Neuronal nicotinic acetylcholine receptors as targets for drug discovery

Displacement and nonlinear chromatographic techniques in the investigation of interaction of noncompetitive inhibitors with an immobilized alpha3beta4 nicotinic acetylcholine receptor liquid chromatographic stationary phase

A liquid chromatographic column containing immobilized alpha3beta4 nicotinic acetylcholine receptors (alpha3beta4-nAChRs) has been used to determine the equilibrium association constants (Ka), desorption rate constants (kd), and adsorption rate constants (ka) for the noncompetitive inhibitors: mecamylamine, ketamine, bupropion, and dextromethorphan. Displacement chromatography, with mecamylamine as the displacer, was used to verify that the four compounds bound to the same site on the immobilized alpha3beta4-nAChRs. Nonlinear chromatographic techniques were then utilized to calculate the Ka, ka, and kd values associated with the formation of the noncompetitive inhibitor-alpha3beta4-nAChR complexes. The ka values determined in this study ranged from 19.7 to 10.5 microM(-1) sec(-1), with a relative order of mecamylamine > dextromethorphan > or = ketamine > bupropion. The kd values determined in this study indicated that dextromethorphan-induced inhibition should produce a longer recovery time than the other three NCIs. This was consistent with results from a previous in vitro study. The data from this study indicate that the immobilized alpha3beta4-nAChR column and nonlinear chromatography can be used in the study of NCIs at the alpha3beta4-nAChR.

6-Chloropyridazin-3-yl derivatives active as nicotinic agents: synthesis, binding, and modeling studies

3,8-Diazabicyclo[3.2.1]octane (1), 2,5-diazabicyclo[2.2.1]heptane (2), piperazine (3), and homopiperazine (4) derivatives, substituted at one nitrogen atom with the 6-chloro-3-pyridazinyl group while the other nitrogen atom was either unsubstituted or mono- or dimethylated, were synthesized and tested for their affinity toward the neuronal nicotinic acetylcholine receptors (nAChRs). All of the compounds had K(i) values in the nanomolar range. A molecular modeling study allowed location of their preferred conformations, the energies of which were recalculated in water with a continuum solvent model. Some of the compounds showed, in their populated conformations, only pharmacophoric distances longer than the values taken into consideration by the Sheridan model for nAChRs receptors. Thus, this SAR study gives support to the hypothesis that these longer distances are still compatible with affinity for alpha4beta2 receptors in the nanomolar range.

Application of natural receptors in sensors and assays

Biosensors are analytical devices that use a biological or biologically derived material immobilized at a physicochemical transducer to measure one or more analytes. Although there are a large number of reviews on biosensors in general, there has been little systematic information presented on the application of natural receptors in sensor technology. This perspective discusses broadly the fundamental properties of natural receptors, which make them an attractive option for use as biorecognition elements in sensor technology. It analyses the current situation by reference to typical examples, such as the application of nicotinic acetylcholine receptor and G protein-linked receptors in affinity sensors and analyses the problems that need to be resolved prior to any commercialization of such devices.

Sensitization of trigeminal caudalis neuronal responses to intraoral acid and salt stimuli and desensitization by nicotine

In human studies, repeated intraoral application of strong acidic or salt stimuli induces irritation that progressively increases across trials (sensitization), whereas irritation elicited by nicotine progressively decreases (desensitization). We investigated whether nociceptive neurons in trigeminal subnucleus caudalis (Vc) exhibit increasing or decreasing patterns of firing to the intraoral application of these irritants. In rats anesthetized with halothane and thiopental, single-unit recordings were made from nociceptive neurons in superficial layers of dorsomedial Vc that responded to mechanical and noxious thermal and chemical stimulation of the tongue. NaCl (5M), citric acid (300 mM), pentanoic acid (300 mM) or nicotine (600 mM) were separately delivered to the tongue by constant flow (0.32 ml/min) for 15 or 25 min. NaCl, citric acid and pentanoic acid each elicited a progressive, significant increase in Vc neuronal firing over the initial 10 min to a plateau level that was maintained for the stimulus duration. Nicotine induced a significant increase in firing rate of Vc neurons within 6 min, followed by a decline back to the baseline level over the ensuing 10 min. Following a rest period, reapplication of nicotine no longer activated Vc neurons, indicative of self-desensitization. We additionally tested for nicotine cross-desensitization to acid. After recording the responses of Vc neurons to pentanoic acid and noxious heat, nicotine was then applied for 15 min. Post-nicotine responses to pentanoic acid were markedly reduced (to 13% of control), indicative of cross-desensitization; responses to noxious heat were also reduced to a lesser degree (to 71% of control). The progressive increase in Vc neuronal firing elicited by NaCl and acid, and the decline in firing after initial nicotinic excitation, resemble psychophysical patterns of sensitization and desensitization, respectively, and support the involvement of Vc neurons in the signaling of oral irritant sensations.

Stereoselective syntheses of the three isomers of ethylene glycol bis(tropane-3-carboxylate)

Three epimers of ethylene glycol bis(tropane-3-carboxylate) (3alpha,3alpha'-, 3alpha,3beta'-, 3beta,3beta'-) have been synthesized by starting from 3-tropinone. 3-Tropinone was converted into the corresponding enol triflate and then subjected to palladium-catalyzed alkoxycarbonylation to provide the key intermediate methyl trop-2-ene-3-carboxylate in good yield. Stereoselective routes were developed to afford the three stereoisomers of ethylene glycol bis(tropane-3-carboxylate).

