Tolerance to morphine analgesia from brief exposure to a palatable solution

Modulation of Pain Sensitivity by Chronic Consumption of Highly Palatable Food Followed by Abstinence: Emerging Role of Fatty Acid Amide Hydrolase

There is a strong relationship between palatable diet and pain sensitivity, and the cannabinoid and opioid systems might play an important role in this correlation. The palatable diet used in many animal models of obesity is the cafeteria (CAF) diet, based on human food with high sugar, salt, and fat content. In this study, we investigated whether long-term exposure to a CAF diet could modify pain sensitivity and explored the role of the cannabinergic system in this modification. Male Sprague–Dawley rats were divided into two groups: one fed with standard chow only (CO) and the other with extended access (EA) to a CAF diet. Hot plate and tail flick tests were used to evaluate pain sensitivity. At the end of a 40-day CAF exposure, EA rats showed a significant increase in the pain threshold compared to CO rats, finding probably due to up-regulation of CB1 and mu-opioid receptors. Instead, during abstinence from palatable foods, EA animals showed a significant increase in pain sensibility, which was ameliorated by repeated treatment with a fatty acid amide hydrolase inhibitor, PF-3845 (10 mg/kg, intraperitoneally), every other day for 28 days. Ex vivo analysis of the brains of these rats clearly showed that this effect was mediated by mu-opioid receptors, which were up-regulated following repeated treatment of PF-3845. Our data add to the knowledge about changes in pain perception in obese subjects, revealing a key role of CB1 and mu-opioid receptors and their possible pharmacological crosstalk and reinforcing the need to consider this modulation in planning effective pain management for obese 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.

You are what you eat: influence of type and amount of food consumed on central dopamine systems and the behavioral effects of direct- and indirect-acting dopamine receptor agonists

The important role of dopamine (DA) in mediating feeding behavior and the positive reinforcing effects of some drugs is well recognized. Less widely studied is how feeding conditions might impact the sensitivity of drugs acting on DA systems. Food restriction, for example, has often been the focus of aging and longevity studies; however, other studies have demonstrated that mild food restriction markedly increases sensitivity to direct- and indirect-acting DA receptor agonists. Moreover, it is becoming clear that not only the amount of food, but the type of food, is an important factor in modifying the effects of drugs. Given the increased consumption of high fat and sugary foods, studies are exploring how consumption of highly palatable food impacts DA neurochemistry and the effects of drugs acting on these systems. For example, eating high fat chow increases sensitivity to some behavioral effects of direct- as well as indirect-acting DA receptor agonists. A compelling mechanistic possibility is that central DA pathways that mediate the effects of some drugs are regulated by one or more of the endocrine hormones (e.g. insulin) that undergo marked changes during food restriction or after consuming high fat or sugary foods. Although traditionally recognized as an important signaling molecule in regulating energy homeostasis, insulin can also regulate DA neurochemistry. Because direct- and indirect-acting DA receptor drugs are used therapeutically and some are abused, a better understanding of how food intake impacts response to these drugs would likely facilitate improved treatment of clinical disorders and provide information that would be relevant to the causes of vulnerability to abuse drugs. This article is part of a Special Issue entitled 'Central Control of Food Intake'.

The relationship between opioid and sugar intake: review of evidence and clinical applications

Opioid dependence poses significant public health risks arising from associated morbidity and mortality caused by accidents, infectious diseases, and social ramifications of crime and unemployment, among other complications. Opioid use, acute and chronic, is also associated with weight gain, glycemic dysregulation, and dental pathology. The literature supporting the connection between opiate use and development of preference for sweet tastes is reviewed, and further association with dental pathology, weight gain, and loss of glycemic control are considered. Additionally, the impact of sweet tastes on the endogenous opioid system, as pertaining to analgesia, is also discussed. The authors discuss the clinical implications in relation to the aforementioned conditions while treating the opiate-dependent patient.

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.

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.

