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Nonspiking interneurons control their synaptic output directly by membrane potential changes caused by synaptic activities. Although these interneurons do not generate spikes, their dendritic membrane is endowed with a variety of voltage-dependent conductances whose functional significance in synaptic integration remains unknown. We quantitatively investigated how the passive and active dendritic properties affect the synaptic integration in an identified nonspiking interneuron of crayfish by computer simulation using its multicompartment model based on electrophysiological measurements and three-dimensional morphometry. At the resting potential level, the attenuation factor (V(s)/V(t)) of a unitary synaptic potential in the course of its spread from a dendritic terminal (V(s)) to other terminals (V(t)) ranged from 4.42 to 6.30 with no substantial difference between hyperpolarizing and depolarizing potentials. The compound synaptic responses to strong mechanosensory stimulation could be reproduced in calculation only as the result of spatial summation of attenuated potentials, not as any single large potential. The characteristic response could be reproduced by assuming that the active conductances were distributed only in the dendritic region where the synaptic summation was carried out. The active conductances in other parts of the cell affected neither the shape of the compound synaptic response nor the dendritic spread of synaptic potentials. These findings suggest that the active membrane conductances do not affect the spatial distribution of synaptic potentials over dendrites but function in sculpting the summed synaptic potential to enhance temporal resolution in the synaptic output of the nonspiking interneuron.

More than half of the identified ascending interneurons originating in the terminal abdominal ganglion of the crayfish received inhibitory sensory inputs from hair afferents innervating the tailfan on the side contralateral to their main branches. Biochemical aspects of this transverse lateral inhibition of ascending interneurons were examined by the use of neurophysiological and pharmacological techniques. Local application of gamma-aminobutyric acid (GABA) and its agonist muscimol into the neuropil induced membrane hyperpolarization of identified ascending interneurons with an increase in membrane conductance. Because the reversal potential of inhibitory postsynaptic potential (IPSPs) in ascending interneurons elicited by the sensory stimulation and GABA injection was similar, and the sensory-stimulated IPSPs of the interneurons were blocked by GABA and muscimol application, this study strongly suggests a GABAergic nature for transverse lateral inhibition of ascending interneurons. According to the response to the GABAA antagonists bicuculline and picrotoxin, ascending interneurons were classified into two types, picrotoxin-sensitive and picrotoxin-insensitive interneurons. Identified ascending interneurons VE-1 and RO-4 showed a pharmacological profile similar to that of the classical GABAA receptor of the vertebrates. Bath application of both bicuculline and picrotoxin reversibly reduced the amplitudes of IPSPs. The other identified ascending interneurons CA-1, RO-1, and RO-2 were not affected significantly by the bath application of GABAA and GABAB antagonists, although bath application of low-chloride saline reversed the sensory-stimulated IPSPs. IPSPs of the picrotoxin-sensitive interneurons had a rather faster time course and shorter duration in comparison with those of the picrotoxin-insensitive interneurons.