The induction of Fos-like immunoreactivity by noxious thermal, mechanical and chemical stimuli in the lumbar spinal cord of infant rats

Endorphins, Sexuality, and Reproduction

Beta-endorphin is secreted from the hypothalamus and pituitary in both mother and newborn. The placenta produces numerous pituitary hormones from the third month of pregnancy, one of which is βE. It has been suggested that βE has a role in the appetitive and precopulatory phase of sexual behavior in animals. An increase in endorphin levels during sexual activity in humans may contribute to attachment and bonding between partners, but contradictory reports in the literature question the association between sexuality and βE levels. The level of βE also increases during pregnancy, rises in early labor, peaks in late labor, and drops in the postpartum period. This fluctuation provides natural analgesia, raises the pain threshold, decreases the sensation of pain, or suppresses pain, and decreases fear levels during labor and birth. Beta-endorphin also protects the fetus from hypoxia during labor and birth and potential neural damage by aiding blood flow to the brain under hypoxic conditions. It has been suggested that a variety of pharmacologic and nonpharmacologic complementary therapies, when used in pregnancy, labor, and birth, activate the opioid receptors in the CNS and alter the sensation of pain during labor and birth, affect the mother-child attachment and affect sexual function. These studies report contradictory results that will be discussed in this chapter.

Tissue Regeneration: The Dark Side of Opioids

Opioids are regarded as among the most effective analgesic drugs and their use for the management of pain is considered standard of care. Despite their systematic administration in the peri-operative period, their impact on tissue repair has been studied mainly in the context of scar healing and is only beginning to be documented in the context of true tissue regeneration. Indeed, in mammals, growing evidence shows that opioids direct tissue repair towards scar healing, with a loss of tissue function, instead of the regenerative process that allows for recovery of both the morphology and function of tissue. Here, we review recent studies that highlight how opioids may prevent a regenerative process by silencing nociceptive nerve activity and a powerful anti-inflammatory effect. These data open up new perspectives for inducing tissue regeneration and argue for opioid-restricted strategies for managing pain associated with tissue injury.

A Combinatorial Approach to Induce Sensory Axon Regeneration into the Dorsal Root Avulsed Spinal Cord

Spinal root injuries result in newly formed glial scar formation, which prevents regeneration of sensory axons causing permanent sensory loss. Previous studies showed that delivery of trophic factors or implantation of human neural progenitor cells supports sensory axon regeneration and partly restores sensory functions. In this study, we elucidate mechanisms underlying stem cell-mediated ingrowth of sensory axons after dorsal root avulsion (DRA). We show that human spinal cord neural stem/progenitor cells (hscNSPC), and also, mesoporous silica particles loaded with growth factor mimetics (MesoMIM), supported sensory axon regeneration. However, when hscNSPC and MesoMIM were combined, sensory axon regeneration failed. Morphological and tracing analysis showed that sensory axons grow through the newly established glial scar along "bridges" formed by migrating stem cells. Coimplantation of MesoMIM prevented stem cell migration, "bridges" were not formed, and sensory axons failed to enter the spinal cord. MesoMIM applied alone supported sensory axons ingrowth, but without affecting glial scar formation. In vitro, the presence of MesoMIM significantly impaired migration of hscNSPC without affecting their level of differentiation. Our data show that (1) the ability of stem cells to migrate into the spinal cord and organize cellular "bridges" in the newly formed interface is crucial for successful sensory axon regeneration, (2) trophic factor mimetics delivered by mesoporous silica may be a convenient alternative way to induce sensory axon regeneration, and (3) a combinatorial approach of individually beneficial components is not necessarily additive, but can be counterproductive for axonal growth.

Pyruvate dehydrogenase kinase 2 and 4 gene deficiency attenuates nociceptive behaviors in a mouse model of acute inflammatory pain

Pyruvate dehydrogenase (PDH) kinases (PDKs) 1-4, expressed in peripheral and central tissues, regulate the activity of the PDH complex (PDC). The PDC is an important mitochondrial gatekeeping enzyme that controls cellular metabolism. The role of PDKs in diverse neurological disorders, including neurometabolic aberrations and neurodegeneration, has been described. Implications for a role of PDKs in inflammation and neurometabolic coupling led us to investigate the effect of genetic ablation of PDK2/4 on nociception in a mouse model of acute inflammatory pain. Deficiency in Pdk2 and/or Pdk4 in mice led to attenuation of formalin-induced nociceptive behaviors (flinching, licking, biting, or lifting of the injected paw). Likewise, the pharmacological inhibition of PDKs substantially diminished the nociceptive responses in the second phase of the formalin test. Furthermore, formalin-provoked paw edema formation and mechanical and thermal hypersensitivities were significantly reduced in Pdk2/4-deficient mice. Formalin-driven neutrophil recruitment at the site of inflammation, spinal glial activation, and neuronal sensitization were substantially lessened in the second or late phase of the formalin test in Pdk2/4-deficient animals. Overall, our results suggest that PDK2/4 can be a potential target for the development of pharmacotherapy for the treatment of acute inflammatory pain. © 2016 Wiley Periodicals, Inc.

Developmental emergence of fear/threat learning: neurobiology, associations and timing

Pavlovian fear or threat conditioning, where a neutral stimulus takes on aversive properties through pairing with an aversive stimulus, has been an important tool for exploring the neurobiology of learning. In the past decades, this neurobehavioral approach has been expanded to include the developing infant. Indeed, protracted postnatal brain development permits the exploration of how incorporating the amygdala, prefrontal cortex and hippocampus into this learning system impacts the acquisition and expression of aversive conditioning. Here, we review the developmental trajectory of these key brain areas involved in aversive conditioning and relate it to pups' transition to independence through weaning. Overall, the data suggests that adult-like features of threat learning emerge as the relevant brain areas become incorporated into this learning. Specifically, the developmental emergence of the amygdala permits cue learning and the emergence of the hippocampus permits context learning. We also describe unique features of learning in early life that block threat learning and enhance interaction with the mother or exploration of the environment. Finally, we describe the development of a sense of time within this learning and its involvement in creating associations. Together these data suggest that the development of threat learning is a useful tool for dissecting adult-like functioning of brain circuits, as well as providing unique insights into ecologically relevant developmental changes.

