Two types of somatostatin-expressing GABAergic interneurons in the superficial layers of the mouse cingulate cortex

Somatostatin-expressing (SOM(+)), inhibitory interneurons represent a heterogeneous group of cells and given their remarkable diversity, classification of SOM(+) interneurons remains a challenging task. Electrophysiological, morphological and neurochemical classes of SOM(+) interneurons have been proposed in the past but it remains unclear as to what extent these classes are congruent. We performed whole-cell patch-clamp recordings from 127 GFP-labeled SOM(+) interneurons ('GIN') of the superficial cingulate cortex with subsequent biocytin-filling and immunocytochemical labeling. Principal component analysis followed by k-means clustering predicted two putative subtypes of SOM(+) interneurons, which we designated as group I and group II GIN. A key finding of our study is the fact that these electrophysiologically and morphologically distinct groups of SOM(+) interneurons can be correlated with two neurochemical subtypes of SOM(+) interneurons described recently in our laboratory. In particular, all SOM(+) interneurons expressing calbindin but no calretinin could be classified as group I GIN, whereas all but one neuropeptide Y- and calretinin-positive interneurons were found in group II.