Nitric oxide (NO) synthase inhibitors abolish cocaine-induced toxicity in mice

The Cardiovascular Effects of Cocaine

Cocaine is the leading cause for drug-abuse-related visits to emergency departments, most of which are due to cardiovascular complaints. Through its diverse pathophysiological mechanisms, cocaine exerts various adverse effects on the cardiovascular system, many times with grave results. Described here are the varied cardiovascular effects of cocaine, areas of controversy, and therapeutic options.

The Role of Inducible-Nitric Oxide in Cocaine-Induced Kindling

Experimentally naive male Sprague Dawley rats (weighing 85–110 g) were used to examine the role of inducible nitric oxide synthase (iNOS) in cocaine-induced kindling. Repeated administration of cocaine (45 mg/kg, ip) to Sprague Dawley male rats for 7 consecutive days produced a progressive increase in the Convulsive responsiveness and death. Pretreatment with iNOS inhibitors, L-N6-(1-iminoethyl)lysine (NIL; 10 mg/kg, ip) and (–)-ePigalloocatechin gallate (EGCG; 10 mg/kg, ip) 30 min before cocaine (45 mg/kg, ip) administration for 7 days attenuated the development of cocaine kindling and blocked cocaine-induced death. Results of NMDA receptor binding assay in the hippocampus showed a significant increase in the affinity without changes in the density in animals treated with cocaine, but there were no changes in these parameters in the cortex. Pretreatment with NIL or EGCG prior to cocaine administration abolished the cocaine-induced effect in the NMDA receptor affinity in the hippocampus. These results suggest that iNOS induction followed by an increase of NMDA receptor affinity in the hippocampus after repeated exposure to cocaine may participate in the process of the development of cocaine kindling.

Role of nitrergic system in behavioral and neurotoxic effects of amphetamine analogs

Several amphetamine analogs are potent psychostimulants and major drugs of abuse. In animal models, the psychomotor and reinforcing effects of amphetamine, methamphetamine (METH), 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy), and methylphenidate (MPD; Ritalin) are thought to be dependent on increased extracellular levels of dopamine (DA) in mesocorticolimbic and mesostriatal pathways. However, amphetamine analogs that increase primarily serotonergic transmission, such as p-chloroamphetamine (PCA) and fenfluramine (FEN), have no potential for abuse. High doses of METH, MDMA, PCA, and FEN produce depletions of dopaminergic and serotonergic nerve terminal markers and are considered as potential neurotoxicants. The first part of this review briefly summarizes the behavioral and neurotoxic effects of amphetamines that have a different spectrum of activity on dopaminergic and serotonergic systems. The second part discusses evidence supporting involvement of the nitrergic system in dopamine-mediated effects of amphetamines. The nitrergic system in this context corresponds to nitric oxide (NO) produced from neuronal nitric oxide synthase (nNOS) that has roles in nonsynaptic interneuronal communication and excitotoxic neuronal injury. Increasing evidence now suggests cross talk between dopamine, glutamate, and NO. Results from our laboratory indicate that dopamine-dependent psychomotor, reinforcing, and neurotoxic effects of amphetamines are diminished by pharmacological blockade of nNOS or deletion of the nNOS gene. These findings, and evidence supporting the role of NO in synaptic plasticity and neurotoxic insults, suggest that NO functions as a neuronal messenger and a neurotoxicant subsequent to exposure to amphetamine-like psychostimulants.

Effects of alcohol on cocaine lethality in rats: acute and chronic assessments

Cocaine and alcohol is a popular, yet toxic, drug combination that results in effects greater than that of either drug alone. The following experiment presents additional evidence that supports this position. Specifically, the lethal effects of acute (1 day) and chronic (4 days) alcohol (0.5 g/kg), cocaine (20, 30, or 40 mg/kg), or the respective alcohol/cocaine combinations were assessed in rats. For acute drug administration, lethality was only evident in those animals administered the combination of 0.5 g/kg alcohol and 40 mg/kg cocaine, supporting the position that the effects of combining alcohol and cocaine are greater than either drug alone. Chronic drug administration resulted in a weak sensitization to cocaine-induced lethality that was increased when alcohol was combined with cocaine. Together, this evidence suggests that combining alcohol and cocaine (acutely) can have lethal consequences in rats that are greater than either drug alone, effects that can be exacerbated with chronic use.

