Biomedical subjects

D S Faber

Publications and source records attributed to D S Faber.

At least 73 records · Page 4Linked to original sources

PubMed1987

Spontaneous quantal currents in a central neuron match predictions from binomial analysis of evoked responses.

Inhibitory postsynaptic currents occurring spontaneously in the teleost Mauthner cell were analyzed with the single-electrode voltage-clamp technique. They were collected during depolarizing steps and were outward-going; this procedure allowed them to be isolated from possible excitatory currents flowing in the opposite direction. Their amplitude histograms were found to exhibit regularly spaced multiple peaks, each of which had a Gaussian distribution of the same width. These compound inhibitory postsynaptic currents represent responses evoked by background firing of presynaptic neurons, and when tetrodotoxin was applied topically, only the first peak in the frequency histogram, which can be attributed to single exocytotic events, remained. The mean conductance of this quantal unit equalled 46.0 nS, which corresponds to the opening of 1000-2000 Cl- channels activated by glycine--the transmitter at these synapses. Its waveform and those of the larger units were essentially the same. Furthermore, in each set of data provided by a given Mauthner cell, the size of a quantum was quite constant, with its variance and those of further peaks being equivalent to that of background noise. These properties, which characterize the quantal events on the basis of spontaneous synaptic activity, were strikingly similar to those of the basic units derived by the simple binomial to those of the basic units derived by the simple binomial analysis of unitary postsynaptic potentials, thus validating the use of this statistical model to quantify the quantal nature of release at central connections. The quite straightforward method used here to extract single miniature currents from complex signals should be applicable to the other systems of the central nervous system, where the pertinence of this probabilistic model of release has yet to be demonstrated.

Animals↗

PubMed1987

Voltage-dependence of glycine-activated Cl- channels: a potentiometer for inhibition?

Single- and double-electrode voltage-clamp techniques have been employed in situ to analyze Mauthner cell inhibitory synaptic responses produced both by activation of the recurrent collateral network and by direct intracellular stimulation of single presynaptic interneurons. The results demonstrate that the synaptically evoked glycinergic postsynaptic currents exhibit a striking voltage sensitivity. Specifically, the time constant of the decay of the synaptic conductance is increased by depolarization and decreased by hyperpolarization. This parameter is exponentially related to membrane potential, changing e-fold for a 45 mV potential shift, regardless of the degree of intracellular chloride loading or the magnitude of the underlying synaptic conductance. In addition, the amplitude of this inhibitory conductance change is decreased by membrane hyperpolarizations of 15 mV or more. Computer modeling demonstrates that the voltage dependence of the kinetics of the synaptic response may serve to enhance the magnitude and duration of inhibitory responses appreciably in the face of increased excitation.

Animals↗

PubMed1986

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.

Afferent Pathways↗

PubMed1986

Instantaneous inward rectification in the Mauthner cell: a postsynaptic booster for excitatory inputs.

Single and double electrode voltage clamp techniques have been used to analyse the leakage conductance of the goldfish Mauthner cell. The results indicate that the input conductance of this neuron is maximal at the resting potential, having an average value of 4.68 microS. Approximately 50% of this conductance is voltage dependent and could be inactivated by large hyperpolarizing command pulses (30-50 mV magnitude) of 20-35 ms duration. The magnitude and apparent time constant of the inactivation are both functions of membrane potential, such that the relaxation of the inward current increased and occurred more rapidly for greater hyperpolarizations. The presence of instantaneous inward tail currents when the imposed voltage steps were terminated and membrane potential was returned to the resting level indicated that the conductance mechanism inactivated during hyperpolarization had generated a small outward current at the resting level. Thus, we propose that a major fraction of the Mauthner cell's input conductance is a voltage-dependent K+ conductance. Hyperpolarizing inactivation is a feature of an inward K+ rectifier in other cell types, and when the recording microelectrode was located in the Mauthner cell lateral dendrite, it was possible to demonstrate characteristics consistent with rectification, namely a low conductance for outward depolarizing currents and a high conductance for inward currents. The inward rectifier of the Mauthner cell is different from that of other neurons in that it is already maximally activated at the resting potential and therefore is a major determinant of that parameter. In addition, its activation and inactivation kinetics are quite fast.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1986

Probabilistic determination of synaptic strength.

