A brainstem substrate for analgesia elicited by intraoral sucrose

Descending GABAergic pathway links brain sugar-sensing to peripheral nociceptive gating in Drosophila

Although painful stimuli elicit defensive responses including escape behavior for survival, starved animals often prioritize feeding over escape even in a noxious environment. This behavioral priority is typically mediated by suppression of noxious inputs through descending control in the brain, yet underlying molecular and cellular mechanisms are incompletely understood. Here we identify a cluster of GABAergic neurons in Drosophila larval brain, designated as SEZ-localized Descending GABAergic neurons (SDGs), that project descending axons onto the axon terminals of the peripheral nociceptive neurons and prevent presynaptic activity through GABAB receptors. Remarkably, glucose feeding to starved larvae causes sustained activation of SDGs through glucose-sensing neurons and subsequent insulin signaling in SDGs, which attenuates nociception and thereby suppresses escape behavior in response to multiple noxious stimuli. These findings illustrate a neural mechanism by which sugar sensing neurons in the brain engages descending GABAergic neurons in nociceptive gating to achieve hierarchical interaction between feeding and escape behavior.

Sucrose analgesia for heel-lance procedures in neonates

BACKGROUND

Sucrose has been examined for calming and pain-relieving effects in neonates for invasive procedures such as heel lance.

OBJECTIVES

To assess the effectiveness of sucrose for relieving pain from heel lance in neonates in terms of immediate and long-term outcomes SEARCH METHODS: We searched (February 2022): CENTRAL, MEDLINE, Embase, PsycINFO, CINAHL, Web of Science, and three trial registries.

SELECTION CRITERIA

We included randomised controlled trials where term and/or preterm neonates received sucrose for heel lances. Comparison treatments included water/placebo/no intervention, non-nutritive sucking (NNS), glucose, breastfeeding, breast milk, music, acupuncture, facilitated tucking, and skin-to-skin care.

DATA COLLECTION AND ANALYSIS

We used standard Cochrane methods. We reported mean differences (MD) with 95% confidence intervals (CI) using the fixed-effect model for continuous outcome measures. We assessed heterogeneity by the I2 test. We used GRADE to assess certainty of evidence.

MAIN RESULTS

We included 55 trials (6273 infants): 29 included term neonates, 22 included preterm neonates, and four included both. Heel lance was investigated in 50 trials; 15 investigated other minor painful procedures in addition to lancing. Sucrose vs control The evidence suggests that sucrose probably results in a reduction in PIPP scores compared to the control group at 30 seconds (MD -1.74 (95% CI -2.11 to -1.37); I2 = 62%; moderate-certainty evidence) and 60 seconds after lancing (MD -2.14, 95% CI -3.34 to -0.94; I2 = 0%; moderate-certainty evidence). The evidence is very uncertain about the effects of sucrose on DAN scores compared to water at 30 seconds after lancing (MD -1.90, 95% CI -8.58 to 4.78; heterogeneity not applicable (N/A); very low-certainty evidence). The evidence suggests that sucrose probably results in a reduction in NIPS scores compared to water immediately after lancing (MD -2.00, 95% CI -2.42 to -1.58; heterogeneity N/A; moderate-certainty evidence). Sucrose vs NNS The evidence is very uncertain about the effect of sucrose on PIPP scores compared to NNS during the recovery period after lancing (MD 0.60, 95% CI -0.30 to 1.50; heterogeneity not applicable; very low-certainty evidence) and on DAN scores at 30 seconds after lancing (MD -1.20, 95% CI -7.87 to 5.47; heterogeneity N/A; very low-certainty evidence). Sucrose + NNS vs NNS The evidence is very uncertain about the effect of sucrose + NNS on PIPP scores compared to NNS during lancing (MD -4.90, 95% CI -5.73 to -4.07; heterogeneity not applicable; very low-certainty evidence) and during recovery after lancing (MD -3.80, 95% CI -4.47 to -3.13; heterogeneity N/A; very low-certainty evidence). The evidence is very uncertain about the effects of sucrose + NNS on NFCS scores compared to water + NNS during lancing (MD -0.60, 95% CI -1.47 to 0.27; heterogeneity N/A; very low-certainty evidence). Sucrose vs glucose The evidence suggests that sucrose results in little to no difference in PIPP scores compared to glucose at 30 seconds (MD 0.26, 95% CI -0.70 to 1.22; heterogeneity not applicable; low-certainty evidence) and 60 seconds after lancing (MD -0.02, 95% CI -0.79 to 0.75; heterogeneity N/A; low-certainty evidence). Sucrose vs breastfeeding The evidence is very uncertain about the effect of sucrose on PIPP scores compared to breastfeeding at 30 seconds after lancing (MD -0.70, 95% CI -0.49 to 1.88; I2 = 94%; very low-certainty evidence). The evidence is very uncertain about the effect of sucrose on COMFORTneo scores compared to breastfeeding after lancing (MD -2.60, 95% CI -3.06 to -2.14; heterogeneity N/A; very low-certainty evidence). Sucrose vs expressed breast milk The evidence suggests that sucrose may result in little to no difference in PIPP-R scores compared to expressed breast milk during (MD 0.3, 95% CI -0.24 to 0.84; heterogeneity not applicable; low-certainty evidence) and at 30 seconds after lancing (MD 0.3, 95% CI -0.11 to 0.71; heterogeneity N/A; low-certainty evidence). The evidence suggests that sucrose probably may result in slightly increased PIPP-R scores compared to expressed breast milk 60 seconds after lancing (MD 1.10, 95% CI 0.34 to 1.86; heterogeneity N/A; low-certainty evidence). The evidence is very uncertain about the effect of sucrose on DAN scores compared to expressed breast milk 30 seconds after lancing (MD -1.80, 95% CI -8.47 to 4.87; heterogeneity N/A; very low-certainty evidence). Sucrose vs laser acupuncture There was no difference in PIPP-R scores between sucrose and music groups; however, data were reported as medians and IQRs. The evidence is very uncertain about the effect of sucrose on NIPS scores compared to laser acupuncture during lancing (MD -0.86, 95% CI -1.43 to -0.29; heterogeneity N/A; very low-certainty evidence). Sucrose vs facilitated tucking The evidence is very uncertain about the effect of sucrose on total BPSN scores compared to facilitated tucking during lancing (MD -2.27, 95% CI -4.66 to 0.12; heterogeneity N/A; very low-certainty evidence) and during recovery after lancing (MD -0.31, 95% CI -1.72 to 1.10; heterogeneity N/A; very low-certainty evidence). Sucrose vs skin-to-skin + water (repeated lancing) The evidence suggests that sucrose results in little to no difference in PIPP scores compared to skin-to-skin + water at 30 seconds after 1st (MD 0.13, 95% CI -0.70 to 0.96); 2nd (MD -0.56, 95% CI -1.57 to 0.45); or 3rd lancing (MD-0.15, 95% CI -1.26 to 0.96); heterogeneity N/A, low-certainty evidence for all comparisons. The evidence suggests that sucrose results in little to no difference in PIPP scores compared to skin-to-skin + water at 60 seconds after 1st (MD -0.61, 95% CI -1.55 to 0.33); 2nd (MD -0.12, 95% CI -0.99 to 0.75); or 3rd lancing (MD-0.40, 95% CI -1.48 to 0.68); heterogeneity N/A, low-certainty evidence for all comparisons. Minor adverse events required no intervention.

AUTHORS' CONCLUSIONS

Sucrose compared to control probably results in a reduction of PIPP scores 30 and 60 seconds after single heel lances (moderate-certainty evidence). Evidence is very uncertain about the effect of sucrose compared to NNS, breastfeeding, laser acupuncture, facilitated tucking, and the effect of sucrose + NNS compared to NNS in reducing pain. Sucrose compared to glucose, expressed breast milk, and skin-to-skin care shows little to no difference in pain scores. Sucrose combined with other nonpharmacologic interventions should be used with caution, given the uncertainty of evidence.

