Effects of aging on signal transmission and transduction systems in the gerbil brain: Morphological and autoradiographic study

Ketogenic diet administration later in life improves memory by modifying the synaptic cortical proteome via the PKA signaling pathway in aging mice

Aging compromises brain function leading to cognitive decline. A cyclic ketogenic diet (KD) improves memory in aged mice after long-term administration; however, short-term effects later in life and the molecular mechanisms that govern such changes remain unclear. Here, we explore the impact of a short-term KD treatment starting at elderly stage on brain function of aged mice. Behavioral testing and long-term potentiation (LTP) recordings reveal that KD improves working memory and hippocampal LTP. Furthermore, the synaptosome proteome of aged mice fed a KD long-term evidence changes predominantly at the presynaptic compartment associated to the protein kinase A (PKA) signaling pathway. These findings were corroborated in vivo by western blot analysis, with high BDNF abundance and PKA substrate phosphorylation. Overall, we show that a KD modifies brain function even when it is administered later in life and recapitulates molecular features of long-term administration, including the PKA signaling pathway, thus promoting synaptic plasticity at advanced age.

Mitochondrial Complex I, a Possible Sensible Site of cAMP Pathway in Aging

In mammals during aging, reactive oxygen species (ROS), produced by the mitochondrial respiratory chain, cause oxidative damage of macromolecules leading to respiratory chain dysfunction, which in turn increases ROS mitochondrial production. Many efforts have been made to understand the role of oxidative stress in aging and age-related diseases. The complex I of the mitochondrial respiratory chain is the major source of ROS production and its dysfunctions have been associated with several forms of neurodegeneration, other common human diseases and aging. Complex I-ROS production and complex I content have been proposed as the major determinants for longevity. The cAMP signal has a role in the regulation of complex I activity and the decrease of ROS production. In the last years, an increasing number of studies have attempted to activate cAMP signaling to treat age-related diseases associated with mitochondrial dysfunctions and ROS production. This idea comes from a long-line of studies showing a main role of cAMP signal in the memory consolidation mechanism and in the regulation of mitochondrial functions. Here, we discuss several evidences on the possible connection between complex I and cAMP pathway in the aging process.

Alterations in cyclic nucleotide signaling are implicated in healthy aging and age-related pathologies of the brain

It is not only important to consider how hormones may change with age, but also how downstream signaling pathways that couple to hormone receptors may change. Among these hormone-coupled signaling pathways are the 3',5'-cyclic guanosine monophosphate (cGMP) and 3',5'-cyclic adenosine monophosphate (cAMP) intracellular second messenger cascades. Here, we test the hypothesis that dysfunction of cAMP and/or cGMP synthesis, execution, and/or degradation occurs in the brain during healthy and pathological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. Although most studies report lower cyclic nucleotide signaling in the aged brain, with further reductions noted in the context of age-related diseases, there are select examples where cAMP signaling may be elevated in select tissues. Thus, therapeutics would need to target cAMP/cGMP in a tissue-specific manner if efficacy for select symptoms is to be achieved without worsening others.

Implication of JAK1/STAT3/SOCS3 Pathway in Aging of Cerebellum of Male Rat: Histological and Molecular study

Aging causes morphological and functional changes in the cerebellum. This work aimed to demonstrate the implication of JAK1/STAT3/SOCS3 on aging-induced changes of rat cerebellum. Thirty male rats were divided into: adult (12 months), early senile (24 months) and late senile (32 months) groups. Immunohistochemical reaction of the cerebellum to GFAP and caspase-3 was assessed and the expression of JAK1, STAT3, SOCS3 proteins was also evaluated. TNFα as well as the activities of malondialdehyde (MDA) and reduced glutathione (GSH) in cerebellar tissue were also measured. The cerebellum of late senile rats revealed more degenerative changes than early senile rats in the form of increase in GFAP and caspase-3 immunoreaction. Additionally, there was decrease in JAK1and STAT3 expression in early and late senile rats and increase in SOCS3 when compare early and late senile groups with adult one. Enhancement of TNFα was noticed with aging as well as significant decrease in GSH and increase in MDA in early senile group. Moreover, late senile group revealed significant decrease in GSH and increase in MDA. It could be concluded that aging resulting in variable changes of the cerebellum as detected by morphological changes, immunohistochemical reactions of caspase-3 and GFAP and expression of JAK1/STAT3/SOCS3 proteins. Additionally, inflammatory marker TNFα and the activity of oxidative/antioxidative stress markers; malondialdehyde (MDA) and reduced glutathione (GSH) were also affected with aging.

Rolipram treatment during consolidation ameliorates long-term object location memory in aged male mice

Normal aging is accompanied by cognitive and memory impairments that negatively impact quality of life for the growing elderly population. Hippocampal function is most vulnerable to the deleterious effects of aging, and deficits in hippocampus-dependent memories are common amongst aged individuals. Moreover, signaling networks such as the cAMP/PKA/CREB pathway, which are critical for memory consolidation, are dampened in healthy aged subjects. Phosphodiesterase (PDE) enzymes that break down cAMP are also affected by aging, and increased break down of cAMP by PDEs may contribute to reduced activity of the cAMP/PKA/CREB signaling network in the brain of aged individuals. Here, we report that the PDE4 inhibitor rolipram administered during consolidation of hippocampus-dependent object location memory improves aged-related spatial memory deficits in aged mice.

