Brain nicotine binding sites

Monospecific antibodies against a synthetic peptide predicted from the alpha-3 nicotinic receptor cDNA inhibit binding of [3H]nicotine to rat brain nicotinic cholinergic receptor

Polyclonal antibodies were raised against a synthetic decapeptide (designated S3) predicted from a segment of the alpha-3 subunit cDNA (amino acid residues 130-139) encoding the rat brain nicotinic cholinergic receptor. This segment was selected because it may be proximate to the nicotine/acetylcholine-binding site of the receptor (1). By radioligand binding assays and sucrose density gradient centrifugation, these monospecific antibodies were shown to inhibit the binding of [3H]nicotine to both the large molecular weight rat brain receptor (240 kDa) and to an SDS-disaggregated nicotine-binding subunit species (80 kDa), in a dose-dependent manner. The neutralizing effect of the anti-S3 antibodies supports the view that this region of the protein is closely related to the agonist binding site.

Relations between structure and nicotine-like activity: X-ray crystal structure analysis of (-)-cytisine and (-)-lobeline hydrochloride and a comparison with (-)-nicotine and other nicotine-like compounds

1. Although (-)-cytisine is a rigid structure, it occurs in the crystal in two distinct but very similar conformations in which the pyridone ring is tilted relative to the charged nitrogen atom at much the same angle as the pyridine ring is in (-)-nicotine hydrogen iodide. The carbonyl group in the pyridone ring of (-)-cytisine, however, is on the side of the ring opposite to pyridine nitrogen in (-)-nicotine. 2. The pKa of (-)-lobeline HCl at 25 degrees C is 8.6 (approx), indicating that (-)-lobeline is at least 90% in the protonated form at physiological pH (7.6). It is probably the phenyl 2-keto-ethyl part of (-)-lobeline, rather than the phenyl 2-hydroxy-ethyl part, which interacts with the receptor. 3. The combination within one molecule of a charged ('onium') nitrogen atom lying out of the plane of, and some distance (4.5-6.5 A) from, an aromatic ring is common to many compounds with nicotine-like activity (e.g. nicotine, cytisine, choline phenyl ether bromide, dimethyl-phenyl-piperazinium (DMPP) iodide, coryneine iodide and m-hydroxyphenylpropyl trimethyl ammonium iodide). In some molecules the aromatic ring can be replaced by an unsaturated group, such as carbonyl (e.g. acetylcholine) or double-bonds (e.g. anatoxin). 4. Activity at nicotinic receptors appears to involve interactions between the positively charged nitrogen atom and a negatively charged group, probably close to cysteine residues 192 and 193 in the receptor. It is suggested that rather than specific groups in the molecule also being involved, activity at nicotinic receptors depends on interactions between a flat part of the drug containing double-bonds, or systems of double bonds, and a planar area in the receptor, possibly tyrosine or phenylalanine residues.

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.

Uptake and regional distribution of (+)-(R)- and (-)-(S)-N-[methyl-11C]-nicotine in the brains of rhesus monkey. An attempt to study nicotinic receptors in vivo

N-[methyl-11C] nicotine (11C-nicotine) was given intravenously to monkeys and the uptake and regional distribution of radioactivity was followed in the brain using positron emission tomography (PET). The 11C-radioactivity in the brain peaked within 1-2 min and then rapidly declined. Pretreatment with unlabelled nicotine (10 micrograms/kg) reduced the uptake of 11C-radioactivity to the brain by 30%. The uptake of radioactivity was higher following (+)11C-nicotine than (-)11C-nicotine. Both enantiomers were distributed in a similar manner within the brain. When animals were infused with a peripheral nicotinic blocker (trimetaphan) the uptake of radioactivity to the brain was lower following (+)11C-nicotine compared to (-)11C-nicotine. The amount of radioactivity was high in the occipital cortex, thalamus, intermediate in the frontal cortex and low in white matter in (-)11C injected monkeys while no regional difference in distribution of 11C-radioactivity was observed after injection of (+)11C-nicotine.

Effect of chronic nicotine treatment on nicotinic autoreceptor function and N-[3H]methylcarbamylcholine binding sites in the rat brain