Synthesis and pharmacological characterization of novel analogues of the nicotinic acetylcholine receptor agonist (+/-)-UB-165

(+/-)-UB-165 (1) is a potent neuronal nicotinic acetylcholine receptor (nAChR) ligand, which displays functional selectivity between nAChR subtypes. Using UB-165 as a lead structure, two classes of racemic ligands were synthesized and assessed in binding assays for three major nAChR subtypes (alpha4beta2, alpha3beta4, and alpha7). The first class of compounds comprises the three pyridine isomers 4-6, corresponding to the 3-, 2-, and 4-substituted pyridine isomers, respectively. Deschloro UB-165 (4) displayed a 2-3-fold decrease in affinity at alpha4beta2 and alpha3beta4 nAChR subtypes, as compared with (+/-)-UB-165, while at the alpha7 subtype a 31-fold increase in affinity was observed. At each of the nAChR subtypes, high affinity binding was dependent on the presence of a 3-substituted pyridine, and the other isomers, 5 and 6, resulted in marked decreases in binding affinities. The second class of compounds is based on replacing the pyridyl unit of 1 with a diazine moiety, giving pyridazine (7), pyrimidine (8), and pyrazine (9), which retain the "3-pyridyl" substructure. Modest reductions in binding affinity were observed for all of the diazine ligands at all nAChR subtypes, with the exception of 7, which retained potency comparable to that of 4 in binding to alpha7 nAChR. In functional assays at the alpha3beta4 nAChR, all analogues 4-9 were less potent, as compared with 1, and the rank order of functional potencies correlated with that of binding potencies. Computational studies indicate that the 3-substituted pyridine 4 and 2-substituted pyridine 5, as well as the diazine analogues 7-9, all conform to a distance-based pharmacophore model recently proposed for the alpha4beta2 receptor. However, the nicotinic potencies of these ligands vary considerably and because 5 lacks appreciable nicotinic activity, it is clear that further refinements of this model are necessary in order to describe adequately the structural and electronic demands associated with this nAChR subtype. This rational series of compounds based on UB-165 presents a systematic approach to defining subtype specific pharmacophores.

Synthesis and biological evaluation at nicotinic acetylcholine receptors of N-arylalkyl- and N-aryl-7-azabicyclo[2.2.1]heptanes

A new series of N-arylalkyl-substituted 7-azabicyclo[2.2.1]heptanes and N-aryl-substituted 7-azabicyclo[2.2.1]heptanes were synthesized and evaluated as potential ligands for neuronal nicotinic acetylcholine receptors. The in vitro binding affinities (K(i)) of the 7-azabicyclo[2.2.1]heptane derivatives were measured by inhibition of [(3)H]cytisine binding to rat brain tissue. The most potent ligand of the series was found to be N-(3-pyridylmethyl)-7-azabicyclo[2.2.1]heptane (5b, K(i) = 98 nM). The chloro analogue (5a, K(i) = 245 nM) 5a and epibatidine (1) produced dose-dependent analgesia in both hotplate and tail-flick tests when administered subcutaneously. However, when compounds 1 and 5a,b were administered intrathecally, all produced analgesia in the tail-flick test but only 5a produced analgesia in the hotplate test.

Characterization of functional nicotinic acetylcholine receptors involved in catecholamine release from the isolated rat adrenal gland

We tried to characterize nicotinic acetylcholine receptors involved in the release of catecholamines from the rat adrenal gland. The isolated adrenal gland was retrogradely perfused via the adrenal vein with Krebs-Ringer solution at a flow rate of 0.5 ml/min. Endogenous catecholamines, adrenaline and noradrenaline, released into the perfusate were electrochemically measured using high-performance liquid chromatography. (-)-Nicotine (3 x 10(-6)-3 x 10(-5) M) evoked the release of catecholamines (adrenaline >> noradrenaline) in a concentration-dependent manner. The (-)-nicotine (10(-5) M)-induced release of catecholamines was effectively attenuated by mecamylamine (10(-7) and 10(-6) M) (a relatively selective antagonist of alpha3beta4 nicotinic receptors), but not influenced by alpha-bungarotoxin (3 x 10(-7) M) (an antagonist of alpha7 nicotinic receptors) and dihydro-beta-erythroidine (10(-5) M) (a relatively selective antagonist of alpha4beta2 nicotinic receptors). (+/-)-Epibatidine (3 x 10(-7) and 10(-6) M) (a non-selective nicotinic receptor agonist), (-)-cytisine (10(-5) and 10(-4) M) (an agonist of beta4 nicotinic receptors) and (+/-)-2-(3-pyridinyl)-1-azabicyclo(2.2.2)octane (RJR-2429) (10(-5) M) (a putative agonist of alpha3beta4 nicotinic receptors) effectively evoked the release of catecholamines (adrenaline >> noradrenaline), while (E)-N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403) (up to 10(-4) M) (a selective agonist of alpha4beta2 nicotinic receptors) had no effect. The efficacies of these agonists are as follows: (+/-) epibatidine >> RJR-2429>(-)-cytisine>(-)-nicotine >> RJR-2403. These results suggest that alpha3beta4 nicotinic receptors are involved in the release of catecholamines from the rat adrenal gland.

Synthesis and evaluation of a novel series of 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine analogues as potential positron emission tomography imaging agents for nicotinic acetylcholine receptors

Reportedly, 2-[(18)F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding affinities for the new series were found to be in the range of K(i) = 9-331 pM. A molecular modeling study showed differences in the comformational profiles and the electronic properties of these compounds, which provides further insight into the structure-activity relationships at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high affinities at nAChRs (K(i) = 28 and 23 pM, respectively) and will be targets for development of (11)CH(3)-labeled derivatives as radiotracers for PET imaging of nAChRs.

Benzylidene anabaseines act as high-affinity agonists for insect nicotinic acetylcholine receptors

Benzylidene anabaseines are agonists selective for vertebrate alpha7-nicotinic acetylcholine receptor (nAChR), while they exhibit antagonist activity toward vertebrate alpha4beta2-nAChR. To investigate the effects of benzylidene anabaseines on insect nAChRs, we performed [3H]epibatidine-binding assays and neurophysiological experiments using American cockroach (Periplaneta americana) nerve-cord preparations. Of the six compounds tested, 3-benzylideneanabaseine (BA) and 6'-chloro-3-benzylideneanabaseine (CBA) displayed the highest potency in the binding assays, with K(i)s of 35.0 and 21.2nM, respectively. The introduction of a nitro group at the 4-position of the phenyl group led to a decrease in affinity by two orders of magnitude, while that of a chlorine atom at the 6'-position had little effect on affinity. In neurophysiological experiments, BA at 3.3 microg/ml increased the spike frequency observed with the nerve preparation, as observed with nicotine at 16.6 microg/ml. These findings suggest that benzylidene anabaseines act as high-affinity agonists in P. americana nAChRs and that they might therefore prove useful as probes for insect nAChRs.

