Morphological and physiological properties of the giant interneuron of the hermit crab (Pagurus pollicaris)

The physiological and morphological properties of the giant interneurons in the hermit crab Pagurus pollicaris are described. The cell bodies are located anteriorly in the supraesophageal ganglion, close to the mid-line. Each cell sends a neurite posteriorly and then laterally, so that they cross over in the center of the ganglion. Each axon then branches: one branch runs laterally while the other travels posteriorly and leaves the ganglion in the circumesophageal connective on the side contralateral to the cell body. The giant axons travel in the circumesophageal connectives and through the thoracic and abdominal ganglia without branching. Each giant axon makes synaptic contact with its ipsilateral giant abdominal flexor motor neuron and with a second flexor motor neuron that has its axon in the contralateral third root. In the supraesophageal ganglion there is a bidirectional synapse between the two giant interneurons. Intracellular recordings from the giant axons show that there is a delay of 0.5 to 0.75 ms that cannot be accounted for by spike propagation along the axons, and may be accounted for by a chemical synapse between the giant interneurons.     

Synaptic potential in the motor giant axon of the crayfish

Some electrical properties of the synapses between central giant axons (presynaptic) and the motor giant axon (postsynaptic) of the crayfish abdominal nerve cord have been investigated. Postsynaptic potential change in response to presynaptic volleys contains two components: a spike potential and a synaptic potential of very long time course. Amplitude of the synaptic potential is graded according to the number of active presynaptic axons. Conductance increase in the synaptic membrane endures over most of the period of potential change, and it is this rather than the "electrical time constant" of the membrane that in large measure determines the form of the synaptic potential. Temporal summation of synaptic potential occurs during repetitive presynaptic stimulation, and after such stimulation the rate of decay of synaptic potential is greatly slowed.     

Spider toxin (JSTX) on the glutamate synapse

A new neurotoxin (JSTX) was separated from spider (Nephila clavata, Joro spider) venom. JSTX irreversibly suppressed the excitatory postsynaptic potential (EPSP) and the glutamate potential in the lobster neuromuscular junction with high degree of specificity. The threshold concentration for suppressing EPSPs corresponds to a small fraction of the toxin in a venom gland, roughly estimated as low as 10(-10) M/l. 10(-10) M/l. In the giant synapse of squid stellate ganglion JSTX suppressed EPSPs without affecting the antidromic response. Glutamate-induced membrane depolarization was blocked by JSTX. In mammalian brain slice preparation, JSTX suppressed the orthodromic spike response but failed to affect on the antidromic spike in the hippocampal pyramidal neuron of CA1 and CA3 region. The above results strongly support the view that the squid giant synapse and synapses in the hippocampal pyramidal neuron are mediated by glutamate.     

Age-associated synapse elimination in mouse parasympathetic ganglia

Little is known about the effects of aging on synapses in the mammalian nervous system. We examined the innervation of individual mouse submandibular ganglion (SMG) neurons for evidence of age-related changes in synapse efficacy and number. For approximately 85% of adult life expectancy (30 months) the efficacy of synaptic transmission, as determined by excitatory postsynaptic potential (EPSP) amplitudes, remains constant. Similarly, the number of synapses contacting individual SMG neurons is also unchanged. After 30 months of age, however, some neurons (23%) dramatically lose synaptic input exhibiting both smaller EPSP amplitude and fewer synaptic boutons. Attenuation of both the amplitude and frequency of miniature EPSPs was also observed in neurons from aged animals. Electron micrographs revealed that, although there were many vesicle-laden preganglionic axonal processes in the vicinity of the postsynaptic membrane, the number of synaptic contacts was significantly lower in old animals. These results demonstrate primary, age-associated synapse elimination with functional consequences that cannot be explained by pre- or postsynaptic cell death.     

The pharmacology of an insect ganglion: actions of carbamylcholine and acetylcholine.

1. Methods for presenting dose-response data for the ganglionic actions of cholinergic agonists (e.g. carbamylcholine) are compared, using the mannitol-gap technique for electrophysiological recording of synaptic events at the cercal nerve, giant fibre synapse of the sixth abdominal ganglion of the cockroach Periplaneta americana. 2. At concentrations around 10(-5)M, carbamylcholine has no effect on ganglionic polarization but potentiates the monosynaptic EPSP. At 10(-4)M and higher concentrations, ganglionic depolarization is accompanied by a reduction of EPSP. 3. Pretreatment with eserine (10(-6) M) considerably shifts the dose-response curve for acetylcholine so that synaptic transmission is consistently sensitive to 10(-6) M acetylcholine.     

