GABAergic and non-GABAergic spiking interneurons of local and intersegmental groups in the crayfish terminal abdominal ganglion

In the first step toward identifying the neurotransmitter released from spi king interneurons of both local and intersegmental groups in the crayfish t erminal abdominal ganglion, the authors examined whether spiking local inte rneurons and ascending intersegmental interneurons contain the transmitter gamma-aminobutyric acid (GABA). In this paper, 17 identified ascending inte rneurons and three spiking local interneurons were stained by intracellular injection of Lucifer yellow and subsequently treated for immunocytochemica l staining against GABA. Double-labeling experiments revealed that six iden tified ascending interneurons are GABAergic, but no spiking local interneur ons show GABA-like immunoreactivity Four ascending interneurons with GABA-l ike immunoreactivity (reciprocal closing ascending neuron 5 [RC-5], recipro cal opening ascending neuron 6 [RO-6], variable-effect ascending interneuro n 1 [VE-1], and no-effect ascending: interneuron 4[NE-4]) had cell bodies t hat formed a duster on the ventral surface of the rostral edge of the gangl ion, whereas two GABAergic interneurons (coinhibiting ascending interneuron 2 [CI-2] and NE-2) had cell bodies in a caudal region around the cell body of the seventh flexor inhibitor (FI) motor neuron. Another four rostral in terneurons (RC-2, RC-3, RC-4, and NE-3) and seven caudal interneurons (CI-3 , RC-7, RO-1, RO-2, RO-3, RO-4, and NE-1) had no GABA-like immunoreactivity . Because VE-1 is known to make direct inhibitory connections with other as cending interneurons, whereas RC-3 and RO-1 are known to make direct excita tory connections, the immunocytochemical results from this study are consis tent with previous physiological studies. Although many spiking local inter neurons (including spiking local interneuron 1 of the anterior group [sp-an t1]) made direct inhibitory connections with nonspiking local interneurons, three spiking local interneurons (sp-ant1, spiking local interneuron 6 of the medial group [sp-med6], and spiking interneuron 5 of the posterior grou p [sp-post]) do not show GABA-like immunoreactivity. These results suggest that the inhibitory transmitter released from spiking local interneurons is not GABA but that another substance mediates the inhibitory action of thes e interneurons. (C) 1999 Wiley-Liss, Inc.