Nonspiking local interneurons mediate surround inhibition of crayfish sustaining fibers

Abstract

1. A class of local, non-spiking interneurons in the second optic lobe of the crayfish is described, and their role in mediation of surround inhibition of sustaining fibers (SFs) is examined. 2. Surround inhibition in SFs is not associated with a measurable IPSP or postsynaptic conductance change in SFs and thus is produced by a decrease in the level of exciatory synaptic input (Fig. 1). 3. Local (amacrine) interneurons in the medulla, the second optic lobe, have biplanar dendritic trees. The proximal plane is coextensive with the single plane of SF dendrites (Fig. 2). 4. The amacrine cells respond to light with a graded depolarization but without impulses (Figs. 3 and 4). Depolarization with extrinsic current strongly reduces ongoing light-induced SF firing, and hyperpolarization enhances SF firing (Fig. 7). 5. Amacrine cell light responses share many properties with SF surround inhibition, including wide receptive fields (Fig. 4), response waveform and time course, and tonic light responses which are proportional to the log of the light intensity (Fig. 5). 6. When sinusoidal current is injected into an amacrine cell (Fig. 9) the SFs exhibit a peak sensitivity to current at modulation frequencies of 2–4 Hz and a flat response above about 10 Hz. SFs exhibit a similar frequency sensitivity to the modulation of illumination in the inhibitory surround. 7. Several results imply that the amacrine cells provide feedback or recurrent inhibition to the excitatory visual pathway.