Circadian rhythms are entrained daily by environmental photic and non-photic cues. The present review describes the anatomy and functional characteristics of the three major input pathways to the circadian clock mediating entrainment, the retino-hypothalamic tract (RHT), the geniculo-hypothalamic tract (GHT) and the midbrain raphe projection.

Zusammenfassung

Rhythmic changes in physiology and behaviour within a 24 h period occur in living organisms on earth to meet the challenges associated with the daily changes in the external environment. The circadian pacemaker responsible for the temporal internal organisation and the generation of endogenous rhythms of approximately 24 h is located in the hypothalamic suprachiasmatic nucleus (SCN) in mammals. The endogenous period generated by the pacemaker is close to, but generally not equal to 24 h and the biological clock therefore needs to be daily adjusted (entrained) by external cues. The daily alteration of light and darkness due to the rotation of our planet on its own axis in relation to the sun is the most prominent "zeitgeber" which adjusts the phase of the circadian rhythms to the astronomical day length, a process known as photoentrainment. In mammals, light is perceived only through photoreceptors located in the retina. Light information is mediated to the SCN via the retinohypothalamic tract (RHT) by activation of the classical photoreceptor system of rods and cones and a more recently identified system of intrinsic photosensitive retinal ganglion cells (ipRGCs) using melanopsin as a photopigment.

Inhalt

Introduction.- The suprachiasmatic nucleus (SCN).- The endogenous rhythm of the SCN.- The SCN molecular clock.- Photic regulation of the circadian rhythm.- Retinal input Neuroanatomy.- Neurotransmitters in the RHT.- Glutamate and PACAP.- Other neurotransmitters of the RHT.- RHT neurotransmitter receptors in the SCN.-Melanopsin - an irradiance detecting photopigment of the RHT.- Retinal input - Physiology.- Non-photic regulation of the circadian rhythms.- Midbrain raphe input - Neuroanatomy.- 5-HT and regulation of the circadian system.- The intergeniculate leaflet and the geniculohypothalamic tract neuroanatomy.- Effects of NPY on circadian rhythm during subjective day.- NPY modulates light-induced phase shift during subjective night.- Summary.