Innervation and target tissue interactions differentially regulate acetylcholine receptor subunit mRNA levels in developing neurons in situ

Spinal cord injury is associated with changes in synaptic properties of the mouse major pelvic ganglion

Spinal cord injury (SCI) has substantial impacts on autonomic function. In part, SCI results in loss of normal autonomic activity that contributes to injury-associated pathology such as neurogenic bladder, bowel, and sexual dysfunction. Yet little is known of the impacts of SCI on peripheral autonomic neurons that directly innervate these target organs. In this study, we measured changes in synaptic properties of neurons of the mouse major pelvic ganglion (MPG) associated with acute and chronic SCI. Our data show that functional and physiological properties of synapses onto MPG neurons are altered after SCI, and differ between acute and chronic injury. After acute injury, excitatory post-synaptic potentials (EPSPs) show increased rise and decay time constants leading to overall broader and longer EPSPs, while in chronic injured animals EPSPs are reduced in amplitude and show faster rise and decay leading to shorter EPSPs. Synaptic depression and low pass filtering are also altered in injured animals. Lastly, cholinergic currents are smaller in acute injured animals, but larger in chronic injured animals relative to controls. These changes in synaptic properties are associated with differences in nicotinic receptor subunit expression as well. MPG CHRNA3 mRNA levels decreased after injury, while CHRNA4 mRNAs increased. Further, changes in the correlations of alpha- and beta-subunit mRNAs suggests that nicotinic receptor subtype composition is altered after injury. Taken together, our data demonstrate that peripheral autonomic neurons are fundamentally altered after SCI, suggesting that longer-term therapeutic approaches could target these neurons directly to potentially help ameliorate neurogenic target organ dysfunction.

Review: Cav2.3 R-type Voltage-Gated Ca2+ Channels - Functional Implications in Convulsive and Non-convulsive Seizure Activity

Background:

Researchers have gained substantial insight into mechanisms of synaptic transmission, hyperexcitability, excitotoxicity and neurodegeneration within the last decades. Voltage-gated Ca²⁺ channels are of central relevance in these processes. In particular, they are key elements in the etiopathogenesis of numerous seizure types and epilepsies. Earlier studies predominantly targeted on Ca_v2.1 P/Q-type and Ca_v3.2 T-type Ca²⁺ channels relevant for absence epileptogenesis. Recent findings bring other channels entities more into focus such as the Ca_v2.3 R-type Ca²⁺ channel which exhibits an intriguing role in ictogenesis and seizure propagation. Ca_v2.3 R-type voltage gated Ca²⁺ channels (VGCC) emerged to be important factors in the pathogenesis of absence epilepsy, human juvenile myoclonic epilepsy (JME), and cellular epileptiform activity, e.g. in CA1 neurons. They also serve as potential target for various antiepileptic drugs, such as lamotrigine and topiramate.

Objective:

This review provides a summary of structure, function and pharmacology of VGCCs and their fundamental role in cellular Ca²⁺ homeostasis. We elaborate the unique modulatory properties of Ca_v2.3 R-type Ca²⁺ channels and point to recent findings in the proictogenic and proneuroapoptotic role of Ca_v2.3 R-type VGCCs in generalized convulsive tonic–clonic and complex-partial hippocampal seizures and its role in non-convulsive absence like seizure activity.

Conclusion:

Development of novel Ca_v2.3 specific modulators can be effective in the pharmacological treatment of epilepsies and other neurological disorders.

A blueprint for the spatiotemporal origins of mouse hippocampal interneuron diversity

Although vastly outnumbered, inhibitory interneurons critically pace and synchronize excitatory principal cell populations to coordinate cortical information processing. Precision in this control relies upon a remarkable diversity of interneurons primarily determined during embryogenesis by genetic restriction of neuronal potential at the progenitor stage. Like their neocortical counterparts, hippocampal interneurons arise from medial and caudal ganglionic eminence (MGE and CGE) precursors. However, while studies of the early specification of neocortical interneurons are rapidly advancing, similar lineage analyses of hippocampal interneurons have lagged. A "hippocampocentric" investigation is necessary as several hippocampal interneuron subtypes remain poorly represented in the neocortical literature. Thus, we investigated the spatiotemporal origins of hippocampal interneurons using transgenic mice that specifically report MGE- and CGE-derived interneurons either constitutively or inducibly. We found that hippocampal interneurons are produced in two neurogenic waves between E9-E12 and E12-E16 from MGE and CGE, respectively, and invade the hippocampus by E14. In the mature hippocampus, CGE-derived interneurons primarily localize to superficial layers in strata lacunosum moleculare and deep radiatum, while MGE-derived interneurons readily populate all layers with preference for strata pyramidale and oriens. Combined molecular, anatomical, and electrophysiological interrogation of MGE/CGE-derived interneurons revealed that MGE produces parvalbumin-, somatostatin-, and nitric oxide synthase-expressing interneurons including fast-spiking basket, bistratified, axo-axonic, oriens-lacunosum moleculare, neurogliaform, and ivy cells. In contrast, CGE-derived interneurons contain cholecystokinin, calretinin, vasoactive intestinal peptide, and reelin including non-fast-spiking basket, Schaffer collateral-associated, mossy fiber-associated, trilaminar, and additional neurogliaform cells. Our findings provide a basic blueprint of the developmental origins of hippocampal interneuron diversity.

From smoking to lung cancer: the CHRNA5/A3/B4 connection

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that modulate key physiological processes ranging from neurotransmission to cancer signaling. These receptors are activated by the neurotransmitter, acetylcholine, and the tobacco alkaloid, nicotine. Recently, the gene cluster encoding the alpha3, alpha5 and beta4 nAChR subunits received heightened interest after a succession of linkage analyses and association studies identified multiple single-nucleotide polymorphisms in these genes that are associated with an increased risk for nicotine dependence and lung cancer. It is not clear whether the risk for lung cancer is direct or an effect of nicotine dependence, as evidence for both scenarios exist. In this study, we summarize the body of work implicating nAChRs in the pathogenesis of lung cancer, with special focus on the clustered nAChR subunits and their emerging role in this disease state.

Absent smooth muscle actin immunoreactivity of the small bowel muscularis propria circular layer in association with chromosome 15q11 deletion in megacystis-microcolon-intestinal hypoperistalsis syndrome

Megacystis-microcolon-intestinal hypoperistalsis syndrome (MMIHS; OMIM%249210) is a rare and severe form of congenital intestinal and urinary dysfunction and malformation. Histologic studies suggest that the predominant intestinal manifestation is smooth muscle myopathy. Molecular observations have linked the disease to the neuronal nicotinic acetylcholine receptor (ηAChR), namely the absence of a functional α3 subunit of the ηAChR in patients with MMIHS. We describe a case of MMIHS in association with a de novo deletion of the proximal long arm of chromosome 15 (15q11.2). Histologic evaluation revealed an appropriate light microscopic appearance of both the circular and longitudinal layers of the small bowel muscularis propria. Immunohistochemical staining for smooth muscle actin, however, was selectively absent in the circular layer, demonstrating isolated absence in a unique and previously undescribed pattern. These observations raise the possibility that the proximal long arm of chromosome 15 (15q11) may be of clinical significance in MMIHS.

The nicotinic acetylcholine receptor CHRNA5/A3/B4 gene cluster: dual role in nicotine addiction and lung cancer

More than 1 billion people around the world smoke, with 10 million cigarettes sold every minute. Cigarettes contain thousands of harmful chemicals including the psychoactive compound, nicotine. Nicotine addiction is initiated by the binding of nicotine to nicotinic acetylcholine receptors, ligand-gated cation channels activated by the endogenous neurotransmitter, acetylcholine. These receptors serve as prototypes for all ligand-gated ion channels and have been extensively studied in an attempt to elucidate their role in nicotine addiction. Many of these studies have focused on heteromeric nicotinic acetylcholine receptors containing α4 and β2 subunits and homomeric nicotinic acetylcholine receptors containing the α7 subunit, two of the most abundant subtypes expressed in the brain. Recently however, a series of linkage analyses, candidate-gene analyses and genome-wide association studies have brought attention to three other members of the nicotinic acetylcholine receptor family: the α5, α3 and β4 subunits. The genes encoding these subunits lie in a genomic cluster that contains variants associated with increased risk for several diseases including nicotine dependence and lung cancer. The underlying mechanisms for these associations have not yet been elucidated but decades of research on the nicotinic receptor gene family as well as emerging data provide insight on how these receptors may function in pathological states. Here, we review this body of work, focusing on the clustered nicotinic acetylcholine receptor genes and evaluating their role in nicotine addiction and lung cancer.

