Autonomic changes elicited by chemical stimulation of mediodorsal nucleus of the thalamus.

Chronic indwelling cannulas were implanted in the mediodorsal nucleus (MD) of the thalamus in New Zealand albino rabbits. Heart rate (HR), blood pressure (BP), respiration and electromyographic activity (EMG) were recorded subsequent to the injection of saline, l-glutamate, or carbachol through the previously implanted cannulas. Dose related decreases in HR and increases in BP were obtained after administration of both l-glutamate and carbachol at doses in the micromolar range. Control injections through cannulas chronically implanted in the overlying hippocampus or adjacent ventricles resulted in dramatic HR and BP baseline changes in a parasympathetic direction (viz HR and BP decreases) suggesting that the effects obtained with MD placements were due to chemical stimulation of MD. Responses elicited from MD in the present experiment were similar, but not identical, to those produced by electrical stimulation of MD, suggesting that the latter effects were not due to stimulation of fibers of passage.