An anatomical study of brainstem projections to the trigeminal motor nucleus of lampreys

This study was undertaken to identify and describe populations of brainstem neurons that project to the area of the nucleus motorius nervi trigemini i n lampreys as a first step in the study of neurons that control feeding beh avior in this species. To identify these neurons, the retrograde tracer cob alt-lysine was injected into the nucleus motorius nervi trigemini on one si de of the in vitro isolated brainstem preparation of seven spawning adult l ampreys (Petromyzon marinus). Transport times ranged from 42 to 48 h. Retro gradely labeled neurons were found within the rostral spinal cord, the rhom bencephalon, the mesencephalon and the caudal diencephalon. This study conc entrates on the labeled neurons in the rhombencephalon, since the essential circuits for mastication and swallowing are confined to this region in hig her vertebrates. Within the rhombencephalon, labeled cells were in the nucl eus sensibilis nervi trigemini on both sides. A densely packed column of la beled neurons was found medial to the nucleus motorius nervi trigemini on t he ipsilateral side, extending further rostrally in the isthmic region. Con tinuous columns of labeled cells were observed in the lateral reticular for mation on each side in the basal plate ventral to rhombencephalic cranial m otor nuclei. They extended from the rostral trigeminal region down into the rostral spinal cord. A comparison with data from cats and rats shows that the distribution of ne urons that project to the nucleus motorius nervi trigemini is very similar in mammals and in agnathes. We conclude that the organization of the motor command network of the trigeminal system is well preserved throughout phylo geny and that the in vitro isolated brainstem of lampreys should be a usefu l model for the study of vertebrate feeding behavior. (C) 1999 IBRO. Publis hed by Elsevier Science Ltd.