Temporally- and spatially-regulated transcriptional activity of the nicotinic acetylcholine receptor beta4 subunit gene promoter

Signaling through nicotinic acetylcholine (nACh) receptors underlies a diverse array of behaviors. In order for appropriate signaling to occur via nACh receptors, it is necessary for the genes encoding the receptor subunits to be expressed in a highly regulated temporal and spatial manner. Here we report a transgenic mouse approach to characterize the transcriptional regulation of the gene encoding the nACh receptor beta4 subunit. nACh receptors containing this subunit play critical roles in both the central and peripheral nervous systems. We demonstrate that a 2.3-kilobase pair fragment of the beta4 5'-flanking region is capable of directing reporter gene expression in transgenic animals. Importantly, the transcriptional activity of the promoter region is cell-type-specific and developmentally regulated and overlaps to a great extent with endogenous beta4 mRNA expression. These data indicate that the 2.3-kilobase pair fragment contains transcriptional regulatory elements critical for appropriate beta4 subunit gene expression.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

Specific subtypes of nicotinic cholinergic receptors involved in sympathetic and parasympathetic cardiovascular responses

Various subtypes of nicotinic cholinergic receptors are expressed in autonomic ganglia. The distinct functional roles of these receptors in autonomic ganglionic transmission to different target organs remain to be elucidated. In this study, we tested the sympathetic and parasympathetic cardiovascular responses to nicotinic agonist and antagonists in urethane-anesthetized mice. Intravenous injection with a nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, induced a brief but pronounced decrease in heart rate, followed by significant increases in heart rate and arterial blood pressure. The bradycardic response was blocked by atropine whereas the pressor response was blocked by prazosine, confirming those responses were parasympathetic and sympathetic activities, respectively. The sympathetic response was blocked by methyllycaconitine citrate, a selective alpha 7 nicotinic cholinergic receptor (nAchR) antagonist. The parasympathetic response was blocked by a selective alpha 4 beta 2 nAchR antagonist, dihydro-beta-erythroidine hydrobromide. Moreover, injection with a selective alpha 4 beta 2 nAchR agonist, RJR2403 oxalate, induced a pronounced parasympathetic response with a smaller sympathetic response. Collectively, these data show that activations of alpha 4 beta 2 nAchRs elicits a parasympathetic cardiovascular response and activation of alpha 7 nAchRs elicits a sympathetic cardiovascular response. These data suggest that specific subtypes of nicotinic receptors at the level of the ganglia may play distinct roles in mediating sympathetic or parasympathetic activation.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

Transcriptional Repression by a Conserved Intronic Sequence in the Nicotinic Receptor α3 Subunit Gene

The genes encoding the nicotinic acetylcholine receptor alpha3, alpha5, and beta4 subunits are genomically clustered. These genes are co-expressed in a variety of cells in the peripheral and central nervous systems. Their gene products assemble in a number of stoichiometries to generate several nicotinic receptor subtypes that have distinct pharmacological and physiological properties. Signaling through these receptors is critical for a variety of fundamental biological processes. Despite their importance, the transcriptional mechanisms underlying their coordinated expression remain to be completely elucidated. By using a bioinformatics approach, we identified a highly conserved intronic sequence within the fifth intron of the alpha3 subunit gene. Reporter gene analysis demonstrated that this sequence, termed "alpha3 intron 5," inhibits the transcriptional activities of the alpha3 and beta4 subunit gene promoters. This repressive activity is position- and orientation-independent. Importantly, repression occurs in a cell type-specific manner, being present in cells that do not express the receptor genes or expresses them at very low levels. Electrophoretic mobility shift assays demonstrate that nuclear proteins specifically interact with alpha3 intron 5 at two distinct sites. We propose that this intronic repressor element is important for the restricted expression patterns of the nicotinic receptor alpha3 and beta4 subunit genes.

Kainate receptors and RNA editing in cholinergic neurons

Parasympathetic ganglia are considered simple relay systems that have cholinergic input and output, with modulation occurring centrally. Greater complexity is suggested, however, by our showing here that avian ciliary ganglion (CG) neurons also express a different excitatory receptor type--ionotropic glutamate receptors of the kainate subtype (KARs). This is the first report of glutamate receptor expression in the CG and KAR expression in any cholinergic neuron. We show that KARs form functional channels on CG neurons. KARs localize to CG neuron axons and somata as well as axons and terminals of pre-synaptic inputs to the CG. Glutamate transporters are expressed on Schwann cells that surround synapses on neuronal somata, and may provide a local source of glutamate. CG neurons express multiple KAR subunit mRNAs (GluR5, GluR7, and KA1), and their relative levels change dramatically during axon outgrowth and synaptic differentiation. The developmental role for KARs may depend upon their calcium permeability, a property regulated by mRNA editing. We show GluR5 editing increases predominantly at the time CG axons contact peripheral targets. Our data suggest that glutamatergic signaling may function as a local circuit mechanism to modulate excitability and calcium signaling during synapse formation and maturation in the CG in vivo.

Shared long-range regulatory elements coordinate expression of a gene cluster encoding nicotinic receptor heteromeric subtypes

The nicotinic acetylcholine receptor (nAChR) beta4/alpha3/alpha5 gene cluster encodes several heteromeric transmitter receptor subtypes that are essential for cholinergic synaptic transmission in adrenal gland, autonomic ganglia, pineal gland, and several nuclei in the central nervous system. However, the transcriptional mechanisms coordinating expression of these subunit genes in different cell populations are unknown. Here, we used transgenic methods to investigate long-range transcriptional control of the cluster. A 132-kb P1-derived artificial chromosome (PAC) encoding the rat cluster recapitulated the neurally- and endocrine-restricted expression patterns of the endogenous beta4/alpha3/alpha5 genes. Mutation of ETS factor binding sites in an enhancer, beta43', embedded in the beta4 3'-untranslated exon resulted in greatly diminished beta4, alpha3, and alpha5 expression in adrenal gland and to a lesser extent in the superior cervical ganglion (SCG) but not in other tissues. Phylogenetic sequence analyses revealed several conserved noncoding regions (CNRs) upstream of beta4 and alpha5. Deletion of one of them (CNR4) located 20 kb upstream of beta4 resulted in a dramatic decrease in beta4 and alpha3 expression in the pineal gland and SCG. CNR4 was sufficient to direct LacZ transgene expression to SCG neurons, which express the endogenous beta4alpha3alpha5 subunits, and pineal cells, which express the endogenous beta4alpha3 combination. Finally, CNR4 was able to direct transgene expression to major sites of expression of the endogenous cluster in the brain. Together, our findings support a model in which cell type-specific shared long-range regulatory elements are required for coordinate expression of clustered nAChR genes.

Developmental regulation of nicotinic synapses on cochlear inner hair cells

In the mature cochlea, inner hair cells (IHCs) transduce acoustic signals into receptor potentials, communicating to the brain by synaptic contacts with afferent fibers. Before the onset of hearing, a transient efferent innervation is found on IHCs, mediated by a nicotinic cholinergic receptor that may contain both alpha9 and alpha10 subunits. Calcium influx through that receptor activates calcium-dependent (SK2-containing) potassium channels. This inhibitory synapse is thought to disappear after the onset of hearing [after postnatal day 12 (P12)]. We documented this developmental transition using whole-cell recordings from IHCs in apical turns of the rat organ of Corti. Acetylcholine elicited ionic currents in 88-100% of IHCs between P3 and P14, but in only 1 of 11 IHCs at P16-P22. Potassium depolarization of efferent terminals caused IPSCs in 67% of IHCs at P3, in 100% at P7-P9, in 93% at P10-P12, but in only 40% at P13-P14 and in none of the IHCs tested between P16 and P22. Earlier work had shown by in situ hybridization that alpha9 mRNA is expressed in adult IHCs but that alpha10 mRNA disappears after the onset of hearing. In the present study, antibodies to alpha10 and to the associated calcium-dependent (SK2) potassium channel showed a similar developmental loss. The correlated expression of these gene products with functional innervation suggests that Alpha10 and SK2, but not Alpha9, are regulated by synaptic activity. Furthermore, this developmental knock-out of alpha10, but not alpha9, supports the hypothesis that functional nicotinic acetylcholine receptors in hair cells are heteromers containing both these subunits.

