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Insulin reduces neuronal excitability by turning on GABA(A) channels that generate tonic current.

Jin Z, Jin Y, Kumar-Mendu S, Degerman E, Groop L, Birnir B - PLoS ONE (2011)

Bottom Line: Here we show in acute rat hippocampal slices that insulin (1 nM) "turns on" new extrasynaptic GABA(A) channels in CA1 pyramidal neurons resulting in decreased frequency of action potential firing.The single-channel current amplitude is related to the GABA concentration resulting in a single-channel GABA affinity (EC(50)) in intact CA1 neurons of 17 pM with the maximal current amplitude reached with 1 nM GABA.The insulin-induced new channels provide a specific target for rescuing cognition in health and disease.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience, Uppsala University, Uppsala, Sweden.

ABSTRACT
Insulin signaling to the brain is important not only for metabolic homeostasis but also for higher brain functions such as cognition. GABA (γ-aminobutyric acid) decreases neuronal excitability by activating GABA(A) channels that generate phasic and tonic currents. The level of tonic inhibition in neurons varies. In the hippocampus, interneurons and dentate gyrus granule cells normally have significant tonic currents under basal conditions in contrast to the CA1 pyramidal neurons where it is minimal. Here we show in acute rat hippocampal slices that insulin (1 nM) "turns on" new extrasynaptic GABA(A) channels in CA1 pyramidal neurons resulting in decreased frequency of action potential firing. The channels are activated by more than million times lower GABA concentrations than synaptic channels, generate tonic currents and show outward rectification. The single-channel current amplitude is related to the GABA concentration resulting in a single-channel GABA affinity (EC(50)) in intact CA1 neurons of 17 pM with the maximal current amplitude reached with 1 nM GABA. They are inhibited by GABA(A) antagonists but have novel pharmacology as the benzodiazepine flumazenil and zolpidem are inverse agonists. The results show that tonic rather than synaptic conductances regulate basal neuronal excitability when significant tonic conductance is expressed and demonstrate an unexpected hormonal control of the inhibitory channel subtypes and excitability of hippocampal neurons. The insulin-induced new channels provide a specific target for rescuing cognition in health and disease.

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Inhibition of insulin (1 nM) induced tonic current by flumazenil and zolpidem.Effects of flumazenil (A, 1 µM), zolpidem (B, 100 nM) and SR-95531 (20 µM) on the currents in an ACSF (a) and an insulin treated neuron (b). c, Amplitudes of tonic currents in individual neurons; A: ACSF (n = 8), insulin incubated (n = 10); B: ACSF (n = 6), insulin-incubated neurons (n = 13). d, A & B, % of total tonic current inhibited by antagonists. C. Tonic currents (a, b) in the presence of L655, 708 (5 µM) or zolpidem (100 nM) or the drugs together (L655, 708,5 µM; zolpidem, 100 nM) in insulin treated neurons. c. Amplitudes of the tonic currents in individual neurons (L655, 708 first applied alone: open circle, n = 4; zolpidem first applied alone: filled circle, n = 4). d. Tonic current inhibited by L655, 708 plus zolpidem (n = 8) normalized to the tonic current inhibited by either L655, 708 or zolpidem in the same cell. ns, no significance.
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pone-0016188-g002: Inhibition of insulin (1 nM) induced tonic current by flumazenil and zolpidem.Effects of flumazenil (A, 1 µM), zolpidem (B, 100 nM) and SR-95531 (20 µM) on the currents in an ACSF (a) and an insulin treated neuron (b). c, Amplitudes of tonic currents in individual neurons; A: ACSF (n = 8), insulin incubated (n = 10); B: ACSF (n = 6), insulin-incubated neurons (n = 13). d, A & B, % of total tonic current inhibited by antagonists. C. Tonic currents (a, b) in the presence of L655, 708 (5 µM) or zolpidem (100 nM) or the drugs together (L655, 708,5 µM; zolpidem, 100 nM) in insulin treated neurons. c. Amplitudes of the tonic currents in individual neurons (L655, 708 first applied alone: open circle, n = 4; zolpidem first applied alone: filled circle, n = 4). d. Tonic current inhibited by L655, 708 plus zolpidem (n = 8) normalized to the tonic current inhibited by either L655, 708 or zolpidem in the same cell. ns, no significance.

