Patrick E. Rothwell

Cocaine Experience Controls Bidirectional Synaptic Plasticity in the Nucleus Accumbens

Plasticity of glutamatergic synapses is a fundamental mechanism through which experience changes neural function to impact future behavior. In animal models of addiction, glutamatergic signaling in the nucleus accumbens (NAc) exerts powerful control over drug-seeking behavior. However, little is known about whether, how or when experience with drugs may trigger synaptic plasticity in this key nucleus. Using whole-cell synaptic physiology in NAc brain slices, we demonstrate that a progression of bidirectional changes in glutamatergic synaptic strength occurs after repeated in vivo exposure to cocaine. During a protracted drug-free period, NAc neurons from cocaine-experienced mice develop a robust potentiation of AMPAR-mediated synaptic transmission. However, a single re-exposure to cocaine during extended withdrawal becomes a potent stimulus for synaptic depression, abruptly reversing the initial potentiation. These enduring modifications in AMPAR-mediated responses and plasticity may provide a neural substrate for disrupted processing of drug-related stimuli in drug-experienced individuals.

Anhedonia requires MC4R-mediated synaptic adaptations in nucleus accumbens

Chronic stress is a strong diathesis for depression in humans and is used to generate animal models of depression. It commonly leads to several major symptoms of depression, including dysregulated feeding behaviour, anhedonia and behavioural despair. Although hypotheses defining the neural pathophysiology of depression have been proposed, the critical synaptic adaptations in key brain circuits that mediate stress-induced depressive symptoms remain poorly understood. Here we show that chronic stress in mice decreases the strength of excitatory synapses on D1 dopamine receptor-expressing nucleus accumbens medium spiny neurons owing to activation of the melanocortin 4 receptor. Stress-elicited increases in behavioural measurements of anhedonia, but not increases in measurements of behavioural despair, are prevented by blocking these melanocortin 4 receptor-mediated synaptic changes in vivo. These results establish that stress-elicited anhedonia requires a neuropeptide-triggered, cell-type-specific synaptic adaptation in the nucleus accumbens and that distinct circuit adaptations mediate other major symptoms of stress-elicited depression.

Autism-Associated Neuroligin-3 Mutations Commonly Impair Striatal Circuits to Boost Repetitive Behaviors

In humans, neuroligin-3 mutations are associated with autism, whereas in mice, the corresponding mutations produce robust synaptic and behavioral changes. However, different neuroligin-3 mutations cause largely distinct phenotypes in mice, and no causal relationship links a specific synaptic dysfunction to a behavioral change. Using rotarod motor learning as a proxy for acquired repetitive behaviors in mice, we found that different neuroligin-3 mutations uniformly enhanced formation of repetitive motor routines. Surprisingly, neuroligin-3 mutations caused this phenotype not via changes in the cerebellum or dorsal striatum but via a selective synaptic impairment in the nucleus accumbens/ventral striatum. Here, neuroligin-3 mutations increased rotarod learning by specifically impeding synaptic inhibition onto D1-dopamine receptor-expressing but not D2-dopamine receptor-expressing medium spiny neurons. Our data thus suggest that different autism-associated neuroligin-3 mutations cause a common increase in acquired repetitive behaviors by impairing a specific striatal synapse and thereby provide a plausible circuit substrate for autism pathophysiology.

Integrating synaptic plasticity and striatal circuit function in addiction.

Exposure to addictive drugs causes changes in synaptic function within the striatal complex, which can either mimic or interfere with the induction of synaptic plasticity. These synaptic adaptations include changes in the nucleus accumbens (NAc), a ventral striatal subregion important for drug reward and reinforcement, as well as the dorsal striatum, which may promote habitual drug use. As the behavioral effects of drugs of abuse are long-lasting, identifying persistent changes in striatal circuits induced by in vivo drug experience is of considerable importance. Within the striatum, drugs of abuse have been shown to induce modifications in dendritic morphology, ionotropic glutamate receptors (iGluR) and the induction of synaptic plasticity. Understanding the detailed molecular mechanisms underlying these changes in striatal circuit function will provide insight into how drugs of abuse usurp normal learning mechanisms to produce pathological behavior.

Protein Kinase C Mediates the Synergistic Interaction Between Agonists Acting at α2-Adrenergic and Delta-Opioid Receptors in Spinal Cord

Coactivation of spinal α2-adrenergic receptors (ARs) and opioid receptors produces antinociceptive synergy. Antinociceptive synergy between intrathecally administered α2AR and opioid agonists is well documented, but the mechanism underlying this synergy remains unclear. The delta-opioid receptor (DOP) and the α2AARs are coexpressed on the terminals of primary afferent fibers in the spinal cord where they may mediate this phenomenon. We evaluated the ability of the DOP-selective agonist deltorphin II (DELT), the α2AR agonist clonidine (CLON) or their combination to inhibit calcitonin gene-related peptide (CGRP) release from spinal cord slices. We then examined the possible underlying signaling mechanisms involved through coadministration of inhibitors of phospholipase C (PLC), protein kinase C (PKC) or protein kinase A (PKA). Potassium-evoked depolarization of spinal cord slices caused concentration-dependent release of CGRP. Coadministration of DELT and CLON inhibited the release of CGRP in a synergistic manner as confirmed statistically by isobolograpic analysis. Synergy was dependent on the activation of PLC and PKC, but not PKA, whereas the effect of agonist administration alone was only dependent on PLC. The importance of these findings was confirmed in vivo, using a thermal nociceptive test, demonstrating the PKC dependence of CLON-DELT antinociceptive synergy in mice. That inhibition of CGRP release by the combination was maintained in the presence of tetrodotoxin in spinal cord slices suggests that synergy does not rely on interneuronal signaling and may occur within single subcellular compartments. The present study reveals a novel signaling pathway underlying the synergistic analgesic interaction between DOP and α2AR agonists in the spinal cord.

