EFFECTS OF VASOACTIVE INTESTINAL POLYPEPTIDE ON THE RESPONSE PROPERTIES OF CELLS IN AREA-17 OF THE CAT VISUAL-CORTEX

1. Vasoactive intestinal polypeptide (VIP) was iontophoretically appli ed to a population of 90 single cells in the primary visual cortex (ar ea 17) of the cat. Response magnitude, response selectivity, spontaneo us activity, and the ratio between the visual response and spontaneous activity (signal-to-noise ratio) of the cells were assessed quantitat ively before and during drug application. 2. VIP had little effect in the absence of visual stimulation, with only 29/90 (32%) of the cells showing a change of even 1 sp/s in their spontaneous activity. In cont rast it had a clear effect on the visual responses of the majority (73 /90, 81%) of the cells tested. 3. VIP produced a substantial change (i .e., greater-than-or-equal-to 40%) in optimal response magnitude for 5 7 of the affected cells. Of these 65% were facilitated, usually with n o change or an improvement in signal-to-noise ratio and direction sele ctivity. The remaining cells were inhibited, with more variable effect s on their visual response characteristics, and were found predominant ly in the superficial laminae. 4. The effects of VIP bore a remarkable resemblance to those reported previously for the muscarinic action of acetylcholine (ACh). VIP and a muscarinic cholinergic agonist, either ACh or acetyl-beta-methacholine (MeCh), were therefore applied in tur n to a group of 40 cells. In 23 cases VIP and the muscarinic agonist w ere also applied simultaneously. 5. The effects of VIP and the choline rgic agonist matched in 92% of the cases where both drugs were effecti ve. That is to say, cells that were facilitated by VIP were facilitate d also by ACh or MeCh, and vice versa. In many instances there was a c lear similarity in the pattern as well as the direction of the effects produced by the two substances. The result of simultaneous applicatio n was generally additive. 6. These data suggest that VIP and ACh activ ate very similar postsynaptic mechanisms, and share a closely related function at the level of individual cortical cells. Thus VIP may facil itate the responses of both the excitatory and the inhibitory componen ts of the cortical circuit, leading to an overall increase in responsi veness and selectivity. In contrast to the cholinergic input from the basal forebrain, however, the VIP-positive cortical cells are likely t o exert a very localized influence, over a circumscribed region of the cortex, in response to the presence of an effective visual stimulus.