The hippocampus in aging and disease: From plasticity to vulnerability

The hippocampus has a pivotal role in learning and in the formation and consolidation of memory and is critically involved in the regulation of emotion, fear, anxiety, and stress. Studies of the hippocampus have been central to the study of memory in humans and in recent years, the regional specialization and organization of hippocampal functions have been elucidated in experimental models and in human neurological and psychiatric diseases. The hippocampus has long been considered a classic model for the study of neuroplasticity as many examples of synaptic plasticity such as long-term potentiation and -depression have been identified and demonstrated in hippocampal circuits. Neuroplasticity is the ability to adapt and reorganize the structure or function to internal or external stimuli and occurs at the cellular, population, network or behavioral level and is reflected in the cytological and network architecture as well as in intrinsic properties of hippocampal neurons and circuits. The high degree of hippocampal neuroplasticity might, however, be also negatively reflected in the pronounced vulnerability of the hippocampus to deleterious conditions such as ischemia, epilepsy, chronic stress, neurodegeneration and aging targeting hippocampal structure and function and leading to cognitive deficits. Considering this framework of plasticity and vulnerability, we here review basic principles of hippocampal anatomy and neuroplasticity on various levels as well as recent findings regarding the functional organization of the hippocampus in light of the regional vulnerability in Alzheimer's disease, ischemia, epilepsy, neuroinflammation and aging.

GABA(A) receptors: structure and function in the basal ganglia

gamma-Aminobutyric acid type A (GABA(A)) receptors, the major inhibitory neurotransmitter receptors responsible for fast inhibition in the basal ganglia, belong to the superfamily of "cys-cys loop" ligand-gated ion channels. GABA(A) receptors form as pentameric assemblies of subunits, with a central Cl(-) permeable pore. On binding of two GABA molecules to the extracellular receptor domain, a conformational change is induced in the oligomer and Cl(-), in most adult neurons, moves into the cell leading to an inhibitory hyperpolarization. Nineteen mammalian subunit genes have been identified, each showing distinct regional and cell-type-specific expression. The combinatorial assembly of the subunits generates considerable functional diversity. Here we place the focus on GABA(A) receptor expression in the basal ganglia: striatum, globus pallidus, substantia nigra and subthalamic nucleus, where, in addition to the standard alpha1beta2/3gamma2 receptor subtype, significant levels of other subunits (alpha2, alpha3, alpha4, gamma1, gamma3 and delta) are expressed in some nuclei.

Loss of zolpidem efficacy in the hippocampus of mice with the GABAA receptor gamma2 F77I point mutation

Zolpidem is a hypnotic benzodiazepine site agonist with some gamma-aminobutyric acid (GABA)(A) receptor subtype selectivity. Here, we have tested the effects of zolpidem on the hippocampus of gamma2 subunit (gamma2F77I) point mutant mice. Analysis of forebrain GABA(A) receptor expression with immunocytochemistry, quantitative [(3)H]muscimol and [(35)S] t-butylbicyclophosphorothionate (TBPS) autoradiography, membrane binding with [(3)H]flunitrazepam and [(3)H]muscimol, and comparison of miniature inhibitory postsynaptic current (mIPSC) parameters did not reveal any differences between homozygous gamma2I77/I77 and gamma2F77/F77 mice. However, quantitative immunoblot analysis of gamma2I77/I77 hippocampi showed some increased levels of gamma2, alpha1, alpha4 and delta subunits, suggesting that differences between strains may exist in unassembled subunit levels, but not in assembled receptors. Zolpidem (1 microm) enhanced the decay of mIPSCs in CA1 pyramidal cells of control (C57BL/6J, gamma2F77/F77) mice by approximately 60%, and peak amplitude by approximately 20% at 33-34 degrees C in vitro. The actions of zolpidem (100 nm or 1 microm) were substantially reduced in gamma2I77/I77 mice, although residual effects included a 9% increase in decay and 5% decrease in peak amplitude. Similar results were observed in CA1 stratum oriens/alveus interneurons. At network level, the effect of zolpidem (10 microm) on carbachol-induced oscillations in the CA3 area of gamma2I77/I77 mice was significantly different compared with controls. Thus, the gamma2F77I point mutation virtually abolished the actions of zolpidem on GABA(A) receptors in the hippocampus. However, some residual effects of zolpidem may involve receptors that do not contain the gamma2 subunit.

Agonistic effects of the β-carboline DMCM revealed in GABAA receptor γ2 subunit F77I point-mutated mice

Affinity of the inverse agonist methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) to the benzodiazepine binding site of the GABA(A) receptor is abolished by a phenylalanine (F) to isoleucine (I) substitution at position 77 of the gamma2 subunit. We tested the effects of DMCM in gene knockin gamma2I77 mice carrying this mutation. Unlike in wild-type mice, DMCM was not able to reverse the GABA-induced reduction of the picrotoxin-sensitive t-butylbicyclophosphoro-[35S]thionate ([35S]TBPS) binding to GABA(A) receptor channels in the forebrain sections of gamma2I77 mice. Accordingly, DMCM was not convulsant in the mutant mice even at doses 20-fold higher (60mg/kg, i.p.) than those producing convulsions in wild-type littermate controls (3 mg/kg, i.p.). Neither did DMCM raise the c-Fos levels in gamma2I77 mouse brain. DMCM additionally exhibits a less well described agonistic effect on GABA(A) receptors that is normally masked by its strong inverse agonist effect. DMCM agonistically enhanced the GABA-induced reduction in [35S]TBPS binding to the cerebellar granule cell layer in control and mutant mice. In vivo DMCM (20-60 mg/kg i.p.) produced modest anxiolytic-like effects in gamma2I77 mice as assessed by elevated plus maze and staircase tests, but no motor impairment was found in the rotarod test. The results suggest only minor agonistic efficacy for the beta-carboline DMCM.

