Role of nitric oxide synthase inhibitors and NMDA receptor antagonist in nicotine-induced behavioral sensitization in the rat

Metabolic hormone action in the VTA: Reward-directed behavior and mechanistic insights

Dysfunctional signaling in midbrain reward circuits perpetuates diseases characterized by compulsive overconsumption of rewarding substances such as substance abuse, binge eating disorder, and obesity. Ventral tegmental area (VTA) dopaminergic activity serves as an index for how rewarding stimuli are perceived and triggers behaviors necessary to obtain future rewards. The evolutionary linking of reward with seeking and consuming palatable foods ensured an organism's survival, and hormone systems that regulate appetite concomitantly developed to regulate motivated behaviors. Today, these same mechanisms serve to regulate reward-directed behavior around food, drugs, alcohol, and social interactions. Understanding how hormonal regulation of VTA dopaminergic output alters motivated behaviors is essential to leveraging therapeutics that target these hormone systems to treat addiction and disordered eating. This review will outline our current understanding of the mechanisms underlying VTA action of the metabolic hormones ghrelin, glucagon-like peptide-1, amylin, leptin, and insulin to regulate behavior around food and drugs of abuse, highlighting commonalities and differences in how these five hormones ultimately modulate VTA dopamine signaling.

Role of Neuronal Nicotinic Acetylcholine Receptors in Cannabinoid Dependence

Cannabis is among the most widely consumed psychoactive drugs around the world and cannabis use disorder (CUD) has no current approved pharmacological treatment. Nicotine and cannabis are commonly co-used which suggests there to be overlapping neurobiological actions supported primarily by the co-distribution of both receptor systems in the brain. There appears to be strong rationale to explore the role that nicotinic receptors play in cannabinoid dependence. Preclinical studies suggest that the ɑ7 nAChR subtype may play a role in modulating the reinforcing and discriminative stimulus effects of cannabinoids, while the ɑ4β2 * nAChR subtype may be involved in modulating the motor and sedative effects of cannabinoids. Preclinical and human genetic studies point towards a potential role of the ɑ5, ɑ3, and β4 nAChR subunits in CUD, while human GWAS studies strongly implicate the ɑ2 subunit as playing a role in CUD susceptibility. Clinical studies suggest that current smoking cessation agents, such as varenicline and bupropion, may also be beneficial in treating CUD, although more controlled studies are necessary. Additional behavioral, molecular, and mechanistic studies investigating the role of nAChR in the modulation of the pharmacological effects of cannabinoids are needed.

Phosphorylation of GluA1-Ser831 by CaMKII Activation in the Caudate and Putamen Is Required for Behavioral Sensitization After Challenge Nicotine in Rats

BACKGROUND

Phosphorylation of the glutamate receptor (GluA1) subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor plays a crucial role in behavioral sensitization after exposure to psychostimulants. The present study determined the potential role of serine 831 (Ser831) phosphorylation in the GluA1 subunit of the caudate and putamen (CPu) in behavioral sensitization after challenge nicotine.

METHODS

Challenge nicotine (0.4 mg/kg) was administered subcutaneously (s.c.) after 7 days of repeated exposure to nicotine (0.4 mg/kg, s.c.) followed by 3 days of withdrawal in rats. Bilateral intra-CPu infusions of drugs were mainly performed to test this hypothesis.

RESULTS

Challenge nicotine increased both phosphorylated (p)Ser831 immunoreactivity (IR) and pCa2+/calmodulin-dependentprotein kinases II (pCaMKII)-IR in the medium spiny neurons (MSNs) of the CPu. These increases were prevented by bilateral intra-CPu infusion of the metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP (0.5 nmol/side) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK801 (2 nmol/side). However, the dopamine D1 receptor (D1R) antagonist SCH23390 (7.5 nmol/side) prevented only pSer831-IR alone. Bilateral intra-CPu infusion of the Tat-GluA1D peptide (25 pmol/side), which interferes with the binding of pCaMKII to GluA1-Ser831, decreased the challenge nicotine-induced increase in locomotor activity.

CONCLUSIONS

These findings suggest that the GluA1-Ser831 phosphorylation in the MSNs of the CPu is required for the challenge nicotine-induced behavioral sensitization in rats. CaMKII activation linked to mGluR5 and NMDA receptors, but not to D1R, is essential for inducing the CaMKII-Ser831 interaction.

Sauchinone Blocks Ethanol Withdrawal-Induced Anxiety but Spares Locomotor Sensitization: Involvement of Nitric Oxide in the Bed Nucleus of the Stria Terminalis

Both the positive (manifested by locomotor sensitization) and negative (withdrawal symptoms) reinforcing effects of ethanol (EtOH) involve central nitric oxide (NO) signaling. Sauchinone (a bioactive lignan in Saururus chinensis) has been shown to improve methamphetamine-induced behavioral and neurochemical changes via the NO signaling pathway. Thus, this study evaluated the effects of sauchinone on locomotor sensitization and anxiety during EtOH withdrawal (EtOHW). Male adult Sprague-Dawley rats were treated with 1.5 g/kg/day of EtOH (20%, vol/vol) via intraperitoneal injection for 28 days, followed by a 3-day withdrawal. During withdrawal, the rats were given intragastric sauchinone (2.5, 7.5, or 25 mg/kg/day) once a day. EtOH locomotor sensitization was determined by challenging EtOHW rats with 0.75 g/kg EtOH, while EtOHW-induced anxiety was assessed using the elevated plus maze (EPM). None of the three doses of sauchinone affected EtOH locomotor sensitization. However, in the EPM, treatment of EtOHW rats with sauchinone at 7.5 or 25 mg/kg/day increased both the number of entries into and the time spent in the open arms. Moreover, the two doses of sauchinone inhibited the oversecretion of plasma corticosterone during EtOHW. In the bed nucleus of the stria terminalis (BNST), EtOHW increased NO production, enhanced gene and protein expression of both inducible nitric oxide synthase (iNOS) and neuronal NOS (nNOS), and also elevated protein levels of corticotropin-releasing factor, which were all inhibited by 25 mg/kg/day sauchinone. In an in vitro experiment, sauchinone (3, 10, and 30 μM) inhibited H₂O₂-stimulated nNOS protein expression in neuronal PC12 cells. Finally, intra-BNST infusion of sodium nitroprusside, a NO donor, after sauchinone (25 mg/kg/day) administration, abolished its expected anxiolytic effect. Taken together, these results indicate that sauchinone attenuates anxiety-like behavior in rats during EtOHW but spares EtOH locomotor sensitization, and the anxiolytic effect is mediated via the NO signaling pathway in the BNST.

