Repeated morphine exposure decreased the nucleus accumbens excitability during short-term withdrawal

Adaptations in Nucleus Accumbens Neuron Subtypes Mediate Negative Affective Behaviors in Fentanyl Abstinence

BACKGROUND

Opioid discontinuation generates a withdrawal syndrome marked by increased negative affect. Increased symptoms of anxiety and dysphoria during opioid discontinuation are significant barriers to achieving long-term abstinence in opioid-dependent individuals. While adaptations in the nucleus accumbens are implicated in opioid abstinence syndrome, the precise neural mechanisms are poorly understood. Additionally, our current knowledge is limited to changes following natural and semisynthetic opioids, despite recent increases in synthetic opioid use and overdose.

METHODS

We used a combination of cell subtype-specific viral labeling and electrophysiology in male and female mice to investigate structural and functional plasticity in nucleus accumbens medium spiny neuron (MSN) subtypes after fentanyl abstinence. We characterized molecular adaptations after fentanyl abstinence with subtype-specific RNA sequencing and weighted gene co-expression network analysis. We used viral-mediated gene transfer to manipulate the molecular signature of fentanyl abstinence in D1-MSNs.

RESULTS

Here, we show that fentanyl abstinence increases anxiety-like behavior, decreases social interaction, and engenders MSN subtype-specific plasticity in both sexes. D1-MSNs, but not D2-MSNs, exhibit dendritic atrophy and an increase in excitatory drive. We identified a cluster of coexpressed dendritic morphology genes downregulated selectively in D1-MSNs that are transcriptionally coregulated by E2F1. E2f1 expression in D1-MSNs protects against loss of dendritic complexity, altered physiology, and negative affect-like behaviors caused by fentanyl abstinence.

CONCLUSIONS

Our findings indicate that fentanyl abstinence causes unique structural, functional, and molecular changes in nucleus accumbens D1-MSNs that can be targeted to alleviate negative affective symptoms during abstinence.

Differential Patterns of Synaptic Plasticity in the Nucleus Accumbens Caused by Continuous and Interrupted Morphine Exposure

Opioid exposure and withdrawal both cause adaptations in brain circuits that may contribute to abuse liability. These adaptations vary in magnitude and direction following different patterns of opioid exposure, but few studies have systematically manipulated the pattern of opioid administration while measuring neurobiological impact. In this study, we compared cellular and synaptic adaptations in the nucleus accumbens shell caused by morphine exposure that was either continuous or interrupted by daily bouts of naloxone-precipitated withdrawal. At the behavioral level, continuous morphine administration caused psychomotor tolerance, which was reversed when the continuity of morphine action was interrupted by naloxone-precipitated withdrawal. Using ex vivo slice electrophysiology in female and male mice, we investigated how these patterns of morphine administration altered intrinsic excitability and synaptic plasticity of medium spiny neurons (MSNs) expressing the D1 or D2 dopamine receptor. We found that morphine-evoked adaptations at excitatory synapses were predominately conserved between patterns of administration, but there were divergent effects on inhibitory synapses and the subsequent balance between excitatory and inhibitory synaptic input. Overall, our data suggest that continuous morphine administration produces adaptations that dampen the output of D1-MSNs, which are canonically thought to promote reward-related behaviors. Interruption of otherwise continuous morphine exposure does not dampen D1-MSN functional output to the same extent, which may enhance behavioral responses to subsequent opioid exposure. Our findings support the hypothesis that maintaining continuity of opioid administration could be an effective therapeutic strategy to minimize the vulnerability to opioid use disorders.SIGNIFICANCE STATEMENT Withdrawal plays a key role in the cycle of addiction to opioids like morphine. We studied how repeated cycles of naloxone-precipitated withdrawal from otherwise continuous opioid exposure can change brain function of the nucleus accumbens, which is an important brain region for reward and addiction. Different patterns of opioid exposure caused unique changes in communication between neurons in the nucleus accumbens, and the nature of these changes depended on the type of neuron being studied. The specific changes in communication between neurons caused by repeated cycles of withdrawal may increase vulnerability to opioid use disorders. This highlights the importance of reducing or preventing the experience of withdrawal during opioid treatment.

