Nicotinic synapses formed between chick ciliary ganglion neurons in culture resemble those present on the neurons in vivo

Segregation of glutamatergic and cholinergic transmission at the mixed motoneuron Renshaw cell synapse

In neonatal mice motoneurons excite Renshaw cells by releasing both acetylcholine (ACh) and glutamate. These two neurotransmitters activate two types of nicotinic receptors (nAChRs) (the homomeric α₇ receptors and the heteromeric α*ß* receptors) as well as the two types of glutamate receptors (GluRs) (AMPARs and NMDARs). Using paired recordings, we confirm that a single motoneuron can release both transmitters on a single post-synaptic Renshaw cell. We then show that co-transmission is preserved in adult animals. Kinetic analysis of miniature EPSCs revealed quantal release of mixed events associating AMPARs and NMDARs, as well as α₇ and α*ß* nAChRs, but no evidence was found for mEPSCs associating nAChRs with GluRs. Bayesian Quantal Analysis (BQA) of evoked EPSCs showed that the number of functional contacts on a single Renshaw cell is more than halved when the nicotinic receptors are blocked, confirming that the two neurotransmitters systems are segregated. Our observations can be explained if ACh and glutamate are released from common vesicles onto spatially segregated post-synaptic receptors clusters, but a pre-synaptic segregation of cholinergic and glutamatergic release sites is also possible.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

N-cadherin induces partial differentiation of cholinergic presynaptic terminals in heterologous cultures of brainstem neurons and CHO cells

N-cadherin is a calcium-sensitive cell adhesion molecule commonly expressed at synaptic junctions and contributes to formation and maturation of synaptic contacts. This study used heterologous cell cultures of brainstem cholinergic neurons and transfected Chinese Hamster Ovary (CHO) cells to examine whether N-cadherin is sufficient to induce differentiation of cholinergic presynaptic terminals. Brainstem nuclei isolated from transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of choline acetyltransferase (ChAT) transcriptional regulatory elements (ChAT^BACEGFP) were cultured as tissue explants for 5 days and cocultured with transfected CHO cells for an additional 2 days. Immunostaining for synaptic vesicle proteins SV2 and synapsin I revealed a ~3-fold increase in the area of SV2 immunolabeling over N-cadherin expressing CHO cells, and this effect was enhanced by coexpression of p120-catenin. Synapsin I immunolabeling per axon length was also increased on N-cadherin expressing CHO cells but required coexpression of p120-catenin. To determine whether N-cadherin induces formation of neurotransmitter release sites, whole-cell voltage-clamp recordings of CHO cells expressing α3 and β4 nicotinic acetylcholine receptor (nAChR) subunits in contact with cholinergic axons were used to monitor excitatory postsynaptic potentials (EPSPs) and miniature EPSPs (mEPSPs). EPSPs and mEPSPs were not detected in both, control and in N-cadherin expressing CHO cells in the absence or presence of tetrodotoxin (TTX). These results indicate that expression of N-cadherin in non-neuronal cells is sufficient to initiate differentiation of presynaptic cholinergic terminals by inducing accumulation of synaptic vesicles; however, development of readily detectable mature cholinergic release sites and/or clustering of postsynaptic nAChR may require expression of additional synaptogenic proteins.

Abelson family tyrosine kinases regulate the function of nicotinic acetylcholine receptors and nicotinic synapses on autonomic neurons

Abelson family kinases (AFKs; Abl1, Abl2) are non-receptor tyrosine kinases (NRTKs) implicated in cancer, but they also have important physiological roles that include regulating synaptic structure and function. Recent studies using Abl-deficient mice and the antileukemia drug STI571 [imatinib mesylate (Gleevec); Novartis], which potently and selectively blocks Abl kinase activity, implicate AFKs in regulating presynaptic neurotransmitter release in hippocampus and postsynaptic clustering of nicotinic acetylcholine receptors (nAChRs) in muscle. Here, we tested whether AFKs are relevant for regulating nAChRs and nAChR-mediated synapses on autonomic neurons. AFK immunoreactivity was detected in ciliary ganglion (CG) lysates and neurons, and STI571 application blocked endogenous Abl tyrosine kinase activity. With similar potency, STI571 specifically reduced whole-cell current responses generated by both nicotinic receptor subtypes present on CG neurons (α3*- and α7-nAChRs) and lowered the frequency and amplitude of α3*-nAChR-mediated excitatory postsynaptic currents. Quantal analysis indicated that the synaptic perturbations were postsynaptic in origin, and confocal imaging experiments revealed they were unaccompanied by changes in nAChR clustering or alignment with presynaptic terminals. The results indicate that in autonomic neurons, Abl kinase activity normally supports postsynaptic nAChR function to sustain nAChR-mediated neurotransmission. Such consequences contrast with the influence of Abl kinase activity on presynaptic function and synaptic structure in hippocampus and muscle, respectively, demonstrating a cell-specific mechanism of action. Finally, because STI571 potently inhibits Abl kinase activity, the autonomic dysfunction side effects associated with its use as a chemotherapeutic agent may result from perturbed α3*- and/or α7-nAChR function.