Isoliquiritigenin Attenuates Anxiety-Like Behavior and Locomotor Sensitization in Rats after Repeated Exposure to Nicotine

As important components of positive and negative reinforcement, locomotor sensitization and withdrawal anxiety following repeated exposure to nicotine (NIC) constitute crucial risk factors for relapse to NIC use after abstinence. Glycyrrhiza radix (G. radix), an important tonic used in traditional Oriental medicine, has not only anxiolytic effects but also reduces NIC-induced locomotor sensitization. Isoliquiritigenin (ISL), a bioactive ingredient of G. radix, also exhibits neuropharmacological effects, including anxiolytic action. Previously, we reported that ISL suppressed cocaine-induced extracellular dopamine release in the nucleus accumbens shell (NaccSh) and attenuated methamphetamine-induced neurotoxicity. The present study was performed to evaluate the effects of ISL on both NIC withdrawal anxiety and locomotor sensitization. Adult male rats received subcutaneous administration of NIC hydrogen tartrate (0.4 mg/kg, twice a day) for 7 days followed by 4 days of withdrawal. During the period of NIC withdrawal, the rats received four intragastric treatments with ISL (3, 10, or 30 mg/kg/day). All three doses of ISL significantly inhibited NIC withdrawal-induced anxiety-like behaviors in the elevated plus maze (EPM) test, but only the 10 mg/kg/day and 30 mg/kg/day ISL doses attenuated locomotor sensitization induced by a challenge dose of NIC. Intracerebroventricular ISL also inhibited both NIC-induced withdrawal anxiety and locomotor sensitization, but intra-NaccSh injection of ISL blocked only NIC locomotor sensitization, which was abolished by post-ISL infusion of tert-butyl hydroperoxide (an oxidant) or N-methyl-d-aspartate (NMDA) into the NaccSh. Moreover, there was increased protein expression of phosphorylated Erk1/2 in the NIC-sensitized NaccSh, which was suppressed by ISL. Taken together, these results suggest that ISL can inhibit repeated NIC-induced withdrawal anxiety and locomotor sensitization, and the latter is mediated by antagonizing accumbal reactive oxygen species and NMDA receptor signaling.

A Proof-of-Mechanism Study to Test Effects of the NMDA Receptor Antagonist Lanicemine on Behavioral Sensitization in Individuals With Symptoms of PTSD

Background: Individuals with post-traumatic stress disorder (PTSD) have a heightened sensitivity to subsequent stressors, addictive drugs, and symptom recurrence, a form of behavioral sensitization. N-methyl-D-aspartate receptors (NMDARs) are involved in the establishment and activation of sensitized behavior. Objective: We describe a protocol of a randomized placebo-controlled Phase 1b proof-of-mechanism trial to examine target engagement, safety, tolerability, and possible efficacy of the NMDAR antagonist lanicemine in individuals with symptoms of PTSD (Clinician Administered PTSD Scale [CAPS-5] score ≥ 25) and evidence of behavioral sensitization measured as enhanced anxiety-potentiated startle (APS; T-score ≥ 2.8). Methods: Subjects (n = 24; age range 21-65) receive three 60-min intravenous infusions of placebo or 100 mg lanicemine over 5 non-consecutive days. Primary endpoint is change in APS from pre-treatment baseline to after the third infusion. NMDAR engagement is probed with resting state EEG gamma band power, 40 Hz auditory steady state response, the mismatch negativity amplitude, and P50 sensory gating. Change in CAPS-5 scores is an exploratory clinical endpoint. Bayesian statistical methods will evaluate endpoints to determine suitability of this agent for further study. Conclusion: In contrast to traditional early-phase trials that use symptom severity to track treatment efficacy, this study tracks engagement of the study drug on expression of behavioral sensitization, a functional mechanism likely to cut across disorders. This experimental therapeutics design is consistent with recent NIMH-industry collaborative studies, and could serve as a template for testing novel pharmacological agents in psychiatry. Clinical Trial Registration: www.ClinicalTrials.gov, identifier NCT03166501.

Nitric oxide blockade in mediodorsal thalamus impaired nicotine/ethanol-induced memory retrieval in rats via inhibition of prefrontal cortical pCREB/CREB signaling pathway

Reciprocal connections between the mediodorsal thalamic nucleus (MD) and the prefrontal cortex (PFC) are important for memory processes. Since the co-abuse of nicotine and ethanol affects memory formation, this study investigated the effect of nitric oxide inhibition in the MD on memory retrieval induced by co-administration of nicotine and ethanol. Subsequently, western blot analysis was used to evaluate how this change would alter the PFC pCREB/CREB signaling pathway. Male Wistar rats were bilaterally cannulated into the MD and the memory retrieval was measured by passive avoidance task. Intraperitoneal (i.p.) administration of ethanol (1 g/kg, i.p) 30 min before the test impaired memory retrieval and caused ethanol-induced amnesia. Subcutaneous (s.c.) administration of nicotine (0.05-0.2 mg/kg, s.c.) prevented ethanol-induced amnesia and improved memory retrieval. Intra-MD microinjection of a nitric oxide synthase (NOS) inhibitor, L-NAME (0.5-1 μg/rat) inhibited the improving effect of nicotine (0.2 mg/kg, s.c.) on ethanol-induced amnesia, while intra-MD microinjection of a precursor of nitric oxide, l-arginine (0.25-1 μg/rat), potentiated such effect. Noteworthy, intra-MD microinjection of the same doses of L-NAME or l-arginine by itself had no effect on memory retrieval. Furthermore, intra-MD microinjection of L-NAME (0.05, 0.1 and 0.3 μg/rat) reversed the l-arginine improving effect on nicotine response. Successful memory retrieval significantly increased the p-CREB/CREB ratio in the PFC tissue. Ethanol-induced amnesia, however, decreased this ratio in the PFC while the co-administration of nicotine and ethanol increased the PFC CREB signaling. Interestingly, the inhibitory effect of L-NAME and the potentiating effect of l-arginine on nicotine response were associated with the decrease and increase of the PFC p-CREB/CREB ratio respectively. It can be concluded that MD-PFC connections are involved in the combined effects of nicotine and ethanol on memory retrieval. The mediodorsal thalamic NO system possibly mediated this interaction via the pCREB/CREB signaling pathways in the PFC.

Evidence for the involvement of NMDA receptors in the antidepressant-like effect of nicotine in mouse forced swimming and tail suspension tests

RATIONALE

The antidepressant action of acute nicotine administration in clinical and animal studies is well recognized. But the underlying mechanism for this effect has not been carefully discovered.

OBJECTIVES

We attempted to evaluate the possible role of N-Methyl-D-aspartate (NMDA) receptors in the antidepressant-like effect of nicotine.

METHODS

After the assessment of locomotor activity in the open-field test, forced swimming test (FST) and tail suspension test (TST) were used to evaluate the antidepressant-like effect of nicotine in mice. We performed intraperitoneal administration of nicotine at different doses and periods before the tests. To assess the possible involvement of NMDA receptors, non-effective doses of NMDA antagonists and an NMDA agonist were obtained and were administered simultaneously with the non-effective and effective doses of nicotine, respectively.

RESULTS

Nicotine (0.2 mg/kg, 30 min before FST/TST) significantly reduced the immobility time of mice similar to fluoxetine (20 mg/kg). Nicotine did not affect the locomotor behavior of mice in open-field test. Co-administration of non-effective doses of NMDA receptor antagonists, ketamine (1 or 0.3 mg/kg), MK-801 (0.05 or 0.005 mg/kg), and magnesium sulfate (10 or 5 mg/kg) with nicotine (0.1 or 0.03 mg/kg) had remarkable synergistic antidepressant effect in both FST and TST. Also, non-effective NMDA (75 or 30 mg/kg) reversed the anti-immobility effect of nicotine (0.2 mg/kg) on mouse FST and TST.

CONCLUSIONS

Our study has for the first time confirmed that the antidepressant-like effect of nicotine on mice is NMDA-mediated, and nicotine presumably exerts this effect by antagonizing the glutamatergic NMDA receptors.

