Presynaptic Inhibition

[Sanaz Tabarestani]

    Hi everybody,
    
    I'm a second year medical student and I have a question about    
    the mechanism of presynaptic inhibtion .I'm wondering if anyone 
    can help me.
    I've read about two different mechanisms and I don't know which 
    one is correct.Both of them are on GABA .
    The first one is that the GABA acts on GABAa receptors to increase
    the conductance to cl(-) ,but as the boutons actively accumulate
    cl(-) the result is an efflux of cl(-) and depolarization of
    the terminal and partial inactivation of the voltage dependent 
    Na(+) mechanism, so the amplitude of any action potential that
    arrives at the axon terminal will reduce.
    The second one is that the GABA opens anion channels ,allowing
    large numbers of chloride ions to diffuse into the terminal
    fibril.The negative charges of these ions cancel much of the
    excitatory effect of the positively charged sodium ions that
    eneter the terminal fibril when an action potential arrives.
    
    Thanks in advance for your help,
    sanaz