Synaptic interactions between primary afferent terminals and GABA and nitric oxide-synthesizing neurons in superficial laminae of the rat spinal cord

Abstract

The superficial laminae (I and II) of the spinal dorsal horn receive small caliber primary afferent fibers responsive to noxious stimulation, and contain local circuit neurons that modulate afferent input. Many of these neurons are GABAergic; about a third of these also synthesize nitric oxide. We identified three main morphological types of primary afferent terminals in superficial laminae after injections of a tracer selective for small caliber afferents into the sciatic nerve of rats. The relative densities of the three types varied through the dorsoventral extent of laminae I and II. Synaptic contacts of each type with GABA- and nitric oxide synthase (NOS)-containing dendrites and axon terminals were determined by preembedding and postembedding immunocytochemistry. Nonglomerular primary afferent terminals, likely to originate from peptidergic unmyelinated fibers, were not seen in synaptic contact with either GABA- or NOS-containing neurons. Primary afferent terminals at the center of type 1 glomeruli (C1) and at the center of type 2 glomeruli (C2) are likely to originate from unmyelinated and small myelinated fibers, respectively. GABAergic terminals contacted more C2 than C1 terminals, suggesting more effective presynaptic inhibition of C2 terminals. Many GABAergic terminals were also positive for NOS, but all GABAergic terminals presynaptic to primary afferent terminals were negative for NOS. Only C2 terminals established frequent synapses with NOS-positive dendrites. These results provide morphological evidence for selective inhibitory gating of input to superficial dorsal horn, and suggest that the link between noxious input and nitric oxide-synthesizing neurons, likely to be involved in nociception, may be provided both by direct synaptic contacts of small myelinated fibers onto dorsally extending NOS-positive dendrites, and by unmyelinated fibers via an oligosynaptic pathway.