Recruitment of hypothalamic orexin neurons after formalin injections in adult male rats exposed to a neonatal immune challenge

Exposure to early life physiological stressors, such as infection, is thought to contribute to the onset of psychopathology in adulthood. In animal models, injections of the bacterial immune challenge, lipopolysaccharide (LPS), during the neonatal period has been shown to alter both neuroendocrine function and behavioral pain responses in adulthood. Interestingly, recent evidence suggests a role for the lateral hypothalamic peptide orexin in stress and nociceptive processing. However, whether neonatal LPS exposure affects the reactivity of the orexin system to formalin-induced inflammatory pain in later life remains to be determined. Male Wistar rats (n = 13) were exposed to either LPS or saline (0.05 mg/kg, i.p) on postnatal days (PND) 3 and 5. On PND 80–97, all rats were exposed to a subcutaneous hindpaw injection of 2.25% formalin. Following behavioral testing, animals were perfused and brains processed for Fos-protein and orexin immunohistochemistry. Rats treated with LPS during the neonatal period exhibited decreased licking behaviors during the interphase of the formalin test, the period typically associated with the active inhibition of pain, and increased grooming responses to formalin in adulthood. Interestingly, these behavioral changes were accompanied by an increase in the percentage of Fos-positive orexin cells in the dorsomedial and perifornical hypothalamus in LPS-exposed animals. Similar increases in Fos-protein were also observed in stress and pain sensitive brain regions that receive orexinergic inputs. These findings highlight a potential role for orexin in the behavioral responses to pain and provide further evidence that early life stress can prime the circuitry responsible for these responses in adulthood.

Persistent at-level thermal hyperalgesia and tactile allodynia accompany chronic neuronal and astrocyte activation in superficial dorsal horn following mouse cervical contusion spinal cord injury

In humans, sensory abnormalities, including neuropathic pain, often result from traumatic spinal cord injury (SCI). SCI can induce cellular changes in the CNS, termed central sensitization, that alter excitability of spinal cord neurons, including those in the dorsal horn involved in pain transmission. Persistently elevated levels of neuronal activity, glial activation, and glutamatergic transmission are thought to contribute to the hyperexcitability of these dorsal horn neurons, which can lead to maladaptive circuitry, aberrant pain processing and, ultimately, chronic neuropathic pain. Here we present a mouse model of SCI-induced neuropathic pain that exhibits a persistent pain phenotype accompanied by chronic neuronal hyperexcitability and glial activation in the spinal cord dorsal horn. We generated a unilateral cervical contusion injury at the C5 or C6 level of the adult mouse spinal cord. Following injury, an increase in the number of neurons expressing ΔFosB (a marker of chronic neuronal activation), persistent astrocyte activation and proliferation (as measured by GFAP and Ki67 expression), and a decrease in the expression of the astrocyte glutamate transporter GLT1 are observed in the ipsilateral superficial dorsal horn of cervical spinal cord. These changes have previously been associated with neuronal hyperexcitability and may contribute to altered pain transmission and chronic neuropathic pain. In our model, they are accompanied by robust at-level hyperaglesia in the ipsilateral forepaw and allodynia in both forepaws that are evident within two weeks following injury and persist for at least six weeks. Furthermore, the pain phenotype occurs in the absence of alterations in forelimb grip strength, suggesting that it represents sensory and not motor abnormalities. Given the importance of transgenic mouse technology, this clinically-relevant model provides a resource that can be used to study the molecular mechanisms contributing to neuropathic pain following SCI and to identify potential therapeutic targets for the treatment of chronic pathological pain.

Altered formalin-induced pain and Fos induction in the periaqueductal grey of preadolescent rats following neonatal LPS exposure

Animal and human studies have demonstrated that early pain experiences can produce alterations in the nociceptive systems later in life including increased sensitivity to mechanical, thermal, and chemical stimuli. However, less is known about the impact of neonatal immune challenge on future responses to noxious stimuli and the reactivity of neural substrates involved in analgesia. Here we demonstrate that rats exposed to Lipopolysaccharide (LPS; 0.05 mg/kg IP, Salmonella enteritidis) during postnatal day (PND) 3 and 5 displayed enhanced formalin-induced flinching but not licking following formalin injection at PND 22. This LPS-induced hyperalgesia was accompanied by distinct recruitment of supra-spinal regions involved in analgesia as indicated by significantly attenuated Fos-protein induction in the rostral dorsal periaqueductal grey (DPAG) as well as rostral and caudal axes of the ventrolateral PAG (VLPAG). Formalin injections were associated with increased Fos-protein labelling in lateral habenula (LHb) as compared to medial habenula (MHb), however the intensity of this labelling did not differ as a result of neonatal immune challenge. These data highlight the importance of neonatal immune priming in programming inflammatory pain sensitivity later in development and highlight the PAG as a possible mediator of this process.

Neuronal Activity Stimulated by Liquid Substrates Injection at Zusanli (ST36) Acupoint: The Possible Mechanism of Aquapuncture

Aquapuncture is a modified acupuncture technique and it is generally accepted that it has a greater therapeutic effect than acupuncture because of the combination of the acupoint stimulation and the pharmacological effect of the drugs. However, to date, the mechanisms underlying the effects of aquapuncture remain unclear. We hypothesized that both the change in the local spatial configuration and the substrate stimulation of aquapuncture would activate neuronal signaling. Thus, bee venom, normal saline, and vitamins B1 and B12 were injected into a Zusanli (ST36) acupoint as substrate of aquapuncture, whereas a dry needle was inserted into ST36 as a control. After aquapuncture, activated neurons expressing Fos protein were mainly observed in the dorsal horn of the spinal cord in lumbar segments L3–5, with the distribution nearly identical among all groups. However, the bee venom injection induced significantly more Fos-expressing neurons than the other substrates. Based on these data, we suggest that changes in the spatial configuration of the acupoint activate neuronal signaling and that bee venom may further strengthen this neuronal activity. In conclusion, the mechanisms for the effects of aquapuncture appear to be the spatial configuration changes occurring within the acupoint and the ability of injected substrates to stimulate neuronal activity.

