Systemic nicotine-induced dopamine release in the rat nucleus accumbens is regulated by nicotinic receptors in the ventral tegmental area

The role of beta 2-subunit-containing nicotinic acetylcholine receptors in the brain explored with a mutant mouse

Neuronal nicotinic receptors comprise a family of pentameric oligomers made up of a combination of 10 different subunits. The beta 2 subunit has the widest pattern of expression in the brain and is thus likely to form a significant fraction of neuronal nicotinic receptors. Using mice lacking the beta 2 subunit, we have shown that nAChRs containing this subunit are responsible for most of the high-affinity binding sites for nicotine, cytisine, and epibatidine in the brain. Functional receptors containing the beta 2-subunit are found in the somatodendritic compartment as well as the axonal compartment of neurons. We have examined the contribution of these receptors to the effects of nicotine on the mesolimbic DA system, which mediates the reinforcing properties of many addictive drugs (including nicotine). Submicromolar doses of nicotine, corresponding to the concentrations of nicotine in vivo in self-administration paradigms, increased the firing rate of dopaminergic neurons in vitro in normal mice but not in mice lacking the beta 2 subunit. Consistently, systemic injection of nicotine induced an increase in extracellular dopamine in normal mice but not in mutant mice, and nicotine self-administration was reduced or suppressed in mutant mice. These data support the view that the beta 2-containing receptors are involved in mediating the reinforcing properties of nicotine.

Nicotine withdrawal in the rat: role of alpha7 nicotinic receptors in the ventral tegmental area

Previous data show that nicotinic receptors in the ventral tegmental area are of importance for the nicotine withdrawal syndrome. Here we have investigated the specific role of alpha7 nicotinic receptors in the ventral tegmental area for the neurobiological and behavioral consequences of nicotine withdrawal. Rats were exposed to nicotine for 14 days via s.c. osmotic minipumps. Bilateral intrategmental injections of the selective alpha7 nicotinic receptor antagonist methyllycaconitine reduced locomotion in the nicotine-treated rats, but not in control animals. Unilateral intrategmental injection of methyllycaconitine reduced dopamine output in the ipsilateral nucleus accumbens of nicotine-treated rats, but not in controls. Our results indicate that alpha7 nicotinic receptors in the ventral tegmental area are involved in the nicotine withdrawal syndrome.

Nicotine self-administration in animals as a dependence model

Various animal models of nicotine dependence now exist. To study the positive reinforcing effects of nicotine, there are choices of animal species, strains, and operant paradigms to use. This manuscript describes the use of one particular paradigm, a model in which work is done by laboratory animals to obtain intravenous infusions of nicotine. This model is particularly useful for examining the mechanisms in the brain that are responsible for the maintenance of drug-taking behavior. Two examples of ongoing studies of the mechanisms of dependence are discussed: the role of cholinergic projections to midbrain dopamine cells, and the influence of opioid receptors in the vicinity of these same dopamine cells.

Operant ethanol self-administration after nicotine treatment and withdrawal

The effects of chronic intermittent administration of nicotine (NIC) and withdrawal on operant ethanol (EtOH) self-administration were tested in Long-Evans rats (n = 8). EtOH self-administration (10% v/v, Fixed Ratio 4 reinforcement schedule) was induced by the sucrose-substitution procedure. Then the animals were divided into two groups of four rats matched on EtOH self-administration and the locomotor activity following an injection of NIC (0.35 mg/kg, SC) or saline was measured. The groups then received 9 days of injection of either NIC (0.35 mg/kg) or saline and then motor activity was retested using the initial NIC dose. This was followed by 17 days of NIC injections (0.6 mg/kg) or saline injections. A final locomotor test using the higher NIC dose was then conducted. The initial acute administration of NIC had no effect on motor activity compared to saline (measured by the number of horizontal movements). However, after the repeated treatment, the group of animals injected chronically and acutely with NIC showed motor activation in comparison with the animals injected chronically with saline and injected acutely with NIC only on the days of activity testing. At the end of the chronic NIC treatment, operant EtOH self-administration was not changed. However, 6 days after the NIC injections were concluded, a change in the pattern of responding for EtOH was observed in the NIC group, showing a decrease in the mean rate of responding during the first half of the operant self-administration session. When both groups were again tested for locomotor activity at the end of the operant self-administration experiment, the increased motor activity in the NIC group was still observed. The results suggest that alterations in the nicotinic system may affect EtOH self-administration, but this appears to be only modulatory, even with a significant change in locomotor response to NIC following chronic NIC administration.

A pharmacologic strategy for the treatment of nicotine addiction

Like many psychostimulant drugs, nicotine elevates extracellular and synaptic dopamine (DA) concentrations in the nucleus accumbens (NAc). This elevation has been linked to its reinforcing properties. Dopaminergic transmission within the NAc is modulated by gamma-aminobutyric acid (GABA). Therefore, we examined the utility of gamma vinyl-GABA (GVG, Vigabatrin) for inhibiting nicotine's biochemical effects on NAc DA as well as its effects on behaviors associated with these biochemical changes. Given 2.5 hours prior to nicotine, GVG (75 mg/kg) had no effect on nicotine-induced increases in extracellular NAc DA. However, at 90 mg/kg, GVG significantly inhibited nicotine-induced increases by approximately 50% while at 100 or 150 mg/kg, GVG completely abolished nicotine-induced increases in both naive and chronically nicotine-treated animals. When given 12 or 24 hours prior to nicotine administration at a dose of 100 mg/kg, GVG-induced inhibition was diminished or abolished, respectively. In addition, at a dose of 18.75 mg/kg GVG abolished the expression of nicotine-induced conditioned place preference (CPP) while a dose of 75 mg/kg abolished the acquisition phase of CPP. Finally, using positron emission tomography (PET) and 11C-raclopride in primates, GVG (100 mg/kg) abolished nicotine-induced increases in synaptic DA while having no effect on the rate of metabolism of the radiotracer or its regional distribution. Together, these data suggest that GVG may be useful for the treatment of nicotine addiction and further support the strategy of targeting the GABAergic system with a suicide inhibitor of GABA-transaminase for the treatment of drug addiction.

Influence of sex and female hormones on nicotine-induced changes in locomotor activity in rats

The acute and chronic effects of nicotine (0.4 mg/kg s.c.) on locomotor activity in photocell cages have been compared in male, female, and ovariectomized hooded rats. In Experiment 1, female rats displayed higher locomotion than males (n = 12); acutely, nicotine-reduced locomotion, and this effect was slightly larger in females than males. Daily administration of nicotine for 21 days produced a similar, gradual increase in activity in both sexes. Tests then confirmed greater activity in females than males and as a function of previous chronic exposure to nicotine (n = 6); there was an activating effect of nicotine challenge but no interaction of nicotine effects with sex. In Experiment 2, ovariectomized rats were primed with 17-beta-estradiol (50 microg/kg s.c.) and progesterone (2.5 mg/kg s.c.) or vehicle only. Acute administration of nicotine reduced activity in both groups similarly (n = 12). After nicotine daily for 21 days, there was increased activity as a function of both chronic nicotine and hormonal priming, and challenge with nicotine increased activity (n = 6). The effects of these challenges with nicotine were also slightly greater, as a function of previous nicotine exposure and priming. As a whole, these experiments showed robust effects of acute and chronic nicotine administration, sex, and hormonal priming; neither sex nor gonadal hormones had marked influences on changes in locomotor activity produced by nicotine.

