Using quantitative autoradiography, we have studied the influence of m
aternal plasma melatonin on the expression and density of melatonin re
ceptors in the brain and pituitary of rat offspring. At birth, the sam
e structures displayed melatonin receptors whether the rats were born
to and reared by intact or pinealectomized dams. The receptor density
was, however, about 20% lower in the group born to pinealectomized dam
s. At postnatal day 9, when the pups of both groups synthetize rhythmi
cally their own melatonin, this difference was suppressed. These resul
ts indicate that melatonin does not appear to be a requirement for the
expression of its receptors, but seems to play a stimulatory role in
their synthesis.