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Methcathinone Intoxication in the Rat: Abrogation by Dextrorphan
LABORATORY I N V E S T I G A T I O N
Methcathinone Intoxication in the Rat: Abrogation by Dextrorphan From the Departments of Emergency Medicine* and Pharmacology and
Toxicology~, University of Mississippi Medical Center, Jackson, Mississippi. Receivedfor publicationMarch4, 1996. Revision received July 15, 1996. Accepted for publication July 24, 1996.
Rob W Rockhold, PhD*
Study objective: Methcathinone, a designer drug, has high
Fredrick B Carlton Jr, MD*
abuse liability. In this study we characterizedacute methcathinone toxicity in rats, attempting to determine whether the excitatory amino acid receptor antagonist dextrorphan can antagonize methcathinone intoxication.
Robert Corkern, MD* Len Derouen, MD* James G Bennett~ Arthur S Hume, PhD*
Portions of the material in this manuscript w e r e presented in abstract
form at the 15th Annual Meetingof the SoutheasternPharmacology
Society, Clearwater Beach, Florida, September 1994; and the
Experimental Biology '95 meeting, Atlanta, April 1995.
Dr Rockhold w a s supported by the American Heart Association, Mississippi affiliate. The work of Dr Derouen and Dr Corkern was performed in partial fulfillment of residency requirementsfor the University of Mississippi Medical Center Department of Emergency Medicine. The assistance ofJames Bennettwas supported by a summer medical student researchfellowship, supported by the Dean of the University of Mississippi Medical School. Copyright © by the American College of Emergency Physicians.
Methods: Intoxication was produced with IV methcathinone infusion (5 mg/kg/minute; 100 mg/mL)in conscious rats. We studied pretreatment, in which dextrorphan or vehicle was injected 30 minutes before methcathinone infusion. In a second protocol, dextrorphan or saline solution was given immediately after the onset of convulsions. Results: Methcathinone caused tachycardia (maximal increase, 131+10 beats/minute), hyperthermia (+2.3 ° C), convulsions, and cardiorespiratory collapse in vehicle-pretreated rats (n=9). Death occurred after 32.0+1.1 minutes of infusion. Dextrorphan pretreatment (25 mg/kg; n=7) significantly reduced hyperthermia (+.1°+.3 ° C) and tachycardia and increased the convulsive (dextrorphan, 134+9 mg/kg; vehicle, 67+4 mg/kg)and lethal doses (dextrorphan, 204+9 mg/kg; vehicle, 160+5 mg/kg). Dextrorphan, given immediately after the initial methcathinone convulsion, reduced hyperthermic and tachycardic responses but not the lethality of methcathinone. Conclusion: Blockade of excitatory amino acid receptors by dextrorphan minimizes acute methcathinone intoxication. [Rockhold RW, Carlton FB, Corkern R, Derouen L, Bennett JG, Hume AS: Methcathinone intoxication in the rat: Abrogation by dextrorphan. Ann EmergMed March 1997;29:383-391 .]
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INTRODUCTION
Methcathinone (2-methylamino-l-phenylpropan-l-one) is the N-methyl derivative of the naturally occurring psychomotor stimulant cathinone. 1 Methcathinone is similar in behavioral effects and pharmacology to other phenylisopropylamines, including methamphetamine and aminorex. 2 Unlike methcathinone, however, cathinone is not considered to have the potential to be a significant drug of abuse in the United States. Methcathinone is more potent than cathinone in terms of psychomotor stimulant activity. ~ Considerable attention has recently been focused on methcathinone with the documentation of human toxic responses following illicit use of methcathinone in the United States. B-6A single clinical report, published in Annals of Emergency Medicine, documented signs and symptoms of acute methcathinone toxicity including confusion, tremors, headaches, abdominal pain, diaphoresis, tachycardia, hyperthermia, and hallucinations.* Episodic bradycardia and hypotension may be seen at varying intervals after drug ingestion. Long-term binge use results in symptoms including paranoid psychosis, weight loss, tremor, hyperreflexia, increases in circulating hepatic enzyme levels, and proteinuria. 4-6 Evidence suggests that physiologic withdrawal responses occur after cessation of long-term use. 5,s Despite the potential clinical and societal impact of methcathinone, very little information is available on acute pharmacologic and toxic responses in laboratory animals under controlled conditions. Methcathinone has been shown to produce locomotor stimulation in the rat 1 and the baboon 7, and avid self-administration of methcathinone has been reported in the baboon, r The psychomotor stimulus imparted by methcathinone is similar to those of cocaine 2 and methamphetamine. ~ Methcathinone, methamphetamine, amphetamine, and cathinone all stimulated release of [3H] dopamine from rat brain tissue, suggesting that these agents exert similar actions as indirect sympathomimetic substances. ~ The apparent similarities of methcathinone, cocaine, and methamphetamine suggest that pharmacologic agents effective in the antagonism of cocaine and methamphetamine toxicity could also be useful in the treatment of methcathinone intoxication. Excitatory amino acid neurotransmitters, including glutamate and aspartate, constitute the major excitatory stimuli in the mammalian central nervous systems and have been implicated in the mediation of respirator/and circulator/ regulation 9-1°, as well as in convulsive activity and neurologic dysfunction. 11-~2 A family of excitatory amino acid receptor subtypes has been described, of which the ionotropic N-methyl-D-aspartate (NMDA) receptor is best characterized, z3 Studies from our laboratorys<~5 and others ~6-2°
3 84
have demonstrated that toxic responses to stimulant drugs of abuse, including cocaine and methamphetamine, can be reduced by excitatory amino acid receptor antagonists. The agents most intensively studied have been the noncompetitive NMDA receptor antagonists MK-801 and dextrorphan. However, MK-801 may exert neurotoxic and behaviorally toxic actions, whereas dextrorphan is relatively innocuous.21-23 In this study we sought to characterize the acute toxic responses to IV methcathinone infusion in conscious rats and to determine whether the excitatory amino acid receptor antagonist dextrorphan effectively minimizes these toxicities. MATERIALS AND METHODS
Male Sprague-Dawley rats weighing 275 to 300 g were purchased from Harlan-Spragne Dawley. The animals were housed, three to five per plastic cage, for 5 to 7 days before the experiments under controlled conditions of light (8 aM to 8 PM), temperature (22 ° to 24 ° C), and humidity (50% to 55%). Access to deionized water and rodent chow was permitted at all times. All experimental handling and treatment of these animals was approved by the University of Mississippi Medical Center Institutional Animal Care and Use Committee. While each animal was under halothane anesthesia (2% to 4% in medical-grade oxygen), polyethylene catheters (Clay Adams) were implanted into the abdominal aorta (PE-50) and vena cava (PE-10) by way of the left femoral vessels. The catheters were tilted with heparinized saline solution (20 U/mL), exteriorized at the nape of the neck, and sealed until use. Aseptic techniques were used for all surgical manipulations. All wound sites were infiltrated with bupivacaine .5 % to provide postoperative pain relief. Each animal was housed individually for 48 hours after surgery in a plastic cage with corncob litter before experimental use. Each animal was placed in a clear Plexiglas restraining tube (internal diameter, 5.5 cm; length, 21.5 cm), after which an electronic thermometer probe was inserted into the rectum and the catheters were connected for the recording of blood pressure and heart rate and the injection of drugs. The Plexiglas tubes were open at both ends, except for restraining posts, and machined with multiple ventilator/ and heatqoss slots. The tubes allowed limited anterior-posterior movement and restricted lateral movement. Movements such as head bobbing, •xion/extension seizures of the limbs, and tail extension were possible and clearly evident. Arterial pressure was measured with Cobe disposable pressure transducers connected to a Grass model 7D polygraph. Mean arterial pressure was determined by electronic
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damping of the pulsatile signal• Pulse was calculated electronically from the pulse interval. Rectal temperature was recorded at discrete intervals. We conducted preliminary studies to choose a concentration of methcathinone (100 mg/mL) and a volume rate of infusion (50 l~L/kg/minute) such that continuous infusion would cause death within approximately 30 minutes in nonpretreated rats. The rate of infusion is identical to that used in previous studies from our laboratory of the acute toxic responses to IV cocaine infusion. ~4,15 The concentration is four times greater than that used with cocaine-infusion regimens. 14,t5 A separate group of six rats was tested with these infusion variables and without pretreatment. In these animals, infusion was continued until the onset of the first visible convulsion. The infusion was immediately discontinued at that time and the animal returned to its home cage. The mean dose required to elicit a convulsion and the mean time to onset of the initial convulsion were 57.4_+4.0 mg/kg and 11.5+.8 minutes, respectively. Survival was assessed 24 hours after methcathinone infusion. Three of the six animals (50%) survived. The 50% lethal dose (LDso) after IV injection (presumably a bolus) of methcathinone in the rat has been reported to be 90_+3 mg/kg4 (see reference 7 of that report). We used two experimental protocols. First, we examined a pretreatment regimen. This protocol involved placing rats in restraining tubes 30 minutes before IV injection of phosphate-buffered saline solution (.9% NaC1, 1 mL/kg) or dextrorphan tartrate (25 mg/kg). All measurements of blood pressure, pulse, and rectal temperature were begun when the rats were placed in the restraining tubes. The dextrorphan dosage regimen has been shown to provide significant protection against cocaine intoxication in a similar model• ~5 Ambient temperature was maintained at 24 ° C. Arterial blood pressure, pulse, and rectal temperature were recorded for an additional 30 minutes before the initiation of a continuous IV infusion of methcathinone hydrochloride at a rate of 5.0 mg/kg/minute (volume rate, 50 laL/kg/minute; 100 mg/mL) with the use of a Sage model 355 infusion pump.
