Targeting morphine-responsive neurons: generation of a knock-in mouse line expressing Cre recombinase from the mu opioid receptor gene locus.

Abstract The mu-opioid receptor (MOR) modulates nociceptive pathways and reward processing, and mediates the strong analgesic and addictive properties of both medicinal as well as abused opioid drugs. MOR function has been extensively studied, and tools to manipulate or visualize the receptor protein are available. However, circuit mechanisms underlying MOR-mediated effects are less known, because genetic access to MOR-expressing neurons is lacking. Here we report the generation of a knock-in Oprm1-Cre mouse line, which allows targeting and manipulating MOR opioid-responsive neurons. A cDNA encoding a T2A cleavable peptide and Cre recombinase fused to enhanced green fluorescent protein (EGFP/Cre) was inserted downstream of the Oprm1 gene sequence. The resulting Oprm1-Cre line shows intact Oprm1 gene transcription. MOR and EGFP/Cre proteins are coexpressed in the same neurons, and localized in cytoplasmic and nuclear compartments, respectively. MOR signaling is unaltered, demonstrated by maintained DAMGO-induced G-protein activation, and in vivo MOR function is preserved as indicated by normal morphine-induced analgesia, hyperlocomotion, and sensitization. The Cre recombinase efficiently drives the expression of Cre-dependent reporter genes, shown by local virally mediated expression in the medial habenula and brain-wide fluorescence on breeding with tdTomato reporter mice, the latter showing a distribution patterns typical of MOR expression. Finally, we demonstrate that optogenetic activation of MOR neurons in the ventral tegmental area of Oprm1-Cre mice evokes strong avoidance behavior, as anticipated from the literature. The Oprm1-Cre line is therefore an excellent tool for both mapping and functional studies of MOR-positive neurons, and will be of broad interest for opioid, pain, and addiction research.


Introduction
The mu-opioid receptor (MOR) is the primary molecular target for medicinal and abused opioids. This receptor mediates both the unrivaled analgesic properties of opioids for pain treatment, and their adverse effects, including notably their strong addictive potential (Matthes et al., 1996;Darcq and Kieffer, 2018), which is driving the current opioid epidemic (World Health Organization, 2017;Volkow and Koroshetz, 2019). Under physiological conditions, MOR is activated by endogenous opioid peptides, and modulates nociceptive pathways (Corder et al., 2018), respiration centers (Levitt and Williams, 2018), and brain circuits that process reward and emotions (Contet et al., 2004;Lutz and Kieffer, 2013). Although the essential role of MOR in pain, drug abuse, and mood disorders is well established, and receptor adaptations to chronic opioid use have been well studied at the cellular level (Williams et al., 2013;Cahill et al., 2016), circuit mechanisms underlying MOR function (Darcq and Kieffer, 2018) and the regulation of neuronal communication driven by MOR (Mechling et al., 2016) and MOR agonists (Nasseef et al., 2019) are poorly understood.
Several genetic mouse tools have been developed to study MOR function, but have not given genetic access to MOR-expressing neurons as yet. A knock-in MOR-mCherry mouse line was developed to map MOR protein expression throughout the nervous system brain (Gardon et al., 2014;Erbs et al., 2015), and it allows speculation about the circuit mechanisms driving MORmediated behaviors. Mice with a floxed Oprm1 gene have permitted receptor deletion in targeted neurons from nociceptive (Weibel et al., 2013) and reward (Charbogne et al., 2017) pathways, uncovering some circuit mechanisms of MOR-mediated pain control and motivation. A next step to understand MOR physiology, and to fully investigate neural dysfunctions associated with opioid drug use, misuse, and abuse, is to study and manipulate the activity of MOR-expressing neurons that directly respond to both exogenous and endogenous mu-opioids.
To this aim, the best approach is to create a mouse line expressing the Cre recombinase in MOR-expressing neurons. Here we report the generation of a line expressing the Cre recombinase under the control of the Oprm1 gene (encoding MOR) promoter, and present molecular and behavioral characterization of this mouse line (Oprm1-Cre line). We also show successful labeling of MOR-positive neurons using a fluorescent Cre-dependent reporter mouse line. We finally demonstrate that optogenetic stimulation of MOR-positive neurons in the ventral tegmental area is sufficient to induce strong avoidance behavior, as anticipated from the literature. The line fulfills all the criteria to successfully study and manipulate MOR neurons.

