Day-to-day decision-making and long-term goal pursuit require circuits that mediate reward and motivation. It is essential that reward-seeking behavior occurs at appropriate times and in the most suitable locations to achieve optimal fitness. Information regarding these diverse factors is integrated with that from other sources in dopaminergic neurons located in the ventral tegmental area (VTA).
One set of inputs that regulates VTA dopaminergic neurons originates from orexin/hypocretin neurons, which are situated exclusively in the lateral hypothalamus (LH; Korotkova et al., 2003). Orexin stimulates arousal and appetite by binding to Gq-coupled orexin 1 receptors (OxR1), which produce excitatory effects in downstream neurons (de Lecea et al., 1998; Sakurai et al., 1998). Intriguingly, lateral hypothalamic neurons that contain and release orexin also contain and release dynorphin. This is surprising because orexin and dynorphin have opposing effects. Dynorphin binds to Gi/o-coupled kappa opioid receptors (KOR), resulting in the inhibition of spiking and neurotransmitter release in target neurons, including dopaminergic neurons in the VTA (Margolis et al., 2003). In general, dynorphin exerts aversive and anxiogenic effects and reduces food consumption, and dysregulation of the dynorphin–KOR has been linked to alcohol and drug addiction (Anderson et al., 2018). Notably, previous work has suggested that orexin and dynorphin neuropeptides are packaged in the same dense core vesicles and are thus coreleased (Chou et al., 2001; Muschamp et al., 2014). Furthermore, both KORs and OxR1 are expressed in several regions associated with motivation and reward, including the VTA, nucleus accumbens (NAc), bed nucleus of the stria terminalis (BNST), central amygdala (CeA), and PFC (Anderson et al., 2018). This raises a puzzling question: How does the corelease of neuropeptides with opposing effects influence the activity of downstream neurons and the behavior they regulate?
Previous work demonstrated that optogenetic stimulation of LH terminals in the VTA resulted in the formation of a place preference, with the animal spending more time in the part of the area where corelease occurred, suggesting that corelease produced a rewarding effect. In addition, this corelease caused the animals to display approach behavior toward food (Thomas et al., 2022). Since the effect was blocked by an OxR1 receptor antagonist, this suggests that the place preference and food seeking were orexin driven. It was also shown that OxR1 activation led to the release of DA in the NAc. A separate study showed that subpopulations of dopaminergic neurons in the VTA that project to different target areas, such as BLA, the medial NAc shell (mAcbSh), and lateral NAc shell (lAcbSh), are both anatomically and electrophysiologically distinct. In brain slices, the different subpopulations further responded distinctly when dynorphin or orexin were administered in bath applications. The effect of this corelease suggests that the two neuropeptides might differentially regulate VTA neurons that project to different targets.
Mohammadkhani et al. (2024) hypothesized that the LHox/dyn release of orexin or dyn, respectively, induces activation or inhibition of distinct projection-dependent subpopulations of VTADA. To test this hypothesis, the authors examined the effects of optically evoked orexin and dynorphin release from LH neurons via selective expression of channelrhodopsin in orexin neurons. This confirmed that modulation by orexin or dynorphin can increase or decrease the firing rate of VTADA. The changes in firing rate persisted in the presence of GABA and glutamate receptor antagonists, indicating that they were produced by direct effects on the postsynaptic neurons rather than by modulation of fast synaptic transmission. The increased firing in VTADA neurons was blocked by the OxR1 antagonist SB334867, while the suppression was blocked by the KOR antagonist NorBNI, indicating that these responses are driven by orexin and dynorphin respectively. Notably, the inhibition of VTADA neurons persisted throughout the experiment, whereas the increase in spiking was more transient.
