Long-term potentiation (LTP) is an activity-dependent enhancement of synaptic efficacy, considered a model of learning and memory. The biochemical cascade producing LTP requires activation of Src, which upregulates the function of NMDA receptors (NMDARs), but how Src becomes activated is unknown. Here, we show that the focal adhesion kinase CAKbeta/Pyk2 upregulated NMDAR function by activating Src in CA1 hippocampal neurons. Induction of LTP was prevented by blocking CAKbeta/Pyk2, and administering CAKbeta/Pyk2 intracellularly mimicked and occluded LTP. Tyrosine phosphorylation of CAKbeta/Pyk2 and its association with Src was increased by stimulation that produced LTP. Finally, CAKbeta/Pyk2-stimulated enhancement of synaptic AMPA responses was prevented by blocking NMDARS, chelating intracellular Ca(2+), or blocking Src. Thus, activating CAKbeta/Pyk2 is required for inducing LTP and may depend upon downstream activation of Src to upregulate NMDA receptors.

Long-term potentiation (LTP) is an activity-dependent strengthening of synaptic efficacy that is considered to be a model of learning and memory. Protein tyrosine phosphorylation is necessary to induce LTP. Here, induction of LTP in CA1 pyramidal cells of rats was prevented by blocking the tyrosine kinase Src, and Src activity was increased by stimulation producing LTP. Directly activating Src in the postsynaptic neuron enhanced excitatory synaptic responses, occluding LTP. Src-induced enhancement of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) receptor-mediated synaptic responses required raised intracellular Ca2+ and N-methyl-D-aspartate (NMDA) receptors. Thus, Src activation is necessary and sufficient for inducing LTP and may function by up-regulating NMDA receptors.

Coincident pre- and postsynaptic activity generates long-term potentiation (LTP), a possible cellular model of learning and memory. LTP has two components: (1) an increase in the excitatory postsynaptic potential (EPSP), and (2) an increase in the ability of the EPSP to generate a spike (E-S coupling of LTP). We have used pharmacological and genetic approaches to address the molecular nature of E-S coupling in CA1 pyramidal neurons. Blockade of the Ca2+-sensitive phosphatase, calcineurin, prevents induction of E-S coupling without interfering with LTP of the EPSP. Calcineurin produces its effect on E-S coupling by inducing a long-lasting depression (LTD) of the GABA(A)-mediated inhibitory postsynaptic potentials (IPSPs). This LTD of the IPSP was prevented by blockade of NMDA receptors. Thus, the tetanus that elicits NMDA-dependent LTP mediates a coordinately regulated double function. It produces LTP of the EPSP and, concomitantly, LTD of the IPSP that leads to enhancement of E-S coupling.

CA1 pyramidal neurons degenerate after transient forebrain ischemia, whereas neurons in other regions of the hippocampus remain intact. Here we show that in rat hippocampal CA1 neurons, forebrain ischemia induces the phosphorylation of the N-methyl-D-aspartate (NMDA) receptor 2A subunit at Ser1232 (phospho-Ser1232). Ser1232 phosphorylation is catalyzed by cyclin-dependent kinase 5 (Cdk5). Inhibiting endogenous Cdk5, or perturbing interactions between Cdk5 and NR2A subunits, abolished NR2A phosphorylation at Ser1232 and protected CA1 pyramidal neurons from ischemic insult. Thus, we conclude that modulation of NMDA receptors by Cdk5 is the primary intracellular event underlying the ischemic injury of CA1 pyramidal neurons.

The generation of new neurons in the adult mammalian hippocampus is thought to play a role in repairing the brain after injury. Here, we show that 7 d after focal cerebral ischemia, newly divided cells in the dentate gyrus of adult rats increased to approximately sevenfold, compared with sham controls. In the same area, this enhanced dentate neurogenesis was associated with activation of inducible nitric oxide synthase (iNOS). Inhibition of iNOS by aminoguanidine prevented ischemia-induced neurogenesis in the dentate gyrus. In null mutant mice lacking the iNOS gene, increased neurogenesis was not observed after focal cerebral ischemia. This study demonstrates that expression of iNOS is necessary for ischemia-stimulated cell birth in the dentate gyrus and indicates that activation of iNOS may provide a possible strategy for functional recovery from cerebral ischemic insult.

