Activation of the parapyramidal region in the ventral medulla stimulates gastric acid secretion through vagal pathways in rats

Neurons synthesizing thyrotropin-releasing hormone, substance P and seroton in in the medullary caudal raphe nuclei project to the dorsal vagal complex and play a role in the central vagal regulation of gastric function. Neuro ns in the parapyramidal region in the ventral medulla share similar biochem ical coding and projections as those in the caudal raphe nuclei. The role o f the parapyramidal region in the autonomic regulation of gastric acid secr etion was investigated in urethane-anesthetized rats. Unilateral microinjec tion of kainate into the parapyramidal region at 10, 15 and 20 ng induced a dose-related stimulation of gastric acid secretion (net increases: 22.2 +/ - 11.2, 40.5 +/- 8.5 and 89.8 +/- 19.4 mu mol/60 min, respectively), while injection of vehicle had no effect (net change: -0.1 +/- 1.4 mu mol/60 min) . Time-course studies showed a nine-fold peak increase over basal at 30 min after parapyramidal injection of kainate (20 ng) and acid secretion return ed to basal level at 70 min. Microinjections of kainate (15-20 ng) outside the parapyramidal region or into the parapyramidal region in vagotomized ra ts had no effect. Exposure to cold (4 degrees C) for 2 h, which is known to induce vagally mediated gastric secretory and motor responses through medu llary thyrotropin-releasing hormone pathways, increased the number of Fos-p ositive cells in the caudal, middle and rostral parts of the parapyramidal region to 4.3 +/- 0.4, 9.4 +/- 0.9 and 18.4 +/- 1.6/section, respectively, compared with 0.1 +/- 0.1, 0.1 +/- 0.0 and 0.7 +/- 0.6/section, respectivel y, in rats maintained at room temperature. Most of the Fos-labeled cells co -expressed pro-thyrotropin-releasing hormone messenger RNA signal and/or we re serotonin immunoreactive. These data show that chemical activation of neurons in the parapyramidal re gion results in a vagal-dependent stimulation of gastric acid secretion and that acute cold exposure activates parapyramidal neurons containing pro-th yrotropin-releasing hormone and/or serotonin, suggesting a potential role o f the parapyramidal region, in addition to the caudal raphe nuclei, as medu llary sites involved in the vagal regulation of gastric function. (C) 1999 IBRO. Published by Elsevier Science Ltd.