Disruption of Electroencephalogram Coherence between Cortex/Striatum and Midbrain Dopaminergic Regions in the Knock-Out Mice with Combined Loss of Alpha, Beta, and Gamma Synucleins.

The malfunctioning of the brain synucleins is associated with pathogenesis of Parkinson's disease. Synucleins' ability to modulate various pre-synaptic processes suggests their modifying effects on the electroencephalogram (EEG) recorded from different brain structures. Disturbances in interrelations between them are critical for the onset and evolution of neurodegenerative diseases. Recently, we have shown that, in mice lacking several synucleins, differences between the frequency spectra of EEG from different brain structures are correlated with specificity of synucleins' combinations. Given that EEG spectra are indirect characteristics of inter-structural relations, in this study, we analyzed a coherence of instantaneous values for EEGs recorded from different structures as a direct measure of "functional connectivity" between them. METHODS: EEG data from seven groups of knock-out (KO) mice with combined deletions of alpha, beta, and gamma synucleins versus a group of wild-type (WT) mice were compared. EEG coherence was estimated between the cortex (MC), putamen (Pt), ventral tegmental area (VTA), and substantia nigra (SN) in all combinations. RESULTS: EEG coherence suppression, predominantly in the beta frequency band, was observed in KO mice versus WT littermates. The suppression was minimal in MC-Pt and VTA-SN interrelations in all KO groups and in all inter-structural relations in mice lacking either all synucleins or only beta synuclein. In other combinations of deleted synucleins, significant EEG coherence suppression in KO mice was dominant in relations with VTA and SN. CONCLUSION: Deletions of the synucleins produced significant attenuation of intra-cerebral EEG coherence depending on the imbalance of different types of synucleins.

Early Effects of Alpha-Synuclein Depletion by Pan-Neuronal Inactivation of Encoding Gene on Electroencephalogram Coherence between Different Brain Regions in Mice.

Inactivation of the Snca gene in young mice by chronic injections of tamoxifen (TAM), a selective estrogen receptor modifier, has been shown to decrease the level of alpha-synuclein, a key peptide in the pathogenesis of Parkinson's disease. In young mice, different time courses of the effect were observed in different brain areas, meaning associated disturbances in the intracerebral relations, namely in brain function after TAM-induced synucleinopathy. METHODS: We analyzed electroencephalogram (EEG) coherence ("functional connectivity") between the cortex (MC), putamen (Pt), and dopamine-producing brain regions (ventral tegmental area, VTA, and substantia nigra, SN) in two groups of two-month-old male mice. We compared EEG coherences in the conditional knockout Sncaflox/flox mice with those in their genetic background (C57Bl6J) one, two, and three months after chronic (for five days) intraperitoneal injections of TAM or the vehicle (corn oil). The EEG coherences in the TAM-treated group were compared with those in the alpha-synuclein knockout mice. RESULTS: A significant suppression of EEG coherence in the TAM-treated mice versus the vehicle group was observed in all inter-structural relations, with the exception of MC-VTA at one and three months and VTA-SN at two months after the injections. Suppressive changes in EEG coherence were observed in the alpha-synuclein knockout mice as well; the changes were similar to those in TAM-treated mice three months after treatment. CONCLUSION: our data demonstrate a combined time-dependent suppressive effect induced by TAM on intracerebral EEG coherence.

Age-Related Modifications of Electroencephalogram Coherence in Mice Models of Alzheimer's Disease and Amyotrophic Lateral Sclerosis.

Evident similarities in pathological features in aging and Alzheimer's disease (AD) raise the question of a role for natural age-related adaptive mechanisms in the prevention/elimination of disturbances in interrelations between different brain areas. In our previous electroencephalogram (EEG) studies on 5xFAD- and FUS-transgenic mice, as models of AD and amyotrophic lateral sclerosis (ALS), this suggestion was indirectly confirmed. In the current study, age-related changes in direct EEG synchrony/coherence between the brain structures were evaluated. METHODS: In 5xFAD mice of 6-, 9-, 12-, and 18-month ages and their wild-type (WT5xFAD) littermates, we analyzed baseline EEG coherence between the cortex, hippocampus/putamen, ventral tegmental area, and substantia nigra. Additionally, EEG coherence between the cortex and putamen was analyzed in 2- and 5-month-old FUS mice. RESULTS: In the 5xFAD mice, suppressed levels of inter-structural coherence vs. those in WT5xFAD littermates were observed at ages of 6, 9, and 12 months. In 18-month-old 5xFAD mice, only the hippocampus ventral tegmental area coherence was significantly reduced. In 2-month-old FUS vs. WTFUS mice, the cortex-putamen coherence suppression, dominated in the right hemisphere, was observed. In 5-month-old mice, EEG coherence was maximal in both groups. CONCLUSION: Neurodegenerative pathologies are accompanied by the significant attenuation of intracerebral EEG coherence. Our data are supportive for the involvement of age-related adaptive mechanisms in intracerebral disturbances produced by neurodegeneration.

