Effects of pharmacological treatment and photoinactivation on the directional responses of an insect neuron.

Soma-ipsilateral branches of the large segmental omega neuron of the phaneropterid bush cricket Ancistrura nigrovittata have smooth endings, which extend through most of the auditory neuropile. Correspondingly, it shows a broad frequency tuning. Large excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are observed when recording from soma-ipsilateral branches. Stimulation from the soma-ipsilateral side leads to a strong excitation. Soma-contralateral branches have a strong, beaded appearance. IPSPs, which seem to be of soma-contralateral origin, can be recorded from these branches. Stimulation from the soma-contralateral side leads to a strong inhibition of the omega neuron. Soma-contralateral stimulation must be 30-40 dB more intense than soma-ipsilateral stimulation to evoke similar spike numbers in the omega neuron. The side-to-side difference is reduced to 10-15 dB after cutting the input from the soma-contralateral leg (tympanic nerve). The thresholds for eliciting IPSPs by soma-contralateral stimulation correspond roughly to excitatory thresholds of the mirror-image omega with the same stimuli. Pharmacological treatment with picrotoxin (PTX) or photoinactivation of the Lucifer Yellow filled mirror-image omega neuron reduces contralateral inhibition considerably and eliminates all visible IPSPs. Nevertheless, an additional contralateral inhibition survives both procedures and is only eliminated after cutting the soma-contralateral tympanic nerve. These results demonstrate that the mirror-image partners of the omega neuron mutually inhibit each other in bush crickets--as in crickets. This mutual inhibition is PTX-sensitive. At least one additional element exerts contralateral PTX-insensitive inhibition on the omega neuron.