Three distinct families of GABAergic neurons in rat visual cortex

In the cortex inhibition is mediated predominantly by GABAergic interneurons. Although all of these neurons use the same neurotransmitter, studies in the rat frontal cortex have shown that they are molecularly and physiologically diverse. It is not known whether similar subgroups of GABAergic neurons exist in primary visual cortex and how these different inhibitory neurons are inserted into specific cortical circuits. We have used immunostaining with antibodies against gamma aminobutyric acid (GABA), parvalbumin (PV), calretinin (CR), somatostatin (SOM), calbindin (CB) and nitric oxide synthase (NOS) to probe for colocalization of known markers of GABAergic interneurons. The results show that the majority of PV (100%), SOM (89.8%) and CR (93.9%) staining neurons are GABA positive. PV immunoreactive neurons constitute a distinct group that show no overlap with CR, SOM and NOS expressing cells and only a minor overlap (5.3%) with CB. PV immunoreactive cells account for 50.8% of GABAergic neurons. A second group of SOM expressing neurons accounts for 16.9% of GABAergic cells. None of these cells colocalize PV or CR, but 1.7% of SOM neurons stain for NOS and 86.3% show CB immunoreactivity. The third distinct group of CR expressing cells accounts for 17.0% of GABAergic neurons. All of these are PV, CB, SOM and NOS negative. CB expressing neurons represent a heterogeneous group that includes GABAergic and non-GABAergic cells. Our findings indicate that GABAergic neurons in rat area 17 are organized in at least three separate families that can be identified by the expression of PV, CR and SOM. These cells account for 84.9% of GABAergic neurons. These results extend previous observations in rat frontal agranular cortex and suggest that in visual cortex the inhibitory network is composed of similar cell types.