Synaptic Transmission In Sympathetic Ganglia Research Articles

Impact of RAAS Receptors and Membrane-Bound Transporter System in the Left Ventricle during the Long-Term Control of Hypertension

The Renin–Angiotensin–Aldosterone System (RAAS) has been implicated in systemic and neurogenic hypertension. The infusion of RAAS inhibitors blunted arterial pressure and efficacy of use-dependent synaptic transmission in sympathetic ganglia. The current investigation aims to elucidate the impact of RAAS-mediated receptors on left ventricular cardiomyocytes and the role of the sarcolemma-bound carrier system in the heart of the hypertensive transgene model. A significant increase in mRNA and the protein expression for angiotensin II (AngII) receptor subtype-1 (AT1R) was observed in (mREN2)27 transgenic compared to the normotensive rodents. Concurrently, there was an upregulation in AT1R and a downregulation in the MAS1 proto-oncogene protein receptor as well as the AngII subtype-2 receptor in hypertensive rodents. There were modifications in the expressions of sarcolemma Na+-K+-ATPase, Na+-Ca2+ exchanger, and Sarcoendoplasmic Reticulum Calcium ATPase in the transgenic hypertensive model. These observations suggest chronic RAAS activation led to a shift in receptor balance favoring augmented cardiac contractility and disruption in calcium handling through modifications of membrane-bound carrier proteins and blood pressure. The study provides insight into mechanisms underlying RAAS-mediated cardiac dysfunction and highlights the potential value of targeting the protective arm of AngII in hypertension.

HCN Channel Blockers Act Pre‐ and Post‐Synaptically to Inhibit Synaptic Transmission and Gain in the Rat Superior Cervical Sympathetic Ganglion

Hyperpolarization‐activated cation channels encoded by HCN subunits are permeable to Na+ and K+ ions and carry inward h‐currents. The h‐current (Ih) was originally discovered in the heart, where it is the principal pacemaker current. Ivabradine, a selective blocker of Ih, was approved in the US in 2015 as an adjunct to β‐adrenergic blockade for clinical management of heart failure. Recently we observed that sympathetic neurons in the rat superior cervical ganglion (SCG) express prominent Ih responses that can be blocked by ZD7288, another selective antagonist. The goal of the present study was to test the hypothesis that Ih modulates synaptic transmission in sympathetic ganglia. We did this using dissociated SCG neurons and using acutely isolated intact ganglia, treated with enzymes to permit whole‐cell recording, together with simultaneous presynaptic stimulation and extracellular recording. When synaptic transmission was elicited by submaximal 1 Hz stimulation in the isolated intact SCG, the extracellular postsynaptic compound action potential was inhibited by about 20% by 10–40 μM Ivabradine and by 10–20 μM ZD7288. In dissociated SCG neurons, we measured Ih under voltage clamp, fit the data to a Hodgkin‐Huxley model and then used the model to implement virtual Ih under dynamic clamp. When neurons were probed with virtual nicotinic fast EPSPs created with dynamic clamp, blocking Ih with 10–15 μM ZD7288 increased the threshold‐synaptic conductance (thresh‐gsyn) from 7.0 ± 0.9 nS to 12.1 ± 2.0 nS (p<0.0001) and decreased synaptic gain from 2.2 to 1.7 in response to noisy patterns of virtual synaptic activity that were designed to mimic physiological activity in vivo. This indicates that the native postsynaptic Ih normally enhances synaptic amplification to increase postsynaptic spike output by the SCG. When the native Ih was reconstituted with virtual Ih, synaptic amplification was restored. Shifting the voltage‐dependence of activation to mimic binding of cyclic AMP to HCN channels further modulated thresh‐gsyn and synaptic gain. To assess presynaptic actions of Ih, we then measured nicotinic cholinergic synaptic currents evoked by minimal presynaptic stimulation of the intact SCG. Ivabradine and ZD7288 both reduced peak EPSC amplitude and total charge by 40 to 50% while also increasing the failure rate. This indicates that native Ih normally enhances the probability of acetylcholine release and the amount of release by preganglionic terminals. Taken together the results suggest that clinical use of Ivabradine may have the heretofore unknown effect of reducing peripheral sympathetic activity through its pre‐ and postsynaptic effects on ganglionic transmission. This may have beneficial therapeutic consequences by countering the sympathetic hyperactivity that can drive human hypertension and that exacerbates the progression of heart failure.Support or Funding InformationSupported by the Great Rivers Affiliate of the American Heart Association through Grant‐in‐Aid 13GRNT1685007

RAGE mediates the inactivation of nAChRs in sympathetic neurons under high glucose conditions.

