Substantia nigra pars reticulata is crucially involved in barbiturate and ethanol withdrawal in mice

A Circuit-Based Information Approach to Substance Abuse Research

Recent animal research on substance-use disorders (SUDs) has emphasized learning models and the identification of 'addiction-prone' animals. Meanwhile, basic neuroscientific research has elucidated molecular, cellular, and circuit functions with increasing sophistication. However, SUD-related research is hampered by continued arguments over which animal models are more 'addiction like', as well as the facile assignment of behaviors to a given brain region and vice versa. We argue that SUD-related research would benefit from a 'bottom-up' approach including: (i) the characterization of different brain circuits to understand their normal function as well as how they respond to drugs and contribute to SUDs; and (ii) a focus on the use patterns and neurobiological effects of different substances to understand the range of critical SUD-related in vivo phenotypes.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

The kappa opioid receptor modulates GABA neuron excitability and synaptic transmission in midbrainprojections from the insular cortex

As an integrative hub, the insular cortex (IC) translates external cues into interoceptive states that generate complex physiological, affective, and behavioral responses. However, the precise circuit and signaling mechanisms in the IC that modulate these processes are unknown. Here, we describe a midbrain-projecting microcircuit in the medial aspect of the agranular IC that signals through the Gαi/o-coupled kappa opioid receptor (KOR) and its endogenous ligand dynorphin (Dyn). Within this microcircuit, Dyn is robustly expressed in layer 2/3, while KOR is localized to deep layer 5, which sends a long-range projection to the substantia nigra (SN). Using ex vivo electrophysiology, we evaluated the functional impact of KOR signaling in layer 5 of the IC. We found that bath application of dynorphin decreased GABA release and increased glutamate release on IC-SN neurons, but did not alter their excitability. Conversely, dynorphin decreased the excitability of GABA neurons without altering synaptic transmission. Pretreatment with the KOR antagonist nor-BNI blocked the effects of dynorphin in IC-SN neurons and GABA neurons, indicating that the changes in synaptic transmission and excitability were selectively mediated through KOR. Selective inhibition of IC GABA neurons using a KOR-derived DREADD recapitulated these effects. This work provides insight into IC microcircuitry and indicates that Dyn/KOR signaling may act to directly reduce activity of layer 5 GABA neurons. In turn, KOR-driven inhibition of GABA promotes disinhibition of IC-SN neurons, which can modulate downstream circuits. Our findings present a potential mechanism whereby chronic upregulation of IC Dyn/KOR signaling can lead to altered subcortical function and downstream activity.

Acute ethanol withdrawal impairs contextual learning and enhances cued learning

BACKGROUND

Alcohol affects many of the brain regions and neural processes that support learning and memory, and these effects are thought to underlie, at least in part, the development of addiction. Although much work has been done regarding the effects of alcohol intoxication on learning and memory, little is known about the effects of acute withdrawal from a single alcohol exposure.

METHODS

We assess the effects of acute ethanol withdrawal (6 hours postinjection with 4 g/kg ethanol) on 2 forms of fear conditioning (delay and trace fear conditioning) in C57BL/6J and DBA/2J mice. The influence of a number of experimental parameters (pre- and post training withdrawal exposure; foreground/background processing; training strength; and nonassociative effects) is also investigated.

RESULTS

Acute ethanol withdrawal during training had a bidirectional effect on fear-conditioned responses, decreasing contextual responses and increasing cued responses. These effects were apparent for both trace and delay conditioning in DBA/2J mice and for trace conditioning in C57BL/6J mice; however, C57BL/6J mice were selectively resistant to the effects of acute withdrawal on delay cued responses.

CONCLUSIONS

Our results show that acute withdrawal from a single, initial ethanol exposure is sufficient to alter long-term learning in mice. In addition, the differences between the strains and conditioning paradigms used suggest that specific learning processes can be differentially affected by acute withdrawal in a manner that is distinct from the reported effects of both alcohol intoxication and withdrawal following chronic alcohol exposure. Thus, our results suggest a unique effect of acute alcohol withdrawal on learning and memory processes.