Arterial blood pressure and pulse were measured at decimal fractions of the time from initiation of the methcathinone infusion until cardiorespiratory arrest and death. The time to onset of seizures and the time to terminal cardiovascular collapse and death were recorded to the nearest second. Convulsions became evident initially with the abrupt onset of a whole-body myoclonic jerk. The interval between seizures progressively decreased. A terminal pattern of seizures included more profound motor activity, which was evident as generalized tonic-clonic forelimb and hindlimb extensions. This pattern coincided with cardiovascular collapse and took the appearance of status epilepticus. Laparotomy was performed in each animal to assess the patency of the venous drug-administration catheter• Any animal with evidence of impaired or diverted venous flow (eg, retroperitoneal hemorrhage) was eliminated from the data set. The volume of the urinary bladder was assessed to the nearest •1 mL by means of needle aspiration of the bladder contents into a 5-mL syringe. In the second protocol we assessed the ability of dextrorphan to antagonize methcathinone toxicity when the drug was administered after the onset of seizure activity Each animal in this protocol was implanted with a second PE-10 venous catheter to permit simultaneous injection of drugs and IV infusion of methcathinone. Both venous catheters were implanted through the same femoral vein, as described previously, Rats were placed in restraining tubes 30 minutes before the initiation of a continuous IV infusion of methcathmone hydrochloride at a rate of 5.0 mg/kg/minute (volume rate, 50 laL/kg/minute; 100 mg/mL) with the use of a Sage model 355 infusion pump. The time elapsed before the onset of the initial seizure was measured to the nearest second. A slow IV injection of phosphate-buffered saline solution (.9% NaC1; 1 mL/kg) or dextrorphan tartrate (25 mg/kg) was administered over 2 minutes. The methcathinone infusion and time measurements were continued throughout these injections. As in the first protocol, values for arterial blood pressure and pulse were measured at decimal fractions of the time from initiation of the methcathi-
Table 1.
Mean arterial blood pressure, pulse, and rectal temperature immediately before the initiation of methcathinone infusion in the two pretreatment groups. Pretreatment Vehicle (n=9) Dextrorphan (n=7) Data expressedas mean _+1 SEM.
MARCH 1997
Arterial Blood Pressure (mm Hg)
Pulse (Beats/Minute)
Rectal Temperature (° Celsius)
117+3 120+7
379_+12 365_+11
37.86+.14 37.89+.19
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none infusion until cardiorespiratory arrest and death. The time to terminal cardiovascular collapse and death was recorded to the nearest second. Necropsy was performed for each animal. The volume of the urinary bladder was assessed to the nearest. 1 mL. Methcathinone hydrochlonde was purchased from Sigma Chemical Company. Dextrorphan D-tartrate was purchased from Research Biochemicals, Incorporated. We assessed statistical significance with ANOVA (oneor two-way, as indicated) and differences between group means with the Neuman-Keuls test. Statistics were perFigure
1.
Effect of IV pretreatment with dextrorphan tartrate (25 mglkg; n=7) or saline solution vehicle (1 mL~g; n=9) on the responses o/A, mean arterial blood pressure and B, pulse to IV infusion of methcathinone (5 mgAglminute).
A
Mean Arterial Blood Pressure(mm Hg)
140 "7 120 ~ 100 4 80 -4 60 -4 /
formed with microcomputer-based statistical packages. 22, 23 Mean values_+1 SEM are presented.
RESULTS The values for mean arterial blood pressure, pulse, and rectal temperature did not differ between the vehicle-pretreated animals (120_+2 mm Hg, 384_+10 beats/minute, and 37.7°_+.1 °C, respectively; n=9) and the dextrorphanpretreated animals (116_+9 mm Hg, 374-_+18 beats/minute, and 38.0°_+.2° C; n=7) groups immediately before vehicle or drug injections. IV injection of dextrorphan produced immediate decreases in pulse and blood pressure in the 2 minutes after injection; these values recovered thereafter. The maximal changes in pulse and blood pressure, respectively, were -105_+13 beats/minute (P<.001 versus vehicle value of 17_+8 beats/minute) and -12_+6 mm Hg (P<.01 versus vehicle value of 6_+3 mm Hg). Rectal temperature was not altered by inj ection of dextrorphan or vehicle. Mild excitation was evident in rats in the 5 to 8 minutes after drug injection. The rats then became quiescent and appeared to be mildly sedated. The values for mean arterial blood pressure, heart rate and rectal temperature did not differ between the two pretreatment groups immediately prior to initiation of methcathinone infusion (Table 1). Infusion of methcathinone resulted in a tachycardia, with little change in blood pressure, and rapidly developing hyperthermia in vehicle-pretreated
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Effect of IV pretreatment with dextrorphan tartrate (25 mg,&g; n=7) or saline solution vehicle (1 rnL/l'eg;n=9) on the dose of methcathinone required to cause convulsions and that required to cause cardiorespiratoU collapse and death.
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The actual times elapsed before death were 40.82+1.81 minutes and 32.02"+1.06 minutes, respectively, in the dextrorphan tartrate and saline solution vehicle groups. Data expressedas mean _+1 SEM.
