Impaired representation of space in the hippocampus of GluR-A knockout mice

To investigate the role of AMPAR-mediated synaptic transmission in the placespecific firing of the hippocampal CA1 pyramidal cells, we have applied multiple tetrode recording techniques in freely behaving mice with a complete knockout of the GluR-A gene. We have found that place-specific activity of the neurons was significantly impaired in the GluR-A KO mice. The pyramidal cells in GluR-A KO mice formed firing (place)fields, that were: significantly larger (160-200%), less location selective (73%), less direction selective (39%), less stable (53%) and carried less information about the animal’s position (47%) than those cells studied in wild-type mice. Despite the observed positive correlation between the firing field’s properties and the degree of cognitive demands of three employed behavioral paradigms, the spatial firing defects were consistent across the paradigms independent from their complexity. We have also found that deletion of the GluR-A gene slightly reduced (5%) the frequency of the hippocampal theta rhythm, but did not affect relative timing of the pyramidal cell spikes in respect to the theta rhythm. These results demonstrate that GluR-A-containing AMPA receptors are necessary for the normal representation of space in the CA1 region of the hippocampus, which might be necessary for the flexible working memory system

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory

Long Homer proteins forge assemblies of signaling components involved in glutamate receptor signaling in postsynaptic excitatory neurons, including those underlying synaptic transmission and plasticity. The short immediate-early gene (IEG) Homer1a can dynamically uncouple these physical associations by functional competition with long Homer isoforms. To examine the consequences of Homer1a-mediated “uncoupling” for synaptic plasticity and behavior, we generated forebrain-specific tetracycline (tet) controlled expression of Venus-tagged Homer1a (H1aV) in mice. We report that sustained overexpression of H1aV impaired spatial working but not reference memory. Most notably, a similar impairment was observed when H1aV expression was restricted to the dorsal hippocampus (HP), which identifies this structure as the principal cortical area for spatial working memory. Interestingly, H1aV overexpression also abolished maintenance of CA3-CA1 long-term potentiation (LTP). These impairments, generated by sustained high Homer1a levels, identify a requirement for long Homer forms in synaptic plasticity and temporal encoding of spatial memory

Speed Controls the Amplitude and Timing of the Hippocampal Gamma Rhythm

Cortical and hippocampal gamma oscillations have been implicated in many behavioral tasks. The hippocampus is required for spatial navigation where animals run at varying speeds. Hence we tested the hypothesis that the gamma rhythm could encode the running speed of mice. We found that the amplitude of slow (20–45 Hz) and fast (45–120 Hz) gamma rhythms in the hippocampal local field potential (LFP) increased with running speed. The speed-dependence of gamma amplitude was restricted to a narrow range of theta phases where gamma amplitude was maximal, called the preferred theta phase of gamma. The preferred phase of slow gamma precessed to lower values with increasing running speed. While maximal fast and slow gamma occurred at coincident phases of theta at low speeds, they became progressively more theta-phase separated with increasing speed. These results demonstrate a novel influence of speed on the amplitude and timing of the hippocampal gamma rhythm which could contribute to learning of temporal sequences and navigation

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-3

Of two runs. During the first (sample) run (upper panel) the mouse is forced to choose one of the two target arms by blocking the other arm. In the second (choice) run (lower panel), it is rewarded if it chooses the previously unvisited arm. () Wild-type ( = 9) but not TgH1aV.Fb ( = 12) learned the task. TgH1aV.Fb mice, however, showed the natural preference of rodents to visit the previously unvisited arm on this task, as shown by significantly better than chance level (50%) of spontaneous alternation during the first training block. () The success rate of wild-type increased over the course of the training. TgH1aV.Fb mice, however, started and finished the training at the same success rate without any significant learning throughout the training. () To question if the working memory deficit can be recovered upon reversal of the transgenic expression in the adult animal, we tested the wild-type ( = 7) and TgH1aV.Fb mice ( = 7) after giving dox for 4 weeks, starting 4 months after birth, and tested them on the T-maze. After five blocks of training, wild-type but not TgH1aV.Fb mice learned the task. () To study if the Homer1a expression in hippocampus alone could cause the working memory deficit, we virally expressed H1aV in hippocampi of adult wild-type mice for 4 weeks before testing the mice on the T-maze. Mice injected with an empty viral construct ( = 6) learned the task just like the uninfected wild-types (gray bars). Mice expressing H1aV in the hippocampus ( = 7, black bars), however, failed to learn the task, even after five blocks of training.<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-0

