Cyclic AMP regulates the proportion of functional acetylcholine receptors on chicken ciliary ganglion neurons

Regulation of nicotinic acetylcholine receptors by post-translational modifications

Nicotinic acetylcholine receptors (nAChRs) comprise a family of pentameric ligand-gated ion channels widely distributed in the central and peripheric nervous system and in non-neuronal cells. nAChRs are involved in chemical synapses and are key actors in vital physiological processes throughout the animal kingdom. They mediate skeletal muscle contraction, autonomic responses, contribute to cognitive processes, and regulate behaviors. Dysregulation of nAChRs is associated with neurological, neurodegenerative, inflammatory and motor disorders. In spite of the great advances in the elucidation of nAChR structure and function, our knowledge about the impact of post-translational modifications (PTMs) on nAChR functional activity and cholinergic signaling has lagged behind. PTMs occur at different steps of protein life cycle, modulating in time and space protein folding, localization, function, and protein-protein interactions, and allow fine-tuned responses to changes in the environment. A large body of evidence demonstrates that PTMs regulate all levels of nAChR life cycle, with key roles in receptor expression, membrane stability and function. However, our knowledge is still limited, restricted to a few PTMs, and many important aspects remain largely unknown. There is thus a long way to go to decipher the association of aberrant PTMs with disorders of cholinergic signaling and to target PTM regulation for novel therapeutic interventions. In this review we provide a comprehensive overview of what is known about how different PTMs regulate nAChR.

cAMP-dependent protein kinase inhibits α7 nicotinic receptor activity in layer 1 cortical interneurons through activation of D1/D5 dopamine receptors

KEY POINTS

Protein kinases can modify the function of many proteins including ion channels. However, the role of protein kinase A in modifying nicotinic receptors in the CNS has never been investigated. We showed through whole-cell recordings of layer 1 prefrontal cortical interneurons that α7 nicotinic responses are negatively modulated by protein kinase A. Furthermore, we show that stimulation of dopamine receptors can similarly attenuate α7 nicotinic responses through the activation of protein kinase A. These results suggest how the interaction of the cholinergic and dopaminergic systems may influence neuronal excitability in the brain.

ABSTRACT

Phosphorylation of ion channels, including nicotinic acetylcholine receptors (nAChRs), by protein kinases plays a key role in the modification of synaptic transmission and neuronal excitability. α7 nAChRs are the second most prevalent nAChR subtype in the CNS following α4β2. Serine 365 in the M3-M4 cytoplasmic loop of the α7 nAChR is a phosphorylation site for protein kinase A (PKA). D1/D5 dopamine receptors signal through the adenylate cyclase-PKA pathway and play a key role in working memory and attention in the prefrontal cortex. Thus, we examined whether the dopaminergic system, mediated through PKA, functionally interacts with the α7-dependent cholinergic neurotransmission. In layer 1 interneurons of mouse prefrontal cortex, α7 nicotinic currents were decreased upon stimulation with 8-Br-cAMP, a PKA activator. In HEK 293T cells, dominant negative PKA abolished 8-Br-cAMP's effect of diminishing α7 nicotinic currents, while a constitutively active PKA catalytic subunit decreased α7 currents. In brain slices, the PKA inhibitor KT-5720 nullified 8-Br-cAMP's effect of attenuating α7 nicotinic responses, while applying a PKA catalytic subunit in the pipette solution decreased α7 currents. 8-Br-cAMP stimulation reduced surface expression of α7 nAChRs, but there was no change in single-channel conductance. The D1/D5 dopamine receptor agonist SKF 83822 similarly attenuated α7 nicotinic currents from layer 1 interneurons and this attenuation of nicotinic current was prevented by KT-5720. These results demonstrate that dopamine receptor-mediated activation of PKA negatively modulates nicotinic neurotransmission in prefrontal cortical interneurons, which may be a contributing mechanism of dopamine modulation of cognitive behaviours such as attention or working memory.