3 86
Death
Data expressed as mean _+SEM. *P<,01,
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rats. Dextrorphan pretreatment blunted the tachycardia and abolished the hyperthermic response. The time courses of methcathinone-induced changes m pulse and blood pressure are presented in Figure 1. A pronounced (often MOO beats/ minute) increase in heart rate developed within 2 minutes of the initiation of methcathinone infusion. The magnitude of this tachycardia did not differ between groups. Pulse was maintained at more than 450 beats/minute in vehicle-wetreated rats for virtually the entire infusion period. In contrast, pulse in the dextrorphan-pretreated rats progressively returned to preinfusion levels. The maximal increases in pulse during methcathinone infusion were 131+10 beats/ minute in vehicle-pretreated and 128+19 beats/minute in dextrorphan-pretreated rats. Mean arterial blood pressure did not differ markedly between the pretreatment groups until the onset of convulsive activity. The maximal increases in blood pressure during methcathinone infusion were 36+3 mm Hg in vehicle-pretreated rats and 20+5 mm Hg (P<.01) in dextrorphan-pretreated rats. These differences, demonstrated in Figure 1, appear to reflect differences in the behavioral responses related to methcathinone-induced convulsions (see below). Each episode of convulsive activity was associated with abrupt bradycardia (often >100 beats/ minutes) and hypotension (15 to 30 mm Hg). These responses were short fived and recovered to preconvulsion levels within 1 minute. Occasionally pulse and blood pressure increased above preconvulsion levels in the 1 to 3 minutes immediately after this recovery from convulsion. The hyperthermic response to methcathinone infusion noted in vehicle-pretreated rats (maximal change, 2.29°+ •17 ° C) was completely prevented by dextrorphan pretreatment (maximal change,. 14°+. 25 ° C; P<.01). Increases in rectal temperature were minimal (usually <.3 ° C) in vehiclepretreated rats before the onset of convulsions but developed very rapidly once convulsions had begun. The mean times to the onset of convulsions and to death, respectively, were 13.48+.87 and 32.02+1.06 minutes in
vehicle-pretreated rats; the corresponding values were 26.72+1.83 (P<.01 versus vehicle) and 40.82+1.81 minutes (P<.01 versus vehicle) in dextrorphan-pretreated rats. Consequently, the dose of methcathinone required to initiate convulsions and that required to cause death were both significantly greater in dextrorphan-pretreated than in vehicle-pretreated rats (Figure 2). Once convulsions began, their frequency in vehicle-pretreated rats increased rapidly, such that the animals appeared to be in status epilepticus for the last third of the infusion period. Convulsive incidents were later in onset and occurred at a much lower frequency in dextrorphan-pretreated rats. The infusion of methcathinone was associated with marked behavioral excitement and increased salivation in vehicle-pretreated rats. Dextrorphan pretreatment did not appear to reduce salivation but did minimize the behavioral response to methcathinone. Bladder volumes did not differ between pretreatment groups. The IV administration of dextrorphan immediately after the onset of methcathinone-induced conwlsive activity significantly reduced pulse, blood pressure, and rectal temperature compared with the responses noted after similar injections of vehicle. The mean values for pulse, blood pressure, and rectal temperature immediately before dextrorphan or vehicle injection, and the maximal changes that ensued, are presented in Table 2. Both pulse and blood pressure began to decrease immediately after dextrorphan injection; they reached nadir values within 5 minutes of injection. The reductions in pulse and blood pressure were maintained until terminal cardiorespiratory collapse occurred (Figure 3). The maximal increase in rectal temperature during methcathinone infusion was reduced by approximately 50% in dextrorphan-treated rats (Table 2). The times to, and doses of methcathinone required for, initiation of convulsive activity were similar between vehicleand dextrorphan-treated groups (Table 3). The injection of dextrorphan minimized convulsive activity, both in fie-
Table 2.
Mean arterial blood pressure, pulse, and rectal temperature in rats injected with dextrorphan (25 mg/kg) or vehicle (1 mg/kg) immediately @er the onset of methcathinone-induced convulsions. Arterial Blood Pressure(ram Hg) Treatment Vehicle (n=9) Dextrorphan (n=7)
Pulse (Beats/Minute)
Rectal Temperature (° Celsius)
Before Injection
Maximal Change
Before Injection
Maximal Change
Before Injection
Maximal Change
124_+6 120-+5
5_+4 -45_+9*
482_+14 523_+11
21_+6 -165_+10*
39.00-+.15 39.06+.25
2.65-+.13 .99_+.22*
Data expressedas mean_+1 SEM. *P<.01 versusvehicle group value.
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DEXTRORPHAN Rockhold et al
quency and in intensity. In addition, the marked behavioral excitement associated with methcathinone infusion was reversed by dextrorphan treatment. Only a modest, and not statistically significant, increase in the dose of methcathinone required to cause death was noted in the dextrorphan treatment group (Table 3). DISCUSSION
There is a dearth of information in the scientific literature about the acute toxic responses to methcathinone adminFigure 3. Changes in mean arterial blood pressure in response to IV infusion of methcathinone (5 mg/kg/minute) in rats in which IV dextrorphan tartrate (25 mg/kg,n=9) or saline solution vehicle (1 mL/kg;n=9) was injected immediately after the onset of methcathinone-induced convulsions.
A
Mean Arterial Blood Pressure (ram Hg)
150 140 130 120 I~0 100 90 80 70
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388
istration and the pharmacologic means by which such adverse effects can be mitigated. The results of this study address these issues and demonstrate that IV infusion of methcathinone results, in the conscious rat, in tachycardia, behavioral excitement, salivation, fulminating hyperthermia, convulsions, and death. Moreover, blockade of excitatory amino acid receptors with dextrorphan can significantly blunt the most severe of these responses. The pattern of cardiovascular response to methcathinone is typical of that which might be expected of an agent with indirect sympathomimetic activity The most remarkable effect was tachycardia, which became evident within 2 minutes of the initiation of methcathinone infusion and was maintained throughout the infusion protocol. The intensity of the tachycardia was surprising, with increases of more than 100 beats/minute. Tachycardia has been reported as a common clinical finding in both short- and long-term abuse 4, but to our knowledge it has not been demonstrated in an animal model. The origin of the tachycardia has not yet been studied. Hyperthermia does not appear to be a significant factor, at least in the early stages; increases in rectal temperature did not occur until around the last half of the infusion period, after the initiation of convulsive activity. A presumption can be made, on the basis of clinical evidence of sympathomimetic responses such as pfloerection and diaphoresis, that the tachycardia is sympathetic in origin. 4 Glennon et al 1 observed that methcathinone increased the release of [3H]dopamine from rat striatal tissue in a manner similar to that evoked by methamphetamine, suggesting that methcathinone can act as an indirect sympathomimetic. The site at which methcathinone acts to generate sympathomimetic responses and the potential involvement of withdrawal of vagal tone remain to be determined. We noted little alteration in mean arterial blood pressure except in the last phases of infusion, when drug-induced convulsive activity may have contributed to increased blood pressure. Pressor responses and hypotension have been reported as responses to human ingestion of methcathinone.4 With the exception of alterations related to acute convulsive episodes, neither increases nor decreases in systemic blood pressure were evident in our study. Both the racemic and the L-isomeric forms of the parent compound of methcathinone, cathinone, have been demonstrated to increase pulse and blood pressure when administered as IV injections to anesthetized rats. 26 Previous studies from our laboratory, in which cocaine was administered using a protocol similar to that used in this study, have demonstrated progressively developing pressor effects with little increase in pulse or mild bradycardic actions in normotensive rats. 14,15 No data
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are available with which to address the differences between the circulatory responses of cocaine and methcathinone. We noted a hyperthermic response, striking in its rate of development, in rats infused with methcathinone. Increases in rectal temperature of more than 2 ° C were observed in a period of less than 15 minutes. Hyperthermic responses to methcathinone have been described in human subjects and in dogs. 6 Rats in this study were confined to observation tubes that limited motion. Nevertheless, behavioral excitement and increases in motor activity were noted after the initiation of methcathinone infusion. Despite these responses, pronounced increases in body temperature were not observed before the onset of convulsive activity The increased muscle activity associated with convulsions appeared to coincide with the period of the most rapid development of hyperthermic responses. In similar protocols, rats have been infused with cocaine rather than methcathinone. The convulsions induced by cocaine, unlike those caused by methcathinone, are not accompanied by increased rectal temperature in normotensive rats. 14,s6 The origin of differences between the effects of methcathinone and cocaine on body temperature merits further study Convulsive activity began at doses of 67+6 mg/kg and 63_+5 mg/kg in the pretreatment and postconvulsion protocols, respectively In a separate protocol, in which 24hour survival was evaluated after the IV infusion of a dose sufficient only to initiate convulsions, a value of 57_+4 mg/kg was noted as the smallest dose to elicit an initial convulsion. That dose resulted in a 50% lethality. Although this method does not yield a classic LDso value, it does not provide data that compare favorably to the LDso value reported for methcathinone after IV injection in the rat (90_+3 mg/kg). 4 In this study we found the mean lethal doses of methcathinone after continuous IV infusion to be 161_+5 mg/kg (pretreatment protocol) and 153_+6 mg/kg (postconvulsion protocol). The slightly greater values for lethality noted in this study may be the result of a fundamental difference in
experimental procedure (ie, the use of a continuous IV infusion rather than bolus IV injection). Comparisons of dose-response relationships between species are fraught with difficulty This is particularly true in comparative evaluations of drugs of abuse, where human data are frequently obscure. Emerson and Cisek4 reported the estimated single intranasal dose of methcathinone to range from 80 to 250 mg in a series of 17 methcathinone users. No data are available on blood levels of methcathinone in human beings relative to observed behavioral or physiologic responses, and the amounts of methcathinone ingested by those patients with toxic responses were not reported.'* The protective effect of dextrorphan against the toxic actions of methcathinone is not surprising in light of the protective influence that excitatory amino acid-receptor blockade has been shown to provide against other stimulant drugs of abuse. Clearly excitatory amino acid receptors mediate some common phase or stage of stimulant intoxication. Anticonvulsant activity of excitatory amino acid receptor antagonists has been demonstrated against a wide range of convulsant stimuli, including administration of excitatory amino acid receptor agonists 27, 2s pentylenetetrazol2< 29, electroshock-induced seizures 29, and organophosphate toxicants) °, 31 More to the point, pretreatment with excitatory amino acid-receptor antagonists abrogates both cocaine- and methamphetamine-induced seizures. The site at which this interaction occurs has yet to be isolated. Death following methcathinone infusion may result from seizure activity, hyperthermia, cardiorespiratory paralysis, or all three. Data from studies involving similar infusions of cocaine suggest that respiratory paralysis is a proximate cause of death in the rat ~5, 19, a response that can be inhibited by pretreatment with excitatory amino acid-receptor antagonists, including dextrorphan. ~5,19,32 Such pretreatment may also protect against cardiac toxicity 15,t7,19 Both dextrorphan and MK-801 block pressor and tachycardic responses to cocaine infusion in the rat. Indeed, pretreat-
Table 3. Time and methcathinone dose required to produce convulsions and death in the two pretreatment groups. Dose (mg/kg) Pretreatment Vehicle (n=9) Dextrorphan (n=7)
Time (Minutes)
Convulsions
Death
Convulsions
Death
63.17+5.46 67.76+7.67
152.69_+6.01 185.13+14.10
12.63+1.09 13.55+1.53
30.53+1.20 37.03+2.82
Data expressedas mean+ ~ SEM. IV methcathinonewas administeredat a rate of 5 mg/kg/minute. IV vehicle (1 mL/kg)or dextrorphan(25 mg/kg) was injected immediatelyafter the start of the first methcathinone-inducedconvulsion.
