The ultrastructural distribution of putative nicotinic receptors on cultured neurons from the rat superior cervical ganglion

A null mutation for the alpha3 nicotinic acetylcholine (ACh) receptor gene abolishes fast synaptic activity in sympathetic ganglia and reveals that ACh output from developing preganglionic terminals is regulated in an activity-dependent retrograde manner

In vertebrates, synaptic activity exerts an important influence on the formation of neural circuits, yet our understanding of its role in directing presynaptic and postsynaptic differentiation during synaptogenesis is incomplete. This study investigates how activity influences synaptic differentiation as synapses mature during early postnatal life. Specifically, we ask what happens to presynaptic terminals when synapses develop without functional postsynaptic receptors and without fast synaptic transmission. To address this issue, we investigated cholinergic nicotinic synapses in sympathetic ganglia of mice with a null mutation for the alpha3 nicotinic ACh receptor gene. Disrupting the alpha3 gene completely eliminates fast excitatory synaptic potentials on postganglionic sympathetic neurons, establishing a crucial role for alpha3-containing postsynaptic receptors in synaptic transmission. Interestingly, the preganglionic nerve terminals form morphologically normal synapses with sympathetic neurons, and these synapses persist without activity in postnatal animals. Surprisingly, when stimulating the preganglionic nerve at physiological rates, we discovered a significant decrease in ACh output from the presynaptic terminals in these alpha3(-/-) sympathetic ganglia. We show that this decrease in ACh output from the presynaptic terminals results, in part, from a lack of functional high-affinity choline transporters. We conclude the following: (1) fast synaptic transmission in mammalian SCG requires alpha3 expression; (2) in the absence of activity, the preganglionic nerve forms synapses that appear morphologically normal and persist for several weeks; and (3) to sustain transmitter release, developing presynaptic terminals require an activity-dependent retrograde signal.

Long-term potentiation of nicotinic synaptic transmission in rat superior cervical ganglia produced by phorbol ester and tetanic stimulation

The long-term potentiation of nicotinic synaptic transmission induced by both active phorbol ester (4beta-phorbol-12,13-dibutyrate, PdBu) and tetanic trains of preganglionic stimulation was studied in single neurons of the superior cervical ganglion (SCG) of the rat using intracellular recording techniques. PdBu significantly increased the mean amplitude of both the unitary evoked fast excitatory postsynaptic potentials (EPSPs) and the fast excitatory postsynaptic currents (EPSCs) to 17.0+/-3.3 mV (control 8.4+/-1.9 mV, n=5) and 2.8+/-0.4 nA (control 0.8+/-0.1 nA, n=10), respectively. There was no significant change in either the resting membrane potential, input resistance, or the threshold for the initiation of an action potential. The response to exogenously applied acetylcholine (ACh) was also not changed following exposure to PdBu. In low-calcium, high-magnesium solutions, PdBu significantly increased the quantal content of EPSPs approximately threefold from a control of 0.9+/-0.2 (n=5) to 2.6+/-0.6 (n=5). The quantal content of EPSCs was also increased to 1.3+/-0.2 (control 0.5+/-0.1, n=10). PdBu increased the frequency of miniature EPSPs (mEPSPs) to 196+/-47% (n=6) of control, while the amplitude, rise time, rate of rise, and decay of mEPSPs were not significantly changed. Tetanic stimulation significantly increased the amplitude of the unitary synaptic EPSPs and EPSCs without significantly changing the resting membrane potential, input resistance, threshold for initiation of an action potential, or the response to exogenously applied ACh. Tetanic stimulation significantly increased quantal content of EPSPs and EPSCs threefold. The results obtained with tetanically induced LTP are similar to the results obtained with phorbol ester-induced LTP in these ganglion neurons. These results suggest that both tetanically induced and phorbol ester-induced LTP, in the rat, share similar mechanisms which involve, at least in part, activation of PKC-dependent mechanisms to increase quantal release from sympathetic preganglionic axon terminals.

