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

Expression of α3β2β4 nicotinic acetylcholine receptors by rat adrenal chromaffin cells determined using novel conopeptide antagonists

Adrenal chromaffin cells release neurotransmitters in response to stress and may be involved in conditions such as post-traumatic stress and anxiety disorders. Neurotransmitter release is triggered, in part, by activation of nicotinic acetylcholine receptors (nAChRs). However, despite decades of use as a model system for studying exocytosis, the nAChR subtypes involved have not been pharmacologically identified. Quantitative real-time PCR of rat adrenal medulla revealed an abundance of mRNAs for α3, α7, β2, and β4 subunits. Whole-cell patch-clamp electrophysiology of chromaffin cells and subtype-selective ligands were used to probe for nAChRs derived from the mRNAs found in adrenal medulla. A novel conopeptide antagonist, PeIA-5469, was created that is highly selective for α3β2 over other nAChR subtypes heterologously expressed in Xenopus laevis oocytes. Experiments using PeIA-5469 and the α3β4-selective α-conotoxin TxID revealed that rat adrenal medulla contain two populations of chromaffin cells that express either α3β4 nAChRs alone or α3β4 together with the α3β2β4 subtype. Conclusions were derived from observations that acetylcholine-gated currents in some cells were sensitive to inhibition by PeIA-5469 and TxID, while in other cells, currents were sensitive only to TxID. Expression of functional α7 nAChRs was determined using three α7-selective ligands: the agonist PNU282987, the positive allosteric modulator PNU120596, and the antagonist α-conotoxin [V11L,V16D]ArIB. The results of these studies identify for the first time the expression of α3β2β4 nAChRs as well as functional α7 nAChRs by rat adrenal chromaffin cells.

Effects of prenatal cigarette smoke exposure on BDNF, PACAP, microglia and gliosis expression in the young male mouse brainstem

Cigarette smoke exposure during pregnancy into infancy affects brain growth and development in both short and long term (into adulthood). Using a mouse model of pre- into post- natal cigarette smoke exposure (SE), we aimed to determine the effects on brain derived neurotrophic factor (BDNF) and its receptor TrkB, neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) and its receptor PAC1, and astrocyte (GFAP) and microglia (Iba-1) immunohistochemical expression, in seven nuclei of the medulla and the facial (FAC) nucleus of the pons. Male pups of dams exposed to two cigarettes (nicotine <1.2 mg, CO <15 mg) twice daily for six weeks prior to mating, during gestation and lactation (n = 5; SE), were compared to pups exposed to air under the same condition (n = 5; SHAM) at postnatal day 20. Expression changes were only evident for BDNF, TrkB and PAC1 and included decreased BDNF in the hypoglossal (XII) nucleus and nucleus of the solitary tract (NTS), increased TrkB in XII but decreased TrkB in the FAC, and increased PAC1 in 4 nuclei of the medulla including the NTS. These results suggest that the effect of SE on the brainstem are region and marker selective, affecting regions of respiratory control (XII and NTS), and restricted to the BDNF system and PAC1, with no effect on activation states of astrocytes or microglia.

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

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

Are nicotinic acetylcholine receptors coupled to G proteins?

It was, until recently, accepted that the two classes of acetylcholine (ACh) receptors are distinct in an important sense: muscarinic ACh receptors signal via heterotrimeric GTP binding proteins (G proteins), whereas nicotinic ACh receptors (nAChRs) open to allow flux of Na+, Ca2+, and K+ ions into the cell after activation. Here we present evidence of direct coupling between G proteins and nAChRs in neurons. Based on proteomic, biophysical, and functional evidence, we hypothesize that binding to G proteins modulates the activity and signaling of nAChRs in cells. It is important to note that while this hypothesis is new for the nAChR, it is consistent with known interactions between G proteins and structurally related ligand-gated ion channels. Therefore, it underscores an evolutionarily conserved metabotropic mechanism of G protein signaling via nAChR channels.

