Overexpression of the NR2A subunit in the forebrain impairs long‐term social recognition and non‐social olfactory memory

Animals must recognize and remember conspecifics and potential mates, and distinguish these animals from potential heterospecific competitors and predators. Despite its necessity, aged animals are known to exhibit impaired social recognition memory. As the brain ages, the ratio of NR2A:NR2B in the brain increases over time and has been postulated to underlie the cognitive decline observed during the aging process. Here, we test the hypothesis that an increased NR2A:NR2B subunit ratio underlies long‐term social recognition memory. Using transgenic overexpression of NR2A in the forebrain regions, we investigated the ability of these mice to learn and remember male and female conspecifics, mice of another strain and animals of another rodent species, the rat. Furthermore, due to the importance of olfaction in social recognition, we tested the olfactory memory in the NR2A transgenic mice. Our series of behavioral experiments revealed significant impairments in the NR2A transgenic mice in long‐term social memory of both male and female conspecifics. Additionally, the NR2A transgenic mice are unable to recognize mice of another strain or rats. The NR2A transgenic mice also exhibited long‐term memory impairments in the olfactory recognition task. Taken together, our results provide evidence that an increased NR2A:NR2B ratio in the forebrain leads to reduced long‐term memory function, including the ethologically important memories such as social recognition and olfactory memory .     

Forebrain NR2B overexpression facilitating the prefrontal cortex long-term potentiation and enhancing working memory function in mice

Prefrontal cortex plays an important role in working memory, attention regulation and behavioral inhibition. Its functions are associated with NMDA receptors. However, there is little information regarding the roles of NMDA receptor NR2B subunit in prefrontal cortical synaptic plasticity and prefrontal cortex-related working memory. Whether the up-regulation of NR2B subunit influences prefrontal cortical synaptic plasticity and working memory is not yet clear. In the present study, we measured prefrontal cortical synaptic plasticity and working memory function in NR2B overexpressing transgenic mice. In vitro electrophysiological data showed that overexpression of NR2B specifically in the forebrain region resulted in enhancement of prefrontal cortical long-term potentiation (LTP) but did not alter long-term depression (LTD). The enhanced LTP was completely abolished by a NR2B subunit selective antagonist, Ro25-6981, indicating that overexpression of NR2B subunit is responsible for enhanced LTP. In addition, NR2B transgenic mice exhibited better performance in a set of working memory paradigms including delay no-match-to-place T-maze, working memory version of water maze and odor span task. Our study provides evidence that NR2B subunit of NMDA receptor in prefrontal cortex is critical for prefrontal cortex LTP and prefrontal cortex-related working memory.     

Impaired social memories in 129P2 inbred mice are rescued by reduced Csk expression

The C-terminal Src kinase (Csk) is an essential signaling factor guiding central nervous system (CNS) development. In the adult brain, Csk-mediated control of Src may also modulate glutamatergic synaptic transmission and N-methyl-d-aspartate receptor (NMDAR)-dependent synaptic plasticity. The regulation of N-methyl-d-aspartate (NMDA)-dependent plasticity by a myriad of kinase cascades has been investigated intensively during spatial and fear learning, while little is known about the regulatory kinases and role of NMDA-dependent plasticity during equally critical forms of social learning. We assessed social memory in Csk(+/+) and Csk(+/-) mice backcrossed onto 129P2, an inbred strain with wild-type impairments in social memory. Reduced Csk expression in Csk(+/-) mice was associated with increased NMDAR subunit 2B (NR2B) phosphorylation in the amygdala (AM) and olfactory bulb (OB), and with markedly improved social recognition memory and social transmission of food preference (STFP). In contrast, phosphorylation of NR2B was only slightly increased in the hippocampus of 129P2/Csk(+/-) mice, and the poor spatial object recognition memory of wild-type 129P2/Csk(+/+) mice was not rescued by reduced Csk expression. The Csk pathway appears to be a critical signaling cascade regulating social learning and memory, and presents a possible therapeutic target in diseases such as autism that are characterized by aberrant social behaviors.     

