Regulatory effects of acutely repeated nicotine treatment towards central muscarinic receptors.

Sensitivity of brain muscarinic acetylcholine receptors to the agonists was examined in nicotine tolerant animals which were developed by acutely repeated injections of nicotine. In conscious rats, the dose-response curves of muscarinic agonists arecoline and pilocarpine, cholinesterase inhibitors soman and physostigmine rather than GABA receptor antagonist pentylenetetrazol or glycine receptor antagonist strychnine for producing EEG seizures were shifted leftwards by acutely repeated injections of nicotine. This phenomenon could be prevented by nicotinic antagonist mecamylamine. Similar results were obtained in acute nicotine tolerant mice and rabbits. In other experiments, the dose-response curve of arecoline-induced convulsions or oxotremorine-induced tremors was also shifted leftwards, and the durations of arecoline- and oxotremorine-induced tremors were prolonged in acute nicotine tolerant mice. In addition, the effects of arecoline for producing down-regulation of muscarinic receptors of rat cerebrum and hippocampus rather than brain stem were potentiated in acute nicotine tolerant rats. It is concluded that the sensitivity of brain muscarinic receptor to its agonists is increased in acute nicotine tolerant rats, mice and rabbits.

Modulation of apomorphine-induced rotations in unilaterally 6-hydroxydopamine lesioned rats by cholinergic agonists and antagonists.

In the present study, we examined the effects of the agonists and antagonists of cholinergic receptors on central dopaminergic function using the 6-hydroxydopamine model of dopamine receptor supersensitivity. Unilateral lesioning of the substantia nigra with 6-hydroxydopamine was carried out in Wistar rats. Two weeks after surgery, the rats were tested for the presence of dopaminergic supersensitivity by their response to the dopamine receptor agonist, apomorphine. Apomorphine-induced rotations were significantly reinforced by the muscarinic receptor antagonist, atropine. In contrast to atropine, the muscarinic receptor agonist oxotremorine attenuated apomorphine's effects. Acute treatment of nicotine significantly reduced apomorphine-induced rotations. However, when increasing doses of nicotine were given for nine days, the rotations of the nicotine-dependent rats were significantly enhanced. So the fact that both muscarinic and nicotinic cholinergic activity could modulate apomorphine-induced rotations was readily apparent in these experiments.

Anticholinergic activity in mice and receptor-binding properties in rats of a series of synthetic tropane derivatives.

A tropane ester, three tropane ethers, atropine and mecamylamine were compared in mice for their anti-muscarinic and anti-nicotinic activity against arecoline-induced tremor and nicotine-induced convulsions, respectively. Their receptor-binding characteristics were studied in neuronal membranes prepared from rat cerebral cortex. The results showed that the tropane ester, 2 alpha R-tropanyl benzylate, was more potent than atropine in its anti-muscarinic activity, but the anti-muscarinic activity of the tropane ethers, 2 alpha-(2',2'-diphenyl-2'-hydroxy-ethoxy)tropane (alpha-DPT) and its two isomers (1R,2 alpha R- and 1S,2 alpha S-) were less potent than that of atropine. In contrast with their anti-muscarinic potency, 2 alpha R-tropanyl benzylate and the three tropane ethers were equipotent in their anti-nicotinic activities. The order of potencies of these compounds to displace the binding of [3H]quinuclidinyl benzylate to brain membranes was similar to that of their anti-muscarinic potencies. The binding of [3H]nicotine to nicotinic receptors from brain was not inhibited by these compounds. Analyses of structure-activity relationships of these compounds suggested that it is the ester groups that determine the anti-muscarinic potencies of 2 alpha R-tropanyl benzylate; their anti-nicotinic activities were independent of the structural changes and of the anti-muscarinic activities of these compounds.

Anti-nicotinic properties of anticholinergic antiparkinson drugs.

The nature of the antagonism by anticholinergic compounds of nicotine-induced convulsion in mice has not been defined clearly. Although, because they do not compete effectively for agonist binding to brain tissue in-vitro, these compounds are thought to be non-competitive antagonists in the brain, pharmacological evidence is lacking. This study describes the anti-nicotinic properties of the clinically used anticholinergic antiparkinson drugs, benztropine, biperiden, caramiphen, ethopropazine, procyclidine and trihexyphenidyl. Nicotine-induced convulsion and arecoline-induced tremor in mice were effectively prevented by these drugs. The concentrations of benztropine, biperiden, caramiphen, ethopropazine, procyclidine and trihexyphenidyl affording 50% prevention of nicotine-induced convulsion (ED50 values) were 7.4, 4.6, 7.8, 4.9, 3.1 and 3.3 mg kg(-1), respectively. The classical muscarinic receptor antagonist atropine had potent anti-muscarinic effects but very weak anti-nicotinic activity. The classical nicotinic receptor antagonist mecamylamine had potent anti-nicotinic activity but no anti-muscarinic effects. The pattern of shift of the dose-response curve for nicotine-induced convulsion in mice was determined in the presence of increasing concentrations of the anticholinergic antiparkinson drugs. These drugs were found to increase the ED50 (0.49 mg kg(-1)) of nicotine-induced convulsion in a dose-related manner. The maximum effect of nicotine and the slope of nicotine dose-response curve were not significantly influenced by either low or high doses of benztropine, procyclidine or trihexylphenidyl, which suggests competitive action. Biperiden, caramiphen and ethopropazine, at low doses which significantly increased the ED50 of nicotine, did not affect the maximum effect of nicotine or the slope of the nicotine dose-response curve; at higher doses, however, they reduced the maximum effect and the slope, which suggests that these drugs have both competitive and non-competitive properties in antagonizing nicotine-induced convulsion in mice. The experiments demonstrate that the anticholinergic antiparkinson drugs and mecamylamine effectively antagonize nicotine-induced convulsion, but atropine does not; some of these drugs have competitive properties whereas others seem to have both competitive and non-competitive properties in antagonizing nicotine-induced convulsion in mice.

