Impaired synaptic plasticity in the prefrontal cortex of mice with developmentally decreased number of interneurons

Effects of working memory training on cognitive flexibility, dendritic spine density and long-term potentiation in female mice

Working memory (WM) is a cognitive ability that allows the short-term maintenance and manipulation of information for goal-directed behavior. The prefrontal cortex (PFC) and the hippocampus (HPC) are two brain regions implicated in WM task performance. Several studies indicate that training in WM (WMT) can enhance performance in various other cognitive tasks. However, our understanding of the neurobiological changes induced by WMT is very limited. Previous work from our lab showed that WMT enhances synaptic and structural plasticity in the PFC and HPC in male mice. In this study, we investigate the effect of WMT on cognitive flexibility and synaptic properties in PFC and HPC in adult female mice. To this end, female adult mice were split into 3 groups: a) mice that remained in their home cage (naïve), b) mice that performed the alternation task in the T-maze (non-adaptive) and c) mice that were trained in the delayed alternation task for 9 days (adaptive). The delayed alternation task was used for WMT. In one cohort, following the delayed alternation task, all mice were tested in the attention set-shifting (AST) task to measure cognitive flexibility, followed by harvesting of the brains for Golgi-Cox staining to study dendritic spine density. Our results showed that in female mice, there were no differences in AST performance among the three groups tested, however, the latency to make a choice was reduced in the adaptive group. With regards to dendritic spine density, no significant differences were identified in PFC while increased dendritic spine density was found in HPC of the adaptive group, compared to the naïve group. In a second cohort, acute brain slices were prepared after completion of the delayed alternation task to investigate the synaptic properties in the PFC and the HPC. Evoked field excitatory post-synaptic potential (fEPSP) recordings were performed in either PFC or HPC brain slices. Our results show that tetanic-induced long-term potentiation (LTP) in the PFC was not different among the three training groups. In the HPC, theta-burst induced LTP was significantly increased in the adaptive group also compared to the non-adaptive and naïve groups. These results reveal both similarities and differences of WMT on performance in the attention set-shifting task, dendritic spine density and LTP in females, compared to males.

The Developmental Changes in Intrinsic and Synaptic Properties of Prefrontal Neurons Enhance Local Network Activity from the Second to the Third Postnatal Weeks in Mice

The prefrontal cortex (PFC) is characterized by protracted maturation. The cellular mechanisms controlling the early development of prefrontal circuits are still largely unknown. Our study delineates the developmental cellular processes in the mouse medial PFC (mPFC) during the second and the third postnatal weeks and characterizes their contribution to the changes in network activity. We show that spontaneous inhibitory postsynaptic currents (sIPSC) are increased, whereas spontaneous excitatory postsynaptic currents (sEPSC) are reduced from the second to the third postnatal week. Drug application suggested that the increased sEPSC frequency in mPFC at postnatal day 10 (P10) is due to depolarizing γ-aminobutyric acid (GABA) type A receptor function. To further validate this, perforated patch-clamp recordings were obtained and the expression levels of K-Cl cotransporter 2 (KCC2) protein were examined. The reversal potential of IPSCs in response to current stimulation was significantly more depolarized at P10 than P20 while KCC2 expression is decreased. Moreover, the number of parvalbumin-expressing GABAergic interneurons increases and their intrinsic electrophysiological properties significantly mature in the mPFC from P10 to P20. Using computational modeling, we show that the developmental changes in synaptic and intrinsic properties of mPFC neurons contribute to the enhanced network activity in the juvenile compared with neonatal mPFC.

Glutamatergic receptor expression changes in the Alzheimer's disease hippocampus and entorhinal cortex

