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

Chronic sucrose intake reduces the antagonist effect of β-funaltrexamine on morphine-induced antinociception in female but not in male rats

Chronic ingestion of a sweet-tasting sucrose solution enhances the pain relieving actions of opioid agonists. These results, taken in conjunction with research demonstrating that sucrose stimulates the production and release of endogenous opioid peptides, have led to the hypothesis that the effects of palatable foods and fluids on pain sensitivity are mediated by the endogenous opioid system. To assess this hypothesis, two studies determined if chronic sucrose intake would block the antagonist effects of the micro-selective opioid antagonist beta-funaltrexamine (beta-FNA) on morphine-induced antinociception. Female and male Long-Evans rats were maintained on chow and water, or chow, water and a 32% sucrose solution. In Experiment 1, after four weeks on the diets, female rats received 0 or 10 mg/kg (subcutaneously (s.c.)) beta-FNA, while in Experiment 2, male and female rats received 0, 5 or 20 mg/kg beta-FNA. Six days later, rats were tested for morphine-induced antinociception using the hot-water tail-withdrawal test. Morphine, administered using a cumulative dose regime (1.0, 3.0, 5.6, 10.0 and 31.0 mg/kg s.c.), led to dose-dependent increases in tail-withdrawal latencies in male and female rats. Males were more sensitive to the pain relieving properties of morphine than females. Sucrose intake increased, while beta-FNA decreased the analgesic actions of morphine in males and females. beta-FNA was less effective in blocking the antinociceptive actions of morphine in sucrose-fed female rats than in females fed only chow. In contrast, diet had minimal effects on responses to beta-FNA in male rats.

Naltrexone antagonism of morphine antinociception in sucrose- and chow-fed rats

Rats given long-term access to a palatable sucrose solution are more sensitive to (1) the analgesic potency of morphine and (2) the anorectic actions of naltrexone. In the present experiment, the ability of naltrexone to antagonize the antinociceptive properties of morphine was examined in sucrose- and chow-fed rats. Twenty adult male Long-Evans rats were maintained on chow and water. Ten rats were also given ad libitum access to a 32% sucrose solution for three weeks. Baseline and post-drug nociceptive thresholds were determined with a warm water (52 degrees C) tail-withdrawal procedure. Complete morphine dose-effect curves (0.32-100.0 mg/kg) were determined alone and in the presence of naltrexone (0.32-0.32 mg/kg). Morphine produced significant increases in tail-withdrawal latencies in both groups of animals. Rats drinking sucrose had significantly longer tail-withdrawal latencies than rats given only chow and water. Naltrexone produced significant and surmountable dose-dependent rightward shifts in the morphine dose-response curves for both groups, but did not differ in potency across diets. The present data support and extend our earlier findings showing sucrose enhancement of morphine-induced antinociception in rats given a palatable sucrose solution to drink.

Chronic sucrose intake augments antinociception induced by injections of mu but not kappa opioid receptor agonists into the periaqueductal gray matter in male and female rats

Chronic intake of a palatable sucrose solution enhances the antinociceptive potency of systemically administered mu, and kappa opioid receptor agonists. To investigate whether the effects of sucrose on the actions of opioid drugs are mediated within the central nervous system (CNS), antinociception was examined following the administration of mu and kappa opioid receptor agonists into the periaqueductal gray area (PAG). Male and female Long-Evans rats consumed either water and ground chow, or water, chow and a 32% (w/v) sucrose solution. After adaptation to the dietary conditions, a guide cannula was stereotaxically implanted into the PAG. Injections of the mu agonist, morphine (0.0, 2.5, 5.0, 10.0 and 20.0 microg), into the PAG led to dose-related increases in antinociceptive responses on a tail flick test in both male and female rats. Rats which had consumed sucrose displayed significantly greater levels of antinociception than rats not given the sugar. Antinociceptive responses to morphine did not differ as a function of sex. Injections of the kappa agonist, spiradoline (0, 100, 300, 600 microg), into the PAG increased tail flick latencies in male and female rats. However, antinociceptive responses did not vary as a function of diet in rats injected with spiradoline. In both diet conditions, spiradoline led to greater levels of antinociception in female rats than in male rats. These results support the hypothesis that intake of palatable foods and fluids act within the CNS to moderate the behavioral actions of opioid drugs.

Potentiation by nitric oxide synthase inhibitor and calcium channel blocker of aspartame-induced antinociception in the mouse formalin test

By applying a 12 day regimen of the non-calorific sweetener, aspartame, in combination with representative compounds of the calcium channel blocker and nitric oxide synthase inhibitor, we tried to investigate using a formalin-test in mice the relative role of aspartame on pain and its mechanism of action. Verapamil (2, 3.5, 5, 7.5 mg/kg) induced significant (P < 0.01) antinociception in both phases of the formalin test. L-Nitro-arginine-methyl-ester (L-NAME) at the doses used, induced significant (P < 0.01) antinociception in early phase (1, 2, 5, 10 mg/kg) and late phase (5, 10 mg/kg). Twelve days of treatment in animals by aspartame (0.16% w/v) significantly induced antinociception in both phases of the formalin test. Both verapamil (5 mg/kg) and L-NAME (10 mg/kg) significantly (P < 0.01) potentiated aspartame-induced antinociception in both phases of formalin test. The present findings support the hypothesis that the activation of NMDA receptors by aspartame modulates pain-related behaviour via a nitric oxide/cGMP/glutamate release cascade. It is concluded that aspartame would be a good analgesic agent if it would be used in combination with a calcium channel blocker or NOS inhibitor.

