Doris Albrecht

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1–8) [Ang II], angiotensin-(3–8) [Ang IV], and angiotensin-(1–7) [Ang-(1–7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1–7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1–7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Capsaicin-Induced Changes in LTP in the Lateral Amygdala Are Mediated by TRPV1

The transient receptor potential vanilloid type 1 (TRPV1) channel is a well recognized polymodal signal detector that is activated by painful stimuli such as capsaicin. Here, we show that TRPV1 is expressed in the lateral nucleus of the amygdala (LA). Despite the fact that the central amygdala displays the highest neuronal density, the highest density of TRPV1 labeled neurons was found within the nuclei of the basolateral complex of the amygdala. Capsaicin specifically changed the magnitude of long-term potentiation (LTP) in the LA in brain slices of mice depending on the anesthetic (ether, isoflurane) used before euthanasia. After ether anesthesia, capsaicin had a suppressive effect on LA-LTP both in patch clamp and in extracellular recordings. The capsaicin-induced reduction of LTP was completely blocked by the nitric oxide synthase (NOS) inhibitor L-NAME and was absent in neuronal NOS as well as in TRPV1 deficient mice. The specific antagonist of cannabinoid receptor type 1 (CB1), AM 251, was also able to reduce the inhibitory effect of capsaicin on LA-LTP, suggesting that stimulation of TRPV1 provokes the generation of anandamide in the brain which seems to inhibit NO synthesis. After isoflurane anesthesia before euthanasia capsaicin caused a TRPV1-mediated increase in the magnitude of LA-LTP. Therefore, our results also indicate that the appropriate choice of the anesthetics used is an important consideration when brain plasticity and the action of endovanilloids will be evaluated. In summary, our results demonstrate that TRPV1 may be involved in the amygdala control of learning mechanisms.

Improved Learning and Memory in Aged Mice Deficient in Amyloid β-Degrading Neutral Endopeptidase

Background Neutral endopeptidase, also known as neprilysin and abbreviated NEP, is considered to be one of the key enzymes in initial human amyloid-β (Aβ) degradation. The aim of our study was to explore the impact of NEP deficiency on the initial development of dementia-like symptoms in mice. Methodology/Principal Findings We found that while endogenous Aβ concentrations were elevated in the brains of NEP-knockout mice at all investigated age groups, immunohistochemical analysis using monoclonal antibodies did not detect any Aβ deposits even in old NEP knockout mice. Surprisingly, tests of learning and memory revealed that the ability to learn was not reduced in old NEP-deficient mice but instead had significantly improved, and sustained learning and memory in the aged mice was congruent with improved long-term potentiation (LTP) in brain slices of the hippocampus and lateral amygdala. Our data suggests a beneficial effect of pharmacological inhibition of cerebral NEP on learning and memory in mice due to the accumulation of peptides other than Aβ degradable by NEP. By conducting degradation studies and peptide measurements in the brain of both genotypes, we identified two neuropeptide candidates, glucagon-like peptide 1 and galanin, as first potential candidates to be involved in the improved learning in aged NEP-deficient mice. Conclusions/Significance Thus, the existence of peptides targeted by NEP that improve learning and memory in older individuals may represent a promising avenue for the treatment of neurodegenerative diseases.

Input-specific long-term potentiation in the rat lateral amygdala of horizontal slices.

Long-term potentiation (LTP) at input synapses to the lateral nucleus of the amygdala (LA) is a candidate mechanism for memory storage during fear learning. Cellular mechanisms of LTP have been nearly exclusively investigated in coronal brain slices. In our experiments, we used a horizontal brain slice preparation of rats that preserved most of the connections to cortical areas and the hippocampus. The stimulation electrodes were located either within the external capsule (EC) or the LA. The aim of the present study was to investigate the mechanisms of LTP induced either by weak theta burst stimulation (TBS) or strong high frequency stimulation (HFS) using the two different stimulation sites. Whereas both TBS and HFS of afferences running through the LA induced stable LTP, TBS failed to induce LTP of EC-inputs to the LA. The present findings also show that LTP in the LA exhibits vulnerability at different time windows after induction. The time window was dependent on the kind of stimulated afferences. Later LTP becomes resistant to disruption by low frequency stimulation. We could show that both used inputs depended on NMDA receptors for LTP-induction. LTP induced by stimulation of fibers within the LA was not altered by nifedipine (10 microM). In contrast, EC-induced LTP was dependent on L-type voltage-gated calcium channels (VGCC). Finally, we found a higher magnitude of LTP in females using TBS, whereas HFS did not cause gender-specific differences. Our study supports the conclusion that the form of LA-LTP depend on which afferences are activated and what pattern of stimulation is used to induce LTP.