Functional coupling between adenosine A1 receptors and G-proteins in rat and postmortem human brain membranes determined with conventional guanosine-5'-O-(3-[35S]thio)triphosphate ([35S]GTPγS) binding or [35S]GTPγS/immunoprecipitation assay

Adenosine signaling plays a complex role in multiple physiological processes in the brain, and its dysfunction has been implicated in pathophysiology of neuropsychiatric diseases such as schizophrenia and affective disorders. In the present study, the coupling between adenosine A₁ receptor and G-protein was assessed by means of two [³⁵S]GTPγS binding assays, i.e., conventional filtration method and [³⁵S]GTPγS binding/immunoprecipitation in rat and human brain membranes. The latter method provides information about adenosine A₁ receptor-mediated Gα_i-3 activation in rat as well as human brain membranes. On the other hand, adenosine-stimulated [³⁵S]GTPγS binding determined with conventional assay derives from functional activation of Gα_i/o proteins (not restricted only to Gα_i-3) coupled to adenosine A₁ receptors. The determination of adenosine concentrations in the samples used in the present study indicates the possibility that the assay mixture under our experimental conditions contains residual endogenous adenosine at nanomolar concentrations, which was also suggested by the results on the effects of adenosine receptor antagonists on basal [³⁵S]GTPγS binding level. The effects of adenosine deaminase (ADA) on basal binding also support the presence of adenosine. Nevertheless, the varied patterns of ADA discouraged us from adding ADA into assay medium routinely. The concentration-dependent increases elicited by adenosine were determined in 40 subjects without any neuropsychiatric disorders. The increases in %E_max values determined by conventional assay according to aging and postmortem delay should be taken into account in future studies focusing on the effects of psychiatric disorders on adenosine A₁ receptor/G-protein interaction in postmortem human brain tissue.

The Past, Present, and Future of Phosphodiesterase-4 Modulation for Age-Induced Memory Loss

The purpose of this chapter is to highlight the state of progress for phosphodiesterase-4 (PDE4) modulation as a potential therapeutic for psychiatric illness, and to draw attention to particular hurdles and obstacles that must be overcome in future studies to develop PDE4-mediated therapeutics. Pathological and non-pathological related memory loss will be the focus of the chapter; however, we will at times also touch upon other psychiatric illnesses like anxiety and depression. First, we will provide a brief background of PDE4, and the rationale for its extensive study in cognition. Second, we will explore fundamental differences in individual PDE4 subtypes, and then begin to address differences between pathological and non-pathological aging. Alterations of cAMP/PDE4 signaling that occur within normal vs. pathological aging, and the potential for PDE4 modulation to combat these alterations within each context will be described. Finally, we will finish the chapter with obstacles that have hindered the field, and future studies and alternative viewpoints that need to be addressed. Overall, we hope this chapter will demonstrate the incredible complexity of PDE4 signaling in the brain, and will be useful in forming a strategy to develop future PDE4-mediated therapeutics for psychiatric illnesses.

Cyclic nucleotide signaling changes associated with normal aging and age-related diseases of the brain

Deficits in brain function that are associated with aging and age-related diseases benefit very little from currently available therapies, suggesting a better understanding of the underlying molecular mechanisms is needed to develop improved drugs. Here, we review the literature to test the hypothesis that a break down in cyclic nucleotide signaling at the level of synthesis, execution, and/or degradation may contribute to these deficits. A number of findings have been reported in both the human and animal model literature that point to brain region-specific changes in Galphas (a.k.a. Gαs or Gsα), adenylyl cyclase, 3',5'-adenosine monophosphate (cAMP) levels, protein kinase A (PKA), cAMP response element binding protein (CREB), exchange protein activated by cAMP (Epac), hyperpolarization-activated cyclic nucleotide-gated ion channels (HCNs), atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), soluble and particulate guanylyl cyclase, 3',5'-guanosine monophosphate (cGMP), protein kinase G (PKG) and phosphodiesterases (PDEs). Among the most reproducible findings are 1) elevated circulating ANP and BNP levels being associated with cognitive dysfunction or dementia independent of cardiovascular effects, 2) reduced basal and/or NMDA-stimulated cGMP levels in brain with aging or Alzheimer's disease (AD), 3) reduced adenylyl cyclase activity in hippocampus and specific cortical regions with aging or AD, 4) reduced expression/activity of PKA in temporal cortex and hippocampus with AD, 5) reduced phosphorylation of CREB in hippocampus with aging or AD, 6) reduced expression/activity of the PDE4 family in brain with aging, 7) reduced expression of PDE10A in the striatum with Huntington's disease (HD) or Parkinson's disease, and 8) beneficial effects of select PDE inhibitors, particularly PDE10 inhibitors in HD models and PDE4 and PDE5 inhibitors in aging and AD models. Although these findings generally point to a reduction in cyclic nucleotide signaling being associated with aging and age-related diseases, there are exceptions. In particular, there is evidence for increased cAMP signaling specifically in aged prefrontal cortex, AD cerebral vessels, and PD hippocampus. Thus, if cyclic nucleotide signaling is going to be targeted effectively for therapeutic gain, it will have to be manipulated in a brain region-specific manner.

