Serotonin‐immunoreactive axon terminals innervate pyramidal cells and interneurons in the rat basolateral amygdala

Abstract

The basolateral nuclear complex of the amygdala (BLC) receives a dense serotonergic innervation that appears to play a critical role in the regulation of mood and anxiety. However, little is known about how serotonergic inputs interface with different neuronal subpopulations in this region. To address this question, dual-labeling immunohistochemical techniques were used at the light and electron microscopic levels to examine inputs from serotonin-immunoreactive (5-HT+) terminals to different neuronal subpopulations in the rat BLC. Pyramidal cells were labeled by using antibodies to calcium/calmodulin-dependent protein kinase II, whereas different interneuronal subpopulations were labeled by using antibodies to a variety of interneuronal markers including parvalbumin (PV), vasoactive intestinal peptide (VIP), calretinin, calbindin, cholecystokinin, and somatostatin. The BLC exhibited a dense innervation by thin 5-HT+ axons. Electron microscopic examination of the anterior basolateral nucleus (BLa) revealed that 5-HT+ axon terminals contained clusters of small synaptic vesicles and a smaller number of larger dense-core vesicles. Serial section reconstruction of 5-HT+ terminals demonstrated that 76% of these terminals formed synaptic junctions. The great majority of these synapses were symmetrical. The main targets of 5-HT+ terminals were spines and distal dendrites of pyramidal cells. However, in light microscopic preparations it was common to observe apparent contacts between 5-HT+ terminals and all subpopulations of BLC interneurons. Electron microscopic analysis of the BLa in sections dual-labeled for 5-HT/PV and 5-HT/VIP revealed that many of these contacts were synapses. These findings suggest that serotonergic axon terminals differentially innervate several neuronal subpopulations in the BLC.

A Genetically Defined Morphologically and Functionally Unique Subset of 5-HT Neurons in the Mouse Raphe Nuclei

“…However, when serial electron microscopic reconstructions are examined, 5-HT terminal boutons make synaptic contacts in several structures (Descarries et al, 2010). Present observations indicated that a sizable contingent of 5-HT terminals make synaptic junctions in the lateral amygdala, in agreement with previous ultrastructural analysis in the rat lateral amygdala (Muller et al, 2007). Unexpectedly, in Pet1 knock-out mice, we found an increased proportion of synaptic junctions made by the spared 5-HT axon terminals in the amygdala, thalamus, and hypothalamus.…”

“…The 5-HT receptors that are present in the areas where 5-HT axon terminals remain in Pet1 knock-out mice may provide interesting clues in this respect. For instance, in the BLA, where 5-HT boutons form synapses, 5-HT2A, 5-HT2C, and 5-HT3 receptors are the main receptors (Wright et al, 1995;McDonald and Mascagni, 2007;Muller et al, 2007). 5-HT2A and 5-HT2C receptors are also strongly expressed in the hypothalamic paraventricular nucleus and intralaminar thalamic nuclei, with preserved innervation in the Pet1 knock-out (Zhang et al, 2002;Heisler et al, 2007).…”

Effects of Repeated Stress on Age-Dependent GABAergic Regulation of the Lateral Nucleus of the Amygdala

“…Amygdala GABAergic interneurons are targets of dopamine, NE, serotonin, acetylcholine, and cannabinoids (Brinley-Reed and McDonald, 1999;Rainnie, 1999;Katona et al, 2001;McDonald and Mascagni, 2001;Azad et al, 2003;Bissiere et al, 2003;Kroner et al, 2005;Muller et al, 2007Muller et al, , 2011Tully et al, 2007;Pinard et al, 2008). Repeated stress may influence amygdala GABAergic transmission by impairing the neuromodulation of amygdala neurons (Braga et al, 2004;Buffalari and Grace, 2009;Hill et al, 2009Hill et al, , 2010, perhaps in an age-dependent manner.…”

Functional anatomy of 5-HT2A receptors in the amygdala and hippocampal complex: relevance to memory functions

“…The highest density of serotonin-IR fibers is observed in the magnocellular division of the basal nucleus and amygdalohippocampal area (Steinbusch 1981). On the other hand, in the monkey's amygdala, the density of serotonin-IR terminals is as follows: (i) low in the accessory basal, anterior cortical, posterior cortical and medial nuclei, and in subregions of the periamygdaloid cortex; (ii) moderate in portions of the basal, lateral, and intercalated nuclei; and (iii) high in the central nucleus, the nucleus of the lateral olfactory tract, the paralaminar nucleus, the anterior amygdaloid area, and a small region of the amygdalohippocampal area (Bauman and Amaral 2005 (Muller et al 2007b). …”

“…In the amygdala and hippocampal region, serotonin operates via conventional synapses as well as local volume neurotransmission, reaching their specific receptors through extra-cellular or paracrine diffusion (Amaral and Lavenex 2007;Muller et al 2007b). …”