Autonomic dysfunction is a serious complication of diabetes and can lead to cardiovascular abnormalities and premature death. It was recently proposed that autonomic dysfunction is triggered by oxidation-mediated inactivation of neuronal nicotinic acetylcholine receptors (nAChRs), impairing synaptic transmission in sympathetic ganglia and resulting in autonomic failure. We investigated whether the receptor for advanced glycation end products (RAGE) and its role in the generation of reactive oxygen species (ROS) could be contributing to the events that initiate sympathetic malfunction under high glucose conditions. Using biochemical, live imaging and electrophysiological tools we demonstrated that exposure of sympathetic neurons to high glucose increases RAGE expression and oxidative markers, and that incubation with RAGE ligands (e.g. AGEs, S100 and HMGB1) mimics both ROS elevation and nAChR inactivation. In contrast, co-treatment with either antioxidants or an anti-RAGE IgG prevented the inactivation of nAChRs. Lastly, a role for RAGE in this context was corroborated by the lack of sensitivity of sympathetic neurons from RAGE knock-out mice to high glucose. These data define a pivotal role for RAGE in initiating the events associated with exposure of sympathetic neurons to high glucose, and strongly support RAGE signaling as a potential therapeutic target in the autonomic complications associated with diabetes.

A Highly Conserved Cytoplasmic Cysteine Residue in the α4 Nicotinic Acetylcholine Receptor Is Palmitoylated and Regulates Protein Expression

Nicotinic acetylcholine receptor (nAChR) cell surface expression levels are modulated during nicotine dependence and multiple disorders of the nervous system, but the mechanisms underlying nAChR trafficking remain unclear. To determine the role of cysteine residues, including their palmitoylation, on neuronal α4 nAChR subunit maturation and cell surface trafficking, the cysteines in the two intracellular regions of the receptor were replaced with serines using site-directed mutagenesis. Palmitoylation is a post-translational modification that regulates membrane receptor trafficking and function. Metabolic labeling with [(3)H]palmitate determined that the cysteine in the cytoplasmic loop between transmembrane domains 1 and 2 (M1-M2) is palmitoylated. When this cysteine is mutated to a serine, producing a depalmitoylated α4 nAChR, total protein expression decreases, but surface expression increases compared with wild-type α4 levels, as determined by Western blotting and enzyme-linked immunoassays, respectively. The cysteines in the M3-M4 cytoplasmic loop do not appear to be palmitoylated, but replacing all of the cysteines in the loop with serines increases total and cell surface expression. When all of the intracellular cysteines in both loops are mutated to serines, there is no change in total expression, but there is an increase in surface expression. Calcium accumulation assays and high affinity binding for [(3)H]epibatidine determined that all mutants retain functional activity. Thus, our results identify a novel palmitoylation site on cysteine 273 in the M1-M2 loop of the α4 nAChR and determine that cysteines in both intracellular loops are regulatory factors in total and cell surface protein expression of the α4β2 nAChR.

Reactive oxygen species inactivate neuronal nicotinic acetylcholine receptors through a highly conserved cysteine near the intracellular mouth of the channel: implications for diseases that involve oxidative stress.

An intriguing feature of several nicotinic acetylcholine receptors (nAChRs) on neurons is that their subunits contain a highly conserved cysteine residue located near the intracellular mouth of the receptor pore. The work summarized in this review indicates that α3β4-containing and α4β2-containing neuronal nAChRs, and possibly other subtypes, are inactivated by elevations in intracellular reactive oxygen species (ROS). This review discusses a model for the molecular mechanisms that underlie this inactivation. In addition, we explore the implications of this mechanism in the context of complications that arise from diabetes. We review the evidence that diabetes elevates cytosolic ROS in sympathetic neurons and inactivates postsynaptic α3β4-containing nAChRs shortly after the onset of diabetes, leading to a depression of synaptic transmission in sympathetic ganglia, an impairment of sympathetic reflexes. These effects of ROS on nAChR function are due to the highly conserved Cys residues in the receptors: replacing the cysteine residues in α3 allow ganglionic transmission and sympathetic reflexes to function normally in diabetes. This example from diabetes suggests that other diseases involving oxidative stress, such as Parkinson's disease, could lead to the inactivation of nAChRs on neurons and disrupt cholinergic nicotinic signalling.

