Vasopressin-mediated slow EPSPs in a mammalian sympathetic ganglion

Vasopressin is one of numerous neuropeptides contained in sympathetic ganglia, but whose function remains unresolved. In this report, we present electrophysiological evidence that arginine-vasopressin (AVP) is a neurotransmitter in guinea pig inferior mesenteric ganglion (IMG). AVP superfused over the IMG, in vitro, produced in a population of neurons a membrane depolarization accompanied by a resistance increase, both of which were blocked by a specific V1 receptor antagonist. Moreover, slow excitatory postsynaptic potentials (EPSPs) elicited by repetitive nerve stimulation were attenuated in 75% of cells tested in the presence of excess AVP, and occasionally in the presence of the antagonist. Thus, AVP joins substance P as a putative transmitter of slow potentials in the guinea pig IMG.     

Fast excitatory postsynaptic currents in voltage-clamped mammalian sympathetic ganglion neurones.

Fast excitatory postsynaptic currents (EPSCs) were recorded in voltage-clamped neurones of isolated superior cervical ganglion of the rabbit. The rise time, decay time and whole duration of EPSC, as well as miniature EPSC, were shorter than those of corresponding postsynaptic potentials. Characteristic impedance for EPSC was 5.5 +/- 1.1 M omega, and was a few times lower than for current evoked by iontophoretic application of ACh. The rise time of EPSC was 2.0 +/- 0.2 msec, the time constant of decay was 3.6 +/- 0.5 msec, and the mean amplitude of EPSC was -5.5 +/- 1.0 nA at the resting potential level (-53.8 +/- 1.4 mV) and at 36 degrees C. Amplitude of EPSC varied with membrane potential almost linearly at negative potentials, non-linearly at positive potentials, and nullified at -8.9 +/- 1.8 mV. The decay of EPSC was exponential over the most of its time course and the rate constant of decay (alpha) varied exponentially with membrane potential according to the relationship alpha(V) = B exp(AV), with A = 0.00716 +/- 0.00101 mV-1 and B = 0.46 +/- 0.07 msec-1. The voltage sensitivity of EPSC decay is interpreted in terms of voltage sensitivity in ionic channel lifetimes.     

Frog sympathetic ganglion cells have local axon collaterals

Amphibian autonomic ganglia have been used as simple models for studies involving the physiology of synaptic transmission. These models assume an anatomical simplicity where the ganglion is a simple relay for central nervous system output to peripheral autonomic targets. Cholinergic preganglionic fibers innervate the soma and proximal axon of the unipolar ganglion cells, which were thought to relay the information to the periphery with little ganglionic processing. However, several different types of synaptic potentials occur in response to preganglionic stimulation. Also, a variety of neuropeptides are found in both preganglionic fibers and ganglion cells; at least one of the peptides found in preganglionic fibers is known to act as a neurotransmitter in the ganglion. Finally, there may be communication between ganglion cells. In the present study, we have explored the morphology of lumbar sympathetic chain ganglion cells by intracellular injection with horseradish peroxidase to determine whether an anatomical substrate exists for processing information within these ganglia. We have shown that 39% of these cells have axons that branch within the ganglion. While both major classes of ganglion cells (B cells and C cells) had intraganglionic axon collaterals, there was a marked difference in the frequency: 65% of the C cell axons had collaterals while only 19% of the B cell axons collateralized within the ganglion. Ultrastructural examination of labeled axon collaterals indicated that these collaterals receive synaptic input; whether the collaterals also make synapses has not been definitively established.     

Distribution of muscarinic receptors in mammalian sympathetic ganglion: autoradiographic and electrophysiological studies

Localization of muscarinic receptors in rabbit superior cervical ganglion (SCG) neurons was investigated by means of light microscopy autoradiographic study of binding sites for the labeled selective muscarinic antagonist atropine, and intracellular recordings were made from single ganglionic neurons to determine their response to local iontophoretic application of the selective muscarinic agonist--5-methylfurmethide (MF). Autoradiography showed that muscarinic receptors are located predominantly on the dendrites of ganglion neurons and on their soma near large processes. The remaining soma areas are usually devoid of muscarinic receptors or their density is much lower than in dendritic areas. MF-elicited depolarization was seen in 34% of neurons studied; an initial hyperpolarization was followed by depolarization in 23% of the neurons and no responses occurred in the remaining 43%. Lowering Ca2+ and increasing Mg2+ content in the external medium abolished MF-hyperpolarization but not MF-depolarization. The following properties of MF-depolarization support the concept of dendritic localization of muscarinic receptors: (i) the time-course of MF-depolarization varies widely in different cells; (ii) its rise time increases with the increase of the MF dose; (iii) MF is more effective in inducing a MF-depolarization if it is applied to cell processes than if it is applied to cell soma. The origin of MF-hyperpolarization is discussed.     

