Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7

doi:10.1016/S0028-3908(00)00188-X

Neuropharmacology

Volume 40, Issue 4, March 2001, Pages 618-623

https://doi.org/10.1016/S0028-3908(00)00188-X Get rights and content

Abstract

Recent evidence demonstrates that the fragment of angiotensin II, angiotensin II (3–8) termed angiotensin IV, binds with high affinity to a specific binding site, the AT₄ receptor. Intracerebroventricular injection of AT₄ receptor agonists improves the performance of rats in passive avoidance and spatial learning paradigms. AT₄ receptors and cholinergic neurons are closely associated in regions involved in cognitive processing, such as the hippocampus and neocortex. We therefore postulated that AT₄ receptors affect cognitive processing by modulating cholinergic neurotransmission. To test this, we examined the effect of AT₄ receptor ligands, angiotensin IV and LVV-hemorphin-7, on potassium-evoked [³H]acetylcholine ([³H]ACh) release from rat hippocampal slices. Hippocampal slices from male Sprague–Dawley rats were incubated with [³H]choline chloride, perfused with Krebs–Henseleit solution and [³H]ACh release was determined. Angiotensin IV and LVV-hemorphin-7 both potentiated depolarisation-induced [³H]ACh release from the rat hippocampus in a concentration-dependent manner with the maximal dose (10⁻⁷M) of each inducing an increase of 45±7.5% (P<0.01) and 95.8±19% (P<0.01) above control, respectively. Potentiation of release by both agonists was attenuated by the AT₄ receptor antagonist, divalinal-Ang IV. Angiotensin IV-induced potentiation was not affected by AT₁ and AT₂ receptor antagonists. These results indicate that stimulation of AT₄ receptors can potentiate depolarisation-induced release of ACh from hippocampal slices and suggest that potentiation of cholinergic transmission may be a mechanism by which AT₄ receptor ligands enhance cognition.

Introduction

Recent evidence suggests that angiotensin II (3–8), termed angiotensin IV (Ang IV), plays a role in memory and learning: intracerebroventricular (icv) administration of Ang IV and its analogues potentiate learning in associative and spatial tasks (Braszko and Wisniewski, 1988, Wright et al., 1993, Pederson et al., 1998, Wright et al., 1999). Furthermore, icv injections of Ang IV stimulate c-fos expression in the CA2–CA3 fields of the rat hippocampus, an area thought to be involved in memory formation (Roberts et al., 1995).

Ang IV binds with high affinity to a pharmacologically distinct binding site which was initially characterized in the bovine adrenal (Swanson et al., 1992). This site has been termed the AT₄ receptor (de Gasparo et al., 1995). In addition to Ang IV, the AT₄ receptor also binds the decapeptide, LVV-hemorphin-7, which we isolated from the sheep brain (Moeller et al., 1997). LVV-hemorphin-7 binds to the AT₄ receptor with high affinity (ic₅₀=4.15 nM), and is present in high concentrations in the central nervous system (Moeller et al., 1997). Hence, LVV-hemorphin-7 may be an endogenous ligand for the AT₄ receptor in the brain.

The AT₄ receptor is widely distributed in the central nervous system (Miller-Wing et al., 1993, Moeller et al., 1996), being abundant in motor nuclei and in areas associated with cognitive and sensorimotor functions, such as the hippocampus, neocortex and cerebellum (Miller-Wing et al., 1993, Moeller et al., 1996). Of particular interest is the occurrence of high AT₄ receptor densities in areas of basal forebrain cholinergic nuclei and their terminal fields, including the medial septal complex, the basal nucleus of Meynert, the CA1 and CA3 fields of the hippocampus, the dentate gyrus and the neocortex (Miller-Wing et al., 1993, Moeller et al., 1996).

