Sucrose ingestion causes opioid analgesia

Validity of mental and physical stress models

Different stress models are employed to enhance our understanding of the underlying mechanisms and explore potential interventions. However, the utility of these models remains a critical concern, as their validities may be limited by the complexity of stress processes. Literature review revealed that both mental and physical stress models possess reasonable construct and criterion validities, respectively reflected in psychometrically assessed stress ratings and in activation of the sympathoadrenal system and the hypothalamic-pituitary-adrenal axis. The findings are less robust, though, in the pharmacological perturbations' domain, including such agents as adenosine or dobutamine. Likewise, stress models' convergent- and discriminant validity vary depending on the stressors' nature. Stress models share similarities, but also have important differences regarding their validities. Specific traits defined by the nature of the stressor stimulus should be taken into consideration when selecting stress models. Doing so can personalize prevention and treatment of stress-related antecedents, its acute processing, and chronic sequelae. Further work is warranted to refine stress models' validity and customize them so they commensurate diverse populations and circumstances.

Sucrose drinking mimics effects of nucleus accumbens µ-opioid receptor stimulation on fat intake and brain c-Fos-expression

Objectives: We have previously shown that the combined consumption of fat and a sucrose solution induces overeating, and there is evidence indicating that sucrose drinking directly stimulates fat intake. One neurochemical pathway by which sucrose may enhance fat intake is through the release of endogenous opioids in the nucleus accumbens (NAC).Methods: To test this hypothesis, we provided rats with a free-choice high-fat diet for two weeks. During the second week, rats had access to an additional bottle of water or a 30% sucrose solution for five minutes per day. After these two weeks, we infused vehicle or the μ-opioid receptor agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) into the NAC 30 min after their daily access to the additional bottle of water or the sucrose solution.Results: Sucrose drinking had two effects, (1) it stimulated fat intake in the absence of DAMGO infusion, (2) it diminished sensitivity to DAMGO, as it prevented the rapid increase in fat intake typically seen upon DAMGO infusion in the nucleus accumbens. In a second experiment, we confirmed that these results are not due to the ingested calories of the sucrose solution. Lastly, we investigated which brain areas are involved in the observed effects on fat intake by assessing c-Fos-expression in brain areas previously linked to DAMGO's effects on food intake. Both intra-NAC DAMGO infusion and sucrose consumption in the absence of DAMGO infusion had no effect on c-Fos-expression in orexin neurons and the central amygdala but increased c-Fos-expression in the NAC as well as the basolateral amygdala.Discussion: In conclusion, we confirm that sucrose drinking stimulates fat intake, likely through the release of endogenous opioids.

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.

A narrative review of highly processed food addiction across the lifespan

Evidence is growing that highly processed (HP) foods (i.e., foods high in refined carbohydrates and fat) are highly effective in activating reward systems and may even be capable of triggering addictive processes. Unlike traditional drugs of abuse, exposure to HP foods is common very early in development. HP food addiction has been associated with negative outcomes, including higher body mass index (BMI), more frequent binge eating, greater failure in weight loss treatment trials, and poorer mental and physical health. Although most research on HP food addiction has been conducted using adult samples, research on this topic now spans across the life span beginning in utero and extending through older adulthood. HP food addiction and related reward-based changes are associated with negative outcomes at every life stage, which has important implications for developmentally tailored prevention and treatment efforts. Using a developmentally informed approach, the current study comprehensively reviews the existing research on HP food addiction across the lifespan and highlights important areas of future research.

Role of OPRM1, clinical and anthropometric variants in neonatal pain reduction

An increased awareness on neonatal pain-associated complications has led to the development of pain scales adequate to assess the level of pain experienced by newborns such as the ABC score. A commonly used analgesic procedure is to administer a 33% oral dextrose solution to newborns prior to the painful intervention. Although this procedure is very successful, not in all subjects it reaches complete efficacy. A possible explanation for the different response to the treatment could be genetic variability. We have investigated the genetic variability of the OPRM1 gene in 1077 newborns in relation to non-pharmacologic pain relief treatment. We observed that the procedure was successful in 966 individuals and there was no association between the genotypes and the analgesic efficacy when comparing individuals that had an ABC score = 0 and ABC score >0. However, considering only the individuals with ABC score>0, we found that the homozygous carriers of the G allele of the missense variant SNP rs1799971 (A118G) showed an interesting association with higher ABC score. We also observed that individuals fed with formula milk were more likely to not respond to the analgesic treatment compared to those that had been breastfed.