Diabetes Depresses Synaptic Transmission in Sympathetic Ganglia by Inactivating nAChRs through a Conserved Intracellular Cysteine Residue

Most people with diabetes develop severe complications of the autonomic nervous system; yet, the underlying causes of many diabetic-induced dysautonomias are poorly understood. Here we explore the idea that these dysautonomias results, in part, from a defect in synaptic transmission. To test this idea, we investigated cultured sympathetic neurons and show that hyperglycemia inactivates nAChRs through a mechanism involving an elevation in reactive oxygen species and an interaction with highly conserved cysteine residues located near the intracellular mouth of the nAChR channel. Consistent with this, we show that diabetic mice have depressed ganglionic transmission and reduced sympathetic reflexes, whereas diabetic mice expressing mutant postsynaptic nAChRs that lack the conserved cysteine residues on the alpha3 subunit have normal synaptic transmission in sympathetic ganglia and normal sympathetic reflexes. Our work suggests a new model for diabetic-induced dysautonomias and identifies ganglionic nAChRs as targets of hyperglycemia-induced downstream signals.

An Activity-Dependent Retrograde Signal Induces the Expression of the High-Affinity Choline Transporter in Cholinergic Neurons

A well-accepted view of developing circuits is that synapses must be active to mature and persist, whereas inactive synapses remain immature and are eventually eliminated. We question this long-standing view by investigating nonfunctional cholinergic nicotinic synapses in the superior cervical ganglia (SCG) of mice with a disruption in the alpha3 nicotinic receptor (nAChR) subunit gene, a gene essential for fast synaptic transmission in sympathetic ganglia. Using imaging and electrophysiology, we show that synapses persist for at least 2-3 months without postsynaptic activity; however, the presynaptic terminals lack high-affinity choline transporters (CHTs), and as a result, they are quickly depleted of transmitter. Moreover, we demonstrate with rescue experiments that CHT is induced by signals downstream of postsynaptic activity, converting immature terminals to mature terminals capable of sustaining transmitter release in response to high-frequency or continuous firing. Importantly, postsynaptic neurons must be continually active to maintain CHT in presynaptic terminals.

The Skok legacy and beyond: Molecular mechanisms of slow synaptic excitation in sympathetic ganglia

Vladimir Skok and his colleagues did much of the pioneering work on fast excitatory synaptic transmission in sympathetic ganglia and on nicotinic acetylcholine receptors that mediate fast transmission. I and my colleagues (including Alex Selyanko, one of Vladimir’s proteges) have studied the additional process of slow synaptic excitation that is mediated by the action of acetylcholine on muscarinic receptors. This results primarily from the closure of “M-channels,” a subset of voltage-gated potassium channels composed of Kv7.2 and Kv7.3 channel subunits. These channels require membrane phosphatidylinositol-4,5-bisphosphate (PIP2) for their opening, and their closure by muscarinic receptor activation is now thought to result from the reduction in PIP2 levels that follows receptor-induced PIP2 hydrolysis. The dynamics of these two forms of synaptic excitation are compared.

Coexpression and Spatial Association of Nicotinic Acetylcholine Receptor Subunits α7 and α10 in Rat Sympathetic Neurons

Fast excitatory synaptic transmission in sympathetic ganglia is mediated by nicotinic acetylcholine receptors (nAChRs). Although it is known that the nAChR alpha7-subunit occurs in sympathetic ganglia, the expression of the recently cloned subunit alpha10 (Elgoyhen et al., 2001; Lustig et al., 2001; Sgard et al., 2002) has not been analyzed. Until now, functional receptors containing alpha10-subunits have been found only in combination with alpha9-subunits (Elgoyhen et al., 2001; Lustig et al., 2001; Sgard et al., 2002). The alpha9-subunit exhibits a restricted expression pattern, whereas the alpha10-subunit is expressed more widely. This broad distribution resembles more closely that known for subunit alpha7 than for subunit alpha9. On this background, we investigated the distribution of nAChR subunits alpha7, alpha9, and alpha10 in rat sympathetic ganglia and studied a possible interaction between subunit alpha7 and potential partners by double-labeling immunofluorescence and fluorescence resonance energy transfer (FRET) (Kam et al., 1995; Jares-Erijman and Jovin, 2003).

Transmission of signals through sympathetic ganglia--modulation, integration or simply distribution?

