Introduction to biological explanations of OCD

Biological explanations suggest that obsessive-compulsive disorder (OCD) arises from factors within the body's biology, such as genes and brain structures. These explanations focus on how genetic inheritance and neural processes might disrupt normal functioning, leading to these symptoms. Understanding this helps explain why OCD affects around 1-2 per cent of the general population and why it often runs in families.

This approach contrasts with psychological explanations, but the evidence suggests that OCD has a strong biological basis.

OCD as a polygenic condition

OCD is considered a polygenic condition, meaning it is influenced by multiple genes rather than a single one. Evidence for this comes from studies comparing OCD rates in families and twins. These studies use concordance rates, and a higher concordance in genetically similar pairs suggests genetic involvement.

Key research: Carey and Gottesman (1981) twin study

• Aim - This study investigated obsessive symptoms in twins to assess genetic influences.
• Method - The researchers examined identical twins and fraternal twins for signs of obsessive symptoms and features.
• Results - Identical twins showed a concordance rate of 87 per cent for obsessive symptoms and features, compared to 47 per cent in fraternal twins. This is much higher than the general population rate of around 1-2 per cent for OCD.
• Conclusions - The difference in concordance rates indicates that genetic factors play a moderate role in OCD, as identical twins are more similar despite sharing similar environments.

Family and twin studies further suggest that genetics contribute to OCD development.

Role of specific genes in OCD

Research has identified certain genes linked to OCD. Two key genes are the SERT gene and the COMT gene, which influence serotonin and dopamine levels, respectively.

The SERT gene and serotonin regulation

The SERT gene, also known as the serotonin transporter gene, affects the transport of serotonin.

How the SERT gene affects OCD:

• In individuals with OCD, the SERT gene is often mutated, leading to more transporter proteins on the neuron's membrane.
• This mutation increases the reuptake of serotonin, where serotonin is pulled back into the neuron after being released.
• As a result, serotonin levels in the synapse decrease, which is linked to OCD symptoms.

The COMT gene and dopamine regulation

The COMT gene (catechol O-methyltransferase gene) regulates dopamine.

How the COMT gene affects OCD:

• A mutated version of the COMT gene is more common in people with OCD, especially males.
• This mutation reduces COMT activity, which normally clears dopamine from synapses.
• Consequently, dopamine levels in the synapses increase.
• High dopamine is thought to affect concentration, potentially causing the intense focus on obsessive thoughts and repetitive behaviours seen in OCD.

Neural circuits involved in OCD

Beyond genes, biological explanations point to specific brain circuits that regulate primitive aspects of our behaviour.

The OCD circuit involves relaying information between three key regions:

• Orbital frontal cortex - Located at the front of the brain.
• Caudate nucleus - Part of the basal ganglia.
• Thalamus - A deeper brain structure.

How the circuit functions in OCD

When this circuit is activated, these impulses are brought to attention and cause an individual to perform a particular behaviour that appropriately addresses the impulse.