Pharmacological characterization of nicotine-induced seizures in mice.

Pharmacological mechanisms involved in nicotine-induced seizures were investigated in mice by testing the ability of several nicotinic agonists in producing seizures after peripheral administration. In addition, nicotinic antagonists such as hexamethonium, mecamylamine, dihydro-beta-erythroidine, and methyllycaconitine citrate (MLA) were used in combination with nicotine. We also examined the involvement of calcium channels, N-methyl-D-aspartate receptors, and nitric oxide formation in nicotine-induced seizures. Our results showed that the peripheral administration of nicotine produced seizures in a stereospecific and mecamylamine-sensitive manner. Nicotine-induced seizures are centrally mediated and involve the activation of alpha7 along with other nicotinic receptor subunits. Indeed, MLA, an alpha7-antagonist, blocked the effects of nicotine after peripheral and central administration. The extent of alpha4beta2-receptor subtype involvement in nicotine-induced seizures was difficult to assess. On one hand, we observed that dihydro-beta-erythroidine (a competitive antagonist) failed to block the effects of nicotine. In addition, a poor correlation was found between binding affinity for (3)H-nicotine-labeled sites (predominantly alpha4beta2) and seizures potency for several nicotinic agonists. On the other hand, mecamylamine, a noncompetitive antagonist, blocked nicotine-induced seizures more potently than MLA. Furthermore, its potency in blocking seizures was in the same general dose range of other nicotinic effects that are not alpha7 mediated. These results suggest that this receptor subtype does not play a major role in nicotine-induced seizures. Our findings also suggest that nicotine enhances the release of glutamate either directly or indirectly (membrane depolarization that opens L-type calcium channels). Glutamate release in turn stimulates N-methyl-D-aspartate receptors, thus triggering the cascade of events leading to nitric oxide formation and possibly seizure production.

Antinociceptive and pharmacological effects of metanicotine, a selective nicotinic agonist.

Metanicotine [N-methyl-4-(3-pyridinyl)-3-butene-1-amine], a novel neuronal nicotinic agonist, was found to bind with high affinity (K(i) = 24 nM) to rat brain [(3)H]nicotine binding sites and it generalized to nicotine in a dose-dependent manner in the drug discrimination procedure. Metanicotine produced significant antinociceptive effects in mice and rats subjected to either acute thermal (tail-flick), mechanical (paw-pressure), chemical (para-phenylquinone), persistent (Formalin), and chronic (arthritis) pain stimuli. Metanicotine was about 5-fold less potent than nicotine in the tail-flick test after s.c administration, but slightly more potent after central administration. Its duration of action was longer than that of nicotine. Nicotinic antagonists, mecamylamine and dihydro-beta-erythroidine, blocked metanicotine-induced antinociception in the different pain models. However, the antinociceptive effect was not affected by pretreatment with either naloxone or by atropine, confirming that metanicotine exerts its antinociceptive effect via nicotinic rather than either opioid or muscarinic mechanisms. In contrast to nicotine, antinociceptive effects of metanicotine were observed at doses that had virtually no effect on spontaneous activity and body temperature in mice. These data indicate that metanicotine is a centrally acting neuronal nicotinic agonist with preferential antinociceptive effects in animals. Thus, metanicotine and related nicotinic agonists may have great potential for development as a new class of analgesics.

Antinociceptive responses to nicotinic acetylcholine receptor ligands after systemic and intrathecal administration in mice.

The objective of this study was to determine which nicotinic receptor subtypes are involved in antinociception and their site of action. For that, the antinociceptive effects of several nicotinic receptor ligands were evaluated in the tail-flick test both after s.c. and intrathecal (i.t.) administration. Nicotine and other nicotine agonists increased tail-flick latencies in a dose-dependent manner after both routes of administration. Epibatidine enantiomers were the most potent agonists examined. Cytisine, a potent nicotinic ligand, failed to elicit antinociception when injected either i.t. or s.c. Despite some similarities in the effects of nicotinic agonists after i.t. and s.c. injections, their rank-order potency was different. In contrast to the s.c. results, the stereoselectivity of nicotine's effect after i.t. administration was minimal. When various nicotinic antagonists were compared after i.t. and s.c. administration, the results showed that mecamylamine and dihydro-beta-erythroidine differ in potency and their degree of antagonism of some of the nicotinic agonists given i.t. These data suggest that different subtypes of nicotinic receptors may exist in the spinal cord. A good correlation was found between binding affinity to [3H]-nicotine binding sites and analgesic potency after i.t. (r = 0.82), suggesting the involvement of alpha 4 beta 2 receptor subunits. In contrast, studies with MLA and alpha-BGTX suggested a minimal role for alpha-BGTX-sensitive receptors in the antinociceptive effect of nicotinic agonists.

