Brain nicotine binding sites

Variation of spacer type and topology of phenyl moiety in 2-pyridone core of 4-oxo-3-N-methylcytisine; effect of synthesized compounds on rat's behavior in conditioned passive avoidance reflex (CPAR) test

Novel derivatives of 4-oxo-3-methylcytisine with phenyl moiety bonded to starting molecule through various spacers were obtained from the 9-amino, -halo, -formyl and 11-halo precursors by reductive alkylation of amines, generation of amide, as well as thio- and carboxamide functions, cross-coupling reactions, aldehyde condensation and reduction of unsaturated 'C-C' bonds. Ability of synthesized compounds to influence the learning and memory was preliminary assessed in conditioned passive avoidance reflex (CPAR) test in rats. It was shown, that derivatives with phenyl group at 11 carbon atom influence the learning and memory in CPAR test more effectively than other compounds. The hit-compound (3-methyl-11-(2-phenylvinyl)-3,5,6-trihydro-2H-1,5-methanopyrido[1,2-a][1,5]diazocine-4,8(1H)-dione) with the best values of 'latency' and 'time spent in the dark compartment' has been identified as a perspective scaffold for synthesis of novel derivatives of (-)-cytisine with potential neuropharmacological activity.

Acetylcholine released from T cells regulates intracellular Ca2+, IL-2 secretion and T cell proliferation through nicotinic acetylcholine receptor

AIMS

T lymphocytes synthesize acetylcholine (ACh) and express muscarinic and nicotinic ACh receptors (mAChR and nAChR, respectively) responsible for increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i). Our aim in the present study was to assess whether autocrine ACh released from T lymphocytes regulates their physiological functions.

MAIN METHODS

MOLT-3 human leukemic cell line and murine splenocytes were loaded with fura-2 to monitor [Ca²⁺]_i changes in the absence or presence of several AChR antagonists, including mecamylamine, methyllycaconitine and scopolamine. Real-time PCR and ELISA were performed to measure interleukin-2 (IL-2) mRNA and protein levels.

KEY FINDINGS

T lymphocytes constitutively produce sufficient amounts of ACh to elicit autocrine changes in [Ca²⁺]_i. These autocrine ACh-evoked [Ca²⁺]_i transients were mediated by nAChRs and then influx of extracellular Ca²⁺. Mecamylamine, a nAChR inhibitor, suppressed not only these [Ca²⁺]_i transients, but also IL-2 release and T cell proliferation.

SIGNIFICANCE

Here, we confirmed that T lymphocytes utilize ACh as a tool to interact with each other and that autocrine ACh-activated nAChRs are involved in cytokine release and cell proliferation. These findings suggest the possibility that nAChR agonists and antagonists and smoking are able to modulate immune function, which in turn suggests the therapeutic potential of immune activation or suppression using nAChR agonists or antagonists.

The expression and functional role of nicotinic acetylcholine receptors in rat adipocytes

To clarify whether nicotine has a direct effect on the function of adipocytes, we evaluated nicotinic acetylcholine receptor (nAChR) expression in adipocytes by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry and the direct effects of nicotine on the production of adipocytokines by enzyme-linked immunosorbent assay and Western blot analysis. Receptor binding assays were performed using [3H]nicotine. RT-PCR studies revealed that alpha1-7, 9, 10, beta1-4, delta, and epsilon subunit mRNAs are expressed in adipocytes. Immunocytochemical experiments also suggested the presence of alpha7 and beta2 subunits. The receptor binding assay revealed a binding site for nicotine (Kd = 39.2 x 10(-9) M) on adipocytes. Adipocytes incubated with nicotine for 12 and 36 h released tumor necrosis factor-alpha (TNF-alpha), adiponectin, and free fatty acid (FFA) into the medium in a dose-dependent manner with increasing nicotine concentration from 6 x 10(-8) to 6 x 10(-4) M. However, TNF-alpha protein levels in adipocytes incubated for 12 and 36 h decreased in a dose-dependent manner with increasing nicotine concentration from 6 x 10(-8) to 6 x 10(-4) M. These results show that adipocytes have functional nAChRs and suggest that nicotine reduces TNF-alpha protein production in adipocytes through the activation of nAChRs. Nicotine may temporarily lower insulin sensitivity by stimulating the secretion of TNF-alpha and FFA, whereas long-term direct stimulation of nAChRs by nicotine in addition to autonomic nervous system stimulation may contribute to better insulin sensitivity in vivo through a modulated secretion of adipocytokines.

Nicotinic cholinergic synaptic mechanisms in the ventral tegmental area contribute to nicotine addiction

Tobacco use is a major health problem that is estimated to cause 4 million deaths a year worldwide. Nicotine is the main addictive component of tobacco. It acts as an agonist to activate and desensitize nicotinic acetylcholine receptors (nAChRs). A component of nicotine's addictive power is attributable to actions on the mesolimbic dopaminergic system, which serves a fundamental role in the acquisition of behaviors that are inappropriately reinforced by addictive drugs. Here we show that nicotine, in the same concentration and time ranges as obtained from tobacco, has three main actions that regulate the activity of midbrain dopamine (DA) neurons. Nicotine first activates and then desensitizes nAChRs on the DA neurons. This process directly excites the DA neurons for a short period of time before the nAChRs desensitize. Nicotine also enhances glutamatergic excitation and decreases GABAergic inhibition onto DA neurons. These events increase the probability for synaptic plasticity, such as long-term potentiation. The short-lived direct excitation of the DA neurons coupled with the enhanced glutamatergic afferent activity provides the presynaptic and postsynaptic coincidence necessary to initiate synaptic potentiation. In total, these synaptic events lead to a relatively long-lasting heightened activity of midbrain DA neurons. Consistent with other summarized studies, this work indicates that the synaptic changes normally associated with learning and memory can be influenced and commandeered during the nicotine addiction process.

Differential desensitization and distribution of nicotinic acetylcholine receptor subtypes in midbrain dopamine areas

Although many psychopharmacological factors contribute to nicotine addiction, midbrain dopaminergic systems have received much attention because of their roles in reinforcement and associative learning. It is generally thought that the mesocorticolimbic dopaminergic system is important for the acquisition of behaviors that are reinforced by the salient drives of the environment or by the inappropriate stimuli of addictive drugs. Nicotine, as obtained from tobacco, can activate nicotinic acetylcholine receptors (nAChRs) and excite midbrain neurons of the mesocorticolimbic system. Using midbrain slices from rats, wild-type mice, and genetically engineered mice, we have found differences in the nAChR currents from the ventral tegmental area (VTA) and the substantia nigra compacta (SNc). Nicotinic AChRs containing the alpha7 subunit (alpha7* nAChRs) have a low expression density. Electrophysiological analysis of nAChR currents, autoradiography of [125I]-alpha-bungarotoxin binding, and in situ hybridization revealed that alpha7* nAChRs are more highly expressed in the VTA than the SNc. In contrast, beta2* nAChRs are move evenly distributed at a higher density in both the VTA and SNc. At the concentration of nicotine obtained by tobacco smokers, the slow components of current (mainly mediated by beta2* nAChRs) become essentially desensitized. However, the minority alpha7* component of the current in the VTA/SNc is not significantly desensitized by nicotine in the range < or =100 nm. These results suggest that nicotine, as obtained from tobacco, can have multiple effects on the midbrain areas by differentially influencing dopamine neurons of the VTA and SNc and differentially desensitizing alpha7* and non-alpha7 nAChRs.

