Injections of 6-hydroxydopamine into the ventral tegmental area destroy mesolimbic dopamine neurons but spare the locomotor activating effects of nicotine in the rat

Electrolytic lesions of the nucleus accumbens enhance locomotor sensitization to nicotine in rats

Electrolytic lesions of the medial core of the nucleus accumbens (NAc) in male Long-Evans rats increased spontaneous locomotion, enhanced the locomotor stimulating effect of acute 5.0 mg/kg cocaine, enhanced the development and subsequent expression of locomotor sensitization produced by repeated injections of 0.4 mg/kg nicotine but not 7.5 mg/kg cocaine, and enhanced the expression of conditioned locomotion. Given that 6-hydroxydopamine lesions of the NAc typically have effects on locomotor-related processes that are opposite of those produced by electrolytic and excitotoxic lesions, these data are consistent with a hypothesis that the NAc output, especially from the core, inhibits a variety of such processes and that the DA input to the NAc enhances these processes by inhibiting this inhibitory output.

Nicotine sensitization of monkey striatal dopamine release

This study with monkeys was designed to answer two questions. 1) Does acute nicotine preferentially release more dopamine in the striatum than in the prefrontal cortex? 2) Do repeated doses of nicotine produce sensitization of striatal dopamine release? Microdialysis techniques were used to measure extracellular dopamine in both brain regions in two separate groups of conscious animals. The acute nicotine i.v. dose schedule was a bolus of 32 microg/kg plus an infusion of +/-0.8 microg/kg/min and a 100 microg/kg bolus plus an infusion of +/-2.53 microg/kg/min for 30 min to mimic human tobacco smoking arterial plasma nicotine concentrations. Acute nicotine given i.v. released more dopamine in the striatum than in the prefrontal cortex. In the second experiment, for convenience, daily nicotine was given i.m. and not i.v. bid in doses of 32 or 100 microg/kg for nine days. Dopamine release was measured after overnight nicotine abstinence using the i.v. dose schedule from the first experiment. Baseline dopamine release was significantly reduced (77.6% of control, P<0.05). With a lowered baseline, a greater facilitation of dopamine release was produced by nicotine compared to that obtained under control conditions when the baseline was higher. The impaired dopamine release with overnight nicotine abstinence was transiently enhanced in a dose dependent manner. These data regarding the striatum are consistent with previous findings in rodents of nicotine sensitization of dopamine release especially in nucleus accumbens following repeated administration.

Irreversible blockade of monoamine oxidases reveals the critical role of 5-HT transmission in locomotor response induced by nicotine in mice

Although nicotine is generally considered as the main compound responsible for addictive properties of tobacco, some experimental data indicate that nicotine does not exhibit all the characteristics of other substances of misuse such as psychostimulants and opiates. For example, nicotine generally fails to induce locomotor response in mice and self-administration of nicotine is difficult to obtain in rats. We have shown recently that a pretreatment with mixed irreversible monoamine oxidase inhibitors (MAOIs), such as tranylcypromine, triggers a locomotor response to nicotine in mice and induces a robust self-administration of nicotine in rats. We show here that when mice were pretreated with enhancers of extracellular levels of noradrenaline, dopamine or serotonin (D-amphetamine, GBR12783 or para-chloro-amphetamine, respectively) and injected with nicotine (1 mg/kg), only those animals pretreated with para-chloro-amphetamine exhibited a specific locomotor response to nicotine. These data indicate a critical role of serotonin in nicotine-induced locomotor activity in mice. This was further confirmed in microdialysis experiments showing that nicotine induces an increase in extracellular serotonin levels in the ventral striatum in mice pretreated with tranylcypromine. This effect of nicotine on extracellular serotonin levels was absent in mice lacking the beta2-subunit of the nicotinic acetylcholine receptor. Our data suggest that mixed irreversible MAOIs contained in tobacco facilitate the effects of nicotine on serotonin release, thus allowing the locomotor and rewarding effects of nicotine.

