GABA-IMMUNOHISTOLOGICAL OBSERVATIONS, AT THE ELECTRON-MICROSCOPIC LEVEL, OF THE NEURONS OF ISTHMIC NUCLEI IN CHICKEN, GALLUS-DOMESTICUS

Following a demonstration of Golgi-impregnated neurons and their termi nal axon arborization in the optic tectum, the neurons of the nucleus parvocellularis and magnocellularis isthmi were studied by means of po stembedded electron-microscopical (EM) gamma-aminobutyric acid (GABA)- immunogold staining. In the parvocellular nucleus, none of the neurona l cell bodies or dendrites displayed GABA-like immunoreactivity in EM preparations stained by postembedded GABA-immunogold. However, numerou s GABA-like immunoreactive and also unlabeled terminals established sy napses with GABA-negative neurons, GABA-like immunoreactive terminals were usually found at the dendritic origin. Around the dendritic profi les, isolated synapses of both GABA-like immunoreactive and immunonega tive terminals established glomerulus-like structures enclosed by glia l processes. All giant and large neurons of the magnocellular nucleus of the isthmi displayed GABA-like immunoreactivity. Their cell surface was completely covered by GABA-like immunoreactive and unlabeled term inals that established synapses with the neurons. These neurons are th ought to send axon collaterals to the parvocellular nucleus; their axo ns enter the tectum opticum. The morphological characteristics of neur ons of both isthmic nuclei are like those of interneurons, because of their numerous axosomatic synapses with both asymmetrical and symmetri cal features. These neurons are not located among their target neurons and exert their modulatory effect on optic transmission in the optic tectum at a distance.