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M V Bennett

Publications and source records attributed to M V Bennett.

At least 127 records · Page 7Linked to original sources
PubMed1984

Determination of synaptic phenotype: insulin and cAMP independently initiate development of electrotonic coupling between cultured sympathetic neurons.

Electrotonic coupling between pairs of sympathetic neurons dissociated from superior cervical ganglia of neonatal rats is rare when cells are cultured for 2 weeks in a nutrient medium plus serum and is common when cells are cultured for the same period in serum-free defined medium. This defined medium is the same nutrient medium with five added factors (progesterone, transferrin, putrescine, insulin, and selenium). When added singly to serum-containing medium, insulin and, to a lesser extent, selenium promote the development of electrotonic and dye coupling. The insulin effect is obtained with doses as low as 0.01 microgram/ml and is maximal after exposures from 3 to 5 days. The incidence of electrotonic coupling is also enhanced by exposure of cells to dibutyryl cAMP. This effect is obtained with doses as low as 0.1 mM, is faster (being maximal at approximately equal to 12 hr exposure), and is prolonged in the presence of the phosphodiesterase inhibitor caffeine. Butyrate itself promotes coupling to a small extent, but cAMP involvement is confirmed by similar effects of other membrane permeant analogues. Endogenous levels of cAMP are significantly elevated in cultures grown in the defined medium but not in those in serum-containing medium to which insulin or selenium are added. We conclude that the promotion of coupling by cAMP and by insulin or selenium are independent. The development of coupling in the defined medium thus seems to be a consequence of the addition of promoting substances (insulin, selenium) and the removal of an inhibitory effect of serum on cAMP levels.

Animals↗
PubMed1984

Substituted benzyl acetates: a new class of compounds that reduce gap junctional conductance by cytoplasmic acidification.

Conductance of gap junctions in many preparations has been shown to be sensitive to cytoplasmic pH, decreasing as pH decreases below 7.5 in fish and amphibian embryos and below 7.1 in crayfish septate axon. We have found a new class of compounds, benzyl acetate derivatives, that reversibly decrease junctional conductance, gj, when applied in low concentration (approximately 1 mM). Simultaneous intracellular pH (pHi) measurements show that the ester effects are attributable to reduction in pHi. The sensitivity of gj to these compounds and the relative lack of side effects make these agents attractive for studies of the role played by gap junctions in normal tissue function. In addition, the finding of cytoplasmic acidification in response to cell exposure to esters suggests caution in interpretation of results obtained using esterified compounds for intracellular loading.

Animals↗
PubMed1983

Gap junctions and septate-like junctions between neurons of the opisthobranch mollusc Navanax inermis.

The buccal ganglia of Navanax inermis were studied by thin section, lanthanum infiltration and freeze fracture. Freeze fracture clearly demonstrates small gap junctions between neuronal processes in the neuropil, many of which are known to be electrotonically coupled. Junctional particles cleave with the P-face. In thin section, gap junctions appeared as small blurred contacts, presumably because of the small size of the junctions. Lanthanum infiltration was poor and failed to aid in identifying gap junctions. However, it did reveal septate-like junctions whose septa were not osmiophilic. Corresponding E-face grooves and ridges were seen in freeze fracture, sometimes adjacent to gap junctions. The septate-like junctions have parallel membranes and may have been mistaken for gap junctions in several other thin section studies of invertebrate neurons.

Animals↗
PubMed1983

Control of intercellular communication by voltage dependence of gap junctional conductance.

The junctional conductance between coupled amphibian blastomeres exhibits a high degree of voltage dependence, as previously described in voltage clamp studies (Spray, D.C., A.L. Harris, and M.V.L. Bennett (1981) J. Gen. Physiol. 77: 77-95; Harris, A.L., D.C. Spray, and M.V.L. Bennett (1981) J. Gen. Physiol. 77: 95-117). The present study examines the properties which this voltage dependence confers on electrotonic coupling between cells. The effects of applied pulses and ramps of current are studied experimentally and are modeled by calculation. During sufficiently large current pulses applied to one cell of a pair, the cells uncouple and then recouple after termination of the pulses. Ramps of current applied to one of the cells can give voltage-current (V-I) relations with a region of hysteresis within which the cells are stably coupled or stably uncoupled depending on previous history. Intrinsically generated currents are able to cause bistability of coupling in the absence of externally applied current. Calculations from the parameters of junctional conductance defined under voltage clamp fully account for these findings and illustrate how junctional and nonjunctional conductances affect the V-I relations in the region of bistability. Recordings from several cells within a small group show that boundaries of intercellular communication can be altered by applied current, a finding that also can be accounted for by voltage dependence of junctional conductance. The "Appendix" examines quantitatively the criteria required for bistability of coupling and the relevance of bistability for intercellular signaling. The plasticity of coupling which the voltage dependence of junctional conductance confers on cells offers an intriguing mechanism by which patterns of intercellular communication could be determined and changed in developing tissues.

