Neuroinflammatory Mechanisms of Depression and the Collapse of the Monoamine Hypothesis

For over four decades, the monoamine hypothesis — and its serotonin-centric variant in particular — served as the dominant explanatory framework for major depressive disorder. The 2022 umbrella review by Moncrieff et al., synthesizing evidence from six major research domains, found no consistent support for the claim that serotonin activity or concentrations are responsible for depression. This paper traces the historical trajectory of the monoamine model from the serendipitous discovery of iproniazid (1952) through the formalization by Schildkraut (1965) and Coppen (1967), the STAR*D trial revealing ~28% first-line remission rates, and the formal repudiation by Moncrieff and colleagues.

The paper then presents the neuroinflammatory framework as a mechanistically grounded alternative. Meta-analytic evidence demonstrates consistent elevations of pro-inflammatory markers (IL-6, TNF-α, CRP, IL-1β) in approximately 25–35% of patients with MDD. The kynurenine pathway is identified as a mechanistic bridge: inflammatory activation of indoleamine 2,3-dioxygenase diverts tryptophan from serotonin synthesis toward neurotoxic quinolinic acid, reframing serotonin depletion as a downstream consequence of immune activation rather than a primary deficiency.

Additional evidence is integrated from gut-brain-immune axis research (microbiome composition, intestinal permeability, vagal signaling) and metabolic psychiatry (the depression–obesity–diabetes triangle, insulin resistance, shared inflammatory pathways). Inflammatory subtyping using C-reactive protein as a stratification biomarker is discussed as a foundation for precision psychiatric intervention: patients with CRP > 5 mg/L show markedly better response to anti-inflammatory agents, while those with low CRP respond preferentially to SSRIs.

The review evaluates emerging therapeutic approaches including celecoxib, minocycline, infliximab (in high-CRP subgroups), ketamine, exercise, and dietary interventions, with effect sizes ranging from 0.28 to 1.00 (Cohen's d). The synthesis argues that depression is not a unitary disorder of neurotransmitter deficiency but a heterogeneous syndrome in which immune dysregulation constitutes a major, and potentially treatable, pathophysiological dimension.