INTERGENICULATE LEAFLET - AN ANATOMICALLY AND FUNCTIONALLY DISTINCT SUBDIVISION OF THE LATERAL GENICULATE COMPLEX

The intergeniculate leaflet (IGL) in the rat is a distinctive subdivis ion of the lateral geniculate complex that participates in the regulat ion of circadian function through its projections to the circadian pac emaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The pre sent investigation was undertaken to provide a precise definition of t he IGL and a characterization of its neuronal organization including n euronal morphology, chemical phenotype, connections, and synaptic orga nization. The IGL extends the entire rostrocaudal length of the genicu late complex and contains a distinct population of small to medium neu rons. In Golgi preparations, the neurons are multipolar with dendrites largely confined to the IGL. The neurons can be subdivided into three groups on the basis of neurotransmitter content and projections: (1) neurons that contain GABA and neuropeptide Y and project to the SCN; ( 2) neurons that contain GABA and enkephalin and project to the contral ateral IGL; and (3) a small group of neurons that projects to the SCN but not characterized as yet by neurotransmitter content. The IGL rece ives dense, bilateral input from retinal ganglion cells and dense subs tance P input of unknown origin. A number of neurons in the anterior h ypothalamic area and, particularly, the retrochiasmatic area project t o the IGL, and there are sparse projections from brainstem monoamine a nd cholinergic neurons. The synaptic organization of the IGL is comple x with afferents terminating in glomerular complexes that include axoa xonic synaptic interactions. Virtually all IGL afferents synapse upon dendrites and spines, with the densest synaptic input occurring on the distal portions of the dendritic arbor. The organization of the IGL a nd its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to prov ide feedback regulation of the SCN pacemaker, (C) 1991 Wiley-Liss, Inc .