Oxytocin increased intragastric pressure in the forestomach of rats via the dorsal vagal complex

We previously reported that appetite-enhancing peptides facilitated phasic contractions of the distal stomach and relaxed the forestomach via the dorsal vagal complex (DVC). The present study investigated the effects of anorectic substances on gastric reservoir function. The effects of oxytocin on the motility of the forestomach were examined in rats anesthetized with urethane–chloralose. Gastric motor responses were measured using an intragastric balloon. The fourth ventricular administration of oxytocin (0.1 - 1.0 nmol) increased intragastric pressure (IGP) in the forestomach in a dose-dependent manner. Conversely, the administration of oxytocin (0.3 nmol) suppressed phasic contractions of the distal stomach. These responses were opposite to those of appetite-enhancing peptides in previous studies. The oxytocin response in the forestomach was not observed after bilateral cervical vagotomy. The effects of oxytocin on forestomach motility were examined in animals that underwent ablation of the area postrema (AP) to clarify its involvement. Although the magnitude of the response to the fourth ventricular administration of oxytocin decreased, a significant response was still observed. A microinjection of oxytocin (3 pmol) into the AP, the left medial nucleus of the nucleus tractus solitarius (mNTS), the left commissural part of the NTS, or the left dorsal motor nucleus of the vagus was performed. The oxytocin injection into the AP and/or mNTS induced a rapid and large increase in IGP in the forestomach. Prior injection of L-368,899, an oxytocin receptor antagonist, into both the AP and mNTS attenuated the oxytocin response of the forestomach induced by fourth ventricular administration of oxytocin. These results indicate that oxytocin acts on the AP and/or mNTS to increase IGP in the forestomach via vagal preganglionic neurons.