Early-Life Iron Deficiency Persistently Alters Nociception in Developing Mice

Clinical association studies have identified early-life iron deficiency (ID) as a risk factor for the development of chronic pain. While preclinical studies have shown that early-life ID persistently alters neuronal function in the central nervous system, a causal relationship between early-life ID and chronic pain has yet to be established. We sought to address this gap in knowledge by characterizing pain sensitivity in developing male and female C57Bl/6 mice that were exposed to dietary ID during early life. Dietary iron was reduced by ∼90% in dams between gestational day 14 and postnatal day (P)10, with dams fed an ingredient-matched, iron-sufficient diet serving as controls. While cutaneous mechanical and thermal withdrawal thresholds were not altered during the acute ID state at P10 and P21, ID mice were more sensitive to mechanical pressure at P21 independent of sex. During adulthood, when signs of ID had resolved, mechanical and thermal thresholds were similar between early-life ID and control groups, although male and female ID mice displayed increased thermal tolerance at an aversive (45 °C) temperature. Interestingly, while adult ID mice showed decreased formalin-induced nocifensive behaviors, they showed exacerbated mechanical hypersensitivity and increased paw guarding in response to hindpaw incision in both sexes. Collectively, these results suggest that early-life ID elicits persistent changes in nociceptive processing and appears capable of priming developing pain pathways. PERSPECTIVE: This study provides novel evidence that early-life ID evokes sex-independent effects on nociception in developing mice, including an exacerbation of postsurgical pain during adulthood. These findings represent a critical first step towards the long-term goal of improving health outcomes for pain patients with a prior history of ID.

Evaluation of the effectiveness of sucrose versus xylitol application in minimizing pain related to dental injection in 5-7-year-old children - A randomized controlled pilot study

Background

Oral application of sweet-tasting solutions has found to be effective in minimizing pain on dental injection in children. Xylitol has never been tested in this regard.

Aim

The aim of this study was to compare the effectiveness of prior application of sucrose versus xylitol solution in minimizing pain on dental injection in 5-7-year-old children.

Settings and Design

This study was a randomized controlled clinical trial.

Materials and Methods

Informed consent was taken. A total of 30 children who required maxillary buccal infiltration injection were selected. The children were randomly allocated into the sucrose group, xylitol group, or control group by lottery method. In the sucrose and xylitol groups, 30% sucrose or 30% xylitol solution was applied on the lateral surface of the tongue for 2 min. In the control group, distilled water was applied. In all the groups, the topical anesthetic was applied at the site of injection followed by buccal infiltration. A video was taken during injection, and the sound eye-motor (SEM) scale was recorded by a blinded examiner. Patients were asked to self-evaluate using the Wong-Baker Faces Pain Scale (WBFPS).

Statistical Analysis

Mann-Whitney test was used to analyze the data.

Results

There was a statistically significant difference in the WBFPS and the SEM score between the sucrose and the control group (P < 0.05) and the xylitol and control group (P < 0.05). No statistically significant difference was found between the sucrose and xylitol group.

Conclusion

The application of xylitol solution before dental injection was as effective as sucrose solution in minimizing pain during the injection.

A Single Dose of Oral Sucrose Is Enough to Control Pain During Venipuncture: A Randomized Controlled Trial

Sucrose is effective in reducing pain during minor procedures in neonates. We evaluated whether a second dose of sucrose was more effective than a single dose during venipuncture. We performed a randomised, double-blind, controlled trial at the NICU of Padua Hospital (August 2016-October 2017). We randomised 72 preterm infants undergoing venipuncture for routine test to a control group, which received a single standard dose of sucrose 2′ before the procedure and a placebo 30″ after the venipuncture, and an experimental group in which they received two doses of 24% sucrose 2′ before and 30″ after the venipuncture. No difference in pain perception was found between the groups at 30″, 60″ and 120″. In conclusion, we do not recommend a second dose of sucrose during venipuncture in prematures.

Acute and Chronic Pain from Facial Skin and Oral Mucosa: Unique Neurobiology and Challenging Treatment

The oral cavity is a portal into the digestive system, which exhibits unique sensory properties. Like facial skin, the oral mucosa needs to be exquisitely sensitive and selective, in order to detect harmful toxins versus edible food. Chemosensation and somatosensation by multiple receptors, including transient receptor potential channels, are well-developed to meet these needs. In contrast to facial skin, however, the oral mucosa rarely exhibits itch responses. Like the gut, the oral cavity performs mechanical and chemical digestion. Therefore, the oral mucosa needs to be insensitive, to some degree, in order to endure noxious irritation. Persistent pain from the oral mucosa is often due to ulcers, involving both tissue injury and infection. Trigeminal nerve injury and trigeminal neuralgia produce intractable pain in the orofacial skin and the oral mucosa, through mechanisms distinct from those seen in the spinal area, which is particularly difficult to predict or treat. The diagnosis and treatment of idiopathic chronic pain, such as atypical odontalgia (idiopathic painful trigeminal neuropathy or post-traumatic trigeminal neuropathy) and burning mouth syndrome, remain especially challenging. The central integration of gustatory inputs might modulate chronic oral and facial pain. A lack of pain in chronic inflammation inside the oral cavity, such as chronic periodontitis, involves the specialized functioning of oral bacteria. A more detailed understanding of the unique neurobiology of pain from the orofacial skin and the oral mucosa should help us develop novel methods for better treating persistent orofacial pain.

Sweet taste does not modulate pain perception in adult humans

Background: Sugar is routinely used to comfort neonates undergoing painful procedures, and animal studies have shown that sucrose increases the time to withdrawal from painful stimuli. However, there are no published studies examining the effects of sweet substances on heat pain thresholds and percept in adult humans. Methods: Healthy adult volunteers (n=27, aged 18-48 years) were recruited to a controlled, double-blind, randomised, cross-over study to characterise the effect of tasting solutions of equivalent sweetness (10% sucrose and 0.016% sucralose) on warm detection and heat pain thresholds and the percept ratings of painfully hot stimuli. The effect of anticipation of a sweet taste on heat pain threshold was also assessed. Results: Tasting either sucrose or sucralose had no significant effect on the percept of an individually titrated hot stimulus (54.5±4.2 and 54.9±3.2 vs 53.2±3.5 for water, 0-100 visual analogue scale), on the warm detection or heat pain threshold (43.3±0.8, 43.2±0.8 vs 43.0±0.8°C). Anticipation of a sweet substance similarly did not affect heat pain thresholds. Conclusions: Sucrose and sucralose solutions had no analgesic effect when assessed using heat detection thresholds and percept ratings of painfully hot stimuli despite being perceived as sweeter and more pleasant than water. These findings are in contrast to results reported from previous animal studies in which thermal analgesia from sweet solutions is robust. Given the ubiquitous availability of sugar rich drinks in the modern environment, the lack of observable effect may be due to an insufficient hedonic value of the test solutions when compared to the experience of a laboratory rodent. Alternatively, sweet tastes may have a specific effect on pain tolerance rather than the threshold and acute percept measures assayed in this study.

Sweet taste does not modulate pain perception in adult humans

Background: It is commonly observed that humans who are in pain or discomfort seek solace in the form of sweet foods and drinks. Sugar is routinely used to comfort neonates undergoing painful procedures, and animal studies have shown that sucrose increases the time to withdrawal from painful stimuli. However, there are no published studies examining the effects of sweet substances on heat pain thresholds and percept in adult humans. Methods: Healthy adult volunteers (n=27, aged 18-48 years) were recruited to a controlled, double-blind, randomised, cross-over study to characterise the effect of tasting solutions of equivalent sweetness (10% sucrose and 0.016% sucralose) on warm detection and heat pain thresholds and the percept ratings of painfully hot stimuli. The effect of anticipation of a sweet taste on heat pain threshold was also assessed. Results: Tasting either sucrose or sucralose had no significant effect on the percept of an individually titrated hot stimulus (54.5±4.2 and 54.9±3.2 vs 53.2±3.5 for water, 0-100 visual analogue scale), on the warm detection or heat pain threshold (43.3±0.8, 43.2±0.8 vs 43.0±0.8°C). Anticipation of a sweet substance similarly did not affect heat pain thresholds. Conclusions: Sucrose and sucralose solutions had no analgesic effect when assessed using heat detection thresholds and percept ratings of painfully hot stimuli despite being perceived as sweeter and more pleasant than water. These findings are in contrast to results reported from previous animal studies in which thermal analgesia from sweet solutions is robust. Given the ubiquitous availability of sugar rich drinks in the modern environment, the lack of observable effect may be due to an insufficient hedonic value of the test solutions when compared to the experience of a laboratory rodent. Alternatively, sweet tastes may have a specific effect on pain tolerance rather than the threshold and acute percept measures assayed in this study.

