Normative values of substance P and neurokinin A in neonates

Background: Substance P (SP) and neurokinin A (NKA) are neuropeptides that have been researched as pain markers in adults, as they are involved in transmission and modulation of pain signals. There is a potential role for them as neurochemical markers of pain in neonates, but this has never previously been investigated.

Aim: To establish normative values of SP and NKA in neonates.

Methods: Longitudinal once-daily morning blood samples were collected over two weeks from 142 neonates, gestation 23-40 weeks. Peptides were extracted, and then quantified using an in-house radioimmunoassay. Infants with presumed painful conditions were excluded.

Results: SP concentrations ranged from <0.98 to 11.2 pmol/L (median 1.7 pmol/L) and NKA concentrations from <1.95 to 74.6 pmol/L (median 6.0 pmol/L). Gestation and birth weight had no significant correlation with peptide concentrations. Postnatally, there was a gradual rise in median SP during the first three days, which decreased again by day 14. Median NKA showed a similar rise, but was not statistically significant. This postnatal rise and fall were more apparent in preterm infants <= 32 weeks gestation. Conclusions: This is the first description of normative values of SP and NKA in neonates. SP and NKA show changes with postnatal age, which are more marked in preterm infants.