The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

The purpose of this review is to summarize the present knowledge on the interplay among the cholinergic system, Extracellular signal-Regulated Kinase (ERK) and Mammalian Target of Rapamycin (mTOR) pathways in the development of short and long term memories during the acquisition and recall of the step-down inhibitory avoidance in the hippocampus. The step-down inhibitory avoidance is a form of associative learning that is acquired in a relatively simple one-trial test through several sensorial inputs. Inhibitory avoidance depends on the integrated activity of hippocampal CA1 and other brain areas. Recall can be performed at different times after acquisition, thus allowing for the study of both short and long term memory. Among the many neurotransmitter systems involved, the cholinergic neurons that originate in the basal forebrain and project to the hippocampus are of crucial importance in inhibitory avoidance processes. Acetylcholine released from cholinergic fibers during acquisition and/or recall of behavioural tasks activates muscarinic and nicotinic acetylcholine receptors and brings about a long-lasting potentiation of the postsynaptic membrane followed by downstream activation of intracellular pathway (ERK, among others) that create conditions favourable for neuronal plasticity. ERK appears to be salient not only in long term memory, but also in the molecular mechanisms underlying short term memory formation in the hippocampus. Since ERK can function as a biochemical coincidence detector in response to extracellular signals in neurons, the activation of ERK-dependent downstream effectors is determined, in part, by the duration of ERK phosphorylation itself. Long term memories require protein synthesis, that in the synapto-dendritic compartment represents a direct mechanism that can produce rapid changes in protein content in response to synaptic activity. mTOR in the brain regulates protein translation in response to neuronal activity, thereby modulating synaptic plasticity and long term memory formation. Some studies demonstrate a complex interplay among the cholinergic system, ERK and mTOR. It has been shown that co-activation of muscarinic acetylcholine receptors and β-adrenergic receptors facilitates the conversion of short term to long term synaptic plasticity through an ERK- and mTOR-dependent mechanism which requires translation initiation. It seems therefore that the complex interplay among the cholinergic system, ERK and mTOR is crucial in the development of new inhibitory avoidance memories in the hippocampus.

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.

Cholinergic modulation of hippocampal network function

Cholinergic septohippocampal projections from the medial septal area to the hippocampus are proposed to have important roles in cognition by modulating properties of the hippocampal network. However, the precise spatial and temporal profile of acetylcholine release in the hippocampus remains unclear making it difficult to define specific roles for cholinergic transmission in hippocampal dependent behaviors. This is partly due to a lack of tools enabling specific intervention in, and recording of, cholinergic transmission. Here, we review the organization of septohippocampal cholinergic projections and hippocampal acetylcholine receptors as well as the role of cholinergic transmission in modulating cellular excitability, synaptic plasticity, and rhythmic network oscillations. We point to a number of open questions that remain unanswered and discuss the potential for recently developed techniques to provide a radical reappraisal of the function of cholinergic inputs to the hippocampus.

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.

Cellular distribution of the nicotinic acetylcholine receptor alpha7 subunit in rat hippocampus

The hippocampus is a region of the mammalian brain that has been extensively studied due to its role in many forms of memory. To better understand hippocampal function, significant attention has focused upon the cellular distribution of ligand-gated ion channels. Despite strong cholinergic innervation from the basal forebrain and a dense expression of nicotinic acetylchoine receptors (nAChRs), the cellular distribution of subunits forming these receptors has received little attention. We used organotypic hippocampal slice cultures (OHSCs) to study native alpha7 subunits, which, unlike other nAChR subunits, form a homomeric receptor. Cell-surface biotinylation, cross-linking of surface proteins, and sub-cellular fractionation all revealed a very limited presence of the subunit at the plasma membrane. In contrast, subunits of other receptors displayed significant surface expression. Notably, subunits in adult hippocampal tissue were distributed in a fashion similar to that observed in OHSCs. To monitor alpha7 subunits contained in functional nAChRs, a colourimetric assay using alpha-bungarotoxin (a specific alpha7 nAChR antagonist) was developed, and revealed a majority of binding at the cell surface. To change alpha7 subunit distribution, OHSCs were treated with compounds known to affect other ionotropic receptors-insulin, genistein, and elevated external K(+); however, neither subunit surface expression nor antagonist binding was affected. Our data reveal that hippocampal neurons possess a large internal population of alpha7 subunits under basal conditions, which persists during stimuli affecting tyrosine phosphorylation or neuronal activity. The nature of the internal pool of alpha7 subunits remains to be determined, but should have important implications for hippocampal activity.

