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The role of the apelin receptor in the HPA axis: a combined genetic and pharmacological approach

Reference	BB/D00196X/1
Principal Investigator / Supervisor	Dr Anne-Marie O'Carroll
Co-Investigators / Co-Supervisors
Institution	University of Bristol
Department	Henry Wellcome LINE
Funding type	Research
Value (£)	340,131
Status	Completed
Type	Research Grant
Start date	01/02/2006
End date	31/01/2010
Duration	48 months

Abstract

Our long-term goal is to elucidate the functions of the apelin receptor in the brain, pituitary and peripheral tissues. The hypothesis behind the proposed research is that apelin, mediating its effects through APJR located on hypothalamic parvocellular (CRH/VP) and magnocellular (VP) PVN neurons, modulates VP and CRH peptide release from within these same neurons to regulate the HPA axis response to stress. This hypothesis is based on our recent work that has established an association between the APJR and regulation of hypothalamic PVN function to modify HPA axis activity. We showed expression of APJR and apelin mRNA in the hypothalamic PVN, co-expression of APJR and VP mRNA in vasopressinergic neurons, increased expression of APJR mRNA in the PVN in response to acute and chronic stress, and activation of hypothalamic vasopressinergic neurons by direct administration of apelin by microdialysis. Based on these observations, the objective of this proposal is to explore the effect of central apelin and the involvement of APJR, VP and CRH receptors in the regulation of the HPA axis response to stress. The specific aims of this work are: (i) to use two complementary approaches, pharmacological studies and the recently developed APJR knockout (APJR-KO) mouse model, to confirm the involvement of APJR in regulation of HPA axis activity (i.e. ACTH and CORT secretion) in response to psychological, physical and systemic stress and, (ii) to determine the involvement of VP and CRH in the hormonal responses elicited by apelin. Our initial studies will confirm pharmacologically, using a selective antagonist, and genetically, using mice that lack the APJR, APJR involvement in co-ordination of HPA axis activity. We will then test whether the HPA axis response to either acute psychological (restraint), physical (forced swim test) or systemic (immune) stress requires the presence of a functional APJR. The induction of c-fos in response to these stressors will then be examined to characterize the pattern of stress-induced activation in the brains of APJR-deficient and wild-type animals. Next, we will investigate the hypothesized involvement of apelin in the regulation of VP and/or CRH neurons by co-localization of APJR with VP- and CRH-expressing neurons. Finally we will determine whether the physiological effects of apelin are mediated through control of VP and/or CRH neurons by investigating the effect of intracerebroventricular injection of apelin on activation of the HPA axis in both wild-type mice pretreated with a CRH receptor selective antagonist, alpha-helical CRH9-41, and V1bR-KO mice. The activity of the HPA axis will be monitored by measuring plasma levels of ACTH and CORT by radioimmunoassay, and assessing the expression of a number of genes that are involved in regulating HPA function by in situ hybridization histochemistry and immunocytochemistry. Those genes include VP, CRH, apelin and APJR in the paraventricular nucleus and pro-opiomelanocortin in the anterior pituitary. c-fos-immunoreactive cells will be examined in the PVN, central amygdala, nucleus of the solitary tract, dorsal raphe nucleus, locus coeruleus and nucleus of the solitary tract. We anticipate that the physiological role of the APJR in the neuroendocrine regulation of the HPA axis in response to stress will be highlighted in these studies, in which we predict reduced HPA responses to stress.

Summary

The neuropeptide apelin is found in the hypothalamus, a specialised part of the brain. This peptide acts through a receptor, the apelin receptor (APJR). The physiological roles of APJR are not fully known but we have shown that the receptor may be involved in the control of stress responses and that it may mediate its effects through regulation of the release of two factors, vasopressin (VP) and/or corticotropin-releasing hormone (CRH). VP and CRH are also synthesised in the hypothalamus and are involved in the body's response to stressful situations by regulating adrenocorticotrophin (ACTH) release from the anterior pituitary gland, which results in the release of the hormone cortisol from the adrenal gland into the blood. It is this hormone which is principally responsible for the body's response to stress. The relative importance of VP and CRH in this response appears to change depending on the type of stress. We have an animal model in which APJR is not functionally active and we propose to use this model to study the role of the receptor in the stress response. If APJR is critical to hypothalamic function, removal of the receptor will affect stress responses (ACTH secretion) in the mutant animal. We also have a compound, an antagonist, which will block the action of the peptide. We propose to use both methods - pharmacologically, using the selective antagonist, and genetically, using mice that lack the APJR, to investigate APJR involvement in acute stress. By using both receptor antagonists and receptor knockout mice, any developmental changes that may result from removal of the receptor gene in the animal will be able to be distinguished from the effects of receptor blockage. We will look at a number of different short term stressors to see if the type of stressor is important for an effect on the APJR. We also want to determine whether apelin mediates its effects through regulation of VP and/or CRH release. We will investigate this by looking at the effect of apelin both in an animal model in which VP can no longer function and also in a situation where the action of CRH is pharmacologically blocked. If apelin exerts its actions through VP we should see a change in the stress response in the non-functional VP animal model. Similarily, if apelin exerts its actions through CRH, we would expect to see a change in the stress response in the situation where CRH is no longer active. This work will result in a deeper understanding of the regulation of stress responses critical to life and will provide unique information about a likely new target system (apelin) that may serve as a novel site for pharmacologic intervention.

Committee	Closed Committee - Animal Sciences (AS)
Research Topics	Neuroscience and Behaviour
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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