Heterogeneous transgene expression in the retinas of the TH-RFP, TH-Cre, TH-BAC-Cre and DAT-Cre mouse lines

Modulating amacrine cell-derived dopamine signaling promotes optic nerve regeneration and preserves visual function

As part of the central nervous system, the optic nerve, composed of axons from retinal ganglion cells (RGCs), generally fails to regenerate on its own when injured in adult mammals. An innovative approach to promoting optic nerve regeneration involves manipulating the interactions between amacrine cells (ACs) and RGCs. Here, we identified a unique AC subtype, dopaminergic ACs (DACs), that responded early after optic nerve crush by down-regulating neuronal activity and reducing retinal dopamine (DA) release. Activating DACs or augmenting DA release with levodopa demonstrated neuroprotective effects and modestly enhanced axon regeneration. Within this context, we pinpointed the DA receptor D1 (DRD1) as a critical mediator of DAC-derived DA and showed that RGC-specific Drd1 overexpression effectively overcame subtype-specific barriers to regeneration. This strategy markedly boosted RGC survival and axon regeneration after crush and preserved vision in a glaucoma model. This study unveils the crucial role of DAC-derived DA signaling in optic nerve regeneration, holding promise for therapeutic insights into neural repair.

The Structural and Functional Integrity of Rod Photoreceptor Ribbon Synapses Depends on Redundant Actions of Dynamins 1 and 3

Vertebrate vision begins with light absorption by rod and cone photoreceptors, which transmit signals from their synaptic terminals to second-order neurons: bipolar and horizontal cells. In mouse rods, there is a single presynaptic ribbon-type active zone at which the release of glutamate occurs tonically in the dark. This tonic glutamatergic signaling requires continuous exo- and endocytosis of synaptic vesicles. At conventional synapses, endocytosis commonly requires dynamins: GTPases encoded by three genes (Dnm1-3), which perform membrane scission. Disrupting endocytosis by dynamin deletions impairs transmission at conventional synapses, but the impact of disrupting endocytosis and the role(s) of specific dynamin isoforms at rod ribbon synapses are understood incompletely. Here, we used cell-specific knock-outs (KOs) of the neuron-specific Dnm1 and Dnm3 to investigate the functional roles of dynamin isoforms in rod photoreceptors in mice of either sex. Analysis of synaptic protein expression, synapse ultrastructure, and retinal function via electroretinograms (ERGs) showed that dynamins 1 and 3 act redundantly and are essential for supporting the structural and functional integrity of rod ribbon synapses. Single Dnm3 KO showed no phenotype, and single Dnm1 KO only modestly reduced synaptic vesicle density without affecting vesicle size and overall synapse integrity, whereas double Dnm1/Dnm3 KO impaired vesicle endocytosis profoundly, causing enlarged vesicles, reduced vesicle density, reduced ERG responses, synaptic terminal degeneration, and disassembly and degeneration of postsynaptic processes. Concurrently, cone function remained intact. These results show the fundamental redundancy of dynamins 1 and 3 in regulating the structure and function of rod ribbon synapses.

Dopamine signaling from ganglion cells directs layer-specific angiogenesis in the retina

During central nervous system (CNS) development, a precisely patterned vasculature emerges to support CNS function. How neurons control angiogenesis is not well understood. Here, we show that the neuromodulator dopamine restricts vascular development in the retina via temporally limited production by an unexpected neuron subset. Our genetic and pharmacological experiments demonstrate that elevating dopamine levels inhibits tip-cell sprouting and vessel growth, whereas reducing dopamine production by all retina neurons increases growth. Dopamine production by canonical dopaminergic amacrine interneurons is dispensable for these events. Instead, we found that temporally restricted dopamine production by retinal ganglion cells (RGCs) modulates vascular development. RGCs produce dopamine precisely during angiogenic periods. Genetically limiting dopamine production by ganglion cells, but not amacrines, decreases angiogenesis. Conversely, elevating ganglion-cell-derived dopamine production inhibits early vessel growth. These vasculature outcomes occur downstream of vascular endothelial growth factor receptor (VEGFR) activation and Notch-Jagged1 signaling. Jagged1 is increased and subsequently inhibits Notch signaling when ganglion cell dopamine production is reduced. Our findings demonstrate that dopaminergic neural activity from a small neuron subset functions upstream of VEGFR to serve as developmental timing cue that regulates vessel growth.

