Abstract
The hypothalamic paraventricular nucleus (PVN) and angiotensin II (AngII) play critical roles in cardiovascular and neurohumoral regulation ascribed in part to vasopressin (VP) release. The AngII actions in the PVN are mediated largely through angiotensin II type 1 (AT1) receptors. However, there is indirect evidence that the functionally elusive central angiotensin II type 2 (AT2) receptors are also mediators of AngII signaling in the PVN. We used electron microscopic dual immunolabeling of antisera recognizing the AT2 receptor and VP to test the hypothesis that mouse PVN neurons expressing VP are among the cellular sites where this receptor has a subcellular distribution conducive to local activation. Immunoreactivity for the AT2 receptor was detected in somatodendritic profiles, of which approximately 60% of the somata and approximately 28% of the dendrites also contained VP. In comparison with somata and dendrites, axons, axon terminals, and glia less frequently contained the AT2 receptor. Somatic labeling for the AT2 receptor was often seen in the cytoplasm near the Golgi lamellae and other endomembrane structures implicated in receptor trafficking. AT2 receptor immunoreactivity in dendrites was commonly localized to cytoplasmic endomembranes, but was occasionally observed on extra- or peri-synaptic portions of the plasma membrane apposed by astrocytic processes or by unlabeled axon terminals. The labeled dendritic plasmalemmal segments containing AT2 receptors received asymmetric excitatory-type or more rarely symmetric inhibitory-type contacts from unlabeled axon terminals containing dense core vesicles, many of which are known to store neuropeptides. These results provide the first ultrastructural evidence that AT2 receptors in PVN neurons expressing VP and other neuromodulators are strategically positioned for surface activation by AngII and/or intracellular trafficking.