Neurotransmitters of the suprachiasmatic nuclei

Abstract

There has been extensive research in the recent past looking into the molecular basis and mechanisms of the biological clock, situated in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus. Neurotransmitters are a very important component of SCN function. Thorough knowledge of neurotransmitters is not only essential for the understanding of the clock but also for the successful manipulation of the clock with experimental chemicals and therapeutical drugs. This article reviews the current knowledge about neurotransmitters in the SCN, including neurotransmitters that have been identified only recently. An attempt was made to describe the neurotransmitters and hormonal/diffusible signals of the SCN efference, which are necessary for the master clock to exert its overt function. The expression of robust circadian rhythms depends on the integrity of the biological clock and on the integration of thousands of individual cellular clocks found in the clock. Neurotransmitters are required at all levels, at the input, in the clock itself, and in its efferent output for the normal function of the clock. The relationship between neurotransmitter function and gene expression is also discussed because clock gene transcription forms the molecular basis of the clock and its working.

Figure 1

Afferent inputs and efferent pathways of the SCN. RHT: Retinohypothalamic tract, GHT: Geniculohypothalamic tract, OC: Optic chiasm, 3V: Third ventricle, IGL: Intergeniculate leaflet, DM: Dorsomedial SCN, VL: Ventrolateral SCN, NPY: Neuropeptide Y, GABA: Gamma amino butyric acid, PACAP: Pituitary adenylate cyclase-activating polypeptide.

Figure 2

Inter-relation and efferent outputs between central and peripheral oscillators. Notice the neural and diffusible control of the central oscillator. Peripheral oscillators respond to signals from SCN as well as to other inputs like periodic food availability. Diffusible output may have AVP, VIP, prokineticin, and TGF-α. RHT: Retinohypothalamic tract, dm: dorsomedial SCN, vl: ventrolateral SCN.