Variations in the stimulus salience of cocaine reward influences drug-associated contextual memory.

Drugs of abuse act as reinforcers because they influence learning and memory processes resulting in long-term memory of drug reward. We have previously shown that mice conditioned by fixed daily dose of cocaine (Fix-C) or daily escalating doses of cocaine (Esc-C) resulted in short- and long-term persistence of drug memory, respectively, suggesting different mechanisms in acquisition of cocaine memory. The present study was undertaken to investigate the differential contribution of N-methyl-D-aspartate receptor (NMDAR) subunits in the formation of Fix-C and Esc-C memory in C57BL/6J mice. Training by Esc-C resulted in marked elevation in hippocampal expression of Grin2b mRNA and NR2B protein levels compared with training by Fix-C. The NR2B-containing NMDAR antagonist ifenprodil had similar attenuating effects on acquisition and reconsolidation of Fix-C and Esc-C memory. However, the NMDAR antagonist MK-801 had differential effects: (1) higher doses of MK-801 were required for post-retrieval disruption of reconsolidation of Esc-C memory than Fix-C memory; and (2) pre-retrieval MK-801 inhibited extinction of Fix-C memory but it had no effect on Esc-C memory. In addition, blockade of NMDAR downstream signaling pathways also showed differential regulation of Fix-C and Esc-C memory. Inhibition of neuronal nitric oxide synthase attenuated acquisition and disrupted reconsolidation of Fix-C but not Esc-C memory. In contrast, the mitogen-activating extracellular kinase inhibitor SL327 attenuated reconsolidation of Esc-C but not Fix-C memory. These results suggest that NMDAR downstream signaling molecules associated with consolidation and reconsolidation of cocaine-associated memory may vary upon changes in the salience of cocaine reward during conditioning.

The strength of aversive and appetitive associations and maladaptive behaviors.

Certain maladaptive behaviors are thought to be acquired through classical Pavlovian conditioning. Exaggerated fear response, which can develop through Pavlovian conditioning, is associated with acquired anxiety disorders such as post-traumatic stress disorders (PTSDs). Inflated reward-seeking behavior, which develops through Pavlovian conditioning, underlies some types of addictive behavior (e.g., addiction to drugs, food, and gambling). These maladaptive behaviors are dependent on associative learning and the development of long-term memory (LTM). In animal models, an aversive reinforcer (fear conditioning) encodes an aversive contextual and cued LTM. On the other hand, an appetitive reinforcer results in conditioned place preference (CPP) that encodes an appetitive contextual LTM. The literature on weak and strong associative learning pertaining to the development of aversive and appetitive LTM is relatively scarce; thus, this review is particularly focused on the strength of associative learning. The strength of associative learning is dependent on the valence of the reinforcer and the salience of the conditioned stimulus that ultimately sways the strength of the memory trace. Our studies suggest that labile (weak) aversive and appetitive LTM may share similar signaling pathways, whereas stable (strong) aversive and appetitive LTM is mediated through different pathways. In addition, we provide some evidence suggesting that extinction of aversive fear memory and appetitive drug memory is likely to be mediated through different signaling molecules. We put forward the importance of studies aimed to investigate the molecular mechanisms underlying the development of weak and strong memories (aversive and appetitive), which would ultimately help in the development of targeted pharmacotherapies for the management of maladaptive behaviors that arise from classical Pavlovian conditioning.

Nitric oxide (NO) signaling as a potential therapeutic modality against psychostimulants.

Abuse of psychostimulants presents a significant health and social problem worldwide. Traditionally, the dopaminergic system has received much attention for its role in the development and manifestation of addictive behavior. The identification of the close interaction between the dopaminergic and glutamatergic pathway and by extension the nitric oxide (NO) signaling pathway (the nitrergic system) have provided a broader scope on the mechanisms underlying the development of addictive behavior following exposure to cocaine and methamphetamine. NO signaling is associated with the acquisition and maintenance of several behavioral phenotypes induced by cocaine and methamphetamine (METH), as well as in METH-induced dopaminergic depletion. Because it appears that NO signaling influences response to reward, memory formation, and free radical-induced neurotoxicity, pharmacotherapies targeting NO signaling pathway may prove beneficial in the treatment of psychostimulants abuse.

