FURTHER OBSERVATIONS ON THE ROLE OF NITRIC-OXIDE IN THE FELINE LATERAL GENICULATE-NUCLEUS

We have examined the responses of a population of 77 cells in the dors al lateral geniculate nucleus (dLGN) of the anaesthetized, paralysed c at. Here the synthetic enzyme for the production of nitric oxide, nitr ic oxide synthase, is found only in the presynaptic terminals of the c holinergic input from the brainstem. In our hands, iontophoretic appli cation of inhibitors of this enzyme resulted both in significant decre ases in visual responses and decreased responses to exogenous applicat ion of NMDA, effects which were reversed by coapplication of the natur al substrate for nitric oxide synthase, L-arginine, but not the biolog ically inactive isomer, D-arginine. Nitroprusside and S-nitroso-Nacety lpenicillamine (SNAP), nitric oxide donors, but not L-arginine, were a ble to increase markedly both spontaneous activity and the responsiven ess to NMDA application. Furthermore, SNAP application facilitated vis ual responses. Responses of cells in animals without retinal, cortical and parabrachial input to the LGN suggest a postsynaptic site of acti on of nitric oxide. This modulation of the gain of visual signals tran smitted to the cortex suggests a completely novel pathway for nitric o xide regulation of function, as yet described only in primary sensory thalamus of the mammalian central nervous system.