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

Exploring the progression of drug dependence in a methamphetamine self-administration rat model through targeted and non-targeted metabolomics analyses

Persistent neurochemical and biological disturbances resulting from repeated cycles of drug reward, withdrawal, and relapse contribute to drug dependence. Methamphetamine (MA) is a psychostimulant with substantial abuse potential and neurotoxic effects, primarily affecting monoamine neurotransmitter systems in the brain. In this study, we aimed to explore the progression of drug dependence in rat models of MA self-administration, extinction, and reinstatement through targeted and non-targeted metabolomics analyses. Metabolic profiles were examined in rat plasma during the following phases: after 16 days of MA self-administration (Group M); after 16 days of self-administration followed by 14 days of extinction (Group MS); and after self-administration and extinction followed by a reinstatement injection of MA (Group MSM). Each group of MA self-administration, extinction, and reinstatement induces distinct changes in the metabolic pathways, particularly those related to the TCA cycle, arginine and proline metabolism, and arginine biosynthesis. Additionally, the downregulation of glycerophospholipids and sphingomyelins in Group MSM suggests their potential role in MA reinstatement. These alterations may signify the progressive deterioration of these metabolic pathways, possibly contributing to drug dependence following repeated cycles of drug reward, withdrawal, and relapse. These results provide valuable insights into the metabolic changes associated with MA use at various stages, potentially facilitating the discovery of early diagnostic biomarkers and therapeutic targets for MA use disorders.

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Nitric oxide and l-arginine have mixed effects on mammalian feeding in condition of a high motivation to feed

Feeding inhibition caused by satiation in rats is partially mediated by the unconventional neurotransmitter nitric oxide (NO). Thus, in satiated rats blocking NO production increases feeding, and treatment with the NO precursor l-arginine or with an NO donor reduces feeding beyond that caused by satiation. Do NO and l-arginine also inhibit feeding when feeding motivation is high? When feeding motivation in satiated animals was hedonically increased by offering a highly attractive food, blocking NO production reduced the quantity eaten, rather than increasing it, indicating that hedonic aspects of food are partially mediated by NO. Increasing NO via an NO donor or l-arginine did not further increase the quantity eaten, indicating a ceiling effect. The NO donor, but not l-arginine, also decreased some motivation-dependent parameters of feeding. When feeding motivation was increased by hunger, quantities of food eaten were unaffected by an NO donor, blocker or precursor, with only the blocker of NO production affecting feeding patterning. We also examined effects on feeding of dissolving l-arginine in drinking water over 3 weeks. Chronic l-arginine administration had different effects during the first and in subsequent weeks, increasing feeding at first, but not later. The data indicate that NO has complex, state dependent effects on both the quantity of food eaten, and on patterns of feeding, probably reflecting different sites and mechanisms of action in the nervous system.

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 Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Estrous cycle and sex affect cocaine-induced behavioural changes in CD1 mice

RATIONALE

Several findings on sex differences in cocaine response suggest a role for hormonal milieu in modulating the subjective effects of cocaine. Nitric oxide (NO) has been involved in the neurochemical, hormonal, and behavioral changes related to stress and anxiety. Within the brain, the anteroventral subdivision of the medial amygdala (MeAV) is an important area involved in processing emotional responses such as anxiety and a high density of NO-producing neurons is observed in this area.

OBJECTIVES

In this study, we hypothesize the possibility of sex/hormonal differences in response to cocaine and that these differences may reflect a change in the MeAV nitrergic system. We have examined cocaine's acute effects on nicotinamide adenine dinucleotide phosphate diaphorase (nadph-d) expression, as well as its effect on motor activity and anxiety in male and estrus and diestrus females.

RESULTS

Our results show that acute cocaine administration produces an increase in both anxiety behaviors and nadph-d expression in the MeAV. Male and diestrus female mice were more susceptible to these effects of cocaine than estrus female mice in which no differences were detected. In addition, we examined individual differences in male and female mice responding to intravenous cocaine reinforcement in a self-administration paradigm. Female mice acquired cocaine self-administration at a faster rate than males and showed a higher motivation to self-administer cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Our data suggest a complex interaction between hormonal milieu and the behavioral and reinforcing effects of cocaine.