Age‐associated synapse elimination in mouse parasympathetic ganglia

Little is known about the effects of aging on synapses in the mammalian nervous system. We examined the innervation of individual mouse submandibular ganglion (SMG) neurons for evidence of age‐related changes in synapse efficacy and number. For approximately 85% of adult life expectancy (30 months) the efficacy of synaptic transmission, as determined by excitatory postsynaptic potential (EPSP) amplitudes, remains constant. Similarly, the number of synapses contacting individual SMG neurons is also unchanged. After 30 months of age, however, some neurons (23%) dramatically lose synaptic input exhibiting both smaller EPSP amplitude and fewer synaptic boutons. Attenuation of both the amplitude and frequency of miniature EPSPs was also observed in neurons from aged animals. Electron micrographs revealed that, although there were many vesicle‐laden preganglionic axonal processes in the vicinity of the postsynaptic membrane, the number of synaptic contacts was significantly lower in old animals. These results demonstrate primary, age‐associated synapse elimination with functional consequences that cannot be explained by pre‐ or postsynaptic cell death. © 2004 Wiley Periodicals, Inc. J Neurobiol 60: 214–226, 2004     

Simulation Studies of Vestibular Macular Afferent-Discharge Patterns Using a New, Quasi-3-D Finite Volume Method

A quasi-three-dimensional finite-volume numerical simulator was developed to study passive voltage spread in vestibular macular afferents. The method, borrowed from computational fluid dynamics, discretizes events transpiring in small volumes over time. The afferent simulated had three calyces with processes. The number of processes and synapses, and direction and timing of synapse activation, were varied. Simultaneous synapse activation resulted in shortest latency, while directional activation (proximal to distal and distal to proximal) yielded most regular discharges. Color-coded visualizations showed that the simulator discretized events and demonstrated that discharge produced a distal spread of voltage from the spike initiator into the ending. The simulations indicate that directional input, morphology, and timing of synapse activation can affect discharge properties, as must also distal spread of voltage from the spike initiator. The finite volume method has generality and can be applied to more complex neurons to explore discrete synaptic effects in four dimensions.     

THREE-DIMENSIONAL ULTRASTRUCTURE OF THE CRAYFISH NEUROMUSCULAR APPARATUS

The synapse-bearing nerve terminals of the opener muscle of the crayfish Procambarus were reconstructed using electron micrographs of regions which had been serially sectioned. The branching patterns of the terminals of excitatory and inhibitory axons and the locations and sizes of neuromuscular and axo-axonal synapses were studied. Excitatory and inhibitory synapses could be distinguished not only on the basis of differences in synaptic vesicles, but also by a difference in density of pre- and postsynaptic membranes. Synapses of both axons usually had one or more sharply localized presynaptic "dense bodies" around which synaptic vesicles appeared to cluster. Some synapses did not have the dense bodies. These structures may be involved in the physiological activity of the synapse. Excitatory axon terminals had more synapses, and a larger percentage of terminal surface area devoted to synaptic contacts, than inhibitory axon terminals. However, the largest synapses of the inhibitory axon exceeded in surface area those of the excitatory axon. Both axons had many side branches coming from the main terminal; often, the side branches were joined to the main terminal by narrow necks. A greater percentage of surface area was devoted to synapses in side branches than in the main terminal. Only a small fraction of total surface area was devoted to axo-axonal synapses, but these were often located at narrow necks or constrictions of the excitatory axon. This arrangement would result in effective blockage of spike invasion of regions of the terminal distal to the synapse, and would allow relatively few synapses to exert a powerful effect on transmitter release from the excitatory axon. A hypothesis to account for the development of the neuromuscular apparatus is presented, in which it is suggested that production of new synapses is more important than enlargement of old ones as a mechanism for allowing the axon to adjust transmitter output to the functional needs of the muscle.     

