Abstract
Purpose: :
Bright light triggers reflexive blinking, tear secretion and pain. It has been proposed that these responses are mediated by a circuit linking the olivary pretectal nucleus (OPN; a major target of intrinsically photosensitive retinal ganglion cells or ipRGCs) and the superior salivatory nucleus (SSN) or facial motor nucleus (FMN). We sought anatomical evidence for this proposed circuit in mice.
Methods: :
Axonal tracers were injected into brains of anesthetized adult mice by stereotaxis. For anterograde tracing from the OPN, we used iontophoretic injection of biotinylated dextran amine (BDA). For retrograde tracing from the SSN/FMN, we pressure-injected Alexa-488-tagged cholera toxin beta subunit. After1-4 days, mice were perfused with 4% paraformaldehyde and their brains sectioned on a vibratome at 50µm. To visualize BDA, sections were incubated overnight in streptavidin-Alexa 488 conjugate.
Results: :
Our data challenge the existence of a direct pathway between the OPN and the SSN or FMN. Large retrograde tracer deposits encompassing SSN/FMN failed to label any OPN neurons, though cells nearby in the periaqueductal grey (PAG) and red nucleus were labeled. Anterograde tracer injections into OPN labeled descending fibers extending as far caudal as the dorsolateral pontine gray, but no fibers reached the SSN/FMN. OPN efferents did terminate in many other targets. Some of the most prominent of these are known to receive input from M1-type ipRGCs, including the ventral lateral geniculate nucleus, intergeniculate leaflet, contralateral OPN and periaqueductal grey (PAG). We confirmed OPN projections to the Edinger-Westphal nucleus, a key link in a circuit by which ipRGCs drive pupillary constriction. Other OPN targets identified included the dorsal lateral geniculate, superior colliculus, accessory oculomotor complex, zona incerta, and midbrain tegmentum just lateral to the red nucleus
Conclusions: :
In the mouse, bright light cannot trigger blinking, tears, or ocular pain thought a direct link between the OPN and the SSN/FMN complex. However, the two structures may be linked by a disynaptic pathway involving the PAG.
Keywords: accessory optic system/pretectum • anatomy