Glutamatergic neurotransmission between the C1 neurons and the parasympathetic preganglionic neurons of the dorsal motor nucleus of the vagus.

The C1 neurons are a nodal point for blood pressure control and other autonomic responses. Here we test whether these rostral ventrolateral medullary catecholaminergic (RVLM-CA) neurons use glutamate as a transmitter in the dorsal motor nucleus of the vagus (DMV). After injecting Cre-dependent adeno-associated virus (AAV2) DIO-Ef1α-channelrhodopsin2(ChR2)-mCherry (AAV2) into the RVLM of dopamine-β-hydroxylase Cre transgenic mice (DβH(Cre/0)), mCherry was detected exclusively in RVLM-CA neurons. Within the DMV >95% mCherry-immunoreactive(ir) axonal varicosities were tyrosine hydroxylase (TH)-ir and the same proportion were vesicular glutamate transporter 2 (VGLUT2)-ir. VGLUT2-mCherry colocalization was virtually absent when AAV2 was injected into the RVLM of DβH(Cre/0);VGLUT2(flox/flox) mice, into the caudal VLM (A1 noradrenergic neuron-rich region) of DβH(Cre/0) mice or into the raphe of ePet(Cre/0) mice. Following injection of AAV2 into RVLM of TH-Cre rats, phenylethanolamine N-methyl transferase and VGLUT2 immunoreactivities were highly colocalized in DMV within EYFP-positive or EYFP-negative axonal varicosities. Ultrastructurally, mCherry terminals from RVLM-CA neurons in DβH(Cre/0) mice made predominantly asymmetric synapses with choline acetyl-transferase-ir DMV neurons. Photostimulation of ChR2-positive axons in DβH(Cre/0) mouse brain slices produced EPSCs in 71% of tested DMV preganglionic neurons (PGNs) but no IPSCs. Photostimulation (20 Hz) activated PGNs up to 8 spikes/s (current-clamp). EPSCs were eliminated by tetrodotoxin, reinstated by 4-aminopyridine, and blocked by ionotropic glutamate receptor blockers. In conclusion, VGLUT2 is expressed by RVLM-CA (C1) neurons in rats and mice regardless of the presence of AAV2, the C1 neurons activate DMV parasympathetic PGNs monosynaptically and this connection uses glutamate as an ionotropic transmitter.