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. 2014 May 21;82(4):797-808.
doi: 10.1016/j.neuron.2014.04.008. Epub 2014 Apr 24.

Chemogenetic Synaptic Silencing of Neural Circuits Localizes a Hypothalamus→midbrain Pathway for Feeding Behavior

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Free PMC article

Chemogenetic Synaptic Silencing of Neural Circuits Localizes a Hypothalamus→midbrain Pathway for Feeding Behavior

Tevye J Stachniak et al. Neuron. .
Free PMC article

Abstract

Brain function is mediated by neural circuit connectivity, and elucidating the role of connections is aided by techniques to block their output. We developed cell-type-selective, reversible synaptic inhibition tools for mammalian neural circuits by leveraging G protein signaling pathways to suppress synaptic vesicle release. Here, we find that the pharmacologically selective designer Gi-protein-coupled receptor hM4D is a presynaptic silencer in the presence of its cognate ligand clozapine-N-oxide (CNO). Activation of hM4D signaling sharply reduced synaptic release probability and synaptic current amplitude. To demonstrate the utility of this tool for neural circuit perturbations, we developed an axon-selective hM4D-neurexin variant and used spatially targeted intracranial CNO injections to localize circuit connections from the hypothalamus to the midbrain responsible for feeding behavior. This synaptic silencing approach is broadly applicable for cell-type-specific and axon projection-selective functional analysis of diverse neural circuits.