Sodium butyrate-induced histone acetylation strengthens the expression of cocaine-associated contextual memory.

The conditioned place preference (CPP) paradigm entails Pavlovian conditioning and allows evaluating the acquisition and extinction of drug-associated memory. Epigenetic regulation of chromatin structure by acetylation and deacetylation of histone proteins is critical for formation of long-term memory (LTM). We have recently shown that a single administration of the histone deacetylase (HDAC) inhibitor sodium butyrate (NaB) facilitated extinction of fear-associated memory in mice. Using the CPP paradigm, the present study investigated the effect of NaB on cocaine-associated memory. C57B/6 mice were conditioned by either fixed daily doses of cocaine (5mg/kg×4 and 15mg/kg×4days) or an escalating schedule (3, 6, 12 and 24mg/kg). Acute administration of NaB (1.2g/kg) prior to conditioning by fixed doses of cocaine increased the expression and impaired the extinction of place preference compared to control subjects. Subjects that were conditioned by 15mg/kg×4 cocaine and received a single injection of NaB following the first or the second CPP test showed impaired extinction compared to control mice that received saline instead of NaB. Subjects that were conditioned by escalating schedule of cocaine and subsequently received repeated injections of NaB during daily reexposure to nonreinforced context showed either enhancement or no effect on place preference. Acute administration of NaB (1.2g/kg) to naïve mice resulted in marked increase in acetylation of histone H3 lysine 14 (H3K14) and histone H4 lysine 8 (H4K8) in hippocampus but not amygdala. Results suggest that regardless of the scheduling of either cocaine or NaB administration, NaB-induced histone hyperacetylation in the hippocampus may strengthen cocaine-associated contextual memory.

The effect of phosphodiesterase inhibitors on the extinction of cocaine-induced conditioned place preference in mice.

Several phosphodiesterase inhibitors (PDEis) improve cognition, suggesting that an increase in brain cAMP and cGMP facilitates learning and memory. Since extinction of drug-seeking behavior requires associative learning, consolidation and formation of new memory, the present study investigated the efficacy of three different PDEis in the extinction of cocaine-induced conditioned place preference (CPP) in B6129S mice. Mice were conditioned by escalating doses of cocaine which was resistant to extinction by free exploration. Immediately following each extinction session mice received (a) saline/vehicle, (b) rolipram (PDE4 inhibitor), (c) BAY-73-6691 (PDE9 inhibitor) or (d) papaverine (PDE10A inhibitor). Mice that received saline/vehicle during extinction training showed no reduction in CPP for >10 days. BAY-73-6691 (a) dose-dependently increased cGMP in hippocampus and amygdala, (b) significantly facilitated extinction and (c) diminished the reinstatement of cocaine CPP. Rolipram, which selectively increased brain cAMP levels, and papaverine which caused increases in both cAMP and cGMP levels, had no significant effect on the extinction of cocaine CPP. The results suggest that increase in hippocampal and amygdalar cGMP levels via blockade of PDE9 has a prominent role in the consolidation of extinction learning.

Histone acetylation rescues contextual fear conditioning in nNOS KO mice and accelerates extinction of cued fear conditioning in wild type mice.

Epigenetic regulation of chromatin structure is an essential molecular mechanism that contributes to the formation of synaptic plasticity and long-term memory (LTM). An important regulatory process of chromatin structure is acetylation and deacetylation of histone proteins. Inhibition of histone deacetylase (HDAC) increases acetylation of histone proteins and facilitate learning and memory. Nitric oxide (NO) signaling pathway has a role in synaptic plasticity, LTM and regulation of histone acetylation. We have previously shown that NO signaling pathway is required for contextual fear conditioning. The present study investigated the effects of systemic administration of the HDAC inhibitor sodium butyrate (NaB) on fear conditioning in neuronal nitric oxide synthase (nNOS) knockout (KO) and wild type (WT) mice. The effect of single administration of NaB on total H3 and H4 histone acetylation in hippocampus and amygdala was also investigated. A single administration of NaB prior to fear conditioning (a) rescued contextual fear conditioning of nNOS KO mice and (b) had long-term (weeks) facilitatory effect on the extinction of cued fear memory of WT mice. The facilitatory effect of NaB on extinction of cued fear memory of WT mice was confirmed in a study whereupon NaB was administered during extinction. Results suggest that (a) the rescue of contextual fear conditioning in nNOS KO mice is associated with NaB-induced increase in H3 histone acetylation and (b) the accelerated extinction of cued fear memory in WT mice is associated with NaB-induced increase in H4 histone acetylation. Hence, a single administration of HDAC inhibitor may rescue NO-dependent cognitive deficits and afford a long-term accelerating effect on extinction of fear memory of WT mice.

