Gender differences in response of hippocampus to chronic glucocorticoid stress: role of glutamate receptors

Glucocorticoids (GC) play critical roles in the pathophysiological reactions to environmental stress. In brain, morphological changes were examined in hippocampal CA3 neurons with 2 weeks of chronic elevation of GC in male and female mice. Molecular correlates and underlying mechanisms paralleling these morphologic changes in hippocampus were investigated. Although the hippocampal neurons in the CA3 area in male mice atrophy with chronically elevated GC, female mice show minimal morphological changes with comparable GC regimens. These sexual morphological differences correlate with differences in the postsynaptic dense protein (PSD95) as well as the spectrum of glutamate receptors induced by GC treatment in male and female mice, including NMDA, AMPA, and KA receptors. These findings suggest that synaptic receptor composition is adapted to the unique physiological requirements of males and females and illuminate underlying mechanisms of GC/stress responses in the brain.     

Effect of aging on psychological stress-induced increases in noradrenaline release in the rat anterior hypothalamus: an in vivo microdialysis study

Basal and psychological stress-induced noradrenaline (NA) release were studied by intracerebral microdialysis in the hypothalamus of rats aged 9 weeks or 12 months. Basal NA output was not significantly different between young (9-week-old; 2.11±0.22 pg/20 min) and aged (12-month-old; 2.29±0.34 pg/20 min) rats. Psychological stress for 20 min significantly increased NA release in both groups (186% and 142% of baseline at 9 weeks old and 12 months old, respectively); however, the increase in aged rats was significantly lower than that in young rats (P<0.01). This finding suggests that the noradrenergic neuronal response to psychological stress is attenuated in aged rats.     

The effects of aging in the hippocampus and cognitive decline

Aging is a natural process that is associated with cognitive decline as well as functional and social impairments. One structure of particular interest when considering aging and cognitive decline is the hippocampus, a brain region known to play an important role in learning and memory consolidation as well as in affective behaviours and mood regulation, and where both functional and structural plasticity (e.g., neurogenesis) occur well into adulthood. Neurobiological alterations seen in the aging hippocampus including increased oxidative stress and neuroinflammation, altered intracellular signalling and gene expression, as well as reduced neurogenesis and synaptic plasticity, are thought to be associated with age-related cognitive decline. Non-invasive strategies such as caloric restriction, physical exercise, and environmental enrichment have been shown to counteract many of the age-induced alterations in hippocampal signalling, structure, and function. Thus, such approaches may have therapeutic value in counteracting the deleterious effects of aging and protecting the brain against age-associated neurodegenerative processes.     

Quantification of synaptic vesicles in hippocampus of aging rats and initial studies of possible relations to neurophysiology

Synaptic vesicle populations were quantified in Schaffer-commissural synapses which terminate on CA1 pyramidal cell apical dendrites, in aging and young-mature rats. Vesicles were found to be reduced with age, and this effect was most pronounced in the oldest animals (e.g., 28 months) within the aged group. Numerical density of synaptic vesicles in aged rat hippocampus was reduced whether expressed as vesicles per terminal, vesicles per μ² of terminal, or vesicles per μ² of terminal corrected for shrinkage or swelling as assessed by mitochondrial cross-sectional diameters. Counted synaptic terminal areas were not significantly different in the aged animals, although a trend toward reduced terminal size with aging was seen. The latter observation apparently rules out increased terminal swelling in aged rat synapses as a basis for reduced vesicle density. In some rats, neurophysiological studies were concomitantly performed. A significant correlation was found between the amplitude of the monosynaptic population spike after 20 min of 4 Hz stimulation of the Schaffer-commissural fibers and synaptic vesicle populations in terminals of these stimulated pathways. However, because of a low n this result must be viewed as preliminary.     

Alterations in the hippocampus of aged mice

Summary The hippocampus of C57B1/6 mice was examined histologically and electron microscopically. Male and female mice at 3 and 8 months of age and female mice at 16 months of age were studied. PAS positive foci, containing particles 1–2 in diameter, were observed in the hippocampal region of 8 and 16 month old mice. These particles were diastase sensitive. Electron microscopically, in similar mice, a vacuolating change was observed in the cytoplasm of perithelial cells (pericytes) in the same area.     

