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

1. In a companion paper (5) we suggested that 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. 2. In order to test this idea more directly, we examined a sensory interneuron in the cricket known as sensory interneuron 10-3 (SI 10-3). We first demonstrated that SI 10-3 was more sensitive to remaining excitatory inputs after partial deafferentation. 3. We then tested the ability of inhibitory inputs to suppress a train of current-evoked action potentials. 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. In order to quantify this effect, current-tone pairs were presented at four 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 (Fig. 7A). 4. 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, we concluded that 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.