Biomedical subjects

J N Reynolds

Publications and source records attributed to J N Reynolds.

At least 37 records · Page 2Linked to original sources

PubMed1999

Subunit dependent modulation of GABAA receptor function by neuroactive steroids.

Neurosteroids are potent, endogenous modulators of GABAA receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABAA receptor-activated membrane current (IGABA). The role of different alpha and gamma subunit subtypes in modulation of IGABA by ALP and AFX was investigated using recombinant GABAA receptor isoforms expressed in Xenopus oocytes. Changing or removal of the alpha subunit subtype altered the efficacy of both ALP and AFX (alpha2beta1gamma2L>alpha1beta1gamma2L>>beta1gamma2L) to potentiate IGABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance IGABA was also dependent on the gamma subunit subtype (alpha1beta1gamma3>alpha1beta1gamma2L = alpha1beta1gamma1). AFX also had higher efficacy in the alpha1beta1gamma3 receptor isoform compared to alpha1beta1gamma1. In contrast to ALP, the potency of AFX was greater in the alpha1beta1gamma3 and alpha1beta1gamma1 receptor isoforms compared to alpha1beta1gamma2L. This study provides evidence that the alpha subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate IGABA. The gamma3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other gamma subunit subtypes. These results suggest that the heteromeric assembly of different GABAA receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABAA receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABAA receptor.

Animals↗

PubMed1999

Chronic ethanol exposure alters MK-801 binding sites in the cerebral cortex of the near-term fetal guinea pig.

The mechanism of ethanol central nervous system (CNS) teratogenesis, resulting from chronic maternal ingestion of high-dose ethanol during pregnancy, is not clearly understood. One of the target sites for ethanol-induced damage in the developing brain is the cerebral cortex. It has been proposed that chronic prenatal ethanol exposure alters NMDA receptors in the developing cerebral cortex. To test this hypothesis, timed pregnant guinea pigs were administered one of the following oral treatments throughout gestation: 4 g ethanol/kg maternal body weight/day; isocaloric sucrose/pair-feeding; water; or no treatment (ad lib). Near-term fetuses were studied at gestational day (GD) 63 (term, about GD 68). This ethanol regimen produced a maternal blood ethanol concentration of 66+/-4 mM (304+/-19 mg/dl) at 1 h after the daily dose on GD 58. The chronic ethanol regimen decreased near-term fetal body weight (12-26% decrease), brain weight (23% decrease), and cerebral cortical weight (21% decrease), compared with the isocaloric sucrose/pair-feeding, and combined water/ad lib experimental groups. Saturation analysis of near-term fetal cerebral cortical membranes using a [3H]MK-801 radioligand binding assay demonstrated a decreased affinity and increased number of MK-801 binding sites for the chronic ethanol regimen compared with the control treatments. These data support the suggestion that upregulation of NMDA receptors in the cerebral cortex after chronic prenatal ethanol exposure could lead to NMDA receptor-mediated excitotoxicity in this brain region.

Animals↗

PubMed1999

Altered GABA(A)-benzodiazepine receptor number and pharmacology in the adult guinea pig cerebral cortex after chronic prenatal ethanol exposure.

