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A J Gibb

Publications and source records attributed to A J Gibb.

At least 19 recordsLinked to original sources
PubMed2001

The single-channel properties of purinergic P2X ATP receptors in outside-out patches from rat hypothalamic paraventricular parvocells.

The aim of this study was to characterize functionally the single-channel properties of rat hypothalamic paraventricular medial parvocell purinergic P2X receptors. Single-channel recordings were made from outside-out patches from coronal whole-brain slices from 4-day-old-rats. A heterogeneous population of functional P2X receptors were observed, possibly reflecting a mixture of homomeric P2X2 receptors and heteromers containing P2X2, P2X3, P2X4 or P2X6 subunits. "Flickery" P2X receptor channels with a high open probability (Po) and chord conductances of 14.4+/-1.75 and 29.6+/-0.5 pS at -80 mV were observed in nine patches. The mean open time was 2.51+/-1.30 ms. In the presence of 1 mM ATP, and excluding long periods of apparent desensitization, these channels had a mean Po of 0.58+/-0.29. "Non-flickery" P2X receptors with chord conductances of 16.9+/-3.48 and 32.4+/-3.75 pS, a mean open time of 0.63+/-0.15 ms and a mean Po of 0.01+/-0.004 at -80 mV were observed in the presence of 1 mM ATP in three patches. P2X receptors showed little apparent concentration dependence in their activity at ATP concentrations of 0.01-1 mM, probably reflecting considerable desensitization. The mean Po (for all data records) at 10 microM was 0.030+/-0.017 and 0.030+/-0.028 at 1 mM.

Adenosine Triphosphate↗
PubMed2001

Characterization of the single-channel properties of NMDA receptors in laminae I and II of the dorsal horn of neonatal rat spinal cord.

The single-channel properties of native NMDA receptors in laminae I and II of the dorsal horn of the neonatal rat spinal cord were studied using outside-out patch-clamp techniques. These receptors were found to have several features that distinguish them from native NMDA receptors elsewhere in the CNS. Single-channel currents activated by NMDA (100 nm) and glycine (10 microm) exhibited five distinct amplitude components with slope-conductance values of 19.9 +/- 0.8, 32.9 +/- 0.6, 42.2 +/- 1.1, 53.0 +/- 1.0 and 68.7 +/- 1.5 pS. Direct transitions were observed between all conductance levels but transitions between 69-pS openings and 20-, 33- and 42-pS openings were rare. There was no significant difference in the frequency of direct transitions from 42- to 20-pS compared to 20- to 42-pS transitions. The Kb (0 mV) for Mg2+ was 89 microm. The Mg2+ unblocking rate constant was similar to other reported values. However, the Mg2+ blocking rate constant was larger than other reported values, suggesting an unusually high sensitivity to Mg2+. The NR2B subunit-selective antagonist, ifenprodil, had no significant effect on overall channel activity but significantly decreased the mean open time of 53-pS openings. These results suggest neonatal laminae I and II NMDA receptors are not simply composed of NR1 and NR2B subunits or NR1 and NR2D subunits. It is possible that these properties are due to an as yet uninvestigated combination of two NR2 subunits with the NR1 subunit or a combination of NR3A, NR2 and NR1 subunits.

Aging↗
PubMed2000

Single channel properties of P2X ATP receptors in outside-out patches from rat hippocampal granule cells.

