13. Evidence. When is the Nac responding to DA and when is it getting there

Question 1

how do you measure DA activity in the NAc (unsuccessful)

Answer 1

• electroencephalography = NAc too deep to record from
• fMRI
• electrophysiology

Question 2

fMRI

Answer 2

= rat NAc is small and there is limited spatial reolution in fMRI

• need rats to be aneathestised which is issuous as need rat to be awake to study reward
• needs to be time sensitive too as its a quick response

Question 3

electrophysiology

Answer 3

= allows recording from populations of brain cells in deep structures

• good spatial and temporal resolutions
• can access both synaptic activity (input) and AP’s (output) of neurons

BUT connectivity of NAc makes it hard due to differing projections in and out
- hippocampal and DA VTA inputs make synaptic connections with the same NAc neuron (McGrinly and Grace, 2009)

• only VTA input to NAc is dopaminergic, other projections rely on glutamate
• how much dopamine is released by the VTW?? = microdialysis

Question 4

microdialysis

Answer 4

• enables sampling and collecting of small-molecular-weight substances from a location at a given time
• provides direct quantification of neurotransmitter levels in behaving animals (through chronic implementation of dialysis probe) from the interstitial space (circulating cerebrospinal fluid)
• microdialysis probe is inserted into target area through which artificial cerebrospinal fluid is constantly perfused (semi permeable to DA and other small molecules) - purfusate
• DA on outside of membrane defuses through semi permeable membrane into the probe along a concentration gradient
• outflow tubing contents can be collected giving identification of the molecules present in the location recorded from (dialysate)
• temporally poor by biochemically very accurate

Question 5

Hernandez and Hoebel (1988) - cocaine

Answer 5

• early evidence showed that local infusions of cocaine increase concentrations of DA in NAc
• also showed that the DA metabolite (formed in metabolism - DOPAC) decreased in concentration because less DA is being metabolised (dopamine is active for longer in cleft and usually gets metabolised in presynaptic neuron)i
• DA is not being broken down because its binding to the post synaptic terminal instead (due to increased DA in cleft)
• DA transporters are blocked so it cant get back into presynaptic terminal so stays active for longer in synapse (more DA)
• very simple methodology

Question 6

Ranaldi et al (1999) - method

Answer 6

• laboratory animals learn to press levers when intravenous injections of cocaine or amph depend on it
• within a few weeks of regular testing the behaviour becomes regulated by the systems drug level
• loading phase
• maintenance phase
• two phases must mean that there is a neural consequence of drug administration (DA level)
• rats also had a microdialysis probe implanted in their NAc, and intravenous catheters for later injections
• DA levels of NAc were measured during MAINTENANCE, EXTINCTION and REINSTATEMENT of amph

Question 7

loading phase

Answer 7

rat hits lever to give higher dose of drug until at a preferred level of high

Question 8

maintenance phase

Answer 8

rat continues to press lever when preferred level of high drops to maintain at same level

Question 9

Ranaldi et al (1999) - results

Answer 9

TIME PHASE
- results highlighted a loading phase with rapid increase in dopamine levels, a clear self maintenance effect where the rat administered amph when DA dropped (not time constricted)
FREQUENCY PHASE
- results highlighted increase in frequency of leveer press when amph was decoupled from rat
- when re-coupled, the rat was given 1 shot to show that it could re-administer
- normal behaviour resumed
behavioural observations aldo made

Question 10

Ranaldi et al (1999) - behavioural observations

Answer 10

in final 30 minutes of extinction all rats had ceases stereotyped behaviour (sniffing, locomotion) and were lying down (thought to represent withdrawal)
- one shot was enough to resume stereotyped drug related behaviour

Question 11

Ranaldi et al (1999) - conclusions

Answer 11

• rate of amph administration is under the control of NAc dopamine concentrations (not timed response habit)
• amph has little rewarding value when DA levels are already elevated (showing saturation in the reward system)
• its the loading thats rewarding = big leap

Question 12

Yi et al, (1998)

Answer 12

1
- explored the effects of heroin self administration on DA levels in the NAc
- used different doses of heroin
- low (.06mg/kg/inj)
- medium (.1)
- high (.2)
- dose dependant increases of DA in the NAc during heroin self administration were witnessed
- frequency of self administration as a function of dose
2
- heroin co-administered with nolaxone (opioid receptor agonist)
- reduces the amount of released in the NAc (effects of opioids blocked)
- when the opioid receotirs were blocked you dont see an increase in DA in NAc

Question 13

Lecca et al (2007)

Answer 13

• master rats (self administered heroin) and yoked rats (administration done for them)
• measuring from core and shell of NAc
• when self administering, the rats showed higher levels of DA in SHELL compared to core
• yoked rats showed no difference
• after 4 weeks of injections, yoked rats show a comparable increase in DA levels within the NAc CORE
• core = premotor control
• shell = hedonic output
• master rat is moving towards rewarding stimuli as its self administering (must have some awareness) so hedonic output is reinforced

Question 14

Hernandez and Hoebel (1988) - natural reward

Answer 14

1
- measured DA levels in the NAc following natural rewards, electrical stimulation and drugs
- extracellular DA concentration was increased when food was made available in hungry rats (rapid increase), persisting for a while to create a stronger association so the rat learns there is food there
- DA levels remained elevated for 40-100 minutes after feeding stops thought to facilitate the effects on memory encoding or emotional states coupled with the food reinforcement
- need time for DA to mediate whether response is at the right level of the NAc. DA gives contextual inout to facilitate memory encoding (how and what - stimuli)
2
- also stimulated the lateral hypothalamus (LH) - DA concentration increased in the NAc whether food was there or not
- if food was available the rat would seek it out and eat it
- descending neurons from LH project to midbrain DA neurons
- VTA projects to NAc (a subset of neurons increase eating when stimulate, more D?A when food is there compared to not there)