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

The role of the L-arginine-NO-cGMP-dependent pathway in the development of sensitization to mephedrone effects on the locomotor activity in mice

Mephedrone, a popular psychostimulating substance widely used illegally in recreational purposes, exerts in rodents that regularly and intermittently were exposed to it a sensitized response to the drug. Behavioral sensitization is one of experimental models of drug dependency/abuse liability. In the present study we evaluated a potential involvement of the L-arginine-NO-cGMP pathway in the development of sensitization to the mephedrone-induced hyperlocomotion. Locomotor activity was measured automatically and experiments were performed on male Albino Swiss mice. We demonstrated that a 5-day administration of 7-nitroindazole (10 or 20 mg/kg/day) and L-NAME (50 mg/kg/day) suppressed the development of sensitization to the mephedrone-induced hyperlocomotion. As for L-arginine (125 or 250 mg/kg/day) and methylene blue (5 or 10 mg/kg/day) the obtained outcomes are inconclusive. Furthermore, the lower dose of L-NAME (25 mg/kg/day) surprisingly potentiated the development of sensitization to the mephedrone-induced effects on the spontaneous locomotor activity in mice. In conclusion, our data demonstrated that modulators of the L-arginine-NO-cGMP pathway may differently affect the development of sensitization to the locomotor stimulant effects of mephedrone. Inhibition of neuronal nitric oxide synthase (NOS) seems to prevent this process quite profoundly, non-selective inhibition of NOS may have a dual effect, whereas inhibition of soluble guanylate cyclase may only partially suppress the development of sensitization to the mephedrone-induced effects.

Pharmacological NOS-1 Inhibition Within the Hippocampus Prevented Expression of Cocaine Sensitization: Correlation with Reduced Synaptic Transmission

Behavioral sensitization to psychostimulants hyperlocomotor effect is a useful model of addiction and craving. Particularly, cocaine sensitization in rats enhanced synaptic plasticity within the hippocampus, an important brain region for the associative learning processes underlying drug addiction. Nitric oxide (NO) is a neurotransmitter involved in both, hippocampal synaptic plasticity and cocaine sensitization. It has been previously demonstrated a key role of NOS-1/NO/sGC/cGMP signaling pathway in the development of cocaine sensitization and in the associated enhancement of hippocampal synaptic plasticity. The aim of the present investigation was to determine whether NOS-1 inhibition after development of cocaine sensitization was able to reverse it, and to characterize the involvement of the hippocampus in this phenomenon. Male Wistar rats were administered only with cocaine (15 mg/kg/day i.p.) for 5 days. Then, animals received 7-nitroindazole (NOS-1 inhibitor) either systemically for the next 5 days or a single intra-hippocampal administration. Development of sensitization and its expression after withdrawal were tested, as well as threshold for long-term potentiation in hippocampus, NOS-1, and CREB protein levels and gene expression. The results showed that NOS-1 protein levels and gene expression were increased only in sensitized animals as well as CREB gene expression. NOS-1 inhibition after sensitization reversed behavioral expression and the highest level of hippocampal synaptic plasticity. In conclusion, NO signaling within the hippocampus is critical for the development and expression of cocaine sensitization. Therefore, NOS-1 inhibition or NO signaling pathways interferences during short-term withdrawal after repeated cocaine administration may represent plausible pharmacological targets to prevent or reduce susceptibility to relapse.

The role of neuronal nitric oxide synthase in cocaine place preference and mu opioid receptor expression in the nucleus accumbens

RATIONALE

There is evidence that central mu opioid receptors (MORs) are implicated in several aspects of cocaine addiction, and that MOR expression is elevated by cocaine in vitro and in the nucleus accumbens (NAc) when administered in vivo.

OBJECTIVE

To understand the cellular mechanisms involved in regulating MOR expression, this study explored whether neuronal nitric oxide synthase (nNOS) modulates the neurochemical and behavioral effects of acute and repeated cocaine administration.

METHODS

Male Sprague-Dawley rats received a single cocaine injection (20 mg/kg, i.p.) in combination with the selective nNOS inhibitor 7-nitroindazole (7-NI) (0, 25, or 50 mg/kg, i.p.), and the expression of MOR and nNOS messenger RNA (mRNA) and protein levels in the NAc were measured. In a separate conditioned place preference (CPP) experiment, 7-NI (0, 25, or 50 mg/kg, i.p.) was administered prior to cocaine (0 or 20 mg/kg, i.p.) conditioning sessions, and levels of MOR and nNOS mRNA and protein in the NAc were measured following CPP test.

RESULTS

Acute cocaine administration significantly enhanced nNOS and MOR mRNA and protein expression in the NAc, and this increase in MOR expression was blocked by 7-NI. Furthermore, in 7-NI pre-treated rats, cocaine-induced CPP was not statistically significant and the increase in MOR mRNA expression in the NAc in these animals was attenuated.

CONCLUSIONS

These findings suggest that nNOS modulates MOR expression following acute cocaine administration, and that cocaine CPP and associated upregulation of MOR expression involve both nNOS-dependent and independent mechanisms. Elucidation of these molecular events may identify useful therapeutic target for cocaine addiction.