Cytisine derivatives as ligands for neuronal nicotine receptors and with various pharmacological activities

Neuronal nicotinic acetylcholine receptors (nAChRs) form a family of ACh-gated cation channels made up of different subtypes. They are widely distributed in peripheral and central nervous systems and are involved in complex cerebral processes as learning, memory, nociception, movement, etc. The possibility that subtype-selective ligands be used in the treatment of CNS disorders promoted the synthesis of a large number of structural analogues of nicotine and epibatidine, two very potent nAChR agonists. Pursuing our long standing research on the structural modification of quinolizidine alkaloids, we devoted our attention to cytisine, another very potent ligand for many nAChR subtypes. Thus a systematic structural modification of cytisine was undertaken in order to obtain compounds of potential therapeutic interest at peripheral as well as central level, with a particular concern for achieving nAChR subtype selective ligands. Up to the present more than 80 cytisine derivatives, mainly of N-substitution and a few by modifying the pyridone ring, have been prepared. The biological results, which concern so far about an half of the prepared compounds, indicate that the introduction of a nitro group in position 3 of the pyridone nucleus further enhances the high affinity of cytisine, while the introduction of substituents on the basic nitrogen, though reducing in different degrees the affinity, gives rise to compounds with a higher selectivity for central (alpha(4)beta(2)) versus gangliar (alpha(3)-containing) receptor subtype. On the other hand, the analgesic, antihypertensive and inotropic activities found in some N-substituted cytisines, represent an attractive starting point for the development of more active compounds.

Desensitization of neuronal nicotinic receptors

The loss of functional response upon continuous or repeated exposure to agonist, desensitization, is an intriguing phenomenon if not as yet a well-defined physiological mechanism. However, detailed evaluation of the properties of desensitization, especially for the superfamily of ligand-gated ion channels, reveals how the nervous system could make important use of this process that goes far beyond simply curtailing excessive receptor stimulation and the prevention of excitotoxicity. Here we will review the mechanistic basis of desensitization and discuss how the subunit-dependent properties and regulation of nicotinic acetylcholine receptor (nAChR) desensitization contribute to the functional diversity of these channels. These studies provide the essential framework for understanding how the physiological regulation of desensitization could be a major determinant of synaptic efficacy by controlling, in both the short and long term, the number of functional receptors. This type of mechanism can be extended to explain how the continuous occupation of desensitized receptors during chronic nicotine exposure contributes to drug addiction, and highlights the potential significance of prolonged nAChR desensitization that would also occur as a result of extended acetylcholine lifetime during treatment of Alzheimer's disease with cholinesterase inhibitors. Thus, a clearer picture of the importance of nAChR desensitization in both normal information processing and in various diseased states is beginning to emerge.

Differential regulation of nicotinic receptor-mediated neurotransmitter release following chronic (-)-nicotine administration

The objective of this study was to compare nAChR-mediated neurotransmitter release from slices of rat striatum, frontal cortex and hippocampus following chronic (-)-nicotine (Nic) administration (tartrate salt, 2 mg/kg twice daily for 10 days). Binding studies were also conducted to measure changes in receptor density. Relative to saline-treated animals, the number of nAChRs measured by [(3)H]-cytisine (CYT) binding was significantly increased in all brain regions examined by 15% to 25% following chronic Nic administration. Using a relatively high throughput method to measure neurotransmitter release, we found that Nic, CYT, and (+/-)-epibatidine (EB) evoked similar concentration-dependent striatal [(3)H]-dopamine (DA) and hippocampal [(3)H]-norepinephrine (NE) release from both saline (rank order of potency for [(3)H]-DA: EB>CYT>Nic; pEC(50) values, EB (9 +/- 0.1), CYT (8 +/- 0.13), Nic (7.3 +/- 0.19); rank order potency for [(3)H]-NE: EB>Nic=CYT; pEC(50) values, EB (8 +/- 0.18), Nic (5.5 +/- 0.09), CYT (5.12 +/- 0.1)) -and Nic-treated animals (pEC(50) values [(3)H]-DA, EB (9.5 +/- 0.15), Nic (8 +/- 0.16, CYT (6.6 +/- 0.52); [(3)H]-NE, EB (8.4 +/- 0.23), Nic (5.19 +/- 0.1), CYT (5.18 +/- 0.29)). Although no change in potency was detected between the two treatment groups, the agonist efficacies in both tissues were significantly reduced by approximately 17-54% following chronic Nic administration. In contrast to striatum, treatment with Nic did not affect the maximal [(3)H]-DA response (efficacy) in the frontal cortex. However, as observed in the striatum, no change in agonist potency was observed in the frontal cortex following chronic Nic administration (pEC(50) values, saline; EB (9.2 +/- 0.2), >CYT (6.95 +/- 0.75) = Nic (6.9 +/- 0.16); Nic-treated, EB (9 +/- 0.42)>CYT (6.88 +/- 0.27) = Nic (7.1 +/- 0.17)). Chronic Nic treatment did not significantly affect KCl-evoked [(3)H]-NE release from hippocampus or [(3)H]-DA release from frontal cortex or striatum. Since previous work has demonstrated that different nAChR subtypes display various sensitivities to chronic Nic exposure, we suggest that the subtypes of nAChRs involved in regulating [(3)H]-DA release may be different in the striatum and frontal cortex. These results support findings from earlier studies comparing the pharmacology of nAChR-evoked striatal versus cortical [(3)H]-DA release.

Synthesis and evaluation of a novel series of 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine analogues as potential positron emission tomography imaging agents for nicotinic acetylcholine receptors

Reportedly, 2-[(18)F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding affinities for the new series were found to be in the range of K(i) = 9-331 pM. A molecular modeling study showed differences in the comformational profiles and the electronic properties of these compounds, which provides further insight into the structure-activity relationships at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high affinities at nAChRs (K(i) = 28 and 23 pM, respectively) and will be targets for development of (11)CH(3)-labeled derivatives as radiotracers for PET imaging of nAChRs.

Investigation of nicotine binding to THP-1 cells: evidence for a non-cholinergic binding site

Nicotine is known to modulate immune function, but reports have produced conflicting evidence as to whether nicotinic acetylcholine (nACh) receptors are responsible for these effects. This study was designed to examine the identity of nicotine-binding sites on immune cells using a human leukaemic monocytic cell line, THP-1, that is known to have functions that are modulated by nicotine. Binding studies were performed on THP-1 whole cells using [3H]nicotine as a probe to analyse any possible nicotine-binding sites on these cells. Saturation analysis of THP-1 cells revealed the presence of 2 distinct binding sites; one with a K(d1) of 3.5 +/- 2.1 x 10(-9) M and a B(max1) of 4100 +/- 560 sites/cell (designated the high-affinity site) and the other with a K(d2) of 27 +/- 9.2 x 10(-9) M and a B(max2) of 11,600 +/- 630 sites/cell (low-affinity site). Competition analysis revealed that one site had an affinity to a range of cholinergic ligands including epibatidine and cytisine. When saturation analysis of [3H](-)-nicotine to THP-1 cells was performed in the presence of 1 x 10(-6) M epibatidine, only one binding site was detected. Comparisons of K(d) and B(max) values showed that the high-affinity site was not occluded by epibatidine. No drugs tested displayed any affinity for the high-affinity site except the two enantiomers of nicotine. The high-affinity site was shown to be stereoselective for the (+)-enantiomer of nicotine as shown by K(i) values produced by competition analysis in the presence of 1 x 10(-6) M epibatidine. These values were 5.7 +/- 0.32 x 10(-11) M and 1.9 +/- 4.9 x 10(-9) M for (+)-nicotine and (-)-nicotine, respectively. This study presents evidence for a possible non-cholinergic binding site that may play a role in the mechanism of immunomodulation by nicotine.

