Magnesium deficiency induces an hyperalgesia reversed by the NMDA receptor antagonist MK801

The Impact of Chronic Magnesium Deficiency on Excitable Tissues-Translational Aspects

Neuromuscular excitability is a vital body function, and Mg2+ is an essential regulatory cation for the function of excitable membranes. Loss of Mg2+ homeostasis disturbs fluxes of other cations across cell membranes, leading to pathophysiological electrogenesis, which can eventually cause vital threat to the patient. Chronic subclinical Mg2+ deficiency is an increasingly prevalent condition in the general population. It is associated with an elevated risk of cardiovascular, respiratory and neurological conditions and an increased mortality. Magnesium favours bronchodilation (by antagonizing Ca2+ channels on airway smooth muscle and inhibiting the release of endogenous bronchoconstrictors). Magnesium exerts antihypertensive effects by reducing peripheral vascular resistance (increasing endothelial NO and PgI2 release and inhibiting Ca2+ influx into vascular smooth muscle). Magnesium deficiency disturbs heart impulse generation and propagation by prolonging cell depolarization (due to Na+/K+ pump and Kir channel dysfunction) and dysregulating cardiac gap junctions, causing arrhythmias, while prolonged diastolic Ca2+ release (through leaky RyRs) disturbs cardiac excitation-contraction coupling, compromising diastolic relaxation and systolic contraction. In the brain, Mg2+ regulates the function of ion channels and neurotransmitters (blocks voltage-gated Ca2+ channel-mediated transmitter release, antagonizes NMDARs, activates GABAARs, suppresses nAChR ion current and modulates gap junction channels) and blocks ACh release at neuromuscular junctions. Magnesium exerts multiple therapeutic neuroactive effects (antiepileptic, antimigraine, analgesic, neuroprotective, antidepressant, anxiolytic, etc.). This review focuses on the effects of Mg2+ on excitable tissues in health and disease. As a natural membrane stabilizer, Mg2+ opposes the development of many conditions of hyperexcitability. Its beneficial recompensation and supplementation help treat hyperexcitability and should therefore be considered wherever needed.

Diet can exert both analgesic and pronociceptive effects in acute and chronic pain models: a systematic review of preclinical studies

BACKGROUND

Although diet is an essential aspect of human health, the link between diet and pain is still not well understood. Preclinical animal research provides information to understand underlying mechanisms that allow identifying the needs for human research.

OBJECTIVES

This study aims to give a systematic overview of the current evidence from preclinical studies regarding the analgesic and pronociceptive effects of various diets in non-neuropathic, non-cancer, or non-visceral acute and chronic pain models.

STUDY DESIGN

A systematic Review.

SETTING

This study examined studies that investigate the analgesic and pronociceptive effects of various diets in non-neuropathic, non-cancer, or non-visceral acute and chronic pain models.

METHODS

This review was conducted following the PRISMA guidelines and was registered in PROSPERO with the registration number CRD42019133473. The certainty of evidence was examined by a modified GRADE approach.

RESULTS

After the screening process twenty-four eligible papers were included in this review. Nineteen studies examined acute pain, nine studies chronic inflammatory pain, and four studies assessed both acute and chronic pain models.

LIMITATIONS

Due to the heterogeneity of the included studies, a meta-analysis was not included in this study.

CONCLUSIONS

In animal models, excessive saturated, monounsaturated or omega-6 polyunsaturated fat ingestion and diets rich in fats and carbohydrates can decrease pain sensitivity in acute nociceptive pain, whereas it can induce mechanical allodynia and heat hyperalgesia in chronic inflammatory pain. Additionally, diets rich in anti-inflammatory ingredients, as well as a calorie-restricted diet can promote recovery from primary mechanical allodynia and heat hyperalgesia in chronic inflammatory pain.

Efficacy of transforaminal epidural magnesium administration when combined with a local anaesthetic and steroid in the management of lower limb radicular pain

BACKGROUND

Lower limb radicular pain resulting from a herniated intervertebral disc is a cause of functional disability and could lead to increased consumption of opioids. We evaluated the efficacy of epidural magnesium combined with a local anaesthetic and steroid in the management of this pain.

