Role of lateral hypothalamic orexin neurons in reward processing and addiction

Abstract

Orexins (also known as hypocretins) are recently discovered neuropeptides made exclusively in hypothalamic neurons that have been shown to be important in narcolepsy/cataplexy and arousal. Here, we conducted behavioral, anatomical and neurophysiological studies that show that a subset of these cells, located specifically in lateral hypothalamus (LH), are involved in reward processing and addictive behaviors. We found that Fos expression in LH orexin neurons varied in proportion to preference for morphine, cocaine or food. This relationship obtained both in drug naïve rats and in animals during protracted morphine withdrawal, when drug preference was elevated but food preference was decreased. Recent studies showed that LH orexin neurons that project to ventral tegmental area (VTA) have greater Fos induction in association with elevated morphine preference during protracted withdrawal than non-VTA-projecting orexin neurons, indicating that the VTA is an important site of action for orexin's role in reward processing. In addition, we found that stimulation of LH orexin neurons, or microinjection of orexin into VTA, reinstated an extinguished morphine preference. Most recently, using a self-administration paradigm we discovered that the Ox1 receptor antagonist SB-334867 (SB) blocks cocaine-seeking induced by discrete or contextual cues, but not by a priming injection of cocaine. Neurophysiological studies revealed that locally applied orexin often augmented responses of VTA dopamine (DA) neurons to activation of the medial prefrontal cortex (mPFC), consistent with the view that orexin facilitates activation of VTA DA neurons by stimulus-reward associations. We also recently showed that orexin in VTA is necessary for learning a morphine place preference. These findings are consistent with results from others showing that orexin facilitates glutamate-mediated responses, and is necessary for glutamate-dependent long-term potentiation, in VTA DA neurons. We surmise from these studies that LH orexin neurons play an important role in reward processing and addiction, and that LH orexin cells are an important input to VTA for behavioral effects associated with reward-paired stimuli.

Figure 1

High-power photomicrograph of the LH showing the double labeling of orexin (brown cytoplasm) and Fos protein (black nuclei) in morphine-conditioned and non-conditioned animals, as indicated. Black arrows indicate double-labeled cells. Taken from (Harris et al., 2005).

Figure 2

Activation of lateral hypothalamus orexin neurons by rPP reinstated an extinguished preference for morphine. a, Preference scores are shown for both rPP- (150 nM) and vehicle-injected groups (mean ± s.e.m. in morphine-paired side minus saline-paired side) during the initial conditioning test, after extinction and during the reinstatement test. b, The selective orexin A antagonist, SB 334867 (20–30 mg kg), blocked reinstatement by rPP (n = 8). Data were included only if rPP injection into lateral hypothalamus on the following day (without the antagonist pretreatment) produced reinstatement of preference. c, Plot of correlation between reinstatement scores and percentages of lateral hypothalamus orexin neurons that were Fos activated in rPP reinstated animals. Taken from (Harris et al., 2005).

Figure 3

SB-334867 (30 mg/kg, ip) attenuated cue- and context-elicited cocaine-seeking. Left graph - Pretreatment with SB-334867 significantly reduced cue-induced reinstatement of lever-pressing as compared to vehicle pretreatment in the same animals (*p<0.01). Right graph -Animals given SB-334867 prior to the first extinction session had significantly reduced lever-pressing as compared to animals given vehicle (*p<0.05). Extinction days 1 and 7 are significantly different for animals given vehicle prior to the first extinction session (p<0.05), whereas extinction days 1 and 7 are not different for animals given SB-334867 prior to the first extinction session.

Figure 4

Fos expression in LH orexin neurons as a function of treatments, as indicated. Morphine- and food-conditioned animals were given morphine or placebo pellets for 2 weeks, and pellets were removed and animals remained in their home cages for 5 weeks before CPP conditioning, as in our previous publications (Aston-Jones and Harris, 2004; Harris and Aston-Jones, 2003c). Note that a higher percentage of LH orexin neurons exhibited Fos in withdrawn animals than in placebo-pelleted animals subjected to morphine CPP. Conversely, a lower percentage of LH orexin neurons exhibited Fos in withdrawn animals than in placebo pelleted animals subjected to food CPP. Cocaine CPP in naïve rats (no prior drug treatment) also increased Fos expression in LH orexin neurons.

Figure 5

Responses of a single DA neuron evoked with mPFC stimulation before and after application of orexin A to the recorded neuron. Left panel shows the responses of a DA neuron evoked with 1 mA stimulation of the mPFC. Right panel shows the responses of the same neuron to the same stimulation approximately 5 min following delivery of 60 nl of 1.4 μM orexin A locally onto the recorded neuron. Dashed line is the onset of stimulation.

Figure 6

(a) Conditioned place preference (CPP) scores and (b) orexin neuronal cell counts for animals given unilateral NMDA injections in the LH and microinjections of SB 334867 in the contralateral VTA during CPP training. (a) Preference scores were calculated by subtracting the time spent in the morphine-paired chamber during the preconditioning day from the time spent in that chamber on the test day (i.e., post-conditioning). Control animals received vehicle instead of NMDA in the LH and received the same SB injections in the contralateral VTA. Scores represent group mean±SEM. (b) Neuronal cell counts of surviving orexin neurons from six adjacent 40 um-thick sections at the level of the neurotoxin injection from animals with effective lesions. Control refers to the number of orexin neurons found on the non-lesioned side in the same slices (*P < .01, n = 9). Taken from (Harris et al., 2007b).

Figure 7

Diagram illustrating a proposed model for the role of orexin in drug-seeking and relapse. Orexin neurons in the lateral hypothalamus (LH) send direct and indirect projections to the ventral tegmental area (VTA). The VTA sends dopaminergic projections to several areas, including the prefrontal cortex (PFC) and nucleus accumbens (NAcc). Psychostimulants cause increased dopamine release in these target areas via direct actions at dopaminergic terminals, whereas opiates and alcohol cause increased dopamine release via actions in the VTA that result in increased dopaminergic cell activation. These differences in acute drug action might explain the ability of SB-334867 to attenuate only certain types of drug-seeking. SB-334867 attenuates increased activation of VTA dopaminergic neurons without effect on baseline firing. Therefore, relapse that depends upon increased VTA dopaminergic cell activation (i.e. opiate-, stress-, and cue-induced reinstatement, and context-elicited drug-seeking) is vulnerable to SB-334867. Relapse that does not require increased activation of VTA dopaminergic cells (i.e. cocaine-induced reinstatement) is unaffected by SB-334867. Illustration modified from Swanson (1992.