The Journal of comparative neurology
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The intergeniculate leaflet (IGL) in the rat is a distinctive subdivision of the lateral geniculate complex that participates in the regulation of circadian function through its projections to the circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. The present investigation was undertaken to provide a precise definition of the IGL and a characterization of its neuronal organization including neuronal morphology, chemical phenotype, connections, and synaptic organization. The IGL extends the entire rostrocaudal length of the geniculate complex and contains a distinct population of small to medium neurons. ⋯ The synaptic organization of the IGL is complex with afferents terminating in glomerular complexes that include axoaxonic 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 and its connections as revealed in this analysis is in accord with its role in the integration of visual input with other information to provide feedback regulation of the SCN pacemaker.