Genetic Enhancement of Memory and Long-Term Potentiation but Not CA1 Long-Term Depression in NR2B Transgenic Rats

One major theory in learning and memory posits that the NR2B gene is a universal genetic factor that acts as rate-limiting molecule in controlling the optimal NMDA receptor''s coincidence-detection property and subsequent learning and memory function across multiple animal species. If so, can memory function be enhanced via transgenic overexpression of NR2B in another species other than the previously reported mouse species? To examine these crucial issues, we generated transgenic rats in which NR2B is overexpressed in the cortex and hippocampus and investigated the role of NR2B gene in NMDA receptor-mediated synaptic plasticity and memory functions by combining electrophysiological technique with behavioral measurements. We found that overexpression of the NR2B subunit had no effect on CA1-LTD, but rather resulted in enhanced CA1-LTP and improved memory performances in novel object recognition test, spatial water maze, and delayed-to-nonmatch working memory test. Our slices recordings using NR2A- and NR2B-selective antagonists further demonstrate that the larger LTP in transgenic hippocampal slices was due to contribution from the increased NR2B-containing NMDARs. Therefore, our genetic experiments suggest that NR2B at CA1 synapses is not designated as a rate-limiting factor for the induction of long-term synaptic depression, but rather plays a crucial role in initiating the synaptic potentiation. Moreover, our studies provide strong evidence that the NR2B subunit represents a universal rate-limiting molecule for gating NMDA receptor''s optimal coincidence-detection property and for enhancing memory function in adulthood across multiple mammalian species.     

Triheteromeric NR1/NR2A/NR2B receptors constitute the major N-methyl-D-aspartate receptor population in adult hippocampal synapses

NMDA receptors (NMDARs), fundamental to learning and memory and implicated in certain neurological disorders, are heterotetrameric complexes composed of two NR1 and two NR2 subunits. The function of synaptic NMDARs in postnatal principal forebrain neurons is typically attributed to diheteromeric NR1/NR2A and NR1/NR2B receptors, despite compelling evidence for triheteromeric NR1/NR2A/NR2B receptors. In synapses, the properties of triheteromeric NMDARs could thus far not be distinguished from those of mixtures of diheteromeric NMDARs. To find a signature of NR1/NR2A/NR2B receptors, we have employed two gene-targeted mouse lines, expressing either NR1/NR2A or NR1/NR2B receptors without NR1/NR2A/NR2B receptors, and compared their synaptic properties with those of wild type. In acute hippocampal slices of mutants older than 4 weeks we found a distinct voltage dependence of NMDA R-mediated excitatory postsynaptic current (NMDA EPSC) decay time for the two diheteromeric NMDARs. In wild-type mice, NMDA EPSCs unveiled the NR1/NR2A characteristic for this voltage-dependent deactivation exclusively, indicating that the contribution of NR1/NR2B receptors to evoked NMDA EPSCs is negligible in adult CA3-to-CA1 synapses. The presence of NR1/NR2A/NR2B receptors was obvious from properties that could not be explained by a mixture of diheteromeric NR1/NR2A and NR1/NR2B receptors or by the presence of NR1/NR2A receptors alone. The decay time for NMDA EPSCs in wild type was slower than that for NR1/NR2A receptors, and the sensitivity of NMDA EPSCs to NR2B-directed NMDAR antagonists was 50%. Thus, NR2B is prominent in adult hippocampal synapses as an integral part of NR1/NR2A/NR2B receptors.     

TNF-Overexpression in Borna Disease Virus-Infected Mouse Brains Triggers Inflammatory Reaction and Epileptic Seizures