Dystrophin stabilizes alpha 3- but not alpha 7-containing nicotinic acetylcholine receptor subtypes at the postsynaptic apparatus in the mouse superior cervical ganglion

The nicotinic acetylcholine receptor (nAChR) subtypes were characterized in the superior cervical ganglion (SCG) of wild-type and dystrophin-lacking mdx mice. The binding of Epibatidine and alphaBungarotoxin, ligands for alpha3- and alpha7-containing receptors, respectively, revealed, for each ligand, a single class of high-affinity binding sites, with similar affinity in both wild-type and mdx mice. The Epibatidine-labeled receptors were immunoprecipitated by antibodies against the alpha3, beta2, and beta4 subunits. Immunocytochemistry showed that the percentage of alpha3-, beta2-, and beta4- but not of alpha7-immunopositive postsynaptic specializations was significantly lower in mdx than in wild-type mouse SCG. These observations suggest that the mouse SCG contains nAChRs, stabilized by dystrophin, in which the alpha3 subunit is associated with the beta2 and/or beta4 subunits. Conversely, dystrophin is not involved in the stabilization of the alpha7-containing nAChRs, as the percentage of alpha7-immunopositive synapses is similar in both wild-type and mdx mouse SCG.

Expression and localization of agrin during sympathetic synapse formation in vitro

Agrin is a motoneuron-derived signaling factor that plays a key organizing role in the initial stages of neuromuscular synapse formation. Agrin is expressed in other regions of the developing central and peripheral nervous systems, however, raising the possibility that it also directs the formation of some interneuronal synapses. To address this question, we have examined the expression and localization of agrin during formation of cholinergic, interneuronal synapses in the sympathetic system. In the superior cervical ganglia (SCG) in vivo, we found that agrin is highly expressed, and that it is present at, but is not limited to, synapses. In SCG neuronal cultures that were treated with ciliary neurotrophic factor to induce a uniform cholinergic phenotype, we found that agrin immunostaining colocalized precisely with cholinergic terminals and aggregates of neuronal acetylcholine receptor on the neuronal cell bodies and dendrites. Moreover, we found that alpha-dystroglycan, which is a potential receptor for agrin, is also concentrated at these cholinergic synaptic contacts. Finally, the SCG neurons expressed the C-terminal isoform of agrin that is neural-specific and highly active in synaptogenesis, and also the N-terminal splice isoform that occurs as a type II transmembrane protein. These findings show that agrin is specifically localized at sympathetic synapses in vitro, and are consistent with it playing a role in interneuronal synapse formation.

Nicotinic acetylcholine receptor subunit proteins alpha7 and beta4 decrease in the superior cervical ganglion after axotomy

Synaptic transmission in the superior cervical ganglion (SCG) is mediated by nicotinic acetylcholine receptors (nAChR). After transection of the postganglionic nerves of the SCG in the adult rat, the transcript levels of four of the five nAChR subunits present in the ganglion, alpha3, alpha5, alpha7, and beta4, decrease dramatically. In the present study, the effect of axotomy on nAChR subunit expression was examined at the protein level, focusing on the alpha7 and beta4 subunits. Immunohistochemistry with monoclonal antibody mAb306 (for the alpha7 subunit) and polyclonal antibody 4886 (for the beta4 subunit) showed that immunoreactivities for both alpha7 and beta4 subunits were concentrated in neurons in the intact ganglion. Results from double staining with antibodies to these subunits and to tyrosine hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of the sympathetic neurotransmitter norepinephrine, demonstrated that most neurons in the SCG express both the alpha7 and beta4 subunits. Three days after axotomy, the number of immunolabeled neurons and the intensity of the immunostaining per labeled neuron were decreased for both subunits. Decreases in subunit levels were also observed by Western blot analysis. Observing changes in these subunits over time after surgery revealed that, while the protein level of the alpha7 subunit recovered substantially within 2 weeks after the lesion, that of the beta4 subunit stayed low. These data demonstrate that decreases in nicotinic receptor subunits are among the changes in proteins that occur in axotomized sympathetic neurons, and suggest that these decreases may contribute to the depression in ganglionic synaptic transmission observed in axotomized ganglia.