Structure-activity studies with ring E analogues of methyllycaconitine. Synthesis and evaluation of enantiopure isomers of selective antagonist at the alpha3 nicotinic receptor

The four diastereomers 4a-d of methyllycaconitine (MLA) analogue 3 ( R =(CH(2))(3)Ph, R'=CH(3)) have been synthesized in enantiomerically pure form by coupling both (S)- and (R)-2-(methylsuccinimido)benzoic acid (5a and 5b) with both (S)- and (R)-3-hydroxymethyl-N-(3-phenyl) propylpiperidine (6a and 6b) using TBTU. These compounds were assayed for potency as nicotinic acetylcholine receptor (nAChRs) antagonist. All the four diastereomers showed the same potency at both the alpha3 and alpha7 receptors as racemic compound 3. This indicates that the binding at nicotine acetylcholine receptors (nAchRs) is probably non-stereospecific.

Cytisine derivatives as high affinity nAChR ligands: synthesis and comparative molecular field analysis

A number of new N-substituted cytisine derivatives were prepared and tested, along with similar compounds already described by us and others, as high affinity neuronal acetylcholine receptor ligands. Structure-affinity relationships were discussed in the light of our recently proposed pharmacophore model for nicotinic receptor agonists. The most significant physicochemical interactions modulating the receptor-ligand binding were detected at the three dimensional (3D) level by means of comparative molecular field analysis (CoMFA). The best predictive PLS model was a single-field steric model showing good statistical figures: n = 17, Q2 = 0.717, s(ev) = 0.566, r2 = 0.942, s = 0.275.

Synthesis and pharmacological evaluation of some (pyridyl)cyclopropylmethyl amines and their methiodides as nicotinic receptor ligands

A series of 3- and (4-pyridyl)cyclopropylmethyl amines and their quaternary ammonium derivatives have been synthesized; they can be considered as rigid analogues of nicotine. The compounds have been tested on rat cerebral cortex to measure the affinity for the central nicotinic receptor. Only the methiodides show affinity in the micromolar range. The results obtained can provide useful information on the topography of the nicotinic receptor-binding site.

Multidimensional on-line screening for ligands to the alpha3beta4 neuronal nicotinic acetylcholine receptor using an immobilized nicotinic receptor liquid chromatographic stationary phase

The alpha3beta4 subtype of the neuronal nicotinic acetylcholine receptor (nAChR) subtype was immobilized on a liquid chromatographic support and the resulting column used for the rapid and direct on-line screening for nAChR ligands. A multidimensional chromatographic system was developed consisting of the immobilized receptor column (NR column) connected via a switching valve to a C(18) column that was, in turn, connected to a single quadrupole mass spectrometer. A mixture of 18 compounds, containing alpha3beta4 nAChR (7) and compounds that are not alpha3beta4 nAChR ligands (11), was injected onto the NR column. The mobile phase consisted of ammonium acetate (10 mM, pH 7.4)-methanol (95:5, v/v) and the flow-rate was 0.2 ml/min. For the first 8 min the eluent was directed to waste. At t=8 min, the switching valve was rotated and the NR column connected to the C(18) column. The eluent from the NR column was directed to the C(18) column for 12 min. At t=20 min, the switching valve was rotated and the NR column was disconnected from the C(18) column. The compounds trapped on the C(18) column were separated and eluted onto the mass spectrometer using a mobile phase of ammonium acetate (10 mM, pH 7.4)-methanol (40:60, v/v) at a flow-rate of 1.0 ml/min. Detection was accomplished using total ion monitoring. The multidimensional system correctly isolated six of the seven alpha3beta4 nAChR ligands and only one of the 11 non-ligands was found with the alpha3beta4 nAChR ligands. The results indicate that the multidimensional liquid chromatographic system can be used for the on-line screening of chemical mixtures for alpha3beta4 nAChR ligands.

Synthesis and nicotinic binding studies on enantiopure diazine analogues of the novel (2-chloro-5-pyridyl)-9-azabicyclo[4.2.1]non-2-ene UB-165

As part of our program aimed at optimizing therapeutic effects over toxic effects (as observed in the naturally occurring nicotinic acetylcholine receptor modulators (-)-nicotine, (-)-epibatidine, (-)-ferruginine, and (+)-anatoxin-a), we investigated the bioisosteric potential of diazines in the field of (+)-anatoxin-a-type structures. In the series of diazine analogues of deschloro-UB-165 (DUB-165, 6), bioisosteric replacement of the 3-pyridyl pharmacophoric element by a 4-pyridazinyl, 5-pyrimidinyl, or 2-pyrazinyl moiety resulted in novel nAChR ligands 7, 8, and 9. A palladium-catalyzed Suzuki cross-coupling of the 3-diethylboranylpyridine (14) and a Stille cross-coupling of the corresponding tributylstannyl diazines 15-17 with the vinyl triflate 13 of the N-protected 9-azabicyclo[4.2.1]nonan-2-one 12 constitute the key steps in the syntheses of these enantiopure anatoxinoids 6-9. Studies of the in vitro affinity for (alpha4)(2)(beta2)(3), alpha3(beta)4, and alpha7 nAChR subtypes by radioligand binding assays demonstrated that the diazine analogues 7-9 can be considered as pharmacologically attractive bioisosteres of DUB-165 (6) but with different effects on the binding affinity with regard to the diazine moiety. The pyrimidine-containing bioisostere 8 turned out to be the most active diazine analogue, which interacts potently (K(i) = 0.14 nM) with the (alpha4)(2)(beta2)(3) subtype and differentiates significantly among the nAChR subtypes investigated. The nitrogens in this anatoxinoid 8 show by far the most negative atomic charges (calculated using the AM1 Hamiltonian). This qualitatively correlates with the highest binding affinity observed for 8 for all subtypes under consideration.