Changes in the magnitude of drug-unconditioned stimulus during conditioning modulate cocaine-induced place preference in mice.

Drug reinforcement learning is relevant for the development of addiction. The present study investigated how changes in the magnitude of drug-unconditioned stimulus during associative learning modulate the acquisition and extinction of cocaine-induced conditioned place preference (CPP). B6;129S F2 mice were conditioned by three dosing schedules of cocaine: (1) ascending, (2) fixed and (3) descending daily doses. Following acquisition of CPP, extinction was induced by (1) context re-exposure, (2) reconditioning by saline and (3) reconditioning by descending doses of cocaine. The magnitude of CPP following conditioning by daily ascending doses of cocaine (2, 4, 8 and 16 mg/kg) was significantly higher than that obtained from conditioning by either a fixed daily dose (16 mg/kg × 4 days) or daily descending doses (24, 12, 6 and 3 mg/kg). Extinction following context re-exposure showed persistent CPP in the 'ascending' group compared to the other two groups. However, extinction via reconditioning by saline was equally effective in all groups. Interestingly, reconditioning by descending doses of cocaine (1) extinguished CPP and (2) resulted in partial resistance to the reinstatement of conditioned response by cocaine priming. Results underscore the significance of daily changes in cocaine dosage in the development and extinction of drug-induced conditioned response. Increase and decrease in cocaine dosage strengthens and weakens cocaine-associated memory, respectively. Moreover, extinction by 'tapering down' drug reward may be superior to extinction by saline.

Pharmacological modulators of nitric oxide signaling and contextual fear conditioning in mice.

RATIONALE: Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) is a retrograde neuronal messenger that participates in synaptic plasticity, including late-phase long-term potentiation (LTP) and long-term memory (LTM) formation. Our recent studies have shown that nNOS knockout (KO) mice have a severe deficit in contextual fear conditioning compared to wild type (WT) counterparts (Kelley et al. 2009). OBJECTIVES: Given the role of the nNOS gene in fear conditioning, we investigated whether systemic administration of modulators of NO signaling affect the formation of contextual and cued fear memories and the effects of these modulators on cyclic 3'5'-guanosine monophosphate (cGMP) levels in the hippocampus and amygdala. METHODS: The preferential nNOS inhibitor S-methyl-L-thiocitrulline (SMTC; 10-200 mg/kg) was administered (IP) to WT mice, and the NO donor molsidomine (10 mg/kg) was administered (IP) to nNOS KO mice either 30 min pretraining or immediately posttraining. RESULTS: Pretraining SMTC administration to WT mice impaired both short- and long-term memories of contextual (36% inhibition) but not cued fear conditioning. Pretraining molsidomine administration to nNOS KO mice improved their deficit in short- and long-term memories of contextual fear conditioning (46% increase). Posttraining drug administration had no effect on WT and nNOS KO mice. The systemic administration of SMTC dose-dependently decreased cGMP concentrations down to 25% of control, while molsidomine increased cGMP concentration (three- and five-fold) in amygdala and hippocampus, respectively. CONCLUSIONS: These findings suggest that neuronal NO and its downstream second messenger cGMP are important for acquisition and subsequent consolidation of LTM of contextual fear conditioning.

Discrimination between cocaine-associated context and cue in a modified conditioned place preference paradigm: role of the nNOS gene in cue conditioning.

