Central actions of nicotine

Nicotine and energy balance: A review examining the effect of nicotine on hormonal appetite regulation and energy expenditure

Nicotine has been shown to decrease appetite, food intake (FI) and body weight, but the mechanisms are unclear. The purpose of this review was to examine research on the effects of nicotine on energy balance by exploring physiological mechanisms and hormone regulation related to FI, subjective appetite and energy expenditure (EE). We searched PubMed and MEDLINE, and included articles investigating the effects of nicotine on central appetite regulation, FI, leptin, peptide-YY (PYY), ghrelin, glucagon-like peptide-1 (GLP-1), adiponectin, cholecystokinin (CCK), orexin, and EE. A total of 65 studies were included in the qualitative synthesis and review. Our findings suggest that the decrease in appetite and FI may be attributed to nicotinic alterations of neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) but the effect of nicotine on FI remains unclear. Furthermore, nicotine increases resting EE (REE) and physical activity EE (PAEE) in both smokers and non-smokers; and these increases may be a result of the catecholaminergic effect of nicotine. Decreases in body weight and appetite experienced by nicotine users results from increased EE and changes in the central hypothalamic regulation of appetite. There is not enough evidence to implicate a relationship between peripheral hormones and changes in appetite or FI after nicotine use. Although nicotine increases REE and PAEE, the effect of nicotine on other components of EE warrants further research. We conclude that further research evaluating the effect of nicotine on appetite hormones, FI and EE in humans is warranted.

Simultaneous and sensitive measurement of nicotine, cotinine, trans-3'-hydroxycotinine and norcotinine in human plasma by liquid chromatography-tandem mass spectrometry

An LC-MS/MS method for the simultaneous quantification of nicotine, cotinine, trans-3'-hydroxycotinine and norcotinine in human plasma was developed and fully validated. Potential endogenous and exogenous interferences were extensively evaluated and limits of quantification were determined by decreasing analyte concentration. Analytical ranges were 1-500 ng/mL for nicotine and cotinine, 5-500 ng/mL for trans-3'-hydroxycotinine and norcotinine. Mean intra- and inter-assay analytical recoveries were between 101.9 and 116.8%, and intra- and inter-assay imprecision were less than 11% RSD for all analytes: parameters were evaluated at three different concentrations across the linear range of the assay. Extraction efficiency was > or = 70% for all analytes. This validated method is useful for the determination of nicotine and metabolites in human plasma to support research on the role of nicotine biomarkers on neuronal systems mediating cognitive and affective processes and to differentiate active, passive and environmental exposure.

Activation of olfactory and trigeminal cortical areas following stimulation of the nasal mucosa with low concentrations of S(-)-nicotine vapor--an fMRI study on chemosensory perception

Applied to the nasal mucosa in low concentrations, nicotine vapor evokes odorous sensations (mediated by the olfactory system) whereas at higher concentrations nicotine vapor additionally produces burning and stinging sensations in the nose (mediated by the trigeminal system). The objective of this study was to determine whether intranasal stimulation with suprathreshold concentrations of S(-)-nicotine vapor causes brain activation in olfactory cortical areas or if trigeminal cortical areas are also activated. Individual olfactory detection thresholds for S(-)-nicotine were determined in 19 healthy occasional smokers using a computer-controlled air-dilution olfactometer. Functional magnetic resonance images were acquired using a 1.5T MR scanner with applications of nicotine in concentrations at or just above the individual's olfactory detection threshold. Subjects reliably perceived the stimuli as being odorous. Accordingly, activation of brain areas known to be involved in processing of olfactory stimuli was identified. Although most of the subjects never or only rarely observed a burning or painful sensation in the nose, brain areas associated with the processing of painful stimuli were activated in all subjects. This indicates that the olfactory and trigeminal systems are activated during perception of nicotine and it is not possible to completely separate olfactory from trigeminal effects by lowering the concentration of the applied nicotine. In conclusion, even at low concentrations that do not consistently lead to painful sensations, intranasally applied nicotine activates both the olfactory and the trigeminal system.

