and suppressing the immune system alleviates pain. Despite this advance in our understanding, virtually nothing is known of the role that the immune system plays in pain processing in infants and children, even though pain is a serious clinical issue in pediatric medicine. This brief review summarizes the existing data on immune-neural interactions in infants, providing evidence for the immaturity of these interactions.

A new formulation of dezocine, Cyc-dezocine, reduces oxycodone self-administration in female and male rats.

Dezocine is a partial mu opioid receptor agonist previously used as an analgesic for perioperative acute pain in the US and is now the most used perioperative analgesic in China. In general, dezocine is well-tolerated, with relatively minimal risk of fatal respiratory depression. To our knowledge, there are no reports of dezocine addiction, which suggests that the abuse liability of dezocine is low. The overarching goal of this study was to determine the efficacy of a novel formulation of dezocine (Cyc-dezocine), developed for intraperitoneal or intranasal administration, to reduce voluntary opioid taking in rats. One cohort of male rats self-administered intravenous oxycodone on a fixed-ratio 5 schedule of reinforcement. Once oxycodone taking stabilized, rats were pretreated with systemic injections of vehicle or Cyc-dezocine. Cyc-dezocine dose-dependently reduced intravenous oxycodone self-administration. A second cohort of male and female rats self-administered oral oxycodone from drinking water. Once oxycodone taking stabilized, rats were pretreated with intra-nasal Cyc-dezocine. Consistent with the effects of i.p. Cyc-dezocine in our intravenous oxycodone studies, intra-nasal Cyc-dezocine attenuated oral oxycodone self-administration. Together, these findings support the need for further studies investigating the therapeutic potential of Cyc-dezocine for treating opioid use disorder.

Infant pain vs. pain with parental suppression: Immediate and enduring impact on brain, pain and affect.

BACKGROUND: In the short term, parental presence while a human infant is in pain buffers the immediate pain responses, although emerging evidence suggests repeated social buffering of pain may have untoward long-term effects. METHODS/FINDING: To explore the short- and long-term impacts of social buffering of pain, we first measured the infant rat pup's [postnatal day (PN) 8, or 12] response to mild tail shock with the mother present compared to shock alone or no shock. Shock with the mother reduced pain-related behavioral activation and USVs of pups at both ages and reduced Fos expression in the periaqueductal gray, hypothalamic paraventricular nucleus, and the amygdala at PN12 only. At PN12, shock with the mother compared to shock alone differentially regulated expression of several hundred genes related to G-protein-coupled receptors (GPCRs) and neural development, whereas PN8 pups showed a less robust and less coherent expression pattern. In a second set of experiments, pups were exposed to daily repeated Shock-mother pairings (or controls) at PN5-9 or PN10-14 (during and after pain sensitive period, respectively) and long-term outcome assessed in adults. Shock+mother pairing at PN5-9 reduced adult carrageenan-induced thermal hyperalgesia and reduced Fos expression, but PN10-14 pairings had minimal impact. The effect of infant treatment on adult affective behavior showed a complex treatment by age dependent effect. Adult social behavior was decreased following Shock+mother pairings at both PN5-9 and PN10-14, whereas shock alone had no effect. Adult fear responses to a predator odor were decreased only by PN10-14 treatment and the infant Shock alone and Shock+mother did not differ. CONCLUSIONS/SIGNIFICANCE: Overall, integrating these results into our understanding of long-term programming by repeated infant pain experiences, the data suggest that pain experienced within a social context impacts infant neurobehavioral responses and initiates an altered developmental trajectory of pain and affect processing that diverges from experiencing pain alone.

The role of the medial prefrontal cortex in innate fear regulation in infants, juveniles, and adolescents.

