PubMed Health⌕ Search

Biomedical subjects

D O Carpenter

Publications and source records attributed to D O Carpenter.

At least 19 recordsLinked to original sources
PubMed1992

Zinc (Zn2+) blocks voltage gated calcium channels in cultured rat dorsal root ganglion cells.

Dorsal root ganglion cells (DRGs) exhibit 3 types of voltage-dependent calcium channels. We have cultured DRGs from 2- to 4-day-old rat pups and obtained whole-cell patch-clamp recordings of calcium-channel currents after 1-5 days in culture. The calcium-channel currents (carried by barium) were recorded with tetrodotoxin (TTX) in the external solution. A cesium-based solution containing Na-ATP, HEPES and EGTA was used in the recording pipette. Cells were held at -80 mV and calcium channel currents were evoked by stepping to depolarized voltages. The divalent cation zinc (Zn2+) blocked sustained and transient voltage sensitive calcium channel currents. Onset of the blockade was fast and a steady-state was reached within 5-15 min, depending upon the concentration used. The IC50 for inhibition of the peak current evoked by a step depolarization from -80 mV to 0 mV (N plus L channels) for 80 ms was 69 microM Zn2+ and the Hill slope about 1. The calcium current evoked by a voltage step from -80 mV to voltages between -40 mV and -15 mV (T-type current) was more sensitive (> 80% block with 20 microM Zn2+). During wash the effect was only partly reversible in 50% of the neurons. Thus, Zn2+ is a potent blocker of voltage dependent calcium currents in mammalian neurons, especially of T-type currents.

Animals↗
PubMed1992

Inhibition of energy metabolism by 3-nitropropionic acid activates ATP-sensitive potassium channels.

3-Nitropropionic acid (1 mM), which inhibits succinate dehydrogenase activity and reduces cellular energy, produces in the pyramidal cell layer of the hippocampal region CA1 a hyperpolarization for variable lengths of time before evoking an irreversible depolarization. Hyperpolarization is caused by an increased potassium conductance that is attenuated by glibenclamide (1-10 microM), a selective antagonist of ATP-sensitive potassium channels; in contrast, diazoxide (0.5 mM), an agonist at this channel, induces a hyperpolarization in CA1 neurons of rat hippocampal slices. The transient hyperpolarization after prolonged (ca. 1 h) application of 3-NPA is followed by a depolarization that is incompletely reversed by brief application of the glutamate antagonists (D-2-amino-5-phosphonopentanoic acid (APV), 6,7-dichloroquinoxaline-2,3-dione (CNQX), 3-(+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), 7-chloro-kynurenic acid (7Cl-KYN)). Early application of glibenclamide (within the initial 5 min) blocked or reduced hyperpolarization and accelerated the depolarization. These data suggest that metabolic inhibition by 3-NPA initially activates ATP-sensitive potassium channels. Events other than activation of glutamate receptors participate in the final depolarization resulting from uncoupling of oxidative phosphorylation.

Adenosine Triphosphate↗
PubMed1992

Distinct receptors for Leu- and Met-enkephalin on the metacerebral giant cell of Aplysia.

1. The effects of D-Ala2-Leu-enkephalin (DALEU), D-Ala2-Met-enkephalin (DAMET), and FMRFamide on the metacerebral cell (MCC) of Aplysia were determined in current- and voltage-clamp experiments. 2. Distinct receptors exist on this neuron for the three substances. 3. DALEU elicited a depolarizing response due to an inward current but not accompanied by a significant change in membrane conductance. 4. In contrast, DAMET elicited a hyperpolarizing response due to an outward current, also not associated with a significant change in membrane conductance. 5. Both the DALEU and the DAMET responses increased with hyperpolarization, decreased with depolarization, but did not reverse at potentials less than -30 mV. Neither response was sensitive to naloxone. 6. FMRFamide induced a voltage-dependent outward current that reversed at about -76 mV. This neuron was responsive to much lower concentrations of FMRFamide than either of the enkephalins, and the response to FMRFamide appears to be a conductance increase to K+. 7. These results suggest that the MCC neuron has distinct receptors for Leu- and Met-enkephalin that activate unusual responses of opposite polarity, as well as more usual inhibitory responses to FMRFamide.

Action Potentials↗
PubMed1992

A topography and ultrastructural characterization of in vivo 5,7-dihydroxytryptamine-labeled serotonin-containing neurons in the central nervous system of Aplysia californica.

