Medullary raphe: a new site for vagally mediated stimulation of gastric motility in cats.

Thyrotropin-releasing hormone (TRH) is clearly implicated in the control of gastric function via interactions in the dorsal motor nucleus of the vagus (DMV) of the cat. The source of the TRH innervation of the DMV is important to determine because this region could be of importance in control of gastric function. TRH-immunoreactive (ir) neurons are located in the raphe obscurus (Ro), raphe pallidus (Rp), and raphe magnus (Rm). Retrograde tracer applied to the DMV resulted in the most numerous labeled neurons in the caudal Ro and Rp in the same region where TRH-ir neurons are located. To address the question whether DMV-projecting neurons in the raphe subnuclei play a role in control of gastric motility, the following experiments were performed in alpha-chloralose-anesthetized cats while recording pyloric motility and blood pressure. Microinjection of a cell body excitant L-glutamate (44-200 nl, 0.5 M) into the caudal Ro and Rp in 15 experiments produced significant increases in pyloric minute motility index (MMI) of 4.9 +/- 1.5 (from 1.6 +/- 0.7 preinjection to 6.5 +/- 1.8 postinjection, P less than 0.05). Mean blood pressure (MBP) decreased significantly in these animals by 12 +/- 7 mmHg (from 100 +/- 6 to 88 +/- 8 mmHg, P less than 0.05). Saline microinjection in the same sites in seven cases resulted in no significant change in pyloric MMI (-1.0 +/- 0.8) or MBP (-4 +/- 11 mmHg). In five of these experiments, a second microinjection of L-glutamate (132-240 nl) was performed into the caudal Ro and Rp after spinal cord transection. This resulted in a significant increase in pyloric MMI of 3.3 +/- 0.9 (from 1.0 +/- 0.5 preinjection to 4.3 +/- 1.1 postinjection, P less than 0.05) but no change in MBP (+1 +/- 1 mmHg). Bilateral vagotomy resulted in the abrupt cessation of the pyloric response to caudal Ro and Rp stimulation. Microinjection of L-glutamate into the rostral Rp and caudal Rm in nine experiments resulted in no significant changes in pyloric MMI (-0.4 +/- 0.8) or MBP (-10 +/- 11 mmHg). These data indicate that a population of neurons in the caudal raphe nuclei, which may contain TRH, project to the DMV. In addition, excitation of these neurons causes an increase in gastric motility that is not caused by inhibition of sympathetic outflow to the gut but rather by excitation of vagal neurons in the DMV.