Promoter directs tTA expression in forebrain principal neurons (Mayford et al., ). The luciferase (luc) and H1aV genes are transcribed from a bidirectional unit activated upon binding of tTA to heptamerized tet operator sequences (tetO) flanked by CMV minimal promoters. Dox prevents tTA binding to tetO, thus switching off expression of luc and H1aV transgenes. () Screening of transgenic founders for dox-dependent gene regulation. Relative light units corresponding to ratios of firefly to renilla luciferase (rlu-FL/rlu-RL) activity, measured in mouse ear fibroblast cell cultures in absence (black) and presence (white) of dox (Hasan et al., ), are plotted logarithmically on the Y-axis. Circles (solid and open) indicate transgenic lines selected for crossing with Tgα-CaMKII-tTA mice. The open circle indicates the line #14 used in this study. () Para-saggital brain section of a P42 mouse positive for both transgenes (TgH1aV.Fb), in absence of dox. Robust Venus epi-fluorescence was detected in cortical and subcortical regions, especially in the striatum (STR) and the hippocampal formation (HP). Boxed regions (CA1, CA3) represent areas for quantitative H1aV expression analysis. Inset upper left: high-power confocal image of the boxed regions in CA1 and CA3. Epi-fluorescence is robust in CA1 pyramidal cell bodies and dendritic trees but sparse in CA3. Note the intense fluorescence of mossy fiber axons. Inset lower right: proximal dendritic shaft segment from a CA1 neuron after deconvolution, with the fusion-protein seen in spines. () Overexpressed H1aV fusion protein has similar binding properties as endogenous Homer1a. Representative IP-Western blot from forebrain lysates of TgH1aV.Fb (N = 2) and wild-type (N = 2) P42 mice. (1), Immunoprecipitation with anti-GFP antibody pulled mGluR1 from lysates of TgH1aV.Fb mice, indicating binding of H1aV to mGluR1. (2), mGluR1 was also detected with Homer1a-specific antibody. (3, 4), No association with mGluR1 was detected with antibodies against Homer2 or Homer1b/c, indicating efficient competition by overexpressed H1aV. (5), In brains of wild-type mice the GFP antibody did not pull mGluR1. (6, 7), Moderate binding was seen with antibodies against Homer1a and Homer2 in lysates of wild-type brains. (8), Antibody against Homer1b/c robustly pulled mGluR1 from lysates of wild-type brains, indicating strong association. P38 was used to show equal overall levels of protein in the lysates (OB, olfactory bulb; CTX, cerebral cortex; STR, striatum; HP, hippocampus; MW., molecular weight).<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-4

Mm from bregma) of a 19 week-old mouse after three weeks on dox (0.1 g/l of drinking water), demonstrating absence of GFP-enhanced H1aV fluorescence. White arrowheads point at the position of the CA1 pyramidal cell layer. − + dox indicate the chronology of dox application. (, ) Comparable sections as in and , showing robust re-induction of GFP-enhanced H1aV fluorescence 3 weeks after dox withdrawal. CA1, CA2, and CA3 mark the three morphologically distinct subregions of the hippocampal pyramidal layer. Zero millimeter indicates the position from the bregma. − + − dox explains the chronology of dox application. (, ) Magnification of the CA1 subregion shown in , and . Strong GFP-enhanced H1aV fluorescence was detected in both, the pyramidal cell bodies of the stratum pyramidale and in the dendrites of the stratum radiatum. () Schematic representation of the transgenes driving H1aV expression in the mouse forebrain. The luc and H1aV genes are expressed from a bidirectional transcription unit (upper and lower chart) activated by binding of tTa, expressed under the control of the mouse αCaMKII promoter (middle chart), to the heptamerized tet operator sequences flanked by CMV minimal promoters. Dox application can shut off transgene expression (, , upper chart), which can be re-induced by withdrawal of dox (–, lower chart).<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-6

O spatial landmarks in the room, a checkerboard pattern was placed in front of the target arm. () TgH1aV.Fb mice ( = 12) did not differ from wild-type ( = 9) during task acquisition and learned the task at similar speed and rate as the wild-type. () Both genotypes reached an asymptotic level of performance that was significantly better than their respective performances during the first training block. () After a 5 weeks break from training, the mice were retested in retrieval sessions. Both genotypes showed similar reduction in performance between the last testing block of the acquisition phase and the first block of the retrieval. () After the mice relearned the task, they were tested in a new room to control for the spatial cue dependence of the mouse behavior. The maze was arranged in the same orientation along the poles, but this time had new landmark cues in the new location. Success rate in the new room was at chance level for both genotypes.<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-1

Ng (TBP) induced LTP in hippocampal CA3-CA1 synapses of wild-type and TgH1aV.Fb mice. In wild-type mice, three TBP trains resulted in LTP in the paired pathway. TBP in slices from TgH1aV.Fb mice resulted in a transient potentiation of synaptic responses, lasting up to 30 minutes postpairing (red circles). TgH1aV.Fb mice completely lack the late component of TBP-induced potentiation found in wild-type mice (black squares). () The same protocol was applied to TgH1aV.Fb mice given dox in the drinking water from P21 until P42. Dox-treated mice (black traces), when compared to untreated ones (red traces), showed robust LTP (black squares), indicating rescue from sustained H1aV overexpression. () Fluorescent cells in hippocampal CA1 region 18 days after stereotaxic injection of lentivirus expressing either wild-type or mutant rat H1aV. LTP induced in hippocampal CA3-CA1 synapses of mice injected with lentivirus expressing H1aV () resulted in a transient potentiation of synaptic responses, whereas H1aV(W234A) expression showed normal LTP of the paired pathway (). Schematic drawing of the lentiviral vectors illustrates LTP profiles after α-CaMKII promoter-dependent transcription of wild-type () and mutant () H1aV. The amino acid substitution in the Homer1a EVH1 domain is indicated below the expression cassette for Homer1a.<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p

Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory-7

Ion of the food reward with one of the two sandpaper samples, before being tested in the long-term retention phase of the experiment. () Both wild-type ( = 7) and TgH1aV.Fb mice ( = 7) learned the task in less than 60 trials at an asymptotic performance level. () The overall success level across the experimental groups was not different at the start (block #1) and end (block #9) of the training. () After all mice reached 100% success rate, they were retested 3 days, 1 week, or 3 weeks after the end of the acquisition phase of the training. Neither group significantly differed from each other at any time point.<p><b>Copyright information:</b></p><p>Taken from "Select Overexpression of Homer1a in Dorsal Hippocampus Impairs Spatial Working Memory"</p><p></p><p>Frontiers in Neuroscience 2007;1(1):97-110.</p><p>Published online Jan 2007</p><p>PMCID:PMC2518050.</p><p></p