Characterization of basal and re-inflammation-associated long-term alteration in pain responsivity following short-lasting neonatal local inflammatory insult

Treatment options for chronic abdominal pain in children and adolescents

Chronic abdominal pain is a common feature of most functional gastrointestinal disorders in children, including functional abdominal pain (FAP) and irritable bowel syndrome (IBS). FAP can impair a child's life and often leads to significant school absences. Although the underlying mechanism is likely multifactorial, early pain experiences during a vulnerable period in the developing nervous system can cause long-term changes in the brain-gut axis and ultimately may result in altered pain pathways and visceral hyperalgesia. Care providers often feel uncomfortable managing patients with chronic abdominal pain, as the pathophysiology is poorly understood, and limited data exist regarding safety and efficacy of therapeutic options in children. The primary goal of therapy in FAP is to alleviate pain symptoms and to help the child return to normal daily activities. Treatment should be individualized and chosen based on the severity of symptoms, the existence of comorbid psychological disorders, and the impact the disorder has on the child's school attendance and normal functioning. Various psychological interventions, such as cognitive-behavioral therapy, hypnosis, and guided imagery, have been successfully used in children with chronic abdominal pain. Pharmacologic therapies such as H(2) blockers, proton-pump inhibitors, tricyclic antidepressants, and various serotonergic drugs have been used, but good controlled trials are lacking. More studies are clearly needed to investigate the benefits and safety of pharmacologic therapy in children. Newer pharmacologic agents that target specific receptors involved in nociception, stress, and neurogenic inflammation currently are being developed. Future targets for visceral hyperalgesia should not only be aimed at alleviating symptoms but also should include prevention, particularly in cases with a suspected sensitizing event such as neonatal pain and postinfectious IBS.

Expression of neurotrophic factors in neonatal rats after peripheral inflammation

Neonatal peripheral inflammatory insult might result in the alteration of neuronal development in the nociceptive circuit. During early postnatal period, neurotrophins play important roles in neural development and sensory nerve innervation in the central and peripheral nervous systems. In this study, we investigated mRNA expression for neurotrophic factors and their receptors in the dorsal root ganglia of rat pups during postnatal life after peripheral inflammation induced by injection of complete Freund's adjuvant (CFA) into hind paw on postnatal day 1. Our results showed that mRNA expression levels of alpha-calcitonin gene-related peptides, tropomyosin-related kinase-A (trkA), p75 neurotrophin receptor (p75(NTR)), and brain-derived neurotrophic factor (BDNF) elevated significantly after CFA treatment. Such an increase began 1 day after CFA treatment and lasted 2 to 3 days for trkA, p75(NTR), and BDNF. In contrast, there was no change in mRNA expression levels for neurotrophin-4/5, beta-nerve growth factor (beta-NGF), trkB, glial cell line-derived neurotrophin factor, and receptor protein tyrosine kinase protein. Our study demonstrated that neonatal peripheral inflammatory insult might result in molecular changes of neurotrophic factors, particularly in NGF receptors and BDNF, in the process of neuronal development and plasticity in primary afferents during early neonatal period.

PERSPECTIVE

Neonatal peripheral inflammation model has been used for the exploration of neuropathic pain mechanism for years. This work provided further detailed information about possible neurotransmitters and peptides involved in this process. This might also lead to future clinical application.

Neonatal nociceptive somatic stimulation differentially modifies the activity of spinal neurons in rats and results in altered somatic and visceral sensation