1. Several weeks after administration of 5,7-dihydroxytryptamine (5,7-DHT) to Aplysia, a dark pigmentation appears in serotonin-containing neurons, and this pigmentation allows visual identification of serotonergic neurons but does not appear to alter their physiology. 2. We have determined the distribution of labeled nerve cell bodies in the various ganglia of Aplysia and have characterized the pigment containing structures in both control and labeled neurons. 3. All neurons in this preparation, whether or not they utilize serotonin as a transmitter, contain pigment granules, and three types of pigment granules can be distinguished. After 5,7-DHT a new type of granule appears in serotonergic neurons, probably reflecting lysosomes that have accumulated serotonergic synaptic vesicles that contain the oxidized 5,7-DHT. 4. It remains unclear why this substance does not cause neurotoxicity in mollusks as it does in mammalian preparations.

5,7-Dihydroxytryptamine↗
PubMed1992

Long-term potentiation is lost in aged rats but preserved by calorie restriction.

Recordings of synaptic population responses, post-tetanic potentiation (PTP) and long-term potentiation (LTP) were made from area CA1 in hippocampal slices from ad libitum-fed rats at about 2 and 24 months of age, and also in animals at about 24 months of age that had been restricted to 60% of the caloric intake of control animals since weaning. Both PTP and LTP were greatly reduced in the old ad-lib animals. Calorically-restricted rats at about 24 months of age showed hippocampal responses with initial peak amplitudes more like those of 2-month controls than the ad-lib animals at 24 months. These observations suggest that calorie restriction preserves nervous-system functions, including indicators of plasticity such as LTP, which are otherwise lost in aging.

Aging↗
PubMed1992

Dynamic changes in neuronal volume resulting from osmotic and sodium transport manipulations.

Electrophysiologic parameters such as input resistance and response to microperfusion of neurotransmitters vary under circumstances where neurons from isolated ganglia of Aplysia californica are subjected to either long-term (several hours) blockade of active sodium transport or to hypo- or hyperosmotic solutions. Since one of multiple possible events under these circumstances is neuronal volume changes, we have developed a system using cultured Aplysia neurons and confocal scanning laser microscopy to directly monitor cell volume when the osmolarity of the perfusing solution is altered and when sodium transport is blocked. Volume changes of greater than 30% were observed, accompanied by changes in surface area of greater than 15%. The volume increase secondary to sodium pump inhibition and hypotonic solutions and the volume decrease secondary to hypertonic solutions were reversible. Our results demonstrate that neuronal volume may change dramatically and raise the possibility that dynamic changes in neuronal cell volume may have physiological importance.

Animals↗
PubMed1991

Pb2+ blocks calcium currents of cultured dorsal root ganglion cells.

The divalent cation lead (Pb2+) blocks sustained and transient voltage sensitive calcium channel currents of cultured rat dorsal root ganglion cells. The IC50 for inhibition of the total peak current evoked by a step depolarization from -80 to 0 mV was 0.6 microM, compared to an IC50 of 2.2 microM for Cd2+. The current activated by a depolarization from -40 to 0 mV was inhibited by 50% by 1.0 microM Pb2+. Low threshold currents activated by a step from -100 to -30 mV were blocked by Pb2+ at higher concentrations (IC50 = 6 microM). The block progressed in the absence of channel activation and showed little voltage dependence. Peak sodium current was reduced by 6.6% at 1 microM Pb2+ while at 20 microM the peak current was reduced by 40% with marked slowing of the time course of activation. The potassium rectifier current was reduced by 4.1% at 1 microM Pb2+. Thus, Pb2+ selectively blocks calcium currents at concentrations in the range of those causing toxicity in man.

Animals↗
PubMed1991

Correlation of 125I-LSD autoradiographic labeling with serotonin voltage clamp responses in Aplysia neurons.

