P. Aimar et al., Nitric oxide-producing islet cells modulate the release of sensory neuropeptides in the rat substantia gelatinosa, J NEUROSC, 18(24), 1998, pp. 10375-10388
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.