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. 2016 Jun 15;90(6):1189-1202.
doi: 10.1016/j.neuron.2016.05.008. Epub 2016 Jun 2.

Sensory-Derived Glutamate Regulates Presynaptic Inhibitory Terminals in Mouse Spinal Cord

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Free PMC article

Sensory-Derived Glutamate Regulates Presynaptic Inhibitory Terminals in Mouse Spinal Cord

Michael Mende et al. Neuron. .
Free PMC article

Abstract

Circuit function in the CNS relies on the balanced interplay of excitatory and inhibitory synaptic signaling. How neuronal activity influences synaptic differentiation to maintain such balance remains unclear. In the mouse spinal cord, a population of GABAergic interneurons, GABApre, forms synapses with the terminals of proprioceptive sensory neurons and controls information transfer at sensory-motor connections through presynaptic inhibition. We show that reducing sensory glutamate release results in decreased expression of GABA-synthesizing enzymes GAD65 and GAD67 in GABApre terminals and decreased presynaptic inhibition. Glutamate directs GAD67 expression via the metabotropic glutamate receptor mGluR1β on GABApre terminals and regulates GAD65 expression via autocrine influence on sensory terminal BDNF. We demonstrate that dual retrograde signals from sensory terminals operate hierarchically to direct the molecular differentiation of GABApre terminals and the efficacy of presynaptic inhibition. These retrograde signals comprise a feedback mechanism by which excitatory sensory activity drives GABAergic inhibition to maintain circuit homeostasis.

Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1. Physiological deficits at sensory afferent terminals in vGluT1−/− mice
(A–B‴) PvON (green) proprioceptive afferent terminals are juxtaposed to postsynaptic Shank1a (blue) in p12 wt (A–A‴) and vGluT1−/− (B–B‴) mice (similar results obtained at p21 (Figures S1K and S1L). (B) Complete absence of vGluT1 immunoreactivity (red) in sensory terminals of vGluT1−/− mice. (C) Quantification of vGluT1 fluorescence intensity in proprioceptive afferent terminals of p12 wt (n = 133 boutons, 3 mice) and vGluT1−/− spinal cords (n = 101 boutons, 3 mice). vGluT1 levels are not detectable in vGluT1−/− mice (Mann-Whitney Rank Sum, p < 0.001 ***). (D) Number of PvON and Runx3ON proprioceptive sensory neurons in L4 dorsal root ganglion (DRG) in wt and vGluT1−/− mice (n(wt) = 1343 neurons; n(vGluT1−/−) = 1427 neurons; Mann-Whitney Rank Sum, p = 1.00 NS). (E) Quantification of the monosynaptic reflex amplitude at different trials as a percentage of the first trial. The reflex is significantly more depressed in vGluT1−/− mice (n(wt) = 5; n(vGluT1−/−) = 4; t-test, p = 0.013 * (2nd trial), 0.043 * (3rd trial), 0.048 * (4th trial), 0.048 * (5th trial)). (F) Fifth lumbar segment (L5) ventral root responses following repetitive stimulation (5 trials at 1 Hz) of the homonymous dorsal root at supramaximal intensities (5x threshold) in p12 wt (black) and vGluT1−/− (red) mice. Black arrows indicate the stimulus artifact. Scale bar: 2.5 μm (A–B‴). Error bars represent s.e.m. Lines and whiskers on box diagrams represent data between 9th and 91st percentile, dots show the 5th and 95th percentile. See also Figure S1.
Figure 2
Figure 2. Reduction of GAD enzyme expression in vGluT1−/− mice
(A–B‴) Immunoreactivity of GAD65 (red) and GAD67 (green) in GABApre terminals contacting CTbON (blue) proprioceptive afferent terminals in p21 wt (A–A‴) and vGluT1−/− (B–B‴) mice. (C) GAD65 and GAD67 intensity measurements in GABApre and GABApost terminals at p21 (GABApre GAD65: n(wt) = 467 boutons, 3 mice; n(vGluT1−/−) = 315 boutons, 3 mice; Mann-Whitney Rank Sum, p < 0.001 ***; GABApre GAD67: n(wt) = 385 boutons, 3 mice; n(vGluT1−/−) = 173 boutons, 3 mice; Mann-Whitney Rank Sum, p < 0.001 ***; GABApost GAD67: n(wt) = 120 boutons, 3 mice; n(vGluT1−/−) = 119 boutons, 3 mice; Mann-Whitney Rank Sum, p = 0.58 NS). Data shown normalized with respect to wt data. Scale bar: 2.5 μm (A–B‴). Lines and whiskers on box diagrams represent data between 9th and 91st percentile, dots show the 5th and 95th percentile. See also Figure S2.
Figure 3
Figure 3. Compromised presynaptic inhibition in vGluT1−/− mice
(A) Experimental protocol to test presynaptic inhibition in the in vitro spinal cord preparation. The dorsal root-to-ventral root reflex in the L5 segment was conditioned by stimulation of the L4 dorsal root (dr-L4) at various time intervals. For simplicity, the key neurons activated in this pathway are color-coded. Insert shows the monosynaptic reflex recorded extracellularly from the L5 ventral root following stimulation of the homonymous dorsal root in p12 wt (black) and vGluT1−/− (red) mice. Black arrows indicates stimulus artifact. (B and C) Averaged (n = 5) response of the monosynaptic reflex recorded from L5 ventral root following dr-L5 stimulation (test stimulus) and after the conditioning stimulus from dr-L4 (conditioned; 800 ms) in wt (blue traces in B) and vGluT1−/− (red traces in C) mice. Frequency of stimulation: 0.1 Hz. The level of presynaptic inhibition of the monosynaptic reflex is highlighted by horizontal arrows in wt (blue) and vGluT1−/− (red) mice. Exposure to bicuculline (grey traces) restored the amplitude of the monosynaptic reflex to test stimulus level (dotted line: assigned as 100% of test response). (D–F) Percentage change of the monosynaptic reflex amplitude during 700–900 ms conditioning intervals (grey boxed region in Figures S3D–S3F) in normal solution (D), under 10 μM bicuculline (E) and under 10 μM bicuculline and 10 μM strychnine (F) in wt (blue, n = 5, p12) and vGluT1−/− (red, n = 4, p12) mice. The level of presynaptic inhibition is significantly reduced in vGluT1−/− preparations under normal solution (D: n(wt) = 4; n(vGluT1−/−) = 4; t-test, p = 0.044 * (700 ms), 0.026 * (800 ms), 0.043 * (900 ms); E: n(wt) = 4; n(vGluT1−/−) = 4; t-test, p = 0.33 NS (700 ms), 0.17 NS (800 ms), 0.57 NS (900 ms); F: n(wt) = 5; n(vGluT1−/−) = 3; t-test, p = 0.97 NS (700 ms), 0.45 NS (800 ms), 0.57 NS (900 ms)). For details see Figures S3D–S3F. Error bars represent s.e.m. See also Figure S3.
Figure 4
Figure 4. mGluR1β expression in GABApre terminals
(A–D) N45GFP immunoperoxidase (N45GFP-ImP; grey immunoreactivity) and mGluR1β immunogold (mGluR1β-ImG; gold particle) double-labeled in GABApre presynaptic terminals (p). mGluR1β ImG particle (red arrowhead) is membrane-associated and in close proximity to sensory afferent terminals (st) on p21 motor neuron dendrites (de). (A′) Color-coded outline of terminals and mGluR1β labeling shown in a. (E–E‴) N45GFPON (green) GABApre terminals contacting vGluT1ON sensory terminals (blue) express mGluR1β (red) in p21 Gad65-N45GFP mice (yellow arrowheads indicate mGluR1β and GFP co-labeling). (F–F‴) mGluR1β (red) overlaps with Syt1 (green) on vGluT1ON sensory terminals (blue) in wt spinal cords (yellow arrowheads indicate mGluR1β and Syt1 co-labeling). (G–G‴) mGluR1β (red) and the GABApre presynaptic marker Syt1 (blue) co-label with N45GFPON (green) GABApre terminals (yellow arrowheads indicate mGluR1β, GFP and Syt1 co-labeling). Scale bars: 200 nm (A, B–D), 2.5 μm (E–G‴). See also Figure S4.
