Indirect synaptic inputs from filiform hair sensory neurons contribute to the receptive fields of giant interneurons in the first-instar cockroach.

Abstract

The first-instar cockroach, Periplaneta americana, detects air movements using four filiform hair sensilla, which make synaptic connections to seven pairs of giant interneurons (GIs) in the terminal abdominal ganglion. The directional sensitivities of some of the GIs, predicted from their patterns of monosynaptic inputs, may not be the same as in the second instar or adult. Intracellular recordings were made to determine the contribution of polysynaptic inputs to the receptive fields of first-instar GIs. The ventral GI1, and the dorsal GI5, GI6, and GI7 were all found to have indirect synaptic inputs from filiform afferents. The indirect inputs were excitatory to GI1, GI5, and GI7, and inhibitory to GI6 and GI7. The indirect excitatory input to GI1 was predicted to alter qualitatively its receptive field, allowing it to respond to wind from the side of the animal, as in the adult. Inhibition was predicted to sharpen the receptive fields of GI6 and GI7. The inhibitory postsynaptic potentials reversed 6-8 mV below resting potential and were blocked by picrotoxin, indicating that they are GABAergic. Indirect excitation also altered the predicted receptive field of GI7, one of the inputs being an unusual "off-response" to movement of a filiform hair in its inhibitory direction.