Bipolar cells containing protein kinase Cα mediate attentional facilitation of the avian retinal ganglion cells by the retinopetal signal

Birds have a well‐developed retinopetal system projecting from the midbrain to the contralateral retina. The signal sent to the retina through the retinopetal system facilitates visual responses of the retinal ganglion cells (RGCs), and the retinopetal signals function as attentional signals during visual search. Thus, the retinopetal signal somehow reaches and facilitates visual responses of the RGCs. However, the tertiary neuron of the retinopetal system, the isthmo‐optic target cell (IOTC), is unlikely to contact most RGCs directly, because the IOTCs form axon terminals localized in the outermost sublayer (lamina 1) of the inner plexiform layer (IPL) where few RGC dendrites terminate. Therefore, some other intrinsic retinal neurons must be involved in the centrifugal attentional enhancement of visual responses of the RGCs. We investigated connections of the target cells of the IOTCs in chicken and quail, using light and electron microscopic immunohistochemistry. We show that axon terminals of the IOTC make synaptic contacts with protein kinase Cα (PKCα)‐immunoreactive (ir) bipolar cells (PKCα‐BCs) in lamina 1 of the IPL. Furthermore, with prolonged electrical stimulation of the isthmo‐optic nucleus (ION) on one side, whose neurons send their axons to the contralateral retina and make synaptic contacts there with IOTCs, phosphorylation of cAMP response element‐binding protein was observed in the PKCα‐BCs in the contralateral retina, but not in the ipsilateral retina. This suggests that electrical stimulation of ION activated PKCα‐BCs through synapses from IOTCs to PKCα‐BCs, thus stimulating transcription in PKCα‐BCs. Thus, centrifugal attentional signals may facilitate visual responses of RGCs via the PKCα‐BCs.