Biomedical subjects

A L Gorman

Publications and source records attributed to A L Gorman.

At least 19 recordsLinked to original sources

PubMed2005

Differences in forebrain activation in two strains of rat at rest and after spinal cord injury.

Forebrain activation patterns in normal and spinal-injured Sprague-Dawley (SD) rats were determined by measuring regional cerebral blood flow as an indicator of neuronal activity. Data are compared to our previously published findings from normal and spinal-injured Long-Evans (LE) rats and reveal a striking degree of overlap, as well as differences, between strains in the basal (unstimulated) forebrain activation in normal animals. Specifically, 81% of the structures sampled showed similar activation in both strains, suggesting a consistent and identifiable pattern of basal cerebral activation in the rat. LE controls showed significantly greater basal activation in the remaining structures compared to SD control group, including the anterior dorsal thalamus, basolateral amygdala, SII cortex, and the hypothalamic paraventricular nucleus. In contrast, spinal cord injury (SCI) resulted in strain-specific changes in forebrain activation categorized by structures that showed significant increases in: (1) only LE SCI rats (posterior, ventrolateral, and ventroposterolateral thalamic nuclei); (2) only SD SCI rats (anterior-dorsal and medial thalamus, basolateral amygdala, cingulate and retrosplenial cortex, habenula, interpeduncular nucleus, hypothalamic paraventricular nucleus, periaqueductal gray); or (3) both strains (arcuate nucleus, ventroposteromedial thalamus, SI and SII somatosensory cortex). These results provide information related to the remote, i.e. supraspinal, effects of spinal cord injury and suggest that genetic differences play an important part in the forebrain response to such injury. Brain activation studies therefore provide a useful tool in understanding the full extent of secondary consequences following spinal injury and for identifying potential central mechanism responsible for the development of pain.

Animals↗

PubMed2001

Conditions affecting the onset, severity, and progression of a spontaneous pain-like behavior after excitotoxic spinal cord injury.

Intraspinal injection of quisqualic acid (QUIS) is associated with the development of spontaneous excessive grooming behavior in male Sprague Dawley rats. To further characterize this pain-like behavior we evaluated the relationship between the onset of this behavior and the rostrocaudal spread of injury-induced neuronal loss in 3 different strains of male rats. The severity and progression of this behavior also were evaluated. Unilateral intraspinal injections of 125 mmol/L QUIS were made in the following groups: Sprague Dawley males (SDMs, n = 21); Long Evans males (LEMs, n = 17); and Wistar Furth males (WFMs, n = 11). Because of differences in grooming characteristics between male and female rats, the modulatory effects of female gonadal hormones also were evaluated in Sprague Dawley females (SDFs, n = 17); bilaterally ovariectomized Sprague Dawley females (OVXs, n = 11); and SDMs treated with either 17-beta-estradiol (50 microg/kg; SDM-Est, n = 9) or progesterone (5 mg/kg; SDM-Pro, n = 11). The results showed that the development of excessive grooming behavior in males of all strains and ovariectomized females is related to the rostrocaudal spread of a specific pattern of neuronal loss in the dorsal horn. Excessive grooming behavior in SDFs was similar in many respects to that found in SDMs; however, SDFs did not show a dependence on the longitudinal extent of injury for the onset of this behavior. The onset, severity, and progression of excessive grooming in OVX females were similar to that found in SDMs. Furthermore, 8 of 9 estradiol-treated SDMs developed severe grooming characterized by an early onset and progressive time course, whereas progesterone treatment delayed the onset of grooming and attenuated its severity and progression. Strain-related differences in some, but not all, grooming characteristics also were observed, eg, WFMs exhibited more aggressive grooming than SDMs or LEMs. In conclusion, the results showed gender, strain, and gonadal hormones influence the onset and progression of injury-induced excessive grooming behavior. A causal relationship also was found between the onset of this behavior and the longitudinal extent of injury.

Journal Article↗

PubMed1999

Oral ketamine is antinociceptive in the rat formalin test: role of the metabolite, norketamine.