Chronic inhibition of the norepinephrine transporter in the brain participates in seizure sensitization to cocaine and local anesthetics

The involvement of chronic inhibition of monoamine transporters (MAT) in the brain with respect to sensitization to cocaine- and local anesthetic-induced seizures was studied in mice. Repeated administration of subconvulsive doses of meprylcaine as well as cocaine, both of which inhibit MAT, but not lidocaine, which does not inhibit MAT, increased seizure activity and produced sensitization to other local anesthetics. The effects of five daily treatments of monoamine transporter inhibitors on lidocaine-induced convulsions were examined 2 or 3 days after the last dose of the inhibitors. Daily treatments of GBR 12935, a specific inhibitor of dopamine uptake, significantly increased the incidence and the intensity of lidocaine-induced convulsions at 20 mg/kg and decreased the threshold of the convulsions. Daily treatments of desipramine and maprotiline, selective norepinephrine uptake inhibitors, markedly increased the incidence and intensity of lidocaine-induced convulsions, and decreased the threshold in a dose-dependent manner at between 5 and 20 mg/kg. Daily treatments of citalopram, a selective serotonin uptake inhibitor, at 10 and 20 mg/kg, produced no significant increase in the incidence or intensity of lidocaine-induced convulsions, but decreased the threshold of the convulsions. These results suggest that the chronic intermittent inhibition of monoamine uptake increases susceptibility to cocaine- and local anesthetic-induced seizures, and the norepinephrine transporter is an integral component of this sensitization.

The effect of nitric oxide synthesis inhibition on intravenous cocaine self-administration

Adult male rats were implanted with intravenous catheters. After a minimum of 10 days recovery from surgery, rats were trained to intravenously self-administer cocaine (1 mg/kg/infusion) during 3-h test sessions. The nitric oxide synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME) was used to determine the effect of nitric oxide (NO) synthesis inhibition on cocaine self-administration. A 5-day protocol was used and on Days 2 and 5, an intraperitoneal injection of L-NAME (0, 3, 30, 300 mg/kg) was administered 45 to 60 min into a 3-h test session. One to two hours following L-NAME administration, there was a dose-dependent decrease in the amount of self-administered cocaine and an increase in the interresponse time (IRT) between successive cocaine injections. L-NAME appeared to prolong the rewarding effect of cocaine possibly through a pharmacokinetic action.

Cocaine-induced kindling is associated with elevated NMDA receptor binding in discrete mouse brain regions

The present study was undertaken to investigate the involvement of N-methyl-D-aspartate (NMDA) type of glutamate receptors in the induction and maintenance of kindling generated by daily cocaine (35 mg/kg) injections to Swiss Webster mice. In addition, the regulation of NMDA receptor binding following the development of sensitization to horizontal locomotor activity produced by daily injections of a low dose of cocaine (15 mg/kg for 5 days) was investigated. Three days following the administration of the high dose of cocaine (35 mg/kg) a marked augmentation in cocaine-induced horizontal and vertical activities was observed (induction phase). Subsequently, after 10 days of cocaine administration, mice developed stage 5 seizures (Racine scale). Binding of [3H]CGP 39653 to the NMDA receptors revealed a marked increase in receptor densities in the striatum, amygdala and hippocampus associated with the induction phase. The elevation of NMDA receptor binding in the striatum and amygdala was sustained for 10 days following the induction phase. The pattern of altered NMDA receptor binding following the expression of cocaine kindled seizures was different. One day after the expression of kindled seizures NMDA receptor binding was elevated in striatum, amygdala, hippocampus and frontal cortex. However, only the elevation of NMDA receptor binding in the amygdala and hippocampus was sustained for 10 days following the expression of cocaine kindled seizures. In the brains of mice sensitized to the low dose of cocaine (15 mg/kg) no change in NMDA receptor binding was observed compared with control values. The present findings suggest the following: (a) The induction of cocaine kindling is associated with increased NMDA receptor binding activity in the striatum, amygdala and hippocampus; (b) the maintenance of cocaine kindling depends on increased NMDA receptor binding in the amygdala and hippocampus; (c) sensitization to cocaine-induced horizontal locomotor activity may be independent of elevation in NMDA receptor binding.

Early withdrawal from repeated cocaine administration upregulates muscarinic and dopaminergic D2-like receptors in rat neostriatum

The present results show an increase in locomotor activity 24 h following repeated cocaine administration only with the higher dose (10 mg/kg, i.p., daily for 1 week) compared to controls (administered with saline). Binding assays were done and the ligands used were [3H]N-methylscopolamine ([3H]-NMS), [3H]-SCH 23390, and [3H]-spiroperidol to determine muscarinic (M1- and M2-like), D1 and D2 receptors, respectively. Scatchard analyses revealed alterations in Bmax not only for muscarinic, but also for D2-like receptors that were significantly increased. On the other hand, no alterations were detected on D1-like receptors densities and dissociation constant values. However, the Kd value was significantly increased for D2 receptors. The changes in muscarinic receptors were observed predominantly on M2-like, which presented an increase of 84% with the 10 mg/kg, i.p., dose only. On D2-like receptors, increases of 63 and 54% were demonstrated with the doses of 5 and 10 mg/kg, i.p.. The preferential effects of cocaine on muscarinic and D2-like receptors were also demonstrated in vitro where decreases in [3H]-NMS and [3H]-spiroperidol binding were observed. The results indicate that the effects of cocaine on muscarinic and dopaminergic postsynaptic receptors are functions of dose, duration of treatment, and time of drug withdrawal.