Effects of Spicy Stimulation and Spicy-Food Consumption on Human Pain Sensitivity: A Healthy Volunteer Study

Spicy-food intake has been shown to affect various human physiological systems and diseases. This study tested the analgesia effect caused by stimulation of a spicy sensation (spicy stimulation) and explored the effect of spicy-food consumption on human basal pain sensitivity. A total of 60 healthy undergraduates were included in the primary study. Placebo and sweet stimulation were used as reference interventions. Pressure and cold-pain thresholds were measured before and after taste stimulation. The frequency of spicy-food intake was also evaluated. An additional 100 subjects were recruited to validate the results. Compared to placebo stimulation, both pressure and cold-pain thresholds increased during spicy stimulation (P < .05). The increased thresholds remained, even when the taste stimulation residue was nearly eliminated (P < .05). The pressure (10.0 [2.1] vs 12.7 [3.0] kg/cm2, P < .001) and cold-pain (4.4 [1.6] vs 6.2 [2.7] seconds, P = .003) thresholds in subjects who consume spicy food ≥3 days/week were significantly lower than in those who consume it <3 days/week. In the validation population, the frequency of spicy-food intake was negatively associated with subjects' pressure (β = -.218, P = .013) and cold-pain (β = -.205, P = .035) thresholds. Spicy stimulation has an analgesia effect on adults that persists even after the taste stimulation stops. Conversely, a long-term spicy diet can reduce the human basal pain threshold. TRIAL REGISTRATION: The study protocol was approved by the Medical Ethics Committee of the Second Affiliated Hospital of Army Medical University, People's Liberation Army (identification No., 2017-023-01), and it was registered on the Chinese Clinical Trial Registry at www.chictr.org.cn (No. ChiCTR1800015053). PERSPECTIVE: This study directly examined the effects of stimulation of a spicy sensation on adult pain sensitivity and was the first to explore the relationship between long-term spicy-food intake and human pain sensitivity. The results provide evidence for future clinical pain intervention and individualized pain treatment.

Opioid-receptor antagonism increases pain and decreases pleasure in obese and non-obese individuals

RATIONALE

Endogenous opioids inhibit nociceptive processing and promote the experience of pleasure. It has been proposed that pain and pleasure lie at opposite ends of an affective spectrum, but the relationship between pain and pleasure and the role of opioids in mediating this relationship has not been tested.

OBJECTIVES

Here, we used obese individuals as a model of a dysfunctional opioid system to assess the role of the endogenous opioid peptide, beta-endorphin, on pain and pleasure sensitivity.

METHODS

Obese (10M/10F) and age- and gender-matched non-obese (10M/10F) controls were included in the study. Pain sensitivity using threshold, tolerance, and subjective rating assessments and perceived sweet pleasantness using sucrose solutions were assessed in two testing sessions with placebo or the opioid antagonist, naltrexone (0.7 mg/kg body weight). Beta-endorphin levels were assessed in both sessions.

RESULTS AND CONCLUSIONS

Despite having higher levels of baseline beta-endorphin and altered beta-endorphin-reactivity to naltrexone, obese individuals reported a similar increase in pain and decrease in pleasantness following naltrexone compared to non-obese individuals. Beta-endorphin levels did not correlate with pain or pleasantness in either group, but naltrexone-induced changes in pain and pleasantness were mildly correlated. Moreover, naltrexone-induced changes in pain were related to depression scores, while naltrexone-induced changes in sweet pleasantness were related to anxiety scores, indicating that pain and pleasantness are related, but influenced by different processes.

Tobacco industry use of flavourings to promote smokeless tobacco products

BACKGROUND

While fruit, candy and alcohol characterising flavours are not allowed in cigarettes in the USA, other flavoured tobacco products such as smokeless tobacco (ST) continue to be sold. We investigated tobacco manufacturers' use of flavoured additives in ST products, the target audience(s) for flavoured products, and marketing strategies promoting products by emphasising their flavour.

METHODS

Qualitative analysis of internal tobacco industry documents triangulated with data from national newspaper articles, trade press and internet.

RESULTS

Internally, flavoured products have been consistently associated with young and inexperienced tobacco users. Internal studies confirmed that candy-like sweeter milder flavours (eg, mint, fruit) could increase appeal to starters by evoking a perception of mildness, blinding the strong tobacco taste and unpleasant mouth feel; or by modifying nicotine delivery by affecting product pH.

DISCUSSION

Similar to cigarettes, flavoured ST is likely to encourage novices to start using tobacco, and regulations limiting or eliminating flavours in cigarettes should be extended to include flavoured ST products.

Heightened eating drive and visual food stimuli attenuate central nociceptive processing

Hunger and pain are basic drives that compete for a behavioral response when experienced together. To investigate the cortical processes underlying hunger-pain interactions, we manipulated participants' hunger and presented photographs of appetizing food or inedible objects in combination with painful laser stimuli. Fourteen healthy participants completed two EEG sessions: one after an overnight fast, the other following a large breakfast. Spatio-temporal patterns of cortical activation underlying the hunger-pain competition were explored with 128-channel EEG recordings and source dipole analysis of laser-evoked potentials (LEPs). We found that initial pain ratings were temporarily reduced when participants were hungry compared with fed. Source activity in parahippocampal gyrus was weaker when participants were hungry, and activations of operculo-insular cortex, anterior cingulate cortex, parahippocampal gyrus, and cerebellum were smaller in the context of appetitive food photographs than in that of inedible object photographs. Cortical processing of noxious stimuli in pain-related brain structures is reduced and pain temporarily attenuated when people are hungry or passively viewing food photographs, suggesting a possible interaction between the opposing motivational forces of the eating drive and pain.

