Quantitative immunogold evidence that glutamate is a neurotransmitter in afferent synaptic terminals within the isthmo-optic nucleus of the pigeon centrifugal visual system

A quantitative electron microscopic analysis of glutamate (GLU) immunoreact ivity using the post-embedding immunogold technique was carried out within the isthmo-optic nucleus (ION) of the pigeon centrifugal visual system (CVS ). Measurements were performed in each of eight different categories of axo n terminals, including those that were GABA-immunoreactive (-ir), considere d representing control profiles and identified using a single or double-lab el immunocytochemical procedure. The results demonstrated that the glutamat e immunogold particle densities for both mitochondrial and vesicular pools and for total surface area of bouton profiles were significantly higher in P1a. P1b and P2b terminals and not significantly different in P4 and P5 ter minals compared to those recorded in control GABA-ir terminals (P2a, P2c, P 3). Moreover, the values measured in GLU-ir positive profiles were all sign ificantly higher than in either P4 or P5 terminals. The results suggest tha t tectal neurons, which provide the main input to the ION cells, are either inhibitory GABA-ir possibly associated with P2c and/or P3 terminals or exc itatory GLU-ir via P1a, P1b and P2b terminals. Such differential effects of tectal afferents may be the basis for the modulation of centrifugal activi ty and consequently of end target retinal ganglion cell responses. The data are relevant to hypotheses Implicating the avian CVS in mechanisms of sele ctive enhancement of visual attention to either novel or meaningful stimuli within the visual held. (C) 2000 Elsevier Science B.V. All rights reserved .