Low formalin concentrations induce fine-tuned responses that are sex and age-dependent: a developmental study

The formalin test is increasingly applied as a model of inflammatory pain using high formalin concentrations (5–15%). However, little is known about the effects of low formalin concentrations on related behavioural responses. To examine this, rat pups were subjected to various concentrations of formalin at four developmental stages: 7, 13, 22, and 82 days of age. At postnatal day (PND) 7, sex differences in flinching but not licking responses were observed with 0.5% formalin evoking higher flinching in males than in females. A dose response was evident in that 0.5% formalin also produced higher licking responses compared to 0.3% or 0.4% formalin. At PND 13, a concentration of 0.8% formalin evoked a biphasic response. At PND 22, a concentration of 1.1% evoked higher flinching and licking responses during the late phase (10–30 min) in both males and females. During the early phase (0–5 min), 1.1% evoked higher licking responses compared to 0.9% or 1% formalin. 1.1% formalin produced a biphasic response that was not evident with 0.9 or 1%. At PND 82, rats displayed a biphasic pattern in response to three formalin concentrations (1.25%, 1.75% and 2.25%) with the presence of an interphase for both 1.75% and 2.25% but not for 1.25%. These data suggest that low formalin concentrations induce fine-tuned responses that are not apparent with the high formalin concentration commonly used in the formalin test. These data also show that the developing nociceptive system is very sensitive to subtle changes in formalin concentrations.

Formalin-induced c-fos expression in the brain of infant rats

In the fetal, infant, and adult rat, injury induces a well-defined behavioral response and induces c-fos expression in the spinal cord dorsal horn. There is more limited information about the processing of noxious stimulation in the infant brain. We describe here the appearance of the Fos protein in the brain of fetal and infant rats following formalin-induced injury. Regions were chosen for analysis with a special focus on brain loci that express c-fos in the adult. No Fos positive cells were found in the brains of fetuses; newborns did not show increased Fos expression after formalin injection in any structure examined. At 3 and 14 days of age, there was a significant increase in Fos staining induced by formalin in the ventral lateral medulla. In contrast, paraventricular and medial dorsal nuclei of the thalamus, the paraventricular nucleus of the hypothalamus, and periaqueductal gray of the midbrain showed increased levels of Fos protein only at 14 days of age. We hypothesize that this developmental pattern is related not only to the maturation of pain perception but also to development of autonomic and defensive reactions to pain in the infant.

PERSPECTIVE

Because the infant processes pain differently than the adult, knowledge of those differences informs pediatric clinical practice. Using Fos expression as a marker of neural activity in the rat, we show that the pattern of brain activation is immature at birth but is in place by 14 days of age.

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.

c-Fos and pERK, which is a better marker for neuronal activation and central sensitization after noxious stimulation and tissue injury?

c-Fos, the protein of the protooncogene c-fos, has been extensively used as a marker for the activation of nociceptive neurons in the spinal cord for more than twenty years since Hunt et al. first reported that peripheral noxious stimulation to a hind paw of rats leads to a marked induction of c-Fos in superficial and deep dorsal horn neurons in 1987. In 1999, Ji et al. reported that phosphorylated extracellular signal-regulated kinase (pERK) is specifically induced by noxious stimulation in superficial dorsal horn neurons. Accumulating evidence indicates that pERK induction or ERK activation in dorsal horn neurons is essential for the development of central sensitization, increased sensitivity of dorsal horn neurons that is responsible for the generation of persistent pain. Further, molecular mechanisms underlying ERK-mediated central sensitization have been revealed. In contrast, direct evidence for c-Fos-mediated central sensitization is not sufficient. After a noxious stimulus (e.g., capsaicin injection) or tissue injury, c-Fos begins to be induced after 30-60 minutes, whereas pERK can be induced within a minute, which can correlate well with the development of pain hypersensitivity. While c-Fos is often induced in the nuclei of neurons, pERK can be induced in different subcellular structures of neurons such as nuclei, cytoplasma, axons, and dendrites. pERK can even be induced in spinal cord microglia and astrocytes after nerve injury. In summary, both c-Fos and pERK can be used as markers for neuronal activation following noxious stimulation and tissue injury, but pERK is much more dynamic and appears to be a better marker for central sensitization.

Threshold-like pattern of neuronal activation in the hypothalamus during treadmill running: Establishment of a minimum running stress (MRS) rat model

Despite the indication that the hypothalamo-pituitary-adrenal (HPA) axis is activated during treadmill running, there have not been any studies focusing on the relationship between exercise intensity and region-specific neural activities in hypothalamus. To address this, rats were subjected to 30 min of running, either at middle (supra-LT, 25 m min(-1)) or low speeds (sub-LT, 15 m min(-1)), and c-Fos-(+) cells were counted and compared with control rats. Significant increases in blood glucose and lactate levels, and plasma ACTH and osmolality levels were observed during supra-LT running. Only supra-LT running significantly increased c-Fos induction in various hypothalamic regions, namely, the medial preoptic area (MPO), periventricular nucleus (Pe), suprachiasmatic nucleus (SCN), supraoptic nucleus (SON), parvocellular division of the paraventricular nucleus (pPVN), anterior hypothalamic area (AH), arcuate nucleus (ARC) and posterior hypothalamic nucleus (PH). However, sub-LT caused no effect on c-Fos accumulation. This indicates that the hypothalamus responds uniquely to running in a threshold-like pattern distinct from the speed-dependent pattern previously reported for the medulla oblongata [Ohiwa et al., 2006a,b]. In addition, these results showed a physiologic basis for mild exercise useful for establishing our minimum running stress (MRS) rat model, or the running conditions that minimize the activation of the HPA axis.

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.

[Neurobiology of the chronicisation of pain in children: the memory of pain and its painful memory]

Reviewing the development of nociceptive circuits provides the rationale behind the need to modify and reduce premature painful experiences, especially during the "plastic" neonatal phase. Indeed, if physiological mechanisms of the functional nociceptive system follow a harmonious and predetermined development, it is the individual personal experience, intrinsically random, which will shape the final reactivity of this system and the later painful experience. If pain would not have been the organism's alarm system, we could have simply compared it by analogy to other sensorial systems, which its development depends exclusively on the presence of environmental stimuli. The eyes wait for light, the ears for sound, the skin to be touched, the tongue to taste and the olfactory bulbs to smell. However with pain it is not the quantitative exposure that determines its development, but rather the context-laden aspects of its affliction which in turn create the complex experience and "memory" of pain. Prolonged, but also "unnecessary" exposure to pain transforms it into a futile sensation, which impacts the individual immediately but also resonates into its future. This article reviews recent neurobiological mechanisms (such as neural circuitry, neurotrophins, peripheral and central sensitization, inhibitory pathways) now known to develop during the chronicisation and apprenticing of pain in the growing individual. Its cognizance is vital for a better comprehension of adult pain.

