Direct chemically mediated synaptic transmission from mechanosensory afferents contributes to habituation of crayfish lateral giant escape reaction.

The neural mechanism of habituation of the crayfish lateral giant-mediated escape reaction was analyzed electrophysiologically and pharmacologically. Upon repeated stimulation of tailfan afferents (at 0.2-1 Hz) lateral giant showed rapid habituation and failed to spike. Upon low-intensity sensory stimulation, the lateral giant responded with two subthreshold excitatory post-synaptic potentials, the alpha and beta components. A third component, the alpha' component, was discriminated at the boundary of excitatory post-synaptic potentials between the late alpha and early beta components with stimulation just subthreshold or suprathreshold to evoke lateral giant spikes. This alpha' component increased in amplitude with hyperpolarizing current injected into the lateral giants, although the amplitude of both the alpha and beta components remained constant. Furthermore, bath application of the nicotinic antagonist, d-tubocurarine caused a rapid reduction in the amplitude of the alpha' component while the amplitude of the beta component was reduced gradually and that of the alpha component remained unchanged. Single-hair stimulation indicated that some sensory afferents made direct connections with the lateral giants mediated by chemical synapses and form the potential of alpha' component of the lateral giants. Since lateral giant inactivation was associated with a reduction of excitatory post-synaptic potential amplitude of the alpha' component, connection from these afferents could contribute, at least in part, to lateral giant habituation.