SYNAPTIC PHYSIOLOGY OF HORIZONTAL AFFERENTS TO LAYER-I IN SLICES OF RAT SI NEOCORTEX

Layer I is a dense synaptic zone ubiquitous in cerebral cortex. Here w e describe a novel in vitro preparation of rat somatosensory (SI) neoc ortical slices that isolates the fibers that extend horizontally throu gh layer I, and allows intracellular and extracellular analysis of syn aptic input to dendrites in layer I. Current source-density analysis o f this isolated horizontal layer I input reveals monophasic current si nks restricted to layer I and the most superficial part of layer II. T he layer I synaptic response of each neuron was correlated with its mo rphology by filling penetrated cells with biocytin. All filled cells t hat responded to horizontal layer I inputs were pyramidal neurons in l ayers II, III, or V with distal apical dendrites in layer I. There was no evidence of antidromic activation from isolated layer I stimulatio n, and HRP injected into layer I was not transported via the isolated layer I pathway to cortical neurons within the slice. Therefore, this preparation provides a unique way to study an extrinsic synaptic input localized to the most distal apical dendrites of many pyramidal neuro ns. In contrast to the EPSP-IPSP sequence characteristically evoked by deep layer stimulation, horizontal layer I inputs evoked long-lasting EPSPs (approximately 50 msec); IPSPs were observed only rarely, in th e most superficial neurons. Horizontal layer I-evoked EPSPs were block ed by the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinox aline-2,3-d-dione. Consistent with the very distal site of layer I inp uts to layer V pyramidal neurons, the amplitudes of initial EPSPs were insensitive to manipulations of the somatic membrane potential. Howev er, these distal EPSPs were greatly attenuated when combined with IPSP s evoked by deep layer stimulation, indicating that the proximal input s may modulate distal EPSPs with shunting inhibition. In many layer V neurons, the initial EPSP evoked by horizontal layer I stimulation was followed by a variable late depolarization that was blocked by the NM DA receptor antagonist 2-amino-5-phosphonovaleric acid. Since these la te depolarizations were enhanced by somatic depolarization and abolish ed by hyperpolarization, they appeared to be generated postsynapticall y at a site more proximal than the initial EPSP that was insensitive t o these manipulations. Synaptic inputs to the distal tufts of pyramida l neurons may trigger active currents along the apical dendrites that amplify the EPSP on its way to the soma.