Influence of acute stress on the pineal activity during day- and nighttime.

Ovariectomized, steroid implanted female ewes were used as a model for studying the effect of acute isolation and confinement stress on the pineal activity during day and nighttime under artificial luteal phase conditions. Male and female intact buffaloes were employed as well, with the aim to establish the influence of another perturbation (venous catheter insertion) on the melatonin levels during daytime. Stress appeared to influence pineal melatonin secretion in controversial manner, namely, decreasing further the low indole levels during the day, while elevating the peripheral concentrations at night, though the initial response to stress during daytime was a transient elevation in melatonin levels. There are no indications that the adrenals are directly involved in the changes observed. Possibilities for different mechanisms of melatonin secretion and release in different species are considered.