Is dopamine important in nicotine dependence?

Nicotine drug discrimination and nicotinic acetylcholine receptors in differentially reared rats

RATIONALE

Individuals vary in sensitivity to the behavioral effects of nicotine, resulting in differences in vulnerability to nicotine addiction. The role of rearing environment in determining individual sensitivity to nicotine is unclear. The neuropharmacological mechanisms mediating the effect of rearing environment on the behavioral actions of nicotine are also poorly understood.

OBJECTIVES

The contribution of rearing environment in determining the sensitivity to the interoceptive effects of nicotine was determined in rats reared in isolated conditions (IC) or enriched conditions (EC). The role of dopamine receptors and α4β2*-nicotinic acetylcholine (nACh) receptors in mediating the differential effect of IC and EC on the interoceptive action of nicotine was determined.

METHODS

The interoceptive action of nicotine was measured as the discriminative stimulus effect of nicotine. Mecamylamine- and eticlopride-inhibition of the nicotine stimulus were used to examine nACh and dopamine receptors, respectively. α4β2*-nACh receptor expression in the mesolimbic dopamine pathway was determined by quantitative autoradiography of [¹²⁵I]-epibatidine binding.

RESULTS

EC-reared rats are less sensitive than IC-reared rats to the discriminative stimulus effects of nicotine at all but maximally effective doses. Mecamylamine inhibited the nicotine stimulus threefold more potently in EC-reared rats (IC₅₀ = 0.25 mg/kg) compared to IC-reared rats (IC₅₀ = 0.75 mg/kg); eticlopride inhibition was not different. [¹²⁵I]-epibatidine binding in the ventral tegmental area of EC-reared rats was reduced (2.8 ± 0.3 fmol) compared to that of IC-reared rats (4.0 ± 0.4 fmol); there was no difference in the nucleus accumbens.

CONCLUSIONS

Rearing environment regulates the sensitivity to the interoceptive effects of nicotine and α4β2*-nACh receptor expression in the mesolimbic dopamine pathway.

D1/D5 receptors and histone deacetylation mediate the Gateway Effect of LTP in hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus is critical for spatial memory and is also thought to be involved in the formation of drug-related associative memory. Here, we attempt to test an aspect of the Gateway Hypothesis, by studying the effect of consecutive exposure to nicotine and cocaine on long-term synaptic potentiation (LTP) in the DG. We find that a single injection of cocaine does not alter LTP. However, pretreatment with nicotine followed by a single injection of cocaine causes a substantial enhancement of LTP. This priming effect of nicotine is unidirectional: There is no enhancement of LTP if cocaine is administrated prior to nicotine. The facilitation induced by nicotine and cocaine can be blocked by oral administration of the dopamine D1/D5 receptor antagonist (SKF 83566) and enhanced by the D1/D5 agonist (SKF 38393). Application of the histone deacetylation inhibitor suberoylanilide hydroxamic acid (SAHA) simulates the priming effect of nicotine on cocaine. By contrast, the priming effect of nicotine on cocaine is blocked in genetically modified mice that are haploinsufficient for the CREB-binding protein (CBP) and possess only one functional CBP allele and therefore exhibit a reduction in histone acetylation. These results demonstrate that the DG of the hippocampus is an important brain region contributing to the priming effect of nicotine on cocaine. Moreover, both activation of dopamine-D1 receptor/PKA signaling pathway and histone deacetylation/CBP mediated transcription are required for the nicotine priming effect in the DG.

The ClockΔ19 mutation in mice fails to alter the primary and secondary reinforcing properties of nicotine

BACKGROUND

Clock genes have been demonstrated to play a role in behavioral responses to a variety of drugs of abuse, including cocaine, amphetamine, morphine, and ethanol. However, no studies to date have examined the role of Clock genes on nicotine-mediated behaviors. We examined the involvement of Clock, one of several Clock genes, on the effects of nicotine by examining mice with the ClockΔ19 mutation in behaviors commonly used to assess drug effects in rodents.