Striatal NOS1 has dimorphic expression and activity under stress and nicotine sensitization

Nicotine exerts its addictive influence through the meso-cortico-limbic reward system, where the striatum is essential. Nicotine addiction involves different neurotransmitters, nitric oxide (NO) being especially important, since it triggers the release of the others by positive feedback. In the nervous system, NO is mainly produced by nitric oxide synthase 1 (NOS1). However, other subtypes of synthases can also synthesize NO, and little is known about the specific role of each isoform in the process of addiction. In parallel, NOS activity and nicotine addiction are also affected by stress and sexual dimorphism. To determine the specific role of this enzyme, we analyzed both NOS expression and NO synthesis in the striatum of wild-type and NOS1-knocked out (KO) mice of both sexes in situations of nicotine sensitization and stress. Our results demonstrated differences between the caudate-putamen (CP) and nucleus accumbens (NA). With respect to NOS1 expression, the CP is a dimorphic region (27.5% lower cell density in males), but with a stable production of NO, exclusively due to this isoform. Thus, the nitrergic system of CP may not be involved in stress or nicotine addiction. Conversely, the NA is much more variable and strongly involved in both situations: its NO synthesis displays dimorphic variations at both basal (68.5% reduction in females) and stress levels (65.9% reduction in males), which disappear when nicotine is infused. Thus, the KO animals showed an increase in NO production (21.7%) in the NA, probably by NOS3, in an attempt to compensate the lack of NOS1.

Sex-influence of nicotine and nitric oxide on motor coordination and anxiety-related neurophysiological responses

RATIONALE

Nitric oxide (NO) is a messenger synthesized in both the neuronal and glial populations by nitric oxide synthase type 1 (NOS1). Nicotine regulates NO production in a sex-dependent manner, both molecules being involved in motor function.

OBJECTIVE

The present study evaluates sex differences in motor coordination, general movement, and anxiety-related responses resulting from both constant and continuous nicotine treatment and the genetic depletion of NOS1 activity.

METHODS

Male and female mice were analyzed with the open-field and the rotarod tests. To understand the role of NO, knockout mice for NOS1 (NOS1-/-) were analyzed. Nicotine was administered continuously at a dose of 24 mg/kg/day via osmotic mini-pumps over 14 days because the behavioral effects elicited are similar to those observed with discontinuous administration.

RESULTS

Data analyses revealed noteworthy sex differences derived from NOS1 depletion. Control NOS1-/- males exhibited an exacerbated anxiety-related response in relation to control NOS1-/- females and control wild-type (WT) males; these differences disappeared in the nicotine-administered NOS1-/- males. Additionally, nicotine administration differentially affected the horizontal movements of NOS1-/- females with respect to WT animals. NO depletion affected male but not female motor coordination improvement along the test days. However, the drug affected female motor coordination only at the end of the administration period.

CONCLUSIONS

We show for the first time that NO affects motor and anxiety behaviors in a sex-dependent manner. Moreover, the behavioral effects of constant nicotine administration are dimorphic and dependent on NO production.

Nicotine-, tobacco particulate matter- and methamphetamine-produced locomotor sensitisation in rats

RATIONALE

Repeated nicotine exposure produces a weak and transient sensitised locomotor response in rats. Since tobacco smoke contains thousands of non-nicotine chemical constituents, these could alter the sensitised response.

OBJECTIVES

This study aims to compare the magnitude, persistence and spatial distribution of locomotor sensitisation produced by repeated doses of nicotine, aqueous tobacco particulate matter (TPM) and a positive methamphetamine control.

METHODS

Male Sprague-Dawley rats received five nicotine (0.0, 0.2 or 0.4 mg/kg), TPM (containing 0.2 or 0.4 mg/kg nicotine) or methamphetamine (0.5 mg/kg) injections every second day, followed by a 4-day withdrawal before the first challenge (Challenge 1, C1). The animals were re-challenged again at 15 days post C1 to test for the persistence of sensitisation (Challenge 2, C2).

RESULTS

There were no major differences in sensitisation profile between nicotine and TPM. At the lowest 0.2 mg/kg nicotine/TPM dose, however, small differences emerged on select test days.

CONCLUSIONS

The results indicated that the non-nicotinic agents in TPM did not greatly impact the nicotine-produced locomotor-sensitised response. These findings might suggest that the differential pharmacological properties of TPM do not have major clinical significance. Alternatively, the locomotor model might not expose effects of non-nicotinic constituents, and furthermore, might not closely relate to human tobacco dependence. Different reward-related behavioural models should also be utilised to assess potential effects of non-nicotinic constituents before a role in dependence is discounted.

Effects of L-arginine and N(ω)-nitro-L-arginine methylester on learning and memory and α7 nAChR expression in the prefrontal cortex and hippocampus of rats

Nitric oxide (NO) is a novel type of neurotransmitter that is closely associated with synaptic plasticity, learning and memory. In the present study, we assessed the effects of L-arginine and N(ω)-nitro-L-arginine methylester (L-NAME, a nitric oxide synthase inhibitor) on learning and memory. Rats were assigned to three groups receiving intracerebroventricular injections of L-Arg (the NO precursor), L-NAME, or 0.9% NaCl (control), once daily for seven consecutive days. Twelve hours after the last injection, they underwent an electric shock-paired Y maze test. Twenty-four hours later, the rats' memory of the safe illuminated arm was tested. After that, the levels of NO and α7 nicotinic acetylcholine receptor (α7 nAChR) in the prefrontal cortex and hippocampus were assessed using an NO assay kit, and immunohistochemistry and Western blots, respectively. We found that, compared to controls, L-Arg-treated rats received fewer foot shocks and made fewer errors to reach the learning criterion, and made fewer errors during the memory-testing session. In contrast, L-NAME-treated rats received more foot shocks and made more errors than controls to reach the learning criterion, and made more errors during the memory-testing session. In parallel, NO content in the prefrontal cortex and hippocampus was higher in L-Arg-treated rats and lower in L-NAME rats, compared to controls. Similarly, α7 nAChR immunoreactivity and protein expression in the prefrontal cortex and hippocampus were higher in L-Arg-treated rats and lower in L-NAME rats, compared to controls. These results suggest that the modulation of NO content in the brain correlates with α7 nAChR distribution and expression in the prefrontal cortex and hippocampus, as well as with learning and memory performance in the Y-maze.

Nicotine withdrawal upregulates nitrergic and galaninergic activity in the rat dorsal raphe nucleus and locus coeruleus

The dorsal raphe nucleus (DRN), a major source of forebrain serotonin, mediates various neural functions including anxiety. The nucleus locus coeruleus (LC) is likewise involved in mediating central components of the stress response and anxiety. An anxiety-reducing effect is widely believed to underlie many cases of nicotine dependence. While much is known about nicotine-serotonin interactions, little is known about how nicotine engages the DRN non-serotonergic domain in specific physiological functions that influence organismal behavior. The aim of this study was to determine how chronic nicotine withdrawal influences neuronal nitric oxide (NO) synthase (nNOS) and galanin immunoreactivity in the DRN and LC of adult rats. Compared with saline, nicotine increased nicotinamide adenine dinucleotide phosphate diaphorase profiles within distinct DRN subregions and also enhanced intensity in nNOS and galanin cell bodies in the rostral DRN as well as galanin in the LC. Nicotine-induced nNOS/galanin staining of somata was abundant in the rostral ventromedial DRN. Galanin-positive terminals surrounded nNOS-containing cell bodies in the DRN lateral wing subregions. These observations suggest that the DRN NOS-galanin domain and galanin in the LC are engaged in the organism's neural adaptation to chronic nicotine exposure. Hence NO and galanin synthesized or released within the DRN and LC or at the respective target sites might regulate the whole animal behavioral response to nicotine exposure.

