The effect of morphine on
melatonin secretion in the domestic pig. In vivo and in vitro study by Bogdan Lewczuk, Barbara Przybylska-Gornowicz & Zygmunt Wyrzykowski
Key words: morphine, melatonin, pig,
in vivo, in vitro
Up till now the results of
performed investigations suggest the involvement of opioids in the regulation of the
pineal gland activity in mammals. On the other hand, they show the existence of large
interspecies differences in the presence of opioid peptides in the pineal gland and in the
effects of opiates on the melatonin secretion. The aim of the present work was to study
the influence of morphine on the melatonin secretion in the domestic pig.
Morphine (about 2.5 mg/kg) was
given intravenously to immature gilts during the day, during the night or during the night
with turned on fluorescent illumination (intensity 500 lx at the level of the animal
heads) and plasma melatonin level was measured. The effect of various concentrations of
morphine on basal or norepinephrine-stimulated melatonin secretion was also investigated
using perfusion culture of pineal glands of immature female pigs.
Morphine did not change plasma melatonin concentration in the
domestic pig when administered in a single dose at the beginning of the light or the dark
phase of the diurnal light-dark cycle. However, morphine administration at the beginning
of the night resulted in significantly decreased the plasma melatonin level in animals
exposed to light, with intensity 500 lx, which was insufficient to block nocturnal rise in
plasma concentration of this pineal hormone in the untreated pigs. Morphine had also no
effect on the level of basal and norepinephrine-stimulated melatonin secretion in vitro.
The obtained results suggest that in immature pigs morphine does
not influence directly the pineal activity, but may modulate the melatonin secretion
indirectly, increasing the sensitivity of the system generating melatonin synthesis and
secretion to the light.