Loss of the Synuclein Family Members Differentially Affects Baseline- and Apomorphine-Associated EEG Determinants in Single-, Double- and Triple-Knockout Mice.

Synucleins comprise a family of small proteins highly expressed in the nervous system of vertebrates and involved in various intraneuronal processes. The malfunction of alpha-synuclein is one of the key events in pathogenesis of Parkinson disease and certain other neurodegenerative diseases, and there is a growing body of evidence that malfunction of other two synucleins might be involved in pathological processes in the nervous system. The modulation of various presynaptic mechanisms of neurotransmission is an important function of synucleins, and therefore, it is feasible that their deficiency might affect global electrical activity detected of the brain. However, the effects of the loss of synucleins on the frequency spectra of electroencephalograms (EEGs) have not been systematically studied so far. In the current study, we assessed changes in such spectra in single-, double- and triple-knockout mice lacking alpha-, beta- and gamma-synucleins in all possible combinations. EEGs were recorded from the motor cortex, the putamen, the ventral tegmental area and the substantia nigra of 78 3-month-old male mice from seven knockout groups maintained on the C57BL/6J genetic background, and 10 wild-type C57BL/6J mice for 30 min before and for 60 min after the systemic injection of a DA receptor agonist, apomorphine (APO). We found that almost any variant of synuclein deficiency causes multiple changes in both basal and APO-induced EEG oscillation profiles. Therefore, it is not the absence of any particular synuclein but rather a disbalance of synucleins that causes widespread changes in EEG spectral profiles.

Cortical and Striatal Electroencephalograms and Apomorphine Effects in the FUS Mouse Model of Amyotrophic Lateral Sclerosis.

BACKGROUND: Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of motor neurons resulting in muscle atrophy. In contrast to the lower motor neurons, the role of upper (cortical) neurons in ALS is yet unclear. Maturation of locomotor networks is supported by dopaminergic (DA) projections from substantia nigra to the spinal cord and striatum. OBJECTIVE: To examine the contribution of DA mediation in the striatum-cortex networks in ALS progression. METHODS: We studied electroencephalogram (EEG) from striatal putamen (Pt) and primary motor cortex (M1) in ΔFUS(1-359)-transgenic (Tg) mice, a model of ALS. EEG from M1 and Pt were recorded in freely moving young (2-month-old) and older (5-month-old) Tg and non-transgenic (nTg) mice. EEG spectra were analyzed for 30 min before and for 60 min after systemic injection of a DA mimetic, apomorphine (APO), and saline. RESULTS: In young Tg versus nTg mice, baseline EEG spectra in M1 were comparable, whereas in Pt, beta activity in Tg mice was enhanced. In older Tg versus nTg mice, beta dominated in EEG from both M1 and Pt, whereas theta and delta 2 activities were reduced. In younger Tg versus nTg mice, APO increased theta and decreased beta 2 predominantly in M1. In older mice, APO effects in these frequency bands were inversed and accompanied by enhanced delta 2 and attenuated alpha in Tg versus nTg mice. CONCLUSION: We suggest that revealed EEG modifications in ΔFUS(1-359)-transgenic mice are associated with early alterations in the striatum-cortex interrelations and DA transmission followed by adaptive intracerebral transformations.

Inflammation/bioenergetics-associated neurodegenerative pathologies and concomitant diseases: a role of mitochondria targeted catalase and xanthophylls.