Cardiovascular Mitochondrial Dysfunction Induced by Cocaine: Biomarkers and Possible Beneficial Effects of Modulators of Oxidative Stress

Cocaine abuse has long been known to cause morbidity and mortality due to its cardiovascular toxic effects. The pathogenesis of the cardiovascular toxicity of cocaine use has been largely reviewed, and the most recent data indicate a fundamental role of oxidative stress in cocaine-induced cardiovascular toxicity, indicating that mitochondrial dysfunction is involved in the mechanisms of oxidative stress. The comprehension of the mechanisms involving mitochondrial dysfunction could help in selecting the most appropriate mitochondria injury biological marker, such as superoxide dismutase-2 activity and glutathionylated hemoglobin. The potential use of modulators of oxidative stress (mitoubiquinone, the short-chain quinone idebenone, and allopurinol) in the treatment of cocaine cardiotoxic effects is also suggested to promote further investigations on these potential mitochondria-targeted antioxidant strategies.

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.

N-acetylcysteine attenuates nicotine-induced kindling in female periadolescent rats

Kindling is a form of behavioral sensitization that is related to the progression of several neuropsychiatric disorders such as bipolar disorder. We recently demonstrated that female periadolescent rats are more vulnerable to nicotine (NIC)-induced kindling than their male counterparts. Furthermore, we evidenced that decreases in brain antioxidative defenses may contribute to this gender difference. Here we aimed to determine the preventive effects of the antioxidant N-acetyl cysteine (NAC) against NIC-kindling in female periadolescent rats. To do this female Wistar rats at postnatal day 30 received repeated injections of NIC 2mg/kg, i.p. every weekday for up to 19 days. NAC90, 180 or 270 mg/kg, i.p. was administered 30 min before NIC. The levels of glutathione (GSH), superoxide dismutase (SOD) activity, lipid peroxidation (LP) and nitrite were determined in the prefrontal cortex (PFC), hippocampus (HC) and striatum (ST). The development of kindling occurred at a median time of 16.5 days with 87.5% of NIC animals presenting stage 5 seizures in the last day of drug administration. NAC270 prevented the occurrence of kindling. NIC-kindled animals presented decreased levels of GSH and increased LP in the PFC, HC and ST, while SOD activity was decreased in the ST. NAC180 or 270 prevented the alterations in GSH induced by NIC, but only NAC270 prevented the alterations in LP. Nitrite levels increased in the ST of NAC270 pretreated NIC-kindled animals. Taken together we demonstrated that NAC presents anti-kindling effects in female animals partially through the restoration of oxidative alterations.

Tolerance to cocaine in brain stimulation reward following continuous cocaine infusions

This study examined tolerance to cocaine's threshold-lowering effect in brain stimulation reward (BSR) following continuous cocaine infusions and secondly, used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) to determine NO's involvement in the development of cocaine tolerance. Animals were continuously infused with saline or cocaine (30 mg/kg per day) via osmotic minipump for 14 days and injected daily with saline or L-NAME (30 mg/kg, i.p.) following BSR testing. Saline-treated animals continuously infused with saline showed stable BSR thresholds across the 14-day infusion period. Saline-treated animals continuously infused with cocaine showed markedly lowered BSR thresholds on Day 1 followed by a progressive increase in BSR thresholds across the infusion period - indicating the development of tolerance. L-NAME-treated animals continuously infused with cocaine showed stimulation thresholds that were not significantly different from saline-treated animals continuously infused with cocaine. A cocaine challenge injection (10 mg/kg, i.p.) administered 3 and again at 10 days following minipump removal revealed that saline-treated animals continuously infused with saline showed lowered BSR thresholds. Saline-treated animals continuously infused with cocaine displayed lowered BSR thresholds that were not significantly different from saline-infused animals. L-NAME treated animals continuously infused with cocaine showed higher BSR thresholds to a challenge 3 days following pump removal. However, stimulation thresholds for this group failed to reach statistical significance on both days (i.e., Days 3 and 10) following pump removal. Results showed that animals continuously infused with cocaine develop robust tolerance to cocaine's threshold-lowering effect during the 14-day infusion period. Tolerance to cocaine's threshold-lowering effect was short-lived and dissipated soon after minipump removal. L-NAME treatment failed to significantly alter the development of tolerance to cocaine's threshold-lowering suggesting that NO does not have a primary role in the development of cocaine tolerance.

Interactions between N-ethylmaleimide-sensitive factor and GluR2 in the nucleus accumbens contribute to the expression of locomotor sensitization to cocaine

Many studies have reported a withdrawal-dependent increase in synaptic AMPA receptor (AMPAR) levels in the nucleus accumbens (NAc) of cocaine-sensitized rats; however, the exact relationship between the expression of sensitization and altered AMPAR surface expression in the NAc has not yet been investigated. We demonstrated that the expression of behavioral sensitization was negatively controlled by N-ethylmaleimide-sensitive factor (NSF)-GluR2 interactions in the NAc. The upregulation of NSF-GluR2 interactions, which may be resulted by the increase in NSF S-nitrosylation after withdrawal from cocaine, was associated with the changes in the expression of behavioral sensitization. Disruption of NSF-GluR2 interactions in the NAc with a specific peptide, TAT-pep-R845A, increased the locomotor response of rats to cocaine by decreasing GluR2 surface insertion. In contrast, prevention of GluR2-containing AMPARs removal from synapses with Pep2-EVKI attenuated the expression of behavioral sensitization. Similarly, treatment with the nitric oxide donor, S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the expression of locomotor sensitization by promoting GluR2 surface expression. This effect was mediated by the binding of S-nitrosylated NSF to GluR2, which promoted the surface expression of AMPARs. Noticeably, exogenous injection of SNAP into NAc also attenuated the expression of cocaine-induced conditioned place preference. Thus, these results indicate that increased NSF-GluR2 interactions in the NAc after withdrawal from cocaine attenuated the expression of behavioral sensitization and serves as a negative regulatory mechanism in drug-exposed individuals.

