Putative excitatory amino acid projections to the suprachiasmatic nucleus in the rat

Retrograde axonal transport of the select neuronal tracer [H-3]D-aspartate was used to demonstrate possible sources of excitatory input to the suprachiasmatic nucleus (SCN) in the albino rat. Following injection of [H-3]D-aspartate into the SCN, neurons were retrogradely labeled in the infralimbic cortex, the lateral septal nucleus, the paraventricular thalamic nucleus, the medial preoptic area, the ventromedial, dorsomedial and posterior hypothalamic nuclei, the zona incerta, the intergeniculate leaflet and the ventral subiculum. Retinal ganglion cells, which project to the SCN and use glutamate as a neurotransmitter, were not labeled in our [H-3]D-aspartate experiments, demonstrating a limitation of this method (i.e., false negatives). Our results show that the [H-3]D-aspartate neuronal tracer labels a subset of areas known to project to the SCN, indicating these areas as likely sources of excitatory input to the SCN.