Tachykinins preferentially excite certain complex cells in the infragranular layers of feline striate cortex

Microiontophoretically administered substance P (SP) affected the visually evoked responses (VER) and the spontaneous firing of 22 (14%) of the 152 neurons recorded from the striate cortex of anaesthetised cats. Enhancing effects were seen in 14 neurons and suppressant actions in 8 neurons. Most of the cells excited by SP were located in infragranular layers and had complex receptive fields; a few belonged to the movement-sensitive class or responded only weakly to visual stimulation. Of the neurons recorded in layer V, about 70% were excited by SP; the respective proportions were 8% in layer VI, and 2% in layer IV. Cells suppressed by SP had either simple or unimodal receptive fields including hypercomplex varieties; most of them were located in layer IVc. The effects of other tachykinins (neurokinin A, neurokinin B) and of the NK-3 receptor agonist Senktide tested in 36 cells were identical to those of SP with respect to types, and intracortical locations, of cells affected. During the enhancement induced by the tachykinins functional parameters of the neurons such as orientation and direction sensitivity were not substantially affected. It seems likely therefore that the effect of tachykinins in the primary visual cortex is not a shaping of receptive field properties, but rather a modulation of the general excitability of neurons projecting to subcortical centers, in particular to the midbrain and pons.