Monoamine oxidase inhibitors allow locomotor and rewarding responses to nicotine

Although nicotine is generally considered to be the main compound responsible for the addictive properties of tobacco, experimental data indicate that nicotine does not exhibit all the characteristics of other abused substances, such as psychostimulants and opiates. For example, nicotine is only a weak locomotor enhancer in rats and generally fails to induce a locomotor response in mice. This observation contradicts the general consensus that all drugs of abuse release dopamine in the nucleus accumbens, a subcortical structure, and thus increase locomotor activity in rodents. Because tobacco smoke contains monoamine oxidase inhibitors (MAOIs) and decreases MAO activity in smokers, we have combined MAOIs with nicotine to determine whether it is possible to obtain a locomotor response to nicotine in C57Bl6 mice. Among 15 individual or combined MAOIs, including harmane, norharmane, moclobemide, selegiline, pargyline, clorgyline, tranylcypromine and phenelzine, only irreversible inhibitors of both MAO-A and -B (tranylcypromine, phenelzine, and clorgyline+selegiline) allowed a locomotor response to nicotine. The locomotor stimulant interaction of tranylcypromine and nicotine was absent in beta2-nicotinic acetylcholine receptor subunit knockout mice. Finally, it was found that, whereas naïve rats did not readily self-administer nicotine (10 microg/kg/injection), a robust self-administration of nicotine occurred when animals were pretreated with tranylcypromine (3 mg/kg). Our data suggest that MAOIs contained in tobacco and tobacco smoke act in synergy with nicotine to enhance its rewarding effects.

Cellular and synaptic mechanisms of nicotine addiction

The tragic health effects of nicotine addiction highlight the importance of investigating the cellular mechanisms of this complex behavioral phenomenon. The chain of cause and effect of nicotine addiction starts with the interaction of this tobacco alkaloid with nicotinic acetylcholine receptors (nAChRs). This interaction leads to activation of reward centers in the CNS, including the mesoaccumbens DA system, which ultimately leads to behavioral reinforcement and addiction. Recent findings from a number of laboratories have provided new insights into the biologic processes that contribute to nicotine self-administration. Examination of the nAChR subtypes expressed within the reward centers has identified potential roles for these receptors in normal physiology, as well as the effects of nicotine exposure. The high nicotine sensitivity of some nAChR subtypes leads to rapid activation followed in many cases by rapid desensitization. Assessing the relative importance of these molecular phenomena in the behavioral effects of nicotine presents an exciting challenge for future research efforts.

Acute and long-term changes in the mesolimbic dopamine pathway after systemic or local single nicotine injections

We have examined several neurochemical and behavioural parameters related to the function of the mesolimbic dopamine (DA) pathway in animals treated with nicotine following three modes of drug administration, i.e. systemic intraperitoneal injection, intra-accumbens (Acb) infusion or intraventral tegmental area (intra-VTA) microinjection. The present modes of systemic, intra-Acb and intra-VTA nicotine administration elicited comparable acute increases in dialysate DA levels from the Acb. The increase in extracellular DA levels was paralleled by a significant enhancement of locomotion in a habituated environment in the case of systemic or intra-VTA nicotine administration, whereas unilateral or bilateral intra-Acb nicotine infusion was ineffective, showing that accumbal DA increase is not sufficient to elicit locomotion in this experimental paradigm. Intra-VTA, but not systemic or intra-Acb, nicotine administration caused a long-term (at least 24-h) increase in basal dialysate DA levels from the Acb. In addition, significant increases in tyrosine hydroxylase (TH) and GluR1 (but not dopamine transporter or NR1) mRNA levels in the VTA were detected 24 h after intra-VTA nicotine administration. Systemic nicotine injection caused only an increase in TH mRNA levels while intra-Acb infusion did not modify any of the mRNAs tested. The long-term increase in basal DA levels in the Acb and TH, and GluR1 mRNA levels in the VTA upon intra-VTA nicotine microinjection indicates that even a single nicotine injection can induce plastic changes of the mesolimbic DA pathway.