The conditioned place preference (CPP) paradigm entails appetitive learning and is utilized to investigate the motivational effects of drug and natural reward in rodents. However, a typical CPP design does not allow dissociation between cue- and context-dependent appetitive learning. In humans, context and cues that had been associated with drug reward can elicit conditioned response and drug craving. Therefore, we investigated (a) methods by which to discriminate between cue- and context-dependent appetitive learning, and (b) the role of the neuronal nitric oxide synthase (nNOS) gene in appetitive learning. Wild-type (WT) and nNOS knockout (KO) mice were trained by cocaine (20 mg/kg) in a discrete context paired with a light cue (a compound context-cue stimulus). In test 1, approach behaviour to either the training context or to the cue in a novel context was determined. WT mice showed robust preference for both cocaine-associated context and cue. nNOS KO mice acquired approach behaviour for the cocaine-associated context but not cue. This finding suggests that the nNOS gene is required for cue-dependent appetitive learning. On the following day (test 2), mice were tested for approach behaviour to the compound context-cue stimulus. Context but not cue exposure in test 1 reduced approach behaviour to the compound context-cue stimulus in test 2, suggesting that repeated context but not cue exposures diminished the conditioned response. Hence, this modified CPP paradigm is useful for the investigation of approach behaviour for both drug-associated context and cue, and allows further investigation of mechanisms underlying cue- and context-dependent appetitive learning.

Impairments in fear conditioning in mice lacking the nNOS gene.

The fear conditioning paradigm is used to investigate the roles of various genes, neurotransmitters, and substrates in the formation of fear learning related to contextual and auditory cues. In the brain, nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) functions as a retrograde neuronal messenger that facilitates synaptic plasticity, including the late phase of long-term potentiation (LTP) and formation of long-term memory (LTM). Evidence has implicated NO signaling in synaptic plasticity and LTM formation following fear conditioning, yet little is known about the role of the nNOS gene in fear learning. Using knockout (KO) mice with targeted mutation of the nNOS gene and their wild-type (WT) counterparts, the role of NO signaling in fear conditioning was investigated. Plasma levels of the stress hormone corticosterone were measured to determine the relationship between physiological and behavioral response to fear conditioning. Contextual fear learning was severely impaired in male and female nNOS KO mice compared with WT counterparts; cued fear learning was slightly impaired in nNOS KO mice. Sex-dependent differences in both contextual and cued fear learning were not observed in either genotype. Deficits in contextual fear learning in nNOS KO mice were partially overcome by multiple trainings. A relationship between increase in plasma corticosterone levels following footshock administration and the magnitude of contextual, but not cued freezing was also observed. Results suggest that the nNOS gene contributes more to optimal contextual fear learning than to cued fear learning, and therefore, inhibition of the nNOS enzyme may ameliorate context-dependent fear response.

The neuronal nitric oxide synthase (nNOS) gene contributes to the regulation of tyrosine hydroxylase (TH) by cocaine.

Recently, we demonstrated that intact nitric oxide (NO) signaling is essential for the development of cocaine behavioral sensitization in adulthood [M.A. Balda, K.L. Anderson, Y. Itzhak, Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice, Psychopharmacology (Berl) 200 (2008) 509-519]. Given the requirement of dopamine (DA) transmission in cocaine-induced behavioral sensitization and the interactions between NO and DA systems, the present study investigated the role of the neuronal nitric oxide synthase (nNOS) gene and the effect of cocaine on the expression of tyrosine hydroxylase (TH)-immunoreactive (-ir) neurons. Adult (postnatal day 80) wild type (WT) and nNOS knockout (KO) mice received saline or a sensitizing regimen of cocaine (20mg/kg) for 5 days. After 24h, TH immunoreactivity was assessed in the ventral tegmental area (VTA) and the dorsal striatum (dST) using stereology and Western blotting, respectively. We report that (a) nNOS KO mice express lower levels of TH-ir neurons in the VTA compared to WT counterparts, (b) cocaine administration to WT mice significantly increased striatal TH expression, and (c) the same cocaine administration to nNOS KO mice significantly decreased striatal TH expression. Thus, the nitrergic system may contribute to cocaine-induced behavioral sensitization by regulating dopaminergic neurotransmission.

Role of the nNOS gene in ethanol-induced conditioned place preference in mice.

