Introduction

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Psychomotor stimulants once in general clinical use in the United States for the treatment of psychiatric disorders, physical and mental fatigue, and counteraction of sedation induced by depressant agents no longer find wide or frequent application in medical practice. The primary impetus for their reduction in clinical use was the profound tolerance and dependence that develops with repeated administration. Amphetamines continue to be of widespread abuse. Methamphetamine, for example, has again found increased illicit use with rates estimated as twice as high as in 1990 (Johnston et al., 1997). Methamphetamine use has been associated with striking proportions of toxic episodes. Recent estimates have indicated a tripling in the methamphetamine-related emergency episodes from 1991 to 1994 (Department of Health and Human Services, 1997). Although this trend has started to reverse, the toxicity remains high.

Sydnocarb (3-(-phenylisopropyl)-N-phenylcarbamoylsydnonimine), synthesized in the Russian Center for Chemistry of Drugs (Moscow) is a psychomotor stimulant in current use in Russia in diverse fields of medical practice (Fig. 1). Sydnocarb is used for the treatment of asthenia, apathy, and adynamia, which can accompany psychiatric disorders such as schizophrenia and manic depression. When combined with hypnotics, anxiolytics, or antipsychotic agents, sydnocarb has been reported to ameliorate the myorelaxing and soporific side effects of these drugs without affecting clinical efficacy (Voronina and Tozhanova, 1981; Valueva and Tozhanova, 1982). No significant toxic episodes have been noted with sydnocarb. Compared with the stimulating effect of amphetamines, the activating effects of sydnocarb develop more gradually, last longer, are accompanied to a lesser extent by stereotypy, and are not accompanied by peripheral sympathomimetic effects, pronounced euphoria, or motor excitation. No behavioral or physical dependence on sydnocarb has been noted (Mashkovsky et al., 1971; Rudenko and Altshuler, 1978).

View larger version (7K): [in this window] [in a new window]   Fig. 1.   Structural formulae for sydnocarb and methamphetamine. Sydnocarb is a psychostimulant in current use in Russia in diverse fields of medical practice.

View larger version (23K): [in this window] [in a new window]   Fig. 2.   Comparison of effects of sydnocarb (squares) and methamphetamine (circles) on horizontally and vertically directed locomotor behavior of mice. S.c. (left) and i.p. (right) routes of administration were evaluated. Drug treatments (closed symbols) were compared with vehicle controls (open symbols) with a one-tailed Dunnett's test; *, significant difference, (p < .05). Data are means ± S.E.M. of groups of at least eight mice per dose.

View larger version (30K): [in this window] [in a new window]   Fig. 3.   Time course of effects of sydnocarb (squares) and methamphetamine (circles) on horizontally and vertically directed locomotor behavior of mice. Other details as in Fig. 2.

View larger version (28K): [in this window] [in a new window]   Fig. 4.   Effects of haloperidol (0.1 mg/kg) alone and in combination with either methamphetamine (left) or sydnocarb (right). Effects of methamphetamine alone, sydnocarb alone, or vehicle alone are shown by the black columns. Gray columns represent effects of haloperidol alone (control) or drugs in combination with haloperidol. Data are means ± S.E.M. of groups of at least eight mice per dose. Treatments were compared with controls with two-tailed Dunnett's tests. *, a significant difference between haloperidol and vehicle; #, a significant difference between vehicle or haloperidol alone controls (p < .05).

View larger version (28K): [in this window] [in a new window]   Fig. 5.   Dose-effect curves for the D1 blocker, SCH 39166, or the D2 blocker, spiperone, alone (open circles) or in combination (filled circles) with methamphetamine (0.3 mg/kg, left) or sydnocarb (10 mg/kg, right) on the horizontally directed locomotor activity of mice. Other details as in Fig. 2.

View larger version (25K): [in this window] [in a new window]   Fig. 6.   Dose effect curves for the D1 blocker, SCH 39166, or the D2 blocker, spiperone, alone (open circles) or in combination (filled circles) with methamphetamine (0.3 mg/kg, left) or sydnocarb (10 mg/kg, right) on the vertically directed locomotor activity of mice. Other details as in Fig. 2.

View larger version (18K): [in this window] [in a new window]   Fig. 7.   Dose-dependent increases in the percentage of mice selecting the methamphetamine-correlated side of a T-maze after testing with increasing doses of methamphetamine (circles) or sydnocarb (squares). Data are derived from experiments in 5 to 8 mice trained to discriminate 3 mg/kg methamphetamine from saline. Data are quantal and do not contain variability estimates.

