Endogenous opioids released by suspending mice by the tail selectively enhance spinal mu opioid analgesia.

In studying the interactions between handling mice and their subsequent analgesic response to an intrathecally (i.t.) administered mu-opioid agonist, DAMGO, it was found that suspending ICR mice by the tail for 1, 5, or 20 sec, 10 min before the tail-flick test, enhanced DAMGO by 5.3-, 7.4- and 23.6-fold, respectively, compared with mice maintained in a level posture. This enhancement was not accompanied by a change in the rostral flow of [3H]-DAMGO (25 ng, i.t.) to the brain (3.7% in 10 min), in its distribution along the neuraxis or in its systemic absorption. However, i.c.v. administration of beta-endorphin (1-27), an antagonist of epsilon opioid receptors, abolished the enhancement of i.t. DAMGO without affecting its basal analgesic potency. Pretreatment with the delta-opioid antagonist naltrindole (5.6 nmol, i.t.,-30 min) also blocked the enhancement of DAMGO without significantly affecting its basal analgesic potency. Alternatively, this same dose of naltrindole injected i.c.v. failed to block the enhancement of DAMGO in suspended mice. A 20-sec suspension failed to enhance i.t. kappa and delta-agonists, but it did enhance i.t. morphine. In mouse strain comparisons, i.t. DAMGO was more potent in C57BL/6J and DBA/2J mice than in C3H/HeJ and ICR mice, but DAMGO was enhanced by a 20-sec suspension in all strains tested. Thus suspending mice by the tail evoked a reflex enhancement of spinal mu agonist-induced analgesia that probably involved both the supraspinal release of beta-endorphin (an endogenous epsilon agonist) and the subsequent spinal release of an endogenous delta-receptor agonist in the reflex pathway.

Effect of intrathecal morphine on the fate of glucose. Comparison with effects of insulin and xanthan gum in mice.

The hypoglycemic effect of morphine (40 micrograms) injected intrathecally (i.t.) was studied with regard to disposition of i.v. [14C]glucose and [3H]2-deoxyglucose and was compared with the effects of two other hypoglycemic agents, insulin (1 IU/kg, s.c.) and xanthan gum (50 mg/kg, i.p.). Mice given i.t. morphine or s.c. insulin exhaled a greater amount of 14CO2 from i.v. [14C]glucose than did control mice given i.t. saline, whereas there was less 14CO2 expiration in xanthan-treated mice. In morphine-treated mice there was less 14C in liver, brain and blood, and more 3H in kidney and hindleg muscle than in control mice. Insulin-treated mice had more 14C in muscle, less 14C in liver, brain, kidney and blood, and less 3H in liver and blood. In xanthan-treated mice, levels of both radiolabels were higher in liver, brain and kidney. Much lower glycogen content in muscle and depletion of liver glycogen occurred in morphine-treated mice, compared with control mice. Spinal transection completely inhibited the hypoglycemic effect of morphine, whereas adrenalectomy caused no inhibition. Morphine, insulin and xanthan appear to be acting by different mechanisms, although the hypoglycemic effects of both morphine and insulin appear to be due largely to an increased glucose uptake by muscle.

Elevated CSF beta-endorphin immunoreactivity in Rett's syndrome: report of 158 cases and comparison with leukemic children.

Because some symptoms of Rett's syndrome are suggestive of excessive endogenous opioid activity, we measured the levels of beta-endorphin-like immunoreactivity in lumbar CSF from 158 affected female patients and from 13 female controls. The mean (+/- SE) control level of beta-endorphin immunoreactivity in CSF was 35.3 +/- 2.8 pg/ml (range, 23 to 48 pg/ml), whereas those with Rett's syndrome had a mean level of 95.3 +/- 3.6 pg/ml (range, 31 to 293 pg/ml). The levels of beta-endorphin immunoreactivity in initial CSF samples exceeded the control range in 90% of the patients with Rett's syndrome. The mean beta-endorphin immunoreactivity was also elevated in CSF from leukemic children (119.2 +/- 16.9 pg/ml; range, 40 to 159 pg/ml), relative to the control group. These results are consistent with the hypothesis that some symptoms of Rett's syndrome may be associated with excessive endogenous opioid levels in the CNS.

