METHODS

Mature male rats of the Long Evans strain, weighing from 250 to 300 g, were used. They were fasted, but given free access to water for 24 hr prior to the experiment. Urethan (0.6 ml/100 g body wt; 25%, weight to volume) was injected intraperitoneally to anesthetize the animals. A small (Clay Adams no. 10) polyethylene tube was inserted into the portal vein just before it branches into the various lobes of the liver. The size of this tube was such that it could lie within the vessel and be held in place by a surgical suture without impeding portal system flow, which approximates 10-l 5 ml/min. It was found that when the food-laden blood could not flow freely through the mesenteric veins, the gut area quickly became black with backed-up blood and the animal did not long survive. The carotid artery was cannulated for blood pressure recording; a Statham pressure transducer and Grass recorder were used for this purpose. Taking advantage of the same ventral incision in the neck, loops of thread were placed around the vagi so that effects of vagotomy could be studied in certain experiments. The trachea was cannulated to permit adequate ventilation after the animal’s head was fixed in the head holder. The stereotaxic apparatus (Baltimore Instrument Company) was so arranged that when the ear plugs were positioned in the external auditory meatus and the upper incisors were fit snugly over the upper incisor bar, 5 mm above the interaural line, the coordinates of the DeGroot atlas (8) could be used for accurate placement of electrodes in the hypothalamus. Once the head was rigidly positioned in the head holder, a midline skin incision was made and the dorsal surface of the brain was exposed in the region of the bregma* Microelectrodes could thus be introduced at several points. In certain experiments, the splanchnic nerves were also