The role ofintramuscular, low pH saline injections during the neonatal period in the development and maintenance of visceral hyperalgesia has not been systematically studied. We aimed to investigate alterations in visceral sensation and neural circuitry that result from noxious stimuli in early life. Neonatal male Sprague-Dawley rats received sterile saline injections of pH 4.0 or 7.4 in the gastrocnemius muscle starting at postnatal day 8. Injections were given unilaterally every other day for 12 days ending on postnatal day 20. A third group received needle prick only on the same shedule as the second group, while a fourth group was left naïve. At 2 months of age, rats underwent assessment of cutaneous and deep somatic sensitivity using von Frey filaments and gastrocnemius muscle pinch, respectively. A visceromotor response (VMR) to graded colorectal distension (CRD; 10-80 mmHg for 30 s with 180 s interstimulus intervals) was recorded. Extracellular single-unit recordings from the thoracolumbar spinal neurons (T13-L1) were performed in adult pH 4.0 injected and naïve controls. There was no difference in the threshold for response to mechanical stimulation of the paw in rats injected with pH 4.0 saline compared to all other groups. Conversely, rats treated with pH 4.0 saline showed a significant bilateral reduction in withdrawal threshold to muscle pinch as adults (P < 0.05). At colorectal distensions > or = 20 mmHg, an increase in the VMR was observed in the pH 4.0 injected group compared to all other groups (P < 0.05). Spinal neurons were classified as short latency abrupt (SL-A) or short latency sustained (SL-S). Spontaneous firing of SL-S (20.6 +/- 2.2 impulses s(-1)), but not SL-A neurons (5.3 +/- 0.9 impulses s(-1)) in the pH 4.0 treated rats was significantly higher than in control rats (SL-S, 2.6 +/- 0.8 impulses s(-1); SL-A, 3.1 +/- 0.7 impulses s(-1)). The response of SL-S neurons to CRD in the pH 4.0 group was significantly higher at distension pressures > or = 20 mmHg. Nociceptive somatic stimulation in neonatal rats results in chronic deep somatic and visceral hyperalgesia in adulthood. Colorectal distension-sensitive SL-S neurons are primarily sensitized to neonatal somatic stimulation.

Steroid receptor antagonist mifepristone inhibits the anti-inflammatory effects of photoradiation

OBJECTIVE

We designed an animal pleurisy study to assess if the anti-inflammatory effect of photoradiation could be affected by concomitant use of the cortisol antagonist mifepristone.

BACKGROUND DATA

Although interactions between photoradiation and pharmacological agents are largely unknown, parallel use of steroids and photoradiation is common in the treatment of inflammatory disorders such as arthritis and tendinitis.

METHODS

Forty BALB/c male mice were randomly divided in five groups. Inflammation was induced by carrageenan administered by intrathoracic injections. Four groups received carrageenan, and one control group received injections of sterile saline solution. At 1, 2, and 3 h after injections, photoradiation irradiation was performed with a dose of 7.5 J/cm(2). Two of the carrageenan-injected groups were pre-treated with orally administered mifepristone.

RESULTS

Total leukocyte cell counts revealed that in carrageenan-induced pleurisy, photoradiation significantly reduced the number of leukocyte cells (p < 0.0001, mean 34.5 [95% CI: 32.8-36.2] versus 87.7 [95% CI: 81.0-94.4]), and that the effect of photoradiation could be totally blocked by adding the cortisol antagonist mifepristone (p < 0.0001, mean 34.5 [95% CI: 32.1-36.9] versus 82.9 [95%CI: 70.5-95.3]).

CONCLUSION

The steroid receptor antagonist mifepristone significantly inhibited the anti-inflammatory effect of photoradiation. Commonly used glucocorticoids are also known to down-regulate steroid receptors, and further clinical studies are necessary to elucidate how this interaction may decrease the effect size of photoradiation over time. For this reason, we also suggest that, until further clinical data can be provided, clinical photoradiation trials should exclude patients who have received steroid therapy within 6 months before recruitment.

Long-term consequences of pain in human neonates

The low tactile threshold in preterm infants when they are in the neonatal intensive care unit (NICU), while their physiological systems are unstable and immature, potentially renders them more vulnerable to the effects of repeated invasive procedures. There is a small but growing literature on pain and tactile responsivity following procedural pain in the NICU, or early surgery. Long-term effects of repeated pain in the neonatal period on neurodevelopment await further research. However, there are multiple sources of stress in the NICU, which contribute to inducing high overall 'allostatic load', therefore determining specific effects of neonatal pain in human infants is challenging.

Early postnatal chronic inflammation produces long-term changes in pain behavior and N-methyl-D-aspartate receptor subtype gene expression in the central nervous system of adult mice