The present study was designed to evaluate the oral efficacy and bioavailability of ketamine. Antinociceptive efficacy was determined with the rat formalin test and oral bioavailability by the measurement of plasma and brain concentrations of ketamine and its major metabolite, norketamine. Oral ketamine in a dose range from 30 to 180 mg/kg or saline was given prior to intraplantar formalin and the flinching behavior was measured. Oral ketamine dose-dependently reduced the flinching during phase 2, while flinching during phase 1 was reduced only with the highest dose given. Following oral ketamine at 100 mg/kg, blood and brain samples were obtained and plasma and brain ketamine and norketamine levels were measured using high-performance liquid chromatography (HPLC). The average concentration ratio of norketamine/ketamine, as expressed by the area under the curve (AUC) value, was 6.4 for plasma and 2.9 for brain. These results demonstrate that a significant amount of norketamine is formed by first pass biotransformation of ketamine and is distributed to the brain. Competition binding assays for the [3H]MK-801-labeled non-competitive site of the N-methyl-D-aspartate receptor (NMDA) receptor revealed that both norketamine and ketamine displaced [3H]MK-801 at low micromolar concentrations with Ki values of 2.5 and 0.3 mM in the forebrain, and 4.2 and 1.0 mM in the spinal cord, respectively. Spinal norketamine was approximately equipotent to ketamine in producing antinociceptive effects during phase 2 of the formalin test. Thus, norketamine appears to contribute to the antinociceptive effects of oral ketamine through its NMDA receptor antagonist activity.

Administration, Oral↗

PubMed1997

The d- and l-isomers of methadone bind to the non-competitive site on the N-methyl-D-aspartate (NMDA) receptor in rat forebrain and spinal cord.

Racemic (dl)-methadone has antagonist activity at the N-methyl-D-aspartate (NMDA) receptor. We evaluated dl-methadone, the opioid active (l-) and the opioid inactive (d-) isomers in competition binding assays. dl-Methadone and its d- and l- isomers exhibited low micromolar affinities for the [3H]MK-801-labeled non-competitive site of the NMDA receptor in both rat forebrain and spinal cord synaptic membranes, with Ki values and displacement curves similar to those of dextromethorphan, an established NMDA receptor antagonist. They lacked affinity at the [3H]CGS-19755-labeled competitive site of the NMDA receptor. Therefore, both methadone and its the d- and l- isomers differ from morphine, hydromorphone, and naltrexone in that they have non-competitive antagonist activity at the NMDA receptor. A non-opioid NMDA receptor antagonist, such as d-methadone, may improve the efficacy of morphine by attenuating the development of tolerance.

Animals↗

PubMed1993

Beta-adrenergic receptors are involved in stress-related behavioral changes.

Cerebral noradrenergic systems have been implicated in stress-related changes in behavior. Previous studies with receptor antagonists suggested that alpha 1-adrenergic receptors were involved in defensive withdrawal in rats and in investigatory behavior in mice tested in the multicompartment chamber. However, beta-adrenoreceptor antagonists attenuated the restraint- and ICV CRF-induced changes in defensive withdrawal, suggesting that beta-adrenergic receptors may also be involved in stress-related responses. To determine whether the beta-adrenergic antagonist effect was limited to rats tested in the defensive withdrawal model, we studied the effects of L-propranolol in two other behavioral models. Propranolol pretreatment (2.5 mg/kg, IP) prevented the restraint-induced changes in the behavior of mice observed in the multicompartment chamber and the elevated plus-maze. It also decreased the plasma corticosterone response measured in restrained mice after plus-maze testing. To investigate further the role of central beta-adrenergic receptors in defensive withdrawal, the effects of the beta-adrenoreceptor agonist isoproterenol were tested. Isoproterenol (0.3-10 micrograms, ICV) produced a dose-dependent increase in defensive withdrawal, statistically significant after 3 and 10 micrograms. Propranolol prevented the isoproterenol-induced defensive withdrawal, suggesting that the effect of isoproterenol resulted from stimulation of beta-adrenergic receptors. These results support earlier data suggesting the involvement of CNS beta-adrenergic receptors in stress-related behavioral changes and suggest that beta-adrenergic agonists exert anxiolytic effects that differ from those of the benzodiazepines.

