Methamphetamine Learning Induces Persistent and Selective Nonmuscle Myosin II-Dependent Spine Motility in the Basolateral Amygdala

Novel therapeutics in development for the treatment of stimulant-use disorder

Misuse and accidental overdoses attributed to stimulants are escalating rapidly. These stimulants include methamphetamine, cocaine, amphetamine, ecstasy-type drugs, and prescription stimulants such as methylphenidate. Unlike opioids and alcohol, there are no therapies approved by the US Food and Drug Administration (FDA) to treat stimulant-use disorder. The high rate of relapse among this population highlights the insufficiency of current treatment options, which are limited to abstinence support programs and behavioral modification therapies. Here, we briefly outline recent regulatory actions taken by FDA to help support the development of new stimulant use disorder treatments and highlight several new therapeutics in the clinical development pipeline.

Targeting the cytoskeleton as a therapeutic approach to substance use disorders

Substance use disorders (SUD) are chronic relapsing disorders governed by continually shifting cycles of positive drug reward experiences and drug withdrawal-induced negative experiences. A large body of research points to plasticity within systems regulating emotional, motivational, and cognitive processes as drivers of continued compulsive pursuit and consumption of substances despite negative consequences. This plasticity is observed at all levels of analysis from molecules to networks, providing multiple avenues for intervention in SUD. The cytoskeleton and its regulatory proteins within neurons and glia are fundamental to the structural and functional integrity of brain processes and are potentially the major drivers of the morphological and behavioral plasticity associated with substance use. In this review, we discuss preclinical studies that provide support for targeting the brain cytoskeleton as a therapeutic approach to SUD. We focus on the interplay between actin cytoskeleton dynamics and exposure to cocaine, methamphetamine, alcohol, opioids, and nicotine and highlight preclinical studies pointing to a wide range of potential therapeutic targets, such as nonmuscle myosin II, Rac1, cofilin, prosapip 1, and drebrin. These studies broaden our understanding of substance-induced plasticity driving behaviors associated with SUD and provide new research directions for the development of SUD therapeutics.

Basolateral amygdala corticotropin releasing factor receptor 2 interacts with nonmuscle myosin II to destabilize memory in males

Preclinical studies show that inhibiting the actin motor ATPase nonmuscle myosin II (NMII) with blebbistatin (Blebb) in the basolateral amgydala (BLA) depolymerizes actin, resulting in an immediate, retrieval-independent disruption of methamphetamine (METH)-associated memory in male and female adult and adolescent rodents. The effect is highly selective, as NMII inhibition has no effect in other relevant brain regions (e.g., dorsal hippocampus [dPHC], nucleus accumbens [NAc]), nor does it interfere with associations for other aversive or appetitive stimuli, including cocaine (COC). To understand the mechanisms responsible for drug specific selectivity we began by investigating, in male mice, the pharmacokinetic differences in METH and COC brain exposure . Replicating METH's longer half-life with COC did not render the COC association susceptible to disruption by NMII inhibition. Therefore, we next assessed transcriptional differences. Comparative RNA-seq profiling in the BLA, dHPC and NAc following METH or COC conditioning identified crhr2, which encodes the corticotropin releasing factor receptor 2 (CRF2), as uniquely upregulated by METH in the BLA. CRF2 antagonism with Astressin-2B (AS2B) had no effect on METH-associated memory after consolidation, allowing for determination of CRF2 influences on NMII-based susceptibility. Pretreatment with AS2B prevented the ability of Blebb to disrupt an established METH-associated memory. Alternatively, combining CRF2 overexpression and agonist treatment, urocortin 3 (UCN3), in the BLA during conditioning rendered COC-associated memory susceptible to disruption by NMII inhibition, mimicking the Blebb-induced, retrieval-independent memory disruption seen with METH. These results suggest that BLA CRF2 receptor activation during memory formation in male mice can prevent stabilization of the actin-myosin cytoskeleton supporting the memory, rendering it vulnerable to disruption by NMII inhibition. CRF2 represents an interesting target for BLA-dependent memory destabilization via downstream effects on NMII.

Neurobiological mechanisms underlying psychostimulant use

Rates of individuals struggling with psychostimulant use disorder (PSUD), defined as chronic use of psychostimulants despite negative consequences, are growing rapidly over the last few decades. However, there are no current pharmacotherapeutics to aid individuals in maintaining drug abstinence. Identifying the underlying neurobiological mechanisms that promote persistent craving and taking of psychostimulants is critical to creating novel pharmacological treatments for PSUD. Psychostimulant use dysregulates processes within the brain that are responsible for decision-making, reward, and memory formation to drive future drug-seeking. Here, we describe novel findings and theories on how psychostimulants impact mechanisms related to transcription, mitochondrial function, and synaptic plasticity within the reward system to drive drug-seeking. We also highlight work examining how psychostimulants impact neural networks through rewiring circuitry to drive addiction-related behaviors. Overall, this review aims to feature the latest progress in understanding the biological basis of PSUD and promising mechanisms for PSUD pharmacotherapeutics.

