Basolateral amygdala GABA-A receptors mediate stress-induced memory retrieval impairment in rats

Memory impairment was ameliorated by corticolimbic microinjections of arachidonylcyclopropylamide (ACPA) and miRNA-regulated lentiviral particles in a streptozotocin-induced Alzheimer's rat model

The present study aimed to investigate the effect of corticolimbic cannabinoid CB1 receptors activity on memory impairment in the intracerebroventricular (ICV)-streptozotocin (STZ) animal model of Alzheimer's like-disease. This study also assessed whether the corticolimbic overexpression of miRNA-137 or -let-7a could increase the endocannabinoids by inhibiting the monoglyceride lipase (MAGL) to ameliorate STZ response. The results showed that ICV microinjection of STZ (3 mg/kg/10 μl) impaired passive avoidance memory retrieval. The chronic microinjection of arachidonylcyclopropylamide (ACPA; 10 ng/0.5 μl), a selective cannabinoid CB1 receptor agonist, into the hippocampal CA1 region, the central amygdala (CeA) or the medial prefrontal cortex (mPFC) ameliorated the amnesic effect of ICV-STZ. Intra-CA1 or -CeA microinjection of ACPA alone did not affect memory retrieval, while its microinjection into the mPFC impaired memory formation. Based on bioinformatics analysis and verification of the MAGL gene, miRNA-137 and -let-7a were chosen to target the expression levels of MAGL in the corticolimbic regions. The chronic corticolimbic microinjection of lentiviral particles containing miRNA-137 or -let-7a ameliorated ICV-STZ-induced memory impairment. The high transfection efficiency was determined for each virus using comparing fluorescent and conventional vision. Corticolimbic overexpression of miRNA-137 or -let-7a decreased the MAGL gene expression that encodes the MAGL enzyme to increase the endocannabinoids. Thus, among the molecular mechanisms and signaling pathways involved in the pathophysiology of Alzheimer's disease (AD), it is worth mentioning the role of endocannabinoids in the corticolimbic regions. CB1 receptor agonists, miRNA-137 or -let-7a, may be potential therapeutic targets against cognitive decline in AD.

Morphine improved stress-induced amnesia and anxiety through interacting with the ventral hippocampal endocannabinoid system in rats

The present study aimed to investigate the possible role of the ventral hippocampal (VH) cannabinoid CB1 receptors in the improving effect of morphine on stress-induced memory formation impairment and anxiety. A step-through type passive avoidance task and a hole-board test were used to measure memory formation and anxiety-like exploratory behavior, respectively. The results showed that the exposure to 10-min stress immediately after the successful training phase impaired memory formation and also produced anxiogenic-like exploratory behaviour in adult male Wistar rats. Moreover, morphine administration before stress exposure improved the adverse effects of stress on memory formation and exploratory behaviour. After training, intra-VH microinjection of cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.01-0.05 μg/rat) enhanced the response of an ineffective dose of morphine (0.5 mg/kg for memory; 5 mg/kg for anxiety, i.p.) on memory impairment and anxiogenic-like exploratory behaviour induced by acute stress. Intra-VH microinjection of the higher dose of WIN 55,212-2 alone impaired memory formation. Post-training microinjection of a cannabinoid CB1 receptor antagonist/inverse agonist, AM-251 (100-150 ng/rat) into the VH attenuated the response of an effective dose of morphine (5 mg/kg for memory; 6 mg/kg for anxiety, i.p.) in stress-exposed rats. Taken together, the present results showed that morphine administration could improve stress-induced memory impairment and anxiety in the rats exposed to the inescapable acute stress. Interestingly, the improving effect of morphine on the adverse effect of stress on memory formation and anxiety-like exploratory behaviour may be mediated through the VH endocannabinoid CB1/CB2 receptors mechanism.