Linking transcriptomes with morphological and functional phenotypes in mammalian retinal ganglion cells

Retinal ganglion cells (RGCs) are the brain's gateway to the visual world. They can be classified into different types on the basis of their electrophysiological, transcriptomic, or morphological characteristics. Here, we characterize the transcriptomic, morphological, and functional features of 472 high-quality RGCs using Patch sequencing (Patch-seq), providing functional and morphological annotation of many transcriptomic-defined cell types of a previously established RGC atlas. We show a convergence of different modalities in defining the RGC identity and reveal the degree of correspondence for well-characterized cell types across multimodal data. Moreover, we complement some RGC types with detailed morphological and functional properties. We also identify differentially expressed genes among ON, OFF, and ON-OFF RGCs such as Vat1l, Slitrk6, and Lmo7, providing candidate marker genes for functional studies. Our research suggests that the molecularly distinct clusters may also differ in their roles of encoding visual information.

Identification of TPBG-Expressing Amacrine Cells in DAT-tdTomato Mouse

Purpose

Neurons are the bricks of the neuronal system and experimental access to certain neuron subtypes will be of great help to decipher neuronal circuits. Here, we identified trophoblast glycoprotein (TPBG)-expressing GABAergic amacrine cells (ACs) that were selectively labeled in DAT-tdTomato transgenic mice.

Methods

Retina and brain sections were prepared for immunostaining with antibodies against various biomarkers. Patch-sequencing was performed to obtain the transcriptomes of tdTomato-positive cells in DAT-tdTomato mice. Whole-cell recordings were conducted to identify responses to light stimulation.

Results

Tyrosine hydroxylase immunoreactive cells were colocalized with tdTomato-positive cells in substantia nigra pars compacta, but not in the retina. Transcriptomes collected from tdTomato-positive cells in retinas via Patch-sequencing exhibited the expression of marker genes of ACs (Pax6 and Slc32a1) and marker genes of GABAergic neurons (Gad1, Gad2, and Slc6a1). Immunostaining with antibodies against relevant proteins (GAD67, GAD65, and GABA) also confirmed transcriptomic results. Furthermore, tdTomato-positive cells in retinas selectively expressed Tpbg, a marker gene for distinct clusters molecularly defined, which was proved with TPBG immunoreactivity in fluorescently labeled cells. Finally, tdTomato-positive cells recorded showed ON-OFF responses to light stimulation.

Conclusions

Ectopic expression occurs in the retina but not in the substantia nigra pars compacta in the DAT-tdTomato mouse, and fluorescently labeled cells in the retina are TPBG-expressing GABAergic ACs. This type of transgenic mice has been proved as an ideal tool to achieve efficient labeling of a distinct subset of ACs that selectively express Tpbg.

PTEN Expression Regulates Gap Junction Connectivity in the Retina

Manipulation of the phosphatase and tensin homolog (PTEN) pathway has been suggested as a therapeutic approach to treat or prevent vision loss due to retinal disease. In this study, we investigated the effects of deleting one copy of Pten in a well-characterized class of retinal ganglion cells called α-ganglion cells in the mouse retina. In Pten +/- retinas, α-ganglion cells did not exhibit major changes in their dendritic structure, although most cells developed a few, unusual loop-forming dendrites. By contrast, α-ganglion cells exhibited a significant decrease in heterologous and homologous gap junction mediated cell coupling with other retinal ganglion and amacrine cells. Additionally, the majority of OFF α-ganglion cells (12/18 cells) formed novel coupling to displaced amacrine cells. The number of connexin36 puncta, the predominant connexin that mediates gap junction communication at electrical synapses, was decreased by at least 50% on OFF α-ganglion cells. Reduced and incorrect gap junction connectivity of α-ganglion cells will affect their functional properties and alter visual image processing in the retina. The anomalous connectivity of retinal ganglion cells would potentially limit future therapeutic approaches involving manipulation of the Pten pathway for treating ganglion cell degeneration in diseases like glaucoma, traumatic brain injury, Parkinson's, and Alzheimer's diseases.