Pharmacological properties and monoaminergic mediation of the slow IPSP, in mammalian sympathetic ganglion

Phenoxybenzamine can selectively eliminate the s-IPSP, in the presence of anti-cholinesterases that enhance s-IPSP and s-EPSP; and the α₂-antagonist, yohimbine, can partially but consistently depress s-IPSP selectively. The results provide positive pharmacological support for the monoaminergic nature of the transmitter for s-IPSP in mammalian sympathetic ganglia and argue against suggestions that the s-IPSP is a direct hyperpolarizing response to acetylcholine.     

Anomalous rectification in rat locus coeruleus neurons

Rat locus coeruleus neurons recorded under current clamp conditions show anomalous rectification (AR) as indicated by a progressive decrease in slope resistance measured with hyperpolarizing current pulses of increasing amplitude. AR was most prominent at potentials more negative than the K+ equilibrium potential. AR was strongly dependent on external K+ concentration and was blocked by external Ba2+ and Cs+, and intracellular injection of acetate.     

5-Hydroxytryptamine controls ACh-receptor sensitivity of bullfrog sympathetic ganglion cells

Experimental evidences showing that 5-hydroxytryptamine (5-HT) is directly interacting with nicotinic acetylcholine (ACh) receptors and thereby depresses the sensitivity of these receptors to ACh, are presented by making use of bullfrog sympathetic ganglion cells and frog skeletal muscle end-plates. It was suggested that 5-HT might decrease the affinity of ACh to nicotinic receptor sites, since the mode of 5-HT action was comparable to that ofd-tubocurarine action.     

Synaptic innervation of sympathetic ganglion cells in the bullfrog.

The synaptic innervation of the ganglion cells in the ninth and tenth paravertebral sympathetic ganglia of the bullfrog was investigated by histochemical and electron microscopic techniques and by intracellular recording. The neurons were unipolar and most ganglion cells were innervated by a single preganglionic axon. The preganglionic fiber stained for acetylcholinesterase and was observed to spiral around the axon hillock of the ganglion cell before arborizing and making synaptic contact with the neuron. Most synapses were located on the soma near the axon hillock region, with features typical for cholinergic junctions. The axosomatic location of the synapses was manifested physiologically by a decrease in membrane resistance (increased conductance) at the peak of the fast EPSP (excitatory postsynaptic potential) and by a demonstrable reversal potential for the fast EPSP.     

Morphological identification and systematic classification of mammalian retinal ganglion cells. I. Rabbit retinal ganglion cells

Retinal ganglion cells (RGCs) convey visual signals to 50 regions of the brain. For reasons of interest and convenience, they constitute an excellent system for the study of brain structure and function. There is general agreement that, absent a complete “parts list,” understanding how the nervous system processes information will remain an elusive goal. Recent studies indicate that there are 30–50 types of ganglion cell in mouse retina, whereas only a few years ago it was still written that mice and the more visually oriented lagomorphs had less than 20 types of RGC. More than 30 years ago, I estimated that rabbits have about 40 types of RGC. The present study indicates that this number is much too low. I have employed the old but powerful method of Golgi-impregnation to rabbit retina, studying the range of component neurons in this already well-studied retinal system. Close quantitative and qualitative analyses of 1,142 RGCs in 26 retinas take into account cell body and dendritic field size, level(s) of dendritic stratification in the retina's inner plexiform layer, and details of dendritic branching. Ninety-one morphologies are recognized. Of these, at least 32 can be correlated with physiologically studied RGCs, dye-injected for morphological analysis. It is unlikely that rabbits have 91 types of RGC, but is argued here that this number lies between 60 and 70. The present study provides a “yardstick” for measuring the output of future molecular studies that may be more definitive in fixing the number of RGC types in rabbit retina.     

