A role for the angiotensin AT4 receptor subtype in overcoming scopolamine-induced spatial memory deficits
Introduction
The cognitive dysfunctions characteristic of Alzheimer's disease (AD) are accompanied by a progressive depletion of central cholinergic neurons [1], [2]. This loss of cholinergic neurons occurs in brain structures devoted to cognitive processing including temporal cortices, hippocampus, septal nuclei, and the nucleus basalis of Meynert (NBM) [3], [4], [5], [6], [7]. Choline acetyltransferase (ChAT), a marker for cholinergic activity, is depleted by up to 90% in these structures [8]. In an attempt to approximate this loss of cholinergic neurotransmission characteristic of AD, pharmacological blockade of the cholinergic system has frequently been employed in animal models. Specifically, peripheral or central treatment with the muscarinic receptor antagonist, scopolamine hydrobromide (scop), produces deficits in acquisition and retrieval of a new task [9], [10], [11], [12].
Several years ago, our laboratory discovered a new angiotensin receptor subtype [13], [14] now termed AT4 [15]. This subtype binds the hexapeptide angiotensin IV (AngIV: Val-Tyr-Ile-His-Pro-Phe) with high specificity and affinity [13], [14], [16]. A role for this AngIV/AT4 system in memory and cognitive processing has been postulated based on the following recent observations. (1) High concentrations of the AT4 receptor site are present in the rat and guinea pig hippocampus, neocortex, and cerebellum [17], and in monkey hippocampus, neocortex, and NBM [18]. (2) Central administration of AngIV (or AngIV analogs) facilitates retention of associative memory (passive avoidance conditioning task) and spatial memory (circular water maze task), whereas AT4 receptor antagonists disrupt these processes [17], [19], [20]. (3) Intracerebroventricular (i.c.v.) infusion of AngIV selectively activates c-Fos immunoreactivity in hippocampal pyramidal and granular cells indicating that i.c.v. application can influence the hippocampus [21]. (4) AngIV potentiates [3H]-acetylcholine (ACh) release from rat hippocampal slices, whereas such release could be attenuated by pretreatment with an AT4 antagonist [22]. And (5), an endogenous ligand from brain, LVV-hemorphin-7, has been identified that binds to the AT4 site [23].
Given the short half-life of native AngIV, we decided to synthesize a high-affinity and more metabolically stable AngIV analog. This was accomplished with Nle1–AngIV prepared with a reduced peptide bond between the first and second amino acids [Nle-(CH2-NH2)-Tyr-Ile-His-Pro-Phe] that has a very high affinity for the AT4 receptor (Kd=3×10−12 M) and very low affinity for the AT1 and AT2 receptor subtypes [16]. Our laboratory also synthesized a new AT4 receptor antagonist, Nle1, Leual3–AngIV, prepared with a reduced peptide bond between the third and fourth amino acids [Nle-Tyr-Leu-(CH2-NH2)-His-Pro-Phe] also with high affinity for the AT4 binding site (two-site model: Kd=1.66×10−10 and 4.74×10−13 M) [24]. The animals utilized in the present experiment were tested in the circular water maze, a measure of spatial memory. This task has been shown to be sensitive to the cognitive disruptions induced by scop [25], and is appropriate given the disruption in spatial memory suffered by AD patients. In an effort to reduce the sensorimotor impairments sometimes reported with the application of muscarinic antagonists (reviewed in Ref. [26]), we included pretraining trials with a visible platform. We hypothesized that scop-induced deficits in spatial memory could be overcome by the i.c.v. infusion of Nle1–AngIV. We further speculated that i.c.v. pretreatment with the AT4 receptor antagonist, Nle1, Leual3–AngIV, would attenuate this Nle1–AngIV-induced reversal of the scop-induced cognitive deficits, suggesting that this effect is specific to the AT4 system.
Section snippets
Animals and surgery
Male Sprague–Dawley rats (Charles River derived; 350–450 g) were housed individually in an AAALAC-approved vivarium maintained at 22±1 °C, with a 12:12 h light/dark cycle initiated at 07:00 h. All animals were provided ad libitum access to water and Purina laboratory rat chow. Food was removed the night before surgery. Each animal was anesthetized with Equithesin (3.5 mg/kg pentobarbital i.p.; Jensen-Salsbury Laboratory) then stereotaxically fitted with an i.c.v. guide cannula (PE 60, Clay
Experiment 1
Fig. 1A presents the mean (±S.E.M.) latencies to find the platform on days 1–8 of testing. In general, members of the group treated with scopolamine and then subsequently treated with aCSF (scop/aCSF) performed poorly as compared with the control group (aCSF/aCSF); while those groups pretreated with scopolamine and then with Nle1–AngIV revealed a dose-dependent improvement in performance. Those animals that received the highest doses (100 and 1000 pmol) exhibited mean latencies to find the
Discussion
Successful acquisition of new spatial memories appears to depend upon a normal functioning hippocampal complex, and intact brain cholinergic system [28], [29], [30], [31], [32], [33]. It is clear that several neuropeptides, including angiotensin, act as modulators of the cholinergic system [34], [35], [36], [37], [38], [39]. Along these lines, the hippocampus has been shown to possess a very high density of AT4 receptors, whereas the level of AT1 receptors is relatively low [40], [41], [42].