Animals
The Oprm1-Cre knock-in mouse line was generated by homologous recombination to express a Cre recombinase under the control of the Oprm1 promoter. In this mouse line, a cDNA encoding a functional EGFP/Cre recombinase fusion protein was inserted into exon 4 of the MOR gene, in frame and 59 of the stop codon, as described in the studies by Gardon et al. (2014) and Erbs et al. (2015). The EGFP/ Cre cDNA was generated by cloning the Cre cDNA [a gift from Daniel Metzger, Institut de Génétique et de Biologie Moléculaire et Cellulaire (IGBMC), Illkirch, France] by PCR into the BglII and EcoRI sites of the pEGFP-C2 plasmid (Clontech/Addgene), resulting in a 7 aa linker SGRTQIS between the two proteins. The cloning of Cre in 39 in phase with EGFP and the absence of mutations were verified by DNA sequencing. The functionality of the EGFP/Cre fusion protein was verified by cotransfecting COS cells with this EGFP/Cre plasmid and with the Cre activity reporter plasmid pCMV-LneoL-Betagal (a gift from Daniel Metzger, IGBMC). Further, a T2A cleavable peptide sequence (Szymczak et al., 2004) was inserted, joining the Oprm1 gene to the EGFP/Cre sequence, so that the EGFP/Cre enzyme is released from the receptor on translation of the MOR-T2A-EGFP/Cre fusion protein. The entire construct was verified by DNA sequencing before homologous recombination was performed. We then verified that the construct had not integrated randomly in the genome. Of note, no DNA sequencing or splicing analysis of the Oprm1 gene was later performed in Oprm1-Cre mice. The genetic background of all mice was 100% C57BL/6N. Mice were group housed (maximum of five mice per cage) in a temperatureand humidity-controlled animal facility (21 6 2°C, 45 6 5% humidity) on a 12 h dark/light cycle with food and water available ad libitum. All experiments were performed in accordance with the Canadian Council of Animal Care and by the Animal Care Committees.

Genotyping PCR
For routine genotyping, the forward primer is ATATTATTTTCCCTGACGCGTTCTG; and the reverse primer is CTGAAGATTGACATTGTATCGAGGA. The PCR product for the Oprm1-Cre wild-type allele is 311 bp, and for the Oprm1-Cre knock-in allele is 387 bp (Extended Data Fig. 1-1).

Dissection for mRNA and signaling testing
Mice were sacrificed by dislocation and the whole brain was quickly extracted and placed upside down in a chilled metal matrix (ASI Instruments). Cold razor blades were inserted into the 1 mm spaced coronal groove with the first most rostral one inserted at the limit of olfactory bulbs. Brain punches were dissected using 1 or 2 mm tissue corers and placed into microtubes, rapidly frozen, and stored at À80°C. A half-brain was used to prepare striatal membranes incubated with different doses of DAMGO in [ 35 S]-GTPg S assay. The other half-brain was used to extract RNA in the following eight regions: dorsal striatum (DS); nucleus accumbens (NAc); habenula (Hb); interpeduncular nucleus (IPN); ventral tegmental area (VTA)/substantia nigra (SN); cerebellum (Cer); periaqueductal gray (PAG); and spinal cord (SC). The gene expression was examined by quantitative PCR.

Quantitative analysis of transcript expression
Quantitative real-time PCR (RT-qPCR) was adapted from the study by Meirsman et al. (2016). Six hundred nanograms of RNA was reverse transcribed using the Invitrogen M-MLV Reverse Transcriptase Kit (Thermo Fisher Scientific) according to the manufacturer instructions. The cDNA was subjected to 45 cycles of amplification using LightCycler 480 SYBR I Green Master Mix (Roche) in the LightCycler 480 II Real-Time PCR System (Roche). cDNA samples were loaded in triplicate, and a no-template control reaction, with just water, was included to check for nonspecific amplification. Relative fold changes were calculated by the comparative Ct method (2 ÀDDCT ; Livak and Schmittgen, 2001) using B2M as a housekeeping gene.

