Dopamine and nicotinic receptor binding and the levels of dopamine and homovanillic acid in human brain related to tobacco use

The dopamine transporter and attention-deficit/hyperactivity disorder

The high incidence of attention-deficit/hyperactivity disorder (ADHD) and escalating use of ADHD medications present a compelling case for clarifying the pathophysiology of, and developing laboratory or radiologic tests for, ADHD. Currently, the majority of specific genes implicated in ADHD encode components of catecholamine signaling systems. Of these, the dopamine transporter (DAT) is a principal target of the most widely used antihyperactivity medications (amphetamine and methylphenidate); the DAT gene is associated with ADHD, and some studies have detected abnormal levels of the DAT in brain striatum of ADHD subjects. Medications for ADHD interfere with dopamine transport by brain-region- and drug-specific mechanisms, indirectly activating dopamine- and possibly norepinephrine-receptor subtypes that are implicated in enhancing attention and experiential salience. The most commonly used DAT-selective ADHD medications raise extracellular dopamine levels in DAT-rich brain regions. In brain regions expressing both the DAT and the norepinephrine transporter (NET), the relative contributions of dopamine and norepinephrine to ADHD pathophysiology and therapeutic response are obfuscated by the capacity of the NET to clear dopamine as well as norepinephrine. Thus, ADHD medications targeting DAT or NET might disperse dopamine widely and consign dopamine storage and release to regulation by noradrenergic, as well as dopaminergic neurons.

Neurochemical alterations produced by daily nicotine exposure in periadolescent vs. adult male rats

Chronic treatment with nicotine differentially alters behavior in adolescent rats compared to adult rats. It is not known, however, whether the effects of nicotine on the neurochemical pathways with which it interacts differ in adolescents vs. adults. In the current study, the effects of a 7-day treatment with nicotine on nicotinic, dopaminergic, and serotonergic neurochemistry were examined in the caudate putamen and nucleus accumbens in periadolescent vs. adult male rats. Nicotine treatment increased dopamine transporter densities and decreased serotonin transporter densities in periadolescent rats. There was no change in nicotinic acetylcholine receptor densities or dopamine D1 or D2 receptor densities in nicotine-pretreated periadolescent rats. In adult rats pretreated with nicotine, there was an increase in nicotinic acetylcholine densities, but no change in dopamine transporter, dopamine D1 or D2 receptor, or serotonin transporter densities. Overall, these findings show that periadolescent rats have neurochemical adaptations to nicotine different from adult rats. These alterations may explain, at least in part, the differential behavioral effects of chronic nicotine in adult and adolescent male rats.

Prolonged nicotine administration results in biphasic, brain-specific changes in kynurenate levels in the rat

The content of the endogenous NMDA and alpha7 nicotinic acetylcholine receptor antagonist kynurenate (KYNA) is increased in the cerebral cortex and cerebrospinal fluid of patients with schizophrenia. In view of the very high incidence of smoking in schizophrenic individuals, a study was designed to examine the effect of acute and prolonged nicotine administration on brain KYNA levels in experimental animals. Adult male rats received subcutaneous nicotine injections twice daily for up to 10 days, and animals were routinely killed 1 h after the last injection. Neither acute treatment nor a 2-day regimen with 1 mg/kg nicotine (= 0.35 mg/kg pure base) caused changes in cerebral KYNA levels. Four- or 6 day-treatment with this dose resulted in 20-40% decreases in cerebral KYNA content. Animals treated with 1 or 10 mg/kg nicotine for 10 days showed dose-dependent, significant increases in KYNA in hippocampus, striatum, and cortex, but not in the serum. Discontinuation of nicotine treatment for 7 days restored brain KYNA to control levels. Separate animals, implanted with osmotic minipumps delivering 2 mg/kg of nicotine/day for 10 days also showed significant elevations in brain KYNA. Hippocampal microdialysis, performed in animals receiving nicotine (1 mg/kg) for 10 days, revealed a significant increase in basal extracellular KYNA levels compared to controls, whereas acute treatment with this dose produced no such change. Measurements of KYNA's bioprecursor kynurenine in brain or blood did not reveal any nicotine-induced changes. These results indicate that nicotine has a brain-specific, biphasic effect on the transamination of kynurenine to KYNA. Such nicotine-induced fluctuations in brain KYNA may cause functional changes in processes that regulate glutamatergic and cholinergic neurotransmission in the normal and diseased brain.

Metabolism and disposition kinetics of nicotine

Nicotine is of importance as the addictive chemical in tobacco, pharmacotherapy for smoking cessation, a potential medication for several diseases, and a useful probe drug for phenotyping cytochrome P450 2A6 (CYP2A6). We review current knowledge about the metabolism and disposition kinetics of nicotine, some other naturally occurring tobacco alkaloids, and nicotine analogs that are under development as potential therapeutic agents. The focus is on studies in humans, but animal data are mentioned when relevant to the interpretation of human data. The pathways of nicotine metabolism are described in detail. Absorption, distribution, metabolism, and excretion of nicotine and related compounds are reviewed. Enzymes involved in nicotine metabolism including cytochrome P450 enzymes, aldehyde oxidase, flavin-containing monooxygenase 3, amine N-methyltransferase, and UDP-glucuronosyltransferases are represented, as well as factors affecting metabolism, such as genetic variations in metabolic enzymes, effects of diet, age, gender, pregnancy, liver and kidney diseases, and racial and ethnic differences. Also effects of smoking and various inhibitors and inducers, including oral contraceptives, on nicotine metabolism are discussed. Due to the significance of the CYP2A6 enzyme in nicotine clearance, special emphasis is given to the effects and population distributions of CYP2A6 alleles and the regulation of CYP2A6 enzyme.

Regional Heterogeneity of Nicotine Effects on Neurotransmitters in Rat Brains in vivo at Low Doses

In our recent studies on nicotine-induced changes in neurotransmitters in brain areas associated with cognitive function using a nicotine dose of 0.5 mg/kg administered subcutaneously to conscious freely moving rats, we found changes in dopamine, norepinephrine, and serotonin, and their metabolites, in the areas examined. For the present report we examined changes in these neurotransmitters following administration of lower nicotine doses, to test regional differences in nicotine response and possible threshold levels for some effects of nicotine. The doses used were 0.15 mg/kg and 0.03 mg/kg nicotine administered subcutaneously. Nicotine levels in the brain reached peak values in less than 10 min and decreased with a half-life of about 60 min (0.15 mg/kg) or 30 min (0.03 mg/kg) to values below detection limits (1 ng/g), by the later time points of the 0.03 mg/kg experiments. Nicotine-induced dopamine (DA) increase (and increase in DA metabolites) and decrease in 5-HT levels at 0.15 mg/kg were significant in the cortex, less so in the hippocampus. Norepinephrine (NE) increase at 0.15 mg/ kg was much less significant than found previously at 0.5 mg/kg. At a low nicotine dose (0.03 mg/kg), the significant changes observed were a decrease in 5-HT in the hippocampus and small increases of DA and NE in the prefrontal cortex and of NE in the medial temporal cortex. In the nucleus accumbens DA, NE, and 5-HT and their metabolites in the ventral tegmental area, mostly DA and metabolites were increased. We conclude that in areas of cognitive function nicotine-induced DA changes are more concentration dependent than changes in NE or 5-HT, and that there are regional differences in neurotransmitter changes induced by nicotine, with NE changes detectable only in the cortex and 5-HT changes only in the hippocampus at a low nicotine dose, indicating significant regional variation in sensitivity to nicotine-induced neurotransmitter changes in brain areas associated with cognitive function. The decrease in 5-HT shows that nicotine also has indirect effects caused by neurotransmitters released by nicotine. The effects of a low nicotine dose are more significant in areas of reward function, indicating differences in sensitivity between cognitive and reward functions.

