Angiotensin II depresses glutamate depolarizations and excitatory postsynaptic potentials in locus coeruleus through angiotensin II subtype 2 receptors

Prostaglandin E2 Induces Long-Lasting Inhibition of Noradrenergic Neurons in the Locus Coeruleus and Moderates the Behavioral Response to Stressors

Neuronal activity is modulated not only by inputs from other neurons but also by various factors, such as bioactive substances. Noradrenergic (NA) neurons in the locus coeruleus (LC-NA neurons) are involved in diverse physiological functions, including sleep/wakefulness and stress responses. Previous studies have identified various substances and receptors that modulate LC-NA neuronal activity through techniques including electrophysiology, calcium imaging, and single-cell RNA sequencing. However, many substances with unknown physiological significance have been overlooked. Here, we established an efficient screening method for identifying substances that modulate LC-NA neuronal activity through intracellular calcium ([Ca2+]i) imaging using brain slices. Using both sexes of mice, we screened 53 bioactive substances, and identified five novel substances: gastrin-releasing peptide, neuromedin U, and angiotensin II, which increase [Ca2+]i, and pancreatic polypeptide and prostaglandin D2, which decrease [Ca2+]i Among them, neuromedin U induced the greatest response in female mice. In terms of the duration of [Ca2+]i change, we focused on prostaglandin E2 (PGE2), since it induces a long-lasting decrease in [Ca2+]i via the EP3 receptor. Conditional knock-out of the receptor in LC-NA neurons resulted in increased depression-like behavior, prolonged wakefulness in the dark period, and increased [Ca2+]i after stress exposure. Our results demonstrate the effectiveness of our screening method for identifying substances that modulate a specific neuronal population in an unbiased manner and suggest that stress-induced prostaglandin E2 can suppress LC-NA neuronal activity to moderate the behavioral response to stressors. Our screening method will contribute to uncovering previously unknown physiological functions of uncharacterized bioactive substances in specific neuronal populations.SIGNIFICANCE STATEMENT Bioactive substances modulate the activity of specific neuronal populations. However, since only a limited number of substances with predicted effects have been investigated, many substances that may modulate neuronal activity have gone unrecognized. Here, we established an unbiased method for identifying modulatory substances by measuring the intracellular calcium signal, which reflects neuronal activity. We examined noradrenergic (NA) neurons in the locus coeruleus (LC-NA neurons), which are involved in diverse physiological functions. We identified five novel substances that modulate LC-NA neuronal activity. We also found that stress-induced prostaglandin E2 (PGE2) may suppress LC-NA neuronal activity and influence behavioral outcomes. Our screening method will help uncover previously overlooked functions of bioactive substances and provide insight into unrecognized roles of specific neuronal populations.

Higher Neuronal Facilitation and Potentiation with APOE4 Suppressed by Angiotensin II

Progressive hippocampal degeneration is a key component of Alzheimer's disease (AD) progression. Therefore, identifying how hippocampal neuronal function is modulated early in AD is an important approach to eventually prevent degeneration. AD-risk factors and signaling molecules likely modulate neuronal function, including APOE genotype and angiotensin II. Compared to APOE3 , APOE4 increases AD risk up to 12-fold, and high levels of angiotensin II are hypothesized to disrupt neuronal function in AD. However, the extent that APOE and angiotensin II modulates the hippocampal neuronal phenotype in AD-relevant models is unknown. To address this issue, we used electrophysiological techniques to assess the impact of APOE genotype and angiotensin II on basal synaptic transmission, presynaptic and post-synaptic activity in mice that express human APOE3 (E3FAD) or APOE4 (E4FAD) and overproduce Aβ. We found that compared to E3FAD mice, E4FAD mice had lower basal synaptic activity, but higher levels of paired pulse facilitation (PPF) and Long-Term Potentiation (LTP) in the Schaffer Collateral Commissural Pathway (SCCP) of the hippocampus. We also found that exogenous angiotensin II has a profound inhibitory effect on hippocampal LTP in both E3FAD and E4FAD mice. Collectively, our data suggests that APOE4 and Aβ are associated with a hippocampal phenotype comprised of lower basal activity and higher responses to high frequency stimulation, the latter of which is suppressed by angiotensin II. These novel data suggest a potential mechanistic link between hippocampal activity, APOE4 genotype and angiotensin II in AD.

Angiotensin II facilitates GABAergic neurotransmission at postsynaptic sites in rat amygdala neurons

The central nucleus of the amygdala (CeA) is critical in the regulation of sodium appetite. Angiotensin II (Ang II) is important in the generation of sodium appetite and may function as a neurotransmitter or modulator to affect the synaptic transmission and the excitability of neurons. However, the role of Ang II in the CeA remains unclear. In this study, we determined the effects of Ang II on the excitatory and inhibitory synaptic inputs to the CeA neurons in brain slices with whole-cell patch-clamp recordings. Ang II (0.5-5 μM) significantly potentiated the amplitude of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) in a concentration-dependent manner. Ang II (2 μM) significantly increased the amplitude of miniature GABAergic inhibitory postsynaptic currents (mIPSCs) without affecting the frequency. This effect was blocked by Ang II type 1 (AT₁) receptor antagonist, losartan. One mM guanosine 5'-O-(-2-thiodiphosphate) (GDP-β-s) in the pipette internal solution eliminated the facilitatory effect of Ang II on GABAergic synaptic transmission. In contrast, Ang II had no effect on the spontaneous glutamatergic excitatory postsynaptic currents (EPSCs) and did not alter the frequency and amplitude of miniature EPSCs at concentrations that facilitated IPSCs. Furthermore, Ang II decreased the firing activity of CeA neurons, and this effect was abolished by losartan and GDP-β-s. In addition, Ang II failed to inhibit CeA neurons in the presence of bicuculline. These data provide substantial new evidence that Ang II inhibits the CeA neurons by facilitation of GABAergic synaptic input efficacy through activation of postsynaptic AT₁ receptors.