Senescent-induced dysregulation of cAMP/CREB signaling and correlations with cognitive decline

It is well known that alongside senescence there is a gradual decline in cognitive ability, most noticeably certain kinds of memory such as working, episodic, spatial, and long term memory. However, until recently, not much has been known regarding the specific mechanisms responsible for the decline in cognitive ability with age. Over the past decades, researchers have become more interested in cAMP signaling, and its downstream transcription factor cAMP response element binding protein (CREB) in the context of senescence. However, there is still a lack of understanding on what ultimately causes the cognitive deficits observed with senescence. This review will focus on the changes in intracellular signaling in the brain, more specifically, alterations in cAMP/CREB signaling in aging. In addition, the downstream effects of altered cAMP signaling on cognitive ability with age will be further discussed. Overall, understanding the senescent-related changes that occur in cAMP/CREB signaling could be important for the development of novel drug targets for both healthy aging, and pathological aging such as Alzheimer's disease.

Phosphorylated extracellular signal-regulated kinase 1/2 immunoreactivity and its protein levels in the gerbil hippocampus during normal aging

Phosphorylated extracellular signal-regulated kinase (pERK) mediates neuronal synaptic plasticity, long-term potentiation, and learning and memory in the hippocampus. In this study, we examined pERK1/2 immunoreactivity and its protein level in the gerbil hippocampus at various ages. In the postnatal month 1 (PM 1) group, very weak pERK1/2 immunoreactivity was detected in the hippocampus. In the CA1 region, pERK1/2 immunoreactivity was considerably increased in the stratum pyramidale in the PM 6 group. Thereafter, pERK1/2 immunoreactivity was decreased. In the CA2/3 region, pERK1/2 immunoreactivity increased in an age-dependent manner until PM 12. Thereafter, numbers of pERK1/2-immunoreactive neurons were decreased. However, in the mossy fiber zone, pERK1/2 immunostaining became stronger with age. In the dentate gyrus, a few pERK1/2-immunoreactive cells were observed until PM 12. In the PM 18 and 24 groups, numbers of pERK1/2-immunoreactive cells were increased, especially in the polymorphic layer. In Western blot analysis, pERK1/2 level in the gerbil hippocampus was increased with age. These results indicate that total pERK1/2 levels are increased in the hippocampus with age. However pERK1/2 immunoreactivity in subregions of the gerbil hippocampus was changed with different pattern during normal aging.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Muscarinic receptor changes in the gerbil thalamus during aging

Here we studied muscarinic receptors in the gerbil thalamus at 8 different ages - from 6 to 36 months - using receptor and functional autoradiography. The pharmacological profile inhibiting [(3)H]N-methyl scopolamine ([(3)H]NMS) binding with 50 and 200 nM pirenzepine, 30 nM pFHHSiD and 100 nM AF-DX 116 revealed the predominance of the M(2) muscarinic subtype in the thalamic nuclei studied, mainly in the anteroventral, anteromedial and paraventricular thalamic nuclei. These data correlated with the highest [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTP gamma S) binding induced in these nuclei by the muscarinic agonist oxotremorine in functional autoradiographic assays. Significant aging-dependent increases in the functional response in these three nuclei were observed, but only the anteroventral and anteromedial thalamic nuclei showed aging-dependent increases in [(3)H]NMS binding. Since these nuclei exert relevant functions, in which cholinergic pathways are involved and acetylcholine release is reported to decrease during aging, we suggest that the anteroventral and anteromedial thalamic nuclei would play critical roles in the cholinergic transmission that require compensatory mechanisms during the aging process and that are not observed in other thalamic nuclei.

Effect of aging on cerebral A1 adenosine receptors: A [18F]CPFPX PET study in humans

Cerebral A(1) adenosine receptors (A(1)AR) fulfill important neuromodulatory and homeostatic functions. The present study examines possible age-related A(1)AR changes in living humans by positron emission tomography (PET) and the A(1)AR ligand [(18)F]CPFPX. Thirty-six healthy volunteers aged 22-74 years were included. The apparent binding potential (BP'2) of [(18)F]CPFPX in various cerebral regions was calculated non-invasively using the cerebellum as reference region. In addition, the total distribution volume (DV't) was assessed in 10 subjects undergoing arterial blood sampling. There was no significant association between regional DV't and age, gender, caffeine consumption or sleep duration. BP'2 showed a significant age-dependent decrease in all regions except cingulate gyrus (p=0.062). The BP'2 decline ranged from -17% (striatum) to -34% (postcentral gyrus), the average cortical decline being -23%. There was no significant effect of gender, caffeine consumption and sleep duration on BP'2. In line with in vitro animal studies, the present in vivo PET study detected an age-dependent A(1)AR loss in humans that may be of pathophysiological importance in various neurological diseases associated with aging. Because of the discrepant results of the invasive (DV't) and the non-invasive (BP'2) analyses the present study needs further validation.

