SYNAPTIC-INTERACTIONS IN NEOCORTICAL LOCAL CIRCUITS - DUAL INTRACELLULAR-RECORDINGS IN-VITRO

Properties of local synaptic connections in neocortex, studied with du al intracellular recordings in vitro and correlated with cell and syna ptic morphology are summarized. The different durations and sensitivit ies to somatic membrane potential of pyramid-pyramid excitatory postsy naptic potentials (EPSPs) apparently reflect the positions of the syna pses on the postsynaptic dendrites. Their time, frequency and voltage- dependent properties enable supra-linear summation of several low-freq uency inputs arising in the same dendritic region, even if only loosel y coincident but they depress during repetitive firing in any one inpu t. Pyramidal input to classical fast spiking and low threshold spiking interneurones are strikingly different. Here low presynaptic firing r ates result in many transmission failures. EPSPs are brief and inputs must be near coincident for summation. However, these synapses display pronounced, frequency-dependent, incrementing facilitation at higher presynaptic frequencies. Once initiated by a brief high-frequency burs t, this facilitation is maintained at lower frequencies. GABA(A) recep tor-mediated inhibitory postsynaptic potentials (IPSPs) arising proxim ally are of very different durations depending on the type of interneu rone activated and can prevent and subsequently synchronize firing in their many postsynaptic partners with very different delays (eg. 10-10 0 ms). Low threshold spiking interneurones, in contrast, generate brie f IPSPs only in more distal dendritic regions and have little effect o n somatic excitability acting to shunt input distally.