Synthesis and In-Vivo Evaluation of [11C]p-PVP-MEMA as a PET Radioligand for Imaging Nicotinic Receptors

Within the class of (4-pyridinyl)vinylpyridines developed by Abbott laboratories as potent neuronal nicotinic acetylcholine receptor ligands, p-PVP-MEMA ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) is the lead compound of a novel series that do not display the traditional nicotinic-like pyrrole-ring but still possessing high subnanomolar affinity (Ki 0.077 nm—displacement of [3H](–)cytisine from whole rat brain synaptic membranes). In the present study, p-PVP-MEMA and its nor-derivative ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) as precursor for labelling with the short-lived positron-emitter carbon-11 (T1/2 20.4 min) were synthesized in 10 chemical steps from 2-hydroxy-5-nitropyridine and Boc-d-alanine. N-Alkylation of nor-p-PVP-MEMA with [11C]methyl iodide afforded [11C]p-PVP-MEMA (>98% radiochemically pure, specific activity of 86.4 GBq μmol–1) in 2% (non-decay corrected and non-optimized) radiochemical yield, in 34 min (including HPLC purification and formulation). Preliminary positron emission tomography (PET) results obtained in a Papio hamadryas baboon showed that [11C]p-PVP-MEMA is not a suitable PET-radioligand.

Effects of nicotine on cerebral metabolism

Interest in identifying brain areas mediating the behavioural effects of nicotine led to autoradiographic studies on the distribution of cerebral metabolic responses to nicotine. The 2-deoxy-D-[1-14C] glucose method was used to map and quantitate nicotine's effects in the rat brain. The method allows simultaneous measurement of the regional cerebral metabolic rate(s) for glucose (rCMRglc), an index of functional activity, throughout the central nervous system. It provides information about sites of initial drug interactions, and of secondary effects propagated via afferents to remote areas. In rats given acute systemic (-)nicotine, stimulation occurs in brain areas which contain specific binding sites for [3H] nicotine, indicating that the sites are true receptors, linked to functional activity. Doses of nicotine that are discriminated by rats and that produce behavioural and physiological effects stimulate rCMRglc. The stimulation is transient and is antagonized by mecamylamine. Affected areas include limbic structures, components of the visual system, brainstem nuclei important in cardiovascular reflexes, and areas involved in motor function. The distribution of nicotine's in vivo effects on rCMRglc implicates various brain regions in the behavioural and physiological effects of nicotine. Future studies employing positron emission tomography will assess relations between nicotine's effects on mood and rCMRglc in man.

3-Pyridyl ethers as SPECT radioligands for imaging nicotinic acetylcholine receptors

To develop a suitable single photon emission computed tomography (SPECT) radioligand for neuronal nicotinic acetylcholine receptors (nAChRs) that displays faster in vivo kinetics than 5-[123I]iodo-A-85380, we synthesised the radioiodinated analogue of A-84543. 5-[123I]Iodo-A-84543 was prepared by electrophilic iododestannylation in a modest yield of 23%. In the baboon brain, 5-[123I]iodo-A-85380 displayed a profile consistent with the known distribution of nAChRs, however, 5-[123I]iodo-A-84543 displayed a homogenous uptake with no preferential localisation in regions known to contain nAChRs. To examine the effect of halogen substitution on the 3-pyridyl ether, A-84543, the 5-chloro, 5-bromo and 5-iodo analogues were synthesised and evaluated with respect to nAChR binding. In vitro binding data revealed that halogen substitution at the 5-position of A-84543 was not well tolerated with an increase in halogen size resulting in lower binding towards nAChRs. The 5-chloro analogue 4 displayed highest affinity, Ki =1.3 nM, compared to the 5-bromo and 5-iodo compounds, 5 Ki =3.3 nM and 3 Ki =40.8 nM, respectively. Taken together, these results clearly indicate that 5-[123I]iodo-A-84543 is not suitable for the study of nAChRs in vivo using SPECT.

