A1-NEURONS AND EXCITATORY AMINO-ACID RECEPTORS IN RAT CAUDAL MEDULLA MEDIATE VAGAL EXCITATION OF SUPRAOPTIC VASOPRESSIN CELLS

Extracellular recordings from the supraoptic nucleus of the rat established that vasopressinergic neurosecretory cells were excited by stimulation of cervical but not abdominal vagal afferents. This response was absent or significantly attenuated after microinjection of gamma-aminobutyric acid into a region of the caudal medulla known to contain the A1 noradrenaline cell group. Consistent with the possible involvement of the A1 group, vagal stimulation approximately doubled the frequency of proto-oncogene expression in Al noradrenaline neurons, as indicated by the occurrence of nuclear Fos-like immunoreactivity in tyrosine hydroxylase-positive neurons of the caudal ventrolateral medulla. Finally, A1 region microinjection of either the N-methyl-D-aspartic acid (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), significantly reduced vasopressin cell responses to vagal stimulation. These findings suggest that: (i) the Al group is an essential component in a pathway which relays facilitatory vagal input of cardiopulmonary origin to neurosecretory vasopressin cells, and (ii) the activation of Al neurons in this pathway involves both NMDA and non-NMDA excitatory amino acid receptors, an observation consistent with an input to Al cells which generates 'mixed' excitatory postsynaptic potentials.