Glutamate, NMDA and NMDA receptor antagonists: cardiovascular effects of intrathecal administration in the rat
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Selected excitatory amino acids and antagonists were tested for their effects on arterial pressure and heart rate when administered intrathecally at the second (T2) or ninth (T9) thoracic spinal levels in urethane-anesthetized Sprague-Dawley rats with spontaneous or artificial respiration. Intrathecal administration of glutamate (1 mumol) and N-methyl-D-aspartic acid (NMDA; 2 nmol) at T9 increased arterial pressure and heart rate. The response began within 1 min, peaked at 2-3 min and persisted for 8-15 min. The maximum changes were 20-25 mm Hg for arterial pressure and 40-50 beats/min for heart rate. These responses were prevented by systemic administration of hexamethonium (10 mg/kg). Responses to administration of NMDA at the two spinal levels were essentially the same. Effects elicited by NMDA but not by glutamate were blocked by pretreatment with the NMDA receptor antagonists, D,L-2-amino-5-phosphonovaleric acid (APV; 10 nmol, intrathecal administration) and ketamine (7 mg/kg, i.v.). Intrathecal administration of APV (10, 50 and 200 nmol) at T2 produced dose-dependent decreases in arterial pressure without changing heart rate. The results support the hypothesis that NMDA receptors are involved in regulation of sympathetic output at the spinal level. They also indicate that in this preparation there is a tonic activation of NMDA receptors in sympathetic pathways to the vessels but not to the heart. Finally, the persistence of the response to glutamate in the presence of NMDA receptor antagonists suggests the involvement of non-NMDA receptors in spinal control of sympathetic output.
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