Proinflammatory state of the brain increases the risk for seizure development. Neonatal Borna disease virus (BDV)-infection of mice with neuronal overexpression of tumor necrosis factor-α (TNF) was used to investigate the complex relationship between enhanced cytokine levels, neurotropic virus infection and reaction pattern of brain cells focusing on its role for seizure induction. Viral antigen and glial markers were visualized by immunohistochemistry. Different levels of TNF in the CNS were provided by the use of heterozygous and homozygous TNF overexpressing mice. Transgenic TNF, total TNF (native and transgenic), TNF-receptor (TNFR1, TNFR2), IL-1 and N-methyl-D-aspartate (NMDA)-receptor subunit 2B (NR2B) mRNA values were measured by real time RT-PCR. BDV-infection of TNF-transgenic mice resulted in non-purulent meningoencephalitis accompanied by epileptic seizures with a higher frequency in homozygous animals. This correlated with lower weight gain, stronger degree and progression of encephalitis and early, strong microglia activation in the TNF-transgenic mice, most obviously in homozygous animals. Activation of astroglia could be more intense and associated with an unusual hypertrophy in the transgenic mice. BDV-antigen distribution and infectivity in the CNS was comparable in TNF-transgenic and wild-type animals. Transgenic TNF mRNA-expression was restricted to forebrain regions as the transgene construct comprised the promoter of NMDA-receptor subunit2B and induced up-regulation of native TNF mRNA. Total TNF mRNA levels did not increase significantly after BDV-infection in the brain of transgenic mice but TNFR1, TNFR2 and IL-1 mRNA values, mainly in the TNF overexpressing brain areas. NR2B mRNA levels were not influenced by transgene expression or BDV-infection. Neuronal TNF-overexpression combined with BDV-infection leads to cytokine up-regulation, CNS inflammation and glial cell activation and confirmed the presensitizing effect of elevated cytokine levels for the development of spontaneous epileptic seizures when exposed to additional infectious noxi.     

Sociability development in mice with cell‐specific deletion of the NMDA receptor NR1 subunit gene

Social affiliative behavior is an important component of everyday life in many species and is likely to be disrupted in disabling ways in various neurodevelopmental and neuropsychiatric disorders. Therefore, determining the mechanisms involved in these processes is crucial. A link between N‐methyl‐d ‐aspartate (NMDA) receptor function and social behaviors has been clearly established. The cell types in which NMDA receptors are critical for social affiliative behavior, however, remain unclear. Here, we use mice carrying a conditional allele of the NMDA R1 subunit to address this question. Mice bearing a floxed NMDAR1 (NR1) allele were crossed with transgenic calcium/calmodulin‐dependent kinase IIα (CaMKIIα)‐Cre mice or parvalbumin (PV)‐Cre mice targeting postnatal excitatory forebrain or PV‐expressing interneurons, respectively, and assessed using the three‐chambered Social Approach Test. We found that deletion of NR1 in PV‐positive interneurons had no effect on social sniffing, but deletion of NR1 in glutamatergic pyramidal cells resulted in a significant increase in social approach behavior, regardless of age or sex. Therefore, forebrain excitatory neurons expressing NR1 play an important role in regulating social affiliative behavior.     

Transient global ischemia in rat brain promotes different NMDA receptor regulation depending on the brain structure studied

The mRNA expression of the major subunits of N-methyl-d-aspartate receptors (NR1, NR2A and NR2B) following ischemia–reperfusion was studied in structures with different vulnerabilities to ischemic insult in the rat brain. The study was performed using quantitative real-time PCR on samples from 3-month-old male Sprague–Dawley rats after global transient forebrain ischemia followed by 48 h of reperfusion. Expression of NMDA receptor subunits mRNAs decreased significantly in all structures studied in the injured animals as compared to the sham-operated ones. The hippocampal subfields (CA1, CA3 and dentate gyrus) as well as the caudate-putamen, both reported to be highly ischemic-vulnerable structures, showed outstandingly lower mRNA levels of NMDA receptor subunits than the cerebral cortex, which is considered a more ischemic-resistant structure. The ratios of the mRNA levels of the different subunits were analyzed as a measure of the NMDA receptor expression pattern for each structure studied. Hippocampal areas showed changes in NMDA receptor expression after the insult, with significant decreases in the NR2A with respect to the NR1 and NR2B subunits. Thus, the NR1:NR2A:NR2B (1:1:2) ratios observed in the sham-operated animals became (2:1:4) in insulted animals. This modified expression pattern was similar in CA1, CA3 and the dentate gyrus, in spite of the different vulnerabilities reported for these hippocampal areas. In contrast, no significant differences in the expression pattern were observed in the caudate-putamen or cerebral cortex on comparing the sham-operated animals with the ischemia-reperfused rats. Our results support the notion that the regulation of NMDA receptor gene expression is dependent on the brain structure rather than on the higher or lower vulnerability of the area studied.     

