Abstract
Norepinephrine (NE) is thought to play a key role in fear and anxiety, but its role in amygdala-dependent Pavlovian fear conditioning, a major model for understanding the neural basis of fear, is poorly understood. The lateral nucleus of the amygdala (LA) is a critical brain region for fear learning and regulating the effects of stress on memory. To understand better the cellular mechanisms of NE and its adrenergic receptors in the LA, we used antibodies directed against dopamine beta-hydroxylase (DβH), the synthetic enzyme for NE, or against two different isoforms of the beta-adrenergic receptors (βARs), one that predominately recognizes neurons (βAR 248) and the other astrocytes (βAR 404), to characterize the microenvironments of DβH and βAR. By electron microscopy, most DβH terminals did not make synapses, but when they did, they formed both asymmetric and symmetric synapses. By light microscopy, βARs were present in both neurons and astrocytes. Confocal microscopy revealed that both excitatory and inhibitory neurons express βAR248. By electron microscopy, βAR 248 was present in neuronal cell bodies, dendritic shafts and spines, and some axon terminals and astrocytes. When in dendrites and spines, βAR 248 was frequently concentrated along plasma membranes and at post-synaptic densities of asymmetric (excitatory) synapses. βAR 404 was expressed predominately in astrocytic cell bodies and processes. These astrocytic processes were frequently interposed between unlabeled terminals or ensheathed asymmetric synapses. Our findings provide a morphological basis for understanding ways in which NE may modulate transmission by acting via synaptic or non-synaptic mechanisms in the LA.
Highlights
The lateral nucleus of the amygdala (LA) is a critical brain region for fear learning (LeDoux, 2000, 2007; Rodrigues et al, 2004; Maren, 2005; Lang and Davis, 2006) and blocking beta-adrenergic receptors within the LA disrupts the acquisition of fear conditioning (Bush et al, 2010)
Using commercial antibodies directed against the β1 and β2 receptor subtypes, one confocal microscopic study found that the β1 and β2 receptor subtypes are widely distributed in basal amygdala neurons but not in astrocytes (Qu et al, 2008)
Light microscopy The LA contains a dense plexus of dopamine beta-hydroxylase fibers The pattern of DβH-immunoreactivity (-ir) in the amygdala has been described previously (Moore and Card, 1984; Fallon and Ciofi, 1992; Asan, 1993, 1998; Li et al, 2001, 2002)
Summary
Norepinephrine (NE) has long been implicated in fear and anxiety (Gray, 1978; Redmond, 1979; Aston-Jones and Bloom, 1981; Aston-Jones et al, 1999, 2000; Sullivan et al, 1999), and is known to play a role in learning and memory (Ferry et al, 1997; Bailey et al, 2000; McGaugh, 2002). Using commercial antibodies directed against the β1 and β2 receptor subtypes, one confocal microscopic study found that the β1 and β2 receptor subtypes are widely distributed in basal amygdala neurons but not in astrocytes (Qu et al, 2008). This group found that while both receptor subtypes were seen in the membranes and cytoplasm of cell bodies, the β2 receptor subtype, but not the β1,was localized to the nucleus. To understand better the cellular mechanisms of NE’s contributions to fear learning, we examined the anatomical
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