Dual intracellular recording was used to examine the interactions between neighbouring spiny (excitatory) and smooth (inhibitory) neurones 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. Fast excitatory inputs onto smooth neurones (basket cells) from spiny cells (spiny stellate or pyramidal cells) (n = 6) produce large excitatory postsynaptic potentials (EPSPs) of up to 4 mV mean amplitude, whereas basket cells evoke slower inhibitory postsynaptic potentials (IPSPs) in their postsynaptic targets (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 may exhibit depression of up to 60 % during short trains of presynaptic spikes. This depression can involve presynaptic and/or postsynaptic factors. 4. One-third (n = 5) of the spiny cell-smooth cell pairs tested were reciprocally connected, and in the one pair for which the suprathreshold interactions were comprehensively investigated, the pattern of basket cell firing was strongly influenced by the activity in the connected excitatory neurone. The basket cell was only effective in inhibiting spiny cell firing when the excitatory neurone was weakly driven.
