Cerebral Cortex, Vol. 9, No. 7, 683-696,
October 1999
© 1999 Oxford University Press
Connectivity of GABAergic Calretinin-immunoreactive Neurons in Rat Primary Visual Cortex
Department of Anatomy and Neurobiology, Washington University School of Medicine, St Louis, MO 63110, USA
In rat visual cortex neurons that are immunoreactive for the calcium-binding protein calretinin (CR+) constitute a distinct family which accounts for 17% of 
-aminobutyric acid (GABA)-expressing cells. It is not clear, however, (i) whether CR is expressed exclusively in GABAergic neurons and (ii) how CR+ neurons are incorporated into neuronal circuits of rat visual cortex. To address these questions we studied synaptic relationships of CR+ neurons with GABA+ and GABA– elements in the neuropil of rat primary visual cortex (area 17). All CR+ neurons are nonpyramidal cells with smooth or sparsely spiny and often beaded dendrites. Of all CR+ neurons, 56% are located in layers 1 and 2/3. In layer 2/3, most CR+ neurons are bipolar-shaped and have vertically oriented dendrites. Many ascending dendritic branches reach layer 1 where they run parallel to pial surface. CR+ axons are thin, highly branched near the cell body and often send descending collaterals to layers 5 and 6. Double immunofluorescence labeling revealed GABA in 94% of CR+ cell bodies in layer 2/3. Electron microscopic analysis shows that all CR+ axon terminals contain elongated vesicles and form symmetric synapses. Postembedding staining shows that 98% of CR+ terminals are GABA+. GABA-immunoreactivity is also present in somata and thick dendrites of CR+ neurons but many thin dendrites are GABA–. CR+ somata, dendrites and axon terminals are enriched in mitochondria. Somata and thick CR+ dendrites are densely innervated. At least 68% of the targets of CR+ terminals in layer 2/3 are GABA+ and 
50% of these are other CR+ neurons. The remainder (32%) of targets of CR+ terminals are thin dendrites of GABA– cells. In contrast, in layers 5 and 6, 60% of CR+ terminals form synapses with GABA– somatic profiles. The preferential interactions of layer 2/3 CR+ neurons with GABAergic neurons, and with CR+ neurons in particular, suggests that these cells play a role in the inhibition of inhibitory neurons of the same layer. Through these interactions CR+ cells may reduce inhibition of pyramidal cells in layers 2/3, 5 and 6 and thus disinhibit a column of neurons.
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