Orexin-1 receptor blockade differentially affects spatial and visual discrimination memory facilitation by intracranial self-stimulation

Spatial memory requires hypocretins to elevate medial entorhinal gamma oscillations

The hypocretin (Hcrt) (also known as orexin) neuropeptidic wakefulness-promoting system is implicated in the regulation of spatial memory, but its specific role and mechanisms remain poorly understood. In this study, we revealed the innervation of the medial entorhinal cortex (MEC) by Hcrt neurons in mice. Using the genetically encoded G-protein-coupled receptor activation-based Hcrt sensor, we observed a significant increase in Hcrt levels in the MEC during novel object-place exploration. We identified the function of Hcrt at presynaptic glutamatergic terminals, where it recruits fast-spiking parvalbumin-positive neurons and promotes gamma oscillations. Bidirectional manipulations of Hcrt neurons' projections from the lateral hypothalamus (LHHcrt) to MEC revealed the essential role of this pathway in regulating object-place memory encoding, but not recall, through the modulation of gamma oscillations. Our findings highlight the significance of the LHHcrt-MEC circuitry in supporting spatial memory and reveal a unique neural basis for the hypothalamic regulation of spatial memory.

Hypothalamic orexins as possible therapeutic agents in threat and spatial memory disorders

Orexin-A and orexin-B, neuropeptides produced exclusively in the lateral hypothalamus, have been implicated in various functions, including memory. Their levels are elevated in certain pathological states, such as PTSD, and lowered in other states, e.g., memory deficits. Recent developments have shown the possibilities of using orexins to modulate memory. Their administration can improve the results of test animals in paradigms such as passive avoidance (PA), cued fear conditioning (CFC), and the Morris water maze (MWM), with differences between the orexin used and the route of drug administration. Blocking orexin receptors in different brain structures produces opposing effects of memory impairments in given paradigms. Therefore, influencing the orexinergic balance of the brain becomes a viable way to ameliorate memory deficits, shift PTSD-induced recall of stressful memories to an extinction path, or regulate other memory processes.

Selective orexin 1 receptor antagonist SB-334867 aggravated cognitive dysfunction in 3xTg-AD mice

Cognitive dysfunction is the main clinical manifestation of Alzheimer's disease (AD). Previous research found that elevated orexin level in the cerebrospinal fluid was closely related to the course of AD, and orexin-A treatment could increase amyloid β protein (Aβ) deposition and aggravate spatial memory impairment in APP/PS1 mice. Furthermore, recent research found that dual orexin receptor (OXR) antagonist might affect Aβ level and cognitive dysfunction in AD, but the effects of OX1R or OX2R alone is unreported until now. Considering that OX1R is highly expressed in the hippocampus and plays important roles in learning and memory, the effects of OX1R in AD cognitive dysfunction and its possible mechanism should be investigated. In the present study, selective OX1R antagonist SB-334867 was used to block OX1R. Then, different behavioral tests were performed to observe the effects of OX1R blockade on cognitive function of 3xTg-AD mice exhibited both Aβ and tau pathology, in vivo electrophysiological recording and western blot were used to investigate the potential mechanism. The results showed that chronic OX1R blockade aggravated the impairments of short-term working memory, long-term spatial memory and synaptic plasticity in 9-month-old female 3xTg-AD mice, increased levels of soluble Aβ oligomers and p-tau, and decreased PSD-95 expression in the hippocampus of 3xTg-AD mice. These results indicate that the detrimental effects of SB-334867 on cognitive behaviors in 3xTg-AD mice are closely related to the decrease of PSD-95 and depression of in vivo long-term potentiation (LTP) caused by increased Aβ oligomers and p-tau.

Protocol to assess rewarding brain stimulation as a learning and memory modulating treatment: Comparison between self-administration and experimenter-administration

Intracranial electrical self-stimulation (ICSS) is a useful procedure in animal research. This form of administration ensures that areas of the brain reward system (BRS) are being functionally activated, since the animals must perform an operant response to self-administer an electrical stimulus. Rewarding post-training ICSS of the medial forebrain bundle (MFB), an important system of the BRS, has been shown to consistently improve rats' acquisition and retention in several learning tasks. In the clinical setting, deep brain stimulation (DBS) of different targets is currently being used to palliate the memory impairment that occurs in some neurodegenerative diseases. However, the stimulation of the MFB has only been used to treat emotional alterations, not memory disorders. Since DBS stimulation treatments in humans are exclusively administered by external sources, studies comparing the efficacy of that form of application to a self-administered stimulation are key to the translationality of ICSS. This protocol compares self-administered (ICSS) and experimenter-administered (EAS) stimulation of the MFB on the spatial Morris Water Maze task (MWM). c-Fos immunohistochemistry procedure was carried out to evaluate neural activation after retention. Results show that the stimulation of the MFB improves the MWM task regardless of the form of administration, although some differences in c-Fos expression were found. Present results suggest that MFB-ICSS is a valid animal model to study the effects of MFB electrical stimulation on memory, which could guide clinical applications of DBS. The present protocol is a useful guide for establishing ICSS behavior in rats, which could be used as a learning and memory-modulating treatment.

Electroacupuncture Enhances Neuroplasticity by Regulating the Orexin A-Mediated cAMP/PKA/CREB Signaling Pathway in Senescence-Accelerated Mouse Prone 8 (SAMP8) Mice

Learning and memory disorders and decreased neuroplasticity are the main clinical manifestations of age-induced cognitive dysfunction. Orexin A (OxA) has been reported to show abnormally elevated expression in the cerebrospinal fluid (CSF) of patients with Alzheimer's disease (AD) and to be associated with cognitive impairment. Here, we further assessed whether the excitatory neurotransmitter OxA is involved in neuroplasticity and cognitive function in senescence-accelerated mouse prone 8 (SAMP8) mice. In this study, we investigated the mechanism of OxA by using behavioral tests, CSF microdialysis, immunofluorescence, toluidine blue staining, gene silencing, transmission electron microscopy, and Western blotting. The results showed that 10 Hz electroacupuncture (EA) effectively alleviated learning and memory impairment in 7-month-old SAMP8 mice, reduced OxA levels in the CSF, increased the level of the neurotransmitter glutamate, alleviated pathological damage to hippocampal tissue, improved the synaptic structure, enhanced synaptic transmission, and regulated the expression of cAMP/PKA/CREB signaling pathway-related proteins. These results suggest that EA enhances neuroplasticity in SAMP8 mice by regulating the OxA-mediated cAMP/PKA/CREB signaling pathway, thus improving cognitive function. These findings suggest that EA may be beneficial for the prevention and treatment of age-induced cognitive impairment.

Intranasal insulin and orexins to treat age-related cognitive decline

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.