Synaptic connectivity between cricket auditory interneurons as studied by selective photoinactivation.

Sound detection and localization are important for crickets. Interneurons located in the prothoracic ganglion play a crucial role in the initial processing of the auditory inputs. Two of the most readily recorded and dye-marked auditory interneurons in the prothoracic ganglion of the cricket are the omega cells (ON/1) and the ascenders (AN/2). By using a new photoinactivation technique to selectively inactivate these cells, the synaptic relationship between them could be studied. Our results indicate that the ON/1 cells are connected to each other with reciprocal inhibitory synapses. An ON/1 cell responds to contralateral stimulation with strong inhibition mediated by the other ON/1 cell. When one cell is killed, this inhibition is removed, and a weak excitatory response is unmasked. Unlike the ON/1 cell, AN/2 produces an inhibitory response when stimulated ipsilaterally to the cell body side, and this response is also removed when the ipsilateral ON/1 cell is killed, providing strong evidence that ON/1 is the source of the inhibition. As with the ON/1, the inhibition is replaced by weak excitation.