The role of neurotransmitter receptors in antipsychotic medication efficacy for Alzheimer’s-related psychosis

AD, sometimes referred to as dementia, is a debilitating neurological condition that mainly affects the elderly [1]. It causes significant cognitive impairment and steadily erodes cognitive abilities [2]. While memory loss is the most well-known indication of AD, it is not the sole manifestation [3]. Psychosis in AD covers a variety of behavioral problems, for example hallucinations, delusions, agitation, and is a significant burden on both patients and caregivers [4]. According to estimates, 50 percent or more of Alzheimer’s victim may suffer from a psychotic episode while they are ill, making it a common and unavoidable feature of the disease’s clinical panorama [5]. The significant effects of ARP include greater percentage of hospitalization, arrangement with healthcare facilities and faster cognitive decline in addition to an increased load and suffering for caregivers [6]. Despite their drawbacks and safety concerns, antipsychotic drugs are often used off-label in this situation and prescribing practices have become cautious due to the possibility of adverse effects, include a higher chance of stroke and death [7, 8]. Although these drugs can provide some respite from the upsetting symptoms of psychosis, there is considerable debate regarding their effectiveness in treating Alzheimer’s patients [9]. Optimizing the management of ARP requires a thorough understanding of the neurological foundations of the disorder as well as the processes via which antipsychotic medicines work [10]. Considerable advancements have been achieved in comprehending complex network of neurotransmitter systems, including as the dopaminergic, glutamatergic, cholinergic, serotonergic, and gamma-aminobutyric acid (GABAergic) pathways, that play a role in psychosis by the discipline of neuropsychopharmacology. These systems are important in determining how ARP develops and are the targets of several antipsychotic drugs [11]. Cognitive neuroscience research has long been drawn to the intricate relationships between cognitive and affective processes and their underlying neural substrates. Finding the neuronal basis of these activities requires studying functional connection throughout the brain, which is the coordinating and synchronization of neural activity among different brain areas [12].

The amygdala, prefrontal cortex (PFC), and hippocampal regions are important for memory formation and learning. The amygdala, which is acknowledged for its importance in the processing of emotions, engages with the hypothalamus and other downstream structures to influence the generation of emotionally charged responses. Specific divisions and nuclei within the amygdala are among the inhibitory processes that impact memory modulation. The hippocampal area directly projects the basolateral amygdala and the infralimbic cortex in the PFC, which is important for spatial navigation and contextual memory and the fact that different hippocampal subregions have been connected to a variety of human behavioral characteristics highlights the varied functions that these regions play in cognitive processes [13]. In addition to exercising top-down management of emotional responses, the PFC is essential to cognitive functions related to the extinction of fear memories. The PFC and amygdala must cooperate within the framework of working neural pathways for adaptive fear management to occur [12], the degree of delusions was found to be negatively correlated with the right hippocampus volume and middle frontal gyrus volume in bilateral frontal, parietal, and striatal areas. Additional research indicates that in people with AD and delusions, the left inferior parietal lobule shows less functional connection than the frontal areas [14].

There has been a little but positive growth in the quantity of chemicals undergoing trials in latest years. Citalopram and pimavanserin were identified as the two utmost auspicious possible treatments in the most recent Delphi consensus [57].

Brexpiprazole is a new third-generation antipsychotic that acts as an antagonist at noradrenaline α1B/α2C receptors, a partial agonist at the serotonin 5-hydroxytryptamine receptor (5-HT) 1A receptor (R) (5-HT1AR), a partial agonist at the dopamine D2 receptor, and an antagonist at the serotonin 5-HT2A/5-HT2B receptor but unlike other atypical antipsychotics this intermediate level aims to stay below the threshold where good sensations brought on by high dopamine development grow, without going too low, preferably to prevent negative consequences like extrapyramidal symptoms (EPS) [70]. The FDA has just approved brexpiprazole for the treatment of schizophrenia and major depressive disorder as an adjuvant to antidepressant medication where akathisia and weight gain were the most often reported side effects associated with therapy.

Brexpiprazole is administered orally once [71] and it is interesting to note that brexpiprazole 2 mg/day has the potential to be an effective, safe, and well-tolerated medication for agitation in individuals with AD, according to two separate phase III clinical trials that included a total of almost 700 adult participants with AD in 2019[72]. Brexpiprazole is hypothesized to have depressive effects due to its significant affinity for inhibiting 5-HT2C-R and 5-HT7-R, blocking 5-HT2C-R probably raises frontocortical norepinephrine and dopamine in a manner similar to that of the antidepressant mirtazapine. 5-HT7-R antagonistic effects, such as those seen in the antidepressant effects of vortioxetine and lurasidone, probably enhance cognition and circadian function. It also exhibits a substantial affinity for 5-HT2A-R Antagonism, which reduces extrapyramidal symptoms (EPS) by enabling higher concentrations of DA to work in the nigrostriatal system [73].