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Rockhold et al
merit with these agents can reverse these effects, resulting in the development of hypotension and bradycardia.*4, ,5 Cardiac responses to cocaine, including ventricular arrhythrajas, have been shown to be blunted by MK-801 pretreatmerit in rats ~5, ,9 and dogs. ~r The pattern of hyperthermic and tachycardic responses to methcathinone infusion appear to resemble that of methamphetamine more closely than that of cocaine. 33 This finding is not surprising; the structural similarity of methamphetamine to methcathinone is greater than that of methcathinone to cocaine. Nevertheless, excitatory amino acid-receptor antagonists are effective in blunting intoxication by all three agents. Two sets of limitations to this study merit commentary The first is related to the methodology for use of the antagonist agent dextrorphan. The protocol involved only a single NMDA receptor blocker, dextrorphan, at a single, almost certainly supramaximal dosage. The capacity of dextrorphan to act as a relatively selective, noncompetitive blocker of the NMDA receptor is well documented 25, but the NMDA receptor is complex and possesses several binding sites. The protocols used herein do not permit discrimination of the relative effectiveness of blockade at each of these sites, nor do they allow determination of the safest and most effective dose regimen for dextrorphan in the prevention or management of acute methcathinone intoxication. In addition, clinical anecdotes ~,3-6 suggest that human symptoms result from long-term binge ingestion of methcathinone more frequently than from acute overdose. Our model is clearly designed to examine only the consequences of massive acute intoxication. Nevertheless, our results indicate that IV infusion of methcathinone produces significant sympathomimetic, hyperthermic, and convulsive effects that mimic the major elements of acute human intoxication with this agent. An involvement of excitatory amino acid receptors in the manifestation of such toxic responses is evident. Continued investigation of these interactions may identify regimens of excitatory amino acid-receptor blockade that can be effective in the management of human methcathinone intoxication. REFERENCES 1. Lennon RA, Yousif M, Naiman N, et al: Methcathinone: A new and potent amphetamine-like agent. PharmacolBiechemBehav1987;26:547-551. 2. Young R, Glennon RA: Cocaine-stimulusgeneralization to two new designer drugs: Methcathinone and 4-methylaminorex. PharmacolBiechemBehav1993;45:229-231. 3. Carroll S: Methcathinona: The next drug epidemic? EmergMedNews 1993;15:1,18-26. 4. EmersonTS, CisekJE: Matbcathinane: A Russiandesigner amphetamine infiltrates the rural Midwest. Ann EmergMed 1993;22:1897-1903. 5. Goldstane MS: "Cat": Methcathinone: A new drug of abuse. JAMA 1993;269:2508. 6. RosenDS: Methcathinone. JAdolesc Health1993;14:426.
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7. Kaminski BJ, Gdffiths RR: Intravenous self-injection of methcathinone in the baboon. PharmacolBiachemBehav1993;47:981-983. 8. FonnumF: Glutamate: A neurotransmitter in mammalian brain. J Neurechem1984;42:1-12. 9. ChalmersJ, Pilowsky P: Brainstem and bulbospinal neurotransmitter systems in the control of blood pressure. Hypertension1991;9:675-694. 10. Saji M, Miura M: Evidencethat glutamate is the transmitter mediating respiratory drive from medullary premotor neurons to phrenic motonaurons:A double labe]ling study in the rat. NeurosciLett 1990;115:177-182. 11. Lipton SA, RosenbergPA: Excitatory amino acids as a final common pathway for neurological disorders. N EnglJ Mad 1994;20:613-622. 12. Wade JV, SamsonFE, Nelson SR, et al: Changesin extraceliular amino acids during somanand kainic acid-induced seizures. J Neurochem1987;49:645-650. 13. Nakanishi S: Molecular diversity of glutamate receptors and implications for brain function. Science19£2;258:597-603. 14. RockholdRW, Surrett RS, Acuff CG, et al: Antagonism of cocaine toxicity by MK-801: Differential effects in spontaneouslyhypertensive and Wistar-Kyoto rats. Neuropharmacology 1992;31:1269-1277. 15. RockholdRW, Byrna M, SpraberyS, et at: Urethane anesthesia reversesthe protective effect of noncompetitive NMDA receptor antagonists against cocaine intoxication. Life Sci 1994;54:321-330. 16. Derlet RW, Albertson TE, Rice P: Antagonism of cocaine, amphetamine, and methamphetamine toxicity. PharmacoIBiachemBehav1990;36:745-749. 17. HagemanGP, Simar T: Attenuation of the cardiac effects of cocaine by dizocilpine. Am J Physio11993;264:H1890-H1895. 18. Seidleck 8K, Thurkauf A, Witkin JM: Evaluationof ADCl against convulsant and locomotor stimulant effects of cocaine: Comparisonwith the structural analogs dizocilpine and carbamazepine. PharmacolBiochamBehav1£94;47:839-844. 19. Tseng CC, Derlet RW, AIbertson TE: Cocaine-inducedrespiratory depression and seizuresare synergistic mechanismsof cocaine-induceddeath in rats. Ann EmergMef11992;21:486-493. 20. Witkin JM, Tortella FC: Modulators of a-methyl-c-aspartate protect against diazepam- or phenobarbital-resistant cocaine convulsions. Life Sci1991;48:PL-51-56. 21. Koek W, Woods JH, Winger GD: MK-gOl, a proposed noncompetitive antagonist of excitatory amino acid neurotransmission,produces phencyciidine-like behavioral effects in pigeons, rats and rhesus monkeys.J PharmacolExp Ther1988;245:969-974. 22. Tallarida RJ, Murray RR: PharmacologicalCalculationSystem.New York: Springer, 1986. 23. Sigmastat. Jande[ Scientific, Incorporated.San Rafael, California, 1992. 24. Olney JW, LabruyereJ, Wang G, at al: NMDA antagonist neuratoxicity: Mechanism and prevention. Science1991;254:1515-1518. 25. Albers GW, Atkinson RP, Kelley RE, ot al: Safety, tolorability, and pharmacokineticsof the NmethyI-D-aspartateantagonist dextrorphan in patients with acute stroke. Stroke1995;26:254258. 26. Yanagita T: Studies on cathinones: Cardiovascularand behavioral affects in rats and selfadministration experiment in Rhesusmonkeys. NIDAResMonographSeries1979;27:326-327. 27. FerkanyJW, BoroskySA, Clissold DB, et al: Dextromethorphaninhibits NMDA-induced convulsions. EurJ Pharmaco11988;151:151-I 54. 28. Vecsei L, Miller J, MacGarveyU, et al: Kynurenineand probenecid inhibit pentylenetetrazoland NMDLA-inducedseizuresand increase kynurenic acid concentrations in the brain. BrainRes Buff 1992;28:233-238. 29. Clineschmidt BV, Martin GE, Bunting PR: Anticonvulsant activity of (+)-8-methyl-lO,11-dihydro-5H-dibenzo[a,d]cyciohepten-5,16-imine(MK-801), a substancewith potent anticonvulsant, central sympathomimetic, and apparent anxiolytic properties. DrugDovRes1982;2:123-134. 30. Braitman DJ, SparenborgS: MK*801 protects against seizuresinduced by the cholinesterase inhibitor soman. BrainResBull 1989;23:145-148. 31. Carpentier P, Foquin-TarriconeA, Badjarian N: Anticenvulsant and antilathal effects of the phencyclidina derivative TOP in soman poisoning. Neurotexicology1994;15:837-852. 32. Tseng C-C, Derlet RW, Stark LG, et el: Cocaine-inducedrespiratory depression in urethaneanesthetized rats: A possible mechanism of cocaine-induceddeath. PharmacolBiochemBehav 1991;39:625-633.