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.

Receptor systems participating in nicotine-specific effects

It is generally accepted that self-administration of drugs is prompted primarily by a reward system driven by an increase in extracellular dopamine in the nucleus accumbens. Recent findings that dopamine increase in the accumbens can be caused by many other factors, among them stress, suggest a more complex mechanism, and possibly differences in the reward system for different compounds. In the present paper we compare the effects of receptor-specific antagonists on the increase of dopamine induced by nicotine with that induced by cocaine in the nucleus accumbens in conscious rats. The compounds alone or together were injected intravenously, and dopamine level changes were measured via microdialysis. When administered together the effect of nicotine and cocaine on the level of dopamine in the accumbens was additive. Apparently there is some interaction between the two compounds, since nicotine had no effect after combined nicotine and cocaine administration. Perhaps the available dopamine pool was exhausted by the prior administration. The nicotinic antagonist mecamylamine, the muscarinic antagonist atropine, and the NMDA glutamate receptor antagonist MK-801 each blocked nicotine-induced dopamine release in the accumbens, indicating the participation of more than a single receptor system in the nicotine-induced effect. These three antagonists did not inhibit cocaine-induced dopamine increase in the accumbens, indicating the lack of a role of these receptors in the cocaine effect under our experimental conditions. SCH-23390, a dopamine D1 receptor antagonist, blocked both nicotine- and cocaine-induced effects, indicating the possible role of this receptor in these reward effects. The results indicate that there are differences in some of the receptors mediating the central effects of the two compounds examined, nicotine and cocaine, although each influences dopamine levels, and that the two compounds interact.

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.

Nicotine and food induced dopamine release in the nucleus accumbens of the rat: putative role of alpha7 nicotinic receptors in the ventral tegmental area

We have recently shown that the stimulatory effect of nicotine on dopamine output in the nucleus accumbens is largely dependent upon an enhanced glutamate transmission via N-methyl-D-aspartate receptors, possibly through stimulation of nicotinic receptors localized presynaptically on glutamatergic afferents in the ventral tegmental area. Given that nicotinic alpha7 receptors have been proposed to be involved in presynaptic regulation of glutamate release, we investigated whether alpha7 receptors underlie such a mechanism in the ventral tegmental area. For this purpose, by utilizing microdialysis we measured dopamine release in the nucleus accumbens in response to systemic nicotine, with, or without, concomitant infusion into the ventral tegmental area of the selective alpha7 receptor antagonist methyllycaconitine. To test also whether alpha7 nicotinic receptor antagonism within the ventral tegmental area affected a natural reward-mediated increase in dopamine release in the nucleus accumbens, we employed a model of schedule-induced feeding. Intrategmental administration of methyllycaconitine decreased both the nicotine-induced and the food-induced dopamine release in the nucleus accumbens. We suggest that alpha7 nicotinic receptors in the ventral tegmental area are involved in the acute effect of nicotine on dopamine release in the nucleus accumbens and conclude that the mechanism, by which nicotine stimulates the mesolimbic dopaminergic system, may be an essential constituent of the natural reward-related circuits in brain.

Transgenic mice in drug dependence research

Transgenic mice with null mutation of specific genes of the central nervous system obtained by homologous recombination, called also knock-out mice, have been recently used by behavioural neuroscientists to understand better the relevance of certain biological mechanisms of drug dependence or addiction. This article reviews some of the main contributions to this fastly developing field. As addictive drugs exert similar reinforcing effects both in humans and other mammals, changes in behavioural performance produced by the motivational effects of the addictive drugs in knock-out mice can give important information about the relevance of that particular gene product (eg a neurotransmitter receptor) for the pathogenicity of substance abuse disorders. In same cases the deletion of a given gene for a neurotransmitter receptor involved in the action of addictive drugs is associated with a phenotype that reproduces the effects obtained by the pharmacological administration of an antagonist for the same receptor. In other instances, surprising results are obtained, the most striking being the evidence that mice lacking the dopamine transporter gene, the most important binding site of cocaine, retain the capability to self-administer cocaine intravenously. Because the gene deletion is operative during embryogenesis, some adaptive compensatory mechanisms may produce unexpected results, suggesting caution in the interpretation of these results. The advent of tissue-specific inducible knock-out mice will soon produce a second revolution in the field of substance abuse research.

Effect of nicotine use and withdrawal on brain preproenkephalin A mRNA

Although the effect of nicotine on brain neurotransmitters and behavior has been studied, the mechanism(s) by which nicotine contributes to tobacco use remains unclear. One transmitter that may relate to long-term nicotine use and its withdrawal is enkephalin, a five-amino acid opioid peptide derived from the proenkephalin A family. In the present study we determined the effect of acute and chronic nicotine treatment and its withdrawal on preproenkephalin A mRNA levels (PPE mRNA) in specific rat brain regions using Northern blot analysis. Acute treatment with nicotine produced a significant increase in PPE mRNA in striatum and hippocampus. Chronic treatment with nicotine caused a significant decrease in PPE mRNA in these brain regions. In both striatum and hippocampus there was a rebound increase in PPE mRNA 24 h after nicotine cessation which approached the saline level 7 days later. Nicotine withdrawal 24 h following nicotine cessation, caused a significant increase in PPE mRNA in both brain regions. These effects of nicotine were blocked by pretreating rats with the nicotinic antagonist, mecamylamine. These data strongly suggest that brain opioid system(s) are involved in mediating nicotinic responses and its withdrawal and may have clinical implications in treating nicotine addiction.

Expression of fos protein in rat brain following administration of a nicotinic acetylcholine receptor agonist epibatidine

Epibatidine (exo-2-(6-chloro-3-pyridyl)-7-azabicyclo-[2.2.1]heptane), an extract of frog skin, is a novel and highly potent agonist for the nicotinic acetylcholine (ACh) receptor. The present study was undertaken to examine the expression of Fos protein in several rat brain regions following an acute administration of epibatidine. Furthermore, we also studied the role of the dopamine D1 and D2 receptors and the N-methyl-d-aspartate (NMDA) receptor, and nicotinic ACh receptor in the expression of Fos protein by epibatidine. A single administration of epibatidine (5, 10, 50 microgram/kg) caused a marked induction of Fos-immunoreactivity in the prefrontal cortex, medial striatum, nucleus accumbens, amygdala and superior colliculus of rat brain. In these regions, pretreatment with SCH 23390 (1.0 mg/kg), a dopamine D1 receptor antagonist, MK-801 (1.0 mg/kg), a NMDA receptor antagonist, and mecamylamine (5. 0 mg/kg), a nicotinic Ach receptor antagonist, inhibited the induction of Fos protein by epibatidine (10 microgram/kg). Pretreatment with sulpiride, a dopamine D2 receptor antagonist, blocked the induction of Fos protein in the prefrontal cortex and the core region of accumbens nucleus, but not in the medial striatum and the shell division of nucleus accumbens of rat brain. These results suggest that epibatidine induced the expression of Fos protein in several regions of rat brain, and that dopamine D1 receptor, NMDA receptor, and nicotinic ACh receptor may play a role in the expression of Fos protein by epibatidine in rat brain. Furthermore, dopamine D2 receptor may, in part, play a role in epibatidine induced expression of Fos protein in the prefrontal cortex and the core region of nucleus accumbens, but not in the medial striatum and the shell division of nucleus accumbens of rat brain.