METHODS

This was a prospective, case-control, randomized, double-blind study. Fifty patients each received 2 ml bupivacaine, 1 ml (40 mg) methylprednisolone and 1 ml saline (0.9%) (group C) or magnesium (200 mg) instead of saline (group M). The primary outcome measure was the improvement in the pain score (assessed using a visual analogue scale (VAS)), and the secondary outcome was the improvement in the functional ability (assessed using the Modified Oswestry Disability Questionnaire (MODQ)). The VAS and MODQ scores were assessed before and at 1 day, 1 week, 1 month and 3 months post-intervention.

RESULTS

The VAS and MODQ scores were significantly better in group M compared to those in group C at all times post-injection (p-value < 0.001). Comparisons within the same group showed that the VAS and MODQ scores were significantly better at all post-injection time points compared to the pre-injection scores in both group C and group M (p-values < 0.0001).

CONCLUSIONS

Adding magnesium to a local anaesthetic and steroid to be injected in the transforaminal epidural space could improve the pain and the quality of life in patients suffering from lower limb radicular pain due to lumbo-sacral disc herniation, and this improvement could last for up to 3 months.

SIGNIFICANCE

Magnesium is efficient when added to local anaesthetics and steroids for management of lower limb radicular pain.

Antinociceptive effects of magnesium sulfate for monitored anesthesia care during hysteroscopy: a randomized controlled study

Background

Opioids are the most effective antinociceptive agents, they have undesirable side effects such as respiratory depressant and postoperative nausea and vomiting. The purpose of the study was to evaluate the antinociceptive efficacy of adjuvant magnesium sulphate to reduce intraoperative and postoperative opioids requirements and their related side effects during hysteroscopy.

Methods

Seventy patients scheduled for hysteroscopy were randomly divided into 2 groups. Patients in the magnesium group (Group M) received intravenous magnesium sulfate 50 mg/kg in 100 ml of isotonic saline over 15 min before anesthesia induction and then 15 mg/kg per hour by continuous intravenous infusion. Patients in the control group (Group C) received an equal volume of isotonic saline as placebo. All patients were anesthetized under a BIS guided monitored anesthesia care with propofol and fentanyl. Intraoperative hemodynamic variables were recorded and postoperative pain scores were assessed with verbal numerical rating scale (VNRS) 1 min, 15 min, 30 min, 1 h, and 4 h after recovery of consciousness. The primary outcome of our study was total amount of intraoperative and postoperative analgesics administered.

Results

Postoperative serum magnesium concentrations in Group C were significantly decreased than preoperative levels (0.86 ± 0.06 to 0.80 ± 0.08 mmol/L, P = 0.001) while there was no statistical change in Group M (0.86 ± 0.07 to 0.89 ± 0.07 mmol/L, P = 0.129). Bradycardia did not occur in either group and the incidence of hypotension was comparable between the two groups. Total dose of fentanyl given to patients in Group M was less than the one administered to Group C [100 (75–150) vs 145 (75–175) μg, median (range); P < 0.001]. In addition, patients receiving magnesium displayed lower VNRS scores at 15 min, 30 min, 1 h, and 4 h postoperatively.

Conclusions

In hysteroscopy, adjuvant magnesium administration is beneficial to reduce intraoperative fentanyl requirement and postoperative pain without cardiovascular side effects. Our study indicates that if surgical patients have risk factors for hypomagnesemia, assessing and correcting magnesium level will be necessary.

Trial registration

ChiCTR1900024596. date of registration: July 18th 2019.

Intravenous magnesium sulfate vs. morphine sulfate in relieving renal colic: A randomized clinical trial

OBJECTIVE

Renal colic emerging from renal stone is virtually the most severe pain which is experienced. Intravenous infusion of morphine sulfate is known as a usual treatment for the disease. This study was designed to compare the efficacy of magnesium sulfate vs morphine sulfate in renal colic relief as for analgesic effect as well as lack of morphine sulfate side effects when using magnesium sulfate.

