Dynamics of learning and recall at excitatory recurrent synapses and cholinergic modulation in rat hippocampal region CA3.

Hippocampal region CA3 contains strong recurrent excitation mediated by synapses of the longitudinal association fibers. These recurrent excitatory connections may play a dominant role in determining the information processing characteristics of this region. However, they result in feedback dynamics that may cause both runaway excitatory activity and runaway synaptic modification. Previous models of recurrent excitation have prevented unbounded activity using biologically unrealistic techniques. Here, the activation of feedback inhibition is shown to prevent unbounded activity, allowing stable activity states during recall and learning. In the model, cholinergic suppression of synaptic transmission at excitatory feedback synapses is shown to determine the extent to which activity depends upon new features of the afferent input versus components of previously stored representations. Experimental work in brain slice preparations of region CA3 demonstrates the cholinergic suppression of synaptic transmission in stratum radiatum, which contains synapses of the longitudinal association fibers.