Repeating a dose of sucrose for heel prick procedure in preterms is not effective in reducing pain: a randomised controlled trial

Oral sucrose is included in almost all recommendations for treatment of pain in newborns, but evidence if multiple doses might be more effective than a single standard dose is lacking. We designed a single-centre, double-blind, randomised, controlled trial. We enrolled preterm infants needing the heel prick procedure. Each enrolled infant was randomised to receive a single standard dose of sucrose 2 min before or a double dose of sucrose 2 min before, and 30 s after heel prick. Primary outcome was the efficacy of the two interventions tested by the premature infant pain profile-PIPP scale obtained at 30 s, 60 s, and 120 s after heel prick. Secondary outcome was the evaluation of the concordance between the PIPP scale and other pain scores more feasible in clinical practice. Seventy-two infants were randomised. No difference in pain perception as measured by the PIPP scale was found between the groups: median PIPP values 4.0(IQR 3.0-4.0) vs 3.0(IQR 3.0-4.0) at baseline; 6.0(IQR 5.0-10.0) vs 6.0(IQR 4.0-8.5) at 30 s; 6.0(IQR 4.0-7.0) vs 5.0(IQR 4.0-8.5) at 60 s and 5.0(IQR 4.0-7.0) vs 5.0(IQR 4.0-7.5) at 2 min, in the experimental and standard treatment groups, respectively (p = 0.9020). There was no correlation between PIPP scores and other pain scales.Conclusion: We do not recommend doubling the dose during heel prick.What is Known:• Oral sucrose is included in almost all international position papers and recommendations for the treatment of mild to moderate pain in newborns, associated with non-nutritive sucking and facilitated tucking• Premature infant pain profile (PIPP) scale is the gold standard for evaluation of pain in preterms but it is difficult to use in clinical practiceWhat is New:• Repeating a dose of 24% sucrose is not effective in reducing pain during the recovery phase of a skin breaking procedure• Other pain scales, easier to use in clinical practice, are not comparable with PIPP for the evaluation of procedural pain in preterms.

Hedonic drinking engages a supraspinal inhibition of thermal nociception in adult rats

The taste of sucrose is commonly used to provide pain relief in newborn humans and is innately analgesic to neonatal rodents. In adulthood, sucrose remains a strong motivator to feed, even in potentially hazardous circumstances (ie, threat of tissue damage). However, the neurobiological mechanisms of this endogenous reward-pain interaction are unclear. We have developed a simple model of sucrose drinking-induced analgesia in Sprague-Dawley rats (6-10 weeks old) and have undertaken a behavioral and pharmacological characterization using the Hargreaves' test of hind-paw thermal sensitivity. Our results reveal an acute, potent, and robust inhibitory effect of sucrose drinking on thermal nociceptive behaviour that unlike the phenomenon in neonates is independent of endogenous opioid signalling and does not seem to operate through classical descending inhibition of the spinal cord circuitry. Experience of sucrose drinking had a conditioning effect whereby the apparent expectancy of sucrose enabled water alone (in euvolemic animals) to elicit a short-lasting placebo-like analgesia. Sweet taste alone, however, was insufficient to elicit analgesia in adult rats intraorally perfused with sucrose. Instead, the sucrose analgesia phenomenon only appeared after conditioning by oral perfusion in chronically cannulated animals. This sucrose analgesia was completely prevented by systemic dosing of the endocannabinoid CB1 receptor antagonist rimonabant. These results indicate the presence of an endogenous supraspinal analgesic circuit that is recruited by the context of rewarding drinking and is dependent on endocannabinoid signalling. We propose that this hedonic sucrose-drinking model may be useful for further investigation of the supraspinal control of pain by appetite and reward.

Adverse Behavioral Changes in Adult Mice Following Neonatal Repeated Exposure to Pain and Sucrose

Sucrose is recommended for the treatment of pain during minor procedures in preterm infants in the neonatal intensive care unit (NICU) and is currently used worldwide as the standard of care. We recently reported that adult mice repetitively exposed to sucrose compared to water during the first week of life, irrespective of exposure to an intervention, had significantly smaller brain volumes in large white matter, cortical and subcortical structures (e.g., hippocampus, striatum, fimbria). These structures are important for stress regulation and memory formation. Here, we report the effects of repeated neonatal exposure to pain and sucrose on adult behavior in mice. Neonatal C57BL/6J mice (N = 160, 47% male) were randomly assigned to one of two treatments (sucrose, water) and one of three interventions (needle-prick, tactile, handling). Pups received 10 interventions daily from postnatal day 1 (P1) to P6. A single dose of 24% sucrose or water was given orally 2 min before each intervention. At adulthood (P60-85) mice underwent behavioral testing to assess spatial memory, anxiety, motor function, pain sensitivity, and sugar preference. We found that mice that had received sucrose and handling only, had poorer short-term memory in adulthood compared to water/handling controls (p < 0.05). When exposed to pain, mice treated with repetitive sucrose or water did not differ on memory performance (p = 0.1). A sugar preference test showed that adult mice that received sucrose before an intervention as pups consumed less sugar solution compared to controls or those that received water before pain (p < 0.05). There were no significant group differences in anxiety, motor, or pain sensitivity. In a mouse model that closely mimics NICU care, we show for the first time that memory in adulthood was poorer for mice exposed to pain during the first week of life, irrespective of sucrose treatment, suggesting that sucrose does not protect memory performance when administered for pain. In the absence of pain, early repetitive sucrose exposure induced poorer short-term memory, highlighting the importance of accurate pain assessment.

Infant Analgesia With a Combination of Breast Milk, Glucose, or Maternal Holding

OBJECTIVES

We studied neonatal cortical brain response to 4 types of nonpharmacological analgesia (oral glucose, expressed breast milk, maternal holding plus oral glucose, breastfeeding). We aimed to assess the differential effect of oral solutions (glucose, breast milk) given alone or combined with the maternal-infant relationship (holding, breastfeeding).

METHODS

Eighty healthy term newborns undergoing a heel stick were randomly assigned to 4 parallel groups of 20 infants each: group 1, infants received a glucose solution on a changing table; group 2, infants received expressed breast milk on a changing table; group 3, infants received a glucose solution in their mothers' arms; and group 4, infants were breastfed by their mothers. Cortical activation in parietal, temporal, and frontal cortices was assessed by multichannel near-infrared spectroscopy. Pain expression was also evaluated.

RESULTS

Oral glucose alone or combined with maternal holding was associated with no cortical activation during heel stick. Expressed breast milk was associated with localized bilateral activation of somatosensory and motor cortices (P < .01). Breastfeeding was associated with extensive bilateral activation of somatomotor, somatosensory, and right parietal cortices (P < .01). Pain expression was lower with the maternal-infant relationship (P = .007).

CONCLUSIONS

Oral glucose, either alone or combined with maternal holding, appears to block or weaken cortical pain processing. Breast milk alone is associated with localized cortical activation. Breastfeeding is associated with extensive activation and may act by extending cortical processing. Maternal relationship, both combined with oral glucose and in breastfeeding, shows the greatest analgesic effect, although the neural patterns involved are distributed differently.

Repeated exposure to sucrose for procedural pain in mouse pups leads to long-term widespread brain alterations

Oral sucrose is administered routinely to reduce pain of minor procedures in premature infants and is recommended as standard care in international guidelines. No human or animal studies on effects of early repeated sucrose exposure on long-term brain development have been done in the context of pain. We evaluated the effects of repeated neonatal sucrose treatment before an intervention on long-term brain structure in mouse pups. Neonatal C57Bl/6J mice (n = 109) were randomly assigned to one of 2 treatments (vehicle vs sucrose) and one of 3 interventions (handling, touch, or needle-prick). Mice received 10 interventions daily from postnatal day 1 to 6 (P1-6). A dose of vehicle or 24% sucrose was given orally 2 minutes before each intervention. At P85-95, brains were scanned using a multichannel 7.0 T MRI. Volumes of 159 independent brain regions were obtained. Early repetitive sucrose exposure in mice (after correcting for whole brain volume and multiple comparisons) lead to smaller white matter volumes in the corpus callosum, stria terminalis, and fimbria (P < 0.0001). Cortical and subcortical gray matter was also affected by sucrose with smaller volumes of hippocampus and cerebellum (P < 0.0001). These significant changes in adult brain were found irrespective of the type of intervention in the neonatal period. This study provides the first evidence of long-term adverse effects of repetitive sucrose exposure and raises concerns for the use of this standard pain management practice during a period of rapid brain development in very preterm infants.

