2 TYPES OF IDENTIFIED ASCENDING INTERNEURONS WITH DISTINCT GABA RECEPTORS IN THE CRAYFISH TERMINAL ABDOMINAL-GANGLION

More than half of the identified ascending interneurons originating in the terminal abdominal ganglion of the crayfish received inhibitory s ensory inputs from hair afferents innervating the tailfan on the side contralateral to their main branches. Biochemical aspects of this tran sverse lateral inhibition of ascending interneurons were examined by t he use of neurophysiological and pharmacological techniques. Local app lication of gamma-aminobutyric acid (GABA) and its agonist muscimol in to the neuropil induced membrane hyperpolarization of identified ascen ding interneurons with an increase in membrane conductance. Because th e reversal potential of inhibitory postsynaptic potentials (IPSPs) in ascending interneurons elicited by the sensory stimulation and GABA in jection was similar, and the sensory-stimulated IPSPs of the interneur ons were blocked by GABA and muscimol application, this study strongly suggests a GABAergic nature for transverse lateral inhibition of asce nding interneurons. According to the response to the GABAA antagonists bicuculline and picrotoxin, ascending interneurons were classified in to two types, picrotoxin-sensitive and picrotoxin-insensitive interneu rons. Identified ascending interneurons VE-1 and RO-S showed a pharmac ological profile similar to that of the classical GABAA receptor of th e vertebrates. Bath application of both bicuculline and picrotoxin rev ersibly reduced the amplitudes of IPSPs. The other identified ascendin g interneurons CA-1, RO-1, and RO-2 were not affected significantly by the bath application of GABA(A) and GABA(B) antagonists, although bat h application of low-chloride saline reversed the sensory-stimulated I PSPs. IPSPs of the picrotoxin-sensitive interneurons had a rather fast er time course and shorter duration in comparison with those of the pi crotoxin-insensitive interneurons.