Adrenergic beta-Agonists↗

PubMed1992

Anxiogenic effects of acute and chronic cocaine administration: neurochemical and behavioral studies.

The effects of cocaine on defensive withdrawal behavior in rats and elevated plus-maze behavior in mice were investigated. Cocaine (20 mg/kg IP) injected daily for 7 or 14 days induced defensive withdrawal; that is, the latency to emerge from a small chamber in an open field and the mean time in the chamber were both significantly increased. Acute cocaine administration also induced defensive withdrawal, and this effect was prevented by prior treatment with chlordiazepoxide (5 mg/kg IP). Both acute and chronic cocaine treatments significantly increased plasma concentrations of corticosterone and reduced the ratios of 3,4-dihydroxyphenylacetic acid to dopamine and 5-hydroxyindoleacetic acid to serotonin in several brain regions. Further evidence for an acute anxiogenic effect of cocaine was obtained from mice studied in the elevated plus-maze. Acute cocaine administration decreased both the number of entries into and the time spent in the open arms of the maze. These results taken together strongly support an anxiogenic action of acute and chronic cocaine administration.

Animals↗

PubMed1990

The involvement of central noradrenergic systems and corticotropin-releasing factor in defensive-withdrawal behavior in rats.

The role of the central noradrenergic systems and corticotropin-releasing factor (CRF) in modulating defensive withdrawal behavior was studied in rats. The apparatus consisted of a small chamber set on one side of a one-meter open field, into which the rat was placed to start the test. When rats were unfamiliar with the apparatus, they displayed species typical defensive withdrawal behavior with long latencies to emerge from and a high proportion of time spent in the small chamber. Intraperitoneal administration of clonidine (0.03 mg/kg), l-propranolol (2.5 micrograms/kg), prazosin (0.1 mg/kg) or chlordiazepoxide (CDP, 5 mg/kg) each significantly decreased the latency to emerge from and the mean time spent in the small chamber (MTIC) and increased the number of chamber entries. When rats were familiar with the apparatus, prior restraint for 20 min significantly increased the latency and MTIC, and decreased the number of chamber entries and rears, but did not alter locomotor activity. Prazosin, clonidine, CDP, l-propranolol and the CRF-antagonist, alpha-helical CRF9-41 (25 micrograms i.c.v.), reversed the restraint-induced increase in the latency and MTIC. CRF (10-100 ng i.c.v.) dose-dependently induced defensive withdrawal behavior in rats familiar with the apparatus; the minimum statistically significant dose was 50 ng. dl-Propranolol (5 mg/kg) and CDP blocked the CRF-induced changes in the latency to emerge and the MTIC; whereas clonidine and prazosin significantly reduced the latency, but had no statistically significant effects on the MTIC. Phenylephrine (25-200 ng i.c.v.) dose-dependently induced defensive withdrawal behavior. This effect of phenylephrine (200 ng) was significantly antagonized by prazosin or alpha-helical CRF9-41 (25 or 50 mg i.c.v.), but not by CDP. Our results suggest that the hyperactivity of the central noradrenergic systems caused by exposure to the novel environment may stimulate the release of CRF, which through some unknown mechanism induces defensive withdrawal behavior in rats. Activation of beta adrenergic receptors may also induce defensive withdrawal.

Animals↗

PubMed1984

Action of quinidine on ionic currents of molluscan pacemaker neurons.

The effects of quinidine on the fast, the delayed, and the Ca2+-activated K+ outward currents, as well as on Na+ and Ca2+ inward currents, were studied at the soma membrane from neurons of the marine mollusk Aplysia californica. External quinidine blocks these current components but to different degrees. Its main effect is on the voltage-dependent, delayed K+ current, and it resembles the block produced by quaternary ammonium ions (Armstrong, C. M., 1975, Membranes, Lipid Bilayers and Biological Membranes: Dynamic Properties, 3:325-358). The apparent dissociation constant is 28 microM at V = +20 mV. The blocking action is voltage and time dependent and increases during maintained depolarization. The data are consistent with the block occurring approximately 70-80% through the membrane electric field. Internal injection of quinidine has an effect similar to that obtained after external application, but its time course of action is faster. External quinidine may therefore have to pass into or through the membrane to reach a blocking site. The Ca2+-activated K+ current is blocked by external quinidine at concentrations 20-50-fold higher compared with the delayed outward K+ current. In addition, it prolongs the time course of decay of the Ca2+-activated K+ current. Na+ and Ca2+ inward currents are also blocked by external quinidine, but again at higher concentrations. The effects of quinidine on membrane currents can be seen from its effect on action potentials and the conversion of repetitive "beating" discharge activity to "bursting" pacemaker activity.