Stimulant-induced exocytosis from neuronal somata, dendrites, and newly formed synaptic nerve terminals in chronically decentralized sympathetic ganglia of the rat

Loss of preganglionic neurones underlies the autonomic failure of human multiple system atrophy. In rat sympathetic ganglia decentralization leads to new synapse formation. We explored whether these synapses are functional, and whether chronically decentralized neurones respond normally to activation, in terms of exocytosis. Potassium depolarization and cholinergic agonists were applied to freshly excised rat superior cervical sympathetic ganglia, preganglionically denervated with prevented reinnervation 5 months earlier. Ganglia were incubated and stimulated in the presence of tannic acid, which stabilizes released vesicle cores for subsequent electron microscopy. In denervated ganglia exocytosis was observed from newly formed synaptic nerve terminals, and from nonsynaptic surfaces of neurone somata and dendrites. The results demonstrated that the new intraganglionic synapses, which are mostly catecholaminergic, can function and that chronically decentralized sympathetic neurones remain capable of stimulant-induced exocytosis from somata and dendrites. The maximal release upon potassium depolarization did not differ significantly between denervated and contralateral ganglia. Relative to this, the exocytotic responses of decentralized somata and dendrites to nicotine resembled those of contralateral ganglia. Responses to muscarine were significantly less in denervated than in contralateral ganglia, indicating inhibition in dendrites. Responses to carbachol suggested interactions between nicotinic and excitatory muscarinic effects. Nerve terminals in denervated ganglia showed high basal release. Their responses to muscarine and carbachol resembled those of the decentralized neurones, from which most may originate. Their response to nicotine evidenced inhibition. Their actions, coupled with nonsynaptic effects of soma-dendritic exocytosis, might modulate responses of the decentralized neurone population to other surviving inputs. This modulation could be influential in disease-induced decentralization in man.

Somatic and prejunctional nicotinic receptors in cultured rat sympathetic neurones show different agonist profiles

1. The release of [3H]-noradrenaline ([3H]-NA) in response to nicotinic acetylcholine receptor (nAChR) agonists was compared with agonist-induced currents in cultured rat superior cervical ganglion (SCG) neurones. 2. [3H]-NA release in response to high concentrations of nicotinic agonists was reduced, but not fully inhibited, by the presence of either tetrodotoxin (TTX) or Cd2+ to block voltage-gated Na+ or Ca2+ channels, respectively. We used the component of transmitter release that remained in the presence of these substances (named TTX- or Cd2+-insensitive release) to pharmacologically characterize nAChRs in proximity to the sites of vesicular exocytosis (prejunctional receptors). Prejunctional nAChRs were activated by nicotinic agonists with a rank order of potency of dimethylphenylpiperazinium iodide (DMPP) > nicotine > cytisine > ACh, and with EC50 values ranging from 22 microM (DMPP) to 110 microM (ACh). 3. [3H]-NA release in response to low concentrations of nAChR agonists was fully inhibited by the presence of either TTX or Cd2+ (named TTX- or Cd2+-sensitive release). TTX-sensitive release was triggered by nicotinic agonists with a rank order of potency of DMPP > cytisine approximately nicotine approximately ACh, which due to its similarity to TTX-insensitive release indicates that it might also be triggered by prejunctional-type nAChRs. The EC50 values for TTX (Cd2+)-sensitive release were less than 10 microM for all four agonists. 4. By contrast to transmitter release, somatic nAChRs as seen by patch clamp recordings were most potently activated by cytisine, with a rank order of potency of cytisine > nicotine approximately DMPP > ACh. EC50 values for the induction of currents exceeded 20 microM for all four agonists. 5. The nicotinic antagonist mecamylamine potently inhibited all transmitter release in response to nicotine. alpha-Bungarotoxin (alpha-BuTX) was, on the other hand, without significant effect on nicotine-induced TTX-insensitive release. The competitive antagonist dihydro-beta-erythroidine (DHbetaE) caused rightward shifts of the dose-response curves for both TTX-sensitive and TTX-insensitive transmitter release as well as for currents in response to nicotine, with pA2 values ranging from 4.03 to 4.58. 6. Due to clear differences in the pharmacology of agonists we propose that nAChRs of distinct subunit composition are differentially targeted to somatic or axonal domains.