Chronic nicotine and smoke treatment modulate dopaminergic activities in ventral tegmental area and nucleus accumbens and the ?-aminobutyric acid type B receptor expression of the rat prefrontal cortex

Dopaminergic afferents from the mesencephalic areas, such as ventral tegmental area (VTA), synapse with the gamma-aminobutyric acid (GABA)-ergic interneurons in the prefrontal cortex (PFC). Pharmacological and electrophysiological data show that the reinforcement, the dependence-producing properties, as well as the psychopharmacologic effects of nicotine depend to a great extent on activation of nicotinic receptors within the mesolimbocortical dopaminergic projection. To explore further the relationship between the mesencephalic dopaminergic neurons and PFC GABAergic neurons, we investigated the effects of nicotine and passive exposure to cigarette smoke on the regulation of tyrosine hydroxylase (TH) in VTA and substantia nigra (SNC) and dopamine (DA) D1 receptor levels in nucleus accumbens (NAc) and caudate-putamen (CPu). Also, the simultaneous changes in GABAB receptors mRNAs in the PFC were studied. The results showed that chronic nicotine and smoking treatment differentially changed the levels of TH protein in VTA and SNC and DA D1 receptor levels in Nac and CPu. GABAB1 and GABAB2 receptor mRNA levels also showed different change patterns. Ten and thirty minutes of smoke exposure increased GABAB1 receptor mRNA to a greater extent than that of GABAB2, whereas GABAB2 was greatly enhanced after 1 hr of smoke exposure. The TH levels in VTA were closely related to DA D1 receptor levels in NAc and with GABAB receptor mRNA changes in PFC. These results suggest that the mesolimbic pathway and GABAB receptor mRNA in PFC are modulated by nicotine and cigarette smoke, implying an important role in nicotine's psychopharmacological effects.

Selective changes in nicotinic acetylcholine receptor subtypes related to tobacco smoking: an immunohistochemical study

Increases in neuronal nicotinic receptors (nAChRs) in response to nicotine exposure have been reported in cell cultures, rodent brains, and in the brains of human smokers. The present study examines alterations in alpha4 and alpha7 nAChR subunit cellular expression in human hippocampus and entorhinal cortex from normal elderly individuals with known smoking history. There were significant increases in the intensity of alpha4 immunoreactive neuropil, but not the number of cell bodies, in many regions of hippocampus and entorhinal cortex in smokers compared to age-matched non-smokers and ex-smokers. There was also an increase in alpha7 immunoreactive perikarya in the granular cell layer of dentate gyrus in smokers but not other regions examined. There was, in contrast, a significant reduction in alpha7 immunoreactive astrocytes in smokers and ex-smokers compared to non-smokers. These findings suggest exposure to tobacco smoke acutely up-regulates alpha4 receptors in axon terminals and dendrites but not perikarya, whereas tobacco smoking induced down-regulation of alpha7 expression on astrocytes is a long-term effect. As the alpha4 subunit decreases with ageing and degenerative diseases such as Alzheimer's disease, whereas alpha7 increases in astrocytes in Alzheimer's disease, the findings further indicate the therapeutic relevance of nicotinic agonists such as nicotine.

Reduced levels of Abeta 40 and Abeta 42 in brains of smoking controls and Alzheimer's patients

The effects of nicotine on levels of Abeta 40 and Abeta 42 and nicotinic receptor binding sites were studied in brains from nonsmoking and smoking patients with Alzheimer's disease (AD) and aged-matched controls. The levels of soluble and insoluble Abeta 40 and Abeta 42 in frontal cortex and Abeta 40 in temporal cortex and hippocampus were significantly decreased in smoking AD patients compared to nonsmokers with AD. In smoking controls the levels of soluble and insoluble Abeta 40 and Abeta 42 in the frontal and temporal cortex were significantly lower than in nonsmoking controls. The binding of [(3)H]cytisine in temporal cortex was significantly increased in smokers with AD compared to nonsmokers with AD. In smoking controls [(3)H]cytisine and [(3)H]epibatidine binding were significantly increased from 1.5- to 2-fold compared to nonsmoking controls whereas binding sites for [(125)I]alpha-bungarotoxin was less up-regulated. These results indicate that selective nicotinic receptor agonists may be a novel protective therapy in AD by reducing Abeta levels as well as the loss of nicotinic receptors in AD brain.

Cortical dopamine D2 receptors in type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

Alcoholism has been associated with lower density of striatal dopamine (DA) D(2) receptors, but there is much less data on cortical DA D(2) receptors. We evaluated the [(125)I]epidepride binding to DA D(2) receptors in Cloninger type 1 and 2 alcoholics and controls in frontal, temporal, and anterior cingulate cortices by using human postmortem whole hemispheric autoradiography, which provides high-resolution images corresponding to positron emission tomographic (PET) studies. Type 1 alcoholics had lower and type 2 alcoholics had higher DA D(2) receptor density in all cortical areas compared to controls. Although the results did not reach statistical significance, the effect sizes were high. The DA D(2) receptor density in type 2 alcoholics decreased statistically significantly with age, and after correcting for age the binding values also fell below the level of controls. A statistically non-significant tendency towards a decrease of cortical DA D(2) receptors was seen in controls, whereas in the type 1 alcoholic group no consistent correlation or even tendency towards increase with age was observed. Our results give preliminary evidence that DA D(2) receptors in cortical areas may be lower among both groups of alcoholics, but not necessarily of same magnitude as in subcortical structures. The rapid decline of cortical DA D(2) receptors among type 2 alcoholics may have some relevance to their antisociality, because this trait tends to diminish with age. The absence of correlation or even tendency towards increase of cortical DA D(2) receptors with age seen in type 1 alcoholics may give further evidence that they have a pre-existing dopaminergic deficit. However, these results especially regarding aging effect must be considered as preliminary due to the different age-range of type 2 alcoholics compared to two other groups.

