Interconnections among nuclei of the subcortical visual shell: The intergeniculate leaflet is a major constituent of the hamster subcortical visual system

Abstract

The intergeniculate leaflet (IGL), a major constituent of the circadian visual system, is one of 12 retinorecipient nuclei forming a "subcortical visual shell" overlying the diencephalic-mesencephalic border. The present investigation evaluated IGL connections with nuclei of the subcortical visual shell and determined the extent of interconnectivity between these nuclei. Male hamsters received stereotaxic, iontophoretic injections of the retrograde tracer, cholera toxin beta fragment, or the anterograde tracer, Phaseolus vulgaris-leucoagglutin, into nuclei of the pretectum (medial, commissural, posterior, olivary, anterior, nucleus of the optic tract, posterior limitans), into the superior colliculus, or into the visual thalamic nuclei (lateral posterior, dorsal lateral geniculate, intergeniculate leaflet, ventral lateral geniculate). Retrogradely labeled cell bodies identified nuclei with afferents projecting to the site of injection, whereas the presence of anterogradely labeled fibers with terminals revealed brain nuclei targeted by neurons at the site of injection. The IGL projects bilaterally to all nuclei of the visual shell except the lateral posterior and dorsal lateral geniculate nuclei. The IGL also has afferents from the same set of nuclei, except the nucleus of the optic tract. The extensive bilateral efferent projections distinguish IGL from the ventral lateral geniculate nucleus. The superior colliculus, commissural pretectal, olivary pretectal, and posterior pretectal nuclei also project bilaterally to the majority of subcortical visual nuclei. The IGL has a well-established role in circadian rhythm regulation, but there is as yet no known function for it in the larger context of the subcortical visual system, much of which is involved in oculomotor control.

Differential firing pattern and response to lighting conditions of rat intergeniculate leaflet neurons projecting to suprachiasmatic nucleus or contralateral intergeniculate leaflet

“…The intergeniculate leaflet (IGL) is a part of the subcortical visual system, formed by the retinorecipient nuclei in the hypothalamus, the lateral geniculate complex, the pretectum and the superior colliculus (Morin and Blanchard, 1998;Marchant and Morin, 1999). The main function of the system is to mediate the acute reactions to light and the adjustment of circadian rhythms by photic and nonphotic cues.…”

“…The latter is, so far, the only well-established function of the IGL (for review see Harrington, 1997;Morin and Allen, 2006). However, extensive, bilateral and reciprocal connections of the IGL with the majority of nuclei of the sleep/arousal and visuomotor systems suggest its participation in other functions and behaviours (Morin and Blanchard, 1998Marchant and Morin, 1999;Horowitz et al, 2004).…”

Neuroanatomy of the extended circadian rhythm system

“…Retrograde tracer injected into the IGL labels sparse cells on or adjacent to the dorsal and lateral borders of the caudal SCN, with more in the retrochiasmatic area (Morin, et al, 1992; Morin and Blanchard, 1999), but not in the SCN proper. Retrogradely labeled neurons are seen in the same areas after tracer injection into the hamster IGL, ventrolateral geniculate (VLG), posterior limitans nucleus (PLi), olivary pretectal nucleus (OPT), medial pretectal nucleus (MPT) and commissural pretectal area (CPT) (Morin and Blanchard, 1998). Visualization of such a SCN-IGL projection by anterograde tracing methods may be a false positive resulting because of tracer spillage from the injection site into adjacent hypothalamus or, just as likely, from uptake by peri-SCN neurons which have dendrites extending into the nucleus.…”

“…Electrical stimulation of the same region greatly attenuates light-induced phase shifts. These subcortical visual areas have substantial interconnections with the IGL (Morin and Blanchard, 1998), possibly accounting for the ability of pretectal/tectal and IGL lesions to block the response to triazolam. In the rat, the phenomenon of dark-induced REM sleep is also prevented by lesions of the pretectum/superior colliculus (Miller, et al, 1998; Miller, et al, 1999), but there is no information as to whether the IGL is a part of the sleep-regulatory circuitry.…”

Normal behavioral responses to light and darkness and the pupillary light reflex are dependent upon the olivary pretectal nucleus in the diurnal Nile grass rat

“…The role of the VLG in masking is currently unknown. This nucleus has extensive connections with visual and non-visual brain regions, including the OPT (Conley and Friederich-Ecsy, 1993; Morin and Blanchard, 1998; Livingston and Mustari, 2000) and participates in the control of visuomotor responses (Harrington, 1997). In laboratory rats the VLG is involved in eye blink conditioning using light as the conditioned stimulus, suggesting its importance for receiving light input to mediate associative cerebellar learning (Halverson and Freeman, 2010).…”