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33. SchindlerCW, ZhengJW, Tella SR, et al: Pharmacologicalmechanismsin the cardiovascular effects of methamphetaminain conscioussquirrelmonkeys.PharmacolBiochemBehav 1992;42:791-796. The secretarial assistance of Mrs Pam Banks and Ms Lisa McCammon is gratefully noted. The professional assistance of faculty from the University of Mississippi Medical Center Division of Biostatistics was freely available when consultation was needed.
Reprint no. 47/1/79856 Address for reprints: Bob W Rockhold, PhD Department of Pharmacology and Toxicology University of Mississippi Medical Center 2500 North State Street Jackson, Mississippi 39216-4505
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Methcathinone Intoxication in the Rat: Abrogation by Dextrorphan From the Departments of Emergency Medicine* and Pharmacology and
Toxicology~, University of Mississippi Medical Center, Jackson, Mississippi. Receivedfor publicationMarch4, 1996. Revision received July 15, 1996. Accepted for publication July 24, 1996.
Rob W Rockhold, PhD*
Study objective: Methcathinone, a designer drug, has high
Fredrick B Carlton Jr, MD*
abuse liability. In this study we characterizedacute methcathinone toxicity in rats, attempting to determine whether the excitatory amino acid receptor antagonist dextrorphan can antagonize methcathinone intoxication.
Robert Corkern, MD* Len Derouen, MD* James G Bennett~ Arthur S Hume, PhD*
Portions of the material in this manuscript w e r e presented in abstract
form at the 15th Annual Meetingof the SoutheasternPharmacology
Society, Clearwater Beach, Florida, September 1994; and the
Experimental Biology '95 meeting, Atlanta, April 1995.
Dr Rockhold w a s supported by the American Heart Association, Mississippi affiliate. The work of Dr Derouen and Dr Corkern was performed in partial fulfillment of residency requirementsfor the University of Mississippi Medical Center Department of Emergency Medicine. The assistance ofJames Bennettwas supported by a summer medical student researchfellowship, supported by the Dean of the University of Mississippi Medical School. Copyright © by the American College of Emergency Physicians.
Methods: Intoxication was produced with IV methcathinone infusion (5 mg/kg/minute; 100 mg/mL)in conscious rats. We studied pretreatment, in which dextrorphan or vehicle was injected 30 minutes before methcathinone infusion. In a second protocol, dextrorphan or saline solution was given immediately after the onset of convulsions. Results: Methcathinone caused tachycardia (maximal increase, 131+10 beats/minute), hyperthermia (+2.3 ° C), convulsions, and cardiorespiratory collapse in vehicle-pretreated rats (n=9). Death occurred after 32.0+1.1 minutes of infusion. Dextrorphan pretreatment (25 mg/kg; n=7) significantly reduced hyperthermia (+.1°+.3 ° C) and tachycardia and increased the convulsive (dextrorphan, 134+9 mg/kg; vehicle, 67+4 mg/kg)and lethal doses (dextrorphan, 204+9 mg/kg; vehicle, 160+5 mg/kg). Dextrorphan, given immediately after the initial methcathinone convulsion, reduced hyperthermic and tachycardic responses but not the lethality of methcathinone. Conclusion: Blockade of excitatory amino acid receptors by dextrorphan minimizes acute methcathinone intoxication. [Rockhold RW, Carlton FB, Corkern R, Derouen L, Bennett JG, Hume AS: Methcathinone intoxication in the rat: Abrogation by dextrorphan. Ann EmergMed March 1997;29:383-391 .]
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DEXTRORPHAN Rockhold et al
INTRODUCTION
Methcathinone (2-methylamino-l-phenylpropan-l-one) is the N-methyl derivative of the naturally occurring psychomotor stimulant cathinone. 1 Methcathinone is similar in behavioral effects and pharmacology to other phenylisopropylamines, including methamphetamine and aminorex. 2 Unlike methcathinone, however, cathinone is not considered to have the potential to be a significant drug of abuse in the United States. Methcathinone is more potent than cathinone in terms of psychomotor stimulant activity. ~ Considerable attention has recently been focused on methcathinone with the documentation of human toxic responses following illicit use of methcathinone in the United States. B-6A single clinical report, published in Annals of Emergency Medicine, documented signs and symptoms of acute methcathinone toxicity including confusion, tremors, headaches, abdominal pain, diaphoresis, tachycardia, hyperthermia, and hallucinations.* Episodic bradycardia and hypotension may be seen at varying intervals after drug ingestion. Long-term binge use results in symptoms including paranoid psychosis, weight loss, tremor, hyperreflexia, increases in circulating hepatic enzyme levels, and proteinuria. 4-6 Evidence suggests that physiologic withdrawal responses occur after cessation of long-term use. 5,s Despite the potential clinical and societal impact of methcathinone, very little information is available on acute pharmacologic and toxic responses in laboratory animals under controlled conditions. Methcathinone has been shown to produce locomotor stimulation in the rat 1 and the baboon 7, and avid self-administration of methcathinone has been reported in the baboon, r The psychomotor stimulus imparted by methcathinone is similar to those of cocaine 2 and methamphetamine. ~ Methcathinone, methamphetamine, amphetamine, and cathinone all stimulated release of [3H] dopamine from rat brain tissue, suggesting that these agents exert similar actions as indirect sympathomimetic substances. ~ The apparent similarities of methcathinone, cocaine, and methamphetamine suggest that pharmacologic agents effective in the antagonism of cocaine and methamphetamine toxicity could also be useful in the treatment of methcathinone intoxication. Excitatory amino acid neurotransmitters, including glutamate and aspartate, constitute the major excitatory stimuli in the mammalian central nervous systems and have been implicated in the mediation of respirator/and circulator/ regulation 9-1°, as well as in convulsive activity and neurologic dysfunction. 11-~2 A family of excitatory amino acid receptor subtypes has been described, of which the ionotropic N-methyl-D-aspartate (NMDA) receptor is best characterized, z3 Studies from our laboratorys<~5 and others ~6-2°
3 84
have demonstrated that toxic responses to stimulant drugs of abuse, including cocaine and methamphetamine, can be reduced by excitatory amino acid receptor antagonists. The agents most intensively studied have been the noncompetitive NMDA receptor antagonists MK-801 and dextrorphan. However, MK-801 may exert neurotoxic and behaviorally toxic actions, whereas dextrorphan is relatively innocuous.21-23 In this study we sought to characterize the acute toxic responses to IV methcathinone infusion in conscious rats and to determine whether the excitatory amino acid receptor antagonist dextrorphan effectively minimizes these toxicities. MATERIALS AND METHODS
Male Sprague-Dawley rats weighing 275 to 300 g were purchased from Harlan-Spragne Dawley. The animals were housed, three to five per plastic cage, for 5 to 7 days before the experiments under controlled conditions of light (8 aM to 8 PM), temperature (22 ° to 24 ° C), and humidity (50% to 55%). Access to deionized water and rodent chow was permitted at all times. All experimental handling and treatment of these animals was approved by the University of Mississippi Medical Center Institutional Animal Care and Use Committee. While each animal was under halothane anesthesia (2% to 4% in medical-grade oxygen), polyethylene catheters (Clay Adams) were implanted into the abdominal aorta (PE-50) and vena cava (PE-10) by way of the left femoral vessels. The catheters were tilted with heparinized saline solution (20 U/mL), exteriorized at the nape of the neck, and sealed until use. Aseptic techniques were used for all surgical manipulations. All wound sites were infiltrated with bupivacaine .5 % to provide postoperative pain relief. Each animal was housed individually for 48 hours after surgery in a plastic cage with corncob litter before experimental use. Each animal was placed in a clear Plexiglas restraining tube (internal diameter, 5.5 cm; length, 21.5 cm), after which an electronic thermometer probe was inserted into the rectum and the catheters were connected for the recording of blood pressure and heart rate and the injection of drugs. The Plexiglas tubes were open at both ends, except for restraining posts, and machined with multiple ventilator/ and heatqoss slots. The tubes allowed limited anterior-posterior movement and restricted lateral movement. Movements such as head bobbing, •xion/extension seizures of the limbs, and tail extension were possible and clearly evident. Arterial pressure was measured with Cobe disposable pressure transducers connected to a Grass model 7D polygraph. Mean arterial pressure was determined by electronic
