Only a subset of heroin users develop addiction, characterized by binge-like heroin use and preference for heroin over other rewards, including social rewards. We recently established a rat model that captures these features. We trained rats to lever-press for social interaction and heroin (or saline, control) infusions and then tested heroin and social seeking and heroin-vs.-social choice. During 3 to 5 abstinence weeks, we used 2-deoxy-2-[ 18 F]fluoro-D-glucose (FDG) positron emission tomography (PET) imaging to assess regional brain metabolic activity at rest (homecage) and during heroin and social seeking. We assessed regional differences in FDG uptake using unbiased voxel-wise analysis and statistical parametric mapping, and correlated FDG uptake with principle-component-analysis-derived addiction severity score incorporating heroin intake, binge-like episodes, and heroin preference. Compared with saline-trained rats, heroin-trained rats showed overall higher FDG uptake across multiple brain regions at rest and during both reward-seeking tests. Comparison of heroin-vs.-social-seeking in heroin-trained rats showed higher uptake in claustrum/striatum and auditory cortex during social seeking. Analysis of individual differences showed that addiction severity was primarily associated with metabolic alterations under resting conditions rather than during heroin or social seeking. At rest, higher addiction severity was associated with lower uptake in piriform cortex and higher uptake in ventral hippocampus, whereas during heroin seeking, addiction severity was associated with lower uptake in postsubiculum and cerebellum. Addiction severity was not associated with differences in social measures or FDG uptake during social-seeking. These findings identify brain metabolic features of heroin addiction vulnerability in a rat model that may serve as brain biomarkers of human opioid addiction.

The brain’s reward-processing circuitry remains sensitive to experience throughout early life and into adulthood, allowing individuals to adapt to their unique environments. Adverse experiences early in life can increase vulnerability to substance use disorders, likely through alterations to this circuitry. Yet, the precise neurobiological mechanisms by which early life adversity acts are incompletely characterized. In this study, we used a limited bedding and nesting (LBN) paradigm as a translationally relevant model of early life adversity in isogenic C57BL/6J mice. After LBN-rearing, we assessed the lasting behavioral and neurobiological impacts of this experience in adulthood. In robust sample sizes, our results validated previous findings of increased risk avoidance, enhanced acute locomotor response to alcohol, and greater voluntary alcohol drinking in socially-housed LBN-reared mice, especially males. Further, using autoradiography, we found LBN-reared mice had increased striatal D1-like receptor binding, skewing D1- to D2-like receptor balance relative to cross-fostered controls. However, after voluntary alcohol drinking, we found a strong downregulation in D1-like, and some D2-like, receptor binding, negating pre-existing differences in striatal dopamine receptor binding. We posit that via both transcriptional and post-transcriptional mechanisms, LBN-rearing upregulates striatal D1-receptor density and alters risk avoidance and acute alcohol stimulation to promote alcohol drinking among adversity-exposed mice. Together, these findings reveal specific neurobiological mechanisms that promote alcohol consumption following early life adversity and suggest complex interactions between early life adversity, sex-related factors, and dopamine receptor regulation in contributing to alcohol use disorder (AUD) vulnerability.

Chemogenetic technologies offer a powerful approach to modulating neural circuits with high precision and hold promise for therapeutic applications in neuropsychiatric disorders. However, translating these tools for noninvasive monitoring in the primate brain has been limited by a lack of suitable positron emission tomography (PET) radioligands. Existing ultrapotent ligands activate PSAM receptors, but the PET ligand [11C]uPSEM792 shows poor brain penetration, and [18F]ASEM lacks receptor specificity. To address this, we developed novel chemogenetic ligands with improved brain permeability and radiolabeled them with fluorine-18. Two candidates, PSG07 and PSN09, showed a high-affinity and potent agonist activity at PSAM4-GlyR and PSAM4-5-HT3 receptors. Monkey PET imaging showed tracer localization at the PSAM4-GlyR expression site, with [18F]PSN09 displaying a detectable signal. Functional imaging with [18F]FDG further confirmed neuronal inhibition following administration of PSG07 and PSN09. These findings highlight PSG07 and PSN09 as promising chemogenetic actuators with the potential as radioligands for translational PET imaging in nonhuman primates.

