Marian L. Logrip

RACK1 and Brain-Derived Neurotrophic Factor: A Homeostatic Pathway That Regulates Alcohol Addiction

Alcoholism is a devastating disease that manifests as uncontrolled drinking. Consumption of alcohol is regulated by neurochemical systems within specific neural circuits, but endogenous systems that may counteract and thus suppress the behavioral effects of ethanol intake are unknown. Here we demonstrate that BDNF plays a role in reducing the behavioral effects of ethanol, including consumption, in rodents. We found that decreasing the levels of BDNF leads to increased behavioral responses to ethanol, whereas increases in the levels of BDNF, mediated by the scaffolding protein RACK1, attenuate these behaviors. Interestingly, we found that acute exposure of neurons to ethanol leads to increased levels of BDNF mRNA via RACK1. Importantly, acute systemic administration of ethanol and voluntary ethanol consumption lead to increased levels of BDNF expression in the dorsal striatum. Taken together, these findings suggest that RACK1 and BDNF are part of a regulatory pathway that opposes adaptations that lead to the development of alcohol addiction.

Corticosteroid-Dependent Plasticity Mediates Compulsive Alcohol Drinking in Rats

Alcoholism is characterized by a compulsion to seek and ingest alcohol, loss of control over intake, and the emergence of a negative emotional state during abstinence. We hypothesized that sustained activation of neuroendocrine stress systems (e.g., corticosteroid release via the hypothalamic-pituitary-adrenal axis) by alcohol intoxication and withdrawal and consequent alterations in glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation drive compulsive alcohol drinking. Our results showed that rats exposed to alcohol vapor to the point of dependence displayed increased alcohol intake, compulsive drinking measured by progressive-ratio responding, and persistent alcohol consumption despite punishment, assessed by adding quinine to the alcohol solution, compared with control rats that were not exposed to alcohol vapor. No group differences were observed in the self-administration of saccharin-sweetened water. Acute alcohol withdrawal was accompanied by downregulated GR mRNA in various stress/reward-related brain regions [i.e., prefrontal cortex, nucleus accumbens (NAc), and bed nucleus of the stria terminalis (BNST)], whereas protracted alcohol abstinence was accompanied by upregulated GR mRNA in the NAc core, ventral BNST, and central nucleus of the amygdala. No significant alterations in MR mRNA levels were found. Chronic GR antagonism with mifepristone (RU38486) prevented the escalation of alcohol intake and compulsive responding induced by chronic, intermittent alcohol vapor exposure. Chronic treatment with mifepristone also blocked escalated alcohol drinking and compulsive responding during protracted abstinence. Thus, the GR system appears to be involved in the development of alcohol dependence and may represent a potential pharmacological target for the treatment of alcoholism.

Glial Cell Line-Derived Neurotrophic Factor Mediates the Desirable Actions of the Anti-Addiction Drug Ibogaine against Alcohol Consumption

Alcohol addiction manifests as uncontrolled drinking despite negative consequences. Few medications are available to treat the disorder. Anecdotal reports suggest that ibogaine, a natural alkaloid, reverses behaviors associated with addiction including alcoholism; however, because of side effects, ibogaine is not used clinically. In this study, we first characterized the actions of ibogaine on ethanol self-administration in rodents. Ibogaine decreased ethanol intake by rats in two-bottle choice and operant self-administration paradigms. Ibogaine also reduced operant self-administration of ethanol in a relapse model. Next, we identified a molecular mechanism that mediates the desirable activities of ibogaine on ethanol intake. Microinjection of ibogaine into the ventral tegmental area (VTA), but not the substantia nigra, reduced self-administration of ethanol, and systemic administration of ibogaine increased the expression of glial cell line-derived neurotrophic factor (GDNF) in a midbrain region that includes the VTA. In dopaminergic neuron-like SHSY5Y cells, ibogaine treatment upregulated the GDNF pathway as indicated by increases in phosphorylation of the GDNF receptor, Ret, and the downstream kinase, ERK1 (extracellular signal-regulated kinase 1). Finally, the ibogaine-mediated decrease in ethanol self-administration was mimicked by intra-VTA microinjection of GDNF and was reduced by intra-VTA delivery of anti-GDNF neutralizing antibodies. Together, these results suggest that GDNF in the VTA mediates the action of ibogaine on ethanol consumption. These findings highlight the importance of GDNF as a new target for drug development for alcoholism that may mimic the effect of ibogaine against alcohol consumption but avoid the negative side effects.

