Responses and afferent pathways of superficial and deeper C-1-C-2 spinal cells to intrapericardial algogenic chemicals in rats

Electrical stimulation of vagal afferents or cardiopulmonary sympathetic af ferent fibers excites C-1-C-2 spinal neurons. The purposes of this study we re to compare the responses of superficial (depth <0.35 mm) and deeper C-1- C-2 spinal neurons to noxious chemical stimulation of cardiac afferents and determine the relative contribution of vagal and sympathetic afferent path ways for transmission of noxious cardiac afferent input to C-1-C-2 neurons. Extracellular potentials of single C-1-C-2 neurons were recorded in pentob arbital anesthetized and paralyzed male rats. A catheter was placed in the pericardial sac to administer a mixture of algogenic chemicals (0.2 ml) tha t contained adenosine (10(-3) M), bradykinin, histamine, serotonin, and pro staglandin E-2 (10(-5) M each). Intrapericardial chemicals changed the acti vity of 20/106 (19%) C-1-C-2 spinal neurons in the superficial laminae, whe reas 76/147 (52%) deeper neurons responded to cardiac noxious input (P < 0. 01). Of 96 neurons responsive to cardiac inputs, 48 (50%) were excited (E), 41 (43%) were inhibited (I), and 7 were excited/inhibited (E-I) by intrape ricardial chemicals. E or I neurons responsive to intrapericardial chemical s were subdivided into two groups: short-lasting (SL) and long-lasting (LL) response patterns. In superficial gray matter, excitatory responses to car diac inputs were more likely to be LL-E than SL-E neurons. Mechanical stimu lation of the somatic field from the head, neck, and shoulder areas excited 85 of 95 (89%) C-1-C-2 spinal neurons that responded to intrapericardial c hemicals; 31 neurons were classified as wide dynamic range, 49 were high th reshold, 5 responded only to joint movement, and no neuron was classified a s low threshold. For superficial neurons, 53% had small somatic fields and 21% had bilateral fields. In contrast, 31% of the deeper neurons had small somatic fields and 46% had bilateral fields. Ipsilateral cervical vagotomy interrupted cardiac noxious input to 8/30 (6 E, 2 I) neurons; sequential tr ansection of the contralateral cervical vagus nerve (bilateral vagotomy) el iminated the responses to intrapericardial chemicals in 4/22 (3 E, 1 I) neu rons. Spinal transection at C-6-C-7 segments to interrupt effects of sympat hetic afferent input abolished responses to cardiac input in 10/10 (7 E, 3 I) neurons that still responded after bilateral vagotomy. Results of this s tudy support the concept that C-1-C-2 superficial and deeper spinal neurons play a role in integrating cardiac noxious inputs that travel in both the cervical vagal and/or thoracic sympathetic afferent nerves.