Sucrose-induced analgesia in mice: role of nitric oxide and opioid receptor-mediated system

Background:

The mechanism of action of sweet substance-induced analgesia is thought to involve activation of the endogenous opioid system. The nitric oxide (NO) pathway has a pivotal role in pain modulation of analgesic compounds such as opioids.

Objectives:

We investigated the role of NO and the opioid receptor-mediated system in the analgesic effect of sucrose ingestion in mice.

Materials and Methods:

We evaluated the effect of intraperitoneal administration of 10 mg/kg of NO synthase inhibitor, N-nitro-L-arginine methyl ester (L-NAME) and 20 mg/kg of opioid receptor antagonist, naltrexone on the tail flick response in sucrose ingesting mice.

Results:

Sucrose ingestion for 12 days induced a statistically significant increase in the latency of tail flick response which was unmodified by L-NAME, but partially inhibited by naltrexone administration.

Conclusions:

Sucrose-induced nociception may be explained by facilitating the release of endogenous opioid peptides. Contrary to some previously studied pain models, the NO/cyclic guanosine monophosphate (cGMP) pathway had no role in thermal hyperalgesia in our study. We recommend further studies on the involvement of NO in other animals and pain models.

The relationship between pain and eating among overweight and obese individuals with osteoarthritis: an ecological momentary study

The relationship between pain and overweight/obesity is bidirectional – excess weight can cause pain, yet eating food high in calories, fat and sugar has been shown to alleviate pain. It appears likely that overweight/obese individuals may turn to these types of foods when they experience pain, leading to weight gain; however, this phenomenon has not been examined to date. The authors of this study set out to elucidate the relationship between pain and food intake in obese and overweight patients with osteoarthritis.

BACKGROUND:

Osteoarthritis (OA) patients who are overweight or obese report higher levels of pain compared with their normal-weight OA counterparts. Evidence suggests that overweight or obese OA patients also experience pain relief from eating foods high in calories, fat or sugar. Eating to alleviate pain may be problematic because it can lead to additional weight gain, which may contribute to heightened pain.

OBJECTIVES:

To investigate the relationship between pain and food intake using ecological momentary assessments in a sample of 71 over-weight and obese OA patients.

METHODS:

Participants completed two consecutive days of diary entries in which they recorded their levels of pain, mood and food intake throughout the day. Data were analyzed using generalized estimating equations that modelled pain as a predictor of calorie, fat and sugar intake. All models were adjusted for sex, body mass index, negative mood, time and treatment history.

RESULTS:

Pain significantly predicted calorie (Z=2.57; P=0.01) and fat intake (Z=1.99; P=0.05).

CONCLUSIONS:

Using ecological momentary assessments as a novel approach, the present study provides preliminary data supporting a relationship between pain and food intake among overweight and obese OA patients. Continued advances in our understanding of the relationship between pain and eating behaviour may help to optimize intervention strategies for these patients.

Effect of chronic intake of sweet substance on nociceptive thresholds and feeding behavior of Rattus norvegicus (Rodentia, Muridae)

The investigation of the influence of sweetened food on feeding behavior targeted to non-sucrose nutrients as well as the sensitivity to painful stimuli in isolated and grouped animals is the aim of the present work. The tail withdrawal latencies in the tail-flick test (a spinal reflex) were measured before and immediately after the treatment with tap water or sucrose (62, 125 or 250 g/l). Our findings suggest that: (a) The analgesic effect of sucrose intake depends on the concentration of sucrose solution and on the time during which the solution is consumed; (b) the most effective concentration of sucrose followed by antinociceptive effect was the one of 250 g/l in both isolated and grouped animals; (c) considering the individually caged rats, the intake of sucrose in the highest concentration (250 g/l) was the smallest as compared with the consumption of sucrose in more diluted solutions (62.5 and 125 g/l), but this higher sweetened solution was followed by antinociception; (d) animals treated with concentrated sucrose solution ate smaller quantities of pellets than animals treated with tap water; (e) tonic intake of highly concentrated sweet substance seems to be crucial for the increase of the nociceptive threshold in our model of sweet substance-induced antinociception.

Paradoxical surrogate markers of dental injury-induced pain in the mouse

Dental pain, including toothache, is one of the most prevalent types of orofacial pain, causing severe, persistent pain that has a significant negative effect on quality of life, including eating disturbances, mood changes, and sleep disruption. As the primary cause of toothache pain is injury to the uniquely innervated dental pulp, rodent models of this injury provide the opportunity to study neurobiological mechanisms of tissue injury-induced persistent pain. Here we evaluated behavioral changes in mice with a dental pulp injury (DPI) produced by mechanically exposing the pulp to the oral environment. We monitored the daily life behaviors of mice with DPI, including measures of eating, drinking, and movement. During the first 48 hours, the only parameter affected by DPI was locomotion, which was reduced. There was also a significant short-term decrease in the amount of weight gained by DPI animals that was not related to food consumption. As cold allodynia is frequently observed in individuals experiencing toothache pain, we tested whether mice with DPI demonstrate an aversion to drinking cold liquids using a cold-sucrose consumption test. Surprisingly, mice with DPI increased their consumption of sucrose solution, to over 150% of baseline, regardless of temperature. Both the weight loss and increased sucrose intake in the first 2 days of injury were reversed by administration of indomethacin. These findings indicate that enhanced sucrose consumption may be a reliable measure of orofacial pain in rodents, and suggest that alterations in energy expenditure and motivational behaviors are under-recognized outcomes of tooth injury.