To determine whether LHox/dyn inputs have different effects on different subpopulations of VTADA neurons, the authors labeled these subpopulations with fluorescent retrobeads. As hypothesized, the responses elicited by LH inputs differed across the subpopulations. LHox/dyn inputs had a diverse effect on VTADA neurons that project to the nucleus accumbens, and these depended on the region of NAc targeted. While the majority of VTADA neurons projecting to the lateral NAc shell were activated by LHox/dyn inputs, a smaller subset showed reduced firing. In contrast, the optogenetic stimulation of LHox/dyn resulted in bidirectional modulation of mAcbSh-projecting VTADA neurons, with similar-sized subsets of cells showing reduced firing and activation. The effect of LHox/dyn inputs on VTADA neurons projecting to the BLA was less variable, with the majority of neurons showing reduced firing after optogenetic stimulation. This reduced firing was blocked by a KOR antagonist, indicating that, dynorphin is responsible for this effect in these projections.
The study conducted by Mohammadkhani et al. (2024) provides valuable insight into the corelease of the neuropeptides orexin and dynorphin onto VTADA neurons and their downstream effects (Figure 1). Although orexin and dynorphin are coreleased, they modulate different subsets of VTADA neurons, with specific effects depending on the target of these neurons. Given the function of the target regions in the NAc and BLA, this adds to our understanding of how these neuropeptides interact to regulate reward-related behaviors. For example, dynorphin signaling from LHox/dyn neurons may suppress dopamine signaling in regions involved in emotional regulation and the stress response, such as the BLA. Conversely, orexin signaling has been shown to promote dopaminergic downstream effects, that may contribute to the regulation of motivation. These findings highlight the complex and multifaceted role of these neuropeptides within the circuits governing reward and emotional regulation.
Importantly, the proportions of neurons in the NAc and BLA that were inhibited or stimulated following bath application are comparable to previous findings using exogenous stimulation (Baimel et al., 2017). This confirms that LHox/dyn neurons are the key mediators of these effects. The consistency of these results with previous studies strengthens the argument that these neurons are crucial regulators of dopamine circuits.
In light of the findings presented in this study and those reported previously, future research could explore how optogenetic stimulation of projection-specific VTADA neurons affects behavior. It would be interesting to see if the animals will exhibit similar behaviors when LHox/dyn inputs are stimulated (Thomas et al., 2022). Will the animals show the same reward-seeking behavior as they did during general stimulation of LHox/dyn inputs, or will different behaviors occur depending on the projection-specific subpopulation?
Further research could also investigate the coordinated increase and decrease in firing of the projection-specific VTADA neurons in vivo. Techniques such as fiber photometry could be used to monitor the downstream effects on regions like the BLA, lAcbSh, and mAcbSh during stimulation of LHox/dyn neurons, thereby providing deeper insight into the temporal and spatial dynamics of these effects.
The findings presented in the study conducted by Mohammadkhani et al. (2024) provide insight into the mechanisms by which orexin and dynorphin interact with downstream targets to regulate the circuits underlying reward-related behaviors. Orexin's role in dopamine signaling and its links to motivational and reward circuits underscore its broader importance in understanding how the brain orchestrates reward-seeking behavior in different contexts. Given the established role of KOR receptor dysregulation in alcohol and drug abuse, this insight into the effects of the dynorphin circuitry may provide new potential therapeutic targets for disorders involving dopamine dysregulation, such as addiction and mood disorders.⇓
Summary of findings by Mohammadkhani et al. (2024). Optogenetic activation of LHox/dyn neurons modulates reward pathways through OxR1- and KOR-dependent regulation of VTADA neurons in a projection-specific manner (created in BioRender.com).
Footnotes
I thank my journal club mentor Dr. Julia C. Lemos (Assistant Professor, Department of Neuroscience, University of Minnesota) and The Journal of Neuroscience editor Teresa Esch for their helpful comments in the writing process of this journal club article.
The authors declare no competing financial interests.
Review of Mohammadkhani et al.
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- Correspondence should be addressed to Nathalie Krauth at nathalie.krauth{at}sund.ku.dk.