Anatomical evidence for brainstem circuits mediating feeding motor programs in the leopard frog, Rana pipiens

Using injections of small molecular weight fluorescein dextran amines, comb ined with activity-dependent uptake of sulforhodamine 101 (SR 101), brainst em circuits presumed to be involved in feeding motor output were investigat ed. As has been shown previously in other studies, projections to the cereb ellar nuclei were identified from the cerebellar cortex, the trigeminal mot or nucleus, and the vestibular nuclei. Results presented here suggest an ad ditional pathway from the hypoglossal motor nuclei to the cerebellar nucleu s as well as an afferent projection from the peripheral hypoglossal nerve t o the Purkinje cell layer of the cerebellar cortex. Injections in the cereb ellar cortex combined with retrograde labeling of the peripheral hypoglossa l nerve demonstrate anatomical convergence at the level of the medial retic ular formation. This suggests a possible integrative region for afferent fe edback from the hypoglossal nerve and information through the Purkinje cell layer of the cerebellar cortex. The activity-dependent uptake of SR101 add itionally suggests a reciprocal, polysynaptic pathway between this same are a of the medial reticular formation and the trigeminal motor nuclei. The tr igeminal motor neurons innervate the m adductor mandibulae, the primary mou th-closing muscle. The SR101 uptake clearly labeled the ventrolateral hypog lossal nuclei, the medial reticular formation, and the Purkinje cell layer of the cerebellar cortex. Unlike retrograde labeling of the peripheral hypo glossal nerve, stimulating the hypoglossal nerve while SR101 was bath-appli ed labeled trigeminal motor neurons. This, combined with the dextran labeli ng, suggests a reciprocal connection between the trigeminal motor nuclei an d the cerebellar nuclei, as well as the medulla. Taken together, these data are important for understanding the neurophysiological pathways used to co ordinate the proper timing of an extremely rapid, goal-directed movement an d may prove useful for elucidating some of the first principles of sensorim otor integration.