Neuronal acetylcholine receptors with alpha7 subunits are concentrated on somatic spines for synaptic signaling in embryonic chick ciliary ganglia

Nicotinic acetylcholine receptors containing alpha7 subunits are widely distributed in the vertebrate nervous system. In the chick ciliary ganglion such receptors generate large synaptic currents but appear to be excluded from postsynaptic densities on the cells. We show here that alpha7-containing receptors are concentrated on somatic spines in close proximity to putative sites of presynaptic transmitter release. Intermediate voltage electron microscopy on thick sections, together with tomographic reconstruction, permitted three-dimensional analysis of finger-like projections emanating from cell bodies. The projections were identified as spines based on their morphology, cytoskeletal content, and proximity to presynaptic elements. Both in situ and after ganglionic dissociation, the spines were grouped on the cell surface and tightly folded into mats. Immunogold labeling of receptors containing alpha7 subunits showed them to be preferentially concentrated on the somatic spines. Postsynaptic densities were present in vivo both on the soma near spines and occasionally on the spines themselves. Synaptic vesicle-filled projections from the presynaptic calyx were interdigitated among the spines. Moreover, the synaptic vesicles often abutted the membrane and sometimes included Omega profiles as if caught in an exocytotic event, even when no postsynaptic densities were juxtaposed on the spine. The results suggest several mechanisms for delivering transmitter to alpha7-containing receptors, and they support new ideas about synaptic signaling via spines. They also indicate that neurons must have specific mechanisms for targeting alpha7-containing receptors to desired locations.

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

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

Synaptic transmission at visualized sympathetic boutons: stochastic interaction between acetylcholine and its receptors

Excitatory postsynaptic currents (EPSCs) were recorded with loose patch electrodes placed over visualized boutons on the surface of rat pelvic ganglion cells. At 34 degrees C the time to peak of the EPSC was about 0.7 ms, and a single exponential described the declining phase with a time constant of about 4.0 ms; these times were not correlated with changes in the amplitude of the EPSC. The amplitude-frequency histogram of the EPSC at individual boutons was well described by a single Gaussian-distribution that possessed a variance similar to that of the electrical noise. Nonstationary fluctuation analysis of the EPSCs at a bouton indicated that about 120 ACh receptor channels were available beneath boutons for interaction with a quantum of ACh. The characteristics of these EPSCs were compared with the results of Monte Carlo simulations of the quantal release of 9000 acetylcholine (ACh) molecules onto receptor patches of density 1400 microns-2 and 0.41 micron diameter, using a kinetic scheme of interaction between ACh and the receptors similar to that observed at the neuromuscular junction. The simulated EPSC generated in this way had temporal characteristics similar to those of the experimental EPSC when either the diffusion of the ACh is slowed or allowance is made for a finite period of transmitter release from the bouton. The amplitude of the simulated EPSC then exhibited stochastic fluctuations similar to those of the experimental EPSC.

Autoradiographic localization of functional muscarinic receptors in the rat superior cervical sympathetic ganglion reveals an extensive distribution over non-synaptic surfaces of neuronal somata, dendrites and nerve endings

