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 proteins calbindin 28kD (CB) and parvalbumin (PV) have differing distributions which match respectively the distribution of W and Y retinal ganglion cell inputs to the cat superior colliculus (SC). In this study we have used electron microscope immunocytochemistry to study directly the synaptic inputs to neurons containing CB and PV. Aspiration lesions of areas 17-18 of visual cortex 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 immunoreactive profiles sampled in both the superficial gray and optic layers (ol) of SC. CB immunoreactive (ir) dendrites were usually of small to medium caliber and were found to receive synaptic input from RTs. These RTs were all small profiles forming a single synaptic contact with asymmetric densifications. CBir profiles also received other synaptic input, including from terminals with dark mitochondria that contained flattened synaptic vesicles (F profiles). No CBir dendrites were found to receive CT input even though degenerating CTs were found in the vicinity of CBir profiles. By contrast, both RT and CT were found to contact PVir dendrites. RT terminals contacting PVir dendrites were both small and larger profiles with round synaptic vesicles and asymmetric synaptic densifications. CT were undergoing electron dense degeneration but still sometimes formed asymmetric synaptic densifications with PV neurons. PV cells also received F profile synaptic input. We conclude that CB neurons receive small RT synapses that are probably of W origin, while PV neurons receive both RT and CT synapses which are likely related to the Y pathway.