Autoradiographic receptor binding studies using 125I-LSD (2-[125I]lysergic acid diethyamide) revealed intense labelling on the soma of a symmetrically located pair of cells in the abdominal ganglion of Aplysia californica. This binding was blocked by micromolar concentrations of serotonin and lower concentrations of the serotonergic antagonists, cyproheptadine and mianserin (Kadan and Hartig, 1988). Electrophysiological investigation of responses to serotonin of neurons in the left upper quadrant, where one of the labeled neurons is located, revealed a range of serotonin responses. Cells L3 and L6 have a K+ conductance increase in response to serotonin that is not blocked by cyproheptadine or mianserin. Cells L2 and L4 have a biphasic response to serotonin: a Na+ conductance increase, which can be blocked by cyproheptadine and mianserin, followed by a voltage dependent Ca2+ conductance which is blocked by Co2+ but not the serotonergic antagonists. Cell L1, and its symmetrical pair, R1, have in addition to the Na+ and Ca2+ responses observed in L2 and L4, a Cl- conductance increase blocked by LSD, cyproheptadine and mianserin. LSD had little effect on the other responses. We conclude that the symmetrically located cells L1 and R1 have a Cl- channel linked to a cyproheptadine- and mianserin-sensitive serotonin receptor that is selectively labelled by 125I-LSD. This receptor has many properties in common with the mammalian serotonin 1C receptor.

Acetylcholine↗
PubMed1991

Appearance of NMDA receptors triggered by anoxia independent of voltage in vivo and in vitro.

Using rat hippocampus we have studied the pattern of neuronal death, abnormal discharge and loss of electrical excitability in slices prepared from animals subjected to bilateral, four-vessel cerebral anoxia and in slices prepared from normal animals that are subjected to anoxia in the recording chamber. As others have reported, pyramidal neurons in area CA1 are lost first after anoxia, while CA3 neurons have an intermediate sensitivity, and those in dentate are relatively anoxia-resistant. After anoxic damage to the intact animal, neurons in both CA1 and CA3 show abnormal bursting discharges in response to synaptic activation for several days, and then the response in CA1 decreases in amplitude and finally the area become unexcitable. While antagonists for N-methyl-D-aspartate (NMDA) receptors have essentially no effect on synaptic responses in control animals, they reduce the bursting responses and greatly depress the small responses in CA1 as neurons are becoming unexcitable after anoxia. With intracellular recording CA1 neurons from animals made transiently anoxic, in contrast to controls, show prolonged synaptic responses, the later components of which are blocked by NMDA antagonists. When slices from normal animals are subjected to anoxia such that excitability is totally lost over a period of about 10 min, there is no significant membrane depolarization during the anoxic episode and recovery of excitability occurs with reoxygenation. However, a period of hyperexcitability and bursting follows and electrical excitability is lost in CA1 but not CA3 neurons after about 90 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

Lead and zinc block a voltage-activated calcium channel of Aplysia neurons.

1. The effects of Pb2+ and Zn2+ on the peak of the voltage-activated calcium current of Aplysia neurons were examined. Calcium currents were reversibly blocked by Pb2+ at concentrations that did not significantly affect potassium and sodium currents and by Zn2+ at concentrations associated with a delay and reduction of peak sodium and potassium currents. 2. The block by both was concentration dependent, and percentage blockade was reduced in elevated Ca2+. The threshold Pb2+ concentration for blockade in 20 mM Ca artificial sea water (ASW) was approximately 1 microM, whereas for Zn2+ it was 2 mM. The Hill coefficient for Pb2+ action was near 1.0 under all conditions, whereas for Zn2+ it was 1.4-1.6. 3. With addition of Pb2+, the voltage at which peak calcium current was generated shifted to hyperpolarized voltages, an effect similar to that caused by reduction of Ca2+ concentration in the absence of Pb2+. Zn2+ shifted the voltage at which peak current was generated in a depolarizing direction. 4. Pb2+ did not significantly change inactivation but shifted the voltage dependence of activation to hyperpolarized voltages in a dose-dependent manner. Zn2+ shifted both activation and inactivation in a depolarizing direction in a dose-dependent fashion. 5. The blockade of calcium currents by Pb2+ but not Zn2+ was highly voltage dependent and increased with depolarization. 6. Our results suggest that Pb2+ is a specific, potent, competitive, and reversible blocker of calcium currents. These observations are consistent with a competition by Pb2+ with Ca2+ at a binding site within the calcium channel. In contrast, the blockade of calcium currents by Zn2+ is probably through actions at fixed charge sites external to the channel.

Animals↗
PubMed1991

Effects of inorganic and triethyl lead and inorganic mercury on the voltage activated calcium channel of Aplysia neurons.

