Tangui Maurice

Amnesia induced in mice by centrally administered β-amyloid peptides involves cholinergic dysfunction

Substantial evidences suggest that the increased cerebral deposition, and neurotoxic action of the beta-amyloid peptide, the major constituent of senile plaques, may represent the underlying cause of the cognitive deficits observed in Alzheimers disease. Herein, we attempted to verify this hypothesis by inducing a potential Alzheimers-type amnesia after direct intracerebroventricular administration of aggregated beta 25-35-amyloid peptide in mice. In this aim, mnesic capacities were evaluated after 6-13 days, using spontaneous alternation in the Y-maze, step-down type passive avoidance and place learning in a water-maze. Pretraining administration of aggregated beta 25-35 peptide induced dose-dependent decreases in both alternation behaviour and passive avoidance, at doses of 3 and 9 nmol/mouse. A reduced but still significant impairment was observed when the peptide was not aggregated, or aged, by preincubation for 4 days at 37 degrees C. The beta 1-28 peptide, at 3 nmol/mouse, also induced a marked decrease in step-down latency. Posttraining, but not preretention, administration of beta 25-35 peptide also significantly impaired learning. The beneficial effects of cholinergic agents on beta 25-35-induced amnesia was examined using the cholinesterase inhibitor tacrine (THA, 1.3 and 4.3 mumol/kg i.p.) and the nicotinic receptor agonist (-)-nicotine (NIC, 0.06 and 0.2 mumol/kg i.p.). Both drugs induced a dose-dependent abrogation of the beta 25-35-induced decreases in alternation behaviour and passive avoidance. Furthermore, THA, at 1.3 mumol/kg, and NIC, at 0.2 mumol/kg, also reversed the beta 25-35-induced impairment of place learning and retention in the water-maze. Histological examination of Cresyl violet-stained brain sections indicated a moderate but significant cell loss within the frontoparietal cortex and the hippocampal formation of mice treated with aged beta 25-35 peptide (9 nmol). Examination of Congo red-stained sections in the same animals demonstrated the presence of numerous amyloid deposits throughout these brain areas. These results confirm that the deposition of beta-amyloid peptide in the brain is in some way related to impairment of learning and cholinergic degeneration and suggest that the [25-35] fragment of the beta-amyloid protein, sufficient to induce neuronal death in cultures, also induces an Alzheimers-type amnesia in mice.

Sigma1 (σ1) receptor agonists and neurosteroids attenuate β25–35-amyloid peptide-induced amnesia in mice through a common mechanism

The sigma1 (sigma 1) receptor agonists exert potent anti-amnesic effects, as they apparently block the learning impairments either induced by the muscarinic receptor antagonist scopolamine, the N-methyl-D-aspartate receptor antagonist dizocilpine or inherently due to the age-related deficits in senescence-accelerated mice. We recently described the amnesia induced by the beta-amyloid-related peptide beta 25-35, administered centrally in an aggregated form, in mice. The deficits were sensitive to cholinomimetics or to N-methyl-D-aspartate/glycine modulatory site agonists. Herein, we examined the effects of sigma 1 receptor ligands on the beta 25-35 peptide-induced amnesia. The effects of neuro(active) steroids, which interact in vitro and in vivo with sigma 1 receptors were examined in parallel. Mnesic capacity was evaluated seven days after administration of aggregated beta 25-35 peptide (3 nmol), using spontaneous alternation in the Y-maze for spatial short-term memory, or after 14 days, using the step-down type passive avoidance test for long-term memory. The sigma 1 receptor agonists (+)-pentazocine, PRE-084, or SA4503 attenuated, in a dose-dependent and bell-shaped manner, the beta 25-35 peptide-induced deficits on both tests. These effects were antagonized by haloperidol or BMY-14802, confirming the sigma 1 receptor pharmacology. Pregnenolone, dehydroepiandrosterone, and their sulphate esters, but not progesterone, also dose-dependently attenuated the beta 25-35 peptide-induced deficits. Progesterone blocked the beneficial effects of each other neurosteroid, behaving as an antagonist. Furthermore, haloperidol blocked the effects induced by neurosteroids, whereas progesterone antagonized the effects of the non-steroidal sigma 1 receptor agonists, showing a clear crossed pharmacology of different drug classes. These results demonstrate that: (i) the anti-amnesic effect of sigma 1 receptor agonists may be of therapeutic relevance in pathological states affecting the cholinergic and/or glutamatergic systems, such as in pathological aging; (ii) neurosteroids play an important role in learning processes and may collectively constitute a therapeutic target; (iii) the interaction between sigma 1 systems and neurosteroids appears indeed of behavioural relevance.

