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B A MacVicar

Publications and source records attributed to B A MacVicar.

At least 55 records · Page 3Linked to original sources
PubMed1989

Identification of a GABA-activated chloride-mediated synaptic potential in rat pars intermedia.

Intracellular recordings from melanotrophs in acutely isolated, intact pituitaries revealed inhibitory postsynaptic potentials (IPSPs) resulting from both pituitary stalk stimulation and exogenous gamma-aminobutyric acid (GABA) application. The stalk-stimulated and GABA-evoked IPSPs displayed identical conductance increases to chloride ions. Both responses reversed in polarity at the same membrane potential and were blocked with bicuculline, a GABAA antagonist. We conclude that activation of this synapse releases GABA which acts on a classical GABAA receptor to increase conductance to chloride in the melanotroph.

Action Potentials↗
PubMed1989

A dopaminergic inhibitory postsynaptic potential mediated by an increased potassium conductance.

Intracellular recordings from intact pituitary melanotrophs show that, in the same cell, inhibitory postsynaptic potentials resulting from either pituitary stalk stimulation or exogenous dopamine are abolished by D2 receptor antagonists, display identical conductance changes, are reversed in polarity at the same membrane potential and are sensitive to pertussis toxin pretreatment. The reversal potential of the inhibitory postsynaptic potential shows a 65 mV shift with a 10-fold change in external potassium concentration, which is close to that predicted by the Nernst equation. We conclude that activation of this synapse releases dopamine which acts on a D2 receptor to increase potassium conductance via a G-protein-mediated mechanism. This is the first characterization of an inhibitory dopaminergic synapse in the mammalian nervous system.

Action Potentials↗
PubMed1989

Local neuronal circuitry underlying cholinergic rhythmical slow activity in CA3 area of rat hippocampal slices.

1. Intracellular and extracellular recordings were obtained from the CA3 area of rat hippocampal slices to study cellular and synaptic mechanisms underlying rhythmic slow activity (RSA). In all impaled CA3 pyramidal neurones, continuous applications of carbachol, a non-hydrolysable cholinergic agonist, induced first a brief non-rhythmic excitation and then periodic bursts of RSA which could persist for several hours. Each burst of RSA consisted of 4-10 Hz oscillatory depolarizations which had a rise time much slower than conventional EPSPs recorded in the same cell. 2. The carbachol-induced RSA was blocked by atropine; therefore the cholinergic stimulation involved muscarinic receptors. 3. Analyses of simultaneous recordings from pairs of neurones, or a neurone and a glial cell, or a neurone and the extracellular field, indicated that carbachol-induced RSA was synchronous in a large population of CA3 pyramidal neurones. 4. Complete removal of the dentate gyrus and CA1 region did not block carbachol-induced RSA in CA3, but applications of tetrodotoxin or inorganic Ca2+ channel blockers (Cd2+, Co2+ or Mn2+) abolished carbachol-induced RSA. This suggested that the RSA involved propagation of action potentials through a local synaptic network in the CA3 area. 5. Carbachol-induced RSA was reversibly blocked by a broad-spectrum excitatory amino acid antagonist (kynurenic acid), but not by two selective N-methyl-D-aspartate (NMDA) antagonists (DL-2-amino-7-phosphonoheptanoic acid or DL-2-amino-5-phosphonovaleric acid), a GABAA antagonist (bicuculline), or a GABAB antagonist (phaclofen), suggesting that carbachol-induced RSA involved primarily non-NMDA excitatory amino acid, but not GABAergic, synapses. 6. Raising extracellular [Ca2+] beyond 7 mM, which should significantly weaken the polysynaptic recurrent excitation among CA3 pyramidal neurones, abolished carbachol-induced RSA. This suggests that the recurrent excitation among CA3 pyramidal neurones is necessary for carbachol-induced RSA in the CA3 area. However, our experiments cannot clarify whether the recurrent excitation, alone, is sufficient for carbachol-induced RSA.

