GABA-immunoreactive neurons in the central and peripheral nervous system of the earthworm, Lumbricus terrestris (Oligochaeta, Annelida)

Abstract.

The distribution of neurons immunoreactive for γ-aminobutyric acid was studied in the nervous system of Lumbricus terrestris (Oligochaeta). In the cerebral ganglion, the 86 cells immunoreactive for γ-aminobutyric acid represented 4.0% of the nerve cells in the brain, had a diameter of 12–50 μm, and were arranged in seven groups. Small-sized (18–30 μm) immunoreactive neurons occurred in the circumpharyngeal connectives. The axons of most immunoreactive neurons of the cerebral ganglion richly arborized in the ventral part of the neuropil and some could also be traced in the circumpharyngeal connectives. The subesophageal ganglion contained 94 immunoreactive cells (6.7% of the cells of this ganglion), also divided into seven groups, and with a diameter of 8–55 μm. The axons of the labeled neurons ran to the central neuropil giving both contra- and ipsilateral processes. Altogether 108 neurons in each ganglion (8.0% of their cells) of the ventral cord were immunopositive. Four labeled cell groups were present in the rostral and caudal part of each ganglion. Axons of these immunoreactive cells arborized in the central neuropil and projected to the segmental nerves. The stomatogastric ganglia and the enteric plexus also contained immunoreactive neurons. Many small elongated immunoreactive cells occurred in the gut epithelium. Postembedding immunogold electron microscopy revealed that immunoreactive varicosities mainly contained small pleomorphic (24 nm) agranular synaptic vesicles and some small granular (50 nm) vesicles.