Kravitz - Research Question(s)
Experiment 1: Does optogenetic excitation of the direct and indirect pathways lead to reinforcement and
punishment respectively?
Experiment 2: Is the effect observed in experiment 1 due to indirect dopamine activation?
Experiment 3: Does optogenetic activation of these pathways affect other behaviours in a similar manner?
Are the effects observed in experiment 1 specific to the behavioural design used?
Kravtiz - Hypothesis
They hypothesized that activation of D1 receptor-expressing dMSNs would facilitate reinforcement while D2 receptor-expressing iMSNs would facilitate punishment.
Kravitz - Experiment 1 Methods
-Genetic targeting of channelrhodopsin (using a virus) in D1 or D2 MSNs
-Optogenetics to activate dMSNs and iMSNs with ChR2
-ICSS:
Two manipulanda (lever-like devices) were inserted on either side of a conditioning box
Capacitive touch devices were used as manipulanda (better for mice than levers)
One manipulanda led to laser activation â laser-paired trigger
Other manipulanda led to no laser activation â non-laser-paired trigger
Mice were trained for three days for 30 min. The fourth day (or more) consisted of âretrainingâ and extinction sessions
Kravitz - Experiment 2 Methods
Same design as experiment 1, yet with a dopamine antagonist given:
Each day of the experiment
Only during the extinction test
Kravitz - Experiment 3 Methods
Conditioned place preference (CPP)
Two 8 x 8 inch chambers, one with black
walls one with white walls
Laser stimulation:
2 seconds on, 8 seconds off while on one
side of box
Kravitz - Results
1 - Optogenetic self-stimulation of medium spiny neurons in the dorsomedial striatum supports positive reinforcement in D1 expressing neurons (dMSNs) and transient negative reinforcement in D2 expressing neurons (iMSNs)
2 - This effect is not dependent on dopamine release
3 - The positive and negative reinforcing effects of D1- and D2-expressing MSNs are also observed in conditioned place preference (CPP*)*
Illango - Research Question
The researchersâ main question was to investigate to what degree dopamine neurons in the substantia nigra pars compacta (SNc) contribute to affective functions.
Illango - Hypothesis
They hypothesized that like dopamine neurons located in the ventral tegmental area, SNc dopamine neurons will produce positive and negative affect if excited or inhibited, respectively.
Illango - Experiment 1 Methods
ChR2
Optogenetics
ICSS (Active lever press/photostimulation)
Operant place preference/photstimulation
Illango - Experiment 2 Methods
Smaller volumes of virus encoding ChR2 injected in SNc and VTA
Sacrifice + looked at CFos counts in the SNc and VTA, both ipsilateral and contralateral, after photostimulation of SNc or VTA
CFos: expressed in recently active cells
Illango - Experiment 3 Methods
NpHR
Operant place preference/photostimulation
Illango - Experiment 4 Methods
Same as 3 but this experiment aims to see if inhibition of neurons in the medial dorsal striatum (mDSTr) and central medial dorsal striatum also leads to aversive effects.
Illango - Results
The present study provides evidence that the excitation and inhi- bition of SNc DA neurons induce reward and aversion, respec- tively, and the extent of these effects is similar to those of the VTA DA neurons. The present study provides first evidence that inhibition of SNc DA neurons or nigrostriatal DA projections causes aversion like that of VTA DA neurons.
Matsumoto - Research Question
Do dopamine neurons respond to cues indicating value and are they inhibited in response to aversive cues?
Matsumoto - Hypothesis
They hypothesized that (only) dopamine neurons in the VTA/SNc would respond to motivational value.
Matsumoto - Methods
Histology: Recording/Locating dopamine neurons in relation to their responses to airpuff-predicting CS
Behavioural paradigm: Pavlovian approach where monkeys were classically conditioned to assess whether dopamine neurons are implicated in motivational value. This procedure involved giving monkeys a reward in the appetitive block or an air puff in the aversive block (unconditioned stimulus) with either 100%, 50% or 0% probability (conditioned stimulus).