Nicotinic acetylcholine receptor distribution in Alzheimer's disease, dementia with Lewy bodies, Parkinson's disease, and vascular dementia: in vitro binding study using 5-[(125)i]-a-85380

Nicotinic acetylcholine receptors (nAChRs) have been implicated in a number of neurological disorders. 5-Iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380) is a novel nAChR marker, binding predominantly to the alpha4beta2 subtype. This in vitro autoradiography study describes the distribution of 5-[(125)I]-A-85380 binding in post-mortem brain tissue from normal elderly individuals and from cases with age-associated dementias of both neurodegenerative and vascular types. The binding distribution of 5-[(125)I]-A-85380 in normal brain tissue was found to be consistent with the reported distribution of other high-affinity nicotinic ligands. In addition to high thalamic and moderate striatal and temporal cortex density, moderate 5-[(125)I]-A-85380 binding was also seen in white matter tracts in cingulate, occipital, and temporal areas, indicating the presence of nAChRs along nerve fiber tracts, which has not been reported in other high-affinity nicotinic agonist distribution studies. In Parkinson's disease (PD), loss of striatal 5-[(125)I]-A-85380 binding closely parallels the loss of nigrostriatal dopaminergic markers previously observed. In dementia with Lewy bodies (DLB) reduced striatal 5-[(125)I]-A-85380 binding density, comparable to that in PD, may be a marker of early degeneration in nigrostriatal inputs, while in Alzheimer's disease (AD) reduced striatal 5-[(125)I]-A-85380 binding could be related to reduced cortical inputs. The reductions of nAChRs seen in AD, DLB, and PD were not apparent in vascular dementia (VaD). In conclusion, 5-I-A-85380 is clearly a useful ligand for both in vitro and in vivo single photon emission tomography human studies investigating disease symptoms and progression, response to acetylcholinesterase-inhibiting drugs and in differentiating primary degenerative dementia from VaD.

Dementia rating and nicotinic receptor expression in the prefrontal cortex in schizophrenia

BACKGROUND

The etiology of dementia that occurs in patients with schizophrenia is not well understood. Nicotinic acetylcholine receptors have been implicated in cognitive function, and deficits in these receptors have been reported in schizophrenia.

METHODS

The present study investigates possible associations of nicotinic receptor subunit expression in the dorsal lateral prefrontal cortex, an area known to be affected in schizophrenia, and dementia rating.

RESULTS

alpha7 immunoreactivity was reduced by 20% to 28% and [(3)H]epibatidine binding was increased twofold in groups of patients with schizophrenia compared to normal control subjects matched for age, postmortem delay, and low levels of brain nicotine and cotinine. In contrast, no significant differences in alpha4, alpha3, or beta2 immunoreactivity or alpha7 messenger RNA expression were observed in schizophrenia patients compared with control subject values. Clinical dementia ratings in patients with schizophrenia were correlated with neither [(3)H]epibatidine binding nor nicotinic receptor subunit expression.

CONCLUSIONS

These data indicate no relationship between the trend for reduced neocortical alpha7 subunit protein expression in schizophrenia and dementia. Further investigations are required to establish whether the reduction in alpha7 protein in the dorsal lateral prefrontal cortex is associated with clinical features other than dementia in schizophrenia.

The dopamine transporter and neuroimaging in attention deficit hyperactivity disorder

There is evidence that abnormalities within the dopamine system in the brain play a major role in the pathophysiology of attention deficit hyperactivity disorder (ADHD). For instance, dopaminergic psychostimulants, the drugs of first choice in ADHD, interact directly with the dopamine transporter (DAT). Molecular genetic studies suggest involvement of a polymorphism of the DAT gene in ADHD. More recent imaging studies show abnormalities in various brain structures, but particularly in striatal regions. In the current paper we review recent studies in this area. First in vivo measurements of DAT with single photon emission computed tomography (SPECT) in ADHD patients revealed an elevation of striatal DAT density. No differences in DAT density between the left and right side and between putamen and caudate nucleus have been found in [99mTc]TRODAT-1 SPECT of ADHD patients. Patients with ADHD and with a history of nicotine abuse both displayed lower values of DAT density in [99mTc]TRODAT-1 SPECT than non-smokers with ADHD. DAT seem to be elevated in non-smoking ADHD patients suffering from the purely inattentive subtype of ADHD as well as in those with the combined or purely hyperactive/impulsive subtype.

Dopamine receptors and transporters in the brain reward circuits of type 1 and 2 alcoholics measured with human whole hemisphere autoradiography

The role of the dopamine system in brain reward mechanisms and development of substance abuse is well-established with nucleus accumbens as a key structure in mediating these effects. Several studies on alcoholism have indicated defects in dopaminergic neurotransmission and alterations in dopamine receptor densities. However, it has remained unclear if the substance abuse-related dopaminergic defect is specifically associated with a certain receptor subtype. The aim of this study was to compare putative alterations of dopamine D(1,) D(2), and D(3) receptors in nucleus accumbens, amygdala, and substantia nigra among alcoholics and controls. We studied the densities of dopamine D(1) and D(3) receptors in brains of 9 type 1 alcoholics, 8 type 2 alcoholics, and 10 healthy controls by using postmortem human whole hemisphere autoradiography. The mean densities of dopamine D(1) and D(3) receptors were at the same level in all study groups. Combining these with our previous results, our data suggest that among type 1 alcoholics dopamine transporters are lower in nucleus accumbens and dopamine D(2), but not D(1) or D(3) receptors in nucleus accumbens and amygdala. Further, the densities of all these dopamine-binding sites among type 2 alcoholics are at the level of healthy controls. The results suggest that lower dopamine receptor density is specific for D(2) receptor and for type 1 alcoholism, which supports Cloninger's neurogenetic model of two alcoholic subtypes, and indicates the importance of classifying these subgroups separately when issues related to dopaminergic activity are studied.

Increased dopamine D3 receptor expression accompanying behavioral sensitization to nicotine in rats

Behavioral sensitization to nicotine, which appears following repeated nicotine administration, has been suggested to take part in the development of smoking habit in humans. The mesolimbic dopaminergic system plays a role in this process and a hypersensitivity of postsynaptic neurons of the nucleus accumbens as been proposed as a mechanism, but changes in dopamine D(1) or D(2) receptors have not been demonstrated to date. A challenge administration of nicotine (0.5 mg/kg s.c.) produced a strong increase in locomotor activity in rats repeatedly pretreated with nicotine (0.5 mg/kg s.c.), but not saline, once a day for 5 days. This behavioral sensitization was accompanied by an increase in D(3) receptor binding and mRNA in the shell of nucleus accumbens. D(3) receptor expression was unchanged in the core of nucleus accumbens and dorsal striatum, as it was in the shell of nucleus accumbens after an acute administration of nicotine to naive rats. In contrast, no changes were noticed in D(1) and D(2) receptor expressions in any brain region examined after chronic or acute treatment with nicotine. In addition, nicotine challenge decreased preprodynorphin and preprotachykinin mRNA levels in naive rats, but only preprotachykinin mRNA levels in rats pretreated with nicotine. These biochemical changes resemble those occurring during behavioral sensitization to levodopa of dopamine-denervated rats, which had been causally related to the induction of D(3) receptor expression. We propose that a similar mechanism is responsible for behavioral sensitization to nicotine.

