NEURONAL SIGNALLING: BS2550 - PAST PAPER QUESTIONS - 2015 Section D Describe the mechanism of action of protein kinase A (PKA). How do adenyl cyclase and phosphodiesterases regulate its activity? Mechanism of PKA - PKA is activated by the stable secondary messenger cyclic AMP which is in turn catalysed by adenylate cyclase - PKA is a CAMP dependent protein kinase - Adenyl cyclase: - Adenyl cyclase activity is controlled by heterotrimeric G proteins - Catalyses the conversion of ATP into cyclic AMP and PPi - AC receptors are present in low abundance on the plasma membrane and consist of 12 trans-membrane domains and two conserved cytosolic loops o Cytosolic loops are located between TM 6-7 and the C-terminal sequence and these form the active site - Forskolin is a general activator of AC BS2550 - PAST PAPER QUESTIONS - 2016 Describe how glutamate changes the membrane potential through its receptors (focus on NMDA and AMPA receptors only) - Glutamate is the main excitatory neurotransmitter in the nervous system specific to ionotropic glutamate receptors Glutamate receptors are permeable to both Na+ and K+ ions thus have the ability to generate excitatory postsynaptic potentials = ESPSPs - Ionotropic glutamate receptors are tetrametric which means they have 4 transmembrane domains which form an ion pore each and as a result four agonist binding sites o N terminal is extracellular whereas the C terminal is intracellular - Two main types of ionotropic glutamate receptors are NMDA and AMPA receptors each having a ligand binding site and ion channel. In response to ligand binding which in this circumstance is the neurotransmitter glutamate, undergo conformational change in their tertiary structure and this initiates a sequence of events intracellularly. - NMDA glutamate receptors expressed in the neurons and glial cells in the nervous system and similar to the name are activated by NMDA
- In order for NMDA receptors to be opened they need to be glutamate and glycine bound and require an overall membrane depolarisation (permeability to calcium, sodium and potassium allows this to occur) o Glutamate binding means NMDA receptors are coincidence detectors. This means the ion channel only opens in response to concurrent presynaptic glutamate release and postsynaptic depolarisation - AMPA glutamate receptors expressed in the neurons and glial cells in the nervous system and similar to the name, are activated by AMPA - When the ligand glutamate binds, the ligand binding domain (LBD) closes. This results in a conformational change in the LBD and Transmembrane domain helices (TMD). These events lead to the opening of ion channel gates - However, unlike NMDA receptors they can open near resting membrane potential and have no requirement for depolarisation. - In regards to both AMPA and NMDA receptors at near resting potential only AMPA receptors are responsive. The binding of glutamate alone allows conformational change that allows Na+ ions to travel through the ion pore generating an excitatory postsynaptic potential - Whereas glutamate cannot activate due the Mg 2+ blockage at the ion pore that can