Fast synaptic transmission in sympathetic ganglia is mediated by acetylcholine, acting on nicotinic receptors, yet muscarinic receptors are also present and are involved in the production of slow postsynaptic potentials. In order further to elucidate the role of muscarinic receptors in ganglionic transmission their distribution in the rat superior cervical sympathetic ganglion was investigated autoradiographically by use of the tritiated irreversible muscarinic ligand propylbenzilylcholine mustard. It was observed that this agent blocked the carbachol-evoked hydrolysis of inositol phospholipids in the ganglion and that this response to carbachol is itself inhibitable by selective muscarinic antagonists with a potency sequence which indicates involvement primarily of M1 receptors. Light microscope autoradiography showed that labelling inhibitable by atropine and by the M1-selective muscarinic antagonist pirenzepine was essentially confined to the margins of neuronal somata and regions of dendritic arborization, which include synaptic contacts. Quantitative electron microscope autoradiography showed that binding of the radioligand, of which approximately 70% was inhibitable by atropine and 68% by pirenzepine, was associated predominantly with surface membranes of neuronal somata, dendrites, other neurites (including axons and uncharacterized dendrites) and nerve terminal profiles, in the approximate ratios 95:85:52:45. Of the inhibitable binding over neuronal membranes in the ganglion little more than 3% was found to be synaptically located, and this involved para- or peri-synaptic regions of nerve terminal contacts rather than the specialized synaptic zone. About 5% of the inhibitable binding over neuronal membranes involved non-synaptic surfaces of nerve terminals and preterminal axon segments; almost 70% was distributed over non-synaptic surfaces of neuronal somata and dendrites, and about 21% upon other neurites. Binding sites were found not to be more highly concentrated at or adjacent to synapses than over other regions of neuronal surface membranes. About 50%, possibly more, of the binding on non-synaptic surfaces of nerve endings, and about 7% of binding upon dendritic membranes, was of non-M1, possibly M2 type, inhibitable by atropine but not by pirenzepine. Non-synaptic neuro-neuronal appositions, which involve dendrites and somata and often lie adjacent to synapses, showed rather more than twice the binding expected for each membrane individually; and neuronal membrane exposed to basal lamina lining ganglionic tissue spaces showed high levels of binding. Little inhibitable binding was seen over membranes of satellite and Schwann cells, or over cytoplasmic territories or ganglionic interstitial tissue. A model was constructed of the distribution of label, which showed that the observed results for total binding could be approximately matched by assuming the following relative densities of ligand binding sites: interstitial tissue space and supporting cells 1, soma cytoplasm 3, cytoplasm of dendrites, neurites and nerve terminals 4.5, surfaces of mesodermal elements 15, surfaces of neurites and nerve endings including sites of synapse 45, surfaces of dendrites 90, surfaces of neuronal somata 120, non-synaptic neuro-neuronal appositions 180. It is concluded that functional muscarinic receptors in this sympathetic ganglion, predominantly of the M1 type linked with slow depolarizations, but including some non-M1 receptors, are widely distributed over non-synaptic surfaces of the neuronal somata and dendrites and are not concentrated at synapses. Presynaptic autoreceptors are also present, of which half or more are of non-M1, possibly M2, type which might be inhibitory. The presence of M4 receptors is not excluded...

alpha 2-Adrenoreceptor-mediated inhibition of acetylcholine-induced noradrenaline release from rat sympathetic neurons: an action at voltage-gated Ca2+ channels

[3H]Noradrenaline release was studied in cultured sympathetic neurons derived from superior cervical ganglia of neonatal rats. Acetylcholine elicited a concentration- and time-dependent increase in 3H outflow which was half-maximal at about 300 microM and within 5 s. The overflow induced by 10 s exposure to 300 micro A acetylcholine was reduced by the nicotinic antagonist hexamethonium, but increased by the muscarinic antagonist atropine. Cd2+ (300 microM) prevented the overflow evoked by electrical field stimulation, but reduced acetylcholine-induced overflow by less than 50%. Removal of extracellular Ca2+ abolished stimulation-evoked tritium overflow irrespective of the stimulus. The selective alpha2-adrenoceptor agonist UK 14,304 inhibited acetylcholine-evoked overflow to a significantly smaller extent (approximately 25% maximal inhibition) than electrically induced overflow ( > or = 45% maximal inhibition). These inhibitory effects were antagonized by the alpha2-adrenoceptor antagonist yohimbine. Noradrenaline (0.1 microM) reduced acetylcholine-evoked overflow to the same extent as did UK 14,304 (0.1 microM). UK 14,304 had no effect when 3H overflow was evoked by acetylcholine in the presence of 300 microM Cd2+. Currents through nicotinic acetylcholine receptors and voltage-activated Ca2+ currents were studied with the whole-cell variant of teh patch-clamp technique. UK 14,304 reduced nicotinic acetylcholine receptor currents and voltage-activated Ca2+ currents with similar potency and efficacy. Yohimbine, however, antagonized only the inhibition of voltage-activated Ca2+ currents, but not the effects of UK 14,304 on nicotinic receptor currents. Furthermore, yohimbine per se reduced currents through nicotinic receptors. Noradrenaline (10 microM) inhibited voltage-dependent Ca2+ currents just as did UK 14,304 (10 microM), but failed to reduce currents through nicotinic acetylcholine receptor channels. Cd2+ (300 microM) abolished voltage-activated Ca2+ currents and reduced nicotinic acetylcholine receptor currents by 65%. These results indicate that acetylcholine evokes noradrenaline release from rat sympathetic neurons by activation of nicotinic receptors and restricts this release via muscarinic receptors. The acetylcholine-induced transmitter release is based on two mechanisms, one involving and the other one bypassing voltage-dependent Ca2+ channels. alpha2-Adrenoceptor activation reduces voltage-activated Ca2+ currents and effects exclusively the component of acetylcholine-induced release which involves voltage-dependent Ca2+ channels. These results support the hypothesis that voltage-activated Ca2+ channels are the sole site of autoinhibitory alpha2-adrenergic effects on transmitter release from rat sympathetic neurons. The inhibitory effects of alpha2-adrenoceptor agonists and antagonists on currents through nicotinic acetylcholine receptors are not mediated by an alpha2-adrenoceptor.