mRNA in situ hybridization
In situ hybridization was performed using Advanced Cell Diagnostics RNAscope probes and reagents according to the manufacturer instruction to detect mRNA encoding MOR (Oprm1) and EGFP (EGFP). Briefly, Oprm1 1/1 and Oprm1 Cre/Cre male mice were killed, and fresh brains were flash frozen in isopentane. The 10-mm-thick coronal sections were cut using a cryostat (Leica), directly mounted on Superfrost slides, and kept at À80°C until processing. Sections were first fixed in chilled 10% neutral buffered formalin for 15 min at 4°C, dehydrated by increasing gradient of ethanol bathes, and left to air dry for 5 min. Endogenous peroxidase activity was quenched with hydrogen peroxide reagent for 10 min, followed by protease digestion for 30 min at room temperature. The following sets of probes were then hybridized for 2 h at 40°C in a humidity-controlled oven (HybEZ II, ACDbio): EGFP-C1 target region 2-707 (catalog #538851, ACDbio) and Oprm1-C2 target region 1135-2162 (catalog #315841-C2, ACDbio). Probes for Oprm1, and eGFP were revealed using respectively Opal Dye 520 and Opal Dye 570-labeled probes. Slides were then coverslipped with Vectashield mounting medium with 49,69-diamidino-2-phenylindole dihydrochloride (DAPI) for nuclear staining (Vector Laboratories) and kept at 4°C until imaging.

[ 35 S]-GTPg S binding assays
The assay was performed as previously described by several studies (Pradhan et al., 2009;Erbs et al., 2015;Meirsman et al., 2016) on membrane preparations from striatum. Striatum was dissected following mouse cervical dislocation, placed on dry ice, and stored at À80°C. To evaluate the MOR function, striatum (n = 2 pools Â four per genotype) were pooled together and membranes were prepared by homogenizing tissues in 0.25 M sucrose with a Polytron, followed by centrifugation at 2500 rpm for 10 min at 4°C. Samples were diluted in TMEN (Tris 50 mM, MgCl2 3 mM, EGTA 0.2 mM, and NaCl 100 mM, pH 7.4) followed by an ultracentrifugation at 40,000 Â g for 30 min at 4°C (MLA-55 rotor, Beckman Coulter). The membrane pellet was resuspended in 0.32 M sucrose by 10 strokes with a potter. Membrane preparations were diluted in 800 ml, aliquoted, and stored at À80°C. Protein concentration was determined by the Bradford assay using a standard curve of bovine serum albumin and triplicate dilution of each sample. For each [ 35 S]-GTPg S binding assay, 5 mg of protein was used per well. Samples were incubated with variable concentration (3 Â 10 À9 to 2 Â 10 À10 M) of DAMGO in assay buffer containing 5 mM GDP and 0.1 nM [ 35 S]-GTPg S for 1 h at 25°C. After wash and filter steps, bound radioactivity was quantified using a liquid scintillation counter (TopCount, PerkinElmer). Nonspecific binding was determined in the absence of agonist. Basal activity was determined in the presence of 10 mM GTPg S. Calculations and sigmoidal dose-response binding curves were done using GraphPad Prism 6 (GraphPad Software).

Image acquisition
Slides were scanned on the Olympus VS120 microscope with a 10Â objective. For fluorescence microscopy, an Olympus IX73 microscope with 10Â or oil-immersion 60Â objective was used. For confocal microscopy imaging, an Olympus FV1200 microscope with 20Â or oil-immersion 60Â objective, was used to take z-stack images.

Behavioral experiments
Morphine-induced analgesia. Mice were intraperitoneally injected with 2.5 or 5 mg/kg morphine or saline, and analgesia was tested using tail immersion (Erbs et al., 2015). Briefly, the mouse was maintained in a cylinder and the tail was immersed in a water bath set at 48°C with a cutoff time of 15 s. Mice were allowed to recover for 1 min followed by tail immersion in a second water bath set at 52°C with a cutoff time of 10 s. The baseline response was measured for tail flick before the first injection, and the morphine-induced analgesia tests were performed 45 min after the injection.
Morphine-induced locomotor sensitization. Locomotor activity was measured in Plexiglas activity boxes (20 Â 20 Â 20 cm) surrounded by horizontal and vertical infrared sensor beams, assisted by VersaMax software. To evaluate the morphine-induced hyperlocomotion at the first session, mice were allowed to explore the boxes for 1 h. Mice were then injected with saline (1 ml/kg) and returned to the boxes for 1 h followed by an intraperitoneal injection of morphine (40 mg/kg) or saline, and activity was recorded for 2 h. For locomotor sensitization, mice were injected twice per week with saline and placed in the boxes for 1 h followed by morphine (40 mg/kg) injection and activity was recorded for 2 h. Measurements were taken on days 1, 4, 8, 11, 14, and 18, as adapted from the study by Darcq et al. (2012).
Real-time place preference. Five weeks after the viral injections, mice were placed in a custom behavioral arena (black Plexiglas, 50 Â 50 Â 25 cm) divided into two identical chambers and allowed to explore each of the two chambers for 20 min. Using an Anymaze hardware controller connected to the laser, light stimulation (473 nm, 10 mW) at 0, 10, 20, or 40 Hz (10 ms pulse width) was delivered through fiber-optic implants during the duration of their time spent in the light stimulation chamber. Mice received no light stimulation in the "no stimulation" chamber. At the start of the session, the mouse was placed in the nonstimulated side of the chamber. The percentage of time spent on the paired stimulation side was recorded via a CCD camera interfaced with the Anymaze (Stoelting) software.

