Calcium antagonists isradipine and nimodipine suppress cocaine and morphine intravenous self-administration in drug-naive mice.

The effect of isradipine and nimodipine, two dihydropyridine calcium antagonists, on intravenous self-administration of cocaine and morphine in naive mice has been investigated. When morphine or cocaine injections were made contingent upon nose-poke response by naive mice, they increased their rate of nose-poking with respect to animals receiving contingent saline injections or yoked control animals, receiving noncontingent cocaine or morphine injections. Pretreatment of mice with isradipine (1.0-3.0 mg/kg, SC) or nimodipine (5-20 mg/kg, SC) inhibited in a dose-related manner self-administration both of cocaine and morphine contingent upon a nose-poke response. The ED50 of isradipine against cocaine and morphine self-administration was 1.7 and 2.1 mg/kg, respectively. The relative values for nimodipine were 14.5 and 11.4 mg/kg, respectively. These data suggest that nimodipine and, especially, isradipine suppress the reinforcing properties of morphine and cocaine and may be an effective pharmacotherapy for treatment of cocaine and heroin abuse.     

Behavioral contingencies determine changes in drug-induced neurotransmitter turnover

Evidence from several research areas suggests that response-dependent and independent presentation of environmental events have different neurobiological consequences. Investigations of neurotransmitter turnover rates and cholinergic muscarinic receptor densities in brain regions of rats receiving contingent and noncontingent intravenous morphineresulted in the identification of two neuronal networks that are involved in response dependent morphine presentation. These circuits are concordant with brain regions showing increased glucose utilization in animals receiving response-dependent electrical stimulation of either the substantia nigra or ventral tegmental area. These data indicate substantial differences between contingent and noncontingent presentation of environmental events and suggest that neurobiological investigations of reinforcement that use response independent delivery of a putative reinforcer must be interpreted with some caution.     

Isolation decreases physical and motivational aspects of morphine withdrawal

Environmental manipulations such as social housing conditions of animals may play a role in the expression of individual differences in response to drugs. This study aimed to evaluate whether isolated and grouped mice develop different degrees of morphine dependence. Isolated and grouped mice were rendered morphine dependent employing two different methods of induction: a fast or slow protocol, both reaching the same maximum daily dose (100 mg/kg). Naloxone-induced morphine withdrawal was assessed using a modified Gellert-Holtzman scale and a conditioned place aversion (CPA) procedure. Isolated animals manifested fewer signs of physical dependence than grouped mice and only those receiving two daily morphine doses presented significantly higher scores on the Gellert-Holtzman scale than controls. Similarly, in CPA, although all morphine-treated animals developed aversion, its intensity was only significantly higher than in controls in grouped animals receiving two daily doses. Analgesic response, measured with the hot-plate test, showed that isolated mice presented longer latencies to lick their paws (even without drug administration), suggesting that they had a higher level of endogenous opiates. It can be argued that isolated animals may be less sensitive to morphine than the non-isolated and therefore tolerate greater quantities or require more drug to produce the same effects. The results suggest that variability in the response to opiates could be affected by environmental manipulations.     

Effects of adrenalectomy and gonadectomy on the antinociceptive effect of morphine and naloxone antagonism in mice

In the present study the effects of adrenalectomy and gonadectomy on the antinociceptive activity of morphine and its antagonism by naloxone were studied in male and female mice. In the female mice, it was found that adrenalectomy enhanced the antinociceptive activity of morphine by two-fold, while the antinociceptive effects of morphine measured in the presence of naloxone were similar to those of the sham-operated controls. The naloxone potency ratios, expressed as the ratios of morphine ED50s with naloxone to that without naloxone, were increased in adrenalectomised animals. Similar effects were observed in oophorectomised female mice though the influence of oophorectomy on naloxone antagonism was not as effective as adrenalectomy. In animals that were both adrenalectomised and oophorectomised the antinociceptive effect of morphine measured was similar to those animals receiving either operative procedure. However, the naloxone potency ratios determined were between those determined in adrenalectomised mice and oophorectomised animals. In male mice, the effect of adrenalectomy was similar to that observed in the female animals, namely both the antinociceptive activity of morphine and naloxone antagonism were enhanced. Orchidectomy enhanced the antinociceptive effect of morphine but not to the same extent as adrenalectomy. Furthermore, the enhancement of naloxone antagonism was also less than that induced by adrenalectomy. The effects of adrenalectomy and orchidectomy in male mice were similar to those of adrenalectomy alone. These results suggest that steroid hormones are involved in the actions of both morphine and naloxone.     

