Under certain conditions, regenerative voltage spikes can be initiated locally in the dendrites of CA1 pyramidal neurons. These are interesting events that could potentially provide neurons with additional computational abilities. Using whole-cell dendritic recordings from the distal apical trunk and proximal tuft regions and realistic computer modeling, we have determined that highly synchronized and moderately clustered inputs are required for dendritic spike initiation: ϳ50 synaptic inputs spread over 100 m of the apical trunk/tuft need to be activated within 3 msec. Dendritic spikes are characterized by a more depolarized voltage threshold than at the soma [Ϫ48 Ϯ 1 mV (n ϭ 30) vs Ϫ56 Ϯ 1 mV (n ϭ 7), respectively] and are mainly generated and shaped by dendritic Na ϩ and K ϩ currents. The relative contribution of AMPA and NMDA currents is also important in determining the actual spatiotemporal requirements for dendritic spike initiation. Once initiated, dendritic spikes can easily reach the soma, but their propagation is only moderately strong, so that it can be modulated by physiologically relevant factors such as changes in the V m and the ionic composition of the extracellular solution. With effective spike propagation, an extremely short-latency neuronal output is produced for greatly reduced input levels. Therefore, dendritic spikes function as efficient detectors of specific input patterns, ensuring that the neuronal response to high levels of input synchrony is a precisely timed action potential output.

Depending on the behavioral state, hippocampal CA1 pyramidal neurons receive very distinct patterns of synaptic input and likewise produce very different output patterns. We have used simultaneous dendritic and somatic recordings and multisite glutamate uncaging to investigate the relationship between synaptic input pattern, the form of dendritic integration, and action potential output in CA1 neurons. We found that when synaptic input arrives asynchronously or highly distributed in space, the dendritic arbor performs a linear integration that allows the action potential rate and timing to vary as a function of the quantity of the input. In contrast, when synaptic input arrives synchronously and spatially clustered, the dendritic compartment receiving the clustered input produces a highly nonlinear integration that leads to an action potential output that is extraordinarily precise and invariant. We also present evidence that both of these forms of information processing may be independently engaged during the two distinct behavioral states of the hippocampus such that individual CA1 pyramidal neurons could perform two different state-dependent computations: input strength encoding during theta states and feature detection during sharp waves.

The effects of ZD 7288, a "bradycardic" agent, in young rat hippocampal slices in vitro were studied. ZD 7288 (1-1000 microM) reduced the hyperpolarization-activated current (Ih) in CA1 pyramidal neurons by a voltage-independent blocking mechanism. Under current-clamp conditions, the bradycardic agent (10 microM) caused membrane hyperpolarization (by 5.9 +/- 0.5 mV) and a reduction of membrane conductance (by 17.9 +/- 4.1%). These data are consistent with the block of an inward current which is active at rest. The drug-induced hyperpolarization depressed the cell's excitability by increasing the threshold current necessary to induce firing. When the drug-induced hyperpolarization was compensated for by injection of a tonic depolarizing current, ZD 7288 caused a reduction of the inhibitory post-synaptic potential (IPSP) in EPSP-IPSP sequences. Since Cs+, another known blocker of Ih, is able to reverse long-term depression (LTD) of the CA3-CA1 synapse in hippocampal slices, we tested the effect of ZD 7288 on synaptic transmission. We found that ZD 7288 did not significantly modify LTD, suggesting that Cs+-induced inhibition of LTD maintenance is not directly related to block of Ih.

At early developmental stages, silent synapses have been commonly found in different brain areas. These synapses are called silent because they do not respond at rest but are functional at positive membrane potentials. A widely accepted interpretation is that N-methyl-D-aspartate (NMDA) but not ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are functionally expressed on the subsynaptic membrane. Here we show that, in both CA3 and CA1 hippocampal regions, AMPA-mediated synaptic responses can be detected already at early stages of postnatal development. However, some synapses appear silent because of a very low probability of glutamate release. They can be converted into functional ones by factors that enhance release probability such as paired-pulse stimulation, increasing the temperature or cyclothiazide (CTZ), a drug that blocks AMPA receptor desensitization and increases transmitter release. Conversely, conducting synapses can be switched off by increasing the frequency of stimulation. Although we cannot exclude that ''latent AMPA receptors'' can become functional after activity-dependent processes, our results clearly indicate that, in the neonatal hippocampus, a proportion of glutamatergic synaptic connections are presynaptically rather than postsynaptically silent.

Extracellular cesium causes synchronous, interictal-like bursting and prevents maintenance of long-term depression (LTD) in the CA1 hippocampal region. We have investigated the cellular mechanisms underlying cesium actions. Whole-cell recordings showed that brief (2 min) bath exposures to cesium caused pyramidal cell hyperpolarization associated with decreased membrane conductance attributable to blockade of an inward h-type current. After prolonged (Ͼ2 min) exposures, a late depolarizing response was observed; this effect was not associated with changes in cell membrane conductance. Recordings from interneurons revealed that I h is expressed in a subpopulation of cells and that cesium effects on interneurons expressing I h are comparable to those observed in pyramidal cells. Consistent with this effect, cesium decreased the early component of the IPSP recorded in pyramidal cells. Interneurons lacking I h were not affected by cesium but developed a depolarizing response when drug applications were paired to orthodromic stimulation. We concluded that cesium actions on LTD and cesium-induced epileptiform activity were not attributable exclusively to its direct effects on neurons. Recordings from hippocampal slice astrocytes revealed that cesium interfered with glial electrical responses during LTD induction. Cesium blocked glial inwardly rectifying potassium channels and increased the amplitude and duration of stimulation-evoked [K ϩ ] out increases. Thus, the effects of cesium on CA1 synchronization and synaptic plasticity appear to be mediated predominantly by blockade of glial voltage-dependent potassium uptake.