In vitro aggregation facilitates β-amyloid peptide-(25–35)-induced amnesia in the rat

Abstract The β-amyloid peptide-(25–35) fragment, but not β-amyloid peptide-(1–28), shares with β-amyloid protein-(1–42) the ability to self-aggregate and to induce neurotoxicity in vitro. This study examined the induction of amnesia in rats given intracerebroventricularly soluble or aggregated β-amyloid peptide-(25–35) (5–45 nmol), or β-amyloid peptide-(1–28) (15 nmol). Memory deficit in the water-maze test, examined 14 days after aggregated β-amyloid peptide-(25–35) injection, was more pronounced than with soluble β-amyloid peptide-(25–35). β-Amyloid peptide-(1–28) only affected retention. These results confirm the direct amnesic properties of β-amyloid peptides in the rat brain and showed that prior peptide aggregation markedly facilitates the appearance of amnesia.

Dehydroepiandrosterone sulfate attenuates dizocilpine-induced learning impairment in mice via σ1-receptors

We previously reported that high-affinity sigma type 1 (sigma 1) ligands attenuate the learning impairment induced in mice by dizocilpine, a non-competitive N-methyl-D-aspartate (NMDA) antagonist. Neurosteroids, such as pregnenolone sulfate, progesterone and dehydroepiandrosterone sulfate (DHEAS), modulate NMDA-evoked responses in the central nervous system. Furthermore, some of them were reported to interact with sigma-receptors. This study was carried out to investigate whether DHEAS, a neurosteroid with memory-enhancing effects, attenuates the dizocilpine-induced learning impairment in mice, and, if so, by a mechanism involving sigma 1-receptors. Learning was evaluated using spontaneous alternation in the Y-maze for spatial working memory and step-down type of passive avoidance for long-term memory. At doses about 10-20 mg/kg s.c., DHEAS significantly attenuated dizocilpine (0.15 mg/kg i.p.)-induced impairment of learning on both tests. The enhancing effect of DHEAS (20 mg/kg s.c.) was antagonized by co-administration of the sigma-antagonist BMY-14802 (5 mg/kg i.p.) and suppressed by a subchronic treatment with haloperidol (4 mg/kg/day s.c. for 7 days). These results indicate that DHEAS attenuates dizocilpine-induced learning impairment via an interaction with sigma 1-receptors.

PRE-084, a σ selective PCP derivative, attenuates MK-801-induced impairment of learning in mice

Abstract We investigated the effect of the σ selective PCP derivative PRE-084 on the impairment of learning induced in mice by the noncompetitive NMDA antagonist MK-801. Learning capacities were evaluated using the spontaneous alternation in a Y-maze test for spatial working memory, the step-down passive avoidance and the elevated plus-maze test for long-term memory. At doses about 1 mg/kg IP, PRE-084 significantly attenuated MK-801 (0.2 mg/kg IP) induced impairment of learning. The dose-response curve was bell-shaped which is typical for cognition enhancers. The effect of PRE-084 was antagonized by BMY-14802 (10 mg/kg IP) and suppressed by a chronic treatment with haloperidol (4 mg/kg/day SC for 7 days). Furthermore, PRE-084 did not affect scopolamine (1 mg/kg SC) induced amnesia but significantly attenuated mecamylamine (10 mg/kg IP) induced amnesia. These results show that σ sites mediate a modulation of the NMDA receptor complex-dependent learning processes and may similarly affect the cholinergic nicotinic memory processes. Moreover, the PCP derivative PRE-084, acting selectively at σ sites, reverses the amnesia induced by a drug acting at the PCP site.