The objective of this study was to test whether postnatal chronic inflammation resulted in altered reactivity to pain later in life when reexposed to the same inflammatory agent and whether this alteration correlated with brain-region-specific patterns of N-methyl-D-aspartate (NMDA) receptor subtype gene expression. Neonatal mouse pups received a single injection of complete Freund's adjuvant (CFA) or saline into the left hind paw on postnatal day 1 or 14. At 12 weeks of age, both neonatal CFA- and saline-treated animals received a unilateral injection of CFA in the left hind paw. Adult behavioral responsiveness of the left paw to a radiant heat source was determined in mice treated neonatally with saline or CFA before and after receiving CFA as adults. Twenty-four hours later, brains were dissected and NMDA receptor subunit gene expression was determined in four different brain areas by using an RNase protection assay. The results indicated that NMDA receptor subtype gene expression in adult mice exposed to persistent neonatal peripheral inflammation was brain region specific and that NMDA gene expression and pain reactivity differed according to the day of neonatal CFA exposure. Similarly, adult behavioral responsiveness to a noxious radiant heat source differed according to the age of neonatal exposure to CFA. The data suggest a possible molecular basis for the hypothesis that chronic persistent inflammation experienced early during development may permanently alter the future behavior and the sensitivity to pain later in life, especially in response to subsequent or recurrent inflammatory events.

Neonatal immune challenge alters nociception in the adult rat

Intense pain or intense peripheral inflammation experienced during development can have pronounced effects upon adult pain sensation. However, little is known about the more commonly encountered mild systemic inflammation, such as that experienced with mild illness. Neonatal exposure to lipopolysaccharide (LPS), an established model of immune system activation, has been shown to affect febrile and cyclooxygenase-2 (COX-2) responses to a similar exposure in adulthood. Adult LPS also elicits a range of sickness behaviours, including enhanced responses to painful stimuli. We, therefore, hypothesized that adult sensation and pain responses could be affected by a neonatal LPS challenge. Male and female Sprague-Dawley rats were administered LPS at postnatal day 14 and were tested in adulthood for nociceptive responses to thermal and mechanical stimuli using, respectively, a plantar test apparatus and von Frey filaments, before and after adult LPS. Expression of dorsal root ganglion and lumbar spinal cord COX-2 was also examined. Animals treated as neonates with saline showed the expected hypersensitivity to painful stimuli after adult LPS as well as enhanced spinal cord COX-2. Neonatally LPS-treated rats, however, showed a significantly different profile. They displayed enhanced baseline nociception and elevated basal spinal cord COX-2 compared with neonatally saline-treated rats. Also, rather than the expected hyperalgesia after adult LPS, no changes in nociceptive responses and a reduction in spinal cord COX-2 expression were observed. These findings have important implications for the understanding of pain and its management and highlight the importance of the neonatal period in the development of pain pathways.

Spontaneous effects of low-level laser therapy (650 nm) in acute inflammatory mouse pleurisy induced by carrageenan

OBJECTIVE

Our aim was to investigate the effect of low-level laser therapy (LLLT), 650-nm wavelength, on acute inflammatory pleurisy.

BACKGROUND DATA

There is only scattered evidence of anti-inflammatory effects from LLLT and dosage characteristics, and the effect on pleurisy inflammation has yet to be investigated.

METHODS

A classical experimental model of pleurisy was used in a sample of 40 Balb male mice, randomly divided into five groups. Inflammation was induced by carrageenan (0.5 mg/cavity) administered by intrathoracic injections. Four groups received the inflammatory agent, and one received injections of sterile saline solution. At 1, 2, and 3 h after injections, LLLT irradiation was performed, with the same power (2.5 mW), but different irradiation times. The energy densities at each of the three treatment sessions were 0 J/cm(2) (placebo), 3 J/cm(2), 7.5 J/cm(2), and 15 J/cm(2), respectively.

RESULTS

Total and differential cell analysis at 4 h after induction of pleurisy showed a significant reduction of inflammatory cell migration for all groups treated with active laser. However, at 4 h after injection, the most significant (p < 0.001) reduction of leukocyte cell migration was seen in the 7.5 J/cm(2) group, at 2.7 (95% CI: 2.5-2.9) x 10(6), versus 7.9 (95% CI: 6.7-9.1) x 10(6) in the placebo control group. The greatest reduction of inflammatory cells was registered for neutrophils.

CONCLUSIONS

LLLT administered at 1-3 h after the induction of inflammatory pleurisy significantly reduces the inflammatory cell migration measured. Under these conditions and at 2.5 mW, 7.5 J/cm(2) was more effective than 3 J/cm(2) and 15 J/cm(2).

