Interactive effects of AM251 and baclofen on synaptic plasticity in the rat dentate gyrus

Underlying mechanisms of long-term potentiation during the inhibition of the cannabinoid CB1 and GABAB receptors in the dentate gyrus of hippocampus

BACKGROUND

The release of various neurotransmitters and thereby the excitability of neuronal circuits are regulated by the endocannabinoid system in an activity-dependent manner. Hippocampal long-term potentiation (LTP) is augmented in cannabinoid type 1 (CB1) receptor-deficient mice. CB1 receptors exist on GABAergic axon terminals in the hippocampus. In our previous work, we showed that CB1 antagonists increased the population spike (PS) amplitude, field excitatory post-synaptic potential (fEPSP), and the LTP induction in the dentate gyrus (DG) of the rat hippocampus while the GABA_B antagonist decreased these parameters. Determining the underlying mechanisms of the pre- and/or postsynaptic locus of LTP expression is of great importance. In this study, we investigated whether LTP alteration acutely caused by CB1 and GABA_B receptor antagonists (AM251 and CGP55845, respectively) happens at the postsynaptic or presynaptic regions, or at both. Therefore, the paired-pulse ratio (PPR) was assessed prior to and following the LTP induction in the studied groups.

METHODS

Male Wistar rats were randomly assigned to the groups of control, AM251, CGP55845, CGP55845 + AM251. A high-frequency stimulation (HFS) of the perforant path (PP) was used to induce LTP in the DG region.

RESULTS

Statistical analysis revealed that AM251 produced significant increase in excitatory postsynaptic potential (EPSP) slope and amplitude of PS. Conversely, administration of CGP55845 produced decrease in slope of EPSP. The current results indicated that the PPR was not influenced by LTP induction in the presence of AM251 or CGP55845 either alone or their combination.

CONCLUSIONS

It can be concluded that the site causing LTP expression is, at least in part, the postsynaptic site because PPR was not influenced by LTP induction in the presence of AM251 or CGP55845 either alone or their combination.

The Role of Acetylcholine on the Effects of Different Doses of Sulfite in Learning and Memory

In this study, the effects of different doses of sulfite on learning, memory, and long term potentiation as well as the relationship of these effects with acetylcholine pathways, Arc and synapsin 1 levels were investigated. Sixty male Wistar albino rats were randomly divided into three groups as control, S100, and S260. Sodiummetabisulfite (S100;100 mg/kg/day, S260;260 mg/kg/day) was given by oral administration. Behavioral changes were evaluated. After long term potentiation recordings from the perforant pathway-dentate gyrus synapses, animals were sacrificed. Acetylcholinesterase activity, choline acetyltransferase activity, acetylcholine level as well as Arc and Synapsin 1 expressions were analyzed on the hippocampi. The total distance and average velocity values in the open field and Morris water maze tests increased in the sulfite groups, while the discrimination index in the novel object recognition test decreased compared to controls. Acetylcholine levels and choline acetyltransferase activity were also increased in the sulfite groups, while acetylcholinesterase activity was decreased compared to controls. Sulfite intake attenuated long term potentiation in the hippocampus. It has been observed that the excitatory postsynaptic potential slope and population spike amplitude of the field potentials obtained in sulfite groups decreased. This impairment was accompanied by a decrease in Arc and synapsin 1 expressions. In conclusion, it has been shown that sulfite intake in adults impairs learning and memory, possibly mediated by the cholinergic pathway. It is considered that the decrement in Arc and synapsin expressions may play a role in the mechanism underlying the impairment in long term potentiation caused by toxicity.