Sweetness and food preference

Human desire for sweet taste spans all ages, races, and cultures. Throughout evolution, sweetness has had a role in human nutrition, helping to orient feeding behavior toward foods providing both energy and essential nutrients. Infants and young children in particular base many of their food choices on familiarity and sweet taste. The low cost and ready availability of energy-containing sweeteners in the food supply has led to concerns that the rising consumption of added sugars is the driving force behind the obesity epidemic. Low-calorie sweeteners are one option for maintaining sweet taste while reducing the energy content of children's diets. However, their use has led to further concerns that dissociating sweetness from energy may disrupt the balance between taste response, appetite, and consumption patterns, especially during development. Further studies, preferably based on longitudinal cohorts, are needed to clarify the developmental trajectory of taste responses to low-calorie sweeteners and their potential impact on the diet quality of children and youth.

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.

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.

Effect of caloric and non-caloric sweet reward solutions on thermal facial operant conditioning

Sweet solutions are commonly used in animal research to deliver drugs to test for addictive capacity and efficacy. In this study we compared the effects of a range of sucrose and saccharin concentrations on the performance of an operant assay. Our findings demonstrate that across a range of sucrose solutions some produce a success ratio which could mistakenly be labeled allodynic demonstrating the importance of choosing the correct reward solution.

Sweet preferences and analgesia during childhood: effects of family history of alcoholism and depression

AIM

To determine whether depression and family history of alcoholism are associated with heightened sweet preferences in children, before they have experienced alcohol or tobacco and at a time during the life-span when sweets are particularly salient. Design Between- and within-subject experimental study.

PARTICIPANTS

Children, 5-12 years old (n = 300), formed four groups based on family history of alcohol dependence up to second-degree relatives [positive (FHP) versus negative (FHN)] and depressive symptoms as determined by the Pictorial Depression Scale [depressed (PDEP) versus non-depressed (NDEP)].

MEASUREMENTS

Children were tested individually to measure sucrose preferences, sweet food liking and, for a subset of the children, the analgesic properties of sucrose versus water during the cold pressor test.

FINDINGS

The co-occurrence of having a family history of alcoholism and self-reports of depressive symptomatology was associated significantly with a preference for a more concentrated sucrose solution, while depressive symptomatology alone was associated with greater liking for sweet-tasting foods and candies and increased pain sensitivity. Depression antagonized the analgesic properties of sucrose.

CONCLUSIONS

While children as a group innately like sweets and feel better after eating them, the present study reveals significant contributions of family history of alcoholism and depression to this effect. Whether the heightened sweet preference and the use of sweets to alleviate depression are markers for developing alcohol-related problems or responses that are protective are important areas for future research.

Adding cocoa to sucrose: the effect on cold pain tolerance

The sweet taste of sucrose acts as an analgesic, whereas the taste of a bitter substance decreases pain tolerance. The present experiment explores the analgesic effect of a complex taste and asks how adding cocoa, a substance often associated with sweet foods but that has a bitter taste, to a sucrose solution affects cold pain tolerance. The 24 male participants were exposed to Cold Pressor Tests (CPTs) while holding 1 of 3 tastants in their mouths: water, sucrose, or sucrose with cocoa added. After each CPT, participants rated pain intensity and tastant qualities. Intraoral sucrose increased the amount of time that men were able to leave their hands in cold water, whereas the cocoa solution did not. Solutions did not differ in pleasantness or sweetness, but the cocoa solution was rated as more bitter. Bitterness ratings of cocoa exceeded the ratings of sucrose (corrected for water) by an average of 16.9% (P = 0.02), which, in turn, produced a 30% reduction in the duration of pain tolerance (P = 0.002). These results suggest that the addition of a bitter substance reduces cues to the nutritive value of sucrose that may drive its analgesic effect.

