Lateral subthalamic area as mediator of bradycardia responses in rabbits

Train stimulation (100 Hz) of lateral zona incerta (LZI) of the subthalamus in anesthetized rabbits greatly decreased heart rate (x = -94 beats/min), which in turn slightly decreased blood pressure (x = -6 mmHg). These decreases were prevented by bilateral vagotomy. To examine the descending pathway of the neurons mediating this decrease, the authors recorded the responses of single neurons in the dorsal medulla that were activated by stimulating both LZI and aortic nerve. Stimulating bradycardia-producing sites in LZI activated 15 neurons in nucleus tractus solitarius or dorsal vagal nucleus. Although mean onset latency was relatively short (6 ms), these neurons did not follow repeated stimuli faithfully. Therefore, the connection between LZI and dorsal medulla is probably not monosynaptic. However, a short train of pulses elicited greater firing rates than single pulses did; thus, some connections between LZI and dorsal medulla may be monosynaptic, but may require a high degree of temporal summation. Of the 15 neurons activated by stimulating LZI and aortic nerve 3 were antidromically activated by stimulating cervical vagus nerve. These 3 neurons were in dorsal vagal nucleus and were probably cardioinhibitory. Finally, 13 neurons in zona incerta responded to stimulation of aortic nerve with latencies as short as 4 ms, raising the possibility that this area participates in the baroreceptor reflex.