A quantitative analysis of parvalbumin neurons in rabbit auditory neocortex

Parvalbumin (PV) is a calcium‐binding protein present in GABAergic cells in the cerebral cortex and in thalamic relay neurons. In the present study, parvalbumin immunocytochemistry (PVi) and stereological methods were used to obtain estimates of cortical volume, total neuron number, laminar density, and the percentage of PV‐immunoreactive neurons in auditory neocortex. PVi clearly delineated the primary auditory cortex (AI), which was characterized by two PV+ bands: dense terminal‐like labeling within lamina III/IV and PV+ somata in lamina VIa. Stereological analysis of Nissl‐stained sections revealed that the total number of neurons in rabbit AI was 1.48 × 10 6 with a mean neuronal density of 55 × 10 3 /mm 3 . Based on a mean cortical thickness of 1.92 mm, there are approximately 106,000 neurons in a 1 mm 2 column of auditory cortex. PVi yields an extraordinary Golgi‐like staining of nonpyramidal cells in all cortical layers. PV+ nonpyramidal cells constitute approximately 7.0% of the neurons in AI. There were significant differences in the morphology and density of PV+ neurons across layers. Although only 5% of cells in lamina I were PV+, three nonpyramidal cell types were present. Lamina II had the highest numerical density within AI but the lowest percentage of PV+ neurons (3.3%). Lamina II, however, contained the greatest diversity of PV+ nonpyramidal cell types, which included small multipolar cells, bipolar cells, and, less frequently, large cells of the bitufted, bipolar, and stellate varieties. Lamina IV had one of the highest numerical densities (67.6 × 10 3 neurons/mm 3 ) and contributed nearly 27% of the total neuron number in AI. The numerical density of PV+ nonpyramidal cells was also greatest within lamina IV (7.1 × 10 3 /mm 3 ) where they formed 10.4% of the neuronal population. PV+ nonpyramidal cells in lamina IV and lamina III were predominantly large basket‐type cells with bitufted dendritic domains and tangentially oriented local axonal plexuses. The terminal‐like label within lamina III/IV derived in part from the basket‐cell axons, which formed pericellular arrays around unstained somata. Cell‐sparse lamina V contained the largest PV+ nonpyramidal cells in AI. These cells, which formed 11% of the neuron population in lamina V, were notable for their tangentially oriented dendritic fields and local axonal arbors. PVi partitioned lamina VI into VIa and VIb. Large multipolar nonpyramidal cells were distributed throughout lamina VI and made up approximately 6% of the total population. Lamina VIa contained a band of lightly labeled PV+ pyramidal neurons that formed 15% of the neuronal population. Double‐labeling experiments revealed that some PV+ pyramidal neurons within VIa also project to the ventral subdivision of the medial geniculate body (MGB). PVi demarcated the three major subdivisions of the MGB: the ventral (vMGB), dorsal (dMGB), and internal (iMGB) nuclei. The vMGB was strongly PV immunoreactive due to dense labeling of the neuropil and moderately labeled somata. The dMGB was characterized by scattered large PV+ cells and coarse PV+ axons. Relative to the vMGB, the neuropil of the dMGB contained only light terminal‐like labeling. The internal MGB contained few, if any, PV+ somata and had the least terminal‐like labeling of all MGB subdivisions. Because calcium‐binding proteins delineate functionally distinct, parallel pathways to sensory neocortex, they will be useful chemoarchitectonic tools for guiding future connectional studies of the MGB with the auditory neocortex and brainstem. © 1994 Wiley‐Liss, Inc.