Saccharin effects on morphine-induced antinociception in the mouse formalin test

This study was performed to investigate the role of sweetness and taste sensations of the non-caloric sweetener saccharin on pain and morphine antinociception by the formalin test in mice. The formalin test was chosen because it measures the response to a long-lasting nociceptive stimulus and thus may closely resemble clinical pain. The total time (seconds) spent licking and biting the injected paw (indices of nociception) during periods of 0-5 min (early phase) and 10-30 min (late phase) were measured as an indicator of pain and inflammatory responses. A 12 days pretreatment of animals with saccharin (0.04%, 0.08%, 0.16%) produced complex effects on the action of morphine. All doses significantly potentiated the low dose (1.5 mgkg(-1)) of morphine-induced analgesia in the early phase significantly but antagonized the effect of morphine (3 mgkg(-1)). The effect of high doses of morphine (6-9 mgkg(-1)) was antagonized by the low dose of saccharin (0.04%), but the effect of morphine (6 mgkg(-1)) was potentiated with high concentrations of saccharin (0.08% and 0.16%). All doses of saccharin decreased the analgesic effect of morphine at a dose of 9 mgkg(-1). Analgesic effects of low doses of morphine (1.5-3 mgkg(-1)) were decreased by all doses of saccharin in the late phase. Different concentrations of saccharin also affected the antagonistic effect of naloxone (0.4 mgkg(-1)) on morphine-induced analgesia in both phases of the formalin test. The high dose of saccharin (0.16%) potentiated the effect of naloxone in the late phase. The results obtained suggest that sweet sensation is an important factor in mediating morphine analgesic properties. It is therefore inappropriate to use different concentrations of sweet saccharin solutions interchangeably.

Effect of VMH lesion on sucrose-Fed nociceptive responses

An initial analgesia followed by hyperalgesia to phasic noxious stimuli occurs after ingestion of sucrose ad libitum. However, the mechanism underlying hyperalgesia is not known. The present study was designed to explore the role of VMH in the mediation of the hyperalgesic effect of sucrose ingestion. Adult male albino rats received sucrose solution (20% p.o.) in addition to laboratory food pellets and tap water ad libitum. Their behavioural responses to various phasic and tonic noxious stimuli were recorded after 6, 12 and 48 h during pre and post-sucrose fed states in both the control and VMH lesion groups of rats. Sucrose feeding to control rats significantly reduced the tail flick latency (TFL) and threshold of vocalization during stimulus (SV) and after discharge (VA) indicating hyperalgesia, while the threshold of tail flick remained unaffected. The average pain rating during the formalin test (tonic pain) decreased significantly indicating analgesia. VMH lesion decreased the latency (mean +/- SD) for tail flick (11.26 +/- 4.65 from 15.61 +/- 5.12 s), threshold (median) for tail flick (0.04 from 0.08 mA), vocalization during stimulus (0.05 from 0.1 mA) and vocalization after discharge (0.15 from 0.2 mA), while the tonic pain rating increased, thereby suggesting a hyperalgesic state. However, sucrose feeding to lesioned rats neither potentiated nor attenuated their hyperalgesia. The results suggest that sucrose feeding for 6-48 h ad libitum produces hyperalgesia to phasic noxious and analgesia to tonic noxious stimuli, while VMH lesion produces hyperalgesia to both phasic and tonic noxious stimuli. Secondly, sucrose ingestion by VMH lesion rats does not affect their responses to pain, suggesting the possible role of VMH in the mediation of sucrose-fed nociceptive responses.

Intake of a palatable sucrose solution modifies the actions of spiradoline, a kappa opioid receptor agonist, on analgesia and feeding behavior in male and female rats

Previous research has shown that rats consuming a sucrose solution and chow are more sensitive to the analgesic actions of morphine, a selective mu opioid agonist, and the anorectic actions of opioid antagonists, than rats eating only chow. However, from these data, it cannot be determined if sucrose intake only modifies the behavioral consequences of drugs that act at the mu opioid receptor, or if the sugar also alters the actions of opioid drugs that act at other opioid receptor subtypes. Thus, the present experiments examined the effects of sucrose intake on the actions of spiradoline, a selective kappa opioid agonist, on analgesia and food intake in male and female Long-Evans rats. In Experiment 1, male and female rats consumed either chow, a 32% sucrose solution and water, or only chow and water. After 3 weeks, antinociceptive responses on the tail-flick test were determined after spiradoline injections (0.0, 0.3, 1.0, and 3.0 mg/ kg, s.c.). Rats fed sucrose were more sensitive to the analgesic actions of spiradoline than rats fed only chow. In Experiment 2, drug-naive male and female rats were maintained under the same dietary conditions as in Experiment 1. Food intake was measured 1, 2, 4, and 6 h after spiradoline injections (0.0, 0.3, 1.0, and 3.0 mg/kg, s.c.). Spiradoline led to significant dose-related decreases in food intake for males and females in both dietary conditions. However, the anorectic effects of the drug were more pronounced in rats fed sucrose than in those eating only chow. These results support the hypothesis that intake of palatable foods and fluids alters the activity of the endogenous opioid system.