BISC 421 1st Edition Lecture 30 Outline of Current Lecture1. Circadian Rhythm and Sleep Part ICurrent LectureCircadian Rhythm and Sleep Part I•Sort of like a 6th sense-‐ the sense of time blue is awake, red is sleepFree running In the absence of cues, period of sleep/wake cycle is 25 - 26 hrs in humans and 23 hrs in mice.•Initally the students were allowed to go to bed and wake up normally and they had external cues•Then they took away the external cues – no alarm clocks no lightng cues-‐ they started sleepinglater and later as the experiment went on-‐ each day slept about an hour later than the previous day•The natural rhythm is 26 hours in absence of any cues (for mice its 23 hours)•If you give back the cues they go back to normal•1. Intrinsic circadian rhythm in the absence of cues•2. In the presence of cues we entrain our system to match up to our environment•This is present in about everyhormone cycle in our body•Circadian rhythm also in temperature-‐ temp at night is slightly lower than during the day•There are also stress hormones•Almost everything in our body is entrained to this 24 hour cycleWhere is the central clock?!•In the absence of cues where is the circadian rhythm of sleep?•This is where it is•Tiny little blue ball that is part of the hypothalamus – called the suprachiasmatc nucleus (SCN)•Right above the optc chiasm and is remarkably smallIsolated SCN neurons maintain 24 hr circadian rhythm!•Evidence that the SCN is out master clock•If we take these cells out and put them on a dish they will fire acton potentals in response to a 24 hour rhythm•The evidence that they are the master clock is that if you take out the SCN you willIsolated SCN neurons maintain 24 hr circadian rhythm!•Can transplant SCNs from one animal to another and adopt the SCN cycle of the other animal•A mutant would show some kind of disrupton in circadian rhythm•The other method that has been used to study circadian rhythm is to look at rodents that have been identfied to have altered circadian rhythms mutatons and this has led to study the conservaton of the genes•Per is one of the genes-‐ some of the first evidence that per plays a role in circadian rhythm•Study showing that the mRNA for per varies in abundance as a functon of the tme of day•Now the mRNA is being translated into the protein and this is when the animal knows to go to sleepmRNAProtein•We can see quantzaton of the mRNA as a functon of circadian rhythm-‐ they go up at the end of the light cycle and then they fall at the end of the night and then go up againTranscription factors can affect expression of many genes•Promoter regions bind the transcripton factors which are what in fact promote the generation of the mRNA-‐ central master control of the transcripton of the mRNA•Per will be transcribed into mRNA and the transcripton factor may bind to manygenes and cause transcripton of many diferent proteins•What is causing this mRNA to cycle like this?•What then trains that cycling to the light-‐dark cycle?•Per is the transcripton factor-‐ important because one element of how this clock is generated is shown here•If we consider this Per gene, its mRNA will be transported out of the nucleus and made into protein and then goes back into the nucleus and inhibits its own transcription-‐ negatve feedback system•This generates some kind of rhythm•Cry forms a heterodimer with Per and they both inhibit their own transcription.What turns on CLK and Bmal•So what turns this all back on again?•There is another pair of transcripton factors called Bmal and CLKthe E box is the binding site for CLK and Bmal and they turn on Perand Cry•When per and cry bind CLK and Bmal and this turns off expression•Heterodimer per and cry are responsible for turning off their own expression•The mammalian version of cycle is called Bmal•Clock and Bmal actvated cry and per and their proteins are generated and then forma hetero dimmer and inhibit clock and Bmal•Just a giant loop•Input from the visual system is actng on this transcriptonal network•Takes 24 hours because the mRNA takes some tme to make the proteins•The light helps it be at the right point in the cycle-‐ why it is messed up without lightInput from the visual system to the SCN-‐ this is through a system meant only for this functon•People show entrainment to light and they show that their circadian rhythm will entrain even with absence of rods and cones-‐ third class of photosensitve cell•Observaton that melanopsin-‐ light sensitve receptor that is actvated by light and was first identfied in frog skin and then later found in a mammalian retna•Evidence of another pigment in the eye that contributes to these non visual responses to light Hattar et al, Science, 2002•Learn the most about these cells through genetc approaches•Which cells express melanopsin? Ganglion cells not photoreceptors•Melanopsin ganglion cells project to the SCN-‐ terminate in the SCN•Cells coming from the retna and project directly to the SCN•First experiment people did was to knock out the melanopsin gene-‐ results were not completely black and white•Entrainment is altered but not completely gone•Whether these cells are the cells that are necessary for entrainment not just melanopsin•Genetcally altered the mice so that the melanopsin cells would be killed if exposed to DT•Found that by killing these melanopsin cells – initally the mouse shows nice entrainment then when injected with DT find that the animals start to show no entrainment to the light• Patents go to sleep at 6 PM and awake at 4 AM• Associated with subtle mutatons in the Per gene thatcause it to accumulate more quickly, hence shortening the clockOther “clock mutatons” may cause delayed sleep phase syndrome •We can see that people with weird sleep behavior have mutatons in their clock genes•One is familial advanced sleep phase syndrome-‐ go to sleep early and wake up early•Mutaton in a per gene that causes it to accumulate more quickly therefore shortening the clock•Delayed sleep phase system-‐ opposite