Synthesis and preliminary pharmacological evaluation of some cytisine derivatives

Thirty-one N-derivatives of cytisine were prepared in order to modify its pharmacological profile and to obtain compounds of potential therapeutic interest either at a peripheral or central level, particularly as nicotinic ligands with improved ability to cross the blood-brain barrier. Actually, with the introduction of different kinds of substituent on the basic nitrogen of cytisine a variety of activities were observed, both in vivo (analgesic, dopamine antagonism, antihypertensive, inhibition of stress-induced ulcers, antiinflammatory, protection from PAF-induced mortality, hypoglycemic) and in vitro (positive cardio-inotropic, beta-adrenergic antagonism, alpha 1- and alpha 2-antagonism, inhibition of PAF-induced platelet aggregation). Six randomly selected compounds were tested for the ability to recognize a central nicotinic receptor and four of them exhibited Ki values in the range 30-163 nM.

Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation.

Nicotine-induced grooming: a possible dopaminergic and/or cholinergic mechanism

The ability of nicotine, to induce grooming in rats was studied. Grooming was induced by i.p. injection of different doses (0.0675-0.5 mg/kg) of nicotine to rats. The effect was dose-dependent. However, the response was decreased with increasing doses of the drug from 0.25-0.5 mg/kg. Administration of the dopamine (DA) D1/D2 receptor agonist apomorphine (0.025-5 mg/kg, i.p.) also caused grooming in a dose-dependent manner. High doses of apomorphine (0.1-0.5 mg/kg, i.p.) also induced a lower degree of response. Combination of a low dose of nicotine (0.0675 mg/kg) with different doses of apomorphine did not show any interaction. However, there was an interaction between a high dose of nicotine and apomorphine. Thus, combination of a higher dose of nicotine (0.125 mg/kg) with apomorphine, reduced apomorphine-induced grooming. The muscarinic receptor antagonist atropine (5 and 10 mg/kg), peripheral nicotinic receptor antagonist hexamethonium (5 and 10 mg/kg), central nicotinic receptor antagonist mecamylamine (1 and 3 mg/kg) and D1 DA receptor antagonist SCH23390 (0.05 and 0.1 mg/kg) all decreased the response to nicotine. Atropine, mecamylamine and SCH23390 by themselves reduced spontaneous grooming. It is concluded that nicotine elicits grooming indirectly through a possible D1 dopaminergic mechanism. However, muscarinic and nicotinic cholinergic mechanism(s) may be involved.

The effect of cholinoceptor agonists and neurotoxins on the release of vasopressin in the rat in relation to the subunit composition of the cholinoceptor

The effects of cholinoceptor agonists and neurotoxins on the release of vasopressin and oxytocin have been investigated in water-loaded rats under ethanol anaesthesia. Release of vasopressin was monitored by antidiuretic responses accompanied by increased urinary excretion of vasopressin. The rate of excretion of oxytocin-like radioimmunoreactivity was measured as an indicator of oxytocin release. Both nicotine and cytisine caused a preferential release of vasopressin. The release by nicotine was not inhibited by alpha- or neuronal-bungarotoxin. Neosurugatoxin blocked the release by cytisine. Comparison with the effects of these agents on combinations of alpha and beta subunits expressed in oocytes suggests that the central cholinoceptors mediating release of vasopressin are similar to those at autonomic ganglia and may contain a beta 4 subunit.

The psychopharmacological basis of nicotine's differential effects on behavior: individual subject variability in the rat

Nicotine, the presumed active pharmacological agent in tobacco, produces variable effects on behavior that are at best described as "paradoxical" in nature. Thus, nicotine, via tobacco use in humans or nicotine administration in experimental animals, tends to transpose behavior depending on predrug baseline rates of behavior. High rates of behavior appear to be reduced, while low rates of behavior appear to be increased by nicotine. This work further proposes that nicotine's variable effects on behavior may be related to its capacity to act as a behavioral agonist and/or antagonist via its ability either to activate or to desensitize distinct central nicotinic acetylcholinergic receptors (nAChR's). Nicotine is portrayed as a neuronal modulating agent that can affect behavior contingent upon the genetic makeup of the individual subject being studied. Depending on the structure, function, and location of distinct nAChR's, nicotine appears to be able to induce a wide range of behavioral effects important to the tobacco user. However, this does not rule out the role the importance that other biogenic amine systems (i.e., serotonin or dopamine) may have in the genetics of tobacco use or nicotine's variable effects on behavior.

Inhibition by dizocilpine (MK-801) of striatal dopamine release induced by MPTP and MPP+: possible action at the dopamine transporter

1. The NMDA-type glutamate receptor antagonist, dizocilpine (MK-801) can protect against neurotoxicity associated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its principal metabolite, the 1-methyl-4-phenylpyridinium ion (MPP+). It has been suggested that these neurotoxic effects may be mediated by release of excitatory amino acids, but possible alternative mechanisms have been little investigated. 2. MPTP and MPP+ (0.1-1000 microM) were tested in superfused rat striatal synaptosomes preloaded with [3H]-dopamine. Both MPTP (10 microM and higher) and MPP+ (1 microM and higher) evoked an immediate and concentration-dependent release of [3H]-dopamine. The maximal effect exceeded that achievable with nicotine. For subsequent experiments, submaximal concentrations of MPTP (50 microM) and MPP+ (10 microM) were tested. 3. MK-801 (0.1-100 microM) inhibited responses to MPTP (50 microM) and MPP+ (10 microM) in a concentration-dependent manner. However, further tests of NMDA-type glutamate receptor involvement proved negative. Responses to MPTP or MPP+ were unaffected by the omission of Mg2+ or Ca2+ and were not reduced by the NMDA receptor antagonists, AP-7 (200 microM) and kynurenic acid (300 microM). In this assay, N-methyl-D-aspartate (even in the absence of Mg2+ and with added glycine and strychnine) did not evoked [3H]-dopamine release. 4. In crude membrane preparations of rat cerebral cortex, MPTP and MPP+ inhibited high-affinity [3H]-nicotine binding to nicotinic cholinoceptors (IC50 1.8 microM and 26 microM, respectively). 5. [3H]-dopamine release evoked by nicotine (1 microM) was blocked by the nicotinic antagonists,mecamylamine and chlorisondamine, and by MK-801 (all at 100 micro M); K+-evoked release was not affected. Release evoked by MPTP and MPP+ was significantly attenuated by MK-801 but not by mecamylamine or chlorisondamine.6. At a high concentration (1O I1M), the selective dopamine uptake inhibitor, nomifensine, completely blocked [3HJ-dopamine release evoked by amphetamine 0.3 microM and MPP+ 10 flM, attenuated responses to MPTP 50 AM and did not affect responses to 12 mM K+. MK-801 100 microM evinced a similar profile but was less effective.7. MK-801 inhibited [3H]-dopamine uptake in striatal synaptosomes with an IC5o of 115 M.8. It is concluded that high concentrations of MK-801 inhibit the acute dopamine release evoked by MPTP and MPP+ in synaptosomes. This antagonism may occur, at least in part, through inhibition of the cell membrane dopamine transporter. MPTP and MPP+ also appear to interact with brain nicotinic cholinoceptors but the functional consequences of this interaction are not yet clear.