BACKGROUND: The effects of chronic prenatal ethanol exposure on GABA(A)-benzodiazepine receptor number and binding pharmacology were examined in the cerebral cortex of the postnatal guinea pig. METHODS: [3H]Flunitrazepam binding to GABA(A)-benzodiazepine receptors was measured in a cerebral cortical cell membrane preparation obtained at postnatal days 11 (preweaning), 21 (postweaning), and 61 (adulthood). Zolpidem, a GABA(A)-benzodiazepine type 1 receptor-selective ligand, was used in a [3H]flunitrazepam competition study. 3a-Hydroxy-5alpha-pregnan-20-one (allopregnanolone) potentiation of [3H]muscimol and [3H]flunitrazepam binding, and GABA potentiation of [3H]flunitrazepam binding were measured in these same animals. RESULTS: At postnatal day 61, but not at the younger ages studied, the following was observed: (1) [3H]Flunitrazepam binding exhibited an increased receptor number (Bmax) and decreased affinity (increased K(D)) in the ethanol-treated offspring compared with isocaloric-sucrose (with pair-feeding) and water-treated controls; and (2) the relative proportion of GABA(A)-benzodiazepine receptors that had high-affinity binding sites for zolpidem was decreased in the ethanol-treated offspring by 31% and 38% compared with the isocaloric-sucrose/pair-fed and water-treated controls, respectively. Chronic prenatal ethanol exposure did not alter the efficiency of coupling between GABA, benzodiazepine, and neurosteroid binding sites at any postnatal ages studied. CONCLUSIONS: These results suggest that, in the cerebral cortex of the adult guinea pig, chronic prenatal exposure to ethanol results in increased GABA(A)-benzodiazepine receptor number, decreased affinity for flunitrazepam, and decreased relative proportion of the GABA(A)-benzodiazepine type 1 receptor.

Animals↗

PubMed1998

Propofol modulation of [3H]flunitrazepam binding to GABAA receptors in guinea pig cerebral cortex.

Binding of the radioligand [3H]flunitrazepam to membranes prepared from the cerebral cortex of adult, male guinea pigs under equilibrium and non-equilibrium conditions was used to investigate the allosteric interaction between the intravenous general anesthetic propofol and the benzodiazepine site of the GABAA receptor. Propofol induced a potentiation of [3H]flunitrazepam binding with an EC50 of 9+/-4 microM. Propofol increased the affinity for [3H]flunitrazepam binding with no change in maximal binding. Propofol did not change the rate constant of association for [3H]flunitrazepam binding to cerebral cortical membranes. In contrast, the rate constant for dissociation of [3H]flunitrazepam was significantly decreased in the presence of propofol. These data demonstrate that propofol increases the affinity of the benzodiazepine site of the GABAA receptor via a selective decrease in the rate constant for dissociation, and suggest a mechanism for the allosteric interaction between propofol and benzodiazepines at the GABAA receptor.

Anesthetics, Intravenous↗

PubMed1998

Modulatory and direct effects of propofol on recombinant GABAA receptors expressed in xenopus oocytes: influence of alpha- and gamma2-subunits.

Propofol (2,6-diisopropylphenol) is an intravenous general anaesthetic which can directly activate and positively modulate the GABAA receptor. The effects of propofol on human recombinant GABAA receptors were studied in Xenopus oocytes expressing either alpha1beta2, alpha1beta2gamma2L, or alpha2beta2gamma2L receptor isoforms. In all receptor isoforms tested, propofol was able to potentiate the GABA-activated currents in a concentration-dependent manner. Although propofol potentiated both alpha1beta2 and alpha1beta2gamma2L receptor isoforms with equal affinity, the efficacy of propofol potentiation was markedly greater in the alpha1beta2 receptor isoform. In contrast, potentiation of the alpha2beta2gamma2L receptor isoform by propofol occurred with higher affinity and lower efficacy than in the alpha1beta2gamma2L receptor isoform. Propofol directly activated all three receptor isoforms in a concentration dependent manner. Addition of the gamma2L subunit subtype to the alpha1beta2 receptor isoform decreased receptor sensitivity to direct activation by propofol. Replacement of the alpha1-subunit subtype with the alpha2-subunit subtype increased receptor sensitivity to propofol's direct effects. These results suggest that the alpha-and gamma2L-subunit subtypes each have the ability to influence both the direct and modulatory actions of propofol on GABAA receptor function.

Anesthetics, Intravenous↗

PubMed1998

Modulation of GABA(A) receptor function by neuroactive steroids: evidence for heterogeneity of steroid sensitivity of recombinant GABA(A) receptor isoforms.

