Sex-dependent effects of monomeric α-synuclein on calcium and cell death of lateral hypothalamic mouse neurons are altered by orexin.

Parkinson's Disease (PD) patients experience sleeping disorders in addition to the disease-defining symptomology of movement dysfunctions. The prevalence of PD is sex-based and presence of sleeping disorders in PD also shows sex bias with a stronger phenotype in males. In addition to loss of dopamine-containing neurons in the striatum, arousal-related, orexin-containing neurons in the lateral hypothalamus (LH) are lost in PD, which could contribute to state-related disorders. As orexin has been shown to be involved in sleeping disorders and to have neuroprotective effects, we asked whether orexin could protect sleep-related LH neurons from damage putatively from the protein α-synuclein (α-syn), which is found at high levels in the PD brain and that we have shown is associated with putatively excitotoxic rises in intracellular calcium in brainstem sleep-controlling nuclei, especially in males. Accordingly, we monitored intracellular calcium transients induced by α-syn and whether concurrent exposure to orexin affected those transients in LH cells of the mouse brain slice using calcium imaging. Further, we used an assay of cell death to determine whether LH cell viability was influenced when α-syn and orexin were co-applied when compared to exposure to α-syn alone. We found that excitatory calcium events induced by α-syn were reduced in amplitude and frequency when orexin was co-applied, and when data were evaluated by sex, this effect was found to be greater in females. In addition, α-syn exposure was associated with cell death that was higher in males, and interestingly, reduced cell death was noted when orexin was present, which did not show a sex bias. We interpret our findings to indicate that orexin is protective to α-syn-mediated damage to hypothalamic neurons, and the actions of orexin on α-syn-induced cellular effects differ between sexes, which could underlie sex-based differences in sleeping disorders in PD.

Chemogenetic inhibition of the lateral hypothalamus effectively reduces food intake in rats in a translational proof-of-concept study.

Despite the therapeutic potential of chemogenetics, the method lacks comprehensive preclinical validation, hindering its progression to human clinical trials. We aimed to validate a robust but simple in vivo efficacy assay in rats which could support chemogenetic drug discovery by providing a quick, simple and reliable animal model. Key methodological parameters such as adeno-associated virus (AAV) serotype, actuator drug, dose, and application routes were investigated by measuring the food-intake-reducing effect of chemogenetic inhibition of the lateral hypothalamus (LH) by hM4D(Gi) designer receptor stimulation. Subcutaneous deschloroclozapine in rats transfected with AAV9 resulted in a substantial reduction of food-intake, comparable to the efficacy of exenatide. We estimated that the effect of deschloroclozapine lasts 1-3 h post-administration. AAV5, oral administration of deschloroclozapine, and clozapine-N-oxide were also effective but with slightly less potency. The strongest effect on food-intake occurred within the first 30 min after re-feeding, suggesting this as the optimal experimental endpoint. This study demonstrates that general chemogenetic silencing of the LH can be utilized as an optimal, fast and reliable in vivo experimental model for conducting preclinical proof-of-concept studies in order to validate the in vivo effectiveness of novel chemogenetic treatments. We also hypothesize based on our results that universal LH silencing with existing and human translatable genetic neuroengineering techniques might be a viable strategy to affect food intake and influence obesity.

The rostromedial tegmental nucleus gates fat overconsumption through ventral tegmental area output in male rats.

Excessive consumption of palatable foods that are rich in fats and sugars has contributed to the increasing prevalence of obesity worldwide. Similar to addictive drugs, such foods activate the brain's reward circuit, involving mesolimbic dopaminergic projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) and the prefrontal cortex. Neuroadaptations occurring in this circuit are hypothesized to contribute to uncontrolled consumption of such foods, a common feature of most of eating disorders and obesity. The rostromedial tegmental nucleus (RMTg), also named tail of the VTA (tVTA), is an inhibitory structure projecting to the VTA and the lateral hypothalamus (LH), two key brain regions in food intake regulation. Prior research has demonstrated that the RMTg responds to addictive drugs and influences their impact on mesolimbic activity and reward-related behaviors. However, the role of the RMTg in food intake regulation remains largely unexplored. The present study aimed to investigate the role of the RMTg and its projections to the VTA and the LH in regulating food intake in rats. To do so, we examined eating patterns of rats with either bilateral excitotoxic lesions of the RMTg or specific lesions of RMTg-VTA and RMTg-LH pathways. Rats were exposed to a 6-week 'free choice high-fat and high-sugar' diet, followed by a 4-week palatable food forced abstinence and a 24 h re-access period. Our results indicate that an RMTg-VTA pathway lesion increases fat consumption following 6 weeks of diet and at time of re-access. The RMTg-LH pathway lesion produces a milder effect with a decrease in global calorie intake. These findings suggest that the RMTg influences palatable food consumption and relapse through its projections to the VTA.

Tesofensine, a novel antiobesity drug, silences GABAergic hypothalamic neurons.