On structural grounds, synaptic transmission in sympathetic ganglia is potentially complex with extensive divergence and convergence between preganglionic and postganglionic neurones. In this review, the focus is on what constitutes a functional synapse in sympathetic ganglia and how intracellular recordings have enabled us to identify how the transmission process operates in vivo. Only one or two suprathreshold or 'strong' inputs are involved in activating each postganglionic neurone. The functional significance of the subthreshold or 'weak' inputs remains obscure. The strong inputs, and sometimes the weak ones as well, respond in the same way during reflexes. The expansion of ineffective weak connections enables the rapid restoration of functional control after lesions that damage preganglionic neurones. These novel connections may generate erroneous reflex responses after spinal injury. Postganglionic discharge in vivo consists of the summed firing of the strong preganglionic inputs limited, at high preganglionic discharge rates, by the properties of the afterhyperpolarization. Preganglionic signals are distributed widely through paravertebral ganglia with little modification.

The effects of ketamine and propofol on neuronal nicotinic acetylcholine receptors and P2x purinoceptors in PC12 cells.

We studied the effects of ketamine and propofol on two ligand-gated ion channels mediating fast synaptic transmission through sympathetic ganglia, neuronal nicotinic acetylcholine receptors (nAchRs), and P2X purinoceptors in a rat pheochromocytoma cell line PC12 using whole cell voltage clamp recording. Ketamine and propofol similarly inhibited the nicotine-induced inward current reversibly and dose-dependently at the membrane potential of -60 mV but had no effects on the adenosine triphosphate-induced current. Both anesthetics accelerated the current decay during agonist application, resulting in greater inhibition on the steady current than the peak current. The 50% inhibition concentration values for the steady current were lower than the clinically relevant concentrations for ketamine (2.8+/-0.6 microM) and higher than those for propofol (5.4+/-0.6 microM). Both anesthetics induced an addition of the fast component to the decay phase and an acceleration of the slow component, which suggests an open channel blockade or an enhancement of desensitization as a mechanism. The effects on closed channels seemed to be small because preincubation with the anesthetics did not significantly augment the block. Inhibition was voltage-independent at membrane potentials between -20 and -70 mV and was consistent with a noncompetitive block. Inhibition of the neuronal nAchR-mediated current may lead to the suppression of synaptic transmission in sympathetic ganglia by ketamine, but not by propofol, at the clinically relevant concentrations. However, these results are not consistent with changes in sympathetic nerve activities reported for animals or humans anesthetized with ketamine or propofol, which suggests effects from other systems, such as the central nervous system in vivo. Ketamine (at smaller than clinically relevant concentrations) and propofol (at larger than clinically relevant concentrations) inhibited neuronal nicotinic acetylcholine receptor-mediated current in PC12 cells, which possess the receptors that resemble those in postganglionic sympathetic neurons. These findings are not consistent with in vivo experiments, which suggests that effects from other systems, such as the central nervous system, are of importance.

Differential effects of thiopental on neuronal nicotinic acetylcholine receptors and P2X purinergic receptors in PC12 cells.

PC12 cells, derived from rat pheochromocytoma, express neuronal nicotinic acetylcholine receptors (nAchRs) and P2X purinergic receptors, both of which resemble the receptors in postganglionic sympathetic neurons. The former is the established and the latter is the putative receptor to mediate fast synaptic transmission. The authors investigated effects of thiopental on these two ligand-gated ion channels. Whole cell currents were recorded in PC12 cells without treatment of nerve growth factor, using conventional whole cell patch clamp technique. Nicotine or adenosine triphosphate (ATP) 30 microM was applied for 4-5 s in the absence or presence of thiopental 3-300 microM. Nicotine induced the rapidly decaying inward current at -60 mV, which exhibited the characteristics of the neuronal nAchR-mediated current. Thiopental inhibited the nicotine-induced inward current and accelerated the current decay in a dose-dependent manner, resulting in the greater effects on the steady current than the peak current. IC50s for the peak and steady current were 56.7 and 7.4 microM, when the anesthetic was coapplied with nicotine. Thiopental's inhibition was not associated with a change in the reversal potential and was voltage-independent at membrane potential of -30 to -70 mV. Most of thiopental's effects seemed to require channel opening. In contrast to the nicotine-induced current, thiopental had little effect on the current elicited by ATP. Thiopental, whose reported EC50 for general anesthesia is 25 microM, inhibited the neuronal nAchR-mediated current but not the P2X receptor-mediated response in PC12 cells at clinically relevant concentrations. Inhibition may result in suppression of synaptic transmission in sympathetic ganglia.

