GABAergic terminals are presynaptic to primary afferent terminals in the substantia gelatinosa of the rat spinal cord

Abstract

Multiple, dorsal rhizotomies were performed unilaterally at lumbar levels L1–L4 in adult rats. Following 24–48 h degeneration periods and fixation by intracardiac perfusions, spinal cord were removed and transversely cut into 150 μm thick sections. These sections were incubated in immunocytochemical reagents for the peroxidase-labeling of glutamic acid decarboxylase (GAD), the enzyme that synthesizes the neurotransmitter γ-aminobutyric acid (GABA). The sections were then prepared for electron microscopic examination, while other sections were processed for light microscopic, GAD immunocytochemistry and for Fink-Heimer staining of degenerating axons and axon terminals. Thirty-six hours following dorsal rhizotomies, the sections that were prepared for the light microscopic study of terminal degeneration showed large numbers of degenerating profiles in the ipsilateral substantia gelatinosa while degenerating profiles were virtually absent contralaterally. In electron microscopic preparations, degenerating primary afferent terminals were commonly observed at the centers of rosettes where they formed synaptic contacts with other axon terminals and with surrounding dendrites. Several types of synaptic relationships were observed in the rosettes which involved both GAD-positive and degenerating primary afferent terminals. Such synaptic relationships included those in which: (a) a single GAD-positive terminal was presynaptic to the central, primary afferent terminal, (b) two different GAD-positive terminals formed synapses with opposite sides of the same central, primary afferent terminal and were also closely apposed to the surrounding dendrites of the rosette, and (c) a GAD-positive terminal was presynaptic to a primary afferent terminal and both types of terminals were presynaptic to the same dendrite of the rosette. The synaptic relationships described in this study are discussed with respect to their possible functional roles in such GABA-mediated phenomena as: (a) primary afferent depolarization, (b) the dorsal root reflex and (c) primary afferent hyperpolarization. Our observations support the concept that GABAergic axon terminals are involved in the synaptic circuits which produce presynaptic inhibition and presynaptic facilitation of the primary afferent input to the dorsal spinal cord. Collectively the observed synaptic relationships could provide a morphological substrate that is compatible with an inhibitory surround system in the substantia gelatinosa.