Comparison of neuronal nicotinic receptors in rat sympathetic neurones with subunit pairs expressed in Xenopus oocytes

1. The agonist sensitivity of nicotinic acetylcholine receptors in rat superior cervical ganglion (SCG) neurones was compared with that of cloned receptors expressed in Xenopus oocytes by pairwise injections of alpha 3-beta 2 or alpha 3-beta 4 neuronal nicotinic subunit combinations. 2. Agonist responses in rat SCG neurones indicated that cytisine was the most potent agonist and lobeline the least potent (rank order of potency: cytisine > dimethylphenylpiperazinium iodide (DMPP) > nicotine > ACh > carbachol > lobeline). 3. Receptors expressed in oocytes by injection of alpha 3 and beta 2 subunits had a relatively high sensitivity to DMPP and low sensitivity to cytisine (rank order of potency: DMPP > ACh > lobeline > carbachol > nicotine > cytisine), whereas receptors composed of alpha 3 and beta 4 subunits had a high sensitivity to cytisine and low sensitivity to DMPP (rank-order of potency: cytisine > nicotine approximately ACh > DMPP > carbachol > lobeline). 4. With the exception of responses to DMPP, agonist sensitivity measurements suggest that nicotinic receptors in the rat SCG are composed of alpha 3 and beta 4 subunits. The results are discussed in terms of the receptor subunit mRNAs known to be expressed in the rat SCG and previous evidence of functional heterogeneity of rat SCG nicotinic acetylcholine receptors.

Phorbol esters and K+ up-regulate alpha-bungarotoxin binding sites in cultured chromaffin cells through a related mechanism

Nicotinic alpha-bungarotoxin (alpha-BGT) receptors in cultured chromaffin cells are up-regulated in response to long term (days) exposure to nicotinic antagonists, elevated K+, and activators of protein kinase C (PKC), such as the phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (PMA). The present experiments were done to determine whether their was any interaction in the ability of PMA and K+ to up-regulate the alpha-BGT receptors. Chromaffin cells were treated for 3 days with both 100 nM PMA and 20 mM K+, concentrations which produce maximal responses on their own. The increase in alpha-BGT binding after the combined treatment was the same as that seen with K+ alone suggesting that K+ inhibited the PMA-mediated effect. The K(+)-induced increase in toxin binding was partially prevented by polymyxin B, an agent which completely inhibited the PMA induced increase. The time courses of the increases in binding induced by both K+ and PMA were similar in that the most marked increases in binding were observed at the later time points. The PMA-induced up-regulation was partially inhibited by an activator of adenylate cyclase, a result similar to that previously seen with K+. The present studies suggest that the up-regulation of alpha-BGT receptors induced by K+ shares similarities with that induced by phorbol esters. The observations that K+ inhibited the PMA induced increase and that a PKC inhibitor partially blocked the K+ response suggest that the K+ and PMA mediated induction of the alpha-BGT sites may be linked with the effects of K+ preceding those of PMA.

