Bradykinin is Involved in the Mediation of Cardiac Nociception during Ischemia through Upper Thoracic Spinal Neurons

Bradykinin is one of metabolites produced during myocardial ischemia and infarction that can activate cardiac spinal (sympathetic) sensory neurons to cause chest pain. The aim of this study was 1) to characterize the responses of thoracic superficial and deeper spinal neurons in rats to intrapericardial administration of bradykinin as a noxious cardiac stimulus; 2) to compare neuronal responses to bradykinin alone and a mixture of algogenic chemicals (serotonin, prostaglandin E-2, histamine, adenosine and bradykinin) used in a previous study. Extracellular potentials of single neurons in the T-3 spinal cord were recorded in pentobarbital anesthetized, paralyzed, and ventilated male rats. A catheter was placed in the pericardial sac to administer 0.2 ml solution of bradykinin (10(-5) M, 1 min). The results showed that 10/33 (30%) superficial and 80/165 (48%) deeper spinal neurons responded to intrapericardial bradykinin. All 10 superficial responsive neurons and 72/80 (90%) deeper neurons were excited; 7 (9%) neurons were inhibited; one neuron showed excitation-inhibition response pattern. Of 72 neurons excited by bradykinin, 35 and 47 neurons exhibited short- and long-lasting responses patterns, respectively. The proportions of response patterns and maximal excitatory responses to bradykinin were similar to effects obtained with a mixture of algogenic chemicals. However, the time to peak (28.3 +/- 3.1 s) and recovery time of long-lasting excitatory responses to bradykinin alone (125.2 +/- 8.9 s, n=47) were significantly shorter than the responses of neurons to the algogenic mixture (38.6 +/- 3.8 s and 187.5 +/- 18.5 s, n=49, P<0.05). In conclusion, bradykinin might play a key role in spinal processing for cardiac nociception, although other components released during ischemia might affect time characteristics of a subtype of thoracic spinal neurons receiving noxious cardiac input.