An insulin-independent mechanism of intrathecal morphine-induced hypoglycemia in mice: mediation through a central alpha-2 adrenergic pathway.

Studies on the mechanism of the hypoglycemia caused by the i.t. administration of morphine (40 micrograms) to nonfasted, unanesthetized mice included assessment of effects of i.t. morphine on circulating levels of immunoreactive insulin and glucagon, on body temperature, as well as testing the effects of i.p. pretreatment with antagonists of cholinergic, beta adrenergic, alpha-1 adrenergic, alpha-2 adrenergic, serotonergic and opioid receptors. Serum levels of insulin were not significantly affected at 30 and 60 min after i.t. morphine, but plasma glucagon levels were significantly increased at both time points. Rectal temperature decreased significantly in mice given either saline or morphine i.t., but there was no significant difference between the two groups over time. Atropine (2 mg/kg), propranolol (10 mg/kg), prazosin (2 mg/kg) and methysergide (2 mg/kg) did not affect morphine-induced hypoglycemia. Naloxone (20 mg/kg) antagonized i.t. morphine. Mecamylamine (2 mg/kg) produced a moderate hypoglycemia in control mice, which appeared to be additive to that caused by i.t. morphine. Yohimbine (1-4 mg/kg i.p.) also produced a moderate hypoglycemia in control mice, but caused a dose-related antagonism of i.t. morphine. The centrally active alpha-2 antagonist, L-657,743 (2S-trans)-1,3,4,5',6,6',7,12b-octahydro-1',3'-dimethylspiro -(2H-benzofuro- (2,3-a)quinolizine-2,4'(1'H)pyrimidin)-2-(3'H)-one hydrochloride) (4-20 mg/kg i.p.), but not the peripherally selective alpha-2 antagonist, L-659,066 (2R-trans)-N-(2-(1,3,4,6,7,12b-he xahydro-2'- oxospiro(2H-benzofuro(2,3-a) quinolizine-2,4-imidazolidin)-3'-yl)ethyl)methanesulfonamide hydrochloride) (4-20 mg/kg i.p.), caused a dose-related antagonism of i.t. morphine. The i.t. coadministration of yohimbine (2 or 10 micrograms) or L-659,066 (10 micrograms) with morphine exhibited no antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diving experience on submersion-induced increases in plasma levels of beta-endorphin in scuba divers.

The submersion of eight male scuba divers in a state of neutral buoyancy for 20 min resulted in a significant increase (mean, 20%) in plasma beta-endorphin immunoreactivity (beta-EIR), and all subjects reported post-submersion feelings of well-being, relaxation or euphoria. The increase in beta-EIR was negatively correlated with age (r = -0.727) and diving experience (r = -0.860), and positively correlated with the amount of air usage during submersion (r = 0.863). Thus, diving experience appears to influence many of the nonsubjective parameters measured.

Stereoselective effect of morphine on antinociception and endogenous opioid peptide levels in plasma but not cerebrospinal fluid of dogs.

Morphine releases endogenous opioids into the circulation of dogs. To test the stereospecificity of this effect, as well as to determine whether morphine also releases endogenous opioids centrally, which might be involved in its antinociceptive action, the effects of (-)-morphine sulfate (10 mg/kg, sc) or (+)-morphine hydrobromide on antinociception in a dog tail-flick test, on semi-quantified morphine-induced signs of salivation, emesis, defecation and ataxia, and on the plasma and cerebrospinal fluid (CSF) levels of endogenous opioid peptides were studied. Plasma and CSF levels of immunoreactive beta-endorphin (i-BE), met-enkephalin (i-ME), leu-enkephalin (i-LE), and dynorphin (i-DY) were quantified by radioimmunoassay in octadecylsilyl-silica cartridge extracts. Immunoreactive morphine (i-M) levels were measured in unextracted samples. (-)-Morphine treatment significantly increased antinociception, morphine-induced signs, i-M levels in plasma and CSF, and i-BE, i-ME, and i-LE levels in plasma, but not CSF. Levels of i-DY remained constant in plasma and CSF. (+)-Morphine treatment did not alter any of these parameters, indicating that the effects of morphine on nociception, behavioral signs, and plasma endogenous opioids in dogs were stereoselective. It is concluded that morphine does not cause an increase in immunoreactive endogenous opioid peptides in the CSF at the time of its peak antinociceptive effect.

