Social stress-potentiated methamphetamine seeking.

Human studies of substance use disorder show that psychological stress and drug availability interact following rehabilitation, contributing to the high relapse potential. Social stressors trigger particularly strong motivation for drug, but how this affects neuronal function to increase relapse is unknown. Animal models, which allow for the dissection of neural mechanisms, primarily utilize physical stressors to trigger relapse. To recapitulate psychosocial post-rehabilitation challenges in animals, we developed a model of social stress-potentiated methamphetamine (METH) seeking. Rats receive a single social defeat (SD) session after completion of self-administration and extinction of lever pressing. While a reminder of the SD was insufficient to reinstate METH seeking on its own, rats that received a reminder of SD followed by a METH-priming injection displayed potentiated reinstatement over METH-priming alone. Examination of neuronal activation patterns of the METH-primed reinstatement session identified c-Fos-immunoreactivity in the basolateral amygdala (BLA) as correlated with SD score, a measure of defeat latency. Rapidly defeated rats showed potentiated METH-primed reinstatement and elevated BLA c-Fos compared with controls. Conversely, rats that were undefeated during the social stress did not show potentiated METH-primed reinstatement or elevated BLA c-Fos. Interestingly, inactivation of the BLA with baclofen/muscimol prior to the stress reminder and METH-priming generated a potentiation of METH seeking in the undefeated rats, suggesting the BLA may mediate resilience to the stressor. This model provides a tool for the further dissection of neural mechanisms mediating social stress-potentiated relapse and for the development of relapse-reducing therapeutics.

Memory disrupting effects of nonmuscle myosin II inhibition depend on the class of abused drug and brain region.

Depolymerizing actin in the amygdala through nonmuscle myosin II inhibition (NMIIi) produces a selective, lasting, and retrieval-independent disruption of the storage of methamphetamine-associated memories. Here we report a similar disruption of memories associated with amphetamine, but not cocaine or morphine, by NMIIi. Reconsolidation appeared to be disrupted with cocaine. Unlike in the amygdala, methamphetamine-associated memory storage was not disrupted by NMIIi in the hippocampus, nucleus accumbens, or orbitofrontal cortex. NMIIi in the hippocampus did appear to disrupt reconsolidation. Identification of the unique mechanisms responsible for NMII-mediated, amygdala-dependent disruption of memory storage associated with the amphetamine class may enable induction of retrieval-independent vulnerability to other pathological memories.

The potential of epigenetics in stress-enhanced fear learning models of PTSD.

Prolonged distress and dysregulated memory processes are the core features of post-traumatic stress disorder (PTSD) and represent the debilitating, persistent nature of the illness. However, the neurobiological mechanisms underlying the expression of these symptoms are challenging to study in human patients. Stress-enhanced fear learning (SEFL) paradigms, which encompass both stress and memory components in rodents, are emerging as valuable preclinical models of PTSD. Rodent models designed to study the long-term mechanisms of either stress or fear memory alone have identified a critical role for numerous epigenetic modifications to DNA and histone proteins. However, the epigenetic modifications underlying SEFL remain largely unknown. This review will provide a brief overview of the epigenetic modifications implicated in stress and fear memory independently, followed by a description of existing SEFL models and the few epigenetic mechanisms found to date to underlie SEFL. The results of the animal studies discussed here highlight neuroepigenetics as an essential area for future research in the context of PTSD through SEFL studies, because of its potential to identify novel candidates for neurotherapeutics targeting stress-induced pathogenic memories.

Role of medial prefrontal cortex Narp in the extinction of morphine conditioned place preference.

Narp knockout (KO) mice demonstrate an impaired extinction of morphine conditioned place preference (CPP). Because the medial prefrontal cortex (mPFC) has been implicated in extinction learning, we tested whether Narp cells in this region play a role in the extinction of morphine CPP. We found that intracranial injections of adenoassociated virus (AAV) expressing wild-type (WT) Narp into the mPFC of Narp KO mice rescued the extinction and the injection of AAV expressing a dominant negative form of Narp (NarpN) into the mPFC of WT mice impaired the extinction of morphine CPP. These findings suggest that Narp in the mPFC mediates the extinction of morphine CPP.