The role of neuronal nitric oxide synthase in cocaine-induced conditioned place preference

Previous studies suggested the involvement of the neuronal nitric oxide synthase (nNOS) in the development of sensitization to psychostimulants. In the present study we investigated the role of nNOS in the rewarding properties of cocaine. Swiss Webster mice treated with cocaine (20 mg/kg) and saline every other day for 8 days (four drug and four saline sessions) developed conditioned place preference (CPP) for the drug-paired compartment of the cage. Pretreatment with the nNOS inhibitor, 7-nitroindazole (7-NI; 25 mg/kg), completely blocked cocaine-induced CPP. Mice deficient for the nNOS gene (homozygote nNOS(-/-) mice) were resistant to cocaine-induced CPP, while wild-type nNOS(+/+) mice developed a marked CPP following cocaine administration. Both, the pharmacological and genetic manipulations of nNOS suggest that nitric oxide (NO) is involved in the rewarding properties of cocaine.

The role of the nitric oxide (NO) pathway in the discriminative stimuli of amphetamine and cocaine

To examine the role of the nitric oxide (NO) pathway in the stimulus effects induced by some psychostimulants, separate groups of rats were trained to discriminate between amphetamine (AMPH; 0.5 mg/kg) and saline, or cocaine (COC; 5 mg/kg) and saline using a standard two-lever operant procedure. Substitution studies showed that AMPH and COC generalized for the training drugs in a dose-dependent manner, their ED50, values being 0.1 mg/kg and 1.2 mg/kg, respectively. The dose-response function of both those psychostimulants did not change in the course of the experiment. Moreover, AMPH and COC induced cross-substitution effects towards each other. Successive combination tests demonstrated that injection of a fixed dose of the NO synthase (NOS) inhibitor 7-nitro indazole (7-NI; 25 mg/kg) plus different doses of AMPH or COC resulted in a leftward shift in the dose-response curves of those psychostimulants and a decrease in their ED50 values. On the other hand, pretreatment with the NO donor molsidomine (MOL), injected in a fixed dose of 100 mg/kg before AMPH and COC, shifted the dose-response curves of the psychostimulants to the right and increased their ED50 values. Our results indicate that NO plays an inhibitory role in the dopamine (DA)-evoked discrimination effects of AMPH and COC in rats.

Anticonvulsant and proconvulsant roles of nitric oxide in experimental epilepsy models

The effect of acute (120 mg/kg) and chronic (25 mg/kg, twice a day, for 4 days) intraperitonial injection of the nitric oxide (NO) synthase (NOS) inhibitor NG-nitro-L-arginine (L-NOARG) was evaluated on seizure induction by drugs such as pilocarpine and pentylenetetrazole (PTZ) and by sound stimulation of audiogenic seizure-resistant (R) and audiogenic seizure-susceptible (S) rats. Seizures were elicited by a subconvulsant dose of pilocarpine (100 mg/kg) only after NOS inhibition. NOS inhibition also simultaneously potentiated the severity of PTZ-induced limbic seizures (60 mg/kg) and protected against PTZ-induced tonic seizures (80 mg/kg). The audiogenic seizure susceptibility of S or R rats did not change after similar treatments. In conclusion, proconvulsant effects of NOS inhibition are suggested to occur in the pilocarpine model and in the limbic components of PTZ-induced seizures, while an anticonvulsant role is suggested for the tonic seizures induced by higher doses of PTZ, revealing inhibitor-specific interactions with convulsant dose and also confirming the hypothesis that the effects of NOS inhibitors vary with the model of seizure.

Free radicals and taste aversion learning in the rat: nitric oxide, radiation and dopamine

1. Injection of sodium nitroprusside (SNP) or N-tert-butyl-alpha-phenyl nitrone (PBN) produces a conditioned taste aversion (CTA) in rats. The CTA can be prevented by pretreatment with N omega-nitro-L-arginine (L-NArg), indicating that nitric oxide (NO) is a behaviorally toxic compound. 2. Radiation-induced CTA learning is not affect by pretreatment with L-NArg or by preexposure to PBN, indicating that a radiation-stimulated formation of NO does not mediate the toxic effects of radiation on behavior. 4. Pretreating rats with the dopamine antagonist haloperidol prevented the acquisition of the CTA produced by SNP and attenuated, but did not eliminate, the PBN-induced CTA. Preexposure to the dopamine agonist amphetamine, attenuated a PBN-induced CTA, although PBN preexposure did not affect an amphetamine-induced CTA. 5. The results are interpreted as supporting a role for NO-stimulated dopamine release in the acquisition of taste aversions following injection of SNP or PBN.

