Eye opening rapidly induces synaptic potentiation and refinement

NMDA receptor (NMDAR)-mediated increases in AMPA receptor (AMPAR) currents are associated with long-term synaptic potentiation (LTP). Here, we provide evidence that similar changes occur in response to normal increases in sensory stimulation during development. Experiments discriminated between eye opening-induced and age-dependent changes in synaptic currents. At 6 hr after eye opening (AEO), a transient population of currents mediated by NR2B-rich NMDARs increase significantly, and silent synapses peak. Sustained increases in evoked and miniature AMPAR currents occur at 12 hr AEO. Significant changes in AMPAR:NMDAR evoked current ratios, contacts per axon, and inputs per cell are present at 24 hr AEO. The AMPAR current changes are those seen in vitro during NMDAR-dependent LTP. Here, they are a consequence of eye opening and are associated with a new wave of synaptic refinement. These data also suggest that new NR2B-rich NMDAR currents precede and may initiate this developmental synaptic potentiation and functional tuning.     

A synaptic strategy for consolidation of convergent visuotopic maps

The mechanisms by which experience guides refinement of converging afferent pathways are poorly understood. We describe a vision-driven refinement of corticocollicular inputs that determines the consolidation of retinal and visual cortical (VC) synapses on individual neurons in the superficial superior colliculus (sSC). Highly refined corticocollicular terminals form 1-2 days after eye-opening (EO), accompanied by VC-dependent filopodia sprouting on proximal dendrites, and PSD-95 and VC-dependent quadrupling of functional synapses. Delayed EO eliminates synapses, corticocollicular terminals, and spines on VC-recipient dendrites. Awake recordings after EO show that VC and retina cooperate to activate sSC neurons, and VC light responses precede sSC responses within intervals promoting potentiation. Eyelid closure is associated with more protracted cortical visual responses, causing the majority of VC spikes to follow those of the colliculus. These data implicate spike-timing plasticity as a mechanism for cortical input survival, and support a cooperative strategy for retinal and cortical coinnervation of the sSC.     

The C. elegans glutamate receptor subunit NMR-1 is required for slow NMDA-activated currents that regulate reversal frequency during locomotion.

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor is important for synaptic plasticity and nervous system development and function. We have used genetic and electrophysiological methods to demonstrate that NMR-1, a Caenorhabditis elegans NMDA-type ionotropic glutamate receptor subunit, plays a role in the control of movement and foraging behavior. nmr-1 mutants show a lower probability of switching from forward to backward movement and a reduced ability to navigate a complex environment. Electrical recordings from the interneuron AVA show that NMDA-dependent currents are selectively disrupted in nmr-1 mutants. We also show that a slowly desensitizing variant of a non-NMDA receptor can rescue the nmr-1 mutant phenotype. We propose that NMDA receptors in C. elegans provide long-lived currents that modulate the frequency of movement reversals during foraging behavior.     

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.     

Selective optical control of synaptic transmission in the subcortical visual pathway by activation of viral vector-expressed halorhodopsin

The superficial layer of the superior colliculus (sSC) receives visual inputs via two different pathways: from the retina and the primary visual cortex. However, the functional significance of each input for the operation of the sSC circuit remains to be identified. As a first step toward understanding the functional role of each of these inputs, we developed an optogenetic method to specifically suppress the synaptic transmission in the retino-tectal pathway. We introduced enhanced halorhodopsin (eNpHR), a yellow light-sensitive, membrane-targeting chloride pump, into mouse retinal ganglion cells (RGCs) by intravitreously injecting an adeno-associated virus serotype-2 vector carrying the CMV-eNpHR-EYFP construct. Several weeks after the injection, whole-cell recordings made from sSC neurons in slice preparations revealed that yellow laser illumination of the eNpHR-expressing retino-tectal axons, putatively synapsing onto the recorded cells, effectively inhibited EPSCs evoked by electrical stimulation of the optic nerve layer. We also showed that sSC spike activities elicited by visual stimulation were significantly reduced by laser illumination of the sSC in anesthetized mice. These results indicate that photo-activation of eNpHR expressed in RGC axons enables selective blockade of retino-tectal synaptic transmission. The method established here can most likely be applied to a variety of brain regions for studying the function of individual inputs to these regions.     