Corticotropin releasing factor: A key role in the neurobiology of addiction

Drug addiction is a chronically relapsing disorder characterized by loss of control over intake and dysregulation of stress-related brain emotional systems. Since the discovery by Wylie Vale and his colleagues of corticotropin-releasing factor (CRF) and the structurally-related urocortins, CRF systems have emerged as mediators of the bodys response to stress. Relatedly, CRF systems have a prominent role in driving addiction via actions in the central extended amygdala, producing anxiety-like behavior, reward deficits, excessive, compulsive-like drug self-administration and stress-induced reinstatement of drug seeking. CRF neuron activation in the medial prefrontal cortex may also contribute to the loss of control. Polymorphisms in CRF system molecules are associated with drug use phenotypes in humans, often in interaction with stress history. Drug discovery efforts have yielded brain-penetrant CRF1 antagonists with activity in preclinical models of addiction. The results support the hypothesis that brain CRF-CRF1 systems contribute to the etiology and maintenance of addiction.

The Dopamine D3 Receptor Is Part of a Homeostatic Pathway Regulating Ethanol Consumption

We recently identified a homeostatic pathway that inhibits ethanol intake. This protective pathway consists of the scaffolding protein RACK1 and brain-derived neurotrophic factor (BDNF). RACK1 translocates to the nucleus after exposure of neurons to ethanol and increases expression of BDNF (McGough et al., 2004). We also found that increasing the levels of BDNF via systemic administration of RACK1 expressed as a Tat-fusion protein (Tat–RACK1) reduces ethanol consumption, whereas reduction of BDNF levels augments this behavior (McGough et al., 2004). Based on these results, we hypothesized that activation of the BDNF receptor TrkB is necessary for the effects of BDNF on ethanol intake and that gene products downstream of BDNF negatively regulate ethanol consumption. Here, we show that inhibition of the BDNF receptor TrkB increases voluntary ethanol consumption in wild-type mice but not in mice lacking one copy of the BDNF gene (BDNF+/−). We also find that increases in the levels of BDNF, mediated by ethanol or RACK1, lead to increased dorsal striatal levels of the dopamine D3 receptor (D3R), a gene downstream of BDNF, via activation of the TrkB receptor. Finally, we show that the Tat–RACK1-mediated reduction of ethanol consumption is attenuated by coinjection with either the Trk inhibitor K252a or the dopamine D3R-prefering antagonist U-99194A [5, 6-dimethoxy-2-(di-n-propylamino)indan], suggesting that activation of the BDNF pathway via RACK1 leads to increased expression of the dopamine D3R, which in turn mediates the attenuation of ethanol consumption.

Escalating ethanol intake is associated with altered corticostriatal BDNF expression

Alcoholism is a chronically relapsing condition, indicative of long‐term neuronal adaptations maintaining the disease even after prolonged abstinence. Previously, we identified brain‐derived neurotrophic factor (BDNF) in the dorsal striatum as the central mediator of a homeostatic mechanism which is activated by acute alcohol (ethanol) exposure and functions to decrease the sensitivity of rodents to ethanol‐related behaviors. We hypothesized that extensive exposure to ethanol would result in dysregulation of this BDNF‐mediated protective mechanism, accompanied by heightened ethanol intake. In this study, we demonstrate that while a single bout of ethanol intake increases BDNF mRNA expression in the dorsal striatum, this effect is no longer observed after 6 weeks of daily ethanol access. Additionally, 6 weeks of ethanol consumption decreases BDNF in the cortex, a main source of BDNF for the striatum. Importantly, these ethanol‐induced changes in BDNF levels are not ameliorated by 2 weeks’ abstinence. Together, these data suggest that the BDNF pathway, which is activated following a single bout of ethanol drinking, breaks down by the end of 6 weeks of access and does not recover its protective function after a 2‐week deprivation period. These results suggest that the persistence of altered BDNF signaling may contribute to the inflexibility of addictive behaviors.