METHODS

We conducted a double-blind randomized clinical trial in renal colic patients who had referred to the emergency department of Shahid Sadoughi Hospital in Yazd, Iran. A total of 80 eligible patients were selected and randomly assigned into two groups; patients in the case group received 50 mg/kg intravenous magnesium sulfate, and those in the control group 0.1 mg/kg intravenous morphine. The primary outcome was the pain score measured on a numerical rating scale at 0, 10 and 20 minutes after infusion. Data were analyzed using SPSS₁₆.

RESULTS

The two groups were similar in terms of demographic features and pain intensity at the time of referral (P <.0001). Ten minutes after drug administration, the pain mean score in the morphine group leveled at 4.88, and in the magnesium group 5.70, which proved to be greater in the morphine group (P- = 0.06). However, the pain mean score turned out to be 3.65 in the morphine group and 3.20 in the magnesium group thus significantly indifferent (P = .48).

CONCLUSIONS

In this study, we concluded that administration of intravenous 50 mg/kg magnesium sulfate could be as effective as morphine in reducing renal colic without any further complications.

High concentration magnesium inhibits extracellular matrix calcification and protects articular cartilage via Erk/autophagy pathway

The significant cytopathological changes of osteoarthritis are chondrocyte hypertrophy, proteoglycan loss, extracellular matrix (ECM) calcification, and terminally, the replacement of cartilage by bone. Meanwhile, magnesium ion (Mg²⁺ ), as the second most abundant divalent cation in the human body, has been proved to inhibit the ECM calcification of hBMSCs (human bone marrow stromal cells), hVSMCs (Human vascular smooth muscle cells), and TDSCs (tendon-derived stem cells) in vitro studies. The ATDC5 cell line, which holds chondrocyte characteristics, was used in this study as an in vitro subject. We found that Mg²⁺ can efficiently suppress the ECM calcification and downregulate both hypertrophy and matrix metalloproteinase-related genes. Meanwhile, Mg²⁺ inhibits the formation of autophagy by inhibiting Erk phosphorylation signaling and lowers the expression of LC3, and eventually effectively reduces the formation of ECM calcification in vitro. In this study, we also used destabilization of the medial meniscus (DMM)-induced osteoarthritis (OA) animal model to further confirm the protective effect of Mg²⁺ on articular cartilage. Compared with the control group (saline-injected), continuous intra-articular magnesium chloride (MgCl₂ ) injection can significantly alleviate the severity of cartilage calcification in OA animal model. Immunofluorescence staining also revealed that saline-injected DMM group had a higher positive rate of LC3 expression in cartilage chondrocytes, compared with MgCl₂ -injected DMM group. In general, Mg²⁺ can significantly downregulate the hypertrophic gene Runx2, MMP13, and Col10α1, upregulate the chondrogenic genes Sox9 and Col1α1, inhibit the Erk phosphorylation signaling, reduce the expression of autophagy protein LC3, and effectively inhibit the ECM calcification of ATDC5. In vivo study also proved that intra-articular injection of Mg²⁺ protected knee cartilage by inhibiting the autophagy formation.

Improved analgesia by correction of hypomagnesaemia?

The role of magnesium as an analgesic in patients is unclear. Hypomagnesaemia is a common electrolyte abnormality, in the chronic state symptoms are insidious and often non-specific. It is often undiagnosed and thus untreated. There is evidence from animal studies that magnesium is involved in pain control including an animal model of hyperalgesia induced by hypomagnesaemia. We report two cases of patients admitted for pain control which improved when hypomagnesaemia was treated. Each case had metastatic cancer. Both were found on admission to have asymptomatic hypomagnesaemia and were treated with intravenous magnesium. Treatment for hypomagnesaemia resulted in an improvement in pain control such that analgesia was decreased. The incidence of hypomagnesaemia in palliative patients is unknown although it is thought to be common. These cases suggest that treating hypomagnesaemia may improve pain control.

Oral Application of Magnesium-L-Threonate Attenuates Vincristine-induced Allodynia and Hyperalgesia by Normalization of Tumor Necrosis Factor-α/Nuclear Factor-κB Signaling

BACKGROUND

Antineoplastic agents, including vincristine, often induce neuropathic pain and magnesium deficiency clinically, but the causal link between them has not been determined. No drug is available for treating this form of neuropathic pain.