Localization of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin binding to nicotinic sites in the chicken forebrain and midbrain

We have previously localized cholinergic cell bodies and fibers within the midbrain of the chicken with choline acetyltransferase immunohistochemistry. In a continuing effort to characterize the central cholinergic system, the present study examines the distribution of various nicotinic acetylcholine receptors in the forebrain and midbrain of the chicken. The binding of 3H-nicotine, 125I-kappa-bungarotoxin, and 125I-alpha-bungarotoxin was localized by film autoradiography in adjacent sections of the adult chicken brain, allowing a comparison of the distribution of different classes of nicotinic binding sites within the brain. Although all three ligands were often co-localized, there were areas that bound 3H-nicotine but not the 125I-neurotoxins, or vice versa. Very high densities of all three ligands were found in the hyperstriatum ventrale; the nucleus geniculatus lateralis, pars ventralis; the griseum tectale; the nucleus dorsolateralis anterior thalami; the nucleus lentiformis mesencephali, pars lateralis and pars medialis; the periventricular organ; and the stratum griseum et fibrosum superficiale, layer f of the optic tectum. The nucleus spiriformis lateralis had the highest levels of 3H-nicotine binding in the chicken brain, but it did not bind either of the two snake neurotoxins. On the other hand, high levels of both 125I-alpha-bungarotoxin and 125I-kappa-bungarotoxin binding were found in the nucleus semilunaris and the nucleus ovoidalis, but these areas contained little or no 3H-nicotine binding. No unique 125I-kappa-bungarotoxin sites, unrecognized by 125I-alpha-bungarotoxin, were identified by the low resolution autoradiography performed in this study. In general, nicotinic receptors were found in areas that have been reported to contain cholinergic cell bodies or fibers. Comparison of our results with the expression of neuronal nicotinic receptor subunits, as determined by in situ hybridization, suggests that many of the high affinity 3H-nicotine sites are localized presynaptically, as, for example, in the retinorecipient nuclei and the nucleus interpeduncularis. The lack of 125I-kappa-bungarotoxin binding in the presence of alpha-bungarotoxin indicates that the chicken brain has only very low levels of a unique kappa-bungarotoxin site. This is in marked contrast to chicken, frog, and rat autonomic ganglia, where a unique kappa-neurotoxin-sensitive receptor has been identified and shown to mediate nicotinic neurotransmission.

The interaction of kappa-bungarotoxin with the nicotinic receptor of bovine chromaffin cells

Whole-cell recording techniques were used to examine acetylcholine-induced nicotinic currents in isolated bovine chromaffin cells. The effects on these currents of kappa-bungarotoxin, a snake venom kappa-neurotoxin, were tested. Exposure of cells to kappa-bungarotoxin (600 nM for 40 min) produced a prolonged blockade of nicotinic currents. The mechanism of this blockade was examined in several ways. Firstly, the pre-exposure of cells to trimetaphan, a competitive nicotinic antagonist, protected against the action of subsequent additions of kappa-bungarotoxin. Secondly, voltage-clamp measurements indicated that the degree of blockade produced by kappa-bungarotoxin was independent of cell membrane potential. Unlike (+)-tubocurarine, kappa-bungarotoxin had no direct agonist effects on nicotinic receptors. It is concluded from the present functional studies and from previously reported binding studies that kappa-bungarotoxin blocks nicotinic responses in bovine chromaffin cells by binding to regions overlying acetylcholine sites on nicotinic receptors.

Differential regulation of muscarinic and nicotinic cholinergic receptors and their mRNAs in cultured sympathetic neurons

Mechanisms regulating expression of neuronal muscarinic and nicotinic receptors were examined in cultures of neonatal rat sympathetic neurons. Two factors known to stimulate cholinergic transmitter development in sympathetic neurons were examined for their effects on cholinergic receptor expression. A membrane associated factor (MANS46) and a diffusible factor produced by cultured rat fibroblasts (RFCM) each decreased muscarinic receptor number. By contrast, neither treatment altered levels of nicotinic receptors. Levels of muscarinic (m2) receptor mRNA were decreased by MANS but not by RFCM, indicating that effects of the two treatments were mediated by different mechanisms. Neither MANS nor RFCM altered levels of nicotinic alpha 3 or beta 2 mRNAs, consistent with the lack of change in numbers of nicotinic receptors. These observations indicate that receptor phenotype in developing neurons is subject to regulation by multiple epigenetic factors. Further, the same signals which regulate transmitter development may also regulate receptor expression in sympathetic neurons.

