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L S Benardo

Publications and source records attributed to L S Benardo.

35 records · Page 2Linked to original sources
PubMed1984

Carbon dioxide sensitivity of dye coupling among glia and neurons of the neocortex.

Among neocortical astrocytes and neurons, intracellular injection of the fluorescent dye Lucifer Yellow CH into single cells will often label multiple adjacent cells. It is possible that this intercellular dye movement occurs through gap junctions, which in several systems are sensitive to cytoplasmic acidification. In the present study we tested the effect of increased CO2 levels, a treatment expected to decrease intracellular pH (pHi), on the prevalence of glial and neuronal dye coupling in neocortical slices. When CO2 levels were raised to 40% or 50%, dye coupling among astrocytes was completely abolished. Under the same conditions, the prevalence of dye coupling among neocortical neurons of adult guinea pigs was significantly reduced to 18.7%, compared to the control level of 32.3%. Dye coupling among immature rat neocortical neurons, which normally occurs at relatively high rates, was not measurably affected by CO2 levels up to 50%. The results suggest that coupling between cortical glia is very sensitive to cytoplasmic acidification. The relative insensitivity of neuronal coupling to CO2 may indicate either that the conductance of neuronal gap junctions is only weakly affected by pHi or that neuron-to-neuron dye passage occurs via other types of intercytoplasmic pathways.

Animals↗
PubMed1983

Coupling between neurons of the developing rat neocortex.

We have estimated the prevalence of coupling between neurons of the rat neocortex during postnatal development. Single intracellular injections of the fluorescent dye Lucifer Yellow CH resulted in dye coupling among 70% of neurons from 1 to 4 days of age. Dye coupling dropped to 30 to 40% by 10 to 18 days and occurred in 20% of injected adult neurons. The number of neurons per dye-coupled aggregate also decreased. Whereas three to seven coupled neurons were common in cortex of 1 to 4 days, aggregates of more than two neurons were exceptionally rare in adults. The frequency of dye coupling did not vary systematically with cortical depth at any age. When chemical synaptic activity was blocked, most 4-day neurons exhibited short latency antidromically evoked depolarizations which were relatively insensitive to repetitive activation and membrane polarization. These depolarizations may represent electrotonically conducted spikes from coupled neurons. No such potentials were found in adult neurons. The results suggest that neuronal coupling is extensive in immature rat neocortex, but that coupling declines at a time just before the numbers of chemical synapses increase most rapidly.

Aging↗
PubMed1982

Cholinergic excitation of mammalian hippocampal pyramidal cells.

Responses of CA1 pyramidal neurons to ACh were recorded with intracellular microelectrodes utilizing the in vitro guinea pig hippocampal slice preparation. ACh was delivered by drop or iontophoretic application to stratum oriens or stratum radiatum. Threshold dose for drop application was 1 mM. An initial hyperpolarization of 3.1 +/- 1.8 (S.D.) mV associated with a decrease in membrane input resistance (RN) of 21 +/- 9% (S.D.) occurred in about half the cells. This result is consistent with a presynaptic action of ACh mediated through excitation of inhibitory interneurons. This interpretation was supported by recordings of cholinergic excitatory responses from presumed interneurons, and repetitive spontaneous IPSPs from pyramidal neurons during the hyperpolarization. ACh evoked a slow depolarization (14.3 +/- 10.8 (S.D.) mV) accompanied by a peak increase in apparent input resistance (Ra) of about 60% in the majority of cells. Large increases in spike frequency were associated with these events but action potential shape was unchanged. Plots of Ra versus membrane potential following ACh application revealed that Ra increases were proportionately higher at depolarized membrane potential levels (less than or equal to -70 mV) in some neurons. In these cells Ra was increased significantly at -60 mV (28%), but only 6% at -75 mV. These results are consistent with the conclusion that ACh reduces a voltage-dependent gK, distinct from delayed rectification. ACh also induced a non-voltage-dependent increase in Ra in some cells. ACh-evoked changes in Ra were long-lasting and gave rise to alterations in firing mode, with development of burst generation. ACh also transiently blocked after hyperpolarizations which followed spike trains in pyramidal neurons and presumed interneurons, an action which may be related to effects on a Ca2+-activated gK.

Acetylcholine↗
PubMed1982

Ionic mechanisms of cholinergic excitation in mammalian hippocampal pyramidal cells.

