Glutamatergic projection to RVLM mediates suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of glutamatergic projection from the parabrachial nucleus (PBN) complex to the rostral ventrolateral medulla (RVLM) in the PB N-induced suppression of reflex bradycardia in adult Sprague-Dawley rats th at were maintained under pentobarbital anesthesia. Under stimulus condition s that did not appreciably alter the baseline systemic arterial pressure an d heart rate, electrical (10-s train of 0.5-ms pulses, at 10-20 mu A and 10 -20 Hz) or chemical (L-glutamate, 1 nmol) stimulation of the ventrolateral regions and Koelliker-Fuse (KF) subnucleus of the PBN complex significantly suppressed the reflex bradycardia in response to transient hypertension ev oked by phenylephrine (5 mu g/kg iv). The PBN-induced suppression of reflex bradycardia was appreciably reversed by bilateral microinjection into the RVLM of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (500 pmo l) or the non-NMDA-receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol). Anatomically, most of the retrogradely labeled neurons in the v entrolateral regions and KF subnucleus of the ipsilateral PBN complex after microinjection of fast blue into the RVLM were also immunoreactive to anti -glutamate antiserum. These results suggest that a direct glutamatergic pro jection to the RVLM from topographically distinct regions of the PBN comple x may participate in the suppression of reflex bradycardia via activation o f both NMDA and non-NMDA receptors at the RVLM.