Neuroactive steroids are potent, selective allosteric modulators of gamma-aminobutyric acid type A (GABA(A)) receptor function in the central nervous system, and may serve as endogenous anxiolytic and analgesic agents. In order to study the influence of subunit subtypes of the GABA(A) receptor on modulation of receptor function by neuroactive steroids, we expressed human recombinant GABA(A) receptors in Xenopus oocytes. GABA-activated membrane current, and the modulatory effects of the endogenous neurosteroid 5alpha-pregnan-3alpha-ol-20-one (allopregnanolone) and the synthetic steroid anesthetic 5alpha-pregnan-3alpha-ol-11,20-dione (alphaxalone) were measured using two-electrode voltage-clamp recording techniques. Allopregnanolone had similar effects to potentiate GABA-activated membrane current in the alpha1beta1gamma2L and alpha1beta2gamma2L receptor isoforms. In contrast, alphaxalone was much more effective as a positive allosteric modulator on the alpha1beta1gamma2L receptor isoform. In the absence of the gamma2L subunit subtype, allopregnanolone had much greater efficacy, but its potency was decreased. Allopregnanolone was much more effective on the alpha1beta1 receptor isoform compared with the alpha1beta2 receptor isoform. The potency for alphaxalone to potentiate the GABA response was not altered in the absence of the gamma2L subunit subtype, although its efficacy was greatly enhanced. Both allopregnanolone and alphaxalone produced nonparallel leftward shifts in the GABA concentration-response relationship in the absence of the gamma2L subunit, decreasing the EC50 concentration of GABA and increasing the maximal response. Only alphaxalone increased the maximal GABA response when the gamma2L subunit subtype was present. The 3beta-pregnane isomers epipregnanolone and isopregnanolone both inhibited the ability of allopregnanolone and alphaxalone to potentiate GABA(A) receptor function. However, the degree of block produced by the 3beta-pregnane steroid isomers was dependent on the type of receptor isoform studied and the neuroactive steroid tested. Isopregnanolone, the 3beta-isomer of allopregnanolone, was significantly more effective as a blocker of potentiation caused by allopregnanolone compared with alphaxalone in all receptor isoforms tested. Epipregnanolone had a greater efficacy as a blocker at the alpha1beta2gamma2L receptor isoform compared with the alpha1beta1gamma2L receptor isoform, and also produced a greater degree of block of potentiation caused by allopregnanolone compared with alphaxalone. Our results support the hypothesis that the heteromeric assembly of different GABA(A) receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABA(A) receptors, which in turn produces distinctly different modulatory interactions between neuroactive steroids acting at the GABA(A) receptor. The alpha and gamma subunit subtypes may have the greatest influence on allopregnanolone modulation of GABA(A) receptor function, whereas the beta and gamma subunit subtypes appear to be most important for the modulatory effects of alphaxalone.

Animals↗

PubMed1996

Propofol and flurazepam act synergistically to potentiate GABAA receptor activation in human recombinant receptors.

The intravenous general anaesthetic propofol (2,6-di-isopropylphenol) is frequently combined with a benzodiazepine. There are clinical reports of synergism between these two agents for induction of general anaesthesia. To investigate a possible mechanism of this synergistic interaction between propofol and benzodiazepines, the effect of propofol and flurazepam on GABAA receptor function was examined in Xenopus oocytes expressing recombinant alpha 1 beta 2 gamma 2L and alpha 2 beta 2 gamma 2L receptor constructs. Potentiation of GABA receptor-activated current by low (1-10 microM) concentrations of propofol together with flurazepam (0.25-0.5 microM) was significantly greater than predicted by an additive response. Isobolographic analysis indicated a strong synergistic interaction between propofol and flurazepam at either of the receptor constructs examined. In contrast, the cyclopyrrolone derivative zopiclone, which produced a similar facilitation of GABA receptor-activated current compared to flurazepam, produced a less than additive potentiation when combined with propofol. Flurazepam significantly decreased the EC50 concentration of propofol for potentiation of GABA responses. Thus, flurazepam, in addition to facilitating GABA receptor activity on its own, also increases on its own, also increases the apparent affinity of the GABAA receptor complex to propofol, resulting in a greater than expected potentiation by the combination of propofol plus flurazepam.