Animals↗

PubMed1984

Intracellular calcium measured with calcium-sensitive micro-electrodes and Arsenazo III in voltage-clamped Aplysia neurones.

Selected neurones of the abdominal ganglion of Aplysia californica were voltage clamped, injected with the Ca2+-indicator dye Arsenazo III, and impaled with Ca2+-selective micro-electrodes. Measurements of the absorbance signal (Arsenazo III) and Ca2+ micro-electrode potential during and following voltage-dependent Ca2+ influx (induced by voltage-clamp pulses) were simultaneously recorded. In neurones held at -50 mV, the mean intracellular free Ca2+ concentration [( Ca]i) measured by the Ca2+ micro-electrode was 0.18 microM, S.D. = 0.22 microM, n = 13. Bathing the cell in 0 Ca2+ artificial sea water (ASW) or intracellularly injecting EGTA decreased the resting [Ca]i. Voltage-clamp pulses, which maximally activated Ca2+ channels (from -50 to +30 mV), transiently increased both the Arsenazo III absorbance and the Ca2+ micro-electrode signals, indicating a rise in [Ca]i. Given the Ca2+ micro-electrode's limited band width, the peak of the Ca2+ signal during the pulse train could not be resolved; however, there was a net deflexion of this signal following the last pulse which slowly decayed to base line. Bathing the cells in 0 Ca2+ ASW, or reducing the driving force for Ca2+ entry (by stepping the voltage-clamp pulses to much higher membrane potentials) dramatically reduced both the absorbance and the Ca2+ micro-electrode signal increases. On the other hand, bathing the cells in 100 mM-Ca2+ ASW increased both signals. The intracellular Ca2+ gradient within the cytoplasm following voltage-clamp pulses was investigated by moving the Ca2+-selective micro-electrode tip in a step-wise manner relative to the membrane surface. The measured rise in [Ca]i was greatest near the membrane and not measurable within 40-50 microns of the membrane surface. The amplitude of the [Ca]i rise at different distances from the membrane could be fitted by a model based on a simple diffusion of Ca2+ from a plane source.

Action Potentials↗

PubMed1983

Localization of neuronal Ca2+ buffering near plasma membrane studied with different divalent cations.

Absorbance changes associated with divalent cation binding to arsenazo III were used to measure changes in Ca2+, Sr2+, and Ba2+ concentrations under a variety of experimental conditions. The rate of the falling phase of an absorbance change signal, measured in nerve cell bodies injected with arsenazo III and under membrane potential control, was taken as an index of divalent cation buffering. With influx of ions through the membrane or with ionophoretic injection, we found the buffering, i.e., the dye-absorbance signal's falling rate, to be greatest for Ca2+ ions: the sequence was Ca2+ greater than Sr2+ much greater than Ba2+. Injecting Ca2+ or Sr2+ into the center of a nerve cell produced a significantly greater amplitude of arsenazo III signal than the same injection near the cell membrane. We did not find this to be the case for Ba2+ or Mg2+ injections. We conclude that the Ca2+ regulatory system binds Ca2+ most strongly compared to the other ions tested, and there is a variable distribution of buffering machinery within the nerve soma, with increased buffer capacity near the plasma membrane of the cell. A preliminary report of some of the results presented in this paper has appeared previously ( Tillotson and Gorman, 1980).

Animals↗

PubMed1983

Ionic and spectral mechanisms of the off response to light in hyperpolarizing photoreceptors of the clam, Lima scabra.

