Investigation of GluA1 and GluA2 AMPA receptor subtype distribution in the hippocampus and anterior cingulate cortex of Long Evans rats during development

Assessment of cFos labeling in the hippocampus and anterior cingulate cortex following recent and remote re-exposure to an unreinforced open field in preadolescent and postadolescent rats

Spatial tasks are often goal-directed or reward-facilitated confounding the assessment of "pure" recent and remote spatial memories. The current work re-exposed preadolescent and postadolescent male rats to a non-reinforced, free exploration task to investigate cFos patterns within the hippocampus and anterior cingulate cortex (ACC) associated with recent and remote periods. Male rats were exposed to an open field task for one, 30 min session on postnatal day (P) 20, 25, or 50 and re-exposed for 30 min at either a recent (24 hours) or remote (3 weeks) timepoint. Distance traveled in the open field was measured as well as cFos labeling. In the P20 age group, there was elevated exploration at the 24-hour and 3-week tests compared to training and compared to the other age groups. In the hippocampus CA1, cFos levels were higher after the remote test than the recent test in the P20 group but higher after the recent test than remote test in the P25 and P50 groups. cFos labeling in the ACC was higher in all remote-tested groups compared to the recent-tested groups across all ages. In the P20, the 24-hour test was associated with less CA1 activity than the other age groups supporting the hypothesis that the hippocampus is not fully developed at this time point. In the P20 group, the remote representation of this task did not seem to be complete as there continued to be CA1 activity along with ACC activity following the remote test associated with elevated exploration. These results indicate the utility of unreinforced spatial navigation tasks for exploring systems consolidation processes over the lifespan and show that a fully developed hippocampus is required for optimal systems consolidation.

Caffeine intake during lactation has a sex-dependent effect on the hippocampal excitatory/inhibitory balance and pups' behavior

During early life, disruptions in glutamatergic and GABAergic synapse development in the hippocampus may contribute to several neurodevelopmental disorders, including cognitive deficits and psychiatric disorders. Caffeine is the most consumed psychoactive drug in the world, and previous work from our group has shown that caffeine disrupts visual system connections at different stages of development. This work aimed to investigate the effects of caffeine consumption during lactation in the glutamatergic and GABAergic synaptic markers in the hippocampus and on the behavior of rat offspring. We found that maternal caffeine intake significantly reduced GluN1 subunits of the NMDA receptor, increased the GluA1/GluA2 ratio of AMPA receptor in the dorsal hippocampus, and decreased GAD content in female pups' ventral hippocampus. On the other hand, an increase in GluN1/GluN2b subunits, a decrease in GAD content in the dorsal hippocampus, and a reduction of the GluA1 content in the ventral hippocampus were observed in male pups. In addition, changes in the behavior of the offspring submitted to indirect caffeine consumption were also sex-dependent, with females developing anxiety-like behavior and males showing anxiety-like behavior and hyper-locomotion. These results highlight that maternal caffeine intake promotes changes in the hippocampal excitatory and inhibitory balance and offspring behavior in a sex-dependent manner, suggesting that the population should be alerted to reduced caffeine consumption by breastfeeding mothers.

Advancements in the study of synaptic plasticity and mitochondrial autophagy relationship

Synapses serve as the points of communication between neurons, consisting primarily of three components: the presynaptic membrane, synaptic cleft, and postsynaptic membrane. They transmit signals through the release and reception of neurotransmitters. Synaptic plasticity, the ability of synapses to undergo structural and functional changes, is influenced by proteins such as growth-associated proteins, synaptic vesicle proteins, postsynaptic density proteins, and neurotrophic growth factors. Furthermore, maintaining synaptic plasticity consumes more than half of the brain's energy, with a significant portion of this energy originating from ATP generated through mitochondrial energy metabolism. Consequently, the quantity, distribution, transport, and function of mitochondria impact the stability of brain energy metabolism, thereby participating in the regulation of fundamental processes in synaptic plasticity, including neuronal differentiation, neurite outgrowth, synapse formation, and neurotransmitter release. This article provides a comprehensive overview of the proteins associated with presynaptic plasticity, postsynaptic plasticity, and common factors between the two, as well as the relationship between mitochondrial energy metabolism and synaptic plasticity.

