Hippocampal Gαq/₁₁ but not Gαo-coupled receptors are altered in aging

Functional approaches to the study of G-protein-coupled receptors in postmortem brain tissue: [35S]GTPγS binding assays combined with immunoprecipitation

G-protein-coupled receptors (GPCRs) have an enormous biochemical importance as they bind to diverse extracellular ligands and regulate a variety of physiological and pathological responses. G-protein activation measures the functional consequence of receptor occupancy at one of the earliest receptor-mediated events. Receptor coupling to G-proteins promotes the GDP/GTP exchange on Gα subunits. Thus, modulation of the binding of the poorly hydrolysable GTP analog [³⁵S]GTPγS to the Gα-protein subunit can be used as a functional approach to quantify GPCR interaction with agonist, antagonist or inverse agonist drugs. In order to determine receptor-mediated selective activation of the different Gα-proteins, [³⁵S]GTPγS binding assays combined with immunodetection by specific antibodies have been developed and applied to physiological and pathological brain conditions. Currently, immunoprecipitation with magnetic beads and scintillation proximity assays are the most habitual techniques for this purpose. The present review summarizes the different procedures, advantages and limitations of the [³⁵S]GTPγS binding assays combined with selective Gα-protein sequestration methods. Experience of functional coupling of several GPCRs to different Gα-proteins and recommendations for optimal performance in brain membranes are described. One of the biggest opportunities opened by these techniques is that they enable evaluation of biased agonism in the native tissue, which results in high interest in drug discovery. The available results derived from application of these functional methodologies to study GPCR dysfunctions in neuro-psychiatric disorders are also described. In conclusion, [³⁵S]GTPγS binding combined with antibody-mediated immunodetection represents an useful method to separately evaluate the functional activity of drugs acting on GPCRs over each Gα-protein subtype.

Luteinizing hormone and the aging brain

Fluctuations in luteinizing hormone (LH) release contribute to the development and maintenance of the reproductive system and become dysregulated during aging. Of note, increasing evidence supports extra-gonadal roles for LH within the CNS, particularly as it relates to cognition and plasticity in aging and age-related degenerative diseases such as Alzheimer's disease (AD). However, despite increasing evidence that supports a link between this hormone and CNS function, the mechanisms underlying LH action within the brain and how they influence cognition and plasticity during the lifespan is poorly understood and, in fact, often in conflict. This chapter aims to provide an up-to-date review of the literature addressing the role of LH signaling in the context of CNS aging and disease and put forward a unifying hypothesis that may explain currently conflicting theories regarding the role of LHCGR signaling in CNS function and dysfunction in aging and disease.

M1 muscarinic acetylcholine receptors: A therapeutic strategy for symptomatic and disease-modifying effects in Alzheimer's disease?

The M1 muscarinic acetylcholine receptor (mAChR) plays a crucial role in learning and memory processes and has long been identified as a promising therapeutic target for the improvement of cognitive decline in Alzheimer's disease (AD). As such, clinical trials with xanomeline, a mAChR orthosteric agonist, showed an improvement in cognitive and behavioral symptoms associated with AD. Despite this, the clinical utility of xanomeline was hampered by a lack of M1 receptor selectivity and adverse cholinergic responses attributed to activation of peripheral M2 and M3 mAChRs. More recently, efforts have focused on developing more selective M1 compounds via targeting the less-conserved allosteric binding pockets. As such, positive allosteric modulators (PAMs) have emerged as an exciting strategy to achieve exquisite selectivity for the M1 mAChR in order to deliver improvements in cognitive function in AD. Furthermore, over recent years it has become increasingly apparent that M1 therapeutics may also offer disease-modifying effects in AD, due to the modulation of pathogenic amyloid processing. This article will review the progress made in the development of M1 selective ligands for the treatment of cognitive decline in AD, and will discuss the current evidence that selective targeting of the M1 mAChR could also have the potential to modify AD progression.

Metabotropic Glutamate Receptors at the Aged Mossy Fiber - CA3 Synapse of the Hippocampus

Metabotropic glutamate receptors (mGluRs) are a group of G-protein-coupled receptors that exert a broad array of modulatory actions at excitatory synapses of the central nervous system. In the hippocampus, the selective activation of the different mGluRs modulates the intrinsic excitability, the strength of synaptic transmission, and induces multiple forms of long-term plasticity. Despite the relevance of mGluRs in the normal function of the hippocampus, we know very little about the changes that mGluRs functionality undergoes during the non-pathological aging. Here, we review data concerning the physiological actions of mGluRs, with particular emphasis on hippocampal area CA3. Later, we examine changes in the expression and functionality of mGluRs during the aging process. We complement this review with original data showing an array of electrophysiological modifications observed in the synaptic transmission and intrinsic excitability of aged CA3 pyramidal cells in response to the pharmacological stimulation of the different mGluRs.