Repeated treatment with nicotine induces phosphorylation of NMDA receptor NR2B subunit in the brain regions involved in behavioral sensitization

Protein phosphorylation is an important mechanism for the post-translational modulation of N-methyl-d-aspartate (NMDA) receptor functions. In the present study, we investigated the levels of NR2B phosphorylation at Tyr1472 and Ser1303 in the nucleus accumbens, striatum, frontal cortex, and hippocampus of rats that exhibit behavioral sensitization to nicotine. Repeated treatment of rats with nicotine (0.6mg/kg, s.c., for 7 days) produced locomotor sensitization accompanied by increased NR2B phosphorylation at Tyr1472 in the nucleus accumbens and striatum, brain regions involved in behavioral sensitization. In contrast, no changes in NR2B phosphorylation were observed after a single treatment with nicotine in these brain regions. In addition, no changes in NR2B phosphorylation at Ser1303 were observed after repeated treatment with nicotine in any examined brain regions. These results suggest that repeated treatment with nicotine induces NR2B phosphorylation at Tyr1472 in the nucleus accumbens and striatum, which might contribute to the development of synaptic and behavioral plasticity in response to nicotine.

Effects of risperidone on development and expression of nicotine-induced locomotor sensitization in rats

It has been shown that atypical antipsychotics significantly reduce smoking and alcohol consumption in schizophrenic patients. However, our knowledge about the effect of risperidone, especially on nicotine abuse is limited. We aimed to test the effects of risperidone in an animal model of nicotine-induced locomotor sensitization, which represents initial neuroadaptations and continued behavioral changes in nicotine-type dependence. To investigate the effect of risperidone on the development of nicotine-induced locomotor sensitization, rats were pretreated with risperidone (0.025 and 0.050 mg kg⁻¹) 30 min before the nicotine (0.5 mg kg⁻¹, base) treatment, and locomotor activity was recorded for 30 min. This procedure was repeated every day for eight sessions. After a 6-day drug-free period, rats were challenged with nicotine (0.5 mg kg⁻¹). To reveal the effect of risperidone on the expression of nicotine-induced locomotor sensitization, rats were injected with nicotine for eight sessions. After a 6-day drug-free period, rats were pretreated with risperidone (0.025 and 0.050 mg kg⁻¹) or vehicle 30 min before the nicotine (0.5 mg kg⁻¹) challenge injection. Repeated administration of nicotine generated robust locomotor sensitization in rats. Risperidone pretreatment (0.050 mg kg⁻¹) blocked the expression but not the development of nicotine-induced locomotor sensitization in rats. Our results suggest that risperidone blocks the continuation of nicotine-type addictive behavior, but it is ineffective on early adaptations in the initiation of nicotine addiction. Thus, this drug may have a limited beneficial effect in treatment of nicotine dependence.

Repeated agmatine treatment attenuates nicotine sensitization in mice: modulation by alpha2-adrenoceptors

Agmatine [2-(4-aminobutyl)guanidine] is an endogenous amine proposed as a neurotransmitter/neuromodulator that binds to multiple target receptors in brain. Besides, many central and peripheral functions, agmatine have been implicated in the process of drug addiction. The purpose of the present study was to examine the effects of centrally injected agmatine on nicotine induced locomotor sensitization in Swiss male mice. Our data shows that repeated injections of nicotine (0.4 mg/kg, sc, twice daily for 7 days) gradually increased locomotion during 7 days development period or after 3 days (nicotine) withdrawal phase challenged with nicotine (0.4 mg/kg, sc) on day 11. Mice were pretreated with agmatine (40-80 microg, icv) or agents known to increase endogenous brain agmatine levels [e.g. an agmatine biosynthetic precursor, L-arginine (80 microg, icv), ornithine decarboxylase inhibitor, difluoromethyl-ornithine (50 microg, icv), diamine oxidase inhibitor, aminoguanidine (25 microg, icv) and agmatinase inhibitor, arcaine (50 microg, icv)] 30 min before daily first nicotine injection or during nicotine withdrawal phase. All these treatments attenuated the development as well as incubation of locomotor sensitization to nicotine. Coadministration of agmatine (20 microg, icv) and alpha(2)-adrenoreceptors agonist, clonidine (0.1 microg, icv) evoked synergistic inhibition of nicotine sensitization. Conversely, prior administration of alpha(2)-adrenoceptor antagonist, yohimbine (5mg/kg, ip) or idazoxan (0.4 mg/kg, ip) reversed the inhibitory effect of agmatine on nicotine sensitization. There was no significant difference in activity between mice injected with any of these agents/saline alone and saline/saline groups. These data indicate that agmatine attenuates nicotine induced locomotor sensitization via a mechanism which may involve alpha(2)-adrenergic receptors. Thus, agmatine might have therapeutic implications in the treatment of nicotine addiction and deserve further investigations.

Critical role of nitric oxide on nicotine-induced hyperactivation of dopaminergic nigrostriatal system: Electrophysiological and neurochemical evidence in rats

Nicotine, the main psychoactive ingredient in tobacco, stimulates dopamine (DA) function, increasing DA neuronal activity and DA release. DA is involved in both motor control and in the rewarding and reinforcing effects of nicotine; however, the complete understanding of its molecular mechanisms is yet to be attained. Substantial evidence indicates that the reinforcing properties of drugs of abuse, including nicotine, can be affected by the nitric oxide (NO) system, which may act by modulating central dopaminergic function. In this study, using single cell recordings in vivo coupled with microiontophoresis and microdialysis in freely moving animals, the role of NO signaling on the hyperactivation elicited by nicotine of the nigrostriatal system was investigated in rats. Nicotine induced a dose-dependent increase of the firing activity of the substantia nigra pars compacta (SNc) DA neurons and DA and 3,4-dihydroxyphenylacetic acid (DOPAC) release in the striatum. Pharmacological manipulation of the NO system did not produce any change under basal condition in terms of neuronal discharge and DA release. In contrast, pretreatments with two NO synthase (NOS) inhibitors, N-omega-nitro-l-arginine methyl ester (l-NAME) and 7-nitroindazole (7-NI) were both capable of blocking the nicotine-induced increase of SNc DA neuron activity and DA striatal levels. The effects of nicotine in l-NAME and 7-NI-pretreated rats were partially restored when rats were pretreated with the NO donor molsidomine. These results further support the evidence of an important role played by NO on modulation of dopaminergic function and drug addiction, thus revealing new pharmacological possibilities in the treatment of nicotine dependence and other DA dysfunctions.

Sensitization to nicotine significantly decreases expression of GABA transporter GAT-1 in the medial prefrontal cortex

This study investigated GABA signaling following induction of behavioural sensitization to nicotine. Rats were repeatedly injected with saline, nicotine or hexamethonium for 18 days and gene expression was measured with qPCR. Nicotine upregulated GABAA alpha1 subunit expression in the nucleus accumbens (p<0.05) while no changes were observed for GABAA alpha3, alpha4 or alpha5. In the medial prefrontal cortex, no change in expression of the GABAA subunits was observed. We found that nicotine significantly decreased expression of the transporter GAT-1/SLC6A1 (p<0.05) in the medial prefrontal cortex while the expression of the GAT-3/SLC6A11 (p<0.05) transporter was increased in the nucleus accumbens. This provides the first evidence of neuroadaptive changes in the GABA system after nicotine sensitization and the first demonstration of an effect on GAT-1 or GAT-3 transporters in the addiction field. The GAT-1 findings also provide evidence for an alternative theory of why most schizophrenic individuals also use tobacco products.