Negative regulation of neurogenesis and spatial memory by NR2B-containing NMDA receptors

Several lines of evidence suggest involvement of NMDA receptors (NMDARs) in the regulation of neurogenesis in adults and the formation of spatial memory. Functional properties of NMDARs are strongly influenced by the type of NR2 subunits incorporated. In adult forebrain regions such as the hippocampus and cortex, only NR2A and NR2B subunits are available to form the receptor complex with NR1 subunit. NR2B is predominant NR2 subunit in any of rat or human neural stem cells (NSCs). Thus, we suppose that NR2B-containing NMDAR should be critical in regulating adult neurogenesis, and thereby playing a role in the formation of spatial memory. In the cultured NSCs derived from the embryonic brain of rats, NR2B subunit-specific NMDAR antagonist Ro25-6981 increased cell proliferation, whereas MK-801, non-selective open-channel blocker of NMDARs, inhibited cell proliferation. Blockade of NR2B-containing NMDAR stimulated neurogenesis in the adult hippocampus and facilitated the formation of spatial memory. The enhanced spatial memory dropped back to base level when the NR2B antagonist-induced neurogenesis was neutralized by 3'-azido-deoxythymidine, a telomerase inhibitor. In addition, blockade of NR2B inhibited neuronal nitric oxide synthase (nNOS) enzymatic activity. In null mutant mice lacking nNOS gene (nNOS−/−), the effects of NR2B antagonist on neurogenesis disappeared. Moreover, nitric oxide donor DETA/NONOate attenuated and nNOS inhibitor 7-nitroindazole enhanced the effect of Ro 25-6981 on NSCs proliferation. Our findings suggest that NR2B-containing NMDAR subtypes negatively regulate neurogenesis in the adult hippocampus by activating nNOS activity and thereby hinder the formation of spatial memory.     

NMDA receptor dysfunction contributes to impaired brain‐derived neurotrophic factor‐induced facilitation of hippocampal synaptic transmission in a Tau transgenic model

While the spatiotemporal development of Tau pathology has been correlated with occurrence of cognitive deficits in Alzheimer's patients, mechanisms underlying these deficits remain unclear. Both brain‐derived neurotrophic factor (BDNF) and its tyrosine kinase receptor TrkB play a critical role in hippocampus‐dependent synaptic plasticity and memory. When applied on hippocampal slices, BDNF is able to enhance AMPA receptor‐dependent hippocampal basal synaptic transmission through a mechanism involving TrkB and N‐methyl‐d‐Aspartate receptors (NMDAR). Using THY‐Tau22 transgenic mice, we demonstrated that hippocampal Tau pathology is associated with loss of synaptic enhancement normally induced by exogenous BDNF. This defective response was concomitant to significant memory impairments. We show here that loss of BDNF response was due to impaired NMDAR function. Indeed, we observed a significant reduction of NMDA‐induced field excitatory postsynaptic potential depression in the hippocampus of Tau mice together with a reduced phosphorylation of NR2B at the Y1472, known to be critical for NMDAR function. Interestingly, we found that both NR2B and Src, one of the NR2B main kinases, interact with Tau and are mislocalized to the insoluble protein fraction rich in pathological Tau species. Defective response to BDNF was thus likely related to abnormal interaction of Src and NR2B with Tau in THY‐Tau22 animals. These are the first data demonstrating a relationship between Tau pathology and synaptic effects of BDNF and supporting a contribution of defective BDNF response and impaired NMDAR function to the cognitive deficits associated with Tauopathies.     

Protective up-regulation of CK2 by mutant huntingtin in cells co-expressing NMDA receptors

Huntington's disease is caused by a polyglutamine expansion in the huntingtin (htt) protein, and previous data indicate that over-activation of NMDA receptors (NMDARs) may be involved in the selective degeneration of cells expressing NR1/NR2B NMDARs. We used Kinetworks™ multi-immunoblotting screens to examine expression of 76 protein kinases, 18 protein phosphatases, 25 heat shock/stress proteins, and 27 apoptosis proteins in human embryonic kidney 293 cells transfected with NR1/NR2B and htt containing 15 (htt-15Q; wild-type) or 138 (htt-138Q; mutant) glutamine repeats. Follow-up experiments revealed several proteins involved in the heat-shock response pathway to be up-regulated in the soluble fraction from cells expressing htt-138Q, including protein phosphatase 5 and cyclin-dependent kinase 5. Increased expression in the soluble fraction of htt-138Q-expressing cells was also noted for the stress- and calcium-activated protein-serine/threonine kinase casein kinase 2, a change which was confirmed in striatal tissue of yeast artificial chromosome transgenic mice expressing full-length mutant htt. Inhibition of casein kinase 2 activity in cultured striatal neurons from these mice significantly exacerbated NMDAR-mediated toxicity, as assessed by labeling of apoptotic nuclei. Our findings are consistent with up-regulation of components of the stress response pathway in the presence of polyglutamine-expanded htt and NR1/NR2B which may reflect an attempt at the cellular level to ameliorate the detrimental effects of mutant htt expression.     