Various inflammatory stimuli are able to modify or even "re-program" the mitochondrial metabolism that results in generation of reactive oxygen species. In noncommunicable chronic diseases such as atherosclerosis and other cardiovascular pathologies, type 2 diabetes and metabolic syndrome, these modifications become systemic and are characterized by chronic inflammation and, in particular, "neuroinflammation" in the central nervous system. The processes associated with chronic inflammation are frequently grouped into "vicious circles" which are able to stimulate each other constantly amplifying the pathological events. These circles are evidently observed in Alzheimer's disease, atherosclerosis, type 2 diabetes, metabolic syndrome and, possibly, other associated pathologies. Furthermore, chronic inflammation in peripheral tissues is frequently concomitant to Alzheimer's disease. This is supposedly associated with some common genetic polymorphisms, for example, Apolipoprotein-E ε4 allele carriers with Alzheimer's disease can also develop atherosclerosis. Notably, in the transgenic mice expressing the recombinant mitochondria targeted catalase, that removes hydrogen peroxide from mitochondria, demonstrates the significant pathology amelioration and health improvements. In addition, the beneficial effects of some natural products from the xanthophyll family, astaxanthin and fucoxanthin, which are able to target the reactive oxygen species at cellular or mitochondrial membranes, have been demonstrated in both animal and human studies. We propose that the normalization of mitochondrial functions could play a key role in the treatment of neurodegenerative disorders and other noncommunicable diseases associated with chronic inflammation in ageing. Furthermore, some prospective drugs based on mitochondria targeted catalase or xanthophylls could be used as an effective treatment of these pathologies, especially at early stages of their development.

Spatial memory deficits initiated by agroclavine injection or olfactory bulbectomy in rats are characterized by different levels of long-term potentiation expression in the hippocampus.

Aim: To clarify whether long-term potentiation (LTP) is the mechanism underpinning mnemonic processes. Mathrials and methods: We studied LTP in hippocampal slices from rats whose spatial memory deficit was produced by either olfactory bulbectomy (OBX) or pretreatment with an ergot alkaloid, agroclavine. OBX is accompanied by cholinergic system inhibition whereas agroclavine predominantly activates dopaminergic mediation. The both have been shown to be involved in learning/memory and LTP mechanisms.Results: In OBX- vs. sham-operated rat, we have revealed significant reduction of LTP in hippocampal CA1 region. In contrast, no LTP differences in agroclavine- vs. vehicle-treated rats were observed. Conclusions: These results demonstrate that LTP expression in the hippocampus is dependent on the origin of spatial memory impairment. Furthermore, they suggest that pharmacological and neurodegenerative models of AD might be useful approach for discovery of both AD mechanisms and mixed pathology dementias.

Loss of Midbrain Dopamine Neurons and Altered Apomorphine EEG Effects in the 5xFAD Mouse Model of Alzheimer's Disease.

Cognitive malfunction, synaptic dysfunction, and disconnections in neural networks are core deficits in Alzheimer's disease (AD). 5xFAD mice, a transgenic model of AD, are characterized by an enhanced level of amyloid-ß and abnormal neurotransmission. The dopaminergic (DA) system has been shown to be involved in amyloid-ß transformations and neuronal plasticity; however, its role in functional network changes in familial AD still remains unclear. In 5xFAD and non-transgenic freely moving mice, electroencephalograms (EEGs) were simultaneously recorded from the secondary motor cortex (MC), superficial layers of the hippocampal CA1 area (HPC), substantia nigra (SN), and ventral tegmental area (VTA). EEGs and their frequency spectra were analyzed before and after systemic injection of a DA receptor agonist, apomorphine (APO). In the baseline EEG from MC and HPC of 5xFAD mice, delta and alpha oscillations were enhanced and beta activity was attenuated, compared to control mice. In VTA and SN of 5xFAD mice, delta-theta activity was decreased and beta oscillations dominated. In control mice, APO suppressed delta activity in VTA to a higher extent than in MC, whereas in 5xFAD mice, this difference was eliminated due to attenuation of the delta suppression in VTA. APO increased beta activity in MC of mice from both groups while significant beta suppression was observed in VTA of 5xFAD mice. These mice were characterized by significant decrease of tyrosine hydroxylase immunopositive cells in both VTA and SN and of DA transporter in MC and hippocampal dentate gyrus. We suggest that the EEG modifications observed in 5xFAD mice are associated with alterations in dopaminergic transmission, resulting in adaptive changes in the cerebral networks in the course of familial AD development.

