Long-term antagonism of κ opioid receptors prevents escalation of and increased motivation for heroin intake.

The abuse of opioid drugs, both illicit and prescription, is a persistent problem in the United States, accounting for >1.2 million users who require treatment each year. Current treatments rely on suppressing immediate withdrawal symptoms and replacing illicit drug use with long-acting opiate drugs. However, the mechanisms that lead to preventing opiate dependence are still poorly understood. We hypothesized that κ opioid receptor (KOR) activation during chronic opioid intake contributes to negative affective states associated with withdrawal and the motivation to take increasing amounts of heroin. Using a 12 h long-access model of heroin self-administration, rats showed escalation of heroin intake over several weeks. This was prevented by a single high dose (30 mg/kg) of the long-acting KOR antagonist norbinaltorphimine (nor-BNI), paralleled by reduced motivation to respond for heroin on a progressive-ratio schedule of reinforcement, a measure of compulsive-like responding. Systemic nor-BNI also significantly decreased heroin withdrawal-associated anxiety-like behavior. Immunohistochemical analysis showed prodynorphin content increased in the nucleus accumbens core in all heroin-exposed rats, but selectively increased in the nucleus accumbens shell in long-access rats. Local infusion of nor-BNI (4 µg/side) into accumbens core altered the initial intake of heroin but not the rate of escalation, while local injection into accumbens shell selectively suppressed increases in heroin intake over time without altering initial intake. These data suggest that dynorphin activity in the nucleus accumbens mediates the increasing motivation for heroin taking and compulsive-like responding for heroin, suggesting that KOR antagonists may be promising targets for the treatment of opioid addiction.

Long-lasting reduction in hippocampal neurogenesis by alcohol consumption in adolescent nonhuman primates.

Binge alcohol consumption in adolescents is increasing, and studies in animal models show that adolescence is a period of high vulnerability to brain insults. The purpose of the present study was to determine the deleterious effects of binge alcohol on hippocampal neurogenesis in adolescent nonhuman primates. Heavy binge alcohol consumption over 11 mo dramatically and persistently decreased hippocampal proliferation and neurogenesis. Combinatorial analysis revealed distinct, actively dividing hippocampal neural progenitor cell types in the subgranular zone of the dentate gyrus that were in transition from stem-like radial glia-like cells (type 1) to immature transiently amplifying neuroblasts (type 2a, type 2b, and type 3), suggesting the evolutionary conservation of milestones of neuronal development in macaque monkeys. Alcohol significantly decreased the number of actively dividing type 1, 2a, and 2b cell types without significantly altering the early neuronal type 3 cells, suggesting that alcohol interferes with the division and migration of hippocampal preneuronal progenitors. Furthermore, the lasting alcohol-induced reduction in hippocampal neurogenesis paralleled an increase in neural degeneration mediated by nonapoptotic pathways. Altogether, these results demonstrate that the hippocampal neurogenic niche during adolescence is highly vulnerable to alcohol and that alcohol decreases neuronal turnover in adolescent nonhuman primate hippocampus by altering the ongoing process of neuronal development. This lasting effect, observed 2 mo after alcohol discontinuation, may underlie the deficits in hippocampus-associated cognitive tasks that are observed in alcoholics.

Permanent impairment of birth and survival of cortical and hippocampal proliferating cells following excessive drinking during alcohol dependence.

Experimenter-delivered alcohol decreases adult hippocampal neurogenesis and hippocampal-dependent learning and memory. The present study used clinically relevant rodent models of nondependent limited access alcohol self-administration and excessive drinking during alcohol dependence (alcohol self-administration followed by intermittent exposure to alcohol vapors over several weeks) to compare alcohol-induced effects on cortical gliogenesis and hippocampal neurogenesis. Alcohol dependence, but not nondependent drinking, reduced proliferation and survival in the medial prefrontal cortex (mPFC). Apoptosis was reduced in both alcohol groups within the mPFC, which may reflect an initiation of a reparative environment following alcohol exposure as decreased proliferation was abolished after prolonged dependence. Reduced proliferation, differentiation, and neurogenesis were observed in the hippocampus of both alcohol groups, and prolonged dependence worsened the effects. Increased hippocampal apoptosis and neuronal degeneration following alcohol exposure suggest a loss in neuronal turnover and indicate that the hippocampal neurogenic niche is highly vulnerable to alcohol.

Corticotropin-releasing factor within the central nucleus of the amygdala mediates enhanced ethanol self-administration in withdrawn, ethanol-dependent rats.

Alcohol dependence is characterized by excessive consumption, loss of control over intake, and the presence of a withdrawal syndrome, including both motivational and physical symptoms. The motivational symptoms, including anxiety, have been hypothesized to be important factors eliciting excessive drinking during abstinence. Previous work has shown that ethanol-dependent rats also display enhanced anxiety-like behaviors and enhanced ethanol self-administration during withdrawal, likely resulting from dysregulation of brain corticotropin-releasing factor (CRF) stress systems. The present study was designed to explore the brain sites within the extended amygdala [central nucleus of the amygdala (CeA), lateral bed nucleus of the stria terminalis (BNST), and nucleus accumbens shell (NAcSh)] that mediate the increased ethanol self-administration observed during withdrawal. Ethanol-dependent animals showed an increase in ethanol self-administration after acute withdrawal relative to nondependent controls. The CRF antagonist D-Phe-CRF(12-41) ([D-Phe(12),Nle(21,38),C alpha MeLeu(37)]-rCRF(12-41)) was administered into the CeA, lateral BNST, or NAcSh of acute-withdrawn dependent and nondependent rats. Administered into the CeA, the antagonist reduced ethanol self-administration in dependent animals, with no effect in nondependent animals. Administration of D-Phe-CRF(12-41) into the lateral BNST and NAcSh was without effect on ethanol self-administration in dependent and nondependent animals. At the same time point of withdrawal, there was a decrease in CRF immunoreactivity within the CeA, suggesting an increased extracellular release of CRF during withdrawal. There was no change in CRF immunoreactivity in the BNST or NAcSh. These results indicate that CRF, specifically within the CeA, plays a role in mediating excessive ethanol consumption in ethanol-dependent animals.

