Aortic Baroreceptor Reflexes Are Mediated by NMDA Receptors in Caudal Ventrolateral Medulla

The purpose of this study was to identify central nervous system pathways and synaptic receptors that participate in baroreflex control of arterial pressure. Microinjections of excitatory amino acids into the caudal ventrolateral medulla (CVM) of anesthetized rats evoked depressor responses analogous to baroreceptor reflexes. Functional inactivation of CVM neurons produced by microinjection of the gamma-aminobutyric acid receptor agonist muscimol completely abolished baroreflex-mediated decreases in arterial pressure elicited by electrical stimulation of the aortic nerve and markedly reduced depressor responses produced by the excitatory amino acid L-glutamate. In contrast, selective blockade of N-methyl-D-aspartic acid (NMDA) receptors in the CVM abolished synaptically mediated depressor responses evoked by aortic nerve stimulation but not those elicited by L-glutamate, kainic acid, or quisqualic acid injected at the same site. These results indicate that the CVM contains an obligatory synapse in the central aortic baroreflex pathway; neural transmission of aortic baroreceptor information in the CVM is mediated by activation of NMDA receptors; and the neurotransmitter released at CVM synapses may be an excitatory amino acid.