GABAB Receptors and Cognitive Processing in Health and Disease

GABA_B receptors are implicated in numerous central nervous system-based behaviours and mechanisms, including cognitive processing in preclinical animal models. Homeostatic changes in the expression and function of these receptors across brain structures have been found to affect cognitive processing. Numerous preclinical studies have focused on the role of GABA_B receptors in learning, memory and cognition per se with some interesting, although sometimes contradictory, findings. The majority of the existing clinical literature focuses on alterations in GABA_B receptor function in conditions and disorders whose main symptomatology includes deficits in cognitive processing. The aim of this chapter is to delineate the role of GABA_B receptors in cognitive processes in health and disease of animal models and human clinical populations. More specifically, this review aims to present literature on the role of GABA_B receptors in animal models with cognitive deficits, especially those of learning and memory. Further, it aims to capture the progress and advances of research studies on the effects of GABA_B receptor compounds in neurodevelopmental and neurodegenerative conditions with cognitive dysfunctions. The neurodevelopmental conditions covered include autism spectrum disorders, fragile X syndrome and Down's syndrome and the neurodegenerative conditions discussed are Alzheimer's disease, epilepsy and autoimmune anti-GABA_B encephalitis. Although some findings are contradictory, results indicate a possible therapeutic role of GABA_B receptor compounds for the treatment of cognitive dysfunction and learning/memory impairments for some of these conditions, especially in neurodegeneration. Moreover, future research efforts should aim to develop selective GABA_B receptor compounds with minimal, if any, side effects.

How do stupendous cannabinoids modulate memory processing via affecting neurotransmitter systems?

In the present study, we wanted to review the role of cannabinoids in learning and memory in animal models, with respect to their interaction effects with six principal neurotransmitters involved in learning and memory including dopamine, glutamate, GABA (γ-aminobutyric acid), serotonin, acetylcholine, and noradrenaline. Cannabinoids induce a wide-range of unpredictable effects on cognitive functions, while their mechanisms are not fully understood. Cannabinoids in different brain regions and in interaction with different neurotransmitters, show diverse responses. Previous findings have shown that cannabinoids agonists and antagonists induce various unpredictable effects such as similar effect, paradoxical effect, or dualistic effect. It should not be forgotten that brain neurotransmitter systems can also play unpredictable roles in mediating cognitive functions. Thus, we aimed to review and discuss the effect of cannabinoids in interaction with neurotransmitters on learning and memory. In addition, we mentioned to the type of interactions between cannabinoids and neurotransmitter systems. We suggested that investigating the type of interactions is a critical neuropharmacological issue that should be considered in future studies.

Density of GABAB Receptors Is Reduced in Granule Cells of the Hippocampus in a Mouse Model of Alzheimer's Disease

Metabotropic γ-aminobutyric acid (GABA_B) receptors contribute to the control of network activity and information processing in hippocampal circuits by regulating neuronal excitability and synaptic transmission. The dysfunction in the dentate gyrus (DG) has been implicated in Alzheimer´s disease (AD). Given the involvement of GABA_B receptors in AD, to determine their subcellular localisation and possible alteration in granule cells of the DG in a mouse model of AD at 12 months of age, we used high-resolution immunoelectron microscopic analysis. Immunohistochemistry at the light microscopic level showed that the regional and cellular expression pattern of GABA_B1 was similar in an AD model mouse expressing mutated human amyloid precursor protein and presenilin1 (APP/PS1) and in age-matched wild type mice. High-resolution immunoelectron microscopy revealed a distance-dependent gradient of immunolabelling for GABA_B receptors, increasing from proximal to distal dendrites in both wild type and APP/PS1 mice. However, the overall density of GABA_B receptors at the neuronal surface of these postsynaptic compartments of granule cells was significantly reduced in APP/PS1 mice. Parallel to this reduction in surface receptors, we found a significant increase in GABA_B1 at cytoplasmic sites. GABA_B receptors were also detected at presynaptic sites in the molecular layer of the DG. We also found a decrease in plasma membrane GABA_B receptors in axon terminals contacting dendritic spines of granule cells, which was more pronounced in the outer than in the inner molecular layer. Altogether, our data showing post- and presynaptic reduction in surface GABA_B receptors in the DG suggest the alteration of the GABA_B-mediated modulation of excitability and synaptic transmission in granule cells, which may contribute to the cognitive dysfunctions in the APP/PS1 model of AD.

