Observation of the ultrastructure and synaptic relationships of angiotensin II-like immunoreactive neurons in the rat area postrema

Brain Angiotensin II and Synaptic Transmission

The renin-angiotensin system is an enzymatic cascade by which angiotensinogen is cleaved by renin and then by angiotensin-converting enzyme to produce angiotensin II (Ang II) and subsequently other angiotensins. Biochemical and neurophysiological studies have documented the presence of the reninangiotensin system and specific Ang II receptors in the brain. Also, circulating Ang II can exert some of its actions, such as blood pressure control and body fluid homeostasis, through stimulation of Ang II receptors in the circumventricular organs that lack a normal blood-brain barrier. In addition to some of the post-synaptic effects of Ang II, recent studies have revealed that Ang II regulates synaptic transmission in several brain regions, especially the nucleus of the solitary tract, hypothalamic paraventricular nucleus, and hippocampus. This review summarizes emerging new evidence on the effect of brain Ang II on glutamatergic and GABAergic synaptic transmission. This previously unrecognized presynaptic action of Ang II is important for the control of neuronal excitability and many physiological functions including autonomic control, hormone secretion, and memory. Future research on the role of brain-derived Ang II and its receptors in synaptic transmission will further enhance our understanding of the cellular mechanisms of Ang II and the relationship between the renin-angiotensin system and brain functions.

Anatomical, molecular and pathological consideration of the circumventricular organs

BACKGROUND AND PURPOSE

Circumventricular organs (CVOs) are a diverse group of specialised structures characterized by peculiar vascular and position around the third and fourth ventricles of the brain. In humans, these organs are present during the fetal period and some become vestigial after birth. Some, such as the pineal gland (PG), subcommissural organ (SCO) and organum vasculosum of the lamina terminalis (OVLT), which are located around the third ventricle, might be the site of origin of periventricular tumours. In contrast to humans, CVOs are present in the adult rat and can be dissected by laser capture microdissection (LCM).

METHODS

In this study, we used LCM and microarrays to analyse the transcriptomes of three CVOs, the SCO, the subfornical organ (SFO) and the PG and the third ventricle ependyma of the adult rat, in order to better characterise these organs at the molecular level. Furthermore, an immunohistochemical study of Claudin-3 (CLDN3), a membrane protein involved in forming cellular tight junctions, was performed at the level of the SCO.

RESULTS

This study highlighted some potentially new or already described specific markers of these structures as Erbb2 and Col11a1 in ependyma, Epcam and CLDN3 in the SCO, Ren1 and Slc22a3 in the SFO and Tph, Anat and Asmt in the PG. Moreover, we found that CLDN3 expression was restricted to the apical pole of ependymocytes in the SCO.

Preproglucagon neurons project widely to autonomic control areas in the mouse brain

Glucagon-like peptide 1 (GLP-1) and its analogue exendin-4 inhibit food intake, reduce blood glucose levels and increase blood pressure and heart rate by acting on GLP-1 receptors in many brain regions. Within the CNS, GLP-1 is produced only by preproglucagon (PPG) neurons. We suggest that PPG neurons mediate the central effects of GLP-1 by modulating sympathetic and vagal outflow. We therefore analysed the projections of PPG neurons to brain sites involved in autonomic control. In transgenic mice expressing yellow fluorescent protein (YFP) under the control of the PPG promoter, we assessed YFP-immunoreactive innervation using an anti-GFP antiserum and avidin-biotin-peroxidase. PPG neurons were intensely YFP-immunoreactive and axons could be easily discriminated from dendrites. YFP-immunoreactive cell bodies occurred primarily within the caudal nucleus tractus solitarius (NTS) with additional somata ventral to the hypoglossal nucleus, in raphé obscurus and in the intermediate reticular nucleus. The caudal NTS contained a dense network of dendrites, some of which extended into the area postrema. Immunoreactive axons were widespread throughout NTS, dorsal vagal nucleus and reticular nucleus with few in the hypoglossal nucleus and pyramids. The dorsomedial and paraventricular hypothalamic nuclei, ventrolateral periaqueductal grey and thalamic paraventricular nucleus exhibited heavy innervation. The area postrema, rostral ventrolateral medulla, pontine central grey, locus coeruleus/Barrington's nucleus, arcuate nucleus and the vascular organ of the lamina terminalis were moderately innervated. Only a few axons occurred in the amygdala and subfornical organ. Our results demonstrate that PPG neurons innervate primarily brain regions involved in autonomic control. Thus, central PPG neurons are ideally situated to modulate sympathetic and parasympathetic outflow through input at a variety of central sites. Our data also highlight that immunohistochemistry improves detection of neurons expressing YFP. Hence, animals in which specific populations of neurons have been genetically-modified to express fluorescent proteins are likely to prove ideal for anatomical studies.

