The Neurobiology of Addiction Research Center (NARC)
Gary Aston-Jones, Ph.D.Project 3 - Role of Lateral Hypothalamic Orexin Neurons in Cocaine-seeking.
A great deal of data indicate that neurons containing the orexin neuropeptides (also known as hypocretins) are involved in arousal and narcolepsy. However, we found that orexin neurons specifically located in lateral hypothalamus (LH) are Fos-activated by drug-associated stimuli in proportion to the preference that animals express on a conditioned place preference (CPP) test day. Our studies also revealed that chemical stimulation of these cells, or microinfusion of orexin into ventral tegmental area (VTA), reinstates an extinguished drug CPP. Other data from our lab, as well as from other investigators, show that orexin neurotransmission is important for stress-induced reinstatement of drug-self administration, for learning and expressing stimulus-drug relationships, and for plasticity in VTA dopamine (DA) neurons. Together, these results provide an important new view on LH orexin projections to midbrain DA neurons, indicating that they play an important role in reward processing and relapse to drug-seeking. The goal of this project is to determine the role of LH orexin neurons and their projections to reward circuitry in the reinstatement of cocaine-seeking following extinction of drug-taking behavior.
Aim 1 – Characterize connections between LH orexin neurons and circuits involved in reinstatement of extinguished cocaine-seeking. Based upon the evidence above and other studies, we proposed that orexin neurons are functionally dichotomous; LH orexin neurons are specifically involved in reward processes whereas more medial orexin cells in the perifornical (PeF) and dorsomedial hypothalamus (DMH) mediate arousal and narcolepsy. This view predicts that these different orexin functions are reflected in different projection targets of these orexin neuronal subgroups. Here, we will test the prediction that LH orexin neurons that are involved in reward processing preferentially innervate brain areas involved in reward function. We will retrogradely label LH orexin neurons from VTA, nucleus accumbens, or medial prefrontal cortex (mPFC), and co-label with Fos following cue-induced reinstatement of extinguished lever pressing to seek cocaine. These results will be compared to results of similar studies examining retrograde labeling from arousal-related areas such as the locus coeruleus and tuberomammillary hypothalamic nucleus. These experiments will identify orexin neurons that are activated with reinstatement of cocaine-seeking, and determine whether reinstatement-related orexin neurons are topographically segregated by their location within the hypothalamus or by their projection targets.
Aim 2 –Determine the role of LH orexin projections to midbrain dopamine neurons in reinstatement of extinguished cocaine-seeking. Our recent work showed that a systemically administered orexin 1 (Ox1) receptor antagonist, SB 334867 (SB), decreased expression of a morphine CPP, indicating a possible role in the recall of stimulus-drug associations. This is consistent with our finding that stimulation of orexin cells, or microinjection of orexin into VTA, induces strong reinstatement of an extinguished preference for morphine. We have recently extended this finding to the self-administration paradigm, finding that systemic SB blocks cue-induced reinstatement of extinguished cocaine-seeking. Here, we will locally manipulate the orexin system to test if cue-induced reinstatement of cocaine-seeking requires orexin transmission specifically in the VTA. In addition, collaborative studies with Projects 1 and 2 will determine whether LH orexin modulation of cortically-projecting midbrain DA neurons induces changes in the mPFC-Nac circuit that are associated with relapse to cocaine-seeking behavior. These experiments will identify circuitry involved in orexin-dependent reinstatement of extinguished cocaine-seeking.
Aim 3 – Characterize cocaine- and orexin-dependent modulation of VTA response to mPFC inputs. DA is important for learning reward-driven behaviors. Recent work has shown that orexin plays a critical role in neural plasticity in VTA DA neurons, augmenting responses of DA neurons to glutamate inputs. Consistent with this, additional recent work from our lab and others shows that orexin inputs to VTA are critical for learning stimulus-drug relationships in behavioral tasks. These various results indicate that orexin-dependent augmentation of responses of VTA DA to glutamate inputs plays an important role in reward learning and drug abuse. It has remained unclear, however, which inputs to VTA are modulated by orexin afferents. The mPFC is a good candidate, given its role in processing complex stimulus-response information and its reciprocal connections with VTA DA neurons. Here, we will use single unit recording techniques to examine the role of mPFC-orexin interactions in VTA and determine if responses of VTA neurons to mPFC inputs are modulated by cocaine exposure or by reinstatement of cocaine-seeking in an orexin-dependent fashion. Such modulation could specify pathways whereby highly processed stimulus-response representations in mPFC lead to cocaine-seeking behaviors.
Together, these studies will determine the role of LH orexin neurons in relapse to cocaine-seeking, and the relationships between these orexin cells and brain circuits shown by others in this Center to be critical for such relapse. Our studies will extend other projects in this Center application by including this important neural system, revealing new mechanisms for plasticity and learning that are critical to addiction and new sites for potential intervention in drug abuse treatment.