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N L Harrison

Publications and source records attributed to N L Harrison.

At least 73 records · Page 4Linked to original sources
PubMed1990

3-Aminopropanephosphinic acid is a potent agonist at peripheral and central presynaptic GABAB receptors.

The actions of the GABA analog 3-aminopropanephosphinic acid (3-APA) were studied in the guinea-pig isolated ileal preparation and at synapses between cultured rat hippocampal neurons. Like the GABAB receptor agonist, baclofen, 3-APA inhibited the electrically evoked ileal twitch. The EC50 for 3-APA was 0.8 microM; the EC50 for baclofen was 9 microM. In addition, the depressant responses to 3-APA and baclofen were blocked by the GABAB receptor antagonists phaclofen, saclofen, 2-hydroxy-saclofen and delta-aminovaleric acid. 3-APA also mimicked the presynaptic action of baclofen at GABAergic synapses between embryonic rat hippocampal neurons in culture. 3-APA reduced the amplitude of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) by greater than 50% at a concentration of 1 microM, while baclofen reduced synaptic transmission to a similar degree at 10 microM. 3-APA did not alter membrane conductance, nor did the drug alter postsynaptic responses to GABA. These data show that 3-APA is a potent agonist at presynaptic GABAB receptors in the periphery and on GABAergic neurons from the central nervous system. The activity of 3-APA at central postsynaptic GABAB receptors remains to be studied.

Amino Acids↗
PubMed1990

On the presynaptic action of baclofen at inhibitory synapses between cultured rat hippocampal neurones.

1. (-)Baclofen reduces inhibitory postsynaptic potentials (IPSPs) and the associated synaptic currents (IPSCs) at inhibitory GABAergic synapses between cultured rat hippocampal neurones. The reversal potential for the IPSC is unaltered. 2. The effect of (-)baclofen is concentration dependent; the EC50 for (-)baclofen is approximately 5 microM. 3. Statistical analyses of the amplitude fluctuations of the IPSC in the presence of (-)baclofen suggested a presynaptic location for the depression of synaptic transmission by (-)baclofen. In control experiments, lowering extracellular Ca2+ produced similar effects. (-)Baclofen has no detectable postsynaptic actions in these cultured neurones. 4. Phaclofen (0.2-0.5 mM) increases IPSC amplitude but does not significantly block the depressant effect of (-)baclofen on synaptic transmission. 5. The effect of (-)baclofen is not blocked by pertussis toxin pre-treatment. 6. It is concluded that (-)baclofen acts presynaptically to reduce the release of GABA. The mechanism by which release is reduced may involve a phaclofen-insensitive GABAB receptor.

Action Potentials↗
PubMed1990

Analogs of cyclic AMP decrease gamma-aminobutyric acidA receptor-mediated chloride current in cultured rat hippocampal neurons via an extracellular site.

We have studied the effects of the membrane-permeant cyclic AMP analogs 8-bromo-cyclic AMP and 8-(4-chlorophenylthio)-cyclic AMP (CPT-cAMP) on the gamma-aminobutyric acidA (GABAA) receptor-mediated chloride current in cultured rat hippocampal neurons. External perfusion with 8-bromo-cyclic AMP or CPT-cAMP caused a reversible, concentration-dependent decrease in the response to GABA. Adding the protein kinase inhibitor H-8 to the perfusing medium or the intracellular recording solution did not affect the response to GABA, which was decreased by CPT-cAMP as before. L858051, a water-soluble derivative of the adenylate cyclase activator forskolin, did not decrease the response to GABA even in the presence of the phosphodiesterase inhibitor 3-isobutylmethylxanthine. External cyclic AMP also caused a reversible, concentration-dependent decrease in the response to GABA with a potency similar to that of 8-Br-cAMP. When cAMP was present in the intracellular recording solution cAMP and CPT-cAMP decreased the response to GABA as before. These experiments suggest that analogs of cAMP decrease GABAA receptor-activated chloride current by acting at an extracellular site.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Blockade of the late IPSP in rat CA1 hippocampal neurons by 2-hydroxy-saclofen.