Ambystoma↗
PubMed1983

alpha-Bungarotoxin labeling and acetylcholinesterase localization at the Mauthner fiber giant synapse in the hatchetfish.

Autoradiographic and histochemical techniques have been used to characterize further the pharmacology of transmission at the Mauthner fiber giant synapse of the South American hatchetfish. [125I]alpha-Bungarotoxin was applied to hatchetfish medullae and a standard autoradiographic procedure was carried out on 3- to 4-microns sections of glutaraldehyde-fixed tissue. All Mauthner fiber giant synapses, as identified by light microscopic criteria, had closely associated silver grains. Labeling was blocked by d-tubocurarine. Glutaraldehyde-fixed slices of hatchetfish medulla were stained histochemically for acetylcholinesterase; all giant synapses that could be identified in the light microscope showed heavy deposits of reaction product. Staining was blocked by diisopropyl-fluorophosphate, which inhibits both pseudocholinesterase and acetylcholinesterase, but was not blocked by tetraisopropylpyrophosphoramide, a specific pseudocholinesterase inhibitor. This evidence strongly supports the suggestion that the Mauthner fiber giant synapse is nicotinic cholinergic.

Acetylcholinesterase↗
PubMed1983

The electromotor system of the stargazer: a model for integrative actions at electrotonic synapses.

The electric organs of Astroscopus are modified from extraocular muscles and innervated by the enlarged oculomotor nuclei. The electromotor neuron somata are contacted by fine processes with which they form gap junctions. Presynaptic vesicles and active zones are also present, although physiological data give no indication of chemically mediated transmission. Antidromic stimulation produces long lasting graded depolarizations in the electromotor neurons. The latency is sufficiently short to indicate that the cells are electrotonically coupled, which was confirmed by direct measurement. Antidromic invasion may normally fail and is easily blocked by hyperpolarization revealing initial segment and axon spikes. Spinal stimulation evokes postsynaptic potentials (PSPs) and orthodromic impulses; the PSPs are not smoothly graded in amplitude. A medullary nucleus innervates the electromotor nucleus; the medullary cells also show short latency graded antidromic depolarizations and presumably are electrotonically coupled. Their coupling accounts for the variability in PSPs evoked by spinal stimulation. Apparent time constants differ greatly for direct stimulation of a single cell, decay of afterhyperpolarization, electrotonic spread from one cell to a neighbor, and decay of PSPs and graded antidromic depolarizations. The differences can be accounted for in terms of a highly interconnected network of electrotonically coupled cells, which was simulated computationally. Because of the long membrane time constant graded antidromic depolarizations summate. Because antidromic invasion is facilitated by depolarization, the antidromic depolarizations can show pronounced facilitation. The observed "plasticity" within this electrotonically coupled system provides a model for integrative actions at other sites of coupling.

Afferent Pathways↗
PubMed1982

Gap junctional conductance: comparison of sensitivities to H and Ca ions.

One cytoplasmic aspect of the junctional membrane between coupled pairs of Fundulus blastomeres was perfused with solutions of known H and Ca ion concentrations. Conductance of junctional membrane was decreased by either ion. The sensitivity to H ions was about 10,000 times greater than that to Ca ions. The results suggest that junctional conductance can be modulated by changes in H ion concentration near physiological pH, but that unphysiologically high concentrations of Ca ion, such as would be reached only on cell death, are required for comparable changes in junctional conductance.

Animals↗
PubMed1981

Gap junctional conductance is a simple and sensitive function of intracellular pH.

