The omega-neuron 2 (ON2) is an auditory interneuron in Acheta domesticus that could play an important role in prothoracic auditory information processing. ON2 neurons were intracellularly recorded and iontophoretically stained with lucifer-yellow.
The morphology of ON2 is similar to that described in other cricket species. Wholemounts and transverse sections of prothoracic ganglia show that ON2 branches cover the same regions as the omega-neuron 1 (ON1) branches except that ON2 has crossover branches in the anterior ring tract from both the ipsilateral and contralateral side. Auditory responses of ON2 are measured with spiking thresholds of about 60 dB SPL at 16 kHz (courtship song frequency) and about 80 dB SPL at 5 kHz (calling song frequency) under open field and closed field (monaural) stimulation. Open and closed field stimulations show almost identical threshold curves and response magnitudes to soma-ipsilateral and soma-contralateral stimulation. These data do not support a function of ON2 for directional hearing.
In addition to excitation, ON2 receives inhibitory inputs from both ears at 5 and 16 kHz. Subthreshold inhibition is obvious in responses with open and closed field stimulation at 5 kHz. Responses show mixed excitation and inhibition at intensities above 80 dB SPL at 5 and 16 kHz. These data confirm previous immunohistochemical results showing GABAergic input to ON2 in Gryllus bimaculatus. Two newly described local neurons ("LN1'', "LN2"), which are probably nonspiking and have low 5-kHz thresholds, could be candidates for the inhibitory input to ON2. J.