Anesthetics, Intravenous↗

PubMed1996

Developmental expression of functional GABAA receptors containing the gamma 2 subunit in neurons derived from embryonal carcinoma (P19) cells.

The expression of the gamma 2 subunit into functional GABAA receptors has been examined in the embryonal carcinoma (EC) cell line P19, a pluripotent cell line which differentiates into a neuronal phenotype after exposure to retinoic acid. Whole-cell voltage-clamp recordings were used to examine the characteristics of the GABA receptors expressed in P19 cells at different times after exposure to retinoic acid. Messenger RNA for both the gamma 2L and gamma 2S splice variants of the GABAA receptor increased dramatically following differentiation of P19 EC cells with retinoic acid. By 12 days after retinoic acid treatment, while both mRNAs were present, there was an approximately 10-fold greater abundance of gamma 2S mRNA compared to gamma 2L. However, at this same time point neurons derived from P19 cells stained intensely with a polyclonal antibody raised against a peptide fragment specific for the gamma 2L subunit. A significant increase in both the affinity for GABA and the maximum current amplitude elicited by GABA occurred between 7 and 12 days after retinoic acid treatment. In contrast, the ability of the benzodiazepine agonist flurazepam to potentiate GABA-induced membrane current was the same at 7 and 12 days after retinoic acid treatment. These data suggest that the gamma 2 subunit of the GABAA receptor is expressed early following differentation of P19 cells into a neuronal phenotype, and that this subunit is incorporated into functional GABAA receptors. Moreover, the gamma 2S and gamma 2L splice variants of this subunit may be co-expressed in neurons derived from P19 cells. The observed affinity change for GABA may reflect a time-dependent change in the expression of alpha and/or beta subunits of the GABAA receptor, as occurs in developing neuronal tissue both in vitro and in vivo.

Amino Acid Sequence↗

PubMed1996

Modulation of GABAA receptor function by alcohols: effects of subunit composition and differential effects of ethanol.

We sought to test the hypotheses that closely related alcohols would have effects on GABAA receptor function that were not predicted by differences in lipid solubility, and that the subunit structure of the GABAA receptor would significantly affect the actions of different alcohols. Cloned subunits of human GABAA receptors were expressed in Xenopus oocytes, and two-electrode voltage-clamp recording was used to quantify the membrane current response to GABA in the presence and absence of different alcohols. 1-Butanol and 2-butanol differentially potentiated the response to 20 microM GABA in oocytes expressing the alpha 1 beta 2 gamma 2L and alpha 2 beta 2 gamma 2L receptor isoforms. In the alpha 1 beta 2 gamma 2L receptor construct, 1-butanol was more potent than 2-butanol to potentiate GABAA receptor function, but 2-butanol had a greater efficacy. In the alpha 2 beta 2 gamma 2L receptor construct, 1-butanol and 2-butanol were equipotent, but 2-butanol again had a greater efficacy. In the alpha 2 beta 2 receptor construct, both 1-butanol and 2-butanol produced large potentiations of the current response to 3 microM GABA. The efficacy for butanol potentiation of GABA responses in the absence of a gamma 2L subunit was greater, but the potency was greatly reduced. Low concentrations (20 mM) of ethanol potentiated GABA responses in the alpha 1 beta 2 gamma 2L receptor construct. Ethanol potentiation of GABAA receptor function was completely blocked by the benzodiazepine receptor partial inverse agonist RO15-4513 at a concentration (0.5 microM) that did not alter the control GABA response. In contrast, RO15-4513 did not block potentiation of GABAA receptor activity induced by n-propanol, 1-butanol, 2-butanol, 1-heptanol, or propofol (2,6-diisopropylphenol). These results suggest that alcohols have specific interactions with GABAA receptors, and that ethanol may have unique effects not shared by other longer chain alcohols.