Neonatal local noxious insult affects gene expression in the spinal dorsal horn of adult rats

Neonatal noxious insult produces a long-term effect on pain processing in adults. Rats subjected to carrageenan (CAR) injection in one hindpaw within the sensitive period develop bilateral hypoalgesia as adults. In the same rats, inflammation of the hindpaw, which was the site of the neonatal injury, induces a localized enhanced hyperalgesia limited to this paw. To gain an insight into the long-term molecular changes involved in the above-described long-term nociceptive effects of neonatal noxious insult at the spinal level, we performed DNA microarray analysis (using microarrays containing oligo-probes for 205 genes encoding receptors and transporters for glutamate, GABA, and amine neurotransmitters, precursors and receptors for neuropeptides, and neurotrophins, cytokines and their receptors) to compare gene expression profiles in the lumbar spinal dorsal horn (LDH) of adult (P60) male rats that received neonatal CAR treatment within (at postnatal day 3; P3) and outside (at postnatal 12; P12) of the sensitive period. The data were obtained both without inflammation (at baseline) and during complete Freund's adjuvant induced inflammation of the neonatally injured paw. The observed changes were verified by real-time RT-PCR. This study revealed significant basal and inflammation-associated aberrations in the expression of multiple genes in the LDH of adult animals receiving CAR injection at P3 as compared to their expression levels in the LDH of animals receiving either no injections or CAR injection at P12. In particular, at baseline, twelve genes (representing GABA, serotonin, adenosine, neuropeptide Y, cholecystokinin, opioid, tachykinin and interleukin systems) were up-regulated in the bilateral LDH of the former animals. The baseline condition in these animals was also characterized by up-regulation of seven genes (encoding members of GABA, cholecystokinin, histamine, serotonin, and neurotensin systems) in the LDH ipsilateral to the neonatally-injured paw. The largest aberration in gene expression, however, was observed during inflammation of the neonatally injured hindpaws in the ipsilateral LDH, which included thirty-six genes (encoding numerous members of glutamate, serotonin, GABA, calcitonin gene-related peptide, neurotrophin, and interleukin systems). These findings suggest that changes in gene expression may be involved in the long-term nociceptive effects of neonatal noxious insult at the spinal level.

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.

Alterations in stress-associated behaviors and neurochemical markers in adult rats after neonatal short-lasting local inflammatory insult

Recently, there has been a growing interest in long-term consequences of neonatal pain because modern neonatal intensive care units routinely employ procedures that cause considerable pain and may be followed by local inflammation and hyperalgesia lasting for several hours or even days. To address this question, we developed a rat model of short lasting (<2 days) early local inflammatory insult produced by a single injection of 0.25% carrageenan (CAR) into the plantar surface of a hindpaw. Previously, we demonstrated that rats receiving this treatment within the first week after birth grow into adults with a global reduction in responsiveness to acute pain. Here, we report that these animals also manifest a low anxiety trait associated with reduced emotional responsiveness to stress. This conclusion is based in the following observations: (a) rats in our model display reduced anxiety on an elevated plus-maze; (b) in the forced swim test, these rats exhibit behavioral characteristics associated with stronger ability for stress coping; and (c) these animals have reduced basal and stress-induced plasma levels of such stress-related neuroendocrine markers as corticotropin-releasing factor, vasopressin, and adrenocorticotrophic hormone. In addition, we used DNA microarray and real-time reverse-transcriptase polymerase chain reaction to profile long-term changes in gene expression in the midbrain periaqueductal gray (PAG; a region involved in both stress and pain modulation) in our animal model. Among the affected genes, serotonergic receptors were particularly well represented. Specifically, we detected increase in the expression of 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C and 5-HT4 receptors. Several of these receptors are known to be involved in the anxiolytic and analgesic activity of the PAG. Finally, to determine whether neonatal inflammatory insult induces elevation in maternal care, which may play a role in generating long-term behavioral alterations seen in our model, we examined maternal behavior for 3 days following CAR injection. Indeed, we observed a substantial increase in maternal attention to the pups at the time of inflammation, but this increase was not without its cost: a period of significant maternal neglect afterward.

Neonatal procedural pain exposure predicts lower cortisol and behavioral reactivity in preterm infants in the NICU