Susceptibility to neuroleptic-induced parkinsonism--age and increased substantia nigra echogenicity as putative risk factors

OBJECTIVE

Patients treated by neuroleptics often develop neuroleptic-induced parkinsonism (NIP) to a varying extent. The reasons for this are discussed controversially in the literature. Previous transcranial sonography (TCS) findings of the substantia nigra (SN) in patients with idiopathic Parkinson's disease suggest a correlation of echogenicity with nigrostriatal dysfunction.

METHOD

One hundred psychiatric patients receiving neuroleptics were included. They underwent clinical examination for NIP (Simpson and Angus-scale) and, independently, TCS of the SN. History of smoking habits and medication were taken from the patient's chart.

RESULTS

We found a significant positive association of the prevalence of NIP with age (P < 0.01) and the echogenic area of the SN (P < 0.05). Neither type nor dosage of the neuroleptics was found to have any significant impact on the occurrence of NIP. Smokers displayed lower prevalence of NIP (P < 0.05) and lower EPS scores (P < 0.01).

CONCLUSIONS

These findings suggest that age and increased size of SN echogenicity are possible risk factors for NIP. In contrast, smoking seems to have a certain protecting effect.

Pharmacological evaluation of a Br-76 analog of epibatidine: a potent ligand for studying brain nicotinic acetylcholine receptors

[(76)Br]-Norchlorobromoepibatidine ([(76)Br]BrPH) is a specific and high affinity radioligand for the nicotinic acetylcholine receptors (nAChRs). In vitro, on rat thalamus membranes [(76)Br]BrPH bound to two sites with apparent affinities of 8 pM and 3 nM. The density of binding sites were 1.9 and 70 fmol/mg protein, respectively. In vivo, in biodistribution and autoradiographic studies in rats the regional distribution of [(76)Br]BrPH paralleled the neuroanatomical localization of nAChRs. Two hours postinjection, the highest concentration in the brain was found in thalamus and colliculi (4% ID/g). Competition experiments with specific nicotinic, muscarinic, dopaminergic, and serotoninergic drugs confirmed that the in vivo binding of [(76)Br]BrPH was consistent with neuronal nicotinic receptors. PET imaging of [(76)Br]BrPH in baboon demonstrated a rapid and high uptake in the brain. Peak uptake occurred at 30-40 min for the thalamus. Due to the constant washout in the cerebellum, the thalamus to cerebellum ratio was 5 at 2 h postinjection. Subcutaneous injection of cytisine (1 mg/kg), 3 h postinjection of [(76)Br]BrPH reduced the radioactivity concentration in thalamus and cortex by 58 and 50%, respectively, as observed 1 h later. Cytisine pretreatment (5 mg/kg s.c.) inhibited completely the radioligand accumulation in the thalamus. Chronic MPTP pretreatment resulted in reduction of [(76)Br]BrPH uptake in all brain regions except in cerebellum. These preliminary results suggest that [(76)Br]BrPH has the potential to be a useful radioligand for studying the pharmacology of nicotinic acetylcholine receptors in preclinical experiments.

Regional cerebral blood flow effects of nicotine in overnight abstinent smokers

BACKGROUND

Most people agree that dependence to tobacco is mediated by the effects of nicotine on the central nervous system, albeit the neural pathways involved are not clearly delineated. We investigated the effect of nasal nicotine spray on regional cerebral blood flow (rCBF) in a sample of habitual smokers, with H2 15O and positron emission tomography (PET).

METHODS

Eighteen volunteer smokers were studied after 12 hours of smoking deprivation. Regional cerebral blood flow measures were obtained with PET and 50 mCi H2 15O in six consecutive scans. Nicotine spray and a placebo spray were administered in a single-blind design, preceded and followed by baseline studies. Images were coregistered and anatomically standardized. Square (9-mm side) regions of interest were placed in 10 preselected brain regions, bilaterally. The effects of the experimental condition and gender were tested with two-way repeated-measures analysis of variance in each of the regions studied.

RESULTS

Nicotine reduced rCBF in the left anterior temporal cortex and in the right amygdala. Increases were noted in the right anterior thalamus.

CONCLUSIONS

In habitual smokers after overnight abstinence, nicotine induced differing effects on regional blood flow relative to whole brain blood flow. Increases were observed in the thalamus, a region rich in nicotinic receptors, and reductions in limbic and paralimbic (amygdala, anterior temporal cortex) regions.