This work was carried on to analyze the presynaptic components of synaptic efficacy, which is designated by the term of synaptic strength. To assess the relations between synaptic strength and innervation density, the properties of unitary Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) evoked in the potentials (IPSPs) evoked in the goldfish Mauthner (M-) cell by single impulses in individual presynaptic cells, including quantal release parameters, were compared with the histological features of the same neurons, determined from their reconstructions after intracellular injection with horseradish peroxidase (HRP). Because the M-cell is a stereotyped target neuron, comparison of the synaptic strength from different experiments was accomplished by using as a quantitative measure of this parameter the mean unitary IPSP amplitude normalized with respect to the reversal potential for Cl- (i.e., the driving force). In 108 experiments at low stimulus frequency, the majority of the normalized responses (63%) were grouped in a rather narrow range, varying about fourfold, or from 1.5 to 6% of the driving force, with results from stained (n = 46) and unstained (n = 62) cells being the same. In contrast, for the same restricted set of responses, the number of presynaptic terminals (histological n) encompassed a larger range, varying from 3 to 52. Impulses in neurons with quite different complements of terminal boutons could evoke similarly sized, normalized IPSPs, and these two parameters were poorly correlated, with there being, at most, a tendency for the responses to increase with histological n for small values of the latter. Quantal fluctuations in IPSP amplitudes were analyzed according to a binomial model having three parameters, p, which is the probability of release, n, which is the number of releasing units and was previously shown to equal the number of presynaptic boutons or active sites, and q, or the quantal size. The normalized quantal size varied randomly, with a mean value of 0.51% (SD = 0.20) and was relatively independent of n. In contrast, the distribution of p, which ranged from 0.17 to 0.74 (mean = 0.40, SD = 0.155), was skewed to the right; this parameter tended to decrease as a function of increasing n. The normalized unitary inhibitory conductance (g'IPSP) underlying an IPSP is equal to the product of npg'q, where g'q is the normalized quantal conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1986

Altered excitability of goldfish mauthner cell following axotomy. I. Characterization and correlations with somatic and axonal morphological reactions.

Axonal transection 7-10 mm distal to the cell body of the goldfish Mauthner (M) cell induced alterations in its excitability; namely, the antidromic spike recorded in the soma was converted from a single-component axon-hillock response to a larger amplitude, two-component impulse. The mean spike amplitude of the axotomized cells was approximately 50% greater (59.6 +/- 15.1 mV, n = 94) than that in controls (39.4 +/- 6.3 mV, n = 73). The onset of the induced increase in spike amplitude occurs at approximately 20 days postaxotomy, and the transition to a reactive spike is complete by approximately 30-35 days. Eighty-three percent of the M-cells axotomized for more than 30 days were physiologically reactive as judged by their large spike amplitudes and/or the presence of an additional spike component. Concomitant with the enhanced spike amplitudes, there was a depression of excitability in the initial segment-axon hillock region of the axotomized cells. This depression was suggested by a decrease in the initial segment (IS) spike height (from 39.4 +/- 6.3 mV, n = 73, in controls to 27.5 +/- 5.6 mV, n = 13, in axotomized cells), a decrease in its maximum rate of rise (from 153.6 +/- 24 V/s, n = 15, to 112.5 +/- 30 V/s, n = 29), and frequent failure of antidromic invasion into the initial segment and axon hillock. These changes in excitability could not be attributed to alterations in passive membrane properties, since the mean resting potential (77.8 +/- 5.2 mV, n = 37, control; 76.9 +/- 7.8 mV, n = 87, axotomized) and input resistance (170 +/- 21.3 K omega, n = 13, control; 176 +/- 26.6 K omega, n = 21, axotomized) were not altered significantly by axotomy. Threshold voltage was also unaffected (13.4 +/- 3.2 mV, n = 11, control; 11.9 +/- 2.5 mV, n = 11, axotomized). Sequential recordings of spike amplitudes from the axon hillock, soma, and lateral dendrite suggest that the generator of the axotomy-induced component is localized to the normally passive soma and proximal dendrite. In addition, the presumed soma-dendritic In addition, the presumed soma-dendritic component contributes very little if anything to the action potentials recorded in the axon. The onset and occurrence of alterations in excitability and cell body morphology (chromatolysis and nuclear associated changes) were compared in different M-cell populations and in the same identified M-cells. The comparisons suggested that these two events tend to occur in parallel.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1986

Altered excitability of goldfish Mauthner cell following axotomy. II. Localization and ionic basis.