The effects of the GABAB receptor antagonist 2-hydroxy-saclofen were studied using intracellular recording of synaptic potential from CA1 hippocampal neurons. 2-Hydroxy-saclofen (50-200 microM) reversibly blocked the late, GABAB receptor-mediated inhibitory postsynaptic potential (IPSP) but not the early, GABAA receptor-mediated IPSP. In addition, the hyperpolarizing response to baclofen was reduced by similar concentrations of 2-hydroxy-saclofen. This suggests that 2-hydroxy-saclofen is a potent antagonist at postsynaptic GABAB receptors on hippocampal neurons.

Action Potentials↗
PubMed1989

Modification of GABAA receptor function by an analog of cyclic AMP.

Chloride-dependent current responses to gamma-aminobutyric acid (GABA) were recorded from cultured rat hippocampal neurons under voltage clamp. In the presence of the membrane-permeable cyclic AMP analog, 8-bromo-cyclic AMP (8-Br-cAMP), the peak current response to GABA was reduced, although the reversal potential for the current evoked by GABA was unaltered; similar concentrations of 8-bromo-cyclic GMP did not alter the GABA response. 8-Br-cAMP also increased spontaneous activity of the neurons and blocked accommodation of firing. It is possible that the alterations in responses to GABA result from the activation of cyclic AMP-dependent protein kinase (cAMP-PK) and subsequent phosphorylation of the GABAA receptor.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1989

Voltage-dependent calcium channels regulate melatonin output from cultured chick pineal cells.

Chick pineal cells maintained in primary culture display a circadian rhythm of melatonin production and release, and the nocturnal increase in melatonin output is enhanced by elevating extracellular K+. The divalent cations, Co2+, Cd2+, and Mn2+, each reduce nocturnal melatonin output. Nitrendipine and nifedipine also prevent the nocturnal rise in melatonin output, while Bay K 8644 increases it, suggesting a role for voltage-dependent Ca2+ channels in regulating melatonin output. The whole-cell patch-clamp technique was used to record from individual chick pineal cells. Under conditions designed to isolate currents through voltage-dependent Ca2+ channels, biphasic inward currents are elicited by large depolarizing commands (e.g., to 0 mV) from a holding potential of -90 mV; from a holding potential of -40 mV, only a sustained inward current is elicited by steps to 0 mV. Both components of the inward current are blocked by Co2+ or Cd2+. The sustained current is increased in amplitude by Bay K 8644 and blocked by nifedipine, while the transient current is unaffected. Since there is no evidence for vesicular release of melatonin, the "L-type" calcium channels mediating the sustained calcium current appear to be involved in the pathways regulating melatonin synthesis in chick pineal cells.

Animals↗
PubMed1988

(-)-Baclofen activates presynaptic GABAB receptors on GABAergic inhibitory neurons from embryonic rat hippocampus.

GABA-mediated monosynaptic inhibitory postsynaptic currents were recorded at synapses between embryonic rat hippocampal neurons in culture. The GABAB receptor agonist L(-)-baclofen reduced inhibitory postsynaptic current amplitude substantially at 10 microM, while the D(+)-isomer of baclofen was without effect. L(-)-Baclofen did not increase postsynaptic membrane conductance or block postsynaptic membrane responses to GABA. It is concluded that L(-)-baclofen depresses GABA-mediated inhibitory postsynaptic potentials by a presynaptic action at GABAB receptors on or near the terminals of the inhibitory neuron. The possible physiological importance of these presynaptic 'autoreceptors' is discussed.

Action Potentials↗
PubMed1988

5,7-Dihydroxytryptamine identifies living dopaminergic neurons in mesencephalic cultures.