Functional diversity among 5-substituted nicotine analogs; in vitro and in vivo investigations

Two 5-substituted derivatives of nicotine (nicotinic acetylcholine receptor: K(i)=2.4 nM) were synthesized and evaluated: 5-bromonicotine (K(i)=6.9 nM) and 5-methoxynicotine (K(i)=14.3 nM). Despite their high affinity, neither 5-bromonicotine nor 5-methoxynicotine mimicked nicotine in producing antinociceptive (tail-flick, hotplate), hypolocomotor, or hypothermic effects in mice. Neither agent antagonized the hypolocomotor actions of nicotine, whereas 5-methoxynicotine, but not 5-bromonicotine, antagonized the antinociceptive (tail-flick) activity of nicotine in a dose-related manner. In tests of stimulus generalization using rats trained to discriminate 0.6 mg/kg of (-)-nicotine from vehicle, 5-bromonicotine substituted for nicotine. Further evaluation of 5-bromonicotine indicated that it might be a partial agonist at alpha4beta2 receptors (stimulation of Rb(+) efflux; alpha4beta2 receptors expressed in oocytes) and at alpha3-containing nicotinic acetylcholine receptors (synaptosomal dopamine release). Thus, 5-bromonicotine might be acting as a partial agonist at alpha4beta2 receptors and/or some of its effects might be related to interactions with non-alpha4beta2 receptors. Clearly, the effects of 5-bromonicotine and 5-methoxynicotine are different from those of nicotine, and from one another. These actions demonstrate that substitution at the 5-position of nicotine exerts a profound influence on the pharmacological profile as well as agonist/antagonist properties of nicotine.

Structural model of nicotinic acetylcholine receptor isotypes bound to acetylcholine and nicotine

BACKGROUND

Nicotine is a psychoactive drug presenting a diverse array of biological activities, some positive, such as enhancement of cognitive performances, others negative, such as addiction liability. Ligands that discriminate between the different isotypes of nicotinic acetylcholine receptors (nAChRs) could present improved pharmacology and toxicity profile.

RESULTS

Based on the recent crystal structure of a soluble acetylcholine binding protein from snails, we have built atomic models of acetylcholine and nicotine bound to the pocket of four different human nAChR subtypes. The structures of the docked ligands correlate with available biochemical data, and reveal that the determinants for isotype selectivity are relying essentially on four residues, providing diversity of the ligand binding pocket both in terms of Van der Waals boundary, and electrostatic potential. We used our models to screen in silico a large compound database and identify a new ligand candidate that could display subtype selectivity.

CONCLUSION

The nAChR-agonist models should be useful for the design of nAChR agonists with diverse specificity profiles.

Nicotinic cholinergic modulation: galantamine as a prototype

Nicotinic acetylcholine receptor pharmacology is becoming increasingly important in the clinical symptomatology of neurodegenerative diseases in general and of cognitive and behavioral aspects in particular. In addition, the concept of allosteric modulation of nicotinic acetylcholine receptors has become a research focus for the development of therapeutic agents. In this review the scientific evidence for changes in nicotinic acetylcholine receptors in Alzheimer's disease is described. Within this context, the pharmacology of galantamine, a recently approved drug for cognition enhancement in Alzheimer's disease, is reviewed along with preclinical studies of its efficacy on learning and memory. Galantamine modestly inhibits acetylcholinesterase and has an allosteric potentiating ligand effect at nicotinic receptors. The data collected in this review suggest that the unique combination of acetylcholinesterase inhibition and nicotinic acetylcholine receptor modulation offers potentially significant benefits over acetylcholinesterase inhibition alone in facilitating acetylcholine neurotransmission.

Syntheses and evaluation of pyridazine and pyrimidine containing bioisosteres of (+/-)-pyrido[3.4-b]homotropane and pyrido-[3.4-b]tropane as novel nAChR ligands

Bioisosteric replacement of the pyridine pharmacophoric element in (+/-)-pyrido[3.4-b]homotropane (PHT) and pyrido[3.4-b]tropane with the pyridazine and pyrimidine nucleus resulted in hitherto unknown nAChR ligands such as 5-8. Inverse type Diels-Alder reactions constitute the key steps in the new routes to the pyridazine- or pyrimidine-annulated bioisosteres. The enantiopure (+)-2-tropinone (11) from the 'chiral pool' is transformed to the ring-expanded silyl enol ether 12 and to the enamine 15. Both proved to be highly dienophilic species in the inverse type [4+2] cycloaddition reactions with the 1,2,4,5-tetrazines 13 and 16a,b or with the 1,3,5-triazine 19 to provide the enantiopure target compounds 5-7. In the same way the racemic pyrimidine-annulated species 8 was obtained from 3-tropanone 21. The new ligands were tested for their in vitro affinity for (alpha4)2(beta2)3 and alpha7* nAChR subtype. In comparison to PHT, well known to exhibit affinity for agonist binding sites in rat brain approximately equivalent to that of (+)-anatoxin-a (1), replacement of the pyridine by the bioisosteric pyridazine resulted in 30-fold lower affinity at the (alpha4)2(beta2)3 subtype. The annulated diazinotropanes 6-8, ligands with ferruginine-like structures more or less retained the affinity of (-)-norferruginine (3) except of compound 7. Remarkably, all of the novel ligands are devoid of affinity at the alpha7* subtype.

Conformational analysis of the eight-membered ring of the oxidized cysteinyl-cysteine unit implicated in nicotinic acetylcholine receptor ligand recognition

Nicotinic acetylcholine receptors (nAChRs) are membrane-bound, pentameric ligand-gated ion channels associated with a variety of human disorders such as Alzheimer's disease, Parkinson's disease, schizophrenia, and pain. Most known nAChRs contain an unusual eight-membered disulfide-containing cysteinyl-cysteine ring, ox-[Cys-Cys], as does the soluble acetylcholine binding protein (AChBP) found in the snail Lymnaea stagnalis. The cysteinyl-cysteine ring is located in a region implicated in ligand binding, and conformational changes involving this ring may be important for modulation of nAChR function. We have studied the preferred conformations of Ac-ox-[Cys-Cys]-NH2 by NMR in water and computationally by Monte Carlo simulations using the OPLS-AA force field and GB/SA water model. ox-[Cys-Cys] adopts four distinct low-energy conformers at slightly above 0 degrees C in water. Two populations are dependent on the peptide omega2 dihedral angle, with the trans amide favored over the cis amide by a ratio of ca. 60:40. Two ox-[Cys-Cys] conformers with a cis amide bond (C+ and C-) differ from each other primarily by variation of the chi3 dihedral angle, which defines the orientation of the helicity about the S-S bond (+/- 90 degrees ). Two trans amide conformers have the same S-S helicity (chi3 approximately -90 degrees ), but are distinguished by a backbone rotation about phi2 and psi1 (T- and T'-). The ratio of T-/T'-/C+/C- is 47:15:29:9. The orientation of the pendant moieties from the eight-membered ring is more compact for the major trans conformer (T-) than for the extended conformations adopted by T'-, C+, and C-. These conformational preferences are also observed in tetrapeptide and undecapeptide fragments of the human alpha7 subtype of the nAChR that contains the ox-[Cys-Cys] unit. Conformer T- is nearly identical to the conformation seen in the X-ray structure of ox-[Cys(187)-Cys(188)] found in the unliganded AChBP, and is a Type VIII beta-turn.