Intracellular recordings from CA1 hippocampal pyramidal neurons were obtained using the in vitro hippocampal slice preparation. Responses to ACh were monitored in the presence of blockers of voltage-dependent conductances including Mn2+, TTX and Ba2+. When Mn2+ was used to block voltage-dependent Ca conductance and possible indirect presynaptic cholinergic actions, ACh still induced a significant voltage-sensitive increase in apparent input resistance (Ra) (29%), but only an insignificant depolarization of membrane potential (Vm). When both voltage-dependent Ca and Na conductances were blocked by application of Mn2+ and TTX, respectively, ACh produced voltage-dependent increases in Ra (31%) without significant depolarization. In solutions containing TTX alone, ACh produced voltage-sensitive increases in Ra (32%) as well as a significant depolarization (6.2 +/- 3.1 mV (S.D.)). ACh transiently blocked the conductance increase which followed presumed Ca spikes, suggesting an action on the Ca-activated K-dependent conductance. The effects of Ba2+ application (100-200 microM) on Ra mimicked those of ACh. When ACh was applied to neurons in the presence of Ba2+, Ra remained unchanged, although Vm depolarization of 5-15 mV was still seen. The data indicate that ACh decreases both a voltage-dependent K conductance (distinct from that of the delayed rectifier) and a Ca-activated K conductance. Muscarinic cholinergic depolarization occurs as a result of blockade of K conductance, and is mediated by voltage-dependent Ca and Na conductances, and perhaps by presynaptic actions.

Acetylcholine↗
PubMed1982

Variations in electrophysiological properties of hippocampal neurons in different subfields.

Intracellular recordings were made from hippocampal pyramidal cells (HPCs) in subregions CA1a, b, c, CA2 and CA3a, b of the guinea pig hippocampal slice. There were significant differences in the mode of spike discharge at various sites. Most neurons in CA1b and CA3b fired single spikes spontaneously, or during intracellular depolarizing current pulses. HPCs in the CA1a and c, as well as CA2 and CA3a subregions usually had a burst mode of discharge under the same conditions. Basic differences in neuronal properties presumably underlie these varieties of behavior between or within various regions. Specification of the site or subregion of recording is important especially in those experiments where the mode of spike discharge or membrane events in HPCs are important variables.

Animals↗
PubMed1982

Lesion-induced synaptogenesis in the dentate gyrus of aged rats: I. Loss and reacquisition of normal synaptic density.

Quantitative electron microscopy was used to examine the ability of aged (2-year-old) and young adult (90-day-old) rats to replace those synapses lost (85-90%) in the outer two-thirds of the molecular layer of the dentate gyrus after a complete unilateral lesion of the entorhinal cortex. In aged rats the synaptic density is significantly lower than that of young adults at 10 days postlesion. Synaptic replacement begins between 2 and 4 days postlesion in young adults, whereas there is a delay until after 10 days postlesion in aged rats. Once synapse replacement begins in aged rats, the rate of synapse reappearance is about equal that of young adults. Thus the initial 10 days postlesion appears critical to growth of responding afferents and reformation of synaptic contacts. Analysis of synapses in terms of noncomplex and complex synaptic types shows that the noncomplex type accounts for the significant synaptic density difference between the two age groups. Replacement of complex synapses is nearly indistinguishable between age groups and is complete by 60 days postlesion. In contrast the initial replacement rate of noncomplex synapses in aged rats is much slower than young adults, though the control synaptic density is achieved by the end of the time course.

Age Factors↗
PubMed1982

Cholinergic pharmacology of mammalian hippocampal pyramidal cells.

Responses of CAl pyramidal cells to cholinergic compounds were recorded with intracellular microelectrodes in guinea-pig hippocampal slices. Perfusion of slices with medium containing the muscarinic antagonists atropine or scopolamine (10(-7)-10(-6)M) blocked all actions of acetylcholine. Properties of control neurons and those from separate populations of neurons impaled in slices exposed to muscarinic blocking agents were compared. 1-2 h of perfusion with atropine-containing media significantly decreased membrane input resistance from 37.6 +/- 8.7 (S.D.) M omega (n = 74) to 21.9 +/- 7.7 (S.D.) M omega (n = 24) without producing significant changes in membrane potential. Muscarinic antagonists also reduced or eliminated the anomalous inward rectification normally seen in hippocampal pyramidal neurons. Exposure of slices to 10(-5)-10(-6)M eserine for about 1 h produced changes in neuronal membrane input resistance and potential and slow after hyperpolarizations similar to those elicited by application of acetylcholine. Bethanechol mimicked the actions of acetylcholine but was effective at lower concentrations and had longer lasting effects on afterhyperpolarizations. Nicotine produced an excitatory response in only one of 7 neurons. These experiments demonstrate that the actions of acetylcholine on hippocampal CAl neurons result from interaction with muscarinic receptors. Acetylcholine has modulatory effects on cell membrane properties which may be mediated through tonic release mechanisms.

Acetylcholine↗
PubMed1982

Dopamine action on hippocampal pyramidal cells.