Activation of midbrain presumed dopaminergic neurones by muscarinic cholinergic receptors: an in vivo electrophysiological study in the rat

1. Extracellular single-unit recording and iontophoresis were used to examine the effects of different cholinoceptor agonists and antagonists on the firing rate and firing pattern of A9 and A10 presumed dopaminergic neurones in the anaesthetized rat. 2. Administration of low currents (1-5 nA) of the selective muscarinic agonists oxotremorine M (Oxo M) and muscarine and of the non-selective muscarinic/nicotinic agonist carbamylcholine (CCh) produced a dose-dependent increase in firing rate in most of the A9 and A10 presumed dopaminergic neurones tested. Oxo M-induced activation could be completely blocked by iontophoretic application of the muscarinic antagonist butyl-scopolamine or systemic administration of the muscarinic antagonist scopolamine (300 microg kg(-1), i.v.). 3. Iontophoretic application of the selective nicotinic agonist methylcarbamylcholine (MCCh), but not nicotine, induced a consistent increase in firing rate. Surprisingly, the excitatory effect of MCCh was significantly reduced by the selective muscarinic antagonist scopolamine (300 microg kg(-1), i.v.), but not by the selective nicotinic antagonist mecamylamine (2.2 mg kg(-1), i.v.). Mecamylamine (3 mg kg(-1), i.v.) was also ineffective in reducing the CCh-induced activation of presumed dopamine neurones, suggesting that both CCh and MCCh increased the activity of dopamine neurones via an interaction with muscarinic receptors. 4. Iontophoretic application of the endogenous agonist acetylcholine (ACh) had no or little effect on the firing activity of A10 presumed dopaminergic neurones. However, concomitant application of neostigmine, a potent cholinesterase inhibitor, with acetylcholine induced a substantial activation of these neurones. This activation consisted of two components; one, which was prevalent, was scopolamine (300 microg kg(-1), i.v.)-sensitive, and the other was mecamylamine (2 mg kg(-1), i.v.)-sensitive. 5. In addition to their effect on firing activity, Oxo M, muscarine and concomitant neostigmine/ACh caused a significant increase in burst firing of A10 neurones, but not of A9 neurones. 6. These data suggest that dopamine cells, both in the A9 and A10 regions, possess functional muscarinic receptors, the activation of which can increase their firing rate and, for A10 neurones, their amount of burst activity. These cholinoceptors would be able to influence the activity of the midbrain dopamine system greatly and may play a role in, and/or be a therapeutic target for, brain disorders in which dopamine is involved (e.g., Parkinson's disease, drug addiction and schizophrenia).

Effects of chronic nicotine treatment and withdrawal on hypothalamic proopiomelanocortin gene expression and neuroendocrine regulation

Considerable evidence suggest that some responses to smoking and nicotine are mediated by forebrain beta-endorphinergic opioid mechanisms. It has also been demonstrated that nicotine stimulates rat tuberoinfundibular dopaminergic activity. Since we have proposed that interactions between mediobasohypothalamic (MBH) dopaminergic and beta-endorphinergic mechanisms have a key role in neuroendocrine integration, we investigated the effects of chronic nicotine treatment and withdrawal on: (1) MBH concentrations of proopiomelanocortin (POMC, precursor for beta-endorphin biosynthesis) mRNA; (2) MBH concentrations of tyrosine hydroxylase (TH, rate limiting enzyme in catecholamine biosynthesis) mRNA; (3) corresponding serum prolacin, corticosterone, luteinizing hormone (LH), and testosterone concentrations. POMC and TH mRNA levels were measured by RNase protection/solution hybridization assay; serum hormone levels were measured by radioimmunoassay. Adult male rats received subcutaneous injections of either nicotine or saline during the dark period of each day on an increasing frequency (1-3 injections/day) and dosage (0.4-0.5 mg nicotine/kg body weight) schedule over 4 weeks. The rats were sacrificed after 4 weeks treatment and at 1, 3, 7, 14 and 21 days withdrawal. Chronic daily nicotine administration induced significant changes in serum corticosterone, serum prolactin, MBH TH mRNA, and MBH POMC mRNA concentrations that tended to persist through day 3 of withdrawal; serum prolactin and MBH POMC mRNA concentrations were suppressed whereas serum corticosterone and MBH TH mRNA concentrations were stimulated. None of the parameters were significantly different from control levels following 7 or more days of withdrawal from nicotine, except for a significant decrease of MBH POMC mRNA concentrations on day 21. Chronic daily nicotine or withdrawal did not significantly alter serum LH or testosterone concentrations. These results suggest that chronic nicotine inhibited POMC gene expression and thus, probably, biosynthesis of beta-endorphin and other opiomelanocortins. We hypothesize that suppression of forebrain beta-endorphin synthesis in response to long-term nicotine exposure produces a chronically opioid deficient condition which may play an important role in maintaining nicotine self-administration and in mediating some changes during the nicotine withdrawal syndrome.

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.

Nicotine increases stress-induced serotonin release by stimulating nicotinic acetylcholine receptor in rat striatum

We used a microdialysis technique to analyze the effects of footshock stress on the release of serotonin (5-hydroxytryptamine: 5-HT) in the striatum or prefrontal cortex (PFC) in rats that were pretreated with nicotine. Neither nicotine administration alone nor stress application alone changed 5-HT release. During stress application, however, both chronic nicotine administration and local infusion of nicotine to the striatum significantly increased 5-HT release in the striatum, though not in the PFC. These increases in 5-HT release were eradicated by a local infusion of mecamylamine. Release of 5-HT increased in the striatum during stress application when nicotine was injected to the striatum, while nicotinic injection to the dorsal raphe nucleus did not increase 5-HT release in the striatum. The present study demonstrates that nicotine induced a release of 5-HT upon stress application by stimulating presynaptic nicotinic receptors in the striatum.

N-methyl-D-aspartate receptor antagonism in the ventral tegmental area diminishes the systemic nicotine-induced dopamine release in the nucleus accumbens

Systemic nicotine enhances burst firing of dopamine neurons in the ventral tegmental area and dopamine release in the nucleus accumbens, mainly via stimulation of nicotinic acetylcholine receptors in the ventral tegmental area. Given that both the neuronal activity of mesolimbic dopamine neurons and terminal dopamine release are regulated by excitatory amino acid inputs to the ventral tegmental area and that nicotine facilitates glutamatergic transmission in brain, we investigated the putative role of ionotropic glutamate receptors within the ventral tegmental area for the effects of nicotine on dopamine release in the nucleus accumbens using microdialysis, with one probe implanted in the ventral tegmental area for drug application and another in the ipsilateral nucleus accumbens for measuring dopamine, in awake rats. Systemic nicotine (0.5 mg/kg, s.c.) and infusion of nicotine (1.0 mM) into the ventral tegmental area increased dopamine output in the nucleus accumbens. Intrategmental infusion of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (0.1 mM) or N-methyl-D-aspartate (0.3 mM) increased accumbal dopamine release; these effects were antagonized by concomitant infusion of a selective antagonist at N-methyl-D-aspartate receptors, 2-amino-5-phosphonopentanoic acid (0.3 mM), and non-N-methyl-D-aspartate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (0.3 mM), respectively. Infusion of either antagonist (0.3 or 1.0 mM) into the ventral tegmental area did not affect basal dopamine levels, whereas infusion of 2-amino-5-phosphonopentanoic acid, but not 6-cyano-7-nitroquinoxaline-2,3-dione, starting 40 min before nicotine injection dose-dependently attenuated the nicotine-induced increase in accumbal dopamine release. Concurrent intrategmental infusion of 2-amino-5-phosphonopentanoic acid and nicotine decreased nicotine-induced dopamine release in the nucleus accumbens. These results indicate that the stimulatory action of nicotine on the mesolimbic dopamine system is to a considerable extent mediated via stimulation of N-methyl-D-aspartate receptors within the ventral tegmental area.

