Immunocytochemical analysis of basket cells in rat piriform cortex

Basket cells, defined by axons that preferentially contact cell bodies, were studied in rat piriform (olfactory) cortex with antisera to gamma-aminobutyric acid (GABA)ergic markers (GABA, glutamate decarboxylase) and to peptides and calcium binding proteins that are expressed by basket cells. Detailed visualization of dendritic and axonal arbors was obtained by silver-gold enhancement of staining for vasoactive intestinal peptide (VIP), cholecystokinin (CCK), parvalbumin, and calbindin. Neuronal features were placed into five categories: soma-dendritic and axonal morphologies, laminar distributions of dendritic and axonal processes, and molecular phenotype. Although comparatively few forms were distinguished within each category, a highly varied co-expression of features from different categories produced a "combinatorial explosion" in the characteristics of individual neurons. Findings of particular functional interest include: dendritic distributions suggesting that somatic inhibition is mediated by feedforward as well as feedback pathways, axonal variations suggesting a differential shaping of the temporal aspects of somatic inhibition from different basket cells, evidence that different principal cell populations receive input from different combinations of basket cells, and a close association between axonal morphology and molecular phenotype. A finding of practical importance is that light microscopic measurements of boutons were diagnostic for the molecular phenotype and certain morphological attributes of basket cells. It is argued that the diversity in basket cell form in the piriform cortex, as in other areas of the cerebral cortex, reflects requirements for large numbers of specifically tailored inhibitory processes for optimal operation that cannot be met by a small number of rigidly defined neuronal populations.