Dual transmission at morphologically mixed synapses: evidence from postsynaptic cobalt injections

A role of electrical inhibition in sensorimotor integration

Although it is accepted that extracellular fields generated by neuronal activity can influence the excitability of neighboring cells, whether this form of neurotransmission has a functional role remains open. In vivo field effects occur in the teleost Mauthner (M)-cell system, where a combination of structural features support the concept of inhibitory electrical synapses. A single spike in one M-cell evoked within as little as 2.2 ms of the onset of an abrupt sound, simulating a predatory strike, initiates a startle-escape behavior [Zottoli SJ (1977) J Exp Biol 66:243-254]. We show that such sounds produce synchronized action potentials in as many as 20 or more interneurons that mediate feed-forward electrical inhibition of the M-cell. The resulting action currents produce an electrical inhibition that coincides with the electrotonic excitatory drive to the M-cell; the amplitude of the peak of the inhibition is approximately 40% of that of the excitation. When electrical inhibition is neutralized with an extracellular cathodal current pulse, subthreshold auditory stimuli are converted into ones that produce an M-spike. Because the timing of electrical inhibition is often the same as the latency of M-cell firing in freely swimming fish, we conclude that electrical inhibition participates in regulating the threshold of the acoustic startle-escape behavior. Therefore, a field effect is likely to be essential to the normal functioning of the neural network.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

A fast synaptic potential mediated by NMDA and non-NMDA receptors

A fast synaptic potential mediated by NMDA and non-NMDA receptors. J. Neurophysiol. 78: 2693-2706, 1997. Excitatory synaptic transmission in the CNS often is mediated by two kinetically distinct glutamate receptor subtypes that frequently are colocalized, the N-methyl--aspartate (NMDA) and non-NMDA receptors. Their synaptic currents are typically very slow and very fast, respectively. We examined the pharmacological and physiological properties of chemical excitatory transmission at the mixed electrical and chemical synapses between auditory afferents and the goldfish Mauthner cell, in vivo. Previous physiological data have suggested the involvement of glutamate receptors in this fast excitatory postsynaptic potential (EPSP), the chemical component of which decays with a time constant of <2 ms. We demonstrate here that the pharmacological and voltage-dependent characteristics of the synaptic currents are consistent with glutamatergic transmission and that both NMDA and non-NMDA receptors are involved. The two components surprisingly exhibit quite similar kinetics even at resting potential, with the NMDA response being only slightly slower. Due to its fast kinetics and characteristic voltage dependence, NMDA receptor-mediated transmission at these first-order synapses contributes significantly to paired pulse and frequency-dependent facilitation of successive fast EPSPs during high-frequency repetitive firing, a presynaptic impulse pattern that induces activity-dependent homosynaptic changes in both electrical and chemical transmission. Thus NMDA receptor kinetics in this intact preparation are suited to its functional requirements, namely speed of information transmission and the ability to trigger changes in synaptic efficacy.

The first developing "mixed" synapses between vestibular sensory neurons mediate glutamate chemical transmission

In the present study, the nature of the synaptic transmission responsible for a monophasic potential generated by vestibular nerve stimulation of the principal cells in the chick tangential nucleus was established. This work was performed in slice preparations at the critical embryonic age of 15-16 days, the time of first observation of morphologically mixed (chemical and electrical) synapses at the axosomatic endings called spoon endings. The spoon endings are formed by the primary vestibular fibers with the largest diameters, the colossal vestibular fibers. This monophasic potential fits the criteria for chemical rather than electrical transmission due to the following responses in most cases: (i) the absence of collision between a direct spike initiated by depolarization in the principal cell and a vestibular-evoked action potential; (ii) failure to follow high frequency stimulation (up to 50 Hz); (iii) sensitivity to low calcium solution (0.1 mM). These tests indicate that strong electrical coupling between spoon endings and principal cells does not prevail at this stage. The recordings were obtained from principal cells injected intracellularly with biocytin, allowing their identification by morphological criteria. The lack of tracer coupling between the stained principal cells and their innervating vestibular fibers (n = 17) is consistent with the absence of electrical coupling. Identification of the neurotransmitter involved in this vestibular response was achieved by bath application of glutamate receptor antagonists, DL-2-amino-5-phosphonovaleric acid (40 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM), which blocked transmission reversibly. These results suggest that at the onset of formation of these "mixed" vestibular synapses, the gap junctions identified morphologically are likely not functional, and that the main response of the principal cells to vestibular nerve stimulation is mediated by glutamate.