The single channel properties of P2X ATP receptors were investigated in outside-out patches from hippocampal granule cells in brain slices from 12-day-old rats. The results demonstrate that functional P2X ATP receptors are expressed in hippocampal granule cells and, combined with previously published information on the P2X subunits expressed in the hippocampus, suggest that the receptors may be heteromers of the P2X4 and P2X6 subunits or P2X1, P2X2, P2X4 and P2X6 subunits. Two distinct types of P2X channel openings were observed. A flickery P2X receptor channel was observed in three patches with a mean chord conductance of 32 +/- 6 pS, a mean open time of 1.0 +/- 0.3 ms and a mean burst length of 11 +/- 5 ms at a membrane potential of -60 mV. A large conductance P2X receptor was observed in 19 out of 98 patches with a mean conductance of 56 +/- 1. 8 pS, a linear current-voltage relationship between -80 and +60 mV with a reversal potential around 0 mV, a mean open time of 2.6 +/- 0. 2 ms and a mean burst length of 8.8 +/- 1.8 ms at -60 mV. At an ATP concentration of 1 mM, these channels exhibited a low steady-state open probability (Popen, 0.07 +/- 0.008; n = 15), little apparent desensitisation and were also activated by alpha,beta-methylene ATP (alpha,beta-meATP, 40 microM; Popen, 0.007 +/- 0.0002; conductance, 57 +/- 1.1 pS; n = 3). No decrease in the single channel conductance was observed on increasing the free extracellular calcium concentration from 0.3 to 0.85 mM. Channel closed time distributions were fitted with five exponential components with time constants (and relative areas) of 90 micros (20 %), 0.77 ms (32 %), 10 ms (15 %), 90 ms (18 %) and 403 ms (15 %) at 1 mM ATP. Of these, the first two components are suggested to represent gaps within single activations of the receptor based on the lack of agonist concentration dependence of these two shut time components between 1 microM and 1 mM ATP. Suramin (40 microM) significantly increased the single channel conductance (19 +/- 7 %; n = 5) and produced a small decrease in Popen (39 +/- 9 %; n = 5) by decreasing mean open time, burst length and total open time per burst. These actions of suramin are not consistent with simple competitive antagonism.

Adenosine Triphosphate↗
PubMed1997

Properties of ATP receptor-mediated synaptic transmission in the rat medial habenula.

The properties of central ATP-mediated synaptic currents were studied using whole-cell patch-clamp recording in rat medial habenula slices. Release was shown to be calcium dependent with a Hill coefficient of approximately 2. The voltage dependence of synaptic current amplitudes was approximately linear. Some reduction of the synaptic current amplitudes was observed at 10 mM extracellular calcium, suggesting calcium block/permeability of the channels. This was confirmed by observation of current-voltage reversal potentials in different calcium concentrations. We estimate that the channels underlying half the synapses showed a negligible calcium permeability. In the other four out of eight synapses the results suggest a very high calcium permeability with an estimated PCa/PCs of > 10. Thus, at -70 mV, in 1 mM calcium, more than 15% of the ATP-mediated synaptic current is estimated to be carried by calcium, but only at synapses with calcium-permeable channels. Net current through these synaptic channels is also controlled by the voltage dependence of synaptic current decay time constants (increasing e-fold for 158 mV depolarization) and by a strong dependence of transmitter release on the frequency of stimulation of the presynaptic neurone, with failure rates increasing 3-fold as stimulation rates were increased from 1 to 10 Hz.

Animals↗
PubMed1995

Persistence: a new statistic for characterizing ion-channel activity.

We introduce and illustrate by examples a new statistical technique, the persistence function, for characterizing ion-channel activity in a single-channel patch-clamp recording. Persistence is a function of both current and time. It is the probability that the current is at a given level (conditional on it having been at that level at an earlier time). Viewed as a function of current it exhibits the prominent conductance levels present in the recording, and viewed as a function of time for a conductance level it portrays the kinetics at that level.

Ion Channels↗
PubMed1995

Heterogeneity of neuronal nicotinic acetylcholine receptors in thin slices of rat medial habenula.