The autofluorescent serotonin analogue 5,7-dihydroxytryptamine (5,7-DHT) was used to identify living catecholaminergic neurons in monolayer cultures derived from the embryonic rat mesencephalon. A high correlation between 5,7-DHT accumulation and aldehyde-induced catecholamine fluorescence as well as tyrosine hydroxylase but not dopamine-beta-hydroxylase or phenylethanolamine-N-methyltransferase immunoreactivity was found. This indicates that these cells were dopamine-containing neurons. Whole-cell patch recordings showed that all mesencephalic neurons had resting membrane potentials of -50 mV or greater and input resistances ranging between 200 and 700 M omega and exhibited spontaneous action potentials and postsynaptic potentials. The duration of the action potential of the dopamine-containing neurons was characteristically longer than that of the non-dopamine-containing mesencephalic cells. In some dopamine-containing neurons, repolarization of the action potential was clearly biphasic, and the slow phase of repolarization was reversibly blocked by local application of Cd2+ or Co2+. This "shoulder" in the action potential was never observed in non-dopamine-containing neurons, where Cd2+ or Co2+ application was always without effect. It is concluded that 5,7-DHT can be used to identify living dopamine-containing neurons in dissociated mesencephalic cultures and these neurons express distinct electrical properties.

5,7-Dihydroxytryptamine↗
PubMed1988

Outward rectification of inhibitory postsynaptic currents in cultured rat hippocampal neurones.

1. Inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) were recorded from cultured hippocampal neurones of the embryonic rat at 22 degrees C, using the whole-cell patch-clamp technique with a low-Cl-, 145 mM-potassium gluconate solution in the patch pipette. Individual synaptic events were elicited at low frequency (0.05-0.1 Hz) by stimulating a presynaptic neurone either by direct intracellular current injection, or by applying a brief pulse of L-glutamate. 2. In target neurones voltage clamped at -40 mV, outwardly directed IPSCs of mean amplitude 0.23 nA were recorded. The IPSCs were depressed by the GABA antagonist bicuculline, and reversed polarity between -50 and -80 mV (mean -64 mV), as did current responses to gamma-aminobutyric acid. The IPSPs and IPSCs reversed as a single phase; no bicuculline-resistant 'late' synaptic event was observed. 3. The IPSCs had variable kinetics, with rise times between 1 and 5 ms (mean 2.9 ms) at -40 mV, and slower, monoexponential, decay phases (decay time constant, tau IPSC, 10-40 ms at -40 mV). In some cells, tau IPSC clearly increased with depolarization. 4. The IPSC reversal potential was -64 +/- 9 mV (n = 23) under the experimental conditions used; this suggests that the synaptically activated channels are approximately 25 times more permeable to Cl- than to the gluconate anion. 5. The peak conductance associated with the IPSC showed outward rectification. The synaptic conductance measured at -40 mV was 1.7 times greater than that measured at -100 mV; at -20 mV, synaptic conductance was 2.5 times greater than at -100 mV. This outward rectification can be explained by a constant field model under these experimental conditions of asymmetric Cl- concentrations.

Action Potentials↗
PubMed1987

Signal transduction mechanisms in cultured CNS neurons and clonal pituitary cells.

The experimental accessibility of monolayer culture has been used to study signal transduction mechanisms in primary CNS neurons and clonal pituitary cells. Here we review results on two signals representative of the emerging diversity of mechanisms discovered in all species studied thus far. One is mediated by micromolar concentrations of the amino acid GABA at postsynaptic membranes throughout the mammalian CNS and involves transient activation of Cl- ion channels whose distribution of conducting periods accounts for the millisecond time course of the signal. This signal serves to depress the probability that the target cell will trigger an action potential. The signal intensifies as the postsynaptic membrane is depolarized and can be modulated by clinically important drugs, primarily through changes in channel kinetics. The other signal involves nanomolar concentrations of the peptide TRH, which stimulates secretion of prolactin from clonal "GH3" pituitary cells. Intracellular recordings of GH3B6 cells show that TRH triggers a complex electrical response lasting several minutes. The response consists of Ca2+-activated K+ conductance followed by Ca2+-action potential activity. Whole-cell patch recordings, which rapidly dialyze the cell, can eliminate the TRH-induced changes in membrane excitability. Inclusion of aqueous lysates of the GH3B6 clone or the soluble second messenger factors inositol trisphosphate (IP3) or protein kinase (PKC) can restore various aspects of the change in membrane excitability. Thus, TRH alters ion conductance mechanisms through a second messenger cascade likely to involve IP3-mediated mobilization of Ca2+ from the endoplasmic reticulum and transient translocation of PKC from cytoplasm to plasma membrane. These synaptic and extrasynaptic signals reflect some of the diversity of transduction mechanisms involved in intercellular communication.