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

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

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.

Up-regulation of nicotinic acetylcholine receptors by central-type acetylcholinesterase inhibitors in rat cortical neurons

We previously reported that donepezil, a central-type acetylcholinesterase inhibitor, showed neuroprotective action via alpha4-and alpha7-nicotinic acetylcholine receptors against glutamate neurotoxicity in rat cortical culture. The present study was performed to investigate whether the neuroprotective action of acetylcholinesterase inhibitors is accompanied by the alteration of expression and function of nicotinic receptors. Four days treatment with acetylcholinesterase inhibitors (10 microM) enhanced the nicotine-induced increase of the intracellular calcium concentration. Immunoblot analysis revealed that donepezil increased both alpha4 and alpha7 subunit proteins. Donepezil and galanthamine increased the number of cells expressing alpha4- and alpha7-nicotinic receptors in immunocytochemical analysis. We examined whether up-regulation of nicotinic receptors affected the neuroprotective action of acetylcholinesterase inhibitors. Under up-regulating conditions, donepezil and galanthamine exerted neuroprotective action at lower concentrations. These results suggest that donepezil and galanthamine up-regulate nicotinic receptors in cortical neurons, and that the up-regulation of nicotinic receptors may make cortical neurons more sensitive to the neuroprotective action of donepezil and galanthamine.

Indirect phosphorylation-dependent modulation of postsynaptic nicotinic acetylcholine responses by 5-hydroxytryptamine

Ionotropic nicotinic acetylcholine (ACh) receptors have been shown to be modulated by protein kinase-mediated phosphorylation in vitro. Here we demonstrate that 5-hydroxytryptamine (5-HT) can downregulate postsynaptic nicotinic ACh responses, elicited in an identified arthropod motoneuron in situ, by a mechanism dependent on protein kinase activity. Serotonergic modulation can be mimicked by perfusion with membrane-permeable analogues of either adenine (cAMP) or guanine (cGMP) cyclic nucleotides, and is prolonged in the presence of phosphodiesterase inhibitors. Furthermore, suppression of the ACh response by 5-HT is blocked by specific competitive inhibitors of protein kinase A and G, as well as the broad specificity protein kinase inhibitor staurosporine. The protein phosphatase inhibitor cantharidin similarly blocks recovery of the ACh response from suppression mediated by 5-HT. Thus, it appears that the nicotinic ACh response is modulated by a cAMP-mediated phosphorylation-dependent intracellular signalling pathway that is distinct from the direct block of mammalian nicotinic ACh receptors by 5-HT previously reported in vitro.

Rapid upregulation of alpha7 nicotinic acetylcholine receptors by tyrosine dephosphorylation

Alpha7 nicotinic acetylcholine receptors (nAChRs) modulate network activity in the CNS. Thus, functional regulation of alpha7 nAChRs could influence the flow of information through various brain nuclei. It is hypothesized here that these receptors are amenable to modulation by tyrosine phosphorylation. In both Xenopus oocytes and rat hippocampal interneurons, brief exposure to a broad-spectrum protein tyrosine kinase inhibitor, genistein, specifically and reversibly potentiated alpha7 nAChR-mediated responses, whereas a protein tyrosine phosphatase inhibitor, pervanadate, caused depression. Potentiation was associated with an increased expression of surface alpha7 subunits and was not accompanied by detectable changes in receptor open probability, implying that the increased function results from an increased number of alpha7 nAChRs. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor-mediated exocytosis was shown to be a plausible mechanism for the rapid delivery of additional alpha7 nAChRs to the plasma membrane. Direct phosphorylation/dephosphorylation of alpha7 subunits was unlikely because mutation of all three cytoplasmic tyrosine residues did not prevent the genistein-mediated facilitation. Overall, these data are consistent with the hypothesis that the number of functional cell surface alpha7 nAChRs is controlled indirectly via processes involving tyrosine phosphorylation.

Sensitivity of neuronal nicotinic acetylcholine receptors to the opiate antagonists naltrexone and naloxone: receptor blockade and up-regulation

In HEK293 cells stably expressing alpha4beta2 nAChRs, naltrexone, but not naloxone, blocked alpha4beta2 nAChRs via an open-channel blocking mechanism. In primary hippocampal cultures, naltrexone inhibited alpha7 nAChRs up-regulated by nicotine, and in organotypic hippocampal cultures naltrexone caused a time-dependent up-regulation of functional alpha7 nAChRs that was detected after removal of the drug. These results indicate that naltrexone could be used as a smoking cessation aid.