Influence of sweet tasting solutions on opioid withdrawal

The effect of the ingestion of palatable fluids on the suppression of opioid withdrawals in the opioid-dependent rats was studied. Physical dependence was induced by administration of morphine over a period of 6 days. Withdrawals were precipitated with naloxone (1 mg/kg, s.c.) 4 h after the last morphine injection on the 6th day. Test drugs (10-30% sucrose solution) were given orally for 2 h prior to naloxone-induced withdrawal in 14 h water deprived rats. Somatic signs of withdrawals were scored by using the global Gellert-Holtzman rating scale. Animals pretreated with low doses of sucrose solution (10-15%) did not produce any effect on the global withdrawal scale whereas a significant decrease on the global withdrawal scores was observed at higher doses of sucrose solution (20-30%) as compared to the controls. All the individual behavioral signs of withdrawals were significantly suppressed in a dose-related manner at higher doses of sucrose solutions whereas minimal suppression was observed for facial fasciculation/teeth chattering. These findings provide support that ingestion of high concentrations of sucrose solutions for shorter duration may activate the endogenous opioid system and appears to have an important role in modifying morphine withdrawals.

A concept of welfare based on reward evaluating mechanisms in the brain: anticipatory behaviour as an indicator for the state of reward systems

In this review we attempt to link the efficiency by which animals behave (economy of animal behaviour) to a neuronal substrate and subjective states to arrive at a definition of animal welfare which broadens the scope of its study. Welfare is defined as the balance between positive (reward, satisfaction) and negative (stress) experiences or affective states. The state of this balance may range from positive (good welfare) to negative (poor welfare). These affective states are momentary or transient states which occur against the background of and are integrated with the state of this balancing system. As will be argued the efficiency in behaviour requires that, for instance, satisfaction is like a moving target: reward provides the necessary feedback to guide behaviour; it is a not steady-state which can be maintained for long. This balancing system is reflected in the brain by the concerted action of opioid and mesolimbic dopaminergic systems. The state of this system reflects the coping capacity of the animal and is determined by previous events. In other words, this integrative approach of behavioural biology and neurobiology aims at understanding how the coping capacity of animals may be affected and measured. We argue that this balancing system underlies the economy of behaviour. Furthermore we argue that among other techniques anticipation in Pavlovian conditioning is an easy and useful tool to assess the state of this balancing system: for estimating the state of an animal in terms of welfare we focus on the conditions when an animal is facing a challenge.

Interaction between calcium channel blockers and sweetening agents on morphine-induced analgesia in mice by formalin test

1. Calcium is known to be an important ion in the modulation of nociception and inflammation. Previous research has shown that mice drinking sweet-tasting solutions such as sucrose, saccharin and aspartame exhibit significant changes in morphine-induced analgesia in both phases of the formalin test. 2. In this study, the role of calcium channel blockers on the effectivity of a 12-day regimen of different sweetening agents (sucrose 32%, saccharin 0.08% and aspartame 0.16%) on the alteration of the morphine response has been investigated. 3. Male albino mice weighing 20-27 g were used for experiments. Animals were given 12 days to adapt to dietary conditions. Animals were given morphine (1.5, 3, 6, 9 mg/kg) subcutaneously 30 min before observation. Nifedipine (5 mg/kg), verapamil (5 mg/kg) and diltiazem (10 mg/kg) were administered intraperitoneally 20 min before morphine injection. 4. Recording of the early phase started immediately and lasted for 10 min after formalin injection. Recording of the late response started 20 min after formalin injection and lasted for 10 min. 5. Calcium channel blockers potentiated the antinociceptive effects of sweetening agents and diminished the antagonistic effects of these compounds on morphine-induced analgesia in the early and late phases of the formalin test. 6. It is proposed that calcium has a role for the interactive effects of sweetening agents and morphine on pain sensitivity.

The relationship between intravenous cocaine self-administration and avidity for saccharin

Two experiments were conducted to determine whether measures of saccharin intake could be used as a predictor of intravenous cocaine self-administration. Saccharin avidity, defined as the ratio of total daily fluid intake when saccharin and water were available to total intake when only water was available, was measured in male rats. Cocaine self-administration (0.4 mg/kg/infusion) was subsequently measured in an initial 18-h session, followed by daily 1-h sessions in which the infusion dose and the reinforcement schedule were varied. In the initial overnight session, some rats obtained the maximum or near-maximum number of infusions; this high level of cocaine intake was unrelated to saccharin avidity. In the remaining rats, there was a pattern somewhat resembling an "inverted-U," in which rats with low or high avidity self-administered less cocaine than those with intermediate avidity. This pattern reemerged later in the experiment when rats were tested at a low cocaine infusion dose combined with a FR-6 reinforcement schedule. In a second experiment, no significant relationship was observed between the self-administration of a lower cocaine dose (0.125 mg/kg/infusion) and avidity for either saccharin or the artificial sweetener SC-45647. Although these results are consistent with a previous report indicating no simple relationship between saccharin preference and the acquisition of cocaine self-administration, they do suggest that a more complex relationship may be observed under some conditions. Additional research with other drugs, as well as with caloric and noncaloric sweeteners, will be needed to determine the usefulness of taste measures in identifying or treating substance abuse.