Oxytocin induces penile erection when injected into the ventral tegmental area of male rats: role of nitric oxide and cyclic GMP

Oxytocin (80 ng) injected into the caudal mesencephalic ventral tegmental area (VTA) of male rats induces penile erection. Such an effect occurs together with an increase in nitric oxide (NO) production, as measured by the augmented concentration of NO(2)(-) and NO(3)(-) found in the dialysate obtained from this brain area by means of intracerebral microdialysis. Both effects are abolished by d(CH(2))(5)Tyr(Me)(2)-Orn(8)-vasotocin (1 microg), an oxytocin receptor antagonist, by S-methyl-l-thiocitrulline acetate (20 microg), a neuronal NO synthase inhibitor, or by omega-conotoxin GVIA (50 ng), a N-type Ca(2+) channel blocker, all injected into the VTA 15 min before oxytocin. In contrast, 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (40 microg), a guanylate cyclase inhibitor, given into the VTA 15 min before oxytocin, abolishes penile erection, but not the increase in NO production, while haemoglobin (40 microg), a NO scavenger, injected immediately before oxytocin reduces the increase in NO production, but not penile erection. 8-Bromo-cyclic guanosine monophosphate (0.5-10 microg) microinjected into the VTA induces penile erection with an inverted U-shaped dose-response curve; the maximal effective dose being 3 microg. Immunohistochemistry reveals that in the caudal VTA oxytocin-containing axons/fibres (originating from the paraventricular nucleus of the hypothalamus) contact cell bodies of mesolimbic dopaminergic (tyrosine hydroxylase-positive) neurons containing both NO synthase and guanylate cyclase. These results suggest that oxytocin injected into the VTA induces penile erection by activating NO synthase in the cell bodies of mesolimbic dopaminergic neurons. NO in turn activates guanylate cyclase present in these neurons, thereby increasing cyclic GMP concentration.

The role of NMDA receptor antagonists in nicotine tolerance, sensitization, and physical dependence: a preclinical review

Nicotine, the primary psychoactive component of tobacco products, produces diverse neurophysiological, motivational, and behavioral effects through several brain regions and neurochemical pathways. Various neurotransmitter systems have been explored to understand the mechanisms behind nicotine tolerance, dependence, and withdrawal. Recent evidence suggests that glutamate neurotransmission has an important role in this phenomenon. The aim of the present review is to discuss preclinical findings concerning the role of N-methyl-D-aspartate (NMDA) receptor neurotransmission in mediating the behavioral effects of nicotine, tolerance, sensitization, dependence, and withdrawal. Based on preclinical findings, it is hypothesized that NMDA receptors mediate the common adaptive processes that are involved in the development, maintenance, and expression of nicotine addiction. Modulation of glutamatergic neurotransmission with NMDA receptor antagonists may prove to be useful in alleviating the symptoms of nicotine abstinence and facilitate tobacco-smoking cessation.

Neurochemical pathways involved in the protective effects of nicotine and ethanol in preventing the development of Parkinson's disease: potential targets for the development of new therapeutic agents

In this short review, neurochemical targets are identified where nicotine, and possibly ethanol, may interact to prevent the occurrence of Parkinson's disease. These are (a) the nicotinic acetycholine receptors present in the nigrostriatal area or on the surface of microglia, (b) monoamine oxidases and (c) inducible nitric oxide synthase. If such induced changes can be verified in clinical studies, this may help in the design of new therapeutic drugs which may be of relevance to diminish the incidence and perhaps the progression of the debilitating condition of Parkinson's disease.

NMDA receptors in the rat VTA: a critical site for social stress to intensify cocaine taking

RATIONALE

Cocaine strengthens behaviors associated with its administration. The stress response by individuals that are defeated in a brief aggressive confrontation can also promote enduring behavioral consequences similar to those of stimulants.

OBJECTIVES

The study intends to find whether intermittent episodes of defeat promote cocaine's reinforcing effects by triggering N-methyl-D: -aspartic acid (NMDA)-receptor-mediated plasticity in the ventral tegmental area (VTA).

MATERIALS AND METHODS

Long-Evans rats were investigated after four social defeats in three experiments. Two experiments examined systemic or intra-VTA antagonism of NMDA receptors during stress on the later expression of behavioral sensitization and cocaine self-administration during fixed and progressive ratio (PR) schedules of reinforcement (0.3 mg/kg/infusion), including a novel 24-h variable-dose continuous access binge (0.2, 0.4, and 0.8 mg/kg/infusion, delivered in an irregular sequence). Third, the expression of receptor proteins NR1 (NMDA) and GluR1 [alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)] were examined in VTA and nucleus accumbens.

RESULTS

Intermittent defeats augment locomotor responses to cocaine and increase cocaine taking. Rates of responding during binges are increased after defeat stress. These effects are prevented when NMDA or AMPA receptor antagonists are administered before defeats. VTA infusions of the NMDA antagonist AP-5 (5 nmol/side) before stress prevents locomotor sensitization to cocaine and intensified responding for cocaine during a PR schedule or binge. Episodic defeats increase GluR1 AMPA subunit protein expression in the VTA.

CONCLUSIONS

Social stress stimulates NMDA receptors in the VTA, and this neural action of defeat may be essential for prompting a later increase in cocaine intake during binges.

Behavioral Cross-Tolerance between Repeated Intracerebellar Nicotine and Acute Δ9-Tetrahydrocannabinol-Induced Cerebellar Ataxia: Role of Cerebellar Nitric Oxide

We have previously demonstrated that acute intracerebellar nicotine or N-methyl-4-(3-pyridinyl)-3-buten-1-amine (RJR-2403), a selective alpha(4)beta(2) nicotinic acetylcholine receptor (nAChR) agonist, dose dependently attenuates Delta(9)-tetrahydrocannabinol (Delta(9)THC)-induced ataxia. Presently, we have shown that intracerebellar nicotine (1.25, 2.5, and 5 ng; once daily for 5 days) and RJR-2403 (250, 500, and 750 ng; once daily for 5 days) significantly attenuate cerebellar Delta(9)-THC-induced ataxia dose dependently, suggesting the development of cross-tolerance between nicotine or RJR-2403 with Delta(9)-THC in male CD-1 mice. Intracerebellar RJR-2403 (750 ng) microinfused for 1, 2, 3, 5, and 7 days (once daily) significantly attenuated Delta(9)-THC-induced ataxia in the 3-, 5-, and 7-day treatment groups; optimal cross-tolerance was evident at day 5 and persisted till 36 h after the last RJR-2403 microinfusion. Intracerebellar microinfusion of hexamethonium (nAChR antagonist; 1 microg) or dihydro-beta-erythroidine hydrobromide (alpha(4)beta(2) nAChR antagonist; 500 ng) for 5 days 10 min before daily intracerebellar nicotine or RJR-2403 microinfusion virtually abolished cross-tolerance between nicotine or RJR-2403 and Delta(9)-THC, indicating nAChR participation. In addition, microinfusion of antagonists 10 min after daily intracerebellar nicotine or RJR-2403 failed to alter the cross-tolerance, suggesting possible involvement of downstream cerebellar second-messenger mechanisms. Finally, the cerebellar concentration of nitric oxide products [total sum of nitrite + nitrate (NO(x))] was increased after 5 days of intracerebellar nicotine or RJR-2403 treatment, which was decreased by acute intracerebellar Delta(9)-THC treatment. The "nicotine or RJR-2403 + Delta(9)-THC" treatments significantly increased cerebellar NO(x) levels compared with treatment with Delta(9)-THC alone, supporting a functional correlation between cerebellar nitric oxide production and cerebellar Delta(9)-THC-induced ataxia and suggesting participation of nitric oxide in the observed cross-tolerance between nicotine/RJR-2403 and Delta(9)-THC.

