Total Number and Ratio of GABAergic Neuron Types in the Mouse Lateral and Basal Amygdala

Abstract

GABAergic neurons are key circuit elements in cortical networks. Despite 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 has 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 sex. 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 (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 pathologic conditions, and to model functioning amygdalar networks in health and disease.

Interneuron Types and Their Circuits in the Basolateral Amygdala

“…Similar to the hippocampus and neocortex, GABAergic cells in the BLA give rise to about 20% of the total neuronal population. Specifically, in the LA 16% of all neurons are inhibitory neurons, whereas in the BA this ratio is significantly higher, 22%, as it has been recently determined (Vereczki et al, 2021 ). These GABAergic neurons may be categorized into four major functional groups defined by their axonal targets: (1) perisomatic region-targeting inhibitory cells innervate the soma, proximal dendrites, or axon initial segment (AIS) of principal cells and provide the most effective control of spiking activity; (2) dendrite-targeting inhibitory cells innervate primarily the dendrites of principal cells, where they can regulate dendritic computation; (3) interneuron-selective interneurons (ISI) form synaptic contacts specifically with other GABAergic cells, and thus interneurons in this category are in a position to disinhibit principal cells temporarily, permitting their activity to increase in an input-specific manner; and (4) GABAergic projection cells giving rise to both local axonal collaterals and long-range projections can control information flow between the BLA and remote regions they project to.…”

“…Marco Capogna’s group was the first to demonstrate the existence of axo-axonic cells in the rat BLA (Bienvenu et al, 2012 ), a finding that was followed by the identification of this cell type in the mouse ( Figures 2A,C ; Veres et al, 2014 ) as well as in the monkey BLA (McDonald and Augustine, 2020 ). In a recent study, we have estimated that 0.8% and 1.3% of all neurons in the mouse LA and BA belong to axo-axonic cells, respectively (Vereczki et al, 2021 ). The majority of them (~70%) express PV, while a small fraction lacks this Ca 2+ binding protein (Vereczki et al, 2021 ).…”

“…In a recent study, we have estimated that 0.8% and 1.3% of all neurons in the mouse LA and BA belong to axo-axonic cells, respectively (Vereczki et al, 2021 ). The majority of them (~70%) express PV, while a small fraction lacks this Ca 2+ binding protein (Vereczki et al, 2021 ). Uniformly, axo-axonic cells do not express calbindin (Calb), another Ca 2+ binding protein (Bienvenu et al, 2012 ; Vereczki et al, 2016 ; Andrasi et al, 2017 ; Rovira-Esteban et al, 2019 ), which is typically present in PV basket cells (Vereczki et al, 2006 ; Bienvenu et al, 2012 ; Andrasi et al, 2017 ; Rovira-Esteban et al, 2019 ).…”

“…The analysis of the distribution of GABAergic axon terminals along the axon initial segments in the BA has uncovered that the largest density of boutons irrespective of their PV content peaked at 20–40 μm measured from the soma (Veres et al, 2014 ; Vereczki et al, 2021 ). So what is special in that region of the axon initial segment covered by GABAergic terminals with the highest density?…”

“…Based on the cell types, their connectivity features, and developmental characteristics, the BLA is a cortical structure. Accordingly, glutamatergic excitatory projection cells expressing vesicular glutamate transporter type 1 (VGluT1; Andrasi et al, 2017 ) are the most numerous neurons in this amygdala region (80–85%; Vereczki et al, 2021 ). The dendrites of the principal cells (PC) are densely decorated with spines and their axon arborizes within the nucleus, giving rise to local collaterals, but they also project to other amygdala regions and remote cortical and/or subcortical areas (Sah et al, 2003 ).…”