Purine P2Y receptors in ATP-mediated regulation of non-quantal acetylcholine release from motor nerve endings of rat diaphragm

We established the effect of ATP, which is released together with acetylcholine (ACh), on the non-quantal ACh release (NQR) in rat diaphragm endplates and checked what kind of purine receptors are involved. NQR was estimated by the amplitude of endplate hyperpolarization (the H-effect) following the blockade of postsynaptic nicotinic receptors and cholinesterase. 100 μM ATP reduced the H-effect to 66% of the control. The action of ATP remained unchanged after the inhibition of ionotropic P2X receptors by Evans blue and PPADS, but disappeared after the application of the broad spectrum P2 receptor antagonist suramin, metabotropic P2Y receptor blocker reactive blue 2 and U73122, an inhibitor of phospholipase C. P2Y-mediated regulation is not coupled to presynaptic voltage-dependent Ca(2+) channels. During the simultaneous application of ATP and glutamate (which is another ACh cotransmitter reducing non-quantal release), the additive depressant effect led to a disappearance of the H-effect. This can be explained by the independence of the action of ATP and glutamate. Unlike the effects of purines on the spontaneous quantal secretion of ACh, its non-quantal release is regulated via P2Y receptors coupled to G(q/11) and PLC. ATP thus regulates the neuromuscular synapse by two different pathways.

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

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

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

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

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

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

Pituitary adenylyl cyclase-activating polypeptide (PACAP) and its receptor (PAC1-R) are positioned to modulate afferent signaling in the cochlea

Pituitary adenylyl cyclase-activating polypeptide (PACAP), via its specific receptor pituitary adenylyl cyclase-activating polypeptide receptor 1 (PAC1-R), is known to have roles in neuromodulation and neuroprotection associated with glutamatergic and cholinergic neurotransmission, which, respectively, are believed to form the primary basis for afferent and efferent signaling in the organ of Corti. Previously, we identified transcripts for PACAP preprotein and multiple splice variants of its receptor, PAC1-R, in microdissected cochlear subfractions. In the present work, neural localizations of PACAP and PAC1-R within the organ of Corti and spiral ganglion were examined, defining sites of PACAP action. Immunolocalization of PACAP and PAC1-R in the organ of Corti and spiral ganglion was compared with immunolocalization of choline acetyltransferase (ChAT) and synaptophysin as efferent neuronal markers, and glutamate receptor 2/3 (GluR2/3) and neurofilament 200 as afferent neuronal markers, for each of the three cochlear turns. Brightfield microscopy giving morphological detail for individual immunolocalizations was followed by immunofluorescence detection of co-localizations. PACAP was found to be co-localized with ChAT in nerve fibers of the intraganglionic spiral bundle and beneath the inner and outer hair cells within the organ of Corti. Further, evidence was obtained that PACAP is expressed in type I afferent axons leaving the spiral ganglion en route to the auditory nerve, potentially serving as a neuromodulator in axonal terminals. In contrast to the efferent localization of PACAP within the organ of Corti, PAC1-R immunoreactivity was co-localized with afferent dendritic neuronal marker GluR2/3 in nerve fibers passing beneath and lateral to the inner hair cell and in fibers at supranuclear and basal sites on outer hair cells. Given the known association of PACAP with catecholaminergic neurotransmission in sympathoadrenal function, we also re-examined the issue of whether the organ of Corti receives adrenergic innervation. We now demonstrate the existence of nerve fibers within the organ of Corti which are immunoreactive for the adrenergic marker dopamine beta-hydroxylase (DBH). DBH immunoreactivity was particularly prominent in nerve fibers both at the base and near the cuticular plate of outer hair cells of the apical turn, extending to the non-sensory Hensen's cell region. Evidence was obtained for limited co-localization of DBH with PAC1-R and PACAP. In the process of this investigation, we obtained evidence that efferent and afferent nerve fibers, in addition to adrenergic nerve fibers, are present at supranuclear sites on outer hair cells and distributed within the non-sensory epithelium of the apical cochlear turn for rat, based upon immunoreactivity for the corresponding neuronal markers. Overall, PACAP is hypothesized to act within the organ of Corti as an efferent neuromodulator of afferent signaling via PAC1-R that is present on type I afferent dendrites, in position to afford protection from excitotoxicity. Additionally, PACAP/PAC1-R may modulate secretion of catecholamines from adrenergic terminals within the organ of Corti.

