IMPORTANCE OF THE VENTROLATERAL REGION OF THE PERIAQUEDUCTAL GRAY ANDADJACENT TEGMENTUM IN THE CONTROL OF PARADOXICAL SLEEP AS STUDIED BY MUSCIMOL MICROINJECTIONS IN THE CAT

It has been demonstrated that coagulation in the area of the periaqued uctal gray induces a marked increase in paradoxical sleep in the cat [ Petitjean F. et al. (1975) Brain Res. 88, 439-453]. This effect was ob tained either by the destruction of ascending or descending fibres or by the lesion of a specific group of local neurons. To assess the role of these neurons, muscimol (0.5 mu g/0.5 mu l) was injected bilateral ly in 31 cats in this area of the periaqueductal gray. Polygraphic rec ordings were performed before and after injections. Following muscimol (GABA(A) agonist) injection, there was a consistent increase in parad oxical sleep lasting 269 +/- 8 min (mean +/- S.E.M.), with a latency o f 31 +/- 2 min. The increase varied from small (20-30%) to medium (30- 50%) to large (50-100% of the recording time), depending on the inject ion site. The intensity of hypersomnia was correlated with the site of the injection. That is, the most profound hypersomnia was obtained wh en muscimol was injected in the vicinity of a target area which lies i n the ventrolateral periaqueductal gray (at the level of the fourth nu cleus) and in the reticular formation situated immediately below. Simi lar effects were also obtained in insomniac cats pretreated with p-chl orophenylalanine and in cats whose brainstem was transected 3 mm rostr al to the injection site. Injections of baclofen, a GABA(B) agonist (0 .25-5 mu g), did not alter the quantity of paradoxical sleep, whereas injections of bicuculline, a GABA(A) antagonist, significantly decreas ed the quantity of paradoxical sleep at the doses of 0.2-2 mu g. It wa s concluded that inactivation of ventrolateral periaqueductal gray neu rons induces a very important increase in paradoxical sleep. The exact mechanisms of this effect remain to be investigated. Copyright (C) 19 96 IBRO. Published by Elsevier Science Ltd.