Involvement of nNOS/NO/sGC/cGMP signaling pathway in cocaine sensitization and in the associated hippocampal alterations: does phosphodiesterase 5 inhibition help to drug vulnerability?

RATIONALE

Repeated cocaine administration induces behavioral sensitization in about 50 % of treated animals. Nitric oxide could be involved in the acquisition and maintenance of behavioral cocaine effects, probably by activation of neuronal nitric oxide synthase (nNOS)/NO/soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP) signaling pathway, since inhibition of the nNOS enzyme attenuates development of sensitization in rats. On the other hand, increased cGMP availability by phosphodiesterase 5 inhibitors has been correlated to the misuse and recreational use of these agents and also to the concomitant use with illicit drugs in humans. Hippocampus is an important brain region for conditioning to general context previously associated to drug availability, influencing drug-seeking behavior and sensitization. Moreover, cocaine and other drugs of abuse can affect the strength of glutamate synapses in this structure, lastly modifying neuronal activity in main regions of the reward circuitry.

OBJECTIVE

The objective of this study is to determine whether the pharmacological manipulation of nNOS/NO/sGC/cGMP signaling pathway altered changes induced by repeated cocaine exposure.

RESULTS

The present investigation showed a relationship between behavioral cocaine sensitization, reduced threshold to generate long-term potentiation (LTP) in hippocampal dentate gyrus, and increased nNOS activity in this structure. However, when nNOS or sGC were inhibited, the number of sensitized animals was reduced, and the threshold to generate LTP was increased. The opposite occurred when cGMP availability was increased.

CONCLUSION

We demonstrate a key role of the nNOS activity and NO/sGC/cGMP signaling pathway in the development of cocaine sensitization and in the associated enhancement of hippocampal synaptic transmission.

Electroacupuncture reduces cocaine-induced seizures and mortality in mice

The aims of this study were to characterize the protective profile of electroacupuncture (EA) on cocaine-induced seizures and mortality in mice. Mice were treated with EA (2 Hz, 50 Hz, and 100 Hz), or they underwent needle insertion without anesthesia at the Dazhui (GV14) and Baihui (GV20) acupoints before cocaine administration. EA at 50 Hz applied to GV14 and GV20 significantly reduced the seizure severity induced by a single dose of cocaine (75 mg/kg; i.p.). Furthermore, needle insertion into GV14 and GV20 and EA at 2 Hz and 50 Hz at both acupoints significantly reduced the mortality rate induced by a single lethal dose of cocaine (125 mg/kg; i.p.). In the sham control group, EA at 50 Hz applied to bilateral Tianzong (SI11) acupoints had no protective effects against cocaine. In addition, EA at 50 Hz applied to GV14 and GV20 failed to reduce the incidence of seizures and mortality induced by the local anesthetic procaine. In an immunohistochemistry study, EA (50 Hz) pretreatment at GV14 and GV20 decreased cocaine (75 mg/kg; i.p.)-induced c-Fos expression in the paraventricular thalamus. While the dopamine D₃ receptor antagonist, SB-277011-A (30 mg/kg; s.c), did not by itself affect cocaine-induced seizure severity, it prevented the effects of EA on cocaine-induced seizures. These results suggest that EA alleviates cocaine-induced seizures and mortality and that the dopamine D₃ receptor is involved, at least in part, in the anticonvulsant effects of EA in mice.

Effect of post-trial L-NAME administration on cocaine sensitization

This study determined if Nω-nitro-L-arginine methyl ester hydrochloride (L-NAME) administered after the cocaine-conditioning trial attenuated the development of sensitization to cocaine's locomotor-stimulating effect and secondly, determined if L-NAME blocked conditioned-locomotor activity (LMA) elicited by a saline-challenge injection. Results revealed that cocaine-injected animals (10 mg/kg, i.p.) showed enhanced locomotor activity across the three conditioning trials (all p's < .05). Cocaine-injected animals administered L-NAME (30 mg/kg, i.p.) after each conditioning trial showed a slight increase in cocaine-stimulated LMA from the first to the second conditioning trial (all p's < .05) and no further increases in LMA thereafter. A saline-challenge injection administered 72 hr after the last conditioning trial revealed that cocaine-injected animals displayed as much locomotor stimulation to a saline injection as they did during their initial exposure to cocaine on the first conditioning trial - indicating the development of cocaine-conditioned LMA. The present findings show that L-NAME administered after the cocaine-conditioning trial attenuates the development of sensitization to cocaine's locomotor-stimulating effect. The failure of L-NAME to block cocaine-conditioned LMA suggests that the pharmacological and conditioning mechanisms of sensitization can be dissociated. It is unlikely that L-NAME's effect is due to a sedative action produced by residual L-NAME since animals administered L-NAME (30 mg/kg, i.p.) for 10 consecutive days exhibited a similar responsiveness to a cocaine challenge administered 3 and 10 days following the termination of L-NAME administration. These data support a role for nitric oxide's involvement in the neuroadaptive responses that result from continued stimulant administration and demonstrate the importance of conditioned drug effects.