Using conventional two electrode voltage clamp techniques we have studied the effects of Pb2+, triethyl lead (TEL) and Hg2+ on voltage-activated calcium channels of Aplysia neurons and found that all three metals are potent inhibitors at micromolar concentrations. However, the time course of current reduction or block and its reversibility vary when comparing Pb2+ to TEL and Hg2+. With application of Pb2+ the calcium current decreases immediately and a steady state is reached within three to seven minutes, depending upon the concentration of Pb2+ (IC50 = 61 microM). The block was easily reversed upon wash out of Pb2+ with a time course similar to that of onset. Perfusion with either TEL (5 to 50 microM) or Hg2+ (5 to 200 microM) resulted only in a small reduction of current when the substances reached the cell membrane but with clear reduction within 2 min. The decrease continued at about the same speed for the total duration of the application. Upon washing there was no recovery of the response. At the onset of washing the rate of current decline stopped for several minutes, but then the current continued to decline at a slower rate in the absence of toxicant. Our data suggest that Pb2+ acts by a direct and reversible blockade of the calcium channel. In contrast TEL and Hg2+ act slowly and irreversibly to block calcium channels at concentrations which do not greatly affect membrane potential or resistance. In spite of the slow time course these substances are probably acting directly on the calcium channel.

Animals↗
PubMed1990

Zn2+ blocks the voltage activated calcium current of Aplysia neurons.

We have investigated the effect of Zn2+ on voltage-activated calcium currents of Aplysia neurons, using conventional two-electrode voltage-clamp techniques. The peak of these currents was reversibly reduced by Zn2+ (50% reduction at 3.75 mM; total block at 20 mM), while the current-voltage relation and the activation and inactivation curves were shifted to depolarized voltages. The effects of Zn2+ were concentration-dependent. The Hill coefficient was 1.62. The high concentrations required, the shift of the current-voltage relation and the effects on activation and inactivation are best explained by a charge-screening effect combined with a specific binding site for Zn2+ near the entrance of the channel.

Animals↗
PubMed1990

Differential effects of baclofen and gamma-aminobutyric acid (GABA) on rat piriform cortex pyramidal neurons in vitro.

1. The effects of baclofen and GABA on rat piriform cortex neurons were investigated electrophysiologically using a brain slice preparation. 2. At resting potential GABA depolarized and baclofen hyperpolarized the cell, probably through activation of Cl and K conductances acting at GABAA and GABAB receptors, respectively. 3. The GABAA receptors were concentrated on the apical and basal dendrites near the cell body, while the baclofen-sensitive GABA receptors were concentrated particularly on the basal dendrites. 4. The different distributions of receptor localization must have functional consequences which remain to be elucidated.

Animals↗
PubMed1990

Influences of trypsin and collagenase on acetylcholine responses of physically isolated single neurons of Aplysia californica.

1. The influences of enzyme treatments (trypsin and collagenase) on responses to perfused acetylcholine were examined on physically isolated single Aplysia neurons, using the voltage-clamp, internal perfusion, and rapid external perfusion technique. 2. During treatment with trypsin (0.025 to 0.1%) for 10 to 30 min at room temperature (22 to 25 degrees C), the peak amplitude of the Na current induced by acetylcholine increased in a time- and dose-dependent manner, and the decay in the continued presence of acetylcholine was slowed. This effect of trypsin treatment was irreversible after washing for 60 min without enzyme. 3. Edrophonium, a cholinesterase inhibitor, has previously been shown to augment the Na acetylcholine response in this preparation by inhibition of acetylcholinesterase. After treatment of the neuron with trypsin, the augmentation after edrophonium was abolished. Furthermore, in the presence of edrophonium, trypsin also failed to increase the response. The dose-response curve for acetylcholine after treatment of trypsin was similar to that in the presence of edrophonium. These results suggest that the modification of the current response by trypsin is a result of removal of cholinesterase activity from the membrane. 4. In contrast to the effects of trypsin, collagenase (0.03 to 0.1%) for 10 to 60 min did not change the current amplitude of the acetylcholine response. However, collagenase treatment did alter the kinetics of the acetylcholine response in a dose-dependent manner, in that the rate of decay was accelerated. A similar acceleration was seen in the acetylcholine responses on other neurons which were due to Cl or K currents, suggesting that the effect was independent on the type of channel. This effect of collagenase was reversible after 30 to 60 min of washing of the neuron. 5. In the presence of edrophonium or after the treatment with trypsin, collagenase still accelerated the current kinetics of the acetylcholine response, indicating that cholinesterase activity is not related to this effect. Furthermore, heated collagenase (presumably inactivated) had a similar action, suggesting that the enzymatic activity of collagenase is not related to the modification of the response. 6. These results suggest that Aplysia acetylcholinesterase is sensitive to trypsin but not to collagenase. However, the preparation of a collagenase used in these studies contains some factor which alters the response to acetylcholine, but this effect is reversible and unrelated to enzymatic activity.

