Neuronal nicotinic receptors: from structure to pathology

More than just a barrier: urothelium as a drug target for urinary bladder pain

Although the urinary bladder urothelium has classically been thought of as a passive barrier to ions/solutes, a number of novel properties have been recently attributed to these cells. Studies have revealed that the urothelium is involved in sensory mechanisms (i.e., ability to express a number of sensor molecules or respond to thermal, mechanical, and chemical stimuli) and can release chemical mediators. Localization of afferent nerves next to the urothelium suggests these cells may be targets for transmitters released from bladder nerves or that chemicals released by urothelial cells may alter afferent excitability. Taken together, these and other findings highlighted in this review suggest a sensory function for the urothelium. Elucidation of mechanisms impacting on urothelial function may provide insights into the pathology of bladder dysfunction.

Introduction of unsaturation into the N-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: effect on affinity and selectivity

N-n-octylnicotinium iodide (NONI) and N-n-decylnicotinium iodide (NDNI) are selective nicotinic receptor (nAChR) antagonists mediating nicotine-evoked striatal dopamine (DA) release, and inhibiting [3H]nicotine binding, respectively. This study evaluated effects of introducing unsaturation into the N-n-alkyl chains of NONI and NDNI on inhibition of [3H]nicotine and [3H]methyllycaconitine binding (alpha4beta2* and alpha7* nAChRs, respectively), (86)Rb+ efflux and [3H]DA release (agonist or antagonist effects at alpha4beta2* and alpha6beta2*-containing nAChRs, respectively). In the NONI series, introduction of a C3-cis- (NONB3c), C3-trans- (NONB3t), C7-double-bond (NONB7e), or C3-triple-bond (NONB3y) afforded a 4-fold to 250-fold increased affinity for [3H]nicotine binding sites compared with NONI. NONB7e and NONB3y inhibited nicotine-evoked 86Rb+ efflux, indicating alpha4beta2* antagonism. NONI analogs exhibited a 3-fold to 8-fold greater potency inhibiting nicotine-evoked [3H]DA overflow compared with NONI (IC50 = 0.62 microM; Imax = 89%), with no change in Imax, except for NONB3y (Imax = 50%). In the NDNI series, introduction of a C4-cis- (NDNB4c), C4-trans-double-bond (NDNB4t), or C3-triple-bond (NDNB3y) afforded a 4-fold to 80-fold decreased affinity for [3H]nicotine binding sites compared with NDNI, whereas introduction of a C9 double-bond (NDNB9e) did not alter affinity. NDNB3y and NDNB4t inhibited nicotine-evoked 86Rb+ efflux, indicating antagonism at alpha4beta2* nAChRs. Although NDNI had no effect, NDNB4t and NDNB9e potently inhibited nicotine-evoked [3H]DA overflow (IC50 = 0.02-0.14 microM, Imax = 90%), as did NDNB4c (IC50 = 0.08 microM; Imax = 50%), whereas NDNB3y showed no inhibition. None of the analogs had significant affinity for alpha7* nAChRs. Thus, unsaturated NONI analogs had enhanced affinity at alpha4beta2*- and alpha6beta2*-containing nAChRs, however a general reduction of affinity at alpha4beta2* and an uncovering of antagonist effects at alpha6beta2*-containing nAChRs were observed with unsaturated NDNI analogs.

Single-channel kinetic analysis of chimeric alpha7-5HT3A receptors

The receptor chimera alpha7-5HT3A has served as a prototype for understanding the pharmacology of alpha7 neuronal nicotinic receptors, yet its low single channel conductance has prevented studies of the activation kinetics of single receptor channels. In this study, we show that introducing mutations in the M3-M4 cytoplasmic linker of the chimera alters neither the apparent affinity for the agonist nor the EC50 but increases the amplitude of agonist-evoked single channel currents to enable kinetic analysis. Channel events appear as single brief openings flanked by long closings or as bursts of several openings in quick succession. Both the open and closed time distributions are described as the sum of multiple exponential components, but these do not change over a wide range of acetylcholine (ACh), nicotine, or choline concentrations. Bursts elicited by a saturating concentration of ACh contain brief and long openings and closings, and a cyclic scheme containing two open and two closed states is found to adequately describe the data. The analysis indicates that once fully occupied, the receptor opens rapidly and efficiently, and closes and reopens several times before it desensitizes. Channel closing and desensitization occur at similar rates and account for the invariant open and closed time distributions.