In adult animals, the medial prefrontal cortex (mPFC) plays a significant role in regulating emotions and projects to the amygdala and periaqueductal gray (PAG) to modulate emotional responses. However, little is known about the development of this neural circuit and its relevance to unlearned fear in pre-adulthood. To address these issues, we examined the mPFC of 14-d-old (infants), 26-d-old (juveniles), and 38- to 42-d-old (adolescents) rats to represent different developmental and social milestones. The expression patterns of the neuronal marker FOS were used to assess neurological activity. Muscimol, a GABA agonist, was used to inactivate the prelimbic and infralimbic mPFC subdivisions (400 ng in 200 nl). Animals were exposed to either a threatening or nonthreatening stimulus that was ecologically relevant and age specific. Freezing was measured as an indicator of innate fear behavior. The data indicated that the mPFC is neither active nor responsive to innate fear in infant rats. In juveniles, the prelimbic mPFC became responsive in processing aversive sensory stimulation but did not regulate freezing behavior. Finally, during adolescence, inactivation of the prelimbic mPFC significantly attenuated freezing and decreased FOS expression in the ventral PAG. Surprisingly, across all ages, there were no significant differences in FOS levels in the medial and basolateral/lateral amygdala when either mPFC subdivision was inactivated. Together, unlearned fear has a unique developmental course with different brain areas involved in unlearned fear in the immature animal than the adult. In particular, the mPFC neural circuitry is different in young animals and progressively develops more capacities as the animal matures.

A Brief Overview of the Neuropharmacology of Opioid Addiction.

The world is in the midst of an opioid crisis. Nearly 92,000 persons in the U.S. alone died from illicit drugs and prescription opioids in 2020 [1]. This number does not include the countless other individuals who die as a result of the violent crime that accompanies the illicit drug trade. To address this crisis, we need to appreciate aspects of drug addiction. The goal of this brief review is to highlight some major facets of addiction neurobiology, focused on opioids, to provide a basic understanding of the research and terminology encountered in more detailed in-depth articles and discussions on addiction.

Dezocine is a Biased Ligand without Significant Beta-Arrestin Activation of the mu Opioid Receptor.

Dezocine is an opioid that was used in clinical practice for acute pain management in the US (1986 to 2011) and is currently in use in China. It is not listed as a controlled substance in the US due to no reported cases of addiction. Dezocine is a partial agonist at the mu opioid receptor (MOR); however, it is unclear whether dezocine can activate both the G protein pathway and the beta-arrestin pathway. In this study we hypothesized that dezocine does not activate the beta-arrestin pathway, which could be the potential molecular mechanism by which dezocine is not addictive or at least less addictive than other classic opioids. Both morphine, a MOR full agonist and buprenorphine, a partial MOR agonist similar to dezocine, were used for comparison purposes. The major side effects of dezocine in clinical usage are its gastrointestinal side effects and first pass effects; therefore, we explored the possibility of administering dezocine intranasally in rodents to demonstrate the feasibility of intranasal administration for new clinical usage purposes. With proper formulation it is possible to administer dezocine intranasally to achieve a high concentration in the brain in the rodent model. The results indicate that dezocine does not activate the beta-arrestin pathway in MOR. Intranasal delivery of dezocine achieves a much higher medication concentration in the blood and brain as compared to intraperitoneal injection. It also persists a longer time before it falls below detection in the blood. This study provides a possible explanation of why dezocine is not addictive or at least less addictive than other commonly used opioids. This study also demonstrates that intranasal administration offers an alternative strategy for its potential clinical applications.

Endogenous Opioid Dynorphin Is a Potential Link between Traumatic Brain Injury, Chronic Pain, and Substance Use Disorder.

Traumatic brain injury (TBI) is a serious public health problem associated with numerous physical and neuropsychiatric comorbidities. Chronic pain is prevalent and interferes with post-injury functioning and quality of life, whereas substance use disorder (SUD) is the third most common neuropsychiatric diagnosis after TBI. Neither of these conditions has a clear mechanistic explanation based on the known pathophysiology of TBI. Dynorphin is an endogenous opioid neuropeptide that is significantly dysregulated after TBI. Both dynorphin and its primary receptor, the ĸ-opioid receptor (KOR), are implicated in the neuropathology of chronic pain and SUD. Here, we review the known roles of dynorphin and KORs in chronic pain and SUDs. We synthesize this information with our current understanding of TBI and highlight potential mechanistic parallels between and across conditions that suggest a role for dynorphin in long-term sequelae after TBI. In pain studies, dynorphin/KOR activation has either antinociceptive or pro-nociceptive effects, and there are similarities between the signaling pathways influenced by dynorphin and those underlying development of chronic pain. Moreover, the dynorphin/KOR system is considered a key regulator of the negative affective state that characterizes drug withdrawal and protracted abstinence in SUD, and molecular and neurochemical changes observed during the development of SUD are mirrored by the pathophysiology of TBI. We conclude by proposing hypotheses and directions for future research aimed at elucidating the potential role of dynorphin/KOR in chronic pain and/or SUD after TBI.