Intracellular recordings were made from distal photoreceptor cells of the file clam Lima scabra in order to examine the ionic and spectral mechanisms which underly the response to light decrement. These receptors are primary sensory neurons that generate nerve impulses in the optic nerve upon light termination without benefit of synaptic interconnections between photoreceptor cells. Microelectrode measurements were made on these cells. Membrane conductance changes were assessed by measuring membrane voltage changes elicited under different conditions while passing constant-current pulses through the microelectrode from an active bridge amplifier. Responses of membrane potential in light and darkness in different concentrations of external potassium ions were fitted to a simplified form of the constant field equation. This analysis allowed an estimation of internal potassium activity (281 mM) as well as changes in PNa/PK in darkness and light. PNa/PK changed from 0.09 in darkness to 0.03 at the peak of the light response. A persistent decrease in membrane conductance at the termination of light is associated with a depolarization that overshoots the dark resting membrane potential. This transient depolarization is dependent on the intensity and duration of the preceding period of light. The amplitude of the dark-dependent depolarization is related to the absorbance of light during the preceding period of light by a long wavelength intermediate of rhodopsin bleaching (metarhodopsin). The frequency of discharge of action potentials with rapid kinetics which occurs following light is proportional to the amplitude of the after depolarizing response. The delay to onset of the discharge is inversely proportional to the amplitude of the after depolarizing response. The sensitivity (response/photon) of distal cells can be modified by background light which passes through a screening pigment found in cells that surround the eye. These data, taken together, provide an explanation for the persistent discharge of action potentials which occurs on termination of light in these cells as well as the visual cells of other gastropod mollusks.

Animals↗

PubMed1983

The cation selectivity and voltage dependence of the light-activated potassium conductance in scallop distal photoreceptor.

Light-dependent voltage and current responses were measured from the distal hyperpolarizing photoreceptors of the scallop (Pecten irradians) retina. In normal external solution, the hyperpolarizing receptor potential was caused by a light-dependent K+ outward current. The magnitude of the hyperpolarizing receptor potential and the light-dependent outward current, measured at the resting potential, was graded with light intensity. In normal external solution, during prolonged illumination the light-dependent K+ outward current was characterized by an early peak and a subsequent plateau. Current responses to brief light flashes were reduced progressively during background illumination. In the absence of external Na+ ions, the reversal potential for the receptor potential changed 58 mV per 10-fold change in the extracellular K+ concentration. The estimated internal K+ concentration was 385 mM. The hyperpolarizing receptor potential produced by prolonged bright illumination consists of an early peak which decays to a plateau. This decay was determined by a decrease in the light-dependent K+ conductance during maintained illumination. The light-dependent conductance pathway passed outward currents better than inward K+ currents. The light-dependent K+ conductance was estimated to increase e-fold per 23-34 mV depolarization at the peak and during the plateau of the light response. The light-dependent conductance pathway was highly selective for K+ ions. The selectivity sequence for monovalent cations was T1+, K+ greater than Rb+ greater than NH4 greater than Cs+, Li+, Na+. External caesium and tetraethylammonium blocked inward but not outward K+ currents through the light-dependent K+ conductance pathway. The data suggest that K+ ions move through an aqueous pore which is controlled by light.

Action Potentials↗

PubMed1983

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor.

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

Action Potentials↗

PubMed1982

The prolonged hyperpolarizing afterpotential in an invertebrate photoreceptor: wavelength and ionic dependence.

A single electrode voltage clamp was used to examine the prolonged hyperpolarizing afterpotential (PHA) which accompanies photoconversion of a substantial fraction of rhodopsin (lambda max = 500 nM) to metarhodopsin (lambda max = 575 nM) in distal photoreceptor cells in the retina of the bay scallop, Pecten irradians. The PHA appears to result from a persistent light-activated outward K+ current passing through the same channels responsible for the normal receptor potential in these cells.

Animals↗

PubMed1982

Selectivity of the Ca2+-activated and light-dependent K+ channels for monovalent cations.

The ionic selectivity of the Ca(2+)-activated K(+) channel of Aplysia neurons and of the light-dependent K(+) channel of Pecten photoreceptors to metal and organic cations was studied. The selectivity sequence determined from reversal potential measurements is T1(+) K(+) > Rb(+) > NH(+) (4) > Cs(+) > Na(+), Li(+) and is identical to the sequence determined previously for voltage-dependent K(+) channels in a variety of tissues. Our results suggest that some physical aspect of the K(+) channel is conserved in phyllogenetically different tissues and cells.