The basolateral amygdala dopaminergic system contributes to the improving effect of nicotine on stress-induced memory impairment in rats

Stress seems to be an important risk factor in the beginning and continuing stages of cigarette tobacco smoking in humans. Considering that both of nicotine administration and stress exposure affect cognitive functions including memory formation, the aim of the present study was 1) to evaluate the effect of subcutaneous (s.c.) administration of nicotine on memory formation under stress and 2) to assess the possible role of the basolateral amygdala (BLA) dopamine D1 and D2 receptors in the effect of nicotine on stress-induced memory retrieval impairment. Adult male wistar rats were bilaterally implanted in the BLA. A step-through type passive avoidance task was used to measure memory retrieval. To induce acute stress, the animals were placed on an elevated platform. The results showed that pre-test exposure to 20 and 30 min stress, but not 10 min, impaired memory retrieval. Nicotine administration (0.05 mg/kg, s.c.) improved stress-induced memory retrieval impairment. The activation of the BLA dopamine receptors via bilateral microinjection of apomorphine (0.025-0.4 μg/rat), a non-selective dopamine receptor agonist, potentiated the effect of nicotine on stress-induced memory retrieval impairment. Interestingly, intra-BLA microinjection of SCH23390 (a selective dopamine D1 receptor antagonist; 0.02-0.5 μg/rat) or sulpiride (a selective dopamine D2 receptor antagonist; 0.02-0.5 μg/rat) dose-dependently inhibited nicotine-induced improvement of the stress amnesic effect. Taken together, it can be concluded that stress-induced impairment of memory retrieval can be improved by nicotine administration. Moreover, the dopaminergic neurotransmission in the BLA through D1 and D2 receptors mediates the improving effect of nicotine on stress-induced memory retrieval impairment.

Brain nicotinic acetylcholine receptors are involved in stress-induced potentiation of nicotine reward in rats

The aim of the present study was to examine the possible role of nicotinic acetylcholine receptors of the dorsal hippocampus (CA1 regions), the medial prefrontal cortex or the basolateral amygdala in the effect of acute or sub-chronic stress on nicotine-induced conditioned place preference. Our results indicated that subcutaneous administration of nicotine (0.2 mg/kg) induced significant conditioned place preference. Exposure to acute or sub-chronic elevated platform stress potentiated the response of an ineffective dose of nicotine. Pre-conditioning intra-CA1 (0.5-4 µg/rat) or intra-medial prefrontal cortex (0.2-0.3 µg/rat) microinjection of mecamylamine (a non-selective nicotinic acetylcholine receptor antagonist) reversed acute stress-induced potentiation of nicotine reward as measured in the conditioned place preference paradigm. By contrast, pre-conditioning intra-basolateral amygdala microinjection of mecamylamine (4 µg/rat) potentiated the effects of acute stress on nicotine reward. Our findings also showed that intra-CA1 or intra-medial prefrontal cortex, but not intra-basolateral amygdala, microinjection of mecamylamine (4 µg/rat) prevented the effect of sub-chronic stress on nicotine reward. These findings suggest that exposure to elevated platform stress potentiates the rewarding effect of nicotine which may be associated with the involvement of nicotinic acetylcholine receptors. It seems that there is a different contribution of the basolateral amygdala, the medial prefrontal cortex or the CA1 nicotinic acetylcholine receptors in stress-induced potentiation of nicotine-induced conditioned place preference.

Harmaline-induced amnesia: Possible role of the amygdala dopaminergic system

In this study, we examined the effect of bilateral intra-basolateral amygdala (intra-BLA) microinjections of dopamine receptor agents on amnesia induced by a β-carboline alkaloid, harmaline in mice. We used a step-down method to assess memory and then, hole-board method to assess exploratory behaviors. The results showed that pre-training intra-BLA injections of dopamine D1 receptor antagonist and agonist (SCH23390 (0.5μg/mouse) and SKF38393 (0.5μg/mouse), respectively) impaired memory acquisition. In contrast, pre-training intra-BLA injections of dopamine D2 receptor antagonist and agonist (sulpiride and quinpirole, respectively) have no significant effect on memory acquisition. Pre-training intra-peritoneal (i.p.) injection of harmaline (1mg/kg) decreased memory acquisition. However, co-administration of SCH 23390 (0.01μg/mouse) with different doses of harmaline did not alter amnesia. Conversely, pre-training intra-BLA injection of SKF38393 (0.1μg/mouse), sulpiride (0.25μg/mouse) or quinpirole (0.1μg/mouse) reversed harmaline (1mg/kg, i.p.)-induced amnesia. Furthermore, all above doses of drugs had no effect on locomotor activity. In conclusion, the dopamine D1 and D2 receptors of the BLA may be involved in the impairment of memory acquisition induced by harmaline.