Slow inhibitory postsynaptic potentials and hyperpolarization evoked by noradrenaline in the neurones of mammalian sympathetic ganglion

Slow IPSPs evoked in the neurones of rabbit isolated superior cervical ganglion by repetitive orthodromic stimulation, and a response evoked in the neurones of this ganglion by perfusion of noradrenaline, were studied using intracellular microelectrodes. Slow IPSPs were observed in 36% of neurones studied, and when investigated after treatment with D-tubocurarine and neostigmine, had a mean amplitude of 4.4 +/- 0.2 mV (mean +/- S.E.) and duration of 5 sec to 1.5 min. Two types of slow IPSPs occurring in different neurones were found. The slow IPSP of the first type was followed by a decrease in cell input resistance, was increased by depolarization and decreased by hyperpolarization of the membrane, with the reversal potential, if estimated by extrapolation method, equal to -77.8 +/- 3.3 mV. The slow IPSP of the second type was not followed by any change in cell input resistance, was increased by hyperpolarization and decreased by depolarization. The slow IPSP of the second type was reversibly blocked by phentolamine (1.4 X 10(-4) M). Noradrenaline (1 X 10(-4) M) evoked hyperpolarization or hyperpolarization followed by depolarization in 55% of the neurones studied. Hyperpolarization evoked by noradrenaline had a mean amplitude to 5.0 +/- 0.2 mV, was not followed by any change in cell input resistance, was reversibly blocked by phentolamine (1.4 X 10(-4) M), and was decreased by both depolarization and hyperpolarization of the cell membrane. It has been concluded that there are two groups of neurones in superior cervical ganglia, different with respect to the ionic mechanisms underlying the slow IPSP. In the first group of neurones the slow IPSP is probably due to an increase in potassium permeability of the membrane. The ionic mechanisms underlying the slow IPSP in the second group of neurones of noradrenaline-induced hyperpolarization remain unclear.     

Modulatory actions of ATP on membrane potentials of bullfrog sympathetic ganglion cells

Adenosine triphosphate (ATP) depolarized the membrane of bullfrog sympathetic ganglion cells by decreasing resting K+ conductance. ATP also depressed the maximum amplitude of after-hyperpolarization of action potentials. Voltage-clamp study revealed that ATP markedly suppressed the TEA-insensitive K+ current which appeared to correspond to the M-current, while it affected less significantly on the delayed rectifier K+ current. It was suggested that ATP depolarized resting membrane by suppressing resting K+ conductances, including the M-current, and also depressed the after-hyperpolarization of action potentials by suppressing both the M-current and delayed rectifier K+ current.     

Morphogenesis and territorial coverage by isolated mammalian retinal ganglion cells

Identified retinal ganglion cells were isolated from postnatal cat retinas and their dendrites were removed by trituration and centrifugation. The denuded cells were placed in a cell culture system and allowed to reexpress dendritic arbors in the absence of afferent input, target tissue, and interactions with neighboring ganglion cells. The retinal ganglion cells were grown above a feeder layer of astrocytes on glass coverslips equipped with paraffin pedestals. The spatial patterns of the reexpressed neurites were quantitatively analyzed using a number of measures, including an estimate of the Hausdorff dimension, H, which was used as a scale-independent metric for how well the neurite patterns filled in a restricted spatial domain. As assessed by the estimation of the Hausdorff dimensions, the neurites from a single cell achieve uniform coverage of a restricted territory independent of the total neurite length or the total number of inter-branch-point segments. A comparison with H values of ganglion cells from the intact retina revealed a similar trend. These results suggest that these cultured ganglion cells can express an intrinsic growth strategy for the uniform coverage of a restricted territory. The arbors expressed in the culture system displayed a limited range of diameters and exhibited morphology similar to the alpha-, beta-, and gamma-ganglion cells of the intact retina in the absence of afferent input or the influences of neighboring cells and target tissue. Time-lapse video data revealed that individual cultured cells showed extensive dendritic remodeling during their growth; however, after about 3 d in culture, this remodeling did not appreciably affect the territorial coverage of a cell. In the intact retina, the existence of dendritic sheets that independently and uniformly sample visual space may result from this intrinsic ability to elaborate dendrites that uniformly cover or fill in a restricted territory.