Protein and mRNA levels of nicotinic receptors in brain of tobacco using controls and patients with Alzheimer's disease

The neuronal nicotinic receptors (nAChRs) are involved in several processes in brain including nicotine dependence and cognitive disorders. While the number of nAChRs in the brain of tobacco smokers is up-regulated, the receptors are reduced in the brain of patients with Alzheimer's disease (AD). The aim of this study was to investigate nAChR mRNA and protein levels in brain of smoking and non-smoking controls and AD patients. Western blotting and RT-PCR techniques were used to quantify different nAChR subunits in autopsy brain. The alpha4 and alpha7 but not the alpha3 nAChR protein levels were significantly increased in the temporal cortex of smoking (SC) compared with non-smoking controls (NSC). The alpha4-protein level was significantly higher in the temporal cortex of smoking AD (SAD) patients compared with non-smoking AD (NSAD). No changes in the alpha3, alpha4 or alpha7 subunits protein level were found in the hippocampus in any of the smoking groups. For both SADs and NSADs the protein levels for the alpha3 and alpha4 in temporal cortex and hippocampus and alpha7 in the hippocampus were significantly lower compared with non-smoking controls. No significant differences in alpha4 and alpha7 mRNA levels were detected in the hippocampus or temporal cortex of smokers compared with non-smokers. In conclusion this study showed an increased level of alpha4 and alpha7 nAChRs subunits in the temporal cortex of SC compared with NSC. This up-regulation was also seen in SAD although the protein levels of nAChR subunits were still lower in smoking AD brain compared with the NSC. The up-regulation of nAChRs in smoking groups and the loss of these receptors in AD patients were not correlated to any changes at the mRNA level suggesting that these changes may reflect post-transcriptional events.

Work exposure to urban pollutants and urinary homovanillic acid

The aim of this study was to evaluate whether traffic policemen exposed to urban pollutants could be at risk of alterations on urinary homovanillic acid in 24h HVA(U) excretion levels, an end product of dopamine catabolism, compared with a control group. Traffic policemen were matched by sex, age, and working life with control group after excluding principal confounding factors; 50 traffic policemen (29 men and 21 women) with outdoor activity exposed to urban pollutants and 50 not exposed subjects (29 men and 21 women) with indoor activity were included in the study. The HVA(U) excretion levels were significantly higher in male and female traffic policemen compared to not exposed subjects (respectively P=0.003; P=0.023). The authors hypothesize an effect on the excretion of HVA(U) in traffic policemen exposed to chemical and physical stressors, according to HVA(U) modifications found by other authors in workers exposed in factories.

Environmental factors in Parkinson's disease

Evidence discussed in this review article lends strong support in favor of an etiologic role of environmentalfactors in Parkinson's disease. First, thanks to the discovery of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), it is now clear that, by targeting the nigrostriatal system, neurotoxicants can reproduce the neurochemical and pathological features of idiopathic parkinsonism. The sequence of toxic events triggered by MPTP has also provided us with intriguing clues concerning mechanisms of toxicant selectivity and nigrostriatal vulnerability. Relevant examples are (i) the role of the plasma membrane dopamine transporter in facilitating the access of potentially toxic species into dopaminergic neurons; (ii) the vulnerability of the nigrostriatal system to failure of mitochondrial energy metabolism; and (iii) the contribution of inflammatory processes to tissue lesioning. Epidemiological and experimental data suggest the potential involvement of specific agents as neurotoxicants (e.g. pesticides) or neuroprotective compounds (e.g. tobacco products) in the pathogenesis of nigrostriatal degeneration, further supporting a relationship between the environment and Parkinson's disease. A likely scenario that emerges from our current knowledge is that neurodegeneration results from multiple events and interactive mechanisms. These may include (i) the synergistic action of endogenous and exogenous toxins (e.g. the ability of the pesticide diethyldithiocarbamate to promote the toxicity of other compounds); (ii) the interactions of toxic agents with endogenous elements (e.g. the protein alpha-synuclein); (iii) the tissue response to an initial toxic insult; and, last but not least, (iv) the effects of environmental factors on the background of genetic predisposition and aging.

Dopamine agonist and antagonist responders as related to types of nicotine craving and facets of extraversion

The role of dopamine (DA) in drug seeking behavior has been a matter of debate in the literature: One theory claims that DA triggers incentive motivational behavior, the other one favors the idea that DA itself is the rewarding property induced by the positive stimuli. The present experiment tries to contribute to a solution of the controversy by the approach of relating DA-associated motivational behavior to constellations of hormone response to a DA agonistic and DA antagonistic challenge performed in the same subjects and by relating their responses to different aspects of personality and smoking motivation. DA agonist (lisuride = LIS), DA antagonist (fluphenazine = FLU), and placebo (P) were applied to 36 male smokers who were deprived from smoking for 3.5 h in a balanced crossover design. Cigarette craving and prolactin (PRL) responses to the drugs were compared under the three pharmacological conditions and related to personality and smoking motivation. Results showed that PRL responder types to LIS and FLU, defined as differences from respective placebo values, emerged as pure agonist or antagonist responders in two-thirds of the cases and as mixed types in one-third. PRL-LIS responders developed more craving in the LIS condition and PRL-FLU responders when exposed to FLU. Furthermore, the first group scored high on the sensation seeking scale (SSS), which related to the concept of incentive motivation and the FLU responders high on extraversion and smoking motivation for stimulating purposes suggesting the endeavor to replace DA. Thus, evidence for the validity of both theories is proven.

Synaptic mechanisms underlie nicotine-induced excitability of brain reward areas

A single nicotine exposure increases dopamine levels in the mesolimbic reward system for hours, but nicotine concentrations experienced by smokers desensitize nAChRs on dopamine neurons in seconds to minutes. Here, we show that persistent modulation of both GABAergic and glutamatergic synaptic transmission by nicotine can contribute to the sustained increase in dopamine neuron excitability. Nicotine enhances GABAergic transmission transiently, which is followed by a persistent depression of these inhibitory inputs due to nAChR desensitization. Simultaneously, nicotine enhances glutamatergic transmission through nAChRs that desensitize less than those on GABA neurons. The net effect is a shift toward excitation of the dopamine reward system. These results suggest that spatial and temporal differences in nicotinic receptor activity on both excitatory and inhibitory neurons in reward areas coordinate to reinforce nicotine self-administration.