Receptors in the ventral tegmental area mediating nicotine-induced dopamine release in the nucleus accumbens

Nicotine or cocaine, when administered intravenously, induces an increase of extracellular dopamine in the nucleus accumbens. The nicotine-mediated increase was shown to occur at least in part through increase of the activity of dopamine neurons in the ventral tegmental area. As part of our continuing studies of the mechanisms of nicotine effects in the brain, in particular, effects on reward and cognitive mechanisms, in the present study we examined the role of various receptors in the ventral tegmental area in nicotine and cocaine reward. We assayed inhibition of the increase of dopamine in the nucleus accumbens induced by intravenous nicotine or cocaine administration by antagonists administered into the ventral tegmental area. Nicotine-induced increase of accumbal dopamine release was inhibited by intrategmental nicotinic (mecamylamine), muscarinic (atropine), dopaminergic (D1: SCH 23390, D2: eticlopride), and NMDA glutamatergic (MK 801) and GABAB (saclofen) antagonists, but not by AMPA-kainate (CNQX, GYKI52466) antagonists under our experimental circumstances. The intravenous cocaine-induced increase of dopamine in the nucleus accumbens was inhibited by muscarinic (atropine), dopamine 2 (eticlopride), and GABAB (saclofen) antagonists but not by antagonists to nicotinic (mecamylamine), dopamine D1 (SCH 23390), glutamate (MK 801), or AMPA-kainate (CNQX, GYKI52466) receptors. Antagonists administered in the ventral tegmental area in the present study had somewhat different effects when they were previously administered intravenously. When administered intravenously atropine did not inhibit cocaine effects. The inhibition by atropine may be indirect, since this compound, when administered intrategmentally, decreased basal dopamine levels in the accumbens. The findings indicate that a number of receptors in the ventral tegmental area mediate nicotine-induced dopamine changes in the nucleus accumbens, a major component of the nicotine reward mechanism. Some, but not all, of these receptors in the ventral tegmental area also seem to participate in the reward mechanism of cocaine. The importance of local receptors in the ventral tegmental area was further indicated by the increase in accumbal dopamine levels after intrategmental administration of nicotine or also cocaine.

Disruption of dopaminergic neurotransmission in nucleus accumbens core inhibits the locomotor stimulant effects of nicotine and D-amphetamine in rats

The locomotor stimulant effects of nicotine and amphetamine appear to be dependent on dopamine transmission in the nucleus accumbens. The present aim was to elucidate the contributions of the accumbens core and medial shell to these effects. In the first experiment, rats received bilateral intra-accumbens infusion of the dopaminergic antagonist eticlopride (or saline) prior to saline or nicotine (0.2 mg/kg s.c.) challenge. Eticlopride inhibited basal and nicotine-induced locomotor activity more effectively when infused into the core (0.0625--0.5 microg/side) than into the medial shell (0.5--1 microg/side). In a second experiment, rats received 6-hydroxydopamine infused into the core or medial shell, and were subsequently tested with saline, nicotine (0.2 mg/kg s.c.) and D-amphetamine (0.75 mg/kg s.c.). Residual dopaminergic innervation was assessed by autoradiographic [(125)I]RTI-55 labelling of the dopamine transporter. [(125)I]RTI-55 labelling in the accumbens core was positively correlated with the locomotor stimulant effects of both nicotine and D-amphetamine. In contrast, [(125)I]RTI-55 labelling in the medial shell was associated negatively with amphetamine-induced activity. Recent evidence suggests that dopamine release in the medial shell may mediate the reinforcing effect of nicotine and D-amphetamine. In contrast, the present findings suggest that dopamine release in the core subregion contributes preferentially to the locomotor stimulant effects of nicotine and D-amphetamine.

Role of dopamine in the behavioural actions of nicotine related to addiction

Experimental impairment of dopamine function by 6-hydroxydopamine lesions or by dopamine receptor antagonists shows that dopamine is involved in nicotine's discriminative stimulus properties, nicotine-induced facilitation of intracranial self-stimulation, intravenous nicotine self-administration, nicotine conditioned place-preference and nicotine-induced disruption of latent inhibition. Therefore, nicotine depends on dopamine for those behavioural effects that are most relevant for its reinforcing properties and are likely to be the basis of the abuse liability of tobacco smoke. On the other hand, in vivo monitoring studies show that nicotine stimulates dopamine transmission in specific brain areas and in particular, in the shell of the nucleus accumbens and in areas of the extended amygdala. These effects of nicotine resemble those of a reward like food except that nicotine-induced release of dopamine does not undergo single-trial, long-lasting habituation. It is speculated that repeated non-habituating stimulation of dopamine release by nicotine in the nucleus accumbens shell abnormally facilitates associative stimulus-reward learning. Acute effects of nicotine on dopamine transmission undergo acute and chronic tolerance; with repeated, discontinuous exposure, sensitization of nicotine-induced stimulation of dopamine release in the nucleus accumbens core takes place while the response in the shell is reduced. It is speculated that these adaptive changes are the substrate of a switch from abnormal incentive responding controlled by consequences (action-outcome responding) into abnormal habit responding, triggered by conditional stimuli and automatically driven by action schemata relatively independent from nicotine reward. These two modalities might coexist, being utilized alternatively in relation to the availability of tobacco. Unavailability of tobacco disrupts the automatic, implicit modality of abnormal habit responding switching responding into the explicit, conscious modality of incentive drug-seeking and craving.