In vivo effects of (-)-nicotine on ethanol-induced increase in glucose utilization in the mouse cerebellum

The purpose of this study was to investigate the possible in vivo effects of (-)-nicotine, ethanol, and an adenosine agonist N6-cyclohexyladenosine (CHA) when injected individually as well as in various combinations on glucose utilization in the fresh cerebellar slices of mice. Mice received ICV (-)-nicotine or CHA followed 5 min later by a test dose of ethanol (2 g/kg; IP). Animals were killed 20 min postethanol treatment and fresh slices (300 microns) of cerebellum were incubated in a glucose medium in Warburg flasks using 14C-glucose as a tracer. Trapped 14CO2 was counted to estimate glucose utilization. Ethanol treatment markedly accentuated glucose utilization, whereas the pretreatment with (-)-nicotine (125 and 250 ng, ICV) resulted in a significant attenuation in the ethanol-induced increase in glucose utilization. However, ICV (-)-nicotine (125 ng) alone did not produce any change in the cerebellar glucose utilization. The attenuation of ethanol-induced increase in glucose utilization by (-)-nicotine was nearly totally blocked by ICV hexamethonium, a purported nicotinic antagonist, suggesting participation of cholinergic-nicotinic receptors. The (-)-nicotine pretreatment also significantly attenuated both the ICV CHA (25 ng)-induced increase in glucose utilization and the accentuation of ethanol-induced increase in glucose utilization by CHA. The antagonistic effect of (-)-nicotine on CHA- and ethanol-induced increase in glucose utilization indicating an interaction between (-)-nicotine and ethanol and between (-)-nicotine and adenosine may suggest involvement of postreceptor (nicotinic and adenosine) mechanisms including ionic channels.

Muscarinic and nicotinic modulation of cortical acetylcholine release monitored by in vivo microdialysis in freely moving adult rats

The aim of the present study was to investigate, using in vivo dialysis, the existence of muscarinic and nicotinic receptors controlling acetylcholine release in the cortex of freely behaving rats. Various muscarinic receptor antagonists, including the nonselective blocker atropine, and a variety of M2 drugs (AF-DX116, AF-DX384, AQ-RA 741) potently stimulated, in a concentration-dependent manner, the in vivo release of acetylcholine in the rat cortex. The effects of all these antagonists were long lasting. The nature of these putative muscarinic autoreceptors is likely of the pharmacologically defined M2 subtype on the basis of the high potency of the antagonists of the AF-DX series and the variability and shorter duration of action of the effects of the prototypic M1 blocker, pirenzepine. 4-DAMP, a purported M3 blocker, also potently stimulated in vivo cortical acetylcholine release, but this likely relates to its now established, rather limited selectivity for any given muscarinic receptor subtypes. Peripheral and central injections of nicotine also induced the in vivo release of acetylcholine in the rat cortex, albeit with a lower potency and shorter duration of action than muscarinic antagonists. Interestingly, the combination of a muscarinic antagonist, such as atropine, AF-DX 116, or AF-DX384, in the presence of nicotine, induced tremendous releases of cortical acetylcholine up to 8- to 10-fold over basal values. This is clearly more than a simply additive effect, and it reveals the great capacity of cortical cholinergic nerve terminals to synthesize and release acetylcholine. Optimal pharmacological manipulations of these putative muscarinic and nicotinic autoreceptors could thus be useful in disorders in which the activity of cholinergic inputs is decreased, such as in Alzheimer's disease.

Central auditory processing in school-age children prenatally exposed to cigarette smoke

One hundred and ten, 6- to 11-year-old children from a low-risk, predominantly middle class sample who are participants in an ongoing longitudinal drug study were assessed using a central auditory processing task (SCAN) that made perceptual rather than linguistic demands. Maternal smoking during pregnancy was linearly associated with poorer performance on the overall SCAN and, particularly, the Competing Words subtest which may be an indication of the child's auditory maturation. The significant associations remained after adjusting for other drug use, demographic variables, and passive smoke exposure both during pregnancy and postnatally. The child's recent second-hand smoke exposure was evaluated by a parental questionnaire and by urine cotinine assay. Neither prenatal nor postnatal passive smoke exposure was statistically significantly associated with the SCAN results. However, among the children of nonsmokers, passive smoke exposure resulted in average scores similar to those of the prenatal light smoking group. The findings are discussed in relation to earlier observations that have reported an association between smoking during pregnancy and altered auditory functioning in the offspring.

Intracerebellar nicotinic-cholinergic participation in the cerebellar adenosinergic modulation of ethanol-induced motor incoordination in mice

Many epidemiological studies have suggested a high correlation between the use of tobacco and ethanol, the two most frequently abused psychoactive drugs. Recently, we reported behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol within the CNS. The present report is a confirmation and an extension of that study. Using a 2 g/kg ethanol-induced motor incoordination (EIMI) as the test response, possible behavioral interactions between (-)-nicotine, (-)-cotinine and ethanol and between (-)-nicotine, (-)-cotinine and adenosine agonist + ethanol in the cerebellum were investigated. (-)-Nicotine, 0.625, 1.25 and 5 ng intracerebellarly (ICB) significantly attenuated EIMI in a dose-related manner. Likewise, ICB injection of 1.25, 2.5, and 5 ng (-)-cotinine, a major metabolite of nicotine, significantly attenuated EIMI after the same i.p. dose of ethanol as in case of (-)-nicotine but less markedly compared to (-)-nicotine. No change in normal motor coordination was observed when the highest dose of (-)-nicotine or (-)-cotinine was injected ICB followed by saline control, suggesting selectivity of their behavioral interactions with ethanol. The attenuation of EIMI by (-)-nicotine and (-)-cotinine was blocked by ICB hexamethonium (1 microgram) and trimethaphan (100 ng), the purported nicotinic-cholinergic antagonists. Finally, the ICB injection of adenosine agonists, N6-cyclohexyladenosine (CHA) or 5'-N-ethylcarboxamidoadenosine (NECA), produced marked accentuation of EIMI which was significantly antagonized by ICB (-)-nicotine and (-)-cotinine. The data obtained in the present study suggested, for the first time, a cerebellar adenosinergic-nicotinic cholinergic interaction and modulation of EIMI. The data also suggested participation of cerebellar nicotinic-cholinergic receptors in EIMI.

