Effects of octopamine on transmission at the first synapse of the crayfish lateral giant escape reaction pathway

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The mechanisms responsible for octopaminecaused increases of excitability of interneuron A of the lateral giant escape reaction circuit of the crayfish, Procambarus clarkii, were investigated. 1. Octopamine increases the amplitude of compound and unitary EPSPs. Neither resting potential, input impedance, nor critical firing level appear to be altered. 2. Increases of unitary EPSP amplitude are accompanied by increases in coefficient of variation as well as by increases in rise and fall times. 3. The above changes suggest that transmission from single afferents to interneuron A occurs at more than one type of synapse or ‘transmission site’. Under control conditions the EPSP is generated largely by sites producing EPSPs having little trial-to-trial variability, while octopamine activates sites having high variability of release. Though the coefficient of variation of EPSP amplitude falls (or remains unchanged) at each type of site individually, the addition of a variable contribution to the relatively non-variable EPSPs of control conditions causes a rise in the coefficient of variation of total amplitude in octopamine. It is also suggested that the less variable sites produce fast EPSPs while the more variable sites produce slow ones.