Distinct Profiles of Anxiety and Dysphoria during Spontaneous Withdrawal from Acute Morphine Exposure

The negative motivational aspects of withdrawal include symptoms of both anxiety and depression, and emerge after termination of chronic drug use as well as after acute drug exposure. States of acute withdrawal are an inherent part of intermittent drug use in humans, but the contribution of acute withdrawal to the development of addiction has received limited systematic investigation, because of a lack of preclinical models for withdrawal states that emerge spontaneously after acute drug exposure. Here, we have characterized a spontaneous increase in the magnitude of the acoustic startle reflex (ie, spontaneous withdrawal-potentiated startle) that emerges after acute morphine administration in rats, and compared the time course of startle potentiation and place conditioning. We find that startle potentiation seems to be related to a decrease in opiate receptor occupancy and reflects an anxiety-like state with a pharmacological profile similar to other signs of opiate withdrawal. Spontaneous startle potentiation emerges before the rewarding effects of morphine have subsided, even though naloxone administration after a single morphine exposure causes both startle potentiation and conditioned place aversion (CPA). These results show that negative emotional signs of withdrawal develop after just one exposure to morphine, and are likely a recurrent aspect of intermittent drug use that may contribute to the earliest adaptations underlying the development of addiction. Furthermore, the dissociation between spontaneous startle potentiation and CPA suggests anxiogenic and dysphoric manifestations of opiate withdrawal may be mediated by distinct neural mechanisms that are progressively engaged as withdrawal unfolds.

An Anatomical Basis for Opponent Process Mechanisms of Opiate Withdrawal

Opponent process theory predicts that the first step in the induction of drug withdrawal is the activation of reward-related circuitry. Using the acoustic startle reflex as a model of anxiety-like behavior in rats, we show the emergence of a negative affective state during withdrawal after direct infusion of morphine into the ventral tegmental area (VTA), the origin of the mesolimbic dopamine system. Potentiation of startle during withdrawal from systemic morphine exposure requires a decrease in opiate receptor stimulation in the VTA and can be relieved by administration of the dopamine receptor agonist apomorphine. Together, our results suggest that the emergence of anxiety during withdrawal from acute opiate exposure begins with activation of VTA mesolimbic dopamine circuitry, providing a mechanism for the opponent process view of withdrawal.

Input- and Output-Specific Regulation of Serial Order Performance by Corticostriatal Circuits

The serial ordering of individual movements into sequential patterns is thought to require synaptic plasticity within corticostriatal circuits that route information through the basal ganglia. We used genetically and anatomically targeted manipulations of specific circuit elements in mice to isolate the source and target of a corticostriatal synapse that regulates the performance of a serial order task. This excitatory synapse originates in secondary motor cortex, terminates on direct pathway medium spiny neurons in the dorsolateral striatum, and is strengthened by serial order learning. This experience-dependent and synapse-specific form of plasticity may sculpt the balance of activity in basal ganglia circuits during sequential movements, driving a disparity in striatal output that favors the direct pathway. This disparity is necessary for execution of responses in serial order, even though both direct and indirect pathways are active during movement initiation, suggesting dynamic modulation of corticostriatal circuitry contributes to the choreography of behavioral routines.

Episodic withdrawal promotes psychomotor sensitization to morphine.

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex—a measure of anxiety induced by opiate withdrawal—was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization—a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

Synaptic Adaptations in the Nucleus Accumbens Caused by Experiences Linked to Relapse

BACKGROUND Excitatory synaptic transmission in the nucleus accumbens (NAc) regulates the reinstatement of drug seeking, an animal model of relapse in human drug addicts. However, the functional adaptations at NAc synapses that mediate reinstatement are not clearly understood. METHODS We assessed the behavioral responses of mice to cocaine administration by measuring locomotor stimulation and the acquisition, extinction, and reinstatement of conditioned place preference. Synaptic function was then examined by preparing acute brain slices and performing whole cell voltage-clamp recordings from individual medium spiny neurons in the NAc shell. RESULTS We find that reduced excitatory synaptic strength in the NAc shell is a common functional adaptation induced by multiple experiences known to cause reinstatement, including stress and drug re-exposure. The same synaptic adaptation is observed shortly after reinstatement of conditioned place preference by a cocaine priming injection. CONCLUSIONS This common synaptic modification associated with stress, drug re-exposure, and reinstatement defines a potential synaptic gateway to relapse.