The ionic basis and spatial localizations of spike generation were examined in normal and axotomized goldfish Mauthner (M-) cells using intra- and extracellular recordings and pharmacological manipulation of ionic conductances, including localized iontophoretic drug applications. Tetrodotoxin (TTX) abolished both the initial segment (IS) spike in normal cells and the larger, two-component action potential in axotomized cells, whereas calcium (Ca2+) blockers did not. Thus, sodium (Na+) appears to be the major inward current carrier in both cases. A shoulder or plateau following the fast-rising Na+-dependent action potential was unmasked in both normal and axotomized M-cells by intracellular injections of tetraethylammonium (TEA), either alone or in conjunction with 4-aminopyridine (4-AP) or cesium (Cs+). This plateau potential was abolished by superfusing with saline containing the Ca2 antagonists, Co2+, Mn2+, or Cd2+. However, barium (Ba2+), which normally substitutes for Ca2+ and also blocks K+ conductances, did not produce a plateau spike, and no action potentials could be evoked in the presence of TTX. Simultaneous extra- and intracellular recordings from the soma and lateral dendrite revealed that both the full-sized axotomized spike and its individual labile components were always maximal at the soma. These data support the earlier suggestion that the axotomy-induced electrogenicity is primarily localized to that region. Iontophoretic application of TTX inside the axon cap, a distinctive neuropil surrounding the initial segment and the axon hillock and circumscribed by a glial border, and at various positions along the lateral dendrite confirmed the Na+-dependency of the action potentials recorded in normal and axotomized cells and further demonstrated that the soma generates the additional spike component in the latter. The results suggest that axotomy causes a persistent change in voltage-gated Na+ channel distribution in the M-cell, with Na+ channels appearing or becoming more numerous in the soma while becoming less concentrated in the initial segment-axon hillock. Possible related shifts in other voltage-dependent conductances are also discussed. Finally, these are the first detailed studies of the ionic basis of axotomy-induced electrogenicity in a vertebrate neuron, central or peripheral, and the similarity to the results obtained with invertebrate neurons suggests common mechanisms underlying the axon reaction.

Action Potentials↗

PubMed1985

Evidence that receptors mediating central synaptic potentials extend beyond the postsynaptic density.

Physiological recordings and computer simulations of unitary inhibitory postsynaptic potentials in the Mauthner cell of the goldfish central nervous system have been used to estimate the expected size of the postsynaptic receptor matrix at individual junctions. Simultaneous pre- and postsynaptic recordings were used to determine the kinetic parameters of the quantal responses under normal conditions and in the presence of strychnine, a competitive antagonist of glycine, which is the putative transmitter at these synapses. Calculations indicate that if the postsynaptic density, which has a radius of 0.1 micron, were to accommodate the population of channels estimated to be opened during a quantal response, the glycine binding site density in that region would be unrealistically high. Computer simulation of the quantal responses included transmitter diffusion, transmitter-receptor interactions, and channel activation under conditions including both normal and lowered binding site densities, the latter corresponding to the experimental data obtained with strychnine. The data indicate that the synaptic receptors involved in generating unitary responses are widely distributed to include regions located outside the junctional area, which directly faces the presynaptic release sites. We further suggest that the receptor matrix is surrounded by a restricted diffusional space; this geometrical organization may underlie the finding that response rise times are relatively independent of receptor binding site densities.

Action Potentials↗

PubMed1984

The axon reaction of the goldfish mauthner cell and factors that influence its morphological variability.

The axon reaction of the goldfish Mauthner cell, elicited by spinal cord transection, included somatic swelling, nuclear eccentricity, chromatolysis, nuclear infolding, and a perinuclear buildup of basophilic material. The latter three changes were found most consistently and showed gradations which were ranked quantitatively. The time of onset of chromatolysis and nucleus-associated changes depended upon the distance of the wound from the Mauthner cell soma. Specifically, for Mauthner axons cut at 5, 10.5, and 20 mm distal to their somata, the approximate postoperative times of onset were 10, 20, and 40 days, respectively. Mauthner cells axotomized 42 mm distally did not display a consistent axon reaction. Cell atrophy and death were not found in cells axotomized 10.5, 20, or 42 mm from their somata up to 285 postoperative days, but were observed at the longer postoperative intervals (421 days) in neurons cut 5 mm distally and were consistently found in neurons axotomized less than 1.6 mm from their somata. The axon reactions of Mauthner cells within a pair were frequently different. This variability cannot be explained by the influence of cut site or postoperative interval and is hypothesized to result from different metabolic conditions of the individual cells.

Animals↗

PubMed1984

Measurement of myelin sheath resistances: implications for axonal conduction and pathophysiology.

