Breathing stimulation mediated by 5-HT1A and 5-HT3 receptors within the preBötzinger complex of the adult rabbit

Abstract Serotonin (5-HT) has been reported to play excitatory effects on respiration by acting on preBotzinger complex (preBotC) neurons in neonatal or juvenile rodents. However, whether its action is circumscribed to the preBotC and present in other animal species, particularly in adult preparations, is unknown. We investigated the respiratory role of 5-HT within the preBotC and neighbouring respiration-related regions. Experiments were performed on α-chloralose-urethane anesthetized, vagotomized, paralyzed and artificially ventilated rabbits making use of bilateral microinjections (30–50 nl). 5-HT caused excitatory effects on respiratory activity only when applied to the preBotC. These effects were mediated by 5-HT 1A and 5-HT 3 receptors as shown by microinjections of specific agonists of the different types of 5-HT receptors. Unexpectedly, the blockade of 5-HT 1A receptors by methysergide or the specific antagonist (S)-WAY 100135 induced excitatory respiratory effects. Microinjections of the 5-HT 3 receptor antagonist ondansetron did not influence respiration, but prevented (S)-WAY 100135-induced responses. The blockade of GABA A receptors by bicuculline within the preBotC prevented the effects of the 5-HT 1A receptor agonist 8-OH-DPAT. The involvement of GABAergic inhibition and 5-HT 1A receptor-mediated disinhibition is also corroborated by immunohistochemical data. The results show for the first time in an adult animal preparation that 5-HT plays a pivotal role in the modulation of the preBotC activity probably via both presynaptic and postsynaptic mechanisms and highlight the importance of disinhibition phenomena. Present findings may be relevant to some respiratory disorders in which an impairment of central 5-HT mechanisms has been reported, such as sleep apnoea and sudden infant death syndrome.