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.

Role of kappa and delta opioid receptors in mediating morphine-induced antinociception in morphine-tolerant infant rats

We have previously noted that the antinociceptive efficacy of morphine was significantly decreased in rat pups chronically infused with morphine from implanted osmotic minipumps. In this study, morphine was fully efficacious (i.e., 100% maximum possible effect, %MPE) in the 52 degrees C tail-immersion test after a 72-h infusion from implanted saline-filled osmotic minipumps. However, administration of up to 1000 mg/kg, s.c. morphine failed to elicit greater than a 27% MPE in rats infused with morphine at 2 mg/kg/h. Morphine was more efficacious when the water bath temperature was decreased to 49 degrees C. Experiments were conducted to determine the mechanisms whereby chronic morphine administration leads to a decrease in antinociceptive efficacy. The kappa-opioid antagonist nor-binalorphimine completely blocked the antinociceptive effects of morphine in morphine-infused rat pups. The kappa agonist U50,488 elicited antinociception; however, the requirement to use higher doses in morphine- than saline-infused rats indicates that kappa cross-tolerance was present. Thus, in tolerant rats the antinociceptive effects of high doses of morphine appear to be mediated through kappa-opioid receptors. The delta-opioid antagonist naltrindole was inactive in both treatment groups. DAMGO-stimulated [(35)S]GTPgammaS and [(3)H]naloxone binding reveals that the anatomical distribution of the mu-opioid receptor was consistent with that of the adult rat brain. In adult rats, the mu-opioid receptor is desensitized during morphine tolerance. However, desensitization was not evident in P17 rats based on the lack of significant decreases in [(35)S]GTPgammaS binding. Furthermore, [(3)H]naloxone binding indicated a lack of mu receptor downregulation in morphine-tolerant rat pups.

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

When pregnant dams are stressed, there is a resultant alteration in brain development and behavior in their offspring. Prior work has shown increased nociceptive responses in adolescent or adult rats born of stressed dams. However, the age at which those changes first occur is not known. The aim of the present study was to evaluate the effect of prenatal stress on pain sensitivity in the formalin test in 7-day-old rats, behaviorally and by fos-like immunoreactivity (Fos-LI) in the lumbar spinal cord dorsal horn. The behavioral response to intraplantar injection of formalin is represented by two nociceptive phases separated by an interphase during which nociceptive responses decrease; the interphase is not seen until the start of the third postnatal week and appears as descending inhibitory monoaminergic systems develop. Prenatally stressed infants showed increased nociceptive responses in the second, tonic phase and a large increase in the number of formalin-induced Fos-LI neurons in the lumbar dorsal horn, a result consistent with the behavioral data. The increased nociception in prenatally stressed 7-day-old pups may be associated with the decrease in the intensity of serotonin-like immunoreactivity and density of serotonergic cells in the lumbar spinal cord dorsal horn and the dorsal raphe nucleus reported earlier.

Isoflurane exerts antinociceptive and hypnotic properties at all ages in Fischer rats

BACKGROUND

Some anaesthetic agents exhibit an age-dependent analgesic effect, for example nitrous oxide, which is ineffective in newborn rats. We investigated whether a similar time dependency existed for the responses to the volatile anaesthetic isoflurane.

METHODS

The analgesic and hypnotic properties of isoflurane at various ages was assessed using four cohorts of Fischer rats aged approximately 7, 16, and 28 days and adults (11-12 weeks old). Intraplantar administration of formalin mimicked inflammatory pain, and its effects were assessed using immunohistochemical (c-Fos staining) and behavioural paradigms. The hypnotic properties of isoflurane were assessed using loss of righting reflex.

RESULTS

Formalin administration produced a typical nociceptive response observed both behaviourally and immunohistochemically in all age groups; these nociceptive responses were significantly attenuated by isoflurane 0.5% at each age (P<0.05). Interestingly 7-day-old animals showed a significantly more potent hypnotic response than older animals (P<0.01): with adult rats being most resistant to isoflurane induced hypnosis (P<0.05).

CONCLUSION

In contrast to nitrous oxide, isoflurane is an effective antinociceptive agent in neonatal rats. If the data can be extrapolated to clinical scenarios these results suggest that isoflurane may be analgesic in newborns as well as adult humans. In addition, isoflurane is a potent hypnotic, especially in the very young, which is in contrast to the neonate's relative resistance to anaesthesia as assessed by minimum alveolar concentration.

Microarray analysis of gene expression following the formalin test in the infant rat☆

Injury and pain experienced by the infant results in immediate changes in pain sensitivity that last into adulthood. These long-term changes are likely initiated by altered gene expression. Here we measured how injury alters gene expression in the lumbar spinal cord early and late in the preweaning period of the rat. The expression of large numbers of genes was altered significantly at 3 days of age, when injury has long-term consequences. The functional classes of altered genes included transcription factors, cell death related and metal ion genes. The intensity of the stimulus in the 3-day-old pups induced changes in different classes of genes. Fewer changes were noted at 21 days of age. The increased expression of transcription factors and decreased expression of genes whose products are protective against cell death are hypothesized to underlie the long-term changes that are seen after injury in the neonate.

The development of nociceptive circuits

The study of pain development has come into its own. Reaping the rewards of years of developmental and molecular biology, it has now become possible to translate fundamental knowledge of signalling pathways and synaptic physiology into a better understanding of infant pain. Research has cast new light on the physiological and pharmacological processes that shape the newborn pain response, which will help us to understand early pain behaviour and to design better treatments. Furthermore, it has shown how developing pain circuitry depends on non-noxious sensory activity in the healthy newborn, and how early injury can permanently alter pain processing.