Age-related changes of structures in cerebellar cortex of cat

We studied the structures of the cerebellar cortex of young adult and old cats for age-related changes, which were statistically analysed. Nissl staining was used to visualize the cortical neurons. The immunohistochemical method was used to display glial fibrillary acidic protein (GFAP)-immunoreactive (IR) astrocytes and neurofilament-immunoreactive (NF-IR) neurons. Under the microscope, the thickness of the cerebellar cortex was measured; and the density of neurons in all the layers as well as that of GFAP-IR cells in the granular layer was analysed. Compared with young adult cats, the thickness of the molecular layer and total cerebellar cortex was significantly decreased in old cats, and that of the granular layer increased. The density of neurons in each layer was significantly lower in old cats than in young adult ones. Astrocytes in old cats were significantly denser than in young adult ones, and accom-panied by evident hypertrophy of the cell bodies and enhanced immunoreaction of GFAP substance. Purkinje cells (PCs) in old cats showed much fewer NF-IR dendrites than those in young adults. The above findings indicate a loss of neurons and decrease in the number of dendrites of the PCs in the aged cerebellar cortex, which might underlie the functional decline of afferent efficacy and information integration in the senescent cerebellum. An age-dependent enhancement of activity of the astrocytes may exert a protective effect on neurons in the aged cerebellum.

Differential effects on [35S]GTPgammaS binding using muscarinic agonists and antagonists in the gerbil brain

In this work, we studied the in vitro G-protein activation induced by muscarinic agonists using [(35)S]guanylyl-5'-O-(gamma-thio)-triphosphate ([(35)S]GTPgammaS) autoradiographic methods to characterize the M(2) and M(4) muscarinic subtypes response. Thus, we describe a detailed characterization of the increases in [(35)S]GTPgammaS binding elicited by carbachol (Cch) and oxotremorine (OXO) (binding in the presence minus binding in the absence of agonist) throughout the gerbil brain (Meriones unguiculatus). For both agonists, the strongest stimulations were found in the superficial gray layer of the superior colliculus, the anteroventral and anteromedial thalamic nuclei, the anterior paraventricular thalamic nucleus, and the caudate-putamen. The comparative study using OXO and Cch suggested that OXO is able to detect differences in the response of structures enriched in M(4) muscarinic receptors, showing a lower potency to stimulate these brain areas. Furthermore, using increasing concentrations of selective M(2) (AF-DX 116) and M(1)/M(4) (pirenzepine) antagonists to inhibit specific Cch- or OXO-induced [(35)S]GTPgammaS binding, significant differences were observed in M(2)-enriched structures but not in M(4)-enriched ones such as the caudate-putamen. These data indicate that appropriate muscarinic agonist stimulation, together with selective inhibition of this effect using functional autoradiography, can be used as a tool to unravel the M(2)- and M(4)-muscarinic subtype-mediated response.

Adenylate Cyclase-Mediated Forms of Neuronal Plasticity in Hippocampal Area CA1 Are Reduced With Aging

Beta-adrenergic receptors and the cyclic AMP signaling pathway play an important role in neuronal plasticity and in learning and memory and are known to change with aging. We examined the effects of β-adrenergic stimulation paired with 5-Hz low frequency stimulation (LFS) of Schaffer collateral-commissural afferents on population spike amplitude in area CA1 of hippocampal slices from young (3 mo) and aged (22 mo) Fischer 344 rats. Application of the β-adrenergic agonist isoproterenol (1 μM) for 10 min followed immediately by 3 min LFS produced long-lasting potentiation in young hippocampi, but the magnitude of potentiation in aged rats was significantly attenuated and was not long-lasting. In slices prepared from young rats, long-term potentiation (LTP) induced by this protocol occludes subsequent attempts to produce conventional high frequency stimulation-induced LTP, and vice versa, suggesting that these two forms of potentiation share one or more molecular mechanisms. Age-related differences in response to LFS alone were not observed, but significant differences in response to β-adrenergic stimulation were apparent. Similarly, significant age-related differences in response to direct activation of adenylate cyclase with forskolin (10 μM) were observed. In both age groups, this enhancement produced by isoproterenol or forskolin is only transient, returning to baseline within 60 or 90 min, respectively. Taken together, these studies of adenylate cyclase-mediated forms of potentiation in area CA1 suggest that there is an age-related defect, either upstream or downstream of adenylate cyclase activation, in this important signaling system. Such changes may contribute to the compromised performance on memory tasks that is often observed with normal aging.

Age-dependent agonist-specific dysregulation of membrane-mediated signal transduction: emergence of the gate theory of aging

Although a general mechanism for the limited responsiveness of senescent cells has yet to be established, reduced responsiveness may in part be ascribed to deficits in the apparatus required for cell surface receptor-mediated signal transduction. Age-related changes of receptor-mediated signal transduction occur at many levels, and are known to include quantitative and qualitative changes in growth factor receptors, G-protein coupled receptors, and many other downstream signaling molecules. Here, we emphasize the prime role of the cellular surface in the perception and transmission of external stimuli in response to the aging process. As major means of cellular signal transduction, the receptor tyrosine kinase (RTK) system and the G protein-coupled receptor (GPCR) system of senescent cells were investigated. We observed that the RTK system was severely damaged, while the GPCR system was only partially inactivated by aging. These results suggest that the agonist-dependent dysregulation of and imbalance of signal transduction pathways might be responsible for the functional deterioration of senescent cells, and indicate a possibility of the functional recovery of senescent cells through agonist-specific signal system activation. Moreover, those data evoke the emerging concept that the senescent phenotype may be modulated by the membrance-associated signal system, implying the gate theory of aging.