Functional nicotinic acetylcholine receptors on subplate neurons in neonatal rat somatosensory cortex

The establishment of cortical synaptic circuits during early development requires the presence of subplate neurons (SPn's), a heterogeneous population of neurons capable to integrate and process synaptic information from the thalamus, cortical plate, and neighboring SPn's. An accumulation of cholinergic afferents and nicotinic acetylcholine receptors (nAChRs) has been documentated in the subplate around birth. To assess the developmental role of the cholinergic innervation onto SPn's, we used whole cell patch-clamp recordings of visually identified and biocytin-labeled SPn's in neonatal rat somatosensory cortex. Functional nAChRs were present in 92% of the investigated SPn's. Activation of postsynaptic nAChRs by local application of agonists elicited a brief membrane depolarization associated with a barrage of action potentials and large inward currents reversing around 0 mV. According to our pharmacological data, excitation of SPn's is mediated by alpha4beta2 receptors. In contrast, functional alpha7 nAChRs could not be identified on SPn's. Activation of nAChRs affected neither the spontaneous synaptic activity of SPn's nor the synaptic connections between thalamus and SPn's and within subplate. Nicotine, at concentrations reaching the developing brain by maternal smoking, induced a severe desensitization of nAChRs and an increase in the baseline noise. These results indicate that nAChR-mediated excitation of SPn's may stabilize the developing synaptic circuits and suggest the involvement of nAChRs located on SPn's in the fetal tobacco syndrome.

Alternate stoichiometries of alpha4beta2 nicotinic acetylcholine receptors

Two functional types of nicotinic acetylcholine receptors (nAChRs) are expressed when human embryonic kidney cells are permanently transfected with equal amounts of human alpha4 and beta2 subunit cDNAs. Most (82%) of these nAChRs exhibit an EC(50) of 74 +/- 6 microM for ACh, a much lower sensitivity than the remaining fraction (EC(50) of 0.7 +/- 0.4 microM) or than expected from expression of equal amounts of alpha4 and beta2 mRNAs in Xenopus laevis oocytes. We have found three conditions that can increase the number of nAChRs with high sensitivity to activation. These are: 1) transient transfection with additional beta2 subunits, 2) overnight incubation in nicotine, or 3) overnight culture at 29 degrees C. Using metabolic labeling with [(35)S]methionine to measure subunit stoichiometry, we found that the majority of nAChRs had a stoichiometry of (alpha4)(3)(beta2)(2). Overnight treatment with nicotine increased the number of nAChRs and increased the proportion of the (alpha4)(2)(beta2)(3) stoichiometry. Alternate alpha4beta2 nAChR stoichiometries with distinct functional properties raise the possibility for an interesting mode of synaptic regulation for nicotinic signaling in the mammalian brain.

Desensitization of neuronal nicotinic receptors

The loss of functional response upon continuous or repeated exposure to agonist, desensitization, is an intriguing phenomenon if not as yet a well-defined physiological mechanism. However, detailed evaluation of the properties of desensitization, especially for the superfamily of ligand-gated ion channels, reveals how the nervous system could make important use of this process that goes far beyond simply curtailing excessive receptor stimulation and the prevention of excitotoxicity. Here we will review the mechanistic basis of desensitization and discuss how the subunit-dependent properties and regulation of nicotinic acetylcholine receptor (nAChR) desensitization contribute to the functional diversity of these channels. These studies provide the essential framework for understanding how the physiological regulation of desensitization could be a major determinant of synaptic efficacy by controlling, in both the short and long term, the number of functional receptors. This type of mechanism can be extended to explain how the continuous occupation of desensitized receptors during chronic nicotine exposure contributes to drug addiction, and highlights the potential significance of prolonged nAChR desensitization that would also occur as a result of extended acetylcholine lifetime during treatment of Alzheimer's disease with cholinesterase inhibitors. Thus, a clearer picture of the importance of nAChR desensitization in both normal information processing and in various diseased states is beginning to emerge.

