The hypotensive effect of activated apelin receptor is correlated with ß-arrestin recruitment.

The apelinergic system is an important player in the regulation of both vascular tone and cardiovascular function, making this physiological system an attractive target for drug development for hypertension, heart failure and ischemic heart disease. Indeed, apelin exerts a positive inotropic effect in humans whilst reducing peripheral vascular resistance. In this study, we investigated the signaling pathways through which apelin exerts its hypotensive action. We synthesized a series of apelin-13 analogs whereby the C-terminal Phe13 residue was replaced by natural or unnatural amino acids. In HEK293 cells expressing APJ, we evaluated the relative efficacy of these compounds to activate Gαi1 and GαoA G-proteins, recruit ß-arrestins 1 and 2 (ßarrs), and inhibit cAMP production. Calculating the transduction ratio for each pathway allowed us to identify several analogs with distinct signaling profiles. Furthermore, we found that these analogs delivered i.v. to Sprague-Dawley rats exerted a wide range of hypotensive responses. Indeed, two compounds lost their ability to lower blood pressure, while other analogs significantly reduced blood pressure as apelin-13. Interestingly, analogs that did not lower blood pressure were less effective at recruiting ßarrs. Finally, using Spearman correlations, we established that the hypotensive response was significantly correlated with ßarr recruitment but not with G protein-dependent signaling. In conclusion, our results demonstrated that the ßarr recruitment potency is involved in the hypotensive efficacy of activated APJ.

Enkephalin knockdown in the central amygdala nucleus reduces unconditioned fear and anxiety.

The endogenous opioid enkephalins (ENK) are highly expressed in the central nucleus of the amygdaloid complex (CeA) where several lines of evidence point to a potential role in the modulation of fear and anxiety. In this study, we aimed to assess the role of CeA ENK using local injections of a lentiviral vector expressing a short hairpin RNA (shRNA) targeting ENK in Sprague-Dawley rats. We injected this vector in the CeA and a 56% downregulation of ENK mRNA was observed in animals when compared with scrambled shRNA animals. Anxiety-like behaviors were also assessed using the elevated plus maze and social interaction test. There was an increase in exploration of open arms of the elevated plus maze in ENK knockdown animals compared with controls, but no change in social interaction. In addition, we used the contextual fear conditioning procedure to assess fear expression and learning in these animals. There was a reduction in freezing induced by acute shocks during the training procedure. Interestingly, associative learning was not affected, and ENK knockdown animals displayed an equivalent freezing when re-exposed to the conditioning chamber 48 h later. These results contrast with knockout mice studies, which ascribed anxiolytic properties to ENK, and they demonstrate the need for a thorough understanding and characterization of neuroanatomically distinct ENK pathways.

C-Terminal modifications of apelin-13 significantly change ligand binding, receptor signaling, and hypotensive action.

Apelin is the endogenous ligand of the APJ receptor, a member of the G protein-coupled receptor family. This system plays an important role in the regulation of blood pressure and cardiovascular functions. To better understand the role of its C-terminal Phe(13) residue on ligand binding, receptor signaling, and hypotension, we report a series of modified analogues in which Phe(13) was substituted by unnatural amino acids. These modifications delivered new compounds exhibiting higher affinity and potency to inhibit cAMP accumulation compared to apelin-13. In particular, analogues Bpa(13) or (α-Me)Phe(13) were 30-fold more potent to inhibit cAMP accumulation than apelin-13. Tyr(OBn)(13) substitution led to a 60-fold improvement in binding affinity and induced stronger and more sustained drop in blood pressure compared to apelin-13. Our study identified new potent analogues of apelin-13, which represent valuable probes to better understand its structure-function relationship.

Enkephalin knockdown in the basolateral amygdala reproduces vulnerable anxiety-like responses to chronic unpredictable stress.

The endogenous enkephalins (ENKs) are potential candidates participating in the naturally occurring variations in coping styles and determining the individual capacities for adaptation during chronic stress exposure. Here we demonstrate that there is a large variance in individual behavioral, as well as in physiological outcomes, in a population of Sprague-Dawley rats subjected to 3 weeks of chronic unpredictable stress (CUS). Separation of resilient and vulnerable subpopulations reveals specific long-term neuroadaptation in the ENKergic brain circuits. ENK mRNA expression was greatly reduced in the posterior basolateral nucleus of amygdala (BLAp) in vulnerable individuals. In contrast, ENK mRNA levels were similar in resilient and control (unstressed) individuals. Another group of rats were used for lentiviral-mediated knockdown of ENK to assess whether a decrease of ENK expression in the BLAp reproduces the behavioral disturbances found in vulnerable individuals. ENK knockdown specifically located in the BLAp was sufficient to increase anxiety in the behavioral tests, such as social interaction and elevated plus maze when compared with control individuals. These results show that specific neuroadaptation mediated by the ENKergic neurotransmission in the BLAp is a key regulator of resilience, whereas a decrease of the ENK in the BLAp is a maladaptation mechanism, which mediates the behavioral dichotomy observed between vulnerable and resilient following 3 weeks of CUS.