Presynaptic involvement in the nicotine prevention of the dopamine loss provoked by 6-OHDA administration in the substantia nigra

While nicotine, through stimulation of a specific sub-population of nicotinic acetylcholine receptors (nAChR) appears to protect cells in culture against a variety of insults, studies in vivo show controversial results. In a previous paper we have shown that in the 6-hydroxydopamine (6-OHDA) model of experimental parkinsonism, an intermittent administration schedule of nicotine (4 h before and 20, 44 and 68 h after 6-OHDA) was able to prevent the decrease of dopamine (DA) concentration in the corpus striatum (CS) provoked by the partial lesion of the substantia nigra (50% neuronal death after 6 micro g of 6-OHDA). To further analyze the mechanisms of nicotine effects, we performed a microdialysis study of striatal extracellular DA concentrations utilizing the nicotine administration schedule that was able to prevent DA decrease. Basal extracellular DA concentrations in the CS were maintained after 6-OHDA and were not modified by nicotine. Basal DOPAC levels were decreased after the neurotoxic administration. The response of extracellular DA to potassium chloride (KCl) challenge was significantly lower after 6-OHDA than in control animals. Nicotine significantly reversed this decrease. As previous studies have shown, the striatal DA terminals surviving the 6-OHDA toxic effect are able to keep extracellular DA concentrations close to normal, likely increasing DA synthesis. Nevertheless, the application of a releasing factor such as KCl shows the fragility of this equilibrium, exposing a decrease in the terminal number. Nicotine, through a further activation of tyrosine hydroxylase and DA synthesis or by prolonging the life of DA terminals, could reverse the effect of 6-OHDA.

Visual hallucinations are associated with lower alpha bungarotoxin binding in dementia with Lewy bodies

Patients with dementia with Lewy bodies (DLB) commonly experience psychotic symptoms, most notably visual hallucinations. Previously, it has been shown that visual hallucinations in DLB are associated with reduced cortical choline acetyltransferase activity, a marker of cholinergic innervation, but not with predominantly postsynaptic muscarinic M1 receptor binding. In the present investigation, nicotinic acetylcholine receptor (nAChR) levels in the temporal cortex (Brodmann's areas [BA] 20 and 36) were measured in a group of 24 prospectively assessed DLB patients; comparisons were made between groups with or without visual and auditory hallucinations and delusional misidentification. Visual hallucinations and delusional misidentification were associated with lower [(125)I]alpha bungarotoxin binding in areas 36 and 20 (P<.05), but not with changes in [(3)H]epibatidine binding. There were no significant associations with auditory hallucinations. [(3)H]epibatidine, but not [(125)I]alpha bungarotoxin, binding for all DLB cases was reduced compared to controls (P<.001). Loss of cortical alpha 7 nicotinic receptors may contribute to hallucinations and delusional misidentification in DLB, with implications for treatment and understanding the mechanisms of psychotic symptoms in dementia.

Enhanced motor activity and brain dopamine turnover in mice during long-term nicotine administration in the drinking water

Nicotine was administered chronically to NMRI mice in their drinking water in gradually increasing concentrations to measure gross motor activity and brain nicotine concentrations over 24 h on the 50th day of nicotine administration. Also, the striatal postmortem tissue concentrations and accumbal extracellular concentrations of dopamine (DA) and its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were measured to study the role of dopaminergic systems in nicotine-induced hyperactivity in mice. The cerebral nicotine concentration was at its highest at the end of the dark period. The activity of nicotine-treated mice and their striatal DA metabolism were parallelly increased at 2 to 3 h after midnight and in the forenoon. Microdialysis experiments carried out in the forenoon showed that the extracellular levels of DA and DOPAC were elevated in the nucleus accumbens of these mice. Nicotine did not alter the circadian rhythmicity of activity in the mice. Rather, our findings suggest that the mice consume more nicotine when active and this might lead to enhanced release and metabolism of DA and further, to enhanced motor behavior. These findings support the suggestions that nicotine's effects on limbic and striatal DA are critical for its stimulating effects.

Nicotine-induced alterations in the expression of nicotinic receptors in primary cultures from human prenatal brain

The nicotinic receptor proteins and gene transcripts for the different nicotinic receptor subunits exist in human prenatal brain already at 4-5 weeks of gestation. The early presence of nicotinic receptors suggests an important role for these receptors in modulating dendritic outgrowth, establishment of neuronal connections and synaptogenesis during development. When measurements of nicotinic receptors using [(3)H]epibatidine (labelling both the alpha3 and alpha4 subtype) and [(3)H]cytisine (labelling the alpha4 subtype) were performed in intact cells from the cortex, subcortical forebrain and mesencephalon (7.5-11 weeks of gestation), the highest specific binding for both ligands was detected in cells from mesencephalon, followed by subcortical forebrain and cortex. The effects of nicotine exposure were studied in primary cultures of prenatal brain (7.5-11 weeks of gestation). Treatment with nicotine (1-100 microM) for 3 days significantly increased the specific binding of [(3)H]epibatidine and [(3)H]cytisine in cortical cells but not in cells from subcortical forebrain and mesencephalon brain regions, indicating region-specific differences in the sensitivity to nicotine exposure. Relative quantification of mRNA showed that the expression of the nicotinic receptor subunits alpha3 and alpha7, but not alpha4, was increased in cortical cells after nicotine treatment. These findings support the assumption of a potential risk of disturbance in the functional role of nicotinic receptors during brain development as a consequence of maternal smoking during pregnancy.

Reduced dopamine D1 receptor binding in the ventral striatum of cigarette smokers

Several drugs of abuse, including nicotine, are thought to exert their reinforcing effects through actions on the mesolimbic dopamine system. Animal and human studies suggest that chronic administration of addictive drugs may lead to impaired dopamine neurotransmission in the nucleus accumbens. We measured D1 receptor density in 11 smokers and 18 nonsmokers using positron emission tomography and the D1 receptor ligand [11C]SCH 23390. Ten of the smokers were scanned twice, once after overnight abstinence from cigarettes, and once while smoking at their usual rate, to account for possible acute effects of cigarette smoking on D1 receptor binding. In addition, eight control subjects were scanned twice to assess the reproducibility of the method. We used compartmental modeling to measure [11C]SCH 23390 binding potential, a measure of D1 receptor density. There were no differences in binding between abstinent and nonabstinent scans in smokers or in the two scans in controls. However, there was a significant reduction in [11C]SCH 23390 binding potential in smokers compared to nonsmokers in the striatum, most prominently in the ventral striatum. This suggests that there is a reduction in dopamine D1 receptor density in the ventral striatum of human cigarette smokers relative to nonsmokers, which implies that the postsynaptic mesolimbic dopamine system may be chronically underactive in smokers, either as an antecedent or consequence of addiction to cigarettes. Such a hypodopaminergic state may play an important role in sustaining nicotine-seeking behavior. Alternatively, an inherited reduction in dopamine receptors in the striatum may be associated with an increased risk of addictive behavior.

