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D O Carpenter

Publications and source records attributed to D O Carpenter.

At least 127 records · Page 7Linked to original sources
PubMed1988

Excitatory amino acid receptors in piriform cortex do not show receptor desensitization.

We have investigated the proposed role of transmitter receptor desensitization as an explanation for the excitotoxicity rank order of several excitatory amino acid agonists as compared to kainic acid, using a brain slice of rat piriform cortex. Responses to glutamate, aspartate, quisqualate, n-methyl aspartate and kainate showed no evidence of receptor desensitization when studied with very long and large ionophoretic pulses, repeated ionophoretic pulses or by bath perfusion. At least in rat piriform cortex, the suggestion that kainate receptors do not desensitize while those to glutamate and quisqualate do, does not apply to nor explain the more potent kainate excitotoxicity.

Animals↗
PubMed1988

Edrophonium-induced inward membrane current in single neurons physically isolated from Aplysia californica.

The action of edrophonium on Aplysia neurons was studied using a concentration clamp technique which combines internal perfusion and a rapid drug application. Edrophonium elicited a dose-dependent inward current in the concentration range 10(-6) to 10(-4) M. At higher concentrations (10(-3) and 10(-2) M), the amplitude of the current often decreased and there was a rapid decay of the current. At these high concentrations, the current increased immediately after washing the neuron with normal solution. These results suggest that edrophonium blocks the ion channel which it opens. Removal of Na+ from the external solution greatly reduced the current amplitude by more than 90%. Removal of Ca2+ also reduced the amplitude of the response; however an increase of Ca2+ did not augment the response. These results suggest that Ca2+ does not carry the current, but is necessary for generation of an Na+-dependent inward current. Edrophonium, 10(-2) M, which completely blocked the current it induced within 20 s, did not significantly affect the voltage-dependent Na+ current. Tetrodotoxin, 1 x 10(-6) M, did not affect the edrophonium response. Hexamethonium, 1 x 10(-4) M, did not change the response elicited by edrophonium, while it significantly reduced the ACh response mediated by Na+. In some neurons edrophonium elicited an inward current, but ACh induced an outward current. Therefore the Na+ channels opened by edrophonium appear to be distinct from both the voltage-gated and ACh receptor-operated Na+ channels.

Acetylcholine↗
PubMed1988

Strychnine decreases the voltage-dependent Ca2+ current of both Aplysia and frog ganglion neurons.

1. The effects of strychnine on the voltage-dependent Ca2+ current (ICa) were studied in physically isolated Aplysia neurons and enzymatically dissociated frog sensory neurons of the dorsal root ganglion. Neurons were studied under the internal perfusion and the voltage clamp condition. 2. Strychnine decreased the ICa with threshold concentrations for effect at 1 to 10 microM. The depression of ICa increased with strychnine dose without effects on the current-voltage relation of ICa. The effects of low concentrations of strychnine were reversible, but recovery was incomplete at higher concentrations. The potency of strychnine was about 10 times less than that of diltiazem, an organic Ca2+ antagonist. At 100 microM the ICa of Aplysia neurons was reduced to about half of the control. This concentration of strychnine also reduced the peak amplitude of ICa of frog sensory neurons. 3. These results indicate that, in addition to its actions on transmitter responses and on Na+ and K+ currents, strychnine has effects on ICa that have not previously been appreciated.

Animals↗
PubMed1988

Piriform cortex brain slices: techniques for isolation of synaptic inputs.

Methods are described for preparation of 3 different slices of piriform cortex which allow convenient study of pyramidal neurons and segregation of synaptic inputs. In slices cut parallel to the pyramidal neurons (perpendicular to the brain surface) one can study chemosensitivity of the various parts of the dendritic tree and the soma. By selected division of this slice the population postsynaptic response to activation of the lateral olfactory tract can be studied without action potential generation. Alternatively the superficial lateral olfactory tract can be removed. Stimulation of deeper regions of the slice under these circumstances elicits a pharmacologically different excitation which appears to be that of association fibers.

Aminobutyrates↗
PubMed1988

Electrophysiological detection of acetylcholinesterase activity using concentration clamp on physically isolated Aplysia neurons.