Differences in vulnerability to nicotine-induced kindling between female and male periadolescent rats

RATIONALE

It has recently been reported that chronic nicotine administration at subconvulsive doses causes seizures, a phenomenon referred to as kindling. Evidence points to the involvement of oxidative stress in pharmacological and electrical kindling, sex is known to influence the brain's response to nicotine.

OBJECTIVES

This study investigated the sex differences in vulnerability to nicotine-induced kindling and the involvement of oxidative stress in this phenomenon.

METHODS

Male and female periadolescent Wistar rats received repeated injections of a subconvulsive dose of nicotine (hemisulfate salt; 2 mg/kg, i.p.) every weekday for up to 25 days. To better understand the influence of oxidative stress in nicotine kindling, the antioxidant vitamin E (200 and 400 mg/kg, p.o.) was administered prior to nicotine administration. The levels of gluthatione (GSH), superoxide dismutase (SOD) activity, and lipid peroxidation were determined in the hippocampus (HC), prefrontal cortex (PFC), and striatum.

RESULTS

Female animals developed kindling more rapidly than male rats. In female rats, kindling was associated with decreases in antioxidant defenses, including GSH levels in the HC and striatum and SOD activity in the PFC and striatum, and increased lipid peroxidation in all brain areas studied. By contrast, male kindled animals presented only with a decrease in the GSH in the HC. Vitamin E prevented the occurrence of kindled seizures by 80 % and 75 % in male and female rats, respectively.

CONCLUSION

These novel findings indicate that female periadolescent rats develop nicotine-kindled seizures earlier than their male counterparts. Differences in the oxidative balance may be involved in this mechanism.

Inhibition of neuronal nitric oxide synthase prevents alterations in medial prefrontal cortex excitability induced by repeated cocaine administration

RATIONALE

The medial prefrontal cortex (mPFC), a forebrain region that regulates cognitive function and reward-motivated behaviors, has been implicated in the neuropathological mechanisms of drug addiction and withdrawal. In cocaine-abstinent human addicts, neuronal activity of the mPFC is increased in response to cocaine re-exposure or drug-associated cues. Additionally, repeated cocaine exposure alters the membrane properties and ion channel function of mPFC pyramidal neurons in drug-withdrawn rats, leading to an increased firing in response to excitatory stimuli. Nitric oxide (NO), a diffusible neuromodulator of neuronal excitability, may play a role in initiating and maintaining behavioral effects of psychostimulants. However, the role of NO in the mechanisms by which cocaine affects membrane excitability is not well clarified.

OBJECTIVES

In this study, we attempted to determine whether inhibition of neuronal nitric oxide synthase (nNOS) altered the changes induced by repeated cocaine exposure and withdrawal.

METHODS

Visualized whole-cell current clamp recordings in brain slices containing the mPFC of rats administered (once per day for 5 days) with either vehicle (10% Cremophor EL in saline 0.9%), cocaine (15 mg/kg, i.p.), or cocaine and the nNOS inhibitor 7-NI (50 mg/kg, i.p.) were employed.

RESULTS

We found that nNOS inhibition prevented cocaine sensitization and the increased membrane excitability of pyramidal cells, evidenced by an increased number of evoked spikes and reductions in inward rectification observed after short-term withdrawal from cocaine.

CONCLUSIONS

These findings suggest that NO plays an important role in chronic cocaine-induced deregulation of the mPFC activity that may contribute to the development of behavioral sensitization and cocaine withdrawal.

Asymmetric dimethylarginine (ADMA)--a modulator of nociception in opiate tolerance and addiction?

Nitric oxide (NO) is generated from l-arginine by NO synthases, of which three forms have been identified: endothelial, inducible and neuronal (eNOS, iNOS and nNOS, respectively). The l-arginine metabolite asymmetric dimethylarginine (ADMA) is a potent, noncompetitive inhibitor of nNOS, while its congener N(G)-monomethyl-l-arginine (l-NMMA) is a less potent, competitive inhibitor. In rat neurons large amounts of ADMA are found, suggesting its importance in modulating neuronal activity. Humans generate approximately 300mumol ( approximately 60mg) ADMA per day. It is released from myelin basic proteins that are highly expressed in neuronal tissue. ADMA is mainly degraded by the action of the enzyme dimethylarginine dimethylaminohydrolase (DDAH), which exists in two isoforms. DDAH1 is highly expressed in brain, suggesting specific function in this area. The presence of nNOS and DDAH1 in brain suggests that ADMA may have specific CNS activity and be more than an unregulated metabolite. Increased NO production-either prior to or concurrently with opioid administration-results in an enhanced rate and extent of development of tolerance to morphine in mice. NO produces an alteration in the mu-opioid receptor that increases constitutive receptor activity. It thereby reduces the ability of a selective mu-opioid agonist to activate the mu-opioid receptor; these in vitro molecular effects occur in a time course consistent with the in vivo development of antinociceptive tolerance in mice. Amongst many other synthetic NOS inhibitors of varying specificity, 7-nitroindazole (7-NI) has been shown to have a high affinity (IC(50) 0.71 microM) to nNOS. Selective blockade of nNOS by 7-NI attenuated morphine withdrawal in opiate dependent rats, suggesting nNOS as a viable target for development of pharmacotherapies. We hypothesize that, by inhibiting nNOS and reducing NO levels, ADMA may decrease mu-opiate receptor constitutive activity, resulting in alteration of the analgesic dose-response curve of morphine.