Angiotensin II AT1 receptors mediate neuronal sensitization and sustained blood pressure response induced by a single injection of amphetamine

A single exposure to amphetamine induces neurochemical sensitization in striatal areas. The neuropeptide angiotensin II, through AT₁ receptors (AT₁-R) activation, is involved in these responses. However, amphetamine-induced alterations can be extended to extra-striatal areas involved in blood pressure control and their physiological outcomes. Our aim for the present study was to analyze the possible role for AT₁-R in these events using a two-injection protocol and to further characterize the proposed AT₁-R antagonism protocol. Central effect of orally administered AT₁-R blocker (Candesartan, 3mg/kg p.o.×5days) in male Wistar rats was analyzed by spontaneous activity of neurons within locus coeruleus. In another group of animals pretreated with the AT₁-R blocker or vehicle, sensitization was achieved by a single administration of amphetamine (5mg/kg i.p. - day 6) followed by a 3-week period off drug. On day 27, after receiving an amphetamine challenge (0.5mg/kg i.p.), we evaluated: (1) the sensitized c-Fos expression in locus coeruleus (LC), nucleus of the solitary tract (NTS), caudal ventrolateral medulla (A1) and central amygdala (CeAmy); and (2) the blood pressure response. AT₁-R blockade decreased LC neurons' spontaneous firing rate. Moreover, sensitized c-Fos immunoreactivity in TH+neurons was found in LC and NTS; and both responses were blunted by the AT₁-R blocker pretreatment. Meanwhile, no differences were found neither in CeAmy nor A1. Sensitized blood pressure response was observed as sustained changes in mean arterial pressure and was effectively prevented by AT₁-R blockade. Our results extend AT₁-R role in amphetamine-induced sensitization over noradrenergic nuclei and their cardiovascular output.

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.

Locus Coeruleus Dysfunction in Transgenic Rats with Low Brain Angiotensinogen

AIMS

Transgenic TGR(ASrAOGEN)680 (TGR) rats with specific downregulation of glial angiotensinogen (AOGEN) synthesis develop cardiovascular deficits, anxiety, altered response to stress, and depression. Here, we evaluated whether these deficits are associated with alteration of the integrity of the noradrenergic system originating from locus coeruleus (LC) neurons.

METHODS

Adult TGR rats were compared to control Sprague Dawley rats in terms of the following: tissue levels of transcripts encoding noradrenergic markers, tissue tyrosine hydroxylase (TH) protein level, in vivo TH activity, density of TH-containing fibers, behavioral response to novelty, locomotor activity, and polysomnography.

RESULTS

TH expression was increased in the LC of TGR rats compared to controls. In LC terminal fields, there was an increase in density of TH-containing fibers in TGR rats that was associated with an elevation of in vivo TH activity. TGR rats also displayed locomotor hyperactivity in response to novelty. Moreover, polysomnographic studies indicated that daily paradoxical sleep duration was increased in TGR rats and that the paradoxical sleep rebound triggered by total sleep deprivation was blunted in these rats.

CONCLUSIONS

Altogether, these results suggest that disruption of astroglial AOGEN synthesis leads to cardiovascular, cognitive, behavioral, and sleep disorders that might be partly due to LC dysfunction.

Effects of angiotensin type 2 receptor on secretion of the locus coeruleus in stress-induced hypertension rats

Locus coeruleus (LC) has noradrenergic nerve terminals projecting to hypothalamus that modulating cardiovascular activity. To study the dynamic characteristics of norepinephrine (NE) release in hypothalamus followed by electrical stimulation in the locus coeruleus in the stress-induced hypertension (SIH) rats, we established the hypertension model rats by stimulations combining noise and foot-shock stress. After the end of modeling, NE release in the hypothalamus by electrical stimulation in LC was studied and NE signal was recorded by carbon fiber electrode. The peak value, the time to peak and half-life period of NE signal in both group rats were analyzed. Furthermore, to clarify the role of angiotensin II type 2 receptors (AT2) in norepinephrine (NE) release and the blood pressure of rat model of stress-induced hypertension, we intraperitoneally administered the AT2 receptor antagonist PD123319 (AT2 receptor antagonist, 0.3mg/kg, i.p.) and intracerebroventricularly injection of CGP42112 (AT2 receptor agonist, 6μg/5μl, i.c.v.) to adult male rats. We found the peak value of NE signal in the hypothalamus followed by electrical stimulation in the LC in SIH rats were higher than that in controls (P<0.01). Intraperitoneal injection of PD123319 (AT2 receptor antagonist) potentiated electrical stimulation in the LC induced NE release in the hypothalamus in SIH rats and elevated blood pressure (P<0.05), whereas intracerebroventricular injection of CGP42112 (AT2 receptor agonist) inhibited the NE release and reduced the heart rate (P<0.05). These results suggest that combining noise and foot-shock stresses increased the blood pressure and the secretion of NE in the hypothalamus followed by electrical stimulation in the LC in rats. AT2 receptors can inhibit the secretion of NE from the LC to the hypothalamus. The attenuation of presynaptic action of AT2 receptor may play a role in the pathophysiological mechanism of SIH in rats.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

How does angiotensin AT(2) receptor activation help neuronal differentiation and improve neuronal pathological situations?

The angiotensin type 2 (AT(2)) receptor of angiotensin II has long been thought to be limited to few tissues, with the primary effect of counteracting the angiotensin type 1 (AT(1)) receptor. Functional studies in neuronal cells have demonstrated AT(2) receptor capability to modulate neuronal excitability, neurite elongation, and neuronal migration, suggesting that it may be an important regulator of brain functions. The observation that the AT(2) receptor was expressed in brain areas implicated in learning and memory led to the hypothesis that it may also be implicated in cognitive functions. However, linking signaling pathways to physiological effects has always proven challenging since information relative to its physiological functions has mainly emerged from indirect observations, either from the blockade of the AT(1) receptor or through the use of transgenic animals. From a mechanistic standpoint, the main intracellular pathways linked to AT(2) receptor stimulation include modulation of phosphorylation by activation of kinases and phosphatases or the production of nitric oxide and cGMP, some of which are associated with the Gi-coupling protein. The receptor can also interact with other receptors, either G protein-coupled such as bradykinin, or growth factor receptors such as nerve growth factor or platelet-derived growth factor receptors. More recently, new advances have also led to identification of various partner proteins, thus providing new insights into this receptor's mechanism of action. This review summarizes the recent advances regarding the signaling pathways induced by the AT(2) receptor in neuronal cells, and discussed the potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT(2) receptor activation by non-peptide and selective agonists could represent new pharmacological tools that may help to improve impaired cognitive performance in Alzheimer's disease and other neurological cognitive disorders.