Neuronal plasticity contributes to postictal death

Repeated generalized tonic-clonic seizures (GTCSs) are the most critical risk factor for sudden unexpected death in epilepsy (SUDEP). GTCSs can cause fatal apnea. We investigated neuronal plasticity mechanisms that precipitate postictal apnea and seizure-induced death. Repeated seizures worsened behavior, precipitated apnea, and enlarged active neuronal circuits, recruiting more neurons in such brainstem nuclei as periaqueductal gray (PAG) and dorsal raphe, indicative of brainstem plasticity. Seizure-activated neurons are more excitable and have enhanced AMPA-mediated excitatory transmission after a seizure. Global deletion of the GluA1 subunit of AMPA receptors abolishes postictal apnea and seizure-induced death. Treatment with a drug that blocks Ca2+-permeable AMPA receptors also renders mice apnea-free with five-fold better survival than untreated mice. Repeated seizures traffic the GluA1 subunit-containing AMPA receptors to synapses, and blocking this mechanism decreases the probability of postictal apnea and seizure-induced death.

GluA1-Shank3 interaction decreases in response to chronic neuronal depolarization

Interactions between AMPA receptors and synaptic scaffolding proteins are key regulators of synaptic receptor density and, thereby, synapse strength. Shank3 is one such scaffolding protein with high clinical relevance, as genetic variants and deletions of this protein have been linked to autism spectrum disorder. Shank3 acts as a master regulator of the postsynaptic density of glutamatergic synapses, interacting with ionotropic and metabotropic glutamate receptors and cytoskeletal elements to modulate synaptic structure. Notably, Shank3 has been shown to interact directly with the AMPAR subunit GluA1, and Shank3 knockout animals show deficits in AMPAR-mediated synaptic transmission. In this study, we sought to characterize the stability of GluA1-Shank3 interaction in response to chronic stimuli using a highly sensitive and specific proximity ligation assay. We found that GluA1-Shank3 interactions decrease in response to prolonged neuronal depolarization induced by elevated extracellular potassium, and that this reduced interaction is blocked by NMDA receptor antagonism. These results firmly establish the close interaction of GluA1 and Shank3 in cortical neurons in vitro, and that this select interaction is subject to modulation by depolarization.

Increased glutamatergic synaptic transmission during development in layer II/III mouse motor cortex pyramidal neurons

Postnatal maturation of the motor cortex is vital to developing a variety of functions, including the capacity for motor learning. The first postnatal weeks involve many neuronal and synaptic changes, which differ by region and layer, likely due to different functions and needs during development. Motor cortex layer II/III is critical to receiving and integrating inputs from somatosensory cortex and generating attentional signals that are important in motor learning and planning. Here, we examined the neuronal and synaptic changes occurring in layer II/III pyramidal neurons of the mouse motor cortex from the neonatal (postnatal day 10) to young adult (postnatal day 30) period, using a combination of electrophysiology and biochemical measures of glutamatergic receptor subunits. There are several changes between p10 and p30 in these neurons, including increased dendritic branching, neuronal excitability, glutamatergic synapse number and synaptic transmission. These changes are critical to ongoing plasticity and capacity for motor learning during development. Understanding these changes will help inform future studies examining the impact of early-life injury and experiences on motor learning and development capacity.

(2R,6R)-hydroxynorketamine acts through GluA1-induced synaptic plasticity to alleviate PTSD-like effects in rat models

Post-traumatic stress disorder (PTSD) is a debilitating mental disorder with high morbidity and great social and economic relevance. However, extant pharmacotherapies of PTSD require long-term use to maintain effectiveness and have enormous side effects. The glutamatergic system, especially the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR), is an important target of current research on the mechanism of PTSD. Postsynaptic AMPAR function and expression are known to be increased by (2R, 6R)-hydronorketamine (HNK), the primary metabolite of ketamine. However, whether (2R,6R)-HNK alleviates PTSD-like effects via AMPAR upregulation is yet to be known. In the present study, rats were exposed to single prolonged stress and electric foot shock (SPS&S). Afterwards, gradient concentrations of (2R,6R)-HNK (20, 50, and 100 μM) were administered by intracerebroventricular (i.c.v.) injection. Open field, elevated plus maze, freezing behavior, and forced swimming tests were used to examine PTSD-like symptoms. In addition, the protein levels of GluA1, BDNF and PSD-95 were analyzed using western blotting and immunofluorescence, and the synaptic ultrastructure of the prefrontal cortex (PFC) was observed by transmission electron microscopy. We found that (2R,6R)-HNK changed SPS&S-induced behavioral expression, such as increasing autonomous activity and residence time in the open arm and decreasing immobility time. Likewise, (2R,6R)-HNK (50 μM) increased GluA1, BDNF, and PSD-95 protein expression in the PFC. Changes in synaptic ultrastructure induced by SPS&S were reversed by administration of (2R,6R)-HNK. Overall, we find that (2R,6R)-HNK can ameliorate SPS&S-induced fear avoidance in rats, as well as rat cognates of anxiety and depression. This may be related to GluA1-mediated synaptic plasticity in the PFC.