Optogenetic Evidence for a Direct Circuit Linking Nociceptive Transmission through the Parabrachial Complex with Pain-Modulating Neurons of the Rostral Ventromedial Medulla (RVM)

The parabrachial complex (PB) is a functionally and anatomically complex structure involved in a range of homeostatic and sensory functions, including nociceptive transmission. There is also evidence that PB can engage descending pain-modulating systems, the best characterized of which is the rostral ventromedial medulla (RVM). Two distinct classes of RVM neurons, “ON-cells” and “OFF-cells,” exert net pronociceptive and anti-nociceptive effects, respectively. PB was recently shown to be a relay of nociceptive information to RVM ON- and OFF-cells. The present experiments used optogenetic methods in a lightly anesthetized rat and an adult RVM slice to determine whether there are direct, functionally relevant inputs to RVM pain-modulating neurons from PB. Whole-cell patch-clamp recordings demonstrated that PB conveys direct glutamatergic and GABAergic inputs to RVM neurons. Consistent with this, in vivo recording showed that nociceptive-evoked responses of ON- and OFF-cells were suppressed by optogenetic inactivation of archaerhodopsin (ArchT)-expressing PB terminals in RVM, demonstrating that a net inhibitory input to OFF-cells and net excitatory input to ON-cells are engaged by acute noxious stimulation. Further, the majority of ON- and OFF-cells responded to optogenetic activation of channelrhodopsin (ChR2)-expressing terminals in the RVM, confirming a direct PB influence on RVM pain-modulating neurons. These data show that a direct connection from the PB to the RVM conveys nociceptive information to the pain-modulating neurons of RVM under basal conditions. They also reveal additional inputs from PB with the capacity to activate both classes of RVM pain-modulating neurons and the potential to be recruited under different physiological and pathophysiological conditions.

What do we really know about newborn infant pain?

NEW FINDINGS

What is the topic of this review? Pain in infancy. What advances does it highlight? New neurophysiological research on pain processing in the human infant brain. Increased awareness of pain in the newborn has led to the development of numerous assessment tools for use in neonatal intensive care units. Here, I argue that we still know too little about the neurophysiological basis for infant pain to interpret data from clinical observational measures. With increased understanding of how the neural activity and CNS connections that underlie pain behaviour and perception develop in the newborn will come better measurement and treatment of their pain. This review focuses upon two interconnected nociceptive circuits, the spinal cord dorsal horn and the somatosensory cortex in the brain, to highlight what we know and what we do not know about infant pain. The effectiveness of oral sucrose, widely used in clinical practice to relieve infant pain, is discussed as a specific example of what we do not know. This 'hot topic review' highlights the importance of new laboratory-based neurophysiological research for the treatment of newborn infant pain.

Sucrose for analgesia in newborn infants undergoing painful procedures

BACKGROUND

Administration of oral sucrose with and without non-nutritive sucking is the most frequently studied non-pharmacological intervention for procedural pain relief in neonates.

OBJECTIVES

To determine the efficacy, effect of dose, method of administration and safety of sucrose for relieving procedural pain in neonates as assessed by validated composite pain scores, physiological pain indicators (heart rate, respiratory rate, saturation of peripheral oxygen in the blood, transcutaneous oxygen and carbon dioxide (gas exchange measured across the skin - TcpO2, TcpCO2), near infrared spectroscopy (NIRS), electroencephalogram (EEG), or behavioural pain indicators (cry duration, proportion of time crying, proportion of time facial actions (e.g. grimace) are present), or a combination of these and long-term neurodevelopmental outcomes.

SEARCH METHODS

We used the standard methods of the Cochrane Neonatal. We performed electronic and manual literature searches in February 2016 for published randomised controlled trials (RCTs) in the Cochrane Central Register of Controlled Trials (CENTRAL; The Cochrane Library, Issue 1, 2016), MEDLINE (1950 to 2016), EMBASE (1980 to 2016), and CINAHL (1982 to 2016). We did not impose language restrictions.

SELECTION CRITERIA

RCTs in which term or preterm neonates (postnatal age maximum of 28 days after reaching 40 weeks' postmenstrual age), or both, received sucrose for procedural pain. Control interventions included no treatment, water, glucose, breast milk, breastfeeding, local anaesthetic, pacifier, positioning/containing or acupuncture.

DATA COLLECTION AND ANALYSIS

Our main outcome measures were composite pain scores (including a combination of behavioural, physiological and contextual indicators). Secondary outcomes included separate physiological and behavioural pain indicators. We reported a mean difference (MD) or weighted MD (WMD) with 95% confidence intervals (CI) using the fixed-effect model for continuous outcome measures. For categorical data we used risk ratio (RR) and risk difference. We assessed heterogeneity by the I(2) test. We assessed the risk of bias of included trials using the Cochrane 'Risk of bias' tool, and assessed the quality of the evidence using the GRADE system.

MAIN RESULTS

Seventy-four studies enrolling 7049 infants were included. Results from only a few studies could be combined in meta-analyses and for most analyses the GRADE assessments indicated low- or moderate-quality evidence. There was high-quality evidence for the beneficial effect of sucrose (24%) with non-nutritive sucking (pacifier dipped in sucrose) or 0.5 mL of sucrose orally in preterm and term infants: Premature Infant Pain Profile (PIPP) 30 s after heel lance WMD -1.70 (95% CI -2.13 to -1.26; I(2) = 0% (no heterogeneity); 3 studies, n = 278); PIPP 60 s after heel lance WMD -2.14 (95% CI -3.34 to -0.94; I(2) = 0% (no heterogeneity; 2 studies, n = 164). There was high-quality evidence for the use of 2 mL 24% sucrose prior to venipuncture: PIPP during venipuncture WMD -2.79 (95% CI -3.76 to -1.83; I(2) = 0% (no heterogeneity; 2 groups in 1 study, n = 213); and intramuscular injections: PIPP during intramuscular injection WMD -1.05 (95% CI -1.98 to -0.12; I(2) = 0% (2 groups in 1 study, n = 232). Evidence from studies that could not be included in RevMan-analyses supported these findings. Reported adverse effects were minor and similar in the sucrose and control groups. Sucrose is not effective in reducing pain from circumcision. The effectiveness of sucrose for reducing pain/stress from other interventions such as arterial puncture, subcutaneous injection, insertion of nasogastric or orogastric tubes, bladder catherization, eye examinations and echocardiography examinations are inconclusive. Most trials indicated some benefit of sucrose use but that the evidence for other painful procedures is of lower quality as it is based on few studies of small sample sizes. The effects of sucrose on long-term neurodevelopmental outcomes are unknown.

AUTHORS' CONCLUSIONS

Sucrose is effective for reducing procedural pain from single events such as heel lance, venipuncture and intramuscular injection in both preterm and term infants. No serious side effects or harms have been documented with this intervention. We could not identify an optimal dose due to inconsistency in effective sucrose dosage among studies. Further investigation of repeated administration of sucrose in neonates is needed. There is some moderate-quality evidence that sucrose in combination with other non-pharmacological interventions such as non-nutritive sucking is more effective than sucrose alone, but more research of this and sucrose in combination with pharmacological interventions is needed. Sucrose use in extremely preterm, unstable, ventilated (or a combination of these) neonates needs to be addressed. Additional research is needed to determine the minimally effective dose of sucrose during a single painful procedure and the effect of repeated sucrose administration on immediate (pain intensity) and long-term (neurodevelopmental) outcomes.

