Comparative density of CCK- and PV-GABA cells within the cortex and hippocampus - PubMed (original) (raw)
Comparative density of CCK- and PV-GABA cells within the cortex and hippocampus
Paul D Whissell et al. Front Neuroanat. 2015.
Abstract
Cholecystokinin (CCK)- and parvalbumin (PV)-expressing neurons constitute the two major populations of perisomatic GABAergic neurons in the cortex and the hippocampus. As CCK- and PV-GABA neurons differ in an array of morphological, biochemical and electrophysiological features, it has been proposed that they form distinct inhibitory ensembles which differentially contribute to network oscillations and behavior. However, the relationship and balance between CCK- and PV-GABA neurons in the inhibitory networks of the brain is currently unclear as the distribution of these cells has never been compared on a large scale. Here, we systemically investigated the distribution of CCK- and PV-GABA cells across a wide number of discrete forebrain regions using an intersectional genetic approach. Our analysis revealed several novel trends in the distribution of these cells. While PV-GABA cells were more abundant overall, CCK-GABA cells outnumbered PV-GABA cells in several subregions of the hippocampus, medial prefrontal cortex and ventrolateral temporal cortex. Interestingly, CCK-GABA cells were relatively more abundant in secondary/association areas of the cortex (V2, S2, M2, and AudD/AudV) than they were in corresponding primary areas (V1, S1, M1, and Aud1). The reverse trend was observed for PV-GABA cells. Our findings suggest that the balance between CCK- and PV-GABA cells in a given cortical region is related to the type of processing that area performs; inhibitory networks in the secondary cortex tend to favor the inclusion of CCK-GABA cells more than networks in the primary cortex. The intersectional genetic labeling approach employed in the current study expands upon the ability to study molecularly defined subsets of GABAergic neurons. This technique can be applied to the investigation of neuropathologies which involve disruptions to the GABAergic system, including schizophrenia, stress, maternal immune activation and autism.
Keywords: Dlx genes; balance; cholecystokinin; cortex; hippocampus; interneuron; intersectional genetics; parvalbumin.
Figures
FIGURE 1
Selective labeling of CCK- and PV-GABA cells using the dual recombinase–based intersectional genetic strategy. (A) Top. A dual recombinase-responsive reporter allele, RC::FrePe, contains two transcriptional stop cassettes. The first stop cassette is flanked by vertically oriented FRT sites (rectangles, denoted with F) and the other by directly oriented loxP sites (triangles, denoted with P). The loxP-flanked stop cassette also contains mCherry-encoding sequences. Middle: Flpe-mediated stop cassette removal results in mCherry expression. The remaining loxP-flanked stop cassette prevents GFP expression. Bottom: Upon removal of both stop cassettes, requiring Flpe- and Cre-mediated excisions, expression of GFP is turned on and expression of mCherry is turned off. The RC::FrePe allele is knocked-in to the Gt(ROSA)26Sor (R26) locus with CAG (chicken β-actin and CMV enhancer) promoter elements. (B) Venn diagrams illustrating intersectional and subtractive cell populations labeled by the intersectional approach using the RC::FrePe allele. The Dlx5/6-Flpe allele is specific to GABAergic cells in the forebrain. In triple transgenic mice inheriting all three alleles (CCK-Cre;Dlx5/6-Flpe;RC::FrePe mice or PV-Cre;Dlx5/6-Flpe;RC::FrePe mice), cells expressing both Cre and Flpe alleles (i.e., intersectional population) represent CCK- or PV-GABA cells, and are labeled with GFP. In contrast, cells expressing only Flpe (i.e., subtractive population) represent nonCCK-GABA or nonPV-GABA neurons and are labeled with mCherry. (C,D) Specificity of labeling GABA cells using the intersectional genetic strategy. (C) Confocal images of the CA1 stratum radiatum in CCK-Frepe mice (top) and the CA1 stratum pyramidale in PV-Frepe mice (bottom). GFP-expressing cells are labeled in green whereas CCK+ cells (top) and PV+ cells (bottom) are shown in blue. The border between the stratum pyramidale and stratum radiatum is denoted by the dotted line. Scale bar = 20 μm. (D) Percentage of GFP-expressing cells in CCK- and PV-Frepe mice that are CCK+ (orange) or PV+ (blue), respectively. (E) Efficacy of GABA cell labeling in the intersectional genetic approach. Confocal image shows the CA1 stratum radiatum in PV-Frepe mice. GFP- and mCherry-expressing cells are shown in green and red, respectively, whereas GAD67+ cells are shown in blue. Scale bar = 20 μm.