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

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

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

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

Septal innervation regulates the function of α7 nicotinic receptors in CA1 hippocampal interneurons

The hippocampus receives substantial input from the medial septum/diagonal band of broca (MS/DB) via the fibria-fornix (FF). Projections from the MS/DB innervate hippocampal interneurons that express alpha7 nicotinic receptors and regulate excitation in principal cell populations. In the present report we used stereotaxic surgery, whole-cell patch clamping, and immunohistochemical techniques to evaluate the effects of FF and MS/DB lesions on alpha7 nicotinic receptors in stratum radiatum interneurons. Focal somatic application of ACh (1 mM) evoked methyllycaconitine (MLA)-sensitive currents that were markedly reduced following aspirative lesions of the FF. Reductions in current amplitudes were prevented or restored to levels not significantly different from controls following in vivo treatment with the alpha7-selective agonist GTS-21, and GTS-21 treatment did not change current amplitudes measured in tissue from unlesioned animals. MS/DB injections of the selective cholinergic neurotoxin 192 IgG-saporin did not affect alpha7 receptor currents, although MS/DB ChAT and hippocampal AChE immunolabeling were significantly reduced. In contrast, kainic acid lesions of the MS/DB, potentially more selective for GABAergic projection neurons, produced significant reductions in current amplitudes. These findings are the first to show functional changes in alpha7 receptors following hippocampal denervation and suggest that MS/DB hippocampal innervation regulates functional aspects of hippocampal alpha7 receptors. The results confirm hippocampal alpha7 nicotinic receptors as viable therapeutic targets in diseases that involve degradation of the septohippocampal pathway and may indicate that GABAergic MS/DB hippocampal input plays a more substantial role in the regulation of alpha7 nicotinic receptor function than MS/DB hippocampal cholinergic input.

Pituitary adenylate cyclase-activating polypeptide (PACAP) mimics neuroendocrine and behavioral manifestations of stress: Evidence for PKA-mediated expression of the corticotropin-releasing hormone (CRH) gene

The physiologic response to stress is highly dependent on the activation of corticotropin-releasing hormone (CRH) neurons by various neurotransmitters. A particularly rich innervation of hypophysiotropic CRH neurons has been detected by nerve fibers containing the neuropeptide PACAP, a potent activator of the cAMP-protein kinase A (PKA) system. Intracerebroventricular (icv) injections of PACAP also elevate steady-state CRH mRNA levels in the paraventricular nucleus (PVN), but it is not known whether PACAP effects can be associated with acute stress responses. Likewise, in cell culture studies, pharmacologic activation of the PKA system has stimulated CRH gene promoter activity through an identified cAMP response element (CRE); however, a direct link between PACAP and CRH promoter activity has not been established. In our present study, icv injection of 150 or 300 pmol PACAP resulted in robust phosphorylation of the transcription factor CREB in the majority of PVN CRH neurons at 15 to 30 min post-injection and induced nuclear Fos labeling at 90 min. Simultaneously, plasma corticosterone concentrations were elevated in PACAP-injected animals, and significant increases were observed in face washing, body grooming, rearing and wet-dog shakes behaviors. We investigated the effect of PACAP on human CRH promoter activity in alphaT3-1 cells, a PACAP-receptor expressing cell line. Cells were transiently transfected with a chloramphenicol acetyltransferase (CAT) reporter vector containing region - 663/+124 of the human CRH gene promoter then treated for with PACAP (100 nM) or with the adenylate cyclase activating agent, forskolin (2.5 muM). Both PACAP and forskolin significantly increased wild-type hCRH promoter activity relative to vehicle controls. The PACAP response was abolished in the CRE-mutant construct. Pretreatment of transfected cells with the PKA blocker, H-89, completely prevented both PACAP- and forskolin-induced increases in CRH promoter activity. Furthermore, CREB overexpression strongly enhanced PACAP-mediated stimulation of hCRH promoter activity, an effect which was also lost with mutation of the CRE. Thus, we demonstrate that icv PACAP administration to rats under non-stressed handling conditions leads to cellular, hormonal and behavioral responses recapitulating manifestations of the acute stress response. Both in vivo and in vitro data point to the importance of PACAP-mediated activation of the cAMP/PKA signaling pathway for stimulation of CRH gene transcription, likely via the CRE.