Effects of nicotine and footshock stress on dopamine release in the striatum and nucleus accumbens

We have used a microdialysis technique to analyze the effects of nicotine administration on the release of dopamine in the striatum and the nucleus accumbens (NAC) in rats under footshock stress. In the striatum, neither chronic systemic nicotine administration alone nor stress alone changed the extent of dopamine release. During stress application, however, chronic nicotine administration significantly increased dopamine release. In the NAC, stress did not induce increase in dopamine release in rats given nicotine chronically. However, in rats subjected to stress alone, dopamine release in the NAC was significantly increased after stress. In the striatum, the local infusion of 1.0 mM nicotine increased dopamine release. Furthermore, stress significantly increased nicotine-induced dopamine release. The local infusion of 1.0 mM nicotine into the NAC significantly increased dopamine release, but the levels returned to the baseline 30 min later. On the other hand, stress alone induced the release of dopamine 30 min later and the combination of stress and nicotine induced the release of dopamine during the stress and the effects lasted for 30 min. These results suggest that the responses of nicotine-induced dopamine release were different in the striatum and in the NAC under the stress. Stress and nicotine (systematically or locally administered) induced an immediate effect on dopamine release in the striatum, but in the nucleus accumbens stress alone and the combination of stress and nicotine induced a lasting release of dopamine (DA).

Acetylcholine receptors containing the beta2 subunit are involved in the reinforcing properties of nicotine

Release of the neurotransmitter dopamine in the mesolimbic system of the brain mediates the reinforcing properties of several drugs of abuse, including nicotine. Here we investigate the contribution of the high-affinity neuronal nicotinic acetylcholine receptor to the effects of nicotine on the mesolimbic dopamine system in mice lacking the beta2 subunit of this receptor. We found that nicotine stimulates dopamine release in the ventral striatum of wild-type mice but not in the ventral striatum of beta2-mutant mice. Using patch-clamp recording, we show that mesencephalic dopaminergic neurons from mice without the beta2 subunit no longer respond to nicotine, and that self-administration of nicotine is attenuated in these mutant mice. Our results strongly support the idea that the beta2-containing neuronal nicotinic acetylcholine receptor is involved in mediating the reinforcing properties of nicotine.

Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

Mesolimbocortical dopamine (DA) neurotransmission is important in the mediation of the dependence-producing actions of nicotine and other drugs of abuse. Withdrawal from chronic treatment with various types of addictive drugs, including amphetamine, cocaine, ethanol and morphine is associated with a decrease in dopaminergic output in the nucleus accumbens (NAC), whereas the effects of withdrawal from these drugs on dopaminergic output in the medial prefrontal cortex (PFC), as yet, remain largely unknown. This study examined putative changes in the extracellular levels of dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC and in the PFC of rats displaying behavioral signs of nicotine withdrawal. Rats were infused for 7 days with nicotine via subcutaneously implanted minipumps, whereas control animals carried saline-containing pumps. On the fifth day of infusion a microdialysis probe was implanted in the NAC or the PFC of the rats. Forty-eight hours later the levels of DA and the monoamine metabolites were assessed in the dialysate. The behavioral and biochemical effects of a saline injection and a subsequent challenge with the nicotinic receptor antagonist mecamylamine (1 mg/kg s.c.) were determined. Following mecamylamine challenge in nicotine-treated animals, the levels of DA, DOPAC and HVA in the NAC, but not in the PFC, decreased below pre-injection levels and in relation to control animals. The score of abstinence signs increased in the nicotine-treated rats, as compared both to the score after saline and to that in control animals. The decreased DA output in the NAC in animals displaying nicotine withdrawal signs is similar to that seen after withdrawal of several other drugs of abuse, and may have bearing on motivational deficits associated with the abstinence reactions.

The role of afferents to the ventral tegmental area in the handling stress-induced increase in the release of dopamine in the medial prefrontal cortex: a dual-probe microdialysis study in the rat brain

This study was aimed to identify the neuronal pathways that mediate the handling stress induced increase in the release of dopamine in the medial prefrontal cortex (mPFC) of the rat brain. For that purpose a microdialysis probe was implanted in the ventral tegmental area (VTA) and a second probe was placed in the ipsilateral mPFC. Receptor specific compounds acting on GABA(A) (20 microM muscimol), GABA(B) (50 microM baclofen), acetylcholine (100 microM atropine, 100 microM mecamylamine), NMDA (30, 100 and 300 microM CPP; 300 microM AP-5, 1 mM (+)-HA-966) and non-NMDA receptors (500 microM CNQX) were infused into the VTA by retrograde dialysis, whereas extracellular dopamine was recorded in the ipsilateral mPFC. Intrategmental infusion of muscimol, baclofen, CPP, AP-5, (+)-HA-966 and CNQX decreased extracellular dopamine in the ipsilateral mPFC; atropine and mecamylamine were without effect on the basal values. During infusion of the various compounds rats were gently handled for 15 min. The infusions of muscimol, atropine, mecamylamine and (+)-HA-966 did not modify the handling stress induced increase in extracellular dopamine in the mPFC. However, during intrategmental infusion of baclofen, CPP, AP-5 and CNQX the handling stress induced increase in extracellular dopamine (expressed as % of controls) in the mPFC was suppressed. These results indicate that a glutamatergic projection to the VTA, acting via both NMDA and non-NMDA-glutamate receptors, play a major role in the handling stress-induced increase in dopamine release in the mPFC. In addition the results suggest a certain role for GABAergic neurones, acting via GABA(B) receptors, in the handling response.

(-)-Nornicotine partially substitutes for (+)-amphetamine in a drug discrimination paradigm in rats

Rats were trained in a two-lever food-reinforced operant task to discriminate (+)-amphetamine (1 mg/kg) from saline. After discrimination training stabilized, test doses of (+)-amphetamine (0.0625-2.0 mg/kg), (-)-nicotine (0.1-1.0 mg/kg), or (-)-nornicotine (1-10 mg/kg) were assessed for their ability to substitute for the (+)-amphetamine training dose during brief test sessions in which food reinforcement was withheld. As expected, as the test dose of (+)-amphetamine increased, there was a dose-related increase in drug-appropriate responding, with both 1 and 2 mg/kg test doses substituting fully for the (+)-amphetamine training dose. Both (-)-nicotine and (-)-nornicotine showed partial substitution (approximately 50% drug-appropriate responding) for the (+)-amphetamine training dose, with (-)-nicotine being more potent than (-)-nornicotine. Rate suppressant effects prevented the assessment of higher doses of (-)-nicotine or (-)-nornicotine. Thus, while (-)-nicotine and (-)-nornicotine share similar discriminative stimulus properties, the mechanism that mediates this effect appears to differ, at least in part, from that activated by (+)-amphetamine.

