Module 6: Figure circadian clock location
Location of the circadian clocks in the suprachiasmatic nucleus (SCN).
The clock neurons within the suprachiasmatic nucleus (SCN) are not homogenous, but fall into two main groups. There are ventral core neurons that receive most of the input (yellow arrow) from the retina that travels along the retinohypothalamic tract (RHT). These neurons send out projections to the dorsal shell SCN neurons, which are mainly responsible for the output signals (red arrow) that leave the SCN. The insets on the right illustrate a simplified version of the neural circuit within the SCN. The RHT neurons release glutamate that acts through the N-methyl-D-aspartate receptor (NMDAR) and metabotropic glutamate receptor 1 (mGluR1) on the core neurons. The latter express a number of transmitters such as γ-aminobutyric acid (GABA), vasoactive intestinal peptide (VIP) and substance P (SP), which act on the dorsal shell neurons. These neurons express vasopressin, which is one of the output signals they release from the neurons that leave the SCN. Both the core and shell neurons have Ca2+ and K+ channels that generate the electrical activity responsible for releasing these neurotransmitters. The ventral neurons stained with an antibody against VIP are shown in panel c in Module 6: Figure suprachiasmatic nucleus.