Changes in the number, function, and regulation of nicotinic acetylcholine receptors during neuronal development

Neuroprotective effect of Liuwei Dihuang decoction on cognition deficits of diabetic encephalopathy in streptozotocin-induced diabetic rat

ETHNOPHARMACOLOGICAL RELEVANCE

Liuwei Dihuang decoction (LWDHD) is a well-known prescription of traditional Chinese medicine (TCM) and consists of six crude drugs including Rehmannia glutinosa Libosch. (family: Scrophulariaceae), Cornus officinalis Sieb. (family: Cornaceae), Dioscorea oppositifolia L. (family: Dioscoreaceae), Paoenia ostii (family: Paeoniaceae), Alisma orientale (G. Samuelsson) Juz (family: Alismataceae) and Poria cocos (Schw.) Wolf (family: Polyporaceae). It has been used for the treatment of "Kidney-Yin" deficiency syndrome in clinic in China for a long time. Recent studies found that LWDHD had a potential benefit for the treatment of diabetic complications. The aim of the present study is to investigate the neuroprotective effect of LWDHD on memory and cognition deficits in streptozotocin (STZ)-induced diabetic encephalopathy (DE) rats.

MATERIALS AND METHODS

Adult male Sprague Dawley (SD) rats were fed with high-glucose-fat diet for 50 days and then received an intraperitoneal injection of STZ (40 mg/kg) to induce DE model. Morris water maze test was used to evaluate the memory and cognition capability of DE rats. Choline acetyltransferase (ChAT), acetylcholinesterase (AChE), Na(+)-K(+)-ATP enzyme, iNOS and GSH kits were used to determine their activities or content in hippocampus. TUNEL staining, immunohistochemistry and Congo red staining were conducted to evaluate the apoptosis, caspase-3 protein expression, insulin-like growth factors 1 (IGF-1) and brain derived neurophic factor (BDNF) expressions, as well as Aβ deposition.

RESULTS

The treatment with LWDHD (1 and 2g/kg, p.o., once daily, 30 days) could significantly reduce the escape latency time and path length, and obviously enhance the spent time in the target quadrant and platform crossings in Morris water maze test compared with model group (P<0.05, P<0.01). LWDHD could also significantly decrease the level of fasting blood glucose, increase Na(+)-K(+)-ATP enzyme and ChAT activities, enhance remarkedly GSH level while decrease significantly AChE and iNOS activities in hippocampus (P<0.05, P<0.01). Furthermore, TUNEL staining, Congo red staining and immunohistochemistry showed that LWDHD significantly improved the expressions of IGF-1 and BDNF, attenuated the neural apoptosis, overexpression of caspase-3 and Aβ deposition in the hippocampus and cerebral cortex of STZ-induced DE rats (P<0.01).

CONCLUSION

Our findings suggested that LWDHD had a neuroprotective effect on DE rats. LWDHD may be of benefit in the treatment of DE.

Mechanism of action of the nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors at peripheral synapses mediate rapid and effective excitatory synaptic transmission. The functional properties of peripheral and central nicotinic acetylcholine receptors are similar, yet in the central nervous system nicotinic receptors do not appear to occur postsynaptically at many excitatory synapses. Two properties of nicotinic receptors are that significant Ca2+ influx can occur through the receptor channel and that at low agonist concentrations steady activation of nicotinic receptors can occur. These are discussed in the context of presynaptic and postsynaptic localizations of nicotinic receptors.

Developmental changes in the expression of nicotinic acetylcholine receptor alpha-subunits in the rat heart

Neuronal nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels that consist of various subunits. During ontogeny, muscular and neuronal nAChR undergo changes in the distribution and subunit composition in skeletal muscle and brain, respectively. Here, we have investigated the occurrence of the ligand-binding alpha-subunits of neuronal nAChR by means of reverse transcription/polymerase chain reaction and immunohistochemistry in the rat heart during prenatal and postnatal development and after capsaicin-induced sensory denervation. mRNAs coding for the alpha4, alpha5, alpha7 and alpha10 subunits were detected throughout all developmental stages. Messenger coding for the alpha2 subunit was first detectable at developmental stage E20; alpha3 subunit mRNA was expressed throughout all prenatal developmental stages, whereas it was restricted postnatally to the atria. mRNA for alpha6 was observed at E14-P8 but was absent thereafter. At no developmental stage could an unequivocal signal for alpha9 nAChR subunit mRNA be obtained. The expression pattern was unchanged by capsaicin treatment. Immunohistochemistry demonstrated alpha7 subunits on cardiac neurons, fibroblasts and cardiomyocytes and alpha2/4 subunits on cardiomyocytes with a postnatal redistribution to intercalated discs, as shown by cryo-immunoelectron microscopy. Our results indicate an additional non-neuronal expression of nAChR subunits in the rat heart that, as in skeletal muscle, precedes functional innervation and then undergoes changes in its distribution on the surface of cells.

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.

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.