Matsumoto - Results
Appetitive = Anticipatory licking increased as probability of reward increased
Aversive Block/Punishment = Anticipatory blinking increased as the probability of airpuff increased
Value coding: VTA & ventromedial SNc = transmit value-related information to ventral striatum (NAc)
Salience: dorsolateral SNc = Response to motivationally salient stimuli
ACS-excited = excited by reward-predicting and airpuff-predicting stimuli. (does not encode motivational value)
â˘ACS-inhibited = excited by reward-predicting stimuli and inhibited by airpuff-predicting stimuli (encodes motivational value)
AUS-inhibited neurons were consistent with Reward Prediction Error whereas AUS-excited neurons were inconsistent with RPE because the measurements weren't in opposite directions as would have been predicted
Namburi - Research Question
How is it possible that potentiation of synapses onto neurons in the BLA underlie learned associations that lead to such different behavioural responses?
Namburi - Hypothesis
BLA projections to the NAc have been implicated in reward-related behaviours, while BLA projections to the CeM have been linked to the expression of conditioned fear.
Namburi - Experiment 1 Methods + Results
Purpose = Examine the synaptic changes in NAc & CeM projectors after fear or reward conditioning.
Methods:
Retrograde tracing
Pavlovian conditioning
Ex vivo electrophysiology (tissue preparation + whole cell patch clamp recording)
Results:
â__Fear conditioning__ = AMPAR/NMDAR ratio decreased (NAc)
â__Reward conditioning__ = AMPAR/NMDAR ratio increased (NAc)
â__Fear conditioning__ = AMPAR/NMDAR ratio increased (CeM)
â__Reward conditioning__ = AMPAR/NMDAR ratio decreased (CeM)
Namburi - Experiment 2 Methods + Results
Purpose = : Determining if there is a causal relationship between populations of projection-identified BLA neurons and behavior
Methods:
Used a retrogradedly infectious rabies viral vector to express ChR2 fused to a fluorescent reporter (Venus) or control virus w/ just Venus in BLA neurons projecting to NAc or CeM.
Intracranial self-stimulation (ICSS) task
-This task has mice engage in nose-pokes in order to receive rewarding pulses of electrical brain stimulation
-However, ICSS only works with positive reinforcement (i.e. they can only measure NAc activity with this task)
To test CeM projectors, the researchers used a closed-loop real-time place avoidance assay (RTPA), where a mouse freely explored two chambers, one in which the they received photostimulation (via laser) of CeM projectors
The mouse should spend less time in the chamber where the laser is turned on
Results:
â most nose-pokes were in response to an active NAc ChR2 = NAc is appetitive and influences behavior
âmouse avoided CeM stimulation = aversive and also influences behavior
Namburi - Experiment 3 Methods + Results
Purpose = Test whether NAc or CeM projectors are necessary in mediating reward or fear conditioning
Methods:
Halorhodopsin + EYFP (fluorescent marker) to inhibit pathways during learning, genes are then double floxed (backwards) and canât be expressed.
Cre-recominase enzyme travels from axon to nucleus flips genes right way (cells now have halorhodopsin + EYFP and can now be inhibited when light is shined on the cell, as haloredopsin is photoinhibitive).
Fear + reward behavioural procedures
Results:
Photoinhibition of CeM projectors during the conditionedâunconditioned stimulus pairing impaired conditioned freezing and enhanced conditioned reward seeking
Tindell - Research Question
The researchersâ main question was to investigate whether neuronal firing in the ventral palladium (VP) codes sensory pleasure.
Tindell - Hypothesis
The Ventral Pallidum (VP) encodes positive hedonic value
Tindell - Methods
Electrode implantation for electrophysiological recording of neurons aligned to behavioural events.
Normal homeostasis testing: habituated to infusions + access to NaCl solution + injected w/ solutions to provoke sodium depletion.
Sodium depletion testing: 24 hours after injections, no NaCl available.
Sodium repletion testing: NaCl access for 3 days, rats euthanized afterward.
Taste reactivity: identify times, duration and latency of positive hedonic reactions (paw licking) and aversive reaction (face washing/gapes) + compare reaction w/ VP firing.
Histology: rats euthanized, cut and stain brains, slice examined under microscope to verify electrode placement.
Tindell - Results
Strong hedonic reaction for sucrose in homeostasis and sodium depletion conditions.
Low Aversive reaction for sucrose in homeostasis and sodium depletion conditions.
Hedonic reactions for NaCl: 3x stronger for depleted condition vs homeostasis (almost equal hedonic response to sucrose).
Aversive reactions for NaCl: disliking during baseline, disliking reduced to 25% below baseline during depletion.
Replenishment of sodium returned to aversive/disliking.
Neural firing due to NaCl had a positive correlational relationship to behavioral hedonic scores.
Strong evidence that aversive tastes can become pleasant if there is a change in the physiological state.