Amino Acids↗
PubMed1987

Potentiation of gamma-aminobutyric-acid-activated chloride conductance by a steroid anaesthetic in cultured rat spinal neurones.

1. Intracellular recordings from cultured rat spinal cord neurones demonstrated that Cl(-)-dependent responses to GABA (gamma-aminobutyric acid) (but not glycine) were increased in amplitude and duration by the steroid anaesthetic alphaxalone (3 alpha-hydroxy-5 alpha-pregnane-11,20-dione) at submicromolar concentrations that produced little or no effect on passive electrical properties. The non-anaesthetic 3 beta-hydroxy analogue was without effect on GABA-evoked responses. 2. Under voltage clamp, membrane currents evoked by GABA were potentiated by alphaxalone without change in the reversal potential for the GABA-evoked response. Fluctuation analysis of GABA-evoked currents suggested that the mean open-time of GABA-activated channels was prolonged from 30 to 74 ms in the presence of the anaesthetic. 3. Higher concentrations of alphaxalone, similar to those reported during surgical anaesthesia, increased membrane conductance in the absence of exogenously applied GABA. Under voltage clamp, current responses to alphaxalone reversed at the same potential as did responses to GABA, suggesting that they result from increased Cl- conductance. 4. Alphaxalone responses were reduced by the GABA antagonist bicuculline. Fluctuation analysis of current responses to the anaesthetic suggest that they result from the activation of ion channels of long (100 ms) open-time and elementary conductance indistinguishable from that of channels activated by GABA (20 pS). Taken together, these findings indicate that the steroid anaesthetic is able to directly activate Cl- conductance normally activated by GABA in spinal neurones. 5. The actions of the steroid at GABA-receptor-Cl(-)-channel complexes are similar to those produced by the anaesthetic barbiturates (e.g. pentobarbitone), although obtained at 50-100-fold lower concentrations. These effects on the inhibitory Cl(-)-conductance mechanism may be partly responsible for the depressant actions of alphaxalone on the mammalian central nervous system.

Anesthetics↗
PubMed1987

Structure-activity relationships for steroid interaction with the gamma-aminobutyric acidA receptor complex.

Certain steroids are potent barbiturate-like modulators of the gamma-aminobutyric acidA (GABA) receptor-chloride ionophore complex in rat brain membranes. At nanomolar to low micromolar concentrations, these steroids stimulate [3H]flunitrazepam and [3H] muscimol binding and displace the convulsant [35S]t-butylbicyclophosphorothionate from its binding site in an allosteric manner, in addition to enhancing Cl- conductance responses to GABA recorded in cultured rat hippocampal and spinal neurons. A stringent structure-activity relationship exists for these interactions of steroids with the GABAA receptor complex. Comparison of the structure-activity relationship data obtained in this study with those for steroid-induced general anesthesia strongly suggests that steroidal anesthesia may result from the interaction between steroids and the GABAA receptor. The essential features of the active structures are a 5 alpha or 5 beta-reduced pregnane skeleton with a hydroxyl at C3 in the alpha-position and a ketone group at C20. These features are all present in some naturally occurring steroids, including metabolites of deoxycorticosterone and progesterone, that show potent activity at the GABAA receptor complex. Two of the compounds investigated are known to be formed in vivo as reduced metabolites of endogenous steroid hormones: 5 alpha-pregnane-3 alpha -ol-20-one and 5 alpha-pregnane-3 alpha,21-diol-20-one, which are derived from progesterone and deoxycorticosterone, respectively. These two steroids produce a striking prolongation of GABA-mediated inhibitory postsynaptic currents recorded at synapses between rat hippocampal neurons in culture and could conceivably regulate GABA-mediated inhibition under some physiologic and pathologic conditions.