AT2 receptor signaling and sympathetic regulation

There is a growing consensus that the balance between Angiotensin Type 1 (AT1R) and Angiotensin Type 2 (AT2R) signaling in many tissues may determine the magnitude and, in some cases the direction, of the biological response. Sympatho-excitation in cardiovascular diseases is mediated by a variety of factors and is, in part, dependent on Angiotensin II signaling in the central nervous system. Recent data have provided evidence that the AT2R can modulate sympatho-excitation in animals with hypertension and heart failure. The evidence for this concept is reviewed and a model is put forward to support the rationale that therapeutic targeting of the central AT2R may be beneficial in the setting of chronic heart failure.

Angiotensin II AT(1) receptor blockade selectively enhances brain AT(2) receptor expression, and abolishes the cold-restraint stress-induced increase in tyrosine hydroxylase mRNA in the locus coeruleus of spontaneously hypertensive rats

Spontaneously hypertensive rats, a stress-sensitive strain, were pretreated orally for 14 days with the AT(1) receptor antagonist candesartan before submission to 2 h of cold-restraint stress. In non-treated rats, stress decreased AT(1) receptor binding in the median eminence and basolateral amygdala, increased AT(2) receptor binding in the medial subnucleus of the inferior olive, decreased AT(2) binding in the ventrolateral thalamic nucleus and increased tyrosine hydroxylase mRNA level in the locus coeruleus. In non-stressed rats, AT(1) receptor blockade reduced AT(1) receptor binding in all areas studied and enhanced AT(2) receptor binding in the medial subnucleus of the inferior olive. Candesartan pretreatment produced a similar decrease in brain AT(1) binding after stress, and prevented the stress-induced AT(2) receptor binding decrease in the ventrolateral thalamic nucleus. In the locus coeruleus and adrenal medulla, AT(1) blockade abolished the stress-induced increase in tyrosine hydroxylase mRNA level. Our results demonstrate that oral administration of candesartan effectively blocked brain AT(1) receptors, selectively increased central AT(2) receptor expression and prevented the stress-induced central stimulation of tyrosine hydroxylase transcription. The present results support a role of brain AT(1) and AT(2) receptors in the regulation of the stress response, and the hypothesis that AT(1) receptor antagonists may be considered as potential therapeutic compounds in stress related disorders in addition to their anti-hypertensive properties.

Central angiotensin II AT1 receptors mediate fetal swallowing and pressor responses in the near-term ovine fetus

Swallowed volumes in the fetus are greater than adult values (per body weight) and serve to regulate amniotic fluid volume. Central ANG II stimulates swallowing, and nonspecific ANG II receptor antagonists inhibit both spontaneous and ANG II-stimulated swallowing. In the adult rat, AT1 receptors mediate both stimulated drinking and pressor activities, while the role of AT2 receptors is controversial. As fetal brain contains increased ANG II receptors compared with the adult brain, we sought to investigate the role of both AT1 and AT2 receptors in mediating fetal swallowing and pressor activities. Five pregnant ewes with singleton fetuses (130 +/- 1 days) were prepared with fetal vascular and lateral ventricle (LV) catheters and electrocorticogram and esophageal electromyogram electrodes and received three studies over 5 days. On day 1 (ANG II), following a 2-h basal period, 1 ml artificial cerebrospinal fluid (aCSF) was injected in the LV. At time 4 h, ANG II (6.4 microg) was injected in the LV, and the fetus was monitored for a final 2 h. On day 3, AT1 receptor blocker (losartan 0.5 mg) was administered at 2 h, and ANG II plus losartan was administered at 4 h. On day 5, AT2 receptor blocker (PD-123319; 0.8 mg was administered at 2 h and ANG II plus PD-123319 at 4 h. In the ANG II study, LV injection of ANG II significantly increased fetal swallowing (0.9 +/- 0.1 to 1.4 +/- 0.1 swallows/min; P < 0.05). In the losartan study, basal fetal swallowing significantly decreased in response to blockade of AT1 receptors (0.9 +/- 0.1 to 0.4 +/- 0.1 swallows/min; P < 0.05), while central injection of ANG II in the presence of AT1 receptor antagonism did not increase fetal swallowing (0.6 +/- 0.1 swallows/min). In the PD-123319 study, basal fetal swallowing did not change in response to blockade of AT2 receptor (0.9 +/- 0.1 swallows/min), while central injection of ANG II in the presence of AT2 blockade significantly increased fetal swallowing (1.5 +/- 0.1 swallows/min; P < 0.05). ANG II caused significant pressor responses in the control and PD-123319 studies but no pressor response in the presence of AT1 blockade. These data demonstrate that in the near-term ovine fetus, AT1 receptor but not AT2 receptors accessible via CSF contribute to dipsogenic and pressor responses.

Angiotensin II attenuates NMDA receptor-mediated neuronal cell death and prevents the associated reduction in Bcl-2 expression