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.

Prolonged stimulation of presynaptic nicotinic acetylcholine receptors in the rat interpeduncular nucleus has differential effects on transmitter release

Alterations in nicotinic acetylcholine (nAChR) receptor number can be induced by chronic exposure to nicotine possibly by stabilization of the desensitized state(s) of the receptor. Since within the central nervous system (CNS), many nAChRs are localized presynaptically, we have investigated the physiological consequences of prolonged nicotine applications on spontaneous transmitter release. In the presence of glutamate receptor antagonists, bicuculline-sensitive spontaneous GABA inhibitory synaptic currents (IPSCs) could be readily resolved in whole-cell recordings from neurons in the interpeduncular nucleus (IPN) maintained as brain slices. Nicotine (300nM) caused a marked enhancement in the frequency of spontaneous events. During a 15min exposure to nicotine, the time course of changes in IPSC frequency could be divided into two groups. In most neurons, there was a fast increase in event frequency followed by a decline to a lower steady-state level that remained above baseline. In the remaining neurons, the effect of nicotine was more slowly developing and outlasted the application. Interestingly, the rapid effect was associated with a shift to higher amplitude events, whereas, no change in the IPSC amplitude histogram was observed during the slow onset effect. These data show that prolonged stimulation of presynaptic nicotinic receptors can have different outcomes that could potentially contribute to the diverse effects of nicotine on central information processing.

Age-related changes of dopamine D1-like and D2-like receptor binding in the F344/N rat striatum revealed by positron emission tomography and in vitro receptor autoradiography

To clarify age-related changes in dopamine D1-like and D2-like receptor binding in the striatum, positron emission tomography (PET) and in vitro receptor autoradiography (in vitro ARG) were performed using F344/N rats of various ages (6, 12, 18, and 24 months). In the PET study, [11C]SCH23390 and [11C]raclopride were used to image dopamine D1-like receptors and dopamine D2-like receptors, respectively, while [3H]SCH23390 and [3H]raclopride were used for the in vitro ARG study. With PET, we calculated the binding potential (= k3/k4, Bmax/Kd) of [11C]SCH23390 and [11C]raclopride in the striatum according to the curve fitting (CF) and the Logan plot (LP) methods. The binding potential of [11C]SCH23390 in the striatum demonstrated significant decrease as a function of age (max. decrease -26%) by the LP method, while this was not observed in the data analyzed by the CF method. In contrast, the binding potential of [11C]raclopride in the striatum decreased significantly with age by both the CF (max. decrease -28%) and the LP (max. decrease -36%) methods. However, no significant difference by either method was observed in rats between 6 and 12 months old using either ligand. In the in vitro ARG study, the specific binding (fmol/mg tissue) of [3H]SCH23390 and [3H]raclopride in the striatum were determined. Both [3H]SCH23390 and [3H]raclopride binding declined considerably with age as noted by comparing 12, 18, and 24-month-old rats against those 6 months old (max. decrease -29% and -31%, respectively). The substantial difference in binding shown in 12-month-olds in comparison with 6-month-olds using either ligand with in vitro ARG was in contrast with the PET results. These distinctions between the PET and the in vitro ARG studies may be attributed to the receptor microenvironment created under these experimental conditions. The results indicate that PET with LP analysis is useful in obtaining age-related changes of D1-like and D2-like receptor binding in the striatum of living rats.

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.

Regulation of the sensitivity of acetylcholine receptors to nicotine in rat habenula neurons

