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A study was made of synaptic transmission between the four median photoreceptors of the giant barnacle (Balanus nubilus) and their post-synaptic cells (I-cells). Simultaneous intracellular recordings were made from the presynaptic terminal region of a photoreceptor and from the soma of an I-cell. The photoreceptor's membrane potential provided feed-back to bath electrodes that passed current into the receptors' axons, permitting the voltage to be controlled at the point of arborization of their presynaptic terminals. Simultaneous recordings from a second photoreceptor showed that its voltage tracked the first. Step depolarizations of the receptors from their dark resting potential (about -60 mV) caused hyperpolarizations of the I-cell that reached a peak, then decayed to a plateau value. The amplitude of the I-cell's response grew with presynaptic depolarizations, saturating at presynaptic values 10-20 mV depolarized from dark rest. Step hyperpolarizations of the receptors from dark rest evoked depolarizations of the I-cell consisting of an initial peak, which varied greatly in amplitude and wave form from preparation to preparation, followed by a plateau. The presence of this post-synaptic response indicates that transmitter is released continuously from the receptors at their dark resting potential. An input-output relation of the synapse was obtained by presenting step depolarizations from a holding potential of -80 mV, where steady-state transmitter release is shut off. The relation is sigmoidal; in the exponentially rising phase of the curve, a 5-11 mV presynaptic change produces a 10-fold change in post-synaptic response. When the presynaptic holding potential was set at values ranging from -80 to -40 mV, the relation between the I-cell's response and the absolute potential to which the receptor was stepped shifted along the presynaptic voltage axis. The slopes of the input-output relations were roughly parallel or increased as the photoreceptors were held more depolarized. This observation limits the possible mechanisms of the shift.

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