Tobacco smoke influence on heart rate, body temperature, and locomotor activity daily rhythms as assessed by radiotelemetry in rats

Using radiotelemetry, this study aimed to evaluate the influence of tobacco smoke on heart rate (H), body temperature (T) and locomotor activity (A) daily rhythms in rats. The tobacco smoke intoxication was produced with a smoking apparatus. H, T, and A data were captured by radiotelemetry. The study was divided into three periods: a 1-week control period (P1), a 1-week stress period (P2), in order to evaluate the stress induced by the animals' restraint in the smoking apparatus, and a 1-week daily tobacco smoke intoxication period (P3). For P1, P2, and P3, a power spectrum analysis was applied in order to determine the dominant period of rhythmicity. Then, characteristics of the rhythms were determined by cosinor analysis. Statistical comparisons were done by ANOVA. Power spectrum analysis showed that neither stress nor tobacco suppressed the daily rhythmicity. Cosinor revealed some modifications: H amplitude was decreased during P2 and P3 with a greater reduction during P3, while T and A amplitudes were decreased during P2 and P3 without difference between P2 and P3. T acrophase was delayed during P2, while A acrophase was delayed during P2 and P3 without any difference between P2 and P3. These perturbations may reflect the effects of stress and tobacco on the suprachiasmatic nucleus by a dopaminergic mechanism.

Contribution of CNS nicotine metabolites to the neuropharmacological effects of nicotine and tobacco smoking

Nicotine, the principal alkaloid in tobacco products, is generally accepted to be the active pharmacological agent responsible for CNS effects resulting from tobacco use. Arguments are presented in this commentary which take issue with this popular dogma, by providing evidence that nicotine metabolites may also be responsible for the CNS effects commonly attributed to nicotine. CNS effects attributed to nicotine include reinforcing effects, mood elevation, arousal, locomotor stimulant effects, and learning and memory enhancement. The reinforcing and locomotor stimulant effects of nicotine have been suggested to be the result of activation of CNS dopaminergic systems, and nicotine-induced modulation of dopaminergic neurotransmission has been studied in detail. Nicotine acts at a family of nicotinic receptor subtypes composed of multiple subunits; however, the exact composition of the subunits in native nicotinic receptors and the functional significance of the receptor subtype diversity are currently unknown. This nicotinic subtype diversity increases the complexity of the potential mechanisms of action of nicotine and its metabolites. Although peripheral metabolism of nicotine has been studied extensively, metabolism in the CNS has not been investigated to any great extent. Recently, studies from our laboratory have demonstrated that several nicotine metabolites are present in the CNS after acute nicotine administration. Moreover, nicotine metabolites are pharmacologically active in neurochemical and behavioral assays. Thus, CNS effects resulting from nicotine exposure may not be due solely to nicotine, but may result, at least in part, from the actions of nicotine metabolites.

Accumbal dopamine overflow after ethanol: localization of the antagonizing effect of mecamylamine

It has been suggested that ethanol exerts its mesolimbic dopamine activating effects and its reinforcing effects via interaction with central nicotinic acetylcholine receptors, thus providing a basis for the often observed covariation between ethanol and nicotine consumption. We have previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine totally counteracts the ethanol-induced elevation of extracellular dopamine in the nucleus accumbens, as measured by in vivo microdialysis. A contribution of peripheral nicotinic receptor blockade could, however, not be excluded. In the present study, mecamylamine (1.0 mg/kg, i.p.) again totally counteracted the ethanol-induced dopamine overflow, as measured by in vivo microdialysis, while the quarternary nicotinic receptor antagonist hexamethonium (10 mg/kg, i.p.) did not. Furthermore, the increase in accumbal dopamine overflow after systemic ethanol (2.5 g/kg, i.p.) was counteracted by local perfusion of mecamylamine (50 microM) in the ipsilateral ventral tegmental area, but not by mecamylamine perfusion in the nucleus accumbens. Ethanol-induced accumbal dopamine overflow was also counteracted by perfusion of hexamethonium (250 microM) in the ventral tegmental area. These results provide further evidence that ethanol-induced activation of the mesolimbic dopamine system is mediated via stimulation of central nicotinic acetylcholine receptors, and that the receptor population within the ventral tegmental area may be the most important in this regard. It is suggested that antagonists of central nicotinic acetylcholine receptors may be useful in the treatment of alcoholism.

Both nicotinic and muscarinic receptors in ventral tegmental area contribute to brain-stimulation reward

Cholinergic neurons of the pedunculopontine tegmental nucleus (Ch5) and laterodorsal tegmental nucleus (Ch6) monosynaptically activate dopamine neurons of the substantia nigra and ventral tegmental area (VTA) via nicotinic and muscarinic receptors. The nicotinic receptors near the VTA have been proposed to be important for nicotine self-administration in rats and for tobacco smoking in humans. Nicotinic and muscarinic blockers were microinjected into the VTA of rats trained to lever-press for lateral hypothalamic stimulation via an ipsilateral electrode. The competitive nicotinic blocker dihydro-beta-erythroidine (DH beta E; 5-60 micrograms) shifted rate-frequency curves to the right by a mean of 6-27% in a dose-related manner; the noncompetitive nicotinic blocker mecamylamine (10-300 micrograms) produced similar shifts of 7-21%. Atropine (30 micrograms) shifted the curves to the right by a mean of 82% in three of the sites tested with DH beta E. All blockers decreased maximum bar-pressing rates significantly in some sites when the shifts were large. Therefore, nicotinic receptors in the VTA make small contributions to the maintained rewarding effect of brain-stimulation reward in rats, but muscarinic receptors in the VTA appear to be more important.

Dopamine receptor antagonists block nerve growth factor-induced hyperactivity

The role of dopamine receptors in mediating nerve growth factor (NGF)-induced locomotor stimulation was investigated by examining the effects of selective dopamine D1 and D2 receptor antagonists on the motor hyperactivity induced by NGF. A single intracerebroventricular administration of NGF (5.1 microg) increased locomotor activity immediately after injection in normal adult rats. This hyperactivity was partly blocked by the dopamine D1 receptor antagonist SCH23390 (R-(+)-7-chloro-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1H-3-benzazepine-8- ol) and by the dopamine D2 antagonist raclopride ((S)-3,5-dichloro-N-((1-ethyl-2-pyrrolidinyl)methyl)-2-hydroxy-6-methoxy benzamide). Effective doses of raclopride did not alter spontaneous levels of activity in control rats. These results suggest that stimulation of both subtypes of dopamine receptors is necessary for eliciting NGF-induced hyperactivity in the rat. The role of the dopamine D2 receptor in mediating the behavioral actions of NGF appears to be more important than that of the dopamine D1 receptor.