Sauchinone blocks methamphetamine-induced hyperlocomotion and place preference in mice

Sauchinone is a phytochemical known as a nitric oxide (NO) inhibitor. NO is a kind of neurotransmitter and involved in psychotic effect of abuse drug. In present, we carried out a study on the effect of sauchinone on methamphetamine-induced alteration of behavior in mice. Locomotory activity and conditioned place preference (CPP) were used to evaluate behavioral changes. As a result, sauchinone inhibited the methamphetamine-induced hyperlocomotion in dose-dependent manner, whereas sauchinone had no effect on normal locomotory activity. The inhibitory effect of sauchinone on methamphetamine-induced hyperlocomotion was reversed by treatment of molsidomine, a NO donor. Sauchinone also significantly blocked the acquisition and expression of CPP induced by methamphetamine in mouse. However, it did not produce place preference or place aversion, when it was treated alone in animals. Taken together, sauchinone blocked drug reward-related behavior as well as acute hyperlocomotion induced by methamphetamine treatment.

AMPA receptor upregulation in the nucleus accumbens shell of cocaine-sensitized rats depends upon S-nitrosylation of stargazin

Behavioral sensitization to cocaine is associated with increased AMPA receptor (AMPAR) surface expression in the nucleus accumbens (NAc). This upregulation is withdrawal-dependent, as it is not detected on withdrawal day (WD) 1, but is observed on WD7-21. Its underlying mechanisms have not been clearly established. Nitric oxide (NO) regulates AMPAR trafficking in the brain by S-nitrosylation of the AMPAR auxiliary subunit, stargazin, leading to increased AMPAR surface expression. Our goal was to determine if stargazin S-nitrosylation contributes to AMPAR upregulation during sensitization. First, we measured stargazin S-nitrosylation in NAc core and shell subregions on WD14 after 8 daily injections of saline or 15 mg/kg cocaine. Stargazin S-nitrosylation was markedly increased in NAc shell but not core. To determine if this is associated with AMPAR upregulation, rats received 8 cocaine or saline injections followed by twice-daily treatments with vehicle or the nitric oxide synthase inhibitor l-NAME (50 mg/kg) on WD1-6, the time when AMPAR upregulation is developing in cocaine-exposed rats. Cocaine/vehicle rats showed elevated stargazin and GluA1 surface expression on WD7 compared to saline/vehicle rats; the GluA1 increase was more robust in core, while stargazin increased more robustly in shell. These effects of cocaine were attenuated in shell but not core when cocaine injections were followed by l-NAME treatment on WD1-6. Together, these results indicate that elevated S-nitrosylation of stargazin contributes to AMPAR upregulation during sensitization selectively in the NAc shell. It is possible that AMPAR upregulation in core involves a different TARP, γ4, which also upregulates in the NAc of sensitized rats.

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.

Acupuncture suppresses reinstatement of morphine-seeking behavior induced by a complex cue in rats

Morphine causes physical and psychological dependence for individuals after repeated-use. Above all, our previous study showed that acupuncture attenuated reinstatement of morphine-seeking behavior induced by pharmacological cue. In this study, we investigated whether acupuncture could suppress the reinstatement of morphine-seeking behavior induced by the combination of environmental and pharmacological cues and the possible neuronal involvement. Male Sprague-Dawley rats were trained to self-administer morphine (1.0 mg/kg) for 3 weeks. Following the withdrawal phase (7 days), the effects of acupuncture on reinstatement of morphine-seeking behavior were investigated. For the investigation of neuronal involvement, the GABAA receptor antagonist bicuculline and the GABAB receptor antagonist SCH 50911 were pre-treated. Morphine-seeking behavior induced by combination of re-exposure to the operant chamber and morphine injection was suppressed perfectly by acupuncture at SI5, but not at the control acupoint LI5 and this effect was blocked by pre-treatment with the GABA receptor antagonists. This study suggests that acupuncture at SI5 can be considered as a predominant therapy for the reinstatement of morphine-seeking behavior in humans.