METHODS

Injection of vincristine (0.1 mg · kg · day, intraperitoneally, for 10 days) was used to induce nociceptive sensitization, which was accessed with von Frey hairs and the plantar tester in adult male Sprague-Dawley rats. Magnesium-L- threonate was administered through drinking water (604 mg · kg · day). Extracellular and intracellular free Mg were measured by Calmagite chromometry and flow cytometry. Molecular biologic and electrophysiologic experiments were performed to expose the underlying mechanisms.

RESULTS

Vincristine injection induced allodynia and hyperalgesia (n = 12), activated tumor necrosis factor-α/nuclear factor-κB signaling, and reduced free Mg in cerebrospinal fluid by 21.7 ± 6.3% (mean ± SD; n = 13) and in dorsal root ganglion neurons by 27 ± 6% (n = 11). Reducing Mg activated tumor necrosis factor-α/nuclear factor-κB signaling in cultured dorsal root ganglion neurons. Oral application of magnesium-L-threonate prevented magnesium deficiency and attenuated both activation of tumor necrosis factor-α/nuclear factor-κB signaling and nociceptive sensitization (n = 12). Mechanistically, vincristine induced long-term potentiation at C-fiber synapses, up-regulated N-methyl-D-aspartate receptor type 2B subunit of N-methyl-D-aspartate receptor, and led to peptidergic C-fiber sprouting in spinal dorsal horn (n = 6 each). The vincristine-induced pathologic plasticity was blocked by intrathecal injection of nuclear factor-κB inhibitor (n = 6), mimicked by tumor necrosis factor-α, and substantially prevented by oral magnesium-L-threonate (n = 5).

CONCLUSIONS

Vincristine may activate tumor necrosis factor-α/nuclear factor-κB pathway by reduction of intracellular magnesium, leading to spinal pathologic plasticity and nociceptive sensitization. Oral magnesium-L-threonate that prevents the magnesium deficiency is a novel approach to prevent neuropathic pain induced by chemotherapy.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Long-term administration of magnesium after acoustic trauma caused by gunshot noise in guinea pigs

In a previous study we observed that a 7-day post-trauma magnesium treatment significantly reduced auditory threshold shifts measured 7 days after gunshot noise exposure. However this improvement was only temporary, suggesting that it could be potentially beneficial to prolong this treatment. The aim of the present study was to evaluate the efficacy of a long-term (1 month) magnesium treatment after an impulse noise trauma, in comparison with either a 7-day magnesium treatment, an administration of methylprednisolone (conventional treatment), or a placebo (NaCl). Guinea pigs were exposed to impulse noise (three blank gunshots, 170 dB SPL peak). They received one of the four treatments, 1 h after the noise exposure. Auditory function was explored by recording the auditory brainstem response (ABR) and measuring the distortion product otoacoustic emissions (DPOAE) over a 3-month recovery period after the gunshot exposure. The functional hearing study was supplemented by a histological analysis. The results showed that a 1-month treatment with magnesium was the most effective treatment in terms of hair cell preservation. The DPOAE confirmed this effectiveness. Methylprednisolone accelerated recovery but its final efficacy remained moderate. It is probable that magnesium acts on the later metabolic processes that occur after noise exposure. Multiple mechanisms could be involved: calcium antagonism, anti-ischaemic effect or NMDA channel blockage. Regardless of the specific mechanism, a 1-month treatment with magnesium clearly attenuates NIHL, and presents the advantage of being safe for use in humans.

Traditional and Evidence‐Based Acupuncture in Headache Management: Theory, Mechanism, and Practice

Acupuncture, traditional Chinese needle therapy, has become widely used for the relief of headache. The history of the practice of acupuncture in the United States and the theoretical framework for acupuncture in Chinese medicine are reviewed. The basic scientific background and clinical application of acupuncture in the headache management are discussed.