Effect of aging on the distribution of basic fibroblast growth factor immunoreactive cells in the rat hippocampus

Hippocampal formation is extremely sensitive to the aging process and appears to be one of the first regions to show structural and physiological changes with advancing age. Basic fibroblast growth factor (bFGF) plays an important role in the stimulation of mitogenesis in glial cells, the support of neuronal survival and the promotion of neurite outgrowth in vitro. In the present study, the effect of aging on the distribution of bFGF immunoreactive (bFGF-ir) cells was investigated. The protein product of bFGF was visualized immunohistochemically in the dorsal hippocampus of Wistar albino rats. bFGF-ir astrocytes in different subfields of hippocampus and neurons in CA2 field were quantified to determine whether changes in immunoreactivity were correlated with advancing age. Aging was accompanied by a decrease in bFGF-ir cell density in subfields of hippocampus. We concluded that aging was associated with a reduction in bFGF-ir cell density that may reflect a decreased expression of bFGF in the rat hippocampus.     

Impaired up-regulation of type II corticosteroid receptors in hippocampus of aged rats

Several recent investigations have reported a decline of rat hippocampal corticosteroid-binding receptors (CSRs) with aging. This decline has been proposed to be an initial cause (through disinhibition) of the elevated adrenal steroid secretion that apparently occurs with aging; however, it could instead be an effect of corticoid elevation (through down-regulation). In order to assess the effects of age on CSR biosynthetic capacity in the absence of down-regulatory influences of endogenous corticoids, as well as to study aging changes in CSR plasticity, we examined the up-regulation of hippocampal CSR that follows adrenalectomy (ADX). The rat hippocampus contains at least two types of CSR binding and differential analysis of types I and II CSR was accomplished by selective displacement of [3H]corticosterone with RU-28362, a specific type II agonist. In young (3 months old) Fischer-344 rat hippocampus, up-regulation of type II binding above 2-day ADX baseline was present by 3-7 days and increased still further by 8-10 days post-ADX; type I CSR density did not change significantly between 1 and 10 days post-ADX. However, in aged (24-26 months old) rats, type II CSR up-regulation did not occur over the 10 day post-ADX period. Thus, the age-related impairment of type II up-regulation may reflect an intrinsic deficit in CSR biosynthesis or lability that is independent of the acute endogenous adrenal steroid environment.     

Modulation of feeding behaviour by blocking purinergic receptors in the rat nucleus accumbens: a combined microdialysis, electroencephalographic and behavioural study

The nonspecific P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), the nonspecific P1 receptor antagonist 8-(p-sulphophenyl)-theophylline (8-SPT) and the combination of both were applied by retrograde microdialysis into the nucleus accumbens (NAc) before and during feeding of 18-h food-deprived rats. In addition to the registration of behavioural parameters, such as the amount and duration of food intake, the feeding-induced changes in dopamine (DA) concentration and the concomitant changes of neuronal activity in the NAc and the ventral tegmental area (VTA) were simultaneously determined. The perfusion with PPADS (20 microm) diminished the amount of food intake and the duration of feeding. Furthermore, the P2 receptor antagonist blocked the feeding-induced DA release and prevented the feeding-elicited changes of the electroencephalography (EEG) power distribution which was characterised by an increase in the power of the 8.0-13.0-Hz frequency band in the NAc and the VTA. The effects of PPADS could be completely prevented by the concomitantly perfused adenosine receptor antagonist 8-SPT (100 microm). When given alone, 8-SPT increased the amount of food ingested, the duration of feeding and the EEG power of the higher frequency range, particularly between 19.0 and 30.0 Hz, in both the NAc and the VTA. The feeding-elicited DA release was supplemented to the enhanced DA level caused by the perfusion with 8-SPT in an additive manner. The P2 and P1 receptor antagonists interact antagonistically in the modulation of feeding behaviour and the feeding-induced changes of EEG activity suggesting that both endogenous extracellular ATP and adenosine are involved in the regulation of the feeding-associated mesolimbic neuronal activity in a functionally antagonistic manner.     