Neurobiology of the nicotine withdrawal syndrome

The aversive aspects of withdrawal from chronic nicotine exposure are thought to be an important motivational factor contributing to the maintenance of the tobacco habit in human smokers. Much emphasis has been placed on delineating the underlying neurobiological mechanisms mediating different components of the nicotine withdrawal syndrome. Recent studies have shown that both central and peripheral populations of nicotinic acetylcholine receptors (nAChRs) are involved in mediating somatic signs of nicotine withdrawal as measured by the rodent nicotine abstinence scale. However, only central populations of nAChRs are involved in mediating affective aspects of nicotine withdrawal, as measured by elevations in brain-stimulation reward thresholds and conditioned place aversion. Nicotine interacts with several neurotransmitter systems, including acetylcholine, dopamine, opioid peptides, serotonin, and glutamate systems. Evidence so far suggests that these neurotransmitters play a role in nicotine dependence and withdrawal processes. The available evidence also suggests that different underlying neurochemical deficits mediate somatic and affective components of nicotine withdrawal. The aim of the present review is to discuss preclinical findings concerning the neuroanatomical and neurochemical substrates involved in these different aspects of nicotine withdrawal.

Antagonism of the anxiolytic effect of nicotine in the dorsal raphe nucleus by dihydro-beta-erythroidine

Nicotine has been reported to reduce anxiety in humans and in a number of animal tests. In the social interaction test of anxiety, administration of low doses of nicotine into the dorsal raphé nucleus (DRN) increases the time spent in social interaction without producing accompanying changes in locomotor activity, suggesting that nicotine acts specifically to reduce anxiety in this brain region. The present study examined the ability of the high-affinity competitive nicotinic receptor antagonist dihydro-beta-erythroidine hydrobromide (DH beta E) to antagonise the anxiolytic effect of nicotine following intra-DRN infusion using the social interaction test. The increase in social interaction observed after administration of nicotine (5 ng) into the DRN was completely reversed by coadministration of 100 ng DH beta E. DH beta E (100 ng), when administered alone into the DRN, did not modify the time spent in social interaction. However, it did significantly increase locomotor activity, and this effect was not antagonised by coadministration of nicotine (5 ng) into the DRN. Because of the pharmacological profile of DH beta E, our results suggest that the anxiolytic effect of nicotine in the DRN is mediated by the alpha 4 beta 2 nicotinic receptor subtype.

Reduced expression of neuronal nicotinic acetylcholine receptors during the early stages of damage by oxidative stress in PC12 cells

The mechanism for a large loss of neuronal nicotinic acetylcholine receptors (nAChRs) in brains with neurodegenerative diseases remains unclear. Based on our previous results of [(3)H]epibatidine binding influenced by lipid peroxidation, we suggest that nAChR deficit in neurodegenerative diseases might be related to the neurons attacked by free radicals. To further understand how free radicals influence the expression of nAChRs, we detected [(125)I]alpha-bungarotoxin binding, nAChR subunit protein and mRNA during the early stage of damage by oxidative stress in PC12 cells in the present study. The results showed that free radical insult (FeSO(4)) within the concentration range (1 -100 microM) used in the study induced dose-dependent increases in lipid peroxidation and toxicity to PC12 cells, but did not result in apoptosis or necrosis. Significant reductions in [(125)I]alpha-bungarotoxin binding site, protein level for the alpha3 and alpha7 subunits, and mRNA level for the alpha7 subunit were observed in PC12 cells treated by FeSO(4) at the concentrations without inducing cell death compared to control. Pretreatment of cultural cells with antioxidant such as Vitamin E and reduced glutathione prevented the inhibiting effect of free radicals on [(125)I]alpha-bungarotoxin and [(3)H]epibatidine bindings. The present results further demonstrate that oxidative stress might reduce the number of [(125)I]alpha-bungarotoxin binding site and selectively suppress the expression of the nAChR subunits at protein and mRNA levels during the early stages of damage in PC12 cells.

Structure-affinity relationships of a unique nicotinic ligand: N(1)-dimethyl-N(4)-phenylpiperazinium iodide (DMPP)

DMPP is a well-known nicotinic agonist that does not fit any proposed pharmacophore for nicotinic binding and represents a unique ligand among the hundreds of nicotinic agonists studied in the past decades. A systematic modulation of the chemical structure of DMPP, aimed to establish its structure-affinity relationships, is reported. The research has allowed to identify molecules such as 11c, 13c, 14c, and 28c, with affinities for alpha(4)beta(2) receptors in the low nanomolar range, some 2 orders of magnitude lower than the lead compound. The agonistic properties of the most interesting compounds have been assessed by measuring their analgesic activity on mice (hot-plate test). Another result of the research was the identification of DMPP analogues, such as 3a (K(i) = 90 nM) and 14b (K(i) = 180 nM), that maintain affinity for the central nicotinic receptor when the ammonium function is changed into an aminic one and are therefore possible leads for drug development in neurodegenerative diseases.

Neuronal nicotinic receptor agonists: a multi-approach development of the pharmacophore

Based on the results obtained with different automated computational approaches as applied to the study of eleven high-affinity agonists of the neuronal nicotine acetylcholine receptor (nAChR), belonging to different chemical classes, new relevant features were detected which complement the existing pharmacophores. Convergent results from DISCO (Distance Comparison), QXP (Quick Explore), Catalyst/HipHop, and MIPSIM (Molecular Interaction Potential Similarity) allowed us to identify and locate, in a well defined spatial arrangement, three geometrically independent key structural features: (i) a positively charged nitrogen atom for ionic or hydrogen bond interactions, (ii) a lone pair of the pyridine nitrogen or a specific lone pair of a carbonyl oxygen, as a hydrogen bond acceptor, and (iii) a centre of a hydrophobic area generally occupied by aliphatic cycles. The pharmacophore presented herein, along with predictive 2D and 3D QSAR models recently developed in our group, could represent valuable computational tools for the design of new nAChR agonists having therapeutical potential.