In vivo imaging of nicotinic receptor upregulation following chronic (-)-nicotine treatment in baboon using SPECT

To quantify changes in neuronal nAChR binding in vivo, quantitative dynamic SPECT studies were performed with 5-[(123)I]-iodo-A-85380 in baboons pre and post chronic treatment with (-)-nicotine or saline control. Infusion of (-)-nicotine at a dose of 2.0 mg/kg/24h for 14 days resulted in plasma (-)-nicotine levels of 27.3 ng/mL. This is equivalent to that found in an average human smoker (20 cigarettes a day). In the baboon brain the regional distribution of 5-[(123)I]-iodo-A-85380 was consistent with the known densities of nAChRs (thalamus > frontal cortex > cerebellum). Changes in nAChR binding were estimated from the volume of distribution (V(d) ) and binding potential (BP) derived from 3-compartment model fits. In the (-)-nicotine treated animal V(d) was significantly increased in the thalamus (52%) and cerebellum (50%) seven days post cessation of (-)-nicotine treatment, suggesting upregulation of nAChRs. The observed 33% increase in the frontal cortex failed to reach significance. A significant increase in BP was seen in the thalamus. In the saline control animal no changes were observed in V(d) or BP under any experimental conditions. In this preliminary study, we have demonstrated for the first time in vivo upregulation of neuronal nAChR binding following chronic (-)-nicotine treatment.

Regulation of AP-1 gene transcription factor binding activity in the rat brain during nicotine dependence

The effects of acute and chronic nicotine treatment on activator protein-1 (AP-1) gene transcription factor binding activity in the rat cortex were investigated. AP-1 DNA binding activity was determined by the electrophoretic gel mobility shift assay. It was observed that 1 h after acute nicotine treatment (single injection) AP-1 DNA binding activity was significantly increased in the rat cortex. On the other hand, AP-1 DNA binding activity in the rat cortex was not altered at 1 and 8 h of nicotine withdrawal after repeated nicotine treatment (10 days). However, at 18 and 24 h of nicotine withdrawal after 10 days of nicotine treatment, AP-1 DNA binding activity was significantly decreased in the rat cortex. Thus, these findings suggest that desensitization of cortical AP-1 DNA binding activity may be involved in the neuroadaptational mechanisms to nicotine dependence.

Pharmacological evaluation of [11C]A-84543: an enantioselective ligand for in vivo studies of neuronal nicotinic acetylcholine receptors

[11C]A-84543, 3-[(1-[11C]methyl-2(S)-pyrrolidinyl)methoxy]pyridine, is a specific and enantioselective neuronal nicotinic acetylcholine receptor (nAChR) radiotracer. The in vivo biodistribution of this radiotracer in mice showed high brain uptake and a distribution consistent with the density of nAChRs. Highest uptake was observed in the thalamus (9.6 %ID/g), cortex (9.9 %ID/g), superior colliculus (7.6 %ID/g) and hippocampus (7.6 %ID/g) at 5 min followed by clearance. As a measure of specificity, the thalamus/cerebellar ratio reached a maximum of 2.3 at 30 min post-injection. Radioactivity in the thalamus and superior colliculus was reduced by 33% by pre-administration of unlabeled A-84543. The nAChR agonists (-)nicotine, cytisine, and (+) epibatidine reduced the radioactivity due to [11C]A-84543 in the superior colliculus by 41%, 38%, and 27%, respectively, while lobeline, which also interacts with central nAChRs, produced a 24% inhibition. The noncompetitive nAChR ligand, mecamylamine displayed no inhibitory effect on [11C]A-84543 accumulation in any brain region. Ketanserin (5-HT2/5-HT2C), scopolamine (mAChR antagonist), (+)butaclamol (DA receptor antagonist), and haloperidol (D2/sigma) also displayed no inhibitory effect in any brain region studied. With the pharmacologically less active enantiomer, 3-[(1-[11C]methyl-2(R)-pyrrolidinyl)methoxy] pyridine, high brain uptake was also observed, but with a low thalamus/cerebellar ratio of 1.4 at 30 min post-injection. [11C]A-84543 displays enantioselectivity for nAChRs and may deserve further investigation as a possible PET radiotracer.