The nerve-heart connection in the pro-oxidant response to Mg-deficiency

Magnesium is a micronutrient essential for the normal functioning of the cardiovascular system, and Mg deficiency (MgD) is frequently associated in the clinical setting with chronic pathologies such as CHF, diabetes, hypertension, and other pathologies. Animal models of MgD have demonstrated a systemic pro-inflammatory/pro-oxidant state, involving multiple tissues/organs including neuronal, hematopoietic, cardiovascular, and gastrointestinal systems; during later stages of MgD, a cardiomyopathy develops which may result from a cascade of inflammatory events. In rodent models of dietary MgD, a significant rise in circulating levels of proinflammatory neuropeptides such as substance P (SP) and calcitonin gene-related peptide among others, was observed within days (1-7) of initiating the Mg-restricted diet, and implicated a neurogenic trigger for the subsequent inflammatory events; this early "neurogenic inflammation" phase may be mediated in part, by the Mg-gated N: -methyl-D-aspartate (NMDA) receptor/channel complex. Deregulation of the NMDA receptor may trigger the abrupt release of neuronal SP from the sensory-motor C-fibers to promote the subsequent pro-inflammatory changes: elevations in circulating inflammatory cells, inflammatory cytokines, histamine, and PGE(2) levels, as well as formation of nitric oxide, reactive oxygen species, lipid peroxidation products, and depletion of key endogenous antioxidants. Concurrent elevations of tissue CD14, a high affinity receptor for lipopolyssacharide, suggest that intestinal permeability may be compromised leading to endotoxemia. If exposure to these early (1-3 weeks MgD) inflammatory/pro-oxidant events becomes prolonged, this might lead to impaired cardiac function, and when co-existing with other pathologies, may enhance the risk of developing chronic heart failure.

Changes in magnesium concentration in the serum and cerebrospinal fluid of neuropathic rats

BACKGROUND

Central sensitization of neuropathic pain is associated with an influx of extracellular calcium via the opening of N-methyl-D-aspartate (NMDA) receptor-gated ion channels, which are usually blocked by magnesium plugs. As magnesium-deficient rats develop a mechanical hyperalgesia and intrathecal or intraperitoneal magnesium suppresses neuropathic pain, the magnesium concentrations in serum and cerebrospinal fluid may be altered in neuropathic pain. We therefore compared the magnesium concentrations in serum and cerebrospinal fluid of neuropathic rats with those in injured rats without symptoms of neuropathic pain and normal rats.

METHODS

Mechanical allodynia was induced in male Sprague-Dawley rats by tight ligature of the left lumbar fifth and sixth spinal nerves. The threshold of paw withdrawal was evaluated by the up-down method using withdrawal response to stimulus with a von Frey filament on the third, seventh and 14th days. Rats with a threshold of less than 4 g were selected as the symptomatic group and compared with an asymptomatic group, an unoperated control group and a sham-operated group. On the 16th day, the Mg2+ concentrations in serum and cerebrospinal fluid were measured.

RESULTS

The magnesium concentrations in the serum and cerebrospinal fluid of symptomatic neuropathic rats did not differ from those in the injured rats without symptoms of neuropathic pain, sham-operated rats and normal rats.

CONCLUSION

Our results suggest that physiologic homeostasis is maintained by active transport through the blood-brain barrier despite the activation of NMDA receptor-gated ion channels. However, rats with neuropathic pain may be in a magnesium-deficient condition at the effector site, such that magnesium treatment can decrease neuropathic pain.

Therapeutic efficacy of magnesium after acoustic trauma caused by gunshot noise in guinea pigs

CONCLUSIONS

The present findings show that magnesium administration can significantly reduce threshold shift 7 days after gunshot noise exposure. However, this improvement seems to be temporary, suggesting a probable advantage in prolonging the treatment.

OBJECTIVE

To evaluate the therapeutic efficacy of magnesium administration after hearing loss induced by gunshot noise.

MATERIAL AND METHODS

Forty-eight guinea pigs were exposed to an impulse noise (blank shot from a rifle; 170 or 176 dB SPL peak). The therapeutic efficacy of magnesium was evaluated by administering either the treatment or a placebo to the traumatized animals for 7 days, beginning 1 h after the trauma. Auditory function was explored for up to 14 days of recovery by recording the compound action potential in the round window. The functional study of hearing was supplemented by histological analysis.