Age-dependent changes in adenosine A1 receptor and uptake site binding in the mouse brain: an autoradiographic study

Ageing is a multifactorial, inevitable event of life span, which affects neurotransmission in the CNS. Since adenosine is a major neuromodulator of the synaptic activity, it was of interest to investigate the possible modification of the adenosinergic system in the brain during ageing. Using "in vitro" quantitative autoradiography and the radioactive ligands [(3)H]Cyclohexyladenosine and [(3)H]Nitrobenzylthioinosine, we have studied the distribution of A1 adenosine receptors and adenosine uptake sites in the aged mice (26 months) compared to the young ones (3 months). Our results showed a widespread reduction in A1 receptor binding in the aged animals, which was brain area-specific, occurring in areas where adenosine plays a significant neuromodulatory role such as the hippocampus, cortex, basal ganglia, and thalamus. Interestingly, the significant reduction in NBI-sensitive adenosine uptake sites was restricted to few areas of the aged brain, mainly in thalamic nuclei. Since the alterations in the density of A1 receptors and adenosine uptake sites showed no regional correlation and since no significant changes in either neuronal or glial cell number are observed, at least in hippocampus and cortex in this mouse strain during ageing, our findings could be explained by a selective age-dependent reduction of these adenosinergic components rather than by a general neuronal cell degeneration. As adenosine depresses electrical activity in hippocampus, a downregulation of adenosinergic function could probably be related to enhanced excitability seen in hippocampal neurons of the CA1 subregion and dentate gyrus of aged animals.

Age-dependent changes in adenosine receptors are not modified by life-long intermittent alcohol administration

Autoradiography and in situ hybridisation were used to examine age-dependent changes in adenosine receptors in male rats and to determine if life-long (94 weeks) intermittent ethanol consumption had any additional effect. Adenosine A2A receptors in striatum, as assessed by [3H]CGS 21680 binding, decreased by approximately 20% between the ages 6 and 99 weeks. Since dopamine D2 receptors and the mRNA for preproenkephalin also decreased there appears to be a loss of A2A-D2 receptor-bearing striatopallidal cells. Life-long ethanol consumption had no additional effect. Adenosine A1 receptors, as determined by [3H]DPCPX binding, did not decrease with age in any region of the brain, but increased slightly in the cerebellum. In substantia nigra, the increase in [3H]DPCPX binding upon addition of GTP was eliminated. Surprisingly, the amount of A1 receptor mRNA decreased significantly with age in most of the examined regions, including the cerebellum. There was no additional effect of ethanol treatment. It is suggested that age alters the number of cells that express A2A receptors, the turnover of A1 receptors, and in some regions their coupling to G proteins, but that life-long intermittent ethanol exposure has little additional effect.

The existence of substance P in Purkinje cells in cerebellum of the gerbil

We performed this study to identify the characteristics of substance P localization in gerbil cerebellum by immunohistochemistry. Substance P immunoreactivity was present in Purkinje cells of cerebellar cortex but not in other structures. It is suggested that the cerebellar mechanism of gerbil may be different from that of other species.

Age-dependent changes of presynaptic neuromodulation via A1-adenosine receptors in rat hippocampal slices

The presynaptic neuromodulation of stimulation-evoked release of [3H]-acetylcholine by endogenous adenosine, via A1-adenosine receptors, was studied in superfused hippocampal slices taken from 4-, 12- and 24-month-old rats. 8-Cyclopentyl-1,3-dimethylxanthine (0.25 microM), a selective A1-receptor antagonist, increased significantly the electrical field stimulation-induced release of [3H]-acetylcholine in slices prepared from 4- and 12-month-old rats, showing a tonic inhibitory action of endogenous adenosine via stimulation of presynaptic A1-adenosine receptors. In contrast, 8-cyclopentyl-1,3-dimethylxanthine had no effect in 24-month-old rats. 2-Chloroadenosine (10 microM), an adenosine receptor agonist decreased the release of [3H]-acetylcholine in slices taken from 4- and 12-month-old rats, and no significant change was observed in slices taken from 24-month-old rats. In order to show whether the number/or affinity of the A1-receptors was affected in aged rats, [3H]-8-cyclopentyl-1,3-dimethylxanthine binding was studied in hippocampal membranes prepared from rats of different ages. Whereas the Bmax value was significantly lower in 2-year-old rats than in younger counterparts, the dissociation constant (Kd) was not affected by aging, indicating that the density rather than the affinity of adenosine receptors was altered. Endogenous adenosine levels present in the extracellular space were also measured in the superfusate by high performance liquid chromatography (HPLC) coupled with ultraviolet detection, and an age-related increase in the adenosine level was found. In summary, our results indicate that during aging the level of adenosine in the extracellular fluid is increased in the hippocampus. There is a downregulation and reduced responsiveness of presynaptic adenosine A1-receptors, and it seems likely that these changes are due to the enhanced adenosine level in the extracellular space.

