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 ascending retino‐tecto‐thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals. We found that the rotundus contains a massive plexus of glutamic acid decarboxylase (GAD)‐immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. The retrograde tracer cholera toxin B fragment was injected into the rotundus to establish the location of the afferent neurons and determine the source of the gamma‐aminobutyric acid (GABA) inputs into the rotundus. 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 (IPS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the SP, SPcd, IPS, 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 reduction in the intensity of the GAD axonal immunoreactivity within the ipsilateral rotundus, but this destruction did not diminish the intensity of the GAD‐immunoreactivity within the contralateral rotundus. Our studies indicated that the source of the massive GAD‐immunoreactive plexus within the rotundus was from the ipsilateral SP, SPcd, IPS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projections upon the rotundus. We suggest that the direct bilateral tecto‐rotundal projections are excitatory, whereas the indirect ipsilateral projections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA‐A receptor