Behavioral and neuropharmacological characterization of nicotine as a conditional stimulus

In rats, the pharmacological (interoceptive) effects of 0.4 mg/kg nicotine can serve as a conditional stimulus in a Pavlovian conditioning task. Nicotine administration is paired with intermittent access to a liquid sucrose unconditional stimulus; sucrose is withheld on saline sessions. An increase in sucrose receptacle entries (goal tracking) on nicotine sessions indicates conditioning. Rats were trained on a nicotine dose ((-)-1-Methyl-2-(3-pyridyl)pyrrolidine; 0.1, 0.2, or 0.4 mg base/kg, s.c.). Generalization was examined using 0.025, 0.05, 0.1, 0.2, and 0.4 mg/kg nicotine and saline. Some behavioral effects of nicotine have been attributed to dopamine and glutamate. Accordingly, potential blockade of the nicotine cue via the dopamine system was examined by administering (R)-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH-23390; 0.005, 0.01, and 0.03 mg/kg), 3-Chloro-5-ethyl-N-[[(2S)-1-ethyl-2-pyrrolidinyl)methyl]-6-hydroxy-2-methoxy-benzamide hydrochloride (eticlopride; 0.01, 0.03, 0.1, and 0.3 mg/kg), or N-[(1-Butyl-2-pyrrolidinyl)methyl]-4-cyano-1-methoxy-2-naphthalenecarboxamide (nafadotride; 0.03, 0.1, 0.3, 1, and 3 mg/kg) before nicotine. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP; 0.3, 1, and 3 mg/kg) and (5S,10R)-(+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801; 0.01, 0.03, 0.1, and 0.2 mg/kg; dizocilpine) were used to examine possible glutamatergic components. Substitution tests were conducted with MPEP and nafadotride. Differential conditioned responding was acquired in the 3 groups. Conditioned responding generally decreased as the nicotine test dose moved away from the training dose; responding increased when 0.4 mg/kg trained rats were tested with 0.2 mg/kg. SCH-23390, eticlopride, nafadotride, and MPEP decreased conditioned responding on nicotine at doses that also decreased chamber activity. In contrast, MK-801 decreased goal tracking on nicotine without decreasing chamber activity, indicating a role for N-methyl-D-aspartate receptors in expression of nicotine-evoked conditioned responding.

Ascorbic acid antagonizes nicotine-induced place preference and behavioral sensitization in mice

In the present study, the influence of ascorbic acid on the nicotine-induced behavioral sensitization and conditioned place preference was investigated in mice. In the place preference paradigm, intraperitoneal (i.p.) nicotine (1 and 1.5 mg/kg, three drug sessions) but not ascorbic acid (1, 10, 100 and 1000 mg/kg) administration induced place preference. Ascorbic acid administration (10, 100 and 1000 mg/kg, i.p.) reduced both the acquisition and expression of nicotine-induced place conditioning. Locomotor sensitization in mice was produced by intraperitoneal injection of nicotine (0.25 mg/kg) for 7 consecutive days. On the 9th day of the experiments, activity of the mice was recorded after challenge with nicotine (0.1 mg/kg, i.p.). Ascorbic acid (10, 100 and 1000 mg/kg, i.p.) was injected 20 min before each injection of nicotine (acquisition of sensitization) or acutely 20 min before a challenge nicotine injection (expression of sensitization). It was shown that ascorbic acid attenuated the acquisition of nicotine sensitization in a dose-independent manner but the expression of nicotine-induced sensitization was not affected by ascorbic acid. In conclusion, it seems that ascorbic acid may interfere with nicotine-induced place preference and behavioral sensitization in mice.

Effect of nitric oxide synthase inhibitor and NMDA receptor antagonist on the development of nicotine sensitization of nucleus accumbens dopamine release: an in vivo microdialysis study

We have previously found that the neuronal nitric oxide synthase inhibitor N-nitro-L-arginine (L-NNA) and the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 prevent behavioral sensitization to nicotine. This study aimed to investigate the effect of L-NNA and MK-801 on a neurochemical component of nicotine sensitization by evaluating the effect of the drugs on nicotine sensitization of nucleus accumbens dopamine (DA) release. Sprague-Dawley rats were pretreated with L-NNA (15 mg/kg, i.p.), MK-801 (0.3mg/kg, i.p.), or saline 30 min before injection of nicotine (0.4 mg/kg, s.c., once daily) for seven consecutive days. Twenty-four hours after the last drug injection, animals were challenged with local perfusion of 5 mM nicotine into the shell of nucleus accumbens for 60 min and DA release was monitored using in vivo microdialysis. In rats treated with repeated nicotine, acute nicotine challenge induced a greater increase of accumbal DA release than in saline-treated animals (maximal DA response=969+/-235% (mean+/-S.E.M.) of basal level versus 520+/-93%, p=0.042). Co-administration of L-NNA or MK-801 with nicotine attenuated an increase of DA release elicited by acute nicotine challenge, compared with nicotine alone (maximal DA response=293+/-58% and 445+/-90% of basal level, respectively versus 969+/-235%, p=0.004 and p=0.013, respectively). These data demonstrate that L-NNA and MK-801 block the development of nicotine sensitization of nucleus accumbens DA release, further supporting the involvement of nitric oxide and NMDA receptors in the development of behavioral sensitization to nicotine.

Involvement of the alpha4beta2 nicotinic receptor subtype in nicotine-induced attenuation of delta9-THC cerebellar ataxia: role of cerebellar nitric oxide

We have recently reported that mediation of intracerebellar nicotine-induced attenuation of cerebellar delta9-THC ataxia was via the alpha4beta2 nAChR. The present study was meant to investigate the role of cerebellar nitric oxide (NO)-guanylyl cyclase (GC) signaling in the alpha4beta2-mediated attenuation in CD-1 male mice. Drugs were given via intracerebellar microinfusion using stereotaxically implanted guide cannulas, with ataxia evaluated by Rotorod. Intracerebellar microinfusion of SNP (sodium nitroprusside, NO donor; 15, 30, 60 pg) and SMT (S-methylisothiourea, inhibitor of inducible NO synthase; 70, 140, 280 fg) significantly enhanced and reduced, respectively, intracerebellar RJR-2403 (selective alpha4beta2 agonist)-induced attenuation of delta9-THC ataxia dose-dependently. Intracerebellar isoliquiritigenin (GC-activator; 1, 2, 4 pg) and ODQ (1H[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, GC inhibitor; 200, 400, 800 fg), significantly enhanced and reduced, respectively, intracerebellar RJR-2403-induced attenuation of delta9-THC ataxia dose-dependently. Further support for the role of NO was evidenced via increases in cerebellar NO(x) (nitrate+nitrite) levels following microinfusion of nicotine or RJR-2403 as compared to control, whereas delta9-THC significantly decreased NO(x) levels. "Nicotine/RJR-2403+delta9-THC" treated mice had cerebellar NO(x) levels significantly increased as compared to mice infused with delta9-THC alone. Results of the present investigation support the role of cerebellar NO-GC signaling in alpha4beta2 nAChR subtype-mediated attenuation of delta9-THC ataxia.

