Response of temperature-sensitive medullary neurons to acetylcholine, norepinephrine, and 5-hydroxytryptamine

Single unit activity was recorded by means of five-barrel micropipettes from temperature-sensitive neurons in the reticular formation of the medulla oblongata in urethanized cats. Acetylcholine (ACh), norepinephrine (NE) and 5-hydroxytryptamine (5-HT) were applied microiontophoretically to the immediate vicinity of the neurons. Both thermally sensitive and insensitive units responded to ACh (82.0%) and 5-HT (93.0%) by increasing discharge rate. Iontophoretically applied atropine, but not hexamethonium, antagonized the excitatory responses to ACh. NE was shown to have different effects on the medullary neurons. Most temperature-insensitive units were either nonresponsive (52.4%) or excited (28.6%) by NE, while a majority of warm-sensitive (61.8%) and cold-sensitive (55.6%) neurons were inhibited by NE. Iontophoretically application of the alpha-adrenergic blocking agent, phentolamine, or the beta-antagonists, propranolol, or sotalol, produced no effect on the inhibitory responses to NE. These results tend to support the current concept of the transmitter role of monoamines in thermoregulation.