Exposure to an enriched environment selectively increases the functional response of the pre-synaptic NMDA receptors which modulate noradrenaline release in mouse hippocampus

We evaluated the impact of environmental training on the functions of pre-synaptic glutamatergic NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and nicotinic receptors expressed by hippocampal noradrenergic nerve terminals. Synaptosomes isolated from the hippocampi of mice housed in enriched (EE) or standard (SE) environment were labeled with [³H]noradrenaline ([³H]NA) and tritium release was monitored during exposure in superfusion to NMDA, AMPA, epibatidine or high K⁺. NMDA -evoked [³H]NA release from EE hippocampal synaptosomes was significantly higher than that from SE synaptosomes, while the [³H]NA overflow elicited by 100 μM AMPA, 1 μM epibatidine or (9, 15, 25 mM) KCl was unchanged. In EE mice, the apparent affinity of NMDA or glycine was unmodified, while the efficacy was significantly augmented. Sensitivity to non-selective or subtype-selective NMDA receptor antagonists (MK-801, ifenprodil and Zn²⁺ ions) was not modified in EE. Finally, the analysis of NMDA receptor subunit mRNA expression in noradrenergic cell bodies of the locus coeruleus showed that NR1, NR2A, NR2B and NR2D subunits were unchanged, while NR2C decreased significantly in EE mice as compared to SE mice. Functional up-regulation of the pre-synaptic NMDA receptors modulating NA release might contribute to the improved learning and memory found in animals exposed to an EE.     

NMDA受体在焦虑症发病机制的研究进展

伴随社会生活和工作压力的增大,常见精神类疾病焦虑症的发病率逐年攀升。焦虑症的发病机制非常复杂,迄今尚未完全阐明。本文概述了焦虑症发病机制与NMDA受体不同亚型的关系。NMDA受体主要广泛分布于脑、脊髓和周围神经系统。NR1广泛分布于中枢神经,在NR2D亚基敲除小鼠中,NR1和NR2D的相互影响可能参与了焦虑样行为。NR2A与NR2B是NMDA受体的两个重要亚基,NR2B的高选择性拮抗剂艾芬地尔在小鼠的高架十字迷宫实验中发挥了抗焦虑功效。将小鼠全脑的NR2C基因用NR2B替代之后,1月龄变异小鼠的高架十字迷宫实验显示有明显的非条件性焦虑行为,表明NR2B和NR2C均可能参与焦虑的发生。因此,深入阐明调控NMDA受体亚基组成的确切作用机制,将有助于探索焦虑症潜在治疗靶点的发现,并针对性地开展新药的研发。     

N-Methyl-d-Aspartate Receptor Proteins NR2A and NR2B Are Differentially Distributed in the Developing Rat Central Nervous System as Revealed by Subunit-Specific Antibodies

Abstract: We have identified the regional distributions and developmental expression of NMDA-receptor proteins NR2A and NR2B in rat CNS, using two subunit-specific affinity-purified polyclonal antibodies that recognize NR2A and NR2B. In western blots of cells transfected with NR2A or NR2B cDNAs, and of brain homogenates, each antibody detects a single predominant 172-kDa protein corresponding to its homologous subunit. Both subunits are glycoproteins that are enriched in synaptic membranes. In adult rat CNS, NR2A and NR2B are enriched in cortex and hippocampus but are present in other forebrain regions. In hindbrain, NR2A is present at low levels but NR2B is barely detectable. These subunits are differentially expressed in postnatal CNS development. In cortex and striatum, NR2A is absent at birth but expression increases thereafter, whereas NR2B is expressed at nearly adult levels during forebrain development. In hindbrain, low levels of NR2A are present throughout development, whereas NR2B is expressed only transiently in the first postnatal weeks. These results suggest that native NMDA receptors are modulated by NR2A and NR2B in adult forebrain but not appreciably in hindbrain. In contrast, during early postnatal development, NR2B may have a more dominant role than NR2A in modulating NMDA receptors throughout the CNS. Thus, transient changes in NMDA-receptor function may occur during maturation of certain neuronal and/or glial populations via differential expression of NR2A and NR2B subunits.     