Mentions: GABAA channels are pentamers formed by subunits from 8 different families (α1–6, β1–4, γ1–3, δ, ε, θ, π, ρ1–3). The channel is thought to be composed of two αs, two βs and a third subunit type that is often the δ or the γ2 subunit [24]. Whether the native channels are homo or heteromeric in terms of the αs and βs subunits is not clear [25]. We examined two compounds that are normally inert at α5βγ2 channels: the benzodiazepine flumazenil (1 µM) and zolpidem (100 and 200 nM), a positive modulator at α1, α2 and α3 in αβγ2 channels. Surprisingly, in slices incubated in insulin both flumazenil (Fig. 2A) and zolpidem (Fig. 2B) shifted the holding current revealing the large insulin-induced tonic current. Similar to L655, 708, flumazenil and zolpidem inhibited the tonic current by 76±6% and 81±5%, respectively (Fig. 2). Flumazenil and zolpidem had minimal effect on the holding current level in the control slices (Flumaznil, n = 8 and zolpidem, n = 6, Fig. 2A & B) consistent with other studies [14], [26]. We examined if L655, 708 and zolpidem affected the same population of channels by first applying one and then the two drugs together and examined the shift in the holding current (Fig. 2C). The results show that it does not matter in which order the drugs are applied, the effects are not additive. Rather, it appears that the first drug applied occludes the effect of the other consistent with that the drugs are acting on the same population of channels. At synapses, flumazenil is normally inert whereas zolpidem is known to potentiate the currents [14], characteristics that are maintained in the insulin incubated slices (Fig. 3A & B).


Insulin reduces neuronal excitability by turning on GABA(A) channels that generate tonic current.

Jin Z, Jin Y, Kumar-Mendu S, Degerman E, Groop L, Birnir B - PLoS ONE (2011)

Inhibition of insulin (1 nM) induced tonic current by flumazenil and zolpidem.Effects of flumazenil (A, 1 µM), zolpidem (B, 100 nM) and SR-95531 (20 µM) on the currents in an ACSF (a) and an insulin treated neuron (b). c, Amplitudes of tonic currents in individual neurons; A: ACSF (n = 8), insulin incubated (n = 10); B: ACSF (n = 6), insulin-incubated neurons (n = 13). d, A & B, % of total tonic current inhibited by antagonists. C. Tonic currents (a, b) in the presence of L655, 708 (5 µM) or zolpidem (100 nM) or the drugs together (L655, 708,5 µM; zolpidem, 100 nM) in insulin treated neurons. c. Amplitudes of the tonic currents in individual neurons (L655, 708 first applied alone: open circle, n = 4; zolpidem first applied alone: filled circle, n = 4). d. Tonic current inhibited by L655, 708 plus zolpidem (n = 8) normalized to the tonic current inhibited by either L655, 708 or zolpidem in the same cell. ns, no significance.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3021545&req=5