Effect of aging on dopaminergic receptors and uptake sites in the rat brain studied by receptor autoradiography

We studied the age-related alterations of dopaminergic receptors in the brain of Fisher 344 rats with various age (3 weeks and 6, 12, 18 and 24 months) using in vitro receptor autoradiography. [3H]SCH 23390, [3H]spiperone and [3H]nemonapride, and [3H]mazindol were used to label dopamine D1 receptors, dopamine D2 receptors and dopamine uptake sites, respectively. In immature rats (3 weeks old), [3H]SCH 23390 binding showed a significant increase in most brain regions compared to adult animals (6 months old), whereas [3H]spiperone and [3H]nemonapride bindings showed no significant alteration in any brain areas. In contrast, [3H]mazindol binding showed a significant decline in most brain regions. On the other hand, the age-related alterations in [3H]SCH 23390 binding were not observed in any brain regions. [3H]Spiperone and [3H]nemonapride bindings also showed no significant alteration in the brain during aging, except for a transient alteration in [3H]spiperone binding in the nucleus accumbens and hippocampus of 12 months old rats. However, [3H]mazindol binding showed a significant reduction in most brain areas of 12 months old rats. Thereafter, the age-related reduction in [3H]mazindol binding was observed in most brain regions of 18 and 24 months old rats. The results demonstrate that dopamine uptake sites are more susceptible to the aging process than both dopamine D1 and D2 receptors. Furthermore, our results suggest that dopaminergic receptors and dopamine uptake sites may develop with different patterns and speeds after birth. Our studies may provide valuable information concerning the effect of aging on dopaminergic systems.

In vivo amino acid release from the striatum of aging rats: adenosine modulation

The release of glutamate, aspartate, GABA, and taurine from the striatum of young (3 months), mature (12 months), and old (22 months), freely moving male rats was investigated by using a microdialysis fiber inserted transversally in the striatum. In old rats basal extracellular glutamate and aspartate levels were decreased vs. young rats (-38 and -49%, respectively). GABA and taurine levels were unmodified by age. In the presence of the adenosine receptor antagonist 8-phenyltheophilline (8-pT) at the concentration of 50 microM, both K(+)-evoked releases of glutamate and aspartate were more than doubled in young, but not in mature and old rats. 8-pT at the concentration of 500 microM significantly decreased glutamate basal levels and K(+)-evoked aspartate release in old rats only. GABA and taurine releases were not affected by 8-pT at either dose. Our findings indicate a modified adenosine modulation on glutamate and aspartate release in aged rats, that could result from a change in the balance between A1 and A2a adenosine receptor density or an alteration of A1 and A2a receptor-effector coupling.

Age-related loss of cholinergic-muscarinic coupling to PLC: comparison with changes in brain regional PLC subtypes mRNA distribution

Activation of phospholipase C (PLC) coupled to phosphoinositide (PtdIns) hydrolysis occurs through one of the two pathways. One of the major pathways for the neurotransmitter signaling involves phosphoinositide (PtdIns) specific and G-protein dependent PLC-beta, which stimulates the formation of inositol triphosphate (IP3) and inositol tetraphosphate (IP4). Another pathway through the stimulation of calcium influx can directly activate all of the PLC isozymes. At least three isozymes of PLC have been characterized in the brain; PLC-A (alpha), PLC-I (beta) and PLC-II (gamma), which are shown to be localized differentially in brain regions. Muscarinic-cholinergic signals are mediated in large part through the hydrolysis of PtdIns by PLC. To investigate changes in muscarinic coupling to PLC during aging, we examined carbachol stimulated and calcium stimulated PtdIns hydrolysis in cerebral cortical membranes in young, middle aged and old rats. In order to determine whether PtdIns hydrolysis changes correspond to PLC isozyme expression in these animals, we examined three subtypes of PLC mRNA expression in brain sections of young and old rats using in situ hybridization technique. Our study indicated decreased carbachol-induced PLC activity in the cerebral cortex and, in contrast, increased PLC-beta mRNA in the frontal cortex and superficial cortical layer of aged rats. PLC-alpha mRNA was decreased in hippocampal regions of older rats. These studies suggest that during aging there is an uncoupling of muscarinic stimulated PtdIns hydrolysis, which is accompanied by an increased PLC-beta mRNA and decreased PLC-alpha mRNA that may represent compensatory changes in PLC expression.

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Phosphoinositide hydrolysis, G alpha q, phospholipase C, and protein kinase C in post mortem human brain: effects of post mortem interval, subject age, and Alzheimer's disease