Alzheimer's Disease (AD) is the leading form of dementia worldwide. Currently, the pathological mechanisms underlying AD are not well understood. Although the glutamatergic system is extensively implicated in its pathophysiology, there is a gap in knowledge regarding the expression of glutamate receptors in the AD brain. This study aimed to characterize the expression of specific glutamate receptor subunits in post‐mortem human brain tissue using immunohistochemistry and confocal microscopy. Free‐floating immunohistochemistry and confocal laser scanning microscopy were used to quantify the density of glutamate receptor subunits GluA2, GluN1, and GluN2A in specific cell layers of the hippocampal sub‐regions, subiculum, entorhinal cortex, and superior temporal gyrus. Quantification of GluA2 expression in human post‐mortem hippocampus revealed a significant increase in the stratum (str.) moleculare of the dentate gyrus (DG) in AD compared with control. Increased GluN1 receptor expression was found in the str. moleculare and hilus of the DG, str. oriens of the CA2 and CA3, str. pyramidale of the CA2, and str. radiatum of the CA1, CA2, and CA3 subregions and the entorhinal cortex. GluN2A expression was significantly increased in AD compared with control in the str. oriens, str. pyramidale, and str. radiatum of the CA1 subregion. These findings indicate that the expression of glutamatergic receptor subunits shows brain region‐specific changes in AD, suggesting possible pathological receptor functioning. These results provide evidence of specific glutamatergic receptor subunit changes in the AD hippocampus and entorhinal cortex, indicating the requirement for further research to elucidate the pathophysiological mechanisms it entails, and further highlight the potential of glutamatergic receptor subunits as therapeutic targets.

Amyloid-beta1-42 induced glutamatergic receptor and transporter expression changes in the mouse hippocampus

Alzheimer's disease (AD) is the leading type of dementia worldwide. With an increasing burden of an aging population coupled with the lack of any foreseeable cure, AD warrants the current intense research effort on the toxic effects of an increased concentration of beta-amyloid (Aβ) in the brain. Glutamate is the main excitatory brain neurotransmitter and it plays an essential role in the function and health of neurons and neuronal excitability. While previous studies have shown alterations in expression of glutamatergic signaling components in AD, the underlying mechanisms of these changes are not well understood. This is the first comprehensive anatomical study to characterize the subregion- and cell layer-specific long-term effect of Aβ_1-42 on the expression of specific glutamate receptors and transporters in the mouse hippocampus, using immunohistochemistry with confocal microscopy. Outcomes are examined 30 days after Aβ_1-42 stereotactic injection in aged male C57BL/6 mice. We report significant decreases in density of the glutamate receptor subunit GluA1 and the vesicular glutamate transporter (VGluT) 1 in the conus ammonis 1 region of the hippocampus in the Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected and naïve controls, notably in the stratum oriens and stratum radiatum. GluA1 subunit density also decreased within the dentate gyrus dorsal stratum moleculare in Aβ_1-42 injected mice compared with artificial cerebrospinal fluid injected controls. These changes are consistent with findings previously reported in the human AD hippocampus. By contrast, glutamate receptor subunits GluA2, GluN1, GluN2A, and VGluT2 showed no changes in expression. These findings indicate that Aβ_1-42 induces brain region and layer specific expression changes of the glutamatergic receptors and transporters, suggesting complex and spatial vulnerability of this pathway during development of AD neuropathology. Read the Editorial Highlight for this article on page 7. Cover Image for this issue: https://doi.org/10.1111/jnc.14763.

The Acute Effects of Amyloid-Beta1-42 on Glutamatergic Receptor and Transporter Expression in the Mouse Hippocampus

Alzheimer’s disease (AD) is the leading type of dementia worldwide. Despite an increasing burden of disease due to a rapidly aging population, there is still a lack of complete understanding of the precise pathological mechanisms which drive its progression. Glutamate is the main excitatory neurotransmitter in the brain and plays an essential role in the normal function and excitability of neuronal networks. While previous studies have shown alterations in the function of the glutamatergic system in AD, the underlying etiology of beta amyloid (Aβ_1–42) induced changes has not been explored. Here we have investigated the acute effects of stereotaxic hippocampal Aβ_1–42 injection on specific glutamatergic receptors and transporters in the mouse hippocampus, using immunohistochemistry and confocal microscopy 3 days after Aβ_1–42 injection in aged male C57BL/6 mice, before the onset of neuronal cell death. We show that acute injection of Aβ_1–42 is sufficient to induce cognitive deficits 3 days post-injection. We also report no significant changes in glutamate receptor subunits GluA1, GluA2, VGluT1, and VGluT2 in response to acute injection of Aβ_1–42 when compared with the ACSF-vehicle injected mice. However, we observed increased expression in the DG hilus and ventral stratum (str.) granulosum, CA3 str. radiatum and str. oriens, and CA1 str. radiatum of the GluN1 subunit, and increased expression within the CA3 str. radiatum and decreased expression within the DG str. granulosum of the GluN2A subunit in Aβ_1–42 injected mice compared to NC, and a similar trend observed when compared to ACSF-injected mice. We also observed alterations in expression patterns of glutamatergic receptor subunits and transporters within specific layers of hippocampal subregions in response to a microinjection stimulus. These findings indicate that the pathological alterations in the glutamatergic system observed in AD are likely to be partially a result of both acute and chronic exposure to Aβ_1–42 and implies a much more complex circuit mechanism associated with glutamatergic dysfunction than simply glutamate-mediated excitotoxic neuronal death.