Intrahippocampal Pathways Involved in Learning/Memory Mechanisms are Affected by Intracerebral Infusions of Amyloid-ß25-35 Peptide and Hydrated Fullerene C60 in Rats.

Primary memory impairments associated with increased level of amyloid-ß (Aß) in the brain have been shown to be linked, partially, with early pathological changes in the entorhinal cortex (EC) which spread on the whole limbic system. While the hippocampus is known to play a key role in learning and memory mechanisms, it is as yet unclear how its structures are involved in the EC pathology. In this study, changes in memory and neuronal morphology in male Wistar rats intrahippocampally injected with Aß25-35 were correlated on days 14 and 45 after the injection to reveal specific cognitive-structural associations. The main focus was on the dentate gyrus (DG) and hippocampal areas of CA1 and CA3 because of their involvement in afferent flows from EC to the hippocampus through tri-synaptic (EC → DG → CA3 → CA1) and/or mono-synaptic (EC → CA1) pathways. Evident memory impairments were observed at both time points after Aß25-35 injection. However, on day 14, populations of morphological intact neurons were decreased in CA3 and, drastically, in CA1, and the DG supramedial bundle was significantly damaged. On day 45, this bundle largely and CA1 neurons partially recovered, whereas CA3 neurons remained damaged. We suggest that Aß25-35 primarily affects the tri-synaptic pathway, destroying the granular cells in the DG supramedial area and neurons in CA3 and, through the Schaffer collaterals, in CA1. Intrahippocampal pretreatment with hydrated fullerene C60 allows the neurons and their connections to survive the amyloidosis, thus supporting the memory mechanisms.

Immunization Against Specific Fragments of Neurotrophin p75 Receptor Protects Forebrain Cholinergic Neurons in the Olfactory Bulbectomized Mice.

Alzheimer's disease (AD) is characterized by progressive cognitive impairment associated with marked cholinergic neuron loss and amyloid-ß (Aß) peptide accumulation in the brain. The cytotoxicity in AD is mediated, at least in part, by Aß binding with the extracellular domain of the p75 neurotrophin receptor (p75NTR), localized predominantly in the membranes of acetylcholine-producing neurons in the basal forebrain. Hypothesizing that an open unstructured loop of p75NTR might be the effective site for Aß binding, we have immunized both olfactory bulbectomized (OBX) and sham-operated (SO) mice (n = 82 and 49, respectively) with synthetic peptides, structurally similar to different parts of the loops, aiming to block them by specific antibodies. OBX-mice have been shown in previous studies, and confirmed in the present one, to be characterized by typical behavioral, morphological, and biochemical AD hallmarks, including cholinergic deficits in forebrain neurons. Immunization of OBX- or SO-mice with KLH conjugated fragments of p75NTR induced high titers of specific serum antibodies for each of nine chosen fragments. However, maximal protective effects on spatial memory, evaluated in a Morris water maze, and on activity of choline acetyltransferase in forebrain neurons, detected by immunoreactivity to specific antibodies, were revealed only for peptides with amino acid residue sequences of 155-164 and 167-176. We conclude that the approach based on immunological blockade of specific p75NTR sites, linked with the cytotoxicity, is a useful and effective tool for study of AD-associated mechanisms and for development of highly selective therapy of cholinergic malfunctioning in AD patients.

Neuroprotective effects of hydrated fullerene C60: cortical and hippocampal EEG interplay in an amyloid-infused rat model of Alzheimer's disease.

We studied the effects of fullerene C60 nanoparticles, namely hydrated fullerene C60 (C60HyFn), on interrelations between EEG frequency spectra from the frontal cortex and the dorsal hippocampus (CA1) on an amyloid-ß (Aß) rat model of Alzheimer's disease (AD). Infusion of Aß1-42 protein (1.5 µl) into the CA1 region two weeks before EEG testing diminished hippocampal theta (3.8-8.4 Hz) predominance and eliminated cortical beta (12.9-26.2 Hz) predominance observed in baseline EEG of rats infused with saline (control) or with C60HyFn alone. In contrast, these Aß1-42 effects were abolished in rats pretreated with C60HyFn, 30 min apart. Dopaminergic mediation in AD has been shown to be involved in neuronal plasticity and Aß transformation in different ways. To clarify its role in the cortex-hippocampus interplay in the Aß model of AD, we used peripheral injection of a dopamine agonist, apomorphine (APO), at a low dose (0.1 mg/kg). In rats infused with C60HyFn or Aß1-42 alone, APO attenuated the cortical beta predominance, with immediate and delayed phases evident in the Aß1-42-rats. Pretreatment with C60HyFn diminished the APO effect in the Aß1-42-treated rats. Thus, we show that intrahippocampal injection of Aß1-42 dramatically disrupts cortical versus hippocampal EEG interrelations and that pretreatment with the fullerene eliminates this abnormality. We suggest that some effects of C60HyFn may be mediated through presynaptic dopamine receptors and that water-soluble C60 fullerenes have a neuroprotective potential.