Fast synaptic transmission in the goldfish CNS mediated by multiple nicotinic receptors

Usually nicotinic receptors in the central nervous system only influence the strength of a signal between neurons. At a few critical connections, for instance some of those involved in the flight response, nicotinic receptors not only modulate the signal, they actually determine whether a signal is conveyed or not. We show at one of the few such connections accessible for study, up to three different nicotinic receptor subtypes mediate the signal. The subtypes appear to be clustered in separate locations. Depending on the number and combination of the subtypes present the signal can range from short to long duration and from low to high amplitude. This provides a critical connection with a built-in plasticity and may enable it to adapt to a changing environment.

Lateral mobility of nicotinic acetylcholine receptors on neurons is determined by receptor composition, local domain, and cell type

The lateral mobility of surface receptors can define the signaling properties of a synapse and rapidly change synaptic function. Here we use single-particle tracking with Quantum Dots to follow nicotinic acetylcholine receptors (nAChRs) on the surface of chick ciliary ganglion neurons in culture. We find that both heteropentameric alpha3-containing receptors (alpha3*-nAChRs) and homopentameric alpha7-containing receptors (alpha7-nAChRs) access synaptic domains by lateral diffusion. They have comparable mobilities and display Brownian motion in extrasynaptic space but are constrained and move more slowly in synaptic space. The two receptor types differ in the nature of their synaptic restraints. Disruption of lipid rafts, PDZ-containing scaffolds, and actin filaments each increase the mobility of alpha7-nAChRs in synaptic space while collapse of microtubules has no effect. The opposite is seen for alpha3*-nAChRs where synaptic mobility is increased only by microtubule collapse and not the other manipulations. Other differences are found for regulation of alpha3*-nAChR and alpha7-nAChR mobilities in extrasynaptic space. Most striking are effects on the immobile populations of alpha7-nAChRs and alpha3*-nAChRs. Disruption of either lipid rafts or PDZ scaffolds renders half of the immobile alpha3*-nAChRs mobile without changing the proportion of immobile alpha7-nAChRs. Similar results were obtained with chick sympathetic ganglion neurons, though regulation of receptor mobility differed in at least one respect from that seen with ciliary ganglion neurons. Control of nAChR lateral mobility, therefore, is determined by mechanisms that are domain specific, receptor subtype dependent, and cell-type constrained. The outcome is a system that could tailor nicotinic signaling capabilities to specific needs of individual locations.

Synchronous and asynchronous transmitter release at nicotinic synapses are differentially regulated by postsynaptic PSD-95 proteins

The rate and timing of information transfer at neuronal synapses are critical for determining synaptic efficacy and higher network function. Both synchronous and asynchronous neurotransmitter release shape the pattern of synaptic influences on a neuron. The PSD-95 family of postsynaptic scaffolding proteins, in addition to organizing postsynaptic components at glutamate synapses, acts transcellularly to regulate synchronous glutamate release. Here we show that PSD-95 family members at nicotinic synapses on chick ciliary ganglion neurons in culture execute multiple functions to enhance transmission. Together, endogenous PSD-95 and SAP102 in the postsynaptic cell appear to regulate transcellularly the synchronous release of transmitter from presynaptic terminals onto the neuron while stabilizing postsynaptic nicotinic receptor clusters under the release sites. Endogenous SAP97, in contrast, has no effect on receptor clusters but acts transcellularly from the postsynaptic cell through N-cadherin to enhance asynchronous release. These separate and parallel regulatory pathways allow postsynaptic scaffold proteins to dictate the pattern of cholinergic input a neuron receives; they also require balancing of PSD-95 protein levels to avoid disruptive competition that can occur through common binding domains.

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC(1)Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC(1)R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC(1)R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC(1)R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.