Effects of hyperbaric simulation of scuba diving pressure on plasma beta-endorphin.

In contrast to our previous studies on the submersion of scuba divers in a state of neutral buoyancy, neither plasma beta-endorphin-like immunoreactivity (beta-EIR) nor affective feelings were significantly changes in scuba divers by mimicking diving pressures of 2 feet (0.6 m) and 50 feet (15.2 m) for 20 min in a hyperbaric chamber. It is concluded that the submersion-induced increase in plasma beta-EIR and accompanying changes in affect reported previously are not due solely to changes in pressure.

Antagonism of the morphine-induced locomotor activation of mice by fructose: comparison with other opiates and sugars, and sugar effects on brain morphine.

The mouse locomotor activation test of opiate action in a 2+2 dose parallel line assay was used in a repeated testing paradigm to determine the test, opiate and hexose specificities of a previously reported antagonism of morphine-induced antinocociception by hyperglycemia. In opiate specificity studies, fructose (5 g/kg, i.p.) significantly reduced the potency ratio for morphine and methadone, but not for levorphanol, meperidine or phenazocine when intragroup comparisons were made. In intergroup comparisons, fructose significantly reduced the potencies of levorphanol and phenazocine, but not methadone or meperidine. In hexose/polyol specificity studies, tagatose and fructose significantly reduced the potency ratio for morphine, whereas glucose, galactose, mannose and the polyols, sorbitol and xylitol, caused no significant decrease in potency. Fructose, tagatose, glucose and mannose (5 g/kg, i.p.) were tested for effects on brain morphine levels 30 min after morphine (60 min after sugar), and all four sugars significantly increased brain morphine relative to saline-pretreated controls. It is concluded that the antagonism of morphine by acute sugar administration shows specificity for certain sugars and occurs despite sugar-induced increases in the distribution of morphine to the brain. Furthermore, the effects of fructose show an opiate specificity similar to that of glucose on antinociception observed previously in our laboratory, except that methadone was also significantly inhibited in the present study, when a repeated-testing experimental design was used.

Is intracellular sodium involved in the mechanism of tolerance to opioid drugs?

Recent studies indicate that one of the more likely mechanisms of opioid tolerance could involve a decrease in the efficiency with which agonists can induce coupling of their specific binding sites in neuronal membranes to the activation (or deactivation) of an effector system. Reports of sodium-induced decreases in opioid receptor agonist binding and in the size of ligand/receptor complexes, as well as modulation of opioid activity by manipulation of sodium in vivo, indicate that sodium might play a physiological role in modulating opioid receptor function. Reports of morphine-induced systemic sodium retention in animals, as well as morphine-induced increases in brain intracellular sodium and decreases in brain Na+, K(+)-ATPase activity, indicate that the development of tolerance may be accompanied by changes in the disposition of sodium. The direction of these sodium- and morphine-induced changes is consistent with the hypothesis that an increase in intracellular sodium could participate in the mechanism(s) of opioid tolerance development.

Hypoglycemia induced by intrathecal opioids in mice: stereospecificity, drug specificity and effect of fasting.