A new FDA-approved drug called lumateperone is used to treat schizophrenia based on a mechanistic approach [74]. It is a novel medication that selectively and simultaneously modifies glutamate, dopamine, and serotonin. Lumateperone is also in phase III clinical for the treatment of bipolar depression and agitation linked to dementia, especially AD, it is also in the approval process for schizophrenia [75]. This medication is effective in treating both positive and negative in addition to cognitive decline, in schizophrenia, with a 60-fold greater affinity for 5-HT2A receptors than D2 receptors, Lumateperone is a high-affinity 5-HT2A receptor antagonist. When the dosage is increased, it acts as a mesolimbic and mesocortical selective post-synaptic antagonist and presynaptic partial agonist at D2 receptors. Furthermore, in a mesolimbic-specific way, lumateperone phosphorylates glutamatergic N-methyl-D-aspartate glutamate receptors (NMDA) GluN2B receptors and inhibits the serotonin transporter [76]. It is considerably less likely than many other atypical antipsychotic medications to result in side effects such as dry mouth, constipation, weariness, insomnia, and drowsiness when sleeping, in addition, the majority-mild side effects of Lumateperone were drowsiness, somnolence, weariness, and constipation which occurred at a clinically significant rate [77]. The dose-dependent clinical impact profile of lumateperone suggests that the medication may have a broader therapeutic range, when used in small dosages, lumateperone has sedative and anti-aggressive properties, and it may be able to accomplish these effects at low doses because it lacks D2 receptor binding and has substantial selective 5HT2A antagonist activity at low concentrations. Raising the doses is necessary to increase the number of D2 receptors, their occupancy, and the affinity and 5HT2A receptor occupancy [78]. Oral administration of the medication in the form of capsules is recommended at a dose of 42 mg. For optimal effectiveness, it is best taken after a full night sleep, and a daily dosage of 120 mg has not been shown to have any statistically significant impact on the result [75]. When used orally, it takes 13 to 21 h to be absorbed and the T-max starts to show results three to four hours after the regimen starts [78]. Lumateperone and its metabolites are hardly ever eliminated by the kidneys or urinary system, some lumateperone users have a range of side effects, from minor to serious [76]. The cytochrome (CY) P450-3A4 isoenzyme metabolizes lumateperone, and it interacts with both its stimulators and inhibitors, if a patient is taking medicine that inhibits or activates CYP3A4, they should not use lumateperone. Even though this medication also has a sedative effect, it probably would not work well with other sedatives like alcohol [79] (Figs. 9, 10).

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Managing psychosis in AD is a difficult task, and using anti-dementia medications in this situation calls for cautious thought [80]. NMDA receptor antagonists memantine, with a maximum oral dosage of 20 mg per day and potential side effects involve headache, nausea, diarrhea, or dizziness [81] and cholinesterase inhibitors (rivastigmine and galantamine) are two anti-dementia medications that are mostly used to alleviate the cognitive symptoms of AD [82]. In which the therapeutic dosage of Rivastigmine delivered by transdermal patch ranges from 4.6 mg to 13.3 mg per 24 h in clinical practice, and the suggested initial dosage of rivastigmine in the form of liquid or pill forms is 1.5 mg twice daily, displaying side symptoms like nausea, vomiting, weight loss, tremors, diarrhea, and disorientation [83]. There are two forms of galantamine that are available: extended-release capsules and tablets with instant release. The recommended dosage for each formulation is 5 mg or 10 mg, to be taken once daily for a duration of 24 weeks [83]. The drug may cause gastrointestinal side effects such as nausea, vomiting, diarrhea, and loss of appetite when starting treatment, and these side effects may worsen when the dosage is increased. Additionally, the drug's cholinergic effects may cause urinary retention and sinus bradycardia [84]. Although cholinesterase inhibitors have a well-established track record in treating cognitive symptoms in AD, it is unknown whether or not these drugs can be therapeutically helpful in treating AD psychosis, and no randomized controlled trials have been done in this regard. However, cholinesterase inhibitor use was associated in the AD subgroup with a decreased chance of starting antipsychotics, according to an analysis from the Swedish Dementia Registry [76]. There is no direct RCT evidence demonstrating a specific advantage in the treatment of clinically severe AD psychosis, despite the fact that donepezil, with highest daily dosage of 23 mg once daily, contributing to diarrhea, nausea, vomiting, vagotonic effects (bradycardia, heart block, syncope), tremor, insomnia, urinary incontinence, and seizure [83] and memantine have been shown to diminish the onset of psychosis in AD [5]. These observations are compatible with the cholinergic pathways reported in AD psychosis, even if this observation does not support a treatment benefit. They may also point to a potential substitute for onerous psychiatric medicines in the management of non-psychotic seizures. Once more, prospective trials are necessary [85] (Figs. 11, 12, 13 and 14).

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Scholars are starting to utilize the correlation between insufficient vitamin D and neurodegenerative illnesses to investigate the potential involvement of vitamin D deficiency in AD psychosis. People with AD or mild cognitive impairment (MCI) who did not experience psychosis were found to utilize vitamin D more frequently, and this use was also associated with a delayed onset of psychosis.[86]. Six of the nine case–control studies found lower serum concentrations of 25-hydroxyvitamin D, a metabolite of vitamin D3, suggesting substantial differences between the AD cases and control groups. As a result, insufficient vitamin D is thought to be a risk factor for AD. On the other hand, vitamin D protective impact against AD + P was recently found, and its mechanism may offer a novel approach to treating and preventing AD + P. Vitamin D can have its protective impact by reducing the oxidative and nitrosative damage caused by high levels of nitric oxide (NO) and Inducible nitric oxide synthase (iNOS) in nerve cells [85]. Additionally, data point to a possible connection between vitamin D pathway abnormalities and amyloid pathology, which helps to explain vitamin D protective function in AD [87].