METHODS

We first measured the locomotor effects of nicotine in mutants and wild type mice in response to repeated nicotine injections (0.175 mg/kg, IP). To assess the secondary properties of nicotine, we measured the ability of nicotine (0.175 mg/kg, IP) to induce a conditioned place preference. Finally, we measured the primary reinforcing properties of nicotine at two doses (0.01 and 0.03 mg/kg/infusion, IV) using the self-administration paradigm.

RESULTS

Mutant mice demonstrated no difference in magnitude of the sensitized response to nicotine as compared to wild-type controls. In the conditioned place preference paradigm, mutant and wild-type mice demonstrated a similar preference for a nicotine-paired environment. And finally, mutant and wild-type mice demonstrated a similar acquisition of nicotine self-administration, as indicated by the number of responses on a nicotine-paired lever and the number of nicotine reinforcers achieved during sessions.

CONCLUSIONS

The ClockΔ19 mutation appears to have no effect on the reinforcing properties of nicotine, in contrast to its demonstrated role in cocaine reinforcement. Further studies are needed to determine the effect of other Clock genes on nicotine reinforcement.

Using drug-discrimination techniques to study the abuse-related effects of psychoactive drugs in rats

Drug-discrimination (DD) techniques can be used to study abuse-related effects by establishing the interoceptive effects of a training drug (e.g., cocaine) as a cue for performing a specific operant response (e.g., lever pressing reinforced by food). During training with this protocol, pressing one lever is reinforced when the training drug is injected before the start of the session, and responding on a second lever is reinforced when vehicle is injected before the session. Lever choice during test sessions can then be used as an indication of whether a novel drug has effects similar to the training drug, or whether a potential therapeutic alters the effects of the training drug. Although training can be lengthy (up to several months), the pharmacological specificity of DD procedures make them a perfect complement to other techniques used to study drug-abuse phenomena, such as intravenous self-administration and conditioned place-preference procedures.

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.

Attenuation of nicotine-induced rewarding effects in A2A knockout mice

The non-selective A2A antagonist caffeine has been reported to modify nicotine-induced locomotor and reinforcing effects. In the present study, we have investigated the specific role of A2A adenosine receptors in the behavioural responses induced by nicotine by using genetically modified mice lacking A2A adenosine receptors. Acute nicotine administration induced a similar decrease of locomotor activity in A2A knockout mice and wild-type littermates. Acute antinociceptive responses elicited by nicotine in the tail-immersion and hot-plate tests were unaffected in these mutant mice. The rewarding properties of nicotine were then investigated using the place-conditioning paradigm. Nicotine-induced conditioned place preference was suppressed in A2A knockout mice. Accordingly, in vivo microdialysis studies revealed that the extracellular levels of dopamine in the nucleus accumbens were not increased after nicotine administration in mutant mice. Wild-type and A2A knockout mice were trained in conditioned taste aversion procedure in which drinking a saccharin or saline solution was paired with nicotine or saline injections. A similar reduction in the intake of nicotine-paired solution in this paradigm was obtained in both genotypes. Finally, the administration of the nicotinic antagonist mecamylamine in nicotine-dependent mice precipitated a similar withdrawal syndrome in both genotypes. Together, the present results identify A2A adenosine receptors as an important factor that contributes to the rewarding properties of nicotine.

