Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are a popular chemogenetic technology for manipulation of neuronal activity in uninstrumented awake animals with potential for human applications as well. The prototypical DREADD agonist clozapine N-oxide (CNO) lacks brain entry and converts to clozapine, making it difficult to apply in basic and translational applications. Here we report the development of two novel DREADD agonists, JHU37152 and JHU37160, and the first dedicated ¹⁸F positron emission tomography (PET) DREADD radiotracer, [¹⁸F]JHU37107. We show that JHU37152 and JHU37160 exhibit high in vivo DREADD potency. [¹⁸F]JHU37107 combined with PET allows for DREADD detection in locally-targeted neurons, and at their long-range projections, enabling noninvasive and longitudinal neuronal projection mapping.

Modern molecular tools make it possible to manipulate neural activity in a reversible and cell-type specific manner. For rhesus monkey research, molecular tools are generally introduced via viral vectors. New instruments designed specifically for use in monkey research are needed to enhance the efficiency and reliability of vector delivery. A suite of multi-channel injection devices was developed to permit efficient and uniform vector delivery to cortical regions of the monkey brain. Manganese was co-infused with virus to allow rapid post-surgical confirmation of targeting accuracy using MRI. A needle guide was designed to increase the accuracy of sub-cortical targeting using stereotaxic co-ordinates. The multi-channel injection devices produced dense, uniform coverage of dorsal surface cortex, ventral surface cortex, and intra-sulcal cortex, respectively. Co-infusion of manganese with the viral vector allowed for immediate verification of injection accuracy. The needle guide improved accuracy of targeting sub-cortical structures by preventing needle deflection. The current methods, hand-held injections or single slow mechanical injection, for surface cortex transduction do not, in our hands, produce the density and uniformity of coverage provided by the injector arrays and associated infusion protocol. The efficiency and reliability of vector delivery has been considerably improved by the development of new methods and instruments. This development should facilitate the translation of chemo- and optogenetic studies performed in smaller animals to larger animals such as rhesus monkeys.

Previous work found that [¹¹C]deschloroclozapine ([¹¹C]DCZ) is superior to [¹¹C]clozapine ([¹¹C]CLZ) for imaging Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). This study used PET to quantitatively and separately measure the signal from transfected receptors, endogenous receptors/targets, and non-displaceable binding in other brain regions to better understand this superiority. A genetically-modified muscarinic type-4 human receptor (hM₄Di) was injected into the right amygdala of a male rhesus macaque. [¹¹C]DCZ and [¹¹C]CLZ PET scans were conducted 2–24 months later. Uptake was quantified relative to the concentration of parent radioligand in arterial plasma at baseline (n = 3 scans/radioligand) and after receptor blockade (n = 3 scans/radioligand). Both radioligands had greater uptake in the transfected region and displaceable uptake in other brain regions. Displaceable uptake was not uniformly distributed, perhaps representing off-target binding to endogenous receptor(s). After correction, [¹¹C]DCZ signal was 19% of that for [¹¹C]CLZ, and background uptake was 10% of that for [¹¹C]CLZ. Despite stronger [¹¹C]CLZ binding, the signal-to-background ratio for [¹¹C]DCZ was almost two-fold greater than for [¹¹C]CLZ. Both radioligands had comparable DREADD selectivity. All reference tissue models underestimated signal-to-background ratio in the transfected region by 40%–50% for both radioligands. Thus, the greater signal-to-background ratio of [¹¹C]DCZ was due to its lower background uptake.

The orbitofrontal cortex (OFC) and its major downstream target within the basal ganglia—the rostromedial caudate nucleus (rmCD)—are involved in reward-value processing and goal-directed behavior. However, a causal contribution of the pathway linking these two structures to goal-directed behavior has not been established. Using the chemogenetic technology of designer receptors exclusively activated by designer drugs with a crossed inactivation design, we functionally and reversibly disrupted interactions between the OFC and rmCD in two male macaque monkeys. We injected an adeno-associated virus vector expressing an inhibitory designer receptor, hM4Di, into the OFC and contralateral rmCD, the expression of which was visualized in vivo by positron emission tomography and confirmed by postmortem immunohistochemistry. Functional disconnection of the OFC and rmCD resulted in a significant and reproducible loss of sensitivity to the cued reward value for goal-directed action. This decreased sensitivity was most prominent when monkeys had accumulated a certain amount of reward. These results provide causal evidence that the interaction between the OFC and the rmCD is needed for motivational control of action on the basis of the relative reward value and internal drive. This finding extends the current understanding of the physiological basis of psychiatric disorders in which goal-directed behavior is affected, such as obsessive-compulsive disorder. SIGNIFICANCE STATEMENT In daily life, we routinely adjust the speed and accuracy of our actions on the basis of the value of expected reward. Abnormalities in these kinds of motivational adjustments might be related to behaviors seen in psychiatric disorders such as obsessive-compulsive disorder. In the current study, we show that the connection from the orbitofrontal cortex to the rostromedial caudate nucleus is essential for motivational control of action in monkeys. This finding expands our knowledge about how the primate brain controls motivation and behavior and provides a particular insight into disorders like obsessive-compulsive disorder in which altered connectivity between the orbitofrontal cortex and the striatum has been implicated.

In primates, visual recognition of complex objects depends on the inferior temporal lobe. By extension, categorizing visual stimuli based on similarity ought to depend on the integrity of the same area. We tested three monkeys before and after bilateral anterior inferior temporal cortex (area TE) removal. Although mildly impaired after the removals, they retained the ability to assign stimuli to previously learned categories, e.g., cats versus dogs, and human versus monkey faces, even with trial-unique exemplars. After the TE removals, they learned in one session to classify members from a new pair of categories, cars versus trucks, as quickly as they had learned the cats versus dogs before the removals. As with the dogs and cats, they generalized across trial-unique exemplars of cars and trucks. However, as seen in earlier studies, these monkeys with TE removals had difficulty learning to discriminate between two simple black and white stimuli. These results raise the possibility that TE is needed for memory of simple conjunctions of basic features, but that it plays only a small role in generalizing overall configural similarity across a large set of stimuli, such as would be needed for perceptual categorical assignment.SIGNIFICANCE STATEMENTThe process of seeing and recognizing objects is attributed to a set of sequentially connected brain regions stretching forward from the primary visual cortex through the temporal lobe to the anterior inferior temporal cortex, a region designated area TE. Area TE is considered the final stage for recognizing complex visual objects, e.g., faces. It has been assumed, but not tested directly, that this area would be critical for visual generalization, i.e., the ability to place objects such as cats and dogs into their correct categories. Here, we demonstrate that monkeys rapidly and seemingly effortlessly categorize large sets of complex images (cats vs dogs, cars vs trucks), surprisingly, even after removal of area TE, leaving a puzzle about how this generalization is done.