Spinal cord excitatory amino acid receptors and plasma catecholamine autonomic responses in the conscious rabbit

Intrathecal administration of the specific glutamate subtype receptor agonist NMDA (N-methyl-D-aspartate), or the non-NMDA receptor agonist AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid), at the level of the lower thoracic spinal cord in the conscious rabbit, produces increased levels of plasma norepinephrine and a rise in blood pressure. These responses are specifically inhibited with prior intrathecal administration of the NMDA receptor antagonist AP-5 (2-amino-5-phosphonovaleric acid) or non-NMDA receptor antagonist DNQX (6, 7-dinitroquinoxaline-2, 3-dione), respectively. In contrast, arterial baroreflex activation induced by transient hypotension following intravenous sodium nitroprusside increases levels of plasma norepinephrine which are inhibited with intrathecal AP-5, but not DNQX. The experiments are consistent with glutamate acting as a neurotransmitter in the spinal cord influencing control of sympathetic nerve function via activation of spinal cord NMDA and non-NMDA subtype receptors. The results support previous work suggesting that baroreflex function is predominantly mediated by spinal NMDA receptors and that spinal glutamate receptors are important in baroreflex control of the circulation.