A dual action of valproic acid upon morphine analgesia and morphine withdrawal

Effects of valproic acid administration on morphine analgesia and on morphine tolerance and dependence were investigated in mice. Valproate increased the reaction time to thermal stimulation in naive animals. This effect was additive with morphine when valproate was administered shortly before the analgesic. However, an antagonism was observed if a 4-hour period elapsed between valproate and morphine administration. When administered to mice receiving a sustained release preparation of morphine, valproate antagonized the development of tolerance to morphine. Valproate elicited a dual action on the abstinence signs observed after naloxone administration in morphine-treated mice. The effect consisted in a reduction of abstinence behavior if the anticonvulsant was administered a few minutes before naloxone; the same treatment increased the severity of the abstinence behavior when valproate was injected 1 h before the precipitating dose of naloxone. In this latter schedule, concomitant administration of gamma-vinyl-GABA failed to reduce the severity of the convulsions observed during the abstinence syndrome. These results suggest that valproate is metabolized to a compound responsible for decreased analgesia and intensified withdrawal signs.     

The effect of pargyline on the toxicity of morphine in the rat

The effect of pargyline on the toxicity of morphine in rats has been investigated. The toxicity of morphine 4 hr after an acute ip injection of pargyline is significantly increased. The toxicity of morphine in rats receiving morphine 4 or 24 hr after the sixth daily ip injection of pargyline is significantly decreased when compared to the acutely pargyline-treated animals. However, the toxicity of morphine is still increased in these animals when they are compared to saline-treated groups. The animals first lost weight 24–48 hr following the administration of pargyline, then gained weight with a slower rate as compared to those receiving saline.     

Morphine action in grouped and isolated rats and mice

The present study determines the analgesic effects of morphine in grouped and isolated rats and mice. Isolated animals developed altered behavioral patterns, including mouse-killing in rats and mutual aggressiveness in mice. The analgesic effect of morphine was tested by tail compression in rats and by the hot plate for mice. Isolated rats developing mouse-killing behavior had a raised pain threshold, while indifferent animals (nonkillers) responded less to morphine. Isolated mice, particularly low aggressors, gave enhanced responses to morphine.     

The effect of morphine pretreatment on the sensitivity of mouse and guinea-pig ileum to acetylcholine and to morphine

Guinea-pigs and mice were subjected to morphine pretreatment for at least 14 days culminating in subcutaneous injections of 1 g/kg daily in 2 divided doses. In mice, tolerance to the effects of morphine on gastrointestinal propulsion had developed by the end of the pretreatment course. The sensitivity to acetylcholine of the ilia from pretreated animals and the effects of morphine on transmural stimulation were compared with the ilia from normal animals. Ilia from pretreated guinea-pigs were marginally more sensitive to frequency of stimulation but significantly less sensitive to the depressant effect of morphine than ilia from normal animals. There was no significant difference in the sensitivity to acetylcholine. There was a highly significant decrease in sensitivity to morphine of ilia from pretreated mice, but this also was not associated with any alteration in sensitivity to acetylcholine. It is concluded that on the isolated ileum preparation the development of tolerance to morphine is not associated with an increased sensitivity to the transmitter whose release morphine inhibits.     