Neuronal nicotinic synapse assembly requires the adenomatous polyposis coli tumor suppressor protein

Normal cognitive and autonomic functions require nicotinic synaptic signaling. Despite the physiological importance of these synapses, little is known about molecular mechanisms that direct their assembly during development. We show here that the tumor-suppressor protein adenomatous polyposis coli (APC) functions in localizing alpha3-nicotinic acetylcholine receptors (nAChRs) to neuronal postsynaptic sites. Our quantitative confocal microscopy studies indicate that APC is selectively enriched at cholinergic synapses; APC surface clusters are juxtaposed to synaptic vesicle clusters and colocalize with alpha3-nAChRs but not with the neighboring synaptic glycine receptors or perisynaptic alpha7-nAChRs on chick ciliary ganglion (CG) neurons. We identify PSD (postsynaptic density)-93, beta-catenin, and microtubule end binding protein EB1 as APC binding partners. PSD-93 and beta-catenin are also enriched at alpha3-nAChR postsynaptic sites. EB1 shows close proximity to and partial overlap with alpha3-nAChR and APC surface clusters. We tested the role of APC in neuronal nicotinic synapse assembly by using retroviral-mediated in vivo overexpression of an APC dominant-negative (APC-dn) peptide to block the interaction of endogenous APC with both EB1 and PSD-93 during synapse formation in CG neurons. The overexpressed APC-dn led to dramatic decreases in alpha3-nAChR surface levels and clusters. Effects were specific to alpha3-nAChR postsynaptic sites; synaptic glycine receptor and perisynaptic alpha7-nAChR clusters were not altered. In addition, APC-dn also reduced surface membrane-associated clusters of PSD-93 and EB1. The results show that APC plays a key role in organizing excitatory cholinergic postsynaptic specializations in CG neurons. We identify APC as the first nonreceptor protein to function in localizing nAChRs to neuronal synapses in vivo.

Target-mediated control of neural differentiation

The development of the nervous system entails the coordination of the spatial and chemical development of both pre- and postsynaptic elements. This coordination is accomplished by signals passing between neurons and the target cells that they innervate. This review focuses on well-characterized examples of target-mediated neuronal differentiation in the central and peripheral nervous systems. These include control of neurogenesis in the leech by male genitalia, presynaptic differentiation induced by postsynaptic molecules expressed by skeletal muscle, postsynaptic adhesion molecules that induce presynaptic differentiation in the central nervous system (CNS), target-mediated control of neurotransmitter phenotype in peripheral neurons, and target-regulated control of neuronal nicotinic acetylcholine receptors (nAChRs) and large conductance calcium-activated potassium channels (BK). The detailed understanding of these processes will uncover signals critical for the directed differentiation of stem cells as well as identify future targets for therapies in neural regeneration that promote the reestablishment of functional connections.

The role of neuronal nicotinic acetylcholine receptor subunits in autonomic ganglia: lessons from knockout mice

Neuronal nicotinic acetylcholine receptors (nAChR), composed of 12 subunits (alpha2-alpha10, beta2-beta4), are expressed in autonomic ganglia, playing a central role in autonomic transmission. The repertoire of nicotinic subunits in autonomic ganglia includes alpha3, alpha5, alpha7, beta2 and beta4 subunits. In the last 10 years, heterologous expression studies have revealed much about the nature of neuronal nAChRs. However, there is only limited understanding of subunit actions in autonomic system. Functional deletions of subunit by gene knockout in animals could overcome these limitations. We review recent studies on nAChRs on autonomic ganglia for physiological and pharmacological properties and potential locations of the subunits.

Regulatory mechanisms that govern nicotinic synapse formation in neurons

Individual cholinoceptive neurons express high levels of different neuronal nicotinic acetylcholine receptor (nAChR) subtypes, and target them to the appropriate synaptic regions for proper function. This review focuses on the intercellular and intracellular processes that regulate nAChR expression in vertebrate peripheral nervous system (PNS) and central nervous system (CNS) neurons. Specifically, we discuss the cellular and molecular mechanisms that govern the induction and maintenance of nAChR expression-innervation, target tissue interactions, soluble factors, and activity. We define the regulatory principles of interneuronal nicotinic synapse differentiation that have emerged from these studies. We also discuss the molecular players that target nAChRs to the surface membrane and the interneuronal synapse.

Extrasynaptic alpha 7-nicotinic acetylcholine receptor expression in developing neurons is regulated by inputs, targets, and activity

Alpha7-nicotinic acetylcholine receptors (nAChRs) are widely expressed in the vertebrate nervous system. alpha7-nAChR functions include postsynaptic transmission, modulating neurotransmitter release, reinforcing nicotine addiction, and a role in neurological disorders, such as schizophrenia and Alzheimer's disease. In chick parasympathetic ciliary ganglion (CG) neurons, alpha7-nAChRs are excluded from the synapse and localize perisynaptically. Despite their extrasynaptic distribution, the highly Ca2+-permeable alpha7-nAChRs have important synapse-related Ca2+-dependent signaling functions in the CG. We show here that the synaptic partners regulate alpha7-nAChR expression during synapse formation in embryonic CG neurons in situ. The absence of inputs and target tissues cause reductions in alpha7-nAChR mRNA and protein levels that primarily resemble those seen for synaptic alpha3-nAChRs. However, there is a difference in their regulation. alpha7-nAChR levels are downregulated by reduced activity, whereas alpha3-nAChR levels are not. We propose that the activity-dependent regulation of extrasynaptic alpha7-nAChR levels may be an important mechanism for postsynaptic CG neurons to detect changes in presynaptic activity levels and respond with Ca2+-dependent plasticity changes in gene expression.

Maturation of postsynaptic nicotinic structures on autonomic neurons requires innervation but not cholinergic transmission

Postsynaptic development at the neuromuscular junction depends on nicotinic transmission and secreted components from the presynaptic motor nerve terminal. Similarly, secreted components and synaptic activity are both thought to guide development of glutamatergic synapses in the CNS. Nicotinic synapses on chick ciliary neurons are structurally complex: a large presynaptic calyx engulfs the postsynaptic neuron and overlays a series of discrete mats of receptor-rich somatic spines tightly interwoven and folded against the soma. We used fluorescence imaging of alpha 7-containing nicotinic receptors and the spine constituent drebrin to monitor postsynaptic development. The results show that surgical disruption of the preganglionic input or removal of the ganglionic synaptic target tissue after synapses form in the ganglion does not disrupt the receptor-rich spine mats. Similarly, removal of the target tissue even prior to synapse formation in the ganglion does not prevent subsequent formation of the receptor clusters and associated spine constituents. Postsynaptic development is arrested, however, if normal innervation is prevented by ablating the preganglionic neurons prior to synapse formation. In this case the neurons express reduced levels of nicotinic receptors and cytoskeletal components and organize them only into early-stage clusters. Even low levels of residual innervation, however, can restore much of the normal postsynaptic receptor patterns. Chronic pharmacological blockade of cholinergic synaptic activity fails to replicate the effects of ablating the preganglionic nucleus. The results indicate that ciliary neurons are programmed to express postsynaptic components and can initiate clustering of alpha 7-containing receptors but need presynaptic guidance for maturation of the postsynaptic structure.