Caffeine and amphetamine produce cross-sensitization to nicotine-induced locomotor activity in mice

Sensitization development is linked to the addictive potential of the drugs. The same mechanisms might play a role in sensitization development to the different addictive drugs. The aim of the study was to investigate the development of cross-sensitization to caffeine and amphetamine in nicotine-induced locomotor sensitization in mice. Caffeine (2.5-20 mg/kg), amphetamine (1-16 mg/kg) or saline were injected to Swiss-Webster mice and locomotor activity was recorded for 30 min. Nicotine (0.5-2 mg/kg) or saline were injected to mice and locomotor activity was recorded for 30 min. Process was applied for 19 days, every other day (10 sessions). Caffeine (5 mg/kg), amphetamine (4 mg/kg) or saline were challenged to the different groups of nicotine-sensitized mice 2 days later on the last nicotine injection, and locomotor activity was recorded. Repetitive injections of nicotine (0.5-2 mg) produced locomotor sensitization in mice. After caffeine and amphetamine challenge injections, locomotor activity of the nicotine-sensitized mice was found to be significantly higher than saline-pretreated mice. Saline challenge did not produce any significant effect in nicotine- or saline-pretreated mice. Our results suggest that a cross-sensitization developed to both caffeine and amphetamine in nicotine-sensitized mice. In conclusion, similar central mechanisms may be responsible for the development of addiction to these substances.

Possible role of mouse cerebellar nitric oxide in the behavioral interaction between chronic intracerebellar nicotine and acute ethanol administration: Observation of cross-tolerance

Many studies have reported cross-tolerance between nicotine and ethanol. Previously we demonstrated that intracerebellar nicotine attenuates ethanol-induced motor impairment. In this study, intracerebellar nicotine (0.625, 2.5, 5 ng; once daily for five days) significantly attenuated ethanol-induced motor impairment in a dose-dependent fashion suggesting the development of cross-tolerance between nicotine and ethanol in male CD-1 mice. Using the same paradigm, intracerebellar nicotine (5 ng) microinfused for 1, 2, 3, 5, 7 days significantly attenuated ethanol-induced motor impairment in all groups except the 1-day treatment group. Cross-tolerance, which developed optimally in 5-day nicotine treatment group, was reversible and detectable up to 40 h post-nicotine microinfusion. Intracerebellar microinfusion of hexamethonium (1 mug once daily for 5 days): (i) did not alter ethanol-induced motor impairment indicating no tonic nicotine receptor involvement; (ii) 10 min prior to daily intracerebellar nicotine treatment virtually abolished the cross-tolerance between nicotine and ethanol indicating nicotinic acetylcholine receptor participation; (iii) when microinfused 10 min after daily intracerebellar nicotine treatment, failed to abolish the cross-tolerance which suggested possible participation of downstream second messenger mechanisms. Chronic intracerebellar microinfusion of nicotine: (i) failed to attenuate acute pentobarbital (25mg/kg i.p.)-induced motor impairment; and (ii) produced no change in normal motor coordination when followed by saline injection. Finally, the cerebellar concentration of total nitric oxide products (nitrite+nitrate; NO(x)); (i) was increased after 5-day intracerebellar nicotine; (ii) was decreased by acute ethanol administration; and (iii) decreased due to acute ethanol administration which was opposed by chronic intracerebellar nicotine treatment. These results support a functional correlation between the cerebellar nitric oxide production and ethanol-induced motor impairment and suggest possible participation of nitric oxide as a factor in the observed cross-tolerance between nicotine and ethanol.

DARPP-32 phosphorylation opposes the behavioral effects of nicotine

BACKGROUND

The addictive properties of nicotine are mediated via dopaminergic pathways and their post-synaptic neurons in the striatum. Because post-synaptic neurons within the striatum contain high levels of the dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32), we hypothesized that DARPP-32 may functionally contribute to the behavioral effects of nicotine.

METHODS

We examined the behavioral effects of nicotine and the phosphorylation state of DARPP-32 in wild-type (WT) and DARPP-32 knockout (KO) mice. In one experiment, we assessed voluntary nicotine intake (0-50 microg/ml) of WT and KO mice in a two-bottle choice paradigm. In a separate experiment, the motor-depressant effects of acute and repeated nicotine injections (0-.8 mg/kg, subcutaneously [SC]) were assessed. The phosphorylation of DARPP-32 at threonine34 and threonine75 were examined using Western blotting.

RESULTS

A heightened responsiveness to nicotine was seen in KO mice when compared with WT mice in oral intake and motor depression. The enhanced responsiveness in KO mice was not due to alterations in taste sensations, fluid intake, or blood nicotine or cotinine levels. Systemic injections of nicotine resulted in increased striatal DARPP-32 phosphorylation at threonine34 and threonine75.

CONCLUSIONS

DARPP-32 opposes the behavioral effects of nicotine possibly via concurrent phosphorylation at the two threonine sites.

Reduced nNOS expression induced by repeated nicotine treatment in μ-opioid receptor knockout mice

To determine whether neuronal nitric oxide synthase (nNOS) is involved in nicotine-induced behavioral sensitization in mu-opioid receptor knockout mice we adopted an immunohistochemical approach. Our results confirm that repeated nicotine administration increased locomotor activity in wild-type mice, but failed to increase locomotor activity in mu-opioid receptor knockout mice, thus suggesting that the mu-opioid receptor is involved in behavioral sensitization. Higher numbers of nNOS-positive cells were observed in the striatum of wild-type mice repeatedly treated with nicotine than in saline-treated wild-type mice. However, mu-opioid receptor knockout mice showed significantly lower nicotine-induced nNOS expression in the striatum versus wild-type mice. No differences were found in the hilus of the dentate gyrus between wild-type and mu-opioid receptor knockout mice. These findings demonstrate that the absence of mu-opioid receptors can cause a significant reduction in the expression of nNOS in the striatum, as induced by repeated nicotine treatment.

The effects of nitric oxide on the acquisition and expression of nicotine-induced conditioned place preference in mice

In the present study, the possible role of nitric oxide on the conditioned place preference (CPP) induced by nicotine in mice was investigated. Intraperitoneal (i.p.) injections of nicotine (1 mg/kg) and the nitric oxide (NO) precursor, L-arginine (200 and 500 mg/kg), produced significant place preference. However, injection of mecamylamine (0.05 and 0.1 mg/kg; i.p.) or the NO synthase (NOS) inhibitor, L-Nitro-amino-methyl-ester, L-NAME (5-20 mg/kg; i.p.), had no effect. Ineffective doses of nicotine in combination with ineffective doses of L-arginine produced significant place preference. Administration of L-arginine (50, 100 and 150 mg/kg; i.p.) on the test day reduced the expression of nicotine-induced place preference. Nicotine injection (0.25, 0.5 and 0.75 mg/kg) on the test day reduced the expression of place preference induced by L-arginine, while both mecamylamine (0.05 and 0.1 mg/kg) and L-NAME (5, 10 and 20 mg/kg) inhibited the acquisition of place preference induced by nicotine (1 mg/kg) and L-arginine (200 mg/kg). Moreover, neither of the antagonists reduced the expression of nicotine- or L-arginine-induced place preference. It is suggested that nitric oxide may play an important role in nicotine-induced place preference.

Evidence for the role of nitric oxide in nicotine-induced locomotor sensitization in mice

RATIONALE

Nitric oxide (NO) is implicated in both acute effects of addictive drugs and development of dependence to them. We investigated the role of NO in nicotine-induced locomotor sensitization.

