Thoracic spinal neuron responses to repeated myocardial ischemia and epicardial bradykinin

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 microg/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.

Silent myocardial ischemia

Although much progress has been made in reducing mortality from ischemic cardiovascular disease, this condition remains the leading cause of death throughout the world. This might in part be due to the fact that over half of patients have a catastrophic event (heart attack or sudden death) as their initial manifestation of coronary disease. Contributing to this statistic is the observation that the majority of myocardial ischemic episodes are silent, indicating an inability or failure to sense ischemic damage or stress on the heart. This review examines the clinical characteristics of silent myocardial ischemia, and explores mechanisms involved in the generation of angina pectoris. Possible mechanisms for the more common manifestation of injurious reductions in coronary flow; namely, silent ischemia, are also explored. A new theory for the mechanism of silent ischemia is proposed. Finally, the prognostic importance of silent ischemia and potential future directions for research are discussed.

Intrapericardiac injections of algogenic chemicals excite primate C1-C2 spinothalamic tract neurons

Extracellular potentials of 38 C1-C2 spinothalamic tract (STT) neurons in anesthetized monkeys (Macaca fascicularis) were examined for responses to intrapericardiac injections of an algogenic chemical mixture (adenosine, 10(-3) M; bradykinin, prostaglandin E(2), serotonin, histamine, each 10(-5) M). Chemical stimulation of cardiac/pericardiac receptors increased activity of 21 cells, decreased activity of 5 cells, and did not change activity of 12 cells. Cells excited by chemical stimuli received input from noxious mechanical stimulation of somatic fields; most receptive fields included the neck, inferior jaw, or head areas. Nerve ablations in 11 cells excited by intrapericardiac chemicals showed that cardiac input activated by algogenic chemicals traveled primarily in vagal afferent fibers to C1-C2 segments; phrenic or cardiopulmonary sympathetic inputs were predominant in 2 of 11 cells. These results supported the concept that activation of cardiac vagal afferents might lead to the production of referred pain sensation in somatic fields innervated from high cervical segments.

Models of Visceral Nociception

Pains arising from the viscera constitute a large portion of clinically treated pains. They are characterized by poor localization; immobility with tonic increases in muscle tone; and vigorous but nonspecific changes in autonomic function, such as changes in respiration, heart rate, and blood pressure. Tissue-damaging stimuli do not reliably produce visceral pain, so the study of visceral nociception in nonhuman animals requires identification of appropriate stimuli and responses. This article defines "noxious" visceral stimuli as those that produce pain in humans, result in aversive behaviors in animals, and evoke responses that are inhibited by manipulations known to be analgesic in humans. To be valid, the measured responses must be reliable, inhibited by known analgesics, and not inhibited by nonanalgesics. Using these criteria as measures of validity, the author examined several visceral pain models. The writhing test (application of intraperitoneal irritants) failed to meet these criteria; however, responses to small bowel distension, colonic-rectal distension, artificial ureteral calculosis, urinary tract distension, and the intravesical application of irritants met most, if not all, of the criteria. Other models, such as responses to biliary system distension, to reproductive organ stimulation, to the focal application of algesic agents onto various viscera, and to ischemic stimuli, met some of these criteria. This information should assist readers in decisions related to the use of visceral pain models.