Data from animal models indicate that neonatal stress or pain can permanently alter subsequent behavioral and/or physiological reactivity to stressors. However, cumulative effects of pain related to acute procedures in the neonatal intensive care unit (NICU) on later stress and/or pain reactivity has received limited attention. The objective of this study is to examine relationships between prior neonatal pain exposure (number of skin breaking procedures), and subsequent stress and pain reactivity in preterm infants in the NICU. Eighty-seven preterm infants were studied at 32 (+/-1 week) postconceptional age (PCA). Infants who received analgesia or sedation in the 72 h prior to each study, or any postnatal dexamethasone, were excluded. Outcomes were infant responses to two different stressors studied on separate days in a repeated measures randomized crossover design: (1) plasma cortisol to stress of a fixed series of nursing procedures; (2) behavioral (Neonatal Facial Coding System; NFCS) and cardiac reactivity to pain of blood collection. Among infants born <or=28 weeks gestational age (GA), but not 29-32 weeks GA, higher cumulative neonatal procedural pain exposure was related to lower cortisol response to stress and to lower facial (but not autonomic) reactivity to pain, at 32 weeks PCA, independent of early illness severity and morphine exposure since birth. Repeated neonatal procedural pain exposure among neurodevelopmentally immature preterm infants was associated with down-regulation of the hypothalamic-pituitary-adrenal axis, which was not counteracted with morphine. Differential effects of early pain on development of behavioral, physiologic and hormonal systems warrant further investigation.

Long-term effects of neonatal surgery on adulthood pain behavior

The long-term consequences of neonatal noxious stimulation on adulthood pain behavior were investigated in male and female mice. On the day of birth, mouse pups were exposed to a laparotomy under cold anesthesia followed by an analgesic dose of morphine (10 mg/kg) post-operatively, or a saline control. An additional group of subjects was exposed to the non-noxious aspects of the surgical procedure (cold exposure, separation from the dam, injection) comprising a 'sham' surgery control group, whereas another group of control subjects was administered an injection of saline or morphine, but was otherwise undisturbed. Behavioral observations of the pups immediately following the procedure indicated that the laparotomy produced increased distress vocalizations in the ultrasonic range (40 kHz) compared to both groups of control subjects. During 90 min observations periods following the surgery and 1-week later, maternal care did not vary among treatment conditions. In adulthood, offspring were tested for nociceptive sensitivity on the hot-plate (HP; 53 degrees C), tail-withdrawal (TW; 50 degrees C) and acetic acid abdominal constriction test (AC). On both the TW and the AC tests, neonatal surgery decreased pain behavior relative to both groups of control subjects, an effect that was reversed by post-operative morphine treatment. On the HP test, both groups of subjects exposed to the stressful aspects of neonatal surgery (laparotomy or sham surgery) exhibited decreased pain behavior in adulthood. These findings suggest that early exposure to noxious and/or stressful stimuli may induce long-lasting changes in pain behavior, perhaps mediated by alterations in the stress-axis and antinociceptive circuitry.

Neonatal hind paw injury alters processing of visceral and somatic nociceptive stimuli in the adult rat

Tissue damage during the first few weeks after birth can have profound effects on sensory processing in the adult. We have recently reported that a short-lasting inflammation of the neonatal rat hind paw produces baseline hypoalgesia and exacerbated hyperalgesia after reinflammation of that hind paw in the adult. Because the contralateral hind paw and forepaws also displayed hypoalgesia, we speculated that effects of the initial injury were not somatotopically restricted and would alter visceral sensory processing as well. In the present study we tested this hypothesis by examining the effects of neonatal hind paw injury at P3 or P14 on visceral and somatic sensitivity in the adult rat. In P3 rats, the visceromotor response evoked by colorectal distention in the absence of colonic inflammation was attenuated in carrageenan-treated neonatal rats compared to naive rats. Colonic inflammation in the adult reversed this hypoalgesia and evoked a level of visceral hyperalgesia similar to naive rats. There were no consequences of the P14 injury observed in the adult. In a second experiment, colonic inflammation in naive rats induced viscerosomatic inhibition to thermal stimulation of the forepaw and hind paw. This inhibition was reversed, and the paw withdrawal latency was slightly decreased in neonatal (P3) carrageenan-treated rats. Rats treated on P14 appeared similar to naive rats. These data support the hypothesis that neonatal hind paw injury during a critical period permanently alters sensory processing of multiple sensory modalities in the adult. Animals develop with greater inhibitory processing of somatic and visceral stimuli throughout the neuraxis. However, inflammation in the adult in previously uninjured tissue reverses the hypoalgesia and evokes development of normal hyperexcitability associated with tissue injury.

PERSPECTIVE

Trauma experienced by premature infants can lead to alterations in sensory processing throughout life. This study shows that short-term somatic tissue injury to neonatal rats during a well-defined critical period alters several aspects of viscerosensory processing in the adult, demonstrating that injury to one tissue affects sensory processing throughout the body.