Comparison of activity characteristics of the cuneate nucleus and thoracic spinal neurons receiving noxious cardiac and/or somatic inputs in rats

Previous studies have shown that the gracile nucleus in postsynaptic dorsal column pathway plays an important role in conveying nociceptive information from pelvic visceral organs. The purpose of this study was to compare effects of a noxious cardiac stimulus on neuronal activity in the cuneate nucleus and upper thoracic spinal cord in rats. Extracellular potentials of single neurons in the cuneate nucleus and upper thoracic (T3) spinal cord were recorded in pentobarbital anesthetized, ventilated and paralyzed male rats. To activate cardiac nociceptors, a silicone tube was placed in the pericardial sac over the left ventricle to administer a solution of bradykinin (10 μg/ml, 0.2 ml, 1 min). The number of cuneate neurons responding to intrapericardial bradykinin (IB, 15.6%, 17/109) was significantly less than for T3 neurons (43.2%, 48/111, P < 0.05). IB excited 9/17 (52.9%) cuneate neurons and inhibited eight neurons. In contrast, IB excited a significantly higher percentage of responding spinal neurons than those in cuneate nucleus (43/48, 89.6%, P < 0.01). The ratio of short latency/long-lasting responses of cuneate neurons to IB (14/3) were significant higher than responses of spinal neurons (26/22, P < 0.05). Spontaneous activity (5.5 ± 0.7 imp/s), response amplitudes (6.0 ± 0.6 imp/s) and durations (83.4 ± 10.8 sec) of cuneate neurons excited by IB were significantly less than for spinal neurons (11.5 ± 1.3 imp/s, 20.4 ± 2.0 imp/s and 104.9 ± 7.0 imp/s, P < 0.01, P < 0.01, P < 0.05), respectively. These results indicate that the cuneate nucleus neurons play a relatively minor role in transmission of cardiac nociceptive information in comparison to upper thoracic spinal neurons.