Zebrafish TARP Cacng2 is required for the expression and normal development of AMPA receptors at excitatory synapses

Fast excitatory synaptic transmission in the CNS is mediated by the neurotransmitter glutamate, binding to and activating AMPA receptors (AMPARs). AMPARs are known to interact with auxiliary proteins that modulate their behavior. One such family of proteins is the transmembrane AMPA receptor‐related proteins, known as TARPs. Little is known about the role of TARPs during development, or about their function in non‐mammalian organisms. Here we report the presence of TARPs, specifically the prototypical TARP, stargazin, in developing zebrafish. We find that zebrafish express two forms of stargazin, Cacng2a and Cacng2b from as early as 12‐h post fertilization (hpf). Knockdown of Cacng2a and Cacng2b via splice‐blocking morpholinos resulted in embryos that exhibited deficits in C‐start escape responses, showing reduced C‐bend angles, smaller tail velocities and aberrant C‐bend turning directions. Injection of the morphants with Cacng2a or 2b mRNA rescued the morphological phenotype and the synaptic deficits. To investigate the effect of reduced Cacng2a and 2b levels on synaptic physiology, we performed whole cell patch clamp recordings of AMPA mEPSCs from zebrafish Mauthner cells. Knockdown of Cacng2a results in reduced AMPA currents and lower mEPSC frequencies, whereas knockdown of Cacng2b displayed no significant change in mEPSC amplitude or frequency. Non‐stationary fluctuation analysis confirmed a reduction in the number of active synaptic receptors in the Cacng2a but not in the Cacng2b morphants. Together, these results suggest that Cacng2a is required for normal trafficking and function of synaptic AMPARs, while Cacng2b is largely non‐functional with respect to the development of AMPA synaptic transmission. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 487–506, 2016     

A protein kinase C site highly conserved in P2X subunits controls the desensitization kinetics of P2X(2) ATP-gated channels

P2X receptors are nonselective cation channels gated by extracellular ATP. Recombinant mammalian P2X subunits assemble in homomeric ionotropic ATP receptors that differ by their agonist sensitivity and desensitization rate in heterologous expression systems. Using site-directed mutagenesis and voltage clamp recording in Xenopus oocytes, we identified the highly conserved protein kinase C site TX(K/R) located in the intracellular N terminus of P2X subunits as a critical determinant of kinetics in slowly desensitizing (time constant, >1 min) rat P2X(2) receptors. Mutant receptors P2X(2)T18A, T18N, and K20T devoid of this consensus site exhibited quickly desensitizing properties (time constant, <1 s). In contrast with wild-type receptors, mutant P2X(2) receptors with truncated C terminus exhibited variable cell-specific kinetics with quickly desensitizing currents converted to slowly desensitizing currents by phorbol ester-mediated stimulation of protein kinase C. Phosphorylation of Thr(18) was demonstrated directly by immunodetection using specific monoclonal antibodies directed against the phosphothreonine-proline motif. Our data indicate that both phosphorylation of the conserved threonine residue in the N-terminal domain by protein kinase C and interaction between the two cytoplasmic domains of P2X(2) subunits are necessary for the full expression of slowly desensitizing ATP-gated channels.     

