Nitric oxide-producing islet cells modulate the release of sensory neuropeptides in the rat substantia gelatinosa

The substantia gelatinosa of the spinal cord (lamina II) is the major site of integration for nociceptive information. Activation of NMDA glutamate re ceptor, production of nitric oxide (NO), and enhanced release of substance P and calcitonin gene-related peptide (CGRP) from primary afferents are key events in pain perception and central hyperexcitability. By combining redu ced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histoche mistry for NO-producing neurons with immunogold labeling for substance P, C GRP, and glutamate, we show that (1) NO-producing neurons in lamina IIi are islet cells; (2) these neurons rarely form synapses onto peptide-immunorea ctive profiles; and (3) NADPH diaphorase-positive dendrites are often in cl ose spatial relationship with peptide-containing terminals and are observed at the periphery of type II glomeruli showing glutamate-immunoreactive cen tral endings. By means of confocal fluorescent microscopy in acute spinal c ord slices loaded with the Ca2+ indicator Indo-1, we also demonstrate that (1) NMDA evokes a substantial [Ca2+](i) increase in a subpopulation of neur ons in laminae I-II, with morphological features similar to those of islet cells; (2) a different neuronal population in laminae I-IIo, unresponsive t o NMDA, displays a significant [Ca2+](i) increase after slice perfusion wit h either substance P and the NO donor 3-morpholinosydnonimine (SIN-1); and (3) the responses to both substance P and SIN-I are either abolished or sig nificantly inhibited by the NK1 receptor antagonist sendide. These results provide compelling evidence that glutamate released at type II glomeruli tr iggers the production of NO in islet cells within lamina IIi after NMDA rec eptor activation. The release of substance P from primary afferents trigger ed by newly synthesized NO may play a crucial role in the cellular mechanis m leading to spinal hyperexcitability and increased pain perception.