Factors determining the efficacy of distal excitatory synapses in rat hippocampal CA1 pyramidal neurones

1. A new preparation of the in vitro rat hippocampal slice has been developed in which the synaptic input to the distal apical dendrites of CA1 pyramidal neurones is isolated. This has been used to investigate the properties of distally evoked synaptic potentials. 2. Distal paired-pulse stimulation (0.1 Hz) evoked a dendritic response consisting of a pair of EPSPs, which showed facilitation. The first EPSP had a rise time (10-90%) of 2.2 +/- 0.05 ms and a half-width of 9.1 +/- 0.13 ms. The EPSPs were greatly reduced by CNQX (10 microM) and the remaining component could be enhanced in Mg(2+)-free Ringer solution and blocked by AP5 (50 microM). In 70% of the dendrites, the EPSPs were followed by a prolonged after-hyperpolarization (AHP) which could be blocked by a selective and potent GABAB antagonist, CGP55845A (2 microM). These results indicate that the EPSPs are primarily mediated by non-NMDA receptors with a small contribution from NMDA receptors, whereas the AHP is a GABAB receptor-mediated slow IPSP. 3. With intrasomatic recordings, the rise time of proximally generated EPSPs (3.4 +/- 0.1 ms) was half that of distally generated EPSPs (6.7 +/- 0.5 ms), whereas the half-widths were similar (19.6 +/- 0.8 ms and 23.8 +/- 1 ms, respectively). These results indicate that propagation through the proximal apical dendrites slows the time-to-peak of distally generated EPSPs. 4. Distal stimulation evoked spikes in 60% of pyramidal neurones. At threshold, the distally evoked spike always appeared on the decaying phase of the dendritic EPSP, indicating that the spike is initiated at some distance proximal to the dendritic recording site. Furthermore, distally and proximally generated threshold spikes had a similar voltage dependency. These results therefore suggest that distally generated threshold spikes are primarily initiated at the initial segment. 5. At threshold, spikes generated by stimulation of distal synapses arose from the decaying phase of the dendritic EPSPs with a latency determined by the time course of the EPSP at the spike initiation zone. With maximal stimulation, however, the spikes arose directly from the peak of the EPSPs with a time-to-spike similar to the time-to-peak of subthreshold dendritic EPSPs. Functionally, this means that the effect of dendritic propagation can be overcome by recruiting more synapses, thereby ensuring a faster response time to distal synaptic inputs. 6. In 42% of the neurones in which distal EPSPs evoked spikes, the relationship between EPSP amplitude and spike latency could be accounted for by a constant dendritic modulation of the EPSP. In the remaining 58%, the change in latency was greater than can be accounted for by a constant dendritic influence. This additional change in latency is best explained by a sudden shift in the spike initiation zone to the proximal dendrites. This would explain the delay observed between the action of somatic application of TTX (10 microM) on antidromically evoked spikes and distally evoked suprathreshold spikes. 7. The present results indicate that full compensation for the electrotonic properties of the main proximal dendrites is not achieved despite the presence of Na+ and Ca2+ currents. Nevertheless, distal excitatory synapses are capable of initiating spiking in most pyramidal neurones, and changes in EPSP amplitude can modulate the spike latency. Furthermore, even though the primary spike initiation zone is in the initial segment, the results suggest that it can move into the proximal apical dendrites under physiological conditions, which has the effect of further shortening the response time to distal excitatory synaptic inputs.