Toxicity and nicotinic acetylcholine receptor interaction of imidacloprid and its metabolites in Apis mellifera (Hymenoptera: Apidae)

Acute oral and contact toxicity tests of imidacloprid, an insecticide acting agonistically on nicotinic acetylcholine receptors (nAChR), to adult honeybees, Apis mellifera L var carnica, were carried out by seven different European research facilities. Results indicated that the 48-h oral LD50 of imidacloprid is between 41 and > 81 ng per bee, and the contact LD50 between 49 and 102 ng per bee. The ingested amount of imidacloprid-containing sucrose solution decreased with increasing imidacloprid concentrations and may be attributed to dose-related sub-lethal intoxication symptoms or to antifeedant responses. Some previously reported imidacloprid metabolites occurring at low levels in planta after seed dressing, i.e. olefine-, 5-OH- and 4,5-OH-imidacloprid, showed lower oral LD50 values (> 36, > 49 and 159 ng per bee, respectively) compared with the concurrently tested parent molecule (41 ng per bee). The urea metabolite and 6-chloronicotinic acid (6-CNA) exhibited LD50 values of > 99,500 and > 121,500 ng per bee, respectively. The pharmacological profile of the [3H]imidacloprid binding site in honeybee head membrane preparations is consistent with that anticipated for a nAChR. IC50 values for the displacement of [3H]imidacloprid by several metabolites such as olefine, 5-OH-, 4,5-OH-imidacloprid, urea and 6-CNA were 0.45, 24, 6600, > 100,000, and > 100,000 nM, respectively. Displacement of [3H]imidacloprid by imidacloprid revealed an IC50 value of 2.9 nM, thus correlating well with the observed acute oral toxicity of the compounds in honeybees. Neurons isolated from the antennal lobe of A mellifera and subjected to whole-cell voltage clamp electrophysiology responded to the application of 100 microM acetylcholine with a fast inward current of between 30 and 1600 pA at -70 mV clamp potential. Imidacloprid and two of the metabolites (olefine- and 5-OH-imidacloprid) acted agonistically on these neurons, whereas the others did not induce currents at test concentrations up to 3 mM. The electrophysiological data revealed Hill coefficients of approximately 1, indicating a single binding site responsible for an activation of the receptor and no direct cooperativity or allosteric interaction with a second binding site.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 2-exo-2-(2'-substituted 5'-pyridinyl)-7-azabicyclo[2.2.1]heptanes. Epibatidine analogues

A convenient, high-yield synthesis of 7-tert-butoxycarbonyl-7-azabicyclo[2.2.1]hept-2-ene (5), which involved the addition of tributyltin hydride to 7-tert-butoxycarbonyl-2-p-toluenesulfonyl-7-azabicyclo[2.2.1]hept-2-ene (4) followed by elimination of the tributyltin and p-tolylsulfonyl groups using tetrabutylammonium fluoride was developed. The addition of 2-amino-5-iodopyridine to 5 under reductive Heck conditions provided 7-tert-butoxycarbonyl-2-exo-(2'-amino-5'-pyridinyl)-7-azabicyclo[2.2.1]heptane (6). Compound 6 was the key intermediate used to prepare epibatidine analogues where the 2'-chloro group on the pyridine ring was replaced with a fluorine (1b), bromine (1c), iodine (1d), hydroxy (1e), amino (1f), dimethylamino (1g), trifluoromethanesulfonate (1h), and hydrogen (1i) group. (+)- and (-)-Epibatidine and compounds 1b-d and 1i all possess similar binding affinities at the alpha(4)beta(2) nAChR receptors labeled by [(3)H]epibatidine. Compound 1f has affinity similar to nicotine, whereas compounds 1e, 1g, and 1h have much lower affinity. The binding affinity appears to be dependent upon the electronic nature of the substituent. However, other factors are also involved. None of the compounds possesses appreciable affinity for the alpha(7) nAChR labeled by [(125)I]iodo-MLA. With the exception of 1f and 1g, all the epibatidine analogues are full agonists (tail flick test) in producing antinociception after intrathecal injection in mice.

Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain

The neuronal nicotinic acetylcholine receptors (nAChR) are involved in functional processes in brain including cognitive function and memory. A severe loss of the nAChRs has been detected in brain of patients with Alzheimer's disease (AD). There is a great interest to image nAChRs noninvasive for detection of receptor impairments even at a presymptomatic stage of AD as well for monitoring outcome of drug treatment. (S) [11C]Nicotine, has so far been the only nAChR ligand used in positron emission tomography (PET) studies for visualizing nAChRs in human brain. In order to develop PET/SPECT nAChRs ligands for detection of subtypes of nAChRs nicotine analogues, epibatidine and A-85380 compounds have been characterized in vitro and investigated in vivo. Epibatidine and A-85380 have been found to have higher specific signals and more favorable kinetic parameters than nicotine and its analogues. The epibatidine and A-85380 compounds can also be radiolabeled with high specific radioactivity, show affinities for the nAChRs in the pM range and readily cross the blood-brain barrier. In addition they reversibly bind to the nAChRs and show low non-specific binding and moderately fast metabolism. Due to a probably high alpha4beta2 nAChR selectivity combined with low toxicity, the A-85380 analogs presently seem to be the most promising nAChR ligand imaging of subtypes of nAChRs in human brain.

Receptor nomenclature guidelines

Receptors are typically characterized via two distinct approaches: (1) the identification and pharmacological characterization of a receptor-mediated response using classical pharmacological and/or radioligand approaches in tissues and animal models using selective agonist and antagonist ligands, and; (2) the cloning and expression of proteins with structural homology to known receptors, the function of which is subsequently established by studying the structure activity relationship (SAR) of receptor-mediated responses. An additional means to characterize receptors proceeded, and evolved, with the structural approach, namely classification in terms of signal transduction mechanisms. The International Union of Pharmacology (IUPHAR) created guidelines and selected working groups for each receptor family to establish a common nomenclature system. Reports from those groups that have reached some degree of consensus have been summarized in this appendix.

