week 9-dopamine Flashcards
dopamine pathways
-Substantial Nigra (SN) Motor Control -Ventral Tegmental Area (VTA) Motivation & emotional response Reward, desire & addiction
Dopamine Synthesis
-Tryrosine an amino acid found in food (Tryrosine Hydroxylase SYNTHESIS ENZYME) -DOPA -Dopamine -Noradrenaline (norepinephrine) ( look at lecture slide)
dopamine synthesis-l-dopa
L-DOPA (Amino Acid decarboxylase SYNTHESIS ENZYME) dopDopamine β-hydroxylase SYNTHESIS & BREAKDOWN ENZYME amine) noradrenaline
Dopamine & Parkinson’s
- Caused by death of dopamine cells in the Substantia Nigra
- Disease initially characterized by motor tremor. Later symptoms include cognitive impairments and dementia. Symptoms often includes reduced “executive function”.
- There is currently NO CURE of the disease, but symptoms can be reduced through drugs and deep brain stimulation.
- Treatment of the disease can cause impulsivity, hypersexuality, gambling, addictive like behaviours (see section on Reward & DA).
Dopamine & Parkinson’s
continued
- Areas of the brain such as the basal ganglia are believed to help inhibit/control motor action.
- In the healthy brain DA release allows selective activation of the basal ganglia & initiation of motor commands.
- In Parkinson’s the lack of DA makes it hard to initiate complex motor actions & control small corrective movements. > patients shake when trying to stay still & have trouble initiating movements like standing, walking, reaching etc
Reward Prediction Error
If an unexpected reward occurs DA neurons become more active and release a burst of DA
-Originally thought every time something good happens there must be dopamine in the brain but it is more dedicate than that,
Each horizontal row bit of a neuron
Burst of dopamine firing
Reward Prediction Error 2
-If a reward is repeatedly given after a stimulus (such as a beep) then the reward will be “expected” & no DA will be released with the reward but will now be released at the time of the beep
-If you keep doing that you get the burst of activity at the beep instead of at the reward
Codes expectation
If you get money and I want to buy shoes, you get excited when you get the money, especially if unexpected
Reward prediction error 3
If a reward is “expected” & not provided DA neurons will be suppressed
Reward - Real & Virtual
Rewards can be - real (e.g. food or sex) - symbolic (e.g. money) - virtual (e.g. points in a game) DA is involved in all cases.
Drug Addiction
- A chronic relapsing disorder which consists of a compulsive pattern of drug-seeking and drug-taking behaviour
- takes place at the expense of other activities
- persists despite adverse consequences
DA Drugs - Cocaine
Reuptake of transmitter (BLOCKED)
DA Drugs - Amphetamine
Drugs = ice (most pure) and speed (less pure)
Reverses uptake transporter actively expelling DA and NA out of the neuron which also prevents DA uptake
Reuptake of transmitter (REVERSED)
Addictive drugs hijack reward response
- Addictive dopamine drugs are ALWAYS coded by the brain as “Better than expected
- compared to normal condition of: When reward is expected NO additional dopamine is released
Addiction & Dopamine
- Reward is coded by the brain if there is increase DA in the Nucleus accumbens (Nac) and other forebrain structures like the amygdala, hippocampus, prefrontal cortex (Pfc).
- Addictive drugs produce extra dopamine release in the Nac (sometimes via different pathways)
- The more DA released the greater the high
- The faster the DA release the more addictive it will be
Addiction and free-will
-1st problem
Drugs initiate “wanting” and in addicted people, this leads to drug urges or cravings.
-2st problem
Cognitive (“top down”) control is reduced by impaired function of the prefrontal cortex (PFC) caused by excessive dopamine.
-imaging studies show PFC abnormalities
-Final Result – addictive behaviour
Failures of “top down” control would contribute to loss of control over the urge to take drugs.