Enkephalin and dynorphin mRNA expression are associated with resilience or vulnerability to chronic social defeat stress.

There are important and enduring differences between individuals in the magnitude of all aspects of the stress response. Among the neuropeptide systems, the endogenous opioids enkephalin (ENK) and dynorphin (DYN), are very interesting candidates to participate in the naturally occurring variations in coping styles and to determine the individual capacity for adaptation during chronic stress exposure. Under chronic social stress exposure, we hypothesize that changes in the ENKergic vs DYNergic neuronal systems within specific nuclei of the basal forebrain contribute to naturally occurring variations in coping styles and will determine individual capacities for stress adaptation. Sprague-Dawley rats were exposed to a resident-intruder model of defeat for 7 days. The average latency to be defeated over seven consecutive days was calculated for each intruder rat. Based on this distribution, we chose an average defeat latency of 350s as a cutoff criterion to define resilient and vulnerable rats. A subpopulation assumed a subordinate posture in a relatively short latency (<350s, SL) and the other subpopulation resisted defeat resulting in longer latencies (>350s, LL) to assume this posture and were identified as being vulnerable and resilient respectively. Rats were euthanized 24h after the last stress session. ENK mRNA expression was lower in the basolateral nucleus of the amygdala in vulnerable compared to control and resilient individuals. In contrast, there was no difference between resilient and control individuals. DYN mRNA is increased only within the dorsal and medial shell of the NAc of vulnerable rats compared to control individuals. There was no difference between resilient and control individuals. DYN mRNA is increased in resilient individuals in the central area of the striatum, caudal part, compared to control individuals. DYN is also increased in medial area of the striatum, caudal part in resilient and vulnerable compared to control individuals. These results have broad implications for understanding the functional roles of opioid neurotransmission following repeated social stress and suggest that ENK could facilitate the adaptation of behavioral responses by opposition to the DYN neurotransmission that appears to promote maladaptive behavioral response to chronic social stress.

Increased anxiety-like behaviors in rats experiencing chronic inflammatory pain.

For many patients, chronic pain is often accompanied, and sometimes amplified, by co-morbidities such as anxiety and depression. Although it represents important challenges, the establishment of appropriate preclinical behavioral models contributes to drug development for treating chronic inflammatory pain and associated psychopathologies. In this study, we investigated whether rats experiencing persistent inflammatory pain induced by intraplantar injection of complete Freund's adjuvant (CFA) developed anxiety-like behaviors, and whether clinically used analgesic and anxiolytic drugs were able to reverse CFA-induced anxiety-related phenotypes. These behaviors were evaluated over 28 days in both CFA- and saline-treated groups with a variety of behavioral tests. CFA-induced mechanical allodynia resulted in increased anxiety-like behaviors as evidenced by: (1) a significant decrease in percentage of time spent and number of entries in open arms of the elevated-plus maze (EPM), (2) a decrease in number of central squares visited in the open field (OF), and (3) a reduction in active social interactions in the social interaction test (SI). The number of entries in closed arms in the EPM and the distance traveled in the OF used as indicators of locomotor performance did not differ between treatments. Our results also reveal that in CFA-treated rats, acute administration of morphine (3mg/kg, s.c.) abolished tactile allodynia and anxiety-like behaviors, whereas acute administration of diazepam (1mg/kg, s.c) solely reversed anxiety-like behaviors. Therefore, pharmacological treatment of anxiety-like behaviors induced by chronic inflammatory pain can be objectively evaluated using multiple behavioral tests. Such a model could help identify/validate alternative potential targets that influence pain and cognitive dimensions of anxiety.

Subacute H2O2, but not poly(IC), upregulates dopamine D2 receptors in retinoic acid differentiated SH-SY5Y neuroblastoma.

Abnormalities of striatal dopaminergic neurotransmission play a significant role in the pathophysiology of central nervous system disorders such as movement disorders, addictions and schizophrenia. The striatum appears to be exposed to intrinsically high levels of oxidative stress (OS). Little is known, however, on the effect of OS on the regulation of the dopamine D2 receptor (DRD2), a key component of striatal dopaminergic neurotransmission. We report here on the effects of H2O2 (a canonical oxidant and non conventional messenger) and polyinosinic-polycytidylic acid (poly(IC) which elicits schizophrenia-like behaviors in newborn rodents and disrupts dopaminergic system development), on DRD2 levels in retinoic acid differentiated SH-SY5Y neuroblastoma. H2O2 elicited a significant increase in DRD2 mRNA and protein levels. Conversely, poly(IC) did not regulate DRD2 levels, although SH-SY5Y cells were confirmed to express TLR3 receptors. Under our conditions, H2O2, but not poly(IC), increased NFkappaB activation (as assessed by p65 nuclear translocation), which paralleled their effects on DRD2 levels regulation.