Time-dependent changes in nicotinic acetylcholine receptor (nAChR) function were studied in acutely isolated medial habenula neurons during whole-cell perfusion. The peak amplitude of inward currents induced by 1 s pulses of nicotinic agonists, applied at 30 s intervals, gradually increased over the first several minutes of whole-cell recording. The ratio of response amplitudes at 1 and 15 min (t15/t1) was 1.9. Run-up of responses occurred independently of channel activation and was specific to nAChRs. The channel blocker chlorisondamine (30 microM), co-applied with nicotine, was used to irreversibly block the majority (91 %) of the nAChRs that opened in the first 2 min of recording. Run-up in the remaining 9 % unblocked channels assessed at 15 min (t15/t2 = 3.4) was similar to that in control cells not exposed to nicotine and chlorisondamine simultaneously, implying that run-up is not due to the incorporation of new receptors. A marked alteration in the sensitivity of nAChRs to extracellular Ca2+ was also observed during whole-cell perfusion. The ratio of current amplitudes obtained in 0.2 and 4.0 mM Ca2+ changed from 0.54 (t = 5 min) to 0.82 (t = 30 min). Inward rectification of nicotine-induced responses was reduced during internal dialysis. Voltages for half-maximal conductance were -23.0 and -13.8 mV at 2 and 15 min, respectively. Inclusion of either free Mg2+ ( approximately 2 mM) or spermine (100 microM) in the internal solution counteracted the change in rectification, but did not prevent run-up. The period of run-up was followed by a use-dependent run-down phase. Little run-down in peak current amplitude was induced provided that agonist was applied infrequently (5 min intervals), whereas applications at 30 s intervals produced a loss of channel function after approximately 15 min whole-cell perfusion. The time at which run-down began ( approximately 5-30 min) was correlated with the initial rate of nAChR desensitization ( approximately 200-4000 ms); slowly desensitizing nicotinic currents demonstrated delayed run-down. We suggest that run-up of nAChR-mediated responses does not require receptor activation and may result from a change in channel open probability. We also hypothesize that channel run-down reflects accumulation of nAChRs in long-lived desensitized/inactivated states.

Opening and closing of KCNKO potassium leak channels is tightly regulated

Potassium-selective leak channels control neuromuscular function through effects on membrane excitability. Nonetheless, their existence as independent molecular entities was established only recently with the cloning of KCNKO from Drosophila melanogaster. Here, the operating mechanism of these 2 P domain leak channels is delineated. Single KCNKO channels switch between two long-lived states (one open and one closed) in a tenaciously regulated fashion. Activation can increase the open probability to approximately 1, and inhibition can reduce it to approximately 0.05. Gating is dictated by a 700-residue carboxy-terminal tail that controls the closed state dwell time but does not form a channel gate; its deletion (to produce a 300-residue subunit with two P domains and four transmembrane segments) yields unregulated leak channels that enter, but do not maintain, the closed state. The tail integrates simultaneous input from multiple regulatory pathways acting via protein kinases C, A, and G.

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.

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.

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.

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.

alpha-bungarotoxin receptors contain alpha7 subunits in two different disulfide-bonded conformations

Neuronal nicotinic alpha7 subunits assemble into cell-surface complexes that neither function nor bind alpha-bungarotoxin when expressed in tsA201 cells. Functional alpha-bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the alpha7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric alpha7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, alpha7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second alpha7/serotonin-3 subunit conformation coincides with the formation of alpha-bungarotoxin-binding sites and intrasubunit disulfide bonding, apparently within the alpha7 domain of the alpha7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional alpha7 receptors, alpha7 subunits undergo a conformational change similar to alpha7/serotonin-3 subunits. alpha7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, alpha-bungarotoxin-binding receptors with two alpha7 conformations. Our results suggest that alpha7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.

Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation.

Upregulation of surface alpha4beta2 nicotinic receptors is initiated by receptor desensitization after chronic exposure to nicotine

It is hypothesized that desensitization of neuronal nicotinic acetylcholine receptors (nAChRs) induced by chronic exposure to nicotine initiates upregulation of nAChR number. To test this hypothesis directly, oocytes expressing alpha4beta2 receptors were chronically incubated (24-48 hr) in nicotine, and the resulting changes in specific [3H]nicotine binding to surface receptors on intact oocytes were compared with functional receptor desensitization. Four lines of evidence strongly support the hypothesis. (1) The half-maximal nicotine concentration necessary to produce desensitization (9.7 nM) was the same as that needed to induce upregulation (9.9 nM). (2) The concentration of [3H]nicotine for half-maximal binding to surface nAChRs on intact oocytes was also similar (11.1 nM), as predicted from cyclical desensitization models. (3) Functional desensitization of alpha3beta4 receptors required 10-fold higher nicotine concentrations, and this was mirrored by a 10-fold shift in concentrations necessary for upregulation. (4) Mutant alpha4beta2 receptors that do not recover fully from desensitization, but not wild-type channels, were upregulated after acute (1 hr) applications of nicotine. Interestingly, the nicotine concentration required for half-maximal binding of alpha4beta2 receptors in total cell membrane homogenates was 20-fold lower than that measured for surface nAChRs in intact oocytes. These data suggest that cell homogenate binding assays may not accurately reflect the in vivo desensitization affinity of surface nAChRs and may account for some of the previously reported differences in the efficacy of nicotine for inducing nAChR desensitization and upregulation.

