Abstracts

RATIONALE: Local synaptic circuits are important for information processing in the brain and are altered during epileptogenesis. The present study examined excitatory synaptic interactions in different areas of the hippocampus.
METHODS: Transverse slices (300 [mu]m) of temporal hippocampus were prepared and perfused in artificial cerebrospinal fluid (ACSF) with 30 [mu]M bicuculline and 250 [mu]M caged glutamate. A combination of whole-cell patch-clamp recording and focal flash photolysis of caged glutamate was used to reveal interconnections among neurons.
RESULTS: Localized photostimulation with a flash lamp (50-100 mJ, 0.5 ms) uncaged glutamate and consistently caused direct depolarization with a burst of action potentials when aimed at the somata or dendrites of a recorded neuron (n=15). The direct excitation of a small population of neurons by flash-induced uncaging of glutamate would be expected to cause excitatory post-synaptic currents (EPSCs) in neurons that are interconnected. In 22 CA1 neurons, 5 exhibited EPSCs in response to photostimulations at other sites in the CA1 pyramidal cell layer (i.e., 150-600 [mu]m from the recorded neurons). When the subiculum was photostimulated while nearby CA1 neurons (i.e., 150-450 [mu]m from the distal end of CA1) were recorded, robust EPSCs were observed in 4 of 13 neurons; however, flashes between the recorded neuron and the distal end of the CA1 layer did not evoke responses. In the CA3 region, 4 of 8 neurons exhibited either evoked EPSCs or an increase in EPSCs to photostimulations in CA3.
CONCLUSIONS: These data provide physiological evidence for excitatory synaptic circuits from the subiculum to CA1, and suggest that hypothetical alterations in local excitatory circuits of CA1 and CA3 associated with epileptogenesis can be studied with this approach.
Support: NS16683