Neurokinin A

Overview

Neurokinin A (NKA), historically referred to as Substance K, is a bioactive neuropeptide belonging to the tachykinin superfamily. NKA comprises 10 amino acids (His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH2) and shares the conserved C-terminal sequence Phe-X-Gly-Leu-Met-NH2 characteristic of all mammalian tachykinins. It was independently discovered in 1983 by multiple laboratories investigating bioactive peptides in the central nervous system.

Gene Organization and Processing

NKA is encoded alongside substance P by the preprotachykinin-A (PPT-A) gene located on chromosome 7q21 in humans. Alternative splicing yields alpha, beta, gamma, and delta mRNA isoforms. The beta isoform encodes both substance P and NKA, while the alpha isoform contains only the NKA sequence. Post-translational processing by prohormone convertases and carboxypeptidase E generates the mature amidated peptide.

Receptor Specificity

NKA demonstrates high-affinity binding to the NK2 receptor (NK2R), with a Ki of approximately 0.1-1 nM. The NK2R couples predominantly to Gq/11 proteins, activating phospholipase C-beta and mobilizing intracellular calcium. NKA exhibits 10-100 fold selectivity for NK2R over NK1R and NK3R, making it the endogenous selective agonist for this receptor subtype.

Physiological Roles

In the respiratory system, NKA causes dose-dependent bronchoconstriction through direct smooth muscle contraction and potentiation of cholinergic neurotransmission. This effect is particularly pronounced in asthmatic airways, where tachykinin levels are elevated. Within the gastrointestinal tract, NKA stimulates propulsive motility and fluid secretion via NK2R activation on myenteric neurons and smooth muscle cells.

Pathophysiological Implications

Elevated NKA levels have been documented in bronchoalveolar lavage fluid from asthmatic patients, contributing to airway hyperreactivity. In irritable bowel syndrome, altered NKA signaling correlates with motility disturbances. NK2R antagonists have shown efficacy in preclinical models of asthma and visceral pain, though clinical translation remains challenging due to species-specific pharmacological differences.

Therapeutic Prospects

Selective NK2 receptor antagonists represent promising targets for respiratory and gastrointestinal disorders. Compounds such as saredutant and SR48968 have demonstrated anxiolytic and bronchodilatory effects, though their clinical development has encountered setbacks related to efficacy and safety.

References

1. Guard, S., & Watson, S.P. (1988). Tachykinin receptors. Neurochemistry International, 12(3), 197-202.
2. Joos, G.F., & Pauwels, R.A. (2000). Pro-inflammatory effects of tachykinins in the lung. Neuroscience Letters, 305(3), 145-148.
3. Steinhoff, M.S., et al. (2004). Tachykinins and their receptors. Biochemical Pharmacology, 67(5), 815-823.