Multiple signal pathways coupling VIP and PACAP receptors to calcium channels in hamster submandibular ganglion neurons

The Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are two novel neuropeptides which produce particular biological effects caused by interaction with G-protein-coupled receptors. We have shown in a previous study where VIP and PACAP 38 inhibit voltage-dependent calcium channel (VDCC) currents (ICa) via G-proteins in hamster submandibular ganglion (SMG) neurons. In this study, we attempt to further characterize the signal transduction pathways of VIP-and PACAP 38-induced modulation of ICa. Application of 1 microM VIP and PACAP 38 inhibited ICa by 33.0 +/- 3.1% and 36.8 +/- 2.6%, respectively (mean +/- S.E.M., n = 8). Application of strong voltage prepulse attenuated PACAP 38-induced inhibition of ICa. Pretreatment of cAMP dependent protein kinase (PKA) activator attenuated VIP-induced inhibition, but not the PACAP 38-induced inhibition. Intracellular dialysis of the PKA inhibitor attenuated the VIP-induced inhibition, but not the PACAP 38-induced inhibition. Pretreatment of protein kinase C (PKC) activator and inhibitor attenuated VIP-induced inhibition, but not the PACAP 38-induced inhibition. Pretreatment of cholera toxin (CTX) attenuated PACAP 38-induced inhibition of ICa. These findings indicate that there are multiple signaling pathways in VIP and PACAP 38-induced inhibitions of ICa: one pathway would be the VPAC1/VPAC2 receptors-induced inhibition involving both the PKA and PKC, and another one concerns the PAC1 receptor-induced inhibition via Gs-protein betagamma subunits. The VIP-and PACAP 38-induced facilitation of ICa can be observed in the SMG neurons in addition to inhibiting of ICa.

Two distinct calcium-sensitive and -insensitive PKC up- and down-regulate an alpha-bungarotoxin-resistant nAChR1 in insect neurosecretory cells (DUM neurons)

While there is mounting knowledge about the structure and diversity of insect neuronal nicotinic acetylcholine receptors, less attention has been directed towards their intracellular regulation by calcium-mediated activation or inhibition of protein phosphorylation. The main goal of this work was to delineate the chain of molecular events that lead to the up- and down-regulation by two protein kinase Cs of an insect neuronal alpha-bungarotoxin-resistant nicotinic acetylcholine receptor (called nAChR1). The native nicotinic acetylcholine receptor intracellular regulation was studied on dissociated adult dorsal unpaired median neurons isolated from the terminal abdominal ganglion of the cockroach Periplaneta americana using whole-cell patch-clamp technique and calcium imaging. We report that under 0.5 micro malpha-bungarotoxin treatment, the inward current produced by pressure ejection application of nicotine onto the cell body was differentially sensitive to specific protein kinase C activators and inhibitors. The phorbol ester PMA produced a calcium-dependent increase in current amplitude blocked by chelerythrine. By contrast, the diacylglycerol analogue 1,2-dioctanoyl-sn-glycerol produced a calcium-independent reduction of the nicotinic response, reversed by rottlerin and chelerythrine. This indicated that two protein kinase C isozymes ('classical' and 'novel' protein kinase C, named PKC1 and PKC2, respectively) up- and down-regulated nicotinic acetylcholine receptor function. PMA and 1,2-dioctanoyl-sn-glycerol effects were mimicked by pirenzepine-sensitive M1 muscarinic receptor subtype coupled to phospholipase C second messenger pathway. Low concentration of muscarine elevated internal calcium levels, which thereby activated PKC1. By contrast, a high concentration of muscarine strongly increased [Ca 2+]i, which induced inhibition of PKC1. This effect was reversed by FK506, suggesting the implication of PP2B which unmasked PKC2 activity mediating down-regulation of nicotinic acetylcholine receptor.

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.

