The effects of acute and repeated nicotine treatment on nucleus accumbens dopamine and locomotor activity

Effect of nicotine, lobeline, and mecamylamine on sensory gating in the rat

In normal subjects, if an acoustic startle stimulus is immediately preceded by a small brief change in background noise intensity, the magnitude of the subsequent startle response is decreased. This prepulse inhibition (PPI) of an acoustic startle response has been shown to be associated with sensorimotor gating. PPI is disrupted in schizophrenic patients and has been linked to attentional disorders characteristic of this disease. We tested the effects of (-)-nicotine, (0.19, 0.62, and 1.9 mumol/kg IP) (equivalent to 0.03, 0.1, and 0.3 mg/kg base) and the nicotinic cholinergic receptor (nAChR) channel blocker, mecamylamine (5.0 and 50 mumol/kg IP) (equivalent to 1.0 and 10.0 mg/kg) on PPI of the acoustic startle response in the rat. Nicotine increased the PPI at the lowest prepulse signal levels but not at the stronger levels. Mecamylamine was without effect at 5.0 mumol/kg, but the 50 mumol/kg dose decreased the inhibition at both weak and strong prepulse (PP) levels. Mecamylamine (5.0 mumol/kg) pretreatment did not block the (-)-nicotine-induced increase in PPI. Lobeline (0.19, 0.62, 1.9, and 6.2 mumol/kg IP) (equivalent to 0.071, 0.23, 0.71, and 2.3 mg/kg) was without effect. These results are consistent with a mecamylamine-insensitive effect of nicotine to improve gating in normal rats. The nAChR subtype involved in producing nicotine's increase of PPI needs further investigation.

Neural mechanisms underlying nicotine dependence

There is little doubt that many habitual smokers find it difficult to quit the habit because they have become addicted to the nicotine present in the smoke. This paper addresses some of the pharmacological mechanisms underlying this addiction and discusses how an understanding of these mechanisms may contribute to the more effective use of nicotine replacement therapy during smoking cessation. It considers critically the evidence that the "rewarding" properties of nicotine, which serve to reinforce drug-seeking behaviour, are related to stimulation of the mesolimbic dopamine system of the brain. The critique focuses specifically on the evidence that many central nicotinic receptors, including those which mediate the effects of the drug on dopamine secretion, are readily desensitized by chronic exposure to agonist and that hypotheses which assume that nicotine inhaled from tobacco smoke invariably results in stimulation of the receptors must be treated with caution. Nicotinic receptors in the brain are, however, heterogeneous in nature with different molecular structures and pharmacologies. It is concluded that the reinforcing properties of nicotine sought by smokers may reflect both stimulation and desensitization of the different nicotinic receptor populations, and that smokers may adjust their smoking habits to achieve the balance of receptor stimulation and desensitization which they find most reinforcing. It seems likely that the efficacy of the different nicotine formulations during the treatment of smoking cessation may also reflect their ability to stimulate or desensitize brain nicotinic receptors.

Effects of aging on acute toxicity of nicotine in rats

Acute toxicity of nicotine was examined in old (24 months) and young (6 weeks) Wistar rats. There were no significant age differences in the mortality and convulsive responses induced by an intraperitoneal injection of nicotine (24.5 mg/kg). The lethal nicotine levels in blood and cortex and the latent period to death in old rats were larger than those in young rats. Cortical and blood nicotine levels 15 min. after the nicotine injection in old rats were also higher than those in young rats. Nicotine significantly increased dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid levels in striatum and hippocampus in young rats, but not those in old rats. Moreover, nicotine-induced elevations in blood levels of corticosterone in old rats were also less than those in young rats. On the other hand, the cytochrome P-450 content, the nicotine oxidase activity and the flavin-containing monooxygenase activity in liver showed age-related decreases in the young, the middle-aged (12 months) and the old rats. These results indicate that the acute toxicity of nicotine in old rats reflects the decreases in hepatic nicotine metabolism and in brain sensitivity to nicotine.