Biochemical Evidence for the Existence of a Pool of Unassembled C2 Exon-Containing NR1 Subunits of the Mammalian Forebrain NMDA Receptor

Abstract: Optimum conditions were determined for the solubilisation of native NMDA receptors of adult mammalian brain with the retention of [³H]MK-801 radioligand binding activity. The most efficient conditions were 1% Triton X-100/1 M NaCl. The efficiency of solubilisation was as follows: cloned NMDA receptors expressed in mammalian cells > forebrain receptors > cerebellar receptors. Triton X-100/1 M NaCl-solubilised forebrain NMDA receptors had a molecular size of 710,000 daltons, but significant NR1 immunoreactivity (41%) migrated as a monomer of 125,000 daltons. Immunoaffinity purification of NMDA receptors from forebrain by anti-NR1 911–920 antibody affinity chromatography from 1% Triton X-100/1 M NaCl solubilised extracts yielded purification of the NR1 M_r 120,000 immunoreactive species, but no detectable NR2A or NR2B immunoreactivity. Immunoprecipitation of NMDA receptors from Triton X-100/1 M NaCl extracts with anti-NR1 911–920 antibodies also resulted in precipitation of NR1 subunits, but with no detectable NR2A or NR2B subunits. In contrast, by immunoprecipitation with anti-NR1 17–35 antibodies, which recognise all forms of NR1, NR1, NR2A, and NR2B immunoreactivities were detected in the immune pellets. Similarly, a coassociation of NR1, NR2A, and NR2B subunits was demonstrated following extraction of forebrain membranes with 1% sodium deoxycholate (pH 9) and purification by anti-NR1 911–920 antibody affinity chromatography. These results are consistent with the identification of a pool of unassembled C2 exon-containing NR1 subunits, i.e., NR1-1a, NR1-1b, NR1-2a, and NR1-2b, selectively solubilised by 1% Triton X-100/1 M NaCl.     

Molecular Dissection of Native Mammalian Forebrain NMDA Receptors Containing the NR1 C2 Exon: Direct Demonstration of NMDA Receptors Comprising NR1, NR2A, and NR2B Subunits Within the Same Complex

Abstract: The subunit compositions of the NR1 C2 exon-containing N -methyl- d -aspartate (NMDA) receptors of adult mammalian forebrain were determined by using a combination of immunoaffinity chromatography and immunoprecipitation studies with NMDA receptor subunit-specific antibodies. NMDA receptors were solubilised by sodium deoxycholate, pH 9, and purified by anti-NR1 C2 antibody affinity chromatography. The purified receptor subpopulation showed immunoreactivity with anti-NR1 C2, anti-NR1 N1, anti-NR1 C2', anti-NR2A, and anti-NR2B NMDA receptor antibodies. The NR1 C2-receptor subpopulation was subjected to immunoprecipitation using anti-NR2B antibodies and the resultant immune pellets analysed by immunoblotting where anti-NR1 C2, anti-NR1 C2', anti-NR2A, and anti-NR2B immunoreactivities were all found. Quantification of the immunoblots showed that 46% of the NR1 C2 immunoreactivity was associated with the NR2B subunit. Of this, 87% (i.e., 40% of total) were NR1 C2/NR2B receptors and 13% (6% of total) were NR1 C2/NR2A/NR2B, thus identifying the triple combination as a minor receptor subset. These results demonstrate directly, for the first time, the coexistence of the NR2A and NR2B subunits in native NMDA receptors. They show the coexistence of two splice forms of the NR1 subunit, i.e., NR1 C2 and NR1 C2', in native receptors and, in addition, they imply an NMDA receptor subpopulation containing four types of NMDA receptor subunit, NR1 C2, NR1 C2', NR2A, and NR2B, which, in accord with molecular size determinations, predicts that the NMDA receptor is at least tetrameric. These results are the first quantitative study of NMDA receptor subtypes and demonstrate molecular heterogeneity for both the NR1 and the NR2 subunits in native forebrain NMDA receptors.     