In this study, we have used electrophysiological techniques to evaluate the acetylcholinesterase (AChE) activity on single neurons physically isolated from the pedal ganglia of Aplysia californica and kurodai. Acetylcholine (ACh) was applied to cells having Na-dependent response using a concentration clamp technique which allows a step-like complete change of external medium around the cell (diameter more than 120 micron) within 30 msec. When the neuron was exposed to ACh (50 microM) initially by rapid and brief (400-800 msec) flow and continuously after stopping the perfusion, the induced current rose to a peak and then decayed in the presence of ACh. However, the current increased again when the rapid flow of the same ACh solution around the cell was resumed. The increase in the current by the resumption of the perfusion was not seen when carbachol was substituted for ACh or when extraordinarily high concentration (10 mM) of ACh was used. Furthermore, this increase in the current was blocked by the antiAChE agent, edrophonium, in a dose-dependent manner. These results suggest that acetylcholinesterase (AChE) causes a local depletion of ACh at the membrane surface where the rate of hydrolysis of ACh exceeds the rate of ACh diffusion from bulk solution and the increase in the current by the resumption of the perfusion resulted from the restoration of ACh concentration at the membrane surface. This electrophysiological indication of AChE activity may be a useful tool for the study of AChE by other than biochemical means.

Acetylcholine↗
PubMed1988

Effects of Ca2+ on the transient outward current of single isolated Helix central neurones.

1. Single Helix neurones were studied under voltage-clamp conditions with internal perfusion in order to examine the contribution of internal and external Ca2+ and the effects of 4-aminopyridine (4-AP) on the transient outward current (IA). 2. In Na+ -free snail Ringer, replacement of external Ca2+ [( Ca2+]o) with equimolar Co2+ reduced the maximum amplitude of IA and induced a shift of the steady-state part of the IA inactivation curve (I-V curve), in a positive direction along the voltage axis when the neurone was internally perfused with K-aspartate. 3. In Ca2+ -free solutions, precipitation or chelation of internal Ca2+ [( Ca2+]i) by internal perfusion with KF or EGTA shifted the curve in a more negative direction without affecting the maximum amplitude of IA. Thus, the kinetics of IA are dependent not only upon [Ca2+]o, as previously suggested, but also upon [Ca2+]i. 4. In the presence of 4-AP the I-V curve for IA shifted in a hyperpolarizing direction and the maximal amplitude was reduced when the neurone was internally perfused with K-aspartate. However, 4-AP had little or no effect on the I-V relationship of IA when the neurone was internally perfused with the Ca2+ precipitating or chelating agent, KF or EGTA. These results suggest that the actions of 4-AP on IA are at least partly dependent upon [Ca2+]i.

4-Aminopyridine↗
PubMed1988

Excitation of area postrema neurons by transmitters, peptides, and cyclic nucleotides.

1. Multiple-barreled microelectrodes were used to record from neurons in the area postrema of anesthetized dogs and to test the responses of the neurons to a variety of substances in this structure, which is known to function as the chemoceptive trigger zone for emesis. 2. The neurons in area postrema were silent at rest but could be "found" by virtue of their response to ionophoretic glutamate. The glutamic response was brief and of short latency with high frequency of discharge. 3. Dog area postrema neurons were also excited by over 20 other substances, including acetylcholine, the biogenic amines, several peptides, and at least two hormones. Not all agents were excitatory, however. 4. The responses to all excitatory agents except glutamate were similar and unusual. All responses showed a relatively long latency (3-20 s), a long duration of excitation (30 s to many minutes), and a low discharge frequency (1-3 Hz). 5. There was a good correlation between substances that were excitatory on area postrema neurons and substances known to cause emesis. Because emesis due to intravenous application of these substances is known to be abolished in animals with ablation of the area postrema, it is very likely that recordings were from the neurons which trigger the response. 6. Because so many substances elicit the same type of response there is a possibility that all utilize a common second messenger. Neurons were not excited by ionophoresis of guanosine 3',5'-cyclic monophosphate (cGMP) but were excited by 8-bromo-adenosine 3',5'-cyclic monophosphate (cAMP) and by forskolin, an activator of adenylate cyclase. 7. Behavioral studies were performed looking for emetic responses in awake dogs following intravenous injection of apomorphine, insulin, angiotensin II, and leucine enkephalin. For each a threshold concentration could be determined, which would consistently evoke emesis. 8. Dogs pretreated with phosphodiesterase inhibitors (theophylline, 3-isobutyl-1-methylxanthine, or RO 1724) showed a shift in the threshold concentration of the above substances that triggered emesis, such that emesis was evoked by lower concentrations than in the control. 9. These results suggest that neurons of the dog area postrema trigger the emetic reflex in response to specific receptors for a great variety of transmitters, peptides, and hormones, and that these receptors act through a common second messenger, cAMP.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1987

Distinct GABA and glutamate receptors may share a common channel in Aplysia neurons.