Pharmacological investigation of (+)- and (-)-cis-2,3,3a,4,5,9b-hexahydro-1-methyl-1H-pyrrolo-[3,2-h]isoq uinoline, a bridged-nicotine analog.

We recently synthesized a bridged-nicotine (BN) analog and its enantiomers. They failed to compete for [3H]nicotine binding in rat brain homogenates, yet they produced nicotine-like effects by decreasing locomotor activity and producing antinociception in the tail-flick, hot-plate and PPQ tests in mice. Therefore, additional in vivo and in vitro studies were undertaken to determine whether these compounds are indeed acting independently of the nicotinic system. Although these analogs did not produce nicotine-like responding when evaluated in rat drug discrimination, the racemate augmented the cue when administered in conjunction with nicotine. Moreover, the antinociceptive measured in the different tests and hypothermic effects of (+)-BN, the more potent enantiomer, were not blocked by the nicotinic antagonists mecamylamine and dihydro-beta-erythroidine. Acute tolerance developed to (+)-BN-induced antinociception but not to hypothermia after subcutaneous administration. In addition, no cross-tolerance was observed between (+)-BN and nicotine in the different tests. The absence of generalization in the discrimination test suggests that the BN analogs do not possess nicotine-like activity. In addition, the failure of mecamylamine and dihydro-beta-erythroidine to antagonize the antinociceptive and hypothermic effects of (+)-BN, on one hand, and the inability of the bridge analogs to stimulate 86Rb+ efflux in brain synaptosomes, on the other hand, provide further evidence that BN analog agonist effects are not mediated by the alpha-4, beta-2 receptor subunit combination. It is unlikely that alpha-7 subunits mediate the agonists effects of BN analogs because their affinity to neuronal [125I]alpha-bungarotoxin binding sites is in the higher micromolar range. Other nicotinic receptor subtypes remain possible candidates because (+/-)-BN augments the generalization of nicotine in drug discrimination and produces some nicotine-like pharmacological effects. BN analogs could represent a novel class of nicotinic analgesics because naloxone and atropine failed to alter the antinociceptive effects of (+)-BN. Alternatively, their actions may be entirely independent of the nicotinic system.

Synthesis, optical resolution, absolute configuration, and preliminary pharmacology of (+)- and (-)-cis-2,3,3a,4,5,9b-hexahydro-1-methyl-1H- pyrrolo-[3,2-h]isoquinoline, a structural analog of nicotine.

Title compound, 8, has been synthesized from isoquinolinone, 1 (an improved preparation for which is presented) and separated into its antipodes with D- and L-di-p-toluoyltartaric acids. These antipodes and the racemic precursor have been evaluated (and found active) in two in vivo systems for their effects. The most potent of the three, (+)-8, has an ED50 of 7.13 mumol/kg for inhibition of spontaneous activity and 7.45 mumol/kg for antinociception compared to 4.44 and 4.81 mumol/kg, respectively, for (S)-(-)-nicotine. Compounds (-)-8 and 7 are about one-fourth as potent. Isomer (+)-8 has the 3aR,9bS configuration, the latter corresponding to (S)-(-)-nicotine as determined by X-ray crystallography. However, (+)-8 failed to compete for [3H]-nicotine binding, and its pharmacological effects were not blocked by mecamylamine. These bridged nicotine analogs either are binding to an as-yet-unidentified nicotinic receptor or they represent a novel class of non-nicotinic analgesics.