Neuroinflammation and Psychiatric Illness

Neuroinflammation refers to immune and inflammatory activity within the brain, including the responses of microglia and the effects of circulating inflammatory signalling molecules called cytokines. A growing body of research links elevated inflammation to depression and other psychiatric conditions, building on the observation that immune activation can produce mood, motivation, and cognitive changes resembling depressive symptoms.

Definition

In the psychiatric context, neuroinflammation denotes activation of the brain's immune machinery (notably microglia) and the influence of peripheral and central inflammatory mediators such as cytokines on neural function and behaviour.

Scope

This topic introduces the immune and inflammatory processes studied in psychiatry — cytokines, microglia, and the brain-immune communication routes — and the evidence connecting inflammation to mood and related disorders. It is reference material describing mechanisms and associations and does not recommend anti-inflammatory or any other treatment.

Core questions

• How can immune activation outside the brain affect mood and behaviour?
• What roles do cytokines and microglia play in proposed inflammatory models of depression?
• Is inflammation a cause, consequence, or marker of psychiatric illness?

Key concepts

• Cytokines and pro-inflammatory signalling
• Microglia
• Sickness behaviour
• Peripheral-to-central immune communication
• Inflammation and the HPA axis
• Stress, immunity, and resilience

Key theories

Cytokine (inflammatory) hypothesis of depression

The proposal that pro-inflammatory cytokines contribute to the pathophysiology of depression by altering neurotransmitter metabolism, neuroendocrine function, and neural circuits, supported by observations that immune activation can induce depressive-like 'sickness behaviour'.

Sickness behaviour as a model of depression

The framework that the coordinated behavioural response to infection — withdrawal, fatigue, anhedonia, and altered appetite, driven by cytokines signalling to the brain — overlaps with depressive symptoms and offers a mechanistic bridge between immune activation and mood disorder.

Mechanisms

Inflammatory signals can reach and influence the brain through several routes, including humoral signalling across circumventricular regions, active transport of cytokines, and neural (vagal) pathways. Within the brain, microglia and cytokine signalling can alter neurotransmitter metabolism — for example shifting tryptophan toward the kynurenine pathway and affecting serotonin and glutamate — and interact with the HPA stress axis and neural circuits that govern mood and motivation. Experimental and clinical immune activation can induce 'sickness behaviour' resembling depressive symptoms, and stress itself can mobilise immune responses, linking psychological stress, inflammation, and vulnerability or resilience (Dantzer et al., 2008; Menard et al., 2017).

Clinical relevance

Inflammation is studied as a contributor to, and potential marker of, depression and related disorders, and it informs research into immune-related subtypes of illness. This entry summarises mechanisms and associations for reference and education; it is not a basis for using anti-inflammatory agents or any other intervention in an individual.

Epidemiology

Elevated levels of inflammatory markers have been reported in subgroups of patients with depression and other psychiatric disorders, and inflammation appears to characterise some but not all affected individuals, consistent with biological heterogeneity within diagnostic categories (Miller & Raison, 2016).

History

Interest in immune mechanisms in psychiatry grew from mid-to-late twentieth-century observations that infections and immune-activating treatments could provoke depressive symptoms, and from animal work on 'sickness behaviour'. From the 1990s and 2000s onward, the cytokine hypothesis of depression formalised these ideas, and the field expanded to encompass microglia, stress-immune interactions, and the search for inflammation-related subtypes of illness.

Debates

Does inflammation cause psychiatric illness or merely accompany it?

Associations between inflammatory markers and depression are well documented, but causality, direction, and which patients are affected remain debated; inflammation appears relevant to a subset of cases rather than to all psychiatric illness.

Key figures

• Andrew Miller
• Robert Dantzer
• Charles Raison
• Scott Russo

Seminal works

• dantzer-2008
• miller-2009
• miller-raison-2016

Frequently asked questions

What is 'sickness behaviour' and how does it relate to depression?

Sickness behaviour is the cluster of fatigue, withdrawal, reduced appetite, and loss of interest that accompanies infection, driven by cytokine signalling to the brain; its overlap with depressive symptoms is a key reason inflammation is studied in mood disorders.

Does everyone with depression have raised inflammation?

No. Elevated inflammatory markers are seen in some but not all people with depression, which fits the view that depression is biologically heterogeneous and that inflammation is relevant to a subgroup of patients.