The long internal loop of the alpha 3 subunit targets nAChRs to subdomains within individual synapses on neurons in vivo

Receptors with opposing functions are in postsynaptic microdomains under one presynaptic terminal

Fast excitatory synaptic transmission through vertebrate autonomic ganglia is mediated by postsynaptic nicotinic acetylcholine receptors (nAChRs). We demonstrate a unique postsynaptic receptor microheterogeneity on chick parasympathetic ciliary ganglion neurons-under one presynaptic terminal, nAChRs and glycine receptors formed separate but proximal clusters. Terminals were loaded with [3H]glycine via the glycine transporter-1 (GlyT-1), which localized to the cholinergic presynaptic terminal membrane; depolarization evoked [3H]glycine release that was calcium independent and blocked by the GlyT-1 inhibitor sarcosine. Ganglionic synaptic transmission mediated by nAChRs was attenuated by glycine. Coexistence of separate clusters of receptors with opposing functions under one terminal contradicts Dale's principle and provides a new mechanism for modulating synaptic activity in vivo.

Rapsyn variants in ciliary ganglia and their possible effects on clustering of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) containing the alpha7 gene product can influence a range of cellular events in neurons, depending on receptor location. On chick ciliary neurons, the receptors are concentrated on somatic spines, but little is known about mechanisms responsible for sequestering them there. Rapsyn is a 43-kDa protein essential for clustering nicotinic receptors at the vertebrate neuromuscular junction. RT-PCR confirmed previous studies showing that the chick ciliary ganglion expresses rapsyn transcripts, including several splice variants lacking part or all of exon 2. Heterologous expression of rapsyn constructs, together with nicotinic receptor constructs, shows that chicken full-length rapsyn can induce clustering of both muscle and neuronal nicotinic receptors. Splice variants lacking one or both leucine zipper motifs of exon 2 are unable to cluster the receptors, though, like full-length rapsyn, they cluster themselves. Immunological analysis demonstrates the presence of full-length rapsyn in chick muscle extracts but fails to detect either full-length or splice-variant versions of rapsyn at significant levels in ganglion extracts. The results suggest that rapsyn does not cluster alpha7-nAChRs on ciliary neurons in any way similar to that of receptors at the neuromuscular junction where rapsyn and the receptors are present in approximately equimolar amounts.

Functional alpha6-containing nicotinic receptors are present in chick retina

Despite the fact that the neuronal chick alpha6 subunit was first cloned several years ago and recently has been shown to form acetylcholine (ACh)-activated channels in heterologous systems, no information is yet available concerning the structure and function of the alpha6-containing nicotinic receptors in neuronal tissues. Using subunit-specific antibodies directed against two different epitopes of the chick alpha6 subunit, we performed immunoprecipitation experiments on immunopurified alpha6-containing receptors radiolabeled with the nicotinic agonist [3H]epibatidine (Epi): almost all of the alpha6 receptors contained the beta4 subunit, 51% the beta3 subunit, 42% the alpha3 subunit, and 7.5% the beta2 subunit. Western blot analyses of the purified receptors confirmed the presence of the alpha3, beta3, beta2, and beta4 subunits, and the absence of the alpha4, alpha5, and alpha7 subunits. The alpha6-containing receptors bind [3H]Epi (Kd = 35 pM) and a number of other nicotinic agonists with very high affinity, the rank order being Epi >> cytisine > nicotine > 1, 1-dimethyl-4-phenylpiperazinium > acetylcholine > carbamylcholine. The alpha6 receptors also have a distinct antagonist pharmacological profile with a rank order of potency of alpha-conotoxin MII > methyllycaconitine > dihydro-beta-erythroydine > MG624 > d-tubocurarine > decamethonium > hexamethonium. When reconstituted in lipid bilayers, the alpha6-containing receptors form functional cationic channels with a main conductance state of 48 pS. These channels are activated by nicotinic agonists in a dose-dependent manner, and blocked by the nicotinic antagonist d-tubocurarine.

Nicotinic acetylcholine receptors assembled from the alpha7 and beta3 subunits

Intracellular recordings were performed in voltage-clamped Xenopus oocytes upon injection with a mixture of cDNAs encoding the beta3 and mutant alpha7 (L247Talpha7) neuronal nicotinic acetylcholine receptor (nAChR) subunits. The expressed receptors maintained sensitivity to methyllycaconitine and to alpha-bungarotoxin but exhibited a functional profile strikingly different from that of the homomeric L247Talpha7 receptor. The heteromeric L247Talpha7beta3 nAChR had a lower apparent affinity and a faster rate of desensitization than L247Talpha7 nAChR, exhibited nonlinearity in the I-V relationship, and was inhibited by 5-hydroxytryptamine, much like wild type alpha7 (WTalpha7) nAChR. Single channel recordings in cell-attached mode revealed unitary events with a slope conductance of 19 picosiemens and a lifetime of 5 ms, both values being much smaller than those of the homomeric receptor channel. Upon injection with a mixture of WTalpha7 and beta3 cDNAs, clear evidence was obtained for the plasma membrane assembly of heteromeric nAChRs, although ACh could not activate these receptors. It is concluded that beta3, long believed to be an orphan subunit, readily co-assembles with other subunits to form heteromeric receptors, some of which may be negative regulators of cholinergic function.

Megacystis, mydriasis, and ion channel defect in mice lacking the alpha3 neuronal nicotinic acetylcholine receptor

The alpha3 subunit of the neuronal nicotinic acetylcholine receptor is widely expressed in autonomic ganglia and in some parts of the brain. The alpha3 subunit can form heteromultimeric ion channels with other alpha subunits and with beta2 and beta4 subunits, but its function in vivo is poorly understood. We prepared a null mutation for the alpha3 gene by deletion of exon 5 and found that homozygous (-/-) mice lacked detectable mRNA on Northern blotting. The -/- mice survive to birth but have impaired growth and increased mortality before and after weaning. The -/- mice have extreme bladder enlargement, dribbling urination, bladder infection, urinary stones, and widely dilated ocular pupils that do not contract in response to light. Detailed histological studies of -/- mice revealed no significant abnormalities in brain or peripheral tissues except urinary bladder, where inflammation was prominent. Ganglion cells and axons were present in bladder and bowel. Bladder strips from -/- mice failed to contract in response to 0.1 mM nicotine, but did contract in response to electrical field stimulation or carbamoylcholine. The number of acetylcholine-activated single-channel currents was severely reduced in the neurons of superior cervical ganglia in -/- mice with five physiologically distinguishable nicotinic acetylcholine receptor subtypes with different conductance and kinetic properties in wild-type mice, all of which were reduced in -/- mice. The findings in the alpha3-null mice suggest that this subunit is an essential component of the nicotinic receptors mediating normal function of the autonomic nervous system. The phenotype in -/- mice may be similar to the rare human genetic disorder of megacystis-microcolon-intestinal hypoperistalsis syndrome.