Role of efferent and afferent vagal nerve activity during reproduction: integrating function of oxytocin on metabolism and behaviour.

The vagal nerves convey information between the brain and the gastrointestinal tract in both afferent and efferent direction. The efferent vagal nerves regulate gastrointestinal secretory and motor function and also the activity in the endocrine system of the gut. Gastrointestinal function and anabolic metabolism is optimized during gestation and lactation as a means of saving energy for this calorie-demanding process. This enhanced activity seems to involve central oxytocinergic transmission which increases efferent vagal nerve activity as reflected by altered levels of gastrointestinal hormones in plasma. In parallel a behaviour pattern characterized by sedation is induced. In addition, vagal afferents transfer information from the gut to the brain, e.g. as to the presence or absence of calories in the gastrointestinal tract. It is well known that satiety and sedation following a meal is in part caused by an activation of vagal afferents in response to food intake. Cholecystokinin released from the intestine is one of the factors that triggers the afferent vagal nerve activity. Also reproductive "interactive behaviours" such as milk ejection and maternal behaviour can be induced by CCK via afferent activation of vagal nerves. In contrast, when no food reaches the gut, milk production and ejection is blocked. Parts of these effects are vagally mediated, since they can be observed in vagotomized rats receiving full amounts of calories.