BK-Type K(Ca) channels in two parasympathetic cell types: differences in kinetic properties and developmental expression

The intrinsic electrical properties of identified choroid and ciliary neurons of the chick ciliary ganglion were examined by patch-clamp recording methods. These neurons are derived from a common pool of mesencephalic neural crest precursor cells but innervate different target tissues and have markedly different action potential waveforms and intrinsic patterns of repetitive spike discharge. Therefore it is important to determine whether these cell types express different types of plasma membrane ionic channels, and to ascertain the developmental stages at which these cell types begin to diverge. This study has focused on large-conductance Ca(2+)-activated K(+) channels (K(Ca)), which are known to regulate spike waveform and repetitive firing in many cell types. Both ciliary ganglion cell types, identified on the basis of size and somatostatin immunoreactivity, express a robust macroscopic K(Ca) carried by a kinetically homogeneous population of large-conductance (BK-type) K(Ca) channels. However, the kinetic properties of these channels are different in the two cell types. Steady-state fluctuation analyses of macroscopic K(Ca) produced power spectra that could be fitted with a single Lorentzian curve in both cell types. However, the resulting corner frequency was significantly lower in choroid neurons than in ciliary neurons, suggesting that the underlying K(Ca) channels have a longer mean open-time in choroid neurons. Consistent with fluctuation analyses, significantly slower gating of K(Ca) channels in choroid neurons was also observed during macroscopic activation and deactivation at membrane potentials positive to -30 mV. Differences in the kinetic properties of K(Ca) channels could also be observed directly in single-channel recordings from identified embryonic day 13 choroid and ciliary neurons. The mean open-time of large-conductance K(Ca) channels was significantly greater in choroid neurons than in ciliary neurons in excised inside-out patches. The developmental expression of functional K(Ca) channels appears to be regulated differently in the two cell types. Although both cell types acquire functional K(Ca) at the same developmental stages (embryonic days 9-13), functional expression of these channels in ciliary neurons requires target-derived trophic factors. In contrast, expression of functional K(Ca) channels proceeds normally in choroid neurons developing in vitro in the absence of target-derived trophic factors. Consistent with this, extracts of ciliary neuron target tissues (striated muscle of the iris/ciliary body) contain K(Ca) stimulatory activity. However, K(Ca) stimulatory activity cannot be detected in extracts of the smooth muscle targets of choroid neurons.

Synaptic interactions regulate gephyrin expression in avian cholinergic neurons in vivo

Our recent studies of chick parasympathetic ciliary ganglion (CG) neurons demonstrate a unique postsynaptic receptor microheterogeneity - under one presynaptic terminal, excitatory nicotinic acetylcholine receptor (nAChR) clusters and separate inhibitory glycine receptor (GlyR) clusters coexist in distinct membrane microregions. Gephyrin, a peripheral membrane protein that is required for GlyR clustering at synapses in the rodent central nervous system, is also expressed in chick CG neurons where it codistributes with GlyRs, but not nAChRs. We now extend these findings by characterizing the regulation of gephyrin expression in chick CG neurons in vivo. We show that developmental increases in gephyrin transcript levels occur during pre- and postganglionic synapse formation. The increases are induced by both innervation and target tissue interactions, with the target tissues having the greater regulatory influence. The time course of the developmental rise in gephyrin mRNA levels most closely resembles that reported for functional GlyR expression, but not that of functional nAChRs nor GABA(A) receptors. We also demonstrate that gephyrin is concentrated in the postsynaptic density of a subset of synapses on both the ciliary and choroid neurons in the CG and is stably expressed from embryonic to adult stages. Altogether, our results suggest that gephyrin is a synapse organizing molecule that functions to localize GlyRs, but not nAChRs, to discrete postsynaptic membrane microregions in chick CG neurons in vivo.

Nicotinic acetylcholine receptor subunit mRNA expression and channel function in medial habenula neurons

Relationships between nicotinic acetylcholine receptor (nAChR) channel function and nAChR subunit mRNA expression were explored in acutely isolated rat medial habenula (MHb) neurons using a combination of whole-cell recording and single cell RT-PCR techniques. Following amplification using subunit-specific primers, subunits could be categorized in one of three ways: (i) present in 95-100% cells: alpha3, alpha4, alpha5, beta2 and beta4; (ii) never present: alpha2; and (iii) sometimes present ( approximately 40% cells): alpha6, alpha7 and beta3. These data imply that alpha2 subunits do not participate in nAChRs on MHb cells, that alpha6, alpha7 and beta3 subunits are not necessary for functional channels but may contribute in some cells, and that nAChRs may require combinations of all or subsets of alpha3, alpha4, alpha5, beta2 and beta4 subunits. Little difference in the patterns of subunit expression between nicotine-sensitive and insensitive cells were revealed based on this qualitative analysis, implying that gene transcription per se may be an insufficient determinant of nAChR channel function. Normalization of nAChR subunit levels to the amount of actin mRNA, however, revealed that cells with functional channels were associated with high levels (>0.78 relative to actin; 11/12 cells) of all of the category (i) subunits: alpha3, alpha4, alpha5, beta2 and beta4. Conversely, one or more of these subunits was always low (<0.40 relative to actin) in all cells with no detectable response to nicotine. Thus the formation of functional nAChR channels on MHb cells may require critical levels of several subunit mRNAs.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

Lithium treatment in ovo: effects on embryonic heart rate, natural death of ciliary ganglion neurons, and brain expression of a highly conserved chicken homolog of human MTG8/ETO

Understanding the action of the mood stabilizer lithium is dependent on availability of experimental models where lithium treatment at clinically relevant concentrations induces marked phenotypic and genotypic changes. Here we report on such changes in the chicken embryo. Lithium chloride (0.6 mM), applied in ovo 60 h after incubation, markedly delayed the heart rate increase observed from ED2.5 to ED5, and induced the brain expression of a new chicken gene cETO from ED7 to ED15. At the same time the overall developmental dynamics and embryo survival, or the expression of chicken gephyrin were not significantly affected. Furthermore, lithium treatment (0.3 mM, 48 h after incubation) abolished the difference in neuronal number between ED12 ciliary ganglia developing in the presence or absence of postganglionic target muscles. We show that cETO is a close homologue of the human transcription factor MTG8/ETO; named after its location on chromosome 8, and participation in chromosomal translocation 8;21 in myeloid leukemia. The mRNA and protein levels of ETO and gephyrin had a parallel course in chicken brain development suggesting that the expression of both genes is regulated mainly at the level of gene transcription. However, the patterns of expression were markedly different. ETO peaked at ED7 and decreased five-fold at ED15. In contrast, gephyrin levels increased five-fold from ED7 to ED15. We propose that the induction of ETO expression, in concert with lithium-induced upregulation of other genes, such as PEBP2beta and bcl-2, is participating in the neuroprotective effect of chronic lithium treatment.

Expression and channel properties of alpha-bungarotoxin-sensitive acetylcholine receptors on chick ciliary and choroid neurons

Cell-specific expression of nicotinic acetylcholine receptors (AChRs) was examined using ciliary and choroid neurons isolated from chick ciliary ganglia. At embryonic days 13 and 14 (E13,14) the neurons can be distinguished by size, with ciliary neuron soma diameters exceeding those of choroid neurons by about twofold. Both neuronal populations are known to express two major AChR types: alpha3*-AChRs recognized by mAb35, that contain alpha3, alpha5, beta4, and occasionally beta2 subunits, and alpha-bungarotoxin (alphaBgt)-AChRs recognized and blocked by alphaBgt, that contain alpha7 subunits. We found that maximal whole cell current densities (I/C(m)) mediated by alphaBgt-AChRs were threefold larger for choroid compared with ciliary neurons, while alpha3*-AChR current densities were similar in the two populations. Different densities of total cell-surface alphaBgt-AChRs could not explain the distinct alphaBgt-AChR response densities associated with ciliary and choroid neurons. Ciliary ganglion neurons display abundant [(125)I]-alphaBgt binding ( approximately 10(6) sites/neuron), but digital fluorescence measurements revealed equivalent site densities on both populations. AChR channel classes having single-channel conductances of approximately 30, 40, 60, and 80 pS were present in patches excised from both ciliary and choroid neurons. Treating the neurons with alphaBgt selectively abolished the 60- and 80-pS events, identifying them as arising from alphaBgt-AChRs. Kinetic measurements revealed brief open and long closed durations for alphaBgt-AChR channel currents, predicting a very low probability of being open (p(o)) when compared with 30- or 40-pS alpha3*-AChR channels. None of the channel parameters associated with the 60- and 80-pS alphaBgt-AChRs differed detectably, however, between choroid and ciliary neurons. Instead calculations based on the combined whole cell and single-channel results indicate that choroid neurons express approximately threefold larger numbers of functional alphaBgt-AChRs (N(F)) per unit area than do ciliary neurons. Comparison with total surface [(125)I]-alphaBgt-AChR sites (N(T)), reveals that N(F)/N(T) << 1 for both neuron populations, suggesting that "silent" alphaBgt-AChRs predominate. Choroid neurons may therefore express a higher density of functional alphaBgt-AChRs by recruiting a larger fraction of receptors from the silent pool than do ciliary neurons.