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damping of the pulsatile signal• Pulse was calculated electronically from the pulse interval. Rectal temperature was recorded at discrete intervals. We conducted preliminary studies to choose a concentration of methcathinone (100 mg/mL) and a volume rate of infusion (50 l~L/kg/minute) such that continuous infusion would cause death within approximately 30 minutes in nonpretreated rats. The rate of infusion is identical to that used in previous studies from our laboratory of the acute toxic responses to IV cocaine infusion. ~4,15 The concentration is four times greater than that used with cocaine-infusion regimens. 14,t5 A separate group of six rats was tested with these infusion variables and without pretreatment. In these animals, infusion was continued until the onset of the first visible convulsion. The infusion was immediately discontinued at that time and the animal returned to its home cage. The mean dose required to elicit a convulsion and the mean time to onset of the initial convulsion were 57.4_+4.0 mg/kg and 11.5+.8 minutes, respectively. Survival was assessed 24 hours after methcathinone infusion. Three of the six animals (50%) survived. The 50% lethal dose (LDso) after IV injection (presumably a bolus) of methcathinone in the rat has been reported to be 90_+3 mg/kg4 (see reference 7 of that report). We used two experimental protocols. First, we examined a pretreatment regimen. This protocol involved placing rats in restraining tubes 30 minutes before IV injection of phosphate-buffered saline solution (.9% NaC1, 1 mL/kg) or dextrorphan tartrate (25 mg/kg). All measurements of blood pressure, pulse, and rectal temperature were begun when the rats were placed in the restraining tubes. The dextrorphan dosage regimen has been shown to provide significant protection against cocaine intoxication in a similar model• ~5 Ambient temperature was maintained at 24 ° C. Arterial blood pressure, pulse, and rectal temperature were recorded for an additional 30 minutes before the initiation of a continuous IV infusion of methcathinone hydrochloride at a rate of 5.0 mg/kg/minute (volume rate, 50 laL/kg/minute; 100 mg/mL) with the use of a Sage model 355 infusion pump.
Arterial blood pressure and pulse were measured at decimal fractions of the time from initiation of the methcathinone infusion until cardiorespiratory arrest and death. The time to onset of seizures and the time to terminal cardiovascular collapse and death were recorded to the nearest second. Convulsions became evident initially with the abrupt onset of a whole-body myoclonic jerk. The interval between seizures progressively decreased. A terminal pattern of seizures included more profound motor activity, which was evident as generalized tonic-clonic forelimb and hindlimb extensions. This pattern coincided with cardiovascular collapse and took the appearance of status epilepticus. Laparotomy was performed in each animal to assess the patency of the venous drug-administration catheter• Any animal with evidence of impaired or diverted venous flow (eg, retroperitoneal hemorrhage) was eliminated from the data set. The volume of the urinary bladder was assessed to the nearest •1 mL by means of needle aspiration of the bladder contents into a 5-mL syringe. In the second protocol we assessed the ability of dextrorphan to antagonize methcathinone toxicity when the drug was administered after the onset of seizure activity Each animal in this protocol was implanted with a second PE-10 venous catheter to permit simultaneous injection of drugs and IV infusion of methcathinone. Both venous catheters were implanted through the same femoral vein, as described previously, Rats were placed in restraining tubes 30 minutes before the initiation of a continuous IV infusion of methcathmone hydrochloride at a rate of 5.0 mg/kg/minute (volume rate, 50 laL/kg/minute; 100 mg/mL) with the use of a Sage model 355 infusion pump. The time elapsed before the onset of the initial seizure was measured to the nearest second. A slow IV injection of phosphate-buffered saline solution (.9% NaC1; 1 mL/kg) or dextrorphan tartrate (25 mg/kg) was administered over 2 minutes. The methcathinone infusion and time measurements were continued throughout these injections. As in the first protocol, values for arterial blood pressure and pulse were measured at decimal fractions of the time from initiation of the methcathi-
Table 1.
Mean arterial blood pressure, pulse, and rectal temperature immediately before the initiation of methcathinone infusion in the two pretreatment groups. Pretreatment Vehicle (n=9) Dextrorphan (n=7) Data expressedas mean _+1 SEM.
MARCH 1997
Arterial Blood Pressure (mm Hg)
Pulse (Beats/Minute)
Rectal Temperature (° Celsius)
117+3 120+7
379_+12 365_+11
37.86+.14 37.89+.19
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DEXTRORPHAN
Rockholdet al
none infusion until cardiorespiratory arrest and death. The time to terminal cardiovascular collapse and death was recorded to the nearest second. Necropsy was performed for each animal. The volume of the urinary bladder was assessed to the nearest. 1 mL. Methcathinone hydrochlonde was purchased from Sigma Chemical Company. Dextrorphan D-tartrate was purchased from Research Biochemicals, Incorporated. We assessed statistical significance with ANOVA (oneor two-way, as indicated) and differences between group means with the Neuman-Keuls test. Statistics were perFigure
1.
Effect of IV pretreatment with dextrorphan tartrate (25 mglkg; n=7) or saline solution vehicle (1 mL~g; n=9) on the responses o/A, mean arterial blood pressure and B, pulse to IV infusion of methcathinone (5 mgAglminute).
A
Mean Arterial Blood Pressure(mm Hg)
140 "7 120 ~ 100 4 80 -4 60 -4 /
formed with microcomputer-based statistical packages. 22, 23 Mean values_+1 SEM are presented.
RESULTS The values for mean arterial blood pressure, pulse, and rectal temperature did not differ between the vehicle-pretreated animals (120_+2 mm Hg, 384_+10 beats/minute, and 37.7°_+.1 °C, respectively; n=9) and the dextrorphanpretreated animals (116_+9 mm Hg, 374-_+18 beats/minute, and 38.0°_+.2° C; n=7) groups immediately before vehicle or drug injections. IV injection of dextrorphan produced immediate decreases in pulse and blood pressure in the 2 minutes after injection; these values recovered thereafter. The maximal changes in pulse and blood pressure, respectively, were -105_+13 beats/minute (P<.001 versus vehicle value of 17_+8 beats/minute) and -12_+6 mm Hg (P<.01 versus vehicle value of 6_+3 mm Hg). Rectal temperature was not altered by inj ection of dextrorphan or vehicle. Mild excitation was evident in rats in the 5 to 8 minutes after drug injection. The rats then became quiescent and appeared to be mildly sedated. The values for mean arterial blood pressure, heart rate and rectal temperature did not differ between the two pretreatment groups immediately prior to initiation of methcathinone infusion (Table 1). Infusion of methcathinone resulted in a tachycardia, with little change in blood pressure, and rapidly developing hyperthermia in vehicle-pretreated
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Fractional Time to Death
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The actual times elapsed before death were 40.82+1.81 minutes and 32.02"+1.06 minutes, respectively, in the dextrorphan tartrate and saline solution vehicle groups. Data expressedas mean _+1 SEM.
