Synaptic interactions between GABA-immunoreactive profiles and the terminals of functionally defined myelinated nociceptors in the monkey and cat spinal cord.
This study analyzes the synaptic interactions between the central terminals of A delta high threshold mechanoreceptors (A delta HTMs) and GABA-immunoreactive profiles. A delta HTM primary afferents from three monkeys and one cat were electrophysiologically identified and intracellularly labeled with HRP, and their terminal arborizations in laminae I and II of the sacrocaudal spinal cord were studied at the ultrastructural level. GABA-immunoreactive profiles in relation to A delta HTM terminals were demonstrated using postembedding colloidal gold techniques. Monkey A delta HTM terminals (n = 131) usually constituted the central element of synaptic glomeruli; they established large asymmetric synaptic contacts with 1-13 dendrites (modal value 2-4) and were surrounded by 0-6 peripheral axon terminals (modal value 2-3). The large majority (around 85%) of the peripheral axon terminals were GABA immunoreactive. They were found presynaptic to the A delta HTM terminal and/or to dendrites postsynaptic to the primary afferent terminal. Furthermore, all peripheral axon terminals found presynaptic to the A delta HTM terminals showed GABA immunoreactivity. Within a single A delta HTM fiber, this synaptic arrangement was found in 20-60% of its boutons. In addition, 28% of the postsynaptic dendritic profiles displayed weak GABA immunoreactivity. Some of them contained vesicles; however, only in a few cases did we observe synapses between a GABA-immunoreactive vesicle-containing dendrite and a dendritic profile postsynaptic to an A delta HTM terminal. Similar synaptology and interactions with GABA-immunoreactive profiles were displayed by the terminals of the single cat A delta HTM fiber studied. Our data support the hypothesis that GABA-containing neurons use both presynaptic and/or postsynaptic mechanisms to exert a powerful control, presumably inhibitory, over the transmission of nociceptive information between A delta HTM afferents and second-order neurons in monkey and cat spinal cord. Our results also imply that GABA may be released within the synaptic glomeruli formed by A delta HTM terminals either by local dendrites or by axon terminals. We discuss the possibility that these GABAergic synapses can be driven by inputs from both primary afferents and/or descending systems to modulate the transmission of nociceptive sensory information.
Alvarez
F J
FJ
Department of Physiology, School of Medicine, University of North Carolina, Chapel Hill 27599.
Kavookjian
A M
AM
Light
A R
AR
D000818:Animals; D002415:Cats / Q000033:anatomy & histology / Q000502:physiology*; D008253:Macaca mulatta / Q000033:anatomy & histology / Q000502:physiology*; D009411:Nerve Endings / Q000502:physiology; D009413:Nerve Fibers, Myelinated / Q000502:physiology* / Q000648:ultrastructure; D009619:Nociceptors / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology / Q000648:ultrastructure; D013569:Synapses / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D005680:gamma-Aminobutyric Acid