A defining feature of substance use disorder is that repeated drug use does not always lead to addiction, motivating the search for biomarkers of vulnerability1. Reduced striatal dopamine D2/3 receptor availability is a robust PET correlate of problematic stimulant use2-5, but the signal may reflect high endogenous dopamine level, and it conflates presynaptic D2 autoreceptors on dopamine axons with postsynaptic D2/3 heteroreceptors on striatal projection neurons. We dissociated these contributions using cell type-specific Drd2 haploinsufficiency in dopamine neurons (autoD2KD), D2-expressing medium spiny neurons (MSN-D2KD), or both. Autoreceptor haploinsufficiency (autoD2KD) weakened presynaptic control of dopamine release, enhanced phasic gain, and prolonged cocaine-evoked dopamine elevations. This was accompanied by a hyper-exploratory trait and altered cocaine adaptation. Specifically, autoD2KD mice showed greater cocaine-seeking behavior, despite intact responses to sucrose reward and punishment. Although all genotypes showed graded reductions in striatal D2/3 binding, D1-like compensations diverged, resulting in different D1:D2/3 ratio in the striatum. The clinical implication is that striatal D1 density and D1:D2/3 balance may emerge as critical biomarkers for distinguishing cell-type-specific D2 reductions relevant to addiction vulnerability.

Synaptic zinc (Zn2+) modulates dopamine and glutamate neurotransmission by binding to the dopamine transporter and glutamate receptors. Among other neurotransmitters, dopamine and glutamate critically regulate physiological processes and behaviors relevant to substance use disorders (SUDs) and addiction. In addition, Zn2+ interacts with inhibitory neurotransmitter systems, including GABA and glycine receptors, further influencing the excitatory-inhibitory balance within circuits relevant to addiction. Nevertheless, the specific involvement of synaptic Zn2+ in such processes is unknown. We propose that synaptic Zn2+ serves as an environmentally derived factor that can influence the vulnerability to and development of SUDs and addiction via its interaction with proteins that regulate dopamine and glutamate neurotransmission in addiction-relevant brain circuits.

Chemical feedback is ubiquitous in physiology but is challenging to study without perturbing basal functions. One example is addictive drugs, which elicit a positive-feedback cycle of drug-seeking and ingestion by acting on the brain to increase dopamine signalling1-3. However, interfering with this process by altering basal dopamine also adversely affects learning, movement, attention and wakefulness4. Here, inspired by physiological control systems, we developed a highly selective synthetic physiology approach to interfere with the positive-feedback cycle of addiction by installing a cocaine-dependent opposing signalling process into this body-brain signalling loop. We used protein engineering to create cocaine-gated ion channels that are selective for cocaine over other drugs and endogenous molecules. Expression of an excitatory cocaine-gated channel in the rat lateral habenula, a brain region that is normally inhibited by cocaine, suppressed cocaine self-administration without affecting food motivation. This artificial cocaine-activated chemogenetic process reduced the cocaine-induced extracellular dopamine rise in the nucleus accumbens. Our results show that cocaine chemogenetics is a selective approach for countering drug reinforcement by clamping dopamine release in the presence of cocaine. In the future, chemogenetic receptors could be developed for additional addictive drugs or hormones and metabolites, which would facilitate efforts to probe their neural circuit mechanisms using a synthetic physiology approach. As these chemogenetic ion channels are specific for cocaine over natural rewards, they may also offer a route towards gene therapies for cocaine addiction.

Deficits in behavioral or cognitive flexibility that are linked to altered activity in both cortical and subcortical brain regions, are often observed across multiple neuropsychiatric disorders. The medial prefrontal cortex (mPFC)-nucleus accumbens (NAc) pathway in rats plays a critical role in flexible control of behavior. However, the modulation of this pathway on activity and functional connectivity with the rest of the brain remains unclear. In this study, we first confirmed the role of the mPFC-NAc pathway in behavioral flexibility using a set-shifting task in rats and then evaluated the causal effects of mPFC-NAc activation induced by chemogenetic stimulation of the terminal axons of the NAc with DREADD expression on whole-brain activity and functional connectivity measured by functional MRI. mPFC-NAc activation improved performance on the set-shifting task by reducing perseverative errors. Additionally, stimulation of this pathway increased activity in a set of brain regions within the basal ganglia-thalamus-cortical loop network including NAc, thalamus, hypothalamus and various connected cortical regions, while also decreased functional connectivity strength of NAc-mPFC, NAc-secondary motor cortex (M2), and various cortical circuits. Moreover, performance on the set-shifting task was related to the functional connectivity strength of the above frontostriatal and cortical circuits. These findings provide insights into the link between specific frontostriatal circuits on decision making flexibility, which may inform potential future interventions for behavioral flexibility deficits.