Altered baseline and amphetamine-mediated behavioral profiles in dopamine transporter Cre (DAT-Ires-Cre) mice compared to tyrosine hydroxylase Cre (TH-Cre) mice

RATIONALE

Transgenic mouse lines expressing Cre-recombinase under the regulation of either dopamine transporter (DAT) or tyrosine hydroxylase (TH) promoters are commonly used to study the dopamine (DA) system. While use of the TH promoter appears to have less liability to changes in native gene expression, transgene insertion in the DAT locus results in reduced DAT expression and function. This confound is sometimes overlooked in genetically targeted behavioral experiments.

OBJECTIVES

We sought to evaluate the suitability of DAT-Ires-Cre and TH-Cre transgenic lines for behavioral pharmacology experiments with DA agonists. We hypothesized that DAT-Ires-Cre expression would impact DAT-mediated behaviors, but no impact of TH-Cre expression would be observed.

METHODS

DAT-Ires-Cre and TH-Cre mice bred on mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds were evaluated for novelty-induced, baseline, and amphetamine (AMPH)-induced locomotion, and for AMPH and D1 agonist (SKF-38393)-induced preservative behaviors.

RESULTS

DAT-Ires-Cre mice on both mixed 129S6/C57BL/6 and pure C57BL/6 backgrounds displayed increased novelty-induced activity and decreased AMPH-induced locomotion, with mixed results for AMPH-induced stereotypy. TH-Cre mice on both backgrounds showed typical baseline activity and AMPH-induced stereotypy, with a difference in AMPH-induced locomotion observed only on the mixed background. Both transgenic lines displayed unaltered SKF-38393-induced grooming behavior.

CONCLUSIONS

Our findings indicate that the DAT-Ires-Cre transgenic line may lead to confounds for experiments that are dependent on DAT expression. The TH-Cre transgenic line studied here may be a more useful option, depending on background strain, because of its lack of baseline and drug-induced phenotypes. These data highlight the importance of appropriate controls in studies employing transgenic mice.

Early diabetes impairs ON sustained ganglion cell light responses and adaptation without cell death or dopamine insensitivity

Retinal signaling under dark-adapted conditions is perturbed during early diabetes. Additionally, dopamine, the main neuromodulator of retinal light adaptation, is diminished in diabetic retinas. However, it is not known if this dopamine deficiency changes how the retina responds to increased light or dopamine. Here we determine whether light adaptation is impaired in the diabetic retina, and investigate potential mechanism(s) of impairment. Diabetes was induced in C57BL/6J male mice via 3 intraperitoneal injections of streptozotocin (75 mg/kg) and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell recordings of light-evoked and spontaneous inhibitory postsynaptic currents (IPSCs) or excitatory postsynaptic currents (EPSCs) were made from rod bipolar cells and ON sustained ganglion cells, respectively. Light responses were recorded before and after D1 receptor (D1R) activation (SKF-38393, 20 μM) or light adaptation (background of 950 photons·μm^-2 ·s^-1). Retinal whole mounts were stained for either tyrosine hydroxylase and activated caspase-3 or GAD65/67, GlyT1 and RBPMS and imaged. D1R activation and light adaptation both decreased inhibition, but the disinhibition was not different between control and diabetic rod bipolar cells. However, diabetic ganglion cell light-evoked EPSCs were increased in the dark and showed reduced light adaptation. No differences were found in light adaptation of spontaneous EPSC parameters, suggesting upstream changes. No changes in cell density were found for dopaminergic, glycinergic or GABAergic amacrine cells, or ganglion cells. Thus, in early diabetes, ON sustained ganglion cells receive excessive excitation under dark- and light-adapted conditions. Our results show that this is not attributable to loss in number or dopamine sensitivity of inhibitory amacrine cells or loss of dopaminergic amacrine cells.

Mouse Retinal Cell Atlas: Molecular Identification of over Sixty Amacrine Cell Types