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity, and evidence suggests its role in a range of effects produced by alcohol in the central nervous system. The aim of the current study was to investigate the role of the nNOS gene in the development of ethanol-induced conditioned place preference (CPP) in mice. The CPP paradigm is designed to investigate the reinforcing properties of drugs of abuse and the development of maladaptive behaviors, such as conditioned response to drug-associated stimuli, after repeated drug exposure. Adult male and female wild type (WT) and nNOS knockout (KO) mice on a mixed B6;129S genetic background were trained by a morning saline session and afternoon ethanol (1, 2, and 3 g/kg; intraperitoneally) session for 4 days. Place preference in a drug-free state was recorded on the following day. Results show that WT males and females developed robust CPP, whereas nNOS KO mice did not (with the exception of female nNOS KO mice conditioned by 2 g/kg ethanol). The differential response of WT and nNOS KO mice was not due to genotypic differences in motor behavior. To investigate if the absence of the nNOS gene causes specific impairment in processing the motivational effect of ethanol or an overall impairment in associative learning, WT and nNOS KO mice were trained by LiCl (150 mg/kg) which causes conditioned place aversion (CPA). Results show that both WT and nNOS KO mice developed significant CPA. The findings that the absence of the nNOS gene impaired ethanol-induced CPP but not LiCl-induced CPA suggest that NO signaling has a specific role in processing the motivational effect of ethanol. Hence, inhibition of nNOS may attenuate the development of maladaptive behaviors associated with alcohol exposure.

Development and persistence of long-lasting behavioral sensitization to cocaine in female mice: role of the nNOS gene.

Our recent studies have shown that the neuronal nitric oxide synthase (nNOS) gene is required for the development and persistence of psychomotor sensitization to cocaine in adult but not adolescent male mice (Balda, M.A., Anderson, K.L., Itzhak, Y., 2008. Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice. Psychopharmacology (Berlin) 200, 509-519.). The aim of the present study was to investigate the contribution of the nNOS gene to cocaine-induced behavioral sensitization in adolescent and adult female mice. Adolescent and adult wild type (WT) and nNOS knockout (KO) mice received saline or cocaine (20 mg/kg) for 5 days and then were challenged with cocaine (20 mg/kg) after a drug-free period of either 10, 30, or 90 days. Context-dependent sensitization was determined by measuring saline-induced locomotor activity in the previously cocaine-paired environment. Results show that adolescent females of both genotypes, like their adult counterparts, developed long-lasting behavioral sensitization to cocaine (a three-month period), suggesting high vulnerability of females to cocaine regardless of age. An effect of genotype was observed in the initiation of sensitization, e.g., delayed onset in the absence of the nNOS gene. The only age-dependent difference observed was that adult, but not adolescent mice developed context-dependent sensitization. The present study suggests that long-term expression of cocaine-induced behavioral sensitization in females (adolescent and adult) is nNOS-independent, unlike our previous findings in adult males.

Role of the NMDA receptor and nitric oxide in memory reconsolidation of cocaine-induced conditioned place preference in mice.

Classical pavlovian conditioning has a major role in the development and persistence of drug addiction. Appetitive conditioning by drug reward, as measured by the conditioned place preference (CPP) paradigm, is an exemplar of classical pavlovian conditioning. Aversive conditioning by footshock involves learning and memory processes similar to those involved in appetitive conditioning. Studies on fear conditioning have shown that long-term fear memory can be extinguished by disruption of reconsolidation of specific memories associated with the fear response. Hence disruption of memory reconsolidation may hold promise for the extinction of maladaptive conditioned behavior. In the present study the effects of the NMDA receptor antagonist, MK-801, and the nNOS inhibitor 7-nitroindazole (7-NI) on memory reconsolidation of cocaine-induced CPP in mice were investigated. We report that, following the acquisition of cocaine CPP, a single acute administration of either MK-801 or 7-NI immediately after retrieval of place preference extinguished subsequent place preference. Moreover, a priming dose of cocaine did not reinstate place preference in the drug-treated groups compared to controls. Male nNOS knockout (KO) mice acquired short-lived cocaine CPP compared to wild-type (WT) mice. A single acute administration of the NO-donor molsidomine to nNOS KO mice immediately after retrieval of CPP prolonged the expression of place preference compared to controls that received saline, suggesting partial strengthening of memory reconsolidation. Taken together, these findings support the role of the NMDAR/NO signaling pathway in memory reconsolidation of cocaine CPP, and suggest that disruption of this pathway during memory reconsolidation may afford resistance to drug-seeking behavior.