While angiotensin II (Ang II) plays a major role in the regulation of blood pressure, fluid homeostasis and neuroendocrine function, recent studies have also implicated the peptide hormone in cell growth, differentiation and apoptosis. In support of this, we have previously demonstrated that Ang II attenuates N-methyl-D-aspartate (NMDA) receptor signaling [Molec. Brain Res. 48 (1997) 197]. To further examine the modulatory role of Ang II on NMDA receptor function, we investigated the effect of angiotensin receptor (AT) activation on NMDA-mediated cell death and the accompanying decrease in Bcl-2 expression. The viability of differentiated N1E-115 and NG108-15 neuronal cell lines was reduced following exposure to NMDA in a dose-dependent manner. MTT analysis (mitochondrial integrity) revealed a decrease in cell survival of 49.4+/-12.3% in NG108 cells and 79.9+/-6.8% in N1E cells following treatment with 10 mM NMDA for 20 h. Cytotoxicity in N1E cells was inhibited by the noncompetitive NMDA receptor antagonist, MK-801. Further, NMDA receptor-mediated cell death in NG108 cells was attenuated by treatment with Ang II. The Ang II effect was inhibited by both AT1 and AT2 receptor antagonists, losartan and PD123319, respectively, suggesting that both receptor subtypes may play a role in the survival effect of Ang II. Since it has been shown that activation of NMDA receptors alters the expression of Bcl-2 family proteins, Western blot analysis was performed in N1E cells to determine whether Ang II alters the NMDA-induced changes in Bcl-2 expression. A concentration-dependent decrease of intracellular Bcl-2 protein levels was observed following treatment with NMDA, and this reduction was inhibited by MK801. Addition of Ang II suppressed the NMDA receptor-mediated reduction in Bcl-2. The Ang II effect on NMDA-mediated changes in Bcl-2 levels was blocked by PD123319, but was not significantly changed by losartan, suggesting AT2 receptor specificity. Taken together, these results suggest that Ang II attenuates NMDA receptor-mediated neurotoxicity and that this effect may be due, in part, to an alteration in Bcl-2 expression.

Chronotropic Effect of Angiotensin II via Type 2 Receptors in Rat Brain Neurons

Previously, we determined that angiotensin II (Ang II) elicits an Ang II type 2 (AT2) receptor–mediated increase of neuronal delayed rectifier K+( I KV) current in neuronal cultures from newborn rat hypothalamus and brain stem. This requires generation of lipoxygenase (LO) metabolites of arachidonic acid (AA) and activation of serine/threonine phosphatase type 2A (PP-2A). Enhancement of I KV results in a decrease in net inward current during the action potential (AP) upstroke as well as shortening of the refractory period, which may lead to alterations in neuronal firing rate. Thus, in the present study, we used whole-cell current clamp recording methods to investigate the AT2 receptor–mediated effects of Ang II on the firing rate of cultured neurons from the hypothalamus and brain stem. At room temperature, these neurons exhibited spontaneous APs with an amplitude of 77.72 ± 2.7 mV ( n = 20) and they fired at a frequency of 0.8 ± 0.1 Hz ( n = 11). Most cells had a prolonged early after-depolarization that followed an initial fully developed AP. Superfusion of Ang II (100 nM) plus losartan (LOS, 1 μM) to block Ang II type 1 receptors elicited a significant chronotropic effect that was reversed by the AT2 receptor inhibitor PD 123,319 (1 μM). LOS alone had no effect on any of the parameters measured. The chronotropic effect of Ang II was reversed by the general LO inhibitor 5,8,11,14-eicosatetraynoic acid (10 μM) or by the selective PP-2A inhibitor okadaic acid (1 nM) and was mimicked by the 12-LO metabolite of AA 12-(S)-hydroxy-(5Z, 8Z, 10E, 14Z)-eicosatetraynoic acid. These data indicate that Ang II elicits an AT2 receptor–mediated increase in neuronal firing rate, an effect that involves generation of LO metabolites of AA and activation of PP-2A.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Hormonal and neurotransmitter roles for angiotensin in the regulation of central autonomic function

In this review we present the case for both hormonal and neurotransmitter actions of angiotensin II (ANG) in the control of neuronal excitability in a simple neural pathway involved in central autonomic regulation. We will present both single-cell and whole-animal data highlighting hormonal roles for ANG in controlling the excitability of subfornical organ (SFO) neurons. More controversially we will also present the case for a neurotransmitter role for ANG in SFO neurons in controlling the excitability of identified neurons in the paraventricular nucleus (PVN) of the hypothalamus. In this review we highlight the similarities between the actions of ANG on these two populations of neurons in an attempt to emphasize that whether we call such actions "hormonal" or "neurotransmitter" is largely semantic. In fact such definitions only refer to the method of delivery of the chemical messenger, in this case ANG, to its cellular site of action, in this case the AT1 receptor. We also described in this review some novel concepts that may underlie synthesis, metabolic processing, and co-transmitter actions of ANG in this pathway. We hope that such suggestions may lead ultimately to the development of broader guiding principles to enhance our understanding of the multiplicity of physiological uses for single chemical messengers.

Angiotensin II increases neuronal delayed rectifier K(+) current: role of 12-lipoxygenase metabolites of arachidonic acid

Angiotensin II (Ang II) elicits an Ang II type 2 (AT(2)) receptor-mediated increase in voltage-dependent delayed rectifier K(+) current (I(KV)) in neurons cultured from newborn rat hypothalamus and brain stem. In previous studies, we have determined that this effect of Ang II is mediated via a Gi protein, activation of phospholipase A(2) (PLA(2)), and generation of arachidonic acid (AA). AA is rapidly metabolized within cells via lipoxygenases (LO), cyclooxygenase (COX) or p450 monooxygenase enzymes, and the metabolic products are known regulators of K(+) currents and channels. Thus in the present study, we have investigated whether the AT(2) receptor-mediated effects of Ang II on neuronal I(KV) require AA metabolism and if so, which metabolic pathways are involved. The data presented here indicate that the stimulatory actions of Ang II and AA on neuronal I(KV) are attenuated by selective blockade of 12-LO enzymes. However, the effects of Ang II are not altered by blockade of 5-LO or p450 monooxygenase enzymes. Furthermore, the actions of Ang II are mimicked by a 12-LO metabolite of AA, but 5-LO metabolites such as leukotriene B(4) and C(4) do not alter neuronal I(KV). These data indicate that the AT(2) receptor-mediated stimulation of neuronal I(KV) is partially mediated through 12-LO metabolites of AA.