Activation of Brainstem Pro-opiomelanocortin Neurons Produces Opioidergic Analgesia, Bradycardia and Bradypnoea

Opioids are widely used medicinally as analgesics and abused for hedonic effects, actions that are each complicated by substantial risks such as cardiorespiratory depression. These drugs mimic peptides such as β-endorphin, which has a key role in endogenous analgesia. The β-endorphin in the central nervous system originates from pro-opiomelanocortin (POMC) neurons in the arcuate nucleus and nucleus of the solitary tract (NTS). Relatively little is known about the NTSPOMC neurons but their position within the sensory nucleus of the vagus led us to test the hypothesis that they play a role in modulation of cardiorespiratory and nociceptive control. The NTSPOMC neurons were targeted using viral vectors in a POMC-Cre mouse line to express either opto-genetic (channelrhodopsin-2) or chemo-genetic (Pharmacologically Selective Actuator Modules). Opto-genetic activation of the NTSPOMC neurons in the working heart brainstem preparation (n = 21) evoked a reliable, titratable and time-locked respiratory inhibition (120% increase in inter-breath interval) with a bradycardia (125±26 beats per minute) and augmented respiratory sinus arrhythmia (58% increase). Chemo-genetic activation of NTSPOMC neurons in vivo was anti-nociceptive in the tail flick assay (latency increased by 126±65%, p<0.001; n = 8). All effects of NTSPOMC activation were blocked by systemic naloxone (opioid antagonist) but not by SHU9119 (melanocortin receptor antagonist). The NTSPOMC neurons were found to project to key brainstem structures involved in cardiorespiratory control (nucleus ambiguus and ventral respiratory group) and endogenous analgesia (periaqueductal gray and midline raphe). Thus the NTSPOMC neurons may be capable of tuning behaviour by an opioidergic modulation of nociceptive, respiratory and cardiac control.

Burning mouth syndrome: Current concepts

Burning mouth syndrome (BMS) is a chronic pain condition. It has been described by the International Headache Society as "an intra-oral burning or dysesthetic sensation, recurring daily for more than 2 h/day for more than 3 months, without clinically evident causative lesions." BMS is frequently seen in women in the peri-menopausal and menopausal age group in an average female/male ratio of 7:1. The site most commonly affected is the anterior two-thirds of the tongue. The patient may also report taste alterations and oral dryness along with the burning. The etiopathogenesis is complex and is not well-comprehended. The more accepted theories point toward a neuropathic etiology, but the gustatory system has also been implicated in this condition. BMS is frequently mismanaged, partly because it is not well-known among healthcare providers. Diagnosis of BMS is made after other local and systemic causes of burning have been ruled out as then; the oral burning is the disease itself. The management of BMS still remains a challenge. Benzodiazepines have been used in clinical practice as the first-line medication in the pharmacological management of BMS. Nonpharmacological management includes cognitive behavioral therapy and complementary and alternative medicine (CAM). The aim of this review is to familiarize healthcare providers with the diagnosis, pathogenesis, and general characteristics of primary BMS while updating them with the current treatment options to better manage this group of patients.

Prevention and Management of Procedural Pain in the Neonate: An Update

The prevention of pain in neonates should be the goal of all pediatricians and health care professionals who work with neonates, not only because it is ethical but also because repeated painful exposures have the potential for deleterious consequences. Neonates at greatest risk of neurodevelopmental impairment as a result of preterm birth (ie, the smallest and sickest) are also those most likely to be exposed to the greatest number of painful stimuli in the NICU. Although there are major gaps in knowledge regarding the most effective way to prevent and relieve pain in neonates, proven and safe therapies are currently underused for routine minor, yet painful procedures. Therefore, every health care facility caring for neonates should implement (1) a pain-prevention program that includes strategies for minimizing the number of painful procedures performed and (2) a pain assessment and management plan that includes routine assessment of pain, pharmacologic and nonpharmacologic therapies for the prevention of pain associated with routine minor procedures, and measures for minimizing pain associated with surgery and other major procedures.

Sucrose-induced analgesia during early life modulates adulthood learning and memory formation

This study is aimed at examining the long-term effects of chronic pain during early life (postnatal day 0 to 8weeks), and intervention using sucrose, on cognitive functions during adulthood in rats. Pain was induced in rat pups via needle pricks of the paws. Sucrose solution or paracetamol was administered for analgesia before the paw prick. Control groups include tactile stimulation to account for handling and touching the paws, and sucrose alone was used. All treatments were started on day one of birth and continued for 8weeks. At the end of the treatments, behavioral studies were conducted to test the spatial learning and memory using radial arm water maze (RAWM), as well as pain threshold via foot-withdrawal response to a hot plate apparatus. Additionally, the hippocampus was dissected, and blood was collected. Levels of neurotrophins (BDNF, IGF-1 and NT-3) and endorphins were assessed using ELISA. The results show that chronic noxious stimulation resulted in comparable foot-withdrawal latency between noxious and tactile groups. On the other hand, pretreatment with sucrose or paracetamol increased pain threshold significantly both in naive rats and noxiously stimulated rats (P<0.05). Chronic pain during early life impaired short-term memory, and sucrose treatment prevented such impairment (P<0.05). Sucrose significantly increased serum levels of endorphin and enkephalin. Chronic pain decreased levels of BDNF in the hippocampus and this decrease was prevented by sucrose and paracetamol treatments. Hippocampal levels of NT-3 and IGF-1 were not affected by any treatment. In conclusion, chronic pain induction during early life induced short memory impairment, and pretreatment with sucrose prevented this impairment via mechanisms that seem to involve BDNF. As evident in the results, sucrose, whether alone or in the presence of pre-noxious stimulation, increases pain threshold in such circumstances; most likely via a mechanism that involves an increase in endogenous opioids.

Energy conservation in infants

Energy acquisition through suckling has been widely studied in rat and human infants. Processes mediating energy conservation, however, have not received the attention that they deserve. This essay, in honor of Professor Jerry Hogan, discusses parallel behaviors used by rat and human mothers to minimize energy loss in their offspring. Parallel mechanisms underlying energy preservation have been identified in rats and humans, suggesting phylogenetic conservation and possibly continuity. This article is part of a Special Issue entitled: In Honor of Jerry Hogan.

[Pain in neonatology. Non-pharmacological treatment]

Diagnostic and therapeutic skin-breaking procedures have become ubiquitous in current medical practice and neonatology does not constitute an exception. One of the main sources of neonatal pain is procedure-induced pain. It has recently become clear that pain prevention must be a health care priority. Non-pharmacological approaches constitute a first option for the analgesia of common procedures performed in neonatology. This article reviews the non-pharmacological treatments most frequently used in this context: swaddling, tucking, containment, sweet solutions, non-nutritive sucking (NNS), breastfeeding analgesia, breast milk and music. In practice, the dose of 1 to 2mL of 24% or 30% sucrose solution or 30% glucose solution immediately followed by NNS can be given for minor painful procedures in term neonates or those weighing more than 2500g. In the preterm, 0.3mL of a sweet solution (sucrose or glucose) can be given for infants weighing less than 1500g and 0.5mL for those weighing between 1500 and 2500g. The synergistic effect of sweet solutions and NNS has been clearly shown and thus their association is largely justified in practice. For breast-fed term neonates, breastfeeding can be given to sooth procedure-induced pain. All these non-pharmacological options can be effective to relieve pain from minor or moderate procedures. However, when more painful procedures are performed, stronger analgesics must be used.

Effects of gustatory nerve transection and/or ovariectomy on oral capsaicin avoidance in rats

The incidence of chronic oral pain such as burning mouth syndrome is greater in peri-menopausal females, and was postulated to be associated with gustatory nerve damage. We investigated whether bilateral transection of the chorda tympani, with or without accompanying ovariectomy, affected oral capsaicin avoidance in rats. Female rats had restricted access to 2 bottles, 1 bottle containing capsaicin (concentration range: 0.33-33 μM/L) and the other vehicle. Percent volume of capsaicin consumption and lick counts were measured. The concentration series was tested before and 0.5, 3, 6, 9, and 12 months after the following surgical procedures: (a) bilateral transection of the chorda tympani (CTx); (b) ovariectomy (OVx); (3) CTx plus OVx; or (4) sham CT surgery. Before surgery there was a concentration-dependent decrease in licks and volume of capsaicin consumed, with a threshold between 0.1 and 0.3 ppm. The majority of drink licks occurred during the first 9 minutes of access. Over the 12-month test period, the CTx group did not exhibit reduced capsaicin consumption, and consumed significantly more capsaicin at 6 and 9 months postsurgery. Rats in the OVx group consistently consumed significantly less capsaicin and exhibited significantly higher counts of capsaicin-evoked Fos-like immunoreactivity in the dorsomedial trigeminal subnucleus caudalis (Vc) compared to all other treatment groups. That CTx, with or without OVx, did not enhance capsaicin avoidance indicates that damage to the gustatory system does not disinhibit trigeminal nociceptive transmission.

