THE EFFECTS OF LESIONS OF THE THALAMIC INTERGENICULATE LEAFLET ON THEPINEAL METABOLISM

The aim of the present work was to study, in rats, the effects of lesi ons of the thalamic intergeniculate leaflet (IGL) and the deep pineal/ lamina intercalaris region (DP) on the diurnal profile of N-acetylsero tonin (NAS) and on the nocturnal pineal reactivity to acute retinal li ght stimulation (1 or 15 min). The 24-h experiment shows that there is no phase-shifting on the diurnal NAS curve of groups of rats with bil ateral IGL lesion compared to the controls. On the other hand there is a significant reduction on the amplitude of pineal NAS content observ ed in every nocturnal point of the curve. The pineal glands of IGL-les ioned rats, after 1 min of retinal light stimulation, keep their NAS c ontent equal to the lesioned dark-killed rats. Nonetheless, after 15 m in of photostimulation, the pineal NAS content is reduced to nearly ze ro equally to the control animals. DP lesion does not modify the conte nt of NAS in the pineal gland of rats killed in the dark. However, the pineal photo-inhibition process induced by 1 min of light exposure is impaired. These results suggest that: (1) the intergeniculate leaflet has a role in regulating the amplitude of the diurnal rhythm of pinea l NAS production rather than its phase entrainment to light-dark cycle . This effect is not dependent on the direct geniculo-pineal connectio ns. (2) The nocturnal pineal photo-inhibition phenomenon could be deco mposed in two processes. One, triggered by short pulses of light and t otally dependent on the IGL and partially dependent on the direct mono synaptic pathway between this structure and the pineal gland. Another one, brought into action by longer lasting light stimulation that is n either dependent on the IGL nor on any direct central neural connectio ns to the pineal gland.