Age and Ketogenic Diet Have Dissociable Effects on Synapse-Related Gene Expression Between Hippocampal Subregions

As the number of individuals living beyond the age of 65 is rapidly increasing, so is the need to develop strategies to combat the age-related cognitive decline that may threaten independent living. Although the link between altered neuronal signaling and age-related cognitive impairments is not completely understood, it is evident that declining cognitive abilities are at least partially due to synaptic dysfunction. Aging is accompanied by well-documented changes in both excitatory and inhibitory synaptic signaling across species. Age-related synaptic alterations are not uniform across the brain, however, with different regions showing unique patterns of vulnerability in advanced age. In the hippocampus, increased activity within the CA3 subregion has been observed across species, and this can be reversed with anti-epileptic medication. In contrast to CA3, the dentate gyrus shows reduced activity with age and declining metabolic activity. Ketogenic diets have been shown to decrease seizure incidence and severity in epilepsy, improve metabolic function in diabetes type II, and improve cognitive function in aged rats. This link between neuronal activity and metabolism suggests that metabolic interventions may be able to ameliorate synaptic signaling deficits accompanying advanced age. We therefore investigated the ability of a dietary regimen capable of inducing nutritional ketosis and improving cognition to alter synapse-related gene expression across the dentate gyrus, CA3 and CA1 subregions of the hippocampus. Following 12 weeks of a ketogenic or calorie-matched standard diet, RTq-PCR was used to quantify expression levels of excitatory and inhibitory synaptic signaling genes within CA1, CA3 and dentate gyrus. While there were no age or diet-related changes in CA1 gene expression, expression levels were significantly altered within CA3 by age and within the dentate gyrus by diet for several genes involved in presynaptic glutamate regulation and postsynaptic excitation and plasticity. These data demonstrate subregion-specific alterations in synaptic signaling with age and the potential for a ketogenic diet to alter these processes in dissociable ways across different brain structures that are uniquely vulnerable in older animals.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

CNS luteinizing hormone receptor activation rescues ovariectomy-related loss of spatial memory and neuronal plasticity

Ovariectomy (OVX), a menopause model, leads to cognition and neuronal plasticity deficits that are rescued by estrogen administration or downregulation of pituitary luteinizing hormone (LH). LH is present in the brain. However, whether LH levels differ across brain regions, change across reproductive stages, or whether brain-specific LHR signaling play a role in OVX-related cognitive and neuroplasticity losses is completely unknown. To address this, we measured brain LH in cycling and OVX C57Bl/6 across brain regions and determined whether OVX-related functional and plasticity deficits could be rescued by intracerebroventricular administration of the LHR agonist (hCG). Here, we show that while pituitary LH is increased in OVX, brain LH is decreased, primarily in spatial memory and navigation areas. Furthermore, intracerebroventricular hCG delivery after OVX rescued dendritic spine density and spatial memory. In vitro, we show that hCG increased neurite outgrowth in primary hippocampal neurons in a receptor-specific manner. Taken together, our data suggest that loss of brain LH signaling is involved in cognitive and plasticity losses associated with OVX and loss of ovarian hormones.

Age-Related Declines in Prefrontal Cortical Expression of Metabotropic Glutamate Receptors that Support Working Memory

Glutamate signaling is essential for the persistent neural activity in prefrontal cortex (PFC) that enables working memory. Metabotropic glutamate receptors (mGluRs) are a diverse class of proteins that modulate excitatory neurotransmission via both presynaptic regulation of extracellular glutamate levels and postsynaptic modulation of ion channels on dendritic spines. This receptor class is of significant therapeutic interest for treatment of cognitive disorders associated with glutamate dysregulation. Working memory impairment and cortical hypoexcitability are both associated with advanced aging. Whether aging modifies PFC mGluR expression, and the extent to which any such alterations are regionally or subtype specific, however, is unknown. Moreover, it is unclear whether specific mGluRs in PFC are critical for working memory, and thus, whether altered mGluR expression in aging or disease is sufficient to play a causative role in working memory decline. Experiments in the current study first evaluated the effects of age on medial PFC (mPFC) mGluR expression using biochemical and molecular approaches in rats. Of the eight mGluRs examined, only mGluR5, mGluR3, and mGluR4 were significantly reduced in the aged PFC. The reductions in mGluR3 and mGluR5 (but not mGluR4) were observed in both mRNA and protein and were selectively localized to the prelimbic (PrL), but not infralimbic (IL), subregion of mPFC. Finally, pharmacological blockade of mGluR5 or mGluR2/3 using selective antagonists directed to PrL significantly impaired working memory without influencing non-mnemonic aspects of task performance. Together, these data implicate attenuated expression of PFC mGluR5 and mGluR3 in the impaired working memory associated with advanced ages.

