Chain-lengthened and imidazoline analogues of nicotine

Analogues of nicotine (1) and azanicotine (3) were prepared with an additional methylene group inserted between the two rings (i.e., homonicotine and homoazanicotine; 6 and 5, respectively). Although 6 (Ki = 3110 nM) and 3 (Ki = 206 nM) bind at nACh receptors with > or = 100-fold lower affinity than nicotine (Ki = 2.1 nM), 5 displays high affinity (Ki = 7.8 nM). Like nicotine (ED50 = 12 microg/mouse), both 3 and 5 (ED50 = 21 and 19 microg/mouse, respectively) produced antinociceptive activity in the tail-flick assay following intrathecal administration. The antinociceptive actions of 3 and 5, unlike those of nicotine, were not antagonized by mecamylamine. Compounds 3 and 5 might represent novel analgesic agents that act via a non-nicotinic mechanism, or via a nicotinic mechanism that is distinct from that mediating the antinociceptive actions of nicotine.

Bupropion is a nicotinic antagonist

Neuronal nicotinic receptors are ligand-gated ion channels of the central and peripheral central nervous system that regulate synaptic activity from both pre- and postsynaptic sites. The present study establishes the acute interaction of bupropion, an antidepressant agent that is also effective in nicotine dependence, with nicotine and nicotinic receptors using different in vivo and in vitro tests. Bupropion was found to block nicotine's antinociception (in two tests), motor effects, hypothermia, and convulsive effects with different potencies in the present investigation, suggesting that bupropion possesses some selectivity for neuronal nicotinic receptors underlying these various nicotinic effects. In addition, bupropion blocks nicotine activation of alpha(3)beta(2), alpha(4)beta(2), and alpha(7) neuronal acetylcholine nicotinic receptors (nAChRs) with some degree of selectivity. It was approximately 50 and 12 times more effective in blocking alpha(3)beta(2) and alpha(4)beta(2) than alpha(7.) This functional blockade was noncompetitive, because it was insurmountable by increasing concentration of ACh in the nAChRs subtypes tested. Furthermore, bupropion at high concentration failed to displace brain [(3)H]nicotine binding sites, a site largely composed of alpha(4)beta(2) subunit combination. Given the observation that bupropion inhibition of alpha(3)beta(2) and alpha(4)beta(2) receptors exhibits voltage-independence properties, bupropion may not be acting as an open channel blocker. These effects may explain in part bupropion's efficacy in nicotine dependence. Our present findings suggest that functional blockade of neuronal nAChRs are useful in nicotine dependence treatment.

The antinociceptive effects of alpha7 nicotinic agonists in an acute pain model

Nicotinic receptors have been found to play a role in modulating pain transmission in the CNS. Activation of cholinergic pathways by nicotine and nicotinic agonists has been shown to elicit antinociceptive effects in a variety of species and pain tests. The involvement of alpha(7) nicotinic receptors in nicotinic analgesia was assessed after spinal (i.t.) and intraventricular (i.c.v.) administration in mice. Dose-dependent antinociceptive effects were seen with the alpha(7) agonist choline after spinal and supraspinal injection using the tail-flick test. Furthermore, alpha(7) antagonists MLA and alpha-BGTX significantly blocked the effects of choline. Dihydro-beta-erythroidine and mecamylamine failed to block choline-induced antinociception. These results strongly support the involvement of alpha(7) subunits in choline's antinociceptive effects. DMXB and 4-OH-DMXB, partial alpha(7) agonists, failed to elicit a significant antinociceptive effect. However, they blocked choline-induced antinociception in a dose-dependent manner following i.t. injection. This antagonism is probably related to their partial agonistic properties of the alpha(7) receptors. These studies suggest that activation of alpha(7) receptors in the CNS elicits antinociceptive effects in an acute thermal pain model.