Nicotinic receptor binding of [3H]cytisine, [3H]nicotine and [3H]methylcarbamylcholine in rat brain

Three radiolabeled nicotinic receptor agonists were examined for their binding characteristics and for inhibition by cholinergic compounds in order to distinguish possible differential affinities for subtypes of neuronal nicotinic acetylcholine receptors. KD and Bmax values for [3H]cytisine, [3H]methylcarbamylcholine and [3H]nicotine were determined from Scatchard analysis using an enriched whole-brain membrane fraction from male Sprague-Dawley rats. Respective KD values were 0.15, 1.07 and 0.89 nM while Bmax values were 99, 64 and 115 fmol/mg protein respectively. All three ligands fit a one-site model of receptor-ligand interaction. Concentration-inhibition curves were used to determine Ki values for 16 cholinergic compounds. The rank order of potencies for displacement of the three ligands was: (-)-cytisine > (-)-nicotine > (-)-lobeline = methylcarbamylcholine > 1,1-dimethyl-4-phenylpiperazinium, (+)-nicotine, dihydro-beta-erythroidine, (+/-)-nornicotine > carbachol > arecoline >> oxotremorine, tetrahydroaminoacridine, AF102B >> (-)-cotinine > RS86 = heptylphysostigmine. Correlations of the affinities of these compounds determined with the three ligands were very near to unity. In contrast, there was a negative correlation of affinities for [3H]cytisine compared to affinities for the muscarinic receptor agonist, [3H]oxotremorine-M, and the muscarinic receptor antagonist, [3H]quinuclidinyl benzoate. Using membranes from whole rat brain yields data suggesting that all three nicotinic ligands bind to the same nicotinic acetylcholine receptor subtype, and are unable to distinguish subtypes of neuronal nicotinic acetylcholine receptor at the level examined.

Blockade of nicotinic receptor-mediated release of dopamine from striatal synaptosomes by chlorisondamine and other nicotinic antagonists administered in vitro

1. Central nicotinic receptor function examined in vitro, by measuring nicotine-induced [3H]-dopamine release from rat striatal synaptosomes. 2. The agonists (-)-nicotine, acetylcholine, 1,1-dimethyl-4-phenylpiperazinium (DMPP) and cytisine (10(-7)-10(-4) M) all increased [3H]-dopamine release in a concentration-dependent manner. Cytisine did not produce a full agonist response, compared to the other agonists. 3. The actions of nicotine, acetylcholine and cytisine were largely dependent on external Ca2+. In contrast, DMPP (10(-5) and 10(-4) M) evoked a marked release of [3H]-dopamine even in the absence of Ca2+. Nevertheless, in the presence of external Ca2+, responses to DMPP were completely blocked by the nicotinic antagonists chlorisondamine and mecamylamine (5 x 10(-5) M); in the absence of external Ca2+, blockade was only partial. 4. Chlorisondamine, mecamylamine and dihydro-beta-erythroidine (10(-8)-10(-4) M) produced a concentration-dependent block of responses to nicotine (10(-6) M). Approximate IC50 values were 1.6, 0.3 and 0.2 x 10(-6), respectively. Chlorisondamine and mecamylamine blocked responses to nicotine (10(-7)-10(-4) M) insurmountably, whereas dihydro-beta-erythroidine behaved in a surmountable fashion. 5. The occurrence of use-dependent block was tested by briefly pre-exposing the synaptosomes to nicotine during superfusion with antagonist, and determining the response to a subsequent nicotine application. Consistent with a possible channel blocking action, brief pre-exposure to agonist increased the antagonist potency of chlorisondamine (approximately 25 fold). No significant use-dependent block was detected with dihydro-beta-erythroidine.

Neurobehavioral mechanisms of nicotine action: role in the initiation and maintenance of tobacco dependence

Basic neuroscience research conducted over the last quarter century has provided us with much information concerning potential biobehavioral and neuromolecular mechanisms involved in the initiation and maintenance of tobacco dependence. Nicotinic-acetylcholinergic receptors (AChRs), in addition to having a primary locus on cholinergic neurons, appear to be also located on a variety of noncholinergic neurons (presynaptic and/or postsynaptic sites). Nicotine therefore appears to be able to affect a variety of neuronal pathways involved in behavioral reward and arousal processes, which appear paramount to tobacco dependence. Nicotine appears to have several unique properties at the cellular level that allow it to act both as an agonist and as a potential antagonist at select AChRs. Nicotine's ability to act as an agonist appears to be contingent on an action at nAChRs, which initially open a receptor-linked cation channel, eliciting the entrance of CA++ (or other cations) into the cell. Cation entrance into the cell, therefore, may be the cellular transducer of nicotine's behavioral and dependence-producing effects. Subsequent to this initial agonist effect, the nicotinic receptor is believed to undergo a refractory period, via a desensitization process, during which Ca++ is prevented from further entrance into the cell. It is this ability to induce receptor desensitization which seems central to nicotine's ability to act as an antagonist. The duration of nAChR desensitization may also be useful in explaining individual variability to nicotine's behavioral effects and may be related to the induction of acute and/or chronic tolerance in both animals and man. Nicotine-induced desensitization may also be important to relapse in the smoker if conditioned stimuli are able to provoke such mechanisms, which could lead to the need to smoke. Finally, a model is presented to account for individual smoking patterns and level of tobacco dependence which is partially based on the proposed cellular mechanisms of nicotine action and desensitization at the nAChR.

Characterization of [3H]cytisine binding to human brain membrane preparations

The binding characteristics of [3H]cytisine, a putative CNS nicotinic receptor ligand, were examined in 4 regions of the human brain. [3H]Cytisine was found to bind non-cooperatively with high affinity to a single site in tissue homogenates and to exhibit low non-specific association. The binding characteristics of this ligand were evaluated in thalamus at 4 degrees C and 24 degrees C. The association constants were found to be 0.234 and 0.308 min-1 nM-1, while the dissociation constants were 0.007 and 0.098 min-1, respectively. Saturation analysis of thalamus revealed the equilibrium Kd to be 147 pM (4 degrees C) and 245 pM (24 degrees C), values in good agreement with those determined kinetically. The Hill coefficient varied slightly between brain regions; however, the mean values in all regions examined were close to 1.0 at 0.95 +/- 0.03 (4 degrees C) and 0.91 +/- 0.04 (24 degrees C). [3H]Cytisine binding could be displaced using both nicotinic agonists and antagonists. Cytisine was the most potent displacer of [3H]cytisine binding with an Ki of 250 pM. Nicotine and acetylcholine were also potent displacers with Ki values of 1.8 and 8.1 nM, respectively. The nicotinic antagonists alpha-bungarotoxin and mecamylamine were ineffective competitors for the [3H]cytisine binding site while dihydro-beta-erythroidine had an Ki value of 109 nM. Thalamus showed the highest density of cytisine binding sites of all the regions examined (48 fmol/mg protein) while the hippocampus, cingulate gyrus and the cortex showed Bmax values of 18.9, 19.3 and 8.8 fmol/mg protein, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Central behavioral interactions between ethanol, (-)-nicotine, and (-)-cotinine in mice

A high correlation between alcohol use and smoking has long been suggested by epidemiological data. We examined the potential behavioral interactions between ethanol and nicotine using ethanol-induced motor incoordination as the test response in mice. Effect of pretreatment of various doses of (-)-nicotine, (-)-cotinine, a major metabolite of nicotine, and (+)-nicotine administered ICV on ethanol (IP)-induced motor incoordination was investigated. (-)-Nicotine (0.19, 0.38, 0.77 and, 1.54 nmoles ICV) produced significant attenuation of motor incoordination due to ethanol (2 g/kg IP) in a nearly dose-related manner which was blocked by ICV hexamethonium and trimethaphan, both purported nicotinic antagonists. (-)-Cotinine (0.35, 0.70, 1.41 nmole ICV) produced similar attenuation but was les potent than (-)-nicotine. Attenuation by (+)-nicotine (0.19, 0.38, and 0.77 nmoles ICV) was also significant but only at 0.77 nmole dose level. (+)-Nicotine-induced attenuation of motor incoordination by ethanol was antagonized by nicotinic antagonists. Data obtained suggest a central behavioral interaction between ethanol and nicotine at least through the participation of cholinergic nicotinic receptors.