Autonomic function in mice lacking alpha5 neuronal nicotinic acetylcholine receptor subunit

Neuronal acetylcholine nicotinic receptors (nAChR) are composed of 12 subunits (alpha2-10, beta2-4), of which alpha3, alpha5, alpha7, beta2 and beta4 subunits are known to exist in the autonomic nervous system (ANS). alpha5 subunits possess unique biophysical and pharmacological properties. The present study was undertaken to examine the functional role and pharmacological properties of the nAChR alpha5 subunits in the ANS using mice lacking alpha5 nAChR subunits (alpha5-/-). These mice grew to normal size showing no obvious physical or neurological deficit. They also showed normality in thermoregulation, pupil size and resting heart rate under physiological conditions. The heart rate and rectal temperature did not differ between alpha5-/- and wild-type mice during exposure to cold stress. An impairment of cardiac parasympathetic ganglionic transmission was observed during high frequency vagal stimulation, which caused cardiac arrest in all wild-type animals while alpha5-/- mice were more resistant. Deficiency of alpha5 subunits strikingly increased the sensitivity to a low concentration of hexamethonium, leading to a nearly complete blockade of bradycardia in response to vagal stimulation. Such a concentration of hexamethonium only slightly depressed the effects of vagal stimulation in control mice. Deficiency of alpha5 subunits significantly increased ileal contractile responses to cytisine and epibatidine. These results suggest that alpha5 subunits may affect the affinity and sensitivity of agonists and antagonists in the native receptors. Previous studies revealed that alpha5 subunits form functional receptors only in combination with other alpha and beta subunits. Thus, the data presented here imply that alpha5 subunits modulate the activity of nAChR in autonomic ganglia in vivo.

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

We studied nicotinic synapses between chick ciliary ganglion neurons in culture to learn more about factors influencing their formation and receptor subtype dependence. After 4--8 days in culture, nearly all neurons displayed spontaneous excitatory postsynaptic currents (sEPSCs), which occurred at about 1 Hz. Neurons treated with tetrodotoxin displayed miniature EPSCs (mEPSCs), but these occurred at low frequency (0.1 Hz), indicating that most sEPSCs are actually impulse driven. The sEPSCs could be classified by decay kinetics as fast, slow, or biexponential and, reminiscent of the situation in vivo, were mediated by two major nicotinic acetylcholine receptor (AChR) subtypes. Fast sEPSCs were blocked by alpha-bungarotoxin (alpha Bgt), indicating dependence on alpha Bgt-AChRs, most of which are alpha 7 subunit homopentamers. Slow sEPSCs were unaffected by alpha Bgt, and were blocked instead by the alpha 3/beta 2-selective alpha-conotoxin-MII (alpha CTx-MII), indicating dependence on alpha 3*-AChRs, which lack alpha 7 and contain alpha 3 subunits. Biexponential sEPSCs were mediated by both alpha Bgt- and alpha 3*-AChRs because they had fast and slow components qualitatively similar to those comprising simple events, and these were reduced by alpha Bgt and blocked by alpha CTx-MII, respectively. Fluorescence labeling experiments revealed both alpha Bgt- and alpha 3*-AChR clusters on neuron somata and neurites. Colabeling with antisynaptic vesicle protein antibody suggested that some alpha 3*-AChR clusters, and a few alpha Bgt-AChR clusters are associated with synaptic sites, as is the case in vivo. These findings demonstrate the utility of ciliary ganglion neuron cultures for studying the regulation of nicotinic synapses, and suggest that mixed AChR subtype synapses characteristic of the neurons in vivo can form in the absence of normal inputs or targets.

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.

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.

Two distinct classes of functional 7-containing nicotinic receptor on rat superior cervical ganglion neurons

Nicotinic acetylcholine receptors (nAChRs) that bind alpha-bungarotoxin (alpha Bgt) were studied on isolated rat superior cervical ganglion (SCG) neurons using whole-cell patch clamp recording techniques. Rapid application of ACh onto the soma of voltage clamped neurons evoked a slowly desensitizing current that was reversibly blocked by alpha Bgt (50 nM). The toxin-sensitive current constituted on average about half of the peak whole-cell response evoked by ACh. Nanomolar concentrations of methyllycaconitine blocked the alpha Bgt-sensitive component of the ACh-evoked current as did intracellular dialysis with an anti-alpha 7 monoclonal antibody. The results indicate that the slowly reversible toxin-sensitive response elicited by ACh arises from activation of an unusual class of alpha 7-containing receptor (alpha 7-nAChR) similar to that reported previously for rat intracardiac ganglion neurons. A second class of functional alpha 7-nAChR was identified on some SCG neurons by using rapid application of choline to elicit responses. In these cases a biphasic response was obtained, which included a rapidly desensitizing component that was blocked by alpha Bgt in a pseudo-irreversible manner. The pharmacology and kinetics of the responses resembled those previously attributed to alpha 7-nAChRs in a number of other neuronal cell types. Experiments measuring the dissociation rate of 125I-labelled alpha Bgt from SCG neurons revealed two classes of toxin-binding site. The times for toxin dissociation were consistent with those required to reverse blockade of the two kinds of alpha Bgt-sensitive response. These results indicate that rat SCG neurons express two types of functional alpha 7-nAChR, differing in pharmacology, desensitization and reversibility of alpha Bgt blockade.

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.

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.

Actin filaments and the opposing actions of CaM kinase II and calcineurin in regulating alpha7-containing nicotinic receptors on chick ciliary ganglion neurons

Nicotinic acetylcholine receptors containing alpha7 subunits have a high relative permeability to calcium and influence numerous calcium-dependent cellular events. On chick ciliary ganglion neurons the receptors are concentrated on somatic spines containing actin filaments. Using conventional whole-cell patch-clamp recording from dissociated ciliary ganglion neurons, we show that responses from alpha7-containing receptors undergo substantial rundown when the receptors are repeatedly challenged with nicotine. Stabilization of actin filaments with phalloidin partially prevents the rundown, whereas collapse of actin filaments with latrunculin A exacerbates it. The rundown depends on calcium influx through the receptors because it requires receptor activation and can be prevented by replacing extracellular calcium with barium or by intracellular dialysis with BAPTA. Thapsigargin and ryanodine each inhibit the rundown, demonstrating further a requirement for calcium release from internal stores. Blockade of calmodulin by calmidazolium or blockade of CaM kinase II with either KN93 or autocamtide-2-related inhibitory peptide each prevents the rundown; blockade of the phosphatase calcineurin with either cyclosporin A or deltamethrin increases the rundown. The results indicate a balance of calcium-dependent kinase and phosphatase activities in regulating the function of alpha7-containing receptors. Manifestation of the rundown depends in part on the loss of intracellular components via dialysis because little rundown is seen if perforated patch-clamp recording is used to monitor receptor responses even in latrunculin A-treated cells. A membrane-permeable calcineurin inhibitor, however, still decreases the nicotinic response in a calcium-dependent manner, confirming that calcium-dependent phosphoregulation of alpha7-containing receptors occurs in the intact cell.