Can you enhance executive control without glucose? The effects of fructose on problem solving

The consumption of glucose can enhance executive control by increasing blood glucose and providing energy for brain processes. However, a glucose mouth rinse also positively affects executive control in the absence of an effect on blood glucose. This observation suggests that glucose can enhance executive control via another mechanism, perhaps by increasing cortical activation in motivational reward pathways. This hypothesis was examined in the current study by having participants consume fructose, glucose or a placebo 10 minutes before solving word anagrams. Fructose was used because it does not affect blood glucose levels, inhibits cortical responding, and cannot entrain a taste preference post-ingestion. The latter evidences that fructose differs from glucose in its ability to elicit activation in motivational reward pathways. It was observed that consuming fructose and glucose resulted in the resolution of more anagrams than the placebo. These results suggest that at least some of the effects of glucose and fructose on problem solving operate through a common peripheral mechanism that is independent of motivational reward pathways.

Gustatory modulation of the responses of trigeminal subnucleus caudalis neurons to noxious stimulation of the tongue in rats

Certain tastants inhibit oral irritation by capsaicin, whereas anesthesia of the chorda tympani (CT) enhances oral capsaicin burn. We tested the hypothesis that tastants activate the CT to suppress responses of trigeminal subnucleus caudalis (Vc) neurons to noxious oral stimuli. In anesthetized rats, we recorded Vc unit responses to noxious electrical, chemical (pentanoic acid, 200 μm) and thermal (55 °C) stimulation of the tongue. Electrically evoked responses were significantly reduced by a tastant mix and individually applied NaCl, monosodium glutamate (MSG), and monopotassium glutamate. Sucrose, citric acid, quinine and water (control) had no effect. Pentanoic acid-evoked responses were similarly attenuated by NaCl and MSG, but not by other tastants. Responses to noxious heat were not affected by any tastant. Transection and/or anesthesia of the CT bilaterally affected neither Vc neuronal responses to electrical or pentanoic acid stimulation, nor the depressant effect of NaCl and MSG on electrically evoked responses. Calcium imaging showed that neither NaCl nor MSG directly excited any trigeminal ganglion cells or affected their responses to pentanoic acid. GABA also had no effect, arguing against peripheral effects of GABA, NaCl or MSG on lingual nocicepive nerve endings. The data also rule out a central mechanism, as the effects of NaCl and MSG were intact following CT transection. We speculate that the effect is mediated peripherally by the release from taste receptor cells (type III) of some mediator(s) other than GABA to indirectly inhibit trigeminal nociceptors. The results also indicate that the CT does not exert a tonic inhibitory effect on nociceptive Vc neurons.

Sucrose for procedural pain management in infants

The use of oral sucrose has been the most extensively studied pain intervention in newborn care to date. More than 150 published studies relating to sweet-taste-induced calming and analgesia in human infants have been identified, of which 100 (65%) include sucrose. With only a few exceptions, sucrose, glucose, or other sweet solutions reduced pain responses during commonly performed painful procedures in diverse populations of infants up to 12 months of age. Sucrose has been widely recommended for routine use during painful procedures in newborn and young infants, yet these recommendations have not been translated into consistent use in clinical practice. One reason may be related to important knowledge and research gaps concerning analgesic effects of sucrose. Notably, the mechanism of sweet-taste-induced analgesia is still not precisely understood, which has implications for using research evidence in practice. The aim of this article is to review what is known about the mechanisms of sucrose-induced analgesia; highlight existing evidence, knowledge gaps, and current controversies; and provide directions for future research and practice.

Sweet taste and chorda tympani transection alter capsaicin-induced lingual pain perception in adult human subjects

Sweetness signals the nutritional value of food and may moreover be accompanied by a sensory suppression that leads to higher pain tolerance. This effect is well documented in infant rats and humans. However, it is still debated whether sensory suppression is also present in adult humans. Thus, we investigated the effects of sweet taste on the perception of the painful trigeminal stimulus capsaicin in two groups of healthy adult human subjects. A solution of 100 μM capsaicin was applied to the tip of the subject's tongues in order to stimulate trigeminal Aδ- and C-fiber nociceptors. When swallowed, 1M sucrose reduced the capsaicin-induced burning sensation by 29% (p ≤ 0.05) whereas a solution of similar taste intensity containing 1 μM quinine did not. Similarly, sucrose application to the frontal hemitongue suppressed the perception of the burning sensation induced by contralaterally applied capsaicin by 25% (p ≤ 0.01). We furthermore investigated the effects of documented unilateral transection of the chorda tympani nerve on capsaicin perception. In accordance with the ipsi-to-contralateral effect of sucrose on capsaicin perception in healthy subjects, hemiageusic subjects were more sensitive for capsaicin on the tongue contralateral to the taste nerve lesion (+38%; p ≤ 0.01). Taken together, these results argue I) for the existence of food intake-induced sensory suppression, if not analgesia, in adult humans and II) a centrally mediated suppression of trigeminal sensation by taste inputs that III) becomes disinhibited upon peripheral taste nerve lesion.

Taste damage (otitis media, tonsillectomy and head and neck cancer), oral sensations and BMI

Otitis media and tonsillectomy are associated with enhanced palatability of energy dense foods and with weight gain. Otitis media can damage the chorda tympani nerve (CN VII); tonsillectomy and head and neck radiation treatment can damage the glossopharyngeal nerve (CN IX). Both of these nerves function prominently in taste sensation. The present study utilizes these sources of damage to study central interactions among the nerves that mediate oral sensations. Mild damage restricted to one of these nerves can actually intensify sensations evoked from undamaged nerves (i.e., whole-mouth taste, oral tactile sensations evoked by fats and irritants). These intensifications may result from disruption of central inhibitory taste circuits, as taste damage appears to disinhibit other oral sensory nerves. In addition, mild damage restricted to one taste nerve can intensify odors perceived from foods in the mouth during chewing and swallowing (i.e., retronasal olfaction); this may be a secondary consequence of the intensification of whole-mouth taste. Damage to both nerves leads to widespread oral sensory loss. At present, the link between sensory alterations and weight gain has not been established for adults (e.g., does increased fat preference occur in individuals with oral sensory intensifications, those with losses, or both?). Finally, pain in non-oral locations is also related to taste loss. When participants rated "the most intense pain of any kind they had ever experienced," those with the greatest taste loss gave the highest ratings. These effects suggest that taste loss significantly influences long-term health outcomes.

Warmth is analgesic in healthy newborns

This study identifies a behavioral and nonpharmacologic means of preventing and reducing newborn pain. Our objective was to determine whether warmth is analgesic in newborn infants undergoing vaccination-a routine painful hospital procedure. We used a prospective randomized controlled trial of 47 healthy full-term newborn infants. Infants were randomized into 1 of 3 conditions prior to vaccination: warmth exposure, pacifier suckling, or sucrose taste. Crying, grimacing, and heart rate differences were analyzed between groups before, during, and after vaccination as outcome measures. Warmer infants cried significantly less than sucrose taste or pacifier suckling after vaccination. Heart rate patterns reflected this analgesia. Core temperature did not differ between study groups. Providing natural warmth to newborn infants during a painful procedure decreases the crying and grimacing on par with the "gold" standard treatments of sucrose or pacifier.

Sweet taste-induced analgesia: an fMRI study

We investigated the brain activation associated with sweet taste-induced analgesia by 3-T functional magnetic resonance imaging, the mechanism of which is considered to involve the central nervous system. After 12 healthy individuals ingested tasteless gelatin (nonsweet condition) or sweet glucose (sweet condition) in a magnetic resonance imaging scanning gantry, the cold pressor test was applied to their medial forearm. Under both conditions, the cold pressor test robustly activated the pain-related neural network, notably the anterior cingulate cortex, insula, posterior parietal cortex, and thalamus, although such activations under the sweet condition weakened with pain threshold increase, compared with those under the nonsweet condition. Together with emotional changes in pain appraisal, our findings provide objective representation of sweet taste-induced analgesia in the human brain.

The role of sensory input of the chorda tympani nerve and the number of fungiform papillae in burning mouth syndrome

OBJECTIVE

The aim of this study was to evaluate patients suffering from burning mouth syndrome (BMS) and control subjects by means of sensory testing and fungiform papillae count.

STUDY DESIGN

The left and right anterior two-thirds of the tongue of of 25 BMS subjects and 20 healthy control subjects were evaluated for electric taste and electric detection threshold. The number of fungiform papillae/cm(2) was evaluated by using close-up digital photography.

