Serotonergic innervation and serotonin receptor expression of NPY-producing neurons in the rat lateral and basolateral amygdaloid nuclei

M Bonn,A Schmitt,K.-P Lesch,E Asan,E J Van Bockstaele

doi:10.1007/s00429-012-0406-5

Abstract

Pharmacobehavioral studies in experimental animals, and imaging studies in humans, indicate that serotonergic transmission in the amygdala plays a key role in emotional processing, especially for anxiety-related stimuli. The lateral and basolateral amygdaloid nuclei receive a dense serotonergic innervation in all species studied to date. We investigated interrelations between serotonergic afferents and neuropeptide Y (NPY)-producing neurons, which are a subpopulation of inhibitory interneurons in the rat lateral and basolateral nuclei with particularly strong anxiolytic properties. Dual light microscopic immunolabeling showed numerous appositions of serotonergic afferents on NPY-immunoreactive somata. Using electron microscopy, direct membrane appositions and synaptic contacts between serotonin-containing axon terminals and NPY-immunoreactive cellular profiles were unequivocally established. Double in situ hybridization documented that more than 50 %, and about 30–40 % of NPY mRNA-producing neurons, co-expressed inhibitory 5-HT1A and excitatory 5-HT2C mRNA receptor subtype mRNA, respectively, in both nuclei with no gender differences. Triple in situ hybridization showed that individual NPY mRNA-producing interneurons co-express both 5-HT1A and 5-HT2C mRNAs. Co-expression of NPY and 5-HT3 mRNA was not observed. The results demonstrate that serotonergic afferents provide substantial innervation of NPY-producing neurons in the rat lateral and basolateral amygdaloid nuclei. Studies of serotonin receptor subtype co-expression indicate a differential impact of the serotonergic innervation on this small, but important, population of anxiolytic interneurons, and provide the basis for future studies of the circuitry underlying serotonergic modulation of emotional stimulus processing in the amygdala.

Highlights

The amygdala is a heterogeneous telencephalic nuclear complex which plays a key role in the processing of emotional, innate anxiety- and fear-related stimuli, including the mediation of adequate autonomous, endocrine and behavioral reactions, and the formation of emotional memories both in rodents and humans (Damsa et al 2009; LeDoux 2000; Roozendaal et al 2009)
We investigated interrelations between serotonergic afferents and neuropeptide Y (NPY)-producing neurons, which are a subpopulation of inhibitory interneurons in the rat lateral and basolateral nuclei with strong anxiolytic properties
NPY, 5-HTT and 5-HT immunoreactivity: LM observations Using single labeling, NPY-immunoreactivity in the central nucleus (Ce) was scarce, but numerous fusiform or round NPY-ir somata (Fig. 1a, insets) and dense plexus of NPY-ir fibers were observed in the lateral nucleus (La) and basolateral nucleus (BL) (Fig. 1a)

Summary

Introduction

The amygdala is a heterogeneous telencephalic nuclear complex which plays a key role in the processing of emotional, innate anxiety- and fear-related stimuli, including the mediation of adequate autonomous, endocrine and behavioral reactions, and the formation of emotional memories both in rodents and humans (Damsa et al 2009; LeDoux 2000; Roozendaal et al 2009). The serotonin receptors 1A (5-HT1A), 2C (5-HT2C) and 3 (5-HT3) are expressed in the La and BL (Clemett et al 2000; Miquel et al 2002; Pazos and Palacios 1985) and seem to be crucially involved in the regulation of anxietyrelated neuronal circuitries These findings render interactions of the serotonergic system with amygdaloid circuits of significant interest for emotion research, and numerous pharmacobehavioral studies have been carried out to elucidate effects of serotonergic agonist or antagonist application into the amygdala, albeit with occasionally controversial results (Barnes and Sharp 1999; Lowry et al 2005; Menard and Treit 1999). Females (Bhatnagar et al 2004), 5-HTR expression studies were performed separately in male and female rats

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