The specificity of the hypoglycemic response to the intrathecal (i.t.) administration of the naturally occurring (-)-enantiomer of morphine previously reported from our laboratory was studied in mice. (+)-Morphine HBr (50 micrograms) caused a behavioral syndrome (scratching, biting, seizures) comparable to that produced by (-)-morphine sulfate (50 micrograms), but did not cause hypoglycemia. Many opioids, at a dose of 50 micrograms i.t. in nonfasted mice, showed either a saline-like hyperglycemic response or no significant effect on blood glucose. (+)-Morphine, ketocyclazocine, U-50,488, (-)- and (+)-N-allyl-normetazocine, beta-endorphin, (-)- and (+)-naloxone and naltrexone caused hyperglycemia. Significant changes from basal blood glucose were not produced by [D-Pen2, L-Pen5]-enkephalin, [D-Ser2]-Leu-enkephalin-Thr or sufentanil in 50-micrograms doses, or by codeine (300 micrograms), levorphanol (400 micrograms) or methadone (200-400 micrograms). Agonists which produced both hypoglycemic and behavioral effects were, in order of decreasing potency, hydromorphone greater than normorphine greater than morphine greater than 6-acetylmorphine greater than oxymorphone much greater than heroin. Morphine-induced hypoglycemia was partially antagonized by the i.t. coadministration of naloxone methobromide (10 micrograms). Fasting for 24 hr increased the sensitivity to hypoglycemic and lethal effects of morphine. D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin (5-50 micrograms i.t.) tended to decrease blood glucose in both nonfasted and fasted mice, but these effects were moderate and appeared to be unrelated to dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Naltrexone therapy of apnea in children with elevated cerebrospinal fluid beta-endorphin.

Previous studies have indicated increased immunoreactivity of the endogenous opioid peptide beta-endorphin in the cerebrospinal fluid (CSF) of infants under 2 years of age with apnea. To assess the role of endogenous opioids in the pathogenesis of apnea in children, the effect of oral treatment with the opioid antagonist naltrexone was studied in apneic infants, as well as in older apneic children, with demonstrated increases in CSF immunoreactive beta-endorphin (i-BE). In the 8 apneic infants with elevated i-BE in lumbar CSF (range, 55-155 pg/ml; normal, 17-52 pg/ml), no further apnea occurred during naltrexone therapy (1 mg/kg/day, by mouth). Five children (2-8 years old) with apnea of unknown cause had elevated CSF i-BE (range, 74-276 pg/ml) compared to 6 age-matched nonapneic children (range, 15-48 pg/ml). No apneic events occurred during naltrexone therapy, except in 1 child during stressful events, but apnea recurred in some patients after attempts to discontinue naltrexone treatment. Adverse effects of naltrexone included complaints of headaches in 2 children and symptoms of a narcotic withdrawal syndrome during the first 3 days of treatment in 1 child. Three children with Leigh's syndrome had elevated CSF i-BE (range, 104-291 pg/ml) and their apnea also responded to naltrexone. We conclude that elevated endogenous opioids contribute to the pathogenesis of apnea in children and may even result in physical dependence.

Studies on the mechanism of hypoglycemia induced by intrathecal morphine: dissociation from behavioral effects, effects of tolerance and depletion of liver glycogen.

To characterize further the hypoglycemic effect of intrathecally (i.t.) administered morphine, species and drug specificity, effects of morphine-induced tolerance and pentobarbital-induced anesthesia and effects on liver glycogen were studied in nonfasted animals. In rats, morphine (125 micrograms i.t.) produced the same behavioral toxicity (scratching, biting, seizures) and hypoglycemia as previously reported in mice. In mice, the glycine antagonist strychnine (5 micrograms i.t.) and the morphine metabolite morphine-3-glucuronide (2 micrograms i.t.) mimicked the behavioral, but not the hypoglycemic, effects of high-dose i.t. morphine. Kainic acid (0.1 micrograms i.t.), which caused high-frequency hindlimb movements, also did not cause hypoglycemia. Naltrexone (1 mg/kg/ s.c.) or the s.c. implantation of morphine pellets for 3 days attenuated the hypoglycemic effect, but not the behavioral effects, of morphine (40 micrograms i.t.). The hyperglycemic effect of s.c. morphine(20 mg/kg) was blocked by i.t. morphine. Anesthesia with pentobarbital (75 mg/kg i.p.) attenuated the hypoglycemic effect of morphine (40 micrograms i.t.). Morphine i.t. also caused a time- and dose-dependent decrease in liver glycogen levels and was more potent in causing glycogenolysis (30 min ED50 = 19 micrograms) than in causing hypoglycemia (30 min ED50 = 30 micrograms). It is concluded that the hypoglycemic effect of i.t. morphine appears to be independent of its behavioral effects, displays tolerance and is accompanied by hepatic glycogen depletion.

