Genetic variations of α(2)-adrenergic receptors illuminate the diversity of receptor functions

Early-Life Stress Influences the Transcriptional Activation of Alpha-2A Adrenergic Receptor and Associated Protein Kinase A Signaling Molecules in the Frontal Cortex of Rats

Early life is a highly sensitive period associated with profound changes in brain structure and function. Adverse experiences of early-life stress (ELS) are prominent risk factors for the precipitation of major depressive disorder (MDD). In recent years, dysfunction of the central noradrenergic (NA) system and subsequent deficits in norepinephrine (NE) signaling have gained increasing attention in the pathophysiology of MDD. However, the role of the α-2A adrenergic receptor and its downstream second messenger signaling system has not been investigated in connection to early-life stress-induced depression, limiting valuable insights into neurobiological mechanisms underlying this disorder. In this study, we used maternal separation (MS) as a rodent model of ELS to investigate whether ELS-induced depressive behavior is related to the α-2A adrenergic receptor and its associated second messenger signaling cascade. To do so, we studied expression levels of the α-2A adrenergic receptor (Adra2a), G alpha proteins (stimulatory subunit-Gαs [Gnas] and inhibitory subunit-Gαi [Gnai1 and Gnai2]), and downstream protein kinase A (PKA) catalytic [Prkarcα and Prkarcβ] and regulatory subunits [Prkar1α, Prkar1β, Prkar2α, and Prkar2β]) in the frontal cortex (FC) of MS rats. We found reduced sucrose preference in MS animals, along with reduced transcript levels of Adra2a, Gnai2, Prkar1β, and Prkarcβ. These findings suggest that ELS exposure may contribute to depression symptomatology via alterations in the expression of key genes involved in the NA system, highlighting potential mechanisms underlying ELS-induced depressive behavior.

β-amyloid redirects norepinephrine signaling to activate the pathogenic GSK3β/tau cascade

The brain noradrenergic system is critical for normal cognition and is affected at early stages in Alzheimer's disease (AD). Here, we reveal a previously unappreciated direct role of norepinephrine signaling in connecting β-amyloid (Aβ) and tau, two key pathological components of AD pathogenesis. Our results show that Aβ oligomers bind to an allosteric site on α2A adrenergic receptor (α2AAR) to redirect norepinephrine-elicited signaling to glycogen synthase kinase 3β (GSK3β) activation and tau hyperphosphorylation. This norepinephrine-dependent mechanism sensitizes pathological GSK3β/tau activation in response to nanomolar accumulations of extracellular Aβ, which is 50- to 100-fold lower than the amount required to activate GSK3β by Aβ alone. The significance of our findings is supported by in vivo evidence in two mouse models, human tissue sample analysis, and longitudinal clinical data. Our study provides translational insights into mechanisms underlying Aβ proteotoxicity, which might have strong implications for the interpretation of Aβ clearance trial results and future drug design and for understanding the selective vulnerability of noradrenergic neurons in AD.

Reciprocal Crosstalk Between Autophagic and Endocrine Signaling in Metabolic Homeostasis

Autophagy is a cellular quality control and energy-providing process that is under strict control by intra- and extracellular stimuli. Recently, there has been an exponential increase in autophagy research and its implications for mammalian physiology. Autophagy deregulation is now being implicated in many human diseases, and its modulation has shown promising results in several preclinical studies. However, despite the initial discovery of autophagy as a hormone-regulated process by De Duve in the early 1960s, endocrine regulation of autophagy still remains poorly understood. In this review, we provide a critical summary of our present understanding of the basic mechanism of autophagy, its regulation by endocrine hormones, and its contribution to endocrine and metabolic homeostasis under physiological and pathological settings. Understanding the cross-regulation of hormones and autophagy on endocrine cell signaling and function will provide new insight into mammalian physiology as well as promote the development of new therapeutic strategies involving modulation of autophagy in endocrine and metabolic disorders.

α(2A) adrenergic receptor promotes amyloidogenesis through disrupting APP-SorLA interaction

Accumulation of amyloid β (Aβ) peptides in the brain is the key pathogenic factor driving Alzheimer's disease (AD). Endocytic sorting of amyloid precursor protein (APP) mediated by the vacuolar protein sorting (Vps10) family of receptors plays a decisive role in controlling the outcome of APP proteolytic processing and Aβ generation. Here we report for the first time to our knowledge that this process is regulated by a G protein-coupled receptor, the α(2A) adrenergic receptor (α(2A)AR). Genetic deficiency of the α(2A)AR significantly reduces, whereas stimulation of this receptor enhances, Aβ generation and AD-related pathology. Activation of α(2A)AR signaling disrupts APP interaction with a Vps10 family receptor, sorting-related receptor with A repeat (SorLA), in cells and in the mouse brain. As a consequence, activation of α(2A)AR reduces Golgi localization of APP and concurrently promotes APP distribution in endosomes and cleavage by β secretase. The α(2A)AR is a key component of the brain noradrenergic system. Profound noradrenergic dysfunction occurs consistently in patients at the early stages of AD. α(2A)AR-promoted Aβ generation provides a novel mechanism underlying the connection between noradrenergic dysfunction and AD. Our study also suggests α(2A)AR as a previously unappreciated therapeutic target for AD. Significantly, pharmacological blockade of the α(2A)AR by a clinically used antagonist reduces AD-related pathology and ameliorates cognitive deficits in an AD transgenic model, suggesting that repurposing clinical α(2A)R antagonists would be an effective therapeutic strategy for AD.