Memory reconsolidation of cocaine-associated context requires nitric oxide signaling

Recent studies suggest that long-term memory (LTM) is labile because retrieval of such memories may undergo a reconsolidation process which is vulnerable to disruption. Nitric oxide (NO) is a retrograde messenger involved in synaptic plasticity and LTM. In the present study the role of NO in reconsolidation of LTM of cocaine-associate context was investigated in wild type (WT) and neuronal nitric oxide synthase (nNOS) deficient mice (knockout; KO). LTM of cocaine-associated context was established in both WT and nNOS KO mice by conditioned place preference learning. Subsequently, the retrieval of place preference in WT mice was challenged by either saline or the selective nNOS inhibitor 7-nitroindazole, and retrieval of place preference in KO mice was challenged by either saline or the NO-donor molsidomine. Results suggest that in the absence of nNOS activity, particularly during the reconsolidation phase, LTM of cocaine-associated context is extinguished.

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.

Differential effects of morphine- and cocaine-induced nNOS immunoreactivity in the dentate gyrus of hippocampus of mice lacking mu-opioid receptors

This study investigated the expression of nNOS after repeated morphine or cocaine administration in order to determine if nNOS (neuronal nitric oxide synthase) is involved in the morphine- or cocaine-induced behavioral sensitization in mu-opioid receptor knockout (MOR(-/-)) mice. Higher numbers of nNOS-positive cells were observed in the dentate gyrus of the hippocampus (DG) of the wild-type (MOR(+/+)) mice repeatedly treated with either morphine or cocaine than in the saline treated MOR(+/+) mice (morphine, +122%; cocaine, +82%). Moreover, the MOR(-/-) mice also showed significantly higher morphine- or cocaine-induced nNOS expression levels in the DG than in the saline treated MOR(+/+) mice (morphine, +234%; cocaine, +54%). The MOR(-/-) mice showed a significantly higher morphine-induced nNOS expression level (+103%) or a lower cocaine-induced nNOS expression level (+38%) in the DG than in the morphine- or cocaine-treated MOR(+/+) mice. These results suggest that morphine and cocaine sensitization is differentially regulated by the mu-opioid receptors in MOR(-/-) mice via the nNOS systems in the DG.

Pharmacological and histological characterisation of nicotine-kindled seizures in mice

The present study reports that it is possible to induce kindling by repeated injections of nicotine. The newly characterised nicotine-kindling model was compared with that of pentylenetetrazole (PTZ) kindling. Mice were kindled by repeated injection of PTZ (37 mg/kg), or nicotine (2.3 mg/kg), and the effect of the anti-epileptic drugs (AED) levetiracetam (LEV), tiagabine (TGB) and phenytoin (PHT) on seizures in kindled and naive mice were investigated. C-Fos immunoreactivity (Fos IR) was used to investigate differences in neuronal activity pattern between PTZ-, nicotine kindled and naive animals. PTZ kindled animals mainly showed increased Fos IR in limbic regions, whereas Fos IR in nicotine kindled animals was increased in the entorhinal cortex, medial habenula and the compact part of substantia nigra. Fully kindled PTZ-induced seizures were inhibited by LEV (ED50=13.6+/-7.8 mg/kg), TGB (ED50=0.3+/-0.04 mg/kg) but not PHT (ED50>40 mg/kg) whereas fully kindled nicotine-induced seizures were inhibited by LEV (ED50=1.4+/-0.4 mg/kg), TGB (ED50=0.3+/-0.06 mg/kg) and PHT (ED50=9.2+/-2.4 mg/kg). These differences in efficacy of AEDs were not due to changes in plasma levels in the various models. In conclusion, repeated administration of nicotine can induce a kindling-like phenomenon and the model showed significantly different Fos IR pattern and pharmacology to that of PTZ kindling.

Fenfluramine-induced serotonergic neurotoxicity in mice: lack of neuroprotection by inhibition/ablation of nNOS

Previous studies have implicated a role for nitric oxide (NO) and peroxynitrite in methamphetamine-induced dopaminergic neurotoxicity. The present study was undertaken to investigate whether NO is involved in serotonergic neurotoxicity caused by fenfluramine. In the first experiment, the effect of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI; 25 mg/kg x 4) on fenfluramine (25 mg/kg x 4)-induced serotonergic neurotoxicity in Swiss Webster mice was investigated. In the second experiment, the effect of fenfluramine (25 mg/kg x 4) on nNOS (-/-) and wild-type (WT) mice was investigated. Fenfluramine induced hypothermia in all three mouse strains, and 7-NI had no thermoregulatory effect. Selective depletion of 5-HT and 5-HT transporter binding sites in the striatum, frontal cortex and hippocampus in all three mouse strains was observed, with no evidence of dopaminergic neurotoxicity. In the first experiment, 7-NI did not attenuate serotonergic neurotoxicity in Swiss Webster mice. In the second experiment, nNOS(-/-) and WT mice were equally sensitive to serotonergic neurotoxicity. These findings suggest that NO and peroxynitrite do not mediate fenfluramine-induced serotonergic neurotoxicity, and that NO is a selective mediator of amphetamines-induced dopaminergic neurotoxicity.

