GABA-IMMUNOREACTIVE NEURONS AND INTERACTIONS OF GABA WITH SEROTONIN AND FMRFAMIDE IN A PERIPHERAL SENSORY GANGLION OF THE POND SNAIL LYMNAEA-STAGNALIS

The osphradium is a putative chemosensory organ of aquatic molluscs. P reviously, we identified cells with serotonin (5-HT) and FMRFamide (FM RFa)-like immunoreactivity in the osphradial ganglion of Lymnaea stagn alis. The present investigation has established the presence of cells immunoreactive to gamma-aminobutyric acid (GABA). Some of these cells send processes to the sensory epithelium and are thus considered to be primary sensory neurones. Colocalisation of GABA and FMRFamide-like i mmunoreactivities was found in some of these and other neurones. The r esponses of the osphradial output electrical activity to the single an d combined application of the above neuroactive substances were examin ed. 5-HT slightly increased and FMRFa decreased the activity. GABA alo ne was generally ineffective; however, it had a consistent stimulating effect after pretreatment with 5-HT. In its turn, pretreatment with G ABA significantly increased the inhibitory action of FMRFa. Primary se nsory neurones giving this kind of responses in the nerve were identif ied electrophysiologically and morphologically in the osphradial gangl ion. Our results indicate that GABA takes part in relay of sensory sig nals into the central nervous system, and transmitter interactions inv olving GABA, 5-HT, and FMRFa are considerable for the final output pat tern of the osphradial sensory network. (C) 1997 Elsevier Science B.V.