COLOCALIZATION OF SEROTONIN AND GABA IN NEURONS OF THE XENOPUS-LAEVISRETINA

Serotonin-synthesizing neurons in the retina of Xenopus laevis have be en identified using anti-phenylalanine hydroxylase (PH) antibody which recognizes tryptophan 5-hydroxylase, the rate-limiting enzyme for ser otonin synthesis. Double-labelling experiments, using anti-PH antibody and anti-serotonin antibody/5,7-dihydroxytryptamine (5,7-DHT) uptake, have shown that some serotonin-like immunoreactive/5,7-DHT-labelled n eurons exhibit PH-like immunoreactivity (PH-LI) (serotonin-synthesizin g neurons), but the others do not (serotonin-accumulating neurons). In the present study, triple-labelling experiments were performed using 5,7-DHT uptake and antibodies raised against GABA and PH, to determine the possible co-localization of y-aminobutyric acid (GABA) in seroton in-synthesizing and/or -accumulating neurons in the Xenopus retina. Al l 5,7-DHT-labelled bipolar cells lacked PH-LI; all of them were immuno reactive to GABA. In contrast, all 5,7-DHT-labelled large amacrine cel ls exhibited PH-LI, but none of them expressed GABA-LI. Small amacrine cells labelled with 5,7-DHT but not PH-LI exhibited GABA-LI, whilst t he small amacrine cells with PH-LI lacked GABA-LI. These observations indicate that GABA is co-localized in serotonin-accumulating amacrine and bipolar cells, whereas serotonin-synthesizing large and small amac rine cells do not contain GABA-LI.