Regional specific effects of nitric oxide donors and cGMP on the electrical activity of neurons in the rat spinal cord

Abstract

Numerous functional studies establish the role of nitric oxide (NO) as a neuromodulator in the central nervous system which affects synaptic transmission. However, there are only a few reports indicating a direct and postsynaptic effect of nitric oxide on the electrical activity of neurons in the central nervous system. The aim of this study was to characterize the effect of nitric oxide on spontaneously active neurons in spinal cord slices using an extracellular recording technique. Because in the lumbar rat spinal cord the NO producing enzyme NO-synthase is primarily located in the superficial dorsal horn (laminae I+II) and around the central canal (lamina X), we restricted our recordings to these areas. While the majority of neurons increased their electrical activity during superfusion with the NO-donor sodium nitroprusside (SNP) in lamina X, neurons in laminae I+II were mainly inhibited by SNP. The excitatory and the inhibitory effects were dose-dependent and reversible and were mimicked by other NO-donors and membrane permeable cyclic guanosine monophosphate (8Br-cGMP) on the same neurons. The spinal cord slice preparation contains functional NO-synthase (NOS), because selective blockade of NOS increased the spontaneous activity of those neurons from laminae I+II which were inhibited by SNP and this effect could be reversed by superfusion with the natural substrate for NOS, l-arginine. It is concluded that NO can activate and inhibit the activity of spinal cord neurons by raising cGMP levels and that these effects are lamina specific. A general consequence of our results is that the NO-induced production of cGMP alone does not allow any prediction about an excitatory or inhibitory effect of NO on the discharge rate of neurons. Thus the NO mediated increase and decrease in neuronal activity is probably the result of intracellular mechanisms downstream from the production of cGMP which results in the activation or inhibition of different ion channels on neurons in laminae I+II and X.