RESULTS

The threshold shifts of the 170-dB SPL group that received magnesium were significantly lower than those of controls after 2 and 7 days of recovery, but no significant difference was evidenced at 14 days in this group, nor at any time in the 176-dB SPL group. Animals treated with magnesium after the 176-dB SPL exposure had a significant reduction in hair cell loss in the basal region.

Identification of a homocysteine receptor in the peripheral endothelium and its role in proliferation

BACKGROUND

Homocysteine, a risk factor for atherosclerosis, increases intimal hyperplasia after carotid endarterectomy with associated smooth muscle cell proliferation and modulation of cytokines. The N-methyl-D-aspartate receptor (NMDAr), a glutamate-gated ion channel receptor, is associated with homocysteine-induced cerebrovascular injury; however, the receptor has not been identified in peripheral vascular cells, nor has any interaction with homocysteine been clarified. Our objectives were first, to identify NMDAr in rat carotid artery and rat aorta endothelial cells (RAEC); and second, to determine whether homocysteine activates NMDAr in the endothelium.

METHODS

NR1 and NR2A, two NMDAr subunits, were probed in rat carotid arteries by immunohistochemistry. RNA was isolated from RAECs, and expression of all NMDAr subunits (NR1, 2A, 2B, 2C, and 2D) were examined by RT-PCR and sequencing. For receptor protein expression, RAEC were incubated with different homocysteine concentrations and incubation times and also were treated with 50 microM homocysteine and/or preincubated with 50 microM dizocilpine MK-801, an NMDAr inhibitor.

RESULTS

Both NR1 and NR2A were expressed in rat carotid arteries. All NMDAr subunits were expressed in the RAECs, and there was 92% to 100% similarity compared with rat NMDAr from the National Center for Biotechnology Information (NCBI) GenBank. Homocysteine upregulated NR1 expression and increased cell proliferation. RAEC pretreatment with MK-801 reduced homocysteine-mediated cell proliferation.

CONCLUSION

This study is the first to show that NMDAr exists in the peripheral vasculature, and that homocysteine may act via NMDAr to increase intimal hyperplasia.

CLINICAL RELEVANCE

Our objectives included the identification of a homocysteine receptor in the peripheral vasculature. The possible inhibition of a homocysteine receptor to prevent intimal hyperplasia rather than treat established stenosis would make a significant clinical impact. This will open further avenues of study in determining the role of homocysteine in the pathogenesis of intimal hyperplasia.

New developments in the pharmacotherapy of tension-type headaches

The first International Headache Society classification defined tension-type headaches (TTHs) by itemising those characteristics of migraines TTHs did not possess [1]. As a result, TTHs, both episodic and chronic, remain the most nonspecific of all the commonly observed primary headaches. Until recently, there has been little impetus on the part of the pharmaceutical industry to investigate TTHs; many of the potentially useful drugs are now generic and unprofitable. In addition, few investigators have pursued the study of TTHs in lieu of its more glamorous neighbour, migraine. As a result, there are few well-designed studies on the pharmacotherapy of TTHs. The few studies that exist support the use of age-old standard drug classes, the tricyclic antidepressants and the NSAIDs. New research is now emerging that points to the potential utility of botulinum toxin type A, NMDA-receptor antagonists including Mg(2+) and nitric oxide synthase inhibitors. More scientifically rigorous clinical studies are needed.

Does Mg2+ deficiency induce a long-term sensitization of the central nociceptive pathways?

In rats, a Mg(2+)-deficient diet, which in a few days dramatically decreased the Mg(2+) concentration in plasma, cerebrospinal fluid (CSF) and spinal cord, was accompanied by a significant lowering of the nociceptive threshold. After reloading, the Mg(2+) concentration was rapidly normalized in both spinal cord and CSF. In parallel, the neurological disturbances induced by Mg(2+) deficiency vanished in less that 24 h, but the reversal of the hyperalgesia was delayed for up to 11 to 20 days. In this model, repeated doses of dizocilpine (MK-801), a non-competitive NMDA receptor antagonist, given at start of the Mg(2+)-depleted diet, prevented hyperalgesia, suggesting the involvement of NMDA receptor channels. The delayed recovery of a normal pain threshold argues for long-term sensitization of the nociceptive pathways.