Animals↗
PubMed1989

GABA-activated Cl- channels in astrocytes of hippocampal slices.

We used kainic acid-lesioned hippocampal slices to examine glial responses to the inhibitory neurotransmitter GABA in a neuron-free environment. Slices were prepared from rats which received intracerebroventricular injections of kainic acid 1 month prior to experiments. Astrocytes (membrane potential averaged 81.4 +/- 5.5 mV; n = 46; mean +/- SD) were impaled in the CA3 region of the slice, which was completely depleted of neurons. GABA (1 mM) application by bath perfusion depolarized membrane potential from 1 to 5 mV. The GABA-induced depolarization was not affected by a tetrodotoxin (1 microM)/high-Mg2+/low-Ca2+ solution. Changing the Cl- equilibrium potential by reducing extracellular Cl- greatly increased the GABA-induced depolarization. Muscimol mimicked the GABA response, while picrotoxin (0.1 mM), an antagonist of the GABA-activated Cl- channel, resulted in a 60% blockade. The barbiturate, pentobarbital (0.1 mM), and the benzodiazepine agonist, flunitrazepam (1 mM), enhanced the depolarization by 60 and 40%, respectively. A blocker of glial GABA uptake, beta-alanine (1 mM), did not affect the GABA-induced membrane depolarization, indicating that the depolarization is not caused by electrogenic uptake of the amino acid. The pharmacological properties of the GABA response of astrocytes from the hippocampal slice is similar to that previously described for cultured astrocytes from rat cerebral hemispheres. Our data suggest that GABA receptors, which are coupled to Cl- channels, are also expressed by astrocytes in an intact tissue.

Animals↗
PubMed1988

Norepinephrine and cyclic adenosine 3':5'-cyclic monophosphate enhance a nifedipine-sensitive calcium current in cultured rat astrocytes.

We employed two microelectrode current-clamp and voltage-clamp methods to examine the modulation of Ca++ channels by norepinephrine and cyclic AMP (cAMP) in cultured astrocytes from the rat cerebral cortex. Currents owing to Ca++ channels were maximized by replacing Ca++ with Ba++ in the extracellular solution and pharmacologically blocking K+ and Na+ currents. In current-clamp experiments, we observed that norepinephrine, isoproterenol (an agonist of beta-receptors for norepinephrine), or dibutyryl cAMP (dbcAMP, a membrane permeant analogue of cAMP) induced or enhanced slow Ba++-dependent action potentials in the cells. In voltage-clamp experiments, we confirmed that the slow action potentials were generated by a voltage-activated and Ba++-dependent inward current. This current was mediated by channels that resembled L-type calcium channels (cf. McCleskey et al., Journal of Experimental Biology 124:177-190, 1986) in their voltage-activation range, slow inactivation, and sensitivity to blockage by Co++, Cd++, and nifedipine. DbcAMP, or isoproterenol, enhanced the Ba++ current. Modulation of Ca++ channel function in glial cells could have functional implications.

Action Potentials↗
PubMed1988

Kainic acid evokes a potassium efflux from astrocytes.

Cultured astrocytes are depolarized by excitatory amino acids such as kainic acid. In these experiments we tested the hypothesis that the kainic acid-induced depolarization also causes an efflux of K+ from astrocytes in the rat. Using K+-sensitive microelectrodes we measured a K+ efflux from cultured astrocytes during the perfusion of kainic acid. The effects of kainic acid were then tested on glial cells in the neuron-free CA3 region of kainic acid-lesioned hippocampus. Glial cells were depolarized by kainic acid and an efflux of K+ was recorded. These effects are most likely due to direct effects on the glial cells because histological examination of the hippocampus showed this area to be free of neurons. Therefore it is hypothesized that kainic acid and any transmitter that depolarizes glial cells, will increase neuronal excitability indirectly by a K+ efflux from glial cells. This will have widespread implications for iontophoretic studies of transmitter actions.

Animals↗
PubMed1987

Morphological differentiation of cultured astrocytes is blocked by cadmium or cobalt.

