Angiotensin Type-2 Receptors Influence the Activity of Vasopressin Neurons in the Paraventricular Nucleus of the Hypothalamus in Male Mice

Abstract

It is known that angiotensin-II acts at its type-1 receptor to stimulate vasopressin (AVP) secretion, which may contribute to angiotensin-II-induced hypertension. Less well known is the impact of angiotensin type-2 receptor (AT2R) activation on these processes. Studies conducted in a transgenic AT2R enhanced green fluorescent protein reporter mouse revealed that although AT2R are not themselves localized to AVP neurons within the paraventricular nucleus of the hypothalamus (PVN), they are localized to neurons that extend processes into the PVN. In the present set of studies, we set out to characterize the origin, phenotype, and function of nerve terminals within the PVN that arise from AT2R-enhanced green fluorescent protein-positive neurons and synapse onto AVP neurons. Initial experiments combined genetic and neuroanatomical techniques to determine that γ-aminobutyric acid (GABA)ergic neurons derived from the peri-PVN area containing AT2R make appositions onto AVP neurons within the PVN, thereby positioning AT2R to negatively regulate neuroendocrine secretion. Subsequent patch-clamp electrophysiological experiments revealed that selective activation of AT2R in the peri-PVN area using compound 21 facilitates inhibitory (ie, GABAergic) neurotransmission and leads to reduced activity of AVP neurons within the PVN. Final experiments determined the functional impact of AT2R activation by testing the effects of compound 21 on plasma AVP levels. Collectively, these experiments revealed that AT2R expressing neurons make GABAergic synapses onto AVP neurons that inhibit AVP neuronal activity and suppress baseline systemic AVP levels. These findings have direct implications in the targeting of AT2R for disorders of AVP secretion and also for the alleviation of high blood pressure.

Centrally Mediated Cardiovascular Actions of the Angiotensin II Type 2 Receptor

“…The same pattern was found by our group using the AT2R-eGFP-BAC-transgenic reporter-mouse: AT2R-positive, fluorescent green staining was not detectable on the soma of AVP neurons within the PVN, but on processes and nerve terminals arising from AT2R-positive cell bodies in the peri-PVN area, which extended into the PVN and synapsed onto AVP neurons [17,19]. The majority of these AT2R-positive neurons turned out to be GABAergic [17,19], and consequently, selective stimulation of these inhibitory neurons by the AT2R agonist C21 led to reduced activity of AVP neurons within the PVN, thus reducing AVP release and eventually decreasing AVP plasma levels.…”

“…All of these nuclei have an impact on blood pressure control by modifying sympathetic and parasympathetic activity [8]. Moreover, AT2R containing neurons are not present within the PVN; however, there are AT2R neurons which surround the PVN (Figure 2), and both these peri-PVN AT2R neurons and AT2R neurons in the median preoptic nucleus extend processes that terminate within the PVN, where they synapse with AVP magnocellular neurons and pre-autonomic neuron cell bodies [17,19]. …”

“…AT2R positive neurites extending into the RVLM have indeed been found in the AT2R-eGFP-BAC-transgenic reporter-mouse [17], but both the origin of these fibers and their functional roles still needs to be identified. However, as will be described in more detail in the following section, experimental evidence has already been provided for an AT2R induced enhancement of the inhibitory effect of GABAergic neurons on AVP release in the PVN (see also Text Box 2) [19]. …”

Protective Angiotensin Type 2 Receptors in the Brain and Hypertension

“…At a regional level, AT2R are present either within or adjacent to brain areas that are directly responsible for modulating sympathetic outflow and baroreflex function. For example, high concentrations of AT2R-containing neurons were found the intermediate NTS (intNTS) and the area postrema (AP), AT2R neuron fibers were present in the rostral ventrolateral medulla (RVLM), and while no AT2R neuron cell bodies were found within the paraventricular nucleus of the hypothalamus (PVN), they were observed on the periphery of this sympathetic control center [••22, ••23]. It was also clear that the PVN contains an abundance of AT2R-positive fibers and terminals, some of which are derived from the peri-PVN AT2R neurons and others from nuclei such as the median preoptic nucleus (MnPO), which is important in metabolism and osmoregulation.…”

“…Furthermore, it has been possible to determine the phenotype of AT2R-containing neurons at cardiovascular control centers. At the intNTS, the AT2R neurons are largely GABAergic, as are the AT2R neurons in the peri-PVN area that terminate in the PVN [••22, ••23]. There are also populations of AT2R-containing glutamate neurons in the intNTS and MnPO [••22].…”