Julio L. Martin

Cocaine-induced Conditioned Place Preference in Mice: Induction, Extinction and Reinstatement by Related Psychostimulants☆

Reinstatement of drug-seeking behavior in animals is relevant to drug relapse in humans. In the present study, we employed the conditioned place preference (CPP) paradigm to investigate the extinction and reinstatement of the place-conditioned response, a model that is consistent with drug-seeking behavior. Cocaine-induced CPP was rendered in Swiss Webster mice and then extinguished after repeated saline injections (8 days) in both the previously cocaine-paired compartment and the saline-paired compartment. Following the extinction phase, the reinstatement of CPP was investigated. Cocaine-experienced mice were challenged with one of the following psychostimulants, cocaine (15 mg/kg), methamphetamine (METH; 0.5 mg/kg), methylphenidate (MPD; 20 mg/kg) and phencyclidine (PCP; 5 mg/kg). The priming injection of cocaine, METH and MPD, unlike PCP, induced a marked preference for the previously cocaine-paired compartment. This finding suggests that all three psychostimulants reinstated the CPP response, and METH and MPD substituted for the reinforcing cue of cocaine. A challenge injection of cocaine administered two and four weeks after the reinstatement of CPP indicated that CPP was maintained up to two weeks. The finding that METH and MPD but not PCP reinstated and supported cocaine-induced CPP suggests that the CPP paradigm may be a useful tool for drug discrimination studies and the reinstatement of drug-seeking behavior.

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.

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 × 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-Nl; 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-Nl) 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.

Resistance of neuronal nitric oxide synthase-deficient mice to cocaine-induced locomotor sensitization

Abstract In brain, nitric oxide (NO) is considered as a retrograde messenger involved in synaptic plasticity. The present study was undertaken to investigate whether mice lacking the neuronal nitric oxide synthase (nNOS) gene are protected from cocaine-induced behavioral sensitization. Mice were administered, IP, either saline or cocaine (15 mg/kg) for 5 days. Sensitization was determined as an increase in cocaine-induced locomotor activity on day 5 compared with day 1 and an amplified response of cocaine-experienced mice to a challenge cocaine injection given after a 10-day drug free period (e.g., on day 15). To investigate the development of a context-dependent locomotion (conditioning), the responses of cocaine- and saline-experienced mice to a saline injection were determined on day 17. Male homozygote nNOS(–/–) mice were sensitive to the acute effect of cocaine (15 mg/kg) on day 1; however, they developed neither a sensitized response to cocaine (on day 5 and 15) nor a conditioned locomotion. Female homozygote nNOS(–/–) mice neither were responsive to 15 mg/kg cocaine on day 1, 5 and 15, nor did they develop a conditioned locomotion. In contrast, the same cocaine regimen delivered to male and female heterozygote nNOS(+/–) mice, and wild type mice (B6 J/sv129, C57BL/6 and sv129) resulted in sensitization to cocaine-induced locomotor activity and context-dependent locomotion. Investigation of [3H]cocaine disposition in the striatum and frontal cortex of the mice revealed neither gender nor strain differences in the drug disposition. Also, no major difference in striatal dopaminergic markers between homozygote nNOS(–/–) and wild type mice was observed. The most significant distinction, however, was the finding that nNOS(–/–) mice are completely deficient in striatal nNOS binding sites. Taken together, our results suggest that the resistance of homozygote nNOS(–/–) mice to cocaine-induced behavioral sensitization is primarily due to the deletion of the nNOS gene. Considering the role of NO in synaptic plasticity, it is conceivable that reduced brain NOS activity blunts the processes that underlie the development of sensitization to cocaine.

Blockade of alcohol-induced locomotor sensitization and conditioned place preference in DBA mice by 7-nitroindazole

Our previous studies indicated that inhibition or ablation of the neuronal nitric oxide synthase (nNOS) prevents the development of sensitization to the locomotor-stimulating effect of cocaine and cocaine-induced conditioned place preference (CPP). The present study was undertaken to investigate the effect of the nNOS inhibitor, 7-nitroindazole (7-NI), on ethanol-induced locomotor sensitization and CPP in DBA/2J mice. Administration of ethanol (1.5 g/kg; i.p.) for 7 days resulted in a progressive increase in the locomotor-stimulating effect of ethanol. Pretreatment with 7-NI (25 mg/kg) blocked the expression of the sensitized response to ethanol. A challenge injection of ethanol given 1 week and then 4 weeks following withdrawal from ethanol indicated that (a) ethanol sensitization was long lasting, and (b) the co-administration of 7-NI and ethanol attenuated the sensitized response to ethanol challenge. The CPP experiments showed that pairing four ethanol (2.5 g/kg) injections with a specific environment resulted in a marked preference for the drug-paired environment. The pretreatment with 7-NI (25 mg/kg) completely blocked ethanol-induced CPP. 7-NI alone produced neither rewarding nor aversive effects. Taken together, results of the present study indicate that blockade of nNOS by 7-NI-attenuated ethanol-induced behavioral sensitization and completely blocked the rewarding effect of ethanol. These findings support the role of NO in ethanol actions and further suggest that the nNOS system is relevant to the rewarding effects of various drugs of abuse.

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.

