GASTRIC DISTENSION MODULATES HYPOTHALAMIC NEURONS VIA A SYMPATHETIC AFFERENT PATH THROUGH THE MESENCEPHALIC PERIAQUEDUCTAL GRAY

The effects of gastric distension on extracellutarly recorded single n euronal activity in the lateral hypothalamus-lateral preoptic area-med ial forebrain bundle (LPA-LH-MFB), other areas of the hypothalamus, me sencephalic periaqueductal gray (FAG), and other areas associated with the mesencephalic reticular formation were determined in the anesthet ized rat. Gastric distension was produced by filling a gastric balloon with water using a calibrated infusion pump. Experimental conditions were based on previous studies that simulated gastric distension durin g fluid consumption in the rat. The effects of stomach distension usin g water at body temperature and room temperature were compared. Neuron s in both the hypothalamus and mesencephalon were modulated by gastric distension. Hypothalamic neurons exhibited responses associated with gastric distension and exhibited interactions between distension and t emperature stimulation of the stomach. Neurons in the mesencephalic pe riaqueductal gray (FAG) and associated reticular formation also were m odulated by these gastric stimuli. When the FAG was electrically stimu lated, similar responses to gastric distension and FAG stimulation wer e observed for hypothalamic neurons. The effects of gastric distension on hypothalamic neurons were reduced or eliminated when the FAG stimu lating electrode site was destroyed by electrocoagulation. In addition , the microiontophoretic application of horseradish peroxidase at hypo thalamic neuronal recording sites where gastric distension effects wer e observed resulted in the retrograde labeling of neurons in the FAG. These gastric stimulation-induced effects on hypothalamic and mesencep halic neuronal activity were attenuated but were not permanently elimi nated by bilateral cervical vagotomy. However, these effects were sign ificantly reduced or eliminated by bilateral transection of the cervic al sympathetic chain or spinal transection at the first cervical level . Because the filling of balloons placed into the abdominal cavity clo se to the stomach had no similar effects on neural activity, these res ults can be attributed primarily to the activation of gastric mechano- and temperature-sensitive receptors. These results indicate that the effects of gastric temperature/distension stimulation under these cond itions are mediated to a large degree by sympathetic afferents. The FA G is clearly involved as one of the mesencephalic relays for gastric a fferent input to the hypothalamus.