GABAERGIC INPUTS TO THE NUCLEUS ROTUNDUS (PULVINAR INFERIOR) OF THE PIGEON (COLUMBA-LIVIA)

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/caudal pulvinar of mammals, is the major target of an as cending retino-tecto-thalamic pathway. Further clarification of the in puts to the rotundus and their functional properties will contribute t o our understanding of the fundamental role of the ascending tectal in puts to the telencephalon in all vertebrates, including mammals. We fo und that the rotundus contains a massive plexus of glutamic acid decar boxylase (GAD)-immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. T he retrograde tracer cholera toxin B fragment was injected into the ro tundus to establish the location of the afferent neurons and determine the source of the gamma aminobutyric acid (GABA) inputs into the rotu ndus. In addition to the recognized bilateral inputs from layer 13 of the tectum, we found intense retrograde labeling of neurons within the ipsilateral nuclei subpretectalis (SP), subpretectalis-caudalis (SPcd ), interstitio-pretecto-subpretectalis (IFS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the S P, SPcd, IFS, and PV were intensely GAD-immunoreactive. The neurons of layer 13 of the tectum were not immunoreactive for GAD. Following the destruction of the ipsilateral SP/IPS complex, we found a major reduc tion in the intensity of the GAD axonal immunoreactivity within the ip silateral rotundus, but this destruction did not diminish the intensit y of the GAD-immunoreactivity within the contralateral rotundus. Our s tudies indicated that the source of the massive GAD-immunoreactive ple xus within the rotundus was from the ipsilateral SP, SPcd, IFS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projecti ons upon the rotundus. We suggest that the direct bilateral tecto-rotu ndal projections are excitatory, whereas the indirect ipsilateral proj ections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA-A receptor. (C) 1996 Wiley-Liss, Inc.