Mechanism based approaches for rescuing and enhancing cognition

Abstract

Progress toward pharmacological means for enhancing memory and cognition has been retarded by the widely discussed failure of behavioral studies in animals to predict human outcomes. As a result, a number of groups have targeted cognition-related neurobiological mechanisms in animal models, with the assumption that these basic processes are highly conserved across mammals. Here we survey one such approach that begins with a form of synaptic plasticity intimately related to memory encoding in animals and likely operative in humans. An initial section will describe a detailed hypothesis concerning the signaling and structural events (a "substrate map") that convert learning associated patterns of afferent activity into extremely stable increases in fast, excitatory transmission. We next describe results suggesting that all instances of intellectual impairment so far tested in rodent models involve a common endpoint failure in the substrate map. This will be followed by a clinically plausible proposal for obviating the ultimate defect in these models. We then take up the question of whether it is reasonable to expect, from either general principles or a very limited set of experimental results, that enhancing memory will expand the cognitive capabilities of high functioning brains. The final section makes several suggestions about how to improve translation of behavioral results from animals to humans. Collectively, the material covered here points to the following: (1) enhancement, in the sense of rescue, is not an unrealistic possibility for a broad array of neuropsychiatric disorders; (2) serendipity aside, developing means for improving memory in normals will likely require integration of information about mechanisms with new behavioral testing strategies; (3) a shift in emphasis from synapses to networks is a next, logical step in the evolution of the cognition enhancement field.

Keywords: LTP; ampakine; animal models; cognitive enhancement; cytoskeleton; learning; long-term potentiation; spaced trials.

Figure 1

Cortical areas expand disproportionately with increases in brain size. Plot shows the relative size of the midbrain (monkeys, apes, human) and frontal cortical area 10 (apes and human, only) plotted as a function of overall brain size. As shown, the slope for midbrain is relatively flat: with a slope of <1 the midbrain occupies a progressively smaller proportion of brain volume as brain sizes increase. In contrast, with a slope of >1, Area 10 grows disproportionately to overall volume with increases in brain volume (Area 10 results from Semendeferi et al., 2001).

Figure 2

Characteristics of long-term potentiation (LTP). Stimulation was applied to two populations of Schaffer-commissural afferents converging on the stratum radiatum of field CA1b. Theta burst stimulation (TBS) delivered to one input (filled circles) causes an immediate increase in the size of the field EPSP which then decays over about 10 min to a plateau at which the responses are 50–60% elevated above the pre-TBS baseline. The potentiation persists unchanged for the duration of the recording session (nearly 5 h in the illustrated case). Note that the second (control) input, which received only 3/min stimulation pulses, was not affected by potentiation of neighboring contacts (open circles). Thus, LTP has the synapse specificity expected for a memory substrate. As with memory encoding, LTP is initially unstable and readily erased by a number of treatments (e.g., 5 Hz stimulation) but then becomes steadily more resistant to disruption. Experimental work indicates that this process involves multiple stages. A rapid phase (“consolidation 1”) has been linked to reorganization of the sub-synaptic cytoskeleton over the 10 min following TBS; this is followed at about 1 h by a newly discovered stabilization event (“consolidation 2”) involving synaptic adhesion receptors. There is also considerable evidence for a still later step that depends on protein synthesis (“consolidation 3”).

Figure 3

Theta burst afferent stimulation (TBS) causes actin polymerization in dendritic spines. Fluorescent-tagged phalloidin was infused into living hippocampal slices to label filamentous (F) actin; after harvest the slices were sectioned for epifluorescence microscopy. (A,B) Images at left show that spine-labeling (white puncta) is very low in a control slice but prominent following TBS. At higher magnification (at right) labeled puncta can be seen to be spines decorating a faintly labeled dendrite. (C,D) Combining phalloidin labeling with green fluorescent protein expression allows confocal visualization of phalloidin-labeled puncta within the heads of clearly defined spines; panels show opaque 3D build of a single spine (C) and a semi-transparent rendering showing the phalloidin-labeled aggregate within the spine's boundaries (D). (E) Quantification of densely phalloidin-labeled spines at different time points following TBS shows that numbers are significantly increased, relative to values in control (cont.) slices, as early as 2 min post-TBS and remain elevated through 60 min after stimulation (p > 0.001 for 2–60 min vs. cont). Modified from Kramar et al. (2012b).