Ethanol-induced behavioral sensitization in adolescent and adult mice: role of the nNOS gene.

BACKGROUND: In the brain, nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity. Recent evidence suggests the role of NO in a variety of effects produced by alcohol in the central nervous system. The current study investigated the role of the nNOS gene in the development of behavioral sensitization to ethanol in adolescent and adult mice. METHODS: Adolescent and adult wild type (WT; B6;129SF2) and nNOS knockout (KO; B6;129S4-Nos1) mice of both sexes received saline or ethanol (1.5 g/kg; intraperitoneally) for 5 consecutive days, and locomotor activity was recorded daily. The locomotor response to challenge ethanol and saline injections was investigated at various time points following withdrawal from ethanol. RESULTS: Adolescent WT but not nNOS KO mice developed a long-lasting sensitized response to ethanol as well as context-dependent hyperlocomotion (in response to saline) from adolescence through adulthood; sex-dependent differences were not observed. Compared to adolescent WT mice, adult WT males developed a short-term sensitized response to ethanol and context-dependent hyperlocomotion; adult WT females showed only short-term context-dependent hyperlocomotion. Adult nNOS KO males (like their adolescent counterparts) did not develop behavioral sensitization; no significant differences between adult nNOS KO and WT females were observed. Blood ethanol concentrations did not show genotype- or sex-dependent differences. CONCLUSIONS: (1) The nNOS gene is required for the development of behavioral sensitization to ethanol in adolescent male and female mice. (2) Adolescent exposure to ethanol results in long-lasting behavioral sensitization through adulthood, while adult exposure to ethanol results in a shorter behavioral sensitization. (3) Sex-dependent differences are observed when ethanol exposure begins in adulthood but not in adolescence. (4) Ethanol-induced behavioral sensitization in adulthood is nNOS-dependent in males but not in females. Taken together, results suggest genotype-, ontogeny-, and sex-dependent differences in the development of behavioral sensitization to ethanol.

Differential role of the nNOS gene in the development of behavioral sensitization to cocaine in adolescent and adult B6;129S mice.

RATIONALE: Previous studies have suggested the involvement of neuronal nitric oxide synthase (nNOS) in the development of sensitization to psychostimulants. Ontogeny-dependent differences in the response to psychostimulants have been reported. OBJECTIVE: The objectives were to investigate (a) the short- and long-term consequences of adolescent and adult cocaine exposure on behavioral sensitization and (b) the role of the nNOS gene in behavioral sensitization in adolescent and adult mice. MATERIALS AND METHODS: Adolescent and adult wild type (WT) and nNOS knockout (KO) mice received saline or cocaine (20 mg/kg) for 5 days and then were challenged with cocaine (20 mg/kg) after a drug-free period of 10 or 30 days. Locomotor activity was recorded by infrared beam interruptions. nNOS immunoreactive (ir) neurons in the dorsal and ventral striatum were quantified 24 h after repeated administration of cocaine to adolescent and adult WT mice. RESULTS: Repeated administration of cocaine to either WT or nNOS KO mice during adolescence resulted in locomotor sensitization, which persisted into adulthood. WT but not KO adult mice developed long-term sensitization to cocaine. Repeated cocaine administration resulted in a 96% increase in the expression of nNOS-ir neurons in the dorsal striatum of adult but not adolescent WT mice. CONCLUSIONS: The nNOS gene is essential for the induction of behavioral sensitization to cocaine in adulthood but not in adolescence. The increased expression of nNOS-ir neurons in the dorsal striatum may underlie the induction of behavioral sensitization in adulthood. Thus, the NO-signaling pathway has an ontogeny-dependent role in the neuroplasticity underlying cocaine behavioral sensitization.

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.

Long-term memory of cocaine-associated context: disruption and reinstatement.