The glutamate/GABA system in the retina of male rats: effects of aging, neurodegeneration, and supplementation with melatonin and antioxidant SkQ1

Glutamate and -aminobutyric acid (GABA) are the most abundant amino acids in the retina. An imbalance of the glutamate/GABA system is involved in the pathogenesis of various neurodegenerative disorders. Here we for the first time analyzed alterations of expression of glutamate- and GABA-synthesizing enzymes, transporters, and relevant receptors in the retina with age in Wistar rats and in senescence-accelerated OXYS rats who develop AMD-like retinopathy. We noted consistent age-dependent expression changes of GABAergic-system proteins (GAD67, GABA-T, and GAT1) in OXYS and Wistar rats: upregulation by age 3 months and downregulation at age 18 months. At a late stage of AMD-like retinopathy in OXYS rats (18 months), there was significant upregulation of glutaminase and downregulation of glutamine synthetase, possibly indicating an increasing level of glutamate in the retina. AMD-like-retinopathy development in the OXYS strain was accompanied by underexpression of glutamate transporter GLAST. Prolonged supplementation with both melatonin and SkQ1 (separately) suppressed the progression of the AMD-like pathology in OXYS rats without affecting the glutamate/GABA system but worsened the condition of the Wistar rat's retina during normal aging. We observed decreasing protein levels of glutamine synthetase, GLAST, and GABAAR1 and an increasing level of glutaminase in Wistar rats. In summary, both melatonin and mitochondrial antioxidant SkQ1 had different effect on the retinal glutamate / GABA in healthy Wistar and senescence-accelerated OXYS rats.

Aβ/tau oligomer interplay at human synapses supports shifting therapeutic targets for Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is characterized by progressive cognitive decline due to accumulating synaptic insults by toxic oligomers of amyloid beta (AβO) and tau (TauO). There is growing consensus that preventing these oligomers from interacting with synapses might be an effective approach to treat AD. However, recent clinical trial failures suggest low effectiveness of targeting Aβ in late-stage AD. Researchers have redirected their attention toward TauO as the levels of this species increase later in disease pathogenesis. Here we show that AβO and TauO differentially target synapses and affect each other's binding dynamics.

METHODS

Binding of labeled, pre-formed Aβ and tau oligomers onto synaptosomes isolated from the hippocampus and frontal cortex of mouse and postmortem cognitively intact elderly human brains was evaluated using flow-cytometry and western blot analyses. Binding of labeled, pre-formed Aβ and tau oligomers onto mouse primary neurons was assessed using immunofluorescence assay. The synaptic dysfunction was measured by fluorescence analysis of single-synapse long-term potentiation (FASS-LTP) assay.

RESULTS

We demonstrated that higher TauO concentrations effectively outcompete AβO and become the prevailing synaptic-associated species. Conversely, high concentrations of AβO facilitate synaptic TauO recruitment. Immunofluorescence analyses of mouse primary cortical neurons confirmed differential synaptic binding dynamics of AβO and TauO. Moreover, in vivo experiments using old 3xTgAD mice ICV injected with either AβO or TauO fully supported these findings. Consistent with these observations, FASS-LTP analyses demonstrated that TauO-induced suppression of chemical LTP was exacerbated by AβO. Finally, predigestion with proteinase K abolished the ability of TauO to compete off AβO without affecting the ability of high AβO levels to increase synaptic TauO recruitment. Thus, unlike AβO, TauO effects on synaptosomes are hampered by the absence of protein substrate in the membrane.

CONCLUSIONS

These results introduce the concept that TauO become the main synaptotoxic species at late AD, thus supporting the hypothesis that TauO may be the most effective therapeutic target for clinically manifest AD.

The temporal origin of dentate granule neurons dictates their role in spatial memory

The dentate gyrus is one of the only brain regions that continues its development after birth in rodents. Adolescence is a very sensitive period during which cognitive competences are programmed. We investigated the role of dentate granule neurons (DGNs) born during adolescence in spatial memory and compared them with those generated earlier in life (in embryos or neonates) or during adulthood by combining functional imaging, retroviral and optogenetic tools to tag and silence DGNs. By imaging DGNs expressing Zif268, a proxy for neuronal activity, we found that neurons generated in adolescent rats (and not embryos or neonates) are transiently involved in spatial memory processing. In contrast, adult-generated DGNs are recruited at a later time point when animals are older. A causal relationship between the temporal origin of DGNs and spatial memory was confirmed by silencing DGNs in behaving animals. Our results demonstrate that the emergence of spatial memory depends on neurons born during adolescence, a function later assumed by neurons generated during adulthood.