Dynorphin is a downstream effector of striatal BDNF regulation of ethanol intake

We recently identified brain‐derived neurotrophic factor (BDNF) in the dorsal striatum to be a major component of a homeostatic pathway controlling ethanol consumption (1, 2). We hypothesized that ethanol‐mediated activation of the BDNF signaling cascade is required for the ethanol‐related function of the neurotrophic factor. Here, we demonstrate that exposure of striatal neurons to ethanol results in the activation of the BDNF receptor TrκB, leading to the activation of the mitogen‐activated protein kinase (MAP kinase) signaling pathway and the subsequent increase in the expression of preprodynorphin (Pdyn) via BDNF. Finally, we show that activation of the dynorphin receptor’ the kappa opioid receptor (KOR), is required for the BDNF‐mediated decrease in ethanol intake, illustrating a function of dynorphin in BDNFs homeostatic control of ethanol consumption. Taken together, these results demonstrate that BDNF regulates ethanol intake by initiation of MAP kinase signaling and the ensuing production of downstream gene products, including Pdyn.—Logrip, M. L., Janak, P. H., Ron, D. Dynorphin is a downstream effector of striatal BDNF regulation of ethanol intake. FASEB J. 22, 2393–2404 (2008)

Role of corticotropin-releasing factor in drug addiction: potential for pharmacological intervention.

Drug dependence is a chronically relapsing disorder that places an enormous strain on healthcare systems. For treatments to have long-term clinical value, they must address the causes of relapse. Corticotropin-releasing factor (CRF), a neuropeptide central to the stress response, may be one key to solving the relapse cycle. CRF is hypothesized to mediate the elevated anxiety and negative emotional states experienced during the development of dependence. This review summarizes existing data on changes in the CRF system produced by drugs of abuse and the function of CRF receptors in regulating behavioural responses to drugs of abuse, with an emphasis on drug dependence. Drug-induced changes in neuronal excitability throughout the limbic system, as well as the reversal of these neuroadaptations by CRF receptor antagonists, are also addressed. CRF receptor antagonists, by reducing the motivational effects of drug withdrawal and protracted abstinence, are proposed to be novel therapeutic targets for drug abuse and addiction.

Glucocorticoid receptor antagonism decreases alcohol seeking in alcohol-dependent individuals

Alcoholism, or alcohol use disorder, is a major public health concern that is a considerable risk factor for morbidity and disability; therefore, effective treatments are urgently needed. Here, we demonstrated that the glucocorticoid receptor (GR) antagonist mifepristone reduces alcohol intake in alcohol-dependent rats but not in nondependent animals. Both systemic delivery and direct administration into the central nucleus of the amygdala, a critical stress-related brain region, were sufficient to reduce alcohol consumption in dependent animals. We also tested the use of mifepristone in 56 alcohol-dependent human subjects as part of a double-blind clinical and laboratory-based study. Relative to placebo, individuals who received mifepristone (600 mg daily taken orally for 1 week) exhibited a substantial reduction in alcohol-cued craving in the laboratory, and naturalistic measures revealed reduced alcohol consumption during the 1-week treatment phase and 1-week post-treatment phase in mifepristone-treated individuals. Mifepristone was well tolerated and improved liver-function markers. Together, these results support further exploration of GR antagonism via mifepristone as a therapeutic strategy for alcoholism.

Blockade of ethanol reward by the kappa opioid receptor agonist U50,488H

Alcoholism is a pervasive social problem, and thus understanding factors that regulate alcohol (ethanol) reward is important for designing effective therapies. One putative regulatory system includes the kappa opioid receptor (KOR) and its endogenous ligand, dynorphin. Previously, we demonstrated that acute ethanol increased preprodynorphin expression via brain-derived neurotrophic factor (BDNF) in striatal neurons, and that blockade of the KOR attenuated decreases in ethanol intake observed following increased expression of BDNF. As high doses of KOR agonists can generate an aversive state, we hypothesized that endogenous dynorphin may regulate ethanol intake by interfering with the rewarding properties of ethanol. We found that low, nonaversive doses of the KOR agonist U50,488H blocked the rewarding properties of ethanol during conditioning, thus impairing the acquisition of conditioned place preference. Importantly, we demonstrate that U50,488H also inhibited the conditioned increase in locomotor activation normally observed in the ethanol-paired chamber on test day. Taken together, these data indicate that the KOR/dynorphin system may acutely regulate ethanol intake via inhibition of the rewarding properties of ethanol.