Loss of inhibitory synaptic input to cricket sensory interneurons as a consequence of partial deafferentation

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Abstract
In a companion paper the enhanced ability of excitatory pathways to drive deafferented interneurons in crickets could be attributed to a reduction in the effectiveness of remaining inhibitory pathways impinging on these neurons. To test this idea more directly, a sensory interneuron was examined in the cricket known as sensory interneuron 10-3 (SI 10-3). SI 10-3 was more sensitive to remaining excitatory inputs after partial deafferentation. The ability of inhibitory inputs to suppress a train of current-evoked action potentials was tested. Brief (100 ms) depolarizing current pulses were injected into the soma of SI 10-3 and the number of action potentials counted. Next, an identical depolarizing pulse was injected into the interneuron coincident with activation of the inhibitory pathway by a tone (75 dB at 450 Hz). Activation of the inhibition decreased the number of current-evoked action potentials. To quantify this effect, current-tone pairs were presented at 4 current intensities. Over the range of current intensities used, the relationship between current intensity and number of action potentials (AP/I curve), elicited with or without tones, was linear. In control preparations, tones always lowered the calculated slope of the observed curve and its y intercept. In treated specimens inhibiting tones decreased the slope of the AP/I curve less than in control specimens. Based on this evidence, the inhibitory pathway had been disrupted by partial deafferentation. This releases the giant neuron from inhibition, allowing it to respond more strongly to remaining excitatory inputs.