Amacrine cells (ACs) are a diverse class of interneurons that modulate input from photoreceptors to retinal ganglion cells (RGCs), rendering each RGC type selectively sensitive to particular visual features, which are then relayed to the brain. While many AC types have been identified morphologically and physiologically, they have not been comprehensively classified or molecularly characterized. We used high-throughput single-cell RNA sequencing to profile >32,000 ACs from mice of both sexes and applied computational methods to identify 63 AC types. We identified molecular markers for each type and used them to characterize the morphology of multiple types. We show that they include nearly all previously known AC types as well as many that had not been described. Consistent with previous studies, most of the AC types expressed markers for the canonical inhibitory neurotransmitters GABA or glycine, but several expressed neither or both. In addition, many expressed one or more neuropeptides, and two expressed glutamatergic markers. We also explored transcriptomic relationships among AC types and identified transcription factors expressed by individual or multiple closely related types. Noteworthy among these were Meis2 and Tcf4, expressed by most GABAergic and most glycinergic types, respectively. Together, these results provide a foundation for developmental and functional studies of ACs, as well as means for genetically accessing them. Along with previous molecular, physiological, and morphologic analyses, they establish the existence of at least 130 neuronal types and nearly 140 cell types in the mouse retina.SIGNIFICANCE STATEMENT The mouse retina is a leading model for analyzing the development, structure, function, and pathology of neural circuits. A complete molecular atlas of retinal cell types provides an important foundation for these studies. We used high-throughput single-cell RNA sequencing to characterize the most heterogeneous class of retinal interneurons, amacrine cells, identifying 63 distinct types. The atlas includes types identified previously as well as many novel types. We provide evidence for the use of multiple neurotransmitters and neuropeptides, and identify transcription factors expressed by groups of closely related types. Combining these results with those obtained previously, we proposed that the mouse retina contains ∼130 neuronal types and is therefore comparable in complexity to other regions of the brain.

nGnG Amacrine Cells and Brn3b-negative M1 ipRGCs are Specifically Labeled in the ChAT-ChR2-EYFP Mouse

Purpose

Experimental access to specific cell subtypes is essential for deciphering the complexity of retinal networks. Here, we characterized the selective labeling, caused by ectopic transgene expression, of two atypical retinal neurons in the ChAT-Channelrhodopsin-2 (ChR2)-EYFP mouse.

Methods

Retinal sections and flat-mounts were prepared for double-staining immunohistochemistry with antibodies against EYFP and various neuronal markers. Sagittal/coronal brain slices were made to visualize EYFP signals in central nuclei. Whole-cell recordings were conducted to test the functionality of ChR2.

Results

Two populations of EYFP-positive retinal cells were observed. The inner nuclear layer (INL)-located one (type I cell) distributed regularly throughout the entire retina, whereas the ganglion cell layer (GCL)-residing one (type II cell) was restricted ventrally. None of them was cholinergic, as evidenced by the complete absence of ChAT immunoreactivity. Type I cells were immunolabeled by the amacrine marker syntaxin. However, the vast majority of them were neither positive to GABA/GAD65, nor to GlyT1/glycine, suggesting that they were non-GABAergic non-glycinergic amacrine cells (nGnG ACs), which was confirmed by double-labeling with the nGnG AC marker PPP1R17. Type II cells were immunopositive to melanopsin, but not to Brn3a or Brn3b. They possessed dendrites stratifying in the outermost inner plexiform layer (IPL) and axons projecting to the suprachiasmatic nucleus (SCN) rather than the olivary pretectal nucleus (OPN), suggesting that they belonged to a Brn3b-negative subset of M1-type intrinsically photosensitive retinal ganglion cells (ipRGCs). Glutamatergic transmission-independent photocurrents were elicited in EYFP-positive cells, indicating the functional expression of ChR2.

Conclusions

The ChAT-ChR2-EYFP retina exhibits ectopic, but functional, transgene expression in nGnG ACs and SCN-innervating M1 ipRGCs, thus providing an ideal tool to achieve efficient labeling and optogenetic manipulation of these cells.

Characterization of transgenic mouse models targeting neuromodulatory systems reveals organizational principles of the dorsal raphe

The dorsal raphe (DR) is a heterogeneous nucleus containing dopamine (DA), serotonin (5HT), γ-aminobutyric acid (GABA) and glutamate neurons. Consequently, investigations of DR circuitry require Cre-driver lines that restrict transgene expression to precisely defined cell populations. Here, we present a systematic evaluation of mouse lines targeting neuromodulatory cells in the DR. We find substantial differences in specificity between lines targeting DA neurons, and in penetrance between lines targeting 5HT neurons. Using these tools to map DR circuits, we show that populations of neurochemically distinct DR neurons are arranged in a stereotyped topographical pattern, send divergent projections to amygdala subnuclei, and differ in their presynaptic inputs. Importantly, targeting DR DA neurons using different mouse lines yielded both structural and functional differences in the neural circuits accessed. These results provide a refined model of DR organization and support a comparative, case-by-case evaluation of the suitability of transgenic tools for any experimental application.