3 86
Death
Data expressed as mean _+SEM. *P<,01,
ANNALS OF EMERGENCY MEDICINE
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DEXTRORPHAN Rockhdd et aZ
rats. Dextrorphan pretreatment blunted the tachycardia and abolished the hyperthermic response. The time courses of methcathinone-induced changes m pulse and blood pressure are presented in Figure 1. A pronounced (often MOO beats/ minute) increase in heart rate developed within 2 minutes of the initiation of methcathinone infusion. The magnitude of this tachycardia did not differ between groups. Pulse was maintained at more than 450 beats/minute in vehicle-wetreated rats for virtually the entire infusion period. In contrast, pulse in the dextrorphan-pretreated rats progressively returned to preinfusion levels. The maximal increases in pulse during methcathinone infusion were 131+10 beats/ minute in vehicle-pretreated and 128+19 beats/minute in dextrorphan-pretreated rats. Mean arterial blood pressure did not differ markedly between the pretreatment groups until the onset of convulsive activity. The maximal increases in blood pressure during methcathinone infusion were 36+3 mm Hg in vehicle-pretreated rats and 20+5 mm Hg (P<.01) in dextrorphan-pretreated rats. These differences, demonstrated in Figure 1, appear to reflect differences in the behavioral responses related to methcathinone-induced convulsions (see below). Each episode of convulsive activity was associated with abrupt bradycardia (often >100 beats/ minutes) and hypotension (15 to 30 mm Hg). These responses were short fived and recovered to preconvulsion levels within 1 minute. Occasionally pulse and blood pressure increased above preconvulsion levels in the 1 to 3 minutes immediately after this recovery from convulsion. The hyperthermic response to methcathinone infusion noted in vehicle-pretreated rats (maximal change, 2.29°+ •17 ° C) was completely prevented by dextrorphan pretreatment (maximal change,. 14°+. 25 ° C; P<.01). Increases in rectal temperature were minimal (usually <.3 ° C) in vehiclepretreated rats before the onset of convulsions but developed very rapidly once convulsions had begun. The mean times to the onset of convulsions and to death, respectively, were 13.48+.87 and 32.02+1.06 minutes in
vehicle-pretreated rats; the corresponding values were 26.72+1.83 (P<.01 versus vehicle) and 40.82+1.81 minutes (P<.01 versus vehicle) in dextrorphan-pretreated rats. Consequently, the dose of methcathinone required to initiate convulsions and that required to cause death were both significantly greater in dextrorphan-pretreated than in vehicle-pretreated rats (Figure 2). Once convulsions began, their frequency in vehicle-pretreated rats increased rapidly, such that the animals appeared to be in status epilepticus for the last third of the infusion period. Convulsive incidents were later in onset and occurred at a much lower frequency in dextrorphan-pretreated rats. The infusion of methcathinone was associated with marked behavioral excitement and increased salivation in vehicle-pretreated rats. Dextrorphan pretreatment did not appear to reduce salivation but did minimize the behavioral response to methcathinone. Bladder volumes did not differ between pretreatment groups. The IV administration of dextrorphan immediately after the onset of methcathinone-induced conwlsive activity significantly reduced pulse, blood pressure, and rectal temperature compared with the responses noted after similar injections of vehicle. The mean values for pulse, blood pressure, and rectal temperature immediately before dextrorphan or vehicle injection, and the maximal changes that ensued, are presented in Table 2. Both pulse and blood pressure began to decrease immediately after dextrorphan injection; they reached nadir values within 5 minutes of injection. The reductions in pulse and blood pressure were maintained until terminal cardiorespiratory collapse occurred (Figure 3). The maximal increase in rectal temperature during methcathinone infusion was reduced by approximately 50% in dextrorphan-treated rats (Table 2). The times to, and doses of methcathinone required for, initiation of convulsive activity were similar between vehicleand dextrorphan-treated groups (Table 3). The injection of dextrorphan minimized convulsive activity, both in fie-
Table 2.
Mean arterial blood pressure, pulse, and rectal temperature in rats injected with dextrorphan (25 mg/kg) or vehicle (1 mg/kg) immediately @er the onset of methcathinone-induced convulsions. Arterial Blood Pressure(ram Hg) Treatment Vehicle (n=9) Dextrorphan (n=7)
Pulse (Beats/Minute)
Rectal Temperature (° Celsius)
Before Injection
Maximal Change
Before Injection
Maximal Change
Before Injection
Maximal Change
124_+6 120-+5
5_+4 -45_+9*
482_+14 523_+11
21_+6 -165_+10*
39.00-+.15 39.06+.25
2.65-+.13 .99_+.22*
Data expressedas mean_+1 SEM. *P<.01 versusvehicle group value.
MARCH 1997 29:3 ANNALS OF EMERGENCY MEDICINE
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DEXTRORPHAN Rockhold et al
quency and in intensity. In addition, the marked behavioral excitement associated with methcathinone infusion was reversed by dextrorphan treatment. Only a modest, and not statistically significant, increase in the dose of methcathinone required to cause death was noted in the dextrorphan treatment group (Table 3). DISCUSSION
There is a dearth of information in the scientific literature about the acute toxic responses to methcathinone adminFigure 3. Changes in mean arterial blood pressure in response to IV infusion of methcathinone (5 mg/kg/minute) in rats in which IV dextrorphan tartrate (25 mg/kg,n=9) or saline solution vehicle (1 mL/kg;n=9) was injected immediately after the onset of methcathinone-induced convulsions.
A
Mean Arterial Blood Pressure (ram Hg)
150 140 130 120 I~0 100 90 80 70
.....I-.-?:,{ • Vehicle • Dextrorphan
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Fractional Time to Death The actualtimes to deathwere 37.03-+2.82minutesand 30.53+120 minutes,respectively,in the dextrorphantartrate and saline solutionvehiclegroups. Data expressedas mean_+1 SEM.