Dexmedetomidine Exerts Dose-Dependent Age-Independent Antinociception but Age-Dependent Hypnosis in Fischer Rats

Dexmedetomidine (Dex), an alpha(2)-adrenoceptor agonist, is an effective analgesic and sedative drug in adults; however, little information is available about its efficacy in pediatric populations. Some anesthetics exhibit an age-dependent analgesic effect, e.g., nitrous oxide, being relatively ineffective in newborn rats. We investigated the analgesic and hypnotic efficacy of Dex using 6 cohorts of Fischer rats aged 7, 15, 19, 23, and 29 days and adults exposed to either Dex (10 or 50 microg/kg) or saline subcutaneously. Formalin plantar testing was used to mimic inflammatory pain, and its effect was assessed using immunohistochemical (c-Fos staining) and behavioral methods. The hypnotic action of Dex was assessed by loss of righting reflex. Formalin administration produced a typical nociceptive response in each age group; these nociceptive responses were significantly attenuated by Dex 50 microg/kg at all ages (P < 0.05), whereas Dex 10 microg/kg had little effect. Neonatal rats showed the greatest hypnotic sensitivity to Dex (P < 0.05).

Does neonatal surgery lead to increased pain sensitivity in later childhood?

Does pain or tissue damage in early life lead to hyperalgesia persisting into childhood? We performed a cross-sectional study in 164 infants to investigate whether major surgery within the first 3 months of life increases pain sensitivity to subsequent surgery and to elucidate whether subsequent surgery in the same dermatome or in a different dermatome leads to differences in pain sensitivity. All infants received standard intraoperative and postoperative pain management, with rescue analgesia guided by a treatment algorithm. Differences in pain sensitivity during surgery were assessed by the intraoperative fentanyl intake and by (nor)epinephrine plasma concentrations. Differences in postoperative pain sensitivity were assessed by the observational pain measures COMFORT and VAS, and by morphine intake and (nor)epinephrine plasma concentrations. Infants previously operated upon in the same dermatome needed more intraoperative fentanyl, had higher COMFORT and VAS scores, had greater (nor)epinephrine plasma concentrations, and needed also more morphine than did infants with no prior surgery. In contrast, infants who previously underwent surgery in another dermatome had only significant higher postoperative analgesic requirements and norepinephrine plasma concentrations in comparison with infants with no prior surgery. These preliminary differences may indicate the occurrence of spinal and supraspinal changes following neonatal surgery. We conclude that the long-term consequences of surgery in early infancy are greater in areas of prior tissue damage and that these effects may portend limited clinical but important neurobiological differences.

Pain and stress in the human fetus

It is not known if the fetus can actually feel pain, but noxious stimulation during fetal life does cause detectable stress responses. These responses cause both short and long-term changes in the central nervous system, which can affect subsequent pain behaviour. Reducing the stress response is known to be beneficial in children and adults and recent evidence suggests this is also true for the fetus. However, the optimal amount of suppression required and the best method of achieving this (opioid or regional anaesthesia techniques) remain unknown. Prevention and treatment of pain is a basic human right, regardless of age, and if the technique of fetal surgery is to progress then a greater understanding of nociception and the stress response is required.

Functional development of neurokinin peptides substance P and neurokinin A in nociception

It is unclear how neonates respond to noxious stimuli. This study examined the role of neurokinin peptides in 3- and 21-day-old rat pups using the preprotachykinin-A (PPTA) knockout mouse, lacking neurokinin A and substance P. We assessed pain behaviors of these mice before the neurokinin system is putatively active, 3 days after birth, and after this system is active, 21 days after birth. The lack of these peptides failed to alter behavioral responses to nociceptive stimulation in the 3-day-old mice. The 21-day-old mice lacking these peptides were less responsive to 5 microl 2% formalin and to high intensity thermal and mechanical stimuli. Thus, the neurokinins appear not to be an important mechanism in the processing of nociceptive information in the infant.

Analgesia induced by local plantar injections of opiates in the formalin test in infant rats

Morphine injected locally to the paw of an adult or an infant rat is analgesic. Opiates specific to micro and kappa opioid receptors, and less consistently to delta opioid receptors, given locally to the site of injury in adult animals are also analgesic in a variety of models of inflammatory pain. To determine which opioid receptor(s) are involved in local analgesia in the immature animal, agonists specific for micro, kappa, and delta opioid receptors were injected into the intraplantar pad in infant rats and the resultant nociceptive behavior and Fos expression assayed in the formalin test. The kappa opioid receptor agonist U50,488 reduced nociceptive behavior in both phases of the formalin test and reduced Fos expression in the dorsal horn of the lumbar spinal cord, at 3 and 21 days of age. Morphiceptin (micro opioid agonist) was analgesic in the 21-day-old pups, but not the 3-day-old pups, measured behaviorally or by Fos expression. DPDPE (delta opioid agonist) was not analgesic at either age. We also tested the effects of opioid receptor antagonists on morphine's local analgesic action. Naltrexone, and to a lesser extent the micro opioid antagonist CTOP, antagonized morphine's analgesic effect. Kappa and delta opioid receptor blockers were inactive. The results demonstrate the ability of the kappa opioid system to mediate analgesia in the neonate at the site of injury in acute and chronic pain models, that the micro opioid agonists are active later in development, but that morphine is analgesic in part through micro opioid receptors.

Nitrous oxide exerts age-dependent antinociceptive effects in Fischer rats

Nitrous oxide (N(2)O) is an inhalational anesthetic/analgesic gas that has been used for clinical practice for more than a century. While its anesthetic mechanisms remain largely unknown, the underlying analgesic mechanisms are now being unraveled. It has been proposed that N(2)O induces opioid peptide release in the midbrain, leading to the activation of descending noradrenergic inhibitory neurons, which modulates pain processing within the spinal cord. Because descending noradrenergic inhibitory neurons are not functional at birth we posit that N(2)O only becomes an effective analgesic/antinociceptive agent in young patients when the descending noradrenergic inhibitory neurons become fully functional. In the present study, we have examined the age-dependence of N(2)O-induced antinociceptive effects on the formalin test. Fischer rats of various ages (7-, 15-, 19-, 23-, and 29-day-old, and adult) were injected 5% formalin into the hind paw during exposure to 75% N(2)O. Both their behavioral responses and changes in Fos-like immunoreactivity in the spinal cord were assessed as markers of N(2)O's antinociceptive effect. Adult-like antinociceptive responses to N(2)O, both behaviorally and immunohistochemically, were only present in rats older than 3 weeks (23- and 29-day-old). These findings support our hypothesis that N(2)O lacks antinociceptive effects in the very young animals.