Recent developments in the field of arrow and dart poisons

Arrow and dart poisons, considered as conventional natural sources for future drug discovery, have already provided numerous biologically active molecules used as drugs in therapeutic applications or in pharmacological research. Plants containing alkaloids or cardiotonic glycosides have generally been the main ingredients responsible for the efficacy of these poisons, although some animals, such as frogs, have also been employed. This paper, without being exhaustive, reports the greater strides made during the past 15 years in the understanding of the chemical nature and biological properties of arrow and dart poison constituents. Examples both of promising biological properties shown by these molecules and of crucial discoveries achieved by their use as pharmacological tools are given. Further studies of these toxic principles are likely to enable scientists to find new valuable lead compounds, useful in many fields of research, like oncology, inflammation and infectious diseases.

Ubiquilin-1 regulates nicotine-induced up-regulation of neuronal nicotinic acetylcholine receptors

Chronic exposure to nicotine, as in tobacco smoking, up-regulates nicotinic acetylcholine receptor surface expression in neurons. This up-regulation has been proposed to play a role in nicotine addiction and withdrawal. The regulatory mechanisms behind nicotine-induced up-regulation of surface nicotinic acetylcholine receptors remain to be determined. It has recently been suggested that nicotine stimulation acts through increased assembly and maturation of receptor subunits into functional pentameric receptors. Studies of muscle nicotinic acetylcholine receptors suggest that the availability of unassembled subunits in the endoplasmic reticulum can be regulated by the ubiquitin-proteosome pathway, resulting in altered surface expression. Here, we describe a role for ubiquilin-1, a ubiquitin-like protein with the capacity to interact with both the proteosome and ubiquitin ligases, in regulating nicotine-induced up-regulation of neuronal nicotinic acetylcholine receptors. Ubiquilin-1 interacts with unassembled alpha3 and alpha4 subunits when coexpressed in heterologous cells and interacts with endogenous nicotinic acetylcholine receptors in neurons. Coexpression of ubiquilin-1 and neuronal nicotinic acetylcholine receptors in heterologous cells dramatically reduces the expression of the receptors on the cell surface. In cultured superior cervical ganglion neurons, expression of ubiquilin-1 abolishes nicotine-induced up-regulation of nicotinic acetylcholine receptors but has no effect on the basal level of surface receptors. Coimmunostaining shows that the interaction of ubiquilin-1 with the alpha3 subunit draws the receptor subunit and proteosome into a complex. These data suggest that ubiquilin-1 limits the availability of unassembled nicotinic acetylcholine receptor subunits in neurons by drawing them to the proteosome, thus regulating nicotine-induced up-regulation.

The vagus nerve and the nicotinic anti-inflammatory pathway

Physiological anti-inflammatory mechanisms are selected by evolution to effectively control the immune system and can be exploited for the treatment of inflammatory disorders. Recent studies indicate that the vagus nerve (which is the longest of the cranial nerves and innervates most of the peripheral organs) can modulate the immune response and control inflammation through a 'nicotinic anti-inflammatory pathway' dependent on the alpha7-nicotinic acetylcholine receptor (alpha7nAChR). Nicotine has been used in clinical trials for the treatment of ulcerative colitis, but its clinical applications are limited by its unspecific effects and subsequent toxicity. This article reviews recent advances supporting the therapeutic potential of selective nicotinic agonists in several diseases. Similar to the development of alpha- and beta-agonists for adrenoceptors, selective agonists for alpha7nAChR could represent a promising pharmacological strategy against infectious and inflammatory diseases.

The Ror receptor tyrosine kinase CAM-1 is required for ACR-16-mediated synaptic transmission at the C. elegans neuromuscular junction

Nicotinic (cholinergic) neurotransmission plays a critical role in the vertebrate nervous system, underlies nicotine addiction, and nicotinic receptor dysfunction leads to neurological disorders. The C. elegans neuromuscular junction (NMJ) shares many characteristics with neuronal synapses, including multiple classes of postsynaptic currents. Here, we identify two genes required for the major excitatory current found at the C. elegans NMJ: acr-16, which encodes a nicotinic AChR subunit homologous to the vertebrate alpha7 subunit, and cam-1, which encodes a Ror receptor tyrosine kinase. acr-16 mutants lack fast cholinergic current at the NMJ and exhibit synthetic behavioral deficits with other known AChR mutants. In cam-1 mutants, ACR-16 is mislocalized and ACR-16-dependent currents are disrupted. The postsynaptic deficit in cam-1 mutants is accompanied by alterations in the distribution of cholinergic vesicles and associated synaptic proteins. We hypothesize that CAM-1 contributes to the localization or stabilization of postsynaptic ACR-16 receptors and presynaptic release sites.

