Enkephalinergic afferents of the centromedial amygdala in the rat

Abstract

The connectivity of the amygdaloid complex has been extensively explored with both anterograde and retrograde tracers. Even though the afferents of the centromedial amygdala [comprising the central (CEA) and medial (MEA) amygdaloid nuclei] are well established, relatively little is known about the neuropeptide phenotype of these connections. In this study, we first examined the distribution of mu-opioid receptor (MOR) and delta-opioid receptor (DOR) in the amygdala via in situ hybridization and immunohistochemistry. We then investigated the distribution of Met-enkephalin (ENK) and Leu-ENK fibers with immunohistochemistry and examined the distribution of preproenkephalin mRNA in the amygdala by using in situ hybridization. Finally, we examined the ENK projections to the CEA and MEA by using stereotaxic injections of the retrograde tracer cholera toxin subunit B or fluorogold revealed by immunohistochemistry combined with in situ hybridization to identify ENKergic neurons. Our results indicate that the centromedial amygdala receives ENK afferents, as indicated by the presence of MOR, DOR, and ENK fibers in the CEA and MEA, originating primarily from the bed nucleus of the stria terminalis (BST) and from other amygdaloid nuclei. The posterior BST, the basomedial nucleus (BMA), and the cortical nucleus of the amygdala (COA) were found to be the major ENK afferents of the MEA, whereas the anterolateral BST, the COA, the MEA, and the BMA provided the main ENKergic innervation of the CEA. In addition, we found that the ventromedial nucleus of the hypothalamus and the pontine parabrachial nucleus provide a moderate ENK input to the CEA and MEA. The functional implications of these connections in stress, anxiety, and nociception are discussed.

The role of delta opioid receptors in the anxiolytic actions of benzodiazepines

“…The amygdala has a high density of GABA A and benzodiazepine receptor sites as well as opioid receptor sites (Shiosaka et al 1983;Mansour et al 1995a;Veinante et al 1997;Wilson et al 2002;Poulin et al 2006). The anxiolytic actions of benzodiazepines are thought to be mediated by neurons expressing the GABA A receptor containing the α2 subunit.…”

“…This particular subunit is largely localized in limbic structures, including the amygdala (Low et al 2000;Rudolph et al 2001). PreproENK mRNA is seen in various nuclei of the amygdala including the central, basolateral and posteroventral portion of the medial nucleus, and most of the ENK afferents to the centromedial amygdala arise from intra-amygdaloid sources or the bed nucleus of the stria terminalis (BNST) (Poulin et al 2006). Combined with our previous data (Kang et al 2000) and the localization of reporter (GFP) expression (Figure 1b), this suggests that viral overexpression most likely changes intraamygdalar ENK projections that subsequently modulate diazepam effects in the plus maze.…”

“…The amygdala has both mu and delta opioid receptor sites (Mansour et al 1995a;Wilson et al 2002;Poulin et al 2006), and met-ENK colocalizes with the GABA synthetic enzyme glutamate decarboxylase (GAD) in the central nucleus of the amygdala (Veinante et al 1997). Morphine injections into the amygdala also produce partial anxiolytic effects in the social interaction test (File and Rodgers 1979a).…”

Distinct opioid circuits determine the palatability and the desirability of rewarding events

“…Using consumption and taste-reactivity measures of palatability in rodents, several groups have reported evidence of an opioid receptor-mediated network within the circuitry of the nucleus accumbens shell and ventral pallidum mediating hedonic processing or reward ''liking'' based on the palatability-enhancing effects of locally infused opioid agonists (5)(6)(7)(8). However, although opioid processes often have been proposed to convey the affective properties of natural rewards (9,10), opioid peptide-containing neurons and receptors are present in multiple basal forebrain regions (11,12) implicated not in reward hedonia but rather in the learning process that mediates goal-directed actions. In particular, several studies have found the opioid receptor-rich basolateral amygdala (13) to be important for encoding the incentive value of the rewards that support the performance of goal-directed actions in rats (14)(15)(16)(17).…”

Occlusal Disharmony Transiently Impairs Learning and Memory in the Mouse by Increasing Dynorphin A Levels in the Amygdala

“…Anatomically, the amygdala is subdivided into central, medial, cortical, basomedial and basolateral nuclei. Among these nuclei, the central nucleus has abundant proenkepha-lin, proopiomelanocortin (POMC) and prodynorphin nerve fibers (Finley et al 1981;Khachaturian et al 1982;Fallon and Leslie 1986;Cassell and Gray 1989;Cassell et al 1999;Poulin et al 2006). The opioid nervous system in the central amygdala associates with pain, fear, anxiety, stress and feeding behavior (Davis et al 1994;Bernard et al 1996;Pitkanen et al 1997;LeDoux 2000;Swanson 2000;Petrovich and Gallagher 2003).…”