Characterizing the Action of Arginine Vasopressin at Hypoglossal Motoneurons In Neonatal Mice

Abstract

Vasopressin, traditionally considered a hormone, also acts as an excitatory neurotransmitter. The effects of vasopressin have been characterized in brainstem motoneurons, including hypoglossal motoneurons (XII MNs). The XII motor nucleus receives inspiratory rhythm from the preBötzinger Complex where it is relayed to the muscles of the tongue via the XII nerve. With some variation across postnatal maturation, the vasopressin V1a receptor gene is expressed from birth to adulthood in the XII motor nucleus, and is enriched in the XII nucleus in adults compared to other cranial motoneuron populations. Previous research demonstrated that vasopressin, acting via V1a receptors on preBötzinger Complex, can stimulate breathing in adult rats by increasing breathing frequency as well as diaphragm and genioglossus (primary tongue protruder) muscle activity. Whether vasopressin can also act directly at XII MNs to potentiate inspiratory bursting has not been well characterized. Thus, the aim of this research was to characterize vasopressin effects on XII MN inspiratory bursting behavior. We generated rhythmic, transverse medullary slices (600μM) from CD1 mice of either sex (postnatal days 0–5), and recorded inspiratory activity from XII nerve rootlets using a suction electrode. We then bath applied arginine vasopressin (0.01μM and 0.1μM, AVP, V1a receptor agonist), or locally injected AVP (0.1μM and 1.0μM). In doing so, we were able to assess the effects of arginine vasopressin on XII inspiratory burst amplitude. Bath applied AVP increased inspiratory burst amplitude by 43.4%±12.5% (n=9) when applied at 0.01μM, and 56.4%±18.1% at 0.1μM. When locally applied (n=7), AVP at 0.1μM increased inspiratory burst amplitude 38.8%±6.2%, and AVP at 1.0μM caused a 26.5%±6.3% increase in inspiratory burst amplitude with a notable increase in tonic activity. In a separate set of experiments (n=8) to test whether AVP acts via V1a receptors, local AVP increased inspiratory burst amplitude by 40.2%±9.3%. Local pre‐application of the V1a receptor antagonist ((d(CH2)51Tyr(Me)2Arg8) Vasopressin, 500nM) for 60 s attenuated the response to AVP (6.7%±14.7% decrease in inspiratory burst amplitude), but the AVP response was recovered after 15 minutes with an increase in XII inspiratory burst amplitude of 22.3%±4.1%. Overall, our results support that AVP can increase XII inspiratory burst amplitude via V1a receptor activation. Further research will need to determine whether AVP plays a role in maintaining airway patency in vivo. Support or Funding Information Kenneth A. Suarez Research Fellowship, Midwestern University