Extracellular calcium regulates responses of both alpha3- and alpha7-containing nicotinic receptors on chick ciliary ganglion neurons

Neuronal nicotinic receptors are generally both permeable to calcium and potentiated by it. We have examined acute calcium regulation of both native alpha7-containing and the less abundant alpha3-containing nicotinic receptors on chick ciliary ganglion neurons. Most of the receptors are concentrated on somatic spines tightly overlaid in situ by a large presynaptic calyx. Whole cell patch-clamp recording from dissociated neurons using perforated patch-clamp techniques indicates that the rapidly desensitizing nicotinic response of alpha7-containing receptors achieves maximum amplitude in 2 mM calcium; both lower and higher concentrations of calcium are less effective. Barium and strontium but not magnesium can substitute for calcium in potentiating the response. Neither calcium current through the receptors nor calcium action at intracellular sites is necessary. These latter conclusions are supported by current-voltage analysis of the nicotine-induced response, ion substitution experiments, and internal perfusion of the cells with 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) via a conventional patch pipette. Varying the agonist concentration indicates that some of the calcium-dependent enhancement may involve a shift in the dose-response curve for agonist binding, but much of the effect is also likely to involve increased receptor responsiveness. Blockade of alpha7-containing receptors with alpha-bungarotoxin showed that the heteromeric alpha3-containing nicotinic receptors also undergo calcium-dependent potentiation. Calcium did not have a major effect on the desensitization rate of either receptor class but did have a selective effect on the rise time of alpha7-containing receptors. Analysis of stably transfected cells expressing an alpha7 gene construct showed that the calcium potentiation observed for native receptors did not require neuron-specific modifications or components and that it could be seen with the natural agonist acetylcholine. Receptor dependence on extracellular calcium may provide a regulatory mechanism for constraining synaptic signaling, avoiding local depletion of external calcium, and limiting calcium buildup in postsynaptic compartments.

Pituitary adenylate cyclase-activating polypeptide activates a phospholipase C-dependent signal pathway in chick ciliary ganglion neurons that selectively inhibits alpha7-containing nicotinic receptors

Neuropeptide receptors couple via G-proteins to two principal signaling pathways that elevate cAMP through adenylate cyclase (AC) or mobilize intracellular Ca(2+) through phospholipase C (PLC)-stimulated inositol phosphate (IP) turnover and production of inositol 1,4,5-trisphosphate (IP(3)). We showed previously that high-affinity receptors for pituitary adenylate cyclase-activating polypeptide (PACAP) are present on chick ciliary ganglion neurons and that receptor occupation increases cAMP production, resulting in enhanced acetylcholine sensitivity. After we suppressed AC activity and cAMP production with 2'-5' dideoxyadenosine, however, PACAP no longer increased acetylcholine sensitivity but instead reduced it, suggesting that an AC-independent signal pathway activated by PACAP inhibits some nicotinic acetylcholine receptors (AChRs). We now use fast-perfusion, imaging, and biochemical methods to identify the AChRs modulated by PACAP and to characterize the signal pathway responsible for their inhibition. Without previous AC block, both the rapidly desensitizing, alpha-bungarotoxin (alphaBgt)-sensitive alpha7-AChRs and the slowly desensitizing, alphaBgt-insensitive alpha3*-AChRs on the neurons were potentiated by PACAP. After AC blockade, however, PACAP inhibited alpha7-AChRs but left alpha3*-AChRs unaffected. The selective inhibition of alpha7-AChRs appeared to use a PLC signaling pathway because it was not seen after lowering PLC activity or buffering intracellular Ca(2+) and was mimicked by dialyzing neurons with an IP(3) receptor agonist. PACAP also induced IP turnover and increased [Ca(2+)](i) assessed directly with Fluo-3AM imaging. Given our previous findings that PACAP receptors couple to AC, the present results demonstrate a remarkable ability of a single neuropeptide to activate two signaling pathways and in so doing selectively regulate two classes of downstream ion channel targets.

Single channel properties of human alpha3 AChRs: impact of beta2, beta4 and alpha5 subunits