Flotillin‐1 promotes formation of glutamatergic synapses in hippocampal neurons

Synapse malformation underlies numerous neurodevelopmental illnesses, including autism spectrum disorders. Here we identify the lipid raft protein flotillin‐1 as a promoter of glutamatergic synapse formation. We cultured neurons from the hippocampus, a brain region important for learning and memory, and examined them at two weeks in vitro, a time period rich with synapse formation. Double‐label immunocytochemistry of native flot‐1 with glutamatergic and GABAergic synapse markers showed that flot‐1 was preferentially colocalized with the glutamatergic presynaptic marker vesicular glutamate transporter 1 (VGLUT1), compared to the GABAergic presynaptic marker glutamic acid decarboxylase‐65 (GAD‐65). Triple‐label immunocytochemistry of native flot‐1, VGLUT1, and NR1, the obligatory subunit of NMDA receptors, indicates that Flot‐1 was preferentially localized to synaptic rather than extrasynaptic NR1. Furthermore, electrophysiological results using whole‐cell patch clamp showed that Flot‐1 increased the frequency of miniature excitatory postsynaptic currents (mEPSCs) but not miniature inhibitory postsynaptic currents (mIPSCs), whereas amplitude and decay kinetics of either type of synaptic current was not affected. Corresponding immunocytochemical data confirmed that the number of glutamatergic synapses increased with flot‐1 overexpression. Overall, our anatomical and physiological results show that flot‐1 enhances the formation of glutamatergic synapses but not GABAergic synapses, suggesting that the role of flot‐1 in neurodevelopmental disorders should be explored. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 875–883, 2010     

Evidence of two populations of GABA(A) receptors in cerebellar granule cells in culture: different desensitization kinetics, pharmacology, serine/threonine kinase sensitivity, and localization

GABA(A) receptors of rat cerebellar granule cells in culture have been studied by the whole cell patch clamp technique. The biphasic desensitization kinetic observed could be due either to different desensitization mechanisms of a single receptor population or to different receptor populations. The overall data indicate that the latter hypothesis is most probably the correct one. In fact, the fast desensitizing component was selectively potentiated by a benzodiazepine agonist and preferentially down-regulated by activation of the protein serine/threonine kinases A and G, as a consequence of the latter characteristic that receptor population was preferentially down-regulated by previous activation of N-methyl-d-aspartate glutamate receptors, via production of nitric oxide and PKG activation, most probably in dendrites. The other population is benzodiazepine insensitive and not influenced by activation of PKA or PKG. This slowly desensitizing population may correspond to the extrasynaptic delta subunit containing GABA(A) receptors described by other authors. Instead, the rapidly desensitizing population appears to represent dendritic synaptic GABA(A) receptors.     

Binding-gating coupling in a nondesensitizing α7 nicotinic receptor: A single channel pharmacological study

The highly conserved αLys145 has been suggested to play an important role in the early steps of activation of the nicotinic acetylcholine receptor (nAChR) by acetylcholine. Both macroscopic and single channel currents were recorded in the slowly desensitizing mutants L248T- and K145A-L248T-α7 receptors expressed in Xenopus oocytes. On ACh-evoked currents, substitution of Lys145 by alanine showed the same effects that in wild type receptors: moderately decreased gating function and a more-than-expected loss of ACh potency, thus validating the experimental model. Single channel analysis quantitatively agreed with macroscopic data and revealed that impaired gating function in the double mutant α7K145A/L248T is the consequence of a slower opening rate, β. Several nicotinic agonists were also studied, showing important features. Particularly, dimethylphenylpiperazinium (DMPP), acting as an antagonist in α7K145A, became a full agonist in α7K145A/L248T. Single channel analysis of DMPP-evoked currents showed effects of Lys145 removal similar to those observed with ACh. Data suggest that α7Lys145 facilitates the early steps of channel activation. Moreover, the slowly desensitizing mutant α7L248T could be an interesting tool for the study of channel activation in α7 receptors. Nevertheless, its extensively altered pharmacology precludes the simple extrapolation of pharmacological data obtained in singly mutated α7 receptors.     

How fast does the gamma-aminobutyric acid receptor channel open? Kinetic investigations in the microsecond time region using a laser-pulse photolysis technique.