Ketamine is a racemic compound and medication comprised of (S)-ketamine and (R)-ketamine enantiomers and its metabolites. It has been used for decades as a dissociative anesthetic, analgesic, and recreational drug. More recently, ketamine, its enantiomers, and its metabolites have been used or are being investigated for the treatment of refractory depression, as well as for comorbid disorders such as anxiety, obsessive-compulsive, and opioid use disorders. Despite its complex pharmacology, ketamine is referred to as an N-methyl-d-aspartate (NMDA) receptor antagonist. In this review, the authors argue that ketamine's pharmacology should be redefined to include opioid receptors and the endogenous opioid system. They also highlight a potential mechanism of action of ketamine for depression that is attributed to bifunctional, synergistic interactions involving NMDA and opioid receptors.

Several dopaminergic compounds, including the clinically used pramipexole, are labelled as preferential dopamine D3 receptor (D3R) agonists based on their moderately higher affinity for the D3R versus other D2-like receptor subtypes. In rodents, these compounds typically produce locomotor depression with low doses and locomotor activation with higher doses, which has been assumed to be mediated by presynaptic D3Rs and postsynaptic striatal D2Rs, respectively. However, studies with selective pharmacological and genetic blockade of each dopamine receptor subtype suggest opposite roles. We address this apparent conundrum by performing a comprehensive in vitro, in vivo and ex vivo pharmacological comparison of several preferential D3R agonists. Their differential properties reveal that their locomotor activating effects in mice are dependent on the striatal postsynaptic D3Rs forming heteromers with D1Rs, via their ability to potentiate β-arrestin recruitment by the D1R in the D1R-D3R heteromer. The results also indicate that the locomotor depressant effects are largely dependent on their ability to activate presynaptic D2Rs. More broadly, it is demonstrated that locomotor activity in mice depends on β-arrestin recruitment by the D1R in the striatal D1R-D3R heteromer. These results can have implications for the treatment of L-dopa-induced dyskinesia and Restless Legs Syndrome.

The anterior hypothalamic area (AHA) is a key brain region for orchestrating defensive behaviors. Using in vivo calcium imaging in mice, we observed that AHA neuronal activity increases during footshock delivery and footshock-associated auditory cues. We found that following shock-induced increases in AHA activity, a decrease in activity coincides with the onset of grooming behavior. Next, we optogenetically activated the projections from the ventromedial hypothalamus (VMH) to the AHA and observed that photoactivation of the VMH→AHA pathway drives avoidance. Interestingly, repetitive grooming behavior occurs following cessation of stimulation. To identify changes in brain-wide activity patterns that occur due to optogenetic VMH→AHA stimulation, we combined optogenetic stimulation with positron emission tomography (PET)-based metabolic mapping. This approach revealed the amygdala as a downstream area activated by the stimulation of this pathway. Our findings show that the rise and fall of AHA neuronal activity triggers repetitive grooming behavior following learned fear and optogenetic stimulation. In addition, activation of the VMH→AHA pathway triggers changes in the activity patterns of downstream brain regions that are reported to be associated with displacement grooming.

Positron emission tomography (PET) reporter systems are a valuable means of estimating the level of expression of a transgene in vivo. For example, the safety and efficacy of gene therapy approaches for the treatment of neurological and neuropsychiatric disorders could be enhanced via the monitoring of exogenous gene expression levels in the brain. The present study evaluated the ability of a newly developed PET reporter system [18F]fluoroestradiol ([18F]FES) and the estrogen receptor-based PET reporter ChRERα, to monitor expression levels of a small hairpin RNA (shRNA) designed to suppress choline acetyltransferase (ChAT) expression in rhesus monkey brain. The ChRERα gene and shRNA were expressed from the same transcript via lentivirus injected into monkey striatum. In two monkeys that received injections of viral vector, [18F]FES binding increased by 70% and 86% at the target sites compared with pre-injection, demonstrating that ChRERα expression could be visualized in vivo with PET imaging. Post-mortem immunohistochemistry confirmed that ChAT expression was significantly suppressed in regions in which [18F]FES uptake was increased. The consistency between PET imaging and immunohistochemical results suggests that [18F]FES and ChRERα can serve as a PET reporter system in rhesus monkey brain for in vivo evaluation of the expression of potential therapeutic agents, such as shRNAs.