Dietary Sucrose Lowers Nociceptive Latencies Independently of Thermogenic Effects

Previous reports indicate that chronic sucrose feeding produces a significant reduction in the latency of response in the radiant heat tail-flick test. Other earlier studies have shown a relationship between tail-skin temperature and tail-flick latency, while others yet have shown an increase in tail-skin temperature following sucrose feeding. Together these previous findings suggest the possibility that dietary-induced alterations in nociceptive latencies occur as an artifact secondary to diet-related changes in tail-skin temperature. The data presented in this study show that chronic sucrose feeding significantly increased tail-skin temperature (p < 0.0001) and decreased tail-flick latency (p < 0.0001) with significant correlations between tail-skin temperatures and tail-flick latencies in both the control and sucrose fed groups. However, while the slopes of the regression lines were similar for both groups, the elevations of the lines were significantly different (p = 0.0068) suggesting a dietary impact on nociceptive thresholds independent of the temperature effect. The data were also subjected to a previously reported temperature-correction procedure and comparisons in the methods of data analysis are discussed.

Effects of sweetening agents on morphine-induced analgesia in mice by formalin test

1. There is evidence that sweet-tasting substances such as sucrose and saccharin can interact with endogenous opioid systems. Further evidence showed that feeding mice different concentrations of sucrose and saccharin alter the latency in the tail-flick test. 2. In the current study, the effects of a 12-day regimen of different sweetening agents [sucrose (32%), saccharin (0.08%) and aspartame (0.16%)] on morphine-induced analgesia with the formalin test were investigated. 3. Male albino mice (20-27 g) were used for the experiments. Animals were given 12 days to adapt to dietary conditions. Animals were first given saline or morphine subcutaneously (1.5, 3.0, 6.0, or 9.0 mg/kg) 30 min before the observation period. The recording of the early phase started immediately and lasted for 10 min. The recording of the late response started 20 min after formalin injection and lasted for 10 min. Statistical analysis was performed by using analysis of variance followed by Newman-Keuls test, and P < or = 0.05 was considered significant. 4. Sucrose and aspartame increased morphine analgesia in the early phase, but saccharin had no effect on the early phase. On the other hand, saccharin and sucrose decreased the effect of morphine in the late phase, but aspartame increased the effect of morphine-induced analgesia. 5. In conclusion, the present data provide further evidence for an important role for dietary variables in determining the effects of exogenous opioids on pain sensitivity.

Dietary modulation of mu and kappa opioid receptor-mediated analgesia

Research has demonstrated that intake of palatable carbohydrates and fats enhanced morphine-induced analgesia (MIA) in Sprague-Dawley rats. To determine if the effects of palatable foods on nociceptive responses would generalize to other strains of animals and other opioid agonists, the present experiments investigated whether intake of palatable foods would: a) alter MIA in Long-Evans rats, and b) alter analgesia produced by drugs acting at kappa opioid receptors. In experiment 1, adult male Long-Evans rats were fed Purina chow alone or chow and either a 32% sucrose solution, a 0.15% saccharin solution, or hydrogenated vegetable fat. Using a tail-flick apparatus, nociceptive responses, measured as percent maximal possible effect (%MPE), were examined after morphine administration [0.0, 1.0, 3.0, and 6.0 mg/kg subcutaneously (SC)]. %MPEs varied directly as a function of dose and were significantly greater for rats fed chow and either sucrose or fat than for rats fed chow alone or chow and saccharin. Experiment 2 compared the analgesic effect of the kappa opioid receptor agonist U50,488H (0, 5.0, 10.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and a 32% sucrose solution. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose. %MPEs of rats fed chow and sucrose were significantly greater than those of rats fed chow alone after injections of 10.0 and 20.0 mg/kg U50,488H. Experiment 3 compared the analgesic effect of U50,488H (5.0, 10.0, 15.0, and 20.0 mg/kg SC) in rats fed chow alone or chow and either a 0.15% saccharin solution or hydrogenated vegetable fat. Administration of U50,488H led to analgesia. However, %MPEs did not vary directly as a function of dose or as a function of diet. %MPEs of rats fed chow and fat were significantly greater than those of rats fed chow alone after injection of 5.0 mg/kg U50,488H.