It has been reported that N-methylcarbamylcholine (MCC), a nicotinic agonist, binds to central nicotinic receptors and causes an increase of acetylcholine (ACh) release from certain central cholinergic nerve terminals. The present experiments determine whether these two phenomena change in response to the chronic administration of nicotine, a procedure known to result in an increase in nicotinic binding sites. Chronic nicotine caused a brain region-specific up-regulation of [3H]MCC sites; binding increased in the frontal cortex, parietal cortex, striatum, and hippocampus, but not in the occipital cortex or cerebellum. The effect of nicotine was selective to nicotinic binding sites, because muscarinic sites, both M1 ([ 3H]pirenzepine) and M2 ([3H]ACh), were unaffected by chronic nicotine treatment. MCC increased the release of ACh from the frontal cortex and hippocampus by a calcium-dependent mechanism; MCC did not alter ACh release from striatum or occipital cortex of control animals. The MCC-induced increase in ACh release was not apparent in those animals which had been treated with nicotine. There was a partial recovery of nicotinic autoreceptor function when animals were allowed to recover (4 days) following chronic nicotine treatment, but the density of binding sites remained increased compared to control. Chronic nicotine did not change the potassium-evoked release of ACh from the frontal cortex or hippocampus, but decreased this measure from striatum. It also decreased the ACh content of the striatum, but not that of the cortex or the hippocampus; the activity of choline acetyltransferase was not altered in any of the regions tested.(ABSTRACT TRUNCATED AT 250 WORDS)

High-affinity and selectivity of neosurugatoxin for the inhibition of 22Na influx via nicotinic receptor-ion channel in cultured bovine adrenal medullary cells: comparative study with histrionicotoxin

In cultured bovine adrenal medullary cells, neosurugatoxin and histrionicotoxin inhibited carbachol-induced influx of 22Na, 45Ca and secretion of catecholamines with IC50 of 27 nM and 3 microM, respectively. The inhibitory effects of neosurugatoxin were reversed by the increased concentrations of carbachol, whereas those of histrionicotoxin were not. Histrionicotoxin at concentrations higher than 10 microM also reduced veratridine-induced influx of 22Na, 45Ca and secretion of catecholamines, while neosurugatoxin had no effects. High K-induced 45Ca influx and catecholamine secretion were not altered by either neosurugatoxin or histrionicotoxin. The present findings suggest (1) neosurugatoxin competitively inhibits nicotinic receptor-ion channel complex at nanomolar concentrations, but has no effects on voltage-dependent Na channel and voltage-dependent Ca channel; (2) histrionicotoxin at micromolar concentrations non-competitively suppresses nicotinic receptor-ion channel complex. Higher concentrations of histrionicotoxin also interferes with voltage-dependent Na channel, but has no effect on voltage-dependent Ca channel; (3) neosurugatoxin, due to its high-affinity and selectivity, may be a useful probe for studying nicotinic receptors in nervous tissues.

Neuroregulatory effects of nicotine

The impact of nicotine on the central nervous system is, in an important sense, neuroregulatory, with cascading effects on physiological and biochemical function as well as on behavioral activity. Accordingly, the neurotransmitter and neuroendocrine effects of nicotine constitute a critical part of its biological action, which includes reinforcing as well as pathophysiological consequences. This review focuses on nicotine's effects on cholinergic and non-cholinergic nicotine receptors and on the responses of catecholamines, monoamines, hypophyseal hormones, and cortisol. The contribution of critical variables, such as timing and duration of neuroregulator release and the patterns that make up the total response, is still largely unknown, particularly with regard to the effects of environmental context, history of nicotine use, and mode of administration. The evidence suggests that by altering the bioavailability of the above-listed neuroregulators, nicotine serves as a pharmacological "coping response", providing immediate though temporary improvement in affect or performance in response to environmental demands. Much of what is known to date is based on studies involving the administration of agonists and antagonists under different environmental conditions. Newer technological approaches such as autoradiography and positron emission tomography show potential for determining the neuroregulatory patterns involved and specifying nicotine's locus of action relevant to its behavioral and physiological effects.

[3H](-)nicotine binding sites in fetal human brain

The development of putative nicotinic binding sites in brains from human fetuses of 12-19 weeks gestation was studied. The binding of [3H](-)nicotine to fetal human brain membranes, using a rapid filtration method, was saturable and stereospecific. Scatchard analysis revealed a single class of high affinity sites with a Kd of 1.5 +/- 0.5 nM and a Bmax of 4.5 +/- 1.9 fmol/mg protein (n = 11). [3H](-)nicotine binding increased between the ages of 12 and 19 weeks in human fetal brain (r = 0.63, n = 20, P less than 0.01). In competition studies nicotinic agonists were the most effective in inhibiting [3H](-)nicotine binding whereas antagonists were relatively ineffective. Ki values for displacing ligands in the presence of [3H](-)nicotine were: cytisine, 1.6 nM; (-)nicotine, 16 nM; (+) nicotine, 510 nM; dihydro-beta-erythroidine, 1.9 microM; dimethyl-4-phenylpiperazinium, 6.5 microM; choline chloride, 25 microM. Atropine and alpha-bungarotoxin failed to inhibit binding up to 50 microM. Comparison of dissected brain regions revealed regional variations in the density of nicotinic binding sites: specific binding of [3H](-)nicotine was greatest in the nucleus basalis of Meynert, globus pallidus, caudate-putamen and thalamus, and lowest in the medulla. These results are interpreted in relation to the development of functional cholinergic transmission in human fetal brain, and the potential vulnerability of this system to maternal tobacco usage.

