Orexinergic modulation of GABAergic neurotransmission to cardiac vagal neurons in the brain stem nucleus ambiguus changes during development.

Cardiac vagal neurons (CVNs) in the nucleus ambiguus (NA) are the major determinant of parasympathetic activity to the heart. Spontaneous GABAergic neurotransmission to CVNs is modulated by hypothalamic neuropeptide orexin-A in postnatal days 2-5 (P5) rats; however, during early postnatal development, orexin expression changes, and the role of orexin-A in modulating CVN activity at other stages of development is unknown. In this study, we compared changes in GABAergic inhibitory postsynaptic currents (IPSCs) in CVNs evoked by orexin-A in P5, P16-20 (P20), and P27-30 (P30) rats using an in vitro brain stem slice preparation. Bath-applied orexin-A enhanced GABAergic IPSCs in all CVNs tested in P5 and P30 animals and in the majority of neurons tested in P20 pups. Focal application of orexin-A ejected from a pipette positioned within 30 μm of the patched CVN did not alter GABAergic signaling in P5 pups. In contrast, in both P20 and P30 rats, focal application of orexin-A inhibited GABAergic IPSCs, and this inhibition persisted in the presence of tetrodotoxin. These results indicate orexin-A facilitates GABAergic IPSCs likely by activating preceding GABAergic neurons that project to CVNs. Orexin-A also likely acts at GABAergic presynaptic terminals surrounding CVNs within the NA to inhibit GABA release. The latter mechanism is absent in P5 pups but occurs in P20 and P30 rats. In conclusion, this study elucidates an important maturation of the parasympathetic cardiac control system. Alterations in these developmental mechanisms may play a role in pathogenesis of disorders related to a specific stage of development maturation.