Estradiol-induced antinociceptive responses on formalin-induced nociception are independent of COX and HPA activation.

Estrogen modulates pain perception but how it does so is not fully understood. The aim of this study was to determine if estradiol reduces nociceptive responses in part via hypothalamic-pituitary-adrenal (HPA) axis regulation of cyclooxygenase (COX)-1/COX-2 activity. The first study examined the effects of estradiol (20%) or vehicle with concurrent injection nonsteroidal antiinflammatory drugs (NSAIDs) on formalin-induced nociceptive responding (flinching) in ovariectomized (OVX) rats. The drugs were ibuprofen (COX-1 and COX-2 inhibitor), SC560 (COX-1 inhibitor), or NS398 (COX-2 inhibitor). In a second study, estradiol's effects on formalin-induced nociception were tested in adrenalectomized (ADX), OVX, and ADX+OVX rats. Serum levels of prostaglandins (PG) PGE(2) and corticosterone were measured. Estradiol significantly decreased nociceptive responses in OVX rats with effects during both the first and the second phase of the formalin test. The nonsteroidal antiinflammatory drugs (NSAIDs) did not alter nociception at the doses used here. Adrenalectomy neither altered flinching responses in female rats nor reversed estradiol-induced antinociceptive responses. Estradiol alone had no effect on corticosterone (CORT) or prostaglandin levels after the formalin test, dissociating the effects of estradiol on behavior and these serum markers. Ibuprofen and NS398 significantly reduced PGE2 levels. CORT was not decreased by OVX surgery or by estradiol below that of ADX. Only IBU significantly increased corticosterone levels. Taken together, our results suggest that estradiol-induced antinociception in female rats is independent of COX activity and HPA axis activation.

The Long-Term Effects of Neonatal Inflammatory Pain on Cognitive Function and Stress Hormones Depend on the Heterogeneity of the Adolescent Period of Development in Male and Female Rats.

Exposure to stress at an early age programs the HPA axis which can lead to cognitive deficits in adults. However, it is not known whether these deficits emerge in adulthood or are expressed earlier in life. The aims of the study were to investigate (1) the immediate effects of early injury-induced stress in one-day-old (P1) and repeated stress on at P1 and P2 rat pups on plasma corticosterone levels; and (2) examine the subsequent long-term effects of this early stress on spatial learning and memory, and stress reactivity in early P26-34 and late P45-53 adolescent male and female rats. Intra-plantar injection of formalin induced prolonged and elevated levels of corticosterone in pups and impaired spatial learning and short- and long-term memory in late adolescent males and long-term memory in early adolescent females. There were sex differences in late adolescence in both learning and short-term memory. Performance on the long-term memory task was better than that on the short-term memory task for all early adolescent male and female control and stressed animals. Short-term memory was better in the late age control rats of both sexes and for formalin treated females as compared with the early age rats. These results are consistent with an impaired function of structures involved in memory (the hippocampus, amygdala, prefrontal cortex) after newborn pain. However, activation of the HPA axis by neonatal pain did not directly correlate with spatial learning and memory outcomes and the consequences of neonatal pain remain are likely multi-determined.

Maternal continuous oral oxycodone self-administration alters pup affective/social communication but not spatial learning or sensory-motor function.

BACKGROUND: The broad use/misuse of prescription opioids during pregnancy has resulted in a surge of infants with Neonatal Opioid Withdrawal Syndrome (NOWS). Short-term irritability and neurological complications are its hallmarks, but the long-term consequences are unknown. METHODS: A newly-developed preclinical model of oxycodone self-administration enables adult female rats to drink oxycodone (∼10/mg/kg/day) before and during pregnancy, and after delivery, and to maintain normal liquid intake, titrate dosing, and avoid withdrawal. RESULTS: Oxycodone was detected in the serum of mothers and pups. Growth parameters in dams and pups and litter mass and size were similar to controls. There were no differences in paw retraction latency to a thermal stimulus between Oxycodone and Control pups at postnatal (PN) 2 or PN14. Oxycodone and Control pups had similar motor coordination, cliff avoidance, righting time, pivoting, and olfactory spatial learning from PN3 through PN13. Separation-induced ultrasonic vocalizations at PN8 revealed higher call frequency in Oxycodone pups relative to Control pups (p<0.031; Cohen's d=1.026). Finally, Oxycodone pups displayed withdrawal behaviors (p's<0.029; Cohen's d's>0.806), and Oxycodone males only vocalized more than Control pups in the first minute of testing (p's<0.050; Cohen's d's>.866). Significant effects were corroborated by estimation plots. CONCLUSIONS: Our rat model of oral oxycodone self-administration in pregnancy shows exacerbated affect/social communication in pups in a sex-dependent manner but spared cognition and sensory-motor behaviors. This preclinical model reproduces selective aspects of human opioid use during pregnancy, enabling longitudinal analysis of how maternal oxycodone changes emotional behavior in the offspring.

