Presynaptic Inhibition

[Darush Pessaran]

Sanaz Tabarestani wrote:

      Hi everybody,

      I'm a second year medical student and I have a question about
      the mechanism of presynaptic inhibition .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

 Reply from Darush Pessaran:
The inhibitory transmitter substance GABA has the specific effect of
opening 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 enter the terminal fibrils when an action potential arrives.
Therefore,the action potential itself in these terminal fibrils becomes
greatly reduced, thus also reducing the degree of excitation of the
postsynaptic neuron.