What brain regions and associated neurotransmitter systems are implicated in mental health disorders?

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Brain Regions and Neurotransmitter Systems in Mental Health Disorders

Mental health disorders involve dysfunction in cortico-striato-thalamo-cortical (CSTC) circuits and frontolimbic networks, with primary neurotransmitter systems including serotonin, dopamine, and glutamate, though the specific brain regions and neurochemical alterations vary substantially by disorder. 1

Key Neural Circuits Across Mental Health Disorders

Cortico-Striato-Thalamo-Cortical (CSTC) Circuits

The CSTC circuits represent the fundamental architecture underlying multiple psychiatric conditions, particularly OCD but also relevant to other disorders 1:

  • Sensorimotor CSTC circuit: Involves supplementary motor area (SMA), pre-SMA, and posterior putamen; mediates stimulus-response-based habitual behaviors
  • Dorsal cognitive CSTC circuit: Includes dorsolateral prefrontal cortex (dlPFC), dorsomedial prefrontal cortex (dmPFC), and dorsal caudate; coordinates cognitive control, working memory, planning, and emotion regulation
  • Ventral cognitive CSTC circuit: Encompasses ventrolateral prefrontal cortex (vlPFC), inferior frontal gyrus, and ventral caudate; manages response inhibition
  • Ventral motivational CSTC circuit: Comprises ventromedial prefrontal cortex (vmPFC), orbitofrontal cortex (OFC), and nucleus accumbens; governs stimulus-outcome-based motivational behavior

Frontolimbic Circuit

This circuit involves the amygdala, hippocampus, and prefrontal regions, playing a critical role in fear extinction and emotional processing 1.

Disorder-Specific Brain Region Alterations

Obsessive-Compulsive Disorder

Structural changes 1:

  • Decreased volumes: Hippocampus, dorsomedial prefrontal cortex, insular opercular region (shared with other psychiatric disorders)
  • Increased volumes: Basal ganglia (putamen, pallidum), particularly pronounced in older patients and those with child-onset disease; cerebellum
  • Thalamus: Enlarged in unmedicated children, suggesting altered brain maturation
  • White matter: Abnormalities in anterior midline tracts, cingulum bundle, corpus callosum, and superior longitudinal fasciculus

Disease stage-specific patterns 1:

  • Early OCD: Alterations in dorsal cognitive, ventral cognitive, and ventral reward CSTC circuits plus frontolimbic circuit → anxiety, uncertainty, goal-directed behaviors
  • Later OCD: Alterations in sensorimotor, dorsal cognitive, and ventral cognitive CSTC circuits → habitual compulsive behaviors

Attention-Deficit/Hyperactivity Disorder

Prominent abnormalities in inferior prefrontal cortex with connections to striatal, cerebellar, and parietal regions 2.

Conduct Disorder

Alterations in the paralimbic system: ventromedial, lateral orbitofrontal, and superior temporal cortices with underlying limbic regions regulating motivation and emotion control 2.

Major Depressive Disorder

  • Structural: Smaller hippocampus (especially with comorbid depression in OCD patients) 1
  • Functional: Alterations in ventral orbital and limbic activity, particularly left hemisphere; medial prefrontal cortex and related limbic structures 2, 3
  • Network involvement: Medial prefrontal network, amygdala, hippocampus, ventromedial basal ganglia 3

Schizophrenia

The disorder converges on disturbances in specific neurobiological systems, primarily involving dopamine and glutamate neurotransmitter systems 4.

Neurotransmitter Systems

Serotonin System

Primary role in OCD 1:

  • Despite selective efficacy of serotonin reuptake inhibitors (SRIs), surprisingly little evidence of primary causal serotonin deficit in OCD
  • Alterations in cerebrospinal fluid serotonin metabolites normalize after successful treatment (sparse evidence)
  • Associations with serotonergic genes (serotonin transporter) reported in some studies
  • Altered serotonin transporter receptor binding in midbrain (inconsistent findings)

Broader role: Implicated across multiple mood disorders 5.

Dopamine System

Key functions and evidence 1:

  • Central role in stereotypic behavior and grooming in animal models
  • Critical for reward processing, which may be altered in OCD
  • Strong evidence for central role in Tourette syndrome (an OCRD)
  • Associations with catecholaminergic genes (COMT) and OCD
  • Decreased striatal dopamine D2 receptors in OCD
  • Therapeutic role for dopamine D2 receptor blockers in OCD augmentation

Broader implications: Key neurotransmitter in schizophrenia pathophysiology 4; involved in depression and mania 5.

Glutamate System

Evidence in OCD 1:

  • Glutamatergic neurons from prefrontal cortex project to striatum (key CSTC component)
  • Alterations in glutamatergic metabolites in cerebrospinal fluid and magnetic resonance spectroscopy (inconsistent findings)
  • Genetic associations: SLC1A1, GRIN2B, GRID2, DLGAP1 variants associated with OCD
  • DLGAP3 (SAPAP3) deletion in mice produces defects in corticostriatal synapses and compulsive grooming

Broader role: Implicated in schizophrenia and depression 4, 5.

Other Neurotransmitter Systems

  • GABA: Implicated in depression and mania 5
  • Norepinephrine: Role in depression; galanin co-exists in noradrenergic locus coeruleus neurons and may participate in resilience mechanisms against depression 6
  • Neuropeptides: Act as auxiliary messengers, particularly when nervous system is challenged by injury, pain, or stress 6

Common vs. Disorder-Specific Alterations

Shared across disorders 1:

  • Smaller hippocampus
  • Smaller dorsomedial prefrontal cortex
  • Smaller insular opercular region
  • White matter abnormalities in frontotemporal and frontoparietal regions (depression, bipolar disorder, OCD, PTSD, social anxiety disorder)

OCD-specific 1:

  • Larger basal ganglia volume (most pronounced in older patients, likely related to disease chronicity and/or medication effects)
  • Larger thalamus in unmedicated children

Critical Clinical Considerations

Medication effects: Many structural brain changes are more pronounced in medicated patients, suggesting neuroplastic changes from disease chronicity and/or long-term medication effects 1. Medicated OCD patients show thinner cortex in multiple regions and smaller frontal surface areas, while unmedicated patients often do not differ from controls.

Comorbidity impact: Hippocampal volume reductions are more pronounced with comorbid depression, as hippocampal changes occur across many disorders 1.

Age and disease onset: Striatal volumetric changes associate with disease onset timing; pallidum enlargement mainly occurs in child-onset OCD 1. Age-related effects show preservation of putamen volume with more pronounced limbic region decreases in older patients 1.

Symptom dimension specificity: Different symptom profiles may reflect different neural mechanisms—harm-related symptoms associate with decreased bilateral anterior temporal poles, while contamination symptoms link to smaller caudate nucleus 1.

References

1
Guideline

obsessive-compulsive disorder.

Nature Reviews Disease Primers, 2019

5
Research

Neurobiological basis of depression: an update.

Metabolism: clinical and experimental, 2005

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