ACTH (Adrenocorticotropic Hormone)

POMC Processing and Related Peptides

ACTH is one of several bioactive peptides produced from the POMC precursor. The tissue-specific processing of POMC determines which peptides are produced: in the anterior pituitary, PC1 cleaves POMC to produce ACTH and beta-lipotropin (β-LPH). In the intermediate lobe (in species with this structure) and hypothalamic neurons, PC2 further cleaves ACTH to produce alpha-MSH (ACTH 1-13 with acetylation and amidation) and corticotropin-like intermediate lobe peptide (CLIP, ACTH 18-39). The complete POMC processing pathway and melanocortin receptor pharmacology are detailed in the MSH-ACTH synergy article.

HPA Axis and Stress Response

The HPA axis is the central neuroendocrine stress response system. CRH neurons in the PVN integrate signals from limbic structures (amygdala, hippocampus, prefrontal cortex), brainstem catecholaminergic nuclei, and circulating inflammatory mediators. ACTH release is stimulated synergistically by CRH (via CRH-R1 on corticotropes) and AVP (via V1b/V3 receptors), with the relative contribution of each varying by stress type and chronicity.

"The HPA axis response to stress involves a coordinated cascade: CRH and AVP from the hypothalamus stimulate ACTH secretion within seconds, ACTH reaches the adrenal cortex within minutes to activate StAR-dependent cholesterol transport, and cortisol rises within 15-30 minutes to begin terminating the stress response through negative feedback at multiple levels." (Herman et al., 2016)

Chronic stress produces sustained HPA axis activation with characteristic changes: elevated basal cortisol, flattened circadian rhythm, resistance to dexamethasone suppression, and eventually adrenal hypertrophy. These alterations are implicated in the pathophysiology of major depressive disorder, post-traumatic stress disorder, and metabolic syndrome.

Cushing's Disease and Ectopic ACTH

Cushing's syndrome results from chronic glucocorticoid excess, and ACTH-dependent forms account for approximately 80% of cases. Cushing's disease (pituitary corticotrope adenoma) and ectopic ACTH syndrome (ACTH secretion by non-pituitary tumors, most commonly small cell lung carcinoma or bronchial carcinoid) must be differentiated for appropriate treatment.

"Inferior petrosal sinus sampling with CRH stimulation remains the gold standard for differentiating pituitary from ectopic ACTH secretion, with a central-to-peripheral ACTH ratio ≥3 after CRH administration having sensitivity >95% for Cushing's disease." (Nieman et al., 2008)

Ectopic ACTH syndrome is characterized by very high ACTH levels, severe hypokalemia (from cortisol-mediated mineralocorticoid receptor activation), and absence of suppression with high-dose dexamethasone. The hyperpigmentation that accompanies ectopic ACTH syndrome reflects ACTH activation of MC1R on melanocytes — the same receptor that alpha-MSH targets for pigmentation.

Addison's Disease (Primary Adrenal Insufficiency)

Thomas Addison first described primary adrenal insufficiency in 1855. The loss of adrenal cortisol production removes negative feedback on the HPA axis, resulting in markedly elevated ACTH levels. The clinical hallmark of Addison's disease — generalized hyperpigmentation, especially of skin creases, buccal mucosa, and scars — results from ACTH and other POMC-derived peptides (alpha-MSH, beta-MSH) activating MC1R on melanocytes.

"The hyperpigmentation of Addison's disease results from elevated POMC-derived peptides, primarily ACTH and alpha-MSH, acting on melanocortin 1 receptors in the skin. ACTH itself has melanotropic activity through its N-terminal sequence, which is identical to alpha-MSH." (Charmandari et al., 2014)

Acthar Gel (Repository Corticotropin Injection)

Acthar Gel is a purified preparation of adrenocorticotropic hormone in a gelatin matrix for prolonged release. It is FDA-approved for multiple indications including infantile spasms (West syndrome), multiple sclerosis exacerbations, nephrotic syndrome, and rheumatic disorders. While ACTH's adrenal effects (cortisol stimulation) contribute to its anti-inflammatory properties, there is evidence that Acthar Gel exerts effects beyond simple cortisol stimulation.

"Repository corticotropin injection (Acthar Gel) demonstrates anti-inflammatory and immunomodulatory effects that are partially independent of adrenal steroidogenesis. ACTH-mediated activation of MC3R and MC5R on immune cells, including macrophages and lymphocytes, contributes to anti-inflammatory signaling through NF-κB suppression and cytokine modulation." (Montero-Melendez et al., 2015)

In infantile spasms, Acthar Gel (or synthetic ACTH) is considered first-line treatment based on evidence of superior efficacy compared to other anticonvulsants. The mechanism may involve both cortisol-mediated effects and direct melanocortin receptor activation in the brain. (Go et al., 2012)

ACTH Fragments and Cognitive Effects

The ACTH molecule contains regions with distinct biological activities. While ACTH 1-24 is required for adrenal stimulation, shorter fragments including ACTH 4-10 and ACTH 4-9 retain cognitive and behavioral effects without cortisol-stimulating activity. This dissociation of cognitive from endocrine effects was first demonstrated by David de Wied in the 1960s and has led to the development of neuropeptide analogs for cognitive enhancement.

"The behavioral effects of ACTH are mediated by the sequence ACTH 4-10 (Met-Glu-His-Phe-Arg-Trp-Gly), which contains the melanocortin core His-Phe-Arg-Trp required for MC3R and MC4R interaction but lacks the N-terminal residues necessary for MC2R activation and adrenal stimulation." (de Wied & Jolles, 1982)

ACTH Stimulation Test (Cosyntropin Test)

The cosyntropin (ACTH 1-24) stimulation test is the gold standard for diagnosing primary adrenal insufficiency and is widely used to evaluate secondary adrenal insufficiency. The standard protocol involves administering 250 μg cosyntropin intravenously or intramuscularly and measuring serum cortisol at baseline, 30 minutes, and 60 minutes. A peak cortisol ≥18 μg/dL (500 nmol/L) is considered a normal response, though newer assay-specific cutoffs are being established.

"The 250 μg cosyntropin stimulation test has sensitivity of 97% and specificity of 95% for primary adrenal insufficiency. The low-dose 1 μg cosyntropin test may have greater sensitivity for detecting partial secondary adrenal insufficiency, though this remains debated." (Bornstein et al., 2016)

The low-dose (1 μg) cosyntropin test was developed to more physiologically assess the adrenal reserve by using a dose that produces ACTH levels closer to the physiological stress range. This test may detect subtle HPA axis suppression from exogenous glucocorticoids that the standard 250 μg test would miss.