JD - Reward and Pleasure Flashcards
What are ‘liking’ and ‘hedonic’ circuits in the brain?
Liking’ circuits, also called hedonic circuits, generate the pleasure of eating food.
- Hedonic feeding is driven by the pleasure of consuming food, not metabolic need
What are 3 features of eating disorders
- Eating disorders can include obesity, bulimia and anorexia
- The number of severely obese and obese children has increased and almost doubled in the last ~10 years
- Obesity is associated with a reduced life expectancy and is a risk factor for a range of diseases
What neural populations are present in the hypothalamus related to appetite? (2)
- Orexigenic (Increase Appetite)
- Anorexigenic (Decrease Appetite)
How was the reward circuit initially discovered?
The reward circuit was discovered through brain stimulation reward studies by Olds and Milner in 1954, where electrodes in certain brain areas of rodents induced self-stimulation behavior
What is the reward circuit?
The reward circuit is the neural network that receives and evaluates the rewarding properties of stimuli.
- It consists of multiple interacting neural circuits.
Self-stimulation is observed from electrodes located in which brain areas? (4)
- Nucleus Accumbens (NAcc)
- Lateral hypothalamus (LH)
- Ventral Tegmental Area (VTA)
- Cortical structures
What are some of the key structures of the reward circuit? (4)
- Medial forebrain bundle
- Reward circuit includes axons that project from the VTA (ventral tegmental area) to the nucleus accumbens (Mesolimbic Dopamine Pathway)
- VTA connects to the amygdala, septum and regions of the cortex
- Hypothalamus receives inputs from structures within the reward circuitry
How can hedonic reactions to food be measured?
Affective orofacial liking reactions can be quantified to provide a readout for how much the subject likes a substance; food
Hedonic Reactions (sweet)
- (Positive facial liking: relaxed facial expression and rhythmic tongue protrusions)
Aversive Reactions (bitter)
- (Facial disliking expressions: gapes, turning away, head shakes)
Endogenous Opoids vs Endogenous endocannabinoids (6)
Endogenous opioids are neuropeptides and include: enkephalins, dynorphins and endorphins
- opioid receptor subtypes include: mu, kappa and delta
- Receptors are G-protein coupled receptors
- Agonists include morphine; Antagonist Naloxone
Endogenous endocannabinoids are lipid molecules: anandamide and 2-arachidonoylglycerol (2-AG)
- 2 cannabinoid receptor subtypes: CB1 and CB2.
- Receptors are G-protein coupled receptors
- CB1 subtype is predominantly expressed in the central nervous system
What role do opioids play in hedonic pathways?
Morphine enhances ‘liking’ responses of rats for palatable foods
Naloxone decreases food intake in rats, especially when sucrose is used
What did the injection of opioid receptor agonist (DAGMO) and measuring facial ‘liking’ reaction identify? (4)
- Role for mu opioid receptors
- Precise map of the nucleus accumbens (NAcc)
- Hedonic hotspot (10% of Nacc)
- A larger region for food intake (‘wanting’)
How was the functional connectivity between different hotspots mapped? (2)
An approach called a Fos plume was used.
- One region of the animal’s brain is injected with the DAMGO and then the brain is removed and immunohistochemical staining was used to quantify the expression of a protein called c-Fos in regions of the brain outside of the injection site.
- c-Fos is a protein that is expressed when neurons are activated, and so increased expression of this protein provides a readout for when neurons are being activated by the drug that is being injected.
Provided evidence that hotspots are functionally connected
What was another experiment to determine functional connectivity of hotspots?
Blocking opioid signaling in one hotspot while simultaneously stimulating another hotspot causes the suppression of enhanced liking responses
- Recruitment of hotspots is an important mechanism for causing increased ‘liking’ responses disruption of one hotspot can disrupt liking reactions and the function of the circuit
How do endocannabinoids contribute to hedonic pathways?
Endocannabinoids like anandamide increase liking reactions when injected into the nucleus accumbens
- There is an endocannabinoid hotspot in the NAcc that overlaps with the opioid hotspot
CB1 receptors and mu receptors have been identified as co-localizing in neurones of the nucleus accumbens
- Function together to co-ordinate the release of neurotransmitters
What is the role of dopamine in reward?
Dopamine was found to be required for the motivational ‘wanting’ aspects of food reward, not the hedonic ‘liking’ component