View larger version (21K): [in this window] [in a new window]   Fig. 8.   Self-administration of sydnocarb in rats trained to self-administer methamphetamine. Data are from successive 2-h experimental sessions in which every 5th response on the active manipulanda produced either methamphetamine (0.06 mg/kg/injection), sydnocarb (0.6 mg/kg/injection), or DMSO (50%). Data from two rats who lost their i.v. catheters during the first two sydnocarb sessions were not included in the data summary. Catheter loss was from excess DMSO solvent infusion. Data are thus means ± S.E.M. for six rats except for experimental sessions 9 to 15 (n = 5) due to the death of one rat after high intake of DMSO solvent on day 9. *p < .05 compared to session 8.

View larger version (13K): [in this window] [in a new window]   Fig. 9.   Comparative effects of sydnocarb (squares) and methamphetamine (circles) on three stereotyped behaviors in mice. Separate groups of mice (n = 8) were pretreated with either drug (filled points) or vehicle (unfilled, unconnected points) 20 min before placement in a wire mesh cage. Visual scoring of sniffing, gnawing, or climbing occurred in 10 consecutive 30-s time periods during which a 1 was scored for each mouse exhibiting the target behavior. Data are means ± S.E.M. * indicates significant difference (p < .05) from control data (open symbols), as calculated by an a priori Dunnett's test.

View larger version (30K): [in this window] [in a new window]   Fig. 10.   Comparative effects of acute (top) or cumulatively-administered (bottom) sydnocarb or methamphetamine on DA concentration in the dorsal striatum. Successive samples of perfusate from microdialysis probes were taken every 25 min.

Amphetamines and other psychomotor stimulants produce a substantial portion of their behavioral effects, including those related to their abuse, through activation of mesolimbic and mesocortical dopaminergic pathways (cf. Di Chiara, 1995). These indirect-acting dopaminergic stimulants are known to produce increases in availability of serotonin, norepinephrine, and dopamine (DA) through release mechanisms or from the inhibition of uptake of these neurotransmitters into presynaptic neurons. Although minimal information is available, sydnocarb appears to differ in its mechanism of action from that of amphetamines, and these differences may underlie the behavioral distinctions thus far uncovered. In contrast with amphetamines (cf. Zetterström et al., 1983), sydnocarb does not decrease extracellular levels of the DA metabolites, homovanillic acid, or of 3,4-dihydroxyphenylacetic acid in rat striatum (Gainetdinov et al., 1997). Sydnocarb releases DA in a tetrodotoxin-sensitive and Ca²⁺-dependent manner (Gainetdinov et al., 1997). In contrast, a rise in extracellular DA after amphetamines occurs via mechanisms that are insensitive to tetrodotoxin and Ca²⁺ (Westernik et al., 1989) through a process of reverse transport (Sulzer et al., 1995). Early neurochemical findings raised the possibility that sydnocarb acts by blocking the uptake of DA into presynaptic neurons. Sydnocarb has a higher affinity than amphetamine for blocking DA uptake and a lower affinity for blocking norepinephrine uptake; both sydnocarb and amphetamine demonstrated equivalent low affinities for inhibiting serotonin uptake (Erdö et al., 1981). Based on these pharmacological profiles of sydnocarb, it has been suggested that a likely mechanism of action of sydnocarb is blockade of DA uptake after its physiologically controlled release via a Ca²⁺-dependent vesicular process (Gainetdinov et al., 1997). Although this proposal remains to be confirmed, the data to date suggest mechanistic differences that may account for some of the observed differences in the effects of sydnocarb and amphetamines reported here and by others.

The purpose of the present study was designed to characterize some of the pharmacological, toxic, and behavioral effects of sydnocarb in comparison to those produced by methamphetamine. The possibility that sydnocarb represents a novel psychomotor stimulant with behavioral effects that can be distinguished from those of the amphetamines was evaluated. An additional goal of this work was the identification of a stimulant already in clinical practice with a reduced propensity for drug abuse and toxicity. Psychostimulants with such profiles have been postulated to be of potential clinical significance in the treatment of cocaine and amphetamine dependence (Witkin, 1994; Rothman and Glowa, 1995). Indeed, some compounds with mild stimulant pharmacological profiles antagonize behavioral stimulation induced by amphetamines (Menon et al., 1973). To date, however, a stimulant with demonstrated safety and efficacy has yet to be recognized for drug abuse therapeutic indications. Lack of efficacy as well as increased toxicity have plagued drug development efforts in this area (cf. Witkin, 1994). The stimulant profile reported for sydnocarb in humans, combined with reports of safety and mild euphoria, were notable in directing our attention to sydnocarb for stimulant drug abuse treatment.