1-Butanol↗

PubMed1995

Hepatitis C prevalence and needle/syringe sharing behaviours in recent onset injecting drug users.

AIM: To investigate the prevalence of hepatitis C antibody (anti-HCV) in injecting drug users (IDU), particularly amongst those of recent onset beginning since the advent of the needle-exchange programme. Secondly this study sought information on needle-sharing practices. METHODS: The records of injecting drug users over 2 years from January 1992 were examined for anti-HCV results and needle-sharing reports on initial assessment at the Wellington drug dependency clinic. RESULTS: Amongst 110 injecting drug users 92 (84%) were tested for anti-HCV, and 71 (77%) were positive. There was a significant (p = 0.02) association between the reported duration of intravenous drug misuse and the proportion of injecting drug users who were positive for anti-HCV. Of those injecting drug users who had reported use of less than 4 years, since the introduction of the New Zealand needle exchange programme, 53% were positive for anti-HCV. Needle-sharing was frequently reported to have ever occurred in 74% of these patients at some time, and in 64% of those who reported use of less than 4 years. These differences were not significant. However, in the period before presenting 67% had not shared over the last 3 months and 57% had not shared over the previous 1 year, which are improvements on previous studies of needle-sharing, considered to be the most important mode of transmission for blood-borne viruses in injecting drug users. CONCLUSIONS: These results suggest considerable potential for HIV infection rates among injecting drug users in New Zealand to increase from the current low level. There needs to be continuing emphasis on public health programmes to eradicate needle-sharing, and to promote safe injecting techniques for injecting drug users.

Adult↗

PubMed1995

Loss of cortical serotonin2A signal transduction in senescent rats: reversal following inhibition of protein kinase C.

Using grease gap recordings, age-related changes in serotonin2A receptors were assessed in sensorimotor regions of the cortex by examining serotonin-induced facilitation of the N-methyl-D-aspartate depolarization in cortical wedges prepared from young adult (3-6 months) and senescent (22-34 months) Fisher 344 rats. Serotonin (10-100 microM) facilitated the N-methyl-D-aspartate depolarization in wedges from young adult rats in a concentration-dependent manner, whereas no facilitation was observed in wedges from senescent rats. Similar results were obtained when +/- 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, a mixed serotonin2A and serotonin2C receptor agonist, was substituted for serotonin. In contrast, agonists at alpha 1A-adrenoceptors, metabotropic glutamate receptors and muscarinic cholinoceptors facilitated the N-methyl-D-aspartate depolarization in wedges from both young adult and senescent rats. Chelerythrine and staurosporine, inhibitors of protein kinase C, but not concanavalin A, myo-inositol or calmodulin antagonists, restored the serotonin facilitation in wedges from senescent animals. In situ hybridization histochemistry revealed that serotonin2A receptor messenger RNA was present in layers II-VI of the cortex, with the highest density of silver grains located in layers III and V of both young adult and senescent rats. Detailed examination of layer V showed that silver grains were significantly higher than background only over pyramidal cells. We conclude that serotonin2A receptors are expressed by pyramidal cells in both young adult and senescent rats and that serotonin acts directly on these receptors to facilitate the N-methyl-D-aspartate depolarization. Moreover, in senescent rats, signal transduction at cortical serotonin2A receptors involved with facilitation of the N-methyl-D-aspartate response is compromised as a result of protein kinase C activation.

Aging↗

PubMed1994

Neurons derived from embryonal carcinoma (P19) cells express multiple GABAA receptor subunits and fully functional GABAA receptors.

