58 Nevertheless, these experiments did not unequivocally discriminate between a pacemaker and a relay function of the SCN. The breakthrough was accomplished by transplantation experiments by Ralph and coworkers, using wild-type and Tau mutant hamsters that free-ran with a period length of 24 and 20 hours, respectively, when kept in selleck chem constant darkness.59 Fetal SCN tissue grafted Into the third ventricle of SCN-lesioned animals rescued clrcadlan rhythms In locomotor activity, and the period length was determined by the donor tissue. These results

clearly Identified the SCN as the central Inhibitors,research,lifescience,medical clrcadlan pacemakers In mammals, several years before the first mammalian clock genes were Identified. Subsequently, organ and cell culture experiments indicated that circadlan rhythm generation is a cell-autonomous 17-DMAG side effects property Inhibitors,research,lifescience,medical of SCN neurons. However, although dissociated SCN neurons displayed

robust rhythms In electrical firing frequency, the Intercellular variability In period lengths was enormous.60, 61 Hence, In Intact animals cellular SCN oscillators must be coupled by Intercellular communication. Oscillator Inhibitors,research,lifescience,medical coupling Is not only important for the synchronization of Individual neurons, but also renders SCN neurons much more resilient to genetic perturbations. Kay and colleagues have recently shown that mPERldeficient SCN neurons lose their rhythm In clock gene expression when cultured as Individual cells, but exhibit robust dally cycles In gene expression when kept In organotypic slice cultures.62 Cellular crosstalk probably Involves both neuronal and paracrine signaling. The Importance Inhibitors,research,lifescience,medical of the latter has been revealed by gene knockout experiments. For example, mice deficient for the vasoactive intestinal peptide (VIP) and pituitary adenylate

cyclase activating peptide (PACAP)receptor VPAC2 are nearly arrhythmic, In spite of ongoing rhythms in Individual cells.63, 64 Since the SCN can measure time only approximately, It must be resynchronized dally. This synchronization Is accomplished by the photoperiod via conventional rod and cone Inhibitors,research,lifescience,medical photoreceptors In the outer retinal layer and, In addition, a very small fraction of melanopsin containing ganglion cells In the Inner retina.65 Mice devoid of rods and cones are visually blind, but owing to melanopsin containing ganglion cells they can still synchronlze their circadian clocks. AV-951 Only when the melanopsin gene Is disrupted In these mice are they free-running with their intrinsic period length, when kept In dally light-dark cycles. Photic cues perceived In the retina are transmitted to the SCN via the retlna-hypothalamic tract. Synaptic release of glutamate and PACAP leads to an Influx of Ca++. This triggers the activation of a variety of protein kinases In postsynaptic SCN neurons, which In turn elicits the activation of immediate early transcription factors, such as cyclic adenosine monophosphate (cAMP) response element binding protein (CREB).