Achieving High-Frequency Optical Control of Synaptic Transmission

Abstract

The optogenetic tool channelrhodopsin-2 (ChR2) is widely used to excite neurons to study neural circuits. Previous optogenetic studies of synapses suggest that light-evoked synaptic responses often exhibit artificial synaptic depression, which has been attributed to either the inability of ChR2 to reliably fire presynaptic axons or to ChR2 elevating the probability of release by depolarizing presynaptic boutons. Here, we compare light-evoked and electrically evoked synaptic responses for high-frequency stimulation at three synapses in the mouse brain. At synapses from Purkinje cells to deep cerebellar nuclei neurons (PC3 DCN), light-and electrically evoked synaptic currents were remarkably similar for ChR2 expressed transgenically or with adeno-associated virus (AAV) expression vectors. For hippocampal CA33 CA1 synapses, AAV expression vectors of serotype 1, 5, and 8 led to light-evoked synaptic currents that depressed much more than electrically evoked currents, even though ChR2 could fire axons reliably at up to 50 Hz. The disparity between optical and electrical stimulation was eliminated when ChR2 was expressed transgenically or with AAV9. For cerebellar granule cell to stellate cell (grc3 SC) synapses, AAV1 also led to artificial synaptic depression and AAV9 provided superior performance. Artificial synaptic depression also occurred when stimulating over presynaptic boutons, rather than axons, at CA33 CA1 synapses, but not at PC3 DCN synapses. These findings indicate that ChR2 expression methods and light stimulation techniques influence synaptic responses in a neuron-specific manner. They also identify pitfalls associated with using ChR2 to study synapses and suggest an approach that allows optogenetics to be applied in a manner that helps to avoid potential complications.

Optogenetic cholinergic modulation of the mouse superior colliculus in vivo

“…Furthermore, previous studies have driven cholinergic neurons in other brain regions with frequencies Ͼ 16 Hz (Herman et al 2014;Ma and Luo 2012;Ren et al 2011;Witten et al 2010). Thus this limit was likely due to the kinetics of activating the ChR2 expressed in the terminals of our ChATCre/ChR2 mice (Jackman et al 2014;Schoenenberger et al 2011). 3) Related to this, the response to the final light pulse of 16-Hz stimulation appears to be larger than the responses to the preceding pulses (Fig.…”

Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

“…Figure 6F), while observing a significant reduction in PPF; the PPR was 1.27 ± 0.06, n = 11 cells and 1.09 ± 0.05, n = 7 cells (p < 0.05, t test) in sham and 6-OHDA lesioned mice, respectively. The differences between electrical and optical stimulation may be because electrical stimulation is less selective than optical stimulation or because ChR2 and AAVs used for gene delivery increase the release probability of synapses (Jackman et al, 2014; Zhang and Oertner, 2007). …”

Transgenic Mice for Intersectional Targeting of Neural Sensors and Effectors with High Specificity and Performance

“…We previously establish a standardized Cre-reporter system in which transgene expression was driven by a strong, ubiquitous CAG promoter targeted to the Rosa26 locus (Madisen et al, 2010; Muzumdar et al, 2007), expressing fluorescent proteins, calcium sensor GCaMP3, and optogenetic activator ChR2(H134R) and silencers Arch and eNpHR3.0 (Madisen et al, 2012; Madisen et al, 2010; Zariwala et al, 2012). Although proved useful in many applications (Ackman et al, 2012; Haddad et al, 2013; Issa et al, 2014; Jackman et al, 2014; Kheirbek et al, 2013; Lee et al, 2014; Nguyen-Vu et al, 2013; Pi et al, 2013), we and others have also identified limitations in the sensitivity or functionality of these reporters in other situations.…”