A number of psychomotor stimulant effects (cf. Harvey, 1987) of sydnocarb were compared to those of methamphetamine. Comparative effects were determined for locomotor activity of mice (Peachey et al., 1976), for drug-induced stereotypies (Cooper and Dourish, 1990; Tirelli and Witkin, 1995), in mice trained to discriminate methamphetamine from saline (Holtzman, 1990), and in rats trained to self-administer i.v. methamphetamine (Pickens et al., 1967; Katz, 1989). Comparative dopaminergic effects were assessed by evaluating the abilities of the stimulants to prevent haloperidol-induced depression of locomotor activity in mice. The abilities of the DA D1 receptor antagonist SCH 31966 (Chipkin et al., 1988) or the D2 receptor antagonist spiperone (Meltzer et al., 1989) to block the locomotor stimulant effects of sydnocarb or methamphetamine were also determined. A comparison of the effects of sydnocarb and methamphetamine on levels of extracellular DA in dorsal striatum of mice in vivo was also evaluated (cf. Zetterström et al., 1983). Because some but not all stimulants enhance the toxicity of acutely administered cocaine (cf. Witkin and Katz, 1992; Acri et al., 1996), sydnocarb was also compared to methamphetamine in this regard. Effects of sydnocarb on locomotor activity and on in vivo microdialysate levels of DA have been reported previously in rats (Gainetdinov et al., 1997). The present systematic replication of these data in mice permitted direct comparison of methamphetamine with sydnocarb and allowed the neurochemical findings in mice to be compared directly to behavioral observations in the same species.

	Materials and Methods

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Subjects. Male Swiss-Webster mice (Taconic Farms, Inc., Germantown, NY), weighing 20 to 45 g, resided in groups of six in a cage with food and water available ad libitum. For the methamphetamine discrimination study, the mice were maintained at ~32 g by postexperimental feeding with Purina Rodent Chow. In the drug self-administration experiments, eight male Sprague-Dawley rats (Charles River Breeding Laboratories, Inc., Wilmington, MA) were used. The rats (300-400 g) were housed individually. Separate groups of mice were used for each of the individual experiments outlined below.

Locomotor Activity. Locomotor experiments were conducted using a group design in which mice were used for one treatment condition only. For the dose-response curves of methamphetamine and sydnocarb, animals were given either a s.c. or i.p. injection of methamphetamine (0, 0.1, 0.3, 1, 3, or 10 mg/kg) or sydnocarb (0, 3, 10, 30, or 100 mg/kg). The mice were not habituated to the locomotor arena prior to testing, a method used routinely in our laboratory (Acri et al., 1996). Directly after the injections, animals were individually placed into Digiscan activity monitors (Omnitech Electronics, Inc., Columbus, OH) with a surface area of 40 × 40 cm and with photoelectric detectors placed 1.8 cm apart along the perimeter. Another set of beams was located 10 cm above the floor for detection of vertically directed movements. Locomotion was recorded for 60 min in 10-min intervals.

Methamphetamine Discrimination. Drug discrimination studies were conducted in a T-maze. The T-maze was located in a dimly illuminated room in the same position every day. The body of the T-maze (7.5-cm wide and 10-cm high) was constructed of opaque Plexiglas and the removable top was clear Plexiglas. The base of the T was 63-cm long and each arm was 30 cm in length. A 7- × 7.5-cm area (pellet box) of opaque Plexiglas was located at the end of each arm. A food pellet (45 mg; BioServe, Frenchtown, NJ) could be placed in the pellet box and obscured from visual access from the junction of the T-maze. Slots for opaque Plexiglas doors were located at the bottom of the base of the T to close off a starting area (7.5 × 10 cm) and entrances to each arm.