Effects of angiotensin II on visual evoked potentials in the superior colliculus of juvenile rats

There are age-related changes in the relative expression of the AT(1)and AT(2)receptors of angiotensin II (Ang II) in brain regions such as the superior colliculus, a midbrain visual structure where both receptor subtypes are found. We investigated the effects of Ang II on gross visual activity in the colliculus of anesthetized rats aged between 15 and 35 post-natal days. Microinjection of Ang II in the superficial layers yielded a strong reduction in the amplitude of visual evoked potentials in a dose-related manner. Injection of the peptide in more ventral collicular layers did not modify the potential confirming the discrete localization of the angiotensinergic receptors in the superficial layers. Preliminary data indicated that the co-injection of Ang II with Losartan or PD 123319 yielded a partial blockade of Ang II suppressive effects, indicating that both AT(1)and AT(2)receptors are likely to be involved in mediating these responses. Overall, this study shows that the inhibitory nature of Ang II action is similar in juvenile and adult animals (Merabet et al. 1994 and Merabet et al. 1997)

Interaction between Mas and the angiotensin AT1 receptor in the amygdala

The Mas-protooncogene is a maternally imprinted gene encoding an orphan G protein-coupled receptor expressed mainly in limbic structures of the rodent CNS. Because Mas and the product of the Mas-related gene enhance the effects of angiotensins on cells expressing angiotensin receptors of the AT1 subtype, we first compared the distribution of cells expressing AT1 receptors in different limbic and thalamic brain structures in Mas-knockout mice and in wildtype mice by an immunohistochemical approach. No significant differences could be found between the two strains. The Mas-protooncogene seems to be implicated in the signal transduction of angiotensin receptors and is expressed in the amygdala. Therefore we then analyzed whether field potentials are altered by angiotensin II in brain slices of the basolateral amygdala. An opposite action of angiotensin II was obtained in mice lacking the Mas-protooncogene in comparison to wildtype mice. The use of different angiotensin receptor antagonists provides the first in vitro evidence for a functional interaction between the Mas-protooncogene and the AT1 receptor.

The angiotensin II type 2 receptor: an enigma with multiple variations

Since it was discovered ten years ago, the angiotensin II (ANG II) type 2 (AT(2)) receptor has been an enigma. This receptor binds ANG II with a high affinity but is not responsible for mediating any of the classical physiological actions of this peptide, all of which involve the ANG II type 1 (AT(1)) receptor. Furthermore, the AT(2) receptor exhibits dramatic differences in biochemical and functional properties and in patterns of expression compared with the AT(1) receptor. During the past decade, much information has been gathered about the AT(2) receptor, and the steadily increasing number of publications indicates a growing interest in this new and independent area of research. A number of studies suggest a role of AT(2) receptors in brain, renal, and cardiovascular functions and in the processes of apoptosis and tissue regeneration. Despite these advances, nothing stands out as the major singular function of these receptors. The study of AT(2) receptors has reached a crossroads, and innovative approaches must be considered so that unifying mechanisms as to the function of these unique receptors can be put forward. In this review we will discuss the advances that have been made in understanding the biology of the AT(2) receptor. Furthermore, we will consider how these discoveries, along with newer experimental approaches, may eventually lead to the elusive physiological and pathophysiological functions of these receptors.

Involvement of central angiotensin receptors in stress adaptation

The present study examined the effects of acute and chronic neurogenic stressors on the expression of two distinct angiotensin receptors in two stress-related brain nuclei: angiotensin type 1A receptor in the paraventricular nucleus of the hypothalamus and angiotensin type 2 receptor in the nucleus locus coeruleus. Male Wistar rats were divided into four experimental groups. The first two groups were subjected once to an acute 90-min immobilization or air-jet stress session, respectively. The other two groups were subjected to 10 days of daily 90-min immobilization sessions and, on the 11th day, one group was exposed to an additional 90-min immobilization and the other to a single air-jet stress (heterotypic but still neurogenic) session. In each group, rats were perfused before stress (0 min), immediately following stress (90 min) or 150, 180, 270 or 360 min (and 24 h in chronic immobilization) after the beginning of the last stress session. Basal expression of both angiotensin receptor subtype 1A and angiotensin receptor subtype 2 messenger RNA was minimal in non-stressed animals. Acute immobilization as well as air-jet stress induced similar patterns (time-course and maximal values) of angiotensin receptor subtype 1A messenger RNA expression in the paraventricular nucleus. Angiotensin receptor subtype 1A messenger RNA expression increased 90-150 min after the beginning of the stress and returned to basal levels by 360 min. Chronic stress immobilization slightly modified the pattern, but not maximal values of angiotensin receptor subtype 1A messenger RNA expression to further immobilization (homotypic) or air-jet stress (heterotypic). Acute immobilization and air-jet stress sessions induced similar locus coeruleus-specific angiotensin receptor subtype 2 messenger RNA expression. This expression increased 90 min following the onset of the stress session and remained elevated for at least 360 min. Chronic immobilization stress increased angiotensin receptor subtype 2 messenger RNA expression to levels comparable to those observed in acute stress conditions. Novel acute exposure to neurogenic stressors did not further increase these levels in either homotypic (immobilization) or in heterotypic (air-jet stress) conditions. These results suggest that central angiotensin receptors are targets of regulation in stress; therefore, stress may modulate angiotensin function in the paraventricular nucleus and locus coeruleus during chronic exposure to neurogenic stressors.

Angiotensin II acts at AT1 receptors in the nucleus of the solitary tract to attenuate the baroreceptor reflex

The object of the current study was to determine if ANG II acts at type 1 (AT1) or type 2 (AT2) receptors in the nucleus of the solitary tract (NTS) to reduce baroreceptor reflex control of renal sympathetic nerve activity (RSNA) and heart rate (HR). Experiments were carried out in urethan-anesthetized Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Reflex changes in RSNA and HR were elicited by intravenous infusion of either phenylephrine or sodium nitroprusside before and after bilateral microinjection of CV-11974 (AT1 receptor antagonist, 10 pmol), PD-123319 (AT2 receptor antagonist, 100 pmol), or artificial cerebrospinal fluid (aCSF, 50 nl) in the NTS. Mean arterial pressure (MAP)-RSNA and MAP-HR data were fit to logistic functions to analyze the baroreceptor reflex. Baroreceptor reflex sensitivities for RSNA and HR were attenuated in SHR compared with those in WKY rats. Bilateral injection of CV-11974, PD-123319, or aCSF in the NTS of either strain had no effect on baseline arterial pressure, HR, or RSNA. However, CV-11974 injected in the NTS increased significantly (P < 0.01) the sensitivities for baroreceptor reflex control of RSNA and HR in SHR and WKY rats. Neither PD-123319 nor aCSF altered baroreceptor reflex control of RSNA and HR in either SHR or WKY rats. These results demonstrate that endogenous ANG II acts at AT1 receptors of the NTS to attenuate the baroreceptor reflex in SHR as well as in WKY rats.