Dopamine (DA) was applied to CA1 region pyramidal cells in slices of guinea pig hippocampus maintained in vitro in order to examine its electrophysiological effect on CNS neurons. DA induced hyperpolarization of membrane potential and an increased conductance in 75% the 21 CA1 neurons to which it was applied. DA also augmented the afterhyperpolarizations and increased conductance which normally follow spike trains in these neurons. These effects were not altered by intracellular injections of Cl- but were blocked when slices were bathed in Mn2+ solutions. The Mn2+ blockade of DA-induced hyperpolarizations could be overcome when large amounts of agonists were applied. The DA effects were long lasting, were mimicked by the dopamine agonists apomorphine and Epinine, and were blocked by the dopamine antagonists flupenthixol and chlorpromazine. Extracellular or intracellular application of cyclic AMP mimicked the effects of DA. The results suggest that DA-induced hyperpolarization and conductance changes are mediated by a Ca2+-activated K+ conductance. DA may increase the intracellular Ca2+ concentration through effects on one of the Ca2+ buffering mechanisms. The long duration of these effects suggest that DA works though some intracellular intermediary, perhaps cyclic AMP, considering that the actions of cyclic AMP on membrane properties are similar to those of DA. The dopaminergic projection to the hippocampus should have a powerful inhibitory action, which would be most effective in modulating the activities of neurons exhibiting high levels of excitability, particularly cells involved in cyclical burst generation.

Action Potentials↗
PubMed1982

Electrophysiology of isolated hippocampal pyramidal dendrites.

In order to study the electrical properties of dendritic membranes independent of the effects of somatic potentials, intracellular recordings in guinea pig hippocampal slices were obtained from the dendrites of CA1 pyramidal neurons (HPCs) which had been isolated from their somata by cuts made through the proximal stratum radiatum. Spikes and subthreshold membrane responses to intracellular current pulses were compared in intact and isolated dendrites and in the residual portions of neurons whose apical dendrites had been severed ("isolated somata"). Isolated dendrites generated both fast, QX-314-sensitive, sodium-mediated spikes, and slow higher threshold spikes which were QX-314-resistant and presumably mediated by Ca2+. Depolarization of "isolated somata" ordinarily evoked only fast (Na+) spikes, but presumed Ca2+ spikes could be elicited after exposure to QX-314 (a local anesthetic). Anomalous inward rectification was depressed by QX-314 in somata but not in dendrites, suggesting that the ionic basis for subthreshold as well as regenerative conductances was different at different sites on the neuron. The dendritic membrane in CA2 HPCs thus generates both Na+- and CA2+-mediated spike potentials and a subthreshold response which probably is mediated primarily by CA2+. Attempts to describe the integrative functions of these neurons must take into account the variety of conductances which are activated nonuniformly in somata and dendrites by changes in membrane potential.

Action Potentials↗
PubMed1981

Acetylcholine induced modulation of hippocampal pyramidal neurons.

Applications of acetylcholine to hippocampal slices maintained in vitro resulted in slow depolarizations and simultaneous increases in membrane resistance (RN)in hippocampal pyramidal neurons. Increases in RN had both voltage dependent and voltage independent components. These effects were associated with increases in cell discharge frequency, and development of spontaneous as well as synaptically and directly evoked burst discharges. The increase in RN and burst firing lasted for hours. Muscarinic antagonists blocked these actions and in addition, produced a decrease in membrane resistance, which appeared to be due to blockade of a tonic effect of acetylcholine on postsynaptic membrane properties. These findings suggest that ACh acts as a neuromodulator in the hippocampus.

Acetylcholine↗
PubMed1980

Decline in reactive fiber growth in the dentate gyrus of aged rats compared to young adult rats following entorhinal cortex removal.

The reaction of septal and commissural-associational afferents in the dentate gyrus was examined at various times following a unilateral entorhinal lesion in 2- and 3-month-old, 12- to 18-month-old and 25--30-month-old rats. The response of septo-hippocampal fibers was examined histochemically by staining for acetylcholinesterase (AChE) activity; and that of commissural-associational fibers by the Holmes' fiber stain. In 2- and 3-month-old rats, AChE staining fibers, which project to the outer three-fourths of the molecular layer of the dentate gyrus, increased their staining intensity within 5--6 days following lesion of the entorhinal cortex. The rate of the response and the eventual magnitude declined progressively with the age of the subject. In 2- and 3-month-old rats, the commissural-associational fiber plexus appeared to expand partially into the entorhinal zone within 6 days following the lesion. This response also decreased progressively in rate and magnitude with age. Animals in the oldest age group showed at 12 days after the lesion a greater variability in the expansion of the commissural-associational fiber plexus than all younger groups. Astrocytes in the dentate molecular layer appeared to become more abundant and more hypertrophied in unoperated animals with age. The appearance of astrocytes in 25- to 30-month-old rats was similar to that seen in 2- and 3-month-old animals following an entorhinal lesion. An entorhinal lesion in the aged animals did not appear to cause a marked change in the appearance of astrocytes.

Aging↗