1. We performed single channel analysis on human alpha3 acetylcholine receptors (AChRs) in Xenopus oocytes and native AChRs from the human neuroblastoma cell line IMR-32. alpha3 AChRs exhibit channel properties that reflect subunit composition. 2. alpha3beta2 AChR open times were 0.71 +/- 0.14 and 3.5 +/- 0.4 ms with a predominant conductance of 26 pS. alpha3beta4 AChRs had open times of 1.4 +/- 0.2 and 6.5 +/- 0.8 ms and a predominant conductance of 31 pS. Burst times were 0.82 +/- 0.12 and 5.3 +/- 0.7 ms for alpha3beta2 and 1.7 +/- 0.1 and 16 +/- 1 ms for alpha3beta4. Desensitization was faster for AChRs with the beta2 subunit than for those with the beta4 subunit. 3. One open time for alpha3alpha5beta2 AChRs (5.5 +/- 0.3 ms) was different from those of alpha3beta2 AChRs. For alpha3alpha5beta4 AChRs, an additional conductance, open time and burst time (36 pS, 22 +/- 3 ms and 43 +/- 4 ms, respectively) were different from those for alpha3beta4 AChRs. 4. alpha3 AChRs were inhibited by hexamethonium or mecamylamine. The rate constants for block of alpha3beta4 by hexamethonium and of alpha3beta2 by mecamylamine were 1.2 x 107 and 4.6 x 107 M-1 s-1, respectively. 5. AChRs from IMR-32 cells had a predominant conductance of 32 pS and open times of 1.5 +/- 0.3 and 9.6 +/- 1.2 ms. These properties were most similar to those of alpha3beta4 AChRs expressed in oocytes. Antibodies revealed that 5 +/- 2 % of IMR-32 alpha3 AChRs contained alpha5 subunits and 6 +/- 2 % contained beta2 subunits. IMR-32 alpha3 AChRs are primarily alpha3beta4 AChRs.

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.

Mammalian nicotinic receptors with alpha7 subunits that slowly desensitize and rapidly recover from alpha-bungarotoxin blockade

One of the most abundant nicotinic receptors in the nervous system is a species that contains the alpha7 gene product, rapidly desensitizes, and binds alpha-bungarotoxin with great affinity. The receptor has a high relative permeability to calcium and performs a variety of functions including presynaptic modulation of transmitter release and postsynaptic generation of synaptic currents. Fast excitatory transmission in mammalian intracardiac ganglia is mediated primarily by nicotinic receptors, and although intracardiac ganglion neurons express the alpha7 gene, no toxin-sensitive response has been detected previously in them. We report here that whole-cell patch-clamp recordings from freshly dissociated intracardiac ganglion neurons reveal a nicotinic response that desensitizes slowly and is blocked by alpha-bungarotoxin in a rapidly reversible manner. The only rat gene previously thought capable of forming such receptors was alpha9, but no evidence suggests that the alpha9 gene is expressed in neurons. We find that reverse transcription (RT)-PCR detects alpha7 but not alpha9 mRNA in the ganglia. In addition, the pharmacology of the nicotinic response is typical of alpha7-containing receptors but differs in several respects from that expected for alpha9. Binding experiments with immunotethered receptors identifies a ganglionic species that contains the alpha7 gene product. Moreover, intracellular perfusion of the cells with an anti-alpha7 monoclonal antibody specifically reduces the amplitude of the toxin-sensitive response. The results indicate that alpha7-containing receptors are responsible for the slowly desensitizing, toxin-reversible response and suggest that the receptors are modified in cell-specific ways to influence their functional properties.

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.

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.

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.

Functional contribution of the alpha7 subunit to multiple subtypes of nicotinic receptors in embryonic chick sympathetic neurones

1. Many studies of the alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) family have demonstrated that this alpha-bungarotoxin (alpha-BgTx)-binding neuronal receptor can participate in ACh-gated channels. Heterologous expression studies reveal that alpha7 subunits form homomeric channels of unusually high Ca2+ permeability. However, the physiological role of the alpha7 subunit in native neuronal nAChR channels is less clear. 2. We present evidence that the alpha7 subunit contributes to the function of at least three subtypes of native nAChR expressed by embryonic chick sympathetic neurones. These subtypes are functionally distinct from heterologously expressed homomeric alpha7 nAChRs as well as homomeric-like currents described in studies of hippocampal and parasympathetic neurones. 3. The proposed nAChRs differ from one another and from homomeric alpha7 nAChRs in their sensitivity to block by alpha7 subunit-specific antagonists: alpha-BgTx and methyllycaconitine (MLA). While MLA blocks 60 % of the macroscopic ACh response, alpha-BgTx inhibits a small component of the macroscopic current described by slow-on and slow-off kinetics. 4. Functional deletion of the alpha7 subunit by antisense oligonucleotide treatment eliminates the susceptibility of the nAChRs to block by both MLA and alpha-BgTx. 5. Single channel recordings combined with pharmacological and antisense-mediated 'deletion' techniques reveal that alpha-BgTx-sensitive alpha7-containing nAChRs have a small unitary conductance (18 pS), brief open time kinetics and relatively low open probability (Po). MLA-sensitive alpha7 nAChRs are characterized by a conductance of approximately 35 pS, intermediate burst duration, and a relatively high Po. 6. The third nAChR subtype deleted by alpha7 antisense treatment is characterized by a unitary conductance of 50 pS and prolonged opening duration. 7. We propose that these three populations of native alpha7-containing nAChRs are distinct heteromeric complexes that include other alpha and/or beta nAChR subunits.

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.

Rapid synaptic transmission in the avian ciliary ganglion is mediated by two distinct classes of nicotinic receptors

We analyzed the kinetics and pharmacology of EPSCs in two kinds of neurons in the embryonic avian ciliary ganglion. Whole-cell voltage-clamp recordings revealed that the singly innervated ciliary neurons had large-amplitude (1.5-8.0 nA) EPSCs that could be classified according to the kinetics of their falling phases. Most of the neurons responded with an EPSC the falling phase of which followed a double exponential time course with time constants of approximately 1 and 10 msec. The EPSCs of the remaining ciliary neurons followed a single time constant ( approximately 8 msec). Multiple innervated choroid neurons had smaller-amplitude responses (0.2-1.5 nA when all inputs were activated) that appeared to contain only a slowly decaying component (tau = 12 msec). The fast and slow components of EPSC decay seen in most ciliary neurons could be pharmacologically isolated with two toxins against nicotinic acetylcholine receptors (AChRs). The fast component was blocked by 50 nM alpha-bungarotoxin (alpha-BuTx), which binds alpha7-subunit-containing AChRs. The slow component was selectively blocked by 50 nM alpha-conotoxin MII (alpha-CTx-MII), which blocks mammalian AChRs containing an alpha3/beta2 subunit interface. A combination of both alpha-BuTx and alpha-CTx-MII abolished nearly all evoked current. Similar pharmacological results were found for ciliary neurons with monoexponentially decaying EPSCs and for choroid neurons. These results suggest that nerve-evoked transmitter acts on at least two different populations of AChRs on autonomic motor neurons in the ciliary ganglion.