The effects of vasopressin and oxytocin on methamphetamine-induced place preference behaviour in rats

Methamphetamine is a highly addictive stimulant drug whose illicit use and resultant addiction has become an alarming global phenomenon. The mesolimbic dopaminergic pathway has been shown to be fundamental to the establishment of addictive behaviour. This pathway, as part of the reward system of the brain, has also been shown to be important in classical conditioning, which is a learnt response. Within the modulation of learning and memory, the neurohypophyseal hormones vasopressin and oxytocin have been reported to play a vital role, with vasopressin exerting a long- term facilitatory effect and oxytocin exerting an inhibitory effect. Therefore we adopted a conditioned place preference model to investigate whether vasopressin V1b receptor antagonist SSR 149415 or oxytocin treatment would cause a decrease in the seeking behaviour in a reinstatement paradigm. Behavioural findings indicated that methamphetamine induced a change in the place preference in the majority of our animals. This change in place preference was not seen when vasopressin was administered during the extinction phase. On the other hand the methamphetamine-induced change in place preference was enhanced during the reinstatement phase in the animals that were treated with oxytocin. Striatal dopamine levels were determined, as methamphetamine is known to increase dopamine transmission in this area. Significant changes in dopamine levels were observed in some of our animals. Rats that received both methamphetamine and oxytocin had significantly higher striatal dopamine than those that received oxytocin alone. Western blot analysis for hippocampal cyclic AMP response element binding protein (CREB) was also conducted as a possible indicator of glutamatergic NMDA receptor activity, a pathway that is important for learning and memory. The Western blot analysis showed no changes in hippocampal pCREB expression. Overall our data led us to conclude that methamphetamine treatment can change place preference behaviour in rats and that this change may be partially restored by vasopressin antagonism, but exaggerated by oxytocin.

Effects of acupuncture on stress-induced relapse to cocaine-seeking in rats

RATIONALE

Cocaine addiction is associated with high rates of relapse, and stress has been identified as a major risk factor. We have previously demonstrated that acupuncture reduces drug self-administration and dopamine release in the nucleus accumbens (NAc), a brain structure implicated in stress-induced reinstatement of drug-seeking behavior.

OBJECTIVE

This study was conducted to investigate the effects of acupuncture on footshock-induced reinstatement of cocaine-seeking and the expression of c-Fos and the transcription factor cAMP response element-binding protein (CREB) in the NAc, used as markers of neuronal activation in conditions of stress-induced reinstatement to cocaine.

METHODS

Male Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg) for 14 days, followed by extinction and then footshock stress. Acupuncture was applied at bilateral Shenmen (HT7) points for 1 min after footshock stress.

RESULTS AND CONCLUSIONS

Acute footshock stress reinstated cocaine-seeking behavior and enhanced c-Fos expression and phosphorylated CREB (pCREB) activation in the NAc shell in cocaine pre-exposed rats. On the other hand, acupuncture at HT7, but not at control point (LI5), markedly reduced reinstatement of cocaine-seeking (86.5 % inhibition vs. control value), c-Fos expression (81.7% inhibition), and pCREB activation (79.3% inhibition) in the NAc shell. These results suggest that acupuncture attenuates stress-induced relapse by regulating neuronal activation in the NAc shell.

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.

Tempol attenuates cocaine-induced death of PC12 cells through decreased oxidative damage

The association between cocaine administration and induction of oxidative stress in different brain regions suggests that oxidative damage is an important factor participating in cocaine disruption of normal central nervous system functions. In order to deal with this topic, brain penetrating exogenous antioxidants were suggested as a tool to prevent cocaine-induced oxidative damage and behavioral changes. Lately, we have shown that Tempol, a stable nitroxide radical reduced oxidative damage and attenuated the development and expression of cocaine psychomotor sensitization. To examine whether nitroxides, represented by Tempol, can exhibit protective effects against cocaine-induced cell death and to elucidate the molecular mechanism of cocaine-induced oxidative damage, we used the well established PC12 cell line model. The results showed that (1) cocaine induced cell death in a dose-dependent manner (2) and that it was reduced significantly by the stable nitroxide radical Tempol. Furthermore, (3) Tempol significantly inhibited oxidative damage induced by cocaine as reflected by mitochondrial superoxide radical and peroxide enhancement. Finally, (4) Tempol restored the total scavenging capacity which was reduced by cocaine in PC12 cells. Cumulatively, these results suggest that nitroxides such as Tempol can attenuate oxidative damage and cell death induced by cocaine and that PC12 cells can be used as an in vitro model to further investigate the precise molecular mechanism of these compounds.