Animals↗

PubMed1982

Ionic requirements for membrane oscillations and their dependence on the calcium concentration in a molluscan pace-maker neurone.

1. Membrane currents from the bursting pace-maker neurone R-15 of Aplysia were measured under conditions designed to simulate membrane oscillations. Changes in the absorbance of the Ca(2+)-sensitive dye arsenazo III were used to monitor changes in the free intracellular Ca(2+) concentration, [Ca](i), under these conditions. In addition, changes in the extracellular K(+), concentration [K](o) were measured with K(+)-sensitive electrodes.2. In normal external ionic conditions the depolarizing phase of pace-maker activity was associated with a slow inward current and the hyperpolarizing phase with a slow outward current.3. In cells where the early inward Na(+) current was blocked by tetrodotoxin and outward K(+) currents were suppressed by intracellular EGTA and extracellular tetraethylammonium and 4-aminopyridine, the slow inward current was significantly larger in amplitude and was suppressed by removal of external Ca(2+) or the addition of external La(3+), but not by the removal of external Na(+).4. The slow inward current was increased when [Ca](o) was raised and decreased when it was reduced in the manner expected for current flow through a Ca(2+) channel. The selectivity of the slow inward current for divalent cations was [Formula: see text].5. The slow inward current was only slightly reduced by a 10 degrees C reduction in temperature.6. In normal external and internal ionic conditions changes in dye absorbance occurred when the membrane was depolarized with slow triangular voltage ramps or long depolarizing steps within the pace-maker oscillation range. The obsorbance change, and thus the increase in Ca(2+), [Ca](i), was well correlated with the appearance of the slow inward current. Moreover, the magnitude of the slow outward current was dependent upon the change in [Ca](i).7. The slow inward current and a substantial fraction of the outward current, as well as the change in [Ca](i), were reduced appreciably by the addition of La(3+) ions (3 mM) to the external medium.8. The increase in [Ca](i) during prolonged depolarization was not affected by external tetrodotoxin or by the removal of external Na(+), but was abolished by a Ca(2+)-free external medium containing EGTA. Nevertheless, significant changes occurred in [Ca](i) during depolarization in 0.1 mM-external Ca(2+).9. In normal external and internal ionic conditions extracellular K(+), [K](o), increased during the depolarizing phase of the pace-maker cycle and decayed during the hyperpolarizing phase.10. There was a measurable increase in [K](o) during small prolonged depolarizing steps which produced a net inward current, indicating that inward and outward currents overlap under normal conditions.11. In the absence of action potential discharge, [Ca](i) increased during the depolarizing phase and decreased during the hyperpolarizing phase of the membrane oscillation.12. It is proposed that pace-maker oscillations depend upon three separate but linked systems which include a voltage-dependent Ca(2+) current, the free intracellular Ca(2+) concentration and the Ca(2+)-activated K(+) current.

Animals↗

PubMed1982

Quantitative differences in the currents of bursting and beating molluscan pace-maker neurones.