Receptor targeting and heterogeneity at interneuronal nicotinic cholinergic synapses in vivo

Within a single neuron the correct targeting of the diverse neurotransmitter receptor types to discrete synaptic regions is crucial for proper function. However, the molecular mechanisms that underlie neuronal receptor clustering and targeting are still largely undefined. Here we report advances in defining the mechanisms that mediate nicotinic acetylcholine receptor (nAChR) targeting to interneuronal synapses. Recent in vivo studies have demonstrated that one subunit plays a critical role in the differentiation of nicotinic cholinergic synapses on vertebrate autonomic neurons. The major cytoplasmic loop of the alpha3 subunit targets specific nAChR subtypes to the synapse. In contrast, nAChR complexes that lack the alpha3 targeting domain are excluded and are perisynaptic. Additional studies have demonstrated a greater complexity to alpha3-nAChR targeting due to a unique postsynaptic receptor microheterogeneity - under one presynaptic terminal, alpha3-nAChR clusters are separate, but proximal to, glycine receptor (GlyR) clusters in discrete postsynaptic membrane microregions. The surprising coexistence under one nerve ending of separate clusters of receptors that respond to different fast-acting transmitters with opposing functions may represent a novel mechanism for modulating synaptic activity. Overall, the receptor targeting and clustering studies reviewed in this issue suggest that a common mechanism underlies the formation of the diverse types of interneuronal synapses but differs from that responsible for neuromuscular junction assembly in vertebrates.

Modulation of GABA(A) receptor subunit mRNA levels in olivocerebellar neurons of purkinje cell degeneration and weaver mutant mice

In olivocerebellar circuits, changes in the subunit composition of GABA(A) receptors occur at a time of extensive synaptic remodeling. In the deep cerebellar nuclei, GABA(A) receptor alpha1, beta2, and gamma2 subunit mRNA expression increases throughout neonatal development, whereas in the inferior olivary complex, the perinatal combination of alpha3, alpha5, beta3, and gamma2 mRNAs switches to the adult combination of alpha2, alpha4, beta3 and gamma1 during postnatal week 2. In situ hybridization was used to examine changes in subunit expression in the olivocerebellar nuclei of Purkinje cell degeneration and weaver mutant mice. In Purkinje cell degeneration, subunit transcripts decreased below control levels in olivary neurons; however, alpha1, beta2, and gamma2 transcript levels were slightly increased in the medial nucleus of the deep cerebellar nuclei. In weaver olivary neurons, although the switch from early- to late-onset subunit mRNAs occurred as in normal mice, transcript levels were differentially modulated by the mutation. Our studies indicate that major alterations in synaptic connectivity do not prevent developmentally programmed switches in GABA(A) receptor gene expression but can modulate the timing and level of transcript expression in afferent and efferent neurons.

Effects of cholinergic deafferentation upon the expression of the alpha2 subunit of the nicotinic acetylcholine receptors in the chick lateral spiriform nucleus

Lesion, immunohistochemical, and immunoblotting methods were used to evaluate the effects of cholinergic deafferentation upon the expression of the alpha2 subunit of the nicotinic acetylcholine receptors in the lateral spiriform nucleus (SpL) of the chick brain. The expression of the alpha2 subunit in the SpL showed biphasic changes after lesion of its cholinergic source (nucleus semilunaris), with an increase after 2 days postlesion and a decrease after 3-7 days. Our results could represent a correlate of the phenomena of nicotinic receptor up- and down-regulation, induced by removal of the cholinergic input.

Nicotinic acetylcholine receptor gene expression in developing chick autonomic ganglia

The developmental expression patterns of ten genes encoding nicotinic acetylcholine receptor subunits were analyzed using Northern blots and in situ hybridization in chick peripheral ganglia of neural crest, placodal and dual embryonic origin. The superior cervical and ciliary ganglia were investigated in detail because they accumulated relatively abundant transcripts of the alpha3, beta4, alpha5 and alpha7 genes. In the superior cervical ganglion, these four mRNA species had similar developmental time-courses. They appeared at embryonic day 8 (E8), increased steadily until E16 and maintained a rather high plateau level until E18. In the ciliary ganglion, alpha7 transcripts were already abundant at E6, increased until E10, and considerably decreased thereafter. High-resolution in situ hybridization showed that alpha7 transcripts were present in all cell types of the E6 ciliary ganglion, whereas they were restricted to large neuronal somas at E16. Transfections with a reporter gene under the control of the alpha7 promoter demonstrated that a sharp developmental divide occurred at E11-12, after which stage the promoter was activatable in neurons exclusively.

Nicotinic acetylcholine receptor agonists promote survival and reduce apoptosis of chick ciliary ganglion neurons

The abundance, diversity, and ubiquitous expression of neuronal nicotinic acetylcholine receptors (AChRs) suggest that many are involved in functions other than synaptic transmission. We now report that a major AChR class promotes neuronal survival. The 10-day survival of ciliary ganglion neurons in basal culture medium (MEM) was approximately 35%, but increased to approximately 75% in MEM containing nicotine (MEM/Nic) or carbachol, an effect similar to that achieved by chronic depolarization with KCl. Pharmacological experiments revealed that agonist-enhanced survival requires activation of AChRs sensitive to alpha-bungarotoxin (alphaBgt). alphaBgt-AChRs partly support neuronal survival by limiting apoptosis since fewer apoptotic neurons were observed in MEM/Nic compared to MEM. Moreover, nicotinic survival support was not further enhanced by fibroblast growth factor, as seen for KCl, but increased to 100% by adding PACAP, a trophic neuropeptide present in the ganglion. These results indicate that alphaBgt-AChR activation regulates neuronal survival and suggest a mechanism involving reduced apoptosis and interaction with an endogenous neuropeptide growth factor.

Effects of nerve growth factor and nicotine on the expression of nicotinic acetylcholine receptor subunits in PC12 cells

The neuroprotective effect of nicotine via nicotinic acetylcholine receptor (nAChR) has been reported in differentiated PC12 cells. We examined the effects of nerve growth factor (NGF) and nicotine on the expressions of the nAChR subunits alpha3, alpha5, alpha7, beta2, beta3 and beta4, in PC12 cells using Northern blot analysis. NGF increased the transcriptions of alpha5 (1,4- to 2,3-fold), alpha7 (1.7- to 2.5-fold) and beta4 (1.9- to 3.0-fold) subunits, but changes in alpha3 and beta2 subunit levels were smaller than alpha5, alpha7, or beta4. Nicotine also increased the levels of mRNA encoding alpha5 and beta2 subunits. The pattern of subunit mRNA changes was different between NGF and nicotine treatment. No signal for beta3 subunit was detected. The results suggest that NGF changes the expression of nAChR in a subtype-specific manner over the course of the differentiation, and the disproportionate subunit expressions might be related to the neuroprotective effect exerted by nicotine.

