Electron microscopic data on the neurons of nuclei subpretectalis and posterior-ventralis thalami.: A combined immunohistochemical study

The nucleus rotundus receives GABA-like immunoreactive fibres from the nuclei subpretectalis and postero-ventralis thalami. This result was confirmed by Phaseolus vulgaris leucoagglutinin (PhA-L) anterograde tracer and with electron microscopic (EM) gamma-aminobutiric acid (GABA)-immunogold staining. The detailed electron microscopic analysis of the structure of the neurons in these nuclei revealed that the neurons in the nucleus subpretectalis displayed GABA-like immunoreactivity. In the postero-ventral thalamic nucleus a group of neurons was GABA-positive. The surface of the neurons was covered both with numerous GABA-negative and GABA-like immunoreactive terminals that established asymmetrical and symmetrical synapses, respectively, with the GABA-positive neurons. The GABA-like immunonegative terminals are supposed to be the axon terminals of the collaterals of tecto-rotundal fibres in the subpretectal nucleus and the collateral terminal branches of contralateral tecto-rotundal fibres in the postero-ventralis thalami. In both nuclei, the GABA-like immunoreactive terminals may be developed by the collaterals of local neurons that establish symmetrical synapses. In the Phaseolus lectin-stained preparations these terminals may be labelled. The morphological characteristics of the neurons in the subpretectal and partly, in the posteroventral nuclei are similar to those of interneurons (local circuit neurons) and the numerous asymmetrical and symmetrical axe-somatic synapses, respectively. But these neurons locate outside of their target nucleus, and exert their modulatory effect on rotundo-ectostriatal transmission. Also, a contralateral influence is present in the nucleus rotundus that may interact in the cooperation of the eyes. The neurons of the subpretectal and posteroventral nuclei, similarly to the neurons of isthmic nuclei, are a special group of modulatory neurons with effects at a distance.