It is critical to perform a comprehensive risk assessment taking into account health history, pharmaceutical history, and cardiovascular risk factors while utilizing antipsychotic drugs to manage psychosis. Baseline evaluations and routine monitoring of mental symptoms, cognitive performance, vital signs, and metabolic indicators should be part of monitoring regimens. Monitoring anthropometric measurements and body compositions (weight, body mass index (BMI), waist circumference (WC), blood pressure (BP), fasting plasma glucose (HbA1c), and lipid profile are among the factors that are advised to be observed [88].

Starting with low dosages, choosing drugs with manageable side effects, and taking into account complementary therapies are all examples of mitigation techniques [89].

Over the past ten years, there have been notable developments in the clinical assessment and management of psychosis and other neuropsychiatric symptoms in dementia. One of these turning points was when antipsychotic prescriptions decreased as a result of the high danger connected with these medications [90]. Aripiprazole, risperidone, amisulpride, and escitalopram are examples of drugs that have been improved for safer precision-based treatment. Other measures that have led to the development of safer, more effective treatments include the development of a clinical trial program for pimavanserin and a strong focus on non-pharmacological interventions like DICE (describe, investigate, create and evaluate) or WHELD (Well-being and Health for People with Dementia) [40]. Studying dynamic changes in receptor activity might shed light on how a treatment is working through neuroimaging techniques like fMRI (functional magnetic resonance imaging) and PET [91]. The best time to start treatment may be determined by longitudinal research monitoring the development of neurotransmitter receptor alterations during Alzheimer's-related psychosis [92]. The creation of The Alzheimer’s Association International Society to Advance Alzheimer's Research and Treatment (ISTAART) psychosis and MBI criteria, as well as the advancement of the IPA (International Psychogeriatric Association) psychosis criteria, should support this endeavor. In fact, these objectives were specifically considered when developing the ISTAART psychosis criteria. These criteria, which frame psychotic symptoms, place late-life emergent psychosis on a spectrum and consider cognitive stages, symptom modality, natural history of symptoms, and biomarkers associated with AD [93]. Frameworks for cross-sectional and longitudinal research could be used to improve our understanding of the clinical, cognitive, and neurobiological aspects of AD psychosis, as well as to support the creation and application of non-pharmacological therapies and medication discovery and repurposing [6]. Also, multiple connections between processes affecting synaptic function and psychosis in AD have been found using functional genomic and post-mortem data findings [49]. The potential of therapies that target neuroinflammation, such as immunomodulators or anti-inflammatory medications, to lessen the symptoms of psychosis in AD is currently being investigated [94]. A non-invasive treatment method called transcranial magnetic stimulation (TMS) stimulates underlying nerve cells by altering the magnetic field. Research is being done on TMS as a potential treatment for dementia and other neurological conditions [95]. The cerebrospinal fluid (CSF) biomarkers play a significant role in the diagnostic process when it comes to identifying AD and other dementia in older individuals who exhibit psychotic symptoms [96]. The muscarinic acetylcholine receptor (mAChR) agonist xanomeline has been shown to reduce psychotic symptoms and enhance cognition in Alzheimer's disease patients and can be targeted as a new approach to potentially treat psychotic disorders [97]. Early postnatal GABAergic neurons experience NMDAR hypofunction, and there is considerable variation in the processes underlying this phenomenon. It would be beneficial to reduce psychotic symptoms if future study could identify the upstream and downstream mechanisms of NMDAR hypofunction [98]. In conclusion, prioritizing patient-centered objectives will result in a thorough understanding of the treatments effects and encompassing quality of life [99].

This study focuses on the complex interactions that exist between neurotransmitter receptors and the effectiveness of antipsychotic medications in treating psychosis associated with AD. The intricate nature of psychosis in AD is highlighted by the neuropathological alterations in neurotransmitter systems, specifically in glutamate, serotonin, and dopamine, which calls for focused therapeutic interventions. The study examined the pharmacodynamic variations across antipsychotic drugs in connection to their receptor affinity, highlighting the necessity of a comprehensive comprehension of their influence on clinical results. Treatment difficulties, such as the possibility of negative cognitive and motor consequences, emphasize the careful balancing act needed when administering antipsychotics to individuals with AD. Although novel tactics targeting receptors show promise in reducing side effects, it is imperative to address current information gaps and conduct customized clinical trials for this population. The analysis concludes that a more tailored approach is required to optimize the treatment of AD-related psychosis and reduce side effects in this vulnerable population. These discoveries should lead to more focused and customized treatment plans as research continues, which will eventually improve the lives of those who have Alzheimer's and the people who care for them.