SYNAPTIC-INTERACTIONS BETWEEN SMOOTH AND SPINY NEURONS IN LAYER-4 OF CAT VISUAL-CORTEX IN-VITRO

1. Dual intracellular recording was used to examine the interactions b etween neighbouring spiny (excitatory) and smooth (inhibitory) neurone s in layer 4 of cat visual cortex in vitro. Synaptic connections were found in seventeen excitatory-inhibitory neurone pairs, along with one inhibitory-inhibitory connection. 2. Past excitatory inputs onto smoo th neurones (basket cells) from spiny cells (spiny stellate or pyramid al cells) (n = 6) produce large excitatory postsynaptic potentials (EP SPs) of up to 4 mV mean amplitude, whereas basket cells evoke slower i nhibitory postsynaptic potentials (IPSPs) in their postsynaptic target s (n = 17), of smaller amplitude (up to 1.6 mV at membrane potentials of -60 mV). 3. Both types of PSP appear to be multiquantal, and both m ay exhibit depression of up to 60% during short trains of presynaptic spikes. This depression can involve presynaptic and/or postsynaptic fa ctors. 4. One-third (n = 5) of the spiny cell-smooth cell pairs tested were reciprocally connected, and in the one pair for which the suprat hreshold interactions were comprehensively investigated, the pattern o f basket cell firing was strongly influenced by the activity in the co nnected excitatory neurone. The basket cell was only effective in inhi biting spiny cell firing when the excitatory neurone was weakly driven .