The involvement of spinal Ca(2+)/calmodulin-protein kinase II in nicotine-induced antinociception in mice

The nature of the signaling process activated by neuronal nicotinic receptors has not been fully defined; however, several recent studies have implicated the involvement of Ca(2+) fluxes in the response to nicotine. In order to assess Ca(2+)-dependent mechanisms in nicotine-induced antinociception, the Ca(2+) channel antagonist nimodipine and several calcium/calmodulin-protein kinase II (CaM kinase II) inhibitors were evaluated for their effects on nicotine-induced antinociception. The results indicate that both of these antagonists dose-dependently blocked nicotine-induced antinociception after intrathecal (i.t.) injection. Indeed, three structurally unrelated CaM kinase II inhibitors blocked nicotine's effects in the tail-flick test in a dose-related manner. A second series of experiments assessed the effect of acute nicotine exposure on [Ca(2+)](i) and CaM kinase II activity in spinal cord tissues. Nicotine increased [Ca(2+)](i) in a concentration-dependent manner after application of the drug to spinal synaptosomes. Furthermore, a dose-dependent increase in the spinal cord membrane CaM kinase II activity was seen after acute injection of nicotine in mice. Taken together, these results are consistent with the hypothesis that nicotine binding to nicotinic receptors leads to channel opening and depolarization responses with an influx of Ca(2+) ions, which would reach sufficient levels to activate Ca(2+)-dependent/CaM kinase II. Neuronal Ca(2+), acting via Ca(2+)-dependent CaM kinase II, appears to mediate nicotine-induced antinociception at the spinal level.

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.

Lobeline: structure-affinity investigation of nicotinic acetylcholinergic receptor binding

(-)Lobeline (1) and (-)nicotine (2) bind at neuronal nicotinic cholinergic (nACh) receptors with high affinity (K(i) = 4 and 2 nM, respectively). Previous attempts to determine whether lobeline fits the currently accepted nicotinic pharmacophore model have led to suggestions that the carbonyl function, rather than the hydroxyl group, is a major contributor to binding. Interestingly, however, it has never been empirically demonstrated that either oxygen function is actually required for interaction with the receptor. In the present investigation we systematically examined a number of abbreviated analogues of lobeline and found that removal of either one or both oxygen functions reduces the affinity of lobeline by at least 25-fold; furthermore, oxidation of the (-)lobeline hydroxyl group (to afford lobelanine) or reduction of the carbonyl group (to afford lobelanidine) also resulted in decreased affinity. Although it is likely that both oxygen functions contribute to the high affinity of (-)lobeline at nACh receptors, it is concluded that the presence of both oxygen functions is not a requirement for binding; that is, replacement of the (-)lobeline hydroxyl group with a chloro group had no effect on affinity. Another finding of the present investigation is that removal of either one or both oxygen functions of lobeline results in compounds that retain the analgesic activity and potency of (-)lobeline, indicating that there is no direct relationship between neuronal nicotinic cholinergic (primarily alpha(4)beta(2) type) receptor affinity and spinal analgesia as measured in the tail-flick assay.

Pharmacological characterization of nicotine's interaction with cocaine and cocaine analogs

Cocaine and a number of 3beta-phenyltropane cocaine analogs were investigated for their potential to block various pharmacological effects of nicotine in animals. They blocked the antinociceptive effect of nicotine in the tail-flick test after systemic administration in a dose-dependent manner. Similarly, cocaine was also able to block nicotine-induced motor impairment in mice. Furthermore, cocaine blocked nicotine-induced seizures at a lower potency than for antinociception, but failed to block nicotine's effect on body temperature and drug discrimination. The antagonistic potencies of the 3beta-phenyltropane cocaine analogs were not correlated with their affinity for monoamines transporters. Additionally, bupropion, nomifensin, GBR 12909, and nisoxetine, but not methylphenidate and fluoxetine, blocked nicotine-induced antinociception; however, their antagonistic potencies were unrelated to their affinities for the transporters. Taken together, these results suggest that the mechanism of cocaine's antagonistic activity is not related to its binding and uptake of inhibition on monoamine neurotransporters. The failure of lidocaine and procaine to antagonize nicotine's effects in the tail-flick assay rules out local anesthetic effects. In addition, cocaine blocked differentially the response of nicotine in the oocyte receptor expression system for the alpha4beta2 and alpha3beta2 subtypes in a dose-dependent manner. Our results suggest that cocaine is a noncompetitive nicotinic antagonist with some selectivity for neuronal nicotinic receptor subtypes. Our studies also demonstrate that 3beta-phenyltropane analogs constitute a new class of nicotinic antagonists. Elucidation of the mechanism of action of this new class of antagonists may provide an explanation for the effectiveness of agents such as bupropion for the treatment of smoking cessation.