Heterogeneity and Selective Targeting of Neuronal Nicotinic Acetylcholine Receptor (nAChR) Subtypes Expressed on Retinal Afferents of the Superior Colliculus and Lateral Geniculate Nucleus: Identification of a New Native nAChR Subtype α3β2(α5 or β3) Enriched in Retinocollicular Afferents

The activation of neuronal nicotinic acetylcholine receptors (nAChRs) has been implicated in the activity-dependent development and plasticity of retina and the refinement of retinal projections. Pharmacological and functional studies have also indicated that different presynaptic nAChRs can have a modulatory function in retinotectal synapses. We biochemically and pharmacologically identified the multiple nAChR subtypes expressed on retinal afferents of the superior colliculus (SC) and lateral geniculate nucleus (LGN). We found that the alpha6beta2(*) and alpha4(nonalpha6)beta2(*) nAChRs are the major receptor populations expressed in both SC and LGN. In addition, the LGN contains two minor populations of alpha2alpha6beta2(*) and alpha3beta2(*) subtypes, whereas the SC contains a relatively large population of a new native subtype, the alpha3beta2(alpha5/beta3) nAChR. This subtype binds the alpha-conotoxin MII with an affinity 50 times lower than that of the native alpha6beta2(*) subtype. Studies of tissues obtained from eye-enucleated animals allowed the identification of nAChRs expressed by retinal afferents: in SC alpha6beta2(*), alpha4alpha6beta2(*), and alpha3beta2(*) (approximately 45, 35, and 20%, respectively), in LGN, alpha4alpha6beta2(*), alpha6beta2(*), alpha4beta2(*), alpha2alpha6beta2(*), and alpha3beta2(*) (approximately 40, 30, 20, 5, and 5%, respectively). In both regions, more than 50% of nAChRs were not expressed by retinal afferents and belonged to the alpha4beta2(*) (90%) or alpha4alpha5beta2(*) (10%) subtypes. Moreover, studies of the SC tissues obtained from wild-type and alpha4, alpha6, and beta3 knockout mice confirmed and extended the data obtained in rat tissue and allowed a comprehensive dissection of the composition of nAChR subtypes present in this retinorecipient area.

Increased severity of experimental colitis in alpha5 nicotinic acetylcholine receptor subunit-deficient mice

Substantial evidence suggests a negative association between cigarette smoking and the incidence and severity of ulcerative colitis, a common human inflammatory bowel disease. Nicotine has been implicated in this association. The detection of nicotinic acetylcholine receptors in colonic epithelium, the primary tissue affected in ulcerative colitis, suggests a role for these receptors in the beneficial effect of nicotine on colonic inflammation. Using an animal model, we demonstrate for the first time that alpha5 nicotinic acetylcholine receptor knockout mice have significantly more severe experimental colitis than wild-type controls and that nicotine significantly ameliorates its course when compared with wild-type controls. These findings suggest that alpha5-containing nicotinic acetylcholine receptors participate in the modulation of colitis in mice, but other nicotinic acetylcholine receptor subunits also mediate the antiinflammatory effects of nicotine.

acr-16 encodes an essential subunit of the levamisole-resistant nicotinic receptor at the Caenorhabditis elegans neuromuscular junction

The Caenorhabditis elegans neuromuscular junction (NMJ) contains three pharmacologically distinct ionotropic receptors: gamma-aminobutyric acid receptors, levamisole-sensitive nicotinic receptors, and levamisole-insensitive nicotinic receptors. The subunit compositions of the gamma-aminobutyric acid- and levamisole-sensitive receptors have been elucidated, but the levamisole-insensitive acetylcholine receptor is uncharacterized. To determine which of the approximately 40 putative nicotinic receptor subunit genes in the C. elegans genome encodes the levamisole-resistant receptor, we utilized MAPCeL, a microarray profiling strategy. Of seven nicotinic receptor subunit transcripts found to be enriched in muscle, five encode the levamisole receptor subunits, leaving two candidates for the levamisole-insensitive receptor: acr-8 and acr-16. Electrophysiological analysis of the acr-16 deletion mutant showed that the levamisole-insensitive muscle acetylcholine current was eliminated, whereas deletion of acr-8 had no effect. These data suggest that ACR-16, like its closest vertebrate homolog, the nicotinic receptor alpha7-subunit, may form homomeric receptors in vivo. Genetic ablation of both the levamisole-sensitive receptor and acr-16 abolished all cholinergic synaptic currents at the NMJ and severely impaired C. elegans locomotion. Therefore, ACR-16-containing receptors account for all non-levamisole-sensitive nicotinic synaptic signaling at the C. elegans NMJ. The determination of subunit composition for all three C. elegans body wall muscle ionotropic receptors provides a critical foundation for future research at this tractable model synapse.