Analgesia accompanying food consumption requires ingestion of hedonic foods

Animals eat rather than react to moderate pain. Here, we examined the behavioral, hedonic, and neural requirements for ingestion analgesia in ad libitum fed rats. Noxious heat-evoked withdrawals were similarly suppressed during self-initiated chocolate eating and ingestion of intraorally infused water, sucrose, or saccharin, demonstrating that ingestion analgesia does not require feeding motivation, self-initiated food procurement, sucrose, or calories. Rather, food hedonics is important because neither salt ingestion nor quinine rejection elicited analgesia. During quinine-induced nausea and lipopolysaccharide (LPS)-induced illness, conditions when chocolate eating was presumably less pleasurable, analgesia accompanying chocolate consumption was attenuated, yet analgesia during water ingestion was preserved in LPS-injected rats who showed enhanced palatability for water within this context. The dependence of ingestion analgesia on the positive hedonics of an ingestate was confirmed in rats with a conditioned taste aversion to sucrose: after paired exposure to sucrose and LPS, rats no longer showed analgesia during sucrose ingestion but continued to show analgesia during chocolate consumption. Eating pauses tended to occur less often and for shorter durations in the presence of ingestion analgesia than in its absence. Therefore, we propose that ingestion analgesia functions to defend eating from ending. Muscimol inactivation of the medullary raphe magnus blocked the analgesia normally observed during water ingestion, showing the involvement of brainstem endogenous pain inhibitory mechanisms in ingestion analgesia. Brainstem-mediated defense of the consumption of palatable foods may explain, at least in part, why overeating tasty foods is so irresistible even in the face of opposing cognitive and motivational forces.

Behavioral approaches to anxiety and pain management for pediatric venous access

Pediatric venous access causes unnecessary anxiety and pain in children and, in turn, can have detrimental consequences. Behavioral approaches to pediatric venous access distress management can be organized temporally. Specifically, preparation before the procedure includes providing children with sensory and procedural information in an age-appropriate manner and providing training in coping skills. It is important to consider the timing, format, and content of the approach to provide optimal preparation for the unique circumstances of the individual patient. In addition to the child patient, preparing parents and teaching them which specific behaviors might be most helpful to their child should prove valuable to both patient and parents. During the procedure, there are benefits to providing secure and comfortable positioning. In addition, researchers recommend that adults encourage children to cope and actively engage children in distracting activities. For infants, there is support for the distress-mitigation properties of swaddling, skin-to-skin contact, breastfeeding, and sucrose. After venous access, distraction and encouragement of coping should speed recovery. In sum, research in behavioral approaches to pediatric pain management has provided recommendations for minimizing children's anxiety and pain associated with venous access.

Kangaroo mother care diminishes pain from heel lance in very preterm neonates: a crossover trial

BACKGROUND

Skin-to-skin contact, or kangaroo mother care (KMC) has been shown to be efficacious in diminishing pain response to heel lance in full term and moderately preterm neonates. The purpose of this study was to determine if KMC would also be efficacious in very preterm neonates.

METHODS

Preterm neonates (n = 61) between 28 0/7 and 31 6/7 weeks gestational age in three Level III NICU's in Canada comprised the sample. A single-blind randomized crossover design was employed. In the experimental condition, the infant was held in KMC for 15 minutes prior to and throughout heel lance procedure. In the control condition, the infant was in prone position swaddled in a blanket in the incubator. The primary outcome was the Premature Infant Pain Profile (PIPP), which is comprised of three facial actions, maximum heart rate, minimum oxygen saturation levels from baseline in 30-second blocks from heel lance. The secondary outcome was time to recover, defined as heart rate return to baseline. Continuous video, heart rate and oxygen saturation monitoring were recorded with event markers during the procedure and were subsequently analyzed. Repeated measures analysis-of-variance was employed to generate results.

RESULTS

PIPP scores at 90 seconds post lance were significantly lower in the KMC condition (8.871 (95%CI 7.852-9.889) versus 10.677 (95%CI 9.563-11.792) p < .001) and non-significant mean differences ranging from 1.2 to1.8. favoring KMC condition at 30, 60 and 120 seconds. Time to recovery was significantly shorter, by a minute(123 seconds (95%CI 103-142) versus 193 seconds (95%CI 158-227). Facial actions were highly significantly lower across all points in time reaching a two-fold difference by 120 seconds post-lance and heart rate was significantly lower across the first 90 seconds in the KMC condition.

CONCLUSION

Very preterm neonates appear to have endogenous mechanisms elicited through skin-to-skin maternal contact that decrease pain response, but not as powerfully as in older preterm neonates. The shorter recovery time in KMC is clinically important in helping maintain homeostasis.

TRIAL REGISTRATION

(Current Controlled Trials) ISRCTN63551708.

Food intake and reward mechanisms in patients with schizophrenia: implications for metabolic disturbances and treatment with second-generation antipsychotic agents

Obesity is highly prevalent among patients with schizophrenia and is associated with detrimental health consequences. Although excessive consumption of fast food and pharmacotherapy with such second-generation antipsychotic agents (SGAs) as clozapine and olanzapine has been implicated in the schizophrenia/obesity comorbidity, the pathophysiology of this link remains unclear. Here, we propose a mechanism based on brain reward function, a relevant etiologic factor in both schizophrenia and overeating. A comprehensive literature search on neurobiology of schizophrenia and of eating behavior was performed. The collected articles were critically reviewed and relevant data were extracted and summarized within four key areas: (1) energy homeostasis, (2) food reward and hedonics, (3) reward function in schizophrenia, and (4) metabolic effects of the SGAs. A mesolimbic hyperdopaminergic state may render motivational/incentive reward system insensitive to low salience/palatability food. This, together with poor cognitive control from hypofunctional prefrontal cortex and enhanced hedonic impact of food, owing to exaggerated opioidergic drive (clinically manifested as pain insensitivity), may underlie unhealthy eating habits in patients with schizophrenia. Treatment with SGAs purportedly improves dopamine-mediated reward aspects, but at the cost of increased appetite and worsened or at least not improved opiodergic capacity. These effects can further deteriorate eating patterns. Pathophysiological and therapeutic implications of these insights need further validation via prospective clinical trials and neuroimaging studies.

