Changes in translation of synaptic excitation to dentate granule cell discharge accompanying long-term potentiation. I. Differences between normal and reinnervated dentate gyrus.

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Abstract
Differences in the way that synaptic excitation evoked by normal and sprouted circuits is translated into granule cell discharge following the induction of LTP [long-term potentiation] were studied in rats. The potentiation of the population EPSP [excitatory postsynaptic potential] and the population spike in both normal and sprouted pathways was examined at 3 intensities of conditioning stimulation, and across a variety of testing stimulus intensities ranging from threshold to that evoking a maximum response. Induction of LTP in each pathway required a threshold intensity of conditioning stimulation. LTP of the population spike was relatively less in the sprouted circuit than in the normal at all conditioning and test stimulus intensities, despite apparently comparable LTP of the population EPSP. There are apparently 2 components of LTP in the normal ipsilateral pathway: a potentiation resulting from increased synaptic potency, and an additional increase in population spike amplitude, which is manifested as spike/EPSP dissociation. The 2nd of these 2 events is absent in the case of LTP induced in the sprouted crossed projection. Possible mechanisms of the spike/EPSP dissociation and the significance of the absence of this property in the sprouted circuit are considered.

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