Mechanisms responsible for changes observed in response properties of partially deafferented insect interneurons

1. Two pairs of large sensory interneurons, the medial and lateral giant interneurons (MGI and LGI), in the abdominal nervous system of crickets were examined after being partially deafferented throughout postembryonic development. Each of these interneurons receives a synaptic input from sensory neurons in a pair of appendages called cerci. The experiment consisted of removing one of these appendages at hatching and examining the morphological and physiological consequences for the MGI and LGI. 2. The morphological consequences of deafferentation were examined by injecting neurons with cobalt acetate and intensifying the profiles either in section or whole mount with a modified Timm's method. We confirmed previous reports that the main dendritic branches were shorter than controls when deafferented. In addition, we demonstrated that the many spinelike processes projecting from the main dendrites were shorter than in controls. The density of spines per unit length of main dendrite was approximately the same on control and treated dendrites. Neither the MGI nor the LGI sprouted new dendrites into normally afferented areas as a result of partial deafferentation. Short periods of deafferentation late in development had similar though less drastic effects on the structure of MGI. 3. The MGI was found to receive stronger than normal excitatory input from the remaining cercus when one cercus was absent throughout postembryonic development. An examination of remaining inhibitory inputs to both the MGI and the LGI demonstrated that these pathways were weaker than normal. The results suggest that the increase in excitability observed in MGI when partially deafferented for long periods is due to removal of inhibition rather than direct changes in the strength of excitatory inputs.