Cross-organ sensitization of thoracic spinal neurons receiving noxious cardiac input in rats with gastroesophageal reflux

Abstract

Gastroesophageal reflux (GER) frequently triggers or worsens cardiac pain or symptoms in patients with coronary heart disease. This study aimed to determine whether GER enhances the activity of upper thoracic spinal neurons receiving noxious cardiac input. Gastric fundus and pyloric ligations as well as a longitudinal myelotomy at the gastroesophageal junction induced acute GER in pentobarbital-anesthetized, paralyzed, and ventilated male Sprague-Dawley rats. Manual manipulations of the stomach and lower esophagus were used as surgical controls in another group. At 4-9 h after GER surgery, extracellular potentials of single neurons were recorded from the T3 spinal segment. Intrapericardial bradykinin (IB) (10 microg/ml, 0.2 ml, 1 min) injections were used to activate cardiac nociceptors, and esophageal distensions were used to activate esophageal afferent fibers. Significantly more spinal neurons in the GER group responded to IB compared with the control group (69.1 vs. 38%, P < 0.01). The proportion of IB-responsive neurons in the superficial laminae of GER animals was significantly different from those in deeper layers (1/8 vs. 46/60, P < 0.01); no difference was found in control animals (7/25 vs. 20/46, P > 0.05). Excitatory responses of spinal neurons to IB in the GER group were greater than in the control group [32.4 +/- 3.5 impulses (imp)/s vs. 13.3 +/- 2.3 imp/s, P < 0.01]. Forty-five of 47 (95.7%) neurons responded to cardiac input and ED, which was higher than the control group (61.5%, P < 0.01). These results indicate that acute GER enhanced the excitatory responses of thoracic spinal neurons in deeper laminae of the dorsal horn to noxious cardiac stimulus.

Fig. 1.

Lesion sites of neurons in representative T3 segment of the rat spinal cord. A: lamina of a T3 spinal segment. I–X, laminae; CC, column of Clarke; IL, intermediolateral nucleus; IM, intermedial nucleus; Liss, Lissauer's tract; LSN, lateral spinal nucleus; Pyr, pyramidal tract. B: neurons with responses to intrapericardial bradykinin (IB) in control animals. ●, Neurons excited by IB; ▲, neurons inhibited by IB. C: neurons with response to IB in gastroesophageal reflux (GER) animals.

Fig. 2.

Response patterns of thoracic spinal neurons to IB in control and GER animals. A–C: examples of neurons with short-lasting excitatory (SL-E, A), long-lasting excitatory (LL-E, B), and inhibitory (C) responses to IB in a control animal. D–F: examples of SL-E (D), LL-E (E), and inhibitory (F) responses to IB in a GER animal. imp, Impulses. In all panels, top trace is the rate histogram and bottom trace is raw cell activity.

Fig. 3.

Example of the cardioesophageal convergent neurons excited by IB (A) and noxious esophageal distension (ED, 0.4 ml, 20 s) (B) in a GER rat.

Fig. 4.

Characteristics of thoracic spinal neurons receiving somatic inputs. A–C: examples of neurons with low-threshold (LT, A), wide-dynamic-range (WDR, B), and high-threshold (HT, C) responses to mechanical stimulation of somatic field in control animals. D–E: examples of neurons with LT (D), WDR (E), and HT (F) responses to mechanical stimulation of somatic field in GER animals. In each panel, top drawing shows location of somatic field and bottom trace shows cell activity. Br, brush; Pr, light pressure; Pi, pinch.

Fig. 5.

Histological examination of esophagitis in rats with the acute GER and control surgery. Digital photomicrographs of cross sections through the thoracic esophagus at 4 (B) and 8 (D) h after GER surgery. B: early basal cell hyperplasia in the epithelium (EP) and thickening and invagination of the lamina propria (LP) compared with control surgery (A). In D, arrows indicate further thickening of the LP at 8 h compared with control surgery (C). Scale bars in A and B = 50 μm; in C and D = 300 μm. Hematoxylin-eosin staining.