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.

Sucrose-induced analgesia is related to sweet preferences in children but not adults

The present study tested the hypothesis that the efficacy of sucrose in reducing pain during the Cold Pressor Test (CPT) was related to its hedonic value. To this aim, we determined the most preferred level of sucrose and the analgesic properties of 24% w/v sucrose during the CPT in 242, 5- to 10-year-old children and their mothers. Outcome measures included pain thresholds (the time at which discomfort was first indicated) and pain tolerance (the length of time the hand was kept in the cold water bath). Although children, as a group, preferred significantly higher sucrose concentrations than adults, there were individual differences that allowed us to group them on the basis of those who preferred sucrose concentrations below that used in the CPT (24% w/v) and those who preferred levels >or=24% w/v sucrose. Regardless of such groupings, sucrose was not an effective analgesic in adult women. Unlike adults, the more children liked sucrose, the better its efficacy as an analgesic. That is, children who preferred >or=24%w/v sucrose exhibited an increased latency to report pain and tolerated pain for significantly longer periods of time when sucrose was held in their mouths relative to water. This effect was more pronounced among normal weight when compared to overweight/at risk for overweight children. The role that dietary habits and individual differences contribute to the preferences for sweet taste and its physiological consequences in children is an important area for future research.

Sugar Solution Analgesia: The Effects of Glucose on Expressed Mu Opioid Receptors

Glucose or sucrose solutions administered orally provide effective analgesia for procedural pain in neonates. Because analgesia with sugar solutions can be decreased by opioid receptor antagonists, we tested the hypothesis that glucose directly activates opioid receptors. Mu opioid receptors (MOR-1) were expressed in Xenopus oocytes, a well recognized expression system, and glucose was tested for possible agonist, antagonist, and modulatory effects on the receptor. In control experiments, 10 nM of Tyr-D-Ala-Gly-Me-Phe-Gly-ol (DAMGO), a synthetic enkephalin and specific mu agonist, activated the MOR-1, whereas 20 mM of glucose had no effect. In addition, glucose had no effect on the activation of the mu receptor by DAMGO. Finally, glucose did not modulate acute receptor desensitization induced by DAMGO. We conclude that glucose does not directly interact with MOR-1 in an in vitro expression system and that the purported interaction between glucose and the opioid system may be an indirect one, involving release of endogenous opioids.

Effect of the blockade of mu1-opioid and 5HT2A-serotonergic/alpha1-noradrenergic receptors on sweet-substance-induced analgesia

RATIONALE

Sweet-substance-induced analgesia has been widely studied, and the investigation of the neurotransmitters involved in this antinociceptive process is an important way for understanding the involvement of the neural system controlling this kind of antinociception.

OBJECTIVE

The aim of this study was to investigate the involvement of opioid and monoaminergic systems in sweet-substance-induced analgesia.

METHODS

The present work was carried out in an animal model with the aim of investigating whether acute (24 h) or chronic (14 days) intake of a sweet substance, such as sucrose (250 g/l), is followed by antinociception. Tail withdrawal latencies in the tail-flick test were measured before and immediately after this treatment. Immediately after the recording of baseline values, independent groups of rats were submitted to sucrose or tap-water intake and, after chronic treatment, they were pretreated with intraperitoneal administration of (1) naltrexone at 0.5, 1, 2 or 3 mg/kg; (2) naloxonazine at 5, 10, 20 or 30 mg/kg; (3) methysergide at 0.5, 1, 2 or 3 mg/kg; (4) ketanserin at 0.5, 1, 2 or 3 mg/kg; or (5) physiological saline.

RESULTS

Naltrexone and methysergide at two major doses decreased sweet-substance-induced analgesia after chronic intake of a sweet substance. These effects were corroborated by peripheral administration of naloxonazine and ketanserin.

CONCLUSIONS

These data give further evidence for: (a) the involvement of endogenous opioids and a mu1-opioid receptor in the sweet-substance-induced antinociception; (b) the involvement of monoamines and 5HT2A serotonergic/alpha1-noradrenergic receptors in the central regulation of the sweet-substance-produced analgesia.