Antinociceptive effect of intracerebroventricular administration of cholecystokinin antagonist in nerve-ligated mice

In the present study we investigated the effects of intracerebroventricular injection of caerulein, the cholecystoki nin receptor agonist and proglumide, the receptor antagonist on morphine response in the sciatic nerve ligation in mice. Subcutaneous administration of morphine induced antinociception in the both intact and ligated mice, however, the response of the opioid was lower in the ligated mice as compared with the intact animals. Caerulein induced antinociception in the non-ligated but not in the nerve-ligated animals. Combination of caerulein with morphine elicited higher response in ligated animals, however, the response induced in ligated animals was much more prominent. Proglumide alone did not elicit any response in both animal groups. The antagonist decreased the response of caerulein in the nonligated mice. Low doses of proglumide in the combination with caerulein induced antinociception in the ligated mice. We conclude that cholecystokinin receptor mechanism(s) may alter morphine resistance induced by nerve ligation.     

Antinociceptive Effect of Intracerebroventricular Administration of Cholecystokinin Receptor Agonist and Antagonist in Nerve‐Ligated Mice

In the present study we investigated the effects of intracerebroventricular injection of caerulein, the cholecystokinin receptor agonist and proglumide, the receptor antagonist on morphine response in the sciatic nerve ligation in mice. Subcutaneous administration of morphine induced antinociception in the both intact and ligated mice, however, the response of the opioid was lower in the ligated mice as compared with the intact animals. Caerulein induced antinociception in the non‐ligated but not in the nerve‐ligated animals. Combination of caerulein with morphine elicited higher response in ligated animals, however, the response induced in ligated animals was much more prominent. Proglumide alone did not elicit any response in both animal groups. The antagonist decreased the response of caerulein in the non‐ligated mice. Low doses of proglumide in the combination with caerulein induced antinociception in the ligated mice. We conclude that cholecystokinin receptor mechanism(s) may alter morphine resistance induced by nerve ligation.     

Morphine, but Not Trauma, Sensitizes to Systemic Acinetobacter baumannii Infection

Acinetobacter baumannii is an important nosocomial pathogen in civilian intensive care units. Recently the incidence has increased in wounded military personnel. Morphine is documented in numerous animal studies to be immunosuppressive and to sensitize to infection. The hypotheses were tested that morphine, administered for analgesia in the battlefield, predisposes to Acinetobacter infection, and that the opioid may have an additive or synergistic effect with trauma. To test these hypotheses, an intraperitoneal infection model was established in mice using several Acinetobacter strains. Morphine administered for 48 h by implantation of a slow-release morphine pellet increased mortality compared to animals receiving a placebo pellet, an effect that was blocked by the mu-opioid receptor antagonist, naltrexone. Acinetobacter burdens in the blood, spleens, livers, and lungs of morphine-treated mice, were significantly higher than those in placebo-treated animals, confirming that mortality was due to potentiated growth of the bacteria. There were also elevated levels of pro-inflammatory cytokines in morphine-treated versus placebo-treated mice. Morphine caused a reduction in the total number of cells in the peritoneal cavity, a decrease in the percentage and total numbers of neutrophils, and a decrease in the total number of macrophages. Morphine treatment also suppressed levels of the neutrophil-inducing molecules, IL-17A and KC/CXCL1. However, IL-17A^−/− mice given morphine were not sensitized to Acintobacter infection to a greater degree than similarly treated wild-type mice. Trauma alone did not sensitize to Acinetobacter infection, and there was no additive effect between morphine and trauma. These results support the hypothesis that morphine potentiates Acinetobacter infection.     

Effects of acute and continuous morphine administration on serum glutamate oxalacetate transaminase and glutamate pyruvate transaminase activities in the mouse.

Mice were exposed continuously to morphine by implanting morphine pellets for 12 hr to 3 days. In mice receiving prolonged morphine pellet implantation, second and third morphine pellets were implanted at days 3 and 6, for a total of 9 days. Results showed morphine pellet implantation for 24 hr caused a 2-fold increase in glutamate oxalacetate transaminase (SGOT) and glutamate pyruvate transaminase (SGPT) activities, as compared with those of the placebo control group. SGOT and SGPT activities were returned to control levels after 3–9 days of morphine pellet implantation. In mice implanted with a morphine pellet for 24 hr, both SGOT and SGPT activities returned to normal levels at 3 and 6 days after the removal of the morphine pellet. In mice receiving morphine pellet implantation for 24 hr with three concurrent administrations of naloxone. 40 mg/kg, s.c., the morphine pellet-induced elevations of SGOT and SGPT activities were attenuated significantly. Acute administration of morphine sulfate by both subcutaneous and intraventricular administration elevated SGOT and SGPT activities. The elevations of SGOT and SGPT activities caused by morphine pellet implantation were prevented completely by hypophysectomy and prevented partially by adrenalectomy. These results substantiate our previous morphological and biochemical findings that initial administration with morphine by either injection or pellet implantation may alter hepatic function and the action may be mediated through the central nervous system.     