Using a microperfusion technique for rapid application of agonists to single identified voltage-clamped neurons of the marine mollusc Aplysia, chloride conductances elicited by gamma-aminobutyric acid (GABA) and L-glutamate were found to differ in rates of activation and desensitization, voltage dependence and dose-response relations. In spite of these marked differences, the two responses showed strong interaction: previous application of GABA could completely block the responses to glutamate while previous application of glutamate decreased the response to GABA. This interaction was not due to transmembrane chloride redistribution, and is probably not cross receptor blockade. Cross-desensitization of GABA and glutamate responses suggest that distinct receptors activate a common ion channel.

Animals↗
PubMed1987

In vivo labeling of serotonin-containing neurons by 5,7-dihydroxytryptamine in Aplysia.

Intrahemocoelial administration of 5,7-dihydroxytryptamine (5,7-DHT) to Aplysia californica induces a transient (less than 4 h) behavioral alteration. About 5 weeks after 5,7-DHT treatment, 5-hydroxytryptamine (5-HT)-containing neurons develop dark brown pigmentation. These labeled 5-HT neurons have normal physiological and pharmacological properties when investigated electrophysiologically. This contrasts with the long-term neurotoxic effect of 5,7-DHT on vertebrate neurons. This technique will greatly facilitate visual identification of 5-HT-containing neurons and study of their physiology and actions.

5,7-Dihydroxytryptamine↗
PubMed1987

Responses of pyriform cortex neurons to excitatory amino acids: voltage dependence, conductance changes, and effects of divalent cations.

The actions of ionophoretically applied N-methyl aspartate (NMA), quisqualate, and kainate, thought to activate three different types of excitatory amino acid receptors, were studied on pyramidal neurons of the rat pyriform cortex, maintained in an isolated, submerged, and perfused brain slice. Intracellular recordings were made with either K acetate or CsCl electrodes. In most neurons all three agonists elicited monophasic responses which could be evoked at 20-sec intervals. Some neurons showed biphasic responses, most commonly to kainate but, on occasion, also for quisqualate. The slower component appeared to be correlated with excitotoxicity and, consequently, was difficult to study. As a result the kainate responses studied were from neurons selected for having a single component. In neurons selected for having a linear current-voltage relationship or neurons loaded with Cs to suppress K conductance and linearize the current-voltage relationship, the average changes in resistance recorded during ionophoretic responses at resting potential were as follows: NMA, 131.2 +/- 6.7% of control; kainate, 104.7 +/- 5.8% of control; and quisqualate, 92.8 +/- 2.8% of control. The magnitude and direction of the conductance change were very reproducible in any one neuron, but especially for kainate some cells showed clear conductance increases, while others showed clear conductance decreases. Using CsCl electrodes it was possible to reduce K+ conductance and depolarize the neurons over a wider range. By passing depolarizing current it was possible to reverse the responses. The response to all three agonists reversed at the same depolarized potential. This observation indicates that while there are differences in the ionic channels associated with the three agonists at resting potential, the channels have similar properties at more depolarized potentials. Responses to all three agonists were influenced by the concentrations of divalent cations in the perfusion medium. The NMA responses were most sensitive to Mg, increasing in amplitude in the absence of Mg and being depressed by Mg elevation. All responses were sensitive to Ca, with discharges being greatly increased by low Ca and depressed by high Ca. The kainate response was most sensitive to Ca concentration changes. Unlike reports from other preparations the apparent conductance decreases to NMA were not altered by the perfusion of solutions with either no added Mg or no added Ca. The NMA response was very much reduced in either Co (1-2 mM) or Zn (100-200 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1987

Multiple mechanisms of antagonism of gamma-aminobutyric acid (GABA) responses.

Gamma-aminobutyric acid (GABA) is one of the most important neurotransmitters in the brain. In an effort to understand the operation of the GABA receptor-ionophore complex, the antagonism of GABA responses by four substances was studied in bullfrog dorsal root ganglion cells by concentration-clamp and internal-perfusion techniques. Two antagonists (bicuculline and Zn2+) were competitive; two (picrotoxin and penicillin) were noncompetitive. However, significant changes in the kinetics of activation and inactivation were produced by the antagonists, including those that were competitive. The causes of these changes may be important clues to the structure and operation of the GABA receptor-ionophore complex.

Animals↗
PubMed1987

Effects of venom from Conus striatus on the delayed rectifier potassium current of molluscan neurons.