Pharmacological properties of purinergic receptors and their effects on proliferation and induction of neuronal differentiation of P19 embryonal carcinoma cells

We have used P19 embryonal carcinoma cells as in vitro model for early neurogenesis to study ionotropic P2X and metabotropic P2Y receptor-induced Ca²⁺ transients and their participation in induction of proliferation and differentiation. In embryonic P19 cells, P2Y₁, P2Y₂ and P2X₄ receptors or P2X-heteromultimers with similar P2X₄ pharmacology were responsible for ATP and ATP analogue-induced Ca²⁺ transients. In neuronal-differentiated cells, P2Y_2, P2Y₆, P2X₂ and possibly P2X₂/P2X₆ heteromeric receptors were the major mediators of the elevations in intracellular free calcium concentration [Ca²⁺]_i. We have collected evidence for the involvement of metabotropic purinergic receptors in proliferation induction of undifferentiated and neural progenitor cells by using a BrdU-incorporation assay. ATP-, UTP-, ADP-, 2-MeS-ATP- and ADP-βS-induced proliferation in P19 cells was mediated by P2Y₁ and P2Y₂ receptors as judged from pharmacological profiles of receptor responses. ATP-provoked acceleration of neuronal differentiation, determined by analysis of nestin and neuron-specific enolase gene and protein expression, also resulted from P2Y₁ and P2Y₂ receptor activation. Proliferation- and differentiation-induction involved the activation of inositol-trisphosphate sensitive intracellular Ca²⁺ stores.     

High-fat diet induces neuroinflammation and reduces the serotonergic response to escitalopram in the hippocampus of obese rats

Clinical studies indicate that obese individuals have an increased risk of developing co-morbid depressive illness and that these patients have reduced responses to antidepressant therapy, including selective serotonin reuptake inhibitors (SSRIs). Obesity, a condition of chronic mild inflammation including obesity-induced neuroinflammation, is proposed to contribute to decreases in synaptic concentrations of neurotransmitters like serotonin (5HT) by decreasing 5HT synthesis in the dorsal raphe nucleus (DRN) and/or affecting 5HT reuptake in DRN target regions like the hippocampus. In view of these observations, the goal of the current study was to examine inflammatory markers and serotonergic dynamics in co-morbid obesity and depression. Biochemical and behavioral assays revealed that high-fat diet produced an obesity and depressive-like phenotype in one cohort of rats and that these changes were marked by increases in key pro-inflammatory cytokines in the hippocampus. In real time using fast scan cyclic voltammetry (FSCV), we observed no changes in basal levels of hippocampal 5HT; however responses to escitalopram were significantly impaired in the hippocampus of obese rats compared to diet resistant rats and control rats. Further studies revealed that these neurochemical observations could be explained by increases in serotonin transporter (SERT) expression in the hippocampus driven by elevated neuroinflammation. Collectively, these results demonstrate that obesity-induced increases in neuroinflammation adversely affect SERT expression in the hippocampus of obese rats, thereby providing a potential synaptic mechanism for reduced SSRI responsiveness in obese subjects with co-morbid depressive illness.     

Cellular reactions to implantation of a microdialysis tube in the rat hippocampus

Summary Microdialysis tubes, used for measurements of extracellular neurotransmitter concentrations, were implanted in rat dorsal hippocampus to study the adjacent tissue reaction. The brain was examined 1–60 days after the implantation. Within the first 2 days, normal neuropil and only occasional hemorrhage surrounded the microdialysis tube. Three days following the implantation astrocytes close to the dialysis tube, hypertrophied. Hypertrophic astrocyte processes invaded the spongy fiber wall. There was no increase in the number of astrocytes. Fourteen days after the fiber insertion layers of reticulin-positive fibers separated astrocytes and the remaining neuropil from the fiber wall. Late tissue changes (1 and 2 months) consisted of collagen deposits and occasional granuloma formation. These results can be used to predict the optimal time for commencing microdialysis after the fiber implantation.Supported by grants 12-5705 and 12-6077 from the Danish Medical Research Council and by the Novo foundation     