Opioid mydriasis: cross-tolerance between morphine and enkephalins

The occurrence of cross-tolerance between morphine and met-enkephalin, and between morphine and DADL (D-Ala-D-Leu-enkephalin) in causing mydriasis in mice was studied. Morphine-tolerant mice treated with met-enkephalin or DADL intracerebroventricularly (ICV) showed marked reduction of the pupillary effect of the endopioids. Maximal mydriasis in tolerant animals was only about 30% for met-enkephalin and 50% for DADL, compared to levels in nontolerant animals. These results are among the first to demonstrate cross-tolerance between morphine and enkephalins in intact animals and may suggest involvement of multiple opiate receptor systems in producing mydriasis.     

Morphine self-administration in µ-opioid receptor-deficient mice

Morphine-induced place preference was demonstrated recently in wild-type mice, whereas this conditioned behaviour was not observed in mu-opioid receptor-deficient mice. In the present study, we investigated locomotor effects of subcutaneously (s.c.) injected morphine as well as intracerebroventricular (i.c.v.) morphine self-administration in mu-opioid receptor-knockout mice. After s.c. morphine injection, locomotor activity significantly increased in wild-type animals. As expected, in the self-administration test the rate of self-administration constantly increased in wild-type mice reflecting reward effects of morphine. This increase was independent of locomotor/motor activity. In contrast, self-administration rates and locomotor/motor activity significantly decreased in the receptor-deficient animals. It was shown that this aversive effect might partly be due to kappa-opioid receptor interaction.     

The physical dependence liability of butorphanol: a comparative study with morphine.

In these studies, the physical dependence liability of butorphanol, a mixed 'agonist/antagonist' opioid analgesic, was compared to that of morphine. Male, Sprague-Dawley rats received i.c.v. infusions of saline (1 microliter/h), or an equimolar dose of butorphanol or morphine (52.3 nmol/h) for 3 days. The physical dependence liabilities of these two compounds were then compared by assessing both behavioral withdrawal signs and weight loss following naloxone-precipitated withdrawal. Body weight loss was also evaluated following abrupt (cessation of infusion) withdrawal from butorphanol or morphine. In animals receiving i.c.v. infusions of butorphanol or morphine, naloxone administration (5 mg/kg s.c.) induced an equivalent degree of body weight loss compared to saline-treated animals. In addition, the ED50 of naloxone to produce wet shakes, escape behavior, teeth chattering, urination and defecation was equivalent in rats receiving butorphanol or morphine. Infusions of butorphanol or morphine also produced an equivalent degree of weight loss in animals undergoing abrupt withdrawal. These results demonstrate then that a substantial degree of physical dependence had developed in rats which received a large dose of butorphanol.     

In vivo opiate binding unchanged in tolerant/dependent mice

The in vivo receptor binding was measured for an agonist and antagonist in the brains of naive mice and of mice made tolerant to and dependent on opiates by morphine pretreatment. There were significant differences between the receptor binding of these substances in the naive and in the tolerant/dependent animals. However, these differences disappeared 8 h after interruption of the morphine supply, although tolerance showed no decline. Therefore, the differences in the binding were obviously not produced by changes in the opiate receptors related to tolerance. Rather, the differences were probably caused by morphine in the brain of the tolerant/dependent mice pretreated with morphine and disappeared because the concentration of the morphine in the brains of these animals dropped during abrupt withdrawal. The fact that tolerance persisted for hours beyond this time indicated that tolerance is a time-dependent phenomena, not directly dependent on receptor occupation by an agonist.     