Requirement of nicotinic acetylcholine receptor subunit beta2 in the maintenance of spiral ganglion neurons during aging

Age-related hearing loss (presbycusis) is a major health concern for the elderly. Loss of spiral ganglion neurons (SGNs), the primary sensory relay of the auditory system, is associated consistently with presbycusis. The causative molecular events responsible for age-related loss of SGNs are unknown. Recent reports directly link age-related neuronal loss in cerebral cortex with the loss of high-affinity nicotine acetylcholine receptors (nAChRs). In cochlea, cholinergic synapses are made by olivocochlear efferent fibers on the outer hair cells that express alpha9 nAChR subunits and on the peripheral projections of SGNs that express alpha2, alpha4-7, and beta2-3 nAChR subunits. A significantly decreased expression of the beta2 nAChR subunit in SGNs was found specifically in mice susceptible to presbycusis. Furthermore, mice lacking the beta2 nAChR subunit (beta2-/-), but not mice lacking the alpha5 nAChR subunit (alpha5-/-), have dramatic hearing loss and significant reduction in the number of SGNs. Our findings clearly established a requirement for beta2 nAChR subunit in the maintenance of SGNs during aging.

Expression of nigrostriatal alpha 6-containing nicotinic acetylcholine receptors is selectively reduced, but not eliminated, by beta 3 subunit gene deletion

mRNAs for the neuronal nicotinic acetylcholine receptor (nAChR) alpha6 and beta3 subunits are abundantly expressed and colocalized in dopaminergic cells of the substantia nigra and ventral tegmental area. Studies using subunit-null mutant mice have shown that alpha6- or beta3-dependent nAChRs bind alpha-conotoxin MII (alpha-CtxMII) with high affinity and modulate striatal dopamine release. This study explores the effects of beta3 subunit-null mutation on striatal and midbrain nAChR expression, composition, and pharmacology. Ligand binding and immunoprecipitation experiments using subunit-specific antibodies indicated that beta3-null mutation selectively reduced striatal alpha6* nAChR expression by 76% versus beta3(+/+) control. Parallel experiments showed a smaller reduction in both midbrain alpha3* and alpha6* nAChRs (34 and 42% versus beta3(+/+) control, respectively). Sedimentation coefficient determinations indicated that residual alpha6* nAChRs in beta3(-/-) striatum were pentameric, like their wild-type counterparts. Immunoprecipitation experiments on immunopurified beta3* nAChRs demonstrated that almost all wild-type striatal beta3* nAChRs also contain alpha4, alpha6, and beta2 subunits, although a small population of non-beta3 alpha6* nAChRs is also expressed. beta3 subunit incorporation seemed to increase alpha4 participation in alpha6beta2* complexes. (125)I-Epibatidine competition binding studies showed that the alpha-CtxMII affinity of alpha6* nAChRs from the striata of beta3(-/-) mice was similar to those isolated from beta3(+/+) animals. Together, the results of these experiments show that the beta3 subunit is important for the correct assembly, stability and/or transport of alpha6* nAChRs in dopaminergic neurons and influences their subunit composition. However, beta3 subunit expression is not essential for the expression of alpha6*, high-affinity alpha-CtxMII binding nAChRs.

Physical activity in relation to cancer of the colon and rectum in a cohort of male smokers

We examined the association between occupational and leisure physical activity and colorectal cancer in a cohort of male smokers. Among the 29,133 men aged 50-69 years in the Alpha-Tocopherol, Beta-Carotene Cancer Prevention study,152 colon and 104 rectal cancers were documented during up to 12 years of follow-up. For colon cancer, compared with sedentary workers, men in light occupational activity had a relative risk (RR) of 0.60 [95% confidence interval (CI), 0.34-1.04], whereas those in moderate/heavy activity had an RR of 0.45 (CI, 0.26-0.78; P for trend, 0.003). Subsite analysis revealed a significant association for moderate/heavy occupational activity in the distal colon (RR, 0.21; CI, 0.09-0.51) but not in the proximal colon (RR, 0.87; CI, 0.40-1.92). There was no significant association between leisure activity and colon cancer (active versus sedentary; RR, 0.82; CI, 0.59-1.13); however, the strongest inverse association was found among those most active in both work and leisure (RR, 0.33; CI, 0.16-0.71). For rectal cancer, there were risk reductions for those in light (RR, 0.71; CI, 0.36-1.37) and moderate/heavy occupational activity (RR, 0.50; CI, 0.26-0.97; P for trend, 0.04), and no association for leisure activity. These data provide evidence for a protective role of physical activity in colon and rectal cancer.