ELECTROPHYSIOLOGICAL IDENTIFICATION OF HORIZONTAL SYNAPTIC CONNECTIONS IN RAT VISUAL-CORTEX IN-VITRO

The presence of intrinsic horizontal synaptic connections in rat visua l cortex was explored electrophysiologically using in vitro slice prep arations. Intracellular recordings were made from pyramidal neurons lo cated in the superficial and deep layers. Electrical stimulation at th e gray matter in the same or different layers but 0.8-2.7 mm apart fro m the recording site evoked compound synaptic potentials composed of e xcitatory and inhibitory postsynaptic potentials of fast and slow time courses. Glutamate blockers, DNQX (5 mu M) and kynurenate (2 mM) redu ced the excitatory postsynaptic potential (EPSP), and GABA(B) receptor antagonist, phaclofen (0.5 mM), abolished the inhibitory postsynaptic potential of the slow time course. EPSP of the fast time course follo wed 20 Hz repetitive stimulation in the medium of high Ca2+ (6.0 mM) a nd Mg2+ (4.0 mM) concentration, suggesting that this fast EPSP was mon osynaptic. Conduction velocity of the fibers mediating the monosynapti c EPSP was estimated to be 0.15-0.55 m/s. These results provide physio logical evidence for the horizontal synaptic connections in the rat vi sual cortex, which had been previously suggested by morphology.