Evidence for hemi-gap junctional channels in isolated horizontal cells of the skate retina

Prolonged depolarization of isolated, voltage-clamped skate retinal horizontal cells produces an outward current that exhibit a late onset and develops slowly with time. This current, which we refer to as the Q-current, is associated with an increase in membrane conductance, and is present when other voltage-gated conductances have been pharmacologically blocked. The reversal potential for the Q-current, obtained using tail current analysis, was close to 0 mV. The magnitude of the current was greatly reduced by superfusion with 25 mM acetate, and by 4 mM cobalt chloride, 2 mM 1-octanol, and a saturated solution of the general anesthetic halothane. In addition, the low-molecular weight fluorescent dye Lucifer yellow, applied extracellularly, entered the cells during activation of the Q-current, whereas a 3 kD dextran-fluorescein complex did not cross the cell membrane. The effects of divalent cations, the non-specific nature of the ionic current suggested by its reversal potential, the entry of Lucifer yellow, and the ability of acetate, halothane, cobalt, and octanol to block the current lead us to hypothesize that the Q-current results from the opening of hemi-gap junctional channels that mediate electrical coupling between skate horizontal cells.

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.

Post-embryonic development of rectifying electrical synapses in the crayfish: ultrastructure

The post-embryonic development of the rectifying Giant Fibre-Motor Giant (GF-MoG) synapse and the Giant Fibre-Segmental Giant (GF-SG) synapse has been investigated using electron-microscopy. In adults, the MoG and SG neurons make contact with the GFs by sending narrow 'finger-like' processes through the glial and connective tissue sheath surrounding each GF. The junctional region is characterized by closely apposed membranes (approximately 4 nm separation) traversed by regularly spaced connections, and large (60-80 nm) spherical vesicles in the presynaptic fibre. In newly hatched crayfish junctional contact is made over extensive areas of flat membrane apposition, due to the absence of a thick connective sheath around the giant fibres. Initially the junctional region is dominated by contacts which are morphologically indistinguishable from chemical synapses, i.e. 1. The apposed membranes are separated by a cleft of approximately 20-30 nm (an order of magnitude larger than the cleft distance at electrotonic synapses). 2. There is pre- and post-synaptic thickening of the junctional membranes with a dense cytoplasmic material. 3. Small (25-40 nm) pleomorphic vesicles are found on the presynaptic side of the junction, commonly in association with a dense presynaptic bar. Regions of junctional contact displaying the adult electronic-type morphology first appear at approximately one week post-hatching. At this age they are limited in distribution and occupy a central position in the area of contact surrounded by a broad 'chemical-like' annulus. During subsequent development these sites with electrotonic-type morphology grow in relative size, so that the 'chemical-like' sites become compressed towards the edges of the regions of contact. The adult type of morphology, in which the 'chemical-like' regions are vestigial, is achieved approximately two months after hatching.

Spinal inputs to the ventral dendrite of the teleost Mauthner cell

Ascending excitatory inputs from the periphery to the ventral dendrite of the goldfish Mauthner (M)-cell are characterized in this report. Direct stimulation of the spinal cord, at strengths suprathreshold for antidromic activation of the M-axon, evoked a graded excitatory postsynaptic potential (EPSP) in the distal ventral dendrite of the cell. This localization was demonstrated by multiple intracellular recordings from the soma and dendritic loci. The EPSP had a relatively long latency (mean = 3.6 ms) and contained multiple components. Furthermore, the EPSP amplitudes were extremely sensitive to frequency, being reduced by more than 50% at frequencies of 1-2 Hz and maximal with interstimulus intervals of 30-60 s. The spinal input is, therefore, likely to be mediated by a polysynaptic pathway. Direct stimulation of the skin surface evoked similar EPSPs, in terms of latency, wave form, graded nature, frequency dependence and spatial distribution on the M-cell ventral dendrite. Thus, the spinal cord and skin inputs probably relay somatosensory information from the trunk to the M-cell ventral dendrite. This notion was further confirmed by an interaction study of the EPSPs evoked from the two sites. We also report that the ventral dendrite does not support active spike electrogenesis, as indicated by the spatial profile of the M-cell antidromic impulse amplitude.