RESULTS

The electric taste/tingling detection threshold ratio was significantly higher in BMS compared with control subjects (P = .041). No difference was found between the number of fungiform papillae/cm(2) in the BMS compared with the control subjects (P = .277). Patients suffering from BMS for a prolonged period of time presented with a significantly elevated electric taste/tingling detection threshold ratio (P = .031).

CONCLUSIONS

BMS may be a neurodegenerative process with chorda tympani nerve hypofunction potentially playing a role in the pathophysiology of this disorder.

Gender differences in pain modulation by a sweet stimulus in adults: A randomized study

This study aimed to examine whether or not there are gender differences in sweet stimulus-induced analgesia for cold pain in adults. In a randomized cross-over design, twenty men and 20 women held either a 24% sucrose solution or distilled water in their mouth before and while they immersed their hand in cold water and their pain response was examined. Unlike the women, when the men held the sucrose solution in their mouth, the latency of the onset of pain significantly increased, compared with the distilled water. Meanwhile, the level of pain tolerance was not significantly different for both sexes. The findings reveal that the analgesic effect of a sweet stimulus on the pain threshold is influenced by gender differences in human adults, indicating that sweet stimulus-induced analgesia has a brief analgesic effect, particularly for men. Although more research is warranted, the sweet stimulus could be put to practical application as an adjunct to acute pain management for men.

Oral sucrose as an analgesic drug for procedural pain in newborn infants: a randomised controlled trial

BACKGROUND

Many infants admitted to hospital undergo repeated invasive procedures. Oral sucrose is frequently given to relieve procedural pain in neonates on the basis of its effect on behavioural and physiological pain scores. We assessed whether sucrose administration reduces pain-specific brain and spinal cord activity after an acute noxious procedure in newborn infants.

METHODS

In this double-blind, randomised controlled trial, 59 newborn infants at University College Hospital (London, UK) were randomly assigned to receive 0·5 mL 24% sucrose solution or 0·5 mL sterile water 2 min before undergoing a clinically required heel lance. Randomisation was by a computer-generated randomisation code, and researchers, clinicians, participants, and parents were masked to the identity of the solutions. The primary outcome was pain-specific brain activity evoked by one time-locked heel lance, recorded with electroencephalography and identified by principal component analysis. Secondary measures were baseline behavioural and physiological measures, observational pain scores (PIPP), and spinal nociceptive reflex withdrawal activity. Data were analysed per protocol. This study is registered, number ISRCTN78390996.

FINDINGS

29 infants were assigned to receive sucrose and 30 to sterilised water; 20 and 24 infants, respectively, were included in the analysis of the primary outcome measure. Nociceptive brain activity after the noxious heel lance did not differ significantly between infants who received sucrose and those who received sterile water (sucrose: mean 0·10, 95% CI 0·04-0·16; sterile water: mean 0·08, 0·04-0·12; p=0·46). No significant difference was recorded between the sucrose and sterile water groups in the magnitude or latency of the spinal nociceptive reflex withdrawal recorded from the biceps femoris of the stimulated leg. The PIPP score was significantly lower in infants given sucrose than in those given sterile water (mean 5·8, 95% CI 3·7-7·8 vs 8·5, 7·3-9·8; p=0·02) and significantly more infants had no change in facial expression after sucrose administration (seven of 20 [35%] vs none of 24; p<0·0001).

INTERPRETATION

Our data suggest that oral sucrose does not significantly affect activity in neonatal brain or spinal cord nociceptive circuits, and therefore might not be an effective analgesic drug. The ability of sucrose to reduce clinical observational scores after noxious events in newborn infants should not be interpreted as pain relief.

FUNDING

Medical Research Council.

Pattern of biphasic response to various noxious stimuli in rats ingesting sucrose ad libitum

Sucrose ingestion is reported to produce an initial (20-30min) analgesia and late (<5h) hyperalgesia. However, the influence of the characteristics of noxious stimuli and sweet substances on the pattern of transition from analgesia to hyperalgesia is not known. Therefore, we investigated the effect of sucrose (20%, sucrose fed group), saccharin (0.1%, saccharin fed group) and water ingestion (control group) on pain responses to various noxious stimuli for 5h. Latency of motor response of tail (TFL), paws to noxious thermal stimuli, threshold for elicitation of motor responses to electrical stimulation of tail nociceptive afferents in 5 sessions (0, 0.25, 1, 3 and 5h) and pain-related behavior to tonic noxious stimulus in 3 sessions at 1, 3 and 5h were recorded. In sucrose fed rats as compared to controls, the TFL sequentially increased (9.29+/-0.47s from 8.41+/-0.25; p<0.01), recovered to base-line and decreased (6.61+/-0.61sec; p<0.0001) in sessions II, III and V indicating analgesia, eualgesia and hyperalgesia, respectively. In saccharin fed rats the initial analgesia extended until session III followed by eualgesia and hyperalgesia in sessions IV and V. Pain related behaviour to tonic noxious stimulus also indicated an initial analgesia (0-5min), intermediate eualgesia and late hyperalgesia (3-5h) in sucrose fed rats, whereas only analgesia in saccharin fed rats. The results of our study suggest that sucrose ingestion for 5h leads to a bi-phasic response to both phasic and tonic noxious stimuli, albeit there are variations in their durations. Therefore, the temporal relationship of the nociceptive responses to palatable food is a function of the stimulus quality of both.

Orosensory deprivation alters taste-elicited c-Fos expression in the parabrachial nucleus of neonatal rats

In the present study we examined the effects of neonatal orosensory deprivation on taste-elicited gustatory activity in the rat parabrachial nucleus (PBN) using the functional anatomical marker c-Fos. Animals in three groups (GG, GO and GM) received gastric cannula implantation surgery on postnatal day 9 (P9). Animals in the fourth group (MR) did not receive any surgery. GG rats were fed by infusion of artificial milk directly into the stomach. GO rats were fed by intraoral infusion of artificial milk. GM and MR rats were reared by their mother with free access to mother's milk, water and rat chow. Rats from all groups were similar in body weight and length by P21. On P21 rats in all groups were intraorally presented with 0.5M sucrose solution and the brains were extracted and processed for c-Fos immunohistochemistry. Taste-elicited c-Fos expression in both the gustatory waist area, and the external lateral subnucleus of the PBN in rats in the GG group was significantly more robust than in the other three groups. These findings suggest a substantial alteration in orosensory-evoked neuronal response in this nucleus, due to sensory or motor deprivation during a critical developmental stage.

The ten commandments of pain assessment and management in preterm neonates

Despite advances in pain assessment and management, nonpharmacologic and pharmacologic analgesic therapies continue to be underutilized in managing both acute and procedural pain in preterm neonates. Untreated acute, recurrent, or chronic pain related to disease or medical care may have significant and lifelong physiologic and psychological consequences. Painful procedures in the NICU may be unavoidable, so it is vital that caregivers balance the painful, medically necessary care with evidence-based nonpharmacologic and pharmacologic strategies to relieve pain and stress. We present Ten Commandments of pain assessment and management in preterm neonates to promote the use of best practices and compassionate care in the NICU.

Release of endogenous opioids from duodenal enteroendocrine cells requires Trpm5

BACKGROUND & AIMS

Enteroendocrine cells, the largest and most diverse population of mammalian endocrine cells, comprise a number of different cell types in the gut mucosa that produce, store, and secrete small molecules, peptides, and/or larger proteins that regulate many aspects of gut physiology. Little is known about less typical endocrine cells in the intestinal mucosa that do not contain secretory granules, such as brush or caveolated cells. We studied a subset of these enteroendocrine cells in duodenum that produce several peptides, including endogenous opioids, and that also express the Trpm5 cation channel.

METHODS

We studied expression patterns of Trpm5 and other molecules by immunohistochemical and enzyme-linked immunosorbent assay analyses of intestinal tissues from transgenic mice that express green fluorescent protein from the Trpm5 promoter, as well as wild-type and Trpm5-null mice.

RESULTS

We describe a type of enteroendocrine cell in mouse duodenum that is defined by the presence of Trpm5 and that does not contain typical secretory granules yet expresses endogenous opioids (beta-endorphin and Met-enkephalin) and uroguanylin in apical compartments close to the lumen of the gut.

CONCLUSIONS

Solitary chemosensory cells that coexpress beta-endorphin, Met-enkephalin, uroguanylin, and Trpm5 exist in mouse duodenum. These cells are likely to secrete the bioactive peptides into the intestinal lumen in response to dietary factors; release of the opioid peptides requires the Trpm5 ion channel.

