Kinetic properties of the GABAA receptor main conductance state of mouse spinal cord neurones in culture.

Macdonald, Robert L.; Rogers, Carl J.; Twyman, Roy E.

doi:10.1113/jphysiol.1989.sp017545

Cited by 206 publications

(205 citation statements)

References 29 publications

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“…However, we found that for each cell type, GABA A R conductance was similar when measured with either mIPSCs or outside-out patch currents (VB: mIPSC, ␥ ϭ 19 Ϯ 1 pS, n ϭ 7; patch, ␥ ϭ 20 Ϯ 1 pS, n ϭ 5; RTN: mIPSC, ␥ ϭ 18 Ϯ 1 pS, n ϭ 6; patch, ␥ ϭ 17 Ϯ 1 pS, n ϭ 5), indicating that similar channels are likely contained at synaptic and somatic sites. These results are consistent with a previous study using single-channel recording tech- niques (Browne et al, 2001), which also found similar GABA A R conductance in RTN and VB neurons, and all values are within the range commonly obtained for GABA A Rs (Macdonald et al, 1989). Additionally, the unitary conductance and number of channels can also be used to calculate the peak open probability (P o-max ) at the maximal current amplitude, a general indicator of the gating efficacy.…”

Section: Nonstationary Variance Analysis Of Mipscs and Outside-out Pasupporting

confidence: 91%

GABA Affinity Shapes IPSCs in Thalamic Nuclei

Schofield¹,

Huguenard²

2007

J. Neurosci.

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Precise neural inhibition in thalamocortical circuits is required for the generation of sleep spindles and suppression of hypersynchrony associated with epileptiform activity. Accordingly, the time course of GABA A receptor-mediated IPSC events is an important parameter influencing the strength of inhibitory signaling. In the thalamus, two distinct types of IPSC kinetics are observed: thalamocortical relay neurons in the ventrobasal nucleus (VB) exhibit a fast decaying IPSC, whereas neurons in the adjacent reticular nucleus (RTN) display a long-lasting, slowly decaying IPSC. Here, we used patch-clamp electrophysiology and computational modeling to elucidate the basis for IPSC kinetic heterogeneity in the thalamus. Rapid application of GABA to excised membrane patches revealed that decay kinetics were attributable to intrinsic differences in GABA A receptor deactivation. Examination of desensitization and gating properties revealed these to be similar in VB and RTN, with the notable lack of fast and long-lasting desensitized states in both cell types. Computational simulations demonstrate that slow GABA binding and unbinding rates could reproduce the characteristic long-lasting IPSCs in RTN cells. These results indicate that within thalamic circuits, a powerful diversity of inhibitory function can result from simple differences in underlying GABA A receptor affinity.

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Section: Nonstationary Variance Analysis Of Mipscs and Outside-out Pasupporting

confidence: 91%

GABA Affinity Shapes IPSCs in Thalamic Nuclei

Schofield¹,

Huguenard²

2007

J. Neurosci.

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“…We adopted and optimized the parameters we proposed previously (27) to reproduce the gating properties of declustered synaptic GABA A receptors. Because the concentrations of GABA used in that study were Ͼ30 M, the transitions between singly bound open and desensitized states could not be resolved (49,50). Therefore, the corresponding rate constants were merely adopted from Jones and Westbrook (48).…”

Section: Block Of Gaba-evoked Currents Induced By Prolonged Expomentioning

confidence: 99%

Clustering of Extrasynaptic GABAA Receptors Modulates Tonic Inhibition in Cultured Hippocampal Neurons

Petrini

Marchionni

Zacchi

et al. 2004

Journal of Biological Chemistry

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Tonic inhibition plays a crucial role in regulating neuronal excitability because it sets the threshold for action potential generation and integrates excitatory signals. Tonic currents are known to be largely mediated by extrasynaptic ␥-aminobutyric acid type A (GABA A ) receptors that are persistently activated by submicromolar concentrations of ambient GABA. We recently reported that, in cultured hippocampal neurons, the clustering of synaptic GABA A receptors significantly affects synaptic transmission (Petrini, E. M., Zacchi, P., Barberis, A., Mozrzymas, J. W., and Cherubini, E. (2003) J. Biol. Chem. 278, 16271-16279). In this work, we demonstrated that the clustering of extrasynaptic GABA A receptors modulated tonic inhibition. Depolymerization of the cytoskeleton with nocodazole promoted the disassembly of extrasynaptic clusters of ␦ and ␥ 2 subunitcontaining GABA A receptors. This effect was associated with a reduction in the amplitude of tonic currents and diminished shunting inhibition. Moreover, diffuse GABA A receptors were less sensitive to the GAT-1 inhibitor NO-711 and to flurazepam. Quantitative analysis of GABA-evoked currents after prolonged exposure to submicromolar concentrations of GABA and model simulations suggest that clustering affects the gating properties of extrasynaptic GABA A receptors. In particular, a larger occupancy of the singly and doubly bound desensitized states can account for the modulation of tonic inhibition recorded after nocodazole treatment. Moreover, comparison of tonic currents recorded during spontaneous activity and those elicited by exogenously applied low agonist concentrations allows estimation of the concentration of ambient GABA. In conclusion, receptor clustering appears to be an additional regulating factor for tonic inhibition.Similar to many other neurotransmitter receptors, ␥-aminobutyric acid type A (GABA A ) 1 receptors are localized at synaptic and extrasynaptic levels. Whereas synaptic GABA A receptors are involved in phasic inhibition (1), extrasynaptic ones are responsible for tonic inhibition (2-9). Tonic inhibition is due to persistent inhibitory conductance that contributes to "signal integration" in the brain since it sets the threshold for action potential generation (10, 11) and shunts excitatory synaptic inputs (2,(12)(13)(14)(15). This conductance is maintained by "ambient" GABA, which represents the amount of neurotransmitter present in the extracellular space. Ambient GABA originates from spillover of the neurotransmitter released at neighboring synapses (3, 5, 11), from astrocytes (16, 17), or from non-vesicular release (18,19). Tonic inhibition has been well characterized in the cerebellum, where ␣ 6 subunit-containing receptors act as high affinity sensors for GABA (4,11,20,21). Persistent GABA conductance has also been identified in other brain regions, including the hippocampus (6,8,9,22,23). However, in this structure, the subunit composition of the receptors involved has not been fully elucidated. In the past years, immunocytochemi...