Reduced antinociception in mice lacking neuronal nicotinic receptor subunits

Nicotine exerts antinociceptive effects by interacting with one or more of the subtypes of nicotinic acetylcholine receptors (nAChRs) that are present throughout the neuronal pathways that respond to pain. To identify the particular subunits involved in this process, we generated mice lacking the alpha4 subunit of the neuronal nAChR by homologous recombination techniques and studied these together with previously generated mutant mice lacking the beta2 nAChR subunit. Here we show that the homozygous alpha4-/- mice no longer express high-affinity [3H]nicotine and [3H]epibatidine binding sites throughout the brain. In addition, both types of mutant mice display a reduced antinociceptive effect of nicotine on the hot-plate test and diminished sensitivity to nicotine in the tail-flick test. Patch-clamp recordings further reveal that raphe magnus and thalamic neurons no longer respond to nicotine. The alpha4 nAChR subunit, possibly associated with the beta2 nAChR subunit, is therefore crucial for nicotine-elicited antinociception.

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.

Pharmacology of lobeline, a nicotinic receptor ligand

In this study we investigated the pharmacology of lobeline, a high affinity nicotinic ligand with a unique pharmacological profile, in different in vitro and in vivo tests. Although lobeline displaced [3H]-nicotine binding sites in the rat brain with a Ki of 4.4 nM, it did not activate alpha4beta2 expressed receptors in frog oocytes. The in vivo pharmacological effects of lobeline were highly complex. Lobeline, at the time of maximal effect, dose-dependently produced motor impairment and decreased locomotor activity and body temperature in mice after s.c. treatment. However, antinociception was present after intrathecal but not after s.c. administration of lobeline in the tail-flick tests. The behavioral effects of lobeline were not blocked by pretreatment with either mecamylamine or dihydro-beta-erythroidine. In addition, lobeline given s.c. enhanced nicotine-induced antinociception in a dose-related manner. No acute tolerance developed to either lobeline's behavioral or antinociceptive effect after s.c. or intrathecal administration, respectively. However, tolerance developed to lobeline's pharmacological effects after chronic treatment with the drug for 10 days (15 mg/kg, s.c. twice a day). Furthermore, cross-tolerance between lobeline and nicotine developed after chronic treatment with either drug. Although the alpha4beta2 receptor is unlikely to mediate the agonist effects of lobeline, our results indicate that lobeline does interact with the nicotinic receptor in a novel fashion.

Altered behavioral sensitivity of Ca(2+)-modulating drugs after chronic nicotine administration in mice

Numerous studies have demonstrated that tolerance develops to the physiological and behavioral effects of nicotine in animals after chronic administration of the drug. However, the mechanisms underlying tolerance to nicotine are not well known. There are several lines of evidence which support a role for Ca2+ in nicotine's acute pharmacological effects. The objective of the study was to determine whether Ca2+ plays a role in the development of tolerance to nicotine by investigating the behavioral activity of several Ca(2+)-modulating drugs after systemic (BAY K 8644: (+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-pheny l] -3-pyridine carboxylic acid methylester) and intrathecal administration (BAY K 8644, Ca2+ and thapsigargin) in nicotine-tolerant mice. The ability of BAY K 8644 to induce motor impairment and hypomotility after i.p. injection was decreased in nicotine-tolerant mice. In addition, tolerance to Ca2+, thapsigargin, and BAY K 8644-induced antinociception after i.t. injection also developed in nicotine-tolerant mice. ED50 values for BAY K 8644 and thapsigargin increased from 3.7 to 12 micrograms/mouse and 0.83 to 19.7 micrograms/mouse, respectively. The greatest tolerance developed to the effects of thapsigargin with an ED50 value that increased from 0.83 to 20 micrograms. Furthermore, chronic nicotine injections did not alter [3H]nitrendipine binding in the brain. These results suggest the involvement of Ca(2+)-dependent mechanisms in nicotine tolerance in mice.