Nicotinic receptors concentrated in the subsynaptic membrane do not contribute significantly to synaptic currents at an embryonic synapse in the chicken ciliary ganglion

Rapid synaptic transmission at the calyciform synapse in the embryonic chicken ciliary ganglion is mediated by two classes of nicotinic receptors: those containing alpha3 subunits [alpha3-nicotinic ACh receptors (nAChRs)] and those containing alpha7 subunits (alpha7-nAChRs). alpha3-nAChRs and alpha7-nAChRs are differentially distributed on the cell surface; alpha3-nAChRs are concentrated at postsynaptic densities, whereas both alpha7-nAChRs and alpha3-nAChRs are found extrasynaptically on somatic spines. I explored the contribution of alpha3-nAChRs and alpha7-nAChRs to uniquantal responses, measured as mEPSCs, or as evoked responses under low release probability conditions. The contribution that each nAChR makes to uniquantal response shape was determined by blocking one nAChR type; pharmacologically isolated alpha7-nAChR responses were kinetically fast (rise time, 0.32 +/- 0.02 ms; decay time, 1.66 +/- 0.18 ms; mean +/- SD; n = 6 cells), whereas pharmacologically isolated alpha3-nAChR responses were slow (rise time, 1.28 +/- 0.35 ms; decay time, 6.71 +/- 1.46 ms; n = 8 cells). In the absence of antagonists, most cells (11 of 14) showed heterogeneity in the kinetics of uniquantal responses, with approximately 25% of events exhibiting fast, alpha7-nAChR-like kinetics and approximately 75% of events exhibiting the kinetics expected of coactivation of alpha7-nAChRs and alpha3-nAChRs. Cells rarely showed significant numbers of uniquantal responses with slow, alpha3-nAChR-like kinetics, which was unexpected given that alpha3-nAChRs alone are concentrated at postsynaptic densities. The only site where ACh quanta can activate both alpha3-nAChRs and alpha7-nAChRs readily is on the somatic spines, where alpha7-nAChRs and alpha3-nAChRs are present extrasynaptically. At the calyciform synapse, rapid synaptic transmission is mediated apparently without participation of ionotropic receptors concentrated at postsynaptic densities.

Pituitary adenylate cyclase-activating polypeptide (PACAP) alters parasympathetic neuron gene expression in a time-dependent fashion

Neuropeptides, including pituitary adenylate cyclase-activating polypeptide (PACAP), can influence diverse cellular processes over a broad temporal range. In ciliary ganglion (CG) neurons, for example, PACAP binding to high-affinity PAC1 receptors triggers transduction cascades that both rapidly modulate nicotinic receptors and synapses and support long-term survival. Since PACAP/PAC1 signaling recruits intracellular messengers and effectors that potently alter transcription, we examined its activation of the transcription factor CREB and then tested for changes in gene expression. PACAP/PAC1 signaling rapidly induced prolonged CREB activation in CG neurons by a phospholipase C -independent mechanism supported by Ca2+-influx, adenylate cyclase, and effectors, including protein kinase C (PKC) and possibly PKA. Since PACAP is abundant in the CG and released from depolarized presynaptic terminals, it is well suited to regulate gene expression relevant to neuronal and synaptic development. Gene array screens conducted using RNA from CG cultures grown with PACAP for 1/4, 24, or 96 h revealed a time-dependent pattern of > 600 regulated transcripts, including several encoding proteins implicated in synaptic function, neuronal survival, and development. The results underscore rapid, neuromodulatory, and long-term, neurotrophic consequences of PAC1 signaling in CG neurons and suggest that PACAP exerts such diverse influences by altering the expression of specific gene transcripts in a time-dependent fashion.

Postsynaptic neuroligin enhances presynaptic inputs at neuronal nicotinic synapses

Neuroligins are cell adhesion molecules that interact with neurexins on adjacent cells to promote glutamatergic and GABAergic synapse formation in culture. We show here that neuroligin enhances nicotinic synapses on neurons in culture, increasing synaptic input. When neuroligin is overexpressed in neurons, the extracellular domain induces presynaptic specializations in adjacent cholinergic neurons as visualized by SV2 puncta. The intracellular domain is required to translate the SV2 puncta into synaptic input as reflected by increases in the frequency of spontaneous mini-synaptic currents. The PDZ-binding motif of neuroligin is not needed for these effects. Together, the extracellular and proximal intracellular domains of neuroligin are sufficient to induce presynaptic specializations, align them over postsynaptic receptor clusters, and increase synaptic function. Manipulation of endogenous neuroligin with beta-neurexin-expressing cells confirms its presence; repressing function with dominant negative constructs and inhibitory shRNA shows that endogenous neuroligin helps confer functionality on existing nicotinic synaptic contacts. Endogenous neuroligin does not appear to be required, however, for initial formation of the contacts, suggesting that other components under these conditions can also initiate synapse formation. The results indicate that postsynaptic neuroligin is important for functional nicotinic synapses on neurons and that the effects achieved will likely depend on neuroligin levels.

