Synthesis and preliminary binding studies of 4,4-ditritio-(-)-nicotine of high specific activity

Disposition kinetics of nicotine and cotinine enantiomers in rabbits and beagle dogs

The disposition kinetics of nicotine and cotinine enantiomers was determined in rabbits. The clearance of (R)-nicotine was similar to that of (S)-nicotine, but clearance of (R)-cotinine was twice that of (S)-cotinine. Fractional conversions of both enantiomers of nicotine to cotinine were approximately 50%. These results suggest that in rabbits the biotransformation pathways of cotinine, but not nicotine, are influenced by stereochemistry. The disposition kinetics of nicotine enantiomers in beagle dogs was also studied. In dogs, the clearance of (R)-nicotine was slightly greater than the clearance of (S)-nicotine. Methods for the synthesis of (R)-nicotine and (R)-cotinine of high enantiomeric purity and a gas chromatographic method for determination of nicotine enantiomeric purity are described.

The spinal cord as a major site for the antinociceptive action of nicotine in the rat

These studies were conducted to localize the antinociceptive action of nicotine within the CNS. Antinociceptive and biodispositional studies were carried out after the injection of [3H]nicotine subcutaneously and intracerebroventricularly into the common carotid and vertebral arteries and into the subarachnoid space. The data indicated that [3H]nicotine was most potent when given into the subarachnoid space than by any of the other route of administration. Further, the disposition studies showed that [3H]nicotine was almost entirely contained in the thoracic and lumbar areas. These results are consistent with the hypothesis that the spinal cord is an important site for antinociception induced by nicotine.

Binding characteristics of (-)- and (+)-nicotine to the rat brain P2 fraction

Saturation studies employing (-)- and (+)-[3H]nicotine indicate that the isomers bind to different very high and high affinity sites since the binding density for (-)-[3H]nicotine is 10 times that for (+)-[3H]nicotine. Both isomers also bind to a low affinity site (KDS = approximately 10(-5) to 10(-4) M). Competition studies employing unlabelled (-)- and (+)-nicotine reveal greater complexities. The isomers also appear to bind to a separate site which enhances binding at the (-)- and (+)-nicotine high affinity sites. (+)-Nicotine is more effective in increasing the binding of (-)-[3H]nicotine at its high affinity site than (-)-nicotine. Further, (+)-nicotine has a greater specificity for enhancing binding than (-)-nicotine in that it enhances (-)-[3H]nicotine binding at lower concentrations and inhibits binding at higher concentrations than (-)-nicotine.

Biochemical characterization of the nicotinic cholinergic receptors in human brain: binding of (-)-[3H]nicotine

(-)-[3H]Nicotine was found to bind specifically to membranes of human brains obtained at autopsy. The binding was stereospecific, (-)-nicotine being 40 times more potent than (+)-nicotine in displacing labeled (-)-nicotine. Saturation binding studies revealed the presence of two binding sites with dissociation constant (KD) values of 8.1 and 86 nM, and maximum binding capacity (Bmax) values of 36 and 90 fmol/mg protein, respectively. In competition studies, nicotinic agonists were 1,000 times more potent than ganglionic, neuromuscular, and muscarinic blocking drugs in displacing labeled (-)-nicotine. IC50 values for cholinergic drugs of (-)-[3H]nicotine binding were as follows: (-)-nicotine, 0.51 nM; acetylcholine, 12.6 nM; (+)-nicotine, 19.9 nM; cytisine, 27.3 nM; and carbachol, 527 nM. IC50 values of alpha-bungarotoxin, hexamethonium, d-tubocurarine, and atropine were larger than 50 microM. (-)-[3H]Nicotine binding was highest in the nucleus basalis of Meynert and thalamus and lowest in the cerebral cortex and caudate in the brain regions tested. These results suggest that nicotinic cholinergic receptors are present in human brain and that there are regional differences in the density of these receptors.