Long-term memory of cocaine-associated context was established by conditioned place preference learning. After 1 week, exposure to context in the absence of cocaine (memory retrieval) was paired with one of the following treatments: saline, scopolamine (muscarinic acetylcholine receptor antagonist), dizocilpine (MK-801; noncompetitive N-methyl-D-aspartate antagonist) or D-cycloserine (partial N-methyl-D-aspartate agonist). In subsequent conditioned place preference tests, place preference was suppressed in the drug-treated groups but not saline-treated groups. Results suggest that the amnesic agents, scopolamine and MK-801, disrupted reconsolidation of cocaine-associated contextual memory. In contrast, the mnemonic agent D-cycloserine might have facilitated extinction learning during context exposure in the absence of cocaine. Challenge administration of cocaine reinstated place preference in all groups except the MK-801 group, suggesting that suppression of conditioned response may or may not suppress memory evoked by drug-context reexposure.

Impairment in consolidation of learned place preference following dopaminergic neurotoxicity in mice is ameliorated by N-acetylcysteine but not D1 and D2 dopamine receptor agonists.

Some of the major concerns related to methamphetamine (METH) abuse are the neuronal damage inflicted at dopamine (DA) nerve terminals and the cognitive deficits observed in human METH abusers. We have shown that a high dose of METH selectively depleted dopaminergic markers in striatum, frontal cortex and amygdala of Swiss Webster mice, and impaired learned place preference. In this study, we investigated whether deficits in consolidation of place learning, as a consequence of METH neurotoxicity, underlie the underperformance of cocaine conditioned place preference (CPP). Administration of METH (5 mg/kg x 3) to Swiss Webster mice decreased striatal tyrosine hydroxylase (TH) immunoreactive neurons and significantly increased glial fibrillary acidic protein (GFAP) expression, confirming the neurotoxic potential of METH in mice. This treatment significantly attenuated the establishment of cocaine (15 mg/kg) CPP compared to control. To investigate whether manipulation of the consolidation phase improves learned place preference, mice were trained by cocaine and received daily post-training injections of DA receptor agonists or N-acetylcysteine (NAC). As memory consolidation occurs shortly after training, drugs were administered either immediately or 2 h post-training. Immediate post-training administration of the D1 DA receptor agonist SKF38393 (5, 10, and 20 mg/kg) or the D2 DA receptor agonist quinpirole (0.25, 0.5, and 1.0 mg/kg) did not improve the establishment of CPP following METH neurotoxicity. However, immediate but not delayed NAC administration (50 and 100 mg/kg) enhanced cocaine CPP following METH neurotoxicity and had no effect on control CPP. The levels of the reduced form of glutathione (GSH) in striatum, amygdala, hippocampus and frontal cortex were significantly lower in METH-treated mice compared to control during the period of CPP training. Acute and repeated administration of NAC to METH-treated mice restored the decreased brain GSH but had no effect on controls. Results suggest that METH-induced dopaminergic neurotoxicity is associated with impairment of consolidation of learned place preference, and that this impairment is improved by immediate post-training administration of the glutathione precursor NAC and not by D1 or D2 DA receptor agonists. Restoration of brain glutathione levels immediately post-training may facilitate the consolidation process.

Methamphetamine-induced selective dopaminergic neurotoxicity is accompanied by an increase in striatal nitrate in the mouse.

Exposure to high doses of methamphetamine (METH), a major drug of abuse, may cause neuronal damage. Previous studies have implicated the role of peroxynitrite, produced by nitric oxide (NO) and reactive oxygen species, in dopaminergic neurotoxicity produced by METH in mice. The present article was undertaken to investigate if a neurotoxic regimen of METH is associated with changes in tissue levels of nitrate and nitrite, which are the stable products of NO. Administration of METH (5 mg/kg x 3) to Swiss Webster mice resulted in marked depletion of dopamine (DA) and DA transporter (DAT) binding sites but no change in 5-hydroxytryptamine (5-HT) and 5-HT transporter (5-HTT) binding sites in the striatum, amygdala, frontal cortex, and hippocampus, suggesting that METH causes selective neurotoxicity to DA nerve terminals. The concentration of nitrate in the striatum was increased by about two-fold after METH administration; however, no changes in nitrate concentration were detected in other brain regions that endured dopaminergic neurotoxicity. These findings suggest that (a) a neurotoxic regimen of METH produces selective increase in NO in the striatum, which may generate toxic species such as peroxynitrite, and (b) toxins other than NO-related derivatives may mediate dopaminergic neurotoxicity in the amygdala and frontal cortex.