As commonly understood, the myelin sheath of axons insulates the internodal axolemma and essentially restricts transmembrane currents to nodal regions. However, recordings obtained from within the myelin sheath showed that its apparent resistance to current generated by action potentials is similar in magnitude to that of the internodal axolemma. This suggests that the sheath does not appreciably limit transmembrane current flow, presumably because there is a longitudinal shunt under the myelin and through the paranodal region. Thus, in some demyelinating diseases and other axonopathies, the safety factor for impulse conduction may be lowered by a loosening or a reduction in the number of paranodal axoglial junctions.

Action Potentials↗

PubMed1984

Regulation of efficacy at central synapses.

The quantal nature of synaptic depression produced by high frequency stimulations has been analyzed at a central synapse for the first time. Simultaneous intracellular recordings were obtained from the Mauthner cell and adjacent identifiable inhibitory interneurons. The presynaptic cells were stimulated at frequencies from 2 to 33 Hz, and the corresponding release parameters were determined using a computational procedure described elsewhere (Korn, H., A. Triller, A. Mallet, and D. S. Faber (1981) Science 213: 898-901). As in our previous studies, these entities were correlated with histological features of the neurons following systematic horseradish peroxidase injections and reconstructions. Evidence was obtained that, in the range of physiological conditions used, the binomial parameter n (number of available quanta for release) remains constant; thus every synaptic bouton continues to function as an independent all-or-none releasing unit. The progressive reduction in amplitude of postsynaptic potentials can be attributed solely to a lower probability of release, as shown by the fall of the binomial parameter p. This evidence supports the concept that p is a critical variable for short-term modifications of synaptic efficacy and may provide insight for instances of synaptic plasticity underlying those behavioral changes which can be attributed to presynaptic loci. The present study also represents a necessary step toward linking mathematical variables of models for transmitter exocytosis with subcellular events.

Animals↗

PubMed1984

Localization of active sites along the myelinated goldfish Mauthner axon: morphological and pharmacological evidence for saltatory conduction.

Injections of Lucifer Yellow (LY) and horseradish peroxidase (HRP) were made within the myelin sheath of the goldfish Mauthner axon to determine the domains of individual oligodendrocytes. Long segments of the myelin sheath were stained with both markers. The lengths and locations of these sheath segments were analyzed in whole mount preparations (LY) or in reconstructions of serial vibratome sections (HRP). The termination sites of individual myelin sheaths, relative to gross anatomical landmarks of the brain, were consistent within and between all fish studied. In particular, the average locations of the termination sites were separated by 2.2 to 2.6 mm and corresponded to the brain regions where active site foci have been previously localized electrophysiologically. Individual sheath segments generally spanned the entire distance between adjacent active sites. The node-internode-node structure of the Mauthner axon that is suggested by these findings was further tested by ejecting tetrodotoxin (TTX) at various discrete rostral-caudal locations just outside the fiber. Large all-or-nothing components of the antidromic action potential were rapidly blocked (within seconds) only when the TTX ejections were made within a few hundred micrometers of the active site foci. The amplitudes of these blocked components are also consistent with predictions based upon previous electrophysiological analyses which demonstrated an active site spacing of 2.2 to 2.6 mm, a space constant of 5.0 mm, and a safety factor of 6 for impulse propagation. It is concluded from these morphological, pharmacological, and electrophysiological observations that the Mauthner axon possesses nodes separated by 2.2 to 2.6 mm and that a single oligodendrocyte spans an internodal region. Although nodal ultrastructure remains to be described, these results rule out the possibility that each of the short (approximately 50 micron), closely spaced (average separation = 155 micron) axon collaterals is a site of action current production.

Action Potentials↗

PubMed1983

Organized projection of the goldfish saccular nerve onto the Mauthner cell lateral dendrite.

Application of horseradish peroxidase (HRP) to the proximal end of the transected saccular nerve in the goldfish results in uptake by fibers projecting to the distal half of the Mauthner (M) cell's lateral dendrite. More discrete HRP injections reveal an organized projection from the saccular nerve onto the lateral dendrite, as small groups of stained fibers terminate there in restricted regions. The possibility that this represents a tonotopic projection onto the M-cell is considered.

Animals↗

PubMed1983

Granular coated vesicles in the goldfish Mauthner cell following axotomy.

Axotomy of the goldfish Mauthner cell induces a marked increase in the occurrence of granular coated vesicles in close association with axosomatic synapses. This phenomenon, which persists for over 200 days, is discussed in relation to other morphological and physiological changes which occur over the same time period.

Animals↗

PubMed1983