Dopamine D4 Receptor Is a Regulator of Morphine-Induced Plasticity in the Rat Dorsal Striatum

Long-term exposition to morphine elicits structural and synaptic plasticity in reward-related regions of the brain, playing a critical role in addiction. However, morphine-induced neuroadaptations in the dorsal striatum have been poorly studied despite its key function in drug-related habit learning. Here, we show that prolonged treatment with morphine triggered the retraction of the dendritic arbor and the loss of dendritic spines in the dorsal striatal projection neurons (MSNs). In an attempt to extend previous findings, we also explored whether the dopamine D4 receptor (D4R) could modulate striatal morphine-induced plasticity. The combined treatment of morphine with the D4R agonist PD168,077 produced an expansion of the MSNs dendritic arbors and restored dendritic spine density. At the electrophysiological level, PD168,077 in combination with morphine altered the electrical properties of the MSNs and decreased their excitability. Finally, results from the sustantia nigra showed that PD168,077 counteracted morphine-induced upregulation of μ opioid receptors (MOR) in striatonigral projections and downregulation of G protein-gated inward rectifier K+ channels (GIRK1 and GIRK2) in dopaminergic cells. The present results highlight the key function of D4R modulating morphine-induced plasticity in the dorsal striatum. Thus, D4R could represent a valuable pharmacological target for the safety use of morphine in pain management.

Morphine Differentially Alters the Synaptic and Intrinsic Properties of D1R- and D2R-Expressing Medium Spiny Neurons in the Nucleus Accumbens

Exposure to opioids reshapes future reward and motivated behaviors partially by altering the functional output of medium spiny neurons (MSNs) in the nucleus accumbens shell. Here, we investigated how morphine, a highly addictive opioid, alters synaptic transmission and intrinsic excitability on dopamine D1-receptor (D1R) expressing and dopamine D2-receptor (D2R) expressing MSNs, the two main output neurons in the nucleus accumbens shell. Using whole-cell electrophysiology recordings, we show, that 24 h abstinence following repeated non-contingent administration of morphine (10 mg/kg, i.p.) in mice reduces the miniature excitatory postsynaptic current (mEPSC) frequency and miniature inhibitory postsynaptic current (mIPSC) frequency on D2R-MSNs, with concomitant increases in D2R-MSN intrinsic membrane excitability. We did not observe any changes in synaptic or intrinsic changes on D1R-MSNs. Last, in an attempt to determine the integrated effect of the synaptic and intrinsic alterations on the overall functional output of D2R-MSNs, we measured the input-output efficacy by measuring synaptically-driven action potential firing. We found that both D1R-MSN and D2R-MSN output was unchanged following morphine treatment.

Altered Activity of SK Channel Underpins Morphine Withdrawal Relevant Psychiatric Deficiency in Infralimbic to Accumbens Shell Pathway

Drug addiction can be viewed as a chronic psychiatric disorder that is related to dysfunction of neural circuits, including reward deficits, stress surfeits, craving changes, and compromised executive function. The nucleus accumbens (NAc) plays a crucial role in regulating craving and relapse, while the medial prefrontal cortex (mPFC) represents a higher cortex projecting into the NAc that is active in the management of executive function. In this study, we investigated the role of the small conductance calcium-activated potassium channels (SK channels) in NAc and mPFC after morphine withdrawal. Action potential (AP) firing of neurons in the NAc shell was enhanced via the downregulations of the SK channels after morphine withdrawal. Furthermore, the expression of SK2 and SK3 subunits in the NAc was significantly reduced after 3 weeks of morphine withdrawal, but was not altered in the dorsal striatum. In mPFC, the SK channel subunits were differentially expressed. To be specific, the expression of SK3 was upregulated, while the expression of SK2 was unchanged. Furthermore, the AP firing in layer 5 pyramidal neurons of the infralimbic (IL) cortex was decreased via the upregulations of the SK channel-related tail current after 3 weeks of morphine withdrawal. These results suggest that the SK channel plays a specific role in reward circuits following morphine exposure and a period of drug withdrawal, making it a potential target for the prevention of relapse.