Alpha subunit composition of nicotinic acetylcholine receptors in the rat autonomic ganglia neurons as determined with subunit-specific anti-alpha(181-192) peptide antibodies

The subunit composition of nicotinic acetylcholine receptors of rat autonomic ganglia neurons was studied by means of antibodies, which differentiated between different alpha subunits and specifically blocked acetylcholine-induced membrane currents. Polyclonal rabbit antibodies and mouse monoclonal antibodies were raised against synthetic peptides matching in sequence the alpha(181-192) region of alpha3, alpha4, alpha5, and alpha7 subunits of rat neuronal nicotinic acetylcholine receptors. The antibodies discriminated among alpha3, alpha4, alpha5, and alpha7 peptides in enzyme-linked immunosorbent assay and bound to native acetylcholine receptors expressed in PC-12 cells. By means of immunoperoxidase staining of cultured rat autonomic neurons followed by transmission, dark-field and phase-contrast microscopy, it was found that all cells of the superior cervical ganglia expressed the alpha3, alpha5, and alpha7 nicotinic acetylcholine receptors, whereas approximately half of the cells were clearly alpha4-positive. In contrast, only about one-third of the intracardiac neurons were alpha3-positive, about 50% were alpha4-positive, one-seventh were alpha5-positive, and one-fifth were alpha7-positive. All antibodies tested blocked acetylcholine-induced currents in the neurons of the superior cervical ganglia as was demonstrated by whole-cell patch-clamp studies. Although each antibody could block up to 80% of the current, the degree of inhibition varied considerably from cell to cell. It is concluded that alpha3, alpha5, and alpha7 subunits are expressed in all neurons of the superior cervical ganglion and in some intracardiac neurons, whereas alpha4 subunits are expressed in some but not all neurons of both tissues. The neurons of the superior cervical ganglion express heterogeneous acetylcholine receptors and differ in relative amounts of acetylcholine receptor subtypes expressed.

Regulation of neuronal K(+) currents by target-derived factors: opposing actions of two different isoforms of TGFbeta

The developmental expression of macroscopic Ca(2+)-activated K(+) currents in chick ciliary ganglion neurons is dependent on an avian ortholog of TGFbeta1, known as TGFbeta4, secreted from target tissues in the eye. Here we report that a different isoform, TGFbeta3, is also expressed in a target tissue of ciliary ganglion neurons. Application of TGFbeta3 inhibits the functional expression of whole-cell Ca(2+)-activated K(+) currents evoked by 12 hour treatment with either TGFbeta1 or beta-neuregulin-1 in ciliary ganglion neurons developing in vitro. TGFbeta3 had no effect on voltage-activated Ca(2+) currents. A neutralizing antiserum specific for TGFbeta3 potentiates stimulation of Ca(2+)-activated K(+) currents evoked by a target tissue (iris) extract in cultured ciliary ganglion neurons, indicating that TGFbeta3 is an inhibitory component of these extracts. Intraocular injection of TGFbeta3 causes a modest but significant inhibition of the expression of Ca(2+)-activated K(+) currents in ciliary ganglion neurons developing in vivo. Further, intraocular injection of a TGFbeta3-neutralizing antiserum stimulates expression of Ca(2+)-activated K(+) currents in ciliary ganglion neurons developing in vivo, indicating that endogenous TGFbeta3 regulates the functional expression of this current. The normal developmental expression of functional Ca(2+)-activated K(+) currents in ciliary ganglion neurons developing in vivo is therefore regulated by two different target-derived isoforms of TGFbeta, which produce opposing effects on the electrophysiological differentiation of these neurons.

Target-specific control of nicotinic receptor expression at developing interneuronal synapses in chick

Neuronal differentiation and development of synaptic specializations are strongly influenced by cellular interactions. We compared the effects of interaction with distinct autonomic targets on the molecular and biophysical differentiation of 'upstream' neuron-neuron synapses. Contact with cardiac tissue induced expression of nicotinic receptor channels (nAChRs) distinct from those induced by renal tissue in presynaptic autonomic neurons. The kinetics of cholinergic currents at interneuronal synapses are dictated by the peripheral target contacted. Analysis of the nAChR channel subtypes and subunits in individual neurons demonstrated that the profile of transmitter receptors expressed at mature neuron-neuron synapses develops from the convergent influences of input-derived (anterograde) and target-specific (retrograde) signals.

Molecular diversity of neuronal nicotinic acetylcholine receptors

The potent behavioral and cognitive effects of nicotine highlight the physiological importance of nicotinic acetylcholine receptors (nAChRs). These receptors are part of the superfamily of neurotransmitter-gated ion channels that are responsible for rapid intercellular communication. Molecular cloning of the protein subunits that make up these receptors has led to greater understanding of the pharmacology and physiology of nAChRs. This review outlines our current understanding of the molecular constituents of these receptors and some of the recent studies of the structural determinants of receptors function.

beta43': An enhancer displaying neural-restricted activity is located in the 3'-untranslated exon of the rat nicotinic acetylcholine receptor beta4 gene

Members of a neuronal nicotinic acetylcholine receptor subunit gene cluster ordered beta4, alpha3, alpha5 in the vertebrate genome are expressed in highly restricted patterns in the PNS and CNS. Nothing is known, however, about the regulatory elements that control transcription of these genes in selected neuronal cell populations. We report here a novel enhancer, designated beta43', that is positioned in the beta4 3'-untranslated exon. It is composed of two nearly identical 37 bp direct repeats that are separated by 6 bp. Multimerization of the enhancer upstream of the alpha3 minimal promoter results in synergistic activation. Analysis in different cell types, including three neural lines and primary keratinocytes, shows that beta43' is preferentially active in the neural line PC12, which expresses all members of the cluster. Mobility shift assays reveal a cell-type-specific complex, which forms with the first repeat of the enhancer and PC12 extracts. Complexes co-migrating with the PC12 cell complex are not detected with extracts from other lines, which suggests that PC12 cells contain a differentially expressed factor that may be important for the restricted activity of beta43'. The cell-type-specific activity of the beta43' enhancer suggests that it is important for regulating restricted expression patterns of one or more clustered neuronal acetylcholine receptor genes. Its location within the beta4 gene may be a selective pressure for maintaining tight linkage of clustered neuronal nAchR genes.

Expression of the GABAA receptor delta subunit is selectively modulated by depolarization in cultured rat cerebellar granule neurons

The levels of several GABAA receptor subunit mRNAs increase as cerebellar granule neurons migrate to their adult positions and receive excitatory mossy fiber input. Despite the temporal similarity of these increases in transcript expression in vivo, studies in cultured granule neurons demonstrated that the subunit mRNAs are differentially regulated. To address the possibility that neuronal activity regulates transcript expression, GABAA receptor subunit mRNA levels were assessed in cultured granule neurons grown in chemically defined, serum-free medium containing either nondepolarizing (5 mM) or depolarizing (25 mM) KCl concentrations. Whereas the delta subunit mRNA was almost undetectable in cultures maintained in nondepolarizing medium, an eightfold increase occurred between days 2 and 4 in cultures grown in depolarizing medium. Furthermore, delta subunit transcript expression was reduced by 76 +/- 6% when neurons in depolarizing medium were switched into nondepolarizing medium. The importance of depolarization in the initiation and maintenance of subunit transcript expression in granule neurons was selective for the GABAA receptor delta subunit. These changes in transcript expression involved calcium entry through L-type calcium channels. Nifedipine treatment (1 microM) both reduced intracellular calcium and decreased delta subunit mRNA expression by 79 +/- 4%. Furthermore, inhibition of Ca2+/calmodulin-dependent protein kinases (CaM kinases) by KN-62 (1 microM) also reduced delta subunit transcript expression. These studies demonstrate that KCl-induced depolarization, a condition that mimics the effects of neuronal activity, selectively modulates GABAA receptor delta subunit mRNA expression through a pathway involving calcium entry and activation of a CaM kinase.

Synapse formation molecules in muscle and autonomic ganglia: the dual constraint hypothesis

In 1970 it was thought that if the motor-nerve supply to a muscle was interrupted and then allowed to regenerate into the muscle, motor-synaptic terminals most often formed presynaptic specializations at random positions over the surface of the constituent muscle fibres, so that the original spatial pattern of synapses was not restored. However, in the early 1970s a systematic series of experiments were carried out showing that if injury to muscles was avoided then either reinnervation or cross-reinnervation reconstituted the pattern of synapses on the muscle fibres according to an analysis using the combined techniques of electrophysiology, electronmicroscopy and histology on the muscles. It was thus shown that motor-synaptic terminals are uniquely restored to their original synaptic positions. This led to the concept of the synaptic site, defined as that region on a muscle fibre that contains molecules for triggering synaptic terminal formation. However, nerves in developing muscles were found to form connections at random positions on the surface of the very short muscle cells, indicating that these molecules are not generated by the muscle but imprinted by the nerves themselves; growth in length of the cells on either side of the imprint creates the mature synaptic site in the approximate middle of the muscle fibres. This process is accompanied at first by the differentiation of an excess number of terminals at the synaptic site, and then the elimination of all but one of the terminals. In the succeeding 25 years, identification of the synaptic site molecules has been a major task of molecular neurobiology. This review presents an historical account of the developments this century of the idea that synaptic-site formation molecules exist in muscle. The properties that these molecules must possess if they are to guide the differentiation and elimination of synaptic terminals is considered in the context of a quantitative model of this process termed the dual-constraint hypothesis. It is suggested that the molecules agrin, ARIA, MuSK and S-laminin have suitable properties according to the dual-constraint hypothesis to subserve this purpose. The extent to which there is evidence for similar molecules at neuronal synapses such as those in autonomic ganglia is also considered.