Statistical analysis
All data are presented as the mean SEM. Statistical analysis was assessed using t tests or repeated-measures ANOVA. When ANOVA reached significance, a Tukey's HSD test was conducted. Nonsignificance was defined as p . 0.05, and significance as pp , 0.05, ppp , 0.01, and pppp , 0.001.

Results
Unsuccessful attempts to develop transgenic mouse lines using both short and bacterial artificial chromosome promoters of the mu-opioid receptor gene (Oprm1), led us to use homologous recombination to insert the Cre recombinase gene into the Oprm1 gene locus. A knock-in strategy for the Oprm1 gene was designed ( Fig. 1a) to generate a large precursor protein, which would be further cleaved to release both the native MOR protein and a functional EGFP/Cre recombinase in cells that normally express the MOR. This approach produced the Oprm1 Cre/Cre (or Oprm1-Cre) mouse line, which we characterized extensively.
We first tested whether the insertion of the T2A-EGFP/Cre cDNA into the Oprm1 gene locus modifies levels of gene transcription. Quantitative mRNA analysis in several brain regions, including most MOR-enriched regions, revealed that the genomic modification does not disrupt Oprm1 transcription (Fig. 1b). Using EGFP primers, we also detected the egfp transcript in Oprm1 Cre/Cre but not Oprm1 1/1 control mice, suggesting that the entire Oprm1-T2A-eGFP/Cre transcript is transcribed in the knock-in line (data not shown). We further performed double in situ hybridization using separate probes for Oprm1 and eGFP/Cre mRNAs in brain sections from Oprm1 Cre/Cre mice (two mice, eight sections) and found colocalization of the two transcripts in all the sections examined (Fig. 1c, level of striatum).
Second, we examined expression of MOR and EGFP/ Cre proteins using immunohistochemistry in brain sections from heterozygous Oprm1 Cre/1 mice. In the MHb (i.e., the best MOR-enriched region), we observed predominant expression of the two proteins in both basolateral and apical subregions (Fig. 1d, left) as expected (Gardon et al., 2014). Higher magnification (Fig. 1d, right panels) showed distinct subcellular distribution of the two proteins, as expected from the natural localization of MOR (cytoplasm/membrane) and EGFP/Cre (nuclear). The latter observation suggests that the T2A-EGFP/Cre Research Article: Methods/New Tools Figure 1. Intact MOR function in the Oprm1-Cre line. a, Targeting strategy: a cDNA encoding a functional EGFP/Cre recombinase fusion protein is inserted in frame and upstream the stop codon of the Oprm1 gene. In addition, a T2A cleavable peptide sequence is joining Oprm1 gene to the EGFP/Cre sequence, so that the EGFP/Cre enzyme is released from the receptor on translation of the fusion protein was mostly cleaved, which was further supported by Western blot (data not shown) and the expected activity for the two proteins (see below).
Third, we investigated whether MOR function is intact in this mouse line. We tested MOR signaling in response to DAMGO, a MOR-selective agonist. The [ 35 S]-GTPg S binding assay showed similar potency and efficacy of DAMGO in samples from the two genotypes, suggesting that MOR-mediated G-protein activation is intact in the Oprm1-Cre line (Fig. 1e). Next, we compared the two best documented in vivo effects of morphine in Oprm1 Cre/Cre and Oprm1 1/1 mice. Morphine-induced hyperlocomotion, as well as locomotor sensitization were measured in Oprm1 Cre/Cre and Oprm1 1/1 mice (Fig. 1f). A two-way, mixed-design ANOVA, with genotype as a between-subjects factor, and days since injection as a within-subjects factor, indicated a main effect only for days (F (5,60) = 35.19, p , 0.001). The interaction term was not significant (F (5,60) = 1.70, p . 0.05). Pairwise comparisons tests were performed using a Tukey's HSD test. The mean total traveled distance at days 11, 14, and 18 was significantly higher compared with days 1, 4, and 8 (p , 0.001), and day 8 was significantly higher than day 1 (p , 0.05). Morphine therefore induced similar hyperlocomotion and sensitization in the two genotypes. Morphine analgesia was assessed using the tail immersion test (Fig. 1g). At 48°C, a two-factor (genotype Â injection treatment) between-subject ANOVA yielded significant main effects for injection treatment (F (2,54) . 53.77, p , 0.0001). The interaction term was not significant (F (2,54) = 1.724, p . 