Autoradiographic distribution of [3H]nicotine binding in human cortex: relative abundance in subicular complex

Distinct patterns of [3H]nicotine (3 nM) binding were apparent in various regions of adult human neo- and archicortex. Receptor binding was greatest in the subicular complex--particularly presubiculum--and entorhinal cortex, where it was prominent in the characteristic parvo- and magnocellular islands of these regions and in middle layers of entorhinal cortex. In somatosensory cortex (Brodmann areas 3, 1 and 2) and occipital (area 17) cortex binding was highest in the upper and lower layers, and relatively sparse in the sensory input, layer IV. In primary motor (area 4) and temporal (area 21) cortex, binding in the outer half of the cortical ribbon was denser than that in the inner half and a distinct band was apparent in temporal and cingulate (area 32) in the lower portion of layer III. In prefrontal association cortex the pattern of binding was less distinct although slightly higher in the lower architectonic layers. There was generally little binding in the hippocampus (areas CA1-4) and dentate gyrus with the exception of the stratum lacunosum moleculare in CA2-3 and, to a lesser extent, supra- and subgranule zones of the dentate. These patterns of reactivity, which are distinct from that of the major cortical cholinergic innervation, suggest that the nicotinic receptor, detected using nanomolar concentrations of [3H]nicotine, may primarily be associated with intracortical circuitry in the neocortex. The relatively high density in entorhinal and subicular regions may be related to the extensive phylogenetic development of these regions which has occurred in conjunction with the development of multimodal association circuitry in the human cortex.

Pre- and postnatal development of high-affinity [3H]nicotine binding sites in rat brain regions: an autoradiographic study

The ontogeny of high affinity nicotinic cholinergic binding sites was studied in Long-Evans rat brain by in vitro autoradiography, using [3H]nicotine (10 nM) and cold (-)nicotine bitartrate to assess specificity. The first binding sites become detectable in spinal cord and caudal medulla oblongata at gestational day (GD) 12. Until GD 14, labelling spreads throughout lower brainstem, mesencephalon and parts of diencephalon, with higher densities in ventral areas (including the area of developing mesencephalic dopamine neurons). Matrix zones remain unlabelled. Receptor sites appear in the cerebellar anlage by GD 15, and in caudal caudate-putamen by GD 16. During development from late gestational to early postnatal stages, labelling is reduced in many lower brainstem areas and increases in forebrain, in particular in neocortex. Receptor density remains high in thalamus. In neocortex, nicotinic receptor sites are first seen in the subplate layer by GD 20. Labelling of this zone remains prominent until PN 14, when an additional band of increased receptor density is seen in cortical layers III/IV which contain high receptor levels in adulthood. At PN 27, the pattern has become similar to the adult one. The development of [3H]nicotine-binding sites in individual brain regions, with a general caudo-rostral gradient, accompanies cell differentiation and early synapse formation, e.g., in neocortex. The ontogenetic pattern differs in detail from that of muscarinic-cholinergic binding sites. The early presence of binding sites provides a basis for specific actions of nicotine on the fetal brain. As a consequence of the ontogenetic changes, different brain structures become targets for the action of this drug at different stages of development.

Central and peripheral effects of arecoline in patients with autonomic failure

Increased plasma adrenalin (A) levels following arecoline in normal subjects and patients with multiple system atrophy (MSA) may result from nicotinic adrenal stimulation. Lack of this response in patients with pure autonomic failure (PAF) is consistent with peripheral sympathetic dysfunction. The mechanisms underlying diminished plasma corticotropin (ACTH) responses to arecoline may differ in patients with autonomic failure. Hypothalamic, cholinergic degeneration could prevent the response in MSA whereas patients with PAF do not manifest the normal increase in A which may be required to elicit an ACTH response. The appearance and exacerbation of tremor, vertigo, and pathological affect in the MSA group suggest that some central cholinergic receptors remain functional.

Purification of a nicotinic acetylcholine receptor from rat brain by affinity chromatography directed at the acetylcholine binding site

We have purified a nicotinic acetylcholine receptor from rat brain by use of an acetylcholine affinity resin commonly employed for the purification of nicotinic acetylcholine receptor from electric tissue. Receptor, specifically eluted with nicotine, bound (-)-[3H]nicotine with a dissociation constant of approximately 21 nM. Binding was inhibited by carbamylcholine but not by alpha-bungarotoxin. Polyacrylamide gel electrophoresis yielded two protein bands, of apparent mol. wts. 80,400 and 52,400. These results provide independent confirmation of the subunit size and composition reported for rat brain nicotinic receptor isolated by immunoaffinity methods and demonstrate a method of purification that can be performed with commercially available reagents.

The relevance of receptor binding studies to tobacco research

The initial event in the process by which nicotine acts on the nervous system is its interaction with specific receptor molecules in the neuronal membrane. This interaction can be characterized in radioligand binding assays, using [3H]nicotine. While this approach has generated a detailed description of recognition sites for nicotine in the brain, caution is necessary in correlating these binding proteins with physiologically relevant sites of action of nicotine. Of particular interest is the disparity between effective nicotine concentrations in binding and functional assays, which may reflect receptor desensitization and could be pertinent to the development of dependence. Receptor heterogeneity, reflecting subtypes of nicotine receptor having different radioligand binding specificities and differing properties, introduces a new dimension in the consideration of nicotine's actions. Thus, receptor binding assays have an important role to play, in conjunction with other approaches, in unravelling the complex mechanisms that can lead to nicotine dependence.

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.

Expression of nicotinic acetylcholine receptor subtypes in brain and retina

Neuronal nicotinic acetylcholine receptors (AChRs) are composed of two types of subunits: ACh-binding (termed alpha 2, alpha 3, alpha 4 ...) and structural (termed beta 2, beta 3, beta 4 ...). AChR subtypes composed of combinations of subunits of these two types encoded by several related genes are expressed in different parts of the nervous system, where they presumably serve different functional roles. Here we identify the ACh-binding subunit of the most prominent chicken brain AChR subtype by N-terminal amino acid sequence and show that it corresponds to the alpha 4 gene. Previously we identified the structural subunit for this AChR subtype from chicken brain as beta 2 by N-terminal amino acid sequence. Thus, this identifies both genes which encode subunits of the major nicotinic AChR subtype in avian brains. By immunoprecipitation, immunohistochemistry, and northern blot analysis we show that alpha 3 (or a very closely related sequence) is expressed at low levels in the brain and relatively high levels in the retina, while alpha 4 is expressed at high levels in the brain and lower levels in the retina. This differential expression indicates that alpha 3-containing 'ganglionic-type' AChRs may be an important AChR subtype in avian retina.