Differential modulation of nicotinic acetylcholine receptor subtypes and synaptic transmission in chick sympathetic ganglia by PGE(2)

The diversity of neuronal nicotinic acetylcholine receptors (nAChRs) is likely an important factor in the modulation of synaptic transmission by acetylcholine and nicotine. We have tested whether postsynaptic nAChRs are modulated in a subtype-specific manner by prostaglandin E(2) (PGE(2)), a regulator of neuronal excitability in both the central and peripheral nervous systems, and examined the effects of PGE(2) on nicotinic transmission. Somatodendritic nAChRs in chick lumbar sympathetic ganglia include four nAChR subtypes distinguished on the basis of conductance and kinetic profile. Nanomolar PGE(2) applied to the extrapatch membrane differentially regulates opening probability (Po), frequency and the opening duration of each nAChR channel subtype in cell-attached patches. PGE(2) decreases the Po of the predominant nAChR subtype (36 pS) and significantly increases Po and open duration of the 23 pS subtype. The 23 pS subtype is gated by the alpha 7-selective agonist choline, and choline-gated currents are inhibited by alpha-bungarotoxin. To examine whether PGE(2) modulates nAChRs at synaptic sites, we studied the effects of PGE(2) on amplitude and decay of synaptic currents in visceral motoneuron-sympathetic neuron co-cultures. PGE(2) significantly decreases the amplitude of miniature excitatory postsynaptic currents (mEPSCs), consistent with the predominant inhibition by PGE(2) of all but the 23 pS subtype. The time constant of mEPSCs at PGE(2)-treated synapses is prolonged, which is also consistent with an increased contribution of the longer open duration of the 23 pS nAChR subtype with PGE(2) treatment. To examine the presynaptic effect of PGE(2), nanomolar nicotine was used. Nicotine induces facilitation of synaptic transmission by increasing mEPSC frequency, an action thought to involve presynaptic, alpha 7-containing nAChRs. In the presence of PGE(2), nicotine-induced synaptic facilitation persists. Thus the net effect of PGE(2) is to alter the profile of nAChRs contributing to synaptic transmission from larger conductance, briefer opening channels to smaller conductance, longer opening events. This subtype-specific modulation of nAChRs by PGE(2) may provide a mechanism for selective activation and suppression of synaptic pathways mediated by different nAChR subtype(s) at both pre- and postsynaptic sites.

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.

VIP and PACAP potentiation of nicotinic ACh-evoked currents in rat parasympathetic neurons is mediated by G-protein activation

The effects of vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on isolated parasympathetic neurons of rat intracardiac and submandibular ganglia were examined under voltage clamp using whole-cell patch-clamp recording techniques. VIP and PACAP (</= 10 nM) selectively and reversibly increased the affinity of nicotinic acetylcholine receptor channels (nAChRs) for their agonists resulting in a potentiation of acetylcholine (ACh)-evoked whole-cell currents at low agonist concentrations. VIP-induced potentiation was observed with either ACh or nicotine as the cholinergic agonist. The VIP- but not the PACAP-induced potentiation of ACh-evoked currents was inhibited by [Ac-Tyr1, D-Phe2]-GRF 1-29, amide (100 nM), a selective antagonist of VPAC1 and VPAC2 receptors; whereas the PACAP38- but not the VIP-induced potentiation was inhibited by 100 nM PACAP6-38, a PAC1 and VPAC2 receptor antagonist. The signal transduction pathway mediating VIP- and PACAP-induced potentiation of nicotinic ACh-evoked currents involves a pertussis toxin (PTX)-sensitive G-protein. Intracellular application of 200 microM GTPgammaS or GDPbetaS inhibited VIP-induced potentiation of ACh-evoked whole-cell currents. GTPgammaS alone potentiated ACh- and nicotine-evoked currents and the magnitude of these currents was not further increased by VIP or PACAP. The G-protein subtype modulating the neuronal nAChRs was examined by intracellular dialysis with antibodies directed against alphao, alphai-1,2, alphai-3 or beta G-protein subunits. Only the anti-Galphao and anti-Gbeta antibodies significantly inhibited the effect of VIP and PACAP on ACh-evoked currents. The potentiation of ACh-evoked currents by VIP and PACAP may be mediated by a membrane-delimited signal transduction cascade involving the PTX-sensitive Go protein.

