Excitation of neurons in the medullary raphe increases gastric acid and pepsin production in cats

The nucleus raphe obscurus (NRO) has recently emerged as an important nucleus for excitation of gastric motor activity through projections to the dorsal motor nucleus of the vagus (DMV) [P.J. Hornby, C.D. Rossiter, R.L. White, W.P. Norman, D.H. Kuhn, and R.A. Gillis. Am. J. Physiol. 258 (Gastrointest. Liver Physiol. 21): G91-G96, 1990; and M.J. McCann, G.E. Herman, and R.C. Rogers. Brain Res. 486: 181-184, 1989]. A neurotransmitter thought to be involved in this NRO-DMV pathway is thyrotropin-releasing hormone (TRH), a peptide that excites gastric activity when microinjected into the DMV. The purpose of the present study was to determine whether gastric acid and pepsin secretion were altered by 1) activation of neurons in the NRO by microinjection of kainic cid and 2) microinjection of TRH into the DMV in chloralose-anesthetized cats. Microinjection of kainic acid into the NRO increased gastric acid secretion [baseline was 6 ± 2 (μeq) H⁺/15 min (n = 7) and increased to 8 ± 2, 26 ± 11 (P < 0.05), and 21 ± 7 μeq/15 min (P < 0.05) during the first, second, and third 15-min periods after microinjection, respectively]. Pepsin output also increased from a baseline of 287 ± 67 pepsin units (PU) (n = 4) to 507 ± 126 PU 15 min postinjection, 541 ± 118 PU 30 min after injection (P < 0.05), 608 ± 92 PU 45 min after injection (P < 0.05), and 700 ± 156 PU 60 min postinjection (P < 0.05). Atropine methyl bromide (administered intravenously) blocked the production of gastric acid resulting from stimulation of the NRO. Microinjection of TRH into the DMV resulted in increases in gastric acid and pepsin secretion similar to that noted after stimulation of the NRO. These results suggest that activation of NRO neurons excites neurons in the DMV and results in an increase in gastric function, which may be due to release of TRH in the DMV.