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Adrenocorticotropin response and nicotine-induced norepinephrine secretion in the rat paraventricular nucleus are mediated through brainstem receptors

Nicotine is a potent stimulus for the secretion of ACTH, and norepinephrinergic neurons originating in the brainstem are involved. Prior reports using in vivo microdialysis in alert rats have shown that nicotine, administered i.p. or into the fourth ventricle, stimulated the release of norepinephrine (NE) into the hypothalamic paraventricular nucleus (PVN), the site of neurons containing CRH. In the present studies, rats received an i.v. infusion of nicotine into the jugular vein on alternate days during their active (dark) phase; therefore, direct correlations between the levels of NE microdialyzed from the PVN and plasma ACTH could be made in each animal. Nicotine administered i.v. (0.045-0.135 mg/kg) elicited dose-dependent increases in both NE and ACTH (P < 0.01). A significant correlation was found between nicotine-stimulated NE release in the PVN and ACTH secretion (r = 0.91, P < 0.01). To address whether the site(s) of action of nicotine was on presynaptic receptors on NE terminals in the PVN or on receptors on neurons in brainstem regions accessible from the fourth ventricle, the nicotinic cholinergic antagonist, mecamylamine (0.1-4.8 microg), was microinjected directly into the PVN or into the fourth ventricle before nicotine infusion. Fourth-ventricular administration of mecamylamine (1.6 microg) or higher, before i.v. nicotine (0.09 mg/kg), completely blocked both NE release in the PVN (IC50 = 0.64 microg) and ACTH secretion (IC50 = 0.40 microg) (P < 0.01, compared with vehicle before nicotine), whereas it was ineffective when injected directly into the PVN. The results demonstrate that the nicotinic cholinergic receptors in the brainstem, rather than presynaptic receptors within the PVN itself, mediate nicotine-stimulated PVN NE release and ACTH secretion.

Regional variation in the effects of nicotine on catecholamine overflow in rat brain

The effects of acute, repeated intermittent and continuous administration of nicotine on the overflow of noradrenaline in the ventral hippocampus and dopamine in the nucleus accumbens and striatum have been studied. Daily injections of nicotine (0.4 mg/kg(-1) for 5 days) enhanced noradrenaline and dopamine overflow in the ventral hippocampus and nucleus accumbens respectively (P < 0.01 and P < 0.05) but not dopamine in the striatum in response to a nicotine challenge. The responses in the ventral hippocampus and nucleus accumbens were attenuated (P < 0.01) by the constant infusion of nicotine at a dose of 1 mg kg(-1) per day; the dopamine response in the striatum required a higher dose (4 mg kg(-1) per day) before desensitisation was observed. The data suggest that the dopamine projections to the striatum are less sensitive to both stimulation and desensitisation by nicotine than the catecholamine projections to the ventral hippocampus and nucleus accumbens.

Nicotinic acetylcholine receptor (nACh-R) agonist-induced changes in brain monoamine turnover in mice

The aim of the present study was to evaluate the effects of nicotinic acetylcholine receptor (nACh-R) agonists such as (-)-nicotine and related compounds on brain monoamine turnover. A single administration of (-)-nicotine (0.04, 0.2, 1.0, and 5.0 mg/kg SC) increased both noradrenaline (NA) and dopamine (DA) turnover in a dose-dependent manner, and the maximum effects were achieved 30 min after treatment with (-)-nicotine (1.0 mg/kg). The effect of (-)-nicotine on serotonin (5-HT) turnover was complicated; 5-HT turnover was increased at a low dose of (-)-nicotine (0.04 mg/kg) but decreased at a high dose (1.0 mg/kg). The (-)-nicotine (1.0 mg/kg)-induced changes in monoamine turnover were blocked by pretreatment with the centrally acting nACh-R channel blocker mecamylamine (2.0 mg/kg i.p.) but not by hexamethonium (2.0 mg/kg i.p.). These findings indicate that systemically administered (-)-nicotine can enhance brain NA and DA turnover and affect 5-HT turnover, both of which are mediated by central nACh-R. The changes in the monoamine turnover induced by (+/-)-anabasine were similar to those induced by (-)-nicotine, while (-)-lobeline and (-)-cytisine had little effect, and 1,1-dimethyl-4-phenyl-piperazinium (DMPP) increased NA and 5-HT turnover but not DA turnover at all doses tested. (S)-3-Methyl-5-(l-methyl-2- pyrrolidinyl)isoxazole (ABT-418), a selective neuronal nACh-R agonist, increased NA, DA and 5-HT turnover, but had a weaker effect on DA turnover than NA and 5-HT turnover. In addition, 9-amino-1,2,3,4-tetrahydroacridine (THA), an acetylcholine esterase inhibitor, also increased monoamine turnover in the brain. Pretreatment with mecamylamine completely blocked the THA-induced increase in NA and 5-HT turnover, but not in DA turnover, suggesting that the nACh-R system is involved in the THA-induced increase in brain NA and 5-HT turnover. On the other hand, (-)-cytisine, a partial agonist for the beta 2 subunit containing nACh-R, completely inhibited the nACh-R agonist- and THA-induced increases in NA turnover, but not in DA turnover, and normalized the changes in 5-HT turnover. In conclusion, the subtypes of nACh-Rs mediating DA turnover may be different from those mediating NA and 5-HT turnover in the CNS.

Presynaptic nicotinic ACh receptors

Nicotinic ACh (nACh) receptors in the CNS are composed of a diverse array of subunits and have a range of pharmacological properties. However, despite the fact that they are ligand-gated cation channels, their physiological functions have not been determined. This has led to increased interest in presynaptic nACh receptors that act to modulate the release of transmitter from presynaptic terminals.

Common neural substrates for the addictive properties of nicotine and cocaine

Regional brain activation was assessed by mapping of Fos-related protein expression in rats trained to self-administration of intravenous nicotine and cocaine. Both drugs produced specific overlapping patterns of activation in the shell and the core of the nucleus accumbens, medial prefrontal cortex, and medial caudate areas, but not in the amygdala. Thus, the reinforcing properties of cocaine and nicotine map on selected structures of the terminal fields of the mesocorticolimbic dopamine system, supporting the idea that common substrates for these addictive drugs exist.

Differential effects of acute and chronic nicotine on dopamine output in the core and shell of the rat nucleus accumbens

Like several drugs of abuse, nicotine increase dopamine (DA) release in the nucleus accumbens (NAC). In the present study, the effects of acute and chronic nicotine on DA output in two subdivisions of the NAC, the core and the shell, which are largely associated with motor control and limbic functions, respectively, were examined by means of in vivo differential normal pulse voltammetry in anesthetized, pargyline-treated rats. In the first experiment, acute administration of nicotine (25, 50 and 100 micrograms/kg, cumulative doses; i.v.) was found to increase DA levels in the NACshell to 163% of baseline, whereas DA output in the NACcore was not significantly affected. In the second experiment, animals were pretreated with twelve daily injections of saline or nicotine (0.5 mg/kg, i.p.); about 24 hours after the last injection, the animals were challenged with nicotine (50 micrograms/kg and 100 micrograms/kg, cumulative doses; i.v.). Under these conditions, nicotine increased DA output in the NACshell in saline-pretreated animals to 248% and in nicotine-pretreated rats to 180%. Also, nicotine increased DA output in the NACcore in saline-pretreated animals to 185%, whereas no significant effect was observed in nicotine-pretreated rats. The results of the present experiments indicate (i) that acutely administered nicotine or nicotine challenge in chronically pretreated animals with either saline or nicotine consistently increases DA release to a greater extent in the NACshell than in the NACcore, and (ii) that chronic nicotine pretreatment reduces the stimulatory-action of nicotine on DA output in either the shell or the core subdivision of the NAC.