The pH of the cytoplasm (pHt) measured with pH-sensitive microelectrodes in cleavage-stage blastomeres of amphibian (Ambystoma) and teleost (Fundulus) embryos is about 7.7. In electrotonically coupled cell pairs, junctional conductance is rapidly and reversibly reduced by acidification of the cytoplasm. The relation between junctional conductance and pHi is the same for increasing and decreasing pH and is independent of the rate of change over a wide range. The relation is well fitted by a Hill curve with K = 50 nM (pK = 7.3) and n = 4 to 5. The closure of gap junction channels at low pHi appears to be a cooperative process involving several charged sites. The absence of hysteresis and identity of effects for fast and slow pHi changes implies that protons act directly on the channel macromolecules and not through an intermediate in the cytoplasm.

Ambystoma↗
PubMed1981

Equilibrium properties of a voltage-dependent junctional conductance.

The conductance of junctions between amphibian blastomeres is strongly voltage dependent. Isolated pairs of blastomeres from embryos of Ambystoma mexicanum, Xenopus laevis, and Rana pipiens were voltage clamped, and junctional current was measured during transjunctional voltage steps. The steady-state junctional conductance decreases as a steep function of transjunctional voltage of either polarity. A voltage-insensitive conductance less than 5% of the maximum remains at large transjunctional voltages. Equal transjunctional voltages of opposite polarities produce equal conductance changes. The conductance is half maximal at a transjunctional voltage of approximately 15 mV. The junctional conductance is insensitive to the potential between the inside and outside of the cells. The changes in steady-state junctional conductance may be accurately modeled for voltages of each polarity as arising from a reversible two-state system in which voltage linearly affects the energy difference between states. The voltage sensitivity can be accounted for by the movement of about six electron charges through the transjunctional voltage. The changes in junctional conductance are not consistent with a current-controlled or ionic accumulation mechanism. We propose that the intramembrane particles that comprise gap junctions in early amphibian embryos are voltage-sensitive channels.

Ambystoma mexicanum↗
PubMed1981

Kinetic properties of a voltage-dependent junctional conductance.

We have proposed that the gap junctions between amphibian blastomeres are comprised of voltage-sensitive channels. The kinetic properties of the junctional conductance are here studied under voltage clamp. When the transjunctional voltage is stepped to a new voltage of the same polarity, the junctional conductance changes as a single exponential to a steady-state level. The time constant of the conductance change is determined by the existing transjunctional voltage and is independent of the previous voltage. For each voltage polarity, the relations between voltage, time constant, and steady-state conductance are well modeled by a reversible two-state reaction scheme in which the calculated rate constants for the transitions between the states are exponential functions of voltage. The calculated rate constant for the transition to the low-conductance state is approximately twice as voltage dependent as that for the transition to the high-conductance state. When the transjunctional voltage polarity is reversed, the junctional conductance undergoes a transient recovery. The polarity reversal data are well modeled by a reaction scheme in which the junctional channel has two gates, each with opposite voltage sensitivity, and in which an open gate may close only if the gate in series with it is open. A simple explanation for this contingent gating is a mechanism in which each gate senses only the local voltage drop within the channel.

Ambystoma mexicanum↗
PubMed1980

Synaptic organization of expansion motoneurons of Navanax inermis.

The opisthobranch mollusc, Navanax, feeds by rapid pharyngeal expansion that sucks in prey followed by peristaltic swallowing that moves prey into the esophagus. Several identifiable neurons on the ventral surface of the buccal ganglia control radial musculature within the pharyngeal wall, contraction of which leads to pharyngeal expansion. These are considered expansion motoneurons because their axons run into the muscle and twitches and EMGs occur one for one with action potentials. The motoneurons are electrotonically coupled. Electrotonic PSPs, the components of spread associated with impulses, can summate with subthreshold DC depolarizations to yield synchronous impulses in coupled cells. During a train of responses the later electrotonic PSPs can be facilitated because of increase in amplitude and duration of the presynaptic impulses. Expansion motoneurons are synaptically connected by two apparently interneuronal pathways: a low threshold pathway activated by subthreshold depolarization of the two largest expansion motoneurons (the G-cells) that inhibits the entire population, and a high threshold pathway that is activated by a train of G-cell impulses and produces largely excitatory PSPs in the smaller expansion motoneurons and an EPSP--IPSP sequence in the G-cells. Coupling among expansion motoneurons can be abolished by chemical inhibitory synaptic inputs that are activated by electrical stimulation of the pharyngeal nerve or tactile stimulation of the pharyngeal wall. This uncoupling phenomenon can be explained by a simple equivalent circuit in which inhibitory synapses along the coupling pathway short circuit electrotonic spread. Uncoupling can outlast the evoking stimulus by several seconds. During uncoupling the smaller expansion motoneurones can fire independently while the G-cell is inhibited, and impulses still propagate from somata to the periphery. The expansion motoneuron population receives excitatory input from the mechanoreceptors in protractor muscles. Mechanical stimulation of the pharyngeal wall activates primary sensory neurons in the buccal ganglia that fire during excitation and during inhibition and uncoupling of expansion motoneurons.