OMICs approaches-assisted identification of macrophages-derived MIP-1γ as the therapeutic target of botanical products TNTL in diabetic retinopathy

BACKGROUND

Inflammatory reaction in the dysfunction of retinal endotheliocytes has been considered to play a vital role in diabetic retinopathy (DR). Anti-inflammatory therapy so far gains poor outcome as DR treatment. This study aims to identify a novel therapeutic target of DR from the OMICs studies of a traditional anti-DR botanical products TNTL.

METHODS

Hyperglycemic mice were treated with TNTL. The anti-hyperglycemic effect of TNTL was validated to confirm the biological consistency of the herbal products from batches. Improvement of DR by TNTL was examined by various assays on the retina. Next-generation transcriptome sequencing and cytokine array was used to identify the therapeutic targets. In vitro study was performed to validate the target.

RESULTS

We observed that TNTL at its high doses possessed anti-hyperglycemic effect in murine type I diabetic model, while at its doses without reducing blood glucose, it suppressed DR incidence. TNTL restored the blood-retina barrier integrity, suppressed retinal neovascularization, and attenuated the retinal ganglion cell degeneration. Transcriptomic analysis on the retina tissue of hyperglycemic mice with or without TNTL revealed that the inflammatory retina microenvironment was significantly repressed. TNTL treatment suppressed pro-inflammatory macrophages in the retina, which resulted in the inactivation of endothelial cell migration, restoration of endothelial cell monolayer integrity, and prevention of leakage. Cytokine array analysis suggested that TNTL could significantly inhibit the secretion of MIP1γ from pro-inflammatory macrophages. Prevention of endothelial dysfunction by TNTL may be mediated by the inhibition of MIP1γ/CCR1 axis. More specifically, TNTL suppressed MIP1γ release from pro-inflammatory macrophages, which in turn inhibited the activation of CCR1-associated signaling pathways in endothelial cells.

CONCLUSION

Our findings demonstrated that TNTL might be an alternative treatment to DR, and the primary source of potential drug candidates against DR targeting MIP1γ/CCR1 axis in the retinal microenvironment.

Light-dependent pathways for dopaminergic amacrine cell development and function

Retinal dopamine is a critical modulator of high acuity, light-adapted vision and photoreceptor coupling in the retina. Dopaminergic amacrine cells (DACs) serve as the sole source of retinal dopamine, and dopamine release in the retina follows a circadian rhythm and is modulated by light exposure. However, the retinal circuits through which light influences the development and function of DACs are still unknown. Intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged as a prime target for influencing retinal dopamine levels because they costratify with DACs in the inner plexiform layer and signal to them in a retrograde manner. Surprisingly, using genetic mouse models lacking specific phototransduction pathways, we find that while light influences the total number of DACs and retinal dopamine levels, this effect does not require ipRGCs. Instead, we find that the rod pathway is a critical modulator of both DAC number and retinal dopamine levels.

Inhibitory Control of Feature Selectivity in an Object Motion Sensitive Circuit of the Retina

Object motion sensitive (OMS) W3-retinal ganglion cells (W3-RGCs) in mice respond to local movements in a visual scene but remain silent during self-generated global image motion. The excitatory inputs that drive responses of W3-RGCs to local motion were recently characterized, but which inhibitory neurons suppress W3-RGCs' responses to global motion, how these neurons encode motion information, and how their connections are organized along the excitatory circuit axis remains unknown. Here, we find that a genetically identified amacrine cell (AC) type, TH2-AC, exhibits fast responses to global motion and slow responses to local motion. Optogenetic stimulation shows that TH2-ACs provide strong GABA_A receptor-mediated input to W3-RGCs but only weak input to upstream excitatory neurons. Cell-type-specific silencing reveals that temporally coded inhibition from TH2-ACs cancels W3-RGC spike responses to global but not local motion stimuli and, thus, controls the feature selectivity of OMS signals sent to the brain.