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...

Mechanism of Action of Local Anesthetics on Synaptic Transmission in the Rat

The mechanism of action of local anesthetics on synaptic transmission and their effects on synaptic components and on electrophysiologic properties of the nerve cell body are not clear. Therefore, the effects of lidocaine and bupivacaine on pre- and postsynaptic mechanisms underlying synaptic transmission in sympathetic ganglia were studied utilizing the techniques of intracellular recording and stimulation on isolated superfused superior cervical ganglia of rats. Lidocaine and bupivacaine either depressed or completely blocked synaptic transmission in sympathetic ganglia in a dose-dependent manner. Blockade of axonal conduction in presynaptic fibers was preceded by increased latency (the latency increased from 11.2 +/- 0.9 to 16.5 +/- 1.4 ms, mean +/- SEM, P less than 0.01) when the drugs were applied to the presynaptic nerves. Application of the drugs directly to the ganglion produced alterations in postsynaptic membrane properties consisting of decreased membrane resistance (from 40 +/- 3 to 32 +/- 3 M omega, P less than 0.01), increased firing threshold (from 14 +/- 0.5 to 18 +/- 0.5 mV, P less than 0.01), and decreased action potential amplitude (P less than 0.01) and/or blockade of action potential generation. Resting postsynaptic membrane potential did not change significantly. These changes were reversible. However, even after the excitatory postsynaptic potential resulting from presynaptic nerve stimulation had fully recovered during washout of the local anesthetic, the threshold for evoking the spike potential (firing level) still remained elevated for both presynaptic and intracellular stimulation of the ganglion cell, suggesting prolonged cell depression.(ABSTRACT TRUNCATED AT 250 WORDS)

Bispyridinium (oxime) compounds antagonize the “ganglion blocking” effect of pyridostigmine in isolated superior cervical ganglia of the rat

The "antidotal effectiveness" of several bispyridinium compounds (HGG 12, HGG 65, HGG 70, HI 6, HLö and HLö 12) against the acetylcholinesterase (AChE) inhibitor pyridostigmine was evaluated in isolated superior cervical ganglia of the rat. Compound action potential amplitudes were inhibited by pyridostigmine in a concentration-dependent manner. HI 6 and atropine proved to be the most effective compounds in antagonizing the "ganglion blocking" action of pyridostigmine. Their relative effectiveness (PE value) was 5.4 and 4.2, respectively. All of the six bispyridinium compounds inhibited carbachol-induced, nicotinic, ganglionic surface depolarizations. The antinicotinic potencies of HI 6 and HLö 7 were about one order of magnitude lower (apparent KI values: 294 and 330 mumol/l) than the antinicotinic potencies of HGG 12, HGG 65, HGG 70 and HLö 12 (apparent KI values ranging from 19 to 41 mumol/l). The antinicotinic potencies of the bispyridinium compounds did not correlate with their in vitro protection of synaptic transmission in sympathetic ganglia. Moreover, the effectiveness of atropine points to the importance of antimuscarinic properties of possible "antidotes" for the maintenance of ganglionic transmission in cases of AChE poisoning.

Brain natriuretic peptide receptors in the rat peripheral sympathetic ganglia

High affinity binding sites for brain natriuretic peptide were characterized in the rat superior cervical ganglia by quantitative autoradiography. In addition, the peptide increased the formation of cyclic GMP in the ganglia in vitro. Brain natriuretic peptide displaced atrial natriuretic peptide from its binding sites. Our results suggest that brain natriuretic peptide and atrial natriuretic peptide may share physiologically active receptors in sympathetic ganglia. Brain natriuretic peptide may modulate the synaptic transmission in sympathetic ganglia, in addition or in conjunction with atrial natriuretic peptide.

