Chemogenetic activation of the lateral hypothalamus reverses early life stress-induced deficits in motivational drive.

Altered motivated behaviour is a cardinal feature of several neuropsychiatric conditions including mood disorders. One well-characterized antecedent to the development of mood disorders is exposure to early life stress (ELS). A key brain substrate controlling motivated behaviour is the lateral hypothalamus (LH). Here, we examined the effect of ELS on LH activation and the motivation to self-administer sucrose. We tested whether chemogenetic activation of LH circuits could modify sucrose responding in ELS rats and examined the impact on LH cell populations. Male rat pups were maternally separated for 0 or 3 h on postnatal days 2-14. During adolescence, rats received bilateral injections of hM3D(Gq), the excitatory designer receptor exclusively activated by designer drugs, into LH. In adulthood, rats were trained to self-administer sucrose and tested under a progressive ratio schedule to determine their motivation for reward following injection with either vehicle or 5 mg/kg clozapine-N-oxide. Brains were processed for Fos-protein immunohistochemistry. ELS significantly suppressed lever responding for sucrose, indicating a long-lasting impact of ELS on motivation circuits. hM3D(Gq) activation of LH increased responding, normalizing deficits in ELS rats, and increased Fos-positive orexin and MCH cell numbers within LH. Our findings indicate that despite being susceptible to environmental stressors, LH circuits retain the capacity to overcome ELS-induced deficits in motivated behaviour.

Cocaine potentiates excitatory drive in the perifornical/lateral hypothalamus.

The hypothalamus is a critical controller of homeostatic responses and plays a fundamental role in reward-seeking behaviour. Recently, hypothalamic neurones in the perifornical/lateral hypothalamic area (PF/LHA) have also been implicated in drug-seeking behaviour through projections to extra-hypothalamic sites such as the ventral tegmental area. For example, a population of neurones that expresses the peptide orexin has been strongly implicated in addiction-relevant behaviours. To date, the effect of addictive drugs on synaptic properties in the hypothalamus remains largely unexplored. Previous studies focusing on the PF/LHA neurones, however, have shown that the orexin system exhibits significant plasticity in response to food or sleep restriction. This neuroadaptive ability suggests that PF/LHA neurones could be highly susceptible to modifications by drug exposure. Here, we sought to determine whether cocaine produces synaptic plasticity in PF/LHA neurones. Whole-cell patch-clamp techniques were used to examine the effects of experimenter-administered (passive) or self-administered (SA) cocaine on glutamatergic synaptic transmission in PF/LHA neurones. These experiments demonstrate that both passive and SA cocaine exposure increases miniature excitatory postsynaptic current (mEPSC) frequency in PF/LHA neurones. In addition, SA cocaine reduced the paired-pulse ratio but the AMPA/NMDA ratio of evoked excitatory inputs was unchanged, indicative of a presynaptic locus for synaptic plasticity. Dual-labelling for orexin and excitatory inputs using the vesicular glutamate transporter (VGLUT2), showed that passive cocaine exposure increased VGLUT2-positive appositions onto orexin neurones. Further, a population of recorded neurones that were filled with neurobiotin and immunolabelled for orexin confirmed that increased excitatory drive occurs in this PF/LHA population. Given the importance of the PF/LHA and the orexin system in modulating drug addiction, we suggest that these cocaine-induced excitatory synapse-remodelling events within the hypothalamus may contribute to persistence in drug-seeking behaviour and relapse.

Orexin-1 receptor signalling within the ventral tegmental area, but not the paraventricular thalamus, is critical to regulating cue-induced reinstatement of cocaine-seeking.

Orexinergic signalling is critical to drug relapse-like behaviour; however, the CNS sites(s) of action remain unknown. Two candidate brain regions are the paraventricular thalamus (PVT) and ventral tegmental area (VTA). We assessed the effect of intra-PVT or -VTA administration of the orexin-1 receptor (OrxR1) antagonist SB-334867 on discriminative cue-induced cocaine-seeking. Animals received either PVT- or VTA-directed SB-334867 (0, 3 or 6 µg; 0, 1 or 3 µg, respectively) prior to reinstatement testing elicited by presenting cocaine-paired stimuli (S+). The effect of VTA-directed injections of SB-334867 (0 or 3 µg) on locomotor activity was also assessed. Intra-VTA, but not -PVT, SB-334867 dose-dependently attenuated S+-induced reinstatement (3 µg dose, p<0.01). Intra-VTA SB-334867 had no effect on locomotor activity. We conclude that OrxR1 signalling within the VTA, but not the PVT, mediates cue-induced cocaine-seeking behaviour. We hypothesize that blockade of VTA OrxR1 signalling may reduce nucleus accumbens dopamine in response to drug cue presentation.

Electrophysiological characteristics of paraventricular thalamic (PVT) neurons in response to cocaine and cocaine- and amphetamine-regulated transcript (CART).