Figures

Figure 1
Figure 1. hM4D is a synaptic silencer
A) In paired whole cell patch clamp recordings of an hM4D–expressing presynaptic neuron (green) and an untransfected postsynaptic neuron (blue), action potentials elicited by current injection (1.5 nA) in the presynaptic cell produce excitatory synaptic currents in the postsynaptic partner. B) In the presence of the hM4D agonist, CNO, the presynaptic action potential is still elicited, but postsynaptic currents are inhibited. C) The average peak amplitude of the postsynaptic current (Base) is reduced with CNO (n = 6). D) Failure rate increases with CNO, indicating a reduction in presynaptic release probability (n = 6). Values are means ± SEM. *p < 0.05, ***p < 0.001.
Figure 2
Figure 2. Efficacy and potency of hM4D/CNO synaptic silencing
A) During loose-seal cell attached recordings in brain slices, pairs of photostimuli were targeted with a focal laser spot (blue starburst) to ChR2/hM4D–transfected L2/3 neurons, which evoked action potentials from somata in L2/3. Application of CNO (1 µM) does not substantially inhibit the fidelity (action potential/photostimulus) of ChR2-evoked action potential initiation. B) In whole cell recordings from postsynaptic L5 neurons, CNO (1 µM) inhibits synaptic currents evoked by photostimulation of presynaptic L2/3 neuron somata (n = 7) with two light pulses. C) CNO potently inhibits synaptic transmission (red diamonds), but does not impair ChR2-induced action potentials (purple squares). D) Paired-pulse photostimulation of ChR2/hM4D–expressing L2/3 neurons (2 × 1 ms, 20Hz) showed increased paired-pulse ratio (P2/P1) for evoked synaptic currents in L5 neurons (30 nM, 100 nM, and 1 µM CNO; n = 8, 6, and 7 respectively), indicating that hM4D/CNO reduces presynaptic release probability. E) Cell attached recordings of back-propagating action potentials elicited by photostimulation of L2/3→L5 axons in L5 (blue starburst). L2/3 neurons co-expressed ChR2 and hM4D. Application of CNO (1–10 µM) did not reduce the fidelity of action potential back-propagation (n = 8). F) In axon-attached recordings from the cut ends of L2/3 neuron transfected axons in L4 and L5, axonal action potentials arising from L2/3 somatic photostimulation were not impaired by hM4D/CNO signaling, demonstrating that hM4D does not suppress action potential propagation. Values are means ± SEM. n.s. p > 0.05, *p < 0.05, **p < 0.01.
Figure 3
Figure 3. Spatially targeted synaptic silencing in vivo
A–C) Schematics for photostimulation of AGRP neurons co-expressing ChR2 and hM4D in the arcuate nucleus (ARC), while spatially targeting CNO microinjection to the PVH. An angled optical fiber is implanted over the ARC to photostimulate AGRP neuron somata (green), which project axons to multiple brain areas. AGRP neuron photostimulation in this configuration robustly evoked feeding (right). CNO or saline were targeted to the PVH through an implanted cannula. (B) Focal microinjection of CNO to the PVH during AGRP neuron photoactivation result in selective silencing of synaptic release from the targeted ARCAGRP→PVH axonal projection field due to hM4D–mediated synaptic inhibition. Evoked food intake is only partially reduced because AGRP neuron axons still transmit action potentials, and non-targeted axon projections remain competent for synaptic transmission. Injection sites were verified postmortem (inset) by injection of FluoroGold (blue) and AGRP immunofluorescence (green). (C) In some mice, cannula placement was outside of the PVH by 300–500 µm (“Miss”). D) Local microinjections of CNO into the PVH (“Hits”) significantly reduced feeding (3 µM CNO, n = 5). No reduction in feeding was observed for CNO injections that missed the PVH (3 µM CNO, n = 4) or with CNO injections into ChR2-expresing mice that lack hM4D (300 µM CNO, n = 5). E) PVH microinjection of CNO reduced feeding by about 50% over a range of doses. Precision of targeted synaptic silencing was reduced with increasing CNO dose, as evidenced by the capacity of injections that miss the PVH to inhibit feeding. Intraperitoneal injection of CNO further reduced evoked feeding to basal levels (18 ± 4% of evoked feeding). Values are means ± SEM. *p < 0.05.
Figure 4
Figure 4. Axon projection-selective silencing with hM4Dnrxn
A) hM4Dnrxn was constructed by addition of an intracellular amino acid sequence from neurexin-1 (a.a. 1425 −1479) to the C-terminus of hM4D that also contained a C-terminal hemagglutinin (HA) tag. B) Cell surface distribution of N-terminal (extracellular) epitope tagged HA-hM4D (Armbruster et al., 2007) and HA-hM4Dnrxn (see Experimental Procedures) was determined in hippocampal neuronal cultures that were co-transfected with EGFP. For surface labeling, anti-HA immunofluorescence was measured without membrane permeabilization. HA-hM4Dnrxn showed reduced intensity in the somatic compartment but not the axonal compartment, compared to HA-hM4D. Label intensity was quantified using linescans (hashed lines) across the cell body or synaptic boutons. EGFP fluorescence (HA/EGFP overlay) was used to trace axons and identify transfected neurons. Representative soma and axon images of anti-HA immunofluorescence were adjusted identically for brightness and contrast for display purposes. Cells transfected with EGFP alone did not show detectable signal for surface label anti-HA immunofluorescence. C) The surface expression of HA-hM4Dnrxn is distributed to the axonal compartment, as illustrated by a reduction in soma:axon (S/A) immunofluorescence ratio, relative to HA-hM4D. D) In cortical brain slices co-expressing hM4Dnrxn and ChR2, synaptic inhibition of L2/3→L5 transmission is as effective with hM4Dnrxn (n = 5) as with hM4D (n = 8). E) The dose response relationship for synaptic inhibition with CNO is similar for hM4Dnrxn and hM4D. F) Hyperpolarization in L2/3 cortical neurons expressing hM4Dnrxn was negligible with CNO (1 µM) (n = 6). G) Conductance change in response to CNO (1 µM) is markedly reduced in L2/3 cortical neurons expressing hM4Dnrxn (n = 6), as compared to hM4D (n = 7). Values are means ± SEM. n.s. p > 0.05, **p < 0.01, ***p < 0.001.
Figure 5
Figure 5. Cell type-selective synaptic silencing localizes a feeding circuit
A) Schematic of descending axon projections from PVHSIM1 neurons, which express hM4Dnrxn. These axon projections were targeted in separate animals for spatially defined synaptic silencing by intracranial microinjection of CNO. Estimated precision of microinjections (based on ±500 µm) at each site is indicated by colored circles (for clarity, cannula schematic is not shown for ‘purple’ injection site). Injection cannula was angled for PAG targeting in order to avoid the sinus confluens (blood vessel, dark red). B) Pre-injection baseline (Base) and evoked feeding response (1 h) from regions targeted by intracranial injections (CNO, 3 µM, typically bilateral) at distinct anterior-posterior positions. The most efficacious feeding responses were observed with microinjections into the PAGvl/DR (light green), which was similar to food consumption evoked by intraperitoneal injection of CNO (IP). Intracranial microinjections that were more ventral or dorsal than the indicated areas were not included in the analysis. C) Schematic of sagittal section showing the location of injection sites, which are color coded to reflect food intake evoked after intracranial 3 µM CNO microinjection, normalized to feeding after IP injection. Two transverse sections are displayed, at the level of PAGvl/DR and the caudal PBN, with the positions of multiple injection sites projected onto a coronal diagram. Bilateral injections are displayed as identically color coded pairs. Asterisks denote bilateral microinjection site at the pedunculopontine tegmental nucleus (PPTg), which did not evoke substantial food intake. D) Dose response for CNO microinjections at different injection sites. Selectivity is reduced with increasing CNO dose, such that microinjection sites outside the PAGvl/DR region can evoke feeding. Values are means ± SEM.

Comment in

  • Silencing synapses with DREADDs.
    Zhu H, Roth BL. Zhu H, et al. Neuron. 2014 May 21;82(4):723-5. doi: 10.1016/j.neuron.2014.05.002. Neuron. 2014. PMID: 24853931 Free PMC article.

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