Codistribution of nicotinic acetylcholine receptor subunit alpha3 and beta4 mRNAs during rat brain development

We have used in situ hybridization to characterize the ontogeny of alpha3 and beta4 nicotinic acetylcholine receptor (nAChR) subunit mRNA expression in rat brain. Transcripts for both subunits were detected in embryonic brain, although overlapping expression of alpha3 mRNA was only evident in areas of strong beta4 mRNA expression, including the medial habenula, locus coeruleus, the cerebellar primordium, and several motor and sensory brainstem nuclei. During the perinatal period, the independent expression of alpha3 mRNA declined, and greater correspondence in the temporal and spatial expression of alpha3 and beta4 subunit mRNAs emerged. In general, beta4 mRNA expression preceded that of alpha3 mRNA by 1 to 2 days. Overlapping expression patterns were transiently detected in the caudate putamen, basal forebrain, frontal and visual cortices, and in the CA3 field of hippocampus. Codistribution that lasted throughout development and into adulthood was noted in a number of brain areas, including the retrosplenial cortex, subiculum, medial habenula, interpeduncular nucleus, locus coeruleus, and brainstem motor nuclei. In many of these regions, alpha5 subunit mRNA was also expressed. Colocalization of alpha3 and beta4 mRNAs with choline acetyltransferase mRNA was detected in cholinergic neurons of the brainstem motor nuclei, nucleus ambiguus, dorsal motor nucleus of the vagus, motor trigeminal nucleus, and facial nucleus, but not in most forebrain cholinergic cells. The extensive correspondence in temporal and spatial distribution of alpha3 and beta4 mRNAs throughout postnatal brain development suggests that these subunits may be coordinately regulated and may form functional neuronal nAChRs with significant developmental roles.

Functional properties of neuronal nicotinic acetylcholine receptor channels expressed in transfected human cells

To study how subunit composition affects the functional properties of neuronal nicotinic acetylcholine receptors (nAChRs), we examined the behaviour of acetylcholine (ACh)-induced single-channel currents in human BOSC 23 cells transiently transfected with various subunit cDNA combinations. For all nAChRs examined (chick and rat alpha 3 beta 4, chick alpha 3 beta 2, alpha 4 beta 2, alpha 7 and alpha 8), expression levels were high enough to allow measurements of acetylcholine-evoked whole-cell currents and nicotine-elicited Ca2+ transients as well as the functional characterization of nAChR channels. Unitary acetylcholine-evoked events of alpha 8 nAChR had a slope conductance of 23 pS, whereas two conductance classes (19-23 and 32-45 pS) were identified for all other nAChR channels. The mean channel open times were significantly longer for homomeric alpha 7 and alpha 8 nAChRs (6-7 ms) than for heteromeric nAChRs (1-3 ms), with the exception of alpha 3 beta 4 nAChRs (8.4 ms for rat, 7 ms for chick). At least two species of heterologously expressed nAChRs (alpha 3 beta 4 and alpha 3 beta 2) exhibited single-channel characteristics similar to those reported for native receptors. The variety of nAChR channel conductance and kinetic properties encountered in human cells transfected with nAChR subunits contributes to the functional diversity of nAChRs in nerve cells.

Changes in the regulatory effects of cell-cell interactions on neuronal AChR subunit transcript levels after synapse formation

Nicotinic acetylcholine receptors (AChRs) mediate excitatory synaptic transmission in the chick ciliary ganglion. AChR protein and mRNA levels are increased by both innervation and retrograde signals from target tissues during synapse formation. We now show that AChR alpha3, beta4, and alpha5 subunit transcript levels stop increasing after synaptogenesis. Moreover, maintenance of these mRNA levels requires the continued presence of regulatory signals from both pre- and postganglionic tissues. Unilateral preganglionic denervation or postganglionic axotomy causes declines in alpha3, beta4, and alpha5 transcript levels, ranging from twofold to 3. 5-fold, relative to contralateral control neuron values in newly hatched chicks. The reductions are not merely an injury response; cbeta4-tubulin mRNA levels are not affected by either axotomy or denervation. Further, similar decreases in AChR mRNA levels are observed after local application of colchicine to the postganglionic nerves, which blocks fast transport without disturbing axonal integrity. These results also demonstrate a developmental change in the regulatory effects of target tissues. Reductions in alpha5 mRNA levels caused by axotomy or colchicine treatment after peripheral synapse formation contrast with the lack of an effect on alpha5 when synapse formation with the target tissue is prevented. The ability of the target tissue to regulate alpha5 mRNA levels after synaptogenesis is interesting, because this subunit may be necessary for the formation of high-conductance AChRs. The specific regulatory effects of target tissues and inputs at different developmental stages demonstrate that neurons continually depend on signals from their pre- and postsynaptic tissues to accomplish mature levels of AChR subunit expression and optimal functioning of that neuronal circuit.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Post-translational regulation of neuronal acetylcholine receptors stably expressed in a mouse fibroblast cell line