The embryonal carcinoma (EC) cell line P19 can be induced to differentiate into neurons and glia by treatment with retinoic acid. Neuronal cells derived from P19 stem cells were found to express messenger RNAs for alpha, beta, and gamma 2 subunits of the GABAA receptor-chloride channel complex. Whole-cell voltage-clamp recording in differentiated P19 cells revealed that these cells possess GABA receptor-activated chloride currents which are blocked by bicuculline and potentiated by flurazepam. P19 EC cells thus represent a stable neuronal cell line which expresses functional GABAA receptors with all of the characteristics of native GABAA receptors.

Animals↗

PubMed1992

Ethanol modulation of GABA receptor-activated Cl- currents in neurons of the chick, rat and mouse central nervous system.

Modulation of gamma-aminobutyric acidA (GABAA) receptor function by drugs such as ethanol may depend on the genetic heterogeneity of GABAA receptor subunits, which vary across species and cell types. For this reason, the effects of ethanol on gamma-aminobutyric acid receptor-activated chloride currents (IGABA) were examined using whole-cell voltage-clamp recordings in primary cultures of neurons obtained from different species (chick, mouse and rat) and from different brain regions (cerebral cortex, hippocampus, cerebellum and spinal cord), and in acutely dissociated neurons from rat neocortical slices. Low concentrations (1-50 mM) of ethanol produced an enhancement of IGABA in some cells from each brain region examined. In cells obtained from the rat and chick cerebral cortex, 40-58% of cells exhibited an ethanol-sensitive IGABA. Moreover, a statistically significant variation in the response to ethanol was found in rat cortical neurons obtained from different litters. In mouse hippocampal neurons, potentiation of IGABA was obtained with ethanol concentrations (1-10 mM) well below those needed to inhibit neuronal responses to N-methyl-D-aspartic acid (30-50 mM), suggesting a differential sensitivity of these two receptor mechanisms to ethanol. Potentiation of IGABA by ethanol was reversed by the benzodiazepine receptor partial inverse agonist RO15-4513 (ethyl 8-azido-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine - 3-carboxylate), but was not affected by chelation of intracellular calcium. Furthermore, high concentrations of GABA attenuated the ability of ethanol to enhance IGABA. These results are consistent with the hypothesis that ethanol facilitates coupling between receptor binding and chloride channel activation.

Animals↗

PubMed1992

Uncoupling of GABA-benzodiazepine receptors in chick cerebral cortical neurons requires co-activation of both receptor sites.

Primary cultures of chick cerebral cortical neurons were exposed to 1 microM flurazepam in vitro, and the effect of flurazepam on GABA-activated membrane current (IGABA) was examined using whole-cell voltage-clamp recording. Exposure of chick cerebral cortical neurons to flurazepam alone for 3-10 days resulted in a significant decrease in the degree of potentiation of IGABA elicited by 0.5 microM flurazepam. When GABA or nipecotic acid (a GABA uptake blocker) were added with flurazepam during chronic drug exposure, neuronal responses to GABA were significantly less sensitive to modulation by 0.5 microM flurazepam compared to flurazepam treatment alone. Furthermore, this effect was significantly reduced by co-administration of the GABAA receptor antagonist bicuculline. These results suggest that tolerance to benzodiazepines in vitro requires activation of both the GABA and the benzodiazepine binding sites on the GABAA receptor-channel complex.

Animals↗

PubMed1991

Ethanol enhances GABAA receptor-activated chloride currents in chick cerebral cortical neurons.

Primary cultures of cerebral cortical neurons were prepared from 7- to 8-day-old chick embryos. The effect of ethanol on GABA-activated membrane current was examined using whole-cell voltage-clamp recording in cells maintained for 3-25 days in vitro. In approximately 60% of neurons examined ethanol caused a potentiation of the membrane current elicited by GABA. The threshold concentration of ethanol was 1 mM, and the potentiating effect of ethanol on GABA-activated currents was maximal at 10 mM. In many cells higher concentration (40-50 mM) of ethanol inhibited GABA-activated currents. These effects of ethanol were all reversible.