pone-0016188-g002: Inhibition of insulin (1 nM) induced tonic current by flumazenil and zolpidem.Effects of flumazenil (A, 1 µM), zolpidem (B, 100 nM) and SR-95531 (20 µM) on the currents in an ACSF (a) and an insulin treated neuron (b). c, Amplitudes of tonic currents in individual neurons; A: ACSF (n = 8), insulin incubated (n = 10); B: ACSF (n = 6), insulin-incubated neurons (n = 13). d, A & B, % of total tonic current inhibited by antagonists. C. Tonic currents (a, b) in the presence of L655, 708 (5 µM) or zolpidem (100 nM) or the drugs together (L655, 708,5 µM; zolpidem, 100 nM) in insulin treated neurons. c. Amplitudes of the tonic currents in individual neurons (L655, 708 first applied alone: open circle, n = 4; zolpidem first applied alone: filled circle, n = 4). d. Tonic current inhibited by L655, 708 plus zolpidem (n = 8) normalized to the tonic current inhibited by either L655, 708 or zolpidem in the same cell. ns, no significance.
Mentions: GABAA channels are pentamers formed by subunits from 8 different families (α1–6, β1–4, γ1–3, δ, ε, θ, π, ρ1–3). The channel is thought to be composed of two αs, two βs and a third subunit type that is often the δ or the γ2 subunit [24]. Whether the native channels are homo or heteromeric in terms of the αs and βs subunits is not clear [25]. We examined two compounds that are normally inert at α5βγ2 channels: the benzodiazepine flumazenil (1 µM) and zolpidem (100 and 200 nM), a positive modulator at α1, α2 and α3 in αβγ2 channels. Surprisingly, in slices incubated in insulin both flumazenil (Fig. 2A) and zolpidem (Fig. 2B) shifted the holding current revealing the large insulin-induced tonic current. Similar to L655, 708, flumazenil and zolpidem inhibited the tonic current by 76±6% and 81±5%, respectively (Fig. 2). Flumazenil and zolpidem had minimal effect on the holding current level in the control slices (Flumaznil, n = 8 and zolpidem, n = 6, Fig. 2A & B) consistent with other studies [14], [26]. We examined if L655, 708 and zolpidem affected the same population of channels by first applying one and then the two drugs together and examined the shift in the holding current (Fig. 2C). The results show that it does not matter in which order the drugs are applied, the effects are not additive. Rather, it appears that the first drug applied occludes the effect of the other consistent with that the drugs are acting on the same population of channels. At synapses, flumazenil is normally inert whereas zolpidem is known to potentiate the currents [14], characteristics that are maintained in the insulin incubated slices (Fig. 3A & B).

Bottom Line: Here we show in acute rat hippocampal slices that insulin (1 nM) "turns on" new extrasynaptic GABA(A) channels in CA1 pyramidal neurons resulting in decreased frequency of action potential firing.The single-channel current amplitude is related to the GABA concentration resulting in a single-channel GABA affinity (EC(50)) in intact CA1 neurons of 17 pM with the maximal current amplitude reached with 1 nM GABA.The insulin-induced new channels provide a specific target for rescuing cognition in health and disease.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience, Uppsala University, Uppsala, Sweden.

ABSTRACT
Insulin signaling to the brain is important not only for metabolic homeostasis but also for higher brain functions such as cognition. GABA (γ-aminobutyric acid) decreases neuronal excitability by activating GABA(A) channels that generate phasic and tonic currents. The level of tonic inhibition in neurons varies. In the hippocampus, interneurons and dentate gyrus granule cells normally have significant tonic currents under basal conditions in contrast to the CA1 pyramidal neurons where it is minimal. Here we show in acute rat hippocampal slices that insulin (1 nM) "turns on" new extrasynaptic GABA(A) channels in CA1 pyramidal neurons resulting in decreased frequency of action potential firing. The channels are activated by more than million times lower GABA concentrations than synaptic channels, generate tonic currents and show outward rectification. The single-channel current amplitude is related to the GABA concentration resulting in a single-channel GABA affinity (EC(50)) in intact CA1 neurons of 17 pM with the maximal current amplitude reached with 1 nM GABA. They are inhibited by GABA(A) antagonists but have novel pharmacology as the benzodiazepine flumazenil and zolpidem are inverse agonists. The results show that tonic rather than synaptic conductances regulate basal neuronal excitability when significant tonic conductance is expressed and demonstrate an unexpected hormonal control of the inhibitory channel subtypes and excitability of hippocampal neurons. The insulin-induced new channels provide a specific target for rescuing cognition in health and disease.

Show MeSH
Related in: MedlinePlus