Anti-relapse medications: preclinical models for drug addiction treatment

Addiction is a chronic relapsing brain disease and treatment of relapse to drug-seeking is considered the most challenging part of treating addictive disorders. Relapse can be modeled in laboratory animals using reinstatement paradigms, whereby behavioral responding for a drug is extinguished and then reinstated by different trigger factors, such as environmental cues or stress. In this review, we first describe currently used animal models of relapse, different relapse triggering factors, and the validity of this model to assess relapse in humans. We further summarize the growing body of pharmacological interventions that have shown some promise in treating relapse to psychostimulant addiction. Moreover, we present an overview on the drugs tested in cocaine or methamphetamine addicts and examine the overlap of existing preclinical and clinical data. Finally, based on recent advances in our understanding of the neurobiology of relapse and published preclinical data, we highlight the most promising areas for future anti-relapse medication development.

Acute cocaine administration increases NO efflux in the rat prefrontal cortex via a neuronal NOS-dependent mechanism

An understanding of the neurochemical changes occurring following exposure to psychostimulants such as cocaine is critical for the development of novel pharmacotherapies aimed at disrupting the addictive cycle. It is well established that the acute effects of cocaine associated with drug-induced blockade of dopamine (DA) reuptake processes occur in reward-related areas of the brain including the medial prefrontal cortex (mPFC). Considerable evidence has accumulated indicating that the interaction between DA, glutamate, and nitric oxide (NO) is likely to play a critical role in the neuroplastic changes associated with psychostimulant exposure. However, the potential impact of cocaine on NO synthase (NOS) activity in the mPFC has not been examined. In this study, NO efflux was measured in the mPFC of anesthetized male rats using a NO-selective electrochemical microsensor. Acute systemic administration of cocaine significantly increased NO efflux in the mPFC in a time-dependent manner. Similar injections using vehicle did not affect NO efflux. The facilitatory effect of cocaine on NO efflux was transient and reproducible. The signal was derived from neuronal sources of NO, because it was attenuated by systemic administration of the neuronal NO synthase inhibitor 7-nitroindazole. These studies support a role for prefrontal cortical NO signaling in cocaine-induced changes in neurotransmission in reward-related circuits involved in addiction.

Activation of the cGMP pathway in dopaminergic structures reduces cocaine-induced EGR-1 expression and locomotor activity

Nitric oxide (NO) and the C-type natriuretic peptide (CNP) exert their action on brain via the cGMP signaling pathway. NO, by activating soluble guanylyl cyclase, and CNP, by stimulating membrane-bound guanylyl cyclase, cause intracellular increases of cGMP, activating cGMP-dependent protein kinases (PKGs). We show here that injection of CNP into the rat ventral tegmental area strongly reduced cocaine-induced egr-1 expression in the nucleus accumbens in a dose-dependent manner. The effect of CNP was reversed by the previous injection of a selective PKG inhibitor, KT5823. Activation of PKG by 8-bromo-cGMP reduced, like CNP, cocaine-induced gene transcription in dopaminergic structures. To confirm the involvement of PKG, this was overexpressed in either the mesencephalon or the caudate-putamen. Using the polyethyleneimine delivery system, an active protein was expressed by injecting a plasmid vector containing the human PKG-Ialpha cDNA. PKG was overexpressed in dopaminergic and GABAergic neurons when the plasmid was injected in the ventral tegmental area, whereas overexpression was observed in medium spiny GABAergic neurons and in both cholinergic and GABAergic interneurons when the PKG vector was injected into the caudate-putamen. Activation of the overexpressed PKG reduced cocaine-induced egr-1 expression in dopaminergic structures and affected behavior (i.e., locomotor activity). These effects were again reversed by previous injection of the selective PKG inhibitor. The current data suggest that NO and the neuropeptide CNP are potential regulators of cocaine-related effects on behavior.

Nitric Oxide Synthesis Inhibition Attenuates Conditioned Reinstatement of Ethanol-Seeking, but Not the Primary Reinforcing Effects of Ethanol

BACKGROUND

Nitric oxide (NO) signaling has been implicated in regulating aspects of the reinforcing and addictive actions of cocaine. These experiments were designed to examine whether NO-dependent neurotransmission also participates in mediating the addictive actions of another drug of abuse, ethanol, with emphasis on both the primary reinforcing effects of ethanol and the incentive motivational effects of ethanol-related contextual stimuli.