Animals↗

PubMed1991

Actions of ketamine, phencyclidine and MK-801 on NMDA receptor currents in cultured mouse hippocampal neurones.

1. Stable N-methyl-D-aspartic acid (NMDA) receptor-mediated currents in cultured mouse hippocampal neurons were evoked by 20 ms pressure pulse applications of L-aspartate, repeatedly applied at 30 or 40 s intervals, to the cell body region of the neurone. We have characterized the voltage- and use-dependent blockade of the currents by three dissociative anaesthetics: ketamine, phencyclidine (PCP) and MK-801 in mouse hippocampal neurones grown in dissociated tissue culture. 2. We have used a simple model of the blockade, based on the 'guarded receptor hypothesis' to interpret our data. The model assumes that receptors are maximally activated at the peak of the response with an open probability (Po) approaching 1, that there is no desensitization and that the blocking drug only associates with, or dissociates from, receptor channels which have been activated by agonist (e.g. open channels). 3. The model allows us to estimate forward and reverse rate constants for binding of the blockers to open channels from measurements of the steady-state level of blockade and the rate of change of the current amplitude per pulse during onset and offset of blockade. As predicted by the model, the estimated reverse rate was independent of blocker concentration while the forward rate increased with concentration. Changing the level of positively charged ketamine (pKa 7.5) tenfold by changing pH from 6.5 to 8.5 caused a corresponding change in the forward rate while having no effect on the reverse rate. Most of the voltage dependence of the blockade could be accounted for by reduction of the reverse rate by depolarization. 4. Estimated forward rate constants for ketamine, PCP and MK-801 were similar to one another when measured under similar conditions and were 3 x 10(4) - 3 x 10(5) M-1 S-1. Most of the differences in potency of the three blockers could be accounted for by differences in the reverse rate constants which were approximately 0.2, 0.03 and 0.003 s-1 for ketamine, PCP and MK-801, respectively. The estimated rate constants actually are the product of the rate constants and 1/Po. Suggestions that maximum Po is much less than 1 for NMDA channels imply that both forward and reverse rate constants of blockade may in fact be larger than we have calculated. However, their magnitudes, relative to one another, are unaffected by this consideration. 5. The reverse rate constant of blockade increased at positive potentials. This increase was prevented when the neurone was loaded with N-methyl-D-glucamine, an impermeant cation which prevented outward currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1990

Kindling-induced epilepsy alters calcium currents in granule cells of rat hippocampal slices.

Single electrode voltage-clamp recordings were obtained from dentate gyrus granule cells (GCs) in hippocampal slices of control and commissurally kindled rats. Two types of calcium currents, a transient and a sustained current, were studied in control and kindled neurons. The threshold of the transient calcium current was lowered in kindled GCs. The sustained calcium current was absent in kindled neurons but it could be restored by the intracellular administration of the calcium chelator EGTA. Our findings are consistent with the hypothesis that the loss of an intraneuronal calcium binding protein (Calbindin-D28K; CaBP) reduces the intraneuronal calcium buffering capacity in kindled neurons and results in the enhanced calcium-dependent inactivation of sustained calcium currents.

Animals↗

PubMed1990

NMDA depolarizations and long-term potentiation are reduced in the aged rat neocortex.

N-Methyl-D-aspartate (NMDA) responses were recorded intracellularly in layer V neocortical neurons in in vitro slices taken from young (4-6 months) and aged (27-29 months) Fischer 344 rats. Increasing amounts of NMDA produced membrane depolarizations in both groups of cells. The regression analysis showed significantly reduced sensitivity to NMDA in old neurons compared to young. A significant long-term potentiation of the field potential evoked by subcortical white matter stimulation was present in young but not in old slices. These results suggest that aging results in a decreased sensitivity to NMDA and impaired synaptic plasticity in the neocortex.

Aging↗