BURSTINGHasaeam Cho

Bio-NanoStructure Lab

of Prof. MC Choi.

I will summary the paper …

Contents

• Introduction• Bursting midbrain DA neurons – beyond RPE signal

• Primate studies• Rodent studies

• Beyond phenomenology – how bursts and pauses are gener-ated in DA neurons

• Afferent inputs controlling bursts in DA neurons• Afferent inputs controlling pauses

• Intrinsic conductances in DA neurons as gates for burst and pause control

• In vitro dynamic clamp approaches to channel function in DA bursting

• Conclusions• References

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STEP 1

INTRODUCTION

DA midbrain neurons

• Dopamine (DA) midbrain neurons• project to several striatal and cortical target areas• Are essentially involved in important brain functions

such as• Action selection• Motor performance• Motivation• Reward-based learning• Working memory• Cognition

Reward Prediction Error RPE

• The reward prediction error (RPE)• The difference between expected and actually de-

livered rewards

Prediction Error = actual reward – expected reward

RPE

• The reward prediction error (RPE)• The difference between expected and actually deliv-

ered rewards

• It’s Positive when the reward > expectation• Positive RPE is expressed as a phasic increase of fir-

ing above the tonic background rate.

• It’s Negative when the reward < expectation

or not delivered at all (reward omission)• Negative RPE is expressed as a transient reduction

of firing frequency below background rate • or even by a period of complete electrical silence

(a pause)

RPE

• Midbrain DA neurons• Short (<500 ms) bursts of high-fre-

quency• In vivo, occur time-locked (>50-300

ms after) to either • unexpected reward delivery or, • after learning, sensory cues that

predict upcoming reward delivery within the next few seconds.

• Quantitative analysis revealed• the cue-related intra-burst firing fre-

quency was associated with both • the expected reward amplitude and

expected probability of delivery, • and the intra-burst frequency after

reward delivery, RPE.

Schultz, 2007

Reinforcement-learning theory

• The reinforcement

-learning

theory

• Both negative and positive RPEs act as teaching signals. • Most likely by changing synaptic weights of glutamater-

gic cortico-striatal synapses on the most prominent tar-get neurons of midbrain DA neurons

• Via altering the occupancy and signaling of postsynaptic D1- and D2-type receptors.

Excitation, Inhibition, Disinhibition

• Excitation• Excitatory postsynaptic

potential (EPSP)• Causes depolarization

• Inhibition• Inhibitory postsynaptic

potential (IPSP)• Causes hyperpolarization

• Disinhibition• A temporary loss of inhi-

bition

STEP 2

BURSTING MIDBRAIN DA NEU-RONS – BEYOND RPE SIGNAL

Burst firing of DA neurons under diverse behavioral contexts in awake animals.

Primate studies

• Burst firing was induced by additional cues.• As aversive, a blow of cold air to the eyes that triggers a protective

blink response

• In many previous studies have characterized dopamine neurons as a functionally homogeneous population.

• However, the largest population 40% of DA neurons did not show phasic response to ACS

• (ACS = air-puff-predictive conditioned stimulus).• But, displayed typical responses to reward-predicting cues.

• An even smaller 10% responses to unexpected airpuffs• dorsolateral DA subpopulation showed large and relatively

short (100 ms) phasic burst.

Primate studies

Matsumoto and Hikosaka, 2009

Primate studies

The midbrain DA population might be indeed relatively uniform in their responses to unexpected reward and reward-predicting cues,

But displays a topographically organized diversity in response to other salient events.

• which might not directly instruct RPE-learning but initiate. E.g. orienting responses.

Primate studies

• The recent wave of studies in awake primates• have significantly widened the functional context for

burst firing• among different types of DA neurons as well as

within the burst firing itself.• However, have remained descriptive and phe-

nomenological • Because pharmacological or even optogenetic tools

have not yet been used.

Rodent studies

• SFB% : the percentage of spikes within a spike train that were fired within bursts spikes fired in bursts

• The degree of “burstiness” of DA neuron• The start (ISI < 80 ms) and stop conditions (ISI > 160 ms)

of burst firing, suggested by Grace and Bunney.• With this criterion, bursting became quantifiable.

• Therefore, alternative burst detection methods have been introduced in recent years

• relate burst firing and pauses to the stochastic properties of the spike train

• are independent of the absolute firing frequency.