Prefrontal-accumbens opioid plasticity: Implications for relapse and dependence

In addiction, an individual's ability to inhibit drug seeking and drug taking is thought to reflect a pathological strengthening of drug-seeking behaviors or impairments in the capacity to control maladaptive behavior. These processes are not mutually exclusive and reflect drug-induced modifications within prefrontal cortical and nucleus accumbens circuits, however unlike psychostimulants such as cocaine, far less is known about the temporal, anatomical, and cellular dynamics of these changes. We discuss what is known regarding opioid-induced adaptations in intrinsic membrane physiology and pre-/postsynaptic neurotransmission in principle pyramidal and medium spiny neurons in the medial prefrontal cortex and nucleus accumbens from electrophysiological studies and explore how circuit specific adaptations may contribute to unique facets of opioid addiction.

Dual Effects of Limbic Seizures on Psychosis-Relevant Behaviors Shown by Nucleus Accumbens Kindling in Rats

BACKGROUND

A paradox in epilepsy and psychiatry is that temporal lobe epilepsy is often predisposed to schizophrenic-like psychosis, whereas convulsive therapy can relieve schizophrenic symptoms. We have previously demonstrated that the nucleus accumbens is a key structure in mediating postictal psychosis induced by a hippocampal electrographic seizure.

OBJECTIVE/HYPOTHESIS

The purpose of this study is to test a hypothesis that accumbens kindling cumulating in a single (1-time) or repeated (5-times) convulsive seizures have different effects on animal models of psychosis.

METHODS

Electrical stimulation at 60 Hz was applied to nucleus accumbens to evoke afterdischarges until one, or five, convulsive seizures that involved the hind limbs (stage 5 seizures) were attained. Behavioral tests, performed at 3 days after the last seizure, included gating of hippocampal auditory evoked potentials (AEP) and prepulse inhibition to an acoustic startle response (PPI), tested without drug injection or after ketamine (3 mg/kg s.c.) injection, as well as locomotion induced by ketamine or methamphetamine (1 mg/kg i.p.).

RESULTS

Compared to non-kindled control rats, 1-time, but not 5-times, convulsive seizures induced PPI deficit and decreased gating of hippocampal AEP, without drug injection. Compared to non-kindled rats, 5-times, but not 1-time, convulsive seizures antagonized ketamine-induced hyperlocomotion, ketamine-induced PPI deficit and AEP gating decrease. However, both 1- and 5-times convulsive seizures significantly enhanced methamphetamine-induced locomotion as compared to non-kindled rats.

CONCLUSIONS

Accumbens kindling ending with 1 convulsive seizure may induce schizophrenic-like behaviors, while repeated (≥5) convulsive seizures induced by accumbens kindling may have therapeutic effects on dopamine independent psychosis.

Regulator of G-Protein Signaling 7 Regulates Reward Behavior by Controlling Opioid Signaling in the Striatum

BACKGROUND

Morphine mediates its euphoric and analgesic effects by acting on the μ-opioid receptor (MOR). MOR belongs to the family of G-protein coupled receptors whose signaling efficiency is controlled by the regulator of G-protein signaling (RGS) proteins. Our understanding of the molecular diversity of RGS proteins that control MOR signaling, their circuit specific actions, and underlying cellular mechanisms is very limited.

METHODS

We used genetic approaches to ablate regulator of G-protein signaling 7 (RGS7) both globally and in specific neuronal populations. We used conditioned place preference and self-administration paradigms to examine reward-related behavior and a battery of tests to assess analgesia, tolerance, and physical dependence to morphine. Electrophysiology approaches were applied to investigate the impact of RGS7 on morphine-induced alterations in neuronal excitability and plasticity of glutamatergic synapses. At least three animals were used for each assessment.