Figure 4

ß1 integrins are required for TBS-induced LTP and increases in spine F-actin. Left: Plot shows that potentiation of the Schaffer-commissural projection to field CA1 is induced with TBS applied in the presence of the control anti-rat IgG but is blocked with local infusion of neutralizing antisera to ß1 integrin: note that with ß1 neutralization there is an initial post-TBS potentiation but the enhanced response rapidly declines to baseline indicating a failure in consolidation. Right: Quantification of spines containing dense F-actin in the CA1 field of afferent stimulation (from in situ phalloidin labeling). As shown, TBS elicits a large increase in spine F-actin if applied alone or in the presence of anti-rat IgG but this effect is totally blocked by neutralizing anti-ß1 (^**p < 0.001).

Figure 5

Hypothesis regarding the links between theta burst stimulation and the cytoskeletal changes underlying rapid consolidation of LTP. The model uses three groups of transmembrane receptors (in blue) for the following: (1) “modifiers” including adenosine (A1), estrogen (ERB), and BDNF (TrkB); (2) released neurotransmitter (Glut); and (3) adhesion proteins (integrins). The last of these, working in conjunction with the modifier group, signal through guanine exchange factors (Gefs) to the small GTPases (violet) which, in turn, activate downstream intermediaries (green) leading to actin regulatory proteins (red) that ultimately control the activity-driven assembly and subsequent stabilization of actin filaments. There is evidence that the RhoA-ROCK-Cofilin path controls F-actin assembly whereas Rac and Ras signaling, including convergence on cortactin, is thought to mediate the stabilization and elaboration of the actin network.

Figure 6

In vivo ampakine treatments rescue TBS-induced spine actin polymerization and LTP in a mouse model of Angelman syndrome. Wild type (WT) mice and mutants with a knockout (KO) of the maternal Ube3a gene (AS mice) were treated with the ampakine CX929 or vehicle daily for 5 days prior to the preparation of acute hippocampal slices. (A) In situ phalloidin labeling was used to assess effects of genotype and treatment on the marked increase in spine F-actin that normally follows TBS. Images and quantification of F-actin rich (phalloidin-labeled) spines show that TBS does not increase the number of such spines in slices prepared from KOs treated with vehicle (veh) relative to counts from slices receiving control (cnt) low frequency stimulation (LFS) only. In contrast, the same stimulation applied to slices from KO mice previously treated with CX929 in vivo (lower right image) elicits a striking increase in F-actin rich spines that is comparable to that obtained in WTs (lower left image). (B) Plots of fEPSP responses show that TBS (arrow) elicits initial potentiation in all groups but the effect slowly decays back to baseline levels in vehicle-treated Ube3a mutants (ube3a veh). However, slices from mutants given CX929 in vivo (ube3a cx929) exhibit the conventional LTP effect. (C) Bar graph shows that context fear conditioning is also rescued in the KOs treated with CX929 (^**p < 0.01 vs. KO veh). Adapted from Baudry et al. (2012).

Figure 7

Spaced stimulation augments LTP but only with delays of about 1 h. Plots show effects of a first (TBS1) and second (TBS2) round of theta stimulation on fEPSP responses. (A) TBS1 (black arrow) reliably increases fEPSPs by about 50% above baseline whereas TBS2 (red arrow) applied 10 (left), 30 (middle) and 40 (right) min later does not augment the level of potentiation. (B) TBS2 applied 60 min after TBS1 doubles the level of potentiation. (C) TBS3 further augments potentiation if delayed by 60 min whereas a similarly delayed TBS4 has little effect suggesting that potentiation approaches ceiling levels after 3 spaced theta trains. Modified from Kramar et al. (2012b).

Figure 8

Enhancing LTP1 blocks further increases in potentiation with a second round of TBS. Theta burst stimulation (TBS) was applied to the Schaffer-commissural afferents to field CA1b in adult rat hippocampal slices. Left: Counts of densely phalloidin-labeled spines in the CA1 field of afferent stimulation show that in the presence of the ampakine CX614 (drug), one round of theta burst stimulation (tbs1) significantly increases the numbers of spines containing this marker of potentiation above that induced by TBS alone (^**p < 0.01). Right: Plot of field EPSPs shows that TBS1 (black arrow) applied in the presence of ampakine infusion (black bar) elicits an LTP effect that is about twice as large as that produced in the absence of the drug (indicated by dashed line) but a second TBS bout (TBS2, red arrow) elicits no further potentiation (in contrast to effects of TBS2 applied under control conditions; see Figure 7B). Modified from Kramar et al. (2012b).