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“…Analysis of these results, based on a model that is similar in structure to the Jones-Westbrook model we have used (Macdonald et al, 1989b), suggested a number of possible anesthetic actions: (1) halothane may slow the microscopic agonist unbinding rate, or (2) increase the microscopic agonist binding rate; 3) halothane may alter gating transition rates, to favor entry into the longer duration open state (although it is possible that this effect may be secondary to a slowing of agonist unbinding rate); and (4) the increase in burst duration may result from a decrease in agonist unbinding rate or a decrease in the entry rate into desensitization. Based on these single channel results, it was not possible to distinguish between these possibilities.…”

Section: Comparison With Single Channel Studiesmentioning

confidence: 99%

Effects of Halothane on GABA_AReceptor Kinetics: Evidence for Slowed Agonist Unbinding

Li¹,

Pearce

2000

J. Neurosci.

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Many anesthetics, including the volatile agent halothane, prolong the decay of GABA A receptor-mediated IPSCs at central synapses. This effect is thought to be a major factor in the production of anesthesia. A variety of different kinetic mechanisms have been proposed for several intravenous agents, but for volatile agents the kinetic mechanisms underlying this change remain unknown. To address this question, we used rapid solution exchange techniques to apply GABA to recombinant GABA A receptors (␣ 1 ␤ 2 ␥ 2s ) expressed in HEK 293 cells, in the absence and presence of halothane. To differentiate between different microscopic kinetic steps that may be altered by the anesthetic, we studied a variety of measures, including peak concentration-response characteristics, macroscopic desensitization, recovery from desensitization, maximal current activation rates, and responses to the low-affinity agonist taurine. Experimentally observed alterations were compared with predictions based on a kinetic scheme that incorporated two agonist binding steps, and open and desensitized states. We found that, in addition to slowing deactivation after a brief pulse of GABA, halothane increased agonist sensitivity and slowed recovery from desensitization but did not alter macroscopic desensitization or maximal activation rate and only slightly slowed rapid deactivation after taurine application. This pattern of responses was found to be consistent with a reduction in the microscopic agonist unbinding rate (k off ) but not with changes in channel gating steps, such as the channel opening rate (␤), closing rate (␣), or microscopic desensitization. We conclude that halothane slows IPSC decay by slowing dissociation of agonist from the receptor. Key words: GABA; halothane; taurine; synaptic inhibition; anesthetics; receptor kineticsThe activity of ligand-gated ion channels can be described in kinetic terms by defining transition rates between individual metastable states of the receptor. Drug action can then be viewed as altering the transition rates between these states, under the assumption that drug binding does not introduce new transitions to the kinetic scheme. Using this approach to study pharmacological modulation of the GABA A receptor, it has been proposed that barbiturates alter transition rates between agonist-bound closed states (Macdonald et al., 1989a;Macdonald and Olsen, 1994), neurosteroids decrease the exit rate from the desensitized state of the receptor (Zhu and Vicini, 1997), and benzodiazepines increase the agonist binding rate (Rogers et al., 1994) or decrease agonist unbinding rate and accelerate desensitization (Mellor and Randall, 1997). In addition, it has been proposed that dephosphorylation of the GABA A receptor reduces the agonist unbinding rate, slowing deactivation and prolonging inhibitory currents (Jones and Westbrook, 1997).The volatile anesthetic halothane prolongs GABA A receptormediated IPSCs (Gage and Robertson, 1985;Mody et al., 1991;Jones and Harrison, 1993;Pearce, 1996), as do a great number o...

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Kinetic properties of the GABAA receptor main conductance state of mouse spinal cord neurones in culture.

Cited by 206 publications

References 29 publications

GABA Affinity Shapes IPSCs in Thalamic Nuclei

GABA Affinity Shapes IPSCs in Thalamic Nuclei

Clustering of Extrasynaptic GABAA Receptors Modulates Tonic Inhibition in Cultured Hippocampal Neurons

Effects of Halothane on GABA_AReceptor Kinetics: Evidence for Slowed Agonist Unbinding

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Kinetic properties of the GABAA receptor main conductance state of mouse spinal cord neurones in culture.

Cited by 206 publications

References 29 publications

GABA Affinity Shapes IPSCs in Thalamic Nuclei

GABA Affinity Shapes IPSCs in Thalamic Nuclei

Clustering of Extrasynaptic GABAA Receptors Modulates Tonic Inhibition in Cultured Hippocampal Neurons

Effects of Halothane on GABAAReceptor Kinetics: Evidence for Slowed Agonist Unbinding

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Effects of Halothane on GABA_AReceptor Kinetics: Evidence for Slowed Agonist Unbinding