Inhibition of neuronal nitric oxide synthase suppresses the maintenance but not the induction of psychomotor sensitization to MDMA (‘Ecstasy’) and p-chloroamphetamine in mice

Repeated exposure to psychostimulants such as cocaine and amphetamines results in behavioral sensitization, a paradigm thought to be relevant to drug craving and addiction in humans. We have previously shown that the induction, expression, and maintenance of psychomotor sensitization to cocaine, methamphetamine, and methylphenidate (indirect dopamine agonists) are blocked by co-administration of the neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI). In the present study, we investigated the effects of 7-NI on the induction, expression, and maintenance of psychomotor sensitization to 3,4-methylenedioxymethamphetamine (MDMA; 'Ecstasy') and p-chloroamphetamine (PCA). The following observations are reported: (a) Repeated administration of MDMA (10 mg/kg) and PCA (5 mg/kg) to Swiss Webster mice for six consecutive days caused a 3-fold increase in the psychomotor stimulating effect of the drugs on day 6 compared to day 1. (b) Pretreatment with 7-NI (25 mg/kg) did not affect the induction and expression of sensitization to MDMA and PCA. (c) Pretreatment with 7-NI did, however, suppress the enduring sensitized response to challenge injections of MDMA and PCA which was observed in mice pretreated with vehicle instead of 7-NI. (d) Unlike other psychostimulants, MDMA and PCA treatment did not produce conditioned (context-dependent) hyperlocomotion. These findings, coupled with our previous studies, suggest the following: (a) The induction and expression of psychomotor sensitization to MDMA and PCA are independent of nNOS activity and involve primarily serotonergic transmission. (b) The maintenance of psychomotor sensitization is dependent on intact nNOS activity and involves primarily dopaminergic transmission.

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.

NMDA sensitization and stimulation by peroxynitrite, nitric oxide, and organic solvents as the mechanism of chemical sensitivity in multiple chemical sensitivity

Multiple chemical sensitivity (MCS) is a condition where previous exposure to hydrophobic organic solvents or pesticides appears to render people hypersensitive to a wide range of chemicals, including organic solvents. The hypersensitivity is often exquisite, with MCS individuals showing sensitivity that appears to be at least two orders of magnitude greater than that of normal individuals. This paper presents a plausible set of interacting mechanisms to explain such heightened sensitivity. It is based on two earlier theories of MCS: the elevated nitric oxide/peroxynitrite theory and the neural sensitization theory. It is also based on evidence implicating excessive NMDA activity in MCS. Four sensitization mechanisms are proposed to act synergistically, each based on known physiological mechanisms: Nitric oxide-mediated stimulation of neurotransmitter (glutamate) release; peroxynitrite-mediated ATP depletion and consequent hypersensitivity of NMDA receptors; peroxynitrite-mediated increased permeability of the blood-brain barrier, producing increased accessibility of organic chemicals to the central nervous system; and nitric oxide inhibition of cytochrome P450 metabolism. Evidence for each of these mechanisms, which may also be involved in Parkinson's disease, is reviewed. These interacting mechanisms provide explanations for diverse aspects of MCS and a framework for hypothesis-driven MCS research.

Effect of the neuronal nitric oxide synthase inhibitor 7-nitroindazole on methylphenidate-induced hyperlocomotion in mice

The psychostimulant methylphenidate (MPD) is used for the treatment of attention-deficit/hyperactivity disorders (ADHD) in adolescents. In the present study we investigated the effect of repeated administration of a low (10 mg/kg) and a high (40 mg/kg) dose of MPD on the locomotor activity of Swiss Webster mice, and the influence of inhibition of the neuronal nitric oxide synthase (nNOS) on MPD-induced hyperlocomotion. In the first experiment, mice were administered either vehicle or the nNOS inhibitor 7-nitroindazole (7-NI; 25 mg/kg), prior to the administration of MPD (10 or 40 mg/kg), for five consecutive days; injections were paired with the test cage ('novel environment') on days 1 and 5. A challenge injection of MPD (10 or 40 mg/kg), given after a 10-day drug-free period, resulted in sensitization to the motor stimulating effect of the low dose of MPD but tolerance to the high dose of MPD. 7-NI blocked the induction of sensitization but had no effect on the development of tolerance. The place-dependent-hyperlocomotion (e.g. conditioning) that developed after the administration of either the low or high dose of MPD was blocked by pretreatment with the nNOS inhibitor. In the second experiment, mice were administered MPD (10 or 40 mg/kg; 5 days) in their home cage and after a 10-day drug-free period were challenged with either vehicle/MPD or 7-NI/MPD. The low dose of MPD elicited a sensitized response that was blocked by the co-administration of 7-NI. The high dose of MPD produced neither sensitization nor tolerance; 7-NI did not affect the response to the high dose of MPD. These findings suggest: (a) MPD-induced sensitization and tolerance are dependent on the dose of the drug and the environment where the drug is delivered (home cage versus test cage); (b) context-dependent hyperlocomotion developed in the absence of a sensitized response to the drug; (c) nNOS is involved in the induction and expression of sensitization to MPD as well as in the conditioned locomotion produced by the drug; (d) no involvement of nNOS in the effects of a high dose of MPD was detected.