Possible hyperendorphinergic pathophysiology of the Rett syndrome.

The Rett syndrome is a postnatal developmental and neurological disorder seen only in girls. Many of the symptoms of this disorder, such as microcephaly, stereotypy, respiratory disturbances and seizures, are analogous to the effects of the administration of beta-endorphin or other opioids in animals. Preliminary reports of elevated beta-endorphin-like immunoreactivity in the cerebrospinal fluid of girls with the Rett syndrome, as well as improvement in some of their symptoms during the administration of the opioid antagonist naltrexone, are suggestive of endorphinergic hyperactivity. Thus, the pathophysiology of the Rett syndrome might involve excessive stimulation of opioid receptors in the central nervous system by beta-endorphin or other endogenous opioids.

Interaction of morphine with intrathecally administered calcium and calcium antagonists: evidence for supraspinal endogenous opioid mediation of intrathecal calcium-induced antinociception in mice.

The interaction between morphine [i.p. and intrathecal (i.t.)] and calcium and its antagonists (i.t. and i.c.v.) was studied in the mouse tail-flick test for antinociception. Calcium (0.66 mumol i.t.) produced antinociception comparable to that of morphine (0.5 microgram i.t.) but had a significantly longer duration. A lower dose of calcium (0.16 mumol i.t.) significantly potentiated morphine (0.2 and 0.5 micrograms i.t.). The antinociceptive effect of i.p. morphine was also potentiated by i.t. calcium, but was antagonized by the i.t. administration of ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (3.7-7.5 nmol), verapamil (15 micrograms), magnesium (9.4 nmol) and barium (1-2 nmol). In contrast, i.t. calcium and i.p. morphine were significantly potentiated by the i.c.v. administration of verapamil (15 micrograms) and antagonized by i.c.v. calcium (0.33 mumol). The antinociceptive effect of i.t. calcium was antagonized by naloxone administered s.c. (1 mg/kg) or i.c.v. (0.5 microgram), but not i.t. (0.5 and 10 microgram). It is concluded that the antinociceptive effect of i.t. calcium is mediated, at least partly, by a reflex supraspinal release of endogenous opioids, and that the administration of calcium and its antagonists modify the antinociceptive effect of morphine in opposite directions, depending upon whether they are administered by the i.t. or i.c.v. routes. Calcium may serve as a useful adjunct for opioid-induced analgesia via the i.t. route.

Differential effects of subcutaneous and intrathecal morphine administration on blood glucose in mice: comparison with intracerebroventricular administration.

The s.c. administration of morphine (2.5-80 mg/kg) produced a dose-dependent hyperglycemia, whereas morphine (12.5-50 micrograms) given i.t. in the lumbar region caused a dose-dependent hypoglycemia in unanesthetized mice. Both effects on blood glucose were antagonized by s.c. naloxone, although inhibition of i.t. morphine required a higher naloxone dose. Naloxone pretreatment with 2 mg/kg, but not 1 mg/kg s.c., potentiated a hyperglycemic response to i.t. saline. High i.t. doses of morphine caused an early (within 2 min) scratching behavior that was not inhibited by naloxone or glucose loading and a later (greater than 20 min) tonic convulsive behavior (opisthotonus). Lethality was partially inhibited by glucose loading which delayed, but did not prevent, the hypoglycemic effect of i.t. morphine. The hypoglycemic effect of i.t. morphine was also delayed in streptozotocin-diabetic ICR mice and diabetic (db/db) C57BL/KsJ mice, but the latter were more sensitive to lethality, which occurred without hypoglycemia or seizures. All these effects of i.t. morphine were completely blocked by acute spinal transection of T10-T11, but the nociceptive, hypoglycemic and opisthotonic effects were not mimicked by i.c.v. morphine (6.25-50 micrograms) in ICR mice, which showed a bell-shaped hyperglycemic dose-response relationship and a brief explosive motor behavior at the higher doses (25-50 micrograms). It is concluded that the effects of morphine on blood glucose and on behavior are dependent upon the route of administration, and that the convulsive effect of i.t. morphine may be facilitated by the production of a profound hypoglycemia, which involves a spinal, rather than supraspinal or systemic, action of morphine.