0.05). Tukey's HSD tests computed on the main effect for treatment indicated that the mean tail withdrawal latencies were significantly increased for both genotypes and at 2.5 and 5 mg/kg morphine doses (all p values , 0.001). Similarly at 52°C, a two-factor (genotype Â injection treatment) between-subject ANOVA yielded a significant main effect for the treatment (F (2,51) . 45.32, p , 0.0001), with no significant interaction (F (2,51) = 0.453, p . 0.05). Tukey's HSD tests indicated that mean tail withdrawal latencies were significantly increased for the two genotypes and at the two morphine doses (all p values , 0.001). Together, results show intact morphine effects on both activity and pain perception in mutant mice, and overall the data suggest that MOR signaling and function in vivo are maintained in the Oprm1-Cre line.
Fourth, we examined whether the EGFP/Cre fusion protein is able to mediate Cre/LoxP recombination in the Oprm1-Cre line. Of note, although the EGFP/Cre fusion has the advantage of allowing detection of the recombinase, non-EGFP fluorophores should be used when combining with other reporters. We therefore used red fluorescent reporters in the subsequent experiments. We first injected a Cre-dependent fluorescent reporter AAV2-mCherry virus in the MHb, and found a strong fluorescent signal (Fig. 2a, left) with a pattern similar to the EGFP/Cre signal (Fig. 1c) in apical and basolateral MHb. Several compartments of the IPN were also strongly labeled, indicating that the fluorescent reporter is transported along fibers to the major projection site (Fig. 2a,  left), as expected (Gardon et al., 2014). To get a brain-wide view of Cre-mediated recombination in the Oprm1-Cre line, we crossed the mice with Cre-dependent reporter Rosa lsltdTomato mice (see Materials and Methods and Movie 1). The tdTomato protein was strongly expressed in MHb, fasciculus retroflexus, and IPN (Fig. 2a, right), with a pattern similar to that observed on Cre-dependent viral reporter expression except that areas out of the MHb-IPN pathway were also labeled. In particular, fluorescence was observed throughout the mesolimbic pathway, including striatal patches, direct pathway projections neurons, and the VTA/SN that are typical of MOR expression (Cui et al., 2014). Recombination also occurred in the central amgydala, intercalated amygdala, and the endopiriform nucleus, but not in the basolateral amygdala, concordant with sites of high MOR expression (Erbs et al., 2015). Together, these anatomic data demonstrate that the EGFP/Cre protein is functional. Further, Cre activity is found at known sites of continued MOR-T2A-EGFP/Cre fusion protein (Extended Data Fig. 1-1, Knockin (KI) genotyping strategy). b, Oprm1 mRNA expression levels are identical between Oprm1 Cre/Cre and Oprm1 1/1 mice. Quantification was done by RT-qPCR in samples from DS, NAc, Hb, IPN, VTA/SN, Cer, PAG, and SC and shows comparable Oprm1 transcript levels across genotypes. c, Confocal imaging of RNAscope probes targeting eGFP (green) and Oprm1 (purple) mRNAs in addition of DAPI staining (blue) shows colocalization of the two transcripts in dorsal striatum sections of Oprm1 Cre/Cre mice (inset). Dashed lines delimit examples of MOR-EGFP/Cre-positive neurons. Magnification, 60Â with immersion oil. Scale bar, 5 mm. d, Left, Immunohistochemistry shows EGFP/Cre and MOR protein expression in the same habenular subdivisions. Coronal brain sections of heterozygote Oprm1 Cre/1 mice were stained with EGFP and MOR antibodies (Extended Data Fig. 1-2, MOR antibody validation), and fluorescence microscopy shows the expected staining mainly in basolateral (bl) and apical (a) parts of the MHb. Bm, Basomedial; sm, stria medullaris tract. Magnification:, 10Â. Scale bar, 50 mm. Right, Four panels at higher magnification (inset, basolateral part of the MHb), staining reveals nuclear DAPI (blue) and EGFP/Cre (green) staining, whereas MOR staining (purple) is exclusively extranuclear. Magnification, 60Â with immersion oil. Scale bar, 5 mm. Dashed lines delimit one example of an MOR-EGFP/Cre-positive neuron. e, MOR signaling is preserved in Oprm1 Cre/Cre mice. G-protein activation was evaluated using [ 35 S]-GTPg S binding: DAMGO-induced G-protein activation is similar in striatal membranes to the two genotypes (n = 2 pools Â 4 striatum; EC 50 , 324 nM for Oprm1 1/1 and 392 nM for Oprm Cre/Cre ; E max , 148 6 7 for Oprm1 1/1 and 156 6 11 for Oprm1 Cre/Cre ). f, Locomotor sensitization of morphine is intact in Oprm1 Cre/Cre mice. Mice were injected at days 1, 4, 8, 11, 