The gamma-aminobuytric acid(A) (GABA(A)) receptor is a membrane-bound protein that mediates signal transmission between neurons through formation of chloride ion channels. GABA is the activating ligand, which upon binding to the receptor triggers channel opening in the microsecond time domain and reversible desensitization of the receptor in the millisecond time region. We have investigated the channel-opening mechanism for this receptor in rat hippocampal neurons before the protein desensitizes by using a rapid flow method (cell-flow) with a 10 ms time resolution and a laser-pulse photolysis technique with a approximately 30 micros time resolution to determine the rate and equilibrium constants for channel opening and closing. Two different forms of the receptor, namely, a rapidly and a slowly desensitizing form, exist in the rat hippocampal cells and are characterized by their different rates for desensitization. At 250 microM GABA the rate constant for desensitization was 2.3 +/- 0.4 s(-)(1) for the rapidly desensitizing form and 0.4 +/- 0.1 s(-)(1) for the slowly desensitizing form. The dissociation constant of GABA from the site controlling channel opening was 100 +/- 40 microM for the rapidly desensitizing form and 120 +/- 60 microM for the slowly desensitizing form. The rate constants for channel closing did not differ significantly for the two forms, 85 +/- 20 s(-)(1) for the rapidly desensitizing and 100 +/- 60 s(-)(1) for the slowly desensitizing form. However, the channel-opening rate constant differed by a factor of 3, 1840 +/- 160 s(-)(1) for the rapidly desensitizing and 6700 +/- 330 s(-)(1) for the slowly desensitizing form. This difference in the rate constant for channel opening for the two forms, determined by the laser-pulse photolysis technique, is reflected as a shift in the channel-opening equilibrium constant, which is 7 +/- 5 and 20 +/- 15 for the rapidly and slowly desensitizing forms respectively, determined by the cell-flow method. These constants, together with the concentration of GABA and the concentration of receptor sites in the membrane, determine the number of channels that open as a function of GABA concentration, and the rate at which they open and close. These constants play an important role in determining the rate of the transmembrane ion flux and, therefore, the receptor-controlled changes in transmembrane voltage that trigger signal transmission.     

The function of GluR1 and GluR2 in cerebellar and hippocampal LTP and LTD is regulated by interplay of phosphorylation and O‐GlcNAc modification

Long‐term potentiation (LTP) and long‐term depression (LTD) are the current models of synaptic plasticity and widely believed to explain how different kinds of memory are stored in different brain regions. Induction of LTP and LTD in different regions of brain undoubtedly involve trafficking of AMPA receptor to and from synapses. Hippocampal LTP involves phosphorylation of GluR1 subunit of AMPA receptor and its delivery to synapse whereas; LTD is the result of dephosphorylation and endocytosis of GluR1 containing AMPA receptor. Conversely the cerebellar LTD is maintained by the phosphorylation of GluR2 which promotes receptor endocytosis while dephosphorylation of GluR2 triggers receptor expression at the cell surface and results in LTP. The interplay of phosphorylation and O‐GlcNAc modification is known as functional switch in many neuronal proteins. In this study it is hypothesized that a same phenomenon underlies as LTD and LTP switching, by predicting the potential of different Ser/Thr residues for phosphorylation, O‐GlcNAc modification and their possible interplay. We suggest the involvement of O‐GlcNAc modification of dephosphorylated GluR1 in maintaining the hippocampal LTD and that of dephosphorylated GluR2 in cerebral LTP. J. Cell. Biochem. 109: 585–597, 2010. © 2010 Wiley‐Liss, Inc.     

Synthesis and characterization of dexamethasone‐conjugated linear polyethylenimine as a gene carrier

Linear polyethylenimine (25 kDa, LPEI25k) has been shown to be an effective non‐viral gene carrier with higher transfection and lower toxicity than branched polyethylenimine (BPEI) of comparable molecular weight. In this study, dexamethasone was conjugated to LPEI25k to improve the efficiency of gene delivery. Dexamethasone is a synthetic glucocorticoid receptor ligand. Dexamethasone‐conjugated LPEI25k (LPEI–Dexa) was evaluated as a gene carrier in various cells. Gel retardation assays showed that LPEI–Dexa completely retarded plasmid DNA (pDNA) at a 0.75:1 weight ratio (LPEI/pDNA). LPEI–Dexa had the highest transfection efficiency at a 2:1 weight ratio (LPEI–Dexa/DNA). At this ratio, the size of the LPEI–Dexa/pDNA complex was approximately 125 nm and the zeta potential was 35 mV. LPEI–Dexa had higher transfection efficiency than LPEI and Lipofectamine 2000. In addition, the cytotoxicity of LPEI–Dexa was much lower than that of BPEI (25 kDa, BPEI25k). In conclusion, LPEI–Dexa has a high transfection efficiency and low toxicity and can therefore be used for non‐viral gene delivery. J. Cell. Biochem. 110: 743–751, 2010. © 2010 Wiley‐Liss, Inc.     