Transmission in squid giant synapses: the importance of oxygen supply and the effects of drugs

Synaptic transmission was studied in giant synapses of the stellate ganglion of the squid. When bathed in air-saturated sea water, the synapses deteriorate in 10 to 20 min.; if the sea water is saturated with 100 per cent oxygen, they function steadily for up to 12 hours. Optimal results probably require a medium with lower magnesium and higher calcium than the sea water used. Of eighteen compounds known to affect other synapses (Table I), none had stimulatory effects when applied to the preparation, but ten produced synaptic depression in concentrations of 10^–3 gm. per ml. or higher. The only exception was procaine, which blocked at 6 x 10^–5 gm. per ml. Intracellular recording with microelectrodes near the synapse showed that the block was associated with a slower rise of the excitatory post-synaptic potential, without a change in the depolarization required to initiate the spike. Procaine was exceptional in also increasing the depolarization at which the spike occurred.     

Fate of ganglionic synapses and ganglion cell axons during normal and induced cell death

In order to understand the significance of cell death in the formation of neural circuits, it is necessary to determine whether before cell death neurons have (a) sent axons to the periphery; (b) reached the proper target organs; and (c) have established synaptic connections with them. Axon counts demonstrated that, after sending out initial axons, ciliary cells sprouted numerous collaterals at the time of peripheral synapse formation. Subsequently, large numbers of axons were lost from the nerves, slightly later than the onset of ganglion cell death. A secondary loss of collaterals later occurred unaccompanied by cell death. Measurements of conduction velocity and axon diameters indicated that all ganglion cell axons grew down the proper pathways from the start, but it was not possible to determine whether all axons had actually formed proper synapses. This was ascertained, however, in the ganglion itself where preganglionic fibres were shown to synapse selectively with all ganglion cells before cell death. During this period, degenerating preganglionic synapses were observed on normal cells. It can therefore be inferred that at least some preganglionics established proper synapses before dying and that a single synapse is not sufficient to prevent cell death. In this system neither preganglionic nor ganglionic cell death seems designed to remove improper connections but rather to remove cells that have not competed effectively for a sufficient number of synapses, resulting in a quantitative matching up of neuron numbers.     

Differential Responses of Crab Neuromuscular Synapses to Cesium Ion

Excitatory postsynaptic potentials (EPSP's) generated in crab muscle fibers by a single motor axon, differ in amplitude and facilitation. Some EPSP's are large at low frequencies of stimulation and show little facilitation; others are smaller and show pronounced facilitation. When K⁺ is replaced by Cs⁺ in the physiological solution, all EPSP's increase in amplitude, but small EPSP's increase proportionately more than large ones. Quantal content of transmission, determined by external recording at single synaptic regions, undergoes a much larger increase at facilitating synapses. The increase in quantal content of transmission is attributable to prolongation of the nerve terminal action potential in Cs⁺. After 1–2 h of Cs⁺ treatment, defacilitation of synaptic potentials occurs at synapses which initially showed facilitation. This indicates that Cs⁺ treatment drastically increases the fraction of the "immediately available" transmitter store released by each nerve impulse, especially at terminals with facilitating synapses. It is proposed that facilitating synapses normally release less of the "immediately available" store of transmitter than poorly facilitating synapses. Possible reasons for this difference in performance are discussed.     

Synapse elimination from the mouse neuromuscular junction in vitro: A non-hebbian activity-dependent process

The effect of action potentials on elimination of mouse neuromuscular junctions (NMJ) was studied in a three compartment cell culture preparation. Axons from superior cervical ganglion or ventral spinal cord neurons in two lateral compartments formed multiple neuromuscular junctions with muscle cells in a central compartment. The loss of synapses over a 2–7-day period was determined by serial electrophysiological recording and a functional assay. Electrical stimulation of axons from one side compartment during this period, using 30-Hz bursts of 2-s duration, repeated at 10-s intervals, caused a significant increase in synapse elimination compared to unstimulated cultures (p< 0.001). The extent of homosynaptic and heterosynaptic elimination was comparable, i. e., of the 226 functional synapses of each type studied, 111 (49%) of the synapses that had been stimulated were eliminated, and 87 (39%) of unstimulated synapses on the same muscle cells were eliminated. Also, simultaneous bilateral stimulation caused significantly greater elimination of synapses than unilateral stimulation (p< 0.005). These observations are contrary to the Hebbian hypothesis of synaptic plasticity. A spatial effect of stimulus-induced synapse elimination was also evident following simultaneous bilateral stimulation. Prior to stimulation, most muscle cells were innervated by axons from both side compartments, but after bilateral stimulation, muscle cells were predominantly unilaterally innervated by axons from the closer compartment. These experiments suggest that synapse elimination at the NMJ is an activity-dependent process, but it does not follow Hebbian or anti-Hebbian rules of synaptic plasticity. Rather, elimination is a consequence of postsynaptic activation and a function of location of the muscle cell relative to the neuron. An interaction between spatial and activity-dependent effects on synapse elimination could help produce optimal refinement of synaptic connections during postnatal development. © 1993 John Wiley & Sons, Inc.     

