Microinjection of substance P and ACh into rat intermediolateral nucleus elicits cardiovascular responses

The effects of substance P (SP) or acetylcholine (ACh) microinjected into the intermediolateral nucleus of the spinal cord (IMLn) on arterial pressure (AP) and heart rate (HR) were investigated in 22 male Wistar rats under urethan and artificial ventilation. L-Glutamate (Glu) was microinjected into the IMLn between C7 and T4 to locate cardiovascular sites. Micropipettes containing Glu were stereotaxically positioned in 82 histologically verified sites in the IMLn between C7 and T4 on both the right and left sides. Microinjection of 4-10 nl of 0.18 M Glu in 30 of 39 explored sites at the T2 level elicited significant increases in HR (+24.2 +/- 3.1 beats/min). These changes were accompanied by significant increases in mean AP (+11.4 +/- 1.2 mmHg) at the T2 level (32/47 sites). Microinjection of 4-10 nl of SP (3 X 10(-7) to 3 X 10(-4) M) or ACh (0.005-0.5 M) in the right IMLn at the T2 level elicited increases in HR but did not affect AP. The duration of the responses to SP or ACh was significantly longer than the duration of the responses to Glu. The responses to ACh could be blocked by prior microinjection of 5 X 10(-2) M atropine. No responses were ever obtained in the left IMLn by microinjection of Glu, SP, or ACh. These results support the hypothesis that Glu, ACh, and SP mediate sympathoexcitation in the IMLn of the rat and that these excitatory responses have different temporal patterns.