Nature of nicotine binding to rat brain P2 fraction

(-)-Nicotine may bind to as many as 5 sites in the rat brain P2 preparation: A very high affinity site (KD approximately 2.2 X 10(-11) M); a positive cooperativity site; a high affinity site (KD approximately 5.2 X 10(-9) M); a low affinity site (KD approximately 4.5 X 10(-5) M) and a very low affinity site. The curvilinear nature of both Scatchard plots and kinetic curves indicates the presence of multiple binding sites. Evidence for a positive cooperativity site includes: (1) The configuration of Scatchard plots (at low concentrations) of saturation as well as inhibition curves for (-)- and (+)-nicotine. (2) The Hill number of 1.37 for the binding of low concentrations of (+/-)-[3H]nicotine. (3) Selectivity among cholinergic drugs for producing positive cooperativity. (4) Markedly different specificities of drugs for the positive cooperativity site. Thus while only (+)- and (-)-nicotine interacted with the very high affinity site, acetylcholine, atropine, mecamylamine, lobeline, carbachol, (+)-nicotine and (-)-nicotine enhanced the binding of (+/-)-[3H]nicotine and cytisine, anabasine, cotinine and choline selectively inhibited binding at the high affinity site. Several lines of evidence indicate that there is stereospecificity. (+)-Nicotine was more potent than (-)-nicotine in inducing positive cooperativity whereas (-)-nicotine was 80 times more potent than (+)-nicotine in inhibiting binding at the high affinity site. Further, the specificity of the binding sites can be altered by changing the concentration of the buffer which gives additional evidence for the lability of the nicotine binding site. Although the pharmacologic significance of the different binding sites has not been determined, these data taken together indicate that (+/-)-[3H]nicotine binds with specificity to multiple sites in the rat brain P2 preparation with a complexity not addressed heretofore.

Antinociceptive action of nicotine and its methiodide derivatives in mice and rats

Three quaternary methiodides of nicotine were prepared and tested for antinociceptive activity in the mouse tail-flick, mouse phenylquinone and rat tail-flick tests. Following peripheral administration, all three methiodides were inactive in the mouse and rat tail-flick procedures, whereas nicotine was active in both tests, which suggested that nicotine was acting centrally. Quaternization of nicotine did not eliminate antinociceptive activity as demonstrated by the intraventricular injection of the methiodides in mice. Nicotine pyrrolidine and bis methiodides were somewhat more potent than nicotine, whereas nicotine pyridine methiodide was considerably less potent than nicotine in the tail-flick procedure. Systemically administered nicotine pyrrolidine methiodine was approximately one-third as active as nicotine in the mouse phenylquinone test; nicotine pyridine methiodide and nicotine bis methiodide were 100 and 300 times less active, respectively. Hexamethonium partially blocked nicotine and nicotine pyrrolidine methiodide, whereas mecamylamine blocked nicotine completely but nicotine pyrrolidine methiodide partially. Nicotine may have both central and peripheral actions in the mouse phenylquinone test, whereas nicotine pyrrolidine methiodide may have both nicotine and non-nicotine like antinociceptive activity. The radiolabelled methiodides were synthesized and their disposition in body tissues studied. The methiodides were found to penetrate brain poorly (plasma-to-brain ratios greater than 20). The methiodides were metabolized to nicotine to a small extent. This metabolism occurred to a greater extent in mice than in rats.

Binding of optically pure (--)-[3H]nicotine to rat brain membranes

With the recent availability of (--)-[3H]nicotine of high specific activity, binding studies were performed on rat brain membranes in the presence of a variety of nicotine analogues and cholinergic drugs. Both a higher affinity (Kd = 2 X 10(-10) M) and a lower affinity (2 X 10(-9) M) site were observed; the stereoselectivity of both sites being similar. A good correlation was observed between IC50-values and psychotropic potency of a series of N'-alkyl substituted nicotine analogues.