Cigarette smoking: effects on cognitive functions and drug-induced parkinsonism in chronic schizophrenia

Epidemiological studies suggest an apparent protective effect of cigarette smoking on the risk of Parkinson's disease. There is also a report suggesting that patients with Parkinson's disease who smoke are less likely to develop dementia. I investigated the relationship of smoking to the severity of cognitive functions and presence of drug-induced parkinsonism in a group of 111 neuroleptic-treated chronic institutionalized schizophrenic patients. Patients who smoked had significantly less cognitive impairment (p < .02) and a lower prevalence of drug-induced Parkinsonism (p < .02) compared to nonsmokers. These findings suggest that cigarette smoking may protect against the development of dementia and drug-induced Parkinsonism in schizophrenia.

Induction of c-fos immunostaining in the rat brain after the systemic administration of nicotine

To search for evidence of altered neuronal gene expression in response to exposure to the highly addictive drug nicotine, rat brains were examined by immunocytochemistry for the fos protein after the systemic administration of nicotine. The drug was administered as an IV infusion over 1 h At a dose of 2 mg/kg, the most dramatic nicotine-induced fos nuclear immunostaining was seen in central visual pathways, including the superficial superior colliculus and the medial terminal nu. of the accessory optic tract, in the interpeduncular nu. Notably, many regions with high levels of nicotine binding sites, including the medial habenula, thalamus, substantia nigra, and ventral tegmental area, failed to express the c-fos gene with this schedule of nicotine administration. A minimal increase in fos immunostaining was seen after a nicotine dose of 0.5 mg/kg, with a much greater response after 1 or 2 mg/kg. The response was seen as soon as 60 min after the beginning of the infusion, was maximal at 2-3 h, and declined thereafter. c-fos expression was substantially attenuated in the superficial gray layer of superior colliculus, medial terminal nucleus of the accessory optic tract, and the interpeduncular nucleus by pretreatment with the centrally acting nicotine antagonist mecamylamine, 5 mg/kg IP, but not with the peripherally acting antagonist hexamethonium, 4 mg/kg IP. These observations identify a subset of central nervous system neurons that respond to nicotine with altered expression of the immediate early gene c-fos. These neurons presumably undergo long-term changes in gene expression as a result of acute exposure to high doses of nicotine.

Smoking and movement disorders in psychiatric patients

Previous studies have suggested that tardive dyskinesia may occur more frequently in patients who smoke. Further evidence of an interaction between smoking and movement disorders includes the low lifetime exposure to cigarettes found in Parkinson's disease patients. In this study 126 patients with chronic psychiatric illnesses were blindly evaluated for tardive dyskinesia, neuroleptic-induced parkinsonism, and akathisia. Patients who smoked received significantly higher doses of neuroleptics but did not have significantly more frequent or more severe tardive dyskinesia or parkinsonism. Female smokers did have significantly more akathisia. These results are discussed with regard to interactions between smoking, central dopaminergic tone, and the psychopathology of extrapyramidal syndromes. The effect of smoking on neuroleptic blood levels as well as clinical symptomatology is also discussed.

Effects of nicotine withdrawal on the local cerebral glucose utilization in conscious rats