Eating is a protected behavior even in the face of persistent pain in male rats

Feeding is critical for survival. Yet, patients with chronic pain often lose their appetite and eat less. We previously showed that ad libitum fed male rats continue to feed rather than withdraw from a brief noxious stimulus. This study examined the effects of a sustained noxious stimulus on feeding by testing ad libitum fed male rats for five eating behaviors--latency to eat, time taken to eat each chip, pauses and scanning during eating, and the number of chocolate chips eaten--during the hour following a sham injection or an injection of a low (0.5%) or moderate (1.5%) dose of formalin into the hind paw. Sham-injected rats showed no pain-related behaviors, rats injected with 0.5% formalin showed very few pain-related behaviors, and rats injected with 1.5% formalin showed favoring, lifting and licking of the injured paw with a characteristic biphasic time course. Besides taking less time to commence eating during the first phase of formalin pain, rats injected with either dose of formalin did not differ from sham-injected rats on any of the other eating measures. Rats injected with 0.5% formalin showed no pain behaviors during eating, whereas those given 1.5% formalin typically ate while not exhibiting any pain behaviors but occasionally ate while favoring the paw, rarely while lifting the paw, and never while licking the paw. These results show that eating is a protected activity even in the presence of persistent pain in male rats.

Inactivation of the periaqueductal gray attenuates antinociception elicited by stimulation of the rat medial preoptic area

The medial preoptic area (MPOA) is a sexually dimorphic structure that plays key roles in gonado-steroidal regulation and thermoregulation. The MPOA may be involved in sex-based differences in nociceptive processing and steroid hormones effect on pain thresholds. Consistent with this, there is evidence that MPOA can produce antinociception or hyperalgesia. MPOA stimulation inhibits spinal cord or trigeminal neuronal responses to noxious stimuli or produces analgesia, yet most of these studies utilized electrical stimulation which antidromically activates periaqueductal gray (PAG) and rostroventromedial medulla (RVM) neurons involved in descending modulation of nociception. Effects of selective activation of MPOA neurons on behavioral indices of antinociception and the site-specificity of such responses are unknown. To address these questions, we examined the influence of MPOA microinjections of d,l homocysteate (DLH) on hindlimb and tail nocifensive reflexes in lightly anesthetized rats. DLH, but not saline, microinjections into several MPOA subregions markedly increased withdrawal response latencies to noxious thermal stimuli. Antinociceptive effects of MPOA activation were abolished by microinjection of lidocaine into PAG. These results suggest that activation of MPOA neurons produces antinociception that is at least partly mediated by projections to PAG.

Pain reduction during pediatric immunizations: evidence-based review and recommendations

The pain associated with immunizations is a source of anxiety and distress for the children receiving the immunizations, their parents, and the providers who must administer them. Preparation of the child before the procedure seems to reduce anxiety and subsequent pain. The limited available data suggest that intramuscular administration of immunizations should occur in the vastus lateralis (anterolateral thigh) for children <18 months of age and in the deltoid (upper arm) for those >36 months of age. Controversy exists in site selection for 18- to 36-month-old children. A number of studies suggest that the ventrogluteal area is the most appropriate for all age groups. Longer needles are usually associated with less pain and less local reaction. During the injection, parental demeanor clearly affects the child's pain behaviors. Excessive parental reassurance, criticism, or apology seems to increase distress, whereas humor and distraction tend to decrease distress. Distraction techniques vary with the age, temperament, and interests of the child, but their efficacy is well supported in the literature. Sucrose solution instilled directly into the mouth or administered on a pacifier reduces evidence of distress reliably in children <6 months of age and should be used routinely. Although there is no perfect topical anesthetic available at this time, selective use for children who are particularly fearful or who have had negative experiences in the past is highly endorsed. Pressure at the site, applied with either a device or a finger, clearly reduces pain. Finally, in the era of multiple injections, it seems that parents prefer that multiple injections be given simultaneously, rather than sequentially, if there are enough personnel available. Immunizations are stressful for many children; until new approaches are developed, systematic use of available techniques can significantly reduce the burden of distress associated with these procedures.

Pain perception and electromagnetic fields

A substantial body of evidence has accumulated showing that exposure to electromagnetic fields (EMFs) affects pain sensitivity (nociception) and pain inhibition (analgesia). Consistent inhibitory effects of acute exposures to various EMFs on analgesia have been demonstrated in most studies. This renders examinations of changes in the expression of analgesia and nociception a particularly valuable means of addressing the biological effects of and mechanisms underlying the actions of EMFs. Here we provide an overview of the effects of various EMFs on nociceptive sensitivity and analgesia, with particular emphasis on opioid-mediated responses. We also describe the analgesic effects of particular specific EMFs, the effects of repeated exposures to EMFs and magnetic shielding, along with the dependence of EMF effects on lighting conditions. We further consider some of the underlying cellular and biophysical mechanisms along with the clinical implications of these effects of various EMFs.

Recent developments in the pharmacological management of pain in children

PURPOSE OF REVIEW

This review explores progress in developmental pharmacokinetics, pharmacogenomics and formulations of analgesic agents, and discusses potential implications for pain therapy.

RECENT FINDINGS

Characterization of the developmental pharmacokinetics of morphine, tramadol, paracetamol and nonsteroidal anti-inflammatory drugs has improved dosing in children. Oral sugar solutions have replaced the brandy/sugar pacifier and are effective for single painful events in neonates. Intravenous paracetamol offers increased dosing accuracy, and avoids absorption and bioavailability variability. New nitric-oxide-releasing versions of paracetamol and nonsteroidal anti-inflammatory drugs offer safer alternatives to their parent drugs with enhanced potency. Ketamine has come under a cloud for its possible effects on the neonatal developing brain, but it is being used increasingly in children to supplement opioids for pain after major surgery. Hopes that morphine analgesia may improve neurological outcome in premature babies have not materialized. Reports concerning chronic pain are generally case series and controlled trials are rare and nearly nonexistent in children.

SUMMARY

Unlicensed drug use in the very young will increase as familiarity increases. Pharmacogenomic studies have the potential to tailor drug therapy to the individual and decrease between-patient variability. Unfortunately, the pharmacodynamic knowledge in children of analgesic agents remains neglected and is usually extrapolated from adult data.

Dorsal raphe nucleus and locus coeruleus neural networks and the elaboration of the sweet-substance-induced antinociception

In order to investigate the effects of monoaminergic neurons of the dorsal raphe nucleus (DRN) and locus coeruleus (LC) on the elaboration and control of sweet-substance-induced antinociception, male albino Wistar rats weighing 180-200 g received sucrose solution (250 g/L) for 7-14 days as their only source of liquid. After the chronic consumption of sucrose solution, each animal was pretreated with unilateral microinjection of ibotenic acid (1.0 microg/0.2 microL) in the DRN or in the LC. The tail withdrawal latencies of the rats in the tail-flick test were measured immediately before and 7 days after this treatment. The neurochemical lesion of locus coeruleus, but not of DRN neural networks with ibotenic acid, after the chronic intake of sweetened solution, decreased the sweet-substance-induced antinociception. These results indicate the involvement of noradrenaline-containing neurons of the LC in the sucrose-induced antinociception. We also consider the possibility of DRN serotonergic neurons exerting some inhibitory effect on the LC neural networks involved with the elaboration of the sweet-substance-induced antinociception.

Sensory suppression during feeding

Feeding is essential for survival, whereas withdrawal and escape reactions are fundamentally protective. These critical behaviors can compete for an animal's resources when an acutely painful stimulus affects the animal during feeding. One solution to the feeding-withdrawal conflict is to optimize feeding by suppressing pain. We examined whether rats continue to feed when challenged with a painful stimulus. During feeding, motor withdrawal responses to noxious paw heat either did not occur or were greatly delayed. To investigate the neural basis of sensory suppression accompanying feeding, we recorded from brainstem pain-modulatory neurons involved in the descending control of pain transmission. During feeding, pain-facilitatory ON cells were inhibited and pain-inhibitory OFF cells were excited. When a nonpainful somatosensory stimulus preactivated ON cells and preinhibited OFF cells, rats interrupted eating to react to painful stimuli. Inactivation of the brainstem region containing ON and OFF cells also blocked pain suppression during eating, demonstrating that brainstem pain-modulatory neurons suppress motor reactions to external stimulation during homeostatic behaviors.