OBJECTIVES

The effects of Nomega-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, and a combination of a NO precursor L-arginine and L-NAME on nicotine-induced locomotor sensitization were investigated in Swiss Webster mice.

METHODS

Sensitization to psychomotor stimulating effect of nicotine was rendered by seven injections of nicotine (1 mg/kg) on every other day. To investigate their effect on the development of sensitization to nicotine, L-NAME (15-60 mg/kg) and L-arginine (1 g/kg) were given before nicotine administration during the first seven sessions. To investigate the effect of these compounds on the expression of nicotine sensitization, after a 4-day drug-free period another group of mice received a challenge injection of nicotine on day 18.

RESULTS

Nicotine (1 mg/kg) produced a robust locomotor sensitization in mice. The doses of 30 mg/kg and 60 mg/kg of L-NAME blocked the development of sensitization to nicotine; and, L-arginine (1 g/kg) pretreatment reversed this effect of L-NAME. Likewise, the doses of 30 mg/kg and 60 mg/kg of L-NAME inhibited the expression of sensitization to nicotine on day 18; and, L-arginine (1 g/kg) pretreatment reversed this inhibitory effect of L-NAME.

CONCLUSIONS

Our results suggest that NO is implicated in the development and expression of nicotine-induced locomotor sensitization in mice.

Rapid delivery of nicotine promotes behavioral sensitization and alters its neurobiological impact

BACKGROUND

Nicotine is highly addictive when it is inhaled from tobacco smoke, whereas nicotine replacement products, which usually deliver nicotine orally or transdermally, rarely lead to addiction. It has been proposed that this is due in part to differences in the rate of nicotine delivery to the brain under the two conditions. However, the mechanism by which rapid nicotine delivery facilitates the transition to addiction is not known. The ability of drugs to alter gene regulation and to produce sensitization has been implicated in addiction. We hypothesized, therefore, that varying the rate of nicotine administration may modulate its ability to elicit this form of plasticity.

METHODS

Animals were treated with repeated intravenous infusions of nicotine over 5, 25, or 100 sec, and their locomotor responses were monitored over treatment days.

RESULTS

We found that increasing the rate of intravenous nicotine infusion potentiated its ability to produce locomotor sensitization, and to induce c-fos and arc mRNA expression in mesocorticolimbic structures.

CONCLUSIONS

We suggest that rapid administration may increase vulnerability to addiction by altering the neurobiological impact of nicotine and promoting a form of neurobehavioral plasticity (i.e., sensitization) that can lead to pathological incentive motivation for drugs.

Effects of voluntary alcohol intake on nicotine-induced behavioural sensitisation in rats

Behavioural sensitisation has been suggested to play a role in the acquisition and maintenance of addictive behaviour. The aim of the present study was to assess nicotine-induced behavioural sensitisation in chronic voluntary alcohol drinking rats. Subjects had free access to alcohol/water or glucose/water solutions since weaning. Rats were pretreated after 2 months of voluntary alcohol drinking. Pretreatment consisted of once-daily intraperitoneal injection of nicotine (0.5 mg/kg) or saline administered for five consecutive days. The nicotine-induced behavioural sensitisation of locomotor activity was tested 3 weeks latter. Horizontal motor activity was monitored for 30 min and expressed as distance travelled (in centimetres). During all the experimental procedure, the animals were maintained under 1-h limited access to alcohol. In glucose-drinking animals, results indicated that nicotine induced locomotor activity sensitization: The locomotor effects of nicotine challenge in the nicotine-pretreated group of rats were significantly enhanced as compared with the saline-pretreated group (Duncan, P<.01). Instead, in the alcohol-drinking animals, no significant differences were observed between the nicotine- and saline-pretreated groups. Thus, chronic alcohol consumption at mild doses prevented the development and/or the long-term expression of the nicotine-induced sensitisation at the doses tested.

Stress during adolescence enhances locomotor sensitization to nicotine in adulthood in female, but not male, rats

A wide body of research has indicated that perinatal exposure to stressors alters the organism, notably by programming behavioral and neuroendocrine responses and sensitivity to drugs of abuse in adulthood. Recent evidence suggests that adolescence also may represent a sensitive period of brain development, and yet there has been little research on the long-lasting effects of stressors during this period. We investigated the effects of pubertal social stress (PS; daily 1-h isolation followed by pairing with a new cage mate on postnatal days 33-48) on locomotor sensitization to injections of nicotine and corticosterone response to restraint stress when the rats were adults (approximately 3 weeks after PS). There were no differences among the groups in locomotor activity to injections of saline. However, PS females had enhanced locomotor sensitization to repeated doses of nicotine compared to control (non-stressed; NS) females, whereas PS males and NS males did not differ. PS enhanced the corticosterone response to restraint in male rats previously sensitized to nicotine and decreased the corticosterone response in nonsensitized male rats. In contrast, PS females and NS females did not differ in plasma corticosterone levels in response to restraint stress, but NS females showed enhanced corticosterone release to restraint after sensitization to nicotine. Thus, during adolescence, social stressors can have long-lasting effects, and the effects appear to differ for males and females.

7-Nitroindazole blocks conditioned place preference but not hyperactivity induced by morphine

The effects of 7-nitroindazole (7-NI), a neural nitric oxide synthase (nNOS) inhibitor, on spontaneous locomotor activity, morphine-induced hyperactivity, acquisition of place conditioning and morphine-induced conditioned place preference (CPP) were evaluated in male mice. In experiment 1, animals treated with 7-NI (25, 50 and 100mg/kg), morphine (40 mg/kg) or morphine (40 mg/kg) plus 7-NI (25, 50 or 100mg/kg) were placed in an actimeter for 3h. In experiment 2, animals treated with the same drugs and doses were conditioned following an unbiased procedure. 7-NI did not affect the spontaneous locomotor activity or hyperactivity induced by morphine. However, the moderate and high doses of 7-NI produced conditioned place aversion (CPA) and the lowest dose blocked morphine-induced CPP. Our results suggest that nitric oxide is involved in the rewarding properties of morphine but not in its motor effects.

Nitric oxide is involved in nicotine-induced burst firing of rat ventral tegmental area dopamine neurons

In the present study, using single cell recordings in vivo and intracellular recordings in vitro from midbrain slices, the role of N-methyl-d-aspartate (NMDA) receptor signaling on firing activity in ventral tegmental area dopamine neurons elicited by nicotine was investigated in the rat. In accordance with previous studies, systemic nicotine (0.5 mg/kg s.c.) increased both firing rate and burst firing of dopamine neurons in vivo, and bath-applied nicotine (10 microM) increased firing rate in vitro. The competitive NMDA receptor antagonist CGP39551 (2.5 mg/kg i.p.) inhibited nicotine's effects on burst firing and also attenuated the nicotine-induced increase in firing rate. Moreover, although the nitric oxide (NO)-synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME; 5.0 mg/kg i.p.) had no effect on cell firing by itself, it prevented the response to nicotine in vivo. In contrast, l-NAME (100 microM) did not influence nicotine's effect on dopamine cell firing in vitro, suggesting that the effect of l-NAME seen in vivo is dependent on presynaptic afferent input. The present study confirms previous results suggesting that the effect of systemically administered nicotine is in part presynaptic and mediated via NMDA receptors. The data also indicate that NO plays an important role in the previously demonstrated, indirect, glutamate-mediated excitation of these neurons by nicotine. By inference, our results provide additional support for the involvement of NO in nicotine dependence.