Effects of nitric oxide donors on the afferent resting activity in the cephalopod statocyst

The effects of bath applications of the nitric oxide (NO) donors sodium nitroprusside (SNP), diethylamine sodium (DEA), 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-penicillamine (SNAP) on the resting activity (RA) of afferent crista fibers were studied in isolated statocysts of the cuttlefish Sepia officinalis. The NO donors had three different effects: inhibition, excitation, and excitation followed by an inhibition. The SNAP analog N-acetyl-DL-penicillamine (xSNAP; with no NO moiety) had no effect. When the preparation was pre-treated with the NO synthase inhibitor N(G)-nitric-L-arginine methyl ester HCl (L-NAME), the NO donors were still effective. When the preparation was pre-treated with the guanylate cyclase inhibitors methylene blue (M-BLU) or cystamine (CYS), NO donors had only excitatory effects, whereas their effects were inhibitory only when pre-treatment was with the adenylate cyclase inhibitors nicotinic acid (NIC-A), 2',3'-dideoxyadenosine (DDA), or MDL-12330A. When pre-treatment was with a guanylate and an adenylate cyclase inhibitor combined, NO donors had no effect; in that situation, the RA of the afferent fibers remained and the preparation still responded to bath applications of GABA. Selective experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus vulgaris gave comparable results. These data indicate that in cephalopod statocysts an inhibitory NO-cGMP and an excitatory NO-cAMP signal transduction pathway exist, that these two pathways are the key pathways for the action of NO, and that they have only modulatory effects on, and are not essential for the generation of, the RA.

Variations in desensitization of nicotinic acetylcholine receptors from hippocampus and midbrain dopamine areas

This study addresses two issues arising from the desensitization of nicotinic acetylcholine receptors from the hippocampus, ventral tegmental area, and substantia nigra. First, biophysical studies can find potent and complete desensitization of nicotinic receptors; but in vivo studies often find that desensitization affecting a behavior is less than complete, or that desensitization is important over a different nicotine concentration range. Our results show that there can be significant differences in desensitization when comparing nearby neurons from the same area of the brain. Thus, nicotinic receptors on a minority of neurons may remain active and maintain a behavior under conditions that can produce significant desensitization. Second, agonist applications that are intended to active nicotinic receptors also cause desensitization. The prevailing conditions and the rate of agonist application and removal will control the degree of activation vs. desensitization. These and other factors regulate the efficacy of nicotinic agonists experimentally and physiologically.

Modulation of stimulus-secretion coupling in porcine adrenal chromaffin cells by receptor-mediated increases in protein kinase C activity

Catecholamine (CAT) secretion by adrenal chromaffin cells is primarily triggered by nicotinic receptor-dependent increases in cytosolic Ca(2+). The principal aim of the present study was to determine whether pituitary adenylate cyclase activating peptide (PACAP), which is coreleased with acetylcholine from the splanchnic nerve, can modulate nicotinic receptor-dependent Ca(2+) signaling and catecholamine secretion in porcine adrenal medullary chromaffin (PAMC) cells. Activation of protein kinase C (PKC) with phorbol myristate acetate (PMA) dose- and time-dependently inhibited nicotine (NIC)-induced Ca(2+) transients. At 100 nM PMA, peak Ca(2+) levels were reduced by 27% +/- 2% (P < 0.05) and 41% +/- 3% (P < 0. 05) after 10 and 20 min exposure, respectively. The inhibitory effects of PMA were significantly reduced by preincubation with the PKC inhibitor staurosporine. KCl-induced Ca(2+) transients were also reduced by 20 min PMA treatment (Delta -27% +/- 4%; P < 0.05), suggesting that PKC affects voltage-gated Ca(2+) channel activity. Pretreatment with PACAP also resulted in both time- and concentration-dependent suppression of Ca(2+) transients. After 20 min exposure to 1 microM PACAP, NIC- and KCl-induced transients were reduced by 36% +/- 5% (P < 0.05) and 51% +/- 6% (P < 0.05), respectively. These effects could also be prevented by staurosporine pretreatment. NIC-induced CAT secretion was significantly reduced by pretreatment with both PMA (Delta -56% +/- 2%; P < 0.05) and PACAP (Delta-53% +/- 7%; P < 0.05). This suppressive effect on secretion could be prevented by pretreatment with staurosporine. These data suggest that, in addition to having direct stimulatory effects on catecholamine synthesis and secretion, PACAP can also negatively modulate nicotinic receptor-dependent Ca(2+) signaling and secretion in PAMC cells.

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.