Duration of sucrose availability differentially alters morphine-induced analgesia in rats

The effects of duration of sucrose consumption on morphine-induced analgesia (MIA) were examined in 20 adult male Long-Evans rats. Ten rats were tested for MIA on a tail-flick apparatus following acute (5 h), chronic (3 weeks) intake, and subsequent removal of a 32% sucrose solution. Ten rats that never received the sucrose solution served as controls. Morphine sulfate was administered according to a cumulative dosing procedure beginning with 2.5 mg/kg morphine. The same dose was administered every 30 min until a total dose of 15 mg/kg was achieved. Tail-flick latencies were measured immediately prior to injections, and 30 min following each injection. After acute intake of sucrose, there was a trend for animals drinking the sugar solution to show suppressed MIA relative to animals drinking water. In contrast, after drinking the sucrose for 3 weeks, rats showed an enhanced MIA relative to rats drinking water. Three weeks after sucrose removal, there were no differences in MIA as a function of prior dietary conditions. The results support the hypothesis that length of exposure to sucrose influences morphine-induced analgesia and suggest that any change in physiology resulting from sucrose exposure may be reversible.

Exercise attenuates oral intake of amphetamine in rats

The effects of wheel running on oral intake of amphetamine were examined in six male Sprague-Dawley rats given a 0.075-mg/ml amphetamine sulfate solution as their sole source of liquid, six rats given a 0.15-mg/ml amphetamine solution, and four rats given water as their sole source of liquid. All animals were housed in Wahmann running wheels and adjoining cages, and had ad lib access to ground Purina Chow. For the first 7 days of the experiment, the doors to the running wheels were closed; the wheels were then opened for 6 days. This cycle was repeated a second time. Animals drinking the 0.15-mg/ml amphetamine solution consumed significantly less food and gained less weight than animals in the other two groups. Although there was no difference in food intake between rats drinking water and rats drinking the 0.075-mg/ml amphetamine solution, rats in the water group gained significantly more weight than rats in the 0.075-mg/ml amphetamine group. With respect to drug intake, rats consumed significantly less amphetamine when running in the wheels than when access to the wheels was prohibited. Access to running wheels did not alter water intake. These latter results suggest that drug intake can be reduced by the provision of an alternate behavior.

Early onset of reduced morphine analgesia by ingestion of sweet solutions

Morphine analgesia can be reduced by prior exposure to food and flavored fluids. The early onset of reduced morphine-induced analgesia (RMA) was studied in 82 male Wistar rats after allowing them access to either a dextrose-saccharin solution or unflavored tap water for 6 or 3 h (Experiment 1, n = 40) or for 3 h, 90, or 45 min (Experiment 2, n = 42). Morphine (4 mg/kg) was injected subcutaneously at the end of the drinking period, and after 25 min a series of tail flick tests was conducted. Morphine produced strong analgesia in all rats that drank unflavored tap water; however, in rats that drank the flavored solution, the analgesic effect of morphine was significantly attenuated following exposures of 6 or 3 h, but not following exposures of 90 or 45 min. Similar quantities of flavored fluid were consumed by groups at all exposure durations; thus, RMA was determined by duration of exposure and not amount consumed. No analgesia attributable to flavor consumption per se was observed. The results suggest that RMA is mediated by endogenous opioid activity in the gustatory and analgesic systems by a mechanism akin to tolerance that requires about 3 h to operate.