Blockade of nitric oxide synthesis reduces responding for cocaine self-administration during extinction and reinstatement

Nitric oxide is a gaseous neurotransmitter that plays a significant role in various forms of synaptic plasticity and may play a role in the behavioral effects of psychostimulant drugs and in cocaine addiction. The course of drug addiction consists of different phases. Relapse into drug-seeking behavior following a period of abstinence is believed to represent one of the major factors leading to the perpetuation of the addictive cycle. In this respect, experimental extinction procedures provide a measure of the motivational properties of drugs as reflected by the persistence of drug-seeking behavior in the absence of the drug and by the reinstatement of responding by non-contingent drug administration. Pretreatment with the nitric oxide synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME, 50 mg/kg IP twice daily for 4 days) impaired responding for cocaine self-administration when the drug was available and the increase of drug-seeking behavior upon abrupt cessation of cocaine availability observed in control rats was significantly reduced after treatment with L-NAME. In addition, the priming effect of a non-contingent injection of cocaine on extinguished cocaine self-administration was also diminished by the same treatment. The acquisition of cocaine self-administration, in contrast, was not affected by treatment with L-NAME. These observations lend further support to the hypothesis of the involvement of nitric oxide in cocaine addiction and extend previous findings to components of the cocaine addictive cycle associated with relapse.

nNOS inhibitors attenuate methamphetamine-induced dopaminergic neurotoxicity but not hyperthermia in mice

Methamphetamine (METH)-induced dopaminergic neurotoxicity is associated with hyperthermia. We investigated the effect of several neuronal nitric oxide synthase (nNOS) inhibitors on METH-induced hyperthermia and striatal dopaminergic neurotoxicity. Administration of METH (5 mg/kg; q. 3 h x 3) to Swiss Webster mice produced marked hyperthermia and 50-60% depletion of striatal dopaminergic markers 72 h after METH administration. Pretreatment with the nNOS inhibitors S-methylthiocitrulline (SMTC; 10 mg/kg) or 3-bromo-7-nitroindazole (3-Br-7-NI; 20 mg/kg) before each METH injection did not affect the persistent hyperthermia produced by METH, but afforded protection against the depletion of dopaminergic markers. A low dose (25 mg/kg) of the nNOS inhibitor 7-nitroindazole (7-NI) did not affect METH-induced hyperthermia, but a high dose (50 mg/kg) produced significant hypothermia. These findings indicate that low dose of selective nNOS inhibitors protect against METH-induced neurotoxicity with no effect on body temperature and support the hypothesis that nitric oxide (NO) and peroxynitrite have a major role in METH-induced dopaminergic neurotoxicity.

The dopamine antagonist sch 23390 reverses dizocilpine-induced blockade of cocaine sensitization

The present work examined the effects of pre-treatment with Sch 23390, a selective D(1) receptor antagonist, on the dizocilpine-induced blockade of sensitization to the locomotor-stimulating effect of cocaine. Rats were given either cocaine [15mgkg(-1)day(-1), intraperitoneally (i.p.)] from day 1 to day 5 (cocaine-experienced rats) or vehicle (cocaine-naïve rats). From day 6 to day 15, animals remained drug-free in their home cages. On day 16 rats received a challenge injection of cocaine (15mgkg(-1)) or vehicle, and were tested for sensitization to the locomotor-stimulating effect of cocaine. In cocaine-naïve rats the acute effect of cocaine was a 1.5 times increase in locomotor activity. In cocaine-experienced rats, the acute effects of cocaine were considerably more pronounced than in cocaine-naïve rats; the stimulating effect of cocaine in these animals was a doubling in locomotor activity. In cocaine-naïve rats, pre-treatment with dizocilpine (100microgkg(-1)), Sch 23390 (100microgkg(-1)) or a combination of the two drugs from day 1 to day 5 changed neither spontaneous locomotor activity nor cocaine stimulant activity. By contrast, cocaine-experienced animals that had been given 100microgkg(-1) dizocilpine from day 1 to day 5 failed to show the increase in locomotor activity when challenged with cocaine on day 16. Pre-treatment with Sch 23390 (100microgkg(-1)day(-1), i.p.) from day 1 to day 5 was found to prevent completely the cocaine anti-sensitization properties of 100microgkg(-1) dizocilpine, but failed to prevent cocaine sensitization. On the other hand, horizontal activity in cocaine-experienced rats that had been given dizocilpine (100microgkg(-1)) 15min before cocaine challenge on day 16 was higher than in corresponding controls. It is concluded that prevention of cocaine sensitization by dizocilpine may be related to the events set into motion by the NMDA antagonist at the level of dopaminergic transmission involving D(1) receptors.

Role of peroxynitrite in methamphetamine-induced dopaminergic neurotoxicity and sensitization in mice

Abstract Methamphetamine (METH)-induced dopaminergic neurotoxicity is thought to be associated with the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Recently, we have reported that copper/zinc(CuZn)-superoxide dismutase transgenic mice are resistant to METH-induced neurotoxicity. In the present study, we examined the role of the neuronal nitric oxide synthase (nNOS), susceptibility of nNOS knockout (KO) mice and sensitization to psychostimulants after neurotoxic doses of METH. Male SwissWebster mice were treated with or without 7-nitroindazole (7-NI) along with METH (5 mg/kg,ip,q 3h x 3) and were sacrificed 72 h after the last METH injection. Dopamine (DA) and dopamine transporter (DAT) binding sites were determined in striatum from saline and METH-treated animals. 7-NI completely protected against the depletion of DA, and DAT in striatum. In follow-up experiments nNOS KO mice along with appropriate control (C57BL/6N, SV129 and B6JSV129) mice were treated with METH (5 mg/kg,ip, q 3h x 3) and were sacrificed 72 h after dosing. This schedule of METH administrations resulted in only 10-20% decrease in tissue content of DA and no apparent change in the number of DAT binding sites in nNOS KO mice. However, this regime of METH resulted in a significant decrease in the content of DA as well as DAT binding sites in the wild-type animals. Pre-exposure to single or multiple doses of METH resulted in a marked locomotion sensitization in response to METH. However, the nNOS KO mice show no sensitization in response to METH after single or multiple injections of METH. Therefore, these studies strongly suggest the role of peroxynitrite, nNOS and DA system in METH-induced neurotoxicity and behavioral sensitization.

