Leptin inhibits swallowing in the rat

Swallowing is under the control of premotoneurons located in the medullary solitary tract nucleus. While rats with transected mid-brain do not seek out food, they are able to ingest food present near the mouth, and acute food deprivation induces an increase in food intake. Leptin is a satiety signal that regulates feeding behavior. Since leptin receptors are found within the caudal brainstem, and since food intake is regulated in mid-brain transected rats, this study tested the hypothesis that leptin is able to modify the activity of premotoneurons involved in swallowing. Leptin was microinjected at the subpostremal level of the medullary solitary tract nucleus, in anaesthetized Wistar rats. Electromyographic electrodes in sublingual muscles allowed recording of swallowing induced by stimulation of sensitive fibers of the superior laryngeal nerve. Repeated stimulation induced rhythmic swallowing. Microinjection of leptin (0.1 pg and 0.1 ng) into the swallowing centre induced an inhibition of rhythmic swallowing (latency of less than 30 sec) as shown by the reduced number and strength of electromyographic activities which could last several minutes. The threshold of the leptin-induced inhibition was close to 0.1 pg. Interestingly, the inhibitory effect of leptin was not observed in leptin receptor deficient Zucker rats. Here we show that in Wistar rat, leptin already known to modulate the discharge of medullary solitary tract nucleus sensitive neurons involved in satiety reflexes, can also modify the activity of swallowing premotoneurons, thereby inhibiting an essential motor component of feeding behavior.