1. Neuronal nicotinic acetylcholine receptors in slices of rat medial habenula were studied using patch clamp recording techniques. 2. Whole cell current responses to cytisine could be blocked by hexamethonium, as expected for nicotinic receptors. The whole cell current-voltage relations were linear at negative membrane potentials, but showed strong inward rectification when chloride currents were minimized. 3. When 1 mM Ca2+ (0 mM Mg2+) was present in the external recording solution, the single channel conductances elicited by acetylcholine or nicotine in twenty patches were in the range 39-58 pS, with a mean of 47 pS. There appeared to be at least two groups of conductances. 4. In the open point amplitude distributions of three patches, the most common amplitude corresponded to 41 pS (81% of the area). In another four patches the most common amplitude corresponded to a mean conductance of 51 pS (83% of the area). Direct transitions between open levels were rare. 5. Channel closed times were not significantly different for the two conductance groups. However, for the four patches with predominantly 51 pS openings, the means of the distributions of open times longer than two filter rise times averaged 5.8 ms. Those patches with predominantly 41 pS openings averaged 14 ms. Also, for patches with predominantly 51 pS openings the overall mean burst length was 5.8 ms, whereas for patches with predominantly 41 pS openings it was 16.1 ms. 6. These observations suggest that 51 and 41 pS openings result from the activity of at least two, but possibly more, different receptor subtypes. We conclude that nicotinic receptors in the rat ventral medial habenula are heterogeneous.

Acetylcholine↗
PubMed1994

Comparison of neuronal nicotinic receptors in rat sympathetic neurones with subunit pairs expressed in Xenopus oocytes.

1. The agonist sensitivity of nicotinic acetylcholine receptors in rat superior cervical ganglion (SCG) neurones was compared with that of cloned receptors expressed in Xenopus oocytes by pairwise injections of alpha 3-beta 2 or alpha 3-beta 4 neuronal nicotinic subunit combinations. 2. Agonist responses in rat SCG neurones indicated that cytisine was the most potent agonist and lobeline the least potent (rank order of potency: cytisine > dimethylphenylpiperazinium iodide (DMPP) > nicotine > ACh > carbachol > lobeline). 3. Receptors expressed in oocytes by injection of alpha 3 and beta 2 subunits had a relatively high sensitivity to DMPP and low sensitivity to cytisine (rank order of potency: DMPP > ACh > lobeline > carbachol > nicotine > cytisine), whereas receptors composed of alpha 3 and beta 4 subunits had a high sensitivity to cytisine and low sensitivity to DMPP (rank-order of potency: cytisine > nicotine approximately ACh > DMPP > carbachol > lobeline). 4. With the exception of responses to DMPP, agonist sensitivity measurements suggest that nicotinic receptors in the rat SCG are composed of alpha 3 and beta 4 subunits. The results are discussed in terms of the receptor subunit mRNAs known to be expressed in the rat SCG and previous evidence of functional heterogeneity of rat SCG nicotinic acetylcholine receptors.

Alkaloids↗
PubMed1993

ATP--a fast neurotransmitter.

ATP receptor-mediated responses in peripheral and central neurons have many characteristics which suggest that ATP may act as a fast neurotransmitter. While the receptors underlying these responses have properties which are similar to other ligand-gated ion channels which mediate fast neurotransmission, the nature of their calcium permeability and the rapid breakdown of ATP to adenosine may confer unique properties on ATP mediated synaptic transmission. The evidence that ATP acts as a fast neurotransmitter is reviewed and the properties of ATP and its receptor channels are discussed in terms of synaptic transmission.

Adenosine Triphosphate↗
PubMed1993

Identification of M-channels in outside-out patches excised from sympathetic ganglion cells.

We have identified the species of K+ channel that underlies the neuronal M-current in rat sympathetic ganglion cells. The channels were kinetically and pharmacologically defined using outside-out and cell-attached patches. They exhibited multiple conductance levels, predominantly 3-9 pS. Their slow gating in response to voltage change in outside-out patches was exhibited only in the presence of AIF-4 or GTP gamma S on the inner membrane surface and when the lower conductance states were dominant. In the absence of AIF-4 or GTP gamma S, the channels exhibited rapid activation and deactivation. We conclude that M-channel gating may be controlled by an associated GTP-binding protein.