Stress experienced in utero reduces sexual dichotomies in neurogenesis, microenvironment, and cell death in the adult rat hippocampus.

Hippocampal function and plasticity differ with gender, but the regulatory mechanisms underlying sex differences remain elusive and may be established early in life. The present study sought to elucidate sex differences in hippocampal plasticity under normal developmental conditions and in response to repetitive, predictable versus varied, unpredictable prenatal stress (PS). Adult male and diestrous female offspring of pregnant rats exposed to no stress (control), repetitive stress (PS-restraint), or a randomized sequence of varied stressors (PS-random) during the last week of pregnancy were examined for hippocampal proliferation, neurogenesis, cell death, and local microenvironment using endogenous markers. Regional volume was also estimated by stereology. Control animals had comparable proliferation and regional volume regardless of sex, but females had lower neurogenesis compared to males. Increased cell death and differential hippocampal precursor kinetics both appear to contribute to reduced neurogenesis in females. Reduced local interleukin-1beta (IL-1beta) immunoreactivity (IR) in females argues for a mechanistic role for the anti-apoptotic cytokine in driving sex differences in cell death. Prenatal stress significantly impacted the hippocampus, with both stress paradigms causing robust decreases in actively proliferating cells in males and females. Several other hippocampal measures were feminized in males such as precursor kinetics, IL-1beta-IR density, and cell death, reducing or abolishing some sex differences. The findings expand our understanding of the mechanisms underlying sex differences and highlight the critical role early stress can play on the balance between proliferation, neurogenesis, cell death, and hippocampal microenvironment in adulthood.

Varied access to intravenous methamphetamine self-administration differentially alters adult hippocampal neurogenesis.

BACKGROUND: Chronic abuse of methamphetamine produces deficits in hippocampal function, perhaps by altering hippocampal neurogenesis and plasticity. We examined how intravenous methamphetamine self-administration modulates active division, proliferation of late progenitors, differentiation, maturation, survival, and mature phenotype of hippocampal subgranular zone (SGZ) progenitors. METHODS: Adult male Wistar rats were given access to methamphetamine 1 hour twice weekly (intermittent short), 1 hour daily (short), or 6 hours daily (long). Rats received one intraperitoneal injection of bromodeoxyuridine (BrdU) to label progenitors in the synthesis (S) phase, and 28-day-old surviving BrdU-immunoreactive (IR) cells were quantified. Ki-67, doublecortin (DCX), and activated caspase-3 (AC-3) were used to visualize and quantify proliferating, differentiating, maturing, and apoptotic cells. Terminal corticosterone was measured to determine changes in adrenal steroids. RESULTS: Intermittent access to methamphetamine increased Ki-67 and DCX-IR cells, but opposing effects on late progenitors and postmitotic neurons resulted in no overall change in neurogenesis. Daily access to methamphetamine decreased all studied aspects of neurogenesis and reduced hippocampal granule neurons and volume, changes that likely are mediated by decreased proliferative and neurogenic capacity of the SGZ. Furthermore, methamphetamine self-administration relative to the amount of methamphetamine intake produced a biphasic effect on hippocampal apoptosis and reduced corticosterone levels. CONCLUSIONS: Intermittent (occasional access) and daily (limited and extended access) self-administration of methamphetamine impact different aspects of neurogenesis, the former producing initial pro-proliferative effects and the latter producing downregulating effects. These findings suggest that altered hippocampal integrity by even modest doses of methamphetamine could account for pronounced pathology linked to methamphetamine abuse.

Chronic morphine treatment alters expression of N-methyl-D-aspartate receptor subunits in the extended amygdala.

The nucleus accumbens (NAcc) and central amygdala (CeA) are parts of the extended amygdala, a complex that plays a key role in drug abuse and dependence. Our previous studies showed that opiates and ethanol alter glutamatergic transmission in these regions. N-methyl-D-aspartate (NMDA) receptors are key components of glutamatergic transmission likely involved in the development of opiate tolerance and dependence. In this study we examined the effects of chronic morphine administration on gene and protein expression of three major NMDA receptors subunits (NR1, NR2A, and NR2B) in NAcc and CeA. Real-time PCR showed no differences in mRNA levels of any of the subunits in the whole NAcc between naïve and morphine-dependent rats. However, at the protein level, immunoblotting revealed that chronic morphine significantly increased levels of NR1 and NR2B subunits. In contrast to the case for NAcc, in CeA we found an increased mRNA level for the NR1 subunit only but unchanged protein levels of all three subunits in morphine-dependent rats. The altered expressions of NMDA receptor subunits, especially in NAcc, of morphine-dependent rats may represent a neuroadaptation to chronic morphine and suggest a mechanism for the changes of glutamatergic transmission found in the extended amygdala in dependent rats. In addition, our results indicate a region-specific response of NMDA receptor subunits to chronic morphine administration at the gene and protein levels.