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.

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

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

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

Long-term desensitization of nicotinic acetylcholine receptors is regulated via protein kinase A-mediated phosphorylation

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electroplax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent (PKA) receptor phosphorylation. To examine the effects of phosphorylation on desensitization of muscle-type ACh receptors, we expressed the frog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the rate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to reduce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors from long-term desensitization. Inactivation of the endogenous PKA activity by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific subunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring several seconds for full recovery and a second state from which recovery requires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitization, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly described effect of phosphorylation on ACh receptor function may serve as an important modulator of postsynaptic receptor sensitivity.

Reappearance of activity in the vestibular neurones of labyrinthectomized guinea-pigs is not delayed by cycloheximide

1. In mammals, unilateral labyrinthectomy induces an immediate depression of the resting discharges in the neurones of the ipsilateral vestibular nuclei. Later on, a spontaneous restoration of this activity occurs. The aim of the present study was to test the possibility that protein synthesis could be involved in the start of this process in the guinea-pig. 2. Cycloheximide (CHX), a protein synthesis inhibitor, was injected intramuscularly 1 h before (30 mg kg-1) and 5 h after (15 mg kg-1) labyrinthectomy. 3. In a first group of animals, CHX was found to induce an inhibition of protein synthesis at levels ranging from 71 to 93% for 9 h after labyrinthectomy. 4. In a second group of alert animals, we studied single unit activity of second-order vestibular neurones. It was found that, in the 12-16 h post-labyrinthectomy period, at a time when restoration began in guinea-pigs not treated with CHX, the discharges in the labyrinthectomized group treated with CHX were not different from those observed in a previous study in labyrinthectomized animals not treated with CHX. 5. We conclude that protein synthesis is not required for the start of restoration of activity in the vestibular neurones deprived of their ipsilateral labyrinthine input.

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.

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.

The alpha4 subunit of rat alpha4beta2 nicotinic receptors is phosphorylated in vivo

The intracellular domains of the alpha4 and beta2 neuronal nicotinic subunits between transmembrane segments 3 and 4 contain a number of predicted phosphorylation sites but there is no direct evidence that any of these sites are actually phosphorylated in vivo. We expressed rat alpha4beta2 nicotinic receptors in Xenopus oocytes, labeled them by an overnight incubation in [32P]orthophosphate, and analyzed the immunoprecipitated receptors by autoradiography and Western blotting. Our results show that the oocytes contained three kinds of alpha4 subunits with apparent weights of 69, 79, and 89 kDa. The 89 kDa alpha4 subunit was the most heavily phosphorylated.

Nicotine increases cytosolic Ca2+ in vasopressin neurons

Strong immunoreactivity for neuronal nicotinic acetylcholine receptor alpha4 subunit was detected in neurons of the supraoptic nucleus (SON). At the ultrastructural level, immunoreactivity for alpha4 was detected in the post-synaptic membranes as well as in the cytoplasmic matrices in the magnocellular neurons. Nicotine (1-10 microM) increased cytosolic Ca2+ concentrations ([Ca2+]i) in isolated arginine-vasopressin (AVP)-containing neurons in the rat SON. Nicotine (10 microM) was less potent in increasing [Ca2+]i in AVP-containing neurons than noradrenaline (1 microM), a known neurotransmitter in the SON. The nicotine-induced [Ca2+]i increase in AVP-containing neurons was markedly reduced when pre-treated with a protein kinase A (PKA) blocker, H89 (40 microM). These findings suggest that nicotine, a known neurotransmitter in the SON, activates AVP-containing neurons via nicotinic acetylcholine receptor which is linked to stimulation of cAMP-PKA-regulated Ca2+ signaling pathway.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Posttranslational regulation of Ca(2+)-activated K+ currents by a target-derived factor in developing parasympathetic neurons