The morphological transformation of astrocytes in primary culture from flat to process bearing is induced by cyclic adenosine monophosphate (cAMP). Calcium currents in cultured astrocytes are also enhanced by cAMP. This correlation suggests the hypothesis that cAMP-induced calcium entry causes the morphological differentiation. The experiments reported here show that cadmium or cobalt, which block astrocyte calcium channels, also block the cAMP-induced morphological differentiation. These results support the hypothesized link between calcium entry and astrocyte differentiation.

Animals↗
PubMed1987

Dye and electrotonic coupling between cultured hippocampal neurons.

Electrotonic coupling was examined between hippocampal neurons in dissociated cell cultures. Injections of Lucifer yellow indicated that approximately 20% of the neurons were dye-coupled. Propionate, which causes cytoplasmic acidification, significantly decreased dye coupling. Simultaneous intracellular recordings from pairs of cultured neurons directly demonstrated electrotonic coupling. Therefore dissociated cell cultures can be used as a model system for the analysis of hippocampal electrotonic coupling.

Animals↗
PubMed1987

Inhibition of synaptic transmission in the hippocampus by cholecystokinin (CCK) and its antagonism by a CCK analog (CCK27-33).

The actions of CCK-8 and putative CCK-8 antagonists were examined on synaptic transmission in the hippocampal slice preparation. CCK-8 (10(-7) M) caused a reversible depression of postsynaptic responses of the Schaffer collateral-CA1 synapse. CCK-mediated depression was unaffected by proglumide (10(-3) M) but was antagonized by a CCK analog (CCK27-33). The CCK analog alone had an excitatory effect on synaptic transmission. These results suggest that CCK has an inhibitory action on synaptic transmission.

Animals↗
PubMed1986

Novel synaptic responses mediated by dopamine and gamma-aminobutyric acid in neuroendocrine cells of the intermediate pituitary.

Neuroendocrine cells of the pituitary pars intermedia contain pro-opiomelanocortin and secrete mainly alpha-melanocyte-stimulating hormone and, in lesser amounts, corticotropin and endorphin. We have recorded intracellularly from rat pars intermedia cells in vitro and analysed the synaptic inputs to these cells. Electrical stimulation of the pituitary stalk evoked a biphasic postsynaptic response in pars intermedia cells. An initial inhibitory postsynaptic potential was most likely mediated by gamma-aminobutyric acid (GABA) because there was a conductance increase, and it was blocked by bicuculline, a GABA antagonist. A more prolonged hyperpolarization was also observed which could last 10-30 s and was associated with a decreased whole cell conductance. This late hyperpolarization was blocked by the dopamine antagonists, chlorpromazine and domperidone, and was therefore most likely dopamine-mediated.

Animals↗
PubMed1986

Calcium activated potassium channels in cultured astrocytes.

The patch clamp technique was used to analyze single channel currents in intact and excised patches of glial cell membranes grown in primary cultures from newborn rat brain. Glial cells were morphologically identified by immunohistochemical staining for glial fibrillary acidic protein. Outward currents due to single channels were observed in recordings from both intact and excised patches obtained from the cell body region. The channel responsible for these currents was preferentially permeable to K+ because the reversal potential for this current was correlated with changes in the potassium equilibrium potential, when experimentally altered. The single channel conductance was 25 pS when measured between -20 and +20 mV in solutions with physiological K+ concentrations (10 degrees C). Channel gating was dependent on both the internal Ca2+ concentration and the membrane potential. Either depolarization of the membrane patch, or the addition of increasing Ca2+ concentrations to the internal surface, increased the probability of channel opening. Tetraethylammonium reversibly blocked the channel whereas 4-aminopyridine had no effect. The characteristics exhibited by this channel indicate that a Ca2+-activated K+ channel is present in the membrane of astrocytes grown in culture. These results, combined with previous evidence for a voltage dependent Ca2+ channel, suggest a dynamic role for glial cells in controlling excitability in the central nervous system. Influx of Ca2+ upon depolarization would increase the membrane permeability to K+ and could increase the "buffering" capacity of glial cells for extracellular K+.