Modulation by nicotine on binding of cerebral muscarinic receptors with muscarinic agonist and antagonist.

AIM: To study the modulatory effects of nicotine on the binding of brain muscarinic receptors. METHODS: The binding of brain muscarinic receptors with the agonist [3H] oxotremorine-M or the antagonist l-[3H]QNB was determined in the presence/absence of nicotine. RESULTS: Pre-incubation of the membrane fraction derived from rat cerebral cortex with nicotine 1.0 mumol.L-1 led to a decrease in the dissociation constant (Kd) for [3H] oxotremorine-M binding to muscarinic receptors, while the maximal binding value (Bmax) was unchanged. The Kd value for binding of the muscarinic antagonist l-[3H]QNB was concentration-dependently increased by preincubation with nicotine 0.1 nmol.L-1-10.0 mumol.L-1, with Bmax unchanged. The effect of nicotine on the Kd for l-[3H]QNB binding was prevented by mecamylamine 10 nmol.L-1, but was enhanced by dithiothreitol 10 mumol.L-1, which by itself was also capable of increasing the Kd value. CONCLUSION: Nicotine increases the affinity of brain muscarinic receptors for muscarinic agonist, but decreases the affinity of brain muscarinic receptors for muscarinic antagonist.

Pharmacological activity and receptor-binding characteristics of 2 beta-(2'-phenyl-2'-cyclopentyl-2'-hydroxy-ethoxy)tropane and its optical isomers.

This study describes the receptor-binding characteristics, antimuscarinic and antinicotinic activities of 2 beta-(2'-phenyl-2'-cyclopentyl-2'-hydroxy-ethoxy)tropane (beta-PCT) and its four optical isomers. Both arecoline-induced tremor and nicotine-induced convulsion in mice were antagonized by beta-PCT and its optical isomers. These compounds were less potent than atropine in their antimuscarinic potencies, but more potent than atropine in their antinicotinic activities. The isomer with the 1S-2 beta-2'R configuration was about one order of magnitude more potent than the isomer with the 1R-2 beta-2'S configuration in their antimuscarinic activity, but the antinicotinic potencies of these compounds did not differ significantly. The order of potencies of beta-PCT and its optical isomers to displace the specific binding of [3H]quinuclidinyl benzilate to muscarinic receptors was similar to that of their antimuscarinic potencies. The binding of [3H]nicotine to central nicotinic receptors was not inhibited by these compounds. The findings indicate that beta-PCT and its optical isomers are useful affinity ligands to examine the biochemical and functional characteristics of brain cholinergic receptors.

Effects of nicotine on 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine-induced depression of striatal dopamine content and spontaneous locomotor activity in C57 black mice.

The present study examined the effects of nicotine on the levels of dopamine and its metabolite 3,4-dihydroxy-phenylacetic acid (DOPAC) in the striatum and on spontaneous locomotor activity of C57 black mice that had been treated with the neurotoxin 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP). The levels of dopamine and DOPAC in the striatum were significantly reduced 10 days after two injections of MPTP (40 mg kg-1, i.p., 24 h apart) in mice. Chronic treatment of nicotine (2.0 mg kg-1, s.c., four injections per day) did not influence the striatal content of dopamine and DOPAC, but it significantly antagonised MPTP-induced depression of dopamine. However, the chronic nicotine treatment did not significantly affect MPTP-induced depression of DOPAC. It was demonstrated that the lethal effect of MPTP was also partly protected by the chronic nicotine treatment. The chronic nicotine treatment also significantly protected MPTP-depressed spontaneous locomotor activity in mice. The results suggest that both MPTP-depressed behaviour and MPTP-induced striatal dopamine depletion in C57 black mice are partly protected by chronic nicotine treatment.

Identification of a C3b/iC3 binding protein of rabbit platelets and leukocytes. A CR1-like candidate for the immune adherence receptor.

Immune adherence is the attachment of C-bearing immune complexes via the major activation fragment of the third component of C(C3b) to C3b binding membrane proteins. On primate E, the C3b-R, termed CR1, mediates immune adherence. In nonprimates, immune adherence involves platelets instead of E. However, these functional data have not been corroborated by the identification of the binding protein. In this work, we have identified a C3b/iC3 binding protein of rabbit platelets and characterized it as a single chain structure with a Mr of 150 kDa (nonreducing) or 175 kDa (reducing). This protein binds to rabbit iC3 or C3b but not C3d. This specificity of binding and the ability to rebind to a second column of iC3- or C3b-thiol-Sepharose are comparable to human CR1. Also, a molecule with the identical Mr as well as other structural and binding characteristics is present on rabbit PBMC. No such protein was isolated from rabbit E. Our data strongly suggest that the C3b/iC3 binding protein of rabbit platelets is the homologue of human CR1. If so, this represents an interesting evolutionary switch in the tissue specific expression of the immune adherence R from platelets in the nonprimate to E in the primate.