Serotonergic connections to the ventral oral pontine reticular nucleus: implication in paradoxical sleep modulation

Cholinergic microstimulation of the ventral part of the oral pontine reticular nucleus (vRPO) in cats generates and maintains paradoxical sleep. The implication of rostral raphe nuclei in modulating the sleep-wakefulness cycle has been based on their serotonergic projections to the pontine structures responsible for the induction of paradoxical sleep. However, serotonergic neurons have also been described in brainstem structures other than the raphe nuclei. The aim of the present work is to trace the origin of the serotonergic afferents to the vRPO and to the locus coeruleus alpha and perilocus coeruleu alpha nuclei, closely related with different paradoxical sleep events. Anterograde and retrograde horseradish peroxidase conjugated with wheat germ agglutinin tracer injections in these nuclei in cats were combined with serotonin antiserum immunohistochemistry. Our results demonstrate that reciprocal connections linking the rostral raphe nuclei to those oral pontine nuclei are scarce. The percentage of double-labeled neurons after injections in the vRPO averaged 18% in rostral raphe nuclei, while a level of 82% was estimated in mesopontine tegmentum structures other than the raphe nuclei. These results showed that the main source of serotonin to the vRPO, implicated in generation and maintenance of paradoxical sleep, arises from these mesopontine tegmentum structures. This indicates that the serotonin modulation of paradoxical sleep could be the result of activation in non-raphe mesopontine tegmentum structures. The existence of a complicated network in the vRPO, which maintains a balance between different neurotransmitters responsible for the generation and alternance of paradoxical sleep episodes, is discussed.