Chronic infusion of nicotine has been shown to result (1) in an increase in nicotine binding sites in the brain and (2) in a distinct pattern of increases in local cerebral metabolic rate for glucose (CMRglc). The present study addresses two questions: (1) whether a one-day withdrawal of nicotine after a two-week exposure is long enough to restore local CMRglc to the preinfusion values and (2) whether an acute nicotine infusion after one day's withdrawal would influence local CMRglc. Chronic infusion of L-nicotine (12.5 micrograms/kg/min) was performed by osmotic minipumps. Local CMRglc was measured using the quantitative 2-deoxyglucose method in conscious rats. The following results were obtained: (1) a one-day withdrawal of nicotine after a two-week chronic infusion restores local CMRglc to a pattern which is close to the control pattern obtained without any nicotine infusion, and (2) an acute infusion of nicotine after a one-day withdrawal of a chronic nicotine infusion induces distinct increases in local CMRglc of several brain structures; these are essentially identical with structures which are activated during an acute nicotine infusion in otherwise untreated rats (no chronic infusion). The data indicate: (1) The main effects of chronic nicotine infusion on local CMRglc have disappeared after one day of nicotine withdrawal. (2) An acute load of nicotine in such nicotine-withdrawal rats has effects on local CMRglc which resemble those found in previously untreated rats during an acute nicotine infusion. (3) In contrast to most binding studies which have shown persisting increases in nicotine binding sites after one day's withdrawal of chronic nicotine, local CMRglc is restored to control values and can be again activated by an acute nicotine infusion.

Acute effects of nicotine injection into the nucleus accumbens on locomotor activity in nicotine-naive and nicotine-tolerant rats

To assess the role of nicotine receptors in the nucleus accumbens on locomotor activity we bilaterally implanted guide cannulae for later injection of (-)-nicotine into the nucleus accumbens of Wistar rats. Motor activity was tested in a complex tunnel maze equipped with photocells for automatic recording. This system of dark tunnels elicits spontaneous exploration even after repeated exposure. Half of the rats were made nicotine-tolerant by daily systemic injections of (-)-nicotine for 15 days (nicotine pretreatment); the other half remained nicotine-naive (saline pretreatment). Whereas (-)-nicotine (40 nmol/0.3 microliter) bilaterally injected into the nucleus accumbens of nicotine-naive animals suppressed locomotor activity, the same amount injected into the nucleus accumbens of nicotine-tolerant rats had no effect on locomotor activity. Systemic injections of nicotine (0.4 mg/kg) induced a depression and stimulation of locomotor activity in saline-pretreated and nicotine-pretreated rats, respectively. Our results support a dual role for nicotine in locomotor activity with the initial depressant effect in nicotine-naive animals due to stimulation of the nucleus accumbens and perhaps other structures.

Stroke and its modification in Parkinson's disease

Previous studies have not agreed on the incidence of ischemic stroke in persons with Parkinson's disease. There are epidemiologic and neurochemical facets of Parkinson's disease that might confer some benefit or protection against ischemic stroke. We used a case-control method to determine the lifetime history of ischemic stroke in 200 patients with Parkinson's disease and 200 controls of a similar age range. Analysis was also carried out for myocardial infarction as a marker of generalized atherosclerotic disease and for stroke risk factors. The cumulative incidence of ischemic stroke was significantly less in the patients with Parkinson's disease than in the controls, as was the cumulative incidence of myocardial infarction. Among risk factors, significantly fewer patients with Parkinson's disease used tobacco than controls. The decreased incidence of ischemic stroke in the patients with Parkinson's disease appears to be related to their less severe generalized atherosclerosis, possibly due to their lower incidence of tobacco use. In view of the known potential for dopamine to exacerbate experimental ischemic tissue damage, the possibility that the dopamine deficiency in the central nervous system of persons with Parkinson's disease confers an additional specific protective benefit against ischemic stroke cannot be excluded and requires further study.

Cigarette smoking and neuroleptic-induced parkinsonism

Several studies have reported an apparent protective effect of cigarette smoking for the risk of idiopathic Parkinson's disease (IPD). These observations are supported by neurochemical studies demonstrating enhancement of central dopaminergic neurotransmission by nicotine. We studied the prevalence and severity of neuroleptic-induced parkinsonism (NIP) in relation to cigarette smoking in a homogeneous sample of 130 psychiatric inpatients receiving long-term neuroleptic treatment. Despite the fact that smokers had significantly higher dosage of neuroleptics during the month prior to evaluation and longer exposure to medication, they presented with significantly less prevalence and severity of NIP than nonsmokers. These findings suggest that the inverse association between smoking and IPD may apply to NIP.

