The dopaminergic reward system

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The dopaminergic reward system

• The mesolimbic dopamine reward pathway is
  composed of dopamine cell bodies in the VTA that
  project to the NAc. The VTA also projects to
  parts of the PFC and the amygdala.
• The dopaminergic system is thought to be
  involved the rewarding aspects of motivated
  behavior.
• Consequently, addiction to drugs is thought to
  involve this system (some drugs directly-cocaine,
  and some indirectly-nicotine).
• The firing patterns of dopaminergic neurons are
  thought to encode reward prediction transition
  from low-frequency tonic to burst firing.
  Accordingly, their firing is inhibited by
  aversive stimuli.

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Whats wrong with the traditional methods?

• Electrical stimulation
• - not confined to a specific subpopulation of
  neurons.
• Lesions
• no temporal control.
• not confined to a specific subpopulation of
  neurons.
• Pharmacology
• - no temporal control.
• not confined to a specific subpopulation of
  neurons.

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Optogenetics

• A conceptually simple trick
• Express light-activated channels in neurons,
  which upon activation with light will
  de/hyper-polarize the neurons.
• Algae/Archae to the rescue
• Channelrhodopsins/halorhodopsins are
  light-activated sensors in algae/archae.
• Channelrhodopsins are non-specific cation
  channels (depolarization).
• Halorhodopsins are chloride transporters
  (hyperpolarizatiom).
• Implementation in neurons
• Remote control of neuronal activity in
  genetically-identified subpopulations.
• Millisecond temporal resolution.

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Now we can have remote control of a
genetically-identified subpopulation of neurons
with fine temporal resolution. Will this bring
us closer to the holy grail of neuroscience
linking specific activity of specific neurons to
specific behaviors?
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Step 1 targeting a specific subpopulation of
neurons

• TH-Cre mice express cre recombinase under the
  tyrosine-hydroxylase (TH) promoter, which is
  expressed specifically in dopaminergic (DA)
  neurons.
• Channelrhodopsin2 (Chr2) expression is
  cre-inducible in the virus.

VTA (source)

• When the virus is injected into the VTA, Chr2
  (tagged with YFP) will only be expressed in DA
  neurons.
• Verification
• gt90 of TH cells were YFP.
• 98 of YFP cells were TH.

NAc (target)
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Step 2 verifying the functionality of Chr2 in DA
neurons

• In vitro
• Whole-cell recordings from live slices.
• Transduced neurons had typical VTA neuron
  resting potential and resistance Chr2
  expression does not affect basic physiology.
• Flashes of blue light cause inward currents
  leading to action-potentials.
• VTA neurons can reliably follow light flashes
  only up to 10 Hz.
• In vivo
• Extracellular recordings reveal light-evoked
  typical VTA neuron spike waveform.
• VTA neurons reliably follow low frequency light
  flashes in vivo.

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Now we can have remote control of a
genetically-identified subpopulation of neurons
(VTA DA neurons) with fine temporal resolution (1
to 50 Hz). Will this bring us closer to the
holy grail of neuroscience linking specific
activity (tonic vs. phasic) of specific neurons
(VTA DA neurons) to specific behaviors?
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Step 3 testing the causal link between VTA DA
neurons to behavior (1)
Conditioned place preference (CPP) Day 1
pre-test to determine default preference (15
min). Day 2 optogenetic conditioning in one
chamber (30 min, 25 flashes at 50 Hz every
min). Day 3 optogenetic conditioning in opposite
chamber (30 min, 25 flashes at 1 Hz every
min). Day 4 post-test to determine conditioned
preference (15 min).
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Step 3 testing the causal link between VTA DA
neurons to behavior (2)
- Optogenetic conditioning with 50 Hz but not 1Hz
produces place preference.

• Control 1 mice receiving no light flashes were
  not conditioned (its not the virus alone).
• Control 2 non-transgenic littermates were not
  conditioned by flashes of light (its not the
  light alone).
• Control 3 the experimental procedure did not
  affect anxiety or locomotion.

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Is this effect due to 1. A positive response to
the 50 Hz stimulation OR 2. A negative response
to the 1 Hz stimulation ???
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Step 3 testing the causal link between VTA DA
neurons to behavior (3)
To differentiate between a positive response to
50 Hz and a negative response to 1 Hz, each mouse
was conditioned only with one of the stimulation
patterns.

• Preference to 50Hz.
• No aversion to 1 Hz.
• A trend to prefer 1 Hz.

VTA DA neuron activation at 50 Hz, but not at 1
Hz conditions place preference due to a positive
response.
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Is this effect actually due to more dopamine
release???
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Step 3 testing the causal link between VTA DA
neurons to behavior (4)

• Dopamine concentration can be measured in vivo
  using fast-scan cyclic voltammetry (oxidation
  with a carbon electrode).
• The phasic and tonic stimuli have the same
  illumination duration and number of light flashes
  (25 x 15 ms light flashes at 1 Hz or 50 Hz).
• - The tonic (50-Hz) light stimulus elicited
  larger DA transients.

Phasic activation causes more DA release than
tonic activation
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Summary and conclusions

• Identity dopamine, but not other
  neurotransmitters, is sufficient to cause place
  preference (although VTA DA neurons can also
  release glutamate).
• Activation pattern phasic activity, but not
  tonic activity, of dopamine neurons is sufficient
  to cause place preference.
• Behavioral effect phasic firing of VTA DA
  neurons causes a positive response, but not a
  negative one.
• These conclusions could not have been reached
  without optogenetics

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Question?