Aluminum↗
PubMed1992

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1992

Activation of N-methyl-D-aspartate receptors by L-glutamate in cells dissociated from adult rat hippocampus.

1. Single channel recording techniques were used to study the ion channel openings resulting from activation of N-methyl-D-aspartate (NMDA) receptors by the agonist glutamate. Patches were from cells acutely dissociated from adult rat hippocampus (CA1). Channel activity was studied at low glutamate concentrations (20-100 nM) with 1 microM-glycine, in the absence of extracellular divalent cations. 2. Channel openings were to two main conductance levels corresponding to 50 pS and 40 pS openings in extracellular solution with 1 mM-Ca2+. Around 80% of openings were to the large conductance level. The single channel conductances increased as extracellular Ca2+ was reduced. 3. Distributions of channel open times were described by three exponential components of 87 microseconds, 0.91 ms and 4.72 ms (relative areas of 51, 31 and 18%). Most long openings were to the large conductance level. 4. The channel closed time distribution was complex, requiring five exponential components to describe it adequately. Of these five components, at least three, with time constants of 68 microseconds, 0.72 ms and 7.6 ms (relative areas of 38, 12 and 17%) represent gaps within single activations of the receptor. The presence of a component with a mean of 7.6 ms is notable because gaps of this length have not previously been identified as being within single NMDA receptor channel activations. 5. Channel activations were identified as including gaps underlying at least the first three closed time components. Activations consisted of clusters of channel openings. Distributions of the length of these clusters had mean time constants of 88 microseconds, 3.4 ms and 32 ms (relative areas of 45, 25 and 30%). Long clusters contained short, intermediate and long duration openings as well as subconductance openings. The open probability within clusters averaged 0.62. Three components were evident in distributions of the number of openings per cluster. These had mean values of 1.22, 3.2 and 11 openings per cluster. 6. An inverse correlation was evident between the length of adjacent open and closed times. When open intervals were separated into groups based on the length of adjacent gaps, the time constants of the exponential components in these conditional open time distributions were independent of the length of the adjacent gap. This supports the idea that the NMDA receptor channel gating has the properties of a discrete Markov process. 7. The long duration of NMDA receptor channel clusters suggests that they contribute to the slow time course of the NMDA receptor-mediated synaptic current.

Animals↗
PubMed1991

Glutamate activation of a single NMDA receptor-channel produces a cluster of channel openings.

Activations of the N-methyl-D-aspartate (NMDA) receptor by glutamate were studied in outside-out patches from CA1 cells in rat hippocampal slices. Very low glutamate concentrations (20-100 nM) were used so that individual receptor activations would be well separated. The shut-time distribution contained at least five components, only the longest component being obviously concentration dependent. The three briefest shut-time components had time constants of 56 microseconds, 0.68 ms and 10.1 ms; all of these were independent of glutamate concentration. An individual activation of the receptor therefore produces a long cluster of channel openings that contains longer gaps than have been reported for receptor activations by other fast neurotransmitters. In addition, (i) some activations may contain still longer (mean 78 ms) shut periods generating 'super clusters', and (ii) a significant amount of NMDA current may be carried by prolonged ('high P(open)') periods during which the channel is open for most of the time. Such periods occur intermittently even at these very low glutamate concentrations. It is suggested that the slow time course of the NMDA receptor-mediated synaptic currents may be determined mainly by the channel activation kinetics.

Animals↗
PubMed1990

Expression of cloned receptor subunits produces multiple receptors.

Expression in Xenopus oocytes of cloned nicotinic acetylcholine receptors (alpha, beta, gamma and delta subunits of BC3H1 receptor), produced more than one sort of functional receptor. This heterogeneity was detectable neither as heterogeneity in single channel conductance, nor as heterogeneity in the burst length. It was seen most obviously as differences from patch to patch in the maximum fraction of time for which the channels are held open at high acetylcholine concentrations, and it was also detectable as differences in the shut time distributions at low acetylcholine concentrations.

Acetylcholine↗