A comparison of the binding of nicotine and nornicotine stereoisomers to nicotinic binding sites in rat brain cortex

Both stereoisomers of nicotine and nornicotine were tested for their ability to competitively displace 3H-(-)-nicotine and 3H-acetylcholine (in the presence of atropine), in rat cortex tissue. 3H-acetylcholine was displaced from two binding sites, super-high and high, by (+)-nicotine, (-)-nornicotine and (+)-nornicotine but from a high affinity site by (-)-nicotine. 3H-nicotine was displaced from two sites, high and low affinity by nicotine and nornicotine stereoisomers. The high-affinity 3H-(-)-nicotine binding site showed similar binding characteristics to one of the sites labelled by 3H-acetylcholine. IC50 values showed (-)-nicotine to be 13 and 25-fold more potent than (+)-nicotine for displacing 3H-(-)-nicotine and 3H-acetylcholine, respectively, but no difference was observed for nornicotine stereoisomers. While (-)-nicotine preferentially bound to the high affinity site of 3H-(-)-nicotine (+)-nicotine preferred the low affinity site. The study provides further evidence for multiple nicotine receptors in brain.

Prejunctional nicotinic receptors involved in facilitation of stimulation-evoked noradrenaline release from the vas deferens of the guinea-pig

In guinea-pig prostatic vas deferens loaded with [3H]-noradrenaline ([3H]-NA), nicotinic receptor agonists, nicotine and dimethylphenylpiperazinium (DMPP) enhanced the resting and facilitated the stimulation-evoked release of [3H]-NA in a concentration-dependent fashion. The effect of nicotine on both contraction of vas deferens and release of NA in response to field stimulation was stereospecific in favour of the naturally occurring (-)-enantiomer. Prolonged (15 min) exposure to (-)-nicotine resulted in a cessation of the facilitatory effect on NA release and on responses of the vas deferens to field stimulation. 2 The rank order of agonist potency in facilitating NA release was DMPP = (-)-nicotine greater than (+)-nicotine. Cytisine had no agonistic activity. The dissociation constants (KD) of antagonists were 9.3 +/- 0.6 and 31.4 +/- 2.4 microM for (+)-tubocurarine and hexamethonium, respectively, when (-)-nicotine was used as agonist. alpha-Bungarotoxin had no antagonistic activity. These findings suggest that nicotinic receptors located on noradrenergic axon terminals are different from those located postsynaptically in striated muscle or ganglia but seem similar to those present on cholinergic axon terminals at the neuromuscular junction. 3. Cotinine, the breakdown product of nicotine failed to have any agonistic activity indicating that nicotine itself is responsible for the effects observed on axon terminals. 4 Stimulation of presynaptic muscarinic receptors by oxotremorine prevented the nicotine-induced facilitation of [3H]-NA release, indicating the presence of both inhibitory muscarinic and facilitatory nicotinic receptors on noradrenergic axon terminals.

Evidence for functional activity of up-regulated nicotine binding sites in rat striatal synaptosomes

A number of studies have found that the chronic administration of nicotine causes an increase in the density of nicotinic binding sites in the brain, but it is not known whether these additional binding sites are functionally active receptors. In this study, the effects of 1-week administration of the potent nicotinic agonist, (+)-anatoxin-a (96 nmol/day via osmotic minipumps), was assessed on [3H]nicotine binding and [3H]dopamine uptake and release in rat striatal synaptosomes. Chronic (+)-anatoxin-a treatment resulted in a 32% increase in the Bmax of [3H]nicotine binding in anatoxin-treated animals compared to control. There was a 43% increase in the activity of 3 microM nicotine to release [3H]dopamine from synaptosomes of anatoxin-treated animals, but the release induced by 20 mM K+ depolarization was unaffected. There was no effect of chronic (+)-anatoxin-a treatment on the uptake of [3H]dopamine. A strong positive correlation (r = 0.64) was found between the density of [3H]nicotine binding sites and the nicotine-induced stimulation of [3H]dopamine release in individual animals. These results indicate that (+)-anatoxin-a, like nicotine, produces an up-regulation of nicotine binding sites following chronic administration, and that these additional sites are functional receptors capable of mediating the release of dopamine from striatal synaptosomes.

Effect of nicotine and tacrine on acetylcholine release from rat cerebral cortical slices

The effect of nicotine (1-10 microM) and tacrine (9-amino-1,2,3,4-tetrahydroacridine; THA) on stimulation evoked release of [3H]acetylcholine from the rat brain slice preparation preincubated with [3H]choline was investigated. In these preparations, nicotine enhanced while tacrine inhibited evoked [3H]acetylcholine release. These effects were blocked by (+)tubocurarine (1 microM) and atropine (0.1 microM) respectively. In the presence of idazoxan (0.3 microM) plus atropine (0.1 microM), nicotine (3 microM) continued to enhance evoked [3H]acetylcholine release while the inhibitory effect of tacrine (1 microM) on evoked [3H]acetylcholine release was reversed to an enhancement. Under these circumstances the effects of both nicotine and tacrine were blocked by (+)tubocurarine (1 microM). These findings demonstrate that tacrine can both inhibit or enhance [3H]acetylcholine release, most likely through its activity as a cholinesterase inhibitor. Under normal circumstances following tacrine the predominant effect of the elevated levels of acetylcholine will be activation of inhibitory presynaptic muscarine receptors on cholinergic nerves and an inhibition of evoked [3H]acetylcholine release. Under conditions where both presynaptic inhibitory muscarine and alpha 2-adrenoceptors are blocked, the elevated levels of acetylcholine produced by tacrine will lead to the activation of facilitatory presynaptic nicotine cholinoceptors on cholinergic nerves and an enhancement of evoked [3H]acetylcholine release.

Molecular cloning of human neuronal nicotinic receptor alpha 3-subunit

Neuronal nicotinic receptors (nAchRs) have been isolated or cloned in insect, bird and mammalian neurons, but no information exists on the primary structure of human neuronal nAchRs. By screening a cDNA library from the human neuroblastoma cell line IMR 32 with a cDNA probe corresponding to the full length of rat alpha 3-nicotinic subunit, we have identified an open reading frame encoding a protein of 502 amino acids. This protein shows all the features of members of the ligand-gated receptor superfamily and has two cysteine residues at positions 192, 193 which are typical of the nicotinic alpha-subunits. Because of its high homology to rat alpha 3 (93% amino acid identity), we conclude that we have cloned the human alpha 3-nicotinic subunit.