• Also, DA recordings in awake rodents, either freely moving or head fixed are increased.

Rodent studies

• In summary,• Cue- and reward-associated burst firing is present in

some, but never all • in recent awake rodent studies even when ruled out by cell

type-specific optogenetic tagging.• Thus, functional diversity among DA VTA neurons appears to

be the norm.

• Burst firing have also been recorded • in behavioral settings not directly related to reward-driven

classic or operant conditioning paradigms.

• Recordings in awake rodents identified• a diverse phenomenology of burst firing associated with

salient sensory cues, delivery of reward and the control of ac-tion sequences.

Ventral tegmental area; VTA, substantia nigra (pars compacta); SN(C)

STEP 3

BEYOND PHENOMENOLOGY – HOW BURSTS AND PAUSES ARE

GENERATED IN DA NEURONSBurst firing of DA neurons by synaptic ex-

citation and GABAergic disinhibition.

Beyond phenomenology

• The behavioral importance of bursts and pauses• Can only be appreciated from studies done in awake behaving ani-

mals• Most underlying mechanisms arise from studies

• in anesthetized rodents • or in vitro brain slice studies.

• DA neurons exhibit spontaneous bursts, even in anesthetized animal.

• Much of studies of these spontaneous bursts.• Mechanisms in anesthetized preparations and in awake behaving

animals• No guarantee that they are identical.• Some evidence that they might be very similar.

• Spontaneous bursts are indistinguishable from reward related bursts in duration, firing frequency, and other structural features.

• The firing rates• Of single spike (non-bursting), about 0-10 Hz.• During bursts in vivo, rates up to 50 Hz.

Afferent inputs controlling bursts in DA neurons• Excitatory synaptic input in vivo is necessary for burst-

ing in DA neurons.

• The synaptic inputs to midbrain DA neurons• Penduculopontine nucleus (PPN)/Lateral dorsal tegmentum

(LDT)• In vivo disinhibition of PPN increased the burst firing of

putative DA neurons in the SN and VTA by about 50%.• PPN inhibition reduced burst firing by about 50%

• Subthalamic nucleus (STN)• GABA-mediated disinhibition of the STN leads to in-

creased burst firing in a subpopulation of DA neurons in an NMDA-sensitive manner.

• In contrast to excitatory input, it has been estimated that 70% of all afferents to DA neurons are inhibitory.

Zweifel et al., 2009

Afferent inputs controlling pauses

• in vivo, IPSPs generated by GABAergic synaptic inputs are the obvious candidate afferents.

• IPSPs are capable of delaying action potentials in a time-dependent way.

• The powerful somatic oscillatory currents drive the sin-gle-spike firing.

The cell will fire again unless there is strong inhibition of long duration.

Pauses are only possible with a synchronized increase in inhibition from significant GABAergic input or a de-crease in tonic excitatory input.

Afferent inputs controlling pauses

Cohen et al., 2012

STEP 4

INTRINSIC CONDUCTANCES IN DA NEURONS AS GATES FOR BURST

AND PAUSE CONTROLControl of burst firing by distinct potas-

sium channels in DA neurons.

Gates for burst and pause control

• Two complementary approaches• used to study functional contribution of postsynaptic

channels to DA bursting.

1. In vivo extracellular approach• is to pharmacologically or molecularly modulate chan-

nel activity• and then analyze the related changes in burst firing pa-

rameters.

2. In vitro intracellular brain slice approach• provides a high level of experimental control• is to induce bursts in DA neurons by stimulation of af-

ferents• and then assess underlying channel mechanisms.

Gates for burst and pause control

• ATP-sensitive potassium (K-ATP) channels• Necessary for bursting in vivo for a medial subpopu-

lation of SNC neurons.• In vitro, selective opening of K-ATP channels in the

presence of tonic NMDA receptor stimulation is suf-ficient to switch medial SNC neurons to a burst-fir-ing mode.

• Enable NMDA-mediated bursting of medial DA SN neuron in vitro and in vivo.

Schiemann et al., 2012

Schiemann et al., 2012

Gates for burst and pause control

• The bio-physical mechanisms is not yet clear• how K-ATP channel opening enables burst firing • which in vivo upstream mechanism controls K-ATP

channel open probabilities in DA neurons

• Behavioral data suggest that K-ATP channel-mediated bursting in medial DA SN neurons is important for ex-plorative behavior.