The reinforcing properties of nicotine are associated with a specific patterning of c-fos expression in the rat brain

Rats were trained for nicotine intravenous infusions in a self-administration paradigm. The effect of nicotine self-administration on regional brain activity was studied by mapping changes of c-fos expression. Specific nicotine effects were determine by comparing the patterning of Fos-like immunoreactivity (Fos-Ll) in nicotine self-administering rats with that in three different control groups. Controls included rats exposed to the same manipulation as nicotine self-administering rats who received intravenous saline instead of nicotine. In addition, two groups of untrained sham-operated rats exposed daily to the same operant boxes were included: one group had the same food restriction used in the operant training, the other was fed ad libitum. Nicotine self-administration, exposure to saline and food restriction increased Fos-Ll in 43, 33 and three brain regions, respectively, when compared with the control group fed ad libitum. Computer-assisted image analysis of Fos-Ll profiles performed on 16 relevant limbic and sensory structures showed that in saline-treated rats a significant (P < 0.01) increase of Fos-Ll profiles was observed in medial prefrontal cortex, lateral septum, core and ventral shell of nucleus accumbens, claustrum, amygdaloid nuclei, paraventricular thalamic nucleus and lateral geniculate nucleus. A significant (P < 0.01) further increase produced by nicotine was found in medial prefrontal cortex and ventral shell of nucleus accumbens. Interestingly, cingulate and piriform cortex, superior colliculus and medial terminal nucleus of the accessory optic tract were specifically activated by nicotine but not saline. These results show that nicotine self-administration activates sensory structures, as well as limbic structures involved in natural rewarding pathways. The results suggest the involvement of restricted terminal regions of the mesocorticolimbic dopaminergic system in the maintenance of nicotine self-administration.

Nicotine injections into the ventral tegmental area increase locomotion and Fos-like immunoreactivity in the nucleus accumbens of the rat

Systemic administration of nicotine has been shown to increase locomotor activity in rats, an effect which is enhanced by chronic pretreatment with the drug. Furthermore, administration of nicotine either systemically, or locally within the ventral tegmental area (VTA), increases extracellular levels of dopamine (DA) in the nucleus accumbens (NAc). In the present study, we examined the effect of local, bilateral injections into the VTA of nicotine (0.02, 0.2, 2.0 and 8.0 micrograms/0.5 microliter/side) on locomotor activity of rats in an open field. Nicotine (8.0 micrograms/side) significantly increased forward locomotion within 20 min after injection, whereas rearing was not affected. The stimulatory effect of locally applied nicotine was completely blocked by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Repeated intra-tegmental injections of a subthreshold dose of nicotine (2.0 micrograms/side every 2 days), gradually increased locomotion, compared to the effect of acute intra-tegmental administration or control injections of saline, after the fifth and sixth injection. The effects of intra-tegmental injections of nicotine were further investigated on cells in several target areas for the VTA-DA neurons through determination of c-fos expression by means of Fos immunohistochemistry. Intra-tegmental injections of nicotine (8.0 micrograms/side) increased Fos-like immunoreactivity in the NAc, but did not affect the number of Fos-positive nuclei in the medial prefrontal cortex or in the dorsolateral striatum. The increase in accumbal Fos-like immunoreactivity was attenuated by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Our data demonstrate that locomotor activating effects similar to those evoked by systemically administered nicotine, including behavioral sensitization, can be produced by intra-tegmental nicotine administration. Moreover, such local VTA administration of the drug was found to significantly affect neurons within DA target areas. Our findings support the notion that the effects of systemically administered nicotine in mesolimbic target areas are largely dependent on stimulation of nicotinic receptors in the VTA.

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.

Brain sites mediating the discriminative stimulus effects of nicotine in rats

Pharmacological studies suggest that the discriminative stimulus (DS) produced by nicotine is mediated centrally. The aim of the present study was to identify neuroanatomical substrates that mediate the DS properties of nicotine. Specifically, the nucleus accumbens, a brain region known to mediate the DS effects of amphetamine and cocaine, was investigated using a two-lever operant drug discrimination paradigm. Male hooded rats were trained to discriminate nicotine (0.2 mg/kg s.c.) from saline with a tandem schedule of food reinforcement. Once stimulus control was attained, a randomised sequence of nicotine microinjections (2-8 micrograms) was tested for generalisation during brief extinction tests. It was confirmed that the stimulus produced by the systemic administration of nicotine generalized to nicotine administered bilaterally into the dorsal hippocampus, with significant decreases in overall response rates. Microinjections of nicotine (1-8 micrograms) into the nucleus accumbens failed to produce any dose-related increases in responding on the nicotine-appropriate lever although these microinjections also produced significant decreases in response rates. Smaller doses (1-4 micrograms) of nicotine administered into the fourth ventricle produced characteristic prostration responses but these microinjections failed to produce generalization in tests carried out 20 min later, when the disabling effects of prostration had dissipated. These results suggest that the DS effects of nicotine may be mediated, at least in part, through the dorsal hippocampus. Results from intra-accumbens and intraventricular injections suggest that these regions may not be important in mediating the DS effects of nicotine.

Nicotinic, muscarinic and dopaminergic actions in the ventral hippocampus and the nucleus accumbens: effects on spatial working memory in rats

Acetylcholine (ACh) systems have been widely shown to be important for memory. In particular, ACh hippocampal neurons are critical for memory formation, though ACh innervation of other areas such as the nucleus accumbens may also be important. There has also been increasing interest in ACh and dopaminergic (DA) interactions with regard to short-term spatial memory. In a series of studies, we have found that ACh and DA agonists and antagonists given systemically interact to influence memory. The critical neural loci of these interactions are not currently known. In the present study, we used local infusion techniques to examine the role of ACh and DA transmitter systems in the nucleus accumbens and the ventral hippocampus on radial-arm maze (RAM) working memory performance. Into the nucleus accumbens of rats, we infused the nicotinic ACh agonist nicotine, the nicotinic ACh antagonist mecamylamine, the DA agonist apomorphine, or the DA antagonist haloperidol. Into the ventral hippocampus, we infused nicotine, mecamylamine, the muscarinic ACh agonist pilocarpine, or the muscarinic ACh antagonist, scopolamine. The nicotinic ACh and DA interaction was tested by a hippocampal infusion of mecamylamine alone or together with the DA D2 agonist quinpirole given via subcutaneous injection. The results confirmed that both nicotinic and muscarinic ACh receptors in the ventral hippocampus play a significant role in spatial working memory. Blockade of either nicotinic or muscarinic ACh receptors caused significant impairments in RAM choice accuracy. However, infusion of either nicotinic or muscarinic agonists failed to improve choice accuracy. The interaction of DA D2 systems in different with hippocampal nicotinic blockade than with general nicotinic blockade. Systemic administration of quinpirole potentiated the amnestic effect of mecamylamine infused into the ventral hippocampus, whereas it was previously found to reverse the amnestic effect of systemically administered mecamylamine. In contrast to the significant effects of mecamylamine in the hippocampus, no effects were found after infusion into the nucleus accumbens. Nicotine also was not found to have a significant effect on memory after intra-accumbens infusion. Neither the DA agonist apomorphine nor the DA antagonist haloperidol had a significant effect on memory after infusion into the nucleus accumbens. This study provides support for the involvement of nicotinic and muscarinic receptors in the ventral hippocampus in memory function. Ventral hippocampal nicotinic systems have significant interactions with D2 systems, but these differ from their systemic interactions. In contrast, nicotinic ACh and DA systems in the nucleus accumbens were not found in the current study to be important for working memory performance in the RAM.