Nicotine releases stereoselectively and Ca2(+)-dependently endogenous 3,4-dihydroxyphenylalanine from rat striatal slices

In superfused slices of rat striatum, nicotine-evoked release of endogenous 3,4-dihydroxyphenylalanine (DOPA) was studied in comparison with that of dopamine (DA). (+/-)-Nicotine (0.1-10 microM) constantly and repetitively released DOPA and DA over a similar time course in a concentration-dependent manner. The ratio of DOPA and DA evoked was approximately 1:2-3. The turnover rate of DOPA was about 300 times higher compared to DA. (+/-)-Nicotine (10 microM)-induced DOPA release was mecamylamine (20 microM)-sensitive, Ca2(+)-dependent and tetrodotoxin (0.3 microM)-insensitive. The (+)-isomer induced no DOPA release. These characteristics of DOPA release were almost the same as those of DA. Nicotine evokes endogenous DOPA via nicotinic cholinergic receptors in a manner similar to the transmitter DA. These findings further support a probable role of DOPA as a neuroactive substance in the rat central nervous system.

Effect of acute and subchronic nicotine treatment on cortical acetylcholine release and on nicotinic receptors in rats and guinea-pigs

1. The effect of acute and chronic (16 days) administration of nicotine on cortical acetylcholine (ACh) release, gross behaviour and brain nicotinic binding sites was investigated in rats and guinea-pigs. 2. The drug, injected either subcutaneously (0.45-0.90 mg kg-1) or intracerebroventricularly (1, 3 and 5 micrograms) increased the cortical ACh release, in a dose-dependent manner, through mecamylamine-sensitive receptors for 1-2 h in both species. 3. Chronic treatment significantly increased basal ACh release in the rat and slightly lowered it in the guinea-pig, but the response to a challenging dose of nicotine was proportionally maintained in both species. 4. The number of nicotinic receptors was four times higher in the rat than in the guinea-pig and was not dependent on the radioligand used ([3H]-nicotine or [3H]-ACh, in the presence of atropine) to determine this. The nicotinic binding sites showed an apparent increase in chronically treated rats but no change in guinea-pigs. 5. Tolerance to the inhibitory effect of the drug, assessed with the T maze test, was found in the rat. No apparent change in gross behaviour was detected in the guinea-pig. 6. It is concluded that chronic nicotine treatment causes evident tolerance to its inhibitory effect on behaviour in the rat, but no adaptation to its excitatory properties on the cholinergic brain structures in rats and guinea-pigs.

The effect of nicotine on energy expenditure during light physical activity

The metabolic effects of nicotine have been implicated in the relation between smoking and lower body weight. This study examined whether the nicotine-induced increase in the metabolic rate observed at rest is also present during physical activity. We compared the energy expenditure of 10 male smokers receiving nicotine (15 micrograms per kilogram of body weight) with that of 10 male smokers receiving placebo on two occasions, each including a period of rest and a period of exercise on a modified bicycle ergometer at workloads designed to simulate and standardize light daily activity. All had abstained from cigarette smoking the night before the study. The excess energy expenditure attributable to nicotine was more than twice as great during exercise (difference between groups, 0.51 kJ per kilogram per hour, or 12.1 percent of the metabolic rate at rest; P less than 0.001) than during rest (0.23 kJ per kilogram per hour, or 5.3 percent of the metabolic rate at rest; P less than 0.05). In contrast, the expenditure was not affected by placebo during exercise or rest in the smokers or in a comparison group of 10 non-smokers, indicating that smoking status has no long-term metabolic effect in the absence of short-term nicotine intake. We conclude that the relatively small metabolic effect of nicotine when the subject is at rest is enhanced during light exercise. Our data also suggest that the weight gain that often follows smoking cessation may be influenced not only by nicotine intake but also by the level of physical activity a smoker typically engages in while smoking.