Analysis of Feedback Signaling from Horizontal Cells to Photoreceptors in Mice

Genetic manipulation of horizontal cells using a Connexin57-iCre mouse (Cx57-iCre) line combined with calcium imaging is proving to be a valuable method to study horizontal cell feedback inhibition onto photoreceptor terminals. While it is accepted that horizontal cells provide lateral inhibitory feedback to photoreceptors, the cellular mechanisms that underlie this feedback inhibition remain only partially elucidated. Feedback inhibition of photoreceptors acts via modulation of their voltage-gated calcium channels at their synaptic terminal. Calcium imaging of photoreceptors in retinal slices, therefore, reflects the impact of inhibitory feedback from horizontal cells. The development of a Cx57-iCre mouse line permits genetic manipulation of horizontal cells. In wild-type mouse retina, depolarization of horizontal cells by kainate provokes a decrease in photoreceptor Ca2+i, whereas hyperpolarization by NBQX elicits an increase in photoreceptor Ca2+i. These responses indicate increased feedback inhibition occurred when horizontal cells are depolarized, and decreased feedback inhibition, when hyperpolarized. This system was used to test the role of GABA release from horizontal cells in feedback inhibition by the selective elimination of VGAT/VIAAT, the inhibitory amino acid transmitter transporter that loads GABA into the synaptic vesicles of horizontal cells. Combined with calcium imaging of photoreceptors in retinal slices, the knockout of specific proteins, e.g., VGAT, provides a robust technique to test the role of GABA in feedback inhibition by horizontal cells.

Transgene is specifically and functionally expressed in retinal inhibitory interneurons in the VGAT-ChR2-EYFP mouse

Ectopic transgene expression in the retina has been reported in various transgenic mice, indicating the importance of characterizing retinal phenotypes. We examined transgene expression in the VGAT-ChR2-EYFP mouse retina by fluorescent immunohistochemistry and electrophysiology, with special emphasis on enhanced yellow fluorescent protein (EYFP) localization in retinal neuronal subtypes identified by specific markers. Strong EYFP signals were detected in both the inner and outer plexiform layers. In addition, the ChR2-EYFP fusion protein was also expressed in somata of the great majority of inhibitory interneurons, including horizontal cells and GABAergic and glycinergic amacrine cells. However, a small population of amacrine cells residing in the ganglion cell layer were not labeled by EYFP, and a part of them were cholinergic ones. In contrast, no EYFP signal was detected in the somata of retinal excitatory neurons: photoreceptors, bipolar and ganglion cells, as well as Müller glial cells. When glutamatergic transmission was blocked, bright blue light stimulation elicited inward photocurrents from amacrine cells, as well as post-synaptic inhibitory currents from ganglion cells, suggesting a functional ChR2 expression. The VGAT-ChR2-EYFP mouse therefore could be a useful animal model for dissecting retinal microcircuits when targeted labeling and/or optogenetic manipulation of retinal inhibitory neurons are required.

Ghrelin receptor antagonism of hyperlocomotion in cocaine-sensitized mice requires βarrestin-2

The "brain-gut" peptide ghrelin, which mediates food-seeking behaviors, is recognized as a very strong endogenous modulator of dopamine (DA) signaling. Ghrelin binds the G protein-coupled receptor GHSR1a, and administration of ghrelin increases the rewarding properties of psychostimulants while ghrelin receptor antagonists decrease them. In addition, the GHSR1a signals through βarrestin-2 to regulate actin/stress fiber rearrangement, suggesting βarrestin-2 participation in the regulation of actin-mediated synaptic plasticity for addictive substances like cocaine. The effects of ghrelin receptor ligands on reward strongly suggest that modulation of ghrelin signaling could provide an effective strategy to ameliorate undesirable behaviors arising from addiction. To investigate this possibility, we tested the effects of ghrelin receptor antagonism in a cocaine behavioral sensitization paradigm using DA neuron-specific βarrestin-2 KO mice. Our results show that these mice sensitize to cocaine as well as wild-type littermates. The βarrestin-2 KO mice, however, no longer respond to the locomotor attenuating effects of the GHSR1a antagonist YIL781. The data presented here suggest that the separate stages of addictive behavior differ in their requirements for βarrestin-2 and show that pharmacological inhibition of βarrestin-2 function through GHSR1a antagonism is not equivalent to the loss of βarrestin-2 function achieved by genetic ablation. These data support targeting GHSR1a signaling in addiction therapy but indicate that using signaling biased compounds that modulate βarrestin-2 activity differentially from G protein activity may be required.

