SYNAPTIC INTEGRATION IN LAYER-IV OF THE FERRET STRIATE CORTEX

1. Whole-cell patch recordings were made with dye-filled electrodes fr om layer IV in slices of the ferret striate cortex. Projections from t he thalamus and layer VI provide most of the extralaminar input to lay er IV, Interactions between these two pathways are thought to play a r ole in the generation of suppressive non-linearities such as end-inhib ition. Thus, synaptic responses evoked by stimulating: each pathway in dividually were compared with those produced by activating both projec tions together. 2. Spiny stellate cells are the majority population in layer IV and were the most frequently patched neurons (n = 23); all f ired adapting trains of large, fast action potentials. About half of t hose tested (n = 13) were progressively inhibited by strengthening the stimulus to layer VI, while the rest became more excited. For the for mer, the response evoked by stimulating both pathways in coincidence w as often more hyperpolarizing than would have been predicted by summin g the responses to either projection alone (n = 4). Hence, the inputs from the thalamus and layer VI are integrated by circuits that can pro duce strong and non-linear inhibition. 3. Recordings from various bask et cells, which are inhibitory, have provided a first view of the supp ressive circuits in layer IV (n = 5). Two cells were excited by stimul ation of both pathways. The remaining three cells were mainly excited by activation of thalamic afferents but were largely inhibited by stim ulation of layer VI. Thus, inhibition seen at the level of the spiny s tellate cells could result from two mechanisms operating via presynapt ic smooth cells: convergent excitation provided by both ascending path ways on the one hand, and a push-pull relationship between pathways on the other.