Rac1 Modulates Excitatory Synaptic Transmission in Mouse Retinal Ganglion Cells

Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the Rho GTPase family which plays important roles in dendritic spine morphology and plasticity, is a key regulator of cytoskeletal reorganization in dendrites and spines. Here, we investigated whether and how Rac1 modulates synaptic transmission in mouse retinal ganglion cells (RGCs) using selective conditional knockout of Rac1 (Rac1-cKO). Rac1-cKO significantly reduced the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents, while glycine/GABA_A receptor-mediated miniature inhibitory postsynaptic currents were not affected. Although the total GluA1 protein level was increased in Rac1-cKO mice, its expression in the membrane component was unchanged. Rac1-cKO did not affect spine-like branch density in single dendrites, but significantly reduced the dendritic complexity, which resulted in a decrease in the total number of dendritic spine-like branches. These results suggest that Rac1 selectively affects excitatory synaptic transmission in RGCs by modulating dendritic complexity.

Mice With Decreased Number of Interneurons Exhibit Aberrant Spontaneous and Oscillatory Activity in the Cortex

GABAergic (γ-aminobutyric acid) neurons are inhibitory neurons and protect neural tissue from excessive excitation. Cortical GABAergic neurons play a pivotal role for the generation of synchronized cortical network oscillations. Imbalance between excitatory and inhibitory mechanisms underlies many neuropsychiatric disorders and is correlated with abnormalities in oscillatory activity, especially in the gamma frequency range (30–80 Hz). We investigated the functional changes in cortical network activity in response to developmentally reduced inhibition in the adult mouse barrel cortex (BC). We used a mouse model that displays ∼50% fewer cortical interneurons due to the loss of Rac1 protein from Nkx2.1/Cre-expressing cells [Rac1 conditional knockout (cKO) mice], to examine how this developmental loss of cortical interneurons may affect basal synaptic transmission, synaptic plasticity, spontaneous activity, and neuronal oscillations in the adult BC. The decrease in the number of interneurons increased basal synaptic transmission, as examined by recording field excitatory postsynaptic potentials (fEPSPs) from layer II networks in the Rac1 cKO mouse cortex, decreased long-term potentiation (LTP) in response to tetanic stimulation but did not alter the pair-pulse ratio (PPR). Furthermore, under spontaneous recording conditions, Rac1 cKO brain slices exhibit enhanced sensitivity and susceptibility to emergent spontaneous activity. We also find that this developmental decrease in the number of cortical interneurons results in local neuronal networks with alterations in neuronal oscillations, exhibiting decreased power in low frequencies (delta, theta, alpha) and gamma frequency range (30–80 Hz) with an extra aberrant peak in high gamma frequency range (80–150 Hz). Therefore, our data show that disruption in GABAergic inhibition alters synaptic properties and plasticity, while it additionally disrupts the cortical neuronal synchronization in the adult BC.