Antagonism of antinociception in mice by glucose and fructose: comparison of subcutaneous and intrathecal morphine.

Determination of the ED50s of glucose and fructose, administered i.p., for antagonizing the antinociceptive action of morphine (4 mg/kg s.c. or 0.5 micrograms i.t.) and determination of the ED50s for i.t. morphine after i.p. pretreatment with saline, glucose (5 g/kg) or fructose (5 g/kg) in the mouse tail-flick test indicated that fructose was more potent than glucose in antagonizing antinociception after either route or morphine administration. It is concluded that the antagonism of morphine-induced antinociception by glucose and fructose is due to a direct effect of these sugars or their metabolic products within the central nervous system.

Increased cerebrospinal fluid beta-endorphin immunoreactivity in infants with apnea and in siblings of victims of sudden infant death syndrome.

To gain further insight into the possible role of endogenous opioid peptides in the respiratory difficulties associated with the apnea of infancy and other disorders possibly related to apnea, the levels of beta-endorphin immunoreactivity were measured in the cerebrospinal fluid (CSF) of five groups of infants: (1) infants with proved apnea, (2) infants with histories of an apparent life-threatening event (ALTE), (3) siblings of victims of the sudden infant death syndrome (SIDS), (4) infants with suspected but unproved apnea, and (5) infants undergoing investigation for other acute illnesses. Twenty-two infants considered at risk for an ALTE (groups 1 to 3) had significantly higher CSF beta-endorphin equivalents (88 +/- 7 pg/mL) than did the 22 control patients in groups 4 and 5 (31 +/- 3 pg/mL). Plasma beta-endorphin immunoreactivity, which was also measured in some of the infants, did not correlate with levels in CSF and, in fact, was significantly lower in the groups at risk for an ALTE (50 +/- 9 pg/mL; n = 14) than in the control subjects (80 +/- 6 pg/mL; n = 11). These studies indicate that elevated beta-endorphin immunoreactivity in CSF may be a marker in infants who have apnea and who may be considered at risk for an ALTE.

Effects of glucose and diabetes on binding of naloxone and dihydromorphine to opiate receptors in mouse brain.

The effects of glucose and diabetes on the high-affinity lofentanil-displaceable opiate-receptor binding in mouse brain membranes were studied to determine if the attenuation of opiate actions by hyperglycemia previously observed in our laboratory was due to a modification of receptor affinity or number. With membranes from normal ICR mice, glucose (100-400 mg/dl) caused small but significant concentration-dependent decreases in receptor affinities for [3H]naloxone and [3H]dihydromorphine, both in the absence and presence of 20 mM NaCl, without changing the maximum number of binding sites. Fructose and the nonmetabolizable sugar 3-O-methylglucose had intermediate effects on naloxone affinity in the presence of NaCl that were not significantly different from control or from the effect of glucose. Similar results were obtained with brain membranes from streptozocin-induced diabetic mice. The binding affinity for [3H]naloxone in the presence of NaCl was not affected by the induction of diabetes in ICR mice via streptozocin or in spontaneously diabetic (db/db) C57BL/KsJ mice compared with their nondiabetic (m+/m+) littermates. These results indicate that the previously observed attenuation of opiate effects by glucose may be partly due to a glucose-induced decrease in opiate-receptor affinity. However, the much greater attenuation of morphine by fructose in vivo cannot be explained by this mechanism.