Self-Administration. Eight standard operant chambers (Coulbourn Instruments, Inc., Lehigh Valley, PA) were used. Each chamber was equipped with two nose-poke keys. Depression of either key produced a brief feedback tone. Nose-poke responses on one of the keys was active in producing drug infusions according to the schedule defined below; responses on the other key were recorded but had no programmed consequences. Catheters were connected to an infusion pump (Harvard Apparatus, South Natick, MA) through a tether and fluid swivel. Experimental events were controlled and responses were recorded by microcomputers operating under MED associates MED-PC software and interfacing (Med Associates, Inc., East Fairfield, VT).

Stereotypies. Male Swiss-Webster mice were given i.p. injections of sydnocarb, methamphetamine, or vehicle and returned to their living cages for 20 min. They were then placed in wire mesh cages (1-cm mesh; 15 × 15 × 26-cm high) where they were visually scored for sniffing, gnawing, or climbing after 1 min. Scoring occurred in 10 consecutive 30-s time periods during which a score of 1 was given for each mouse (n = 8) for the occurrence of the target behavior. Sniffing was defined as repetitive, rapid snout movements back and forth on the floor or grid surface. Gnawing was scored if the incisors were over a grid. Climbing was scored when a mouse had all four paws on the wire grid. For each behavior, a total theoretical score of 80 could be achieved (all 8 mice exhibiting the behavior for each of the 10 observation periods). The mouse strain, route of administration, and pretreatment times were selected to be comparable to the parameters used in the neurochemical studies (described below) in which striatal DA efflux was measured.

Neurochemistry. Mice were anesthetized with urethane (1.6 mg/g i.p.) 30 min before mounting in a stereotaxic apparatus. Each animal was stabilized in a flat skull orientation using cheek bars and a tooth bar modified for the mouse (David Kopf Instruments, Topanga, CA). A CMA 11 guide cannula (CMA Microdialysis, Acton, MA) was implanted in the dorsal striatum (A/P 0.7 mm; lateral 1.7 mm; D/V 2.1 mm, relative to bregma, using a correction factor derived from bregma/lambda distance differences from atlas measures (Slotnick and Leonard, 1975) and fixed in place with dental cement. One-half hour later, a CMA 11 (2-mm) probe was placed through the guide cannula into the dorsal striatum. During a stabilizing period (2 h), probes were perfused with artificial cerebrospinal fluid (145 mM NaCl, 2.8 mM KCl, 1.2 mM MgCl₂, 1.2 mM CaCl₂, 0.25 mM ascorbate, 5.4 mM glucose, pH 7.2-7.4) at a flow rate of 0.5 µl/min for the first hour and 1.0 µl/min for the second hour. Samples were collected every 25 min at a flow rate of 1.0 µl/min. Animals received i.p. injections of either methamphetamine (1, 3, and 10 mg/kg, cumulatively, every 75 min) or sydnocarb (10, 30, and 100 mg/kg, cumulatively, every 75 min) after the collection of three baseline samples. An additional group of animals received an acute i.p. injection of the highest dose of methamphetamine (10 mg/kg) or sydnocarb (100 mg/kg). Dose ranges were determined from behavioral data collected as described above. During the experimental procedures, mice were placed on a heating pad and body temperature was maintained between 36 and 37°C. All samples were collected on ice and subsequently frozen on dry ice.

Convulsions. Vehicle (s.c.), methamphetamine (10 mg/kg s.c.), or sydnocarb (100 mg/kg s.c.) was administered 30 min before cocaine (55 mg/kg i.p.). Immediately after cocaine, mice were individually placed in clear Plexiglas observation chambers (14 × 25 × 36-cm high). Mice were observed for 30 min for the occurrence of convulsions, defined as a loss of the righting posture for at least 5 s, and clonic limb convulsions.

Drugs. D-Methamphetamine HCl (Sigma Chemical Co. St. Louis, MO), SCH39166 HCl (Schering-Plow, Bloomfield, NJ), spiperone HCl (Research Biochemicals International, Natick, MA) and ()cocaine HCl (Sigma) were dissolved in 0.9% NaCl. Sydnocarb (synthesized in the Institutes of Pharmaceutical Chemistry and Pharmacology, Russian Academy of Medical Sciences) was dissolved in propylene glycol (Sigma)/water (50% v/v) with heat and sonication. Haloperidol (McNeil Laboratories Inc., Fort Washington, PA) was dissolved in sterile water with a minimal amount of 1 N HCl for dissolution. Injections were given as 0.1 ml/10 g b.wt. Drug doses are in terms of the forms noted above. For the self-administration studies, methamphetamine was prepared in a final concentration of 0.24 mg/ml. Sydnocarb was dissolved in 50% DMSO in 0.9%NaCl to a final concentration of 2.4 mg/ml for self-administration experiments. Drugs were prepared fresh daily before the experimental session.

