MATERNAL PLASMA-CATECHOLAMINES IN THE RHESUS MACAQUE DURING LATE GESTATION - EFFECT OF PHOTOPERIOD AND TIMED MELATONIN INFUSION

This study tested the hypothesis that changes in photoperiod alter plasma catecholamine concentrations in the rhesus monkey during late gestation, Twelve chronically catheterized pregnant rhesus macaques were acclimated to a 12-h photoperiod (lights-on, 0700-1900 h). Under the control L:D cycle, blood samples were collected at 3-h intervals over 24 h for catecholamine analysis. Plasma concentrations (mean +/- SEM, pg/ml) ranged from 678 +/- 90 to 928 +/- 142 for norepinephrine; 230 +/- 22 to 631 +/- 141 for epinephrine; and 282 +/- 70 to 1090 +/- 362 for dopamine. A diurnal rhythm was observed in epinephrine with peak concentrations during tights-on (0900-1800 h; p < 0.05, compared to lights-off). After the first sampling protocol, the animals were divided equally between two groups: phase shift, in which tights-on was shifted 11 h (2000-0800 h) and constant light, with lights on continuously. After the phase shift, a parallel shift in the plasma epinephrine rhythm was noted, with peak levels observed between 2200 and 0700 h (p < 0.05). Constant light abolished the rhythm in epinephrine, with an overall reduction in mean basal levels of all three catecholamines. Daily melatonin infusions (0.2 mug/kg/h, 1900-0630 h) under constant light failed to restore the epinephrine rhythm or to return basal catecholamine concentrations to control photoperiod levels. These data suggest that photoperiod entrains the rhythm in epinephrine secretion, but the rhythm is ablated under constant conditions. Further, melatonin does not appear to play a role in the regulation of catecholamine secretion in the pregnant rhesus macaque.