Microinjections of norepinephrine into the intermediolateral cell column of the spinal cord exert excitatory as well as inhibitory effects on the cardiac function

Cardiac responses to microinjections of norepinephrine (NE) into the intermediolateral column of the spinal cord (IML) at T₂ level were studied in pentobarbital-anesthetized, immobilized and artificially ventilated, male Wistar rats. For describing the effects of NE conveniently, the doses of NE were divided into two ranges. The small dose-range consisted of 20 nl volumes of 50, 75 and 100 micromolar (μM) solutions (i.e. 1, 1.5 and 2 pmole in 20 nl, respectively). The larger dose-range consisted of 20 nl volumes of 2.5, 25, 40 and 50 millimolar (mM) solutions (i.e. 0.05, 0.5, 0.8 and 1 nmole in 20 nl, respectively). Injections of small doses of NE (1-2 pmole) into the IML increased heart rate (HR); intravenous injections of these doses did not alter either blood pressure (BP) or HR. Larger doses of NE (0.05-1 nmole) elicited a decrease in HR; intravenous injections of these doses increased HR and BP. Maximum increase in HR was produced by injections of 1.5 pmole of NE into the IML; this effect was blocked by prior injections of prazosin (an α_-1adrenergic receptor antagonist; 50 pmole) but not idazoxan (an α_-2 adrenergic receptor blocker; 10 pmole) into the IML. Maximum decrease in HR was elicited by injections of 0.8 nmole of NE into the IML; this effect was blocked by idazoxan (10 pmole) but not prazosin (50 pmole). Microinjections of idazoxan (10 pmole) alone increased HR while prazosin (50 pmole) alone was ineffective. Intravenous injections of chlorisondamine (a ganglion blocker) blocked the increase in HR elicited by injections of 1.5 pmole of NE into the IML. Injections of NE, prazosin or idazoxan into the IML did not alter the responses to subsequent injections of glutamate at the same site. The effects of NE remained unchanged in midcollicular decerebrate rats. Vehicles in which NE, prazosin or idazoxan were dissolved, did not elicit a response when injected into the IML. Injections of NE outside the IML failed to elicit any response. These results suggest that NE may have a role to play as a neuromodulator/neurotransmitter in the IML. The pathways involved in these actions of NE in the IML remain to be identified.