Modulation by neurosteroids of the in vivo (+)-[3H]SKF-10,047 binding to σ1 receptors in the mouse forebrain

Recent reports suggest an interaction between neuro‐(active)steroids and sigma1 (σ1) receptors, affecting biochemical parameters as well as physiological responses mediated by σ1 ligands in the rodent brain. In this study, we examined the modulation by neurosteroids of the haloperidol‐sensitive in vivo (+)‐[3H]SKF‐10,047 binding to σ1 sites in the mouse hippocampus and cortex. Progesterone (PROG; 2–40 mg/kg), pregnenolone sulfate (PREGS; 10–40 mg/kg), and dehydroepiandrosterone sulfate (DHEAS; 10–40 mg/kg) were administered systemically 10 min before the radioactive tracer. The total amount of (+)‐[3H]SKF‐10,047 bound in each structure was significantly affected by PROG and PREGS only at the highest dose tested and was unaffected by DHEAS. However, bound to free (B/F) radioactivity ratios were highly significantly decreased by 30–40% in each structure by PROG and PREGS. DHEAS, at 40 mg/kg, induced a significant 20% decrease in the hippocampus. Furthermore, the in vivo (+)‐[3H]SKF‐10,047 binding parameters were diminished in pregnant female mice compared to non‐pregnant or male mice. These results confirm the in vitro binding results, bring a direct in vivo demonstration of the interaction between neurosteroids and σ1 receptors, and show that physiologic modulations of the steroidal concentrations affect the σ1 systems.

Reversion of β25–35-amyloid peptide-induced amnesia by NMDA receptor-associated glycine site agonists

The effects of D-cycloserine (DCS), a N-methyl-D-aspartate receptor-associated glycine site agonist, and milacemide (MIL), a glycine prodrug, were examined on learning impairments induced by administration of beta 25-35-amyloid peptide (3 nmol i.c.v.). Mice were examined for spontaneous alternation and step-down passive avoidance, 7 and 14 days after beta 25-35, respectively. The beta 25-35-induced deficits were reversed by DCS, 1-30 mg/kg i.p., or MIL, 3-100 mg/kg i.p., each drug being ineffective on control mice behaviours. These observations strengthen the therapeutic potential of glycine site agonists against the memory impairments induced by beta-amyloid peptides.

Modulation of steroidal levels by adrenalectomy/castration and inhibition of neurosteroid synthesis enzymes affect sigma1 receptor-mediated behaviour in mice.

The interaction between neurosteroids and sigma1 (σ1) receptors may be of therapeutic interest during physiological or pathological ageing, particularly concerning their neuromodulatory role on cognitive functions. Neurosteroids modulate memory processes through a mechanism involving interactions with GABAA, N‐methyl‐ d‐aspartate and/or σ1 receptors. To measure the contribution of endogenous neurosteroid levels to the antiamnesic effects of σ1 agonists, we investigated the effects of inhibitors of key enzymes involved in neurosteroid synthesis, in adrenalectomized/castrated (AdX/CX) mice to avoid the effect of circulating steroids. Trilostane, a 3β‐hydroxysteroid‐deshydrogenase inhibitor, blocks the pregnenolone to progesterone conversion and leads to a decrease of progesterone. Finasteride, a 5α‐reductase inhibitor, blocks the progesterone to 5α‐pregnane‐3,20‐dione conversion and leads to an accumulation of progesterone. The in vivo binding of (+)‐[3H]SKF‐10u2003047 to σ1 sites was measured in the mouse hippocampus and cortex. The attenuating effect of the selective σ1 agonist PRE‐084 (0.1–3u2003mg/kg) against dizocilpine (0.15u2003mg/kg)‐induced learning impairment was examined using spontaneous alternation behaviour, step‐down passive avoidance and place learning in the elevated plus‐maze. The in vivo (+)‐[3H]SKF‐10u2003047 binding appeared significantly increased in AdX/CX mice and after trilostane treatment (10u2003mg/kg twice a day, 7u2003days), compared with sham‐operated animals. The finasteride treatment (25u2003mg/kg, 7u2003days) significantly decreased binding levels. The learning deficits induced by dizocilpine were not affected by the treatments. The antiamnesic effect of PRE‐084 was facilitated in AdX/CX mice and even more after trilostane treatment, as several parameters for animals treated with both PRE‐084 and dizocilpine returned to control values. The PRE‐084 effect was blocked after finasteride. These results confirmed that endogenous neurosteroidal levels modulate σ1 receptor‐mediated behaviour directly, and revealed that, among neurosteroids, progesterone may be the main modulator of σ1 receptors.

