BISC 421 1st Edition Lecture 28 Outline of Current LectureI.Olfactory System II. TasteCurrent Lecture•Some of the outstanding mysteries in the olfactory system •What causes those olfactory sensory neurons to converge in the olfactory bulb? Oneof the main questions •Example of how specific cells wire up to each other •One hint is that there is some evidence that the odorant receptor itself is a key factor•The olfactory epithelium projectng to the olfactory bulbhigher brain centers•Injected a olfactory bulb that corresponds to partcular odorant receptors to see where the glomeruli projected toMitral/tufted cells connected to a single glomerulus project to several areas of the olfactorycortex.•Labeled the cells that project to just one glomerulus and can see the projecton of this microcell as it projects to the brainHow many tastes?• Western culture – Five tastes: Bitter, Sweet, Umami, Sour, Salty• We don’t know when the first four tastes were identfied: Aristotle wrote that all tastes derived from mixtures of bitter and sweet (but he also thought all colors came from mixtures of black and white).• Umami (meaty/savory) discovered in 1908 by Japanese researcher Kilunae Ikeda who isolatedthe actve ingredient from seaweed and found it was glutatmate.• Chinese culture – five tastes (water, fire, wood, metal and earth) credited to Yi Yin (16th C BC).• based on work of a German Ph.D. student at the turn of the century is probably wrong.•When we talk about taste we are not talking about complex taste but rather the 5 basic tastes on the tongue or the palate•How do we know that there are 5 tastes? Not clear•Umami is a savory taste that is not consciously perceived-‐ makes things taste more delicious•Seaweed is high in glutamate and this is what causes the taste of umami•Taste map shows that diferent parts of the tongue are more sensitve to certain tastes than others: IT IS WRONG and oversimplified•2 basic categories: attractve (sweet umami and salty) and aversive (sour and bitter)•in other animals sour is very aversive but in humans it is attractve because we need the vitaminC in citrus•bitter is aversive in all animals because it means there is something toxic in what we are eating•The tongue is not uniformly covered with taste buds (a lot in the back, some on sides, some on front, and some on roof•The taste bud itself is called a bud because it is a ball of 50-‐100 modified epithelial cells – do not have an axons•Send a projecton to the taste core which has microvillae (similar to cilia in that they increase surface area)•Specific subset of cells that respond to diferent tastes•G protein coupled receptors for sweet and umami (amino acid)•Know that the sweet receptor is composed of 2 subunits (T1R2 and T1R3)•Umami also has 2 subunits (T1R1 and T1R3)•How do we know that these partcular molecules are the taste receptors?•Sweet receptor: cloned using genetc research•Identfied from a mouse that couldn't taste sweet-‐ figured out which gene was defectve in this mouse•Can tranfect things into tssue culture cells (don't respond to a lot of things) introduce the receptor that you think is the sweet receptor and see if it is sweet•Need some way of recording if the cell is responding to sweet-‐ use elevaton of calcium in response to sweet•This pathway leads to the opening of an ion channel in the cell and released ca2+ that is stored in the cell•This is how the experiment looks•See the rise in calcium released from stores by the appearance of green•Low calcium = blue•T1R3 is just a co subunit-‐ needs T1R2 to actvate these cells-‐ need both•If you have high concentratons of Sucrose, T1R3 will act alone though•Need very high concentratons of sucrose to get a robust response•Eliminate the gene in the mouth and see if it efects taste•These are experiments where they are recording from a nerve•These are all of the sweeteners that the mouse responds to•if you knock out T1R2 the response is eliminated•If you knock out T1R3 the response is also eliminated• Receptors are T2Rs (family of~30 GPCRs)• Subset of cells express bitter receptors –T2Rs• ~30 receptors, each sensitve to different bitter tastes (quinine, caffeine, etc).• All receptors are expressed in the same cells (so we cannot distnguish different bitter tastes).•Bitter is also coupled with a G protein coupled receptor•There are a lot of these receptors (30 diferent types of T2Rs receptors-‐ # of genes)•Not all cells make bitter receptors only some•All of the receptors are expressed in the same cell – in this way we can't distnguish between bitter tastes because when the cell binds that ligand it doesn't know which one it found because it has receptors for all of the diferent bitter tastes•Probably because we don't need to discriminate, we just need to avoid it.•Made a mouse missing TRPM5 gene-‐ they are normal except for taste where the efect is very dramatic•See that normal wild type animals like sucrose and glutamate a lot where as the knock out animals don't•The opposite happens with the bitter taste, wild type avoids and the knock out doesn't have a preference as compared to water•Cell type and not receptor identty determines the behavioral outcome (attractve or aversion)•Taste is hard wired-‐ not learned•First they recorded responses in wild type animals, see that sweet and umami are attractve and bitter is aversive•Then knock out PLCB2 and then have a T2R rescue (bitter receptor)-‐ use the promoter of the bitter receptor to drive transcripton of PLCB2-‐ make and animal in which for all of the cells that are making T2R, they will also express PLCB2-‐ in this way only bitter cells will express PLCB2 and sweet and umami will not and therefore the cell should respond to bitter but not the others•Found that this was exactly what happened•Clear that the bitter receptors in diferent cells than the sweet receptors and umami