Effects of single swim stress on changes in TRPV1-mediated plasticity in the amygdala

By examining the involvement of transient receptor potential vanilloid type 1 (TRPV1) in the stress modulation of learning and memory processes in mice, we evaluated the effects of endovanilloid N-oleoyldopamine (OLDA) on the long-term potentiation (LTP) of the lateral nucleus of the amygdala (LA). After high-frequency stimulation of external capsule fibers we found that LA-LTP is reduced in OLDA-treated slices derived from adult C57BL/6 control mice. The specificity of the TRPV1 receptor activation by OLDA was confirmed by blocking the OLDA-induced inhibitory effect on LA-LTP with the specific TRPV1 receptor antagonist AMG 9810. The specificity of OLDA was further supported by using TRPV1 deficient mice, where the effect of OLDA on LA-LTP was missing. Following exposure to a forced swim test (FST) OLDA enhanced LA-LTP in control but not TRPV1-deficient mice. The results also show that a short period of acute stress significantly impairs LA-LTP. Since we have recently shown the involvement of cannabinoid CB1 receptors in the mediation of capsaicin-induced inhibitory effects on LA-LTP ([23] Zschenderlein et al., 2011), it is reasonable to assume that the OLDA-induced enhancement of LA-LTP after the forced swim test can be attributed to the up-regulation of TRPV1 and the action of ligands such as anandamide on TRPV1. As a result, stimulation of TRPV1 receptors rescues LTP in slices derived from swim-stressed mice.

Angiotensin II suppresses long-term depression in the lateral amygdala of mice via L-type calcium channels

Previously we have shown that angiotensin II (Ang II) suppresses long-term potentiation (LTP) in the lateral nucleus of the amygdala (LA) of horizontal slices. This study examines the effect of Ang II on long-term depression (LTD) in the LA. Low frequency stimulation (1 Hz, 15 min; LFS) applied to fibers running within the LA induced a long-lasting reduction of the amplitudes of field potentials in the LA of mice. We have previously shown that this LTD is sensitive to the NMDA receptor blocker D-AP5 and is dependent on group II mGlu receptors. Ang II blocked dose-dependent LTD. Losartan, an AT1 receptor antagonist, blocked the Ang II-induced suppression of LTD, whereas PD 123 319, an AT2 receptor antagonist, had no effect. Co-administration of nifedipine, an L-type calcium channel antagonist, abolished Ang II-induced suppression of LTD. When applied alone nifedipine reduced the magnitude of LA-LTD. According to our previous results, stimulation of external capsule (EC) fibers in rats did not cause LTD in mice. Similarly, Ang II did not induce long-lasting changes of activity when EC stimulation site was used. The results support the view that angiotensins are involved in mechanisms of learning and memory including the plasticity changes in the LA.

Very slow oscillations of activity in geniculate neurones of urethane-anaesthetized rats.

The spontaneous activity of neurones of the dorsal lateral geniculate nucleus (dLGN) in urethane-anaesthetized rats was examined for the presence of very slow oscillatory activity. Fifty-four of the 86 dLGN neurones (63%) recorded extracellularly displayed oscillatory activity in the 0.02-0.03 Hz range. Similar very slow oscillations were observed in the ventral part of the LGN and the nucleus lateralis posterior (LP), but not in the hippocampus. Diffuse light stimuli dampened or blocked these oscillations in 23 of the 29 neurones tested. In a second group of experiments (iontophoretic studies) the very slow oscillatory activity was efficiently blocked by N-methyl-D-aspartate.

Different isoforms of nitric oxide synthase are involved in angiotensin-(1-7)-mediated plasticity changes in the amygdala in a gender-dependent manner.

