Importance of the ventrolateral region of the periaqueductal gray and adjacent tegmentum in the control of paradoxical sleep as studied by muscimol microinjections in the cat
Section snippets
Experimental procedures
A total of 31 cats, male and female (2.2–4.7 kg), were employed in this study. Non-pretreated normal cats received muscimol (n = 23), saline (n = 3), bicuculline (n = 6) or baclofen (n = 6) microinjections. In addition, five cats were pretreated with p-chlorophenylalanine (PCPA) before muscimol microinjections, whereas three other cats received muscimol in the target area three to seven days after a total transection of the brainstem at the A4 plane and removal of the cerebrum (see below in Pontine
Physiological and clinical effects
In all cases the cats had to be restrained for 6 min during the bilateral injection of muscimol. We performed a total of 86 injections: 53 were followed by hypersomnia, and when injections were performed either above, below, lateral or medial to the target area (33 cases), we obtained either no effect upon PS or a 2–3 h suppression of PS or a specific increase in SWS2. In some cases, the cat appeared to be looking and searching for unknown objects. This resembles the “hallucinatory behaviour”
Discussion
The present study demonstrates that muscimol microinjections in the ventrolateral portion of the posterior PAG and its immediate vicinity are followed by a very large increase in PS (up to 100% of the recording time during a 5-h period). To the best of our knowledge, this is the only place in the brain in which the injection of a small amount of muscimol is followed by such a dramatic hypersomnia in the cat.26, 43
Conclusions
The bilateral inactivation of the caudal intermediate ventrolateral area of the PAG by microinjections of muscimol is followed by a dramatic increase in PS. This effect is mediated by descending pathways acting upon either executive or permissive mechanisms of PS.
Acknowledgements
We are grateful for the assistance of G. Debilly with statistical data analysis. We also thank M. Schmidt and C. Limoge for correction of the English. Part of this work was presented at the 12th Congress of the European Sleep Research Society held in Florence in May 1994. This study was supported by CNRS URA 1195, DRET (91130) and INSERM U52.
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