Effects of Hypericum scabrum extract on dentate gyrus synaptic plasticity in high fat diet-fed rats

High-fat diet (HFD) can induce deficits in neural function, oxidative stress, and decrease hippocampal neurogenesis. Hypericum (H.) scabrum extract (Ext) contains compounds that could treat neurological disorders. This study aimed to examine the neuroprotective impacts of the H. scabrum Ext on hippocampal synaptic plasticity in rats that were fed HFD. Fifty-four male Wistar rats (220 ± 10 g) were randomly arranged in six groups: (1) HFD group; (2) HFD + Ext300 group; (3) HFD + Ext100 group; (4) Control group; (5) Ext 300 mg/kg group; (6) Ext 100 mg/kg group. These protocols were administrated for 3 months. After this stage, a stimulating electrode was implanted in the perforant pathway (PP), and a bipolar recording electrode was embedded into the dentate gyrus (DG). Long-term potentiation (LTP) was provoked by high-frequency stimulation (HFS) of the PP. Field excitatory postsynaptic potentials (EPSP) and population spikes (PS) were recorded at 5, 30, and 60 min after HFS. The HFD group exhibited a large and significant decrease in their PS amplitude and EPSP slope as compared to the control and extract groups. In reverse, H. scabrum administration in the HFD + Ext rats reversed the effect of HFD on the PS amplitude and EPSP slope. The results of the study support that H. scabrum Ext can inhibit diminished synaptic plasticity caused by the HFD. These effects are probably due to the extreme antioxidant impacts of the Ext and its capability to scavenge free radicals.

Coenzyme Q10 supplementation reverses diabetes-related impairments in long-term potentiation induction in hippocampal dentate gyrus granular cells: An in vivo study

Diabetes mellitus (DM) is associated with impaired hippocampal synaptic plasticity. Coenzyme Q10 (CoQ10) acts as an antioxidant and exerts neuroprotective effects. Accordingly, this study aimed at evaluating the effects of CoQ10 on hippocampal long-term potentiation (LTP) and paired-pulse facilitation (PPF) in streptozotocin (STZ)-induced diabetic rats. Male Wistar rats were randomly divided into six groups (n = 8 per group) as follows and treated for 90 days: the control, control + low dose of CoQ10 (100 mg/kg), control + high dose of CoQ10 (600 mg/kg), diabetic, diabetic + low dose of CoQ10, and diabetic + high dose of CoQ10 groups. Diabetes was induced by a single intraperitoneal injection of 50 mg/kg STZ. The population spike (PS) amplitude and slope of excitatory post synaptic potentials (EPSPs) were measured in dentate gyrus (DG) area in response to the stimulation applied to the perforant path (PP). The results showed that the STZ-induced diabetes impaired LTP induction in the PP-DG synapses. This finding is supported by the decreased EPSP slope and PS amplitude of LTP (P < 0.05). Both low- and high-dose CoQ10 supplementation in the control and diabetic animals enhanced EPSP slope and PS amplitude of LTP in the granular cells of DG (P < 0.05). PPF was affected by LTP induction in diabetic animals receiving the high dose of CoQ10 (P < 0.05). It is suggested that CoQ10 administration could attenuate deteriorative effect of STZ-induced diabetes on in vivo LTP in the DG. The enhanced transmitter release can be partly one of the possible underlying mechanism(s) responsible for the LTP induction in the diabetic animals treated with CoQ10.