Adenosine A2A Receptors Control Glutamatergic Synaptic Plasticity in Fast Spiking Interneurons of the Prefrontal Cortex

Adenosine A_2A receptors (A_2AR) are activated upon increased synaptic activity to assist in the implementation of long-term plastic changes at synapses. While it is reported that A_2AR are involved in the control of prefrontal cortex (PFC)-dependent behavior such as working memory, reversal learning and effort-based decision making, it is not known whether A_2AR control glutamatergic synapse plasticity within the medial PFC (mPFC). To elucidate that, we tested whether A_2AR blockade affects long-term plasticity (LTP) of excitatory post-synaptic potentials in pyramidal neurons and fast spiking (FS) interneurons in layer 5 of the mPFC and of population spikes. Our results show that A_2AR are enriched at mPFC synapses, where their blockade reversed the direction of plasticity at excitatory synapses onto layer 5 FS interneurons from LTP to long-term depression, while their blockade had no effect on the induction of LTP at excitatory synapses onto layer 5 pyramidal neurons. At the network level, extracellularly induced LTP of population spikes was reduced by A_2AR blockade. The interneuron-specificity of A_2AR in controlling glutamatergic synapse LTP may ensure that during periods of high synaptic activity, a proper excitation/inhibition balance is maintained within the mPFC.

Prefrontal cortical-specific differences in behavior and synaptic plasticity between adolescent and adult mice

Adolescence is a highly vulnerable period for the emergence of major neuropsychological disorders and is characterized by decreased cognitive control and increased risk-taking behavior and novelty-seeking. The prefrontal cortex (PFC) is involved in the cognitive control of impulsive and risky behavior. Although the PFC is known to reach maturation later than other cortical areas, little information is available regarding the functional changes from adolescence to adulthood in PFC, particularly compared with other primary cortical areas. This study aims to understand the development of PFC-mediated, compared with non-PFC-mediated, cognitive functions. Toward this aim, we performed cognitive behavioral tasks in adolescent and adult mice and subsequently investigated synaptic plasticity in two different cortical areas. Our results showed that adolescent mice exhibit impaired performance in PFC-dependent cognitive tasks compared with adult mice, whereas their performance in non-PFC-dependent tasks is similar to that of adults. Furthermore, adolescent mice exhibited decreased long-term potentiation (LTP) within upper-layer synapses of the PFC but not the barrel cortex. Blocking GABAA receptor function significantly augments LTP in both the adolescent and adult PFC. No change in intrinsic excitability of PFC pyramidal neurons was observed between adolescent and adult mice. Finally, increased expression of the NR2A subunit of the N-methyl-d-aspartate receptors is found only in the adult PFC, a change that could underlie the emergence of LTP. In conclusion, our results demonstrate physiological and behavioral changes during adolescence that are specific to the PFC and could underlie the reduced cognitive control in adolescents.

NEW & NOTEWORTHY This study reports that adolescent mice exhibit impaired performance in cognitive functions dependent on the prefrontal cortex but not in cognitive functions dependent on other cortical regions. The current results propose reduced synaptic plasticity in the upper layers of the prefrontal cortex as a cellular correlate of this weakened cognitive function. This decreased synaptic plasticity is due to reduced N-methyl-d-aspartate receptor expression but not due to dampened intrinsic excitability or enhanced GABAergic signaling during adolescence.

Gene-environment interactions in cortical interneuron development and dysfunction: A review of preclinical studies

Cortical interneurons (cINs) are a diverse group of locally projecting neurons essential to the organization and regulation of neural networks. Though they comprise only ∼20% of neurons in the neocortex, their dynamic modulation of cortical activity is requisite for normal cognition and underlies multiple aspects of learning and memory. While displaying significant morphological, molecular, and electrophysiological variability, cINs collectively function to maintain the excitatory-inhibitory balance in the cortex by dampening hyperexcitability and synchronizing activity of projection neurons, primarily through use of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Disruption of the excitatory-inhibitory balance is a common pathophysiological feature of multiple seizure and neuropsychiatric disorders, including epilepsy, schizophrenia, and autism. While most studies have focused on genetic disruption of cIN development in these conditions, emerging evidence indicates that cIN development is exquisitely sensitive to teratogenic disruption. Here, we review key aspects of cIN development, including specification, migration, and integration into neural circuits. Additionally, we examine the mechanisms by which prenatal exposure to common chemical and environmental agents disrupt these events in preclinical models. Understanding how genetic and environmental factors interact to disrupt cIN development and function has tremendous potential to advance prevention and treatment of prevalent seizure and neuropsychiatric illnesses.