Acetylcholine↗
PubMed1990

Lead inhibits the voltage-activated calcium current of Aplysia neurons.

Lead is a potent and reversible inhibitor of the voltage-dependent calcium current of Aplysia neurons in a concentration range (1-1000 microM) similar to that which elicits toxic effects in man. The threshold for inhibition is 1 microM and the dissociation constant about 90 microM. The inhibition is due to reduction of the peak current amplitude, and not due to alteration of the voltage dependence of the activation or inactivation. The effect of lead is specific to the calcium current as the delayed rectifier potassium and the sodium currents are not affected by concentrations of lead (200 microM) which give near-maximal inhibition. The calcium current of Aplysia neurons appears similar to the current flowing through the mammalian L-type calcium channel. This suggests that inhibition of the investigated calcium channel may contribute to the toxic effects of lead in mammals.

Animals↗
PubMed1990

Neural mechanisms of emesis.

Emesis is a reflex, developed to different degrees in different species, that allows an animal to rid itself of ingested toxins or poisons. The reflex can be elicited either by direct neuronal connections from visceral afferent fibers, especially those from the gastrointestinal tract, or from humoral factors. Emesis from humoral factors depends on the integrity of the area postrema; neurons in the area postrema have excitatory receptors for emetic agents. Emesis from gastrointestinal afferents does not depend on the area postrema, but probably the reflex is triggered by projections to some part of the nucleus tractus solitarius. As with a variety of other complex motor functions regulated by the brain stem, it is likely that the sequence of muscle excitation and inhibition is controlled by a central pattern generator located in the nucleus tractus solitarius, and that information from humoral factors via the area postrema and visceral afferents via the vagus nerve converge at this point. This central pattern generator, like those for motor functions such as swallowing, presumably projects to the various motor nuclei, perhaps through interneuronal pathways, to elicit the sequential excitation and inhibition that controls the reflex.

Animals↗
PubMed1989

Desensitization kinetics of a K+ acetylcholine response in Aplysia.

We have studied the process of acetylcholine receptor desensitization on Aplysia medial pleural neurons under voltage clamp conditions. Acetylcholine, applied by microperfusion, elicits a biphasic response on these neurons, a rapid component which reverses polarity at about -60 mV and is Cl-dependent, and a slower component which reverses at about -85 mV and is K-dependent. Both components show desensitization, and the present study focuses on the K-dependent component, which could be isolated by maintaining membrane potential at the Cl equilibrium potential or by blocking the Cl component pharmacologically. K-dependent acetylcholine responses on these neurons varied in regard to time to peak of response and rate of desensitization. While the rising phase of the response was always fitted by a single exponential process, times to peak were divided somewhat arbitrarily into three broad groups of fast (less than 3 s), medium (3-6 s) and slow (greater than 6 s). Desensitization of fast responses was best described by two exponential processes plus a constant, medium responses by a double exponential, and slow responses by single exponential plus a constant. The apparent dissociation constant of acetylcholine was 17.3 +/- 1.6 microM. The best fit of responses for a given cell remained constant over a range of acetylcholine doses, but the kinetics of both fast and slow components accelerated with dose and depolarization. The fast component of desensitization was very temperature dependent. In neurons where it was present it was abolished by cooling, while in neurons with no fast component at room temperature it would appear with warming. The time constant of the fast component varied inversely with temperature. The time constant of the slow component was maximal at 22-24 degrees C, and fell on either side of this temperature. These results suggest that receptor desensitization for acetylcholine K responses is, like Na-dependent responses, composed of two independent processes. When responses to the acetylcholine agonists, carbachol and arecoline, were compared to those of acetylcholine on fast-type neurons, the times to peak varied in the order acetylcholine less than carbachol less than arecoline. The carbachol response was best fitted by two exponential functions, while arecoline was best fitted by a single exponential plus a constant.

Acetylcholine↗