Sequential interplay of nicotinic and GABAergic signaling guides neuronal development

GABA (gamma-aminobutyric acid), the major inhibitory transmitter in the brain, goes through a transitory phase of excitation during development. The excitatory phase promotes neuronal growth and integration into circuits. We show here that spontaneous nicotinic cholinergic activity is responsible for terminating GABAergic excitation and initiating inhibition. It does so by changing chloride transporter levels, shifting the driving force on GABA-induced currents. The timing of the transition is critical, because the two phases of GABAergic signaling provide contrasting developmental instructions. Synergistic with nicotinic excitation, GABAergic inhibition constrains neuronal morphology and innervation. The results reveal a multitiered activity-dependent strategy controlling neuronal development.

Depolarization promotes survival of ciliary ganglion neurons by BDNF-dependent and -independent mechanisms

Membrane activity upregulates brain derived neurotrophic factor (BDNF) expression to coordinately support neuronal survival in many systems. In parasympathetic ciliary ganglion (CG) neurons, activity mimicked by KCl depolarization provides nearly full trophic support. While BDNF has been considered unable to influence CG neuronal survival, we now document its expression during CG development and show that low concentrations do support survival via high-affinity TrkB receptors. Furthermore, a contribution of BDNF to activity-induced trophic support was demonstrated by showing that KCl depolarization increased BDNF mRNA and protein in, and release of BDNF from, CG neuron cultures. Application of anti-BDNF blocking antibody or mitogen activated protein kinase (MAPK) kinase inhibitor, attenuated depolarization-supported survival, implicating canonical BDNF/TrkB signaling. Ca2+-Calmodulin kinase II (CaMKII) was also required since its inhibition combined with anti-BDNF or MAPK kinase inhibitor abolished or greatly reduced the trophic effects of depolarization. Membrane activity may thus support CG neuronal survival both by stimulating release of BDNF that binds high-affinity TrkB receptors to activate MAPK and by recruiting CaMKII. This mechanism could have relevance late in development in vivo as ganglionic transmission and the effectiveness of BDNF over other growth factors both increase.

PACAP support of neuronal survival requires MAPK- and activity-generated signals

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the parasympathetic ciliary ganglion (CG) and modulates nicotinic acetylcholine receptor function. PACAP also provides trophic support, promoting partial survival of CG neurons in culture and full survival when accompanied by membrane depolarization. We probed the adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP to determine their respective roles in supporting neuronal survival and examined their interaction with signals generated by membrane activity. While PLC-dependent signaling was dispensable, AC-generated signals proved critical for PACAP to support neuronal survival. Specifically, PACAP-supported survival was mimicked by 8Br-cAMP and blocked by inhibiting either PKA or the phosphorylation of mitogen-activated protein kinase (MAPK). The ability of PACAP to promote survival was additionally dependent on spontaneous activity as blocking Na+ or Ca2+ channel currents completely abrogated trophic effects. Our results underscore the importance of coordinated MAPK- and activity-generated signals in transducing neuropeptide-mediated parasympathetic neuronal survival.

Effect of chronic nicotine treatment on localization of neuronal nicotinic acetylcholine receptors at cellular level

Chronic nicotine treatment increases the number of neuronal nicotinic acetylcholine receptors (nAChRs). Localization of nAChRs at a cellular level determines their functional role. However, changes in the localization of nAChRs caused by chronic nicotine treatment are not well known. In this study, we have examined the effects of chronic nicotine treatment on alpha7 and beta2 nAChR subunits in vitro in cell lines and in vivo in mouse striatum. In vitro, two different cell lines were used, SH-SY5Y cells endogenously expressing several nAChR subtypes and SH-EP1-halpha7 cells, transfected with the human alpha7 nAChR subunit gene. Effects of chronic nicotine treatment (10 microM, 3 days) were studied in vitro by using confocal and electron microscopy and calcium fluorometry. In vitro in SH-SY5Y cells, alpha7 and beta2 subunits formed groups, unlike alpha7 subunits in SH-EP1-halpha7 cells, which were partially localized on endoplastic reticulum. Chronic nicotine treatment did not change the localization of nAChRs in endosomes, but caused clustering of alpha7 subunits in SH-EP1-halpha7 cells. In vivo, nicotine was given to mice in their drinking water for 7 weeks. Results showed that alpha7 and beta2 subunits formed groups, and that chronic nicotine treatment increased the size of the clusters. As a conclusion, our data show that there are large intracellular pools of nAChR subunits, which are partially localized on endoplastic reticulum. Chronic nicotine treatment does not change endocytotic trafficking of nAChRs. Chronic nicotine treatment increased clustering of nAChRs, which could have a role in the release of dopamine (DA) evoked by nicotine.