Sensory circumventricular organs in health and disease

Circumventricular organs (CVOs) are specialized brain structures located around the third and fourth ventricles. They differ from the rest of the brain parenchyma in that they are highly vascularised areas that lack a blood-brain barrier. These neurohaemal organs are classified as "sensory", when they contain neurons that can receive chemical inputs from the bloodstream. This review focuses on the sensory CVOs to describe their unique structure, and their functional roles in the maintenance of body fluid homeostasis and cardiovascular regulation, and in the generation of central acute immune and febrile responses. In doing so, the main neural connections to visceral regulatory centres such as the hypothalamus, the medulla oblongata and the endocrine hypothalamic-pituitary axis, as well as some of the relevant chemical substances involved, are described. The CVOs are vulnerable to circulating pathogens and can be portals for their entry in the brain. This review highlights recent investigations that show that the CVOs and related structures are involved in pathological conditions such as sepsis, stress, trypanosomiasis, autoimmune encephalitis, systemic amyloidosis and prion infections, while detailed information on their role in other neurodegenerative diseases such as Alzheimer's disease or multiple sclerosis is lacking. It is concluded that studies of the CVOs and related structures may help in the early diagnosis and treatment of such disorders.

Angiotensin II type 2 receptors have a major somatodendritic distribution in vasopressin-containing neurons in the mouse hypothalamic paraventricular nucleus

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.

Angiotensin II increases excitability and inhibits a transient potassium current in vagal primary sensory neurons

The octapeptide angiotensin II (ANG II) plays a pivotal role in the maintenance of blood pressure by activating ANG II receptors located in variety of cell types including neurons housed in the central nervous system (CNS) and in the peripheral nervous system (PNS). ANG II (100 nM) blocked spike frequency accommodation (SFA) recorded with whole-cell patch technique in acutely isolated nodose ganglion neurons (NGN) from adult rats. ANG II increased the frequency of action potentials (AP) produced by supramaximal 500 ms depolarizing currents recorded in both tonic (16 Hz vs. 58 Hz, control vs. ANG II perfusion respectively, n=9) and phasic (1Hz vs. 38 Hz, n=13) NGNs. ANG II produced no significant changes in: the resting membrane potential (-51 mV vs. -50 mV, n=65), AP overshoot (46 mV vs. 41 mV, n=25), AP undershoot (-65 mV vs. -61 mV, n=25), AP duration (1 ms vs. 1.2 ms, n=25), and AP threshold (-40 mV vs. -43 mV, n=19). CV-11974 (600 nM), a specific AT1 receptor antagonist, prevented ANG II-evoked changes SFA (n=10). ANG II (100 nM) had no significant effect on total outward potassium current (I(K)) but inhibited a fast activating and fast inactivating I(K) recorded in the presence of TEA. A kinetically similar I(K) was also inhibited by 4-AP (3mM). In phasic NGNs, 4-AP occluded the effects of 100 nM ANG II on SFA. Our results indicate that ANG II can block an A-type of I(K) and that this effect may underlie the ANG II-mediated change in SFA.