Role of spinal NMDA receptors, protein kinase C and nitric oxide synthase in the hyperalgesia induced by magnesium deficiency in rats

1. Magnesium (Mg)-deficient rats develop a mechanical hyperalgesia which is reversed by a N-Methyl-D-Aspartate (NMDA) receptor antagonist. Given that functioning of this receptor-channel is modulated by Mg, we wondered whether facilitated activation of NMDA receptors in Mg deficiency state may in turn trigger a cascade of specific intracellular events present in persistent pain. Hence, we tested several antagonists of NMDA and non-NMDA receptors as well as compounds interfering with the functioning of intracellular second messengers for effects on hyperalgesia in Mg-deficient rats. 2. Hyperalgesic Mg-deficient rats were administered intrathecally (10 microl) or intraperitoneally with different antagonists. After drug injection, pain sensitivity was evaluated by assessing the vocalization threshold in response to a mechanical stimulus (paw pressure test) over 2 h. 3. Intrathecal administration of MgSO4 (1.6, 3.2, 4.8, 6.6 micromol) as well as NMDA receptor antagonists such as MK-801 (0.6, 6.0, 60 nmol), AP-5 (10.2, 40.6, 162.3 nmol) and DCKA (0.97, 9.7, 97 nmol) dose-dependently reversed the hyperalgesia. Chelerythrine chloride, a protein kinase C (PKC) inhibitor (1, 10.4, 104.2 nmol) and 7-NI, a specific nitric oxide (NO) synthase inhibitor (37.5, 75, 150 micromol x kg(-1), i.p.) induced an anti-hyperalgesic effect in a dose-dependent manner. SR-140333 (0.15, 1.5, 15 nmol) and SR-48968 (0.17, 1.7, 17 nmol), antagonists of neurokinin receptors, produced a significant, but moderate, increase in vocalization threshold. 4. These results demonstrate that Mg-deficiency induces a sensitization of nociceptive pathways in the spinal cord which involves NMDA and non-NMDA receptors. Furthermore, the data is consistent with an active role of PKC, NO and, to a lesser extent substance P in the intracellular mechanisms leading to hyperalgesia.

A comparative study with oral nifedipine, intravenous nimodipine, and magnesium sulfate in postoperative analgesia

We tested the ability of two L-type calcium channel blockers (nifedipine and nimodipine) and the N-methyl D-aspartate natural antagonist magnesium to decrease morphine requirements and pain in the postoperative period in 92 patients undergoing elective colorectal surgery. In a randomized, double-blinded study, patients were assigned to one of four groups. The control group received placebo. The nifedipine group received 60 mg of oral nifedipine. The magnesium group received an initial dose of 30 mg/kg followed by 10 mg x kg(-1) x h(-1) of magnesium sulfate over 20 h. The nimodipine group received 30 microg x kg(-1) x h(-1) of nimodipine over 20 h. Postoperative morphine consumption was assessed for 48 h. Pain at rest and pain on movement were assessed up to the fifth day postsurgery. There were no differences among groups in postoperative morphine consumption at 12 and 24 h. The nifedipine group consumed more morphine than the control and nimodipine groups during 24-48 h. Pain at rest scores were higher at 16 and 24 h in the nifedipine group than in the other three groups. Pain on movement scores were lower at 72 h in the nimodipine group than in the control and nifedipine groups. In conclusion, the perioperative application of oral nifedipine, IV nimodipine, or IV magnesium sulfate failed to decrease postoperative morphine requirements after colorectal surgery.

IMPLICATIONS

The increase of intracellular calcium plays a key role in spinal transmission of pain and in the establishment of central sensitization. We examined the effects of nifedipine, nimodipine, and magnesium sulfate in postoperative analgesia after colorectal surgery. We found no differences in morphine consumption with the administration of each drug alone.