Influences of post mortem time interval, subject age and Alzheimer's disease were investigated on several components of the phosphoinositide second messenger system, including stimulation of [3H]phosphatidylinositol hydrolysis by GTP[S] and several receptor agonists and the levels of Galphaq, beta, delta and gamma subtypes of phospholipase C, and five protein kinase C isoforms, in membranes prepared from post mortem human prefrontal cortex. Most of these components were stable with post mortem delays in the range of 5-21 h, but decreases of Galphaq and the alpha and xi protein kinase C subtypes were detected. Within the subject age range of 19-100 years, G-protein- and agonist-induced [3H]phosphatidylinositol hydrolysis decreased, as did levels of Galphaq, but the levels of phospholipase C and protein kinase C subtypes were generally unchanged. In Alzheimer's disease, compared with age- and post mortem interval-matched controls, there was a decrease in [3H]phosphatidylinositol hydrolysis stimulated by G-proteins and by several receptor agonists, but the levels of Galphaq and most of the phospholipase C and protein kinase C isoforms were unaffected. The greatest deficits, which were >50%, occurred with GTP[S]- and carbachol-induced [3H]phosphatidylinositol hydrolysis, indicating that this G-protein function and the response to cholinergic stimulation are significantly impaired in Alzheimer's disease. In summary a comprehensive assessment of several components of the phosphoinositide second messenger system was made in post mortem human brain. Most elements were stable within the post mortem interval range of 5-21 h, lending validity to measurements using these tissues. Significant age-related reductions in several components were identified, indicating loss of responses with increasing age. Most importantly, severe reductions in responses to several stimuli were found in Alzheimer's disease brain, deficits in signal transduction which may contribute to impaired cognition and to the limited therapeutic responses to drugs, such as those used to activate cholinergic receptors coupled with the phosphoinositide system.

Protein kinase C activity, translocation, and conventional isoforms in aging rat brain

Protein kinase C was studied in various brain areas in aging Wistar rats. Histone-directed kinase activity from the cortex, hippocampus and cerebellum did not change with aging. Using purified protein B-50 as a substrate, between 3 and 8 months a decrease in in vitro phosphorylation was detected in the membrane fraction of the cortex but after this age values remained stable. In hippocampal membranes, B-50 phosphorylation was increased in aged rats. PKC translocation was impaired in aged rats in both the cortex and the hippocampus. PKC alpha and beta mRNA decreased in the cortex between 3 and 8 months with no further decline in aged animals. Hippocampal mRNA for calcium-dependent PKC isoforms was not modified during aging, as assessed by Northern and in situ hybridization. Western blot analysis revealed a change in PKC gamma protein only, which was increased in hippocampal membranes from aged rats. The data indicate that the key PKC function that is impaired in aged rats is enzyme translocation irrespective of the brain area investigated.

Bromocriptine protects against delayed neuronal death of hippocampal neurons following cerebral ischemia in the gerbil

Bromocriptine, a dopamine D2 receptor agonist, has widely been used for patients with Parkinson's disease. In this study, we examined its neuroprotective effects against neuronal damage in the CA1 subfield of the hippocampus following experimental cerebral ischemia in the Mongolian gerbil. Forebrain ischemia was induced by occlusion of bilateral common carotid arteries for 3 min. Bromocriptine, at a dose of 0.3 or 3 mg/kg, was injected i.p. 30 min before the onset of ischemia. Histopathological observations showed that neuronal damage to hippocampal CA1 neurons, which was seen 7 days after ischemia in vehicle-treated animals, was prevented by bromocriptine treatment. Immunohistochemical staining for copper/zinc superoxide dismutase and manganese superoxide dismutase decreased markedly in the CA1 neurons of vehicle-treated animals 2 days after ischemia when histological neuronal destruction was not yet seen, but was well preserved in bromocriptine-treated animals. The present findings show that bromocriptine protects against ischemia-induced neuronal damage, and that the mechanism of the neuroprotection may relate to the preservation of SODs. Bromocriptine, which was recently shown to be a potent free radical scavenger, may have a potent neuroprotective action against disorders including ischemic stroke.

Cerebrovascular, neuronal, and behavioral effects of long-term Ca2+ channel blockade in aging normotensive and hypertensive rat strains

The pathogenesis of essential hypertension is not fully understood, but most of the cardiovascular, metabolic, neurogenic, and humoral abnormalities are explained by dysfunctions in the control of intracellular Ca2+ concentrations in the cells of the vascular wall. Most theories of disturbed calcium regulation focus on the calcium concentration within vascular smooth muscle cells. The implications of hypertension for the increased calcium content of aging arteries seem to be clear, but were only studied in the peripheral circulation; hypertension prominently augments the aging-related accumulation of calcium in the vessel wall. Although the contribution of calcium overload in hypertensive cerebrovascular damage is well documented, it is not clear yet if hypertension per se is the main cause of hypertension-associated calcium-dependent cerebral damage. Thus far, the hypotensive effects of most calcium antagonists were extensively described, and their efficacy in stroke prevention was proven. Earlier studies indicated that chronic administration of nimodipine revealed a protective effect in the occurrence of strokes in SHR-SP rats, yielding a decreased mortality rate. Because nimodipine did not lower the extremely high blood pressure of these animals, the mechanisms behind such nimodipine-induced stroke prevention may be attributed to a direct cerebrovascular and/or neuronal action of nimodipine. Hypertension is generally considered a vascular pathologic condition, and most research has been directed towards the influences of hypertension on large peripheral arteries such as the aorta and coronary artery. The influence of the CNS on the regulation of cardiovascular system and blood pressure regulation was described in detail, and the role of the CNS in hypertension also was the subject of study. The increased risk of stroke in hypertensive subjects generated numerous studies on the precise nature of compromised cerebrovascular functioning under hypertensive conditions. Few data are available on Ca2+ alterations in cerebral neurons during hypertension. Honda et al. demonstrated that voltage-dependent Ca2+ uptake was higher in cortical synaptosomes from SHR than form normotensive animals and suggested that an important alteration in Ca2+ channel characteristics may occur in SHR brain synaptosomes. Although the density of L-type calcium channels was shown to be higher in the hippocampus of SHR rats, others reported that the number of L-type calcium channels was significantly lower in the brain of SHR rats than WKY normotensive controls. The latter data suggest that hypertension may be associated with similar alterations in neuronal calcium homeostasis as demonstrated for aging in normotensive subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Age-related decrease in GABAB receptor binding in the Fischer 344 rat inferior colliculus

