Magnesium influence on morphine ‐‐ induced pharmacodependence in rats

ARTICLE

Auteur(s) : Mihai Nechifor¹, Dan Chelarescu¹, Maria Miftode²

¹ Department of Pharmacology ; 
² Department of Inorganic Chemistry, University of Medicine and Pharmacy Gr. T. Popa, Universitatii 16, Iasi 6600, Romania

Introduction

Opiate-induced pharmacodependence is a more and more frequently met phenomena, with complex mechanisms and affecting a large number of people. Pharmacodependence was characterized in essence as a phenomenon with a compulsive use of a substance (in this case opiates) and existence of craving. Pharmacodependence on opiates is a complex process involving various cerebral molecular mechanisms. A detailed understanding of cerebral molecular mechanisms that are keystones of opiate-induced pharmacodependence is considered to be essential for treating and prevention of this phenomena.

Magnesium has many involvements in the central nervous system (CNS) function. Changes in magnesium concentrations exert diverse influences on neuronal functions [1]. These changes in extra and intracellular magnesium concentrations are found in different psychiatric diseases : depression, bipolar psychosis, schizophrenia, etc. [2, 3, 4]. On the other hand an association was proved between chronic deficiency of Mg and clinical manifestations such as hyperemotivity, decreasing convulsive threshold, anxiety, dizziness, etc [1].

Materials and method

We tested the influence of magnesium acetate (MgAc) on morphine-induced pharmacodependence in rats. We worked on 5 groups, eight male adult Wistar rats in each, bred in normal laboratory conditions, weighing 180-210 g. First group was for control and did not receive any substance. 2^nd group received morphine after the following schedule : 5 mg/kg i.p. on the I^st day, 10 mg/kg/day on the 2^nd day. The dose was raised by 10 mg/day until 90 mg/kg/day i.p on the 10^th day. Daily dose was split in 2 administrations at 9 a.m and 5 p.m. [5]. On 11^th day, 2 hours after the last morphine administration (90 mg/kg), all animals received naloxone 1 mg/kg i.p. The 3^rd group received morphine on the same schedule as the 2^nd group but 1 hour before morphine they received MgAc 0.5 mEq/kg/day. The 4^th group got moprphine as the 2^nd group but each day received MgAc 0.1 mEq/kg/day i.p. 1 hour before morphine. The 5^th group received only MgAc 0.5 mEq/kg/day, 10 days. A schedule of the experiment is shown in the figure 1.

Animals' weight was measured before starting the experiment and on day 2,4,6,8,10. Morphine administration was adapted to each animal's weight variations during experiment. 15 min after naloxone administration, symptoms from withdrawal syndrome were followed on each animal for 25 minutes. Survey was performed with a video camera linked with the computer. We searched for symptoms of withdrawal syndrome, teeth chattering, compulsive mastication, exploration behavior, jumping, aggressive postures, diarrhea, grooming, loss of weight for each animal during a 25 minute period.

All animal groups were surveyed in the same conditions. Plasmatic magnesium level was determined by spectrophotometry before magnesium administration, and at 2 h, 4 h, and 6 h, 24 h after magnesium administration. Data were statistically interpreted with ANOVA test.

Results

The group that got only morphine evolved a strong pharmacodependence with symptoms of withdrawal syndrome vs. control group and MgAc group. These last groups did not develop pharmacodependence and did not change behavior after naloxone administration. The influence of MgAc on some symptoms (erections, toilets, aggressive postures, locomotor activity-explorations) of withdrawal syndrome is shown in figure 2. Magnesium influence on jumping, grooming, teeth chattering, diarrhea is shown in figure 3. Numbers of jump were not significantly influenced at any concentrations of MgAc used. There is no significant difference between weight loss of animals that got only morphine vs. morphine (M) + MgAc group. Evolution of animal weight during withdrawal is shown in figure 4. Serum Mg concentrations increase significantly after MgAc administration at 2, 4 and 6 h. Evolution of serum magnesium level before and after MgAc administration is shown in figure 5.

Discussion

Opiates act at CNS level on membrane receptors (µ κ δ). Consequent to these receptors' stimulation molecular processes appear that lead (among a others) to sustained release of dopamine on ventral tegmentum and nucleus accumbens. This is considered to be essential for pharmacodependence emergence. Dopamine is the most important but not the only neuromediator involved in opiate-induced pharmacodependence [6]. Dopamine is the main neuromediator for the reward system in human and animal brains [7].

