The rat ponto-medullary network responsible for paradoxical sleep onset and maintenance: a combined microinjection and functional neuroanatomical study

The neuronal network responsible for paradoxical sleep (PS) onset and maintenance has not previously been identified in the rat, unlike the cat. To fill this gap, this study has developed a new technique involving the recording of sleep-wake states in unanaesthetized head-restrained rats whilst locally administering pharmacological agents by microiontophoresis from glass multibarrel micropipettes, into the dorsal pontine tegmentum and combining this with functional neuroanatomy. Pharmacological agents used for iontophoretic administration included carbachol, kainic acid, bicuculline and gabazine. The injection sites and their efferents were then identified by injections of anterograde (phaseolus vulgaris leucoagglutinin) or retrograde (cholera toxin B subunit) tracers through an adjacent barrel of the micropipette assembly and by C-Fos immunostaining. Bicuculline, gabazine and kainic acid ejections specifically into the pontine sublaterodorsal nucleus (SLD) induced within a few minutes a PS-like state characterized by a continuous muscle atonia, low voltage EEG and a lack of reaction to stimuli. In contrast, carbachol ejections into the SLD induced wakefulness. In PHA-L, glycine and C-Fos multiple double-labelling experiments, anterogradely labelled fibres originating from the SLD were seen apposed on glycine and C-Fos positive neurons (labelled after 90 min of pharmacologically induced PS-like state) from the ventral gigantocellular and parvicellular reticular nuclei. Altogether, these data indicate that the SLD nuclei contain a population of neurons playing a crucial role in PS onset and maintenance. Furthermore, they suggest that GABAergic disinhibition and glutamate excitation of these neurons might also play a crucial role in the onset of PS.