Behavioural and neurochemical adaptations to nicotine in rats: influence of NMDA antagonists

1. The repeated co-administration of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (0.1 and 0.3 mg kg-1, i.p.) with nicotine (0.4 mg kg-1, s.c.) attenuated the development of tolerance to the locomotor depressant effect of the nicotine in rats. 2. The repeated co-administration of the competitive NMDA antagonist D-CPPene (SDZ EAA 494; 3-(2-carboxypiperazin-4-yl)-1-propenyl-1-phosphonic acid, 2 and 8 mg kg-1, i.p.) also attenuated tolerance to the locomotor depressant effect of nicotine. 3. Dizocilpine (0.3 mg kg-1, i.p.) pretreatment attenuated sensitization to the locomotor stimulant effect of nicotine (0.4 mg kg-1, s.c.) and prevented sensitization of nicotine-induced dopamine release in the nucleus accumbens. However, pretreatment with dizocilpine alone caused a modest enhancement of the behavioural response to a subsequent acute dose of nicotine. 4. D-CPPene (2.0 mg kg-1, i.p.) pretreatment prevented sensitization to the nicotine-induced dopamine release in the nucleus accumbens. There was no enhanced locomotor response that could be attributed to nicotine pretreatment when D-CPPene was co-administered with nicotine. However, pretreatment with D-CPPene alone enhanced the locomotor response to an acute dose of nicotine. 5. The results suggest the involvement of NMDA receptors in adaptations of the behavioural and neurochemical effects of nicotine that occur as a result of repeated administration of the drug.

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

The locomotor response to nicotine was assessed four weeks following destruction of mesolimbic dopamine (DA) neurons in rats by infusion of 6-hydroxydopamine into the ventral tegmental area. Resulting depletions of nucleus accumbens (N.Acc.) DA of up to 100% of control concentrations did not block the acute locomotor response to nicotine (0.4 mg/kg, base, s.c.). Such depletions also did not prevent the progressive enhancement of nicotine's locomotor effects when injections were repeated daily for nine days. These results suggest that mesolimbic DA is not necessary for the elicitation of locomotor activation by nicotine.

Studies on the influence of nicotine infusions on mesolimbic dopamine and locomotor responses to nicotine

The present study examined the effects of constant nicotine infusions on dopamine overflow in the nucleus accumbens and on locomotor activity and compared them with the changes evoked by repeated daily injections (one injection per day for 5 days) of the drug. The putative anxiolytic properties of nicotine have also been examined using the elevated plus-maze test of anxiety. Repetitive daily subcutaneous injections of nicotine (0.4 mg/kg) enhanced (P < 0.01) the overflow of dopamine evoked by a challenge dose of the drug (0.4 mg/kg) and increased (P < 0.01) its stimulatory effects on locomotor activity. The constant infusion of nicotine, at doses of 1 and 4 mg/kg per day, abolished (P < 0.05) the effects of a bolus injection of nicotine on extracellular dopamine and attenuated (P < 0.01) the enhanced locomotor response evoked by daily pretreatment with nicotine boli. The mesolimbic dopamine response to a bolus injection of nicotine was not significantly attenuated by nicotine infusions when the dose was reduced to 0.25 mg/kg per day. The locomotor responses in these rats were significantly (P < 0.05) less than those seen in the animals pretreated with nicotine injections alone but were also higher (P < 0.05) than those seen in saline-treated control rats given a bolus injection of nicotine. Neither the constant infusion (4 mg/kg per day) nor the injection of nicotine (0.4 mg/kg) evoked an anxiolytic or anxiogenic effect in the elevated plus-maze test. However, the nicotine infusions did abolish the locomotor stimulant effects of the drug in this apparatus.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of voluntary alcohol intake in the rat by modulation of the dopaminergic mesolimbic system: transplantation of ventral mesencephalic cell suspensions

The dopaminergic mesolimbic system plays a major role in the mechanisms of reward and positive reinforcement, and is also known to be a primary target for the action of substances that are self-administered and are considered drugs of abuse. Even though alcohol administration has been shown, by physiological and pharmacological manipulations, to cause changes in the mesolimbic dopaminergic system, it has not yet been determined whether, conversely, experimentally induced changes in this system are effective in regulating the voluntary intake of ethanol. In the present study we assessed the effects of the intrastriatal transplantation of fetal dopaminergic grafts on the regulation of voluntary alcohol intake in the rat. Fetal dopaminergic transplants from ventral mesencephalon--but not dopamine-poor transplants or sham-operated animals--reduced the voluntary intake of ethanol by about 40-50%. These results indicate that the effects obtained are due to the dopaminergic nature of the grafts, and not the consequence of a non-specific effect of the graft, or of the surgical procedure itself. These results support the hypothesis that the dopaminergic mesolimbic system plays an important role in the regulation of the voluntary intake of ethanol.