Nicotine alone and in combination with L-DOPA methyl ester or the D(2) agonist N-0923 in MPTP-induced chronic hemiparkinsonian monkeys

Nicotine, the soluble methyl ester of L-DOPA, and the D(2) agonist N-0923 were given alone and in combination im to five hemiparkinsonian monkeys. Daily nicotine in doses of 32-320 micrograms/kg for 6 days each, surprisingly, had slight effects on motor activity. When combined with N-0923, nicotine did not further enhance its effects. However, L-DOPA methyl ester plus nicotine produced greater contraversive circling than L-DOPA methyl ester plus 0.9% NaCl. Similar effects were obtained on significant motor movements of both the affected (contralateral) and normal (ipsilateral) arm and hand. The results indicate that nicotine is synergistic with l-DOPA methyl ester, but not with the postsynaptic D(2) agonist N-0923.

Effect of ventral tegmental 6-hydroxydopamine lesions on the locomotor stimulant action of nicotine in rats

Convergent evidence suggests that the locomotor stimulant effect of nicotine is mediated by nicotinic receptors located on mesolimbic dopaminergic neurons. However, 6-hydroxydopamine lesions of the ventral tegmental area, resulting in substantial depletion of nucleus accumbens dopamine, were recently reported to have no effect on nicotine-induced locomotion. The present study sought to re-examine this issue. Rats received bilateral infusions of 6-hydroxydopamine or vehicle into the ventral tegmental area. Starting 3 weeks later, locomotor activity was tested after subcutaneous injection of saline, nicotine (0.4 mg/kg base), amphetamine (0.5 mg/kg) or scopolamine (0.5 mg/kg). In lesioned animals, the locomotor stimulant effects of nicotine and amphetamine were greatly reduced, whereas saline and scopolamine-induced activity was scarcely affected. Dopamine denervation was assessed by autoradiography, using [125I]RTI-55 to label plasmalemmal dopamine transporters. Labelling was reduced in nucleus accumbens core and shell and in the ventral tegmental area (by 87, 81 and 70%, respectively), and in nigrostriatal areas (52-77%). The locomotor stimulant effects of nicotine and amphetamine were correlated with residual [125I]RTI-55 labelling in mesolimbic and nigrostriatal regions (r=0.6-0.8). The present results provide further evidence that the locomotor stimulant effect of nicotine is dependent on the integrity of ascending dopamine neurons.

The influence of lobeline on nucleus accumbens dopamine and locomotor responses to nicotine in nicotine-pretreated rats

In vivo brain microdialysis was used to investigate the influence of lobeline on dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) overflow in the core of the nucleus accumbens of freely-moving rats pretreated with nicotine (0.4 mg x kg(-1), s.c., once per day for 5 days). Locomotion was also recorded. Lobeline, at doses of 0.7, 4.0 and 10.0 mg x kg(-1), i.p., failed to elicit any significant changes in extracellular dopamine or dihydroxyphenylacetic acid levels during the 60 min following its administration and did not stimulate locomotor. The dopamine responses to nicotine (0.4 mg x kg(-1), s.c.), were abolished (P<0.01) if the nicotine challenge was administered 10 min but not 60 min, after lobeline doses of 4.0 and 10.0 mg kg(-1), i.p., but were unaffected following lobeline at the lowest dose tested (0.7 mg x kg(-1), i.p.) at either time. The increase in locomotor activity was significantly attenuated (P<0.01), to a similar extent, when the nicotine was injected 10 min, but not 60 min, after all three doses of lobeline (0.7, 4.0 and 10.0 mg kg(-1), i.p.) when compared with the saline-treated rats. The results suggest that lobeline is a short-acting antagonist of the nicotinic AChRs which mediate the effects of nicotine on mesolimbic dopamine activity and locomotor stimulation.