METHODS

Male Wistar rats were operantly trained to orally self-administer 10% (w/v) ethanol in daily 30-min sessions and to associate distinct discriminative stimuli with the availability of ethanol (S+) versus nonreward (S-). Rats were treated with the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 0, 10, or 40 mg/kg intraperitoneally) 30 min before self-administration tests that were conducted after establishment of stable levels of daily ethanol intake and conditioned reinstatement tests that were performed after extinction of ethanol-maintained operant responding.

RESULTS

L-NAME did not alter the primary reinforcing effects of ethanol in self-administration tests. In contrast, L-NAME dose-dependently attenuated the recovery of extinguished responding induced by the ethanol S in the absence of ethanol availability during reinstatement tests.

CONCLUSIONS

These results suggest that the NO system does not play a role in behavior reinforced directly by ethanol. However, the results implicate NO-dependent neurotransmission in alcohol-seeking responses elicited by drug-related contextual stimuli.

Effects of nitric oxide on morphine self-administration in rat

Previous studies have reported that morphine exerts its effects in part through the release of nitric oxide (NO). In the present study, the effects of acute and chronic administration of the NO precursor, L-arginine and NO synthase (NOS) inhibitor, L-nitro-amino-methyl-ester (L-NAME) on morphine self-administration in rats were investigated. The animals were initially trained to press a lever using food as reinforcer. Rats were surgically prepared with a chronic Silastic catheter implanted in the external jugular vein. Five days after surgery, they were trained to press a lever for drug self-administration. The present data indicate that L-arginine (0.05, 0.1, and 0.15 mg/kg/injection) but not L-NAME (0.05, 0.1, and 0.15 mg/kg/injection) induced self-administration behavior and increased locomotion. The response induced by L-arginine (0.1 mg/kg/injection) was reduced by pretreatment with L-NAME (5, 10, and 15 mg/kg ip). Both the acute (5, 10, and 15 mg/kg ip) and the chronic (200 mg/kg ip; twice daily for 4 days) administration of L-arginine reduced morphine self-administration. However, acute (5, 10, and 20 mg/kg ip) and chronic (50 mg/kg ip; twice daily for 4 days) administration of L-NAME increased morphine self-administration significantly. It can be concluded that NO may have a role in morphine self-administration.

Effects of agmatine on the escalation of intravenous cocaine and fentanyl self-administration in rats

Escalation of drug intake reliably occurs when animals are allowed extended self-administration access. As a form of plasticity, escalation of drug intake may be accompanied by neuroadaptive changes that are related to the transition from controlled use to addiction. The purpose of the present experiment was to examine the effects of agmatine (decarboxylated L-arginine) on the escalation of intravenous (iv) fentanyl and cocaine self-administration in rats. Subjects were allowed 12 h of daily access to fentanyl (2.5 microg/kg) or cocaine (0.2 mg/kg) under a fixed-ratio (FR) 1 schedule of reinforcement for 30 days. Animals self-administering fentanyl were distributed into three groups: (1) low-dose agmatine (10 mg/kg) throughout self-administration; (2) high-dose agmatine (30 mg/kg) throughout self-administration; and (3) high-dose agmatine after significant escalation (Day 18) of drug intake had occurred. Animals in a fourth group were pretreated with a high dose of agmatine throughout 30 days of cocaine self-administration. Both doses of agmatine, when given throughout self-administration, significantly decreased the escalation of responding that occurred for fentanyl but not cocaine. In the group that received agmatine after significant escalation had occurred, fentanyl-maintained responding was not significantly altered. These data indicate that agmatine attenuates the escalation of fentanyl self-administration if administered before the escalation begins and may mediate neuroadaptive events related to chronic opioid self-administration.

Effect of 7-nitroindazole on drug-priming reinstatement of D-methamphetamine-induced conditioned place preference

The study investigated the role of nitric oxide (NO) in the relapse to drug-seeking behavior induced by D-methamphetamine. After the induction of D-methamphetamine (1 mg/kg) conditioned place preference, the rewarding effect became extinct 6 weeks later. The extinguished place preference was reinstated by D-methamphetamine (0.125 mg/kg) injection, an effect which was attenuated by 7-nitroindazole (12.5 and 25 mg/kg) pretreatment. The results demonstrate that NO is involved in relapse primed by D-methamphetamine injection.