Feeding behavior in mammals: corticobulbar projection is reorganized during conversion from sucking to chewing.

It is known that repetitive stimulation of the frontal cortex (cortical masticatory area, CMA) induces rhythmical jaw movements similar to chewing in adult mammals. In the present study we were able to induce rhythmical jaw movements similar to sucking by repetitive stimulation of the frontal cortex in neonatal guinea pigs. This area, which we named the cortical sucking area (CSA), was located rostral to the CMA which was later formed upon maturation. Neurons of the CSA were shown electrophysiologically and morphologically to project primarily to the dorsal part of the paragigantocellular reticular nucleus of the contralateral side. This was the site which the CMA neurons, later, projected to induce chewing. It is generally thought that tooth eruption triggers the conversion from sucking to chewing. However, guinea pigs are born with a complete permanent dentition and therefore devoid of this peripheral trigger for the conversion to chewing. Accordingly we propose that shift of the cortical projection area from the CSA to the CMA during the maturation causes the conversion of the mammalian feeding behavior. It is discussed that this transition involves extensive reorganization of the cortical efferent system including the pyramidal tract during early postnatal development.