Prolactin, tryptophan, and branch chain amino acids, and relationships with psychomotor performance during heat stress

Changes in Prolactin (PRL) and the ratio of Tryptophan (TRYP) to Branch Chain Amino Acids (BCAA) during stress have been identified as indicators of central fatigue. Research suggests that an increase in rectal temperature (Tre) impairs psychomotor function, due to increased perceived discomfort and fatigue; however the relationship to the changes in purported neuroendocrine markers of central fatigue is unclear, particularly during uncompensable heat stress. PURPOSE: The purpose was to examine the relationship between psychomotor performance and neuroendocrine markers of central fatigue during acute uncompensable heat stress to exhaustion. METHODS: Twelve trained and eleven untrained males (Mean\ub1SE: VO2peak=70\ub12 and 50\ub11 mL\ub7kgLBM\u207b \ub9\ub7min\u207b \ub9, 11\ub11 and 16\ub11% fat, respectively) walked to exhaustion (Tre= 39.7 and 39.0\ub0C, respectively) on a treadmill (4.5 km\ub7h\u207b \ub9, 2% grade) in 40\ub0C and 30% R.H., wearing combat clothing and a protective hooded overgarment. At a Tre of 38.0, 38.5, 39.0\ub0C, and exhaustion (EXH) venous blood samples were obtained and subjects performed a Psychomotor Vigilance Task (PVT) for reaction time (RT), false starts, and lapses (RT>0.5s). Serum was analyzed for PRL using a chemiluminescent immunoassay, and plasma analyzed for BCAA and TRYP using gas chromatography/mass spectrometry. Perceptual Strain Index (PeSI) was calculated based on thermal comfort and perceived exertion ratings. RESULTS: PRL, TRYP, and PeSI increased with Tre from 38\ub0C to EXH (p < 0.05) but were not related to changes in reaction time, lapses, or false starts for either group. BCAA decreased (38\ub0C to EXH: 374 \ub1 23 to 348 \ub1 21 nmol/mL) with increasing Tre and was related to the increase of reaction time for UT (r= 120.62, p < 0.05). There was a trend toward slower reaction times in TR and UT from 38\ub0C (257 \ub1 9 and 277 \ub1 21 msec, respectively) to EXH (270 \ub1 9 and 314 \ub1 29 msec, respectively) (p < 0.06). No relationship was observed between the change in TRYP/BCAA and measures of psychomotor performance for either group. CONCLUSIONS: Changes in PRL or TRYP/BCAA, as indicators of central fatigue, were not related to measures of psychomotor performance in TR or UT, however changes in BCAA alone were related to changes in psychomotor performance for UT.Peer reviewed: YesNRC publication: N