Involvement of pre- and post-synaptic serotonergic receptors of dorsal raphe nucleus neural network in the control of the sweet-substance-induced analgesia in adult Rattus norvegicus (Rodentia, Muridae)

In order to investigate the effects of monoaminergic mechanisms of the dorsal raphe nucleus 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 14 days as their only source of liquid. After the chronic consumption of sucrose solution, each animal was pretreated with unilateral microinjection of methiothepin mesylate (5.0 microg/0.2 microL), or methysergide maleate (5.0 microg/0.2 microL) in the dorsal raphe nucleus. Each rat consumed an average of 15.6g sucrose/day. Their tail withdrawal latencies in the tail-flick test were measured immediately before and after this treatment. An analgesia index was calculated from the withdrawal latencies before and after the pharmacological treatment. The blockade of serotonergic receptor in the dorsal raphe nucleus with methysergide after the chronic intake of sucrose decreased the sweet-induced antinociception. However, microinjections of methiothepin in the dorsal raphe nucleus did not cause a similar effect on the tail-flick latencies after the chronic intake of sucrose solution, increasing the sweet-substance-induced analgesia. These results indicate the involvement of serotonin as a neurotransmitter in the sucrose-produced antinociception. Considering that the blockade of pre-synaptic serotonergic receptors of the neural networks of the dorsal raphe nucleus with methiothepin did not decrease the sweet-substance-induced antinociception, and the central blockade of post-synaptic serotonergic receptors decreased the sucrose-induced analgesia, the modulation of the release of serotonin in the neural substrate of the dorsal raphe nucleus seems to be crucial for the organization of this interesting antinociceptive process.

Sugar-dependent rats show enhanced intake of unsweetened ethanol

Rats show signs of dependence on sugar when it is available intermittently, including bingeing, withdrawal, and cross-sensitization with amphetamine. In the current study, we sought to determine whether sugar-dependent rats would show increased intake of unsweetened ethanol and, conversely, whether intermittent access to ethanol would augment sugar consumption. In Experiment 1, with intermittent versus ad libitum access to ethanol, Sprague-Dawley rats were given escalating concentrations of ethanol (1%, 2%, 4%, 7%, and 9%) over the course of 20 days. Rats in the intermittent ethanol access group, with 12-h daily access, consumed more 4%, 7%, and 9% ethanol during the first hour of access, and more 9% ethanol daily, than did rats in the ad libitum ethanol access group. In Experiment 2, with ethanol as a gateway to sugar intake, the rats from Experiment 1 were switched to 10% sucrose with 12-h daily access for 1 week. Rats in the intermittent ethanol access group consumed significantly more sugar than was consumed by rats in a control group with no prior ethanol experience. In Experiment 3, with sugar as a gateway to ethanol to determine whether sugar dependence leads to increased ethanol intake, four groups were maintained for 21 days according to the following designations: intermittent access to sugar and chow, ad libitum access to sugar and chow, intermittent access to chow, or ad libitum access to chow. Four days later, all groups were switched to intermittent ethanol access, as described in Experiment 1. The group with intermittent access to sugar and chow consumed the most 9% ethanol, supporting the suggestion that sugar dependence alters a rat's proclivity to drink ethanol. These results may relate to the co-morbidity between binge-eating disorders and alcohol intake and the tendency of people abstaining from alcohol to consume excessive amounts of sugar. In conclusion, bingeing on either ethanol or sugar fosters intake of the other.

Taste modulation of nociception differently affects chronically stressed rats

Stress responses cover a wide range of physiological changes, including alterations in the perception of and response to pain. Animals submitted to repeated stress present altered nociception and this effect is part of this process of adaptation; in addition pleasant and unpleasant experiences with tastes and odors have been shown to affect distinct behavioral aspects, such as pain perception. The aim of the present study is to verify the responses of repeatedly stressed rats (1 h of daily immobilization during 40 days) to pleasant and unpleasant tastes on nociception, when compared to control animals. An increase in the tail-flick latency (TFL) was observed 5 min after exposure to a sweet taste in the control group, whereas no effect was observed in chronically stressed animals. When submitted to an unpleasant taste (5% acetic acid), the chronically stressed group presented an increase in TFL, whereas no effect was observed in the control group. In conclusion, chronically stressed animals present different nociceptive responses to sweet and acid tastes; although control animals suitably respond to a sweet stimulus, stressed animals seem to be more apt to react to the unpleasant stimulus.

[Treatment of postoperative pain in elderly oncology patients with intravenous administration of a 50% glucose solution]

