The nucleus raphe obscurus controls pancreatic hormone secretion in the rat

Until recently, the dorsal vagal complex (DVC) was considered as the only brain stem regulatory center for the vagal control of the endocrine pancreas. Because the nucleus raphe obscurus (NRO) maintains anatomic connections via the DVC to the pancreas, a functional significance of these findings was investigated in the present study. Kainic acid and vehicle were microinjected into the right DVC and the NRO of alpha-chloralose-anesthetized rats, and plasma concentrations of rat insulin, glucagon, and glucose were determined before and 5, 15, 30, and 60 min after injections. Chemical stimulation of neurons in the DVC by kainic acid at a dose of 200 pmol evoked increases in concentrations of insulin, with a peak at 15 min, and glucagon, with a peak at 30 min. Microinjection of kainic acid into the NRO at a dose of 200 pmol, but not at a dose of 20 pmol, produced increases in plasma concentrations of insulin, with a peak at 30 min, and glucagon, with a peak at 60 min. Plasma glucose levels on microinjection of kainic acid into the NRO at a dose of 20 pmol were decreased, whereas no changes on microinjection of kainic acid at a dose of 200 pmol were observed. The effects of kainic acid on insulin and glucagon secretion in the NRO were abolished by bilateral vagotomy. The study demonstrates for the first time that the NRO can contribute to vagal control of pancreatic endocrine function, although the exact circuitry and neurotransmitters involved in this response remain unknown.