The sensory circumventricular organs: brain targets for circulating signals controlling ingestive behavior

Sensory circumventricular organs (CVOs) are specialized areas of the brain that lack a normal blood-brain barrier, and therefore are in constant contact with signaling molecules circulating in the bloodstream. Neurons of the CVOs are well endowed with a wide spectrum of receptors for hormones and other signaling molecules, and they have strong connections to hypothalamic and brainstem nuclei. Therefore, lying at the blood-brain interface, the sensory CVOs are in a unique position of being able to detect and integrate humoral and neural information and relay the resulting signals to autonomic control centers of the hypothalamus and medulla. This review focuses primarily on the roles played by the sensory CVOs in fluid balance and energy metabolism.

Angiotensin II inhibition of Ca2+ currents is independent of ATR1 angiotensin II receptor activation in rat adult vagal afferent neurons

Angiotensin II (ANG II) has the ability to modulate the activity of neurons involved in the cardiovascular regulation. One effective way of doing that is by changing calcium currents. In the present study, we investigated the effects of ANG II on high-voltage-activated (HVA) Ca2+ currents measured in adult vagal afferent neurons using the whole-cell patch-clamp technique. In addition, we demonstrated the presence of ATR1 and ATR2 receptors mRNA at nodose neurons using conventional reverse transcriptase-polymerase chain reaction (RT-PCR). ANG II (100 nM) decreased the HVA Ca2+ current (peak current recorded at 0 mV: -60.9+/-8.7 pA/pF in control conditions versus -31.9+/-5.7 pA/pF in the presence of ANG II) and shifted the Ca2+ current activation to a more negative membrane potential (control V0.5=-12.5+/-1.5 mV versus -18.4+/-2.8 mV during perfusion with ANG II). Losartan (500 nM) was not able to prevent the ANG II effect on the HVA Ca2+ current making unlikely the involvement of the ATR1 receptor. When ANG II was perfused in the continuous presence of saralasin, a non-selective ANG II receptor antagonist, we observed a faster but transient inhibition of HVA Ca2+ current. The inhibition was not sustained as observed when we applied ANG II alone and the HVA Ca2+ current recovered with time reaching levels close to the control. Unexpectedly, treatment with the ATR2 blocker PD 123,319 (500 nM) caused a significant inhibition on the HVA Ca2+ current making rather difficult any further conclusions. The above results allow us to conclude that ANG II induced inhibition on the HVA Ca2+ current is probably not via ATR1 receptor activation.

Sensory circumventricular organs: central roles in integrated autonomic regulation

Circumventricular organs (CVO) play a critical role as transducers of information between the blood, neurons and the cerebral spinal fluid (CSF). They permit both the release and sensing of hormones without disrupting the blood-brain barrier (BBB) and as a consequence of such abilities the CVOs are now well established to have essential regulatory actions in diverse physiological functions. The sensory CVOs are essential signal transducers located at the blood-brain interface regulating autonomic function. They have a proven role in the control of cardiovascular function and body fluid regulation, and have significant involvement in central immune response, feeding behavior and reproduction, the extent of which is still to be determined. This review will attempt to summarize the research on these topics to date. The complexities associated with sensory CVO exploration are intense, but should continue to result in valuable contributions to our understanding of brain function.

Angiotensin II subtype 1A (AT1A) receptors in the rat sensory vagal complex: subcellular localization and association with endogenous angiotensin