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.

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).

Female and male rats readily consume and prefer oxycodone to water in a chronic, continuous access, two-bottle oral voluntary paradigm.

The increasing abuse of opioids - such as oxycodone - poses major challenges for health and socioeconomic systems. Human prescription opioid abuse is marked by chronic, voluntary, oral intake and sex differences. To develop interventions, the field would benefit from a preclinical paradigm that similarly provides rodents with chronic, continuous, oral, voluntary and free-choice access to oxycodone. Here we show female and male rats voluntarily ingest and choose oxycodone over water and show both dependence and motivation to take oxycodone during a chronic oral voluntary, two-bottle choice, continuous access paradigm. Adult female and male Long-Evans rats were given unlimited, continuous homecage access to two bottles containing water (Control) or one bottle of water and one bottle of oxycodone dissolved in water (Experimental). Virtually all experimental rats voluntarily drank oxycodone (~10 mg/kg/day) and escalated their intake over 22 weeks. Females self-administered twice as much oxycodone by body weight (leading to higher blood levels of oxycodone) and engaged in more gnawing behavior of wooden blocks relative to males. Precipitated withdrawal revealed high levels of dependence in both sexes. Reflecting motivation to drink oxycodone, ascending concentrations of citric acid suppressed the intake of oxycodone (Experimental) and the intake of water (Control); however, Experimental rats returned to pre-citric acid preference levels whereas Controls rats did not. Pre-screening behaviors of rats on open field exploration predicted oxycodone intake. Thus, rats consumed and preferred oxycodone over time in this chronic two-bottle oral choice paradigm and both sexes displayed many features of human oxycodone abuse.

Regional Fos expression induced by morphine withdrawal in the 7-day-old rat.

Human infants are often exposed to opiates chronically but the mechanisms by which opiates induce dependence in the infant are not well studied. In the adult the brain regions involved in the physical signs of opiate withdrawal include the periaqueductal gray area, the locus coeruleus, amygdala, ventral tegmental area, nucleus accumbens, hypothalamus, and spinal cord. Microinjection studies show that many of these brain regions are involved in opiate withdrawal in the infant rat. Our goal here was to determine if these regions become metabolically active during physical withdrawal from morphine in the infant rat as they do in the adult. Following chronic morphine or saline treatment, withdrawal was precipitated in 7-day-old pups with the opiate antagonist naltrexone. Cells positive for Fos-like immunoreactivity were quantified within select brain regions. Increased Fos-like labeled cells were found in the periaqueductal gray, nucleus accumbens, locus coeruleus, and spinal cord. These are consistent with other studies showing that the neural circuits underlying the physical signs of opiate withdrawal are similar in the infant and adult.

Differences Between the Prenatal Effects of Fluoxetine or Buspirone Alone or in Combination on Pain and Affective Behaviors in Prenatally Stressed Male and Female Rats.