Abolition of zolpidem sensitivity in mice with a point mutation in the GABAA receptor gamma2 subunit

Agonists of the allosteric benzodiazepine site of GABAA receptors bind at the interface of the alpha and gamma subunits. Here, we tested the in vivo contribution of the gamma2 subunit to the actions of zolpidem, an alpha1 subunit selective benzodiazepine agonist, by generating mice with a phenylalanine (F) to isoleucine (I) substitution at position 77 in the gamma2 subunit. The gamma2F77I mutation has no major effect on the expression of GABAA receptor subunits in the cerebellum. The potency of zolpidem, but not that of flurazepam, for the inhibition of [3H]flunitrazepam binding to cerebellar membranes is greatly reduced in gamma2I77/I77 mice. Zolpidem (1 microM) increased both the amplitude and decay of miniature inhibitory postsynaptic currents (mIPSCs) in Purkinje cells of control C57BL/6 (34% and 92%, respectively) and gamma2F77/F77 (20% and 84%) mice, but not in those of gamma2F77I mice. Zolpidem tartrate had no effect on exploratory activity (staircase test) or motor performance (rotarod test) in gamma2I77/I77 mice at doses up to 30 mg/kg (i.p.) that strongly sedated or impaired the control mice. Flurazepam was equally effective in enhancing mIPSCs and disrupting performance in the rotarod test in control and gamma2I77/I77 mice. These results show that the effect of zolpidem, but not flurazepam, is selectively eliminated in the brain by the gamma2F77I point mutation.

Arg3.1/Arc mRNA induction by Ca2+ and cAMP requires protein kinase A and mitogen-activated protein kinase/extracellular regulated kinase activation

Long-term potentiation (LTP) is a cellular model for persistent synaptic plasticity in the mammalian brain. Like several forms of memory, long-lasting LTP requires cAMP-mediated activation of protein kinase A (PKA) and is dependent on gene transcription. Consequently, activity-dependent genes such as c-fos that contain cAMP response elements (CREs) in their 5' regulatory region have been studied intensely. More recently, arg3.1/arc became of interest, because after synaptic stimulation, arg3.1/arc mRNA is rapidly induced and distributed to dendritic processes and may be locally translated there to facilitate synapse-specific modifications. However, to date nothing is known about the signaling mechanisms involved in the induction of this gene. Here we report that arg3.1/arc is robustly induced with LTP stimulation even at intensities that are not sufficient to activate c-fos expression. Unlike c-fos, the 5' regulatory region of arg3.1/arc does not contain a CRE consensus sequence and arg3.1/arc is unresponsive to cAMP in NIH3T3 and Neuro2a cells. However, in PC12 cells and primary cultures of hippocampal neurons, arg3.1/arc can be induced by cAMP and calcium. This induction requires the activity of PKA and mitogen-activated protein kinase, suggesting a neuron-specific pathway for the activation of arg3.1/arc expression.

The polo-like protein kinases Fnk and Snk associate with a Ca(2+)- and integrin-binding protein and are regulated dynamically with synaptic plasticity

In order to stabilize changes in synaptic strength, neurons activate a program of gene expression that results in alterations of their molecular composition and structure. Here we demonstrate that Fnk and Snk, two members of the polo family of cell cycle associated kinases, are co-opted by the brain to serve in this program. Stimuli that produce synaptic plasticity, including those that evoke long-term potentiation (LTP), dramatically increase levels of both kinase mRNAs. Induced Fnk and Snk proteins are targeted to the dendrites of activated neurons, suggesting that they mediate phosphorylation of proteins in this compartment. Moreover, a conserved C-terminal domain in these kinases is shown to interact specifically with Cib, a Ca(2+)- and integrin-binding protein. Together, these studies suggest a novel signal transduction mechanism in the stabilization of long-term synaptic plasticity.

Minimizing inter city transportation risk--a matter of perspective

Car, aeroplane and train travel have different risk characteristics that influence the safety ranking between them. The ranking depends on the range of distances compared. It also depends on what we mean by risk. Three reasonable interpretations of risk are presented. They are applied to Swedish transportation statistics. The results reveal that there is a safety niche for each of the three alternatives. That is, car, aeroplane and train each have a certain claim to being the safest means of transportation.

Nutritional response in a Drosophila yolk protein gene promoter

The yolk protein genes (yps) of Drosophila melanogaster are only expressed in the ovary and fat body of female flies if they are supplied with proteinaceous food. This nutritional response is specific to the yp genes. We have used transgenic flies transformed with a series of constructs bearing deletions in the upstream region of the yp1 and yp2 genes attached to a reporter gene to search for DNA sequences responsible for the nutritional induction specific for yp1 and yp2 genes. Several regions were shown independently of each other to confer nutritional regulation on the expression of the yp1 and yp2 genes. This regulation can be induced both on the yp promoter and the heterologous Drosophila heat-shock 70 (hsp 70) promoter. The redundancy of sequences conferring a nutritional response on the yp genes is similar to that observed for the female specificity of these genes and suggest that several DNA binding proteins interact to provide the correct regulation of these genes. These results suggest that nutrition acts to modify the level of a trans-acting factor in the fat body. Northern blot analysis showed that the transcript levels from the dsx gene are not affected by nutrition, indicating that the response is not mediated via the dsx gene.