Mpdz expression in the caudolateral substantia nigra pars reticulata is crucially involved in alcohol withdrawal

Association studies implicate the multiple PDZ domain protein (MUPP1/MPDZ) gene in risk for alcoholism in humans and alcohol withdrawal in mice. Although manipulation of the Mpdz gene by homologous recombination and bacterial artificial chromosome transgenesis has suggested that its expression affects alcohol withdrawal risk, the potential confounding effects of linked genes and developmental compensation currently limit interpretation. Here, using RNA interference (RNAi), we directly test the impact of Mpdz expression on alcohol withdrawal severity and provide brain regional mechanistic information. Lentiviral-mediated delivery of Mpdz short hairpin RNA (shRNA) to the caudolateral substantia nigra pars reticulata (clSNr) significantly reduces Mpdz expression and exacerbates alcohol withdrawal convulsions compared with control mice that delivered a scrambled shRNA. Neither baseline nor pentylenetetrazol-enhanced convulsions differed between Mpdz shRNA and control animals, indicating Mpdz expression in the clSNr does not generally affect seizure susceptibility. To our knowledge, these represent the first in vivo Mpdz RNAi analyses, and provide the first direct evidence that Mpdz expression impacts behavior. Our results confirm that Mpdz is a quantitative trait gene for alcohol withdrawal and demonstrate that its expression in the clSNr is crucially involved in risk for alcohol withdrawal.

Genetics and genomics of alcohol sensitivity

Alcohol abuse and alcoholism incur a heavy socioeconomic cost in many countries. Both genetic and environmental factors contribute to variation in the inebriating effects of alcohol and alcohol addiction among individuals within and across populations. From a genetics perspective, alcohol sensitivity is a quantitative trait determined by the cumulative effects of multiple segregating genes and their interactions with the environment. This review summarizes insights from model organisms as well as human populations that represent our current understanding of the genetic and genomic underpinnings that govern alcohol metabolism and the sedative and addictive effects of alcohol on the nervous system.

Striatal involvement in human alcoholism and alcohol consumption, and withdrawal in animal models

BACKGROUND

Different regions of the striatum may have distinct roles in acute intoxication, alcohol seeking, dependence, and withdrawal.

METHODS

The recent advances are reviewed and discussed in our understanding of the role of the dorsolateral striatum (DLS), dorsomedial striatum (DMS), and ventral striatum in behavioral responses to alcohol, including alcohol craving in abstinent alcoholics, and alcohol consumption and withdrawal in rat, mouse, and nonhuman primate models.

RESULTS

Reduced neuronal activity as well as dysfunctional connectivity between the ventral striatum and the dorsolateral prefrontal cortex is associated with alcohol craving and impairment of new learning processes in abstinent alcoholics. Within the DLS of mice and nonhuman primates withdrawn from alcohol after chronic exposure, glutamatergic transmission in striatal projection neurons is increased, while GABAergic transmission is decreased. Glutamatergic transmission in DMS projection neurons is also increased in ethanol withdrawn rats. Ex vivo or in vivo ethanol exposure and withdrawal causes a long-lasting increase in NR2B subunit-containing NMDA receptor activity in the DMS, contributing to ethanol drinking. Analyses of neuronal activation associated with alcohol withdrawal and site-directed lesions in mice implicate the rostroventral caudate putamen, a ventrolateral segment of the DMS, in genetically determined differences in risk for alcohol withdrawal involved in physical association of the multi-PDZ domain protein, MPDZ, with 5-HT(2C) receptors and/or NR2B.

CONCLUSIONS

Alterations of dopaminergic, glutamatergic, and GABAergic signaling within different regions of the striatum by alcohol is critical for alcohol craving, consumption, dependence, and withdrawal in humans and animal models.