Subpopulations of somatostatin-immunoreactive non-pyramidal neurons in the amygdala and adjacent external capsule project to the basal forebrain: evidence for the existence of GABAergic projection neurons in the cortical nuclei and basolateral nuclear complex

Alexander J Mcdonald,Violeta Zaric,Franco Mascagni

doi:10.3389/fncir.2012.00046

Abstract

The hippocampus and amygdala are key structures of the limbic system whose connections include reciprocal interactions with the basal forebrain (BF). The hippocampus receives both cholinergic and GABAergic afferents from the medial septal area of the BF. Hippocampal projections back to the medial septal area arise from non-pyramidal GABAergic neurons that express somatostatin (SOM), calbindin (CB), and neuropeptide Y (NPY). Recent experiments in our lab have demonstrated that the basolateral amygdala, like the hippocampus, receives both cholinergic and GABAergic afferents from the BF. These projections arise from neurons in the substantia innominata (SI) and ventral pallidum (VP). It remained to be determined, however, whether the amygdala has projections back to the BF that arise from GABAergic non-pyramidal neurons. This question was investigated in the present study by combining Fluorogold (FG) retrograde tract tracing with immunohistochemistry for GABAergic non-pyramidal cell markers, including SOM, CB, NPY, parvalbumin, calretinin, and glutamic acid decarboxylase (GAD). FG injections into the BF produced a diffuse array of retrogradely labeled neurons in many nuclei of the amygdala. The great majority of amygdalar FG+ neurons did not express non-pyramidal cell markers. However, a subpopulation of non-pyramidal SOM+ neurons, termed “long-range non-pyramidal neurons” (LRNP neurons), in the external capsule, basolateral amygdala, and cortical and medial amygdalar nuclei were FG+. About one-third of the SOM+ LRNP neurons were CB+ or NPY+, and one-half were GAD+. It remains to be determined if these inhibitory amygdalar projections to the BF, like those from the hippocampus, are important for regulating synchronous oscillations in the amygdalar-BF network.

Highlights

The amygdala is one of the most important brain regions for the generation of emotional behavior and in the formation and retrieval of emotional memories, those related to fear and anxiety (Sah et al, 2003; Pape and Pare, 2010)
Since oscillatory activity in the amygdala is important for emotional arousal and emotional memory (Paré and Collins, 2000; Paré et al, 2002; Pape et al, 2005; Lesting et al, 2011), it is important to determine if there are GABAergic non-pyramidal neurons in the amygdala that project to the basal forebrain (BF). This question was investigated in the present study by combining Fluorogold (FG) retrograde tract tracing with immunohistochemistry for GABAergic non-pyramidal cell markers, including somatostatin (SOM), calbindin (CB), neuropeptide Y (NPY), parvalbumin (PV), calretinin (CR), and glutamic acid decarboxylase (GAD, the synthetic enzyme for GABA)
Examination of sections stained for immunofluorescence or immunoperoxidase revealed that the distribution of retrogradely labeled FG+ neurons in the amygdala was similar in most of the cases examined in this study and consisted of a diffuse array of neurons extending through many nuclei of the ipsilateral amygdala

Summary

Introduction

The amygdala is one of the most important brain regions for the generation of emotional behavior and in the formation and retrieval of emotional memories, those related to fear and anxiety (Sah et al, 2003; Pape and Pare, 2010). The BF contains a diffuse array of cholinergic and noncholinergic neurons that extends through a continuous region which includes the medial septal area, diagonal band of Broca, ventral pallidum (VP), and substantia innominata (SI) (Mesulam et al, 1983; Woolf, 1991). This complex has topographically organized connections with different forebrain regions including the hippocampus, neocortex, and amygdala (Mesulam et al, 1983; Zaborszky et al, 1999). In all three areas the cholinergic BF neurons innervate both pyramidal cells and GABAergic non-pyramidal cells (Frotscher and Léránth, 1985; Beaulieu and Somogyi, 1991; Henny and Jones, 2008; Muller et al, 2011), whereas the axons of GABAergic BF neurons form multiple synaptic contacts with individual GABAergic non-pyramidal cells (Freund and Gulyás, 1991; Freund and Meskenaite, 1992; Freund and Buzsáki, 1996; McDonald et al, 2011)

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Results

Conclusion