GABA-IMMUNOREACTIVE NEURONS IN THE CENTRAL AND PERIPHERAL NERVOUS-SYSTEM OF THE EARTHWORM, LUMBRICUS-TERRESTRIS (OLIGOCHAETA, ANNELIDA)

The distribution of neurons immunoreactive for gamma-aminobutyric acid was studied in the nervous system of Lumbricus terrestris (Oligochaet a). In the cerebral ganglion, the 86 cells immunoreactive for gamma-am inobutyric acid represented 4.0% of the nerve cells in the brain, had a diameter of 12-50 mu m, and were arranged in seven groups. Small-siz ed (18-30 mu m) immunoreactive neurons occurred in the circumpharyngea l connectives. The axons of most immunoreactive neurons of the cerebra l ganglion richly arborized in the ventral part of the neuropil and so me could also be traced in the circumpharyngeal connectives. The subes ophageal ganglion contained 94 immunoreactive cells (6.7% of the cells of this ganglion), also divided into seven groups, and with a diamete r of 8-55 mu m The axons of the labeled neurons ran to the central neu ropil giving both contra- and ipsilateral processes. Altogether 108 ne urons in each ganglion (8.0% of their cells) of the ventral cord were immunopositive. Four labeled cell groups were present in the rostral a nd caudal part of each ganglion. Axons of these immunoreactive cells a rborized in the central neuropil and projected to the segmental nerves . The stomatogastric ganglia and the enteric plexus also contained imm unoreactive neurons. Many small elongated immunoreactive cells occurre d in the gut epithelium. Postembedding immunogold electron microscopy revealed that immunoreactive varicosities mainly contained small pleom orphic (24 nm) agranular synaptic vesicles and some small granular (50 nm) vesicles.