Ontogeny of plasma, csf and brainstem ACTH in piglets: Effects of hypoxia and anesthesia

This study was designed to assess the postnatal maturation of adrenocorticotropin (ACTH) levels in piglets under basal conditions and in response to single, acute stressors. ACTH levels were measured by radioimmunoassay in plasma, cerebro-spinal fluid (CSF) and in a dorsal medullary slice containing the nucleus tractus solitarii (dmscNTS) of young and older piglets (1.5-6 and 35-43 days old, respectively) under the following experimental conditions: (1) normoxia (both groups); (2) hypoxia, 10% O₂/N₂ for ≥ 30 min (both groups); (3) sham anesthesia, i.p. saline in normoxia (young group); (4) anesthesia, 25 mg/kg i.p. pentobarbital in normoxia (young group), and (5) anesthesia combined with hypoxia (young group). During normoxia, ACTH levels in young, as compared to older piglets, were higher in CSF (p<0.0l) and plasma (0.05 < p < 0.10) and not different in dmscNTS. Hypoxia produced no ACTH changes in CSF, increased ACTH in plasma of young (p = 0.03) and older piglets (p = 0.09), and decreased ACTH in dmscNTS of older (p = 0.01) and young piglets (p = 0.07). As compared to sham anesthesia, anesthesia did not alter any ACTH levels. Combined hypoxia and anesthesia increased ACTH levels in plasma when compared to normoxia (p<0.05), sham anesthesia (p<0.05) or anesthesia alone (p<0.05), but not when compared to hypoxia alone. We conclude that neonatal swine have high basal ACTH levels and mount significant plasma ACTH responses to a single, acute hypoxic stressor. The presence of ACTH in the region of the NTS supports its possible role as a neuromodulator in the brain.