Nicotinic modulation of [3H]dopamine release from striatal synaptosomes: pharmacological characterisation

Presynaptic nicotinic acetylcholine receptors on striatal nerve terminals modulate the release of dopamine. We have compared the effects of a number of nicotinic agonists and antagonists on a perfused synaptosome preparation preloaded with [3H]dopamine. (-)-Nicotine, acetylcholine, and the nicotinic agonists cytisine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), at micromolar concentrations, stimulated the release of [3H]dopamine from striatal nerve terminals. Carbamylcholine was a much weaker agonist. The actions of (-)-nicotine, cytisine, and DMPP were inhibited by low concentrations of the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, pempidine, and neosurugatoxin; alpha-bungarotoxin was without effect, and extending the time of exposure to this toxin resulted in only very modest inhibition. This pharmacology points to a specific nicotinic receptor mechanism that is clearly distinct from that at the neuromuscular junction. Atropine failed to antagonise the effects of acetylcholine and carbamylcholine, suggesting that no muscarinic component is involved. The nicotinic receptor ligands (-)-[3H]nicotine and 125I-alpha-bungarotoxin bound to specific sites enriched in the synaptosome preparation. Drugs tested on the perfused synaptosomes were examined for their ability to interact with these two ligand binding sites in brain membranes. The differential sensitivity to the neurotoxins alpha-bungarotoxin and neosurugatoxin of the 125I-alpha-bungarotoxin and (-)-[3H]nicotine binding sites, respectively, leads to a tentative correlation of the (-)-[3H]nicotine site with the presynaptic nicotinic receptor on striatal nerve terminals.

Desensitization of central cholinergic mechanisms and neuroadaptation to nicotine

This review focuses on neuroadaptation to nicotine. The first part of the paper delineates some possible general mechanisms subserving neuroadaptation to commonly abused drugs. The postulated role of the mesocorticolimbic neuroanatomical pathway and drug-receptor desensitization mechanisms in the establishment of tolerance to, dependence on, and withdrawal from psychoactive drugs are discussed. The second part of the review deals with the pharmacological effects of nicotine at both pre- and postsynaptic locations within the central nervous system, and the still-perplexing upregulation of brain nicotine-binding sites seen after chronic nicotine administration. A special emphasis has been put on desensitization of presynaptic cholinergic mechanisms, and postsynaptic neuronal nicotinic-receptor function and its modulation by endogenous substances. A comparison with the inactivation process occurring at peripheral nicotinic receptors is also included. Finally, a hypothesis on the possible connections between desensitization of central cholinergic mechanisms and neuroadaptation to nicotine is advanced. A brief comment on the necessity of fully understanding the effects of nicotine on the developing nervous system closes this work.

Postnatal development of two nicotinic cholinergic receptors in seven mouse brain regions

The developmental profiles for binding of alpha-[125I]bungarotoxin and L-[3H]nicotine to putative nicotinic cholinergic receptors were determined in seven mouse brain regions. The overall pattern of development of alpha-bungarotoxin binding was similar in all of the regions. Neonatal binding values tended to be greater than those observed in adult brain regions. Maximal binding occurred within 10 days of birth and adult binding values were reached by 20 days of age. The patterns of development of nicotine binding in each of the seven brain regions differed according to region. Gross similarities in developmental profiles for nicotine binding were found among the more caudal and among the more rostral regions. In hindbrain and cerebellum, maximal nicotine binding was found at birth (5 days of age in cerebellum); binding declined approximately 4-fold by 20 days and remained relatively constant thereafter. In midbrain and hypothalamus, a less extensive decrease in nicotine binding occurred from birth to adulthood (midbrain, 25%; hypothalamus, 50%). Nicotine binding in hippocampus and cortex remained unchanged between birth and adulthood. The developmental pattern for nicotine binding in striatum differed from that found in the other brain regions. At 5 days of age, binding was about 65% of adult binding, which was reached at 30 days of age. In most of the brain regions the developmental profile for alpha-bungarotoxin binding was different from that of nicotine. This difference was especially notable in striatum, where adult nicotine binding was higher than neonatal nicotine binding, whereas adult alpha-bungarotoxin binding was lower than neonatal alpha-bungarotoxin binding.

Purification of L-[3H]nicotine eliminates low affinity binding

Some studies of L-[3H]nicotine binding to rodent and human brain tissue have detected two binding sites as evidenced by nonlinear Scatchard plots. Evidence presented here indicates that the low affinity binding site is not stereospecific, is not inhibited by low concentrations of cholinergic agonists and is probably due to breakdown products of nicotine since purification of the L-[3H]nicotine eliminates the low affinity site.

Influence of development and aging on nicotinic receptor subtypes in rodent brain

The influence of development and aging on nicotinic receptor subtypes in rodent brain was investigated. 3H-nicotine and 3H-acetylcholine (3H-ACh) were used as receptor ligands. Specific binding sites for 3H-nicotine and 3H-ACh were detected in mouse brain during the late prenatal period. A drop in the number of 3H-nicotine and 3H-ACh binding sites was measured shortly after birth. The 3H-nicotine and 3H-ACh binding sites showed different time courses during prenatal development. Competition experiments using unlabelled (-)nicotine and 3H-nicotine revealed one population of high affinity nicotinic binding sites in the cortex of 1-day and 5-day-old mice whereas both a set of high and low affinity binding sites were found in adult mice. The proportion of cortical high and low affinity nicotinic binding sites did not change with aging although the absolute amount of high affinity nicotinic binding sites decreased. The 3H-nicotine binding showed different temperature dependence in rat brain of different ages. The results illustrate dynamic changes in nicotinic receptor properties during life span of rodents.

Presynaptic cholinergic mechanisms in the rat cerebellum: evidence for nicotinic, but not muscarinic autoreceptors

The present study shows that N-[3H]methylcarbamylcholine ([3H]MCC) binds to a single population of high-affinity/low-density (KD = 5.0 nM; Bmax = 8.2 fmol/mg of protein) nicotinic binding sites in the rat cerebellum. Also, there exists a single class of high-affinity binding sites (KD = 4.8 nM; Bmax = 24.2 fmol/mg of protein) in the cerebellum for the M1 specific muscarinic ligand [3H]pirenzepine. In contrast, the M2 ligand, [3H]AF-DX 116, appears to bind to two classes of binding sites, i.e., a high-affinity (KD = 3 nM)/low-capacity (Bmax = 11.7 fmol/mg of protein) class, and a second class of lower affinity (KD = 28.4 nM) and higher capacity (Bmax = 36.3 fmol/mg of protein) sites. The putative M3 selective ligand [3H]4-diphenylacetoxy-N-methylpiperidine also binds to two distinct classes of binding sites in cerebellar homogenates, one of high affinity (KD = 0.5 nM)/low capacity (Bmax = 19.5 fmol/mg of protein) and one of low affinity (KD = 57.5 nM)/high capacity (Bmax = 140.6 fmol/mg of protein). In experiments which tested the effects of cholinergic drugs on acetylcholine release from cerebellar brain slices, the nicotinic agonist MCC enhanced spontaneous acetylcholine release in a concentration-dependent manner, and the maximal increase in acetylcholine release (59.0-68.0%) occurred at 10(-7) M. The effect of MCC to increase acetylcholine release was Ca2+-dependent and tetrodotoxin-insensitive, suggesting an action on cholinergic terminals. Also, the MCC-induced increase in acetylcholine release was effectively antagonized by dihydro-beta-erythroidine, d-tubocurarine, and kappa-bungarotoxin, but was insensitive to either atropine or alpha-bungarotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)