Sex differences in [123I]beta-CIT SPECT measures of dopamine and serotonin transporter availability in healthy smokers and nonsmokers

Nicotine and other constituents of tobacco smoke elevate dopamine (DA) and serotonin (5-HT) levels in brain and may cause homeostatic adaptations in DA and 5-HT transporters. Since sex steroids alter DA and 5-HT transporter expression, the effects of smoking on DA and 5-HT transporter availability may differ between sexes. In the present study, DA and 5-HT transporter availabilities were quantitated using single photon emission computed tomography (SPECT) imaging approximately 22 h after bolus administration of [123I]beta-CIT, an analog of cocaine which labels DA and 5-HT transporters. Forty-two subjects including 21 pairs of age-, race-, and gender-matched healthy smokers and nonsmokers (12 female and 9 male pairs) were imaged. Regional uptake was assessed by the outcome measures, V3", which is the ratio of specific (i.e., ROI-cerebellar activity) to nondisplaceable (cerebellar) activity, and V3, the ratio of specific to free plasma parent. Overall, striatal and diencephalic [123I]beta-CIT uptake was not altered by smoking, whereas brainstem [123I]beta-CIT uptake was modestly higher (10%) in smokers vs. nonsmokers. When subgrouped by sex, regardless of smoking status, [123I]beta-CIT uptake was higher in the striatum (10%), diencephalon (15%), and brainstem (15%) in females vs. males. The sex*smoking interaction was not significant in the striatum, diencephalon, or brainstem, despite the observation of 20% higher brainstem [123I]beta-CIT uptake in male smokers vs. nonsmokers and less than a 5% difference between female smokers and nonsmokers. The results demonstrate higher DA and 5-HT transporter availability in females vs. males and no overall effect of smoking with the exception of a modest elevation in brainstem 5-HT transporters in male smokers. Although these findings are preliminary and need validation with a more selective 5-HT transporter radiotracer, the results suggest that brainstem 5-HT transporters may be regulated by smoking in a sex-specific manner.

Laminar distribution of nicotinic receptor subtypes in cortical regions in schizophrenia

The laminar cortical distribution of the [125I]alpha-bungarotoxin, [3H]cytisine and [3H]epibatidine nicotinic acetylcholine receptor ligands was investigated by quantitative autoradiography in autopsy tissue from the cingulate, orbitofrontal and temporal cortices of control and schizophrenia subjects matched for age and smoking history. Different laminar binding patterns were observed for the various nicotinic ligands both in schizophrenic and control brains. [125I]alpha-Bungarotoxin binding was distributed homogeneously across all cortical layers in all three brain regions, with highest binding densities in the cingulate cortex. [3H]Cytisine and [3H]epibatidine binding varied across the cortical ribbon, with high binding in layers I, III, V and VI, within the three cortical regions. A significantly reduced [125I] alpha-bungarotoxin binding (-54%) was observed in the cingulate cortex of schizophrenia subjects, in comparison with normal individuals who smoked tobacco. In the same brain region also a significantly higher [3H]cytisine binding (48-77%) was observed in nearly all layers, except for layer I of the schizophrenia subjects, when compared to normal individuals with a history of tobacco use. No significant changes in [3H]epibatidine binding was observed within the individual cortical layers between control subjects and patients with schizophrenia, but when calculated as a whole region (i.e. measurements performed across the whole cortical ribbon), the temporal cortex showed a significant increase in [3H]epibatidine binding in schizophrenia subjects compared to control subjects. The results suggest opposite changes of the alpha4beta2 and alpha7 nicotinic receptor subtypes in the cingulate cortex of patients with schizophrenia which might reflect involvement of two different nicotinic receptor mechanisms in schizophrenia brain.

Galantamine, a cholinesterase inhibitor that allosterically modulates nicotinic receptors: effects on the course of Alzheimer's disease

Despite the proven efficacy of acetylcholinesterase inhibitors in Alzheimer's disease, there is a need for new and more effective treatments. Galantamine is a novel treatment for Alzheimer's disease that inhibits acetylcholinesterase and modulates nicotinic receptors. In randomized, double-blind, placebo-controlled studies of up to 6 months duration, galantamine significantly improved cognitive function. Galantamine also had beneficial effects on instrumental and basic activities of daily living, and postponed the progression of behavioral symptoms. Patients who completed one of the 6-month, placebo-controlled studies were eligible to enter a 6-month, open-extension study of the 24-mg/day dose of galantamine. At the end of 12 months, cognitive function and activities of daily living were preserved in those patients who had been treated throughout the study with galantamine 24 mg/day. At 12 months, this group of patients had significantly better cognitive functions than patients who had been treated with a placebo for 6 months before receiving galantamine. These studies indicate that galantamine postpones the progression of symptoms in Alzheimer's disease. Since galantamine shows the greatest benefits when treatment is started early, its long-term benefits may result from an effect on the underlying disease process; such an effect might be mediated by galantamine's concomitant action on nicotinic receptors.

Nicotine prevents striatal dopamine loss produced by 6-hydroxydopamine lesion in the substantia nigra

While the work of several groups has shown the neuroprotective effects of nicotine in vitro, evidences for the same effects in vivo are controversial, mainly regarding neuroprotection in experimental models of Parkinson's disease. In this context, we investigated the capability of various systemic administration schedules of nicotine to prevent the loss of striatal dopamine levels produced by partial or extensive 6-hydroxydopamine (6-OHDA) lesion of rat substantia nigra (SN). Eight days after 6- and 10-microg injections of 6-OHDA in the SN there was a significant decrease of dopamine concentrations in the corpus striatum (CS) and a concomitant increase in dopamine turnover. While 10 microg 6-OHDA produced an almost complete depletion of dopamine in the SN, 6 microg decreased dopamine levels by 50%. Subcutaneous nicotine (1 mg/kg) administered 4 h before and 20, 44 and 68 h after 6 microg 6-OHDA, prevented significantly the striatal dopamine loss. Administered only 18 or 4 h before or only 20, 44 and 68 h after, nicotine failed to counteract the loss of dopamine or the increase in dopamine turnover observed in the CS. Nicotine also failed to prevent significantly the decrease of striatal dopamine levels produced by the 10-microg 6-OHDA intranigral dose. Chlorisondamine, a long-lasting nicotinic acetylcholine receptor antagonist, reverted significantly the nicotinic protective effects on dopamine concentrations. These results are showing that putative neuroprotective effects of nicotine in vivo depend on an acute intermittent administration schedule and on the extent of the brain lesion.