Morphine administration in rats (according to our schedule for inducing pharmacodependence), determines the emergence of strong pharmacodependence [5]. The main mechanisms involved in morphine (and other opiates) induced pharmacodependence are dysfunctions in the cerebral dopaminergic system functioning at the level of nucleus accumbens and ventral tegmentum. Enhanced and prolonged release of biological active amines (especially dopamine) is a main mechanism for stimulation of the reward system by opiates (but dopamine is not the only neuromediator involved in the induction of pharmacodependence). Magnesium is a bivalent cation involved in many processes at CNS level. Magnesium has multiple actions at neuronal level (including synapses). Among other actions, Mg²⁺ decreases the presynaptic release of cathecolamines and acetylcholine (ACh) [2]. Enhanced release of excitatory neuroaminoacids (especially glutamate) plays an important role in the mechanism of clinical symptoms from withdrawal syndrome [6]. Our results show that MgAc in dose of 0.5 mEq/kg/day i.p. administrated during induction of morphine pharmacodependence decreases the intensity of symptoms of withdrawal syndrome. We consider that Mg is an important factor for decreasing morphine pharmacodependence.

Our data are in agreement with Miller-Reiter et al. [8], which show administration of magnesium salts during withdrawal syndrome in opiate-dependent patients decreased the intensity of withdrawal symptoms and with data about favorable effect in patients with delirium tremens [9]. In opposition to the above studies where an alleviation of withdrawal symptoms appear (without Mg action during induction of morphine/ethanol dependence) in our experiment an action on the mechanism of pharmacodependence induction, because magnesium was not administrated after the induction of withdrawal syndrome.

Mg²⁺ may decrease the intensity of morphine-induced pharmacodependence in rats and consequently the intensity of withdrawal symptoms by many mechanisms :

a) by decreasing presynaptic release of cathecolamines (including dopamine, essential for the molecular mechanism of morphine-induced addiction) [10, 11].

b) by decreasing glutamate action on NMDA receptors into the brain. Glutamate action as the main excitatory aminoacid is important in emerging symptoms of withdrawal syndrome. Magnesium plays an antagonist role toward Ca²⁺ entrance into neurons through Ca²⁺ channels coupled with NMDA receptors [12].

c) by increasing synaptic concentrations of GABA, decreasing GABA decarboxilation and stimulating its release [13].

Possible mechanisms through which Mg may decrease morphine-induced pharmacodependence are shown in figure 6.

Data regarding magnesium concentration in plasma and cerebrospinal fluid (CSF) in patients with delirium tremens are different [14, 15] but in most situations hypomagnesemia was found to be significant. This might be an argument for magnesium deficiency contribution in the emergence of ethanol pharmacodependence and also in withdrawal syndrome. During morphine administration a decrease in magnesium concentration was observed. Dopamine release from nucleus accumbens is a phenomenon stimulated and dependent on calcium. This is an argument for magnesium effect in decreasing intensity of morphine-induced pharmacodependence, magnesium acting as a calcium antagonist and decreasing dopamine release. Without increasing dopamine release in nucleus accumbens and ventral tegmentum, pharmacodependence does not evolve [6].

It was proved that glutamate-induced activation of NMDA presynaptic receptors has a facilitating effect for dopamine presynaptic release [16]. We consider that magnesium acts as an antagonist of Ca²⁺ entrance into the neuron after NMDA receptor activation to decrease morphine-induced dopamine release and also release of other substances with pharmacodependence potential. Data obtained are in agreement with our previous data [17], when we used another Mg compound (magnesium acexamate) and a different experimental pharmacodependence model.

Data obtained show the possibility that magnesium intake during morphine-induced pharmacodependence may decrease the intensity of pharmacodependence.

Conclusion

Results show that MgAc administrated during pharmacodependence induction significantly decreased the intensity of signs of withdrawal syndrome in animals. The effect is not related to a direct magnesium action during withdrawal syndrome, Mg administration being ceased 24 h before naloxone-induced withdrawal syndrome.

We consider that it might be possible to decrease the intensity of withdrawal syndrome from opiate-induced pharmacodependence by therapeutic magnesium supplementation.

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