Neuronal nicotinic acetylcholine receptor binding affinities of boron-containing nicotine analogues

A series of boron-containing nicotine (NIC) analogues 7-9 was synthesized and evaluated for binding to alpha4beta2 and alpha7 nicotinic receptors. Compound ACME-B inhibited [3H]methyllycaconitine binding to rat brain membranes with a similar potency compared to NIC (Ki = 2.4 and 0.77 microM, respectively), but was markedly less potent in inhibiting [3H]NIC binding when compared to NIC (Ki = 0.60 microM and 1.0 nM, respectively). Thus, tethering a two-carbon bridge between the 2-pyridyl and 3'-pyrrolidino carbons of NIC or 7 affords analogues that bind to the alpha7 receptor in a manner similar to NIC, but with a dramatic loss of affinity for the alpha4beta2 receptor.

Nicotinic acetylcholine receptor assays

Nicotinic acetylcholine receptors (nAChRs) in peripheral tissues are localized almost exclusively to autonomic nerves and the motor end plates of striated musculature. Pharmacologic analyses of nicotinic receptor antagonist potencies can be conducted by assessing the ability of these compounds to inhibit responses elicited by preganglionic autonomic nerve stimulation or stimulation of the motor nerves innervating striated muscle in isolated tissue preparations. In addition, in some isolated tissues innervated by autonomic nerves, nicotinic receptor mediated responses can be elicited by exogenously administered agonists, and the effects of antagonists on these responses can be assessed using pharmacologic analyses. This unit describes the guinea pig trachea/esophagus preparation, in which nicotinic receptor pharmacology can be studied at synapses of the parasympathetic and sympathetic nervous system and the striated musculature of the esophagus. In addition, a preparation whereby the nicotinic receptors of the striated musculature of the diaphragm can be studied is described as are techniques for studying exogenous nicotinic agonist mediated effects in two smooth muscle preparations.Nicotinic acetylcholine receptors (nAChRs) in peripheral tissues are localized almost exclusively to autonomic nerves.

Syntheses and evaluation of halogenated cytisine derivatives and of bioisosteric thiocytisine as potent and selective nAChR ligands

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.

An improved nicotinic pharmacophore and a stereoselective CoMFA-model for nicotinic agonists acting at the central nicotinic acetylcholine receptors labelled by

A study of a series of compounds with agonistic effect at the alpha4beta2 nicotinic acetylcholine receptors resulted in an improved pharmacophore model as well as a CoMFA model. The pharmacophore was composed of three pharmacophoric elements: (1) a site point (a) corresponding to a protonated nitrogen atom, (2) a site point (b) corresponding to an electronegative atom capable of forming a hydrogen bond, and (3) the centre of a heteroaromatic ring or a C=O bond (c). The pharmacophoric elements were related by the following parameters: (a-b) 7.3-8.0 A, (a-c) 6.5-7.4 A, and the angle between the two distance vectors (delta bac) 30.4-35.8 degrees. In addition to this, a stereoselective CoMFA model was developed, which showed good predictability even for compound classes not present in the training set.

exo-2-(Pyridazin-4-yl)-7-azabicyclo[2.2.1]heptanes: syntheses and nicotinic acetylcholine receptor agonist activity of potent pyridazine analogues of (+/-)-epibatidine

A new strategy for the straightforward synthesis of novel racemic epibatidine analogues is presented, in which the 2-chloropyridinyl moiety of epibatidine is bioisosterically replaced by differently substituted pyridazine rings. A key step of the new syntheses is the inverse type Diels-Alder reaction of the electron-rich enol ether 13 with the electron-deficient diazadiene systems of the 1,2,4, 5-tetrazines 14a-d to yield the novel pyridazine analogues of (+/-)-epibatidine 18, 19, 22, and 24. In addition preparation of the N-substituted derivatives, such as 26 and 28, is described. The structures of the novel epibatidine analogues were assigned on the basis of spectral data, that of compound 24 being additionally verified by X-ray crystallography exhibiting two racemic solid-state conformations in the crystal lattice and representing the first X-ray structure of an unprotected 7-azabicyclo[2.2.1]heptane moiety. The nAChR agonist activity of the racemic compounds 18, 19, 22, 24, and 28 was assayed in vitro by whole-cell current recordings from Xenopus oocytes expressing different recombinant nicotinic receptors from the rat. Among the compounds synthesized and tested, the pyridazine analogue 24 of (+/-)-epibatidine and its N-methyl derivative 28 were found to be the most active ones retaining much of the potency of natural epibatidine but with a substantially improved selectivity ratio between the alpha4beta2 and alpha3beta4 subtypes.

5-Azidoimidacloprid and an acyclic analogue as candidate photoaffinity probes for mammalian and insect nicotinic acetylcholine receptors

The 5-azido analogue of the major insecticide imidacloprid, 1-(5-azido-6-chloropyridin-3-ylmethyl)-2-nitroiminoimidaz olidine (1), and an acyclic analogue, N-(5-azido-6-chloropyridin-3-ylmethyl)-N'-methyl-N' '-nitroguanidine (2), were prepared in good yields as candidate photoaffinity probes for mammalian and insect nicotinic acetylcholine receptors (nAChRs). The essential intermediate was 5-azido-6-chloropyridin-3-ylmethyl chloride (3) prepared in two ways: from 6-chloro-5-nitronicotinic acid by selective reduction and then diazotization, and from N-(6-chloropyridin-3-ylmethyl)morpholine by an electrophilic azide introduction with lithium diisopropylamide followed by chlorine substitution of morpholine with ethyl chloroformate. Coupling of 3 with 2-nitroiminoimidazolidine gave 1. Conversion of 3 to 2 was achieved in good yields via the hexahydrotriazine intermediate 14. Fortuitously, the azido substituent in 1 and 2 increases the affinity 7-79-fold for rat brain and recombinant alpha4beta2 nAChRs (K(i)s 4.4-60 nM competing with [(3)H](-)-nicotine) while maintaining high potency on both insect nAChRs (Drosophila and Myzus) (K(i)s 1-15 nM competing with [(3)H]imidacloprid). Azidopyridinyl compounds 1 and 2 are therefore candidate photoaffinity probes for characterization of both mammalian and insect receptors.