The effect of morphine pretreatment on hypothermia induced by morphine in mice.

Morphine caused an apparently dose-dependent hypothermia in mice. Co-administration of naloxone antagonised this effect. Pretreatment with a single dose of morphine induced detectable tolerance to the hypothermic effect of a second dose of morphine given 3 h later and naloxone was more effective in antagonising the hypothermic effect of morphine in morphine-pretreated mice than in saline-pretreated animals. The present study has shown that morphine pretreatment can augment the antagonistic effect of naloxone towards the hypothermic action of morphine.     

Secondary reinforcement property of a stimulus paired with morphine administration in the rat

Rats learned to run to the correct arm of a Y-maze. Correct responses were reinforced with morphine injection paired with a conditional tone stimulus. After the maze response was well established, extinction trials were run. During extinction half of the animals received neither morphine nor tone as a consequence of a correct response, while the other half received the tone but no morphine. Rats receiving the tone during extinction required significantly more trials to reach the extinction criteria than rats not receiving tone presentations. Extinction with the tone also facilitated relearning of the maze response. The results support the view that morphine is a potent reinforcer, and that stimuli paired with morphine administration acquire the properties of a secondary reinforcer.     

Modification of morphine-induced analgesia, tolerance and dependence by bromocriptine

The effect of two doses of bromocriptine, a dopamine agonist, on morphine-induced analgesia, tolerance and dependence was investigated in mice. Bromocriptine at doses of 0.04 and 0.08 mg/kg did not affect the baseline tail flick latency of mice but potentiated the morphine analgesia. Pretreatment of mice with 5 mg/kg of sulpiride, a D-2 antagonist, not only blocked the effect of 0.08 mg/kg of bromocriptine but also antagonized the morphine analgesia. Control animals given daily injections of 10 mg/kg of morphine rapidly developed tolerance to the analgesic effect. A combined treatment of bromocriptine with morphine given daily suppressed the development of tolerance to morphine analgesia. However, development of tolerance to morphine analgesia was not significantly modified in the animals treated daily with bromocriptine (0.08 mg/kg) plus sulpiride (5 mg/kg). Acute dependence was induced by the administration of 100 mg/kg of morphine. The administration of bromocriptine 30 min before naloxone significantly decreased the ED₅₀ value for naloxone for inducing jumping in mice. Coadministration of sulpiride and bromocriptine attenuated the ability of bromocriptine to potentiate the withdrawal syndrome of morphine dependence. The results indicate that bromocriptine potentiates morphine analgesia, suppresses the development of tolerance to morphine analgesia but exacerbates opiate withdrawal signs in morphine-dependent mice. These effects of bromocriptine appear to be mediated via D-2 receptors.     

Overexpression of type 7 adenylyl cyclase in the mouse brain enhances acute and chronic actions of morphine

The mechanisms by which morphine-induced analgesia and tolerance and physical dependence on morphine arise have been the subject of intense study, and much work has pointed to the involvement of cAMP-mediated events in the neuroadaptive phenomena leading to morphine tolerance and/or dependence. We overexpressed an opioid receptor-stimulatable form of adenylyl cyclase (type 7) in the central nervous system of mice and demonstrated significant effects of this manipulation on the animals' acute response to morphine, the development of morphine tolerance, and development of sensitization to morphine. Measurements of the acute analgesic response to morphine demonstrated that the ED(50) values for the transgenic mice were significantly lower than the ED(50) values determined for the "wild-type" animals. During chronic treatment with morphine, the transgenic mice developed tolerance more rapidly than the wild-type mice, and transgenic animals of the C57BL/6xSJL background showed a larger sensitization to morphine's effects on locomotor activity than did wild-type mice of the same background. These results indicated that cAMP-generating systems may simultaneously modulate the development of tolerance and sensitization. Interestingly, the signs of physical dependence on morphine in the transgenic mice did not differ from those in their wild-type litter mates, indicating that separate mechanisms may modulate opiate tolerance and opiate dependence.