Data Analysis. For the locomotor studies, data for the horizontal and vertical activity were summed for the recorded time intervals, and means and S.E.M.s were calculated. The experimental data were compared to the appropriate control by a one- or two-tailed Dunnett's test. Occasional additional comparisons were done with a one-tailed Student's t test. The ED₅₀ values with 95% CL of methamphetamine and sydnocarb alone and of SCH 39166 and spiperone for the inhibition locomotor stimulation were calculated from linear regression analysis.

	Results

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Locomotor Activity. Regardless of the route of administration (s.c. or i.p.), both methamphetamine (circles) and sydnocarb (squares) increased horizontal locomotor activity (Fig. 2, top). Methamphetamine was more potent than sydnocarb in increasing locomotion (see Table 1 for ED₅₀ values). Methamphetamine was also more efficacious than sydnocarb, increasing activity to approximately twice the levels achieved by sydnocarb. Peak increases occurred at 3 mg/kg methamphetamine and at 30 mg/kg sydnocarb. Methamphetamine produced large, dose-dependent increases in vertical activity (Fig. 2, circles, bottom). Increases in vertical activity were observed only after s.c. administration of sydnocarb. As with horizontal activity, methamphetamine was both more potent (Table 1) and more efficacious than sydnocarb in increasing vertical activity. Higher doses of sydnocarb could not be evaluated because of limitations in solubility. As the route of administration was not a major determinant of behavioral effects of these compounds, subsequent experiments used either i.p. or s.c. routes with the stipulation that identical routes for both sydnocarb and methamphetamine were used with each experiment.

Reversal of Haloperidol-Induced Locomotor Depression. Fig. 4 presents the comparative abilities of methamphetamine (left) and sydnocarb (right) to reverse behavioral effects of haloperidol (0.1 mg/kg). Haloperidol decreased both horizontal (top) and vertical (bottom) activity. The haloperidol-induced decreases in horizontal activity (gray columns above control) were reversed by 0.3 mg/kg methamphetamine, a dose that increased activity when given alone (+ saline; black columns). Sydnocarb completely reversed this effect of haloperidol at 10 mg/kg, a dose which was inactive when given alone. The decreases in vertical activity produced by haloperidol were only modestly reversed by 0.3 mg/kg methamphetamine.

Blockade of the Locomotor Stimulant Effects by DA Antagonists. Doses of methamphetamine and sydnocarb were selected so as to achieve generally equivalent and submaximal effects. Significant increases in horizontal (Fig. 5) and vertical (Fig. 6) activity were produced by 0.3 mg/kg methamphetamine or by 10 mg/kg sydnocarb. When given alone, both the D1 antagonist, SCH 39166, and the D2 antagonist, spiperone, dose-dependently decreased horizontal and vertical locomotion (open symbols). Both antagonists also produced dose-dependent decreases in the stimulant effects of each of the psychomotor stimulant drugs (filled symbols). The separation between doses of the DA antagonists that blocked locomotor stimulation and doses that decreased activity when given alone was not substantial (Figs. 5 and 6). Nonetheless, potency comparisons (ED₅₀ of antagonist/ED₅₀ of drug combination) indicated that SCH 39166 produced a more selective blockade for horizontal locomotor stimulation than spiperone. Spiperone was a bit more selective in its blockade of sydnocarb-induced stimulant effects than of methamphetamine-induced behavioral effects (Table 2).

Methamphetamine Discrimination. Both methamphetamine and sydnocarb engendered dose-dependent increases in the percentage of mice turning toward the methamphetamine-appropriate side of the T-maze (Fig. 7). Mice generally completed each trial in less than 30 s and never took more than 2 min. A dose of 1 mg/kg methamphetamine and a dose of 3 mg/kg sydnocarb produced 100% methamphetamine-appropriate responding. ED₅₀ values (Table 1) indicated that methamphetamine was about 9 times more potent than sydnocarb; the relative potency of sydnocarb to methamphetamine was 8.8 (95% CL, 2.4-33).