Intake of sweet water attenuates restraint-stress-induced potentiation of morphine analgesia in rats

The analgesia induced in rats by morphine is potentiated by restraint-stress exposure and is reduced in rats that have been consuming a sweet solution. The purpose of the present study was to determine whether the potentiation of morphine-induced analgesia following restraint immobilization would be attenuated in rats consuming a sweet solution. Groups of rats were maintained on unsweetened water or allowed 2 h of daily access to a solution of saccharin and glucose (SG). Half of the rats in each of these groups were subjected to 1 h of restraint stress (groups RS and RS+SG) and the other half in each group were not stressed (groups NS and NS+SG). Rats then underwent 1 h of RS treatment or were nonstressed (NS). The next day all rats were injected subcutaneously with morphine (0.0, 4.0, 8.0, or 16 mg/kg) and analgesia was assessed using the tail flick assay. ED50S (mg/kg) were calculated for each treatment group; NS = 5.8, RS = 1.6, NS+SG = 6.4, and RS+SG = 4.4. Our results demonstrate that RS potentiated morphine-induced analgesia in rats given access to SG as well as non-SG exposed rats that displayed ED50S 1.5 and 3.9 times lower than their respective controls. RS-treated rats that consumed SG solution had significantly lower tail flick latencies than did non-SG exposed rats. Additionally, tail flick latencies of rats in the nonstressed and NS+SG groups did not significantly differ. We conclude that the brain mechanism(s) responsible for RS-induced potentiation of morphine antinociception are attenuated by intake of a sweet solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary influences on morphine-induced analgesia in rats

Morphine-induced analgesia was examined using a tail-flick apparatus in 36 adult male Sprague-Dawley rats. Rats were given ad lib access to Purina Chow alone (N = 9) or given a choice of Purina Chow and either a 0.15% saccharin solution (N = 9), a 32% sucrose solution (N = 9), or hydrogenated vegetable fat (Crisco) (N = 9). Analgesic testing was conducted immediately preceding and at 30, 60 and 90 minutes following intraperitoneal administration of morphine sulfate (0.0, 2.5, 5.0 and 10.0 mg/kg). No differences in analgesic responsiveness were observed as a function of diet preceding morphine administration. However, dietary variables did alter morphine-induced analgesia. At 30 minutes following injections of the highest dose of morphine, animals fed saccharin, sucrose or Crisco had significantly longer tail-flick latencies than rats given only Purina Chow. Sixty minutes following injections, rats fed Crisco continued to display a significantly longer tail-flick latency than rats fed only Chow. These data demonstrate that palatable substances can enhance the analgesic properties of exogenous opioids.

Discontinuation of sustained sucrose-feeding aggravates morphine withdrawal

Sprague-Dawley rats were allowed ad lib access to a 20% sucrose solution in addition to their normal diet to investigate the relationship between the prolonged consumption of a high carbohydrate diet and opioid function as evidenced by opioid dependence and withdrawal. Morphine dependence, assayed by tailflick, was induced, followed by naloxone-precipitated withdrawal, gauged by weight loss. Sucrose-fed animals developed lowered pain thresholds prior to dependence induction relative to those of control animals, but failed to exhibit any differences from controls in the development of dependence. Weight loss during withdrawal was increased by the discontinuation of sustained sucrose-feeding. In addition, the induction of dependence first decreased, then increased the animals' preference for sucrose. It is concluded that changes in opioid function caused by sustained sucrose-feeding are insufficient to affect the development of tolerance to morphine analgesia, but do aggravate the symptoms of precipitated withdrawal when access to sucrose is denied prior to the injection of naloxone.

Relationships between sustained sucrose-feeding and opioid tolerance and withdrawal

This study examines the effect of sustained sucrose consumption on the development of tolerance to morphine analgesia (20 mg/kg IP injections) and subsequent, naloxone-precipitated withdrawal (2 mg/kg IP). Food intakes are also measured. Sprague-Dawley rats were allowed ad lib access to a 20% sucrose solution in addition to their normal diet. Pain thresholds and intakes were monitored for two weeks, then morphine tolerance was induced, followed by precipitated withdrawal. Tolerance was assayed by the tailflick method, and withdrawal was gauged by weight loss. The animals given access to sucrose developed lowered pain thresholds prior to tolerance induction relative to those of control animals, but they failed to exhibit any differences from controls in tolerance development of severity of withdrawal. The induction of tolerance first decreased, then increased sucrose consumption and steadily decreased chow consumption. Naloxone-precipitated withdrawal decreased chow consumption, but failed to affect the ingestion of the sucrose solution. It is concluded that changes in opioid function caused by sustained sucrose-feeding are insufficient to affect the development of tolerance to morphine analgesia; however, tolerance induction biphasically alters sucrose consumption.