Plasticity of expression of a synaptic vesicle antigen in adult rat superior cervical ganglion

The effects of deafferentation and alterations of synaptic activity on levels of a synaptic vesicle-specific membrane protein (SV) were studied in the adult rat superior cervical ganglion (SCG) in vivo, using a monoclonal antibody directed against the protein. Levels of SV were quantified by radioimmunoassay. Deafferentation of the SCG results in a transient increase in SV levels in the SCG on days 7 and 10 after surgery, with levels then dropping below control levels on days 14, 21, and 30 after surgery. Immunohistochemical labeling of deafferented ganglia indicates that the increase is confined to the perikarya of principal ganglionic neurons. Levels of SV in an SCG target tissue, the iris, do not differ from control levels on day 7 after deafferentation, but are elevated at days 10, 14, and 30 after surgery. After reinnervation of the SCG, levels of SV in the SCG are elevated above control values, but do not differ from control values in the iris. Treatment with chlorisondamine, which blocks synaptic transmission in sympathetic ganglia, produces a significant increase in SV levels in the SCG after 7 d of treatment. Long-term chlorisondamine treatment results in reductions in SV in the SCG after 14 and 28 d. Treatment with phenoxybenzamine for 6 d, which reflexly increases synaptic activity, produces a marked decrease in SV in the SCG. These results suggest that activity, mediated by transsynaptic factors, contributes to the regulation of synthesis of a synaptic vesicle protein in the SCG. The results further suggest that accumulation of synaptic vesicles in terminals of the principal ganglion neurons may help regulate the maintenance of normal synaptic vesicle pools within sympathetic neurons.

Effects of kynurenate on root potentials evoked by synaptic activity and amino acids in the frog spinal cord

The effects of kynurenate (kyn) on synaptic- and excitatory amino acid-mediated responses in isolated, hemisected spinal cords of frog were examined. Kyn (0.5 mM) rapidly and reversibly blocked >90% of the synaptically mediated ventral root potential (VRP) produced by stimulation of the dorsal root. Spontaneous activities recorded from both ventral and dorsal roots were also reversibly blocked by Kyn. However, Kyn had no effect on action potentials or excitability per se, nor was it a general inhinitor of synaptic transmission since Kyn concentrations as high as 2.5 mM had no effect on synaptically mediated dorsal root potentials produced by stimulation of the ventral root. In addition, Kyn had no effect on synaptic transmission in sympathetic ganglia of frog. Although Kyn (2.5 mM) by itself produced no ventral root response in spinal cords treated with tetrodotoxin, it antagonized those induced by the excitatory amino acids N-methyl- d,l-aspartate, quisqualite, kainate, aspartate, and glutamate. The ventral responsed to all concentrations of quisqualate tested were depressed by 2.5 mM Kyn. In addition, when Kyn was washed out, the rate of recovery from Kyn block was accelerated by the presence of quisqualate. These results indicate that quisqualate and Kyn compete for common binding sites. However, low concentrations of Kyn (e.g. 0.1 mM) potentiated the peak of the response to saturating concentrations of quisqualate by as much as 30%. The durations of the potentiated quisqualate responses were significantly shorter than the control responses. Thus, Kyn does not act simply as a competitive inhibitor of quisqualate. Incontrast, Kyn appears to simply block N-methyl- d,l-aspartate responses with no signs of poptentiation or charges in kinetics.

Conduction of excitation through the superior cervical ganglion during early postnatal development

The action of hexamethonium, D-tubocurarine, phentolamine, and atropine on synaptic transmission in the superior cervical ganglion was studied in the early stage of postnatal development (1–8 days after birth) and in the adult period in cats, rabbits, and rats. Hexamethonium and D-tubocurarine, if injected intravenously or added to the Krebs' solution surrounding the ganglion, were shown to inhibit the conduction of excitation through the ganglion effectively in both newborn and adult animals. No significant difference in the action of phentolamine and atropine on synaptic transmission in the ganglia could be found in these groups of animals. It is concluded that synaptic transmission in sympathetic ganglia is cholinergic in the early stage of postnatal development of animals blind at birth.

Roles of cyclic nucleotides in the synaptic transmission in sympathetic ganglia of rabbits

1. Two kinds of slow postsynaptic potentials can be elicitable in the rabbit superior cervical ganglion: a dopaminergic hyperpolarizing (slow IPSP) one and a muscarinic depolarizing (slow EPSP) one, respectively. 2. Cyclic AMP may not be involved in the generation of slow IPSP, whereas it certainly mediates the dopamine-induced enduring modulatory enhancement of the slow EPSP response. 3. Cyclic GMP appears to mediate at least one of the components in the mechanism underlying the slow EPSP by generating a depolarization with no change in membrane conductance. 4. Cyclic GMP does additionally regulate the dopamine (or cyclic AMP)-induced modulatory enhancement of the slow EPSP by antagonizing it in a uniquely time-dependent fashion.