Covariant maturation of nocifensive oral behaviour and c-fos expression in rat superior colliculus

Injections of formalin into the rodent paw elicit a rapid orientation of the head and mouth to the source of discomfort, followed by licking and biting the injected area. Previous work has shown this response is dependent on the integrity of the midbrain superior colliculus. The present experiments were initiated to examine the ontogeny of this oral nocifensive reaction and to determine whether it is correlated with the functional maturation of collicular responses to noxious stimuli (as indicated by c-fos immunohistochemistry). Rat pups at various postnatal ages received formalin injections in either the hindpaw or perioral regions. Behaviour was videotaped, and after 120 min, animals were killed and the brain and spinal cord processed for Fos-like immunoreactivity. Uninjected controls were treated identically. Formalin-induced oral responses following injections into the hindpaw and the expression of Fos in the superior colliculus were virtually absent until 10 days postnatal, despite the presence of Fos-like immunoreactivity in many other structures (e.g. spinal cord, parabrachial area, periaqueductal grey). In contrast, animals from day 1 were able to use limbs to localise the perioral injection site. From day 10 onward, there was a progressive increase in oral nocifensive behaviours and Fos expression in the superior colliculus. Our observations are consistent with the hypothesis that the normal elaboration of pain-induced oral behaviour is initiated only after a functionally active superior colliculus has developed, and support previous observations that link the colliculus particularly with oral nocifensive behaviours.

A role for protein kinase intracellular messengers in substance P- and nociceptor afferent-mediated excitation and expression of the transcription factor Fos in rat dorsal horn neurons in vitro

Expression of the inducible transcription factor Fos in the spinal dorsal horn in vivo is associated with nociceptive afferent activation, but the underlying stimulation-transcription pathway is less clear. This in vitro spinal cord study concerns the role of protein kinase A and C second messengers in substance P receptor (NK1R)-mediated or nociceptive afferent-evoked neuronal excitation and Fos expression. Nociceptive afferent (dorsal root) stimulation of isolated spinal cords (10-14 day old rats) evoked a 'prolonged' excitatory polysynaptic potential (DR-EPSP) that was attenuated (P < 0.05) by: the protein kinase A inhibitor, Rp-cAMP; the protein kinase C inhibitor, bisindolymaleimide I; and the selective NK1R antagonist, GR82334. Neuronal excitations induced by the NK1R agonist [Sar9,Met(O2)11]-SP were attenuated by Rp-cAMP, bisindolymaleimide I and GR82334. Effects of the protein kinase A and C inhibitors on the DR-EPSP or the [Sar9,Met(O2)11]-SP-induced depolarization were nonadditive, suggesting convergence of these intracellular signalling pathways onto a common final target. Nociceptor afferent-induced Fos, detected by immunohistochemistry in superficial and deep dorsal horn laminae, was attenuated by Rp-cAMP, bisindolymaleimide I and GR82334. In spinal cords pretreated with TTX to eliminate indirect neuronal activation, [Sar9,Met(O2)11]-SP (1-20 microM) elicited a dose-related expression of Fos that was reduced by Rp-cAMP, bisindolymaleimide I and GR82334. The effects of these inhibitors were most pronounced in the deep laminae. These data support a causal relationship between protein kinase A- or C-dependent signal transduction, nociceptive afferent- or NK1R-induced neuronal excitation and Fos expression in dorsal horn. Implications for short- versus long-term modulation of nociceptive circuitry are discussed.

Analgesic efficacy of ketorolac and morphine in neonatal rats

Ketorolac is a potent nonsteroidal antiinflammatory drug (NSAID). In adult humans and animals, its analgesic efficacy can be comparable to opiates. However, it has not been studied in neonatal animals. We conducted a blinded, controlled study comparing the effects of ketorolac and morphine in neonatal rats using the formalin model. Animals were given intraperitoneal (i.p.) injections of ketorolac or morphine at 3 or 21 days of age. Ketorolac had an analgesic and antiinflammatory effect in 21-day-old pups, but not in the 3-day-olds. Morphine had a significant analgesic, but no antiinflammatory effect at both ages. These results indicate that ketorolac is an effective analgesic agent in preweaning, but not neonatal rats. Opiates may be more appropriate analgesics in neonates.

Maturation of NK1 receptor involvement in the nociceptive response to formalin

Administration of NK1 antagonists in adult animals attenuates the nociceptive response in the formalin test, indicating that the neurokinins and the NK1 receptor play a role in mediating this pain response. The number and distribution of NK1 receptors change dramatically during development, and the age at which they become involved in pain processing is not known. We examined the role of NK1 receptors in the formalin model in rats ranging in age between 3- and 21-days old. An NK1 antagonist, CP99,994, and its less active enantiomer CP100,263 were administered to the spinal cord (intrathecal), systemically (subcutaneous), or locally (intraplantar). Intrathecal administration of CP99,994, but not CP100,263, attenuated pain behaviors in the second phase of the formalin response in 14-day and 21-day old rats, but did not alter the pain response in 3-day or 10-day old rats. CP99,994 also reduced the expression of the c-fos protein in the superficial dorsal horn of 21-day old rats. Systemic and intraplantar injection of either CP99,994 or CP100,263 reduced the pain response to formalin in 3-day and 21-day old rats, suggesting a non-NK1 mediated mechanism of action. These results indicate that, within the spinal cord, NK1 receptors start to play a role in the pain response to formalin between 10 and 21 days. Moreover, analgesia induced by systemic or local injection of NK1 antagonists involves mechanisms other than, or in addition to, the NK1 receptor.

Effects of perinatal pain and stress

Neonatal intensive care exposes preterm neonates to a series of repeated, randomly occurring invasive procedures and handling, resulting in acute pain, chronic pain, and prolonged stress during a critical window associated with epochal brain development. Characteristics of the immature pain system in preterm neonates (such as a low pain threshold, prolonged periods of windup, overlapping receptive fields, immature descending inhibition) predisposes them to greater clinical and behavioral sequelae from inadequately treated pain than older age groups. Evidence for developmental plasticity in the neonatal brain suggests that repetitive painful experiences during this period or prolonged exposure to analgesic drugs may alter neuronal and synaptic organization permanently. Traditionally, clinicians have chosen the perspective that routine use of analgesic or sedative drugs in preterm neonates may create more problems than minimal therapy. However, the immediate and long-term consequences of inadequately treated pain have forced them to reconsider the risk-benefit ratios for such therapy. Whereas the short-term consequences of prolonged analgesic therapy in human neonates are well-known (tolerance, withdrawal, ventilator dependency), long-term consequences are relatively unknown. Advances in the study of repetitive pain associated with routine NICU care have challenged the perspective that prolonged pain and stress were inevitable consequences of premature birth.