Second messenger regulation of neuronal acetylcholine receptors (AChRs) was investigated in a mouse fibroblast cell line, M10, stably transfected with chicken alpha 4 and beta 2 cDNAs. Both forskolin and 8-bromo-cyclic adenosine 3',5'-monophosphate (cAMP) induced large increases in the numbers of AChRs. The increases were due in part to increased transcription and translation of the alpha 4 and beta 2 genes. Blockade of protein synthesis with cycloheximide, however, revealed that forskolin also exerts a post-translational effect, increasing the number of surface receptors by twofold. Immunoblot analysis of sucrose gradient fractions confirmed that the cells had a large fraction of unassembled subunits potentially available for receptor assembly. The post-translational effect of forskolin was blocked by H-89, an inhibitor of cAMP-dependent protein kinase, and by okadaic acid, an inhibitor of phosphatases 1 and 2A. Nicotine also acted post-translationally to induce a twofold increase in the number of surface receptors, but the mechanism differed from that utilized by forskolin, since the effects of the two agents were additive and were differentially affected by okadaic acid. The results suggest that protein phosphorylation-dephosphorylation mechanisms act post-translationally to increase the number of neuronal AChRs maintained on the cell surface. This could be achieved by increasing the efficiency of receptor assembly, transport, or stabilization on the cell surface.

Muscarinic receptor agonist-induced increases in cytosolic Ca2+ concentrations in chick ciliary ganglion cells

We used fura-2 microfluorometry to examine the mechanism underlying the muscarinic receptor agonist-induced increase in the cytosolic Ca2+ concentration ([Ca]in) in acutely isolated chick ciliary ganglion neurons. The order of potencies of muscarinic agonists in increasing [Ca]in was: oxotremorine M > muscarine > methacholine > oxotremorine > bethanechol. The muscarine-induced increase in [Ca]in persisted after treatment with thapsigargin, which blocked caffeine- and muscarinic agonist-induced Ca2+ release. The muscarine-sensitive [Ca]in increase was inhibited by both L- and N-type Ca2+ channel blockers but potentiated by an L-type Ca2+ channel agonist. Muscarine was effective in increasing [Ca]in in the presence of a desensitizing concentration of nicotine, and simultaneous addition of maximal doses of muscarine and nicotine caused an additive increase in [Ca]in. On the other hand, nicotine-, ATP-, and high K(+)-induced increase in [Ca]in was markedly potentiated during continuous stimulation with muscarine. These results suggest that muscarinic receptor stimulation increases Ca2+ influx passing through voltage-dependent Ca2+ channels. However, the muscarine-induced Mn2+ influx was observed in only some muscarine-sensitive cells, suggesting that muscarine-induced depolarization is too weak to overcome the inhibitory effect of Mn2+ on Ca2+ channels.

Patch-clamp analysis of glycine-induced currents in chick ciliary ganglion neurons

1. The whole-cell configuration of the patch-clamp technique was used to analyse currents induced by glycine in chick ciliary ganglion neurons freshly dissociated from 14- to 15-day-old embryos. 2. Application of glycine to cells voltage-clamped at -60 mV induced inward currents in all neurons tested. Dose-response curves yielded an EC50 of about 50 microM. Similar responses were elicited by beta-alanine and taurine though higher concentrations were required. 3. Strychnine reversibly inhibited the glycine-induced responses. The effect was dose dependent with a half-maximal effect being obtained with 20 nM strychnine. 4. Glycine-induced currents were inhibited by 100 microM Zn2+. The inhibition had slow rates of onset and recovery, in contrast to Zn2+ inhibition of GABA responses. Both bicuculline and d-tubocurarine inhibited glycine responses in a dose-dependent manner. 5. The steady-state I-V curve for glycine-induced currents was linear over the range -60 to +60 mV, but showed an outward rectification at very hyperpolarized membrane potentials. The reversal potential of glycine-induced currents shifted with changes in intracellular chloride concentration in a manner expected for chloride-selective channels. 6. Expression of functional glycine receptors during development was examined at embryonic day 8 (E8), 11, 14, and 18. The mean peak current was about 60-fold larger at E14 than at E8 and vastly exceeded changes in cell size. During the same period, responses to GABA increased only 2-fold in amplitude and correlated with changes in cell size.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine differentially affects intracellular calcium via nicotinic and muscarinic receptors on the same population of neurons

Multiple receptor subtypes activated by the same ligand but coupled to different second messengers can produce divergent signaling in a cell, while receptors activated by different ligands but sharing the same second messenger can produce convergent signaling. We show here that chick ciliary ganglion neurons have three classes of receptors activated by the same neurotransmitter, acetylcholine, and that all three regulate the same second messenger, intracellular free calcium. Activation of muscarinic receptors on the neurons stimulates phosphatidylinositol turnover and induces calcium oscillations that are initiated and maintained by calcium release from caffeine/ryanodine-insensitive intracellular stores. Extracellular calcium is required to sustain the oscillations, while cadmium abolishes them. Activation of either of two classes of nicotinic receptors, distinguished both by location on the neurons and by subunit composition, induces a single, rapid elevation in intracellular calcium without inducing phosphatidylinositol turnover. The nicotinic responses are entirely dependent on extracellular calcium, show no dependence on release from internal stores, and do not display oscillations. Low concentrations of the native agonist, acetylcholine, induce repetitive calcium spikes in the neurons characteristic of muscarinic receptors, while higher concentrations induce nonoscillating increases in intracellular calcium that include contributions from nicotinic receptors. The three classes of receptors also differ in the acetylcholine concentration required to elicit a response. These differences, together with differences in receptor location and sources of calcium mobilized, may enable the receptor subtypes to target different sets of calcium-dependent processes for regulation.