Self-Administration. Results of substitution tests in which sydnocarb was substituted for methamphetamine in rats trained to self-administer methamphetamine are shown in Fig. 8. Methamphetamine (0.06 mg/kg/injection) maintained a stable level of injections of methamphetamine (mean = 24.7 ± 0.8 infusions/2-h session or 1.48 mg/kg). A unit dose of sydnocarb for substitution experiments was selected on the basis of the drug discrimination experiments (~10-fold potency difference). When 0.6 mg/kg/injection of sydnocarb was substituted for methamphetamine, responding was maintained at levels comparable to those maintained by methamphetamine [F_(5,25) = 1.61, p > .05]. Over the five experimental sessions in which sydnocarb was available, a mean of 36.1 ± 2.8 infusions per session were delivered. Substitution of the 50% DMSO vehicle for sydnocarb resulted in a session-by-session decline in the number of infusions (mean = 13.0 ± 3.6). ANOVA revealed a significant decrease across sessions: F_(5,20) = 5.303, p < .01. Post hoc analysis revealed significant differences in the number of infusions in sessions 11 to 13 compared with the last session with sydnocarb available (session 8). One rat died after the first day of DMSO substitution, probably as a result of excessive systemic levels of DMSO. Reinstatement of methamphetamine resulted in increased levels of responding which were not statistically different than levels when methamphetamine was previously available (session 3) [F_(4,8) = 3.73, p > .05). Rates of responding on the inactive response manipulanda (upon which responses did not produce i.v. infusions) were always less than 10 responses per session with the exception of the first days of DMSO substitution when the number of responses on the inactive manipulanda transiently increased in some rats (data not shown).

Stereotypies. To directly compare behavioral effects of methamphetamine or sydnocarb with a neurochemical marker of dopaminergic function (see below), the same strain of mouse, pretreatment times, and routes of administration were used in both sets of experiments. Vehicle administration engendered low levels of sniffing, gnawing, and climbing (Fig. 9). Climbing occurred at a significantly higher level in saline than in propylene glycol-treated mice. Methamphetamine produced dose-dependent increases in sniffing. At 30 and 100 mg/kg, methamphetamine produced sniffing in all mice for the entire 10-min observation period. In contrast, sydnocarb significantly increased sniffing only at 30 mg/kg, and the maximal effect was much less than that induced by methamphetamine. Gnawing and climbing were affected in a biphasic manner by methamphetamine with maximal effects occurring at 10 mg/kg. Sydnocarb produced linear increases in gnawing and climbing. As with sniffing, sydnocarb was less efficacious in engendering gnawing and climbing than was methamphetamine.

Neurochemistry. The basal concentration of DA in mice dialysates was 2.78 ± 0.25 nM and the administration of saline or propylene glycol vehicle did not significantly modify basal levels. The acute administration of the highest dose of methamphetamine (10 mg/kg) produced a robust increase in striatal DA concentration relative to the basal level with a peak increase of 600%. The magnitude of this increase was approximately twice as large as that induced by the highest dose of sydnocarb (100 mg/kg; Fig.10, top). Cumulative administration of methamphetamine every 75 min dose-dependently increased DA levels to a maximum of 500% (Fig. 10, bottom). ANOVA revealed that these changes were significant at all three doses of methamphetamine (F_{(1, 46)} = 33.1, p < .001; F_{(1, 46)} = 53.2, p < .001; F_{(1, 46)} = 138.3, p < .001 for 1, 3, and 10 mg/kg, respectively). In contrast, the lowest sydnocarb dose tested (10 mg/kg) failed to modify basal DA levels (F_{(1, 40)} = 3.99, p > .05). Higher doses resulted in a moderate but statistically significant increase of extracellular DA concentration relative to baseline values (F_{(1, 40)} = 4.13, p < .05 and F_{(1, 27)} = 15.48, p < .01 at 30 and 100 mg/kg, respectively). The maximum increase after cumulative sydnocarb administration was only 150% (see Fig.10, bottom).

Convulsions. As shown in Table 3, the coadministration of methamphetamine with cocaine resulted in a significant increase in the percentage of mice exhibiting clonic convulsions. Sydnocarb had no potentiating effect. When given alone, both drugs produced 0% convulsions in the doses tested (i.e., up to 30 mg/kg for methamphetamine and up to 100 mg/kg for sydnocarb).