The development of the nociceptive responses in neurokinin-1 receptor knockout mice

An important yet unanswered question is how neonates respond to painful stimuli, given the immaturity of their neural pathways. We examined the development of the neurokinin system using a novel approach, examining changes of this system by observing the pain responses of mice lacking the NK1 receptor at different stages of development We show that the NK1 receptor is not involved in nociception to heat, mechanical or chemical stimuli, at 3 days. In contrast, the NK1 receptor is involved in nociceptive responses to high intensity heat and mechanical stimuli, and mediates the second phase of the formalin response in 21-day-old mice. This indicates that nociception in neonates does not require the NK1 receptor and that the functional maturation of the NK1 receptor allows diversity in both the type of stimuli that activate the pain system and the types of responses elicited by nociceptive stimuli.

Alterations in the distribution of stimulus-evoked c-fos in the spinal cord after neonatal peripheral nerve injury in the rat

Neonatal peripheral nerve injury results in a significant rearrangement of the central terminals of surviving axotomized and adjacent intact primary afferents in the dorsal horn of the spinal cord. This study investigates the ability of these afferents to make functional contacts with dorsal horn cells, using c-fos expression as a marker of synaptic activation. Graded electrical stimulation at A- or C-fiber strength of either the neonatally axotomized sciatic nerve or the adjacent uninjured saphenous nerve was performed in adult rats. Stimulation of the contralateral uninjured nerve served as a control. Quantitative examination of the number and distribution of c-fos-labeled cells in the spinal cord laminae was performed. Electrical stimulation of the previously axotomized sciatic nerve at A-fiber intensity resulted in many labeled profiles in laminae I-V of the lumbar spinal cord on the experimental as compared to the contralateral side. Electrical stimulation of uninjured saphenous nerve or saphenous-nerve-innervated skin (using pin electrodes) at A-fiber intensity did not evoke c-fos. Stimulation of the saphenous nerve at C-fiber intensity, however, resulted in a significant increase in the number and distribution of c-fos-labeled profiles in laminae I-V on the experimental side as compared to the contralateral control side. The results show that the distribution of c-fos-expressing cells after neonatal nerve injury is compatible with the previously demonstrated distribution of sprouting of primary afferents belonging to an uninjured nerve adjacent to an injured nerve, and that the surviving axotomized afferents are capable of transmitting signals to postsynaptic cells. These findings indicate that Abeta afferent stimulation of injured but not uninjured afferents elicits c-fos expression in postsynaptic cells. This may reflect an injury-induced maintenance of a normal developmental process whereby Abeta stimulation elicits c-fos in dorsal horn neurons.

Suckling- and sucrose-induced analgesia in human newborns

This experiment had three goals: 1. To identify the basis of sucking-induced analgesia in healthy, term, newborn humans undergoing the painful, routine, procedure of heel lance and blood collection. 2. To evaluate how taste-induced and sucking-induced analgesias combine to combat pain. 3. To determine whether facial grimacing was an accurate index of diminished pain, or whether it was linked to tissue trauma. We report that: 1. Sucking an unflavored pacifier was analgesic when and only when suck rate exceeded 30 sucks/min. 2. The combination of sucrose and nonnutritive sucking was remarkably analgesic; we saw no behavioral indication in nine of the ten infants that the heel lance had even occurred. 3. Grimacing was reduced to almost naught by procedures that essentially eliminated crying and markedly reduced heart rate during the blood harvesting procedure.

Modern concepts of paediatric analgesia

Laboratory data, economic pressures, and the wish for humane treatment have been some of the driving forces behind improvements in paediatric pain management. Within the space of 10 years, there have been dramatic changes in the quality of treatment received by children undergoing surgical operations. Moreover, those receiving medical treatment, for example, sickle cell disease, have also benefited from increased experience in pain management. Children receiving care in specialised centres can now expect to benefit from up-to-date techniques of pain management, such as patient-controlled analgesia, nurse-controlled analgesia, and epidural infusions. They will be managed by ward nurses experienced and trained in paediatric pain relief, they will be attended by nurses whose special interest and training is the management of children's pain, and they will be provided with the techniques of analgesia by competent, trained anaesthetic staff. Improved care, with close attention to pain relief, is not only humane, but improves the patient turnaround by enhancing rapid discharge. Further education is required to spread these benefits to children being managed outside highly specialised centres. Not only education, but investment, is needed also to ensure that all children receive a standard of care second to none.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

The formalin test: a dose-response analysis at three developmental stages

We investigated the behavioral response of rat pups to intraplantar injection of varying formalin concentrations using a time-sampling method. At 3 days of age, the response was monophasic and persisted for the whole hour, even at low formalin concentrations. Flexion, shaking and licking the injected limb and hind-limb kicking correlated strongly with log formalin concentration (r = 0.82); behavioral state was altered only at the highest concentration. The response on day 15 was also monophasic, but it waned in 30 min, even at the highest formalin concentration tested. Flexion, shaking and licking of the injected limb were strong pain measures (r = 0.83). The response at 25 days was biphasic, and the adult measures, paw lifting and licking, produced a good formalin concentration-effect relationship (r = 0.80). The log concentration-effect relationships for formalin at the three developmental stages and for adult rats were parallel, but between 3 days and 15 days of age, the relationship shifted to the right by 2.5-fold, and by a further 4-fold between 15 and 25 days, when the sensitivity to formalin-induced pain was similar to that in adults. The data describe efficient, quantitative measures of formalin-induced pain for developing rats, show that the pain response is log-linearly related to formalin concentration throughout development, and demonstrate that the sensitivity to formalin-induced pain is about 10-fold higher in neonatal rats than in weanlings. the data imply that there are major qualitative changes in pain processing as the nervous system develops.