Animals↗
PubMed1985

Uncoupling of CA3 pyramidal neurons by propionate.

The influence of cytoplasmic acidification on dye-coupling between CA3 pyramidal neurons was examined by Lucifer yellow injections in guinea pig hippocampal brain slices. Neurons were believed to be acidified by exposure to 100 mM propionate. Dye-coupling was reduced significantly to 6% (n = 35) in propionate versus 28% (n = 39) in control solution (P less than 2%). Propionate may be a useful tool for experimental manipulation of coupling in the mammalian CNS.

Animals↗
PubMed1985

Barium action potentials in regenerating axons of the lamprey spinal cord.

Intracellular recordings were obtained from growing tips of regenerating giant axons in the lamprey spinal cord, the recording sites verified by Lucifer yellow injection. In the presence of extracellular Ba++ (3-6 mM), tetraethylammonium (10-15 mM), and 4-aminopyridine (4-6 mM), action potentials showed prolonged plateaus. The fast initial phase of the action potential, but not the plateau (Ba++-spike), was blocked by tetrodotoxin (10(-6) gm/ml). The Ba++ spike was associated with increased membrane conductance and could be terminated with hyperpolarizing current pulses. Normal axons did not generate similar Ba++ spikes. However, TTX-resistant, voltage-dependant conductance changes could be elicited in normal axons if much higher concentrations of Ba++ (18-30 mM) were used. Their rate of rise was slower than in regenerating axons (0.6 V/sec vs 3.2 V/sec; n = 5), and the response did not outlast the current pulse. The Ba++ responses in normal and regenerating axons were blocked by ions known to block voltage-gated Ca++ conductances (Co++, Ni++, or Cd++). Therefore, these spikes probably represent Ba++ entry through voltage-dependent Ca++ channels, suggesting the presence of a higher-than-average voltage-dependent Ca++ conductance in the growing axon. However, Ca++-dependent spikes could not be obtained under any conditions in either normal or regenerating axons. Simultaneous intracellular recordings from growth cones and axons indicated that the Ba++ spike was initiated, in most cases, at the growth cone. The Ba++ spikes were recorded in regenerating axons for as long as 50 days following cord transection and were not correlatable with the "dying-back" phenomenon in cut axons, which usually is over before day 6. The concept of a higher-than-average voltage-dependent Ca++ conductance in growing tips of regenerating axons is in agreement with the hypothesis that Ca++ is important in regeneration and that regeneration may be related to the process of chemical synaptic transmission.

4-Aminopyridine↗
PubMed1984

Voltage-dependent calcium channels in glial cells.

The electrophysiological properties of glial cells were examined in primary culture in the presence of tetraethylammonium and Ba2+, a treatment that reduces K+ permeability of the membrane and enhances currents through voltage-dependent Ca2+ channels. Under these conditions, glial cells showed both spontaneous action potentials and action potentials evoked by the injections of current. These responses appear to represent entry of Ba2+ through Ca2+ channels because they were resistant to tetrodotoxin but were blocked by Mn2+ or Cd2+.

Action Potentials↗
PubMed1984

A reliable method for immunocytochemical identification of Lucifer Yellow injected, peptide-containing mammalian central neurons.

An immunocytochemical procedure is described for reliably determining the hormone content of magnocellular neuroendocrine neurons that have been injected with Lucifer Yellow in slices of rat hypothalamus. The chief advantages of this procedure over others currently available are: (a) it permits whole mount observation of the tissue, and thus, of the morphology of filled cell(s) as well as of such phenomena as dye-coupling; (b) the reliability of tissue preparation and peptide determination has been optimized so that about 85% of injected cells are identified immunocytochemically; and (c) the final immunostained product is permanent, permitting bright-field examination of the injected cell. Relative advantages and limitations of this and other recently published methods are discussed.

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