Field potential analysis of the cortical projection of the central lateral nucleus in the cat

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Abstract
A previous field potential study has indicated a monosynaptic projection of fibres from the central lateral nucleus (CL) to the mid-suprasylvian gyrus (MSSG). The present study, which is based on an analysis of current source density (CSD), aims to investigate further the sites of major localized synaptic activities in different layers of the MSSG after electrical stimulation in the CL. An initial positive surface potential was evoked in the MSSG with a latency of 3-5 ms and followed by a large negative potential with a peak latency of 8-15 ms. The initial positivity was only found in the rostral part of the MSSG, which corresponds to area 5. The positivity reversed in deeper layers. The CSD analysis showed a sink at a depth from 650 to 1050 .mu.m. A corresponding source was found more superficially at 400-600 .mu.m. This indicates that CL fibres have an excitatory synaptic termination on teh soma or proximal dendrites of neurons in layers III and IV. The surface negative potential reversed at the border between layers II and III, suggesting a sueprficial CL projection. The CSD analysis of potentials in superficial layers showed a sink appearing between the pial surface and a depth of 350 .mu.m, and a source lying in layers below. This indicates a depolarization of apical dendrites of cells in layers II and III. The superficial sink appeared in a large part of the MSSG. Application of a solutin of 0.5% gamma-aminobutyric acid (GABA) on the surface of the cortex blocked the suprficial sink and source and revealed a prominent sink current in layers III and IV in agreement with a deep termination CL fibres. Application of a solution of 25 mM DL-1-amino-5-phosphono-valeric acid (APV) abolished CL-evoked cortical responses indicating that N-methyl-D-aspartate (NMDA) receptors are involved in the cortical activation. The CSD analysis confirms that CL has a wide superficial projection to the MSSG. It also confirms a deeper monosynaptic projection from CL to area 5.