The aim of the present investigation was to study the effect of neurotoxic
ibotenic acid lesion of the retrochiasmatic area on the daily profile of pi
neal N-acetylserotonin and melatonin synthesis and on the pineal metabolic
reactivity to nocturnal short-term retinal photostimulation. Groups of rats
were killed 6 h after lights off either in the dark of immediately after b
eing photostimulated for 1 or 15 min. Additionally, groups of rats were sac
rificed at six different time points throughout the 24-hour light-dark cycl
e. The results suggested the presence of two functionally distinct territor
ies in the retrochiasmatic area. The basal retrochiasmatic area, an area si
tuated immediately ventral to the third ventricle, behind the suprachiasmat
ic nuclei and in front of the arcuate nucleus, is implicated in the nocturn
al inhibitory process induced by short-term retinal photostimulation. The l
ateral retrochiasmatic area, which is situated immediately lateral to the a
nterior periventricular nucleus, below the anterior hypothalamic nucleus an
d in front of the ventromedial hypothalamic nucleus, is importantly involve
d in the control of the peak amplitude of the daily production of N-acetyls
erotonin and melatonin by the pineal gland.