14, and 18 with morphine (40 mg/kg, i.p.). Total traveled distances recorded for 2 h are comparable in Oprm1 1/1 and Oprm1 Cre/Cre mice (n = 7 animals/group). Data are presented as the mean 6 SEM. g, Analgesic effects of morphine are intact in Oprm1 Cre/Cre mice. Analgesia was assessed by tail immersion test: identical tail withdrawal latencies were measured at 48°C and 52°C, in wild-type Oprm1 1/1 and Oprm1 Cre/Cre mice 45 min after a single saline or morphine injection (2.5 or 5 mg/kg; n = 10 animals/group). Dashed horizontal lines show cutoff at 15 s for 48°C and 10 s for 52°C. Data are presented as the mean 6 SEM. pppp , 0.001 morphine effect compared with saline.  Diagram showing viral delivery of AAV2.EF1a.DIO.ChR2-mCherry (channelorhodopsin) or AAV2.EF1a.DIO.mCherry (control) into the VTA and fiber-optic implantation above the VTA, a well as a timeline for the experimental procedure. b, Representative image of viral expression (red), optic fiber implantation, and VTA dopamine cells immunostained with anti-tyrosine hydroxylase (TH)/Alexa Fluor 647 (blue). Tissues were observed on an inverted epifluorescence microscope. c, Confocal imaging of VTA sections stained with GABA antibody show overlapping with viral expression. Scale bar, 10 mm. d, Occupancy plots for representative individual at 20 Hz. Mice were free to explore the two-chamber RTPT apparatus for 20 min. Mice then received a blue stimulation when entering the light-paired side at 0, 10, 20, and 40 Hz (473 nm, 10 mW, 10 ms pulse width) on 4 consecutive days, as described by Seo et al. (2016). e, Activation of VTA MOR neurons produces place avoidance. Light stimulation in Oprm1-Cre VTA-VTA::ChR2 mice induced significant behavioral avoidance to the light-paired side compared with control group (n = 7-8/group). The graph shows frequency responses of mice receiving a blue stimulation when entering the light-paired side (473 nm, 10 mW, 10 ms pulse width at 0, 10, 20, and 40 Hz; n = 9-10, control vs ChR2). Data are expressed as the mean 6 SEM. ppp , 0.01, pppp , 0.001. f, Further analysis of the 20 Hz simulation condition indicated significant avoidance for the light-paired side without affecting the total distance traveled (p . 0.05).
MOR expression, indicating accurate transcriptional control of Cre-egfp under the Oprm1 promoter. Staining patterns observed here differ from those previously reported in MOR-mCherry mice (Gardon et al., 2014;Erbs et al., 2015), as two distinct knock-in strategies were used to label either MOR-expressing cells (MOR-Cre mice; this study) or the receptor itself (MOR-mCherry mice).
Finally, we tested whether Oprm1 Cre/Cre mice can be used for cell-specific optogenetic manipulation of MOR neurons. Earlier studies have demonstrated that optical stimulation of GABAergic interneurons in the VTA induces avoidance behavior, as a consequence of dopamine (DA) neuron inhibition (Tan et al., 2012). MOR is a G i -coupled receptor expressed in these interneurons, and a best known mechanism for MOR-mediated reward is through the inhibition of these VTA GABA interneurons, an activity that in turn disinhibits DA neurons (Johnson and North, 1992;Fields and Margolis, 2015). We therefore hypothesized that optogenetic activation of MOR-EGFP/Cre-expressing neurons in the VTA of Oprm1 Cre/Cre mice would produce a place avoidance, as did the stimulation of the entire population of VTA GABAergic neurons in the study by Tan et al. (2012). We injected a Cre-dependent AAV2channelrhodopsin virus in the VTA (Fig. 3a,b) to express ChR2 mainly in MOR/GABAergic interneurons (Fig. 3c) and tested animals using a real-time place-testing (RTPT) paradigm (Fig. 3d). A two-way ANOVA with virus as a between-subject factor and frequency as a within-subject factor yielded a significant interaction (F (3,51) = 16.93, p , 0.001). Simple effect test revealed that the Oprm1-Cre VTA-VTA::ChR2 mice spent significantly less time on the side paired with light stimulation than control mice at 10 Hz (ppp , 0.01), 20 Hz, and 40 Hz (pppp , 0.001) compared with control mice (Fig. 3d,e), indicating that optical stimulation triggered place avoidance. The stimulation did not affect total activity measured at 20 Hz (unpaired t test, t (14) = 1.497, p . 0.05; Fig. 3f). Together, data demonstrate that the activation of MOR-positive neurons in the VTA produce avoidance, as predicted. The Oprm1-Cre line, therefore, efficiently drives Cre-mediated recombination to modulate behavior.