Contribution of neuroinflammation and immunity to brain aging and the mitigating effects of physical and cognitive interventions

It is widely accepted that the brain and the immune system continuously interact during normal as well as pathological functioning. Human aging is commonly accompanied by low-grade inflammation in both the immune and central nervous systems, thought to contribute to many age-related diseases. This review of the current literature focuses first on the normal neuroimmune interactions occurring in the brain, which promote learning, memory and neuroplasticity. Further, we discuss the protective and dynamic role of barriers to neuroimmune interactions, which have become clearer with the recent discovery of the meningeal lymphatic system. Next, we consider age-related changes of the immune system and possible deleterious influences of immunosenescence and low-grade inflammation (inflammaging) on neurodegenerative processes in the normally aging brain. We survey the major immunomodulators and neuroregulators in the aging brain and their highly tuned dynamic and reciprocal interactions. Finally, we consider our current understanding of how physical activity, as well as a combination of physical and cognitive interventions, may mediate anti-inflammatory effects and thus positively impact brain aging.     

Inhibiting neuroinflammation: The role and therapeutic potential of GABA in neuro-immune interactions

The central nervous system, once thought to be a site of immunological privilege, has since been found to harbour immunocompetent cells and to communicate with the peripheral nervous system. In the central nervous system (CNS), glial cells display immunological responses to pathological and physiological stimuli through pro- and anti-inflammatory cytokine and chemokine signalling, antigen presentation and the clearing of cellular debris through phagocytosis. While this neuroinflammatory signalling can act to reduce neuronal damage and comprises a key facet of CNS homeostasis, persistent inflammation or auto-antigen-mediated immunoreactivity can induce a positive feedback cycle of neuroinflammation that ultimately results in necrosis of glia and neurons. Persistent neuroinflammation has been recognised as a major pathological component of virtually all neurodegenerative diseases and has also been a focus of research into the pathology underlying psychiatric disorders. Thus, pharmacological strategies to curb the pathological effects of persistent neuroinflammation are of interest for many disorders of the CNS. Accumulating evidence suggests that GABAergic activities are closely bound to immune processes and signals, and thus the GABAergic neurotransmitter system might represent an important therapeutic target in modulating neuroinflammation. Here, we review evidence that inflammation induces changes in the GABA neurotransmitter system in the CNS and that GABAergic signalling exerts a reciprocal influence over neuroinflammatory processes. Together, the data support the hypothesis that the GABA system is a potential therapeutic target in the modulation of central inflammation.     

Regulation of neurogenesis in the aging vertebrate brain: role of oxidative stress and neuropsychiatric factors

Neurogenesis persists in the subventricular zone of the lateral ventricles and in the dentate gyrus of the hippocampus of the vertebrate brain throughout adulthood. Though the rate of neurogenesis decreases dramatically with increasing age, neurons continue to be born throughout old age in rodents, primates, and human beings. This decline, however, is not irreversible and may be prevented in part by the action of numerous trophic and other regulatory factors. Among these are FGF-2 and BDNF as well as several psychotrophic factors used in the treatment of neuropsychiatric disorders, including serotonin, lithium, and a number of antidepressants. In addition, exposure to a challenging and complex environment (‘enriched environment’) has been demonstrated to have striking neuroprotective effects. In the context of a link between neurogenesis and neuropsychiatric disorders, we propose one possible mechanism, oxidative stress, through which substances such as alcohol may impair neurogenesis. We also explore the potential functional significance of adult neurogenesis in the hippocampus and consider the evidence suggesting a role in the cognitive functions of learning and memory.     

