Total number and ratio of GABAergic neuron types in the mouse lateral and basal amygdala.

GABAergic neurons are key circuit elements in cortical networks. In spite of growing evidence showing that inhibitory cells play a critical role in the lateral (LA) and basal (BA) amygdala functions, neither the number of GABAergic neurons nor the ratio of their distinct types have been determined in these amygdalar nuclei. Using unbiased stereology, we found that the ratio of GABAergic neurons in the BA (22%) is significantly higher than in the LA (16%) in both male and female mice. No difference was observed between the right and left hemispheres in either sexes. In addition, we assessed the ratio of the major inhibitory cell types in both amygdalar nuclei. Using transgenic mice and a viral strategy for visualizing inhibitory cells combined with immunocytochemistry, we estimated that the following cell types together compose the vast majority of GABAergic cells in the LA and BA: axo-axonic cells (5.5-6%), basket cells expressing parvalbumin (17-20%) or cholecystokinin (7-9%), dendrite-targeting inhibitory cells expressing somatostatin (10-16%), NPY-containing neurogliaform cells (14-15%), VIP and/or calretinin-expressing interneuron-selective interneurons (29-38%) and GABAergic projection neurons expressing somatostatin and neuronal nitric oxide synthase (nNOS, 5.5-8%). Our results show that these amygdalar nuclei contain all major GABAergic neuron types as found in other cortical regions. Furthermore, our data offer an essential reference for future studies aiming to reveal changes in GABAergic cell number and in inhibitory cell types typically observed under different pathological conditions, and to model functioning amygdalar networks in health and disease.SIGNIFICANCE STATEMENTGABAergic cells in cortical structures, like in the lateral and basal nucleus of the amygdala, have a determinant role in controlling circuit operation. In this study, we provide the first estimate for the total number of inhibitory cells in these two amygdalar nuclei. In addition, our study is the first to define the ratio of the major GABAergic cell types present in these cortical networks. Taking into account that hyper-excitability in the amygdala, arising from the imbalance between excitation and inhibition typifies many altered brain functions including anxiety, posttraumatic stress disorder, schizophrenia and autism, uncovering the number and ratio of distinct amygdalar inhibitory cell types offers a solid base for comparing the changes in inhibition in pathological brain states.