Neurons can maintain multiple classes of nicotinic acetylcholine receptors distinguished by different subunit compositions

Although 10 genes have been cloned encoding putative subunits of neuronal nicotinic acetylcholine receptors, little is known about the variety or subunit composition of such receptors expressed by individual neurons. Chick ciliary ganglion neurons express five of the known genes and assemble a class of synaptic-type receptors collectively containing gene products from three of them: alpha 3, beta 4, and alpha 5. Using subunit-specific monoclonal antibodies, we show here that all of the synaptic-type acetylcholine receptors having alpha 3 also have beta 4 subunits and vice versa. In addition, most, if not all, of the alpha 5 gene product present in fully assembled receptors is associated with both alpha 3 and beta 4 subunits. Although the receptors may be homogeneous in these respects, only about 20% of them also contain the fourth gene product, beta 2, newly identified in the ganglion; essentially all of the neurons express the beta 2 gene. No beta 2 subunits are found coassembled with the fifth acetylcholine receptor gene product expressed by the neurons, alpha 7, which has been shown previously to comprise a class of abundant, nonsynaptic receptors on the cells. The identification of three acetylcholine receptor subtypes distinguished by subunit composition on the same neurons provokes questions about their individual physiological roles.

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

Neurons engage in two distinct types of cell-cell interactions: they receive innervation and establish synapses on target tissues. Regulatory events that influence synapse formation and function on developing neurons are largely undefined. We show here that nicotinic acetylcholine receptor (AChR) subunit transcript levels are differentially regulated by innervation and target tissue interactions in developing chick ciliary ganglion neurons in situ. Using ganglia that have developed in the absence of pre- or postganglionic tissues and quantitative RT-PCR, we demonstrate that alpha 3 and beta 4 transcript levels are increased by innervation and target tissue interactions. In contrast, alpha 5 transcript levels are increased by innervation, but target tissues have little effect. Whole-cell ACh-induced currents, used to estimate the number of functional AChRs, change in correlation with alpha 3 and beta 4, but not alpha 5, transcript levels. A model is proposed in which the changes in AChR subunit expression regulate levels of synaptic activity, which is a critical determinant of synapse stabilization and elimination, and neuronal cell death.

Developmental regulation of multiple nicotinic AChR channel subtypes in embryonic chick habenula neurons: contributions of both the alpha 2 and alpha 4 subunit genes

Habenula neurons from both early and late stage embryonic chickens express multiple subtypes of nicotinic acetylcholine receptor channels (nAChRs). The channel subtypes expressed by habenula neurons are similar in functional properties, but apparently distinct in subunit composition, from their peripheral counterparts in autonomic ganglia. Early in development, nicotine activates four classes of neuronal bungarotoxin (nBGT)-sensitive channels (approx. conductance = 15, 30, 50, 60pS) that are intermingled on the surface of habenula neuronal somata. In neurons removed from older animals, nAChR channel activity has increased 4- to 40-fold and channel subtypes have become spatially segregated from one another. Analysis of the profile of nAChR subunit gene expression by polymerase chain reaction indicates that several of the alpha-type subunit genes, including alpha 2,3,4,5,7, and alpha 8, as well as both beta 2 and beta 4, are expressed. Treatment of the neurons with subunit specific antisense oligonucleotides reveals that the alpha 2 and alpha 4 (but not alpha 3) subunits contribute to the functional profile of native nAChRs expressed by habenula neurons. Consideration of the functional properties and apparent subunit composition of autonomic ganglion nAChRs in the chick suggests that habenula neurons may utilize a very distinct set of subunit combinations to produce an array of nAChR channel subtypes similar in both conductance and pharmacological profile to those expressed by sympathetic neurons.

Cell-specific regulation of agrin RNA splicing in the chick ciliary ganglion

Alternative splicing results in production of four agrin proteins (agrin0, agrin8, agrin11, and agrin19) with different AChR aggregating activities. However, the cellular origin of mRNAs encoding each agrin isoform remains unknown. Using single-cell PCR, we demonstrate that in the chick ciliary ganglion, nonneuronal cells express only mRNA encoding agrin0, whereas neurons express one or any combination of agrin mRNAs. Moreover, significant differences were observed between the agrin mRNA profiles of ciliary and choroid neurons in the ganglion. The abundance of each agrin mRNA, the fraction of neurons expressing each transcript, and the combinations of transcripts expressed by neurons also change during development. Our results demonstrate that transcripts encoding agrin proteins with high AChR aggregating activity are expressed exclusively by neurons in the ciliary ganglion and that alternative splicing of agrin mRNA is regulated during development and in a cell-specific manner.

Neuronal acetylcholine receptors that bind alpha-bungarotoxin with high affinity function as ligand-gated ion channels

Neuronal membrane components that bind alpha-bungarotoxin with high affinity can increase intracellular levels of free calcium, demonstrating the components function as nicotinic receptors. Though such receptors often contain the alpha 7 gene product, which by itself can produce ionotropic receptors in Xenopus oocytes, numerous attempts have failed to demonstrate an ion channel function for the native receptors on neurons. Using rapid application of agonist, we show here that the native receptors are ligand-gated ion channels which are cation selective, prefer nicotine over acetylcholine, and rapidly desensitize. Much of the calcium increase caused in neurons by the receptors under physiological conditions appears to result from their depolarizing the membrane sufficiently to trigger calcium influx through voltage-gated channels.