Some aspects of the structural organization of the spinal cord of Gymnotus carapo (Teleostei, Gymnotiformes). I. The electromotor neurons

The spinal electromotor neurons (EMNs) of Gymnotus carapo from a distinct column dorsal to the central canal. When massively retrograde-labeled with horseradish peroxidase, these neurons show a well-developed dendritic arborization. Dendrites run along the longitudinal axis of the cord and also project toward the dorsal gray and dorsolateral funiculi. Input to the EMNs is mediated by scarce synaptic contacts which show the fine structural characteristics of the so-called "morphologically mixed" junctions. Serial section reconstructions of the junctional areas revealed the occurrence of "gap junctions," dense membranes facing cumuli of microvesicles, and relatively large zones of undifferentiated membranes. Comparative electrophysiological and morphological data suggest that EMN activity may be mediated by electrical transmission. Since neither dendro-dendritic nor dendro-somatic junctions have been observed, other circuitry alternatives are proposed to account for the expected synchronized firing of the EMNs.

Freeze-fracture study of the large myelinated club ending synapse on the goldfish Mauthner cell: special reference to the quantitative analysis of gap junctions

The large myelinated club endings (LMCEs) of primary eighth nerve afferents form mixed synapses on the lateral dendrite of the giant Mauthner cell. The double replica freeze-fracture technique was employed to examine the intramembrane fine structure of these LMCE synapses. Morphological correlates of both chemical and electrical transmission were found at the LMCE synapses. Electrical synaptic junctions, or gap junctions, were located over much (10-20%) of the synaptic contact. These were seen in both pre-and postsynaptic membrane as tightly packed P face particle aggregates and corresponding aggregates of E face pits. Specializations characteristic of chemical synaptic junctions were most prominent at the periphery of the synaptic contact. These specializations consisted of postsynaptic E face particle aggregates which were subjacent to presynaptic active zones. The active zones were distinguishable as regions with an increased density of large particles and vesicle attachment sites represented by P face depressions and E face protuberances. Quantitative analysis of gap junction particle (connexon) number at five LMCEs revealed 24,000-106,000 connexons per LMCE. Comparison with data from electrophysiological studies of single LMCEs indicates that only a small fraction of the connexon channels are open at any given time during electrotonic transmission at an LMCE synapse.

Electrophysiological and morphological correlates of axotomy-induced deafferentation of the goldfish Mauthner cell

Axotomy-induced changes in afferent synapses to the goldfish Mauthner cell have been analyzed with intracellular recordings and with electron microscopy. The studies encompassed 7-208 days after cervical spinal cord transection. The physiological findings suggest a persistent and specific reduction in excitatory chemical inputs to the soma and proximal lateral dendrite, with no changes in somatic inhibition or in the electrotonic and chemical inputs to the more distal regions of the lateral dendrite. Corroborative morphological evidence includes swelling of the M-cell soma, as indicated by a 35% increase in the length of its minor diameter, an increased spacing and a quantitatively lower density of terminals on the soma, and the appearance of astrocytic processes partially or completely engulfing the terminals in that region. Similar changes were observed on the inferior dendrites projecting from the ventral surface of the soma, although these dendrites do not exhibit the chromatolytic changes observed at the soma. In contrast, there are no noticeable changes in either the synaptic investment of the lateral dendrite or its ultrastructure. Quantitative and qualitative data support the conclusion that there is a restricted and specific reduction in the proximal excitatory inputs to the M-cell. The evidence also suggests that electrotonic junctions between afferents and the M-cell remain intact, functionally and structurally.

Large myelinated club endings on the Mauthner cell in the goldfish. A study with thin sectioning and freeze-fracturing

Thin sectioning and freeze-fracturing have revealed the distribution of gap junctions and chemical synapses in the synaptic interface of the large myelinated club endings on the lateral dendrite of the goldfish Mauthner cell. In 12 samples of club endings fractured completely or nearly completely, the apposed synaptic membrane area averaged 39.090 microns2, of which 16.6% was occupied by gap junctions and about 4 to 5% by the active zones of chemical synapses. The numerical profile density (number per unit area of the synaptic membrane) of gap junctions varied greatly, from 1.78 to 6.30, and was mostly in inverse proportion to their size. The chemical synapses were located mainly in two places: in the circumferential rim of the synaptic membrane next to the widened extracellular space, and in the margins of intraterminal invaginations of the synaptic cleft. The axoplasm of the preterminal axon, just after losing its myelin sheath, was filled with microtubules, among which neurofilaments gathered into many small bundles. The correlation between the areas of gap junctions and the chemical synapses and the amplitude of the excitatory postsynaptic potentials (EPSP) is discussed.

