Cardiac responses to the microinjections of excitatory amino acids into the intermediolateral cell column of the rat spinal cord.

Sympathoexcitatory cardiovascular responses to the microinjections of L-glutamate into the intermediolateral cell column (IML) of the upper thoracic cord (C8 to T4) were studied. Mean arterial pressure (MAP), heart rate (HR), the rate of increase in the left ventricular pressure (dP/dt) and contractility index were monitored in immobilized and artificially ventilated male Wistar rats anesthetized with pentobarbital or isoflurane. On the right side, microinjections (10-20 nl) of L-glutamate (0.9-1.77 nmol in 0.9% sodium chloride solution, pH 7.4) into the IML at T2 level produced marked tachycardiac responses with relatively small changes in contractility. On the left side, similar microinjections produced marked increase in dP/dt and contractility index with relatively small increase in HR. On either side, the responses were smaller at T1 and T3 level and absent at C8 and T4 level. No changes in blood pressure were observed with microinjections of L-glutamate on either side. Microinjections of N-methyl-D-aspartic acid (NMDA), 1-100 pmol, into the IML elicited responses similar to those of L-glutamate. These amino acids failed to evoke any response when microinjected into the adjacent areas (e.g. 0.5 mm lateral or medial to the IML). The effects of glutamate and NMDA in the IML were blocked by microinjections of glutamic acid diethylester (GDEE) and D-2-amino-7-phosphonoheptanoic acid (D-AP7), respectively. Control microinjections of physiological saline into the IML produced no responses. These results indicate that excitatory amino acids, in small doses and volumes, can be used to identify cardiac sympathoexcitatory neuronal pools in the IML. This preparation may prove useful in characterizing pharmacological actions of various putative neurotransmitters in this region of the spinal cord.