Cholinergic interneurons mediate fast VGluT3-dependent glutamatergic transmission in the striatum

Abstract

The neurotransmitter glutamate is released by excitatory projection neurons throughout the brain. However, non-glutamatergic cells, including cholinergic and monoaminergic neurons, express markers that suggest that they are also capable of vesicular glutamate release. Striatal cholinergic interneurons (CINs) express the Type-3 vesicular glutamate transporter (VGluT3), although whether they form functional glutamatergic synapses is unclear. To examine this possibility, we utilized mice expressing Cre-recombinase under control of the endogenous choline acetyltransferase locus and conditionally expressed light-activated Channelrhodopsin2 in CINs. Optical stimulation evoked action potentials in CINs and produced postsynaptic responses in medium spiny neurons that were blocked by glutamate receptor antagonists. CIN-mediated glutamatergic responses exhibited a large contribution of NMDA-type glutamate receptors, distinguishing them from corticostriatal inputs. CIN-mediated glutamatergic responses were insensitive to antagonists of acetylcholine receptors and were not seen in mice lacking VGluT3. Our results indicate that CINs are capable of mediating fast glutamatergic transmission, suggesting a new role for these cells in regulating striatal activity.

Figure 1. ChR2-mediated activation of striatal cholinergic interneurons.

(Ai) Confocal image of mCherry-positive ChR2-expressing neurons in the dorsal striatum of a ChAT-IRES-Cre mouse injected with AAV encoding DFI-ChR2-mCherry. (Aii) Fluorescence immunohistochemical staining for ChAT reveals cholinergic neurons. (Aiii) Merged image. (B) inset, Schematic diagram of the recording conditions: cell-attached recordings were made from ChR2-expressing CINs (red) in the dorsal striatum and blue light was delivered to the surrounding area (blue circle) through the microscope objective. main panel, Example action potential recorded in cell-attached mode from a ChR2-expressing CIN evoked by a 4 ms pulse of 473 nm light.

Figure 2. Light-evoked CIN action potentials evoke glutamatergic responses in MSNs.

(A) inset, Schematic diagram of the recording conditions: whole-cell recordings were made from MSNs that neighbored ChR2-expressing CINs (red). The blue circle indicates the region stimulated by blue light. main panel, Example EPSP recorded in an MSN in response to a 4 ms light pulse (blue bar). (B) Amplitudes of light-evoked EPSPs in the presence of antagonists of GABA_A (Picrotoxin, Ptx), muscarinic (Scopolamine, Scop), and nicotinic (Mecamylamine, Mec) receptors and following application of the AMPA/kainate glutamate receptor antagonist NBQX. (C) Example of light evoked (blue bar) CIN-mediated EPSCs in a voltage-clamped MSN at holding potentials of −70 and +40 mV demonstrating the large current that is visible more than 100 ms after the light pulse at +40 mV. The amplitudes of the rapid −70 and prolonged +40 mV EPSC components were measured in the periods indicated by the gray bars. (D) Confocal image of mCherry-positive ChR2-expressing fibers in the motor cortex, white matter (WM) and underlying striatum (Str). Large bundles of corticofugal fibers (white arrowheads) and diffuse small axonal collaterals are visible throughout the dorsolateral striatum. Scale bar 200 µm. (E) inset, Schematic diagram of the recording conditions: whole-cell recordings were made from MSNs neighboring ChR2-expressing corticostriatal fibers (red), and blue light was delivered to the region indicated by the blue circle. main panel, Example EPSCs recorded in an MSN in response to a blue light pulse at the indicated holding potentials. (F) left, Average amplitudes of light-evoked EPSCs measured in MSNs held at either −70 or +40 mV in response to ChR2-mediated activation of either CINs or corticostriatal fibers (Cx). right, Average ratio of +40/−70 mV current amplitudes measured following ChR2-mediated activation of either CINs or corticostriatal fibers.

Figure 3. CIN-mediated glutamatergic currents in MSNs require VGluT3 expression.

(A) Example light-evoked EPSCs at 32–34°C in an MSN held at −70 or +40 mV in an acute striatal slice of a DFI-ChR2-mCherry AAV injected ChAT-IRES-Cre mouse. (B) As in Panel A, showing recordings at 32–34°C obtained from an MSN in an acute slice of a DFI-ChR2-mCherry AAV injected GM60 mouse that expresses Cre under control of a BAC spanning the ChAT genomic locus. (C) As in Panel B, showing failures to evoke EPSCs in an MSN of a DFI-ChR2-mCherry AAV injected GM60; VGluT3^−/− mouse. (D) Average light-evoked EPSC amplitudes measured in MSNs at holding potentials of −70 and +40 mV in acute slices prepared from mice of the indicated genotypes.