Demonstration of the facilitatory role of 8-OH-DPAT on cholinergic transmission in the rat hippocampus using in vivo microdialysis

The role of the serotonin (5-HT)_1A receptor in the regulation of acetylcholine (ACh) release in the hippocampus was investigated using an in vivo microdialysis technique and a sensitive radioimmunoassay specific for ACh. The mean (±S.E.M.) basal ACh contents in the hippocampal perfusate of conscious, freely moving rats was 60 ± 4 (n = 29) and 3691 ± 265 fmol/30 min (n = 31), respectively, in the absence and presence of physostigmine (Phy) in the perfusion fluid. Systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.5 mg/kg, s.c.), a 5-HT_1A agonist, significantly enhanced ACh release both in the presence and absence of Phy. Local application of 8-OH-DPAT (3–30 μM) into the hippocampus through the microdialysis probe significantly potentiated ACh release only in the presence of Phy, whereas no significant effect was observed in its absence. Pretreatment with NAN-190 (3 mg/kg, i.p.), a 5-HT_1A antagonist, eliminated the increasing effect of systemically applied 8-OH-DPAT on ACh release, while NAN-190 alone had no effect on basal ACh release either in the absence or presence of Phy. Consistent with the time course of ACh release, systemic administration of 8-OH-DPAT evoked hyperlocomotion, which was reversed by NAN-190. However, local hippocampal application of 8-OH-DPAT did not affect the locomotor activity of the rats. These findings suggest that at least two different sites are involved in the 8-OH-DPAT-induced increase in the release of ACh in the rat hippocampus in vivo.     

银杏叶片对衰老大鼠海马CREB表达影响的实验研究

目的研究银杏叶片对衰老大鼠海马CREB、pCREB表达的影响。方法雄性Wistar大鼠分为青年对照组(3月龄)、老年对照组(13月龄)和银杏叶组(13月龄)。银杏叶组大鼠每日灌胃银杏叶片,其余两组动物灌胃等量蒸馏水,共8w。Morris水迷宫实验观察大鼠空间记忆能力变化,Western Blot检测海马CREB、pCREB表达。结果 (1)与青年对照组相比,老年大鼠逃避潜伏期延长,穿越平台次数及平台象限停留时间减少(P<0.05);与老年对照组比较,银杏叶组大鼠逃避潜伏期缩短,穿越平台次数增多,平台象限停留时间延长(P<0.05)。(2)老年对照组大鼠海马CREB、pCREB表达水平低于青年对照组(P<0.05);与老年对照组相比,银杏叶组大鼠海马CREB、pCREB表达上升(P<0.05)。结论银杏叶片可改善老龄大鼠空间记忆功能,其机制可能与上调海马CA1区CREB、PCREB有关。     

The effects of aging on muscarinic receptor/G-protein coupling in the rat hippocampus and striatum

In the striatum and hippocampus, there is a loss of sensitivity to muscarinic agonists with age which has been traced to events early in the signal transduction pathway. Our laboratory has therefore focussed on investigations at this level. The current experiments investigate the effects of age on G-protein/receptor interactions by using competitive binding assays to measure the ability of GppNHp to decrease the proportion of receptors bound to G-proteins in the absence and the presence of added Mg2+. L-[3H]Quinuclidinyl benzilate was used as a nonselective ligand and [3H]pirenzepine as an M1 selective ligand. We find that: (1) muscarinic receptors and G-proteins in the striatum appear to become loosely coupled with age, with no change in Mg2+ sensitivity. (2) M1-receptor/G-protein complexes in the hippocampus display increased sensitivity to the presence of Mg2+ with age, with those from old but not young tissue requiring added Mg2+ in order to uncouple. This effect, however, may not be M1 specific.     

Region-specific stability of dendritic extent in normal human aging and regression in Alzheimer's disease. I. CA1 of hippocampus

Pyramidal neurons in two subdivisions of CA1 (CA1 and CA1a+b) of hippocampus from human brains obtained at autopsy were studied in Golgi Cox-stained tissue. Seventeen cases were a part of a normal aging series, ranging in age from 43 to 95 years; and 5 cases had Alzeiheimer'z disease (AD). Dendritic extent of apical and basal trees was found to be stable in normal aging. In AD there was a significant loss of total dendritic length and/or average segment length for the apical and basal trees of both subdivisions of CA1. This finding is consistent with the findings of severe pathology in CA1 reported by others. The reductions in overall dendritic extent in CA1a+b in AD could be attributed largely to alterations in the lenghts of the terminal segments. Apical and basal trees of CA1c were more severely affected by AD than those of CA1a+b and showed more widespread reductions in numbers of segments as well as lengths of segments.