Role of calcineurin in activity‐dependent pattern formation in the dorsal lateral geniculate nucleus of the ferret

In the retinogeniculate pathway of the ferret, in addition to the separation of the inputs from the two eyes to form eye‐specific layers, there is also an anatomical segregation of the terminal arbors of on‐center retinal ganglion cells from the terminal arbors of off‐center retinal ganglion cell axons to form on/off sublaminae. Sublamination normally occurs during postnatal weeks 3–4 and requires the activity of retinal afferents, N‐methyl‐D‐aspartate receptors, nitric oxide synthase, and a target of nitric oxide, cyclic guanosine monophosphate. Calcineurin is a calcium/calmodulin dependent serine, threonine protein phosphatase suggested to mediate NMDA‐receptor dependent synaptic plasticity in the hippocampus. We have examined whether calcineurin plays a role during on/off sublamination in the dorsal lateral geniculate nucleus (dLGN) of the ferret. Immunohistochemistry showed that calcineurin expression is transiently up‐regulated in dLGN cells and neuropil during the period of on/off sublamination. A functional role for calcineurin during sublamination was investigated by blocking the enzyme locally via intracranial infusion of FK506. Treatment with FK506 during postnatal weeks 3–4 disrupted the appearance of sublaminae. These results suggest that calcineurin may play a role during this process of activity‐dependent pattern formation in the visual pathway. © 2003 Wiley Periodicals, Inc. J Neurobiol 56: 153–162, 2003     

Single-channel study of the binding-gating coupling in the slowly desensitizing chimeric α7-5HT3A receptor

We have studied the role of the highly conserved residue αLysine145 in the early steps of activation by acetylcholine of the nicotinic acetylcholine receptor (nAChR). Both macroscopic and single-channel currents were recorded in the slowly desensitizing chimeric mutant receptor α7V201-5HT3A/R432Q/R436D/R440A, made of α7 nAChRs and serotonin receptors of subtype 3A (ch1), and its corresponding mutant K145A (ch1/K145A) expressed in Xenopus oocytes. Mutant ch1/K145A receptors had a reduced gating function similar to that produced by the same mutation in the wild type receptor α7. The mutated receptor has reduced opening rate constants, β, and increased closing rate constants, α.     

Dual Gating Mechanism and Function of P2X7 Receptor Channels

The ATP-gated P2X7 receptor channel (P2X7R) operates as a cytolytic and apoptotic receptor but also controls sustained cellular responses, including cell growth and proliferation. However, it has not been clarified how the same receptor mediates such opposing effects. To address this question, we have combined electrophysiological, imaging, and mathematical studies using wild-type and mutant rat P2X7Rs. Activation of naïve (not previously stimulated) receptors by low agonist concentrations caused monophasic slow desensitizing currents and internalization of receptors without other changes in the cellular morphology, much like other P2XRs. In contrast, saturating agonist concentrations induced high-amplitude biphasic currents, reflecting pore dilation and causing rapid cell swelling and lysis. The existence of these two signaling patterns was accounted for using a revised Markov-state model that included, in addition to naïve and sensitized states, desensitized states. Occupancy of one or two ATP-binding sites of naïve receptors favored a slow transition to desensitized states, whereas occupancy of the third binding site favored a transition to sensitized/dilated states. Consistent with model predictions, nondilating P2X7R mutants always generated desensitizing currents. These results suggest that the level of saturation of the ligand binding sites determines the nature of the P2X7R gating and cellular actions.     

