Postsynaptic action potentials do not alter short-term potentiation in the dentate gyrus

The dendrites of neocortical neurons have been shown to support active action potentials which back-propagate from the soma after an output spike has been initiated. This observation has led to speculation that dendritic action potentials may participate in various forms of synaptic plasticity. The contribution of dendritic spikes to paired-pulse facilitation (PPF), a form of short-term plasticity, was investigated in the dentate gyrus of hippocampal slices. Paired orthodromic stimulation of the perforant path produced an average facilitation of the test population spike (PS) amplitude of 167% (n = 16, conditioning response = 100%). There was also a small but significant increase in slope of the field EPSP (fEPSP) of 108%. To determine whether increased presynaptic drive could account for this facilitation, the relationship between fEPSP slope and spike amplitude (I-O) was determined for a range of stimulus intensities. An increase in fEPSP slope of 171% was associated with an increase in PS amplitude equal to the facilitation produced by paired-pulse stimulation (167%), suggesting a postsynaptic component in PPF. Electric field effects were then used as a tool to alter the excitability of granule cells during the conditioning response without changing synaptic drive. Any change in the test response associated with manipulation of the conditioning population spike amplitude would suggest that dendritic spikes may contribute to the postsynaptic component of PPF. Surprisingly, altering the number of neurons responding to the conditioning stimulus with an action potential had no effect on the test response, suggesting that dendritic action potentials do not participate in this form of short-term synaptic plasticity.