Neuronal activity-regulated pentraxin expressed in medial prefrontal cortex neurons is not necessary for extinction of heroin self-administration.

The medial prefrontal cortex (mPFC) plays a key role in extinction learning. Previously, we found that expression of a neuronal activity-regulated pentraxin (Narp) dominant-negative construct in the mPFC of mice blocked extinction of morphine-conditioned place preference. To further investigate the role of mPFC Narp in the extinction of drug seeking, we tested whether mPFC Narp is necessary for the extinction of heroin self-administration in rats. Specifically, we injected an adeno-associated viral vector expressing a dominant-negative form of Narp (NarpN) into the infralimbic region of the mPFC of rats and compared lever presses during extinction to those of rats injected with a control virus. In contrast to our previous study, we found that injection of NarpN did not affect extinction of heroin self-administration. Our findings suggest that mPFC Narp is necessary for extinction of opiate seeking in the Pavlovian-conditioned place preference paradigm but not in the operant paradigm of drug self-administration.

Localized disruption of Narp in medial prefrontal cortex blocks reinforcer devaluation performance.

Neuronal activity regulated pentraxin (Narp) is a secreted protein that regulates α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors (AMPAR) aggregation and synaptogenesis. Mapping of Narp-positive neurons in brain has revealed it is prominently expressed in several limbic system projection pathways. Consistent with this localization pattern, Narp knockout mice show deficits in using the current value of a reinforcer to guide behavior, a critical function of the limbic system. To help assess whether this behavioral deficit is due to impairment of synaptogenesis during development or in modulating synaptic signaling in the mature brain, we have used a dominant negative Narp viral construct which blocks trafficking of endogenous Narp to axons. Focal injection of this viral construct into the medial prefrontal cortex (mPFC) of adult mice, a region containing Narp-positive projection neurons, blocked reinforcer devaluation. Thus, these results indicate that Narp released from mPFC neurons plays a key role in mediating synaptic changes underlying instrumental reinforcer devaluation.

L-type voltage-gated calcium channels are required for extinction, but not for acquisition or expression, of conditional fear in mice.

It has been shown recently that extinction of conditional fear does not depend acutely on NMDA-type glutamate receptors, although other evidence has led to the hypothesis that L-type voltage-gated calcium channels (LVGCCs) play a role in conditional fear. We therefore tested the role of LVGCCs in the acquisition, expression, and extinction of conditional fear of cue and context in mice. Using systemic injections of two LVGCC inhibitors, nifedipine and nimodipine, which both effectively cross the blood-brain barrier, we show that LVGCCs are essential for the extinction, but not for the acquisition or expression, of conditional fear in mice.

Temporally massed CS presentations generate more fear extinction than spaced presentations.

Rodent fear conditioning models both excitatory learning and the pathogenesis of human anxiety, whereas extinction of conditional fear is a paradigm of inhibitory learning and the explicit model for behavior therapy. Many studies support a general learning rule for acquisition: Temporally spaced training is more effective than massed training. The authors asked whether this rule applies to extinction of conditional fear in mice. The authors find that both short- and long-term fear extinction are greater with temporally massed presentations of the conditional stimulus (CS). The data also indicate that once CS presentations are sufficiently massed to initiate, or "induce," extinction learning, further CS presentations are more effective when spaced.

Narp knockout mice show normal reactivity to novelty but attenuated recovery from neophobia.

Narp knockout (KO) mice demonstrate cognitive inflexibility and addictive behavior, which are associated with abnormal reactivity to a novel stimulus. To assess reactivity to novelty, we tested Narp KO and wild-type (WT) mice on a neophobia procedure. Both Narp KO and WT mice showed a similar decrease in consumption upon initial exposure to a novel flavor, but Narp KO mice did not increase consumption with subsequent exposures to the novel flavor like the WT mice. Therefore, Narp KO mice do not have abnormal reactivity to novelty but show deficits in adapting behavior to reflect the updated value of a stimulus.

