Pharmacology in the age of circuit neuroscience: Illuminating the neural mechanisms of reward, drug use and addiction and enlightening the future of translational research

The brain remains one of the last frontiers in biology and medicine. Since its first recorded mention on an Egyptian papyrus in the 17th century BCE (Kamp et al., 2012), understanding brain function, its impact on our daily lives, and its role in the course of psychiatric diseases has been a fixation that has captivated medical and scientific professionals and the public alike. How does this enormously complex system, sculpted across millennia of evolution out of the will to survive, govern all that we think, remember, feel, and do? What changes in this labyrinth of soft tissue underlie the profound disturbances in our experience resulting from neuropsychiatric conditions such as depression, anxiety, dementia, and addiction? While these questions have been pondered for centuries, it is only during the last few decades that we have made great strides towards finding the answers.

The 21st century is an extraordinary time to study brain function and neuroscience. Technological advances—spanning from structural and functional neuroimaging, microscopy, Cre-Lox recombination, CRISPR/Cas9 and other tools to manipulate the genome, macromolecule engineering of G protein-coupled receptors and the discovery of light-sensitive opsins for neuromodulation, and electrophysiology, fiber photometry and calcium imaging protocols for monitoring neuronal activity—have led to rapid shifts in neuroscience research and exciting new discoveries of the neural circuitry underlying brain function, behavior, and neuropsychiatric conditions. In the 20th century we worked with the basic notions of anatomy, electrophysiology, pharmacology, neurochemistry, physiological psychology. Today, a new generation of investigators, trained with a foundation in cellular biology, uses light—optogenetics and fluorescent-labelled calcium—instead of electricity for their power. However, the advent and rapid adoption of new “circuit neuroscience” technologies has posed important challenges as well. The sub-optimal selectivity of some chemogenetic ligands in driving neuronal activity (Goutaudier et al., 2019), weak correlations between calcium fluorescence and spike activity in some parts of the brain (Legaria et al., 2021), and reports of brain activity detected in functional magnetic resonance imaging (fMRI) of a dead salmon (Bennett et al., 2009) illustrate just some of the issues with embracing new technology. Nevertheless, there are solutions to these challenges.

Combining circuit neuroscience with well-established pharmacological tools can help validate and anchor new techniques with experimental manipulations that have withstood decades of testing. New technologies in circuit neuroscience offer unprecedented cell- and system-type specificity. The marriage of these techniques with canonical pharmacological and biochemical manipulations strengthens reliability and lends credence to novel study findings. This special issue highlights recent research that leverages neural circuitry manipulations alongside pharmacological and biochemical techniques to elucidate the complexities of neural function underlying behavior and neuropsychiatric disorders. Although this special issue is not meant to be comprehensive, it includes four original research papers and 11 reviews that provide an overview of the state-of-the-art research accumulating in pharmacology and neuroscience, with a particular emphasis on translational studies of the neural substrates underlying reward, drug use, and addiction.

The first six papers included in this special issue provide compelling reviews of the latest technological developments in circuit neuroscience, nested in the context of traditional experimental techniques. Authors Yocky and Covey provide an overview of the evolution and latest developments in dopamine monitoring techniques. Readers are carried from seminal discoveries of dopamine’s role in neuropsychiatric conditions to recent technical advancements involving increasingly spatiotemporally-defined measurements of real-time dopamine signaling in freely behaving animals. Next, Wang, DeMarco, Witzel and Keighron review the strengths and limitations of neuronal activity monitoring techniques ranging from electrophysiology to fast scan cyclic voltammetry, amperometry, fiber photometry, and electrochemical and genetically encoded biosensors. While techniques like fast scan cyclic voltammetry measure phasic dopamine release, both older (e.g., microdialysis) and newer (e.g., photometry) methods yield important insights into other aspects of dopamine function. Further, the union of cell-type specific activity measurements with global brain mapping techniques can powerfully illuminate the contribution of neuronal subpopulations to system-wide brain activity.

The next two papers address major challenges in the implementation of circuit neuroscience techniques, including standardizing analyses of the massive data sets that are collected during neural activity monitoring, and establishing the selectively and specificity of neural activity manipulations. To address the former, Bruno, O’Brien, Bryant, Mejaes, Estrin, Pizzano and Barker developed the photometry Modular Analysis Tool (pMAT), an open-source, user-friendly software that enables fiber photometry data analysis across multiple fluorescent sensors and platforms. Next, Boehm, Bonaventura, Gomez, Solis, Stein, Bradberry and Michaelides describe the utility of in vivo Positron Emission Tomography (PET), which can be performed repeatedly over the lifetime with a wide variety of radiotracers to examine brain activity, glucose metabolism, neurotransmitter function and more. The combination of PET with chemogenetics and other neural activity manipulations offers a robust way to validate the selectivity and localization of designer receptors and understand the impact of cell-type specific neuronal activity on system-wide brain function.

The next review in this special issue further highlights the utility of brain imaging in uncovering circuits involved in psychiatric conditions. Scarlata, Keeley and Stein explore key insights gleaned from fMRI studies into the neural substrates underlying nicotine addiction. By providing direct comparisons between fMRI studies in humans and other animals, the authors elegantly demonstrate the correlational vs. causative or predictive utility of each and the importance of forward and reverse translation in understanding psychiatric conditions.