Effect of melatonin on methamphetamine- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic neurotoxicity and methamphetamine-induced behavioral sensitization

Methamphetamine (METH)- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic neurotoxicity is thought to be associated with the formation of free radicals. Since evidence suggests that melatonin may act as a free radical scavenger and antioxidant, the present study was undertaken to investigate the effect of melatonin on METH- and MPTP-induced neurotoxicity. In addition, the effect of melatonin on METH-induced locomotor sensitization was investigated. The administration of METH (5 mg kg(-1) x 3) or MPTP (20 mg kg(-1) x 3) to Swiss Webster mice resulted in 45-57% depletion in the content of striatal dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, and 57-59% depletion in dopamine transporter binding sites. The administration of melatonin (10 mg kg(-1)) before each of the three injections of the neurotoxic agents (on day 1), and thereafter for two additional days, afforded a full protection against METH-induced depletion of dopamine and its metabolites and dopamine transporter binding sites. In addition, melatonin significantly diminished METH-induced hyperthermia. However, the treatment with melatonin had no significant effect on MPTP-induced depletion of the dopaminergic markers tested. In the set of behavioral experiments, we found that the administration of 1 mg kg(-1) METH to Swiss Webster mice for 5 days resulted in marked locomotor sensitization to a subsequent challenge injection of METH, as well as context-dependent sensitization (conditioning). The pretreatment with melatonin (10 mg kg(-1)) prevented neither the sensitized response to METH nor the development of conditioned locomotion. Results of the present study indicate that melatonin has a differential effect on the dopaminergic neurotoxicity produced by METH and MPTP. Since it is postulated that METH-induced hyperthermia is related to its neurotoxic effect, while regulation of body temperature is unrelated to MPTP-induced neurotoxicity or METH-induced locomotor sensitization, the protective effect of melatonin observed in the present study may be due primarily to diminishing METH-induced hyperthermia.

Methamphetamine-induced dopaminergic neurotoxicity in mice: long-lasting sensitization to the locomotor stimulation and desensitization to the rewarding effects of methamphetamine.

High doses of methamphetamine (METH) cause the depletion of striatal dopaminergic markers; however, little is known about the behavioral consequences of METH-induced neurotoxicity. In the present study, the authors investigated the effect of a neurotoxic dose of METH (5 mg/kg; every 3 h x3) on the subsequent response of Swiss Webster mice to (a) the psychomotor-stimulating effect of METH and (b) the acquisition and maintenance of conditioned place preference (CPP) by METH. The latter is a paradigm for the assessment of the rewarding properties of abused substances. The administration of the high dose of METH resulted in significant depletion of dopamine (DA) and its metabolites and dopamine transporter (DAT) binding sites in the striatum. The dopaminergic markers were below control levels until the 95th day after METH administration. METH-pretreated mice were sensitized to the psychomotor-stimulating effect of METH (1 mg/kg) as determined on Days 3 and 74 after the initial exposure to the neurotoxic dose of METH. However, the acquisition of CPP by METH (0.5 mg/kg) was markedly reduced in the mice pretreated with the neurotoxic dose of METH compared with the control group. The CPP was maintained for 8 weeks in the control group but not in the METH group. A priming injection of METH (0.5 mg/kg) caused marked reinstatement of place preference in the control group; this response was maintained for three additional weeks. However, the priming injection of METH resulted in diminished place preference in the METH group and the conditioned response dissipated within 3 weeks. These findings suggest that METH-induced striatal dopaminergic neurotoxicity is associated with two opposing and long-lasting behavioral outcomes: (a) sensitization to the psychomotor-stimulating effect of the drug and (b) desensitization to the rewarding properties of the drug. These consequences may be relevant to the psychopathology of METH abuse.

Effect of riluzole and gabapentin on cocaine- and methamphetamine-induced behavioral sensitization in mice

Abstract  Rationale: Recent studies have suggested the involvement of excitatory amino acid (EAA) and inhibitory gamma amino butyric acid (GABA) transmission in the effects of psychostimulants such as cocaine and amphetamines. Objectives: The present study was undertaken to investigate whether drugs that are considered to inhibit glutamate release (e.g., riluzole) or increase GABAergic transmission (e.g., gabapentin) attenuate the induction and expression of sensitization to cocaine and methamphetamine (METH) in Swiss Webster mice. Methods: Sensitizationto the psychomotor stimulating effect of cocaine and METH was rendered by five daily injections of cocaine (20 mg/kg) or METH (1.0 mg/kg). Locomotor activity was measured by infrared beam interrupts. Results: Pretreatment with riluzole (2.5–20.0 mg/kg) affected neither the expression nor the induction of sensitization to cocaine. The pretreatment with riluzole (20 mg/kg) blocked the acute response to METH on day 1 and the expression of the sensitized response on day 5 but not the induction of sensitization to METH. Pretreatment with gabapentin (10 mg/kg and30 mg/kg) affected neither the expression nor the induction of sensitization to cocaine. The pretreatment with gabapentin attenuated the acute response to METH on day 1 and the expression of the sensitized response on day 5, but it failed to block the induction of sensitization to METH. Psychostimulant-induced conditioned locomotion was affected neither by riluzole nor by gabapentin. Conclusions: Riluzole and gabapentin had no effect on the induction of sensitization to cocaine and METH; however, they attenuated the expression of sensitization to METH but not to cocaine. These findings suggest that riluzole- and gabapentin-mediated changes in EAA and GABAergic transmission, respectively, had no effect on mechanisms associated with the induction of sensitization, but they may affect the expression of the sensitized response to METH.

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.