Dopamine and drug addiction: the nucleus accumbens shell connection

Microdialysis studies in animals have shown that addictive drugs preferentially increase extracellular dopamine (DA) in the n. accumbens (NAc). Brain imaging studies, while extending these finding to humans, have shown a correlation between psychostimulant-induced increase of extracellular DA in the striatum and self-reported measures of liking and 'high' (euphoria). Although a correlate of drug reward independent from associative learning and performance is difficult to obtain in animals, conditioned taste avoidance (CTA) might meet these requirements. Addictive drugs induce CTA to saccharin most likely as a result of anticipatory contrast of saccharin over drug reward. Consistently with a role of DA in drug reward, D2 or combined D1/D2 receptor blockade abolishes cocaine, amphetamine and nicotine CTA. Intracranial self-administration studies with mixtures of D1 and D2 receptor agonists point to the NAc shell as the critical site of DA reward. NAc shell DA acting on D1 receptors is also involved in Pavlovian learning through pre-trial and post-trial consolidation mechanisms and in the utilization of spatial short-term memory for goal-directed behavior. Stimulation of NAc shell DA transmission by addictive drugs is shared by a natural reward like food but lacks its adaptive properties (habituation and inhibition by predictive stimuli). These peculiarities of drug-induced stimulation of DA transmission in the NAc shell result in striking differences in the impact of drug-conditioned stimuli on DA transmission. It is speculated that drug addiction results from the impact exerted on behavior by the abnormal DA stimulant properties acquired by drug-conditioned stimuli as a result of their association with addictive drugs.

The role of 5-HT1A receptors in mediating the anxiogenic effects of nicotine following lateral septal administration

The purpose of the present study was to determine the role of the 5-HT1A receptors in the lateral septum in the mediation of the anxiogenic effects of nicotine in the social interaction and elevated plus maze tests of anxiety in the rat. Bilateral infusion of (-)-nicotine (4 and 8 microg) and of the 5-HT1A receptor agonist 8-OH-DPAT (200 and 500 ng) into the lateral septum decreased the time spent in social interaction, indicating anxiogenic effects. The anxiogenic effect of 8-OH-DPAT (500 ng) was completely reversed by coadministration of a behaviourally inactive dose of the 5-HT1A receptor antagonist, WAY 100635 (200 ng). The anxiogenic effect of the lower dose of (-)-nicotine (4 microg) was completely reversed by WAY 100635 (200 ng), but the reversal was only partial following administration of 8 microg nicotine. In a second test of anxiety, the elevated plus maze, lateral septal administration of 8-OH-DPAT (500 ng) and nicotine (4 microg) induced anxiogenic effects. In this test, the anxiogenic effect of nicotine (4 microg) was completely reversed by coadministration of WAY 100635 (200 ng). The effects of 8-OH-DPAT demonstrate that stimulation of 5-HT1A receptors in the lateral septum has anxiogenic effects in two animal tests and that the anxiogenic effects of nicotine are mediated at least in part by these 5-HT1A receptors.

Differential effects of nicotine against stress-induced changes in dopaminergic system in rat striatum and hippocampus

A number of studies have shown an increase in nicotine self-administration among smokers when exposed to stress. Since it is well known that nicotine or stress alter the dopaminergic system, we examined the effect of chronic nicotine administration on the dopamine level and its metabolism in the striatum and the hippocampus during stressful conditions in rats. Nicotine (0.4 mg/kg, i.p. for 14 days) increased the dopamine level in the striatum (P<0. 05) and decreased it in the hippocampus (P<0.05) in comparison with the effect of saline. Three hours of water-immersion restraint stress sharply elevated the dopamine level (P<0.05) and reduced the 3-methoxytyramine level (P ranged from 0.05 to 0.001 depending on the area and time point) in both brain regions studied, while dihydroxyphenylacetic acid and homovanilic acid levels were not altered. Nicotine pretreatment attenuated some of these changes in a region- and time-dependent manner. However, stress induced a decrease in dopamine turnover in the hippocampus (P<0.05) but not in the striatum, and nicotine failed to prevent this effect. Stress-induced alterations gradually returned toward normal during the 48-h observation period, and in some cases this was facilitated by nicotine. Thus, we demonstrated differential, region- and time-dependent protective effects of chronic nicotine administration against stress-induced changes in dopamine levels and release in brain regions critically affected by stress.