The efficiency of information transfer from pre to postsynaptic neurons can be acutely modulated by recent presynaptic activity. Such short term synaptic plasticity figures prominently in the neuro-computational capabilities of cortical neurons; moreover, long term potentiation can act to modify short term plasticity, rather than to alter a uniform synaptic weight. Hence, transient changes in synaptic efficacy may be fundamental to how we process and store information. Because P/Q and N type calcium channels trigger neurotransmitter release, dynamic modulation of channels by G proteins and voltage inactivation could provide biophysical and molecular mechanisms for activity dependent changes in synaptic efficacy. This project will investigate this intriguing possibility by deepening the biophysical and structural understanding of G protein inhibition and voltage inactivation of neuronal Ca channels, and by bridging this new understanding to the operation of channels during activation by action potential waveforms. Electrophysiology, recombinant expression of P/Q and N type Ca channels, and molecular biological approaches will be combined in 5 specific aims, each of which targets a salient gap in understanding channel performance during action potentials. Because many Ca channel subunit combinations comprise the broad categories of P/Q and N type channels, recombinant Ca channels will be studied to permit deepened understanding of individual subunit combinations. 1) Test whether channels mainly open according to one gating pathway, despite a possible multiplicity of opening pathways in currently proposed mechanisms. The one pathway scenario would simplify prediction of Ca2+ current during action potentials. 2) Define and characterize fast and slow forms of voltage inactivation. Two forms are unmistakable in our preliminary studies, but have been poorly appreciated. 3) Establish whether G protein inhibition and voltage inactivation interact, or proceed independently. Because both processes evolve smultaneously, this is a key issue. 4) Determine whether specified domains of different Ca channel beta subunits have selective control over fast inactivation, slow inactivation , or G protein inhibition. The existence of such domains would argue that the three processes are distinct in terms of not only function, but molecular mechanism. 5) Understand the impact of G protein inhibition and voltage inactivation on channel performance during trains of action potential waveforms.
This aim explicitly links the deepened basic understanding to channel performance during physiological paradigms of activation. Overall, this proposal will usher in new awareness and understanding of the potential role of Ca channels in short term changes of synaptic efficacy.
