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We engineered light-gated channelrhodopsins (ChRs) whose current strength and light sensitivity enable minimally invasive neuronal circuit interrogation. Current ChR tools applied to the mammalian brain require intracranial surgery for transgene delivery and implantation of fiber-optic cables to produce light-dependent activation of a small volume of tissue. To facilitate expansive optogenetics without the need for invasive implants, our engineering approach leverages the substantial literature of ChR variants to train statistical models for the design of high-performance ChRs. With Gaussian process models trained on a limited experimental set of 102 functionally characterized ChRs, we designed high-photocurrent ChRs with high light sensitivity. Three of these, ChRger1–3, enable optogenetic activation of the nervous system via systemic transgene delivery. ChRger2 enables light-induced neuronal excitation without fiber-optic implantation; that is, this opsin enables transcranial optogenetics.

The prefrontal cortex is a critical neuroanatomical hub for controlling motivated behaviours across mammalian species 1 , 2 , 3 . In addition to intra-cortical connectivity, prefrontal projection neurons innervate subcortical structures that contribute to reward-seeking behaviours, such as the ventral striatum and midline thalamus 4 . While connectivity among these structures contributes to appetitive behaviours 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , how projection-specific prefrontal neurons encode reward-relevant information to guide reward seeking is unknown. Here we use in vivo two-photon calcium imaging to monitor the activity of dorsomedial prefrontal neurons in mice during an appetitive Pavlovian conditioning task. At the population level, these neurons display diverse activity patterns during the presentation of reward-predictive cues. However, recordings from prefrontal neurons with resolved projection targets reveal that individual corticostriatal neurons show response tuning to reward-predictive cues, such that excitatory cue responses are amplified across learning. By contrast, corticothalamic neurons gradually develop new, primarily inhibitory responses to reward-predictive cues across learning. Furthermore, bidirectional optogenetic manipulation of these neurons reveals that stimulation of corticostriatal neurons promotes conditioned reward-seeking behaviour after learning, while activity in corticothalamic neurons suppresses both the acquisition and expression of conditioned reward seeking. These data show how prefrontal circuitry can dynamically control reward-seeking behaviour through the opposing activities of projection-specific cell populations. Neurons that project from the prefrontal cortex to either the nucleus accumbens or paraventricular thalamus receive different inputs, differentially encode reward-predictive cues, and have opposing effects on reward seeking during cue presentation. Control of reward-seeking behaviour Projections from the prefrontal cortex to the nucleus accumbens and paraventricular thalamus contribute to reward-seeking behaviours, but the type of reward-relevant information that these prefrontal-cortex neurons encode is unknown. Garret Stuber and colleagues show that these two populations of projection neuron receive different inputs, differentially encode reward-predictive cues, and have opposing effects on reward seeking when cues are presented. These findings show how the prefrontal cortex can dynamically control reward-seeking behaviour through the opposing activities of anatomically segregated, projection-specific cell populations.

Social behaviors require neural circuits to process social cues and orchestrate motivational states. This study identifies a subpopulation of hypothalamic neurons expressing neurotensin that are engaged by social and hormonal signals. These neurons project to midbrain dopaminergic reward systems to promote and reinforce social and motivated behavior in a hormone-sensitive manner. Neural networks that control reproduction must integrate social and hormonal signals, tune motivation, and coordinate social interactions. However, the neural circuit mechanisms for these processes remain unresolved. The medial preoptic area (mPOA), an essential node for social behaviors, comprises molecularly diverse neurons with widespread projections. Here we identify a steroid-responsive subset of neurotensin ( Nts )-expressing mPOA neurons that interface with the ventral tegmental area (VTA) to form a socially engaged reward circuit. Using in vivo two-photon imaging in female mice, we show that mPOA Nts neurons preferentially encode attractive male cues compared to nonsocial appetitive stimuli. Ovarian hormone signals regulate both the physiological and cue-encoding properties of these cells. Furthermore, optogenetic stimulation of mPOA Nts –VTA circuitry promotes rewarding phenotypes, social approach and striatal dopamine release. Collectively, these data demonstrate that steroid-sensitive mPOA neurons encode ethologically relevant stimuli and co-opt midbrain reward circuits to promote prosocial behaviors critical for species survival.