Recent work has established that the paraventricular thalamus (PVT) is a central node in the brain reward-seeking pathway. This role is mediated in part through projections from hypothalamic peptide transmitter systems such as cocaine- and amphetamine-regulated transcript (CART). Consistent with this proposition, we previously found that inactivation of the PVT or infusions of CART into the PVT suppressed drug-seeking behavior in an animal model of contingent cocaine self-administration. Despite this work, few studies have assessed how the basic physiological properties of PVT neurons are influenced by exposure to drugs such as cocaine. Further, our previous work did not assess how infusions of CART, which we found to decrease cocaine-seeking, altered the activity of PVT neurons. In the current study we address these issues by recording from anterior PVT (aPVT) neurons in acutely prepared brain slices from cocaine-treated (15 mg/ml, n = 8) and saline-treated (control) animals (n = 8). The excitability of aPVT neurons was assessed by injecting a series of depolarizing and hyperpolarizing current steps and characterizing the resulting action potential (AP) discharge properties. This analysis indicated that the majority of aPVT neurons exhibit tonic firing (TF), and initial bursting (IB) consistent with previous studies. However, we also identified PVT neurons that exhibited delayed firing (DF), single spiking (SS) and reluctant firing (RF) patterns. Interestingly, cocaine exposure significantly increased the proportion of aPVT neurons that exhibited TF. We then investigated the effects of CART on excitatory synaptic inputs to aPVT neurons. Application of CART significantly suppressed excitatory synaptic drive to PVT neurons in both cocaine-treated and control recordings. This finding is consistent with our previous behavioral data, which showed that CART signaling in the PVT negatively regulates drug-seeking behavior. Together, these studies suggest that cocaine exposure shifts aPVT neurons to a more excitable state (TF). We propose that the capacity of CART to reduce excitatory drive to this population balances the enhanced aPVT excitability to restore the net output of this region in the reward-seeking pathway. This is in line with previous anatomical evidence that the PVT can integrate reward-relevant information and provides a putative mechanism through which drugs of abuse can dysregulate this system in addiction.

Orexin antagonists for neuropsychiatric disease: progress and potential pitfalls.

The tight regulation of sleep/wake states is critical for mental and physiological wellbeing. For example, dysregulation of sleep/wake systems predisposes individuals to metabolic disorders such as obesity and psychiatric problems, including depression. Contributing to this understanding, the last decade has seen significant advances in our appreciation of the complex interactions between brain systems that control the transition between sleep and wake states. Pivotal to our increased understanding of this pathway was the description of a group of neurons in the lateral hypothalamus (LH) that express the neuropeptides orexin A and B (hypocretin, Hcrt-1 and Hcrt-2). Orexin neurons were quickly placed at center stage with the demonstration that loss of normal orexin function is associated with the development of narcolepsy-a condition in which sufferers fail to maintain normal levels of daytime wakefulness. Since these initial seminal findings, much progress has been made in our understanding of the physiology and function of the orexin system. For example, the orexin system has been identified as a key modulator of autonomic and neuroendocrine function, arousal, reward and attention. Notably, studies in animals suggest that dysregulation of orexin function is associated with neuropsychiatric states such as addiction and mood disorders including depression and anxiety. This review discusses the progress associated with therapeutic attempts to restore orexin system function and treat neuropsychiatric conditions such as addiction, depression and anxiety. We also highlight potential pitfalls and challenges associated with targeting this system to treat these neuropsychiatric states.

Cocaine- and amphetamine-regulated transcript (CART) signaling within the paraventricular thalamus modulates cocaine-seeking behaviour.

BACKGROUND: Cocaine- and amphetamine-regulated transcript (CART) has been demonstrated to play a role in regulating the rewarding and reinforcing effects of various drugs of abuse. A recent study demonstrated that i.c.v. administration of CART negatively modulates reinstatement of alcohol seeking, however, the site(s) of action remains unclear. We investigated the paraventricular thalamus (PVT) as a potential site of relapse-relevant CART signaling, as this region is known to receive dense innervation from CART-containing hypothalamic cells and to project to a number of regions known to be involved in mediating reinstatement, including the nucleus accumbens (NAC), medial prefrontal cortex (mPFC) and basolateral amygdala (BLA). METHODOLOGY/PRINCIPAL FINDINGS: Male rats were trained to self-administer cocaine before being extinguished to a set criterion. One day following extinction, animals received intra-PVT infusions of saline, tetrodotoxin (TTX; 2.5 ng), CART (0.625 µg or 2.5 µg) or no injection, followed by a cocaine prime (10 mg/kg, i.p.). Animals were then tested under extinction conditions for one hour. Treatment with either TTX or CART resulted in a significant attenuation of drug-seeking behaviour following cocaine-prime, with the 2.5 µg dose of CART having the greatest effect. This effect was specific to the PVT region, as misplaced injections of both TTX and CART resulted in responding that was identical to controls. CONCLUSIONS/SIGNIFICANCE: We show for the first time that CART signaling within the PVT acts to inhibit drug-primed reinstatement of cocaine seeking behaviour, presumably by negatively modulating PVT efferents that are important for drug seeking, including the NAC, mPFC and BLA. In this way, we identify a possible target for future pharmacological interventions designed to suppress drug seeking.