Mechanism for increased hippocampal synaptic strength following differential experience

Exposure to novel environments or behavioral training is associated with in creased strength at hippocampal synapses. The present study employed quanta l analysis techniques to examine the mechanism supporting changes in synapt ic transmission that occur following differential behavioral experience. Me asures of CA1 synaptic strength were obtained from hippocampal slices of ra ts exposed to novel environments or maintained in individual cages. The inp ut/output (I/O) curve of extracellularly recorded population excitatory pos tsynaptic potentials (EPSPs) increased for animals exposed to enrichment. T he amplitude of the synaptic response of the field potential was related to the fiber potential amplitude and the paired-pulse ratio, however, these m easures were not altered by differential experience. Estimates of biophysic al parameters of transmission were determined for intracellularly recorded unitary responses of CA1 pyramidal cells. Enrichment was associated with an increase in the mean unitary synaptic response, an increase in quantal siz e, and a trend for decreased input resistance and reduction in the stimulat ion threshold to elicit a unitary response. Paired-pulse facilitation, the percent of response failures, coefficient of variance, and estimates of qua ntal content were not altered by experience but correlated well with the me an unitary response amplitude. The results suggest that baseline synaptic s trength is determined, to a large extent, by presynaptic release mechanisms . However, increased synaptic transmission following environmental enrichme nt is likely due to an increase in the number or efficacy of receptors at s ome synapses and the emergence of functional synaptic contacts between prev iously unconnected CA3 and CA1 cells.