Brain-derived neurotrophic factor and trkB signaling in parasympathetic neurons: relevance to regulating alpha7-containing nicotinic receptors and synaptic function

Parasympathetic neurons do not require neurotrophins for survival and are thought to lack high-affinity neurotrophin receptors (i.e., trks). We report here, however, that mRNAs encoding both brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tropomyosin-related kinase B (trkB) are expressed in the parasympathetic chick ciliary ganglion (CG) and that BDNF-like protein is present in the ganglion and in the iris, an important peripheral target of ciliary neurons. Moreover, CG neurons express surface trkB and exogenous BDNF not only initiates trk-dependent signaling, but also alters nicotinic acetylcholine receptor (nAChR) expression and synaptic transmission. In particular, BDNF applied to CG neurons rapidly activates cAMP-dependent response element-binding protein (CREB), and over the long-term selectively upregulates expression of alpha7-subunit-containing, homomeric nAChRs (alpha7-nAChRs), increasing alpha7-subunit mRNA levels, alpha7-nAChR surface sites, and alpha7-nAChR-mediated whole-cell currents. At nicotinic synapses formed on CG neurons in culture, brief and long-term BDNF treatments also increase the frequency of spontaneous EPSCs, most of which are mediated by heteromeric nAChRs containing alpha3, alpha5, beta4, and beta2 subunits (alpha3*-nAChRs) with a minor contribution from alpha7-nAChRs. Our findings demonstrate unexpected roles for BDNF-induced, trk-dependent signaling in CG neurons, both in regulating expression of alpha7-nAChRs and in enhancing transmission at alpha3*-nAChR-mediated synapses. The presence of BDNF-like protein in CG and iris target coupled with that of functional trkB on CG neurons raise the possibility that signals generated by endogenous BDNF similarly influence alpha7-nAChRs and nicotinic synapses in vivo.

Dissection and culturing of chick ciliary ganglion neurons: a system well suited to synaptic study

This chapter describes the function and development of the ciliary ganglion, the potential of ciliary ganglion neurons as a cell biological tool, and their dissection, dissociation, and culturing. Ciliary ganglion neurons grow unusually rapidly on a laminin-based substratum and develop large, thin calyx terminals in culture in less than 12 h. The two neuronal classes present in the cultures can be identified by size alone. The limited number of ganglia per animal renders this ganglion a poor choice for biochemical studies based on the extraction of cultured cells. However, they are ideally suited for studies based on single-cell observation, particularly investigation of presynaptic mechanisms using fluorescence microscopy.

PDZ-containing proteins provide a functional postsynaptic scaffold for nicotinic receptors in neurons

Protein scaffolds are essential for specific and efficient downstream signaling at synapses. Though nicotinic receptors are widely expressed in the nervous system and influence numerous cellular events due in part to their calcium permeability, no scaffolds have yet been identified for the receptors in neurons. Here we show that specific members of the PSD-95 family of PDZ-containing proteins are associated with specific nicotinic receptor subtypes. At postsynaptic sites, the PDZ scaffolds are essential for maturation of functional nicotinic synapses on neurons. They also help mediate downstream signaling as exemplified by activation of transcription factors. By tethering components to postsynaptic nicotinic receptors, PDZ scaffolds can organize synaptic structure and determine which calcium-dependent processes will be subject to nicotinic modulation.