Sensitization of the mesoaccumbens dopamine response to nicotine

This article reviews the evidence that pretreatment with nicotine causes a regionally selective sensitization of its stimulatory effects on a pathway, the mesoaccumbens dopamine (DA) system, which has been implicated in the locomotor stimulant response to nicotine and its ability to reinforce self-administration. The sensitization evoked by daily injections of nicotine is associated with a regionally selective downregulation of the control of mesoaccumbens DA neurons by inhibitory autoreceptors and depends upon co-stimulation of NMDA glutamatergic receptors. It is suggested that the sensitization is related to enhanced burst firing of mesoaccumbens neurons, which results in an enhancement of DA release into the extracellular space between the cells where it acts upon putative extrasynaptic dopamine receptors. The studies with NMDA receptor antagonists revealed a dissociation between the expression of sensitized mesoaccumbens DA and locomotor responses to nicotine. It is proposed, therefore, that the sensitized mesoaccumbens DA responses to nicotine may be implicated in psychopharmacological responses to drug concerned more closely with nicotine dependence.

Partial reversal of stress-induced behavioral sensitization to amphetamine following metyrapone treatment

Several studies indicate that blockade of stress-induced corticosterone secretion prevents the development of stress-induced sensitization to the behavioral effects of stimulants. The present study examined whether chronic blockade of corticosterone synthesis with metyrapone could reverse stress-induced amphetamine sensitization in rats. Restraint stress in cylindrical chambers, 2 times 30 min/day for 5 days over an 8-day schedule, was used as the stressor. Following completion of the stress protocol, animals were cannulated with microdialysis guide cannulae over the nucleus accumbens, and then treated with either metyrapone (50 mg/kg, i.p.) or vehicle (1 ml/kg) for 7 days. On the seventh day, animals were implanted with microdialysis probes in the nucleus accumbens, and on the following day, all animals were tested for their locomotor, stereotypy, and nucleus accumbens dopamine and DOPAC release responses to an injection of saline followed 60 min later by d-amphetamine (1.5 mg/kg, i.p.). Neither stress or metyrapone treatment had an effect on the behavioral or dopamine release response to saline. However, amphetamine-stimulated locomotion and stereotypy were strongly enhanced, while amphetamine-stimulated dopamine release response was slightly enhanced (significant only by drug x time interaction), in stressed animals. Metyrapone treatment reduced the stress-induced increase in the locomotor, but not stereotypy, response to amphetamine. In contrast, the dopamine release response to amphetamine was enhanced in metyrapone-treated animals, in both stressed and non-stressed groups, while DOPAC levels were unaffected by treatment group. This augmentation was particularly evident in the stressed-metyrapone-treated animals. Furthermore, non-stressed animals showed an increased locomotor and stereotypy response to amphetamine after treatment with metyrapone. These findings indicate that metyrapone treatment can reverse, or inhibit the expression of, stress-induced sensitization to the behavioral effects of amphetamine. However, the ability of metyrapone treatment to enhance the behavioral (in non-stressed animals) and dopamine release (in non-stressed and stressed animals) responses to amphetamine indicate that chronic metyrapone treatment will produce stimulant sensitization when given alone.

Modulatory role of catecholamines in the transsynaptic expression of c-fos in the rat medial prefrontal cortex induced by disinhibition of the mediodorsal thalamus: a study employing microdialysis and immunohistochemistry

