| r-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the central nervous system (CNS), acting on two receptor subtypes in the rat spinal dorsal horn: ligand-gated GABAa and metabotropic GABAB receptors (GABABR). In the spinal dorsal horn, previous studies have demonstrated that: (1)GABABR concentrates in the presynaptic fine nociceptive primary afferent fibers and dorsal horn neurons; (2) activation of which inhibits the release of both excitatory neurotransmitter glutamate and inhibitory neurotransmitter GABA; and (3) behavior study has shown that GABABR mediated antinociception in the spinal level, leading to a possibility that GABABR agonist, baclofen could be considered as a potential analgesic drug. The morphological and functional properties for GABABR, especially its relationship with analgesia, however, remain unclear. The main questions are: (1)precise morphological localization for GABAeR; (2) special modulatory effects of GABABR-mediated depression on different primary afferent inputs related to the antinociception; (3) cellular mechanisms and related ion channels for inhibition mediated by GABAeR; and (4) the effect and mechanism of GABABR for the modulation on the descending pain control system.In the present study, we addressed the above issues by methods of morphologyand electrophysiology. The main results are:(1) Employing immunohistochemical staining method, we found that under light microscope GABABR-like-immunoreactive (GABABR-LI) neuronal cell bodies, libers and terminals were densely located in the spinal dorsal horn, particularly in lamina I and lamina II (substantia gelatinosa, SG), as well as in the dorsal root ganglion (DRG), where many neurons of various diameters showed GABABR-immunopositive staining. Electron microscopy further revealed that GABABR-like immunoreactivities were localized in terminals of myelinated (Ap, A) and unmyelinated (C) fibers in the superficial laminae (lamina I and lamina II) of the dorsal horn. Many GABAbR-LI neuronal cell bodies and dendritic processes were also observed in the superficial laminae. These results indicate that GABAbR locates in the superficial dorsal horn, at both pre- and postsynaptic sites Additionally, GABAbR-like immunopositive products were densely encountered in the DRG (Paper 1).(2) We further investigated the origins of GABABR-LI neuropil in rat spinal dorsal horn. After either lumbar dorsal root rhizotomy or thoracic cord hemisection, the densities of GABAbR-LI terminals in the neuropil of the dorsal horn were reduced in the ipsilateral superficial dorsal horn of relevant segments. The density declined remarkably when the above two cuts being done on the same animal, even so, there were some GABAbR-LI neurons and sparsely distributed fibers and terminals still remained. These results indicate that GABAbR-LI terminals in the spinal dorsal horn originate from peripheral afferent fibers, supraspinal structures and interneurons within the spinal dorsal horn. Combining Fluoro-Gold retrograde tracing with immunofluorescence histochemical staining, we further found that GABABR-LI neurons, mainly in the rostral ventromedialmedulla (RVM), raphe nuclei and midbrain periaqueductal gray (PAG), project to the spinal dorsal horn. Considering that PAG, RVM and raphe nuclei are important structures for the descending pain modulation system, our data suggest that the descending pain modulation system might be mediated, at least in part, by GABAB receptors (Paper 2).(3) We improved the method of making dorsal root-attached spinal cord slice preparation. By using blind whole cell patch-clamp recording method on the spinal cord slice, it is possible to evaluate synaptic transmission by analyzing the postsynaptic currents (Paper 3).(4) Whole-cell voltage-clamp recordings on slice preparation revealed that baclofen, a GABABR agonist, induced a slow outward (inhibitory) current in dorsal horn neurons. This slow current was depressed when the postsynaptic G protein-coupled receptors or K+ channels being inhibi... |
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