The effect of crude venom extracted from venom ducts of Conus striatus on the delayed rectifier potassium current (IK) of the marine mollusc Aplysia californica was studied using voltage clamp techniques. Initial experiments indicated that the venom had phospholipase activity which destroyed the cells. The use of phospholipase inhibitors prevented destruction of the cell and permitted long-term electrophysiological measurements to be made. Application of the venom to unclamped cells caused a dramatic increase in the frequency of action potentials associated with a depolarization of the membrane potential. A broadening of the action potential was also observed. Three separate effects of the venom were observed on IK in voltage clamped cells: an increase in peak current (effect I), a slowing of both the activation and inactivation kinetics (effect K) and a decrease in the peak current (effect D). All three effects were dose dependent and both effects on peak current were greater at more depolarized membrane potentials. The data suggest that the three effects on IK are caused by different components of the venom. Effect D appears to be caused by a heat-labile compound of molecular weight greater than 50,000, effect I by a heat-stable compound of less than 50,000 and effect K by a heat-stable compound of intermediate molecular size.

Action Potentials↗
PubMed1987

Antagonists of GABA responses, studied using internally perfused frog dorsal root ganglion neurons.

Responses of frog dorsal root ganglion neurons to GABA were studied under conditions of internal perfusion. Conductances to Na, Ca and K were pharmacologically blocked, C1 concentrations were maintained equal on both sides of the membrane and a small holding potential was used. Under these conditions GABA-induced C1 currents could be studied in isolation without shifts in EC1 occurring after GABA application. GABA currents were blocked by a variety of agents. The blockade by bicuculline and Zn was competitive, while that to penicillin was competitive at low concentrations (6 x 10(-5) M) and non-competitive at high concentrations (3 x 10(-4) M). Picrotoxin was non-competitive at all concentrations studied. The time course of the GABA-induced currents was changed in the presence of antagonists, including those that were competitive. These actions appear to be due to a change in the rates of receptor desensitization rather than shifts in EC1. Pretreatment with antagonists increased the degree of inhibition only for picrotoxin as compared to simultaneous application of GABA plus antagonist. The voltage dependence of the GABA response was altered by penicillin but not by other antagonists. GABA responses on frog dorsal root ganglion cell were also depressed by a variety of other metal ions (Cd, Ni, Cu, Co, Mn) and other drugs (strychnine, curare, 4-acetamide, 4'-isothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt, 4,4'-diisothiocyano-stilbene-2,2'-dilsulfonic acid disodium salt trihydrate, bemegride and folic acid). In this preparation bicuculline and the heavy metal ions appear to block at or very near to the agonist binding site, while penicillin probably blocks the ion channel. The non-competitive action of picrotoxin appears not to be channel blockade, but to be due to a slowly equilibrating action at a site different from either the agonist binding site or the channel.

Animals↗
PubMed1986

Insulin excites neurons of the area postrema and causes emesis.

Responses of neurons of the canine area postrema were recorded to ionophoretic application of insulin, apomorphine, leucine-enkephalin and glutamate. Each excited the neurons directly in a dose-dependent fashion. Like apomorphine and leucine enkephalin, which are known to induce emesis by activation of area postrema neurons, insulin given systemically induced emesis in intact dogs but not in animals with area postrema ablations. These results provide further support for a critical role of the area postrema in triggering the emetic reflex, and are the first definitive demonstration of a direct excitatory action of insulin on mammalian neurons.

Animals↗
PubMed1986

Multiple interactions of anticholinesterases with Aplysia acetylcholine responses.

The effects of the carbamate anticholinesterases neostigmine and pyridostigmine on the kinetics of desensitization of responses of isolated, voltage-clamped Aplysia neurons to microperfused acetylcholine (ACh) was examined. The peak ACh-induced current was potentiated at low carbamate doses and antagonized at higher doses (greater than 10(-5) M); neostigmine was more potent than pyridostigmine in producing both effects. These effects suggest two mechanisms of action of these compounds: (a) inhibition of acetylcholinesterase at low doses, which increases the effective ACh dose, and (b) direct antagonism of the response at higher concentrations, which is associated with a slowing of both the activation and desensitization of the ACh response. These compounds may therefore have direct actions on the excitatory ACh receptor in Aplysia neurons which are similar to the effects of these drugs at the vertebrate endplate.

Acetylcholine↗
PubMed1986

Spectra of neurotransmitter receptors and ionic responses on cerebral A and B neurons in Aplysia californica.

Receptors to putative transmitters on A and B cells of Aplysia californica were identified and characterized. Neurons within each cluster were similar in responses to transmitters, but the neurons in A and B clusters differed. Both exhibited receptors to acetylcholine, dopamine, gamma-aminobutyric acid, glutamate, histamine and serotonin but not to octopamine or phenylethanolamine. Bi- or multiphasic responses to a single transmitter were common on both cell types. Inhibitory responses were more common on A than B cells where glutamate, histamine and serotonin were all excitatory. Each of these clusters appear homogeneous both in terms of presence of receptors and the ionic channels activated by the receptors.

Acetylcholine↗