Attenuation of cocaine kindling by 7-nitroindazole, an inhibitor of brain nitric oxide synthase

Recent studies suggest the involvement of the N-methyl-D-aspartate (NMDA) type of glutamate receptors and nitric oxide synthase (NOS) in the process of increased sensitivity to the convulsive effect of cocaine ("cocaine kindling"). The present study was undertaken to analyze the various behavioral stages in the development of cocaine kindling and to investigate the effect of 7-nitroindazole (7-NI), a relatively selective inhibitor of the neuronal NOS isoform, on the induction and expression of sensitization to the convulsive effect of cocaine. Also, the effect of 7-NI on responses produced by acute systemic administration of cocaine or N-methyl-D,L-aspartate (NMDLA) was investigated. Cocaine kindling was assessed on a five-stage scale following the administration of a sub-convulsant dose of the drug (35 mg/kg/day; i.p.) to Swiss Webster mice for 10 days. Stage 5 seizures developed following the 9th day of cocaine administration. Pre-treatment with 7-NI (25 mg/kg/day; i.p.) 15 min before cocaine for 10 days completely prevented the appearance of stage 4 and 5 seizures, and it significantly attenuated stage 3 behavior in response to a challenge cocaine dose (35 mg/kg) given either 24 hr or 10 days after 7-NI/cocaine administration was stopped. A single injection of 7-NI (25 mg/kg; i.p.) completely prevented the expression of cocaine kindled seizures. Whereas 7-NI had no effect on the responses elicited by acute cocaine administration (60 mg/kg; i.p.), this agent partially attenuated the effects induced by systemic administration of the NMDA receptor agonist NMDLA (250 mg/kg; i.p.). The present study indicates that 7-NI attenuates both the induction and expression of sensitization to the convulsive effect of cocaine. The findings that 7-NI attenuated cocaine kindling and partially blocked the effects produced by activation of the NMDA receptor, but not the effects induced by acute cocaine administration, support the role of the NMDA receptor and brain NOS in the development of cocaine kindling rather than in the acute effects of the drug.

Cocaine kindling in mice. Responses to N-methyl-D,L-aspartate (NMDLA) and L-arginine

Previous studies proposed the involvement of the N-methyl-D-aspartate (NMDA) type of glutamate receptors in the development of sensitization to the convulsive effect of cocaine (cocaine kindling). The present study was undertaken to determine, first, if cocaine kindling is associated with enhanced sensitivity of the NMDA receptor to the convulsive response of N-methyl-D,L-aspartate (NMDLA), and second, whether in vivo modulation of nitric oxide synthase (NOS) function regulates the development of cocaine kindling. The following results were observed: 1. Cocaine-kindled animals were significantly more susceptible to the convulsive effect of the NMDA receptor agonist NMDLA than saline controls; 2. Pretreatment with the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 100 mg/kg; ip) blocked the development of cocaine kindling; 3. The protective effect of L-NAME was partially reversed with the coadministration of the NOS substrate, L-arginine (300 mg/kg; ip), but not D-arginine; and 4. L-Arginine (300 mg/kg; ip), but not D-arginine, amplified the development of cocaine kindling. Taken together, these findings suggest that supersensitivity of the NMDA receptor and activation of NOS may underlie the development of cocaine kindling.

Nitric oxide synthesis, epileptic seizures and kindling

Nitric oxide (NO) has been implicated in synaptic changes underlying long-term potentiation and some forms of learning. It is unclear, however, whether NO contributes to long-term changes associated with the kindling of epileptic seizures. In the present study rats were treated, on the first 6 days of kindling, with L-arginine (L-Arg), the endogenous donor from which NO derives, or with L-nitro-arginine (L-No-Arg), a competitive inhibitor of NO synthesis, or with vehicle. Drugs were given in doses previously shown to affect learning or other behaviour. L-Arg (750 mg/kg IP) did not affect kindling or seizure severity. L-No-Arg (100 mg/kg) prolonged the duration of afterdischarges and convulsions on treatment days but did not advance kindling or affect seizures on subsequent days. A second experiment examined the possible role of NO in the development of resistance to seizures following prior seizures. Six or more stimuli were administered at 10-min intervals to fully-kindled rats after injection of L-No-Arg or vehicle. Vehicle-treated rats became progressively more resistant to afterdischarges and convulsions with successive stimulations but L-No-Arg-treated rats failed to do so. Rats injected with L-NO-Arg also showed an unexpected high mortality in the ensuing 24 h. L-No-Arg appeared to have no direct effect on the course of kindling but impaired the development of postictal resistance, and increased the duration and lethal after-effects of closely repeated seizures. The results do not support suggestions that antagonists of NO might prove clinically useful as anticonvulsants.