Obesity is a major global health epidemic that has adverse effects on both the people affected as well as the cost to society. Several anti-obesity drugs that target GLP-1 receptors have recently come to the market. Here, we describe the effects of tesofensine, a novel anti-obesity drug that acts as a triple monoamine neurotransmitter reuptake inhibitor. Using various techniques, we investigated its effects on weight loss and underlying neuronal mechanisms in mice and rats. These include behavioral tasks, DeepLabCut videotaped analysis, electrophysiological ensemble recordings, optogenetic activation, and chemogenetic silencing of GABAergic neurons in the Lateral Hypothalamus (LH). We found that tesofensine induces a greater weight loss in obese rats than lean rats, while differentially modulating the neuronal ensembles and population activity in LH. In Vgat-ChR2 and Vgat-IRES-cre transgenic mice, we found for the first time that tesofensine inhibited a subset of LH GABAergic neurons, reducing their ability to promote feeding behavior, and chemogenetically silencing them enhanced tesofensine's food-suppressing effects. Unlike phentermine, a dopaminergic appetite suppressant, tesofensine causes few, if any, head-weaving stereotypy at therapeutic doses. Most importantly, we found that tesofensine prolonged the weight loss induced by 5-HTP, a serotonin precursor, and blocked the body weight rebound that often occurs after weight loss. Behavioral studies on rats with the tastant sucrose indicated that tesofensine's appetite suppressant effects are independent of taste aversion and do not directly affect the perception of sweetness or palatability of sucrose. In summary, our data provide new insights into the effects of tesofensine on weight loss and the underlying neuronal mechanisms, suggesting that tesofensine may be an effective treatment for obesity and that it may be a valuable adjunct to other appetite suppressants to prevent body weight rebound.

Effect of leptin on nitrergic neurons in the lateral hypothalamic area and the supraoptic nucleus of rats.

The adipocyte-derived hormone, leptin, plays a key role in the maintenance of energy homeostasis. Leptin binds to the long form of its receptor, which is predominantly expressed in various hypothalamic regions, including the lateral hypothalamic area (LH) and supraoptic nucleus (SO). Several studies have suggested that leptin directly activates neuronal nitric oxide synthase, leading to increased nitric oxide production. We used histochemistry for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) as a marker for nitric oxide synthase activity and assessed the effect of leptin on nitrergic neurons in the LH and SO of rats. We found that intraperitoneal administration of leptin led to a significant increase in the number of NADPH-d-positive neurons in the LH and SO. In addition, the intensity (optical density) of NADPH-d staining in LH and SO neurons was significantly elevated in rats that received leptin compared with saline-treated rats. These findings suggest that nitrergic neurons in the LH and SO may be implicated in mediating the central effects of leptin.

Chemogenetic activation of histamine neurons promotes retrieval of apparently lost memories.

Memory retrieval can become difficult over time, but it is important to note that memories that appear to be forgotten might still be stored in the brain, as shown by their occasional spontaneous retrieval. Histamine in the central nervous system is a promising target for facilitating the recovery of memory retrieval. Our previous study demonstrated that histamine H3 receptor (H3R) inverse agonists/antagonists, activating histamine synthesis and release, enhance activity in the perirhinal cortex and help in retrieving forgotten long-term object recognition memories. However, it is unclear whether enhancing histaminergic activity alone is enough for the recovery of memory retrieval, considering that H3Rs are also located in other neuron types and affect the release of multiple neurotransmitters. In this study, we employed a chemogenetic method to determine whether specifically activating histamine neurons in the tuberomammillary nucleus facilitates memory retrieval. In the novel object recognition test, control mice did not show a preference for objects based on memory 1 week after training, but chemogenetic activation of histamine neurons before testing improved memory retrieval. This selective activation did not affect the locomotor activity or anxiety-related behavior. Administering an H2R antagonist directly into the perirhinal cortex inhibited the recovery of memory retrieval induced by the activation of histamine neurons. Furthermore, we utilized the Barnes maze test to investigate whether chemogenetic activation of histamine neurons influences the retrieval of forgotten spatial memories. Control mice explored all the holes in the maze equally 1 week after training, whereas mice with chemogenetically activated histamine neurons spent more time around the target hole. These findings indicate that chemogenetic activation of histamine neurons in the tuberomammillary nucleus can promote retrieval of seemingly forgotten object recognition and spatial memories.

Lesion of NPY Receptor-expressing Neurons in Perifornical Lateral Hypothalamus Attenuates Glucoprivic Feeding.

Glucoprivic feeding is one of several counterregulatory responses (CRRs) that facilitates restoration of euglycemia following acute glucose deficit (glucoprivation). Our previous work established that glucoprivic feeding requires ventrolateral medullary (VLM) catecholamine (CA) neurons that coexpress neuropeptide Y (NPY). However, the connections by which VLM CA/NPY neurons trigger increased feeding are uncertain. We have previously shown that glucoprivation, induced by an anti-glycolygic agent 2-deoxy-D-glucose (2DG), activates perifornical lateral hypothalamus (PeFLH) neurons and that expression of NPY in the VLM CA/NPY neurons is required for glucoprivic feeding. We therefore hypothesized that glucoprivic feeding and possibly other CRRs require NPY-sensitive PeFLH neurons. To test this, we used the ribosomal toxin conjugate NPY-saporin (NPY-SAP) to selectively lesion NPY receptor-expressing neurons in the PeFLH of male rats. We found that NPY-SAP destroyed a significant number of PeFLH neurons, including those expressing orexin, but not those expressing melanin-concentrating hormone. The PeFLH NPY-SAP lesions attenuated 2DG-induced feeding but did not affect 2DG-induced increase in locomotor activity, sympathoadrenal hyperglycemia, or corticosterone release. The 2DG-induced feeding response was also significantly attenuated in NPY-SAP-treated female rats. Interestingly, PeFLH NPY-SAP lesioned male rats had reduced body weights and decreased dark cycle feeding, but this effect was not seen in female rats. We conclude that a NPY projection to the PeFLH is necessary for glucoprivic feeding, but not locomotor activity, hyperglycemia, or corticosterone release, in both male and female rats.