Calbindin 28kD and parvalbumin immunoreactive neurons receive different patterns of synaptic input in the cat superior colliculus

Recent evidence suggests that neurons containing the calcium binding protei ns calbindin 28kD (CB) and parvalbumin (PV) have differing distributions wh ich match respectively the distribution of W and Y retinal ganglion cell in puts to the cat superior colliculus (SC). In this study we have used electr on microscope immunocytochemistry to study directly the synaptic inputs to neurons containing CB and PV. Aspiration lesions of areas 17-18 of visual c ortex were made 4 days prior to sacrifice in order to identify degenerating cortical terminals (CT). Retinal terminals (RTs) were identified by their characteristic morphology including large round synaptic vesicles and pale mitochondria. We photographed RTs and CTs that were in contact with immunor eactive profiles sampled in both the superficial gray and optic lavers (ol) of SC. CB immunoreactive (ir) dendrites were usually of small to medium ca liber and were found to receive synaptic input from RTs. These RTs were all small profiles forming a single synaptic contact with asymmetric densifica tions. CBir profiles also received other synaptic input, including from ter minals with dark mitochondria that contained flattened synaptic vesicles (F profiles). No CBir dendrites were found to receive CT input even though de generating CTs were found in the vicinity of CBir profiles. By contrast, bo th RT and CT were found to contact PVir dendrites. RT terminals contacting PVir dendrites were both small and larger profiles with round synaptic vesi cles and asymmetric synaptic densifications. CT were undergoing electron de nse degeneration but still sometimes formed asymmetric synaptic densificati ons with PV neurons. PV cells also received F profile synaptic input. We co nclude that CB neurons receive small RT synapses that are probably of W ori gin, while PV neurons receive both RT and CT synapses which are likely rela ted to the Y pathway. (C) 1999 Elsevier Science B.V. All rights reserved.