Animals↗
PubMed1987

A steroid anesthetic prolongs inhibitory postsynaptic currents in cultured rat hippocampal neurons.

Whole-cell patch-clamp recordings were made from cultured rat hippocampal neurons to examine the effects of the steroidal general anesthetic alphaxalone (3 alpha-hydroxy 5 alpha-pregnane 11,20-dione) on responses to pharmacologically applied and physiologically released GABA. At low micromolar concentrations in the anesthetic range, alphaxalone potentiated Cl- conductance responses elicited by GABA and also prolonged evoked GABA-mediated postsynaptic potentials. Under voltage clamp at -40 mV, rapid outwardly directed synaptic currents were evoked that decayed with single exponential kinetics; mean decay time constant was 24 msec at room temperature. Alphaxalone prolonged the decay of these inhibitory postsynaptic currents by 5- to 8-fold, with no increase in peak amplitude or change in growth time. This substantial prolongation of GABA-mediated inhibitory synaptic conductance at clinically effective concentrations may contribute significantly to the anesthetic activity of alphaxalone.

Anesthetics↗
PubMed1986

Benzodiazepine pharmacology of cultured mammalian CNS neurons.

Many neurons cultured from the embryonic mammalian central nervous system (CNS) express benzodiazepine receptors while some neurons differentiate specific transmitter phenotypes like glutamic acid decarboxylase (GAD), the synthetic enzyme for gamma-aminobutyric acid (GABA). The benzodiazepine receptors in these cultured neurons are often, if not always coupled to a practically ubiquitous GABA-mediated function, activation of Cl- ion conductance. The transmitter signal serves to inhibit neuronal excitability and is facilitated by clinically important benzodiazepines. Here we review some details regarding the pharmacological actions of benzodiazepines on membrane excitability.

Animals↗
PubMed1986

Steroid hormone metabolites are barbiturate-like modulators of the GABA receptor.

Two metabolites of the steroid hormones progesterone and deoxycorticosterone, 3 alpha-hydroxy-5 alpha-dihydroprogesterone and 3 alpha, 5 alpha-tetrahydrodeoxycorticosterone, are potent barbiturate-like ligands of the gamma-aminobutyric acid (GABA) receptor-chloride ion channel complex. At concentrations between 10(-7) and 10(-5)M both steroids inhibited binding of the convulsant t-butylbicyclophosphorothionate to the GABA-receptor complex and increased the binding of the benzodiazepine flunitrazepam; they also stimulated chloride uptake (as measured by uptake of 36Cl-) into isolated brain vesicles, and potentiated the inhibitory actions of GABA in cultured rat hippocampal and spinal cord neurons. These data may explain the ability of certain steroid hormones to rapidly alter neuronal excitability and may provide a mechanism for the anesthetic and hypnotic actions of naturally occurring and synthetic anesthetic steroids.

20-alpha-Dihydroprogesterone↗
PubMed1986

Spontaneous paroxysmal activity induced by zero magnesium and bicuculline: suppression by NMDA antagonists and GABA mimetics.

Slices of rat cerebral cortex developed spontaneous paroxysmal discharges when superfused with Krebs medium containing zero Mg2+ or 50 microM bicuculline. In both situations, the N-methyl-D-aspartate (NMDA) antagonists APV, 100 microM, and ketamine, 100 microM substantially reduced the frequency of the paroxysmal events, the reduction being greater in zero Mg2+. gamma-Aminobutyric acid (GABA) 1 mM, the GABA-A agonist muscimol 2 microM and the GABA-B receptor agonist baclofen 10 microM, each reduced the frequency of events in zero Mg2+ while muscimol and GABA also reduced the amplitude of the events. GABA and baclofen were similarly effective against bicuculline-induced events but the muscimol concentration required was 5-10-fold higher. These results suggest that, under our vitro conditions, neocortical cells are normally restrained from paroxysmal discharges by Mg2+. Inhibition by GABA through GABA-A receptors and inhibition by GABA through GABA-B receptors, may also contribute to this restraint.

Animals↗