We studied the interaction of catecholaminergic and thalamic afferents of the medial prefrontal cortex (PFC) by analyzing the effects of catecholamine depletion on thalamus-induced c-fos expression in the PFC of freely moving rats. Thalamic projections to the PFC were pharmacologically activated by perfusing the GABA-A receptor antagonist bicuculline (0.03 mM or 0.1 mM) through a dialysis probe implanted into the mediodorsal thalamic nucleus. Bicuculline perfusion induced Fos-like immunoreactivity in the thalamic projection areas, including the PFC, and in the thalamic nuclei surrounding the dialysis probe. 6-Hydroxydopamine lesions of the ventral tegmental area causing a 70-80% depletion of catecholamines in the PFC did not influence the increase in the number of Fos-like immunoreactive nuclei in the prefrontal cortex in response to thalamic stimulation. However, densitometric image analysis revealed that the intensity of Fos-like immunoreactivity in the PFC of lesioned rats perfused with 0.1 mM bicuculline was higher than in correspondingly treated controls. The behavioral activity to bicuculline perfusion, an increase of non-ambulatory activity (0.03 mM) followed by locomotion and rearing (0.1 mM), was not changed in 6-hydroxydopamine-lesioned rats. It is suggested that the thalamically induced c-fos response is directly mediated by excitatory, presumably glutamatergic, transmission and not indirectly by an activation of catecholaminergic afferents of the PFC. The increase in the intensity of Fos-like immunostaining in strongly stimulated, catecholamine-depleted rats suggests that catecholamines modulate the degree to which thalamic activity can activate the PFC of awake animals.

Real-time measurement of stimulated 5-hydroxytryptamine release in rat substantia nigra pars reticulata brain slices

Fast cyclic voltammetry at a carbon fibre microelectrode was used to measure 5-HT signals following electrical or chemical stimulation in rat substantia nigra pars reticulata slices. Chemical stimulation with (+)-amphetamine or veratrine gave signals which were indistinguishable from those of exogenous 5-HT. Electrical stimulation of sufficient duration gave voltammetric signals which were characteristic of 5-HT. Release of dopamine was not detected following either chemical or electrical stimulation. The 5-HT signals were attenuated by TTX and enhanced by fluvoxamine. It was not possible to demonstrate regulation of 5-HT release in the SNr by 5-HT1B autoreceptors using CGS 12066A or methiothepin. Signal following electrical stimulation were not enhanced by either benztropine or GBR12909, or modified in the presence of either quinpirole or sulpiride. We conclude that 5-HT release can be detected voltammetrically in the SNr; 5-HT release is likely to be from axon terminals, but somatodendritic DA release could not be detected.

Ventral tegmental injection of nicotine induces locomotor activity and L-DOPA release from nucleus accumbens

Effects of nicotine systemically or locally on locomotor activity and L-3,4-dihydroxyphenylalanine (L-DOPA) release were studied using microdialysis in the nucleus accumbens of freely moving rats. The basal L-DOPA release was Ca2(+)-dependent and tetrodotoxin-sensitive. Systemic nicotine (1 mg/kg s.c.) increased locomotor activity and L-DOPA release preferentially in the nucleus accumbens as compared with the striatum. Injection of nicotine (30 micrograms) into the ventral tegmental area increased locomotor activity and L-DOPA release from the nucleus accumbens. These increases were antagonized by prior injection of mecamylamine into the ventral tegmental area. Nicotine induces locomotor activity and L-DOPA release from the nucleus accumbens via nicotinic receptors in the ventral tegmental area. The release may be relevant to behavioral actions of nicotine.

Effects of environmental conditioning on the development of nicotine sensitization: behavioral and neurochemical analysis

We have investigated the effects of environmental conditioning on the induction of nicotine sensitization of locomotion, stereotypy and nucleus accumbens dopamine release. Sprague-Dawley rats, some of which had been previously implanted with a microdialysis guide cannula over the nucleus accumbens, were sensitized with 5 days of repeated nicotine (0.6 mg/kg per day, SC) or saline injections (1 ml/kg per day). During nicotine treatment the drug administration was either paired with the microdialysis/activity monitor testing chamber (conditioned) (n = 6) or with the animal's home cage (unconditioned) (n = 6) and after 60 min the animal was returned to home cage and received a second injection of saline 15 min later. A third group received saline in the testing apparatus followed by nicotine in the home cage (pseudo-conditioned) (n = 6). In the guide cannulated animals, 2 mm microdialysis probes were inserted after completing day 5 of treatment and all animals were tested for their response to nicotine (0.6 mg/kg, SC) on day 6. Both locomotor activity and nucleus accumbens dopamine release showed a larger response subsequent to nicotine challenge in the nicotine versus saline pretreated animals in the conditioned group, but not in the unconditioned group. In the pseudo-conditioned group there was an increase in the stereotypy responses to nicotine, however the locomotor and dopamine release responses were not significantly enhanced. The results from the conditioned group were confirmed in animals which were tested for behavioral activation and dopamine release simultaneously (n = 5). These findings indicate that nicotine sensitization of locomotor activity and nucleus accumbens dopamine release (using a 5-day pretreatment protocol) is dependent on conditioning the animal to the testing environment during nicotine pretreatment.