Considerable evidence implicates cholinergic neurons in cognition (for review see Woolf, 1998). Cholinergic neurons, located in the medial septal nucleus and in the nuclei of the diagonal band, project to the hippocampal formation (Woolf et al., 1984). This septo-hippocampal cholinergic projection has a critical role in the generation and maintenance of theta rhythms, which are functionally important in learning and memory (Vinogradova, 1995). Many studies have demonstrated that anticholinergic agents such as scopolamine and atropine can disrupt both the acquisition and performance of a variety of learned behaviours (Bammer, 1982, Molchan et al., 1992). In line with this, loss of basal forebrain cholinergic neurons has been associated with cognitive disorders, such as Alzheimer's disease (Coyle et al., 1983).

Given the abundance of AT₄ receptors in regions containing cholinergic cell bodies or their terminal fields and the demonstration that the scopolamine-induced spatial learning impairments could be attenuated by Ang IV analogues (Pederson et al., 1998), we postulated that the activation of AT₄ receptors potentiate cognitive processes via modulation of cholinergic transmission. Thus, the aim of this study was to examine the effect of AT₄ receptor stimulation on acetylcholine (ACh) release in the rat hippocampus.

Section snippets

Materials and methods

All experiments were performed in accordance with the Australian National Health and Medical Research Council code of practice for the care and use of animals for scientific purposes and were approved by the Animal Experimentation Ethics Committee of the Howard Florey Institute.

Potassium-evoked release of [³H]ACh from rat hippocampal slices

Basal release of ACh was linear and proceeded on average at a rate of 0.2% per 5 min. Depolarisation of rat hippocampal slices by 30 mM KCl evoked [³H]ACh release, which was at least twice basal release. Evoked release was Ca²⁺-dependent since it was blocked by the omission of Ca²⁺ and inclusion of 0.1 mM EGTA in the superfusion buffer (data not shown).

Effect of Ang IV on evoked release of [³H]ACh from rat hippocampal slices

The addition of Ang IV (10⁻⁹–10⁻⁶M) to the superfusion buffer potentiated the evoked release of [³H]ACh in a concentration-dependent manner (

Discussion

The present study demonstrates that stimulation of the AT₄ receptor, by either Ang IV or LVV-hemorphin-7, can potentiate depolarisation-evoked release of ACh in rat hippocampal slices. The potentiating effects of Ang IV and LVV-hemorphin-7 on ACh release appear to be mediated via the AT₄ receptor, since these effects were significantly attenuated in the presence of the AT₄ receptor antagonist, divalinal-Ang IV. Divalinal-Ang IV, given alone, did not modify evoked or basal ACh release,

Acknowledgments

This study was supported by the Australian National Health and Medical Research Council No. 983001. The authors thank Ms A. Gibson for technical assistance.

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In fact facilitation of the AT4 receptor subtype results in increased internalization of calcium via at least three different calcium channels, suggesting a rapid and salient cell signaling event (Davis et al., 2006) and agrees with the observation that HGF-induced responses, including LTP, also depend upon the internalization of calcium (Tyndall et al., 2007). ( 3) Conversion of AngII to AngIV appears to be necessary for AngII-induced DA release in the striatum (Stragier et al., 2004), and acetylcholine release in the hippocampus (Lee et al., 2001). ( 4) Increased neural intracellular calcium coupled with matrix metalloproteinases released into the extracellular space suggests a neural plasticity function (Meighan et al., 2006, 2007), consistent with similar observations for HGF (Gutierrez et al., 2004; Hashimoto et al., 2001; Tyndall et al., 2007).
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Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7

Abstract

Introduction

Section snippets

Materials and methods

Potassium-evoked release of [3H]ACh from rat hippocampal slices

Effect of Ang IV on evoked release of [3H]ACh from rat hippocampal slices

Discussion

Acknowledgments

Neurosci. Behav. Rev.

Brain Res.

Peptides

Neuropeptides

Brain Res.

Brain Res. Rev.

Regul. Pep.

Biochem. Biophys. Res. Commun.

Brain Res.

Life Sci.

Regul. Pep.

Prog. Neurobiol.

Potassium-evoked release of [³H]ACh from rat hippocampal slices

Effect of Ang IV on evoked release of [³H]ACh from rat hippocampal slices