CELLULAR NEUROPHYSIOLOGY

Transcription

1 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-1: GABA A receptor-mediated current and potential changes

2 GABAergic synaptic transmission 1 Constance Hammond

3 The inhibitory postsynaptic potential (IPSP) 2 whole-cell configuration current-clamp mode [Cl - ] i = 14 mm; [Cl - ] o = 146 mm E Cl = -59 mv V m (mv) In these experimental conditions, stimulation of the GABAergic presynaptic neuron causes a transient hyperpolarization of the postsynaptic membrane I (pa)

4 The inhibitory postsynaptic potential (IPSP) 3 whole-cell configuration current-clamp mode [Cl - ] i = 14 mm; [Cl - ] o = 146 mm E Cl = -59 mv Definition Benzodiazepines are psychoactive drugs used for instance in the treatment of anxiety (anxiolytic). V m (mv) I (pa)

5 The inhibitory postsynaptic potential (IPSP) 4 whole-cell configuration current-clamp mode [Cl - ] i = 14 mm; [Cl - ] o = 146 mm E Cl = -59 mv V m (mv) I (pa)

6 The inhibitory postsynaptic potential (IPSP) 5 What ion channel is involved? To which ion(s) is it permeable? How do benzodiazepines work?

7 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-2: GABA A receptors and unitary GABA A receptor-mediated current

8 GABA receptors 6 type A (GABA A ) - ionotropic type B (GABA B ) - metabotropic

9 GABA A receptor-channel 7 α, β or subunit 5 subunits

10 GABA A receptor-channel 8 α, β or subunit 2 GABA + R GABA 2 -R GABA 2 R*

11 Unitary GABA A receptor-mediated current 9 cell-attached configuration voltage-clamp mode [Cl - ] intrapipette = 146 mm V H outward current = positive charges flow outside the cell OR negative charges flow inside

12 Unitary GABA A current 10 cell-attached configuration voltage-clamp mode V H

13 Unitary GABA A current 11 cell-attached configuration voltage-clamp mode [Cl - ] i 14 mm [Cl - ] o = 146 mm

14 Unitary GABA A current 12 cell-attached configuration outside-out configuration [Cl - ] i 14 mm [Cl - ] o = 146 mm [Cl - ] i = 146 mm [Cl - ] o = 146 mm E Cl = 0 mv

15 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-3: The total GABA A receptor-mediated current

16 Total GABA A receptor-mediated current 13 whole-cell configuration voltage-clamp mode E Cl = 0 mv spontaneous currents glutamate channel blockers (10 µm CNQX + 40 µm APV) V rest = -70 mv Variable amplitude (1-30 pa), depending on the number of activated GABA A receptors

17 Total GABA A receptor-mediated current 14 Rising phase: rise time = ms summation of the unitary GABA A currents GABA release and binding receptorchannels open Decay phase: duration = hundreds of milliseconds GABA A receptor-channels close (GABA unbinding and re-uptake)

18 Take home message 15 GABA A receptor channels are made up of five subunits (a, b, ) Both a & b subunits delimit the receptor sites for GABA GABA A receptor channels are permeable to chloride ions (anions) The amplitude of the GABA A current is close to zero at resting membrane potential

19 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-4: GABA A receptor-channel and benzodiazepines

20 Unitary GABA A current and benzodiazepines 16 How do benzodiazepines work? Benzodiazepines are psychoactive drugs (anxiolytic and muscle relaxant). E.g.: diazepam

21 Unitary GABA A current and benzodiazepines 17 outside-out configuration voltage-clamp mode (V H = -75 mv) [Cl - ] i = [Cl - ] e ; E Cl = 0 mv I = Np o i GABA A channel opening frequency increases intensity of the total GABA A current increases

22 Benzodiazepine action on the GABA A channel 18

23 Take home message 19 Both a & subunits delimit the receptor site for benzodiazepines Benzodiazepines increase the total GABA A (chloride) current by increasing the probability of opening of the GABA A channel

24 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-5: GABA A receptor-mediated postsynaptic potential

25 GABA A postsynaptic potential 20 whole-cell configuration current-clamp mode spontaneous postsynaptic potentials 146 mm 14 mm V rest = -50 mv E Cl = -59 mv inibitory postsynaptic potential (IPSP)

26 GABA A postsynaptic potential 21 whole-cell configuration current-clamp mode spontaneous postsynaptic potentials truncated action potentials

27 Take home message 22 The GABA A current reverses at E Cl E Cl varies from neurons to neurons: therefore GABA sometimes hyperpolarizes sometimes depolarizes the postsynaptic membrane

28 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND 5. SYNAPTIC TRANSMISSION II: GABAERGIC TRANSMISSION Video 5-6: Conclusion

29 Overview of GABAergic transmission 23 Type A receptor-channels (GABA A ): 5 subunits In general: 2 α subunits 2 β subunits 1 subunits 2 GABA receptor sites at the interface between α and β subunits 1 receptor site for benzodiazepines, allosteric agonists of the GABA A receptor Essentially permeable to chloride ions (Cl - ) Adult neurons: no or slightly hyperpolarizing current Immature neurons: high [Cl - ] i depolarizing current

30 Overview of GABAergic transmission 24 Presynaptic membrane: - Ca 2+ channel - anchoring proteins - synaptic vesicle fusion sites (GABA release) - GABA transporters (re-uptake) Postsynaptic membrane: - GABA A receptor-channels - GABA B receptors (not shown) Glial cell - GABA transporters (re-uptake) presynaptic element postsynaptic element

31 Overview of GABAergic transmission 25 (1) GABA binds to the GABA A receptor the channels open (2) GABA binds to transporters recapture in presynaptic terminals and glial cells (3) GABA spontaneously unbinds the channels close presynaptic element postsynaptic element

32 CELLULAR NEUROPHYSIOLOGY CONSTANCE HAMMOND Professor Constance Hammond Project manager Isabelle Virard Graphic designer Géraldine Fohr Movie director Marine Chabrolin Music Tetoma - Hicham Chahidi Translation Isabelle Virard et Atenao 2016