Non-psychostimulant drugs of abuse and anxiogenic drugs activate with differential selectivity dopamine transmission in the nucleus accumbens and in the medial prefrontal cortex of the rat

In rats vertically implanted with concentric dialysis probes in the medial prefrontal cortex and in the medial nucleus accumbens, morphine, ethanol and nicotine failed to modify extracellular dopamine in the medial prefrontal cortex at doses that were fully effective in raising extracellular dopamine in the nucleus accumbens. Conversely, the aversive/anxiogenic drugs picrotoxin, pentylenetetrazol and FG 7142, administered at subconvulsant doses, increased extracellular dopamine in the medial prefrontal cortex but failed to do so in the nucleus accumbens. Systemic administration of low doses of the 5HT3 antagonist ICS 205930, previously reported to prevent the increase of extracellular dopamine in the nucleus accumbens elicited by morphine, nicotine, ethanol and haloperidol (Carboni et al. 1989) as well as by stress (Imperato et al. 1990), also prevented the increase of extracellular dopamine elicited in the prefrontal cortex by anxiogenic drugs. Therefore, mesocortical and mesolimbic dopamine neurons show clear-cut differences in the reactivity to drugs of abuse and to aversive drugs but are both modulated by a facilitatory serotonergic input mediated by 5HT3 receptors.

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.

Sensitization to the conditioned rewarding effects of morphine: pharmacology and temporal characteristics

An unbiased place preference conditioning procedure was used to determine whether the repeated administration of morphine results in sensitization to its conditioned rewarding effects. Rats received once daily injections of saline or morphine (5.0 mg/kg; i.p.) for 5 days in a room distinct from that where conditioning would occur. Place preference conditioning commenced 72 h later. A minimum of three drug conditioning sessions was necessary for the establishment of morphine-induced conditioned place preferences in saline-pretreated rats. The minimum dose producing this effect was 5.0 mg/kg. In animals pre-exposed to morphine, significant place preferences occurred after only two drug conditioning sessions and in response to doses of 3.0 mg/kg and greater. The augmented response to morphine was apparent when conditioning commenced 3, 10 or 21 days after the cessation of morphine pretreatment. It was not apparent when conditioning commenced 1 day after treatment cessation. An enhanced response to morphine was also observed in rats which had previously received either fentanyl (0.016 mg/kg/day) or nicotine (0.4 mg/kg/day) for 5 days. Animals which received morphine or fentanyl in combination with naloxone (0.5 mg/kg; s.c.) for 5 days failed to exhibit a conditioned response to morphine. When, however, naloxone was administered in combination with nicotine, significant morphine-induced place preferences were still seen. These data demonstrate that both sensitization and cross-sensitization develop to the conditioned rewarding effects of morphine. Furthermore, they indicate that the sensitization induced by morphine and fentanyl, but not nicotine, is opioid-receptor mediated.

Differential role of delta-opioid receptors in the development and expression of behavioral sensitization to cocaine

The present study was designed to determine whether the selective delta-opioid receptor antagonist naltrindole hydrochloride can prevent the expression and development of sensitization to the locomotor-activating effects of cocaine. Rats were sensitized to the motor stimulant effects of cocaine (20 mg/kg i.p. x 3 days). 48 h after withdrawal of pretreatment, rats were pretreated with naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle and 15 min later challenged with either saline or the sensitizing dose of cocaine. In a second set of experiments, naltrindole (0.1-1.0 mg/kg s.c.) or its vehicle were given in combination with either saline or cocaine (20 mg/kg i.p.) for 3 days. Activity in response to saline and to cocaine (20 mg/kg i.p.) was assessed on days 4 and 5, respectively. Additional experiments determined whether naltrindole prevents the development of sensitization to the locomotor-activating effects of nicotine: naltrindole (0.3, 1.0 mg/kg s.c.) or its vehicle were given in combination with nicotine (0.6 mg/kg s.c.) for 3 days. Naltrindole blocked the development but not expression of sensitization to the locomotor-activating effects of cocaine. In contrast, naltrindole failed to modify nicotine-induced sensitization in nicotine-treated animals. These data suggest that delta-opioid receptors are involved in the development but not expression of behavioral sensitization to cocaine.

Pharmacology of nicotine and its therapeutic use in smoking cessation and neurodegenerative disorders

During the last decade, nicotine has been used increasingly as an aid to smoking cessation and has been found to be a safe and efficacious treatment for the symptoms of nicotine withdrawal. This period has also seen significant advances in our understanding of the mechanisms underlying the psychopharmacological responses to nicotine, including, particularly, those that have been implicated in nicotine addiction. This paper reviews this decade of progress in the specific context of the therapeutic application of nicotine to the treatment of smoking cessation. Other putative future applications, particularly in the treatment of neurodegenerative disorders, are also reviewed.

5-Hydroxytryptamine-mediated effects of nicotine on endogenous GABA efflux from guinea-pig cortical slices

1. The effect of nicotine on endogenous basal GABA outflow was studied in guinea-pig cerebral cortex slices. 2. Nicotine 1.86-18.6 mumol l-1 significantly decreased the basal, tetrodotoxin-sensitive GABA efflux, whereas at higher concentrations (186-620 mumol l-1) nicotine increased it. The inhibition was prevented by mecamylamine while the facilitation was blocked by mecamylamine, (+)-tubocurarine and tetrodotoxin. 3. The effect of nicotine was due to an indirect 5-hydroxytryptaminergic action. In fact, MDL 72222 (1 mumol l-1) completely prevented the alkaloid inhibition and methysergide (1 mumol l-1) reversed the facilitation into inhibition; concomitant treatment with methysergide and MDL 72222 antagonized the effect of nicotine at 186 mumol l-1 4. Lower concentrations of 5-HT (3-10 mumol l-1) decreased, whereas higher concentrations (30-100 mumol l-1) increased, spontaneous GABA outflow. The inhibition of GABA efflux was prevented by MDL 72222 whereas the facilitation was reversed by methysergide (1 mumol l-1) into inhibition, and prevented by MDL 72222 1 mumol l-11. 5. These results suggest that, by activating nicotinic receptors present on 5-hydroxytryptaminergic terminals, nicotine releases 5-HT which, in turn, inhibits or increases the secretory activity of cortical GABA interneurones via 5-HT3 and methysergide-sensitive receptors, respectively.

The role of mesolimbic dopaminergic and retrohippocampal afferents to the nucleus accumbens in latent inhibition: implications for schizophrenia

Latent inhibition (LI) consists in a retardation of conditioning seen when the to-be-conditioned stimulus is first presented a number of times without other consequence. Disruption of LI has been proposed as a possible model of the cognitive abnormality that underlies the positive psychotic symptoms of acute schizophrenia. We review here evidence in support of the model, including experiments tending to show that: (1) disruption of LI is characteristic of acute, positively-symptomatic schizophrenia; (2) LI depends upon dopaminergic activity; (3) LI depends specifically upon dopamine release in n. accumbens; (4) LI depends upon the integrity of the hippocampal formation and the retrohippocampal region reciprocally connected to the hippocampal formation; (5) the roles of n. accumbens and the hippocampal system in LI are interconnected.

Effects of transderman nicotine on mood and sleep in nonsmoking major depressed patients

The role of nicotine as an indirect cholinergic agent in sleep has been studied in normal subjects. There are no studies of its effects on sleep in depressed patients. Nicotine transdermal patches (17.5 mg), were studied in eight depressed patients (DSM-III-R) and eight normal volunteers. Subjects wore placebo and nicotine patches for 24 h. Depressed patients showed increased REM sleep without changes in other sleep variables. They also showed a short term improvement of mood. Normal volunteers had sleep fragmentation, and reduction of REM sleep time. No major side effects were reported in either group.