Neuronal ion channel signalling pathways: modulation by angiotensin II

The brain contains both angiotensin II (Ang II) type 1 (AT1) and Ang II type 2 (AT2) receptors. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. The physiological actions of Ang II in the brain, whether mediated by AT1 or AT2 receptors, involve changes in neuronal activity that are initiated by changes in the activity of membrane ionic currents and channels. This review focusses on the intracellular signalling pathways that couple neuronal AT1 and AT2 receptors to changes in the activity of membrane K+ and Ca2+ currents and channels. As will become clear from our discussion, the signalling pathways that are modulated by neuronal AT1 and AT2 receptors are quite distinct.

Angiotensin receptors and norepinephrine neuromodulation: implications of functional coupling

The objective of this review is to examine the role of neuronal angiotensin II (Ang II) receptors in vitro. Two types of G protein-coupled Ang II receptors have been identified in cardiovascularly relevant areas of the brain: the AT1 and the AT2. We have utilized neurons in culture to study the signaling mechanisms of AT1 and AT2 receptors. Neuronal AT1 receptors are involved in norepinephrine (NE) neuromodulation. NE neuromodulation can be either evoked or enhanced. Evoked NE neuromodulation involves AT1 receptor-mediated, losartan-dependent, rapid NE release, inhibition of K+ channels and stimulation of Ca2+ channels. AT1 receptor-mediated enhanced NE neuromodulation involves the Ras-Raf-MAP kinase cascade and ultimately leads to an increase in NE transporter, tyrosine hydroxylase and dopamine beta-hydroxylase mRNA transcription. Neuronal AT2 receptors signal via a Gi protein and are coupled to activation of PP2A and PLA2 and stimulation of K+ channels. Finally, putative cross-talk pathways between AT1 and AT2 receptors will be discussed.

Analysis of angiotensin type 2 receptors in vasopressinergic neurons and pituitary in the rat

Previous functional studies indicated that an angiotensin type 2 (AT2) receptor subtype may participate in the regulation of vasopressin release by angiotensin II (AngII). In the present study, AT2 receptor-directed antiserum immunohistochemically detected AT2 receptors within the hypothalamic paraventricular (PVN) and the supraoptic nuclei (SON) of the rat brain, more specifically, in identified vasopressinergic neurons. Considering the lack of AT2 binding in the PVN and the SON using receptor autoradiography, we tested the hypothesis that these AT2 receptors are transported to the posterior pituitary. Western blot analysis detected AT2 immunoreactivity in the posterior pituitary. However, no AT2 binding was detected in posterior pituitary membranes, and no AT2 binding was detected with quantitative receptor autoradiography in the neurohypophysis. Thus, if AT2 receptors are transported from the magnocellular vasopressin neurons to the posterior pituitary, their role in AngII regulation of vasopressin release at the neurohypophyseal terminals remains to be clarified.

Angiotensin II inhibits long-term potentiation within the lateral nucleus of the amygdala through AT1 receptors

Long-term potentiation (LTP) of field potentials in the lateral nucleus of the amygdala (LA) evoked by brief tetanic stimuli of the LA were observed in horizontal rat brain slices. The amplitude of field potentials was significantly enhanced after repetitive stimulation. When angiotensin II was administrated the induction of LTP was suppressed. This inhibition of LTP was mediated by angiotensin II because it could be blocked by the coadministration of saralasin, an nonspecific angiotensin II antagonist. The coadministration of losartan, a competitive antagonist of the AT1 receptor subtype, concomitant with angiotensin II was also able to block this inhibition. The coadministration of the AT2 receptor antagonist PD 123,319 failed to block the inhibition of LTP.

Angiotensin receptors and norepinephrine neuromodulation: implications of functional coupling

The objective of this review is to examine the role of neuronal angiotensin II (Ang II) receptors in vitro. Two types of G protein-coupled Ang II receptors have been identified in cardiovascularly relevant areas of the brain: the AT1 and the AT2. We have utilized neurons in culture to study the signaling mechanisms of AT1 and AT2 receptors. Neuronal AT1 receptors are involved in norepinephrine (NE) neuromodulation. NE neuromodulation can be either evoked or enhanced. Evoked NE neuromodulation involves AT1 receptor-mediated, losartan-dependent, rapid NE release, inhibition of K+ channels and stimulation of Ca2+ channels. AT1 receptor-mediated enhanced NE neuromodulation involves the Ras-Raf-MAP kinase cascade and ultimately leads to an increase in NE transporter, tyrosine hydroxylase and dopamine beta-hydroxylase mRNA transcription. Neuronal AT2 receptors signal via a Gi protein and are coupled to activation of PP2A and PLA2 and stimulation of K+ channels. Finally, putative cross-talk pathways between AT1 and AT2 receptors will be discussed.

Dose-dependent inhibitory effects of angiotensin II on visual responses of the rat superior colliculus: AT1 and AT2 receptor contributions

Angiotensin II (Ang II) has traditionally been regarded as a peripherally circulating and acting hormone involved in fluid homeostasis and blood pressure regulation. With the rather recent localization of Ang II receptors within the mammalian brain, renewed interest has emerged in the hope of elucidating the central impact and function of this hormone. One region that has been clearly demonstrated to express Ang II receptors is the superior colliculus (SC). This mesencephalic structure plays an important role in sensory visuomotor integration. Receptors for Ang II (of both the AT1 and AT2 subtypes) have been localized within the superficial layers of this structure, i.e. the areas that are visually responsive. In the hopes of characterizing the role of Ang II in the SC, we have attempted to physiologically activate these receptors in vivo and observe the effects of Ang II on visually evoked responses. In the attempt to identify the receptor subtype(s) responsible in mediating these effects, Ang II was injected concomitantly with selective receptor ligands. Experiments were performed on adult rats prepared in classical fashion for electrophysiological studies. Through microinjection of Ang II, and the simultaneous recording of visually evoked potentials to flash stimulation, we have observed that this peptide yields a strong suppressive effect on visual neuronal activity. By injecting Ang II at various concentrations (10(-3)-10(-10) M), we have further observed that the effects of this peptide express a dose-related dependency. Injection of Ang II in progressively more ventral layers yielded less pronounced effects, demonstrating physiologically the discrete localization of these receptors in the stratum griseum superficiale. Coinjection of Ang II with Losartan yielded a near complete blockade of Ang II suppressive effects, suggesting that AT1 receptors play a prominent role in mediating these responses. However, coinjection of Ang II with PD 123,319 yielded a slight, yet significant partial blockade. Coinjection of Ang II with both the AT1 and AT2 receptor antagonists yielded a complete blockade of the Ang II effect. Finally, some of the results suggest that the AT2 receptor ligand CGP 42,112 may possess agonist properties. Taken together, these findings suggest that the AT1 receptor is predominantly involved in mediating Ang II responses in the SC and there also appears to be some indication of AT2 receptor involvement. However, the underlying mechanisms (such as receptor interactions), the exact specificity of the ligands used, and the possibility of other receptor subtype implication have yet to be explored fully.