Nicotinic receptors in human brain: topography and pathology

Brain nicotinic acetylcholine receptors (nAChR) are a class of ligand-gated channels composed of alpha and beta subunits with specific structural, functional and pharmacological properties. They participate in the physiological and behavioural effects of acetylcholine and mediate responses to nicotine. They are associated with numerous transmitter systems and their expression is altered during development and ageing as well as in diseases such as autism, schizophrenia, Alzheimer's disease, Parkinson's disease and Lewy body dementia. Nicotinic receptors containing a number of different subunits are highly expressed during early human development. Disorders believed to be associated with abnormal brain maturation involve deficits in both alpha4beta2, in the case of autism, and alpha7 possibly in addition to alpha4beta2 nAChRs in the case of schizophrenia. In ageing and age-related neurodegenerative disorders nAChR deficits are predominantly associated with alpha4-containing receptors, although some studies also indicate the involvement of alpha3 and alpha7 subunits. Whilst ageing appears to be associated with reductions in subunit mRNA as well as protein expression, in Alzheimer's disease only protein loss is apparent. Nicotinic therapy may be of benefit in a number of neurological conditions, however studies evaluating further both the distribution of specific subunit involvement and the correlation of nAChR deficits with clinical symptoms are required to inform therapeutic strategy.

Alpha and beta nicotinic acetylcholine receptors subunits and synaptophysin in putamen from Parkinson's disease

It is well established that nicotinic receptors in the mammalian striatum are involved in modulation of the release of several neurotransmitters, including dopamine. In addition, nicotinic receptors with high affinity for agonists have generally been found to be reduced in the striatum in Parkinson's disease. In the present study antibodies have been used to examine which subunits contribute to the striatal nicotinic receptor loss in Parkinson's disease, and whether the reduction in [(3)H]nicotine binding correlates with synaptic loss. Autopsy tissue from the putamen of 12 Parkinson's disease cases and 12 age-matched control subjects was analysed by immunoblotting using antibodies against recombinant peptides specific for alpha3, alpha4, alpha7, beta2 and beta4 nicotinic acetylcholine receptor (nAChR) subunits and the synaptic marker synaptophysin, in conjunction with assessment of [(3)H]nicotine binding by autoradiography. The data indicate that there is no loss of alpha3, alpha4, alpha7 and beta2 immunoreactivity in the putamen in Parkinson's disease, despite a highly significant reduction in [(3)H]nicotine binding. An intense signal of beta4 immunoreactivity was found in human dorsal root ganglia, but not in temporal cortex or putamen samples. Synaptophysin immunoreactivities were also similar in Parkinson's disease and control cases. These results suggest that the loss of nicotine binding in the putamen in Parkinson's disease may involve an nAChR subunit (e.g., alpha5 and/or alpha6) other than those investigated. Alternatively, the results could reflect impaired subunit assembly at the plasma membrane.

The opioid antagonist naltrexone inhibits activity and alters expression of alpha7 and alpha4beta2 nicotinic receptors in hippocampal neurons: implications for smoking cessation programs

This study was designed to investigate whether naltrexone, an opioid antagonist that has been evaluated clinically as a co-adjuvant in smoking cessation programs, affects function and expression of neuronal nicotinic receptors (nAChRs). Whole-cell current recordings from rat hippocampal neurons in culture and in slices demonstrated that alpha7 nAChRs can be inhibited non-competitively by naltrexone (IC(50) approximately 25 microM). The voltage dependence of the effect suggested that naltrexone acts as an open-channel blocker of alpha7 nAChRs. Naltrexone also inhibited activation of alpha4beta2 nAChRs in hippocampal neurons; however its IC(50) was higher ( approximately 141 microM). At a concentration as high as 300 microM (which is sufficient to block by 100% and 70% the activity of alpha7 and alpha4beta2 nAChRs, respectively), naltrexone had no effect on kainate and AMPA receptors, blocked by no more than 20% the activity of NMDA and glycine receptors, and reduced by 35% the activity of GABA(A) receptors. A 3-day exposure of cultured hippocampal neurons to naltrexone (30 microM) or nicotine (10 microM, a concentration that fully desensitized alpha7 nAChRs) resulted in a 2-fold increase in the average amplitude of alpha7 nAChR-subserved currents. Naltrexone did not augment the maximal up-regulation of alpha7 nAChRs induced by nicotine, indicating that both drugs act via a common mechanism. In addition to increasing alpha7 nAChRs-mediated responses per neuron, nicotine increased the number of neurons expressing functional non-alpha7 nAChRs (probably alpha4beta2 nAChRs); this effect was blocked by naltrexone (0.3 and 30 microM). Therefore, naltrexone may affect dependence on cigarette smoking by differentially altering function and expression of alpha7 and alpha4beta2 nAChRs in the central nervous system.

Nicotinic acetylcholine receptors during prenatal development and brain pathology in human aging

Nicotinic acetylcholine receptor (nAChRs) proteins and gene transcripts are already present in human prenatal brain and spinal cord at 4-6 weeks gestation, and a clear age-related increase in number of nAChRs was apparent during first trimester. In pons, there was also a parallel increase in the alpha7 mRNA level with age. The highest specific binding of [3H]epibatidine and [3H]cytisine was detected in spinal cord, pons and medulla oblongata, and binding of [125I]alpha-bungarotoxin was highest in spinal cord, medulla oblongata and mesencephalon. From the late fetal stage brain nAChRs have been shown to fall with increasing age. During aging (between 40 and 100 years) high affinity nicotine binding in the frontal cortex decreases in parallel with glutamate NMDA receptor binding ([3H]MK801). In the hippocampal formation and entorhinal cortex nicotine binding also declines with age, in common with [125I]alpha-bungarotoxin in the entorhinal cortex, but NMDA receptor binding remains unchanged. These reductions in nicotine binding with age may predispose the neo- and archicortex to the loss of nAChRs observed in age-associated neurodegenerative conditions. By contrast no loss in nAChR binding with aging is observed in the thalamus and only after the 70th decade in the striatum, although in Alzheimer's disease, Parkinson's disease and Lewy body dementia deficits in nAChRs are observed in these areas and may be associated with specific disease-related processes.