Effect of sedation on pain perception

BACKGROUND

Sedation or anesthesia is used to facilitate many cases of an estimated 45 million diagnostic and therapeutic medical procedures in the United States. Preclinical studies have called attention to the possibility that sedative-hypnotic drugs can increase pain perception, but whether this observation holds true in humans and whether pain-modulating effects are agent-specific or characteristic of IV sedation in general remain unclear.

METHODS

To study this important clinical question, the authors recruited 86 healthy volunteers and randomly assigned them to receive one of three sedative drugs: midazolam, propofol, or dexmedetomidine. The authors asked participants to rate their pain in response to four experimental pain tasks (i.e., cold, heat, ischemic, or electrical pain) before and during moderate sedation.

RESULTS

Midazolam increased cold, heat, and electrical pain perception significantly (10-point pain rating scale change, 0.82 ± 0.29, mean ± SEM). Propofol reduced ischemic pain and dexmedetomidine reduced both cold and ischemic pain significantly (-1.58 ± 0.28, mean ± SEM). The authors observed a gender-by-race interaction for dexmedetomidine. In addition to these drug-specific effects, the authors observed gender effects on pain perception; female subjects rated identical experimental pain stimuli higher than male subjects. The authors also noted race-drug interaction effects for dexmedetomidine, with higher doses of drug needed to sedate Caucasians compared with African Americans.

CONCLUSIONS

The results of the authors' study call attention to the fact that IV sedatives may increase pain perception. The effect of sedation on pain perception is agent- and pain type-specific. Knowledge of these effects provides a rational basis for analgesia and sedation to facilitate medical procedures.

α2 adrenergic receptor dysregulation in depressive disorders: implications for the neurobiology of depression and antidepressant therapy

Dysfunction in noradrenergic neurotransmission has long been theorized to occur in depressive disorders. The α2 adrenergic receptor (AR) family, as a group of key players in regulating the noradrenergic system, has been investigated for involvement in the neurobiology of depression and mechanisms of antidepressant therapies. However, a clear picture of the α2ARs in depressive disorders has not been established due to the existence of apparently conflicting findings in the literature. In this article, we report that a careful accounting of methodological differences within the literature can resolve the present lack of consensus on involvement of α2ARs in depression. In particular, the pharmacological properties of the radioligand (e.g. agonist versus antagonist) utilized for determining receptor density are crucial in determining study outcome. Upregulation of α2AR density detected by radiolabeled agonists but not by antagonists in patients with depressive disorders suggests a selective increase in the density of high-affinity conformational state α2ARs, which is indicative of enhanced G protein coupling to the receptor. Importantly, this high-affinity state α2AR upregulation can be normalized with antidepressant treatments. Thus, depressive disorders appear to be associated with increased α2AR sensitivity and responsiveness, which may represent a physiological basis for the putative noradrenergic dysfunction in depressive disorders. In addition, we review changes in some key α2AR accessory proteins in depressive disorders and discuss their potential contribution to α2AR dysfunction.

Desipramine selectively potentiates norepinephrine-elicited ERK1/2 activation through the α2A adrenergic receptor

The precise physiological effects of antidepressant drugs, and in particular their actions at non-monoamine transporter targets, are largely unknown. We have recently identified the tricyclic antidepressant drug desipramine (DMI) as a direct ligand at the α(2A) adrenergic receptor (AR) without itself driving heterotrimeric G protein/downstream effector activation [5]. In this study, we report our novel finding that DMI modulates α(2A)AR signaling in response to the endogenous agonist norepinephrine (NE). DMI acted as a signaling potentiator, selectively enhancing NE-induced α(2A)AR-mediated ERK1/2 MAPK signaling. This potentiation of ERK1/2 activation was observed as an increase in NE response sensitivity and a prolongation of the activation kinetics. DMI in a physiologically relevant ratio with NE effectively turned on ERK1/2 signaling that is lacking in response to physiological NE alone. Further, the DMI-induced ERK1/2 potentiation relied on heterotrimeric G(i/o) proteins and was arrestin-independent. This modulatory effect of DMI on NE signaling provides novel insight into the effects of this antidepressant drug on the noradrenergic system which it regulates, insight which enhances our understanding of the therapeutic mechanism for DMI.