Functional Synaptic Integration of Forebrain GABAergic Precursors into the Adult Spinal Cord

Abstract

Spinal cord transplants of embryonic cortical GABAergic progenitor cells derived from the medial ganglionic eminence (MGE) can reverse mechanical hypersensitivity in the mouse models of peripheral nerve injury-and paclitaxel-induced neuropathic pain. Here, we used electrophysiology, immunohistochemistry, and electron microscopy to examine the extent to which MGE cells integrate into host circuitry and recapitulate endogenous inhibitory circuits. Whether the transplants were performed before or after nerve injury, the MGE cells developed into mature neurons and exhibited firing patterns characteristic of subpopulations of cortical and spinal cord inhibitory interneurons. Conversely, the transplanted cells preserved cortical morphological and neurochemical properties. We also observed a robust anatomical and functional synaptic integration of the transplanted cells into host circuitry in both injured and uninjured animals. The MGE cells were activated by primary afferents, including TRPV1-expressing nociceptors, and formed GABAergic, bicucullinesensitive, synapses onto host neurons. Unexpectedly, MGE cells transplanted before injury prevented the development of mechanical hypersensitivity. Together, our findings provide direct confirmation of an extensive, functional synaptic integration of MGE cells into host spinal cord circuits. This integration underlies normalization of the dorsal horn inhibitory tone after injury and may be responsible for the prophylactic effect of preinjury transplants.

Long‐term, dynamic synaptic reorganization after GABAergic precursor cell transplantation into adult mouse spinal cord

“…These contacts involved multiple, unlabeled dendrites, a glomerular synaptic assembly characteristic of primary afferent terminals (Basbaum et al, ; Ribeiro‐da‐Silva et al, ). This ultrastructural arrangement is consistent with our recent finding that optogenetic stimulation of TRPV1‐expressing, unmyelinated afferents monosynaptically activates transplanted cells (Etlin et al, ). Conversely, we also observed axoaxonic appositions between MGE and presumptive host terminals, including a GFP‐immunoreactive terminal that likely formed a synapse onto a scalloped terminal at the center of a synaptic glomerulus.…”

“…Silver et al, ; Sun & He, ). Taken together with the fact that transplants performed in uninjured spinal cord can prevent the mechanical hypersensitivity produced by subsequent nerve injury (Etlin et al, ), our demonstration of extensive synaptic reorganization in the largely intact spinal cord provides some of the most powerful evidence that structural plasticity in the adult spinal cord can have significant behavioral consequences.…”

“…The structural plasticity that we observed is particularly impressive given that the normal environment of the transplanted cells is the cerebral cortex and that the cells appear to retain their cortical phenotypes. Specifically, the MGE cells express somatostatin (J. M. Braz et al, ; Etlin et al, ), which is characteristic of some cortical, but not spinal cord, GABAergic interneurons. In the present report we also demonstrate that the morphology of the transplanted cells is comparable to cortical GAD‐GFP interneurons in transgenic GAD‐GFP mice, with round cell bodies and widely ramifying dendrites.…”

“…Specifically, the MGE cells express somatostatin (J. M. Braz et al, 2012;Etlin et al, 2016), which is characteristic of some cortical, but not spinal cord, GABAergic interneurons. In the present report we also demonstrate that the morphology of the transplanted cells is comparable to cortical GAD-GFP interneurons in transgenic GAD-GFP mice, with round cell bodies and widely ramifying dendrites.…”

“…Based on Fos expression in transplanted neurons induced by noxious stimuli, we suggested that the functional improvement resulted from integration of the transplanted cells into host dorsal horn circuitry. Consistent with this conclusion, we recently demonstrated that optogenetic stimulation of sensory neurons activates transplanted neurons and that optogenetic stimulation of the transplants results in GABA A receptor‐mediated inhibition of host neurons (Etlin et al, ). Finally, we studied mice in which the gene that encodes the vesicular GABA transporter had been deleted.…”