Tolerance to the antinociceptive effect of epibatidine after acute and chronic administration in mice

Epibatidine is a novel potent antinociceptive agent that acts through nicotinic receptors. In this study we investigated the development of tolerance to the antinociceptive effects of epibatidine enantiomers after acute and chronic administration in mice using the tail-flick test. Contrary to nicotine, mice treated with epibatidine enantiomers at different times and doses did not develop significant tolerance after s.c. acute nor after repeated injections. In mice that chronically received (+)-epibatidine, no significant tolerance was seen after acute challenge with (+)-epibatidine. However, a significant shift in (-)-epibatidine's dose-response curve was obtained in animals that received (-)-epibatidine. In nicotine-tolerant animals, no significant tolerance was seen after acute challenge with (+)-epibatidine. However, the animals were less sensitive to the acute (-)-epibatidine challenge. Our results show that development of tolerance to epibatidine antinociceptive effects has a different profile and characteristics than that found for nicotine.

Characterization and modulation of acute tolerance to nicotine in mice

Acute tolerance to the effects of nicotine is believed to play an important role in the development and maintenance of dependence to this drug. The objective of this study was to investigate and characterize the development of acute tolerance to nicotine after systemic and intrathecal administrations. Acute tolerance developed to several centrally mediated pharmacological effects of nicotine after systemic (motor coordination, body temperature, antinociception) and intrathecal (antinociception) injection of the drug. The appearance and the magnitude of acute tolerance varied depending on the response measured. Development of acute tolerance to nicotine-induced hypothermia and motor impairment was blocked after intraperitoneal pretreatment with nimodipine. Similarly, an intrathecal injection of nimodipine blocked the development of acute tolerance to nicotine-induced antinociception. On the other hand, intrathecal administration of calcium and thapsigargin enhanced the acute tolerance to nicotine-induced antinociception. Characterization of the acute tolerance to nicotine in several animal models revealed time and dose dependencies that are consistent with receptor-mediated events. More importantly, acute tolerance was modulated by agents that influence cellular calcium homeostasis.

Comparative pharmacology of nicotine and ABT-418, a new nicotinic agonist

ABT-418, a novel cholinergic ligand, was reported to possess potent cognitive-enhancing and anxiolytic properties in animal models with reduced side effects (Decker et al. 1994; Garvey et al. 1994) suggesting selectivity of effects. In this study, the binding properties of ABT-418 to [3H]-nicotine sites were evaluated and its pharmacology investigated in different tests in laboratory animals. ABT-418 binds with high affinity to 3H-nicotine binding sites in the brain with, however, a Ki (6 nM) less than that of nicotine (four-fold). In addition, it acts as a full nicotinic agonist in producing hypomotility, hypothermia and antinociception in mice and engendering nicotine-like responding in rat drug discrimination. The potency of ABT-418 is three to four times less than that of nicotine in all of the animal models, except for hypothermia. In addition, its behavioral effects are completely blocked by mecamylamine, a non-competitive nicotinic antagonist. Although activation of nicotinic receptors by ABT-418 produced several behavioral and pharmacological effects, our results do not suggest high selectivity of different effects as reported by Decker et al. (1994) and Garvey et al. (1994). However, it should be noted that we did not perform some of these tests that produced effects at low doses (Decker et al. 1994) and additional pharmacological studies are needed to establish its selectivity at multiple nicotinic receptors.

In vivo pharmacological effects of dihydro-beta-erythroidine, a nicotinic antagonist, in mice

The comparative in vivo pharmacology of mecamylamine and dihydro-beta-erythroidine (DH beta E) in mice was studied. Modulation of the behavioral effects (antinociception, hypomotility, motor impairment and hypothermia) of nicotine in mice by DH beta E and mecamylamine were carried out. After SC administration, DH beta E and mecamylamine were nearly equipotent in blocking nicotine's effects except for antinociception, in which mecamylamine was clearly more potent. Intrathecal injection of DH beta E was also effective in blocking the antinociceptive effect of nicotine. In vivo interaction of DH beta E with calcium and calcium channels, involved in the central actions of nicotine, showed that intrathecal administration of DH beta E failed to reduce the antinociception induced by diverse drugs which increase intracellular calcium such as thapsigargin, (+/-)-BAYK 8644 and calcium, indicating that this antagonist does not affect calcium-dependent mechanisms involved in antinociception. On the other hand, mecamylamine blocked the antinociceptive effect of the calcium modulatory drugs, suggesting that it may be acting on calcium-dependent mechanisms involved in the intracellular signaling process. We conclude that DH beta E, a nicotinic neuromuscular antagonist, is able to block some of the central actions of nicotine after systemic and intrathecal administration. The mechanism of blockade is different from that of mecamylamine, a classical ganglionic antagonist, and may involve a direct action of DH beta E on nicotine receptor.