Human hypocretin and melanin-concentrating hormone levels are linked to emotion and social interaction.

The neurochemical changes underlying human emotions and social behaviour are largely unknown. Here we report on the changes in the levels of two hypothalamic neuropeptides, hypocretin-1 and melanin-concentrating hormone, measured in the human amygdala. We show that hypocretin-1 levels are maximal during positive emotion, social interaction and anger, behaviours that induce cataplexy in human narcoleptics. In contrast, melanin-concentrating hormone levels are minimal during social interaction, but are increased after eating. Both peptides are at minimal levels during periods of postoperative pain despite high levels of arousal. Melanin-concentrating hormone levels increase at sleep onset, consistent with a role in sleep induction, whereas hypocretin-1 levels increase at wake onset, consistent with a role in wake induction. Levels of these two peptides in humans are not simply linked to arousal, but rather to specific emotions and state transitions. Other arousal systems may be similarly emotionally specialized.

Mediating the effects of drug abuse: the role of Narp in synaptic plasticity.

There has been remarkable progress in deciphering the molecular mechanisms that mediate synaptic plasticity. Advances have stimulated interest in determining whether these plasticity mechanisms also mediate the long-lasting behavioral effects induced by drugs of abuse. The observation that drugs of abuse, such as cocaine or morphine, can elicit robust immediate early gene (IEG) responses similar to those induced by long-term potentiation stimulation has provided important support for this hypothesis. Evidence that repeated administration of cocaine produces alterations in expression and trafficking of AMPA receptors, processes that play a central role in synaptic plasticity, has also bolstered this view. Neuronal activity-regulated pentraxin (Narp), an IEG, has emerged as an attractive candidate to mediate long-term effects of drugs of abuse because it encodes a secreted protein that binds to the extracellular surface of AMPA receptors and regulates their trafficking. In this review we provide background information on Narp and closely related proteins, the neuronal pentraxins, and summarize studies of Narp knockout mice demonstrating that this IEG modulates long-term behavioral responses to drugs of abuse.

Like extinction, latent inhibition of conditioned fear in mice is blocked by systemic inhibition of L-type voltage-gated calcium channels.

Having recently shown that extinction of conditioned fear depends on L-type voltage-gated calcium channels (LVGCCs), we have been seeking other protocols that require this unusual induction mechanism. We tested latent inhibition (LI) of fear, because LI resembles extinction except that cue exposures precede, rather than follow, cue-shock pairing. Systemic injections of two LVGCC inhibitors, nifedipine and diltiazem, before pre-exposure blocked LI completely with no evidence of state-dependent learning. The results indicate that extinction and LI share a common molecular requirement and may support the notion that LI, like extinction, is a form of inhibitory learning.

Adrenergic transmission facilitates extinction of conditional fear in mice.

Extinction of classically conditioned fear, like its acquisition, is active learning, but little is known about its molecular mechanisms. We recently reported that temporal massing of conditional stimulus (CS) presentations improves extinction memory acquisition, and suggested that temporal spacing was less effective because individual CS exposures trigger two opposing processes: (1) fear extinction, which is favored by CS massing, and (2) fear incubation (increase), which is favored by spacing. We here report the effects of manipulating the adrenergic system during massed or spaced CS presentations in fear-conditioned mice. We administered yohimbine (5 mg/kg), an alpha(2)-receptor antagonist, or propranolol (10 mg/kg), a beta-receptor antagonist, systemically prior to CS presentation sessions and recorded both short- and long-term changes in conditional freezing. Yohimbine treatment facilitated extinction of both cue and context fear with massed protocols. When given before spaced CS presentations, propranolol led to a persistent incubation of cue fear, whereas yohimbine led to persistent extinction, compared with vehicle-treated animals, which showed no change in fear. These results suggest that norepinephrine positively modulates the formation of fear extinction memories in mice. They also provide clear evidence that spaced CS presentations trigger both fear-reducing (extinction) and fear-increasing (incubation) mechanisms.