Nicotine binding sites and their localization in the central nervous system

The resolution of racemic nicotine to provide optically pure (+)-nicotine and the synthesis of radiolabeled nicotine with high specific activity have facilitated the study of nicotine binding in brain. The actions of the stereoisomers of nicotine on the central nervous system are qualitatively similar in most tests but (-)-nicotine is more potent than the unnatural (+)-isomer by 10-fold or greater. Binding of radiolabeled nicotine to brain has both saturable and nonsaturable components. Only saturable binding is affected by incubation conditions such as time, temperature, pH and ion concentration. Excess concentrations of the stereoisomers are equally effective in displacing (-)-[3H]-nicotine from brain homogenates. Nevertheless, a direct comparison of (+)-[3H]-nicotine and (-)-[3H]-nicotine binding shows that the latter has a KD three times lower than the former. (-)-[3H]-Nicotine is bound to the greatest degree in hypothalamus and hippocampus, areas that also exhibited the most stereoselectivity for nicotine. However, differences in the binding affinities of the two isomers were far less than the pharmacological stereospecificity observed.

Synthesis of 4,4-ditritio-(+)-nicotine: comparative binding and distribution studies with natural enantiomer

The preparation of 4,4-ditritio-(+)-nicotine (Vb) (specific activity 10.3 Ci/mmole)from (+)-nicotine (Ib) via (-) 4,4-dibromocotinine (IIIb) is described. Although Ib is 10-30 times less potent than (-)-nicotine (Ia) in the CNS, its binding affinity for the crude mitochondrial or nuclear fraction of whole rat brain is only three times less than that of Ia. However, distribution studies showed that the maximum brain levels of (-)-[3H] nicotine are nearly twice those of (+)-[3H]-nicotine following administration of a 2-micrograms/kg dose. Binding affinity and disposition of the stereoisomers account for a portion of the pharmacological stereospecificity of nicotine.

Quantification of phencyclidine in mainstream smoke and identification of phenylcyclohex-1-ene as pyrolysis product

Parsley cigarettes containing [3H]phencyclidine were machine smoked, and the mainstream smoke was trapped in glass wool filters. Radioactivity was extracted from these filters with chloroform. The average recoveries of radioactivity were 76, 85, 70, and 69% for cigarettes containing 3, 10, 30, and 50 mg of [3H]phencyclidine hydrochloride, respectively. TLC and GLC-mass spectrometry were employed to identify and quantify compounds in the filter extracts. Approximately one-half of the recovered radioactivity represented a pyrolysis product, phenylcyclohex-1-ene. Formation of this product involved loss of piperidine from phencyclidine. Piperidine, which was not radiolabeled, also may appear in smoke intact. The remainder of the radiolabeled material represented unchanged phencyclidine. Therefore, the percentage of [3H]phencyclidine delivered was approximately 40% of the amount smoked. This result was independent of puff frequency and quantity of phencyclidine hydrochloride smoked over the range tested. The [3H]phencyclidine delivery was compared to the quantities of [3H]-delta 9-tetrahydrocannabinol and [3H]nicotine delivered in mainstream smoke. The recovery of unchanged [3H]-delta 9-tetrahydrocannabinol from placebo marijuana cigarettes injected with a solution containing 3 mg of delta 9-tetrahydrocannabinol was 60%. Tobacco cigarettes injected with [3H]nicotine yielded 70% unchanged nicotine in mainstream smoke.

Noncholinergic, saturable binding of (+/-)-[3H]nicotine to mouse brain

(+/-)-[3H]Nicotine was bound saturably to crude particulate, and synaptosomal-mitochondrial fraction from mouse brain. Scatchard and Hill plots of the binding data are in agreement with the existence of two independent classes of binding sites with high (Kd of 0.1-0.4 microM) and low(Kd is 20 microM) affinities, although negative cooperativity or a two-step model of ligand-receptor interaction cannot be ruled out. Nicotinic or muscarinic agonists and antagonists had little or no affinity for the nicotine binding sites, suggesting that nicotine binds in brain to noncholinergic sites. The binding did not display stereospecificity; this is consistent with the similarity in the pharmacological effects of (-)- and (+)-nicotine. Our results indicate that binding studies with [3H]nicotine should be interpreted with extreme caution.