A subpopulation of efferent neurons in the mouse suprachiasmatic nucleus is also light responsive.

The suprachiasmatic nucleus (SCN) is the site of a circadian clock with input (afferent) pathways for photic entrainment and output (efferent) pathways for expression of overt, measurable rhythms. To determine whether there are individual neurons in the mouse SCN that might be part of both pathways, we performed double-label immunohistochemistry for light-induced c-Fos and the retrograde tracer cholera toxin subunit B (CtB), 2 weeks after CtB was iontophoresed into the subparaventricular area (subPVA). A minority of neurons was found that were both efferent to the subPVA and responsive to light. This cellular subset may function as a direct channel through the SCN for photic inputs to influence neural outputs, and its existence highlights the topographical heterogeneity of SCN tissue.