Developmental changes in the nicotinic responses of ciliary ganglion neurons

The accumulation of functional neurotransmitter receptors by neurons during development is an essential part of synapse formation. Chick ciliary ganglion neurons express two kinds of nicotinic receptors. One is abundant, contains the alpha7 gene product, rapidly desensitizes, and binds alpha-bungarotoxin. The other is less abundant, contains multiple gene products (alpha3, beta4, alpha5, and beta2 subunits), slowly desensitizes, and binds the monoclonal antibody mAb 35. Rapid application of agonist to freshly dissociated neurons elicits responses from both classes of receptors. Between embryonic days 8 and 15, the whole cell response of alpha3-containing receptors increases fivefold in peak amplitude and, normalized for cell growth, 1.7-fold in current density. In addition, the response decays more slowly in older neurons, suggesting a developmental decrease in the rate of desensitization. The whole cell response of alpha7-containing receptors increases 10-fold in peak amplitude over the same period and 3-fold in current density. No change in the rate of desensitization was apparent for alpha7-containing receptors with developmental age, but analysis was limited by overlap in responses from the two kinds of receptors. Indirect immunofluorescence measurements on dissociated neurons showed that the relative levels of alpha7-containing receptors on the soma increased during development to the same extent as the whole cell response attributed to them. In contrast, the relative levels of alpha3-containing receptors increased more during the same time period than did the whole cell response they generated. The immunofluorescence analysis also showed that both classes of receptors become distributed in prominent clusters on the cell surface as a function of developmental age. The results indicate that during this period of synaptic consolidation on the neurons, the two major classes of functional nicotinic receptors undergo substantial upregulation; alpha3-containing receptors as a class may undergo changes in receptor properties as well.

Regulation of ion channel expression in neural cells by hormones and growth factors

Voltage-and ligand-gated ion channels are key players in synaptic transmission and neuron-glia communication in the nervous system. Expression of these proteins can be regulated at several levels (transcriptional, translational, or posttranslational) and by multiple extracellular factors in the developing and mature nervous system. A wide variety of hormones and growth factors have been identified as important in neural cell differentiation, which is a complex process involving the acquisition of cell-type-specific ion channel phenotypes. Much literature has already accumulated describing the structural and functional characteristics of ion channels, but relatively little is known about the factors that influence their synthesis and cell surface expression, although this area has generated considerable interest in the context of neural cell development. This article reviews several examples of regulated expression of these channels by cellular factors, namely peptide growth factors and steroid hormones, and discusses, where applicable, current understanding of molecular mechanisms underlying such regulation of voltage-and neurotransmitter-gated ion channels.

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

Different types of neurotransmitter receptors coexist within single neurons and must be targeted to discrete synaptic regions for proper function. In chick ciliary ganglion neurons, nicotinic acetylcholine receptors (nAChRs) containing alpha 3 and alpha 5 subunits are concentrated in the postsynaptic membrane, whereas alpha-bungarotoxin receptors composed of alpha 7 subunits are localized perisynaptically and excluded from the synapse. Using retroviral vector-mediated gene transfer in vivo, we show that the long cytoplasmic loop of alpha 3 targets chimeric alpha 7 subunits to the synapse and reduces endogenous nAChR surface levels, whereas the alpha 5 loop does neither. These results show that a particular domain of one subunit targets specific receptor subtypes to the interneuronal synapse in vivo. Moreover, our findings suggest a difference in the mechanisms that govern assembly of interneuronal synapses as compared to the neuromuscular junction in vertebrates.

Role of cell-cell interactions in the developmental regulation of Ca2+-activated K+ currents in vertebrate neurons

The functional expression of the Ca2+-activated K+ current (IK[Ca]) is dependent on cell-cell interactions in developing chick autonomic neurons. In chick ciliary ganglion (CG) neurons, expression of macroscopic IK[Ca] coincides with the formation of synapses with target tissues. CG neurons that develop in vivo in the absence of normal target tissues fail to express functional IK[Ca], although voltage-activated Ca2+ currents and most other ionic currents are expressed at normal amplitudes and densities. CG neurons placed in cell culture prior to formation of synapses with target tissues also fail to express macroscopic IK[Ca]. However, CG neurons cultured in the presence of a heat- and trypsin-sensitive extract of target tissues express IK[Ca] at normal levels. Similarly, interactions with target tissue appear to regulate the expression of whole-cell IK[Ca] in developing chick sympathetic ganglion neurons, although the relevant trophic factors appear to be different from those required by CG neurons. In addition to target tissue interactions, an intact preganglionic innervation is required for the normal in vivo development of IK[Ca] in chick CG neurons. The trophic effects of the afferent innervation do not require synaptic activation of the CG neurons, indicating secretion of a trophic factor, possibly an isoform of beta-neuregulin. The results are consistent with the hypothesis that target- and nerve terminal-derived trophic factors interact at a posttranslational level in the regulation of a functional IK[Ca]. Together, this body of data demonstrates an essential role for cell-cell interactions in the differentiation of neuronal excitability.

Innervation and target tissue interactions induce Rab-GDP dissociation inhibitor (GDI) expression during peripheral synapse formation in developing chick ciliary ganglion neurons in situ

Regulated exocytosis of neurotransmitter from synaptic vesicles involves the function of a small GTP-binding protein, Rab3A. Rab-GDP dissociation inhibitor (GDI) is an important modulator of Rab function and subcellular distribution. We have characterized the respective roles of innervation and target tissue interactions in regulating GDI expression during synapse formation in chick ciliary ganglion (CG) neurons developing in situ. Here we report the first full-length chick GDI cDNA sequence. It is highly homologous to mammalian GDI isoforms and includes all of the sequence-conserved regions critical for Rab3A binding. This chick GDI mRNA is predominantly expressed in neurons as judged by Northern blot analysis of tissue distribution and by in situ hybridization of CG sections. Developmental increases in CG GDI mRNA levels occur in two phases as determined by reverse transcription (RT)-PCR and by Northern analysis of both normal-developing and input- or target tissue-deprived ganglia. The initial phase appears to be independent of cell-cell interactions. In contrast, the second, larger increase is induced by both presynaptic inputs and postganglionic target tissues but does not occur until target tissue innervation. Synaptic interaction with the target seems necessary for the regulatory response to both inputs and target tissues. GDI protein levels show similar changes. The developmentally delayed ability of inputs and targets to influence GDI levels differs from the regulation of neurotransmitter receptor expression in CG neurons. These results suggest that distinct extrinsic regulatory signals influence the expression of synapse-related components at the presynaptic axon terminal versus postsynaptic membrane in an individual neuron.

Differential regulation of synaptic vesicle protein genes by target and synaptic activity

Differentiation of presynaptic nerve terminals involves changes in gene expression; these may be regulated by synaptic transmission and/or by contact with the target muscle. To gain insight into the control of presynaptic differentiation, we examined the regulation by target and synaptic activity of synaptic vesicle protein (SVP) genes in the chick ciliary ganglion (CG). In the CG, two SVP genes, synaptotagmin I (syt I) and synaptophysin II (syp II), are coordinately upregulated at the time of target contact. To test the hypothesis that this upregulation is induced by target contact, we examined mRNA levels of syt I and syp II in CGs from embryos in which one eye had been removed before axon outgrowth. As expected, target removal prevented the normal upregulation of syt I mRNA in the deprived ganglion. In contrast, and unexpectedly, syp II mRNA upregulation was not affected. The target dependence of syt I upregulation was not attributable to nerve-muscle transmission, because blockade of this transmission had no effect on SVP mRNA levels. Surprisingly, blockade of synapses onto CG neurons from the brain also did not affect syt I mRNA levels but increased levels of syp II mRNA. We conclude that contact with target induces upregulation of syt I mRNA, which is the case for spinal motor neurons. However, the normal upregulation of syp II mRNA is not controlled by the same signal(s). Instead, our results suggest that these two SVP genes are differentially regulated, both by target contact and by blockade of synaptic transmission.