Modulation of excitatory synaptic transmission in locus coeruleus by multiple presynaptic metabotropic glutamate receptors

Metabotropic glutamate receptors have been implicated in modulation of synaptic transmission in many different systems. This study reports the effects of selective activation of metabotropic glutamate receptors on synaptic transmission in intracellularly recorded locus coeruleus neurons in brain slice preparations. Perfusion of either L-2-amino-4-phosphonobutyric acid (L-AP4; 0.1-500 microM) or (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD; 0.1-500 microM) caused a depression of excitatory postsynaptic potentials in a dose-dependent fashion to about 70% inhibition. Both agonists exerted their effects at relatively low concentrations with estimated EC50s of 2.6 microM and 11.5 microM for L-AP4 and t-ACPD, respectively. This inhibition was not observed with the potent group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG; 100 microM). Conversely, (R)-4-carboxy-3-hydroxyphenyl-glycine (4C-3H-PG), a group I antagonist/group II agonist, and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC), a novel and specific group II agonist, also caused an inhibition of excitatory postsynaptic potentials. Both t-ACPD and L-AP4 produced an increase in paired-pulse facilitation, and failed to change the locus coeruleus response to focally applied glutamate, indicating a presynaptic locus of action. The L-AP4 inhibition was antagonized by (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP4: group III antagonist) but not by (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG; mixed antagonist], suggesting that this agonist acts through a type 4 metabotropic glutamate receptor. Conversely, t-ACPD was antagonized by MCPG and by ethyl glutamate (group II antagonist), but not by aminoindan dicarboxylic acid (AIDA; group I antagonist) or MAP4, suggesting that this agonist acts on a type 2 or 3 metabotropic glutamate receptor. Taken together, these results suggest that two pharmacologically distinct presynaptic metabotropic glutamate receptors function in an additive fashion to inhibit excitatory synaptic transmission in locus coeruleus neurons. These receptors may be involved in a feedback mechanism and as such may function as autoreceptors for excitatory amino acids.

Effects of angiotensin II and IV on geniculate activity in nontransgenic and transgenic rats

Microiontophoretic ejection of angiotensin II and angiotensin IV in the vicinity of geniculate neurons was used to study the effects of these peptides on the discharge rate and the discharge pattern of extracellularly recorded activity. The main aim of the experiments was to study the effects of angiotensins in different strains of rats anesthetized with urethane (normotensive Wistar, normotensive Sprague-Dawley and hypertensive, transgenic (TGR(mREN2)27) rats). Both angiotensins mostly increased the spontaneous activity of angiotensin-sensitive geniculate neurons in all strains. Angiotensin II reduced the number of bursts in most neurons, whereas angiotensin IV significantly enhanced it. Inhibitory effects of angiotensins on spontaneous as well as on light-evoked activity could be effectively blocked by GABA(A) or GABA(B) receptor antagonists. Therefore, it can be supposed that angiotensin-containing afferent fibers innervate both projection and local circuit neurons of the dorsal lateral geniculate nucleus. In addition, angiotensin II suppressed excitation induced by glutamate receptor agonists in most neurons tested. Angiotensin-induced effects could be blocked by specific receptor antagonists. There were no significant differences in the effects of angiotensins in the various strains of rats, except for the latencies of the neuronal responses to the iontophoretic ejection of angiotensins.

Modulation of membrane potential and ionic currents by the AT1 and AT2 receptors of angiotensin II

Angiotensin II, the principal effector of the renin-angiotensin system, modulates various ionic currents. Its effects on potassium currents, including outward transient potassium current, the inward or outward rectifiers, as well as Ca(2+)- activated potassium currents, is well described. Other ionic currents, such as voltage-dependent calcium currents, cationic or chloride currents, are also altered by the hormone. All these effects provoke changes in membrane potential, such as modulation of action potential firing or resting membrane potential and control intracellular calcium concentration. Summarized here are the results obtained on these membrane electrical properties using electrophysiological recordings.

Effects of non-peptide angiotensin II-receptor antagonists on pentylenetetrazol kindling in mice

The effects of non-peptide AT1- and AT2-receptor antagonists DuP 753 (losartan) and PD 123319 on the intensity of pentylenetetrazol (PTZ)-kindled seizures in mice were studied. PTZ was injected intraperitoneally at a subconvulsive dose of 40 mg/kg at 48 h until the appearance of clonic seizures. DuP 753 administered intracerebroventricularly (i.c.v.) tended to decrease seizure intensity. Successive administration of ineffective doses of DuP 753 (losartan) and AT2 (angiotensin II) significantly decreased seizure intensity. PD 123319 (i.c.v.) decreased seizure intensity. Combination of ineffective doses of PD 123319 and AT2 also significantly decreased seizure intensity. The results suggest the role of AT2 receptor and its subtypes in PTZ-kindled seizures as well as an action of DuP 753 and PD 123319 similar to the action of AT2, an AT2-receptor agonist.