Both electrical and chemical transmission between the 'lobula giant movement detector' and the 'descending contralateral movement detector' neurons of locusts are supported by electron microscopy

Conventional electron microscopy combined with cobalt staining techniques has revealed chemical synapses and gap junction-like areas denoting specific regions of contact between two large, uniquely identifiable visual interneurons in the brain of the locust Schistocerca gregaria. The morphological demonstration of chemical synapses suggests that one of the two neurons, the 'descending contralateral movement detector', receives a chemically mediated input from its main presynaptic element, the 'lobula giant movement detector'. This observation supports recent electrophysiological studies demonstrating synaptic delays between the two cells, characteristic of chemical synapses. However, regions with the appearance of gap junctions are also observed. This corroborates earlier work which suggested that these two neurons are coupled electrically.

Ultrastructural correlates of electrical-chemical synaptic transmission in goldfish cranial motor nuclei

The output connections of the cranial relay neurons, part of the Mauthner cell network, were examined in goldfish with light and electron microscopic techniques. Either lucifer yellow or horseradish peroxidase (HRP) was injected into cranial relay neuron axons to demonstrate that they diverge to several motor nuclei and to many motoneurons within one nucleus. Retrograde transport of the enzyme from injections of mandibular muscles was used to label the trigeminal motoneurons. In the electron microscope, cranial relay neuron processes were distinguished by the granular appearance of the electron-opaque polymer formed enzymatically by HRP, while the retrogradely labeled motoneurons had the polymer enclosed in lysosomes. The cranial relay neuron terminals contained many presynaptic vesicles which concentrated the HRP reaction product. Active zones and synaptic clefts were evident. At some synapses, both gap junctions and presynaptic vesicles were found. The mechanism of synaptic transmission was investigated by simultaneous recording with two intracellular microelectrodes from cranial relay neuron-motoneuron pairs. Composite postsynaptic potentials in a trigeminal motoneuron were evoked by intracellular stimulation of a cranial relay neuron axon. The earliest excitatory postsynaptic potential (EPSP) component had a latency of 0.25 msec and had a peak amplitude that was not depressed by repetitive stimulation. A second component had larger peak amplitudes which were reduced easily by repetitive stimulation. Antidromic action potentials were not transmitted from motoneurons to the cranial relay neuron axons. Thus, both electrical and chemical transmission probably occur at the cranial relay neuron-motoneuron synapses. Since the cranial relay neurons fire synchronously and receive excitatory chemical synapses, the function of the gap junctions and electrical transmission is unclear. Perhaps the importance of these gap junctions is more for transport of small molecules than for impulse transmission.

Cobalt-coupled neurons of a giant fibre system in Diptera

Certain intact nerve cells in flies can be filled with cobalt from presynaptic or postsynaptic neurons. This cobalt coupling is best demonstrated in giant fibre systems where the phenomenon was originally termed 'transsynaptic staining'. Fine structural analysis of silver-intensified, cobalt-coupled neurons indicates that the passage of cobalt ions occurs at gap junctions that are accompanied by conventional chemical synapses. Cobalt-coupled systems in dipterous insects are uniquely identifiable and can always be detected between the same kinds of neurons. The visualization of cobalt-coupled neurons allows the identification of functional pathways linking the brain to motor neuropils.

Synaptic excitation of the second and third order auditory neurons in the avian brain stem

Synaptic potentials were examined in the second- and third-order auditory neurons of nucleus magnocellularis and nucleus laminaris in the chick. Brain stems of mature chick embryos were explanted and maintained in vitro for 4 to 8 h. Field potentials, extracellular spike potentials and intracellular potentials evoked by 8th-nerve stimulation were examined. Eighth-nerve stimulation reliability elicited four identifiable field potentials which could be attributed to: (i) the afferent volley of the 8th-nerve axons, (ii) postsynaptic responses of n. magnocellularis neurons, and (iii) ipsilaterally and, (iv) contralaterally-evoked n. laminaris postsynaptic responses. Intracellular-recorded postsynaptic potentials were characterized by a rapid rise time and short duration. They were apparently monosynaptic with a synaptic delay of 0.4 ms. In each n. magnocellularis neuron the 'fast' excitatory postsynaptic potentials were composed of 1 to 3 all-or-none components. 'Slow' excitatory postsynaptic potentials were characterized by a longer latency, a longer duration and graded amplitude variation in proportion to the intensity of 8th-nerve stimulation. Both 'fast' and 'slow' excitatory postsynaptic potentials had similar reversal potentials. Since the 8th nerve makes monosynaptic connection with n. magnocellularis neurons, it is likely that at this synapse the 'fast' excitatory postsynaptic potentials were produced, while the 'slow' potential may be attributable to the convergence of many boutonal synapses of unknown origin. Intracellular injections of horseradish peroxidase into n. magnocellularis revealed that its efferents bifurcate below the nucleus and send one axon to the contralateral n. laminaris while the other axon forms a highly divergent projection to the ipsilateral laminar nucleus. The intracellular records obtained from n. laminaris are consistent with this anatomical finding in that graded excitatory postsynaptic potentials were elicited by 8th-nerve stimulation.

