GLUTAMATE IMMUNOREACTIVITY IN TERMINALS OF THE RETINOHYPOTHALAMIC TRACT OF THE BROWN NORWEGIAN RAT

The mammalian circadian pacemaker of the suprachiasmatic nucleus (SCN) is entrained to the environmental light-dark cycle via a retinal proj ection, the retinohypothalamic tract (RHT). Several studies suggest th at an excitatory amino acid, possibly glutamate, is involved in photic entrainment. However, it has not yet been established whether glutama te is a transmitter of the RHT itself. We have now identified terminal s of the RHT in the SCN of brown Norwegian rats by intravitreous injec tions of horse radish peroxidase conjugated to cholera toxin. To detec t glutamate immunoreactivity (IR), post-embedding immunocytochemistry was performed with polyclonal antibodies which were visualized for ele ctron microscopy with colloidal gold particles. Retinal terminals had a significantly 82% higher glutamate-IR than their post-synaptic dendr ites and a significantly 76% higher glutamate-IR than non-retinal term inals. These observations provide ultrastructural evidence that glutam ate is a transmitter of the RHT.