Angiotensin II induction of neurite outgrowth by AT2 receptors in NG108-15 cells. Effect counteracted by the AT1 receptors

In the present study, 3-day treatment of nondifferentiated NG108-15 cells with 100 nM angiotensin II (Ang II) induces morphological differentiation of neuronal cells characterized by the outgrowth of neurites. These morphological changes are correlated with an increase in the level of polymerized tubulin and in the level of the microtubule-associated protein, MAP2c. Mediation by the AT2 receptor may be inferred since: (a) these cells contain only AT2 receptors; (b) the effects are mimicked by CGP 42112 (an AT2 receptor agonist); (c) they are not suppressed by the addition of DUP 753 (an AT1 receptor antagonist); and (d) are abolished by co-incubation with PD 123319 (an AT2 receptor antagonist). Application of Ang II in dibutyryl cAMP-differentiated cells (which contain both types of receptors) induces neurite retraction, an effect mediated by the AT1 receptor. These results indicate that the AT2 receptor of Ang II induces neuronal differentiation, which is initiated through an increase in the levels of MAP2c associated with tubulin. Moreover, our results demonstrate that the AT1 receptor inhibit the process of differentiation induced by dibutyryl cAMP, whereas the AT2 receptors potentiate this effect, illustrating negative cross-talk interaction between the two types of Ang II receptors.

Localization of AT2 angiotensin II receptor gene expression in rat brain by in situ hybridization histochemistry

To localize the gene expression of AT2 angiotensin II receptors in rat brain we performed in situ hybridization histochemistry using 35S-labeled antisense riboprobes. The AT2 receptor mRNA expression pattern was compared in consecutive brain sections, from 2 week old rats, with the receptor expression by means of [125I]Sar1-ANG II binding and displacement with AT2 selective ligands followed by autoradiography. Expression of AT2 receptor mRNA was found in several thalamic nuclei (ventral posterolateral, mediodorsal, central medial, paracentral, and paraventricular), the medial geniculate nuclei, the nucleus of the optic tract, the subthalamic nucleus, the interposed nucleus of the cerebellum, and in the inferior olive. In these areas the AT2 receptor gene expression corresponds well with [125I]Sar1-ANG II binding. In addition, AT2 receptor mRNA expression was found in the red nucleus where no [125I]Sar1-ANG II binding was present. No significant hybridization of the AT2 receptor antisense probe was found in septal nuclei, the locus coeruleus, the dorsolateral geniculate nucleus, or the cerebellar cortex, areas rich in [125I]Sar1-ANG II binding. Our results indicate that some brain regions may be involved in AT2 receptor formation, transporting the receptor protein to other brain areas. However, in most structures, both the formation and expression of receptors occur, suggesting the existence of local AT2 receptor circuits, or that of AT2 autoreceptors. Other structures express only the receptor protein, indicating that these AT2 receptors are produced elsewhere. Our present data are the basis for further studies on the clarification of AT2 receptor pathways in the brain.

Heterogeneity of angiotensin type 2 (AT2) receptors

Evidence continues to accumulate that strengthens the proposal of heterogeneity within both the AT1 and the AT2 receptor subtypes. Pharmacologic, biochemical and immunological studies of AT2 receptors expressed in N1E-115 cells strengthen the hypothesis of AT2 receptor heterogeneity. However, it is important to reassess these studies, especially in terms of how these results correlate with other reports of AT2 receptor heterogeneity. For example, AT2 receptor immunoreactivity was absent in some neuronal regions which have previously been proposed to express the AT2 receptor subtype. In particular, AT2 receptor staining was not seen in the inferior olive, a region which is known to express a high density of AT2 receptors. Upon first examination, these results were somewhat troubling. However, when compared with earlier reports, these results should not have been unexpected. For instance, Tsutsumi and Saaverdra previously have shown that AT2 receptors in the locus coeruleus are sensitive to the actions of guanine nucleotides, while AT2 receptors in the inferior olive are insensitive (21). These antisera were raised against a population of AT2 receptors which are sensitive to GTP gamma S and therefore, the lack of AT2 receptor staining in the inferior olive, as well as the presence of AT2 receptor immunoreactivity in the locus coeruleus, confirms and extends these earlier reports. In addition the AT2 receptors expressed in the locus coeruleus have been shown to be functionally distinct from AT2 receptors in the inferior olive. In this regard, Ang II has been shown to depress glutamate-induced EPSPs in the locus coeruleus, an effect which is mediated through the AT2 receptor (19). Conversely, AT2 receptors have been shown to increase the firing rate of neurons in the inferior olive (20). Collectively, these results would predict that staining should be absent in the inferior olive using these AT2-directed antisera. Indeed, in view of these earlier physiological and pharmacological studies, the presence of AT2 receptor immunoreactivity in the inferior olive would have been surprising. The most convincing example of AT2 receptor heterogeneity is the characterization of AT2 receptors present in N1E-115 cells. Separation of solubilized N1E-115 membranes by heparin-Sepharose chromatography generates two populations of AT2 receptors which are pharmacologically and biochemically distinct. In particular, CGP42112A was approximately 2 orders of magnitude more selective for Peak III AT2 receptors than was PD123319. Binding activity of Peak I and Peak III AT2 receptor populations also differed in their responses to GTP gamma S and DTT treatment. Lastly, the AT2-directed antisera, raised against the Peak I population of AT2 receptors, were not able to immunodetect the Peak III population of AT2 receptors in immunoblot analysis, or immunoprecipiatate AT2 binding activity from Peak III material. Pharmacological, biochemical and immunological analysis of the AT2 receptor clone isolated from N1E-115 cells revealed that it has the identical characteristics or properties of the Peak III receptor. The AT2 receptor isolated from N1E-115 cells exhibited a similar pharmacology as the Peak III AT2 receptor, in that CGP42112A was more effective at displacing 125I-Ang II binding activity than was PD123319. The AT2 receptor clone was also shown to be insensitive to the actions of GTP gamma S, as well as demonstrated increased binding activity in the presence of DTT, identical to the Peak III AT2 receptor. Lastly, immunoblot analysis of membranes prepared from COS-1 cells transfected with the AT2 receptor cDNA from N1E-115 cells did not demonstrate any immune-specific bands with the AT2-directed antisera. Characterization of an AT2 receptor cDNA isolated from N1E-115 cells reveals that this clone is identical to the Peak III type of AT2 receptor.