Acknowledgements
The authors thank Mrs. Ruth Day for excellent secretarial assistance with the preparation of this manuscript. This research was supported by funds provided by the Edward E. and Lucille I. Laing Endowment, and Washington State University.
References (53)
Acetylcholine, aging, and Alzheimer's disease
Pharmacol., Biochem. Behav.
(1997)- et al.
Alzheimer plaques and cortical cholinergic innervation
Neuroscience
(1986) - et al.
Neuronal loss in different parts of the nucleus basalis is related to neuritic plaque formation in cortical target areas in Alzheimer's disease
Neuroscience
(1985) - et al.
Plaque-like lesions in the basal forebrain in Alzheimer's disease
Neurosci. Lett.
(1985) - et al.
The cholinergic system in Alzheimer's disease
Prog. Neurobiol.
(1997) - et al.
Scopolamine-disruption of radial arm maze performance: modification by noradrenergic depletion
Brain Res.
(1987) - et al.
Scopolamine impairs both working and reference memory in rats: a replication and extension
Pharmacol., Biochem. Behav.
(1989) - et al.
Effects of scopolamine infusions into the anterior and posterior cingulate on passive avoidance and water maze navigation
Brain Res.
(1995) - et al.
Identification of an AII(3–8) [AngIV] binding site in guinea pig hippocampus
Brain Res.
(1992) - et al.
Discovery of a distinct binding site for angiotensin II(3–8), a putative angiotensin IV receptor
Regul. Pept.
(1992)
AT4 receptor structure-binding relationship: N-terminal-modified angiotensin IV analogues
Peptides
Angiotensin II(3–8) (AngIV) hippocampal binding: potential role in the facilitation of memory
Brain Res. Bull.
Distribution of AT4 receptors in the Macaca fascicularis brain
Brain Res.
The angiotensin IV system: functional implications
Front. Neuroendocrinol.
Autoradiographic identification of brain angiotensin IV binding sites and differential c-Fos expression following intracerebroventricular injection of angiotensin II and IV in rats
Brain Res.
Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7
Neuropharmacology
Testing the NMDA, long-term potentiation, and cholinergic hypothesis of spatial learning
Neurosci. Biobehav. Rev.
Attenuation of scopolamine-induced spatial learning impairments by an angiotensin IV analog
Regul. Pept.
Spatial location does not require the presence of local cues
Learn Motiv.
Hippocampal, granule cell and CA3–4 lesions impair formation of a place learning-set in the rat and induce reflex epilepsy
Behav. Brain Res.
Angiotensin II inhibits acetylcholine release from human temporal cortex: implications for cognition
Brain Res.
Co-existence of galanin and acetylcholine: is galanin involved in memory processes and dementia?
Trends Neurosci.
Naloxone enhancement of memory
Behav. Neural Biol.
Enhanced inhibitory avoidance learning produced by post-trial injections of substance P into the basal forebrain
Behav. Neural Biol.
Brain angiotensin receptor subtypes in the control of physiological and behavioral responses
Neurosci. Biobehav. Rev.
Brain angiotensin receptor subtypes AT1, AT2, and AT4 and their functions
Regul. Pept.
Cited by (77)
Cognitive benefits of angiotensin IV and angiotensin-(1–7): A systematic review of experimental studies
2018, Neuroscience and Biobehavioral ReviewsThe development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases
2015, Progress in NeurobiologyCentral administration of angiotensin IV rapidly enhances novel object recognition among mice
2013, NeuropharmacologyCitation Excerpt :Consistent with this, central administration of Ang IV induced cognitive improvements equivalent to Ang II in rats (Braszko et al., 1988), and the blockade of AT4 receptors produced deficits in spatial learning (Wilson et al., 2009; Wright et al., 1999). Agonism of AT4 receptors has been reported to promote hippocampal long-term potentiation (LTP; Kramár et al., 2001; Wayner et al., 2001), spatial learning (Lee et al., 2004; Wright et al., 1999), and the restoration of spatial cognitive function following a variety of central insults (Stubley-Weatherly et al., 1996; Wright et al., 1996, 1999; Pederson et al., 2001). In the present investigation, we assessed the dose–response relationship of Ang IV-enhanced learning and memory using a truncated version of the novel object recognition task.
Brain renin-angiotensin-A new look at an old system
2011, Progress in NeurobiologyIntrahippocampal Norleucine<sup>1</sup>-Angiotensin IV mitigates scopolamine-induced spatial working memory deficits
2010, PeptidesCitation Excerpt :Thus no animals were excluded from our study on the basis of improper injection site. We observed a delay between the first injection and behavioral improvements in the Scop/Nle1-AngIV group (Fig. 3) which is consistent with many studies assessing the behavioral effects of this compound [3,23,26]. Therefore, to allow time for Nle1-AngIV to have an effect on all of our measures, we analyzed our day 1 data separately from our day 2–3 data.