Nicotine-induced place preferences following prior nicotine exposure in rats

The motivational properties of morphine and nicotine were investigated in an automated conditioned place preference (CPP) procedure using a two-compartment apparatus. The accuracy of the photocell recording system was assessed by correlation with direct observation. In a counterbalanced conditioning design, graded doses of morphine (0.1-3.2 mg/kg SC) produced dose-related CPP. Under similar conditions, a dose of nicotine (0.6 mg/kg SC) previously reported to produce CPP failed to show an effect. Increasing the number of conditioning trials from 4 to 12 did not facilitate CPP with nicotine. After pretreatment with nicotine (0.4 mg/kg SC) daily for 7 days prior to conditioning, nicotine (0.4-0.8 mg/kg) produced increasing magnitudes of CPP. Locomotor activity was assessed during both conditioning and extinction tests. During conditioning, nicotine but not morphine decreased activity in the first conditioning trial, but by the fourth trial, marked stimulation was apparent following administration of either drug. Activity in the drug-paired compartment was not increased during tests for CPP carried out in the undrugged state following 4 conditioning trials with either morphine or nicotine, but there was evidence for conditioned hyperactivity after 12 conditioning trials with nicotine. The results suggest that motivational properties of nicotine can be detected in counterbalanced CPP procedures, but only in subjects with a history of nicotine exposure. The CPP produced by morphine or nicotine does not appear to be an artefact associated with conditioned changes in locomotor activity.

Effects of systemic nicotine on serotonin release in rat brain

We used in vivo microdialysis to examine the acute effects of systemically administered nicotine (0.8-8.0 mg/kg, s.c.) on extracellular levels of serotonin (5-HT) in the frontal cortex of awake rats and animals anesthetized with chloralose/urethane. In anesthetized animals, 5-HT efflux was elevated during the initial 15 min after nicotine administration (2-8 mg/kg), but then returned to baseline values. All of the effective nicotine doses also lowered and then raised blood pressure in these animals. However, other drugs which raised (methoxamine, 0.07 mg/kg, i.v.) or lowered (mecamylamine, 5 mg/kg, i.p.) blood pressure without directly activating nicotinic receptors failed to alter 5-HT release. Moreover, pretreatment with a centrally active dose of mecamylamine, a known nicotinic antagonist, blocked the effects of nicotine (4 mg/kg) on 5-HT release. For studies on awake rats the perfusion fluid also contained fluoxetine, since basal 5-HT levels were barely detectable without this uptake blocker. In such animals, 1.6 mg/kg of nicotine significantly increased 5-HT release, an effect apparent in the initial 20 min after treatment and persisting for at least 2 h. These observations demonstrate that systemically administered nicotine increases frontocortical 5-HT release, that this effect is independent of the cardiovascular responses to the drug, and that it probably results from the activation of previously described nicotinic receptors on raphe neurons. The present findings are consistent with the hypothesis that the appetitive and mood disturbances associated with nicotine withdrawal may be mediated by diminished serotoninergic transmission.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

Influence of tetrodotoxin and calcium on changes in extracellular dopamine levels evoked by systemic nicotine

The influence of tetrodotoxin (TTX) and calcium on the increase of extracellular dopamine (DA) levels in the nucleus accumbens (NAcc), evoked by the systemic administration of nicotine, cocaine and d-amphetamine, have been studied in conscious, freely moving rats using in vivo microdialysis. TTX (10(-6) M), administered via the dialysis probe, completely abolished (P < 0.01) the elevations in extracellular DA, DOPAC and HVA seen following nicotine (0.4 mg/kg SC). The removal of calcium with the inclusion of diaminoethanetetraacetic acid (EDTA 10(-4) M) in the Ringer solution was also associated with inhibition (P < 0.01) of the nicotine-induced changes in these parameters. The systemic administration of cocaine (15 mg/kg IP) and d-amphetamine (0.5 mg/kg SC) caused elevations in extracellular DA (P < 0.01) accompanied by significant decreases (P < 0.01) in HVA levels. DOPAC levels were also significantly (P < 0.01) lowered by d-amphetamine treatment. The presence of TTX and removal of calcium with addition of EDTA completely abolished the changes in NAcc DA and HVA induced by cocaine. TTX had no influence on the d-amphetamine evoked responses in NAcc DA. However, the metabolites, which were markedly reduced by the TTX, were not further decreased by the systemic administration of d-amphetamine. NAcc DA was significantly (P < 0.01) raised following d-amphetamine in the absence of calcium and presence of EDTA. However, this was significantly (P < 0.01) attenuated in comparison to that seen in the presence of calcium. The results support the conclusion that, at the dose tested, nicotine evokes increases in extracellular NAcc DA levels by calcium and impulse-dependent mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)