In the last technique-focused review of this special issue, Kohut and Kaufman discuss magnetic resonance spectroscopy (MRS), which measures neuronal metabolites (e.g., glutamate, GABA, glutamine, ATP, markers of oxidative stress), has high translational value, and can be used alongside MRI or diffusion tensor imaging (DTI) to detect relationships between biochemical changes and functional or structural connectivity across the brain. MRS inspires many avenues for new translational research to understand the neurochemical and physiological consequences of substance use disorders and other mental health conditions.

The next four papers in this special issue provide examples of the union of novel circuit neuroscience techniques with pharmacological manipulations in original research reports. Rohan, Lowen, Rock and Andersen use lentiviral vectors to overexpress dopamine D₁ receptors in glutamate neurons of the prelimbic prefrontal cortex of juvenile female rats. By combining behavioral phenotyping with fMRI, the authors discover important sex differences in the impact of prefrontal cortex D₁ receptor overexpression on novelty seeking, cocaine place preferences, and blood-oxygen-level-dependent (BOLD) responses to cocaine-paired odors in neural salience networks.

Next, Davis, Coldren and Li present a research investigation on methamphetamine seeking after an extended abstinence period using an incubation of craving model. This group coupled intravenous methamphetamine self-administration behavioral assessments with retrograde neuronal tract tracing and c-Fos immediate early gene double labeling, discovering that methamphetamine craving is linked to increased neuronal activation and projections within several cortical, thalamic and amygdalar brain regions.

Shen, Chen, Marino, McDevitt, and Xi provide another unique example of the combination of pharmacology with circuit neuroscience to elucidate the role of the vesicular glutamate transporter type 2 (VGluT2) in mediating dopamine and glutamate responses to methamphetamine in midbrain dopamine neurons. By pairing Cre-Lox recombination with RNAscope in situ hybridization, in vivo microdialysis, and open field locomotor testing, the authors show that VGluT2 deletion critically modulates methamphetamine-induced changes in dopamine and glutamate levels in the nucleus accumbens as well as locomotor activity.

In the last original research report of this special issue, McDevitt, Marino, Tejeda, and Bonci deploy several circuit neuroscience and pharmacological techniques to explore the role of serotonin in cue-related reward learning in mice. Using classical behavioral lever pressing experiments in combination with serotonin reuptake inhibitor administration and region-specific knock out of a key serotonin synthesis enzyme, the authors reveal an inhibitory effect of serotonin on reward learning and dissect differential roles of the raphe nuclei in mediating the attribution of motivational value or salience to a reward-predictive cue. In context with earlier work showing that some serotonin projections may activate the dopamine system, these findings demonstrate that serotonin plays many roles in mediating reward. Careful differentiation of serotonergic neuronal phenotypes is needed to fully understand its effects.

The final set of articles in this special issue comprise five review papers that explore the role of specific neural circuits in neuropsychiatric conditions and behaviors. Alonso-Caraballo, Guha, and Chartoff examine common neural substrates underlying abstinence-induced food and drug reward seeking in both males and females, with a focus on hypothalamic-thalamic-striatal circuitry. By examining prior studies on the role of gonadal hormones in food and opioid seeking and highlighting the paucity of work that has included both males and females, the authors provide motivation for future research on sex differences in the neural circuity underlying reward-related behaviors. Altshuler, Lin and Li further emphasize the need for studies involving both sexes in methamphetamine craving, providing a deep dive into the neural mechanisms underlying incubation of methamphetamine seeking, including genetic, epigenetic and protein expression changes in key brain areas such as the striatum, amygdala and prefrontal cortex.

Elvig, McGinn, Smith, Arends, Koob and Vendruscolo next review alcohol tolerance, or the reduced effect of alcohol following repeated exposures. The authors outline key preclinical behavioral studies on pharmacological changes that occur during the development of alcohol tolerance. Future opportunities involving the combination of established behavioral models with new innovations in optogenetics, chemogenetics and calcium imaging to further elucidate the circuits contributing to alcohol tolerance and its treatment are thoughtfully explored.

Kesner, Calva, and Ikemoto illustrate the union of pharmacology and circuit neuroscience techniques in their review of a previously unrecognized reward circuit involving the supramammillary region, medial septal nucleus, ventral tegmental area, and beyond. Through a series of studies involving pharmacological microinjections, intravenous and intracranial drug self-administration, electrical intracranial self-stimulation (ICSS), optogenetics, and fiber photometry, new pathways involving glutamatergic neurons in the supramammillary region are revealed to be critical in guiding the search for motivationally-relevant information, a behavior that is fundamental to survival.

In addition to facilitating new discoveries, the union pharmacology and circuit neuroscience has challenged old dogmas and theories. In the final review of this special issue, Galaj and Xi explore recent progress in understanding the neural substrates of opioid reward. A prevailing hypothesis has been that opioid reward results from disinhibition of dopamine neurons via inhibition of local GABAergic neurons. However, recent work utilizing pharmacological manipulations, autoradiography, RNAscope in situ hybridization, and optogenetics provides a new circuit-based analysis that establishes central roles for the substantia nigra, rostromedial tegmental nucleus, nucleus accumbens, ventral pallidum and dorsal striatum in opioid reward and opioid use disorders.

As pharmacology and circuit neuroscience continue to shed light on brain function and neuropsychiatric conditions, we will inevitably find that every answer leads to many more questions. We hope that this collection will be of interest to a wide audience of readers and will inspire exciting new avenues of scientific pursuit in generations to come.