Structure-activity relationships in a peptidic alpha7 nicotinic acetylcholine receptor antagonist

alpha-Conotoxins are small disulfide-constrained peptide toxins which act as antagonists at specific subtypes of nicotinic acetylcholine receptors (nACh receptors). In this study, we analyzed the structures and activities of three mutants of alpha-conotoxin ImI, a 12 amino acid peptide active at alpha7 nACh receptors, in order to gain insight into the primary and tertiary structural requirements of neuronal alpha-conotoxin specificity. NMR solution structures were determined for mutants R11E, R7L, and D5N, resulting in representative ensembles of 20 conformers with average pairwise RMSD values of 0.46, 0.52, and 0.62 A from their mean structures, respectively, for the backbone atoms N, C(alpha), and C' of residues 2-11. The R11E mutant was found to have activity near that of wild-type ImI, while R7L and D5N demonstrated activities reduced by at least two orders of magnitude. Comparison of the structures reveals a common two-loop architecture, with variations observed in backbone and side-chain dihedral angles as well as surface electrostatic potentials upon mutation. Correlation of these structures and activities with those from previously published studies emphasizes that existing hypotheses regarding the molecular determinants of alpha-conotoxin specificity are not adequate for explaining peptide activity, and suggests that more subtle features, visualized here at the atomic level, are important for receptor binding. These data, in conjunction with reported characterizations of the acetylcholine binding site, support a model of toxin activity in which a single solvent-accessible toxin side-chain anchors the complex, with supporting weak interactions determining both the efficacy and the subtype specificity of the inhibitory activity.

Neonicotinoid insecticides: molecular features conferring selectivity for insect versus mammalian nicotinic receptors

The favorable selective toxicity of neonicotinoid insecticides (represented here by imidacloprid, thiacloprid, and a nitromethylene analogue) for insects versus mammals is not shared by three of their N-unsubstituted imine derivatives or by nicotine or epibatidine. The same selectivity pattern is evident at the receptor level, i.e., the insect nicotinic acetylcholine receptor (nAChR) versus mammalian nAChR subtypes (alpha1, alpha3, alpha4, and alpha7) assayed independently. The insect-selective compounds are not protonated with a nitroimine, cyanoimine, or nitromethylene group and the mammalian-selective compounds are ionized at physiological pH. We propose that the negatively charged tip of the nitro or cyano group (not a partial positive charge at imidazolidine N-1 as suggested earlier) interacts with a putative cationic subsite of the insect nAChR. This contrasts with the mammalian nAChRs where the iminium cation (+C-NH2 <--> C =+NH2) of the neonicotinoid imine derivatives or ammonium nitrogen of nicotine or epibatidine interacts with the anionic subsite.

(-)-Spiro[1-azabicyclo[2.2.2]octane-3,5'-oxazolidin-2'-one], a conformationally restricted analogue of acetylcholine, is a highly selective full agonist at the alpha 7 nicotinic acetylcholine receptor

Neuronal nicotinic acetylcholine receptors are members of the ligand-gated ion channel receptor superfamily and may play important roles in modulating neurotransmission, cognition, sensory gating, and anxiety. Because of its distribution and abundance in the CNS, the alpha 7 nicotinic receptor is a strong candidate to be involved in some of these functions. In this paper we describe the synthesis and in vitro profile of AR-R17779, (-)-spiro[1-azabicyclo[2.2. 2]octane-3,5'-oxazolidin-2'-one] (4a), a potent full agonist at the rat alpha 7 nicotinic receptor, which is highly selective for the rat alpha 7 nicotinic receptor over the alpha 4 beta 2 subtype. Preliminary SAR of AR-R17779 presented here indicate that there is little scope for modification of this rigid molecule as even minor changes result in significant loss of the alpha 7 nicotinic receptor affinity.

Synthesis and activity of 3-pyridylamine ligands at central nicotinic receptors

A series of thirty 2-(3-pyridylaminomethyl)azetidine, pyrrolidine and piperidine analogues as nicotinic acetylcholine receptor (nAChR) ligands was explored. In general, pyrrolidinyl and many azetidinyl compounds were found to bind with enhanced affinity relative to the piperidines. In the three series, the parallel structural changes (stereochemistry, N-methylation and/or chloro substitution) do not consistently lead to parallel shifts in affinity. The more active compounds (K(i) affinity values ranging from 8.9 to 90 nM) were about as analgesic as nicotine in a tail-flick assay in mice after subcutaneous injections.

Sensitization, desensitization and stimulus-induced recovery of trigeminal neuronal responses to oral capsaicin and nicotine

Repeated application of capsaicin at a 1-min interstimulus interval (ISI) to the tongue induces a progressively increasing irritant sensation (sensitization), followed after a rest period by reduced sensitivity to further capsaicin (desensitization). Sequential reapplication of capsaicin induces irritation that eventually increases to initial levels: stimulus-induced recovery (SIR). In contrast, repeated application of nicotine elicits a declining irritant sensation across trials. To investigate possible neural correlates of these phenomena, we recorded from single units in superficial laminae of the dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal (54 degrees C) and chemical (1 M pentanoic acid) stimulation of the tongue of anesthetized rats. We then recorded responses to either capsaicin (330 microM) or nicotine (0.6 M), delivered either once, repeatedly at 1-min ISI, or continually by constant flow. After the initial capsaicin application and a rest period, the capsaicin was reapplied in the identical manner to test for SIR. The mean response of 14 Vc units to sequential application of pentanoic acid did not vary significantly across trials, indicating lack of tachyphylaxis or sensitization. The averaged response of 11 Vc units to repeated capsaicin increased significantly across the first eight trials and then plateaued. Following the rest period, spontaneous firing had returned to the precapsaicin level. With capsaicin reapplication, the averaged response increased again after a significant delay (due to desensitization), but did not reattain the peak firing rate achieved in the initial series (partial SIR). Constant-flow application of capsaicin induced an identical sensitization followed by nearly complete SIR. A single application of capsaicin induced a significant rise in firing in eight other units, but the rate of rise and maximal firing rate were both much lower compared with repetitive or constant-flow capsaicin. When capsaicin was reapplied once after the rest period, there was no change in firing rate indicating absence of SIR. These results indicate that maintenance of the capsaicin concentration induces a progressive increase in neuronal response that parallels sensitization. With recurrent capsaicin application, desensitization can be overcome to result in a delayed recovery of Vc responses similar to SIR. In contrast, the averaged response of 17 Vc units to repeated or constant-flow application of nicotine increased only over the first 3 min, and then decreased to spontaneous levels even as nicotine was still being applied. These results are consistent with the decrease in the perceived irritation elicited by sequential application of nicotine in humans.