Influence of hippocampal GABAB receptor inhibition on memory in rats with acute β-amyloid toxicity

The neurotransmitter γ-aminobutyric acid (GABA) is involved in the process of memory. It has been reported that the inhibition of GABA_B receptors has beneficial effects on cognition. The aim of this study was to investigate the role of CGP35348 (a GABA_B receptor antagonist) on dentate gyrus GABA_B receptor inhibition and its effects on learning and memory impairments that had been induced in adult male rats by microinjection of β-amyloid (Aβ). Seventy Wistar male rats were randomly divided into seven groups: control, sham (receiving the Aβ vehicle only), Aβ, Aβ + CGP35348 (1, 10, and 100 μg/μL), and CGP35348 alone (10 μg/μL). Memory impairment was induced by unilateral interventricular microinjection of Aβ (6 μg/6 μL). Rats were cannulated bilaterally in the dentate gyrus, and then, they were treated for 20 consecutive days. Learning and memory were assessed using the novel object recognition and passive avoidance learning tests. The discrimination index and the step-through latency were significantly increased in the Aβ + CGP35348 group in comparison to the Aβ only group (P < 0.05 and P < 0.01, respectively). Data showed that the discrimination index was decreased in the Aβ + CGP35348 group in comparison with the control group (P < 0.05) and sham group (P < 0.01). Moreover, the step-through latency was significantly decreased in the Aβ + CGP35348 group in comparison to the control and sham groups (P < 0.01). Data from this study indicated that intra-hippocampal microinjection of the GABA_B receptor antagonist counteracts the learning, memory, and cognitive impairments induced by Aβ. It can be concluded that the GABA_B receptor antagonist is a possible therapeutic agent against the progression of acute Aβ toxicity-induced memory impairment.

Effect of a 5-HT1D receptor agonist on the reinstatement phase of the conditioned place preference test and hippocampal long-term potentiation in methamphetamine-treated rats

Methamphetamine (METH)-seeking relapse is associated with memory and synaptic plasticity changes. Serotonin is a key neuromodulator in this process. While there is a known distribution of 5-HT_1D receptors in reward and memory areas, such as the hippocampus, its physiological function is currently unknown. Here, we evaluated effect of a 5-HT_1D receptor agonist, PNU142633, on the reinstatement of METH-seeking behavior and long-term potentiation. Rats were implanted with a cannula into lateral ventricle, then treated with saline or METH (5 mg/kg) during the acquisition phase of the conditioned place preference (CPP) test. On day 13 of the extinction phase, METH groups were divided into four groups: METH (0: saline, 1, or 2.5 (priming METH) mg/kg; i.p.) + vehicle (5 µl/rat) or a priming dose of METH (2.5 mg/kg; i.p.) + PNU (2 µg/5 µl; i.c.v.) and their preference scores were calculated on reinstatement day (day 14). Immediately following this, electrophysiology was performed to assay the field excitatory postsynaptic potential (fEPSP) slope and population spike (PS) amplitude between groups. The results showed that CPP induction by METH gradually declined to extinction on days 12 and 13. A priming METH treatment significantly increased preference for the METH-paired chamber when compared with other groups, but pre-treatment with PNU significantly attenuated this effect. PS amplitude and fEPSP slopes in vehicle + priming METH rats were greater when compared with other groups. Furthermore, PNU attenuated the priming METH-induced increase in PS amplitude. These findings suggest that PNU can decrease synaptic transmission and prevent METH reinstatement in rats.

Effects of thymol on amyloid-β-induced impairments in hippocampal synaptic plasticity in rats fed a high-fat diet

