Efferent projections of the intergeniculate leaflet and the ventral lateral geniculate nucleus in the hat

The intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleu s (VLG) are ventral thalamic derivatives within the lateral geniculate comp lex. In this study, IGL and VLG efferent projections mere compared by using anterograde transport of Phaseolus vulgaris-leucoagglutinin and retrograde transport of FluoroGold. Projections from the IGL and VLG leave the genicu late in four pathways. A dorsal pathway innervates the thalamic lateral dor sal nucleus (VLG), the reuniens and rhomboid nuclei (VLG and IGL), and the paraventricular nucleus (IGL). A ventral pathway runs through the geniculoh ypothalamic tract to the suprachiasmatic nucleus and the anterior hypothala mus (IGL). A medial pathway innervates the zona incerta and dorsal hypothal amus (VLG and IGL); the lateral hypothalamus and perifornical area (VLG); a nd the retrochiasmatic area (RCA), dorsomedial hypothalamic nucleus, and su bparaventricular zone (IGL). A caudal pathway projects medially to the post erior hypothalamic area and periaqueductal gray and caudally along the brac hium of the superior colliculus to the medial pretectal area and the nucleu s of the optic tract (IGL and VLG). Caudal IGL axons also terminate in the olivary pretectal nucleus, the superficial gray of the superior colliculus, and the lateral and dorsal terminal nuclei of the accessory optic system. Caudal VLG projections innervate the lateral posterior nucleus, the anterio r pretectal nucleus, the intermediate and deep gray of the superior collicu lus, the dorsal terminal nucleus, the midbrain lateral tegmental field, the interpeduncular nucleus, the ventral pontine reticular formation, the medi al and lateral pontine gray, the parabrachial region, and the accessory inf erior olive. This pattern of IGL and VLG projections is consistent with our understanding of the distinct functions of each of these ventral thalamic derivatives. J. Comp. Neurol. 420:398-418, 2000. 2000 Wiley-Liss, Inc.