RESULTS

Elimination of RGS7 enhanced reward, increased analgesia, delayed tolerance, and heightened withdrawal in response to morphine administration. RGS7 in striatal neurons was selectively responsible for determining the sensitivity of rewarding and reinforcing behaviors to morphine without affecting analgesia, tolerance, and withdrawal. In contrast, deletion of RGS7 in dopaminergic neurons did not influence morphine reward. RGS7 exerted its effects by controlling morphine-induced changes in excitability of medium spiny neurons in nucleus accumbens and gating the compositional plasticity of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors.

CONCLUSIONS

This study identifies RGS7 as a novel regulator of MOR signaling by dissecting its circuit specific actions and pinpointing its role in regulating morphine reward by controlling the activity of nucleus accumbens neurons.

Drug-primed reinstatement of cocaine seeking in mice: increased excitability of medium-sized spiny neurons in the nucleus accumbens

To examine the mechanisms of drug relapse, we first established a model for cocaine IVSA (intravenous self-administration) in mice, and subsequently examined electrophysiological alterations of MSNs (medium-sized spiny neurons) in the NAc (nucleus accumbens) before and after acute application of cocaine in slices. Three groups were included: master mice trained by AL (active lever) pressings followed by IV (intravenous) cocaine delivery, yoked mice that received passive IV cocaine administration initiated by paired master mice, and saline controls. MSNs recorded in the NAc shell in master mice exhibited higher membrane input resistances but lower frequencies and smaller amplitudes of sEPSCs (spontaneous excitatory postsynaptic currents) compared with neurons recorded from saline control mice, whereas cells in the NAc core had higher sEPSCs frequencies and larger amplitudes. Furthermore, sEPSCs in MSNs of the shell compartment displayed longer decay times, suggesting that both pre- and postsynaptic mechanisms were involved. After acute re-exposure to a low-dose of cocaine in vitro, an AP (action potential)-dependent, persistent increase in sEPSC frequency was observed in both NAc shell and core MSNs from master, but not yoked or saline control mice. Furthermore, re-exposure to cocaine induced membrane hyperpolarization, but concomitantly increased excitability of MSNs from master mice, as evidenced by increased membrane input resistance, decreased depolarizing current to generate APs, and a more negative Thr (threshold) for firing. These data demonstrate functional differences in NAc MSNs after chronic contingent versus non-contingent IV cocaine administration in mice, as well as synaptic adaptations of MSNs before and after acute re-exposure to cocaine. Reversing these functional alterations in NAc could represent a rational target for the treatment of some reward-related behaviors, including drug addiction.

Regional and cell-type-specific effects of DAMGO on striatal D1 and D2 dopamine receptor-expressing medium-sized spiny neurons

The striatum can be divided into the DLS (dorsolateral striatum) and the VMS (ventromedial striatum), which includes NAcC (nucleus accumbens core) and NAcS (nucleus accumbens shell). Here, we examined differences in electrophysiological properties of MSSNs (medium-sized spiny neurons) based on their location, expression of DA (dopamine) D1/D2 receptors and responses to the μ-opioid receptor agonist, DAMGO {[D-Ala²-MePhe⁴-Gly(ol)⁵]enkephalin}. The main differences in morphological and biophysical membrane properties occurred among striatal sub-regions. MSSNs in the DLS were larger, had higher membrane capacitances and lower Rin (input resistances) compared with cells in the VMS. RMPs (resting membrane potentials) were similar among regions except for D2 cells in the NAcC, which displayed a significantly more depolarized RMP. In contrast, differences in frequency of spontaneous excitatory synaptic inputs were more prominent between cell types, with D2 cells receiving significantly more excitatory inputs than D1 cells, particularly in the VMS. Inhibitory inputs were not different between D1 and D2 cells. However, MSSNs in the VMS received more inhibitory inputs than those in the DLS. Acute application of DAMGO reduced the frequency of spontaneous excitatory and inhibitory postsynaptic currents, but the effect was greater in the VMS, in particular in the NAcS, where excitatory currents from D2 cells and inhibitory currents from D1 cells were inhibited by the largest amount. DAMGO also increased cellular excitability in the VMS, as shown by reduced threshold for evoking APs (action potentials). Together the present findings help elucidate the regional and cell-type-specific substrate of opioid actions in the striatum and point to the VMS as a critical mediator of DAMGO effects.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).