Pharmacological effects of epibatidine optical enantiomers

The pharmacology of synthetic D- and L-epibatidine, an alkaloid originally characterized from frog skin, were studied in different behavioral tests in mice and rats. The two enantiomers have potent antinociceptive activity in mice using the tail-flick test, with an ED50 of 6.1 and 6.6 micrograms/kg for L- and D-epibatidine respectively. Epibatidine enantiomers were 200 x more potent than L-nicotine as an antinociceptive agent in mice after s.c. administration. Their analgesic effect was blocked by mecamylamine but not naloxone, an opiate antagonist. Both D- and L-epibatidine have high affinity (Ki 54.7 and 55.0 pM, respectively) for [3H]nicotine binding site in rat brain. In addition, they reduced mice locomotor activity and body temperature in a dose-dependent manner. In rats trained with nicotine (0.4 mg/kg), epibatidine enantiomers engendered nicotine-like responding in a dose-related manner with an ED50 of 1.00 and 0.93 micrograms/kg for D and L, respectively. The discriminative effect of L- and D-epibatidine in rats was blocked by mecamylamine but not by hexamethonium. As in binding results, there was no significant enantioselectivity for these effects in our study.

Nicotine-induced antinociception in mice: role of G-proteins and adenylate cyclase

The effects of pertussis toxin, forskolin, and cAMP analogues on the antinociceptive action of nicotine were examined to investigate the possible involvement of adenylate cyclase and G-proteins in nicotine's antinociceptive effect. Intrathecal injection of pertussis toxin (0.25 and 0.50 micrograms) in mice inhibited nicotine-induced antinociception in the tail-flick test. The effect of the toxin was dose and time dependent. Forskolin, a potent adenylate cyclase activator, and 8-(-4-chlorophenylthio) adenosine-3':5' monophosphate, cyclic (8-CPT-cAMP), a cAMP analogue, inhibited the antinociceptive effects of nicotine in a dose-dependent manner. EGTA reversal of 8-CPT-cAMP's inhibitory effects suggests that calcium may to be involved. These data implicate the possible involvement of a G-protein and a second messenger system (activation of a cAMP-dependent protein kinase and increase in cyclic AMP levels) in nicotine-induced analgesia in mice.

Involvement of the serotonergic system in the hypoactive and antinociceptive effects of nicotine in mice

Pretreatment with the 5-HT2 antagonist ketanserin and the 5-HT1A/2 antagonist spiperone did not reduce nicotine-induced hypomotility in mice, nor did MDL 7222, a selective 5-HT3 antagonist. In addition, 8-OH-DPAT and buspirone, 5-HT1A agonists, had no significant effects on nicotine-induced hypomotility. However, 8-OH-DPAT and buspirone did reduce the antinociceptive effects of nicotine in a dose-dependent manner. 8-OH-DPAT blockade of this nicotine effect was reversed by spiperone, a 5-HT1A/2 antagonist. Nicotine's ED50 was increased from 1.00 mg/kg (0.90-1.68) to 2.00 mg/kg (1.6-2.55) and 2.66 (1.7-3.51) by buspirone and 8-OH-DPAT, respectively. Ketanserin, spiperone and MDL 7222 had no significant effect on nicotine-induced antinociception. The present data suggest an important role of 5-HT1A receptors in the modulation of antinociceptive actions of nicotine.

Involvement of calcium and L-type channels in nicotine-induced antinociception

The nature of the signaling process activated by neuronal nicotinic receptors has not been fully defined; however, several recent studies have implicated the involvement of calcium ion fluxes in the response to nicotine on a cellular level. Alteration of nicotine-induced antinociception in mice after systemic administration was therefore investigated in the presence of several drugs that increase intracellular calcium. Calcium, (+/-)-BAYK 8644, thapsigargin, glyburide and A23187 administered intrathecally (i.t.) were found to enhance nicotine-induced antinociception by shifting its dose-response curve to the left. Conversely, i.t. administration of agents which decrease intracellular calcium, such as EGTA and alpha-calcitonin gene-related peptide, blocked nicotine-induced antinociception. These findings support a role for spinal intracellular calcium in the pharmacological effects of nicotine. Additionally, blockade of antinociception by nimodipine and nifedipine indicates that a L-type calcium channel is involved in nicotine's effect. However, nicotine did not compete for [3H] nitrendipine binding. Intrathecal administration of mecamylamine, a nicotinic antagonist, resulted in a blockade of antinociception produced by the i.t. injection of thapsigargin, A23187, calcium and (+/-)-BAYK 8644. The mechanism of mecamylamine's antagonism of nicotine is uncertain. However, these results suggest that mecamylamine blocks the effects of drugs which increase intracellular calcium by either a modulation of intracellular calcium-dependent mechanisms or a blockade of calcium channels. Thus, mecamylamine could modulate a calcium signaling process secondary to receptor activation resulting in blockade of antinociception produced by diverse agents.