Pharmacologically Selective Actuator Molecules(PSAMs) are advanced chemogenetic tools used to manipulate neuronal activity in brain. A triple-mutant PSAM is called PSAM4. Here, PSAM4-GlyR is a chimeric ligand-gated ion channel comprising a mutated a7 nicotinic acetylcholine receptor(α7-nAChR) ligand-binding domain with an inhibitory chloride-selective glycine receptor ion pore. To create an improved PET radioligand for verifying PSAM4 expression, we synthesized two analogs of varenicline, an anti-smoking drug that activates PSAM4 and evaluated their behavior as PET radioligands in rats, mice, and monkeys.

Anxiety disorders are among the most prevalent psychiatric disorders, with symptoms often beginning early in life. To model the pathophysiology of human pathological anxiety, we utilized Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) in a nonhuman primate model of anxious temperament to selectively increase neuronal activity of the amygdala. Subjects included 10 young rhesus macaques; 5 received bilateral infusions of AAV5-hSyn-HA-hM3Dq into the dorsal amygdala, and 5 served as controls. Subjects underwent behavioral testing in the human intruder paradigm following clozapine or vehicle administration, prior to and following surgery. Behavioral results indicated that clozapine treatment post-surgery increased freezing across different threat-related contexts in hM3Dq subjects. This effect was again observed approximately 1.9 years following surgery, indicating the long-term functional capacity of DREADD-induced neuronal activation. [11C]deschloroclozapine PET imaging demonstrated amygdala hM3Dq-HA specific binding, and immunohistochemistry revealed that hM3Dq-HA expression was most prominent in basolateral nuclei. Electron microscopy confirmed expression was predominantly on neuronal membranes. Together, these data demonstrate that activation of primate amygdala neurons is sufficient to induce increased anxiety-related behaviors, which could serve as a model to investigate pathological anxiety in humans.

Mu opioid receptor (MOR) agonists comprise the most effective analgesics, but their therapeutic utility is limited by adverse effects. One approach for limiting such effects has been to develop "biased" MOR agonists that show preference for activating G protein over β-Arrestin signaling. However, the notion of biased agonism has been challenged by recent studies. Oliceridine (Olinvyk®, TRV-130, OLC) is a selective MOR agonist approved by the FDA in 2020 for pain management in controlled clinical settings. Oliceridine purportedly demonstrates diminished adverse effects compared to morphine or other MOR agonists, a profile attributed to its biased agonism. However, recent studies suggest that oliceridine does not display biased agonism but instead weak intrinsic efficacy for G protein and β-Arrestin activation. Nevertheless, these insights have been derived from in vitro studies. To better understand oliceridine's in vivo efficacy profile, we performed a comprehensive assessment of its in vitro and in vivo pharmacology using both cultured cells and rodents. In vitro, oliceridine displayed high MOR affinity and weak intrinsic efficacy. In vivo, oliceridine showed impaired brain penetrance and rapid clearance, effects we attributed to its interaction with the P-glycoprotein (P-gp) efflux transporter. Moreover, we found that P-gp was essential for oliceridine's in vivo efficacy and adverse effect profiles. Taken together with prior studies, our results suggest that oliceridine's in vivo efficacy and adverse effect profiles are not attributed solely to its weak intrinsic efficacy or biased agonism but, to a large extent, its interaction with P-gp as well.

Transgenic neuromodulation tools have transformed the field of neuroscience over the past two decades by enabling targeted manipulation of neuronal populations and circuits with unprecedented specificity. Chemogenetic and optogenetic neuromodulation systems are among the most widely used and allow targeted control of neuronal activity through the administration of a selective compound or light, respectively. Innovative genetic targeting strategies are utilized to transduce specific cells to express transgenic receptors and opsins capable of manipulating neuronal activity. These allow mapping of neuroanatomical projection sites and link cellular manipulations with brain circuit functions and behavior. As these tools continue to expand knowledge of the nervous system in preclinical models, developing translational applications for human therapies is becoming increasingly possible. However, new strategies for implementing and monitoring transgenic tools are needed for safe and effective use in translational research and potential clinical applications. A major challenge for such applications is the need to track the location and function of chemogenetic receptors and opsins in vivo, and new developments in positron emission tomography (PET) imaging techniques offer promising solutions. The goal of this review is to summarize current research combining transgenic tools with PET for in vivo mapping and manipulation of brain circuits and to propose future directions for translational applications.