The hamster intergeniculate leaflet (IGL), part of
the circadian rhythm regulatory system, has very extensive
interconnections with subcortical visual nuclei. The present
investigation describes IGL connections with the hamster
diencephalon and telencephalon and compares them with ventral
lateral geniculate nucleus (VLG) connections and retinal
projections. Connections of the geniculate nuclei were
evaluated using anterograde transport of iontophoretically
injected Phaseolus vulgaris leucoagglutinin and
by retrograde transport of cholera toxin β fragment.
The cholera fragment was also injected intraocularly to
trace retinal efferents. The IGL has ipsilateral and contralateral
projections to the anterior and posterior hypothalamic
nuclei, the ventral preoptic, lateral and dorsal hypothalamic
areas, but not to the core ventromedial nucleus and very
sparsely to the paraventricular nucleus. There are also
IGL projections to the medial and lateral zona incerta,
anteroventral, anterodorsal, reuniens, parataenial, paraventricular,
centrolateral, central medial, and laterodorsal thalamic
nuclei. IGL projections to the telencephalon are found
in the horizontal limb of the diagonal band, olfactory
tubercle, nucleus of the lateral olfactory tract, posterior
bed nucleus of the stria terminalis, ventral pallidum,
and in nuclei of the medial amygdala. The only substantial
VLG projections are to bed nucleus of the stria terminalis,
IGL, medial zona incerta, central medial and laterodorsal
thalamic nuclei. Several of the IGL targets, the bed nucleus
of the stria terminalis and zona incerta in particular,
send projections back to the IGL and VLG. In addition,
cells are present in the caudal cingulate cortex that project
to both nuclei. Retinal projections are found in many of
the regions receiving IGL innervation, including nuclei
of the medial basal telencephalon, the posteromedial bed
nucleus of the stria terminalis, and nuclei of the hypothalamus.
A retinal projection is also visible in the lateral olfactory
tract from which it extends rostrally, then medially along
the base of the rhinal fissure. Fibers also extend caudally,
in a superficial location, to perirhinal cortex. The results
further demonstrate the widespread connections of the IGL
and support the idea that the IGL modulates olfactory,
photic, and circadian rhythm regulation of regulatory physiology
and behavior.
Thalamic Input to Orbitofrontal Cortex Drives Brain-wide, Frequency-Dependent Inhibition Mediated by GABA and Zona Incerta
