Chemical activation of cardiac receptors affects activity of superficial and deeper T3-T4 spinal neurons in rats.

The purposes of this study were to examine responses of superficial (depth <300 microm) and deeper thoracic spinal neurons to chemical stimulation of cardiac afferents and effects of descending influences on these neurons. Extracellular potentials of single T(3)-T(4) neurons were recorded in pentobarbital anesthetized, paralyzed and ventilated male rats. A catheter was placed in the pericardial sac to administer 0.2 ml of a mixture of algogenic chemicals that contained adenosine (10(-3) M), bradykinin, histamine, serotonin, prostaglandin E(2) (10(-5) M). Fifteen of 55 (27%) superficial neurons responsive to intrapericardial chemicals were compared to 80/169 (47%) deeper neurons. All 15 superficial neurons that responded to cardiac afferents were excited (E), whereas 66 deeper neurons were excited, ten were inhibited and four showed excitation-inhibition. Spontaneous activity of superficial neurons with short-lasting excitatory responses was significantly lower than that of deeper neurons (P<0.05). Somatic receptive fields on chest, axilla, arm and upper back areas were found for 77/95 (81%) neurons that responded to intrapericardial chemicals. The proportion of somatic field properties and their sizes in superficial neurons were similar to deeper neurons. After cervical spinal transection, both spontaneous activity and responses to chemical stimulation of cardiac afferents significantly increased in six out of six neurons excited by intrapericardial injections. Results showed that chemical stimulation of cardiac afferents excited superficial T(3)-T(4) spinal neurons, whereas deeper neurons exhibited multiple patterns of responses. Some characteristics of subgroups of superficial neurons were quantitatively different from deeper neurons. Thoracic spinal neurons processing cardiac nociceptive information were under tonic descending inhibition.