Rectification of currents activated by nicotinic acetylcholine receptors in rat sympathetic ganglion neurones

1. The inward rectification of the whole-cell current evoked by acetylcholine (ACh) and other nicotinic agonists in rat sympathetic ganglion neurones has been studied using patch-clamp recording techniques. The selective nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (20 microM) induced an average peak current of -367 pA at -50 mV but no detectable outward current at +50 mV. Similar observations were made with ACh and carbachol. 2. The current-voltage relation of the whole-cell response induced by DMPP was linear in the negative voltage range; however, there was no detectable outward current in the voltage range 0 to about +70 mV. Above +70 mV an outward current became clearly detectable. Rapid depolarizing jumps in the holding potential failed to reveal any rapidly decaying outward current. 3. The rectification was not alleviated by changing the main permeant cation, by removal of divalent cations from the intracellular or extracellular solutions or by altering the pH buffer in the extracellular solution from HEPES to Tris. 4. Intracellular magnesium ions can block the channel. This effect increases with depolarization, but dissociation outwards (i.e. permeation by Mg2+) appears to relieve the block at more extreme positive potentials. This effect alone, or in combination with the voltage dependence of the burst length, is unlikely to be able to account for the whole-cell rectification in intact cells, much less that seen in cells perfused with Mg2(+)-free intracellular medium. 5. When the reversal potential was shifted to approximately -50 mV (by the use of impermeant cations) nicotinic agonists produced small outward currents in the membrane potential range -20 to +10 mV while shifting it to about +40 mV produced small inward currents in the potential range 0 to +20 mV. The rectification therefore appears to be independent of the direction of current flow and is maximum at a potential positive to 0 mV. 6. At positive potentials the receptors desensitized much less than at negative potentials in the continued presence of agonist. Thus, exposure of the cells to a steady application of 30 microM-ACh produced no detectable response if the cell was at a positive potential, but when the cell was stepped to a negative potential in the continued presence of ACh (at a time when much of the ACh current would be expected to have desensitized), ACh induced a large inward current. The onset of the ACh current had a time constant of 10 ms. It then decayed with a time constant of 790 ms as desensitization developed.(ABSTRACT TRUNCATED AT 400 WORDS)

Nicotinic acetylcholine receptors in porcine hypophyseal intermediate lobe cells

1. Acetylcholine (ACh) was found to depolarize isolated porcine intermediate lobe cells maintained in primary cells culture. We investigated the ACh-induced responses in both whole-cell and cell-attached configurations of the patch-clamp technique. 2. From noise analysis of ACh-evoked whole-cell currents, we estimated an elementary conductance of 20 pS and a channel open duration of about 1.7 ms at -60 mV. From single-channel recordings, we obtained a slope conductance of 26 pS and a mean open time of 1.8 ms at membrane potentials between -60 and -80 mV. 3. ACh-evoked responses were blocked by d-tubocurarine (d-TC), hexamethonium and mecamylamine, but were insensitive to alpha-bungarotoxin. These characteristics define a neuronal type of nicotinic receptors. 4. The whole-cell current induced by ACh showed a strong inward rectification with no outward current being obtained. This phenomenon was observed when the intracellular ion is either sodium or caesium, and even when Ca2+ and Mg2+ were totally removed from the intracellular medium. 5. ACh-gated channels in intermediate lobe cells were cation selective and were permeable to Na+ and Cs+. In Ca2(+)-free extracellular solution, single-channel conductances were much larger (46 pS) than in the presence of 2 mM-Ca2+ (26 pS). 6. The possibility of an excitatory cholinergic control of intermediate lobe cells is discussed.

Neuronal bungarotoxin blocks the nicotinic stimulation of endogenous dopamine release from rat striatum

Nicotinic receptors in the brain are receiving increased attention due in part to the recent cloning of receptor subunits and to postmortem studies revealing alterations in receptor density associated with Alzheimer's disease. The peptide neurotoxin neuronal bungarotoxin (NBT) has been shown to block nicotinic cholinergic responses in autonomic ganglia and in retinal ganglion cells. These findings suggest that NBT may be a useful probe for studying nicotinic receptors in the brain. Therefore, we have investigated the effects of NBT on the nicotine-mediated enhancement of endogenous dopamine release from rat striatal slices. It was found that the transient increase in dopamine release caused by 100 microM nicotine was completely blocked by 100 nM NBT, indicating that NBT is a functional nicotinic antagonist in this system.