Basolateral Amygdala Corticotrophin Releasing Factor Receptor 2 Interacts with Nonmuscle Myosin II to Destabilize Memory

Inhibiting the actin motor ATPase nonmuscle myosin II (NMII) with blebbistatin (Blebb) in the basolateral amgydala (BLA) depolymerizes actin, resulting in an immediate, retrieval-independent disruption of methamphetamine (METH)-associated memory. The effect is highly selective, as NMII inhibition has no effect in other relevant brain regions (e.g. dorsal hippocampus [dPHC], nucleus accumbens [NAc]), nor does it interfere with associations for other aversive or appetitive stimuli, including cocaine (COC). To investigate a potential source of this specificity, pharmacokinetic differences in METH and COC brain exposure were examined. Replicating METH's longer half-life with COC did not render the COC association susceptible to disruption by NMII inhibition. Therefore, transcriptional differences were next assessed. Comparative RNA-seq profiling in the BLA, dHPC and NAc following METH or COC conditioning identified crhr2, which encodes the corticotrophin releasing factor receptor 2 (CRF2), as uniquely upregulated by METH in the BLA. CRF2 antagonism with Astressin-2B (AS2B) had no effect on METH-associated memory after consolidation, allowing for determination of CRF2 influences on NMII-based susceptibility after METH conditioning. Pretreatment with AS2B occluded the ability of Blebb to disrupt an established METH-associated memory. Alternatively, the Blebb-induced, retrieval-independent memory disruption seen with METH was mimicked for COC when combined with CRF2 overexpression in the BLA and its ligand, UCN3 during conditioning. These results indicate that BLA CRF2 receptor activation during learning can prevent stabilization of the actin-myosin cytoskeleton supporting the memory, rendering it vulnerable to disruption via NMII inhibition. CRF2 represents an interesting target for BLA-dependent memory destabilization via downstream effects on NMII.

Neuromechanobiology: An Expanding Field Driven by the Force of Greater Focus

The brain processes information by transmitting signals through highly connected and dynamic networks of neurons. Neurons use specific cellular structures, including axons, dendrites and synapses, and specific molecules, including cell adhesion molecules, ion channels and chemical receptors to form, maintain and communicate among cells in the networks. These cellular and molecular processes take place in environments rich of mechanical cues, thus offering ample opportunities for mechanical regulation of neural development and function. Recent studies have suggested the importance of mechanical cues and their potential regulatory roles in the development and maintenance of these neuronal structures. Also suggested are the importance of mechanical cues and their potential regulatory roles in the interaction and function of molecules mediating the interneuronal communications. In this review, the current understanding is integrated and promising future directions of neuromechanobiology are suggested at the cellular and molecular levels. Several neuronal processes where mechanics likely plays a role are examined and how forces affect ligand binding, conformational change, and signal induction of molecules key to these neuronal processes are indicated, especially at the synapse. The disease relevance of neuromechanobiology as well as therapies and engineering solutions to neurological disorders stemmed from this emergent field of study are also discussed.

The Mechanisms and Boundary Conditions of Drug Memory Reconsolidation

Drug addiction can be seen as a disorder of maladaptive learning characterized by relapse. Therefore, disrupting drug-related memories could be an approach to improving therapies for addiction. Pioneering studies over the last two decades have revealed that consolidated memories are not static, but can be reconsolidated after retrieval, thereby providing candidate pathways for the treatment of addiction. The limbic-corticostriatal system is known to play a vital role in encoding the drug memory engram. Specific structures within this system contribute differently to the process of memory reconsolidation, making it a potential target for preventing relapse. In addition, as molecular processes are also active during memory reconsolidation, amnestic agents can be used to attenuate drug memory. In this review, we focus primarily on the brain structures involved in storing the drug memory engram, as well as the molecular processes involved in drug memory reconsolidation. Notably, we describe reports regarding boundary conditions constraining the therapeutic potential of memory reconsolidation. Furthermore, we discuss the principles that could be employed to modify stored memories. Finally, we emphasize the challenge of reconsolidation-based strategies, but end with an optimistic view on the development of reconsolidation theory for drug relapse prevention.

Spectral power and theta-gamma coupling in the basolateral amygdala related with methamphetamine conditioned place preference in mice

The basolateral amygdala (BLA) plays a crucial role in conditioned place preference (CPP) for addictive drugs. However, neural signaling associated with methamphetamine (METH) craving and seeking remained to be investigated. This study characterized local field potential (LFP) oscillatory patterns in the BLA and conditioned place preference induced by METH-related context. Male Swiss albino ICR mice were deeply anesthetized for LFP intracranial electrode implantation in the BLA. Control and METH groups received sessions to learn to associate saline-paired and METH-paired compartments of the CPP apparatus with saline and METH injections, respectively, for 10 days. LFP signals and exploring behavior were recorded simultaneously during pre- and post-conditioning phases. Time spent in METH-paired compartment was normalized and expressed as CPP scores. Fast Fourier Transform (FFT) algorithm was used to analyze LFP powers of 8 discrete frequency ranges (delta, theta, alpha, beta, gamma I-IV). During post-conditioning phase of METH CPP with METH cues, statistical analysis revealed that METH group significantly increased time spent in METH-paired compartment. Significant suppressions of theta and alpha powers were observed. Phase-amplitude cross frequency coupling analyses confirmed significant increases in maximal modulation index (MI), frequency for phase of slow wave and MI of theta-gamma II coupling. Taken together, LFP oscillation in the BLA was sensitive in association with METH CPP. These research findings might suggest the underlying mechanisms of drug reward learning and adaptive changes in the BLA in acquisition of METH CPP and dependence.