Figure 5
Figure 5. Reduction of GAD67 levels in GABApre terminals in mGluR1−/− mice
(A and B) Images of vGluT1ON (red) proprioceptive afferent terminals (white arrowheads) and cholinergic terminals (vesicular acetylcholine transporter (vAChT) and ChAT (green)) in p21 wt (A) and mGluR1−/− (B) mice. Motor neurons labeled for neuronal nuclei (NeuN, blue), and ChAT (green). ChAT and vAChT visualized in green. (C–D‴) GAD65ON (red) and GAD67ON (green) GABApre terminals contacting vGluT1ON proprioceptive afferent terminals (blue) in p21 wt and mGluR1−/− mice. (E and F) Quantification of GABApre and non-GABApre synaptic marker intensities. GAD67 levels are significantly reduced in mGluR1−/− GABApre terminals as compared to wt mice (E; n(wt) = 564 boutons, 5 mice; n(mGluR1−/−) = 647 boutons, 6 mice; Mann-Whitney Rank Sum, p < 0.001 ***). No significant change in GAD65 protein levels (E; n(wt) = 777 boutons, 5 mice; n(mGluR1−/−) = 898 boutons, 6 mice; Mann-Whitney Rank Sum, p = 0.065 NS). vGluT1 and GAD67 levels in (non-GABApre) terminals on motor neurons are unaffected in mGluR1−/− mice (F; wt (non-GABApre vGluT1): n = 256 boutons, 3 mice; mGluR1−/− (non-GABApre vGluT1): n = 248 boutons, 4 mice; Mann-Whitney Rank Sum, p = 0.29 NS; wt (non-GABApre GAD67): n = 262 boutons, 3 mice; mGluR1−/− (non-GABApre GAD67): n = 226 boutons, 4 mice; Mann-Whitney Rank Sum, p = 0.10 NS). Scale bars: 10 μm (A, B), 2.5 μm (C–D‴). Lines and whiskers on box diagrams represent data between 9th and 91st percentile, dots show the 5th and 95th percentile. See also Figure S5.
Figure 6
Figure 6. Loss of mGluR1 results in reduced presynaptic inhibition
(A and B) Averaged responses of the monosynaptic reflex recorded from vr-L5 following dr-L5 stimulation (test stimulus) and after conditioning stimulus from dr-L4 (conditioned) in wt (A) and mGluR1−/− (B) mice. Horizontal arrows (blue for wt and brown for mGluR1−/− mice) indicate the level of presynaptic inhibition. (C) Percent change of monosynaptic reflex amplitude at three conditioning intervals (700–900 ms), in which presynaptic inhibition in mGluR1−/− mice (brown bars) was significantly reduced compared to wt mice (blue bars) in normal solution (n = 4 for all groups; t-test compared to wt, p = 0.048 * (700 ms), 0.046 * (800 ms), 0.050 * (900 ms)). (D) In the presence of 50 μM CPCCOEt, the strength of presynaptic inhibition is not significantly reduced (green bars) (n = 3 in both groups; t-test, p = 0.393 NS (700 ms), 0.585 NS (800 ms), 0.107 NS (900 ms)). Error bars represent s.e.m.
Figure 7
Figure 7. GAD65 in GABApre terminals is regulated by sensory-derived BDNF
(A and B) BDNF ELISA of L3–L5 mouse spinal cords at p12 shows significantly reduced BDNF in vGluT1−/− as compared to wt mice (A; n = 4 in both groups; t-test, p = 0.005 **). BDNF levels are not significantly reduced in mGluR1 mutant mice (B; n = 4 in both groups; t-test, p = 0.157 NS). (C–F‴) ICV injection of AAV2/9-CMV-GFP (+GFP) results in GFP labeling of sensory neurons in the DRG (C) and a subset of motor neurons (D). GFP is detectable in terminals of proprioceptive sensory afferents labeled with Pv and vGluT1 (E–F‴). GAD expression levels were analyzed in GABApre terminals on GFPON sensory terminals. (G–P) GAD65 and GAD67 expression in wt and vGluT1−/− mice injected with AAV2/9-CMV-GFP (+GFP) or AAV2/9-CMV-GFP-BDNF (+BDNF). Reduced GAD65 levels are rescued in vGluT1−/− mice overexpressing BDNF (G–K). GAD67 levels remain unchanged (L–P). Data shown normalized with respect to [wt +GFP] data (GAD65: n = 300 boutons, 3 mice for all conditions; One-way ANOVA with Tukey post test, [wt +GFP] vs [vGluT1−/− +GFP] p < 0.001 ***; [wt +GFP] vs [wt +BDNF] NS; [wt +GFP] vs [vGluT1−/− +BDNF] NS; [wt +BDNF] vs [vGluT1−/− +GFP] p < 0.001 ***; [wt +BDNF] vs [vGluT1−/− +BDNF] p < 0.001 ***; [vGluT1−/− +GFP] vs [vGluT1−/− +BDNF] p < 0.001 ***; GAD67: n = 300 boutons, 3 mice for all conditions; One-way ANOVA with Tukey post test, [wt +GFP] vs [vGluT1−/− +GFP] p < 0.001 ***; [wt +GFP] vs [wt +BDNF] NS; [wt +GFP] vs [vGluT1−/− +BDNF] p < 0.001 ***; [wt +BDNF] vs [vGluT1−/− +GFP] p < 0.001 ***; [wt +BDNF] vs [vGluT1−/− +BDNF] p<0.001 ***; [vGluT1−/− +GFP] vs [vGluT1−/− +BDNF] NS). (Q) Percentage change of monosynaptic reflex amplitude at three conditioning intervals (700 – 900 ms) in Ptf1aCre; TrkBflox/flox mice (orange bars) resulted in moderate reduction in presynaptic inhibition compared with wt mice (blue bars) (n = 3 for all groups; t-test, p = 0.01 * (700 ms), 0.11 NS (800 ms), 0.08 NS (900 ms)). (R) The model proposed for regulation of presynaptic inhibition, whereby glutamate (red) release at sensory terminals controls GAD65 and GAD67 levels in GABApre terminals via two distinct pathways. Glutamate controls the level of GAD67 (red) directly via mGluR1β on GABApre terminals. Glutamate also regulates in an autocrine manner sensory terminal expression of BDNF (green), which acts via TrkB receptors on GABApre terminals to control presynaptic levels of GAD65 (green). Consequently, in vGluT1−/− mice, reduced glutamate release from proprioceptive afferent terminals leads to decreased levels of both GAD65 and GAD67. In mGluR1−/− spinal cords, GABApre synapses are impaired in sensing glutamate release, resulting in reduced levels of GAD67 expression, while GAD65 levels remain normal. Scale bars: 50 μm (C), 20 μm (D), 2 μm (E–J′ and L–O′). Lines and whiskers on box diagrams represent data between 9th and 91st percentile, dots show the 5th and 95th percentile. Error bars represent s.e.m. See also Figure S6.

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