Trial-by-trial reliability of responses in the primary visual cortex on binocular disparity depends on stimulus order.

An association of the detrimental effect of monocular deprivation on binocular vision with reduced reliability of neuronal responses in the primary visual cortex has been shown on randomly presented binocular stimuli [V. Vorobyov et al. (2007) Eur J Neurosci. 26(12), 3553-3563]. To examine this effect on biologically relevant signals, binocular gratings of varying relative phase disparity were presented in sequential order, simulating motion, to 55 cats with various types of daily visual experience. During sequential stimulation, the proportions of 'unstable' cells (with phase differences exceeding 22.5 ° between peak binocular responses in two consecutive trials) were similar in cats with exclusively binocular experience and with short periods of daily monocular vision (≤ 3.25 h), in mixed binocular-monocular conditions. In contrast, random stimulation was characterized by a significantly enlarged population of 'unstable' cells in the latter. After a longer period of monocular vision (6.5 h) or exclusively discordant binocular experience (strabismus), sequential stimulation was accompanied by a significant increase of this population, whereas during randomized stimulation it was very similar to that in cats with short periods of daily monocular vision. Finally, there were no differences in populations of 'unstable' cells in cats with long monocular or strabismic vision and those with exclusive monocular experience during sequential stimulation, in contrast with a significant increase in the latter during randomized stimulation. I propose that the detrimental effect of abnormal binocular experience on binocular processing in the primary visual cortex is associated with a disruption of the mechanisms involved in both discrimination of binocular disparity signals and evaluation of their temporal profiles.

Effects of digesting chondroitin sulfate proteoglycans on plasticity in cat primary visual cortex.

Monocular deprivation (MD) during a critical period of postnatal development produces significant changes in the anatomy and physiology of the visual cortex, and the deprived eye becomes amblyopic. Extracellular matrix molecules have a major role in restricting plasticity such that the ability to recover from MD decreases with age. Chondroitin sulfate proteoglycans (CSPGs) act as barriers to cell migration and axon growth. Previous studies showing that degradation of CSPGs by the bacterial enzyme chondroitinase can restore plasticity in the adult rat visual cortex suggest a potential treatment for amblyopia. Here MD was imposed in cats from the start of the critical period until 3.5 months of age. The deprived eye was reopened, the functional architecture of the visual cortex was assessed by optical imaging of intrinsic signals, and chondroitinase was injected into one hemisphere. Imaging was repeated 1 and 2 weeks postinjection, and visually evoked potentials (VEPs) and single-cell activity were recorded. Immunohistochemistry showed that digestion of CSPGs had been successful. After 2 weeks of binocular exposure, some recovery of deprived-eye responses occurred when chondroitinase had been injected into the hemisphere contralateral to that eye; when injected into the ipsilateral hemisphere, no recovery was seen. Deprived-eye VEPs were no larger in the injected hemisphere than in the opposite hemisphere. The small number of neurons dominated by the deprived eye exhibited poor tuning characteristics. These results suggest that despite structural effects of chondroitinase in adult cat V1, plasticity was not sufficiently restored to enable significant functional recovery of the deprived eye.

Effects of different forms of monocular deprivation on primary visual cortex maps.