Calcium agonists and antagonists of the dihydropyridine type: effect on nicotine-induced antinociception and hypomotility

The influence of a calcium agonist (BAYK 8644) and several calcium channel blockers on nicotine-induced antinociception was investigated in mice. The effect of nicotine was sharply increased by BAYK 8644. This potentiation by BAYK 8644 was blocked by pretreating the animals with nifedipine at 2 and 10 mg/kg. The calcium channel antagonists, nifedipine and nimodipine at doses that had no effect on tail-flick time, reduced significantly the antinociception induced by nicotine (1.5 mg/kg, s.c.). However, the effect of verapamil on nicotine was not significant. These results indicate that DHP calcium channels (type L-channel) play a role in some of the pharmacological effects of nicotine, particularly, locomotor activity and antinociception.

Is the dopaminergic system involved in the central effects of nicotine in mice?

Pretreatment with ineffective doses of the D1 antagonist SCH23390 but not the D2 antagonist sulpiride reduced hyperactivity induced by nicotine in mice habituated to the test cage. On the other hand, the D1 and D2 antagonists were ineffective in blocking nicotine-induced hypoactivity in naive mice. Finally, SCH23390 and sulpiride did not block the antinociception induced by nicotine. Our data indicate that the dopamine receptors D1 and D2 are not involved in all the central effects of nicotine in mice, but seems to be a substrate for locomotor activation induced by nicotine under specific experimental conditions.

In vivo evidence for carrier-mediated brain uptake of a new 2-amino-2-oxazoline (COR3224) via the purine transport system in rat

We studied the brain uptake of a new 2-amino-2-oxazolamines derivative (COR3224) in the rat by means of the rapid intracarotid injection technique described by Oldendorf. The brain uptake index (BUI) of labelled COR3224 decreased progressively from 10% to 5% when concentrations of unlabelled compound were increased. The effect of various compounds indicated that COR3224 is transported into the brain by the purine carrier. The affinity of COR3224 for this carrier (Km = 5.68 microM) was higher than that of adenine.

Determination of a new 2-amino-2-oxazoline (COR 3224) in plasma and brain tissue of the rat by high-performance liquid chromatography with electrochemical detection

A reversed-phase (CN as stationary phase) liquid chromatographic method with electrochemical detection is described for the quantitation of COR 3224, a new 2-amino-2-oxazoline in plasma and brain tissue of the rat. Extraction was performed with dichloromethane and detection was achieved at a working electrode potential of +0.85 V versus an Ag/AgCl reference electrode. The recovery of the method is about 80 and 60% for plasma and brain, respectively. The limit of detection was less than 10 ng/ml for both plasma and brain, five times lower than that with ultraviolet detection.

High-performance liquid chromatography of a new 2-amino-2-oxazoline: application to pharmacokinetic studies in dogs

5-(1'-Phenyl-4'-piperazinomethyl)-2-amino-2-oxazoline (1; COR3224), a derivative of 2-amino-2-oxazolines with antidepressant properties in rats, was assayed in plasma by high-performance liquid chromatography. After back extraction, 1 and a ortho-O-methyl derivative of 1 as the internal standard were separated by reversed-phase liquid chromatography and measured by UV detection (235 nm). The method is rapid and specific: the detection was linear in the range 125-1000 micrograms.L-1, with a detection limit of 50 micrograms.L-1. This method allowed the determination of pharmacokinetic parameters in six beagle dogs after intravenous and oral administration of 14C-labeled 1 ([14C]1) in a crossover study. The comparison of the results obtained from total radioactivity counting and unchanged product evaluated by HPLC-UV suggest the presence of metabolites.