Quantitative receptor autoradiography was used to assess GABAB receptor binding in three primary subdivisions of the inferior colliculus (IC): dorsal cortex (DCIC), external cortex (ECIC), and the central nucleus (CIC) of 3-, 18-20-, and 26-month-old Fischer 344 rats. GABAB binding sites were localized using [3H]GABA in the presence of a saturating concentration of isoguvacine, a selective GABAA receptor agonist, to displace [3H]GABA bound to GABAA receptor sites. In the three IC subdivisions examined, GABAB receptor binding was significantly reduced in 26-month-old rats when compared to 3-month-old rats (DCIC, -44%; ECIC, -36%; CIC, -32%; p < 0.05). For comparison, GABAB binding was determined in the portion of cerebellum located in the recess of the IC. In the molecular layer of this region, there was no statistically significant differences in receptor binding between 3, 18-20-, and 26-month-old rats. In addition, there was not a significant age-related change in the cross-sectional area of the IC. These findings provide additional evidence to support the existence of selective age-related changes in GABA neurotransmitter function in the rat IC.

Effects of naftidrofuryl oxalate, a 5-HT2 antagonist, on neurotransmission and transduction systems in the gerbil hippocampus

We investigated the effects of age and nafidrofuryl oxalate (Naftidrofuryl), a 5-HT2 antagonist, on neurotransmission and transduction systems in the gerbil hippocampus using quantitative autoradiography. [3H]Quinuclidinyl benzilate (QNB), [3H]cyclohexyl-adenosine (CHA), [3H]MK-801, and [3H]muscimol were used to label muscarinic acetylcholine, adenosine A1, N-methyl-D-aspartate (NMDA), and gamma-aminobutyric acid-A (GABAA) receptors, respectively. [3H]PN200-110 labeled L-type Ca2+ channels. [3H]Forskolin, [3H]cyclic adenosine monophosphate (cAMP), [3H]phorbol 12,13-dibutyrate (PDBu), and [3H]inositol 1,4,5-triphosphate (IP3) were used to label adenylate cyclase, cAMP-dependent protein kinase, protein kinase C (PKC), and IP3 receptors, respectively. Approximately 20% reductions in [3H]QNB, [3H]forskolin, and [3H]PDBu binding were observed in the hippocampus of 9-month-old gerbils in comparison with 5-week-old gerbils. Treatment with Naftidrofuryl (10 mg/kg, i.p., once a day for 7 days) ameliorated these reductions. No changes were found in [3H]CHA, [3H]MK-801, [3H]muscimol, [3H]PN200-110, [3H]cAMP, and [3H]IP3 binding. The results suggest that Naftidrofuryl may have beneficial effects on the age-related alterations in signal transmission and transduction systems in the brain. Because the acetylcholine system, adenylate cyclase, and PKC are considered to be involved in learning and memory processes, the result may have clinical implications.

Alterations of inositol phosphate turnover in striatum of aged rats

Age-related inositol phosphate turnover in the rat central nervous system was investigated. Higher phospholipase-C activity and drastically higher (almost 2.5-fold) inositol 1,4,5-trisphosphate concentration in the corpus striatum (caudate-putamen) of extremely old (approximately 40 months) female Wistar rats in comparison to the young adult (approximately 3.5 months) rats were observed. Dopamine seems to slightly inhibit total inositol phosphate formation and this effect was antagonized by (-)-sulpiride.

Effects of aging on spatial learning and hippocampal protein kinase C in mice

C57BL/6Nia and F1(B6xD2)Nia mice were tested on the Morris water maze task for 5 days followed by 12 days of testing on the place learning-set task (8 trials/day with each task). Mice were tested at 3, 14, and 25 months of age. C57 mice, 25 months of age, were significantly impaired in both the Morris and place learning-set task probe trial performance compared to mice 3 months of age (p < 0.05). These aged C57 mice also demonstrated a significant reduction in membrane-bound hippocampal protein kinase C (PKC) activity (p < 0.05) with no significant change in cytosolic PKC activity. F1 mice, however, showed no effect of age on probe trial performance on the spatial learning tasks. In addition, in a comparison of C57 and F1 mice within each age group, F1 mice demonstrated superior learning performance which was accompanied by a significant elevation in PKC activity (p < 0.05). Spatial learning performance of both strains significantly correlated with membrane-bound PKC activity (p < 0.01). These data provide additional support for our previous hypothesis of an involvement of hippocampal PKC activity in spatial learning and suggest that the amount of membrane-bound PKC activity may be a determinant of age-related decline in spatial learning.