Electrophysiological properties of neurons and neuronal organization of the crayfish somatogastric ganglion

Experiments with intracellular recording from neurons of the isolated crayfish somatogastric ganglion established that the membrane potential of the neurons is 53±3 mV. Single stimulation of the central branches of the ganglion evoked EPSP and a spike in the neurons. The spike amplitude was 7.5±0.6 mV. The small amplitude of the spike is explained by the fact that it arises at some distance from the body of the neuron and propagates electrotonically in it. Summation of several EPSP is necessary in most cases for initiation of the spike. When the orthodromic stimulus was strong enough, and IPSP occurred in some cells in addition to the EPSP and spike. Stimulation of the peripheral nerves of the ganglion induced in most neurons antidromic excitation and in some neurons orthodromic excitation. Some neurons spontaneously discharged rhythmically with an unstable frequency (11–27 impulses/sec). An investigation of the effect on neurons of chemical agents [acetylcholine, adrenalin, noradrenalin, gamma-aminobutyric acid (GABA), glutamic acid, and dopamine] showed that acetylcholine has the strongest and most stable depolarizing action and apparently is a synaptic transmitter in the ganglion. The other agents excited some neurons — depolarized them and evoked rhythmic discharges — and, coversely, hyperpolarized and suppressed the rhythmic activity of other neurons. A scheme of neuronal organization of the somatogastric ganglion of the crayfish is proposed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 307–313, May–June, 1970.     

Some observations on the fine structure of the giant synapse in the stellate ganglk of the squid,Doryteuphis bleekeri

Summary The fine structure of the synapse between the second-order giant fibre and the third order-giant fibre of the squid Doryteuphis bleekeri was studied by means of electron microscope. In the synaptic region, the two giant fibres are arranged side by side. Many small processes from the third-order giant fibre penetrate the common sheath which separats the adjacent giant axons making synaptic contact with the second order giant axon.The contact surface consists of opposing two plasma membranes of adjacent axons separated by a narrow space of 20–30 m in width. The synaptic membranes are more electron dense and thicker than the other part of the axon membrane. The synaptic vesicles are concentrated exclusively in the presynaptic axon.The fine structural differences between giant synapse in the stellate ganglion of the squid and the giant-to-motor giant synapse of the crayfish were discussed.This work was supported by Grant Number B-3348 from the National Institutes of Health, United States Public Health Service, Department of Health, Education and Welfare.     

Sensory expectations and anti-Hebbian synaptic plasticity in cerebellum-like structures

Cerebellum-like sensory structures in different groups of fish have been shown to generate a negative image of predictable features of the sensory input. We show here that anti-Hebbian plasticity is present at the synapse between parallel fibers and Purkinje-like cells which could mediate the generation of these negative images. We also show that this synapse is capable of bidirectional changes in synaptic efficacy with the direction of change depending on the precise temporal relation of presynaptic input and postsynaptic spike during pairing. Parallel fiber-evoked EPSPs are depressed after pairings in which the EPSP begins between 0 and 60 ms before the postsynaptic spike but are enhanced at other delays, including those in which the postsysnaptic spike occurs just before the EPSP.     

Electron microscope investigation of synapses,synapse-like structures,and possible sites of release of neurosecretory material in the buccal ganglia of Helix pomatia (Mollusca)

Summary In the buccal ganglia of Helix pomatia synapses and sites of possible release of neurosecretory material were investigated electron microscopically. There is one chemical synapse and one electrotonic synapse in the neuropile of the ganglion. No synapses could be detected in the buccal nerves, cerebro-buccal connectives, or in the buccal commissure. The synaptic cleft of the chemical synapse is about 25 nm wide and contains electron-dense material whereas the cleft of the electrotonic synapse is only 5 nm wide. The presynaptic fibre of the chemical synapse contains clear vesicles and dense core vesicles. The release sites of neurosecretory material are found at the initial segment of the axons, at perikarya of neurones, and at the perineurium of the ganglion. If the terminals are located at the plasmalemma of a nerve cell, these release sites are called synapse-like structures according to Roubos and Moorer-van Delft (1979). The synapse-like structures show all structural elements of synapses, except the 25 nm cleft containing dense material; the cleft is only 15–20 nm wide here like the normal cleft between neurones and glial cells or between two fibres. If the secretory material is released at the periphery through the perineurium the terminal is called synaptoid according to Scharrer (1970). In all cases, i.e. synapses, synapse-like structures, and synaptoids, clear vesicles were found in the axon terminal. This finding provides further evidence that clear vesicles always accompany the release of substances from axon endings.     