Enhancement of the N-methyl-D-aspartate response in spinal dorsal horn neurons by cAMP-dependent protein kinase

Glutamate-gated ion channels mediate excitatory synaptic transmission in the central nervous system and are involved in synaptic plasticity, neuronal development and excitotoxicity (5,24). These ionotropic glutamate receptors were classified according to their preferred agonists as AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid), KA (kainate), and NMDA (N-methyl-D-aspartate) receptors [Trends Pharmacol. Sci., 11 (1990) 25-33]. The present study of NMDA receptor channels expressed in acutely isolated spinal dorsal horn (DH) neurons of young rat reveals that they are subject to modulation through the adenylate cyclase cascade. Whole-cell voltage-clamp recording mode was used to examine the effect of adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase (PKA) on the responses of DH neurons to NMDA. Whole-cell current response to NMDA was enhanced by 8 Br-cAMP, a membrane permeant analog of cAMP or by intracellular application of cAMP or catalytic subunit of PKA.

Muscle membrane preparation restores sensitivity to acetylcholine in cultured chick ciliary ganglion neurons

Ciliary ganglion (CG) neurons grown in culture in the absence of muscle cells rapidly lose sensitivity to acetylcholine (ACh), while neurons grown in the presence of muscle or muscle cell membranes maintain sensitivity to ACh for extended periods of time. The present study examined whether exposure to muscle membrane preparation or stimulation of cAMP-dependent processes could restore sensitivity to ACh in cultured neurons which had lost responsiveness to ACh. CG neurons from 11- to 14-day-old chick embryos were grown on collagen substrate in the absence of muscle cells. Sensitivity to ACh was assessed by measuring peak current responses following application of ACh (IACh) to neurons under whole-cell voltage clamp. In control cultures IACh decreased from an average of 837 pA the day of plating to 145 pA following 4 days in culture. Stimulation of cAMP-dependent processes with forskolin and 3-isobutyl-1-methylxanthine (IBMX) or 8'Br-cAMP and IBMX had variable effects on IACh. These treatments increased peak IACh in some neurons maintained in culture for less than 48 h. Treatment with these agents decreased peak IACh in cultures which were more than 48 h old. Exposure of neurons, which had lost sensitivity to ACh in culture, to muscle membranes increased IACh 2- to 3-fold over 24 to 48 h. This membrane-induced restoration of sensitivity to ACh was blocked by exposure to the protein synthesis inhibitor cycloheximide. Stimulation of cAMP-dependent processes in neurons exposed to muscle membrane decreased IACh. In conclusion, these results indicate that some element associated with the membranes of muscle cells has the ability to restore ACh responsiveness to CG neurons which have become insensitive to ACh in culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Allosteric modulations of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor behaves as an allosteric protein with multiple, interconvertible conformations: a resting state, an open channel state and several desensitized states. A variety of pharmacological agents and physiological ligands regulate the transitions between these states when they bind to sites topographically distinct from the acetylcholine binding site. The physiological significance of this type of regulation is discussed and its potential role in the modulation of synaptic efficacy suggested.

Co-existent expression of GABAA receptor beta 2, beta 3 and gamma 2 subunit messenger RNAs during embryogenesis and early postnatal development of the rat central nervous system

The expression of beta 1, beta 2, beta 3, gamma 2 and delta subunit messenger RNAs of the GABAA receptor was followed by in situ hybridization histochemistry using radiolabeled oligodeoxynucleotide probes in sections of embryonic (E12-21) and early postnatal (P1-5) rat. beta 2, beta 3 and gamma 2 subunit messenger RNAs were first detectable at E15 in the spinal cord (ventral > dorsal) and lower central nervous system regions (e.g. pons, medulla and thalamus). beta 3 subunit messenger RNA was abundantly expressed in olfactory bulb neurons at E15. At E17, the expression pattern of these subunit messenger RNAs continued in the lower central nervous system. In the upper central nervous system, beta 2, beta 3, and gamma 2 subunit messenger RNAs were first detectable in the outer layer of the hippocampal and entire cortical neuroepithelium. The expression for both beta 3 and gamma 2 subunit messenger RNAs increased significantly over that observed at E15, whereas beta 2 subunit messenger RNA increased to a lesser extent and was more discretely expressed in inferior colliculus, cerebellar neuroepithelium and spinal cord (ventral = dorsal). By E19, messenger RNAs for beta 2, beta 3 and gamma 2 subunits a widespread and abundant co-existent distribution throughout the central nervous system. Exceptions to this co-expression were the absence of beta 2 messenger RNA in the dentate gyrus and beta 3 messenger RNA in entorhinal cortex, areas in which they are present in adult. There was also a differential distribution of subunit messenger RNAs in developing olfactory bulb at E19-20: the glomerular cells preferentially expressed beta 3 and gamma 2 subunit messenger RNAs; the mitral cells preferentially expressed beta 2 subunit messenger RNA; inner granule cells expressed moderate levels of beta 2, beta 3 and gamma 2 subunit messenger RNAs. Expression of beta 2, beta 3 and gamma 2 messenger RNAs was also anatomically co-existent at P5. In addition, significant expression of beta 1 and delta subunit messenger RNAs was apparent in hippocampus and entorhinal cortex. The identity of the gamma 2 expressed between E15 and E21 was shown to be mostly the short isoform of gamma 2 subunit messenger RNA. Expression of both forms was evident beginning around P3-5. These results indicate that during the late embryonic and early postnatal period of development, beta 2, beta 3 and gamma 2 subunit messenger RNAs are abundantly expressed and co-localized to most central nervous system regions.(ABSTRACT TRUNCATED AT 400 WORDS)