	Discussion

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Indirect-acting dopaminergic stimulants (uptake blockers, releasers) display a common set of behavioral effects. These effects include behavioral stimulation at low to moderate doses and behavioral stereotypies at higher doses (cf. Peachey et al., 1976; Tirelli and Witkin, 1995). These drugs also appear to share common subjective effects (Schuster and Johanson, 1988) as suggested by drug discrimination experiments in preclinical and clinical studies (cf. Holtzman, 1990). Indirect-acting DA agonists are also generally self-administered by humans and experimental animals (cf. Katz, 1989). Neurochemically, indirect-acting dopaminergic stimulants give rise to increased mesolimbic and striatal levels of DA, the major neurotransmitter implicated in their psychomotor stimulant effects (Westernik et al., 1989; Di Chiara, 1995). A comparison of the effects of sydnocarb with those of methamphetamine revealed that sydnocarb and methamphetamine share a host of common behavioral effects. Both drugs increased locomotor activity and substituted for methamphetamine in mice trained to discriminate methamphetamine from vehicle or in rats trained to self-administer methamphetamine. Both compounds also engendered behavioral stereotypies. In general, sydnocarb was less potent in producing these effects. In the case of locomotor activity and behavioral stereotypies, sydnocarb was also less efficacious. The lower potency and efficacy of sydnocarb compared to racemic amphetamine has been reported for effects on feeding and body temperature (Rudenko and Altshuler, 1978). The data reported here document the psychomotor stimulant effects of sydnocarb and its overlap in behavioral pharmacology with effects of amphetamines (cf. Harvey, 1987; Witkin et al., 1990). These comparative data with methamphetamine are also consistent with the clinical appraisal of sydnocarb as a central nervous system stimulant (Altshuler, 1973).

Results of drug discrimination and self-administration experiments in which sydnocarb substituted fully for methamphetamine suggest that sydnocarb has subjective effects and abuse liability comparable to that of amphetamines. Preliminary clinical reports of only modest euphoria or abuse potential with sydnocarb (Mashkovsky et al., 1971; Rudenko and Altshuler, 1978) suggest, however, that the preclinical models may not be fully adequate as predictors of these effects in humans. This discrepancy between preclinical data and clinical observation has also been noted for some other molecules (cf. Rothman and Glowa, 1995). There are several drugs that produce central dopaminergic facilitation (e.g., mazindol, GBR 12909, benztropine) and yet do not fully mimic the behavioral effects, subjective states or abuse potential of the amphetamines, the reasons for which are currently only speculative (cf. Rothman and Glowa, 1995; Acri et al., 1996). Mazindol, for example, is a catecholamine uptake blocker that mimics the discriminative stimulus effects and self-administration profile of abused substances (e.g., cocaine, amphetamines) and mimics the sequela of toxic effects in animal models (cf. Bergman et al., 1989; Witkin et al., 1991; Witkin and Katz, 1992). In humans, mazindol is not self-administered and is not euphorogenic (cf. Chait et al., 1987). Such discrepancies depend, however, upon a host of factors other than the intrinsic pharmacological effects of the drugs including pharmacokinetics and considerations of drug availability (cf. Katz, 1990). Thus, although sydnocarb may display milder euphorogenic effects than amphetamines (Mashkovsky et al., 1971; Rudenko and Altshuler, 1978), these effects may be sufficient to engender abuse if the drug were available in a form for i.v. or inhalational use by a drug-abusing community. However, it is possible that without a history of methamphetamine self-administration, sydnocarb would not have maintained responding, an effect that could not be tested here because of limitations in drug availability and water solubility.

In addition to its reduced potency and efficacy in some behavioral tests, sydnocarb also displayed a lower propensity for producing stereotypies, suggesting that sydnocarb produces less behavioral toxicity than methamphetamine. That is, sydnocarb may have a reduced tendency to produce gross behavioral changes that interfere with normal, ongoing activity (e.g., locomotion). Methamphetamine also produced high rates of abortive grooming and intense sniffing in C57BL/6J mice (Tirelli and Witkin, 1995), as quantified in Swiss-Webster mice in the present study. However, when the potencies of the stimulants to affect behavior (e.g., locomotion) versus stereotypies are taken into account, a different picture is seen. Methamphetamine demonstrates a 25-fold separation in doses that increase locomotor activity versus doses producing stereotypy (e.g., sniffing). Sydnocarb was only 1.7 times more potent in increasing locomotor activity over induction of sniffing. Nonetheless, the low efficacy of sydnocarb to induce behavioral stereotypies is predictive of a wider window of safety than that of methamphetamine. Thus, although sydnocarb may begin to produce unwanted behavioral effects at doses closer to the therapeutic range than predicted for methamphetamine, such unwanted behavioral effects may be of minimal clinical significance given the markedly reduced maximal stereotypic effects produced by sydnocarb. Such preclinical data are consistent with available clinical information where behavioral toxicity has not been significant (Mashkovsky et al., 1971; Rudenko and Altshuler, 1978).