Regional and cellular induction of nicotine-metabolizing CYP2B1 in rat brain by chronic nicotine treatment

In the rat, nicotine is metabolized to cotinine primarily by hepatic cytochrome P450 (CYP) 2B1. This enzyme is also found in other organs such as the lung and the brain. Hepatic nicotine metabolism is unaltered after nicotine exposure; however, nicotine may regulate CYP2B1 in other tissues. We hypothesized that nicotine induces its own metabolism in brain by increasing CYP2B1. Male rats were treated with nicotine (0.0, 0.1, 0.3, or 1.0 mg base/kg in saline) s.c. daily for 7 days. CYP2B1 mRNA and protein were assayed in the brain and liver by reverse transcriptase-polymerase chain reaction (RT-PCR), immunoblotting, and immunocytochemistry. In control rats, CYP2B1 mRNA and protein expression were brain region- and cell-specific. CYP2B1 was not induced in the liver, but CYP2B1 mRNA and protein showed dose-dependent, region- and cell-specific patterns of induction across brain regions. At 1.0 mg nicotine/kg, the largest increase in protein was in the brain stem (5.8-fold, P < 0.05) with a corresponding increase in CYP2B1 mRNA (7.6-fold, P < 0.05). Induction of CYP2B1 was also observed in the frontal cortex, striatum, and olfactory tubercle. Immunocytochemistry showed that induction was restricted principally to neurons. These data indicate that nicotine may alter its own metabolism in the brain through transcriptional regulation, perhaps contributing to central tolerance to the effects of nicotine. CYP2B1 and its human homologue CYP2B6 also activate tobacco smoke procarcinogens such as NNK [4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone]. Highly localized increases in CYP2B could result in increased mutagenesis. These data suggest roles for nicotine-induced CYP2B in central metabolic tolerance, nicotine-induced neurotoxicity, neuroplasticity, and carcinogenesis.

Modulation of beta-amyloid precursor protein processing and tau phosphorylation by acetylcholine receptors

Neurofibrillary lesions and senile plaques that are composed mainly of hyperphosphorylated tau protein and the amyloid-beta peptide derived from the amyloid precursor protein, respectively, are classical hallmarks of Alzheimer's disease. A number of studies strongly suggests that amyloid-beta formation and amyloid depositions are linked to the pathogenesis of Alzheimer's disease. Recent findings suggest that very low concentrations of the amyloid-beta can inhibit various cholinergic neurotransmitter functions independently of apparent neurotoxicity. Many factors have been shown to influence the processing of amyloid precursor protein, including activation of muscarinic and nicotinic receptors. This review focus on some recent studies concerning the regulation of amyloid precursor protein processing and modulation of tau phosphorylation by acetylcholine receptor stimulation and how cholinergic deficits and amyloid-beta might be related to one another.

Nicotinic receptor subtypes in human brain ageing, Alzheimer and Lewy body diseases

Human brain ageing is associated with reductions in a variety of nicotinic receptors subtypes, whereas changes in age-related disorders including Alzheimer's disease or Parkinson's disease are more selective. In Alzheimer's disease, in the cortex there is a selective loss of the alpha4 (but not alpha3 or 7) subunit immunoreactivity and of nicotine or epibatidine binding but not alpha-bungarotoxin binding. Epibatidine binding is inversely correlated with clinical dementia ratings and with the level of Abeta1-42, but not related to plaque or tangle densities. In contrast, alpha-bungarotoxin binding is positively correlated with plaque densities in the entorhinal cortex. In human temporal cortex loss of acetylcholinesterase catalytic activity is positively correlated with decreased epibatidine binding and in a transgenic mouse model over expressing acetylcholinesterase, epibatidine binding is elevated. In Parkinson's disease, loss of striatal nicotine binding appears to occur early but is not associated with a loss of alpha4 subunit immunoreactivity. Tobacco use in normal elderly individuals is associated with increased alpha4 immunoreactivity in the cortex and lower densities of amyloid-beta plaques, and with greater numbers of dopaminergic neurons in the substantia nigra pars compacta. These findings indicate an early involvement of the alpha4 subunit in beta-amyloidosis but not in nigro-striatal dopaminergic degeneration.

Nicotine binding in human striatum: elevation in schizophrenia and reductions in dementia with Lewy bodies, Parkinson's disease and Alzheimer's disease and in relation to neuroleptic medication

Striatal nicotinic acetylcholine receptors with high affinity for nicotinic agonists are involved with the release of a number of neurotransmitters, including dopamine. Previous findings as to whether these receptors are changed in Parkinson's disease and Alzheimer's disease are inconsistent and no previous investigations have focused on these receptors in dementia with Lewy bodies and schizophrenia, which are also associated with disorders of movement. The present autoradiographic study of striatal [3H]nicotine binding in Alzheimer's and Parkinson's diseases, dementia with Lewy bodies and schizophrenia was conducted with particular reference to the potentially confounding variables of tobacco use and neuroleptic medication. [3H]Nicotine binding in both dorsal and ventral caudate and putamen was significantly reduced in Parkinson's disease (43-67%, n=13), Alzheimer's disease (29-37%, n=13) and dementia with Lewy bodies (50-61%, n=20) compared to age-matched controls (n=42). Although tobacco use in the control group was associated with increased [3H]nicotine binding (21-38%), and neuroleptic treatment in dementia with Lewy bodies and Alzheimer's disease was associated with reduced [3H]nicotine binding (up to 29%), differences between neurodegenerative disease groups and controls persisted in subgroups of Alzheimer's disease cases (26-33%, n=6, in the ventral striatum) and dementia with Lewy body cases (30-49%, n=7, in both dorsal and ventral striatum) who had received no neuroleptic medication compared to controls who had not smoked (n=10). In contrast, striatal [3H]nicotine binding in a group of elderly (56-85 years) chronically medicated individuals with schizophrenia (n=6) was elevated compared with the entire control group (48-78%, n=42) and with a subgroup that had smoked (24-49%, n=8). The changes observed in [3H]nicotine binding are likely to reflect the presence of these receptors on multiple sites within the striatum, which may be differentially modulated in the different diseases. Further study is warranted to explore which nicotinic receptor subunits and which neuronal compartments are involved in the changes in [3H]nicotine binding reported, to aid development of potential nicotinic receptor therapy.

Neuronal nicotinic receptors in dementia with Lewy bodies and schizophrenia: alpha-bungarotoxin and nicotine binding in the thalamus

Neuronal nicotinic receptors have been implicated in schizophrenia on the basis of the high incidence of tobacco smoking in patients, abnormalities in cytisine and alpha-bungarotoxin (alphaBGT) binding in the hippocampus, and linkage between auditory P50 deficits and the region of chromosome 15 coding the alpha7 subunit. In another disease associated with psychosis, dementia with Lewy bodies (DLB), in which visual hallucinations predominate, reductions in nicotine binding have been identified in various cortical and subcortical regions. We investigated both alphaBGT and nicotine binding autoradiographically in different thalamic nuclei in autopsy brain tissue from patients with schizophrenia and DLB. AlphaBGT binding in the reticular nucleus was moderately reduced (25%) in schizophrenia and more extensively reduced (50%) in DLB. There were no significant alterations in nicotine binding in schizophrenia, and in DLB, a trend towards moderate reductions in most nuclei reached significance in the lateral dorsal nucleus. It is concluded that widespread abnormalities of thalamic nicotine are not implicated in schizophrenia or DLB, but that reticular alphaBGT binding may be involved to a lesser and greater extent in the pathophysiology or psychopathology of both disorders.