STEP 5

IN VITRO DYNAMIC CLAMP AP-PROACHES TO CHANNEL FUNC-

TION IN DA BURSTINGRealistic burst firing dissected by dynami-

cal clamp techniques.

Dynamic Clamp technique

• A method that uses computer simulations to introduce virtual conductances into real neurons.

• representing a hybrid between computational models and bio-logical neurons.

• can be used to study specific parameters of NMDA, AMPA, GABA, and other channel conductances in shaping bursts and pauses in DA neurons.

In vitro dynamic clamp approaches to channel function• NMDA receptor

• conductance is capable of following the activity of DA neurons at bursting rates.

• undergoes block, following each AP within a burst.helps to remove sodium channel inactivation.allows full hyperpolarization during the repolarization

following each AP within a burst.unblocks during the depolarizing phase prior to the

next AP, increasing the speed of depolarization.

• Therefore, the voltage dependence of NMDA re-ceptors allows dopaminergic neurons to fire at rates higher than those possible.

N-methyl-D-aspartate (NMDA)

In vitro dynamic clamp approaches to channel function

Deister et al., 2009

In vitro dynamic clamp approaches to channel function

Lobb et al., 2010

In vitro dynamic clamp approaches to channel function• Dynamic clamp experiments show that

• Realistic burst firings• the NMDA receptor conductance is capable of following the

activity of DA neurons at bursting rates .• Intrinsic cellular dynamics are an important part of bursting in

the dopaminergic neuron.

• Future studies• More complete burst mechanism by incorporating synaptic ki-

netics with the ion channel kinetics• how specific excitatory inputs may differentially affect the firing

pattern of DA neurons.• more data regarding dendritic ion channel distributions.

In vivo study

• In vivo studies show that • local application of a variety of GAB antagonists • by pressure ejection onto the recorded neuron shifts the firing

pattern of the DA neuron from a single-spiking mode to a bursting one

tonic GAB receptor activation suppresses burst firing.

• However, it would need to overcome a substantially in-creased conductance and block.

• These limitations can be overcome by a mechanism whereby bursting is controlled by disinhibition, and pausing is controlled by phasic removal of excitation.

STEP 6

CONCLUSION

Highlights

STEP 2

• Burst firing of DA neurons under diverse behavioral contexts in awake animals.

STEP 3

• Burst firing of DA neurons by synaptic excitation and GABAergic disinhibition.

STEP 4

• Control of burst firing by distinct potassium channels in DA neurons.

STEP 5

• Realistic burst firing dissected by dynamic clamp tech-niques.

CONCLUSION

• Transient changes of firing in midbrain DA neurons have re-ceived great attention.

• Recent work broadened the behavioral framework of these changes both in non-human primates and rodents.

• However, up to now most mechanistic studies are currently limited to in vitro preparations.

• Future in vivo studies are likely to advance mechanistic un-derstanding.

• With the advent of optogenetic tagging of distinct DA neurons in the midbrain and the potential to selectively drive synaptic inputs,

• However, intracellular in vivo recordings of DA neurons are needed to improve our mechanistic understanding of burst and pause firing of defined DA neurons in awake behaving animals.

REFERENCES• Figure 1

• Generating bursts (and pauses) in the dopamine midbrain neurons, C. A. Paladini and J. Roeper, Neuroscience (2014)

• Step 1 Introduction - Figure 1, 2, 3, 4, 5, 6, 7• http://www.medizinische-fakultaet-hd.uni-heidelberg.de/Simon.102039.0.html• PPTs of Brain Science Fundamental• Figure 2 of Schultz, 2007a

• Step 2 - Figure 7, 8• Figure 1, 2 of Matsumoto and Hikosaka, 2009

• Step 3 – Figure 9• Figure 3 of Cohen et al., 2012

• Step 4 – Figure 10, 11• Figure 2, 3 of Schiemann et al., 2012

• Figure 12• http://blog.gametize.com/2014/03/7-smart-ways-to-improve-learning-with-gamification-part-2/bori

ng/

• Step 5 The dynamic clamp – Figure 13, 14, 15, 16• http://rtxi.org/docs/tutorials/2014/12/05/dynamic-clamp/• Figure 1 of Dynamic clamp with StdpC software, Ildiko Kemenes et al., 2011, Nature Protocols• Figure 6 of Diester et al., 2009• Figure 1 of Lobb et al., 2010

TANK YOU!