Stereotypy induced by dopaminergic drugs is controlled through activation of dopaminergic transmission in nigrostriatal, mesolimbic, and ventral thalamic circuits (cf. Cooper and Dourish, 1990). In vivo microdialysis in dorsal striatum of mice demonstrated increased DA efflux after administration of methamphetamine in keeping with the documented pharmacology of amphetamines (cf. Zetterström et al., 1983; Westernik et al., 1989; Camp et al., 1994). The marked increases in striatal DA levels after methamphetamine corresponded to the intense stereotypies that developed with this compound. Sydnocarb demonstrated smaller increases in striatal levels of DA and, correspondingly, smaller degrees of stereotypy in this mouse strain. In addition to differences in efficacy, the mouse microdialysis experiments also revealed differences in potency between methamphetamine and sydnocarb, which are in accord with the greater behavioral potency of methamphetamine. Sydnocarb increased DA levels in dialysate samples to a maximum of 350% of control in dorsal striatum in nonanesthetized rats; sydnocarb was equipotent in producing increases in DA dialysate levels in the dorsal striatum and in the nucleus accumbens (Gainetdinov et al., 1997). Comparative microdialysis data on sydnocarb and amphetamines is not available in the rat so it is not possible to directly compare the sydnocarb data presented here with the rat data of Gainetdinov et al. (1997). The increases in DA dialysate levels in nucleus accumbens after sydnocarb in both rat and mouse, however, are in accord with the findings of the methamphetamine-like self-administration and discriminative stimulus effects of this compound reported in the present study (cf. Di Chiara, 1995).

Interactions of sydnocarb with DA antagonists provided additional evidence for the dopaminergic actions of this compound that are relevant to the production of behavioral effects. Both the D1 receptor antagonist, SCH 39166, and the D2 antagonist, spiperone, dose-dependently attenuated the locomotor stimulant effects of sydnocarb and methamphetamine. Blockade was selective in that there was a separation between doses of the antagonists that decreased behavior when given alone and doses that blocked the locomotor stimulation induced by sydnocarb or methamphetamine. Blockade of behavioral effects of amphetamines by subtype-selective DA antagonists has been reported previously (cf. Ross et al., 1989). In the converse experiment, sydnocarb and methamphetamine attenuated the locomotor depressant effects of haloperidol. Blockade of this behavioral effect of haloperidol is consistent with reports that both amphetamine and sydnocarb reverse trifluoperazine-induced catalepsy in rats and neuroleptic-induced behavioral depression in humans (Mashkovsky et al., 1971; Rudenko and Altshuler, 1978).

Pharmacological treatment of psychomotor stimulant abuse continues to be imperfect, with the absence of any drugs with proven efficacy (see introduction). The possible introduction of a nonabused psychomotor stimulant into this therapeutic arena has been suggested for a number of years, although such an agent has yet to be recognized (cf. Witkin, 1994; Rothman and Glowa, 1995). Given the reduced efficacy of sydnocarb in some behavioral tests and in its behavioral toxicity (stereotypies), along with the reported safety and lack of abuse of sydnocarb in clinical practice (Rudenko and Altshuler, 1978), this novel stimulant may be a candidate for investigation as a potential stimulant abuse therapeutic agent. The lack of combined toxicity of sydnocarb and cocaine which contrasts with that of methamphetamine (reported here) or mazindol (Witkin and Katz, 1992) is an additional piece of promising data in this regard.

In summary, the present data on behavioral, neurochemical, and toxic effects of sydnocarb indicate that this compound produces psychomotor stimulant effects that overlap with those of methamphetamine, as well as effects that are both quantitatively and qualitatively distinct. Further investigations of the mechanism of action of sydnocarb will be needed to evaluate any potential differences in neurochemistry that may differentiate these stimulants. Nonetheless, the current preclinical findings are generally consistent with the effects of sydnocarb that have been described in humans. Given the continued safety of sydnocarb in clinical practice, this compound may offer a new opportunity to more broadly integrate a central nervous stimulant into use by the therapeutic community.