Developmental regulation of NMDA receptor 2B subunit mRNA and ifenprodil binding in the zebra finch anterior forebrain

In passerine songbirds, song learning often is restricted to an early sensitive period and requires the participation of several discrete regions within the anterior forebrain. Activation of N-methyl-D-aspartate (NMDA) receptors is implicated in song learning and in one forebrain song region, the lateral magnocellular nucleus of the anterior neostriatum (IMAN), NMDA receptors decrease in density, their affinity for the antagonist MK-801 increases, and their currents decay more quickly as young male zebra finches lose the ability to imitate new song elements. These developmental changes in NMDA receptor pharmacology and physiology suggest that the subunit composition of NMDA receptors changes developmentally. Here, we have used in situ hybridization and [3H]ifenprodil receptor autoradiography to study the developmental regulation of the NMDA receptor 2B subunit (NR2B) within the anterior forebrain of male zebra finches. NR2B mRNA expression within the IMAN was twice as great in 30-day-old males (early in the sensitive period for song learning) as in adult males, and this developmental decrease in NR2B mRNA expression was mirrored by a decrease in high-affinity (NR2B-associated) [3H]ifenprodil binding within this song region. In another anterior forebrain song region, Area X, NR2B mRNA also declined significantly after 30 days posthatch, but this decline was not accompanied by a significant decrease in [3H]ifenprodil binding. The results are consistent with the hypothesis that developmental changes in NMDA receptor function mediated by regulation of subunit composition contribute to the sensitive period for vocal learning in birds.     

Cholinergic cells in the nucleus basalis of mice express the N-methyl-D-aspartate-receptor subunit NR2C and its replacement by the NR2B subunit enhances frontal and amygdaloid acetylcholine levels

It is known that glutamatergic and cholinergic systems interact functionally at the level of the cholinergic basal forebrain. The N-methyl-d-aspartate receptor (NMDA-R) is a multiprotein complex composed of NR1, NR2 and/or NR3 subunits. The subunit composition of NMDA-R of cholinergic cells in the nucleus basalis has not yet been investigated. Here, by means of choline acetyl transferase and NR2B or NR2C double staining, we demonstrate that mice express both the NR2C and NR2B subunits in nucleus basalis cholinergic cells. We generated NR2C-2B mutant mice in which an insertion of NR2B cDNA into the gene locus of the NR2C gene replaced NR2C by NR2B expression throughout the brain. This NR2C-2B mutant was used to examine whether a subunit exchange in cholinergic neurons would affect acetylcholine (ACh) content in several brain structures. We found increased ACh levels in the frontal cortex and amygdala in the brains of NR2C-2B mutant mice. Brain ACh has been implicated in neuroplasticity, novelty-induced arousal and encoding of novel stimuli. We therefore assessed behavioral habituation to novel environments and objects as well as object recognition in NR2C-2B subunit exchange mice. The behavioral analysis did not indicate any gross behavioral alteration in the mutant mice compared with the wildtype mice. Our results show that the NR2C by NR2B subunit exchange in mice affects ACh content in two target areas of the nucleus basalis.     

Decades-long social memory in bottlenose dolphins

Long-term social memory is important, because it is an ecologically relevant test of cognitive capacity, it helps us understand which social relationships are remembered and it relates two seemingly disparate disciplines: cognition and sociality. For dolphins, long-term memory for conspecifics could help assess social threats as well as potential social or hunting alliances in a very fluid and complex fission–fusion social system, yet we have no idea how long dolphins can remember each other. Through a playback study conducted within a multi-institution dolphin breeding consortium (where animals are moved between different facilities), recognition of unfamiliar versus familiar signature whistles of former tank mates was assessed. This research shows that dolphins have the potential for lifelong memory for each other regardless of relatedness, sex or duration of association. This is, to my knowledge, the first study to show that social recognition can last for at least 20 years in a non-human species and the first large-scale study to address long-term memory in a cetacean. These results, paired with evidence from elephants and humans, provide suggestive evidence that sociality and cognition could be related, as a good memory is necessary in a fluid social system.