GABA-like immunoreactivity in the pedal ganglia of Mytilus galloprovincialis: light and electron microscopic study.

The distribution of gamma-aminobutyric acid (GABA)-like immunoreactivity was investigated in the pedal ganglia (PG) of Mytilus galloprovincialis (Mollusca, Bivalvia) with the aid of an antiserum raised against GABA coupled to bovine serum albumin. Examination of whole-mount preparations and serial vibratome and semithin sections showed the presence of different types of immunoreactive neurons. Small unipolar neurons were the most numerous, and were located mainly in the lateral ganglion cortex. A few bipolar and small multipolar neurons were scattered in the cortex, and, more rarely, in the neuropile. Furthermore, two large symmetrical multipolar neurons, the processes of which extended over large fields in the ipsilateral and contralateral neuropile, were consistently observed in each ganglion. Immunoreactive fibers formed networks in the neuropile and ran parallel in the commissure and in all nerves and connectives. The morphology and distribution of neurons and fibers immunostained by the anti-GABA serum were similar to those of GAD-like immunoreactive elements, which indicates that the neurotransmitter and its biosynthetic enzyme are present in the same neurons. Moreover, comparison of serial semithin sections alternatively incubated in postembedding with anti-GABA and antiserotonin sera revealed that immunoreactivity for these two substances was present in different neuronal populations. However, close association between serotoninlike and GABA-like immunoreactive elements was observed in a few PG areas. GABA-like immunoreactivity was demonstrated on ultrathin sections by using secondary antiserum coupled to colloidal gold particles. Labeling was found over somata, fibers, and varicosities containing a distinct type of small (63 nm), pleomorphic, dense-cored vesicle.