Background: The amygdala receives afferent sensory input and processes information related to hydromineral balance. Angiotensin acts on and through the amygdala to stimulate thirst and sodium appetite. In addition, different angiotensins seem to play a role in cognition and learning mechanisms by acting on and through the amygdala. Recently, we showed that angiotensin-(1–7) (Ang-(1–7)) enhances the magnitude of long-term potentiation (LTP) in the lateral nucleus of the amygdala (LA) via the Mas receptor. Methods: Extracellular field potentials were measured in the LA. Results: LA-LTP induced by stimulation of the external capsule was nitric oxide (NO)-dependent because the NO synthase (NOS) inhibitor L-NAME reduced LA-LTP. The LA-LTP was also reduced in both male and female nNOS and eNOS knockout mice. In male eNOS–/– mice, Ang-(1–7) enhanced LA-LTP, whereas the LTP-enhancing effect of Ang-(1–7) was missing in female eNOS–/– mice. Therefore, the LTP-enhancing effect of Ang-(1–7) was mediated by eNOS in females. In contrast, Ang-(1–7) strongly enhanced the LTP in nNOS–/– females, whereas the effect of Ang-(1–7) was missing in nNOS–/– males. Thus, Ang-(1–7) induced an increase in the magnitude of LTP via the involvement of nNOS in males. Conclusion: Our data support not only the hypothesis that NO contributes to plasticity changes in the lateral amygdala, but also show for the first time a gender-dependent involvement of different isoforms of NOS in the mediation of Ang-(1–7) on LTP in the amygdala.

Physiological and pathophysiological functions of different angiotensins in the brain

The renin–angiotensin system (RAS) is better known for its role in the control of blood pressure, but evidence obtained from animal experiments and clinical trials suggests that it is involved in complex brain functions. It is now well accepted that neuronal AT1 receptors mediate the stimulatory actions of angiotensin II regarding blood pressure and the intake of water and salt. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic AT1 receptors. Angiotensin‐(1‐7) [Ang‐(1‐7)] mediates its antihypertensive effects by stimulating synthesis and release of vasodilator prostaglandins and nitric oxide. New data concerning the receptor types binding angiotensin IV or Ang‐(1‐7) also support the existence of complex site‐specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Different angiotensin receptors (AT1, AT2, AT4, Mas) are also present in memory‐relevant structures. The effects of different angiotensins on cognition initiated the search for their mechanisms of action. Studies looking for a possible link between the RAS and brain disorders (stress, anxiety, depression, alcohol abuse, epilepsy, Alzheimers disease) either inherited or acquired have been reviewed. The therapeutic potential of different angiotensins, as well as the potential use of agents known to influence the RAS, will be considered.

Gender-dependent ATPA-induced changes in long-term potentiation in the rat lateral amygdala

There is increasing evidence that kainate receptors contribute to both postsynaptic and presynaptic signaling not only in the hippocampus but also in the amygdala. The present study demonstrates that low concentrations of the specific kainate GLUK5 receptor agonist, ATPA, depressed baseline activity in the lateral nucleus of the rat amygdala (LA), induced by stimulation of external capsule fibers or by intranuclear stimulation in horizontal brain slices. ATPA reduced high‐frequency‐induced long‐term potentiation (LTP) in males while it enhanced LTP in females during certain phases of the estrus cycle. In untreated slices from females, LA‐LTP differed depending on the phase of the estrus cycle. In addition, we show for the first time that the p38 mitogen‐activated protein (MAP) kinase inhibitor, SKF 86002, reduced LA‐LTP. In males, the effects of ATPA and SKF 86002 were not additive. To the contrary, in females, the exposure to ATPA in control plus SKF 86002 increases LTP relative to control plus SKF 86002 alone. Thus, we demonstrate that the effectiveness of GLUK5 stimulation on plasticity changes in the amygdala is gender‐dependent and that the MAP kinase pathway might be involved in males. Schubert, M., Drephal, C., Albrecht, D. Gender‐dependent ATPA‐induced changes in long‐term potentiation in the rat lateral amygdala. FASEB J. 22, 1268–1274 (2008)