Retinohypothalamic tract

Figure 22.3 Possible links in the induction of circadian rhythm between daylight, the suprachiasmatic nucleus and melatonin release from the pineal gland. Some fibres in the optic nerve, projecting from the eye to the lateral geniculate nucleus (LGN) in the thalamus, innervate the suprachiasmatic nucleus (SCN) in the anterior hypothalamus, via the retinohypothalamic tract (RHT). Others project to the SCN from the LGN in the geniculohypothalamic tract (GHT). The release of melatonin into the circulation from the pineal gland (PG) is maximal at night and appears to be controlled partly by noradrenaline released from sympathetic nerves originating in the superior cervical ganglion (SCG). Melatonin receptors are found in the SCN, the removal of which dampens melatonin secretion...

Since most excitatory transmission is mediated by glutamate this must be involved in the sleep-waking cycle. It certainly mediates the input of the retinohypothalamic tract to the SCN, apart from afferent inputs more generally to the ARAS, etc. So far, specific in vivo manipulation of the direct glutamate input to the SCN has not been possible. 

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the retina exclusively within the RGCs of the retinohypothalamic tract (RHT), and melanopsin was found to co-localize with PACAP in the retina (Hannibal et al... 

Hannibal J, Hindersson P, Knudsen SM, Georg B, Fahrenkrug J 2002 The photopigment melanopsin is exclusively present in pituitary adenylate cyclase-activating polypeptide-containing retinal ganglion cells of the retinohypothalamic tract. J Neurosci 22 RC191... 

Moore RY, Speh JC, Card JP 1995 The retinohypothalamic tract originates from a distinct subset of retinal ganglion cells. J Comp Neurol 352 351—366... 

Moore RY, Speh JC, Card JP 1995 The retinohypothalamic tract originates from a distinct subset of retinal ganglion cells. J Comp Neurol 352 351—366 Morris ME, Viswanathan N, Kuhlman S, Davis PC, Weitz CJ 1998 A screen for genes induced in the suprachiasmatic nucleus by light. Science 279 1544—1547 Mrosovsky N 1999 Masking history, definitions, and measurement. Chronobiol Int 16 415-429... 

It is now known that circadian rhythms are controlled by clock genes which are found in species as wide apart as insects and mammals. It would appear that the clock genes are activated by light falling on the retina. The activated retina neurons then stimulate the retinohypothalamic tract which projects to the suprachiasmatic nucleus and thence to the anterior pituitary. This pathway is responsible for coupling the circadian rhythm with the... 

De Vries MJ, Nunes-Cardoso B, Van der Want J, De Wolf A, Meijer JH (1993) Glutamate immunoreactivity in terminals of the retinohypothalamic tract of the brown Norwegian rat. Brain Res 672 231-237. 

Miyamoto, Y., and Sancar, A. (1998). Vitamin B2-based blue-light photoreceptors in the retinohypothalamic tract as the photoactive pigments for setting the circadian clock in mammals. Proc. Natl. Acad. Sci. USA 95, 6097-6102. 

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