Dual Gating Mechanism and Function of P2X7 Receptor Channels

The ATP-gated P2X7 receptor channel (P2X7R) operates as a cytolytic and apoptotic receptor but also controls sustained cellular responses, including cell growth and proliferation. However, it has not been clarified how the same receptor mediates such opposing effects. To address this question, we have combined electrophysiological, imaging, and mathematical studies using wild-type and mutant rat P2X7Rs. Activation of naïve (not previously stimulated) receptors by low agonist concentrations caused monophasic slow desensitizing currents and internalization of receptors without other changes in the cellular morphology, much like other P2XRs. In contrast, saturating agonist concentrations induced high-amplitude biphasic currents, reflecting pore dilation and causing rapid cell swelling and lysis. The existence of these two signaling patterns was accounted for using a revised Markov-state model that included, in addition to naïve and sensitized states, desensitized states. Occupancy of one or two ATP-binding sites of naïve receptors favored a slow transition to desensitized states, whereas occupancy of the third binding site favored a transition to sensitized/dilated states. Consistent with model predictions, nondilating P2X7R mutants always generated desensitizing currents. These results suggest that the level of saturation of the ligand binding sites determines the nature of the P2X7R gating and cellular actions.     

Motor neuropathy‐associated mutation impairs Seipin functions in neurotransmission

Gain‐of‐toxic‐function mutations in Seipin (Asparagine 88 to Serine (N88S) and Serine 90 to Leucine (S90L) mutations, both of which disrupt the N‐glycosylation) cause autosomal dominant motor neuron diseases. However, the mechanism of how these missense mutations lead to motor neuropathy is unclear. Here, we analyze the impact of disruption of N‐glycosylation of Seipin on synaptic transmission by over‐expressing mutant Seipin in cultured cortical neurons via lentiviral infection. Immunostaining shows that over‐expressed Seipin is partly colocalized with synaptic vesicle marker synaptophysin. Electrophysiological recordings reveal that the Seipin mutation significantly decreases the frequency, but not the amplitudes of miniature excitatory post‐synaptic currents and miniature inhibitory post‐synaptic currents. The amplitude of both evoked excitatory post‐synaptic currents and inhibitory post‐synaptic current is also compromised by mutant Seipin over‐expression. The readily releasable pool and vesicular release probability of synaptic vesicles are both altered in neurons over‐expressing Seipin‐N88S, whereas neither γ‐amino butyric acid (GABA) nor α‐Amino‐3‐hydroxy‐5‐methyl‐4‐ isoxazolepropionic acid (AMPA) induced whole cell currents are affected. Moreover, electron microscopy analysis reveals decreased number of morphologically docked synaptic vesicles in Seipin‐N88S‐expressing neurons. These data demonstrate that Seipin‐N88S mutation impairs synaptic neurotransmission, possibly by regulating the priming and docking of synaptic vesicles at the synapse.

     

A new semisynthetic derivative of sauroine induces LTP in hippocampal slices and improves learning performance in the Morris Water Maze

Two semisynthetic acetyl derivatives of the alkaloid sauroine from Huperzia saururus, monoacetyl sauroine, and diacetyl sauroine (DAS) were obtained and their chemical structures were analyzed by NMR. While monoacetyl sauroine is the typical product of acetylation, DAS is an unexpected derivative related to the keto‐enol formation of sauroine. Recordings of field excitatory post‐synaptic potentials from the CA1 region of rat hippocampal slices showed that only DAS acutely applied induced chemical long‐term potentiation (LTP) in a dose‐dependent manner with an EC₅₀ of 1.15 ± 0.09 μM. This effect was blocked by 10 μM D(‐)‐2‐amino‐5‐phosphonopentanoic acid (AP5), suggesting dependence on the NMDA receptor. DAS significantly increased NMDA receptor‐dependent excitatory post‐synaptic currents without affecting α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate receptor‐dependent currents. Repetitive administration of DAS improved visuo‐spatial learning in the Morris Water Maze. In slices from rats tested in the Morris Water Maze, LTP resulting from electrical synaptic stimulation was 2.5 times larger than in controls. Concentration of DAS measured in the brain after repetitive administration was 29.5 μM. We conclude that slices perfused with DAS display a robust NMDA receptor‐dependent chemical LTP. During chronic treatment, DAS enhances learning abilities through a metaplastic mechanism as revealed by the augmentation of LTP in slices. DAS, therefore, may be a promising compound as a nootropic therapeutic drug.