Monocular deprivation (MD) by lid suture is one of the classic paradigms for the study of developmental plasticity in the cerebral cortex, and we have detailed knowledge of its anatomical and physiological consequences as well as underlying molecular and cellular mechanisms. However, the effects of other forms of manipulating visual input through one eye on the functional architecture of the primary visual cortex (V1) have not yet been examined directly. We compared MD by lid suture with the effects of daily monocular lens wear using either a frosted lens or a neutral density (ND) filter. We used optical imaging of intrinsic signals and visually evoked potentials (VEPs) to assess responses in V1 to monocular stimulation. We found that loss of stimulus contrast through monocular frosted lens wear resulted in marked takeover of cortical territory by the nondeprived eye (NDE) similar to that caused by classic MD, and in virtual absence of orientation-selective responses following stimulation of the deprived eye (DE). Furthermore, amplitudes of VEPs in response to gratings of a range of spatial frequencies were significantly reduced in the DE compared to the NDE. In contrast, differences in luminance between two eyes caused by an ND filter in front of one eye did not affect ocular dominance and orientation maps, and there was no significant difference in the amplitude of VEPs elicited through the two eyes. Our results are consistent with previous electrophysiological studies in demonstrating that binocular pattern information is necessary to maintain normal functional maps in both eyes, while reduced luminance in one eye has little effect on the overall functional architecture and visual responses in V1.

Experience-dependent regulation of functional maps and synaptic protein expression in the cat visual cortex.

Although the basis of our knowledge of experience-dependent plasticity comes from studies on carnivores and primates, studies examining the physiological and molecular mechanisms that underlie development and plasticity have increasingly employed mice. We have used several common rearing paradigms, such as dark-rearing and monocular deprivation (MD), to examine the timing of the physiological and molecular changes to altered experience in the cat primary visual cortex. Dark-rearing from birth or for 1 week starting at 4 weeks of age produced a similar reduction in the amplitude of responses measured through intrinsic signal imaging and a reduction in orientation selectivity. One week of visual experience following dark-rearing until 4 weeks of age yielded normal responses in both amplitude and orientation selectivity. The depression of deprived-eye responses was similar in magnitude after 2 and 7 days of MD. In contrast, non-deprived-eye responses almost doubled in magnitude after 7 days compared with 2 days of MD. These changes in the functional properties of primary visual cortex neurons were mirrored by specific changes in synaptic protein expression. Changes in proteins such as the NR2A and NR2B subunits of the N-methyl-D-aspartate receptor, postsynaptic density protein 95, alpha-CA(2+) /calmodulin-dependent protein kinase II (αCaMKII), and GABA(A) α1a indicated that the levels of sensory activity regulated mechanisms associated with both excitatory (NR2A and NR2B) and inhibitory (GABA(A) α1a) transmission so as to maintain response homeostasis. Additionally, we found that MD regulated the AMPA receptor glutamate (GluR1) subunit as well as signalling molecules (αCaMKII and synaptic Ras GTPase activating protein, SynGAP) downstream of N-methyl-D-aspartate receptors. Proteins in a common signalling pathway appeared to have similar developmental expression profiles that were broadly similar between cats and rodents.

Effects of nootropics on the EEG in conscious rats and their modification by glutamatergic inhibitors.

To study the effects of acute and repeated injections of nootropics and to learn how glutamate receptors might be involved in their mediation, the frequency spectra of cortical and hippocampal electroencephalogram (EEG) were analyzed in non-narcotized rats subcutaneously injected repeatedly with Piracetam (400mg/kg) or its analogue, Noopept (0.2mg/kg), after intracerebroventricular infusions of saline (5 µl) or the antagonists of NMDA and quisqualate/AMPA receptors: CPP (0.1 nmol) and GDEE (1 µmol), respectively. Piracetam increased alpha/beta1 EEG activity in the left frontal cortex, and alpha activity in both the right cortex and hippocampus, with a 10-min latency and 40-min duration. Noopept increased alpha/beta1 activity, with 30-min latency and 40-min duration in all brain areas. CPP pretreatment eliminated Piracetam EEG effects; reduced Noopept effects in the cortex and completely suppressed them in the hippocampus. After four injections of Piracetam, EEG effects were very small in the cortex, and completely lacking in the hippocampus, while GDEE pretreatment partially recovered them. The effect of Noopept in the alpha/beta1 ranges was replaced by increased beta2 activity after the eighth injection, while no effects were observed after the ninth one. GDEE pretreatment restored the effect of Noopept in the beta2 frequency range. These results demonstrate similarities in EEG effects and their mediatory mechanisms for Piracetam and its much more effective analogue, Noopept. Activation of NMDA receptors is involved in the effects of a single injection of the nootropics, whereas activation of quisqualate/AMPA receptors is associated with the decrease in their efficacy after repeated use.