Obesity and a high-fat diet (HFD) are known to increase the incidence of Alzheimer's disease (AD). Oxidative stress, a major risk factor for AD, is increased with HFD consumption. Thymol (Thy) has antioxidant properties. Therefore, in the present study, we examined the protective and therapeutic effects of Thy on amyloid-β (Aβ)-induced impairments in the hippocampal synaptic plasticity of HFD-fed rats. In this study, 72 adult male Wistar rats were randomly assigned to 9 groups (n = 8 rats/group): Group 1 (control; standard diet); Group 2: Control + phosphate-buffered saline (PBS) + Oil (Thy vehicle); Group 3 (HFD + PBS); Group 4: (HFD + Aβ); Group 5: Control + PBS + Thy; Group 6: (HFD + Aβ + Oil); Group 7: Control + Aβ + Thy; Group 8: HFD + PBS + Thy; Group 9: (HFD + Aβ + Thy). After stereotaxic surgery, the field potentials were recorded after the implantation of the recording and stimulating electrodes in the dentate gyrus (DG) and perforant pathway, respectively. Following high-frequency stimulation, the long-term potentiation (LTP) of the population spike (PS) amplitude and the slope of the excitatory postsynaptic potentials (EPSPs) were measured in the DG. The HFD rats that received Aβ exhibited a significant decrease in their EPSP slope and PS amplitude as compared to the control group. In contrast, Thy administration in the HFD + Aβ rats reduced the decrease in the EPSP slope and PS amplitude. Thy decreased the Aβ-induced LTP impairments in HFD rats. The HFD significantly increased serum malondialdehyde levels and decreased total antioxidant capacity and total glutathione levels; whereas, Thy supplementation significantly reversed these parameters. Therefore, these results suggest that Thy, a natural antioxidant, can be therapeutic against high risk factors for AD, such as HFD.

The interactive role of CB1 receptors and L-type calcium channels in hippocampal long-term potentiation in rats

Long-term potentiation (LTP) of synaptic responses is a widely researched model of synaptic plasticity that occurs during learning and memory. The cannabinoid system is an endogenous system that modulate this kind of synaptic plasticity. In addition, voltage dependent calcium channels is essential for induction of LTP at some synapses in the hippocampus. However, there is currently debate over the interaction between L-type calcium channels and cannabinoid system on the synaptic plasticity. In this study, we examined the effects of an acute administration of the cannabinoid antagonist AM251 following a chronic administration of the Ca²⁺ channel blocker verapamil on LTP induction in the hippocampal dentate gyrus(DG) of rats. Male Wistar rats were administered verapamil(10,25,50mg/kg) or saline intraperitoneally(IP) daily for 13days(n=10/group). After this treatment period, animals were anesthetized with an IP injection of urethane; the recording and stimulating electrodes were positioned in the DG and the perforant pathway. After obtaining a steady state baseline response, a single IP injection of saline or AM251(1 or 5mg/kg) was administered. LTP was induced by high-frequency stimulation(HFS). The population spike(PS) amplitude and the slope of excitatory postsynaptic potentials(EPSP) were compared between the experimental groups. The acute administration of the CB1 antagonist AM251 increased LTP induction. The EPSP slopes and PS amplitude in the verapamil and AM251 groups differed after HFS, such that AM251 increased LTP, whereas verapamil decreased LTP induction. These findings suggest that there are functional interactions between the L-type calcium channels and cannabinoid system in this model of synaptic plasticity in the hippocampus.

Effects of cannabinoid and vanilloid receptor agonists and their interaction on learning and memory in rats

Despite previous findings on the effects of cannabinoid and vanilloid systems on learning and memory, the effects of the combined stimulation of these 2 systems on learning and memory have not been studied. Therefore, in this study, we tested the interactive effects of cannabinoid and vanilloid systems on learning and memory in rats by using passive avoidance learning (PAL) tests. Forty male Wistar rats were divided into the following 4 groups: (1) control (DMSO+saline), (2) WIN55,212–2, (3) capsaicin, and (4) WIN55,212–2 + capsaicin. On test day, capsaicin, a vanilloid receptor type 1 (TRPV1) agonist, or WIN55,212–2, a cannabinoid receptor (CB1/CB2) agonist, or both substances were injected intraperitoneally. Compared to the control group, the group treated with capsaicin (TRPV1 agonist) had better scores in the PAL acquisition and retention test, whereas treatment with WIN55,212–2 (CB1/CB2 agonist) decreased the test scores. Capsaicin partly reduced the effects of WIN55,212–2 on PAL and memory. We conclude that the acute administration of a TRPV1 agonist improves the rats’ cognitive performance in PAL tasks and that a vanilloid-related mechanism may underlie the agonistic effect of WIN55,212–2 on learning and memory.