Emotions, Mood and Affect

Question 1

Outline the Papez circuit and its role in emotional processing.
(6 marks)

Answer 1

• Cingulate gyrus projects to the hippocampus via the cingulum bundle.
• Hippocampus projects to the mammillary bodies of the hypothalamus via the fornix.
• Hypothalamus projects to the anterior thalamic nuclei via the mammillothalamic tract.
• Mammillothalamic tract projects back to the cingulate gyrus via the cingulum bundle.
• The hypothalamus governs the behavioural expression of emotion.
• The cingulate cortex regulates emotional experience.

Question 2

Outline one of the neurochemical theories of depression.

4 marks

Answer 2

Serotonin theory-

• A reduction in the serotonin transporter 5-HTT in the brain
• Leads to a reduction of serotonin being transported across the synapse
• Such a reduction has been shown to be associated with depressive symptoms
• Some studies have shown up to a 10% reduction of 5-HTT in those with Major Depressive Disorder

Dopamine receptor theory

• The VTA is the reward and pleasure centre of the brain
• It is dense with dopamine receptors
• If these dopamine receptors are inhibited depressive symptoms are increased
• Mouse studies using optogenetics have demonstrated these abnormalities

Question 3

Outline one of the epigenetic theories of depression

Answer 3

Rac1 Theory

• Chronic stress leads to the downregulation of the Rac1 gene
• The lack of Rac1 leads to an increase in immature dendrites on the medium spiny neurons of the Nucleus Accumbens
• The dendrites not reaching maturity reduces the number of available synapses and therefore excitability
• An increased amount of dopamine is therefore required to generate feelings of euphoria. This can generate anhedonia, a depressive symptom

∆FosB Theory

• ∆FosB is a splice variant of the FosB gene
• An increase in ∆FosB leads to an increase in AMPA receptors with 2 GluA1 subunits within medium spiny neurons in the Nucleus Accumbens
• The increase in AMPA receptors leads to a reduction in excitability in the medium spiny neurons.
• An increased amount of dopamine is therefore required to generate feelings of euphoria. This can generate anhedonia, a depressive symptom

BDNF Theory

• Stress causes the upregulation of opioid receptors on the GABA neurons of the VTA
• The upregulation of GABA neurons leads to a decrease in the inhibition of dopaminergic neurons.
• This in turn leads to the upregulation of brain derived neurotrophic factor (BDNF) which binds to the TrkB receptors on the medium spiny neurons
• Excess binding of BDNF to TrkB receptors leads to apoptosis of the medium spiny neurons
• Due to a decrease in medium spiny neurons, an increased amount of dopamine is therefore required to generate feelings of euphoria. This can generate anhedonia, a depressive symptom