Action Potential Propagation in Mitral Cell Dendrites
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For my examination in the course Cellular Neuroscience at the University of Copenhagen I presented this classic paper by Bischofberger and Jonas (1997).
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Action Potential Propagation in Mitral Cell Dendrites
- 1. Department of Neuroscience and Pharmacology Copenhagen, 19th of June, 2013 Cellular Neuroscience examine Rune Rasmussen BSc. Student, Sport and Exercise Sciences Action potential propagation into the presynaptic dendrites of rat mitral cells (Bischofberger and Jonas, 1997)
- 2. Mitral cells (MC) Pre-synaptic neurotransmitter release to interneurons (glutamate) Principal projection neurons Dendrodendritic synapses contain L- and N-type Ca2+ channels Department of Neuroscience and Pharmacology (Illustration from Neuroscience, fifth edition, 2011)
- 3. Mitral cells (MC) (Illustration from S. Cajal, 1894: La fine structures des centres nerveux) Department of Neuroscience and Pharmacology Large soma Primary dendrite
- 4. Aim of study Department of Neuroscience and Pharmacology Examine directly the presynaptic voltage signal that triggers transmitter release at dendrodendritic synapse in MC
- 5. Evoked action potential (AP) – fast time course Whole-cell current-clamp Somatic current injection (Illustration from Neuroscience, fifth edition, 2011) Department of Neuroscience and Pharmacology
- 6. Where is the AP initiated? Evoked AP - fast time course Department of Neuroscience and Pharmacology
- 7. APs initiated near soma Whole-cell current-clamp Somatic current injection Department of Neuroscience and Pharmacology
- 8. APs initiated near soma Whole-cell current-clamp Dendritic current injection Department of Neuroscience and Pharmacology
- 9. APs amplitude conserved Amplitude and time course maintained during back-propagation Which channels mediates this? Department of Neuroscience and Pharmacology
- 10. Voltage-gated Na+ channels present in dendrites Dendritic outside-out voltage clamp (Illustration from Neuroscience, fifth edition, 2011) Department of Neuroscience and Pharmacology K+ replaced by Cs+
- 11. Department of Neuroscience and Pharmacology • Fast voltage-dependent activation • Rapid inactivation • Sensitive to TTX Voltage-gated Na+ channels present in dendrites
- 12. Na+ channels support AP back-propagation Somatic current injection Somatic AP-like voltage command Current clamp dendrite Department of Neuroscience and Pharmacology
- 13. Small HD, which channels mediates this? Half-duration (HD)* from baseline, not lowest value ½ height *Half-duration defined in: Nature Reviews Neuroscience 8, 451-465 (June 2007) Department of Neuroscience and Pharmacology Fast repolarization phase
- 14. Voltage-gated K+ channels present in dendrites Dendritic outside-out voltage clamp from -60 to 30 mV Department of Neuroscience and Pharmacology Glutathione added
- 15. Department of Neuroscience and Pharmacology • Voltage-dependent activation • No inactivation • Sensitive to TEA Voltage-gated K+ channels present in dendrites
- 16. Department of Neuroscience and Pharmacology K+ channels mediate repolarization phase Somatic current injection Somatic AP-like voltage command Current clamp dendrite
- 17. Summary Amplitude and time course of somatic AP maintained during AP back-propagation in MC Voltage-gated Na+ channels support this back- propagation into MC dendrites Voltage-gated K+ channels mediate AP repolarization in MC dendrites Department of Neuroscience and Pharmacology
- 18. Department of Neuroscience and Pharmacology Copenhagen, 19th of June, 2013 Cellular Neuroscience examine Rune Rasmussen BSc. Student, Sport and Exercise Sciences
- 19. Additional diagrams Department of Neuroscience and Pharmacology MC filled with biocytinand rhodamine stained.