Condition-independent sensitization of locomotor stimulation and mesocortical dopamine release following chronic nicotine treatment in the rat

Chronic nicotine (NIC) pretreatment has been shown to enhance NIC-induced locomotor stimulation, an effect that seems critically dependent on activation of brain dopamine (DA) systems. In the present study the effects of chronic, intermittent NIC treatment were examined in the rat to establish whether such behavioral sensitization is associated with specific, regional changes in brain dopaminergic activity. Male rats received daily injections in their home cage with either saline (SAL) or NIC (0.5 mg/kg, s.c.) for 12 days. Twenty-four hours later, the locomotor activity of the animals subjected to NIC challenge as well as the functional responsiveness of the mesolimbocortical dopaminergic system were assessed. To this end, microdialysis experiments were performed in awake animals, measuring extracellular concentrations of DA and its metabolites in the prefrontal cortex (PFC) and the nucleus accumbens (NAC). Extracellular single cell recordings from DA neurons in the ventral tegmental area (VTA) were also performed in anesthetized animals. NIC (0.5 mg/kg, s.c.) increased all measured parameters of locomotor activity, with the exception of rearing, in SAL-pretreated animals; these effects were substantially enhanced after pretreatment with NIC. Nicotine (0.5 mg/kg, s.c.) increased DA release in both the PFC and the NAC in SAL-treated animals. Nicotine pretreatment significantly enhanced this effect in the PFC, whereas it did not affect the response in the NAC. Low doses of intravenously administered NIC dose-dependently increased burst activity, starting at 12 micrograms/kg in the SAL pretreated animals and at 6 micrograms/kg in the NIC-pretreated animals, and also dose-dependently increased firing rate in SAL as well as NIC-pretreated animals, although starting at a higher dose level, i.e., 25 micrograms/kg. These results demonstrate that behavioral sensitization after chronic NIC treatment is accompanied by an enhanced dopamine release specifically within the PFC. This phenomenon may be highly significant for the dependence-producing effects of NIC, particularly in association with major psychiatric disorder, such as schizophrenia.

Comparison of somatodendritic and axon terminal dopamine release in the ventral tegmental area and the nucleus accumbens

Fast cyclic voltammetry at a carbon fibre microelectrode was used to measure dopamine release following electrical or chemical stimulation in rat brain slices incorporating either the ventral tegmental area or the core region of the nucleus accumbens. Electrical or chemical stimulation gave clear voltammetric signals which corresponded to dopamine; less dopamine was released in the ventral tegmental area than in the nucleus accumbens. In contrast to the nucleus accumbens, electrically stimulated dopamine release in the ventral tegmental area was not sensitive to tetrodotoxin, was not modified by the presence of dopamine uptake inhibitors, or agonist or blockers acting at dopamine D2 autoreceptors.

The pharmacology of (-)-nicotine and novel cholinergic channel modulators

Advances in the understanding of the molecular biology and pharmacology of nAChRs may provide targets for the development of novel and selective modulators of nAChRs in the brain. This contention is supported by the dissimilar behavioral effects observed following systemic administration of currently available nicotinic ligands. The concept of multiple subtypes of nAChRs is not unique, as evidenced by the pharmacology of other ligand-gated ion channels, such as GABA-A receptor, which also exist in multiple subtypes. At present, with respect to the nAChRs, relatively few of the subtypes identified have been cloned from human tissue and pharmacologically evaluated, but several groups are focusing their research efforts in this direction. With a thorough understanding of the pharmacological and functional characteristics of more of the putative human nAChR subtypes, this knowledge will facilitate the discovery of more efficacious and less toxic ChCMs that may provide potential novel therapeutic agents for a variety of CNS conditions.