Multiple cell types form the VIP amacrine cell population

Amacrine cells are a heterogeneous group of interneurons that form microcircuits with bipolar, amacrine and ganglion cells to process visual information in the inner retina. This study has characterized the morphology, neurochemistry and major cell types of a VIP-ires-Cre amacrine cell population. VIP-tdTomato and -Confetti (Brainbow2.1) mouse lines were generated by crossing a VIP-ires-Cre line with either a Cre-dependent tdTomato or Brainbow2.1 reporter line. Retinal sections and whole-mounts were evaluated by quantitative, immunohistochemical, and intracellular labeling approaches. The majority of tdTomato and Confetti fluorescent cell bodies were in the inner nuclear layer (INL) and a few cell bodies were in the ganglion cell layer (GCL). Fluorescent processes ramified in strata 1, 3, 4, and 5 of the inner plexiform layer (IPL). All tdTomato fluorescent cells expressed syntaxin 1A and GABA-immunoreactivity indicating they were amacrine cells. The average VIP-tdTomato fluorescent cell density in the INL and GCL was 535 and 24 cells/mm² , respectively. TdTomato fluorescent cells in the INL and GCL contained VIP-immunoreactivity. The VIP-ires-Cre amacrine cell types were identified in VIP-Brainbow2.1 retinas or by intracellular labeling in VIP-tdTomato retinas. VIP-1 amacrine cells are bistratified, wide-field cells that ramify in strata 1, 4, and 5, VIP-2A and 2B amacrine cells are medium-field cells that mainly ramify in strata 3 and 4, and VIP-3 displaced amacrine cells are medium-field cells that ramify in strata 4 and 5 of the IPL. VIP-ires-Cre amacrine cells form a neuropeptide-expressing cell population with multiple cell types, which are likely to have distinct roles in visual processing.

Prox1 Is a Marker for AII Amacrine Cells in the Mouse Retina

The transcription factor Prox1 is expressed in multiple cells in the retina during eye development. This study has focused on neuronal Prox1 expression in the inner nuclear layer (INL) of the adult mouse retina. Prox1 immunostaining was evaluated in vertical retinal sections and whole mount preparations using a specific antibody directed to the C-terminus of Prox1. Strong immunostaining was observed in numerous amacrine cell bodies and in all horizontal cell bodies in the proximal and distal INL, respectively. Some bipolar cells were also weakly immunostained. Prox1-immunoreactive amacrine cells expressed glycine, and they formed 35 ± 3% of all glycinergic amacrine cells. Intracellular Neurobiotin injections into AII amacrine cells showed that all gap junction-coupled AII amacrine cells express Prox1, and no other Prox1-immunostained amacrine cells were in the immediate area surrounding the injected AII amacrine cell. Prox1-immunoreactive amacrine cell bodies were distributed across the retina, with their highest density (3887 ± 160 cells/mm2) in the central retina, 0.5 mm from the optic nerve head, and their lowest density (3133 ± 350 cells/mm2) in the mid-peripheral retina, 2 mm from the optic nerve head. Prox1-immunoreactive amacrine cell bodies comprised ~9.8% of the total amacrine cell population, and they formed a non-random mosaic with a regularity index (RI) of 3.4, similar to AII amacrine cells in the retinas of other mammals. Together, these findings indicate that AII amacrine cells are the predominant and likely only amacrine cell type strongly expressing Prox1 in the adult mouse retina, and establish Prox1 as a marker of AII amacrine cells.

Viral vector strategies for investigating midbrain dopamine circuits underlying motivated behaviors

Midbrain dopamine (DA) neurons have received significant attention in brain research because of their central role in reward processing and their dysfunction in neuropsychiatric disorders such as Parkinson's disease, drug addiction, depression and schizophrenia. Until recently, it has been thought that DA neurons form a homogeneous population whose primary function is the computation of reward prediction errors. However, through the implementation of viral vector strategies, an unexpected complexity and diversity has been revealed at the anatomical, molecular and functional level. In this review, we discuss recent viral vector approaches that have been leveraged to dissect how different circuits involving distinct DA neuron subpopulations may contribute to the role of DA in reward- and aversion-related behaviors. We focus on studies that have used cell type- and projection-specific optogenetic manipulations, discuss the strengths and limitations of each approach, and critically examine emergent organizational principles that have led to a reclassification of midbrain DA neurons.