- 20. Additional diagrams Department of Neuroscience and Pharmacology Primary dendrite – Cell attached configuration
- 21. Additional diagrams Department of Neuroscience and Pharmacology Soma – Before seal formation
- 22. Additional diagrams Department of Neuroscience and Pharmacology Sodium current vs. distance Slope = pA/um
- 23. Additional diagrams Department of Neuroscience and Pharmacology Amplitude of AP
- 24. Additional diagrams Department of Neuroscience and Pharmacology Potassium current vs. distance Slope = pA/um
- 25. Additional diagrams Department of Neuroscience and Pharmacology Half-duration of AP
Notas del editor
- Part of the olfactory system. Located in olfactory bulb. Principal projection neurons to other brain regions. Dendrodendritic synapses – dendrites able to release neurotransmitter like axonal terminals.
- Part of the olfactory system. Located in olfactory bulb. Principal projection neurons to other brain regions. Dendrodendritic synapses – dendrites able to release neurotransmitter like axonal terminals.
- Part of the olfactory system. Located in olfactory bulb. Principal projection neurons to other brain regions. Dendrodendritic synapses – dendrites able to release neurotransmitter like axonal terminals.
- Whole cell current clamp – measures regional potentials though numerous channels. APs is equal in amplitude and time course in primary dendrite and soma when current injected in soma.
- So where is the AP initiated first? In the primary dendrites? The soma? or in the axon initial segment?
- Current injected in soma => AP initiated in the soma first.
- Current injected in primary dendrite => AP initiated in the soma first. => The AP is initiated close to the soma, most likely initial axonal segment.
- As mentioned both the amplitude and time course is conserved with increasing distance away from AP initiation site => Which channels are mediating this back-propagation into the primary dendrite?
- Outside-out voltage clamp recording mode. Depolarizing voltage pulses from -90 mV to -10 mV with and without the sodium channel blocker Tetrodotoxin (TTX) Cesium blocks potassium channels
- In response to membrane depolarizing step a fast inward current which quickly inactivates and it is sensitive to TTX => Bears all the characteristics for a voltage-gated sodium current. They conclude that there are voltage-gated sodium channels in the primary dendritic membrane of MC.
- Is it the voltage-gated sodium channels that mediates the fast back-propagation in MC dendrites? Control situation; current clamp mode and initiates an AP in soma and primary dendrite in equal size. TTX in extracellular solution; Not able to trigger an AP in the soma region so they use an AP-like voltage camp (Voltage clamp mode) and measures the potential at the primary dendrite (current clamp). Rate of rise is decreased as well as the peak amplitude of the AP. => Sodium channels are supporting the AP back-propagation into primary dendrites in MC.
- In the control situation we notice a small half-duration of the AP in the primary dendrite. Which channels are mediating this fast repolarization of the membrane?
- Outside-out voltage clamp mode from a patch of the dendritic plasma membrane. Apply voltage steps from -60 mV to 30 mV and measures the current in the pipette under native conditions and in the presence of TEA, a potassium channel blocker. Glutathione inactivates voltage-gated sodium channels.
- The current-amplitude is voltage-dependent as with increasing depolarizing steps the outward current is increased. No sign of inactivation when the voltage step is maintained. TEA decreases the amplitude of the outward current even if the same voltage steps is applied. They conclude that there are voltage-gated potassium channels in the primary dendritic membrane of MC. Normally the potassium channel activation time-constant is relatively fixed, but in this case they are highly voltage-sensitive, as with increasing depolarization the time-constant for activation decreases, and is only submillisecond at +30 mV => they only have a very small effect on the AP amplitude.
- Is the the voltage-gated potassium channels that mediates the repolarization in MC dendrites? Control situation; current clamp mode and initiates an AP in soma and primary dendrite in equal size. TEA in extracellular solution; They use an AP-like voltage camp (Voltage clamp mode) and measures the potential at the primary dendrite (current clamp). The half-duration of the AP is increased. => Potassium channels are mediating the fast repolarization in the primary dendrites of MC.
- Sodium currents all the way from soma to distal dendritic site. Perhaps a bit increase in conductance density with increasing distance from soma?! High variability in recordings.
- Equal AP amplitude at both soma and dendritc site under control conditions, but TTX decreases the amplitude at dendrites markedly => Sodium channels are supporting the active back-propagation into MC dendrites.
- Potassium currents all the way from soma to distal dendritic sites. Perhaps a little bigger conductance density at more distal site than at soma?!
- Potassium currents all the way from soma to distal dendritic sites. Perhaps a little bigger conductance density at more distal site than at soma?!