Molecular aspects of age-related cognitive decline: the role of GABA signaling

Alterations in inhibitory interneurons contribute to cognitive deficits associated with several psychiatric and neurological diseases. Phasic and tonic inhibition imparted by γ-aminobutyric acid (GABA) receptors regulates neural activity and helps to establish the appropriate network dynamics in cortical circuits that support normal cognition. This review highlights basic science demonstrating that inhibitory signaling is altered in aging, and discusses the impact of age-related shifts in inhibition on different forms of memory function, including hippocampus-dependent spatial reference memory and prefrontal cortex (PFC)-dependent working memory. The clinical appropriateness and tractability of select therapeutic candidates for cognitive aging that target receptors mediating inhibition are also discussed.

Deletion of Gαq in the telencephalon alters specific neurobehavioral outcomes

G(αq) -coupled receptors are ubiquitously expressed throughout the brain and body, and it has been shown that these receptors and associated signaling cascades are involved in a number of functional outputs, including motor function and learning and memory. Genetic alterations to G(αq) have been implicated in neurodevelopmental disorders such as Sturge-Weber syndrome. Some of these associated disease outcomes have been modeled in laboratory animals, but as G(αq) is expressed in all cell types, it is difficult to differentiate the underlying circuitry or causative neuronal population. To begin to address neuronal cell type diversity in G(αq) function, we utilized a conditional knockout mouse whereby G(αq) was eliminated from telencephalic glutamatergic neurons. Unlike the global G(αq) knockout mouse, we found that these conditional knockout mice were not physically different from control mice, nor did they exhibit any gross motor abnormalities. However, similarly to the constitutive knockout animal, G(αq) conditional knockout mice demonstrated apparent deficits in spatial working memory. Loss of G(αq) from glutamatergic neurons also produced enhanced sensitivity to cocaine-induced locomotion, suggesting that cortical G(αq) signaling may limit behavioral responses to psychostimulants. Screening for a variety of markers of forebrain neuronal architecture revealed no obvious differences in the conditional knockouts, suggesting that the loss of G(αq) in telencephalic excitatory neurons does not result in major alterations in brain structure or neuronal differentiation. Taken together, our results define specific modulation of spatial working memory and psychostimulant responses through disruptions in G(αq) signaling within cerebral cortical glutamatergic neurons.

Spatial reference memory in normal aging Fischer 344 × Brown Norway F1 hybrid rats

Fischer 344 × Brown Norway F1 (F344 × BN-F1) hybrid rats express greater longevity with improved health relative to aging rodents of other strains; however, few behavioral reports have thoroughly evaluated cognition across the F344 × BN-F1 lifespan. Consequently, this study evaluated spatial reference memory in F344 × BN-F1 rats at 6, 18, 24, or 28 months of age in the Morris water maze. Reference memory decrements were observed between 6 and 18 months and 18 and 24 months. At 28 months, spatial learning was not worse than 24 months, but swim speed was significantly slower. Reliable individual differences revealed that ∼50% of 24- to 28-month-old rats performed similarly to 6 months, whereas others were spatial learning impaired. Aged rats were impaired at learning within daily training sessions but not impaired at retaining information between days of training. Aged rats were also slower to learn to escape onto the platform, regardless of strategy. In summary, these data clarify the trajectory of cognitive decline in aging F344 × BN-F1 rats and elucidate relevant behavioral parameters.

Interaction of DHPG-LTD and synaptic-LTD at senescent CA3-CA1 hippocampal synapses

The susceptibility, but not the magnitude, of long-term depression (LTD) induced by hippocampal CA3-CA1 synaptic activity (synaptic-LTD) increases with advanced age. In contrast, the magnitude of LTD induced by pharmacological activation of CA3-CA1 group I metabotropic glutamate receptors (mGluRs) increases during aging. This study examined the signaling pathways involved in induction of LTD and the interaction between paired-pulse low frequency stimulation-induced synaptic-LTD and group I mGluR selective agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG, 100 µM)-induced DHPG-LTD in hippocampal slices obtained from aged (22-24 months) male Fischer 344 rats. Prior induction of synaptic-LTD did not affect induction of DHPG-LTD; however, prior induction of the DHPG-LTD occluded synaptic-LTD suggesting that expression of DHPG-LTD may incorporate synaptic-LTD mechanisms. Application of individual antagonist for the group I mGluR (AIDA), the N-methyl-d-aspartate receptor (NMDAR) (AP-5), or L-type voltage-dependent Ca(2+) channel (VDCC) (nifedipine) failed to block synaptic-LTD and any two antagonists severely impaired synaptic-LTD induction, indicating that activation of any two mechanisms is sufficient to induce synaptic-LTD in aged animals. For DHPG-LTD, AIDA blocked DHPG-LTD and individually applied NMDAR or VDCC attenuated but did not block DHPG-LTD, indicating that the magnitude of DHPG-LTD depends on all three mechanisms.