Feedback synaptic interaction in the dragonfly ocellar retina.

Abstract

The intracellular response of the ocellar nerve dendrite, the second order neuron in the retina of the dragonfly ocellus, was modified by application of various drugs and a model developed to explain certain features of that response. Curare blocked the response completely. Picrotoxin and bicuculline eliminated the off overshoot. Bicuculline decreased the size of response and the sensitivity, while GABA increased the size of response. The receptor transmitter seems to be acetylcholine which is inhibitory to the ocellar nerve dendrite, whereas the feedback transmitter from the ocellar nerve dendrite may be GABA which is facilitory to receptor transmitter release. The model of synaptic feedback interaction developed to be consistent with these results has certain important features. The feedback transmitter is probably released in the dark to increase input sensitivity from receptors in response to dim light. This implies that the dark potential of the ocellar nerve dendrite may be determined by a dynamic equilibrium established by synaptic interaction between it and the receptor terminals. Such a system is well suited to signalling phasic information about changes in level of illumination over a wide range of intensities, a characteristic which appears to be a significant feature of the dragonfly median ocellar response. [Species studied were Aeshna tuberculifera, Libellula pulchella, Plathenis lydia, Erythemis simplicicollis and Anax junius].