Glutamate stimulation of raphe pallidus attenuates the cardiopulmonary reflex in anaesthetised rats

Serotonin has been implicated as having a modulatory action on NTS neurones mediating vagal inputs controlling the cardiorespiratory system. Since raphe pallidus and raphe obscurus both send serotonergic projections to the NTS, we have investigated a neuromodulatory role for these structures on the cardiopulmonary reflex. A multibarrelled microelectrode positioned around the level of the area postrema was placed at varying depths into mid-line brainstem structures and the effect of glutamate stimulation on the cardiopulmonary reflex tested. Excitatory chemical stimulation in the region of raphe pallidus, but not raphe obscurus, attenuated significantly the respiratory and bradycardic components of the cardiopulmonary reflex. This attenuation was reversed by an NTS microinjection of RS-39604, a selective 5-HT4 receptor antagonist. We propose that neurones in raphe pallidus that project to the NTS can release serotonin which acts via 5-HT4 receptors to attenuate the reflex phrenic nerve activity and heart rate components of the cardiopulmonary reflex.