The selective serotonin reuptake inhibitor fluoxetine and the 5-HT1A receptor agonist buspirone are used to treat depression and anxiety. Previously we demonstrated that chronic stress during pregnancy (prenatal stress) in rats, used as a model of maternal depression risk, increased inflammatory pain and depressive-like behavior in the offspring; buspirone injected to pregnant dams was protective. Clinically, the addition of buspirone to fluoxetine increases the latter's efficacy in treating depression in patients. Here, we investigated the influence of repeated prenatal injections of fluoxetine, buspirone or their combination on pain- and depressive-like behaviors in prenatally stressed young male and female rats. Prenatal stress augmented depressive-like behavior and both thermal and inflammatory pain (formalin test), replicating our prior findings, and increased basal levels of corticosterone in the blood plasma. Both drugs and their combination reduced the effects of prenatal stress on thermal pain and depressive-like behavior independently of sex. The combination of fluoxetine and buspirone, compared with fluoxetine, was more antinociceptive in the hot plate test in both sexes, and when compared with buspirone, was more antinociceptive only in males. A detailed study of the time-course of formalin-induced pain showed a nuanced effect of these drugs that was sex-dependent. The combination of the two drugs was less effective in females than males during the initial acute phase of nociceptive behavior in flexing + shaking behaviors, whereas that combination was more effective than fluoxetine alone in the first acute phase of licking behavior in females. The antinociceptive effect of buspirone dominated that of the drug combination and of fluoxetine alone, especially during the interphase of the formalin test in both sexes for both flexing + shaking and licking, suggesting a more effective prenatal action of buspirone on the development of a descending serotonergic inhibitory system modulating pain in the spinal cord dorsal horn neurons. Our results indicate that inflammatory pain-like responses integrated at the spinal level in males were more vulnerable to prenatal stress than females. In licking, the antinociceptive effect of fluoxetine and drug combination in the interphase was more in males than females. The data underscore the importance of considering sexual dimorphism when using drug therapy.

Trigeminal Pain Responses in Obese ob/ob Mice Are Modality-Specific.

How obesity exacerbates migraine and other pain disorders remains unknown. Trigeminal nociceptive processing, crucial in migraine pathophysiology, is abnormal in mice with diet induced obesity. However, it is not known if this is also true in genetic models of obesity. We hypothesized that obese mice, regardless of the model, have trigeminal hyperalgesia. To test this, we first evaluated trigeminal thermal nociception in leptin deficient (ob/ob) and control mice using an operant thermal assay. Unexpectedly, we found significant hypoalgesia in ob/ob mice. Because thermal hypoalgesia also occurs in mice lacking the transient receptor potential vanilloid 1 channel (TRPV1), we tested capsaicin-evoked trigeminal nociception. Ob/ob and control mice had similar capsaicin-evoked nocifensive behaviors, but ob/ob mice were significantly less active after a facial injection of capsaicin than were diet-induced obese mice or lean controls. Conditioned place aversion in response to trigeminal stimulation with capsaicin was similar in both genotypes, indicating normal negative affect and pain avoidance. Supporting this, we found no difference in TRPV1 expression in the trigeminal ganglia of ob/ob and control mice. Finally, we assessed the possible contribution of hyperphagia, a hallmark of leptin deficiency, to the behavior observed in the operant assay. Ob/ob and lean control mice had similar reduction of intake when quinine or capsaicin was added to the sweetened milk, excluding a significant contribution of hyperphagia. In summary, ob/ob mice, unlike mice with diet-induced obesity, have trigeminal thermal hypoalgesia but normal responses to capsaicin, suggesting specificity in the mechanisms by which leptin acts in pain processing.

Spinal cord ionotropic glutamate receptors function in formalin-induced nociception in preweaning rats.

RATIONALE: Neonates respond to noxious stimuli at or before birth, but the organization of nociceptive systems changes well into postnatal life. It is unknown how nociceptive information is processed in the immature animal and, specifically, whether noxious stimulation is transmitted by glutamatergic circuits, known to play an important role in nociception in the adult. Both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are found within the neonatal spinal cord, but in immature form, and when they become involved in pain processing in vivo is not known. OBJECTIVES: The objective was to determine the age-related changes in the involvement of spinal NMDA and AMPA receptors in formalin-induced nociception during early life. Because the formalin test provides a measure of immediate nociceptive responding (first phase) and of peripheral and central sensitization (second phase), a second aim was to determine if there is specificity of the effects to either phase. MATERIALS AND METHODS: NMDA antagonists (MK801, AP5) or an AMPA antagonist (YM872) was administered intrathecally, and pups were assessed in the formalin test behaviorally and by Fos expression within the spinal cords of 3-, 10-, and 21-day-old rats. RESULTS: The NMDA antagonists attenuated formalin-induced behavioral responses at the youngest age tested with some selectivity for the second phase of responding. MK-801 did not induce motor impairment at any age. YM872 also attenuated formalin-induced nociceptive responses at all ages throughout the test session, although there was some motor impairment in the 3-day-old subjects. Spinal administration of either YM872 or MK-801 reduced Fos expression in the spinal cord at all ages. CONCLUSIONS: These data suggest that spinal NMDA and AMPA receptor are functional and involved in formalin-induced nociception throughout development.