Macroscopic IK[Ca is not expressed in normal levels in chick ciliary ganglion (CG) neurons prior to synapse formation with target tissues, or in neurons developing in vitro or in situ in the absence of target tissues. Here, two chick CG slo partial cDNAs encoding IK[Ca channels were isolated, cloned, and sequenced. Both slo transcripts were readily detected in developing CG neurons prior to or in the absence of target tissue interactions. When CG neurons developed in vitro in the presence of target tissue (iris) extracts, a normal whole-cell IK[Ca was expressed. These effects did not require protein synthesis, and the activity was detectable throughout the stages of synapse formation in the iris. The active component has an apparent molecular weight of 40-60 kDa.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

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.

Inhibition of nicotinic acetylcholine receptor channels in bovine adrenal chromaffin cells by Y3-type neuropeptide Y receptors via the adenylate cyclase/protein kinase A system

The effect of neuropeptide Y [NPY(1-36)] and related peptides on the voltage-dependent currents and the nicotinic acetylcholine receptor (nAChR) currents (IACh) of bovine adrenal chromaffin cells was investigated using the whole-cell patch clamp technique. Catecholamine release from single chromaffin cells was measured by means of fast cyclic voltammetry. The potency order of these peptides in inhibiting IACh evoked by nicotine was NPY(1-36), NPY (16-36) > peptide YY(PYY) > [Leu31, Pro34]NPY. NPY(16-36) produced a similar degree of inhibition, irrespective of whether nicotine or an equipotent concentration of acetylcholine was used to evoke IACh. NPY(16-36) failed to alter voltage-dependent inward or outward currents. Intracellular cAMP, and extracellular dibutyryl-cAMP, produced a slowly developing increase in IACh. Intracellular cAMP, extracellular 8-Br-cAMP or dibutyryl-cAMP, and an inhibitor of cyclic nucleotide phosphodiesterases 3-isobutyl-1-methyl-xanthine (IBMX), decreased the inhibitory effect of NPY(16-36) on IACh. Although the intracellular application of the cAMP-dependent protein kinase A inhibitor [PKI(14-24)amide] alone did not alter IACh, it potentiated the effect of NPY(16-36) in interaction experiments. While the NPY(16-36)-induced inhibition of IACh was reversed on washout of the peptide, the slightly shorter C-terminal fragment NPY(18-36) caused a long-lasting depression of both IACh and catecholamine secretion evoked by nicotine. This depression was smaller in the presence of extracellular 8-Br-cAMP than in its absence. NPY(18-36) did not alter the secretory activity induced by a high concentration of potassium. It appears that, by activating Y3-receptors, NPY inhibits nAChR-current and the resulting secretion of catecholamines from bovine chromaffin cells. This process may involve a G protein-mediated decrease in intracellular cAMP with a subsequent decrease in the degree of phosphorylation of the nAChR-channel.

Modulation of AMPA/kainate receptors in cultured murine hippocampal neurones by protein kinase C

1. The patch clamp technique, together with intracellular perfusion of the catalytic fragment of protein kinase C (PKCM), was employed to investigate the role of this enzyme in the intracellular regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptors in cultured hippocampal neurones. 2. The responses evoked by near-maximal concentrations of kainate (250 microM) and AMPA (100 microM) were potentiated by the introduction of PKCM, whilst co-application of the inhibitory peptide fragment PKCI(19-36) prevented this action. 3. Modulation of kainate responses by PKCM was dependent upon the concentration of agonist applied. Currents evoked by kainate were potentiated at concentrations above those which caused 50% of the maximal response (EC50) and depressed at lower concentrations. Furthermore, okadaic acid, a specific inhibitor of phosphatases 1 and 2A, had a similar effect upon concentration-response relationships when currents activated by kainate were recorded using the perforated patch technique. 4. In addition, the mean amplitude and/or time constant of decay of miniature excitatory synaptic currents (mediated by AMPA/kainate receptors) was increased by the intracellular injection of PKCM. 5. These observations suggest that the function of postsynaptic excitatory amino acid receptors can be modulated by the activity of PKC as well as by endogenous phosphatases. This regulation may contribute to some forms of synaptic plasticity within the central nervous system.