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Neuroscience Journal Club 19 February 2008 Henry Lester . H Alle and J R. P. Geiger (2006) Combined Analog and Action Potential Coding in Hippocampal Mossy Fibers Science 311: 1290. Y Shu, A Hasenstaub, A Duque, Y Yu & D. A. McCormick (2006).

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slide1

Neuroscience Journal Club

19 February 2008

Henry Lester

slide2

H Alle and J R. P. Geiger (2006)

Combined Analog and Action Potential Coding in Hippocampal Mossy Fibers

Science 311: 1290

Y Shu, A Hasenstaub, A Duque, Y Yu & D. A. McCormick (2006).

Modulation of intracortical synaptic potentials by presynaptic somatic membrane potential

Nature 441: 761 and PNAS, 2007

M. H. P. Kole, J. J. Letzkus, G. J. Stuart (2007)

Axon initial segment Kv1 channels control axonal action potential waveform and synaptic efficacy

Neuron, 2007

Hideki Kawai, Ronit Lazar & Raju Metherate (2007)

Nicotinic control of axon excitability regulates thalamocortical transmission

Nature Neurosci 10, 1168

slide3

Classical studies (Katz & Miledi, squid synapse) show . . .

Presynaptic voltage-dependent Ca channels are the major source of Ca influx leading to transmitter release. These Ca channels do not inactivate strongly on a ms time scale.

On a ms time scale, the Ca sensors seem to integrate Ca influx.

In nonsaturating regimes, release depends strongly on the Ca influx—probably to the 3rd or 4th power.

Therefore transmitter release depends strongly on the duration of the presynaptic AP.

slide4

The prevailing mode to encode information in the mammalian central nervous system is to convert an analog signal resulting from graded synaptic inputs into patterns of action potentials, which are transmitted as all-or-none signals along the axons.

By contrast, in primary sensory systems and central neural circuits of small invertebrates, analog signals are used directly to transmit information.

In many brain regions the axonal distances from the cell body to a

large fraction of the corresponding presynaptic boutons are rather short and somatic subthreshold signals can be large in amplitude.

The question arises whether analog axonal signaling contributes to information transmission in the mammalian brain.

Paraphrased from Alle & Geiger, intro

slide6

Caltech Neuroscience Journal Club, February 1986

Presented by Jeanne Nerbonne

O Belluzi, O Sacchi, E. Wanke (1985). A fast transient outward current in the rat sympathetic neurone studied under voltage-clamp conditions.

J Physiol. 358:91-108

slide7

The Basic Phenomenon:

Depolarizing the Presynaptic Soma  Larger EPSP

Yu et al, Figure 1

slide8

The mechanism:

Changes in somatic membrane potential affect

amplitude & duration of somatic and axonal action potentials

Yu et al, Figure 3

slide10

EPreSPs: recorded in mossy fiber boutons;

generated upstream of the CA3 region.

Alle & Geiger

Figs 1 & 2

slide11

Electrotonic propagation of somatic depolarizations underlies EPreSPs.

Alle & Geiger

Fig 3

slide12

Dual pre- & postsynaptic V-clamp:

nerve terminal depolarizations do increase EPSCs

Alle & Geiger

Fig 3

slide15

Kv1.2* channels appear to control AP waveform

α-DTX specifically blocks Kv1.1*, 1.2*, or 1.6* channels; low doses of 4-APblock less specifically.

Tityustoxin-Kα, which is relativelyspecific for K+ channels containing Kv1.2 subunits stronglyblocked the slowly inactivating K+ current in cortical axons,whereas dendrotoxin-K, which is relatively specific for Kv1.1subunits, exhibited only marginal effects.

 Kv1.2* channels arethe mediators of the D-current responsible for axonal spikerepolarization.

Immunocytochemical studies reveal the prominentexpression of Kv 1.2 channels in axons and axonal terminal fieldsthroughout the brain. In the neocortex, Kv1.2is of particularly high density in the more distal portionsof the axon initial segment of cortical pyramidal cells.

Yu et al PNAS 2007

slide18

. . . because nicotine enhances success rate for thalamocortical axonal transmission

Subcortical white matter

axons

Local nicotine perfusion

STR =superior thalamic radiation

Kawai et al

slide19

Chronic nicotine:

increases perforant path a4* nicotinic receptor fluorescence ~ 2-fold

Alveus

Py

Or

Rad

    • TV Bliss, T Lömo (1973)
  • Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.
    • J Physiol. 232:331-56.

LMol

200 mm

Temperoammonic Path

Medial Perforant Path

Nashmi et al, 2007

slide20

Acute Nicotine

Acute Saline

80 min

10 min

80 min

10 min

Chronic

Chronic

1 mV

Saline

Saline

10 ms

0.5 mV

0.5 mV

10 ms

5 ms

Nicotine

1mV

Nicotine

1 mV

10 ms

10 ms

Simple model for

cognitive

sensitization:

chronic nicotine

+

acute nicotine

lowers the threshold for perforant pathway LTP

Acute Nicotine

Acute Saline

Chronic Nicotine

Acute

Nashmi et al, 2007

Chronic

Chronic