Differential actions of central alloxan upon opioid and nonopioid antinociception in rats

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.

2-Deoxy-D-glucose antinociception and serotonin receptor subtype antagonists: test-specific effects in rats

The antinociceptive actions of 2-deoxy-D-glucose (2-DG) are mediated in part by endogenous opioid, dopaminergic, cholinergic, histaminergic, and neurohormonal influences. Although 2-DG antinociception was not affected by tryptophan hydroxylase inhibition, a possible serotonergic role in 2-DG antinociception was investigated because of the existence of serotonin [5-hydroxytryptamine (5-HT)] receptor subtypes. The present study examined the effects of general (methysergide: 5 and 10 mg/kg), 5-HT2 (ritanserin: 2.5 mg/kg), and 5-HT3 (ICS-205,930: 0.25-5 mg/kg) receptor subtype antagonists upon 2-DG antinociception on the tail-flick and jump tests in rats. On the tail-flick test, 2-DG (450 mg/kg) antinociception was significantly reduced by all ICS-205,930 doses (48-58%) but unaffected by either methysergide (22-29% reduction) or ritanserin (6% reduction). In contrast, 2-DG antinociception on the jump test was significantly potentiated across the 120-min time course and across the 2-DG dose-response curve (100-650 mg/kg) by methysergide, ritanserin, and ICS-205,930 pretreatment. Each of the three antagonists produced significant leftward shifts in the peak and total 2-DG dose-response curve for the jump test. These data suggest different sites of action for 2-DG antinociception as a function of the pain test employed and a differential modulation by serotonin receptor subtypes at those sites.

Differential actions of central alloxan upon opioid and nonopioid antinociception in rats: a further examination

Previous work demonstrated that central pretreatment with alloxan significantly reduced antinociception induced by morphine and 2-deoxy-D-glucose (2DG), an opioid-mediated stressor, but not induced by continuous cold-water swims (CCWS), a nonopioid-mediated stressor. The alloxan-induced deficits in 2DG antinociception were ameliorated by coadministration of D-glucose (3 M, 3M-DG). The present study evaluated this relationship further by: a) examining whether central alloxan reduced morphine antinociception following either simultaneous 3M-DG and alloxan coadministration, alloxan followed 10 days later by 3M-DG and 3M-DG alone, and b) determining whether central alloxan pretreatment altered nonopioid antinociception induced by the muscarinic cholinergic agonist, pilocarpine. Morphine (2.5-5 mg/kg, SC) antinociception on the tail-flick and jump tests was significantly reduced by central alloxan. In contrast, simultaneous coadministration of 3M-DG and alloxan failed to alter morphine antinociception. This ameliorative effect of 3M-DG was not due to its ability to affect morphine antinociception, and was time-dependent in that delays in 3M-DG administration failed to affect the alloxan-induced deficit. Central alloxan pretreatment failed to alter pilocarpine antinociception on the tail-flick test, and increased pilocarpine antinociception on the jump test. That central alloxan reduced opioid (e.g., morphine and 2DG), but not nonopioid (e.g., CCWS, pilocarpine) forms of antinociception suggests a specific mode of action, possibly through disruptions of glucoprivic control mechanisms which is in keeping with the suggestion that opioid systems are sensitive to changes in central glucose function.

Mediation of insulin hyperphagia by specific central opiate receptor antagonists

The hyperphagic properties of insulin (10 U/kg, s.c.) were transiently (2h) and dose-dependently inhibited (30%) by central pretreatment with naltrexone (20-50 micrograms, i.c.v.). The irreversible mu opioid antagonist, beta-funaltrexamine (B-FNA, 20 micrograms, i.c.v.) significantly inhibited insulin hyperphagia by 28-54% over the 6-h time course. In contrast, insulin hyperphagia was only transiently (2 h) inhibited (27-30%) by either the irreversible mu 1 antagonist, naloxonazine (50 micrograms, i.c.v.) or the selective kappa antagonist, nor-binaltorphamine (NorBNI, 20 micrograms, i.c.v.). The delta-antagonistic actions of [D-Ala2, Leu5, Cys6]-enkephalin (DALCE, 40 micrograms, i.c.v.) failed to affect insulin hyperphagia. These data suggest that the mu 2 opioid receptor subtype modulates insulin hyperphagia.

Kappa-opioid receptor-mediated thermonociceptive mechanisms in streptozotocin diabetes

The effects of the benzomorphan kappa-opiate antagonist MR 2266 and its dextro enantiomer MR 2267 were assessed on thermonociception in male Swiss Webster mice. Experimental diabetes was induced by injecting streptozotocin (200 mg/kg IP, 7-8 days before). Animals with dextrose treatment (5 g/kg, IP, at the time of opiate injection) were used as acute hyperglycemic controls. Nociception was assessed by supraspinal nociceptive reflex (licking and jumping in hot plate test) indicative of higher cognitive process as well as a predominantly lower spinal monosynaptic reflex (tail immersion test). In normoglycemic, acute hyperglycemic and diabetic mice, MR 2266 decreased, while MR 2267 increased, the reaction latencies. The results indicate tonic stereospecific kappa-opiate receptor-mediated spinal and supraspinal thermonociceptive reactions are not modulated by experimental diabetes and thus are distinct from those of naloxone-sensitive mu-opiate receptor sites.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.

Inhibition of deprivation-induced feeding by naloxone and cholecystokinin in rats: effects of central alloxan

Central administration of alloxan reduces the hyperphagic, but not the hyperglycemic response to glucoprivation by presumably acting upon brain glucoreceptors or a glucoprivic control mechanism. The present study evaluated whether central alloxan pretreatment respectively altered the dose-dependent suppressant effects upon deprivation (24-hr)-induced feeding of naloxone (0.01-10 mg/kg, IP) and cholecystokinin octapeptide (CCK-8: 1-8 micrograms/kg, IP) in rats. Central alloxan (200 micrograms, ICV) failed to alter body weight, free-feeding and deprivation-induced feeding. Both naloxone and CCK-8 produced significant dose-dependent inhibitions of deprivation-induced feeding in control rats. Central alloxan treatment significantly diminished peak naloxone hypophagia induced by 2.5 and 10 mg/kg doses, and CCK-8 hypophagia induced by the 1 and 4 micrograms/kg doses. Coadministration of 3 M D-glucose, which acts as a cytoprotectant against alloxan-induced diabetes, blocked the attenuating actions of alloxan upon naloxone and CCK-8 hypophagia. These data indicate the effectiveness of central alloxan in restricting the ability of pharmacological agents to either stimulate or inhibit food intake in rats without altering basal intake or body weight maintenance.