Postoperative pain is the most important factor od so called "tumor-promotive effect of surgery" i.e. of endocrine-metabolic changes having the consequence drop in immune, antiinfective and antitumor defense. Due to presence of organic involutive changes, old people (= 65 years), often have serious side effects during application of usual analgetics. Since hypertonic glucose (33%) given i.v. or per os, works analgesically in small children there is assumption that it can be used in treatment of postoperative pain in old oncology patients. We tested the hypothesis that postoperative pain in old oncology patients can be treated with i.v. application of 50% of glucose solution. 37 oncology patients over 65 years, 26 females and 11 males, operated for breast cancer and soft tissue cancer, were investigated. Average age of the patients was 72 +/- 4 years. 50% Glucose solution was given in two boluses of 20 ml each: the first bolus was given to all patients at the end of anesthesia, and the other bolus was given individually after appearance of post-operative pain. Pain intensity (in coefficients of the visual analogue scale VAK = 1-100) and its characteristics were tested by oral testing of operated patients; after weakening from anesthesia, after the first appearance of the pain and 15 minutes after giving of the second glucose bolus. None patient had pain weakening from anesthesia. All tested patients experienced pain during the first 70 minutes and it could be categorized as very strong pain (= 82 VAK). The pain was decreased with another glucose bolus by approximately (= 56% VAK) so it was classifies in category of bearable pains (= 36 VAK). In 9 patients (24.3%) the pain had neuropathic component (filing of "burning") which could not be eliminated by hypertonic glucose, but only with application of tramadol. Activation of the central cholinergic transmission is the most significant mechanism of analgesic glucose effect, but, probably there is another one: facilitation of entrance of formerly given analgesics in the brain cells. As energetic substrate, entering all organism cells, glucose could make easier intracell breakthrough of any other analgesic drug, of the peripheral or central action, and final antipain effect could be potential or additional one. It was concluded that 40 ml of 50% glucose solution given in two identical boluses, has good analgesic effect in treatment of postoperative pain in old oncology patient: the pain was not completely eliminated, but it was significantly decreased and became tolerable. Hypertonic glucose neither eliminates, nor decreases neuropathic component of the pain, so, when the pain appears the therapy should be supplemented with other drugs, which may completely eliminate all pain components.

Age-dependency of analgesia elicited by intraoral sucrose in acute and persistent pain models

Treatment of pain in newborns is associated with problematic drug side effects. Previous studies demonstrate that an intraoral infusion of sucrose and other sweet components of mother's milk are effective in alleviating pain in infant rats and humans. These findings are of considerable significance, as sweet tastants are used in pain and stress management in a number of clinical procedures performed in human infants. The ability of sweet stimuli to induce analgesia is absent in adult rats, suggesting that this is a developmentally transient phenomenon. However, the age range over which intraoral sucrose is capable of producing analgesia is not known. We investigated the effects of intraoral sucrose (7.5%) on nocifensive withdrawal responses to thermal and mechanical stimuli in naive and inflamed rats at postnatal days (P) P0-21. In some rats, Complete Freund's adjuvant (CFA) was injected in a fore- or hindpaw to produce inflammation. In non-inflamed animals, for noxious thermal stimuli, sucrose-induced analgesia emerged at P3, peaked at P7-10, then progressively declined and was absent at P17. For mechanical forepaw stimuli, sucrose-induced analgesia emerged, and was maximal at approximately P10, then declined and was absent at P17. By contrast, maximal sucrose-induced analgesia for mechanical hindpaw stimuli was delayed (P13) compared to that for the forepaw, although it was also absent at P17. In inflamed animals, sucrose reduced hyperesthesia and hyperalgesia assessed with mechanical stimuli. Sucrose-induced analgesia in inflamed animals was initially present at P3 for the forepaw and P13 for the hindpaw, and was absent by P17 for both limbs. Intraoral sucrose produced significantly greater effects on responses in fore- and hindpaws in inflamed rats than in naive rats indicating that it reduces hyperalgesia and allodynia beyond its effects on responses in naive animals. These findings support the hypothesis that sucrose has a selective influence on analgesic mechanisms and that an enhanced sucrose effect takes place in hyperalgesic, inflamed animals as compared to naive animals. Taken together, these results indicate that intraoral sucrose alleviates transient pain in response to thermal and mechanical stimuli, and also effectively reduces inflammatory hyperalgesia and allodynia. Sucrose-induced analgesia is age-dependent and limited to the pre-weaning period in rats. The age-dependency of sucrose-induced analgesia and its differential maturation for the fore- and hindpaw may be due to developmental changes in endogenous analgesic mechanisms and developmental modulation of the interaction between gustatory and pain modulatory pathways.

Psychopharmacological evidences for the involvement of muscarinic and nicotinic cholinergic receptors on sweet substance-induced analgesia in Rattus norvegicus

In order to investigate the effects of sweet substance intake on pain modulation, male albino Wistar rats weighing 180-200 g received either tap water or sucrose solutions (250 g/l) for 14 days as their only source of liquid. Each rat consumed an average of 15.6 g sucrose/day. Their tail withdrawal latencies in the tail-flick test (probably a spinal reflex) were measured immediately before and after this treatment. An analgesia index was calculated from the withdrawal latencies before and after treatment. The index (mean +/- SEM, N = 8) for the groups receiving sucrose solution plus saline (NaCl; 0.9%) for 14 days was 0.70 +/- 0.01. Atropine (1 and 2 mg/kg)-treated rats (N = 8) after intake of sucrose exhibited an analgesia index of 0.39 +/- 0.09 and 0.39 +/- 0.08, respectively, while mecamylamine (1 and 2 mg/kg)-treated rats (N = 10) after intake of sucrose had an index of -0.02 +/- 0.07 and 0.03 +/- 0.07, respectively. These results indicate that the effect of sucrose intake on nociceptive thresholds is controlled by neurotransmission of acetylcholine and depends on the nicotinic cholinergic receptors for its major analgesic effect, although muscarinic receptors were also involved in this antinociceptive process.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.