Neuropeptide Y and the Calcitonin Gene‐related Peptide Attenuate Learning Impairments Induced by MK‐801 via a Sigma Receptor‐related mechanism

It has been shown recently that low doses of sigma (σ) receptor ligands like 1,3‐di‐(2‐tolyl)guanidine (DTG), (+)N‐allylnormetazocine [(+)SKF 10 047] and (+)pentazocine can antagonize learning impairments induced by dizocilpine (MK‐801), a non‐competitive antagonist at the NMDA receptor channel. This antagonism has been proposed to involve σ receptor sites since it is blocked by the administration of purported o antagonists such as NE‐100 and BMY‐14802. It has also been demonstrated that peptides of the neuropeptide Y (NPY) and calcitonin gene‐related peptide (CGRP) families modulate, in vivo, é labelling and electrophysiological effects in the hippocampal formation. Accordingly, we investigated if NPY‐ and CGRP‐related peptides modulate cognitive processes by interacting with σ sites in mice. In order to test this hypothesis, a step‐down passive avoidance task was used. Interestingly, similarly to various σ agonists, NPY, peptide YY (PYY) and the Y1 agonist [Leu31Pro34]NPY (but not NPY13–36, a purported Y2 agonist), as well as hCGRPα and the purported CGRP2 agonist [CyS(ACM)2–7]hCGRPα (but not CGRP8–37, a CGRP1 receptor antagonist), significantly attenuated learning impairments induced by MK‐801. Furthermore, the effects of NPY, [Leu31 Pro34]NPY, hCGRPα and [CYS(ACM)2–7]hCGRPα were blocked by the administration of the σ antagonist, BMY‐14802. The present data suggest that NPY‐ and CGRP‐related peptides can indirectly interact in vivo with σ receptors to modulate cognitive processes associated with NMDA receptor function.

The attenuation of learning impairments induced after exposure to CO or trimethyltin in mice by sigma (σ) receptor ligands involves both σ1 and σ2 sites

Sigma (σ) receptor ligands were previously reported to alleviate learning and memory impairments on several pharmacological and pathological rodent models of amnesia. Such effect was demonstrated as involving the σ1 subtype of σ receptor. In this study, we characterized the pharmacological effect mediated by σ ligands on two lesional models of amnesia in mice: (1) the hypoxia‐related learning and memory impairment model induced by repeated exposure to carbon monoxide (CO) gas; and (2) the intoxication with trimethyltin (1u2003mgu2003kg−1). The selective σ1 ligand PRE‐084 (1u2003mgu2003kg−1) or the non‐selective σ1/σ2 compounds DTG (0.1u2003mgu2003kg−1), BD1008 (3u2003mgu2003kg−1), and haloperidol (0.1u2003mgu2003kg−1) reversed significantly the spontaneous alternation deficits observed 7 days after exposure to CO or 14 days after intoxication with trimethyltin. The selective σ1 receptor antagonist NE‐100 (1u2003mgu2003kg−1) was ineffective by itself, but blocked completely the PRE‐084 effects, partially the DTG effects, and did not affect the effects induced by BD1008 or haloperidol. A similar pharmacological profile was observed in the step‐down type passive avoidance test performed 8 days after exposure to CO. These results show that, in contrast to the previously reported amnesia models, the impairments induced after exposure to CO or intoxication with trimethyltin could be alleviated not only by σ1 receptor agonists but also by σ2 agonists. The particular pattern of neurodegeneration observed in these lesional models may explain these differences.