7-Nitroindazole blocks nicotine-induced conditioned place preference but not LiCl-induced conditioned place aversion

We have shown previously that the neuronal nitric oxide synthase (nNOS) is involved in the rewarding effect of cocaine as determined by the conditioned place preference (CPP) paradigm. In the present study we investigated the effect of the nNOS inhibitor 7-nitroindazole (7-NI) on nicotine-induced CPP and LiCl-induced conditioned place aversion (CPA) in Swiss Webster mice. Mice treated with nicotine (0.5 mg/kg) or saline every other day for 8 days (four drug and four saline sessions) developed CPP that was completely blocked by pretreatment with 7-NI (25 mg/kg). Mice treated with LiCl (150 mg/kg) developed marked aversion to the LiCl-paired compartment. LiCl-induced CPA was not affected by the pretreatment with 7-NI. These findings suggest that nitric oxide (NO) plays a role in the acquisition of reward but not of aversion and that the blockade of nicotine-induced CPP is probably not due to impairment of learning and memory.

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.

Influence of the non-competitive NMDA receptor antagonist MK-801 on 2-deoxy-D-glucose-induced hyperphagia in rats

The effects of the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5, 10-imine (MK-801) on 2-deoxy-D-glucose-induced hyperphagia were investigated in rats. MK-801 significantly increased 2-deoxy-D-glucose-elicited eating. The facilitating effects of MK-801 on 2-deoxy-D-glucose-elicited feeding were not affected by coadministration of a nitric oxide (NO) precursor, L-arginine. Because NO synthase inhibitors inhibit 2-deoxy-D-glucose-induced hyperphagia and activation of the NMDA receptor leads to NO formation, our results suggest that blockade of the NMDA receptor increases 2-deoxy-D-glucose-induced hyperphagia, which is unrelated to inhibition of NO, and that NMDA receptors may play a role in satiety.

A neuronal nitric oxide synthase inhibitor 7-nitroindazole reduces the 5-HT1A receptor against 8-OH-DPAT-elicited hyperphagia in rats

The effects of the neuronal nitric oxide (NO) synthase inhibitor 7-nitroindazole on 8-hydroxy-2-di-n-(propylamino)tetralin (8-OH-DPAT)-induced hyperphagia, which is mediated by the 5-HT1A autoreceptor, were investigated in rats. 7-Nitroindazole suppressed 8-OH-DPAT-elicited increases in food intake. The inhibitory effects of 7-nitroindazole on 8-OH-DPAT-induced feeding were prevented by the NO precursor L-arginine. Although 8-OH-DPAT decreases 5-hydroxytryptamine (5-HT) synthesis, 7-nitroindazole did not reverse the 8-OH-DPAT-elicited decrease in 5-HT synthesis. Therefore, these results indicate that NO formed in the brain is involved in 8-OH-DPAT-induced hyperphagia and that the hypophagic effects of 7-nitroindazole are not dependent on 5-HT synthesis.

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 excitatory amino acids in behavioral sensitization to psychomotor stimulants

Behavioral sensitization refers to the progressive augmentation of behavioral responses to psychomotor stimulants that develops during their repeated administration and persists even after long periods of withdrawal. It provides an animal model for the intensification of drug craving believed to underlie addiction in humans. Mechanistic similarities between sensitization and other forms of neuronal plasticity were first suggested on the basis of the ability of N-methyl-D-aspartate (NMDA) receptor antagonists to prevent the development of sensitization [Karler, R., Calder, L. D., Chaudhry, I. A. and Turkanis, S. A. (1989) Blockade of "reverse tolerance" to cocaine and amphetamine by MK-801. Life Sci., 45, 599-606]. This article will review the large number of subsequent studies addressing: (1) the roles of NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors in the development and expression of behavioral sensitization, (2) excitatory amino acids (EAAs) and the role of conditioning in sensitization, (3) controversies regarding EAA involvement in behavioral sensitization based on studies with MK-801, (4) the effects of acute and repeated stimulant administration on EAA neurochemistry and EAA receptor expression, and (5) the neuroanatomy of EAA involvement in sensitization. To summarize, NMDA, AMPA metabotropic glutamate receptors all participate in the development of sensitization, while maintenance of the sensitized state involves alterations in neurochemical measures of EAA transmission as well as in the expression and sensitivity of AMPA and NMDA receptors. While behavioral sensitization likely involves complex neuronal circuits, with EAAs participating at several points within this circuitry, EAA projections originating in prefrontal cortex may play a particularly important role in the development of sensitization, perhaps via their regulatory effects on midbrain dopamine neurons. The review concludes by critically evaluating various hypotheses to account for EAA involvement in the development of behavioral sensitization, and considering the question of whether EAA receptors are involved in mediating the rewarding effects of psychomotor stimulants and sensitization of such rewarding effects.