Discussion
Originally developed to allow gene targeting in specific cells using Cre/LoxP recombination (Gavériaux-Ruff and Kieffer, 2007), Cre driver lines are also extensively used to label specific neuronal populations and visualize their connectivity patterns when combined with Cre-dependent fluorescent reporter mouse lines or viruses (Cai et al., 2013;Soden et al., 2014), or to monitor the activity of phenotypically defined neuronal ensembles using Cre-dependent calcium indicators (Russell, 2011). The utility of mouse Cre lines has further expanded with the advent of optogenetic and chemogenetic approaches, which allow manipulating Cre-expressing neurons to understand the circuit mechanisms underlying behavior (Fenno et al., 2011;Sternson and Roth, 2014). Transgenic Cre lines have now been generated by individual laboratories, as well as large initiatives, including the GENSAT program (Gong et al., 2007) the NIH Blueprint Cre driver Network (Taniguchi et al., 2011) and the Allen Brain Institute.
Lines providing access to the opioidergic circuitry remain limited (for review, see Darcq and Kieffer, 2018), and targeting cells responding to medicinal and abused opioid drugs is a desirable goal. To study opioid peptide-expressing cells, Cre lines using Pdyn, Penk, and Pomc gene promoters have been created (Harris et al., 2014, for Penk and Pomc lines), and Pdyn-Cre mice were used to characterize subpopulations of striatal neurons in the direct pathway (Al-Hasani et al., 2015) study D 1 /D 2 -type neuron activity balance in the nucleus accumbens (Tejeda et al., 2017), rescue MOR expression in the striatum (Cui et al., 2014), or investigate amygdala circuitry (Crowley et al., 2016). On the receptor side, one knock-in Cre line was created to gain genetic access to kappa-opioid receptor (KOR)-expressing cells (Cai et al., 2016) and used to study peripheral KOR neuron terminals in pain control (Snyder et al., 2018). This work provides a tool to access MOR neurons.
Full characterization of this novel Oprm1-Cre driver line demonstrates the following: (1) MOR signaling and function are preserved, as shown by intact G-protein activation and normal morphine-induced analgesia, locomotor stimulation, and sensitization; (2) EGFP/Cre is detectable, and the expression pattern matches MOR expression; and (3) the EGFP/Cre recombinase is functional and effectively drives both the expression of Cre-dependent reporter genes and optogenetic sensors in MOR-expressing neurons. Together with a recently published inducible MOR-CreER mouse line (Okunomiya et al., 2020), this Oprm1-Cre line is a unique tool for both mapping and functional studies of MOR-positive neurons, and will be of broad interest for opioid, pain, reward, and addiction research.