FIGURE 2
Distribution of CCK- and PV-GABA cells within the hippocampus. (A) Hippocampal sections from CCK-Frepe (left) and PV-Frepe mice (right). GFP-labeled cells represent CCK- and PV-GABA cells, respectively. (B) Percentage contribution of CCK-GABA cells (orange) and PV-GABA cells (blue) to the GABA neuron population in each hippocampal subregion. CCK-GABA cells were comparatively more abundant in the dCA1 and vCA1 regions whereas PV-GABA cells were more abundant in the dSub region. (C) The total density of GABA cells, defined as the sum of GFP- and mCherry-labeled cell density, did not differ between CCK- and PV-Frepe mice. (D,E) Distribution of CCK- and PV-GABA cells by hippocampal layer. CCK-GABA cells were most common in the sr layer but were also found in the sp and so layers. PV-GABA cells were more concentrated in the sp layer, but were also found in the so layer. Abbreviations by subregion: dCA1, dorsal CA1, dCA3, dorsal CA3, dDG, dorsal dentate gyrus, dSub, dorsal subiculum, vCA1, ventral CA1, vCA3, ventral CA3 and vDG, ventral dentate gyrus. Abbreviations by layer: hi, hilus, sg, stratum granulosum, sl, stratum lucidum, slm, stratum lacunosum-moleculare, sm, stratum moleculare, so, stratum oriens, sp, stratum pyramidale and sr, stratum radiatum. Significance at the p < 0.05 level is denoted with an asterisk. Scale bar = 500 μM.
FIGURE 3
CCK-GABA cells are most numerous within the medial prefrontal cortex. (Top) Sections of the prefrontal cortex in CCK- and PV-Frepe mice. (Bottom) Percentage contribution of CCK- and PV-GABA cells to the total GABA neuron population by subregion of the frontal cortex. In the DP and IL regions, CCK-GABA cells were more abundant than PV-GABA cells. CCK-GABA cells also tended to be more abundant in the PL region. PV-GABA cells were more abundant in M1 region and tended to be more abundant in the M2 region. Abbreviations: Cg, cingulate cortex, DP, dorsal peduncular region, IL, infralimbic cortex, M1, primary motor cortex, M2, secondary motor cortex, PL, prelimbic cortex. Significance at the p < 0.05 level is denoted with an asterisk. Scale bar = 500 μM.
FIGURE 4
Abundance of PV-GABA cells in the parietal cortex. (Top) Sections of the intermediate parietal cortex from CCK- and PV-Frepe mice. (Bottom) Relative contribution of CCK- and PV-GABA cells to the total GABA neuron population by subregion of the parietal cortex. PV-GABA cells were comparatively more numerous in every subregion but the PtaM. Abbreviations: PtaL, lateral parietal association area, PtaM, medial parietal association area, PtaP, posterior parietal association area, S1, primary somatosensory area, S1bf, somatosensory area (barrel), S1tr, somatosensory area (trunk), S1ulp, somatosensory area (upper lip region), S2, secondary somatosensory area. Significance at the p < 0.05 level is denoted with an asterisk. Scale bar = 500 μM.
FIGURE 5
Abundance of PV-GABA cells in the occipital cortex. (Top) Sections of the occipital cortex from CCK- and PV-Frepe mice. (Bottom) Percentage contribution of CCK- and PV-GABA cells to the total GABA cell population by subregion of the occipital cortex. PV-GABA cells are comparatively more numerous in every subregion but the V2mm, where they tended to be more numerous. Abbreviations: V1b, primary visual cortex, basal region; V1m, primary visual cortex, medial region; V2l, secondary visual cortex, lateral region; V2ml, secondary visual cortex, mediolateral region; V2mm, secondary visual cortex, mediomedial region. Significance at the p < 0.05 level is denoted with an asterisk. Scale bar = 500 μM.
FIGURE 6
Contrasting distribution of CCK- and PV-GABA cells in the temporal cortex. (Top) Sections of the temporal cortex from CCK- and PV-Frepe mice. (Bottom) Percentage contribution of CCK- and PV-GABA cells to the total GABA cell population by subregion of the temporal cortex. PV-GABA cells are significantly more numerous in auditory cortex regions. Ventral to the rhinal fissure, PV-GABA cells become less common and CCK-GABA cells become more numerous. Abbreviations: V1b, primary visual cortex, basal region; V1m, primary visual cortex, medial region; V2l, secondary visual cortex, lateral region; V2ml, secondary visual cortex, mediolateral region; V2mm, secondary visual cortex, mediomedial region. Significance at the p < 0.05 level is denoted with an asterisk. Scale bar = 500 μM.
FIGURE 7
Perisomatic interneuron balance and its relationship to cortical function. Cell balance was estimated using the delta score (D), which was the subtractive difference between CCK-GABA and PV-GABA cell percentage. Brain regions with D > 0 had more CCK-GABA cells, whereas regions with D < 0 had more PV-GABA cells. Moving right along _x_-axis of the graph, a decreasing D reflects a relatively increasing PV-GABA cell content. Regions of the primary sensory cortex or primary motor cortex (red) had negative D, indicating relatively higher PV-GABA cell content. Interestingly, secondary regions (green) had less negative D than their primary counterparts, indicating relatively lower PV-GABA cell content. For comparison, other non-motor and non-sensory cortical subregions are included (gray).
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