New neurons are generated in adult mammalians and may contribute to repairing the brain after injury. Here, we show that the number of new neurons in the dentate gyrus of adult rats increased in cerebral ischemic stroke and correlated with activation of the cAMP-response-element-binding protein (CREB). Inhibition of endogenous CREB by expression of a dominant-negative mutant of CREB (CREB-S133A or CREB-R287L) blocked ischemia-induced neurogenesis in the dentate gyrus of adult rats, whereas expression of constitutively active CREB, VP16-CREB, increased the number of new neurons. Thus, our findings provide roles and regulatory mechanisms for CREB in adult neurogenesis and possibly suggest a practical strategy for replacing dead neurons in brain injury. Progenitor cells in the adult dentate gyrus can proliferate and differentiate into mature neurons when maintained in culture medium containing growth factors in vitro (1) or when grafted into the adult brain in vivo (2), suggesting that residential neuronal progenitors are capable of responding to environmental factors in the adult host (2, 3). Consistent with this hypothesis, several recent studies show that neurogenesis in the dentate gyrus of adults is regulated by stress (4), exercise (5), and learning (6, 7).There is also precedent for neuronal injury's modifying the fate of immature precursor cells. An earlier report showed that the number of BrdUrd-labeled cells in the dentate gyrus of the gerbil is increased on the day after transient global ischemia (8). Consistent with this finding, our studies, as well as others, show that focal cerebral ischemia increases the number of newly generated neurons that migrate from the subgranule zone into the granule cell layer of the dentate gyrus in adult rats (9, 10). A more recent study extended these findings to demonstrate that activation of endogenous progenitors after transient forebrain ischemia leads to massive regeneration of pyramidal neurons in the CA1 area of the hippocampus (11). These results have been interpreted as evidence for the direct migration of neuronal precursors toward injured areas, possibly to trigger brain repair (11).Because neurons that die in adulthood can be replaced by neurons of the same class (12-14), it is crucial to determine what signaling molecules promote the production of replacement neurons. Several signals control the proliferation, differentiation, and survival of endogenous progenitors (15,16). In this study, we examined cAMP-response-element-binding protein (CREB) in regulation of adult neurogenesis and found that CREB activation is responsible for recruiting new neurons into the dentate circuits of adult rats that have been subjected to cerebral ischemic stroke. Materials and MethodsFocal Cerebral Ischemia and Stereotaxic Operation. In our preliminary studies, we found that adult male rats after focal cerebral ischemia revealed greater increase in the number of BrdUrdlabeled cells than did female rats (Fig. 5, which is published as supporting information on the PNAS web site)....

The photoinduced nonadiabatic decay dynamics of 9H-adenine (hereafter, adenine) in aqueous solution were investigated by surface-hopping simulations within a quantum mechanical/molecular mechanical (QM/MM) framework. The QM subsystem (adenine) was treated at the semiempirical OM2/MRCI level, whereas the MM solvent (water) was described by the TIP3P force field model. Classical molecular dynamics (MD) simulations were used to generate snapshots with different solvent configurations and geometries. For a representative number of these snapshots, the energy minima of the lowest electronic states and the most important conical intersections were located by QM/MM geometry optimization. Surface-hopping QM/MM MD simulations were performed for all selected snapshots to study the nonadiabatic dynamics after photoexcitation, including the two lowest excited singlet states, which are both populated in the initial photoexcitation due to strong vibronic coupling in the Franck-Condon region. The simulations yield ultrafast S(2)-S(1) decay within 40 fs and S(1)-S(0) internal conversion to the ground state within 410 fs, which is consistent with recent experimental results from time-resolved spectroscopy.

Long-term depression (LTD) is an activity-dependent weakening of synaptic efficacy at individual inhibitory synapses, a possible cellular model of learning and memory. Here, we show that the induction of LTD of inhibitory transmission recruits activated calcineurin (CaN) to dephosphorylate type-A GABA receptor (GABAARs) via the direct binding of CaN catalytic domain to the second intracellular domain of the GABAAR-γ2subunits. Prevention of the CaN–GABAAreceptor complex formation by expression of an autoinhibitory domain of CaN in the hippocampus of transgenic mice blocks the induction of LTD. Conversely, genetic expression of the CaN catalytic domain in the hippocampus depresses inhibitory synaptic responses, occluding LTD. Thus, an activity-dependent physical and functional interaction between CaN and GABAAreceptors is both necessary and sufficient for inducing LTD at CA1 individual inhibitory synapses.