While TGF-β signaling is essential for microglial function, the cellular source of TGF-β1 ligand and its spatial regulation remains unclear in the adult CNS. Our data supports that microglia but not astrocytes or neurons are the primary producers of TGF-β1 ligands needed for microglial homeostasis. Microglia- Tgfb1 KO leads to the activation of microglia featuring a dyshomeostatic transcriptome that resembles disease-associated, injury-associated, and aged microglia, suggesting microglial self-produced TGF-β1 ligands are important in the adult CNS. Astrocytes in MG-Tgfb1 inducible (i)KO mice show a transcriptome profile that is closely aligned with an LPS-associated astrocyte profile. Additionally, using sparse mosaic single-cell microglia KO of TGF-β1 ligand we established an autocrine mechanism for signaling. Here we show that MG- Tgfb1 iKO mice present cognitive deficits, supporting that precise spatial regulation of TGF-β1 ligand derived from microglia is required for the maintenance of brain homeostasis and normal cognitive function in the adult brain. TGF-β signaling is required for microglial homeostasis, however the source of ligands in the adult brain is unknown. Here, the authors show that microglial homeostasis relies on microglia-derived TGF-β1 ligand via an autocrine mechanism, which is also important for astrocyte homeostasis.

The mesolimbic dopamine system is an evolutionarily conserved set of brain circuits that play a role in attention, appetitive behavior, and reward processing. In this circuitry, ascending dopaminergic projections from the ventral midbrain innervate targets throughout the limbic forebrain, such as the ventral striatum/nucleus accumbens (NAc). Dopaminergic signaling in the NAc has been widely studied for its role in behavioral reinforcement, reward prediction error encoding, and motivational salience. Less well characterized is the role of dopaminergic neurotransmission in the response to surprising or alerting sensory events. To address this, we used the genetically encoded dopamine sensor dLight1 and fiber photometry to explore the ability of striatal dopamine release to encode the properties of salient sensory stimuli in mice, such as threatening looming discs. Here, we report that lateral NAc (LNAc) dopamine release encodes the rate and magnitude of environmental luminance changes rather than the visual stimulus threat level. This encoding is highly sensitive, as LNAc dopamine could be evoked by light intensities that were imperceptible to human experimenters. We also found that light-evoked dopamine responses are wavelength-dependent at low irradiances, independent of the circadian cycle, robust to previous exposure history, and involve multiple phototransduction pathways. Thus, we have further elaborated the mesolimbic dopamine system’s ability to encode visual information in mice, which is likely relevant to a wide body of scientists employing light sources or optical methods in behavioral research involving rodents.

Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder whose neurodevelopmental symptoms include impaired executive function, attention, and spatial learning and could be due to perturbed mesolimbic dopaminergic circuitry. However, these circuits have never been directly assayed in vivo. We employed the genetically encoded optical dopamine sensor dLight1 to monitor dopaminergic neurotransmission in the ventral striatum of NF1 mice during motivated behavior. Additionally, we developed novel systemic AAV vectors to facilitate morphological reconstruction of dopaminergic populations in cleared tissue. We found that NF1 mice exhibit reduced spontaneous dopaminergic neurotransmission that was associated with excitation/inhibition imbalance in the ventral tegmental area and abnormal neuronal morphology. NF1 mice also had more robust dopaminergic and behavioral responses to salient visual stimuli, which were independent of learning, and rescued by optogenetic inhibition of non-dopaminergic neurons in the VTA. Overall, these studies provide a first in vivo characterization of dopaminergic circuit function in the context of NF1 and reveal novel pathophysiological mechanisms. About one in 3,500 people have a genetic disorder called neurofibromatosis type 1, often shortened to NF1, making it one of the most common inherited diseases. People with NF1 may have benign and cancerous tumors throughout the body, learning disabilities, developmental delays, curvature of the spine and bone abnormalities. Children with NF1 often experience difficulties with attention, hyperactivity, speech and language delays and impulsivity. They may also have autism spectrum disorder, or display symptoms associated with this condition. Studies in mice with a genetic mutation that mimics NF1 suggest that abnormal development in cells in the middle of the brain may cause the cognitive symptoms. These midbrain neurons produce a chemical called dopamine and send it throughout the brain. Dopamine is essential for concentration and it is involved in how the brain processes pleasurable experiences. Now, Robinson et al. show that, at rest, the NF1 model mice release dopamine less often than typical mice. This happens because, when there are no stimuli to respond to, neighboring cells slow down the activity of dopamine-producing neurons in NF1 model mice. In the experiments, both NF1 model mice and typical mice were taught to associate environmental cues with rewards or punishments. Robinson et al. then measured the release of dopamine in the mice using a sensor called dLight1, which produces different intensities of fluorescent light depending on the amount of dopamine present. This revealed that the NF1 model mice produced more dopamine in response to visual cues and had enhanced behavioral responses to these stimuli. For example, when a looming disc that mimics predators approached them from above, the NF1 model mice tried to hide in an exaggerated way compared to the typical mice. Previously, it had been shown that this type of behavior is due to the activity of the dopamine-producing neurons' neighboring cells, which Robinson et al. found is greater in NF1 model mice. Next, Robinson et al. stopped neighboring cells from interfering with the dopamine-producing neurons in NF1 model mice. This restored dopamine release to normal levels at rest, and stopped the mice from overreacting to the looming disc. The experiments help explain how the NF1 model mice process visual information. Further study of the role dopamine plays in cognitive symptoms in people with NF1 may help scientists develop treatments for the condition.

The OPRM1 A118G polymorphism is the most widely studied μ-opioid receptor (MOR) variant. Although its involvement in acute alcohol effects is well characterized, less is known about the extent to which it alters responses to opioids. Prior work has shown that both electrophysiological and analgesic responses to morphine but not to fentanyl are moderated by OPRM1 A118G variation, but the mechanism behind this dissociation is not known. Here we found that humanized mice carrying the 118GG allele (h/mOPRM1-118GG) were less sensitive than h/mOPRM1-118AA littermates to the rewarding effects of morphine and hydrocodone but not those of other opioids measured with intracranial self-stimulation. Reduced morphine reward in 118GG mice was associated with decreased dopamine release in the nucleus accumbens and reduced effects on GABA release in the ventral tegmental area that were not due to changes in drug potency or efficacy in vitro or receptor-binding affinity. Fewer MOR-binding sites were observed in h/mOPRM1-118GG mice, and pharmacological reduction of MOR availability unmasked genotypic differences in fentanyl sensitivity. These findings suggest that the OPRM1 A118G polymorphism decreases sensitivity to low-potency agonists by decreasing receptor reserve without significantly altering receptor function.