Functional contribution of the alpha5 subunit to neuronal nicotinic channels expressed by chick sympathetic ganglion neurones

1. Heterologous expression studies of the alpha5 subunit of the neuronal acetylcholine receptor (nAChR) gene family have demonstrated that it can participate in the function of ACh-gated channels if co-expressed with another alpha- and a beta-subunit. Previous studies also indicate prominent expression of alpha5 in both central and peripheral nervous systems. The participation of alpha5 in native nAChRs and its functional role in these channels is, however, unknown. 2. In this study, we present evidence that alpha5 has a role in at least two distinct subtypes of nAChR complexes expressed by embryonic chick sympathetic neurones. 3. alpha5 contributes not only to agonist but also to antagonist sensitivity of natively expressed nAChR channels. Functional deletion of the alpha5 subunit by antisense oligonucleotide treatment removes the nAChRs with relatively low affinity to ACh and cytisine. Deletion of alpha5 also eliminates channels that are blocked by the alpha7-specific antagonist methyllycaconitine (MLA) while increasing the percentage of current carried by nAChRs that are sensitive to alpha-bungarotoxin (alpha-BgTx). 4. Single channel analyses indicate that functional deletion of alpha5 results in the deletion of both the 'brief' and 'long' open duration, 50 pS subtypes of nAChR channels while increasing the expression of the 18 pS, alpha-BgTx-sensitive native nAChRs normally detected in sympathetic neurones at later developmental stages. 5. The biophysical and pharmacological profiles of native nAChRs revealed by this study and previous work are discussed in the context of a proposed model of the nAChR channels expressed by chick sympathetic neurones throughout development.

Mechanisms of increased NGF production in vascular smooth muscle of the spontaneously hypertensive rat

The spontaneously hypertensive rat (SHR) was developed as a genetic model of essential hypertension. In vivo and in vitro evidence demonstrates that vascular smooth muscle cells (VSMCs) from the SHR produce more nerve growth factor (NGF) than the normotensive Wistar-Kyoto (WKY) control strain. This increased NGF production is accompanied by excessive innervation of target tissues in the SHR. In the present study, a sensitive, competitive, quantitative, reverse-transcriptase polymerase chain reaction (C Q RT-PCR) assay is characterized and used to analyze levels of NGF mRNA in cultured VSMCs derived from the SHR and WKY strains as well as bladder tissue. Differences in NGF secretion rates between SHR and WKY VSMCs were partially due to an increased stability of NGF mRNA in SHR VSMCs. Following treatment with platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta 1) to elevate NGF production, the half-life of the NGF mRNA was 104.5 +/- 18.0 min in SHR VSMCs, compared to only 36.5 +/- 11.6 min in WKY VSMCs. Sequence analysis of the 3' untranslated region (UTR) revealed no strain differences in cis-acting sequences potentially involved in determining mRNA stability. Thus, it seems unlikely to be a 3'UTR mutation that prolongs mRNA lifetime. Rather, differential regulation of an RNA-binding protein may play a role in the abnormal NGF mRNA stability in SHR VSMCs. SHR VSMCs also demonstrate an increased translational efficiency of NGF protein; more NGF protein is synthesized per unit of NGF mRNA. The use of a C Q RT-PCR assay has allowed the determination that abnormal NGF mRNA stabilization as well as altered translational efficiency may contribute to excess NGF synthesis and progressive hypertension in the SHR.

Differential regulation of levels of nicotinic receptor subunit transcripts in adult sympathetic neurons after axotomy

Axotomy of adult peripheral neurons produces decreases in the levels of transcripts for a number of proteins involved in synaptic transmission. For example, tyrosine hydroxylase and neuropeptide Y mRNA decrease in axotomized sympathetic neurons in the superior cervical ganglion (SCG). In the present study, the effects of axotomy on the expression of nicotinic receptor subunit transcripts were examined in the SCG and the results were compared to those produced by deafferentation and explantation. Normally, neurons in the SCG express five different nicotinic subunits: alpha3, alpha5, alpha7, beta2, and beta4. Forty-eight hours after axotomy in vivo or explantation, dramatic decreases in these transcripts were seen, except for beta2, which increased. In contrast, deafferentation of the SCG had negligible effects on any of these transcripts. Both leukemia inhibitory factor (LIF) and nerve growth factor (NGF) have been shown to play a role in the decrease in neuropeptide Y mRNA expression after axotomy. In the cases of these nicotinic receptor transcripts, however, similar decreases were seen in wild-type and LIF knockout animals. Furthermore, administration of an antiserum to NGF in intact animals produced no changes in transcript levels. On the other hand, providing exogenous NGF to axotomized SCG in vivo or in explant cultures partially prevented the decreases in the transcripts for alpha3, alpha5, alpha7, and beta4. These data indicate that axotomy produces dramatic decreases in the expression of several nicotinic receptor subunit transcripts, and that the molecular signals underlying these changes differ from those previously shown to mediate the decrease in neuropeptide Y expression.

Strychnine-sensitive stabilization of postsynaptic glycine receptor clusters

The cellular and molecular mechanisms underlying the postsynaptic aggregation of ionotropic receptors in the central nervous system are not understood. The glycine receptor (GlyR) and its cytoplasmic domain-associated protein, gephyrin, are clustered at the postsynaptic membrane and constitute a good model for addressing these questions. The glycine receptor is inhibited by strychnine. The effects of chronic strychnine treatment on the expression and cellular distribution of gephyrin and glycine receptor were therefore tested using primary cultures of spinal cord neurons. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis revealed that the glycine receptor alpha1, alpha2, beta subunits and gephyrin mRNAs were expressed at comparable levels in strychnine-treated and untreated cultures. The number of immunoreactive cells and the subcellular distribution of gephyrin and GlyR subunits was determined with standard and confocal immunofluorescence. The proportion of gephyrin and glycine receptor-immunoreactive (IR) cells was unaffected by strychnine treatment. Confocal microscopy revealed that the glycine receptor was mainly localized intracellularly near the nucleus. This cytoplasmic glycine receptor was not associated with the Golgi apparatus nor with the rough endoplasmic reticulum and therefore is not likely to correspond to neosynthesized proteins. The number of GlyR clusters on the somato-dendritic membrane was dramatically reduced on neurons displaying intracellular staining. In contrast, the subcellular distribution and the number of gephyrin clusters was not modified by the treatment. The fact that gephyrin postsynaptic localization was not modified by strychnine suggests that the aggregation of glycine receptor and gephyrin is governed by different mechanisms. The distribution of other cell surface molecules such as NCAM or GABAA receptor beta2/3 subunits was not modified by strychnine treatment. Chronic exposure of the cultures to tetrodotoxin did not affect gephyrin or glycine receptor cluster formation. Taken together, these results indicate that functional glycine receptor, but not electrical synaptic activity, is required for the formation of glycine receptor clusters.

A cysteine-rich isoform of neuregulin controls the level of expression of neuronal nicotinic receptor channels during synaptogenesis

We report here that neuregulin (NRG) isoforms with a conserved cysteine-rich domain (CRD) in their N terminus regulate expression of nicotinic acetylcholine receptors (nAChRs) at developing interneuronal synapses and report the isolation of transmembrane NRG isoforms with this CRD within the N-terminal portion. CRD-NRG mRNA and immunoreactive protein are detected early in developing presynaptic (visceral motor) neurons. The levels of expression of CRD-NRG peak prior to the formation of synapses with their postsynaptic partners, the ganglionic sympathetic neurons. Recombinant CRD-NRG mimics the effects of presynaptic input on target neurons. Functional deletion of CRD-NRG from presynaptic neurons abolishes the upregulation of nAChR expression induced by input-derived soluble material. Thus, CRD-NRG appears to be both a necessary and a sufficient signal for the control of neuronal nAChR expression during synaptogenesis.