Maturation of the biphasic behavioral and heart rate response in the formalin test

The biological processes that mediate and modulate the perception of pain in the infant animal are not well studied and thus nociception during early development is poorly understood. In the adult animal, injection of formalin into the hind paw produces distinct phases of behavioral and autonomic responses: an early nociceptive response followed by a period of quiescence and a later second phase that matches or exceeds the initial response. The delayed reaction of the second phase has been suggested to be a model of inflammation-induced changes in neuronal sensitivity. Studies in the infant rat have demonstrated that the first phase is present in the fetus and neonate but the onset of the second phase is later maturing. We report here that the first phase occurs in 7- to 35-day-old pups in the formalin test when measured behaviorally and in 14- to 35-day-old pups when assessed by increased heart rate. However, the behavioral response in second phase is greatly attenuated or absent in 7- or 14-day-old pups, a finding consistent with that of others, appearing first at 21 days of age. The biphasic tachycardic response was not noted until even later, at 35 days of age. These data confirm that the neural mechanisms that mediate the secondary behavioral phase in the formalin test are late maturing, that the biphasic cardiovascular response does not occur until substantially later, after weaning, and that the behavioral and cardiovascular responses are dissociated developmentally.

Ontogeny of defensive behavior and analgesia in rat pups exposed to an adult male rat

Aversive situations may reduce nociception. The mechanism underlying such analgesia has been suggested to involve the interaction between the two separate but interconnected motivational systems "defense" and "pain." To determine the developmental course of defense and nociception, these processes were analyzed during early ontogeny in rats. To elicit a defensive reaction, a huddle of preweanling rat pups was exposed to an unfamiliar, unrelated adult male, or, for comparison, to the mother. On postnatal Day 7 the pups did not show a behavioral reaction to the presence of the mother or the male, and no reduction in nociceptive threshold in a thermal paw withdrawal test. On Day 14, pups in the presence of the male stopped ongoing behaviors and became immobile, and showed reduced paw withdrawal after the exposure. At Day 21, 22 pups of 32 became immobile when exposed to the male, whereas 10 pups explored the partition separating them from the male. Neither group showed reduced paw withdrawal. Immobility was considered a defensive reaction because it reduces auditory and visual cues and therefore the probability of being detected. The developmental course of immobility seems to reflect both the changes in threat imposed on the pups by a potentially infanticidal male and the ability of pups to react to that threat. The reduction in paw withdrawal that followed male exposure indicates an inhibitory mechanism. It is discussed whether the activation of the defense system results in an inhibition of nociception.

Suckling and sucrose ingestion suppress persistent hyperalgesia and spinal Fos expression after forepaw inflammation in infant rats

Sweet taste and nonnutritive suckling produce analgesia to transient noxious stimuli in infant rats and humans. The present study evaluated the pain-modulating effects of sucrose and suckling in a rat model of persistent pain and hyperalgesia that mimics the response to tissue injury in humans. Fore- and hindpaw withdrawal latencies from a 30 degrees or 48 degrees C brass stylus were determined in 10-day-old rats following paw inflammation induced by complete Freund's adjuvant (CFA; 1:1 injected s.c. in a 0.01 ml volume). CFA markedly decreased escape latencies to both 48 degrees and 30 degrees C stimulation, thereby demonstrating thermal hyperalgesia and mechanical allodynia. The combination of nonnutritive suckling and sucrose (7.5%, 0.01-0.06 ml/min) infusion markedly increased escape latencies to forepaw stimulation in both CFA-treated and control rats. In contrast, intraoral sucrose and suckling did not increase hindpaw withdrawal latencies in either control or CFA-inflamed rats. The effect was specific to sweet taste because neither water nor isotonic saline infusion affected forepaw escape latencies. Parallel findings were obtained for CFA-induced Fos-like immunoreactivity (Fos-LI), a marker of neuronal activation. Fos-LI was selectively induced in cervical and lumbar regions ipsilateral to forepaw and hindpaw inflammation, respectively. Suckling-sucrose treatment significantly reduced Fos-LI at the cervical but not at the lumbar regions. These findings demonstrate: (i) the development of persistent pain and hyperalgesia in 10-day-old rats that can be attenuated by endogenous pain-modulating systems activated by taste and nonnutritive suckling; (ii) the mediation of the sucrose-suckling analgesia and antihyperalgesia at the spinal level; and (iii) a differential rostrocaudal maturation of descending pain-modulating systems to the spinal cord of 10-day-old rats. These findings may provide new clinical approaches for engaging endogenous analgesic mechanisms in infants following tissue injury and inflammation.

C-fos can be induced in the neonatal rat spinal cord by both noxious and innocuous peripheral stimulation

The development of spinal cord nociceptive pathways has been investigated in neonatal rat pups using the expression of Fos immunoreactivity in laminae I and II cells produced by high and low intensity skin stimulation. Noxious pinch of the hindpaw evoked a clear response in the newborn rat pup, which was not significantly different from that seen at postnatal day (P) 21. Low intensity touch stimulation also produced a significant fos response in laminae I and II cells at P3 which was 60% that of the pinch response. This was reduced to 27% of the pinch response by P10 and was gone by P21. Electrical stimulation through percutaneous electrodes showed that A beta fibre stimulation also produced a fos response at P3 that was not significantly different from that produced by C fibre stimulation. By P21 and P30 the response to C fibre stimulation was much greater and the response to A fibre stimulation was not significantly above background. The results suggest that in the neonatal spinal cord, low threshold A fibres are able to activate pathways in lamina I and II of the dorsal horn that in the adult are predominantly nociceptive.

The suppression of formalin-induced fos expression by different anesthetic agents in the infant rat

Despite the importance of pediatric anesthesiology, the sites and mechanisms of anesthetic action in the neonate are not well described in either human or nonhuman species. This experiment investigated suppression produced by different anesthetic agents of neuronal activity in the lumbar spinal cord of the 3-day-old rat. The expression of the c-fos immediate early gene following formalin injection into the hindpaw was used as a marker for neuronal activity. Pups were anesthetized by one of the following often-used agents: methoxyflurane, acepromazine, a mixture of ketamine and xylazine, and hypothermia. All treatments induced behavioral anesthesia. Despite the behavioral anesthesia, the ketamine-xylazine mixture was completely ineffective in suppressing formalin-induced-Fos expression. In contrast, methoxyflurane and hypothermia blocked the appearance of the Fos protein. Similarly, acepromazine was effective in eliminating some of the Fos-labeled nuclei. These data suggest that, in the infant rat, both hypothermia and methoxyflurane act in part at the spinal level by depressing either primary afferents or dorsal horn neuronal activity whereas the site of action for ketamine-xylazine may be located supraspinally.