Synaptic organization in the pacemaker nucleus of a medium-frequency weakly electric fish, Eigenmannia sp

The medullary command nucleus (MCN) of the medium-frequency weakly electric fish, Eigenmannia sp., contains two types of neurones, namely large and small cells, which are embedded in a neuropile of large and small myelinated fibers. Using serial semi-thin and ultra-thin sectioning, combined with HRP labelling it has been established that both cell types possess rich dendritic arborization and large myelinated axons. Only the axons of the large cells leave the nucleus and these constitute the unique output of the MCN. Axon branching has been observed only in the axons of small cells and their collaterals show an exclusively intranuclear course. Two types of synaptic terminals have been found on large as well as on small cells: (1) large club endings forming both gap (electrotonic) junctions and polarized chemical synapses, which often appear at the same junction constituting morphologically mixed synapses; and (2) small bouton-like terminals forming exclusively chemical synaptic contacts. No differences between the two neuron types could be detected with respect to the arrangement of the synaptic contacts: club endings and small bouton-like terminals synapse on dendritic processes as well as on perikarya, while the unmyelinated initial segments were always found to be free of synaptic contacts. Large and small cells were found to be simultaneously connected by the same club ending or small bouton-like terminal: in the case of club endings by means of gap junctions and chemical synapses, whereas in the case of boutons by chemical synapses only. Club endings sometimes form gap junctions with each other. The possible role of these unusual synaptic connections in local synchronization is suggested. Club endings originate from the large axons of small cells, while small bouton-like terminals originate from the fine myelinated fibers of extranuclear origin. In Eigenmannia, small cells, being connected to large cells as well as to each other by axo-somatic and axodendritic synapses, can be considered as the pacemaker cells of the MCN whereas large cells are relay cells. Small bouton-like terminals may convey exogeneous impulses towards the MCN exerting modulatory effects at both pacemaker and relay cell levels. The greater variety of ultrastructural correlates established in the MCN of Eigenmannia, in comparison with Sternarchus (see also ref. 16), suggests increased modulation possibilities in the former fish's EOD behaviour.

Axotomy-induced changes in cell structure and membrane excitability are sustained in a vertebrate central neuron

The retrograde reactions of the goldfish Mauthner neuron to axotomy 8-10 mm caudal to its soma are detectable within a few weeks and persist for more than 200 days. Morphological changes include chromatolysis, reflecting a redistribution of cytoplasmic ribosomes, and infolding of the nuclear membrane. At the time, the normally inexcitable soma-dendritic membrane becomes capable of impulse initiation; this induced excitability also persists for at least 200 days.

Electrophysiological abnormalities in chronic epileptogenic foci: an intracellular study

Intracellular recordings were performed from chronic cobalt epileptogenic foci in rats. Two main features were found: (a) in 29% of the neurons, depolarizations of long duration occurred coinciding with cortical surface paroxysms; and (b) in 13% of the neurons, partial spikes, whose amplitude varied little with membrane DC potential changes, occurred. It is suggested that these phenomena have a dendritic origin, and possible mechanisms underlying the generation of paroxysmal events are discussed.

Transneuronal labeling with horseradish peroxidase in the visual system of the house fly

Horseradish peroxidase (HRP) is taken up by lesioned neurons in the fly CNS and passes from these into certain sets of adjoining neurons which are in synaptic contact. This transneuronal labeling resolves neurons in fine detail. Electron microscopy shows reaction product in secondary neurons associated with plasmalemma and microtubules. In some cases also vesicles were found containing reaction product. It is suggested that the transneuronal passage takes place in vivo.