Presynaptic inhibition and histamine in cerebral ganglion of Aplysia.

: In the cerebral ganglion of Aplysia californica synaptic transmission between identified neurones undergoes a prolonged decrease of efficacy following activation of a separate afferent pathway. Synaptic efficacy was measured as the amplitude of a monosynaptic and unitary excitatory response (test excitatory postsynaptic potential = test EPSP) generated by intracellular presynaptic stimulation. Test EPSP depression occurs with a delay of about 20 s, and may have a total duration of several minutes. Test EPSP depression is associated with a hyperpolarization of the presynaptic neurone. It is concluded that the test EPSP depression is due to presynaptic inhibition. Histamine mimics the pre- and postsynaptic effects obtained with activation of secondary afferent pathways.