The Suprachiasmatic Nucleus and the Circadian Timing System - Progress in Molecular Biology and Translational Science - page 7

driving rhythmicity originated in the forebrain and raised the question in my
mind of whether we could approach the issue of localizing the control of
mammalian circadian function. A number of studies had shown that the lat-
eral eyes are necessary for entrainment in mammals. The general outline for a
circadian system in Fig. 1.1 requires that the brain mechanisms for mamma-
lian circadian rhythm generation and regulation must have three parts: pho-
toreceptors, entrainment pathways that are a component of central retinal
projections, and brain pacemakers with output to systems under circadian
control. As no other optic photoreceptors were known at the time, we
assumed that rod and/or cone photoreception was the mechanism for con-
verting light to neural information. The localization question, then, was
identification of the retinal projections mediating entrainment. It seemed
likely that the retinal projection would be to hypothalamus as circadian
timing is clearly a regulatory function. The most recent study of retinal pro-
jections in the ra t 40 available at that time had used the Nauta stain (Refs.
41,147 ) . This was the most powerful method for tract tracing, and it had
not revealed evidence for retinohypothalamic projections. Indeed, there
was general consensus among experts on the projections of retinal ganglion
cells that there were no projections to hypothalamus. 42-44 We employed a
new variant of the Nauta stain, the Fink-Heimer method, 45 and found tan-
talizing hints but no definitive evidence for a projection. It was at this time,
however, that axoplasmic transport was discovered using the technique of
applying tritiated amino acids near or in a neuronal population and following
their transport, after incorporation into protein, to axon terminals by mea-
suring either radioactivity or autoradiography. 46,47 Grafstein's work using
intraocular injections of a tritiated amino acid to trace retinal projections
in the frog, in particular, suggested that the existence of retinohypothalamic
projections could be explored with this methodology in mammals. With
Nicholas Lenn, then a graduate student, we carried out the autoradiographic
experiment and found evidence for a retinal projection to the ventral por-
tion of the SCN, but not to other medial hypothalamic nuclei, and con-
firmed the projection by showing degenerating axon terminals by
electron microscopy in the same area after eye removal. 48 Similar findings
were published at the same time by Hendrickson. 49 During this period,
I also demonstrated that the retinohypothalamic tract (RHT) was a consis-
tent feature of the mammalian visual system finding it in six other mamma-
lian species from metatherians to primates. 50 This did not prove definitively
that the RHT was the entrainment pathway but a selective transection of the
RHT was not possible technically at the time, and it seemed to me that the
next obvious experiment was to ablate the SCN and study the effects on