Hippocampal signaling pathways are involved in stress-induced impairment of memory formation in rats

Stress is a potent modulator of hippocampal-dependent memory formation. The aim of the present study was to assess the role of hippocampal signaling pathways in stress-induced memory impairment in male Wistar rats. The animals were exposed to acute elevated platform (EP) stress and memory formation was measured by a step-through type passive avoidance task. The results indicated that post-training or pre-test exposure to EP stress impaired memory consolidation or retrieval respectively. Using western blot analysis, it was found that memory retrieval was associated with the increase in the levels of phosphorylated cAMP-responsive element binding protein (P-CREB), peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) and its downstream targets in the hippocampus. In contrast, the stress exposure decreased the hippocampal levels of these proteins. In addition, stress-induced impairment of memory consolidation or retrieval was associated with the decrease in the P-CREB/CREB ratio and the PGC-1α level in the hippocampus. On the other hand, the hippocampal level of nuclear factor E2-related factor 2 (Nrf2) and gamma-glutamylcysteine synthetase (γ-GCS) which are the master regulators of defense system were decreased by the stress exposure. The increased hippocampal levels of Nrf2 and it׳s downstream was observed during memory retrieval, while stress-induced impairment of memory consolidation or retrieval inhibited this hippocampal signaling pathway. Overall, these findings suggest that down-regulation of CREB/PGC-1α signaling cascade and Nrf2 antioxidant pathways in the hippocampus may be associated with memory impairment induced by stress.

Ventral hippocampal nicotinic acetylcholine receptors mediate stress-induced analgesia in mice

Evidence suggests that various stressful procedures induce an analgesic effect in laboratory animals commonly referred to as stress-induced analgesia (SIA). The aim of the present study was to assess the role of ventral hippocampal (VH) nicotinic acetylcholine receptors (nAChRs) in SIA in adult male NMRI mice. The VHs of animals were bilaterally cannulated and nociceptive threshold was measured using infrared source in a tail-flick apparatus. Acute stress was evoked by placing the animals on an elevated platform for 10, 20 and 30 min. The results showed that exposure to 20 and 30 min acute stress produced analgesia, while exposure to 10 min stress had no effect on the pain response. Intra-VH microinjection of nicotine (0.001-0.1 μg/mouse), 5 min before an ineffective stress (10 min stress), induced analgesia, suggesting the potentiative effect of nicotine on SIA. It is important to note that bilateral intra-VH microinjections of the same doses of nicotine without stress had no effect on the tail-flick test. On the other hand, intra-VH microinjection of mecamylamine (0.5-1 μg/mouse) 5 min before 20-min stress inhibited SIA. However, bilateral intra-VH microinjections of the same doses of mecamylamine without stress had no effect on the tail-flick response. In addition, the microinjection of mecamylamine into the VH reversed the potentiative effect of nicotine on SIA. Taken together, it can be concluded that exposure to acute stress induces SIA in a time-dependent manner and the ventral hippocampal cholinergic system may be involved in SIA via nAChRs.

Involvement of neurotransmitters in the action of the nociceptin/orphanin FQ peptide-receptor system on passive avoidance learning in rats

The nociceptin/orphanin FQ peptide (NOP) receptor and its endogenous ligand plays role in several physiologic functions of the central nervous system, including pain, locomotion, anxiety and depression, reward and drug addiction, learning and memory. Previous studies demonstrated that the NOP-receptor system induces impairment in memory and learning. However, we have little evidence about the underlying neuromodulation. The aim of the present study was to investigate the involvement of distinct neurotransmitters in the action of the selective NOP receptor agonist orphan G protein-coupled receptor (GPCR) SP9155 P550 on memory consolidation in a passive avoidance learning test in rats. Accordingly, rats were pretreated with a nonselective muscarinic acetylcholine receptor antagonist, atropine, a γ-aminobutyric acid subunit A (GABA-A) receptor antagonist, bicuculline, a D2, D3, D4 dopamine receptor antagonist, haloperidol, a nonselective opioid receptor antagonist, naloxone, a non-specific nitric oxide synthase inhibitor, nitro-L-arginine, a nonselective α-adrenergic receptor antagonist, phenoxybenzamine and a β-adrenergic receptor antagonist, propranolol. Atropine, bicuculline, naloxone and phenoxybenzamine reversed the orphan GPCR SP9155 P550-induced memory impairment, whereas propranolol, haloperidol and nitro-L-arginine were ineffective. Our results suggest that the NOP system-induced impairment of memory consolidation is mediated through muscarinic cholinergic, GABA-A-ergic, opioid and α-adrenergic receptors, whereas β-adrenergic, D2, D3, D4-dopaminergic and nitrergic mechanisms are not be implicated.