Animals↗
PubMed1980

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish, Sternarchus albifrons.

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adrendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 micrometers in diameter) and large neurons (approx. 30 micrometers in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called "relay cells" of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to "pacemaker cells." The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the elctromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and SV-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.

Animals↗
PubMed1980

Peripheral fields and branching patterns of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis.

A population of about 75 primary sensory cells were identified on the dorsal surface of each buccal ganglion of Navanax. Each sensory cell possesses at least one mechanosensitive field in the pharyngeal wall or lips that correlates somatotopically with its position in the buccal ganglia. Many cells had additional fields that could be widely separated, requiring that their afferent processes branch. Cells were found with processes in more than one buccal nerve and with multiple processes in the same nerve. Hyperpolarization of the soma or repetitive electrical or physiological stimulation could cause failure of centripetal propagation of impulses. Impulses initiated in different branches could fail at different distances from the soma. Axon spikes that fail to invade the soma may or may not invade other branches. Axon spikes in separate branches that fail to invade the soma can summate to initiate an invading impulse. These findings suggest that integration of information from different branches may occur in a single sensory neuron.

Animals↗
PubMed1980

Synaptic connections of buccal mechanosensory neurons in the opisthobranch mollusc, Navanax inermis.

Mechanical stimulation of various areas of the pharyngeal wall and lips can produce EPSPs and IPSPs, as well as abruptly rising impulses, in primary sensory cells. IPSP fields are generally larger than EPSP fields and these fields are distributed without obvious order around fields from which afferent spikes are evoked. Apparently monosynaptic excitatory and inhibitory contacts are formed between primary sensory neurons. These synapses are blocked by high Mg2+ indicating chemical transmission. IPSPs are inverted by Cl- injection. Excitatory inputs can be electrically far from the soma. Sensory cells form apparently monosynsptic excitatory or inhibitory contacts on motoneurons mediating pharyngeal expansion. Brief sensory excitation can initiate sustained firing within this neuronal population and sustained synaptic activity in motoneurons. Interactions of sensory neurons may be important in information processing and in generating motor paterns. These neurons serve both primary sensory and interneuronal functions.

Animals↗
PubMed1980

Gap junctions in early amphibian embryos.

Cell pairs from late cleavage and early blastula Ambystoma mexicanum embryos were found to be electrotonically coupled. Thin-section electron microscopy revealed typical gap junctions between the cells. Freeze-fracture electron microscopy showed the gap junctions to be comprised of aggregations of 8.5-nm P-face particles and corresponding E-face pits. The conductance of the amphibian embryonic gap junction is known to be voltage-dependent, but no obvious gating structure was resolved by these morphological techniques.

Ambystoma mexicanum↗
PubMed1980

Formation of gap and tight junctions between reaggregated blastomeres of the killifish, Fundulus.

Blastomeres from eggs of the killifish, Fundulus, were mechanically dissociated and reaggregated by pelleting in a simple saline solution. Formation of gap and tight junctions was followed by electron microscopy of freeze-fracture replicas. Five to eight min after pelleting, neither new nor old junctions were observed. After 10-15 min, small gap junctions were found, but these were not associated with distinct formation plaques. Larger gap junctions were observed after 45 min, and the images were consistent with growth by accretion of intramembrane particles. In aggregates, after 20 min or more, tight junctions were much more commonly found than in intact blastulae, and it seemed likely that they were being formed by cells that were not doing so in the intact embryo. Initial stages consisted of short strands that appeared to grow in length. Also, more elaborate junctions were seen than occur in situ. Particle-free membrane often occurred near incomplete junctions, and large junctions like those in situ separated particle-rich from particle-free membrane. In this system, the formation of both gap and tight junctions occurs with shorter latency, and is more precisely timed, than heretofore described.

Animals↗