Fragile X syndrome (FXS) is a leading cause of intellectual disability. FXS is caused by loss of function of the FMR1 gene, and mice in which Fmr1 has been inactivated have been used extensively as a preclinical model for FXS. We investigated the behavioral pharmacology of drugs acting through dopaminergic, glutamatergic, and cholinergic systems in fragile X (Fmr1 (-/Y)) mice with intracranial self-stimulation (ICSS) and locomotor activity measurements. We also measured brain expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis. Fmr1 (-/Y) mice were more sensitive than wild type mice to the rewarding effects of cocaine, but less sensitive to its locomotor stimulating effects. Anhedonic but not motor depressant effects of the atypical neuroleptic, aripiprazole, were reduced in Fmr1 (-/Y) mice. The mGluR5-selective antagonist, 6-methyl-2-(phenylethynyl)pyridine (MPEP), was more rewarding and the preferential M1 antagonist, trihexyphenidyl, was less rewarding in Fmr1 (-/Y) than wild type mice. Motor stimulation by MPEP was unchanged, but stimulation by trihexyphenidyl was markedly increased, in Fmr1 (-/Y) mice. Numbers of midbrain TH+ neurons in the ventral tegmental area were unchanged, but were lower in the substantia nigra of Fmr1 (-/Y) mice, although no changes in TH levels were found in their forebrain targets. The data are discussed in the context of known changes in the synaptic physiology and pharmacology of limbic motor systems in the Fmr1 (-/Y) mouse model. Preclinical findings suggest that drugs acting through multiple neurotransmitter systems may be necessary to fully address abnormal behaviors in individuals with FXS.

Rationale The neuroactive steroid (3α,5α)-3-hydroxy-pregnan-20-one (3α,5α-THP, allopregnanolone) has effects on reward-related behaviors in mice and rats that suggest that it may activate brain reward circuits. Intracranial self-stimulation (ICSS) is an operant behavioral technique that detects changes in the sensitivity of brain reward circuitry following drug administration. Objective To examine the effects of the neuroactive steroid allopregnanolone on ICSS and to compare these effects to those of cocaine. Methods Male C57BL/6J mice implanted with stimulating electrodes implanted into the medial forebrain bundle responded for reinforcement by electrical stimulation (brain stimulation reward (BSR)). Mice received cocaine ( n  = 11, 3.0–30.0 mg/kg, intraperitoneal (i.p.)) or the neuroactive steroid allopregnanolone ( n  = 11, 3.0–17.0 mg/kg, i.p.). BSR thresholds ( θ 0 ) and maximum (MAX) operant response rates after drug treatments were compared to those after vehicle injections. Results Cocaine and allopregnanolone dose dependently lowered BSR thresholds relative to vehicle injections. Cocaine was maximally effective (80 % reduction) in the second 15 min following the 30 mg/kg dose, while allopregnanolone was maximally effective (30 % reduction) 15–45 min after the 17 mg/kg dose. Neither drug had significant effects on MAX response rates. Conclusions The effects of allopregnanolone on BSR thresholds are consistent with the previously reported effects of benzodiazepines and alcohol, suggesting that positive modulation of GABA A receptors can facilitate reward-related behaviors in C57BL/6J mice.

Rationale Sensitivity to the stimulant and rewarding effects of alcohol may be genetically correlated traits that predispose individuals to develop an alcohol use disorder. Objective This study aimed to examine the effects of alcohol and cocaine on intracranial self-stimulation (ICSS) in FAST and SLOW mice, which were selectively bred for extremes in alcohol stimulation. Methods Male FAST and SLOW mice were conditioned to respond for reinforcement by direct electrical stimulation of the medial forebrain bundle (i.e., brain stimulation reward). ICSS responses were determined immediately before and after oral gavage with water or alcohol (0.3–2.4 g/kg) or intraperitoneal injection with saline or cocaine (1.0–30.0 mg/kg). In separate FAST and SLOW mice, the locomotor effects of these treatments were measured in activity chambers. Results Alcohol dose-dependently lowered the threshold for self-stimulation ( θ 0 ) and the frequency that maintained 50% of maximal responding (EF50) in FAST mice but did not significantly affect these parameters in SLOW mice. The largest effects of alcohol were after the 1.7- and 2.4-g/kg doses and were about 40% compared to water injection. Alcohol did not affect MAX response rates, but dose-dependently stimulated locomotor activity in FAST mice. Cocaine lowered thresholds equally in FAST and SLOW mice, although cocaine-stimulated locomotor activity was higher in the FAST than in the SLOW mice. Conclusions Selective breeding for alcohol locomotor stimulation also renders the mice more sensitive to the effects of alcohol, but not cocaine, on ICSS.