Peripheral induction of burst firing in locus coeruleus neurons by nicotine mediated via excitatory amino acids

The effect of systemic nicotine administration (50 micrograms kg-1 i.v.) on the activity of brain noradrenaline neurons in the locus coeruleus (LC) of chloral hydrate-anesthetized rats was analyzed with single cell recording techniques and quantitative computer assessment of firing rate, degree of bursting, and regularity of firing. Nicotine caused an increased firing rate of the cells, with an average time of onset of 1.7 s. An increase in burst activity was observed, as well as deregularization of the firing pattern. Intraventricularly administered kynurenic acid (1 mumol), an antagonist of excitatory amino acids (EAA), did not change the firing rate of the LC cells, but did induce a marked regularization of their firing pattern into a pacemaker-like activity and completely abolish burst firing. The EAA antagonist also blocked all of the above effects of nicotine on the LC neurons as well as their typical burst-activation response to a peripheral, noxious stimulus such as paw-pinch. Since the circulation time in the rat is about 20 s, these results provide unequivocal evidence for a peripheral site of origin for the rapid LC activation induced by systemic nicotine administration. The data also allow the conclusion that the nicotine-induced LC activation is indirect and dependent on EAA in brain. Our results provide evidence for a tonically active EAA input to the LC, being of importance for induction of changes in the spontaneous, pacemaker activity of LC neurons into burst firing or more irregular firing patterns. It is suggested that the LC activation by nicotine may be significant in relation to tobacco dependence.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

The effect of an acute nicotine infusion on the local cerebral glucose utilization of the awake rat

The effect of acute infusion of nicotine on local cerebral glucose utilization (LCGU) was studied in discrete regions of the central nervous system of the rat by means of the quantitative autoradiographic 2-deoxy-D-[1-14C]glucose method described by Sokoloff et al. Nicotine was administered in 3 dosages: 0.5 microgram/kg/min, 1.58 micrograms/kg/min and 5 micrograms/kg/min. The resulting plasma concentrations of nicotine were 10/39/114 ng/ml plasma. During the experiment, blood pressure, heart rate, body temperature, hematocrit, acid-base status and plasma glucose concentration showed no--or minor--changes. Nicotine significantly increased LCGU in a dose-dependent manner in the following 9 of 45 examined structures: substantia nigra (compact part), superior colliculus (superficial grey layer), interpeduncular nucleus and cingulate cortex (P less than 0.01); lateral geniculate body, optic chiasm, anteroventral and anteromedial nucleus of thalamus and mamillary body (P less than 0.05). For most of these structures with increased LCGU, other groups have reported a high regional receptor binding of nicotine (exception: mamillary body and optic chiasm). It is concluded that nicotine has distinct effects on the functional activity of localized brain areas.

Autoradiographic localization of [3H]nicotine binding sites in the rat brain

In vitro autoradiography was used to visualize [3H]nicotine binding sites in the rat brain. Labeling was densest in the interpeduncular nucleus and medial habenula but was also detected in thalamic nuclei, areas related to sensory function, the cerebral cortex, and molecular layer of the dentate gyrus. Specific binding was sparse in the hypothalamus and caudate-putamen, and not detected in Ammon's horn of the hippocampus or in the periaqueductal grey matter. These findings may relate to the distribution of nicotine's cerebral loci of action.

Isolation of a nicotine binding site from rat brain by affinity chromatography

With the use of affinity chromatography, a [3H]-nicotine binding site was purified almost 1,000-fold from a Triton X-100-solubilized extract of rat brain neural membranes. The affinity column was prepared by conjugation of (R,S)-6-(2-hydroxyethyl)nicotine to epoxy-activated Sepharose. Further purification of the material from the affinity column was resolved by using another column of the same affinity gel, resulting in the isolation of a major protein (about 95% purity) that had a Mr of 56,000, as determined by NaDodSO4/polyacrylamide gel electrophoresis, with very minor components ranging in Mr from 47,000 to 83,000. With the use of various nicotine analogues, it was shown that the purified material exhibited nearly identical binding characteristics to rat brain membrane preparations, including stereoselectivity for the nicotine enantiomers. The Kd of the purified site, 3.5 x 10(-9) M, was similar to that observed with membrane and Triton X-100-soluble preparations, whereas the binding capacity was greater than 25 pmol/mg of protein, as compared to 0.07 pmol/mg of protein in the starting material. The results are discussed in relation to the purified nicotinic cholinergic receptor from electroplax. It was concluded that the nicotine site in rat brain was different from the cholinergic receptor of electroplax or calf skeletal muscle.