Neural pathways and innervation of cnidocytes in tentacles of sea anemones

Our previously published studies are here reviewed detailing neuro-cnidocyte synapses, demonstrating putative neurotransmitter substances, and identifying complex neural pathways in sea anemones. Synapses were traced to their contacts on nematocytes and spirocytes by transmission electron microscopy of serial thin sections of tentacles. In five animals, cells containing microbasic p-mastigophores had synapses with clear vesicles, whereas cells containing basitrichous isorhizas had synapses with dense-cored vesicles, providing preliminary evidence for a selectivity of neurotransmitter types for different nematocysts. Either clear or dense-cored synaptic vesicles were also present at neuro-spirocyte contacts. Antho-RFamide immunoreactivity occurred in some anthozoan synaptic vesicles and immunogold labeling of serotonin was found at a neuro-spirocyte synapse. Neural pathways included direct innervation of spirocytes by sensory cells, sequential neuro-neuro-spirocyte and neuro-neuro-nematocyte synapses and reciprocal synapses involving axons of both sensory cells and ganglion cells. Such synaptic patterns resemble neuro-effector pathways found in higher animals and lay to rest the independent effector hypothesis for cnidocyte discharge in tentacles of sea anemones.     

Spine calcium transients induced by synaptically-evoked action potentials can predict synapse location and establish synaptic democracy

CA1 pyramidal neurons receive hundreds of synaptic inputs at different distances from the soma. Distance-dependent synaptic scaling enables distal and proximal synapses to influence the somatic membrane equally, a phenomenon called "synaptic democracy". How this is established is unclear. The backpropagating action potential (BAP) is hypothesised to provide distance-dependent information to synapses, allowing synaptic strengths to scale accordingly. Experimental measurements show that a BAP evoked by current injection at the soma causes calcium currents in the apical shaft whose amplitudes decay with distance from the soma. However, in vivo action potentials are not induced by somatic current injection but by synaptic inputs along the dendrites, which creates a different excitable state of the dendrites. Due to technical limitations, it is not possible to study experimentally whether distance information can also be provided by synaptically-evoked BAPs. Therefore we adapted a realistic morphological and electrophysiological model to measure BAP-induced voltage and calcium signals in spines after Schaffer collateral synapse stimulation. We show that peak calcium concentration is highly correlated with soma-synapse distance under a number of physiologically-realistic suprathreshold stimulation regimes and for a range of dendritic morphologies. Peak calcium levels also predicted the attenuation of the EPSP across the dendritic tree. Furthermore, we show that peak calcium can be used to set up a synaptic democracy in a homeostatic manner, whereby synapses regulate their synaptic strength on the basis of the difference between peak calcium and a uniform target value. We conclude that information derived from synaptically-generated BAPs can indicate synapse location and can subsequently be utilised to implement a synaptic democracy.     

Fast recruitment of recurrent inhibition in the cat visual cortex

Neurons of the same column in L4 of the cat visual cortex are likely to share the same sensory input from the same region of the visual field. Using visually-guided patch clamp recordings we investigated the biophysical properties of the synapses of neighboring layer 4 neurons. We recorded synaptic connections between all types of excitatory and inhibitory neurons in L4. The E-E, E-I, and I-E connections had moderate CVs and failure rates. However, E-I connections had larger amplitudes, faster rise-times, and shorter latencies. Identification of the sites of putative synaptic contacts together with compartmental simulations on 3D reconstructed cells, suggested that E-I synapses tended to be located on proximal dendritic branches, which would explain their larger EPSP amplitudes and faster kinetics. Excitatory and inhibitory synapses were located at the same distance on distal dendrites of excitatory neurons. We hypothesize that this co-localization and the fast recruitment of local inhibition provides an efficient means of modulating excitation in a precisely timed way.