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Calcium binding protein (calbindin D28k) immunoreactivity in the hamster superior colliculus: ultrastructure and lack
M Behan1, A Jourdain, G M Bray
1Center for Research in Neuroscience, McGill University, Montreal, Canada.
Insights
Calbindin D28k protein expression marks specific neuron types in the hamster superior colliculus (SC). This study found calbindin D28k neurons are distinct from GABA neurons in the SC, unlike in the neocortex.
Area of Science:
- Neuroscience
- Neuroanatomy
- Cell Biology
Background:
- Calcium-binding proteins are increasingly used as neuroanatomical markers for functionally similar neurons.
- Calbindin D28k is a specific calcium-binding protein whose distribution can indicate neuronal function.
Purpose of the Study:
- To investigate the distribution and cellular localization of calbindin D28k in the adult hamster superior colliculus (SC).
- To determine the relationship between calbindin D28k-immunoreactive neurons and GABA-immunoreactive neurons in the SC.
Main Methods:
- Light and electron microscopy were used to examine calbindin D28k distribution in the hamster SC.
- Double fluorescent labeling and confocal laser microscopy were employed to assess colocalization with GABA.
- Neuron size distribution was compared with GABA-immunoreactive and Nissl-stained neurons.
Main Results:
- Calbindin D28k immunoreactivity was prominent in specific SC regions and laminae, labeling horizontal, vertical, and stellate cells.
- Electron microscopy revealed calbindin D28k-immunoreactive profiles were mainly dendrites, some postsynaptic to retinal ganglion cells.
- No neurons in the hamster SC exhibited co-localization of calbindin D28k and GABA, despite size distribution overlap.
Conclusions:
- Calbindin D28k serves as a marker for distinct neuronal subpopulations within the SC.
- The SC differs from the neocortex in the lack of calbindin D28k and GABA co-localization.
- Distinct calcium-binding proteins may signify different functional properties and neurotransmitter associations in neurons.
Abstract:
The expression of specific calcium binding proteins is being used increasingly as a potential neuroanatomical marker for neurons with similar functions. In this study, the distribution of calbindin D28k in the superior colliculus (SC) of adult hamsters was examined by light and electron microscopy. Calbindin immunoreactivity was prominent in specific regions and laminae of the SC throughout its rostrocaudal extent, and was found to label horizontal, vertical and stellate cell types. In addition, calbindin label highlighted "bridges" of neuronal processes in the intermediate layers. The most frequent calbindin-immunoreactive profiles seen in the electron microscope were dendrites, some of which were post-synaptic to apparent retinal ganglion cell axon terminals. Labelled axons and axon terminals were less frequently encountered. There was considerable overlap between the size distribution of calbindin D28k-immunoreactive neurons and that of GABA-immunoreactive or Nissl stained neurons in the SC. However, using a double fluorescent labelling technique, and examination of the tissue with confocal laser microscopy, no neurons were observed in the hamster SC that showed immunoreactivity for both calbindin and GABA. In this regard, the SC is similar to the mammalian lateral geniculate nucleus and the pretectum, but differs from the neocortex, where calbindin and GABA are colocalized. The demonstration in the SC, as well as other parts of the nervous system, of sub-populations of neurons that contain distinct calcium-binding proteins suggests that these neurons have different functional properties. Correlative studies may clarify the relevance of these cytoplasmic components as cell markers, as well as their different patterns of association with neurotransmitters and peptides.