388
istration and the pharmacologic means by which such adverse effects can be mitigated. The results of this study address these issues and demonstrate that IV infusion of methcathinone results, in the conscious rat, in tachycardia, behavioral excitement, salivation, fulminating hyperthermia, convulsions, and death. Moreover, blockade of excitatory amino acid receptors with dextrorphan can significantly blunt the most severe of these responses. The pattern of cardiovascular response to methcathinone is typical of that which might be expected of an agent with indirect sympathomimetic activity The most remarkable effect was tachycardia, which became evident within 2 minutes of the initiation of methcathinone infusion and was maintained throughout the infusion protocol. The intensity of the tachycardia was surprising, with increases of more than 100 beats/minute. Tachycardia has been reported as a common clinical finding in both short- and long-term abuse 4, but to our knowledge it has not been demonstrated in an animal model. The origin of the tachycardia has not yet been studied. Hyperthermia does not appear to be a significant factor, at least in the early stages; increases in rectal temperature did not occur until around the last half of the infusion period, after the initiation of convulsive activity. A presumption can be made, on the basis of clinical evidence of sympathomimetic responses such as pfloerection and diaphoresis, that the tachycardia is sympathetic in origin. 4 Glennon et al 1 observed that methcathinone increased the release of [3H]dopamine from rat striatal tissue in a manner similar to that evoked by methamphetamine, suggesting that methcathinone can act as an indirect sympathomimetic. The site at which methcathinone acts to generate sympathomimetic responses and the potential involvement of withdrawal of vagal tone remain to be determined. We noted little alteration in mean arterial blood pressure except in the last phases of infusion, when drug-induced convulsive activity may have contributed to increased blood pressure. Pressor responses and hypotension have been reported as responses to human ingestion of methcathinone.4 With the exception of alterations related to acute convulsive episodes, neither increases nor decreases in systemic blood pressure were evident in our study. Both the racemic and the L-isomeric forms of the parent compound of methcathinone, cathinone, have been demonstrated to increase pulse and blood pressure when administered as IV injections to anesthetized rats. 26 Previous studies from our laboratory, in which cocaine was administered using a protocol similar to that used in this study, have demonstrated progressively developing pressor effects with little increase in pulse or mild bradycardic actions in normotensive rats. 14,15 No data
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are available with which to address the differences between the circulatory responses of cocaine and methcathinone. We noted a hyperthermic response, striking in its rate of development, in rats infused with methcathinone. Increases in rectal temperature of more than 2 ° C were observed in a period of less than 15 minutes. Hyperthermic responses to methcathinone have been described in human subjects and in dogs. 6 Rats in this study were confined to observation tubes that limited motion. Nevertheless, behavioral excitement and increases in motor activity were noted after the initiation of methcathinone infusion. Despite these responses, pronounced increases in body temperature were not observed before the onset of convulsive activity The increased muscle activity associated with convulsions appeared to coincide with the period of the most rapid development of hyperthermic responses. In similar protocols, rats have been infused with cocaine rather than methcathinone. The convulsions induced by cocaine, unlike those caused by methcathinone, are not accompanied by increased rectal temperature in normotensive rats. 14,s6 The origin of differences between the effects of methcathinone and cocaine on body temperature merits further study Convulsive activity began at doses of 67+6 mg/kg and 63_+5 mg/kg in the pretreatment and postconvulsion protocols, respectively In a separate protocol, in which 24hour survival was evaluated after the IV infusion of a dose sufficient only to initiate convulsions, a value of 57_+4 mg/kg was noted as the smallest dose to elicit an initial convulsion. That dose resulted in a 50% lethality. Although this method does not yield a classic LDso value, it does not provide data that compare favorably to the LDso value reported for methcathinone after IV injection in the rat (90_+3 mg/kg). 4 In this study we found the mean lethal doses of methcathinone after continuous IV infusion to be 161_+5 mg/kg (pretreatment protocol) and 153_+6 mg/kg (postconvulsion protocol). The slightly greater values for lethality noted in this study may be the result of a fundamental difference in
experimental procedure (ie, the use of a continuous IV infusion rather than bolus IV injection). Comparisons of dose-response relationships between species are fraught with difficulty This is particularly true in comparative evaluations of drugs of abuse, where human data are frequently obscure. Emerson and Cisek4 reported the estimated single intranasal dose of methcathinone to range from 80 to 250 mg in a series of 17 methcathinone users. No data are available on blood levels of methcathinone in human beings relative to observed behavioral or physiologic responses, and the amounts of methcathinone ingested by those patients with toxic responses were not reported.'* The protective effect of dextrorphan against the toxic actions of methcathinone is not surprising in light of the protective influence that excitatory amino acid-receptor blockade has been shown to provide against other stimulant drugs of abuse. Clearly excitatory amino acid receptors mediate some common phase or stage of stimulant intoxication. Anticonvulsant activity of excitatory amino acid receptor antagonists has been demonstrated against a wide range of convulsant stimuli, including administration of excitatory amino acid receptor agonists 27, 2s pentylenetetrazol2< 29, electroshock-induced seizures 29, and organophosphate toxicants) °, 31 More to the point, pretreatment with excitatory amino acid-receptor antagonists abrogates both cocaine- and methamphetamine-induced seizures. The site at which this interaction occurs has yet to be isolated. Death following methcathinone infusion may result from seizure activity, hyperthermia, cardiorespiratory paralysis, or all three. Data from studies involving similar infusions of cocaine suggest that respiratory paralysis is a proximate cause of death in the rat ~5, 19, a response that can be inhibited by pretreatment with excitatory amino acid-receptor antagonists, including dextrorphan. ~5,19,32 Such pretreatment may also protect against cardiac toxicity 15,t7,19 Both dextrorphan and MK-801 block pressor and tachycardic responses to cocaine infusion in the rat. Indeed, pretreat-
Table 3. Time and methcathinone dose required to produce convulsions and death in the two pretreatment groups. Dose (mg/kg) Pretreatment Vehicle (n=9) Dextrorphan (n=7)
Time (Minutes)
Convulsions
Death
Convulsions
Death
63.17+5.46 67.76+7.67
152.69_+6.01 185.13+14.10
12.63+1.09 13.55+1.53
30.53+1.20 37.03+2.82
Data expressedas mean+ ~ SEM. IV methcathinonewas administeredat a rate of 5 mg/kg/minute. IV vehicle (1 mL/kg)or dextrorphan(25 mg/kg) was injected immediatelyafter the start of the first methcathinone-inducedconvulsion.
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merit with these agents can reverse these effects, resulting in the development of hypotension and bradycardia.*4, ,5 Cardiac responses to cocaine, including ventricular arrhythrajas, have been shown to be blunted by MK-801 pretreatmerit in rats ~5, ,9 and dogs. ~r The pattern of hyperthermic and tachycardic responses to methcathinone infusion appear to resemble that of methamphetamine more closely than that of cocaine. 33 This finding is not surprising; the structural similarity of methamphetamine to methcathinone is greater than that of methcathinone to cocaine. Nevertheless, excitatory amino acid-receptor antagonists are effective in blunting intoxication by all three agents. Two sets of limitations to this study merit commentary The first is related to the methodology for use of the antagonist agent dextrorphan. The protocol involved only a single NMDA receptor blocker, dextrorphan, at a single, almost certainly supramaximal dosage. The capacity of dextrorphan to act as a relatively selective, noncompetitive blocker of the NMDA receptor is well documented 25, but the NMDA receptor is complex and possesses several binding sites. The protocols used herein do not permit discrimination of the relative effectiveness of blockade at each of these sites, nor do they allow determination of the safest and most effective dose regimen for dextrorphan in the prevention or management of acute methcathinone intoxication. In addition, clinical anecdotes ~,3-6 suggest that human symptoms result from long-term binge ingestion of methcathinone more frequently than from acute overdose. Our model is clearly designed to examine only the consequences of massive acute intoxication. Nevertheless, our results indicate that IV infusion of methcathinone produces significant sympathomimetic, hyperthermic, and convulsive effects that mimic the major elements of acute human intoxication with this agent. An involvement of excitatory amino acid receptors in the manifestation of such toxic responses is evident. Continued investigation of these interactions may identify regimens of excitatory amino acid-receptor blockade that can be effective in the management of human methcathinone intoxication. REFERENCES 1. Lennon RA, Yousif M, Naiman N, et al: Methcathinone: A new and potent amphetamine-like agent. PharmacolBiechemBehav1987;26:547-551. 2. Young R, Glennon RA: Cocaine-stimulusgeneralization to two new designer drugs: Methcathinone and 4-methylaminorex. PharmacolBiechemBehav1993;45:229-231. 3. Carroll S: Methcathinona: The next drug epidemic? EmergMedNews 1993;15:1,18-26. 4. EmersonTS, CisekJE: Matbcathinane: A Russiandesigner amphetamine infiltrates the rural Midwest. Ann EmergMed 1993;22:1897-1903. 5. Goldstane MS: "Cat": Methcathinone: A new drug of abuse. JAMA 1993;269:2508. 6. RosenDS: Methcathinone. JAdolesc Health1993;14:426.
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33. SchindlerCW, ZhengJW, Tella SR, et al: Pharmacologicalmechanismsin the cardiovascular effects of methamphetaminain conscioussquirrelmonkeys.PharmacolBiochemBehav 1992;42:791-796. The secretarial assistance of Mrs Pam Banks and Ms Lisa McCammon is gratefully noted. The professional assistance of faculty from the University of Mississippi Medical Center Division of Biostatistics was freely available when consultation was needed.
Reprint no. 47/1/79856 Address for reprints: Bob W Rockhold, PhD Department of Pharmacology and Toxicology University of Mississippi Medical Center 2500 North State Street Jackson, Mississippi 39216-4505
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