1. The spontaneous activity and the membrane conductances to Na(+), Ca(2+) and K(+) ions of the bursting pace-maker neurone R-15 and the repetitively discharging (beating) pace-maker neurone L-11 in the abdominal ganglion of the marine mollusc, Aplysia californica, were compared.2. The bursting pace-maker R-15 can be converted to a beating pace-maker neurone by the removal of external Ca(2+) or by the injection of EGTA intracellularly. Bursting pace-maker activity is not restored by changes in the resting potential.3. Spontaneous action potentials of cell R-15 are reduced, but not abolished, by the addition of tetrodotoxin (TTX) to block Na(+) currents or by the removal of external Ca(2+) to abolish Ca(2+) currents, whereas the spontaneous action potentials of cell L-11 are abolished by external TTX, but are unaffected by external Ca(2+) removal.4. The membranes of both cells contain Na(+) and Ca(2+) inward currents. The specific Na(+) conductance of both cells is of similar magnitude, whereas the specific Ca(2+) conductance is about half the Na(+) conductance in R-15 cells and an order of magnitude smaller in L-11 cells.5. The delayed K(+) conductance of cell L-11 is about 1.2 times greater than this conductance in cell R-15. The transient K(+) currents of the two cells are about the same magnitude.6. The Ca(2+)-activated K(+) conductance of cell R-15 and cell L-11 was estimated using two methods. The Ca(2+)-activated K(+) conductance of cell R-15 estimated from the difference in the total outward current in normal external solution and the delayed K(+) current in Ca(2+)-free solution (to preclude Ca(2+) influx) or after internal EGTA injection (to prevent Ca(2+) accumulation) is about 23 times greater than this conductance in cell L-11. The Ca(2+)-activated K(+) conductance of cell R-15, estimated from local internal Ca(2+) injections in Ca(2+)-free solution, is about 3 times greater than this conductance in cell L-11.7. The leakage conductance of cell L-11 is about 1.3 times greater than this conductance in cell R-15. This conductance increases by a factor of about 2 in both cells in Ca(2+)-free external solutions containing 1 mM-EGTA, but is unchanged or is decreased slightly by injection of EGTA internally.8. It is concluded that the Ca(2+) conductance and the Ca(2+)-activated K(+) conductance are appreciably greater in the bursting pace-maker neurone R-15 than in the beating pace-maker neurone L-11, whereas other voltage-dependent conductances to Na(+) and K(+) ions as well as the leakage conductance are quite similar. These quantitative differences provide a basis for understanding the different spontaneous activities of the two cells.

Action Potentials↗

PubMed1982

Responses of cortical neurons to stimulation of corpus callosum in vitro.

1. An in vitro slice preparation of rat cingulate cortex was used to analyze the responses of layer V neurons to electrical stimulation of the corpus callosum (CC). In addition, synaptic termination of callosal afferents with layer V neurons was evaluated electron microscopically to provide a structural basis for interpreting some of the observed response sequences. 2. Layer V neurons had a resting membrane potential (RMP) of 60 +/- 0.68 (SE) mV, an input resistance of 47 +/- 4.74 M omega, a membrane time constant of 4.37 +/- 0.51 ms, an electrotonic length constant of 1.38 +/- 0.25, and produced spontaneous action potentials that were 50 +/- 0.3 mV in amplitude. Intracellular depolarizing current pulses evoked spikes that were sometimes associated with low-amplitude (2-5 mV) depolarizing (5-10 ms in duration) and hyperpolarizing (10-20 ms in duration) afterpotentials. 3. A single stimulus of increasing intensities to the CC produced one of the following response sequences: a) antidromic spike and an excitatory postsynaptic potential (EPSP), which initiated one or more spikes; b) antidromic spike, EPSP-evoked action potentials, and a hyperpolarization, which may have represented an intrinsic cell property or inhibitory synaptic activity; c) EPSP and evoked spikes only; d) high-amplitude EPSP with an all-or-none burst of action potentials. 4. Antidromically activated (AA) neurons always produced EPSPs in response to CC stimulation. When compared with nonantidromically activated neurons, AA cells had a more negative RMP, greater electrotonic length constant (LN), higher ratio of dendritic to somatic conductance (rho), and formed shorter duration, callosal-evoked EPSPs. 5. Neurons in anterior cingulate cortex produced EPSPs of longer duration than did those in posterior cortex (50 +/- 3.57 versus 26 +/- 1.56 ms, respectively). EPSPs in anterior neurons also had a higher maximum amplitude (20.5 +/- 1.0 versus 11.5 +/- 0.79 mV) and longer time to peak (11.6 +/- 2.2 versus 8.2 +/- 0.8 ms). 6. Electron microscopy of Golgi-impregnated neurons following contralateral lesions demonstrated that both pyramidal and nonpyramidal neurons received direct callosal afferents. Synaptic termination of callosal axons with the apical dendritic trees of anterior pyramidal cells was 6 times greater than it was with posterior pyramidal neurons. 7. EPSP shape differences in anterior and posterior neurons may be partially accounted for by the density and distribution of callosal afferents to these two cortices.

Animals↗