Parallel Inhibition of Dopamine Amacrine Cells and Intrinsically Photosensitive Retinal Ganglion Cells in a Non-Image-Forming Visual Circuit of the Mouse Retina

An inner retinal microcircuit composed of dopamine (DA)-containing amacrine cells and melanopsin-containing, intrinsically photosensitive retinal ganglion cells (M1 ipRGCs) process information about the duration and intensity of light exposures, mediating light adaptation, circadian entrainment, pupillary reflexes, and other aspects of non-image-forming vision. The neural interaction is reciprocal: M1 ipRGCs excite DA amacrine cells, and these, in turn, feed inhibition back onto M1 ipRGCs. We found that the neuropeptide somatostatin [somatotropin release inhibiting factor (SRIF)] also inhibits the intrinsic light response of M1 ipRGCs and postulated that, to tune the bidirectional interaction of M1 ipRGCs and DA amacrine cells, SRIF amacrine cells would provide inhibitory modulation to both cell types. SRIF amacrine cells, DA amacrine cells, and M1 ipRGCs form numerous contacts. DA amacrine cells and M1 ipRGCs express the SRIF receptor subtypes sst(2A) and sst4 respectively. SRIF modulation of the microcircuit was investigated with targeted patch-clamp recordings of DA amacrine cells in TH-RFP mice and M1 ipRGCs in OPN4-EGFP mice. SRIF increases K(+) currents, decreases Ca(2+) currents, and inhibits spike activity in both cell types, actions reproduced by the selective sst(2A) agonist L-054,264 (N-[(1R)-2-[[[(1S*,3R*)-3-(aminomethyl)cyclohexyl]methyl]amino]-1-(1H-indol-3-ylmethyl)-2-oxoethyl]spiro[1H-indene-1,4'-piperidine]-1'-carboxamide) in DA amacrine cells and the selective sst4 agonist L-803,087 (N(2)-[4-(5,7-difluoro-2-phenyl-1H-indol-3-yl)-1-oxobutyl]-L-arginine methyl ester trifluoroacetate) in M1 ipRGCs. These parallel actions of SRIF may serve to counteract the disinhibition of M1 ipRGCs caused by SRIF inhibition of DA amacrine cells. This allows the actions of SRIF on DA amacrine cells to proceed with adjusting retinal DA levels without destabilizing light responses by M1 ipRGCs, which project to non-image-forming targets in the brain.

Ectopic transgene expression in the retina of four transgenic mouse lines

Retinal expression of transgenes was examined in four mouse lines. Two constructs were driven by the choline acetyltransferase (ChAT) promoter: green fluorescent protein conjugated to tau protein (tau-GFP) or cytosolic yellow fluorescent protein (YFP) generated through CRE recombinase-induced expression of Rosa26 (ChAT-CRE/Rosa26YFP). Two other constructs targeted inhibitory interneurons: GABAergic horizontal and amacrine cells identified by glutamic acid decarboxylase (GAD65-GFP) or parvalbumin (PV) cells (PV-CRE/Rosa26YFP). Animals were transcardially perfused and retinal sections prepared. Antibodies against PV, calretinin (CALR), calbindin (CALB), and tyrosine hydroxylase (TH) were used to counterstain transgene-expressing cells. In PVxRosa and ChAT-tauGFP constructs, staining appeared in vertically oriented row of processes resembling Müller cells. In the ChATxRosa construct, populations of amacrine cells and neurons in the ganglion cell layer were labeled. Some cones also exhibited GFP fluorescence. CALR, PV and TH were found in none of these cells. Occasionally, we found GFP/CALR and GFP/PV double-stained cells in the ganglion cell layer (GCL). In the GAD65-GFP construct, all layers of the neuroretina were labeled, except photoreceptors. Not all horizontal cells expressed GFP. We did not find GFP/TH double-labeled cells and GFP was rarely present in CALR- and CALB-containing cells. Many PV-positive neurons were also labeled for GFP, including small diameter amacrines. In the GCL, single labeling for GFP and PV was ascertained, as well as several CALR/PV double-stained neurons. In the GCL, cells triple labeled with GFP/CALR/CALB were sparse. In conclusion, only one of the four transgenic constructs exhibited an expression pattern consistent with endogenous retinal protein expression, while the others strongly suggested ectopic gene expression.