Angiotensin II (Ang II) type 1 (AT1) receptors are prevalent in the sensory vagal complex including the nucleus tractus solitarii (NTS) and area postrema, each of which has been implicated in the central cardiovascular effects produced by Ang II. In rodents, these actions prominently involve the AT1A receptor. Thus, we examined the electron microscopic dual immunolabeling of antisera recognizing the AT1A receptor and Ang II to determine interactive sites in the sensory vagal complex of rat brain. In both the area postrema and adjacent dorsomedial NTS, many somatodendritic profiles were dually labeled for the AT1A receptor and Ang II. In these profiles, AT1A receptor-immunoreactivity was often seen in the cytoplasm beneath labeled portions of the plasma membrane and in endosome-like granules as well as Golgi lamellae and outer nuclear membranes. In addition, AT1A receptor labeling was detected on the plasma membrane and in association with cytoplasmic membranes in many small axons and axon terminals. These terminals were morphologically heterogeneous containing multiple types of vesicles and forming either inhibitory- or excitatory-type synapses. In the area postrema, AT1A receptor labeling also was detected in many non-neuronal cells including glia, capillary endothelial cells and perivascular fibroblasts that were less prevalent in the NTS. We conclude that in the rat sensory vagal complex, AT1A receptors are strategically positioned for involvement in modulation of the postsynaptic excitability and intracrine hormone-like effects of Ang II. In addition, these receptors have distributions consistent with diverse roles in regulation of transmitter release, regional blood flow and/or vascular permeability.

Reciprocal synaptic relationships between angiotensin II-containing neurons and enkephalinergic neurons in the rat area postrema

A preembedding double immunostaining technique was used to study synaptic relationships between angiotensin-II-like immunoreactive and enkephalin-like immunoreactive neurons in the rat area postrema. The angiotensin-II-like immunoreactive neurons were detected by silver-gold intensification of the DAB reaction results while the enkephalin-like immunoreactive neurons were detected by simple ABC-DAB reaction. The synaptic relationships were reciprocal between the two neurons. Most of the synapses found between these two neurons were the presynaptic enkephalin-like immunoreactive axon terminals that made synapses on the angiotensin-II-like immunoreactive perikarya and dendrites. Both the axo-somatic and axo-dendritic synapses were symmetrical. However, although angiotensin-II-like immunoreactive axon terminals also made synapses on enkephalin-like perikarya and dendrites, the axo-somatic synapses were symmetrical, while the axo-dendritic synapses were asymmetrical. The present results confirm the presence of angiotensin-II-like immunoreactive neurons in the area postrema and suggest that these angiotensinergic neurons in the area postrema may play a role in the regulation of blood pressure via coordinated synaptic interactions with enkephalinergic neurons.

Atp-binding cassette transporter ABC2/ABCA2 in the rat brain: a novel mammalian lysosome-associated membrane protein and a specific marker for oligodendrocytes but not for myelin sheaths

We recently cloned a full-length cDNA of the rat ATP-binding cassette transporter 2 (ABC2, or ABCA2) protein, a member of the ABC1 (or ABCA) subfamily (-ABC1/ABCA1 is a causal gene for Tangier disease) and found it to be strongly expressed in the rat brain. In this study, we identified ABC2 as a lysosome-associated membrane protein that is being localized specifically in oligodendrocytes. The ABC2-immunolabeled cells were detected mainly in the white matter but were also scattered in gray matter throughout the whole brain. In addition, these cells were found to be colocalized with 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) immunoreactivity when the marker antibody for oligodendrocytes was used. However, no such colocalization was observed with markers for other kinds of glial cells. Unlike the CNP antibody, which also intensely stains myelin sheaths in the white matter, ABC2 immunoreactivity was detected only in the cell bodies of oligodendrocytes. At the ultrastructural level, ABC2 immunoreactivity was detected mostly around lysosome and partly in Golgi apparatus by electron microscopy. This was confirmed by immunocolocalization of ABC2 and lysosomal markers in a neuroblastoma cell line. Immunoblotting analysis of ABC2 from the whole brain and the ABC2-transfected cell line revealed bands at approximately 260 kDa. The result of in situ hybridization with a riboprobe for ABC2 matched the results obtained from immunostaining. These findings strongly suggest that ABC2 is a specific marker for oligodendrocytes but not for myelinsheaths and that it is as a novel mammalian lysosome-associated membrane protein involved in myelinization or other kinds of metabolism in the CNS.