Characterization of N-[3H]methylcarbamylcholine binding sites and effect of N-methylcarbamylcholine on acetylcholine release in rat brain

The present experiments show that N-[3H]-methylcarbamylcholine ([3H]MCC) binds specifically and with high affinity to rat hippocampus, frontal cortex, and striatum. The highest maximal density of binding sites was apparent in frontal cortex and the lowest in hippocampus. [3H]MCC binding was potently inhibited by nicotinic, but not muscarinic, agonists and by the nicotinic antagonist dihydro-beta-erythroidine in all three brain regions studied. The effect of unlabeled MCC on acetylcholine (ACh) release from slices of rat brain was tested. The drug significantly enhanced spontaneous ACh release from slices of hippocampus and frontal cortex, but not from striatal slices. This effect of MCC to increase ACh release from rat hippocampus and frontal cortex was antagonized by the nicotinic antagonists dihydro-beta-erythroidine and d-tubocurarine, but not by alpha-bungarotoxin or by the muscarinic antagonist atropine. The MCC-induced increase in spontaneous ACh release from hippocampal and frontal cortical slices was not affected by tetrodotoxin. The results suggest that MCC might alter cholinergic transmission in rat brain by a direct activation of presynaptic nicotinic receptors on the cholinergic terminals. That this alteration of ACh release is apparent in hippocampus and frontal cortex, but not in striatum, suggests that there may be a regional specificity in the regulation of ACh by nicotinic receptors in rat brain.

Stereoselective nicotine-induced release of dopamine from striatal synaptosomes: concentration dependence and repetitive stimulation

Using a sensitive perfusion system we have studied the nicotine-induced release of [3H]dopamine ([( 3H]DA) from striatal synaptosomes. Nicotine-evoked release was concentration dependent with an EC50 of 3.8 microM. The response to 1 microM nicotine was comparable to that to 16 mM K+; 10 microM veratridine evoked a larger response. All three stimuli were Ca2+ dependent but only the response to veratridine was blocked by tetrodotoxin. Repetitive stimulations by 1 microM (-)-nicotine (100 microliters) at 30-min intervals resulted in similar levels of [3H]DA release; higher concentrations of (-)-nicotine resulted in an attenuation of the response particularly following the third stimulation. This may reflect desensitisation or tachyphylaxis of the presynaptic nicotinic receptor. The action of nicotine was markedly stereoselective: a 100-fold higher concentration of (+)-nicotine was necessary to evoke the same level of response as 1 microM (-)-nicotine. It is proposed that these presynaptic nicotinic receptors on striatal terminals are equivalent to high-affinity nicotine binding sites described in mammalian brain.

High affinity binding of [3H] (-)-nicotine to rat brain membranes and its inhibition by analogues of nicotine

Analysis of the characteristics of cerebral binding of [3H] (-)-nicotine revealed a single population of sites with high affinity (KD = 6.0 +/- 0.6 nM). The regional distribution of the binding of [3H] (-)-nicotine was heterogeneous with the largest concentration of binding sites being in the thalamus, cortex and striatum, and a low level of binding activity in the cerebellum and hypothalamus. Competition studies showed that several metabolites and congeners of nicotine potently competed with [3H] (-)-nicotine and their in vitro activity was correlated with behavioural activity, as estimated from previously published data for rats trained to discriminate central effects of nicotine. Conversely, nicotine antagonists, with the exception of dihydro-beta-erythroidine, were weak or inactive in this binding assay. It is concluded that the binding site for [3H] (-)-nicotine investigated probably mediates at least one of the behavioural effects of nicotine, the nicotine discrimative stimulus.

Methyllycaconitine and (+)-anatoxin-a differentiate between nicotinic receptors in vertebrate and invertebrate nervous systems

Specific high-affinity binding sites for 125I-alpha-bungarotoxin and (-)-[3H]nicotine have been measured in rat brain and locust (Schistocerca gregaria) ganglia. The binding sites for 125I-alpha-bungarotoxin had similar Kd values of 1.5 x 10(-9) and 0.8 x 10(-9) M for rat and locust preparations, respectively; the corresponding values for the (-)-[3H]nicotine-binding site were 9.3 x 10(-9) and 1.7 x 10(-7) M. Methyllycaconitine (MLA) potently inhibited 125I-alpha-bungarotoxin binding in both rat and locust. MLA was a less effective inhibitor of (-)-[3H]nicotine binding whereas (+)-anatoxin-a was a very potent inhibitor at this site in the rat but not in the locust. These data suggest that (+)-anatoxin-a is a useful probe for the high-affinity nicotine-binding receptor in vertebrate brain, whereas MLA is a preferential probe for the subclass of receptor that binds alpha-bungarotoxin.

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.