Potentiation of alpha7-containing nicotinic acetylcholine receptors by select albumins

Nicotinic receptors containing alpha7 subunits are ligand-gated ion channels widely distributed in the nervous system; they influence a diverse array of events because of their high relative calcium permeability. We show here that nicotine-induced whole-cell responses generated by such receptors can be dramatically potentiated in a rapidly reversible manner by some but not all albumins. The potentiation involves increases both in potency and efficacy with no obvious differences in rise and fall times of the response. The potentiation is not reduced by removing absorbed components; it is abolished by proteolysis, suggesting that the albumin protein backbone is essential. The fact that some albumins are ineffective indicates that minor differences in amino acid sequence may be critical. Experiments with open channel blockers indicate that the potentiation involves increased responses from active receptors rather than recruitment of receptors from a previously silent pool. Single channel recordings reveal that the potentiation correlates with increased single channel opening probability, reflected in increased frequency of channel opening and increased mean channel open time. The potentiation can be exploited to overcome blockade by noncompetitive inhibitors such as beta-amyloid peptide. The results raise the possibility that endogenous compounds use the site to modulate receptor function in vivo, and suggest that the receptors may represent useful targets for therapeutic intervention in cases where they have been implicated in neuropathologies such as Alzheimer's disease.

Relating neuronal nicotinic acetylcholine receptor subtypes defined by subunit composition and channel function

Neuronal nicotinic acetylcholine receptors (nAChRs) are widespread, diverse ion channels involved in synaptic signaling, addiction, and disease. Despite their importance, the relationship between native nAChR subunit composition and function remains poorly defined. Chick ciliary ganglion neurons express two major nAChR types: those recognized by alpha-bungarotoxin (alphaBgt), nearly all of which contain only alpha7 subunits (alpha7-nAChRs) and those insensitive to alphaBgt, which contain alpha3, alpha5, beta4, and, in some cases, beta2 subunits (alpha3*-nAChRs). We explored the relationship between nAChR composition and channel function using toxins recognizing alpha7 subunits (alphaBgt), and alpha3/beta4 (alpha-conotoxin-AuIB), or alpha3/beta2 (alpha-conotoxin-MII) subunit interfaces to perturb responses induced by nicotine, alpha7-, or alpha3-selective agonists (GTS-21 or epibatidine, respectively). Using these reagents, fast-decaying whole-cell current components were attributed solely to alpha7-nAChRs, and slow-decaying components mostly to alpha3*-nAChRs. In outside-out patches, nicotine activated brief 60- and 80-pS single nAChR channel events, and mixed-duration 25- and 40-pS nAChR events. Subsequently, 60- and 80-pS nAChR events and most brief 25- and 40-pS events were attributed to alpha7-nAChRs, and long 25- and 40-pS events to alpha3*-nAChRs. alpha3*-nAChRs lacking beta2 subunits seemed responsible for long 25 pS nAChR events, whereas those containing beta2 subunits mediated the long 40 pS nAChR events that dominate single-channel records. These results reveal greater functional heterogeneity for alpha7-nAChRs than previously expected and indicate that beta2 subunits contribute importantly to alpha3*-nAChR function. By linking structural to functional nAChR subtypes, the findings also illustrate a useful pharmacological strategy for selectively targeting nAChRs.

Synaptic vesicle recycling at two classes of release sites in giant nerve terminals of the embryonic chicken ciliary ganglion

Rapid synaptic transmission in the embryonic chicken ciliary ganglion occurs through the activation of two distinct classes of nicotinic acetylcholine receptors (AChRs): those containing alpha3 subunits (alpha 3*-AChRs) and those containing alpha7 subunits (alpha 7*-AChRs). alpha3*-AChRs are found on ciliary neurons in clusters at synaptic sites on the cell body, whereas alpha7* -AChRs are found on somatic spines, which historically were thought not to have release sites in the embryo. However, Shoop et al. (Shoop et al. [1999] J. Neurosci. 19:692-704) recently described release sites having pre- and postsynaptic densities on somatic spines. We used transmission electron microscopy to compare the structure of synaptic sites on spines with those on the smooth surfaced part of the cell. We find that the two populations of sites are similar in active zone length, number of vesicles, and distance between vesicles and active zone. To study the functional properties of these sites, we examined their stimulation-dependent uptake and release of the extracellular tracer horseradish peroxidase (HRP). We found that each class of release sites both took up and released HRP in a stimulation- and calcium-dependent manner. The mean fraction of synaptic vesicles labeled with tracer was similar for the two populations, both after loading ( approximately 45%) and after unloading ( approximately 7%). Thus we detect no differences between these two anatomically distinct classes of release sites, other than their incidence: sites on spines occurred only 12% as often as those on the cell body. The release sites on somatic spines presumably underlie synaptic responses attributable to alpha7*-AChRs.