Long-term potentiation (LTP) requires postsynaptic depolarization that can result from EPSPs paired with action potentials or largerEPSPs that trigger dendritic spikes. We explored the relative contribution of these sources of depolarization to LTP induction during synaptically driven action potential firing in hippocampal CA1 pyramidal neurons. Pairing of a weak test input with a strong input resulted in large LTP (ϳ75% increase) when the weak and strong inputs were both located in the apical dendrites. This form of LTP did not require somatic action potentials. When the strong input was located in the basal dendrites, the resulting LTP was smaller (Յ25% increase). Pairing the test input with somatically evoked action potentials mimicked this form of LTP. Thus, back-propagating action potentials may contribute to modest LTP, but local synaptic depolarization and/or dendritic spikes mediate a stronger form of LTP that requires spatial proximity of the associated synaptic inputs.

Development of GABA A receptor-mediated inhibitory postsynaptic currents in hippocampus. J Neurophysiol 88: 3097-3107, 2002; 10.1152/jn.00026.2002. Hippocampal CA1 pyramidal cells receive two kinetic classes of GABA A receptor-mediated inhibition: slow dendritic inhibitory postsynaptic currents (GABA A,slow IPSCs) and fast perisomatic (GABA A,fast ) IPSCs. These two classes of IPSCs are likely generated by two distinct groups of interneurons, and we have previously shown that the kinetics of the IPSCs have important functional consequences for generating synchronous firing patterns. Here, we studied developmental changes in the properties of GABA A,fast and GABA A,slow spontaneous, miniature, and evoked IPSCs (sIPSCs, mIPSCs, and eIPSCs, respectively) using whole cell voltage-clamp recordings in brain slices from animals aged P10 -P35. We found that the rate of GABA A,slow sIPSCs increased by over 70-fold between P11 and P35 (from 0.0017 to 0.12 s Ϫ1 ). Over this same age range, we observed a Ͼ3.5-fold increase in the maximal amplitude of GABA A,slow eIPSCs evoked by stratum lacunosummoleculare (SL-M) stimuli. However, the rate and amplitude of GABA A,slow mIPSCs remained unchanged between P10 and P30, suggesting that the properties of GABA A,slow synapses remained stable over this age range, and that the increase in sIPSC rate and in eIPSC amplitude was due to increased excitability or excitation of GABA A,slow interneurons. This hypothesis was tested using bath application of norepinephrine (NE), which we found at low concentrations (1 M) selectively increased the rate of GABA A,slow sIPSCs while leaving GABA A,fast sIPSCs unchanged. This effect was observed in animals as young as P13 and was blocked by coapplication of tetrodotoxin, suggesting that NE was acting to increase the spontaneous firing rate of GABA A,slow interneurons and consistent with our hypothesis that developmental changes in GABA A,slow IPSCs are due to changes in presynaptic excitability. In contrast to the changes we observed in GABA A,slow IPSCs, the properties of GABA A,fast sIPSCs remained largely constant between P11 and P35, whereas the rate, amplitude, and kinetics of GABA A,fast mIPSCs showed significant changes between P10 and P30, suggesting counterbalancing changes in action potential-dependent GABA A,fast sIPSCs. These observations suggest differential developmental regulation of the firing properties of GABA A,fast and GABA A,slow interneurons in CA1 between P10 and P35.

The interaction of ATP with the active site of hexokinase is unknown since the crystal structure of the hexokinase-ATP complex is unavailable. It was found that the ATP binding site of brain hexokinase is homologous to that of actin, heat shock protein hsc70, and glycerol kinase. On the basis of these similarities, the ATP molecule was positioned in the catalytic domain of human brain hexokinase, which was modeled from the X-ray structure of yeast hexokinase. Site-directed mutagenesis was performed to test the function of residues presumably involved in interaction with the tripolyphosphoryl moiety of ATP. Asp532, which is though to be involved in binding the Mg2+ ion of the MgATP2- complex, was mutated to Lys and Glu. The kcat values decreased 1000- and 200-fold, respectively, for the two mutants. Another residue, Thr680 was proposed to interact with the gamma-phosphoryl group of ATP through hydrogen bonds and was mutated to Val and Ser. The kcat value of the Thr680Val mutant decreased 2000-fold, whereas the kcat value of the Thr680Ser decreased only 2.5-fold, implying the importance of the hydroxyl group. The Km and dissociation constant values for either ATP or glucose of all the above mutants showed little or no change relative to the wild-type enzyme. The Ki values for the glucose 6-phosphate analogue 1,5-anhydroglucitol 6-phosphate, were the same as that of the wild-type enzyme, and the inhibition was reversed by inorganic phosphate (Pi) for all four mutants. The circular dichroism spectra of the mutants were the same as that of the wild-type enzyme. The results from the site-directed mutagenesis demonstrate that the presumed interactions of investigated residues with ATP are important for the stabilization of the transition state.

The impact of synaptic inhibition depends on the passive and active properties of the neuronal membrane as well as on the characteristics of the underlying synaptic conductances. Here, we evaluated the contributions of these different factors to the IPSPs produced by two kinetically and anatomically distinct inhibitory synapses onto hippocampal CA1 pyramidal neurons: somatic GABA A,fast and dendritic GABA A,slow . Using combined current-clamp and voltage-clamp recordings from neurons in hippocampal brain slices, we found that despite pronounced differences in kinetics and only weak voltage dependence of the underlying synaptic conductances, there were much smaller differences in duration but strong voltage dependence of IPSPs arising from somatic and dendritic synapses. Pharmacologic tests and compartmental modeling showed that these effects were produced by the hyperpolarization-activated cation current, I H , which accelerated IPSP decay over a broad range of membrane potentials and reduced IPSP amplitudes at hyperpolarized potentials, and the persistent sodium current, I NaP , which prolonged and amplified IPSPs at depolarized subthreshold potentials. The relative magnitudes of their influences depended on the kinetics of the underlying synaptic conductances: the effect on duration was greater for GABA A,fast and on amplitude was greater for GABA A,slow . Passive and active factors thus influence the impact of synaptic inhibition in a location-and voltage-dependent manner.