Age-dependent changes in projections from locus coeruleus to hippocampus dentate gyrus and frontal cortex

Multisensory Stimulation Reverses Memory Impairment in Adrβ3KO Male Mice

Norepinephrine plays an important role in modulating memory through its beta-adrenergic receptors (Adrβ: β₁, β₂ and β₃). Here, we hypothesized that multisensory stimulation would reverse memory impairment caused by the inactivation of Adrβ₃ (Adrβ₃KO) with consequent inhibition of sustained glial-mediated inflammation. To test this, 21- and 86-day-old Adrβ₃KO mice were exposed to an 8-week multisensory stimulation (MS) protocol that comprised gustatory and olfactory stimuli of positive and negative valence; intellectual challenges to reach food; the use of hidden objects; and the presentation of food in ways that prompted foraging, which was followed by analysis of GFAP, Iba-1 and EAAT2 protein expression in the hippocampus (HC) and amygdala (AMY). The MS protocol reduced GFAP and Iba-1 expression in the HC of young mice but not in older mice. While this protocol restored memory impairment when applied to Adrβ₃KO animals immediately after weaning, it had no effect when applied to adult animals. In fact, we observed that aging worsened the memory of Adrβ₃KO mice. In the AMY of Adrβ3KO older mice, we observed an increase in GFAP and EAAT2 expression when compared to wild-type (WT) mice that MS was unable to reduce. These results suggest that a richer and more diverse environment helps to correct memory impairment when applied immediately after weaning in Adrβ₃KO animals and indicates that the control of neuroinflammation mediates this response.

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Age-related differences in event-related potentials and pupillary responses in cued reaction time tasks

Deficits in the noradrenergic system are associated with age-related cognitive decline, yet how healthy aging influences the functional properties of this arousal system is still poorly understood. We addressed this question in humans using pupillometry, a well-established indicator of activity levels in the locus coeruleus (LC), the main noradrenergic center in the brain. We recorded the pupillogram and the electroencephalogram of 36 young and 39 older adults, while they were engaged in cued reaction time tasks known to elicit LC responses in monkeys. Event-related potentials (ERPs) revealed significant group differences. Older adults showed higher cortical activation during preparatory processes reflected in enhanced cue-evoked frontocentral ERPs and reduced parietal ERPs at the time of the motor response. In contrast, the amplitude of the task-related pupillary responses did not show a significant group effect. Our findings suggest that aging-related changes in cortical processing during motor preparation and execution, as documented by electroencephalogram, are not accompanied by changes in the amplitude of activation of the LC, as documented by pupillography.

The Noradrenergic System of Aged GDNF Heterozygous Mice

Glial cell line-derived neurotrophic factor (GDNF) is a trophic factor for noradrenergic (NE) neurons of the pontine nucleus locus coeruleus (LC). Decreased function of the LC-NE neurons has been found during normal aging and in neurodegenerative disorders. We have previously shown that GDNF participates in the differentiation of LC-NE neurons during development. However, the continued role of GDNF for LC-NE neurons during maturation and aging has not been addressed. We examined alterations in aged mice that were heterozygous for the GDNF gene (Gdnf+/–). Wild-type (Gdnf+/+) and Gdnf+/– mice (18 months old) were tested for locomotor activity and brain tissues were collected for measuring norepinephrine levels and uptake, as well as for morphological analysis. Spontaneous locomotion was reduced in Gdnf+/– mice in comparison with Gdnf+/+ mice. The reduced locomotor activity of Gdnf +/– mice was accompanied by reductions in NE transporter activity in the cerebellum and brain stem as well as decreased norepinephrine tissue levels in the LC. Tyrosine hydroxylase (TH) immunostaining demonstrated morphological alterations of LC-NE cell bodies and abnormal TH-positive fibers in the hippocampus, cerebellum, and frontal cortex of Gdnf+/– mice. These findings suggest that the LC-NE system of Gdnf+/– mice is impaired and suggest that GDNF plays an important role in continued maintenance of this neuronal system throughout life.

Loss of Catecholaminergic Neuromodulation of Persistent Forms of Hippocampal Synaptic Plasticity with Increasing Age

Neuromodulation by means of the catecholaminergic system is a key component of motivation-driven learning and behaviorally modulated hippocampal synaptic plasticity. In particular, dopamine acting on D1/D5 receptors and noradrenaline acting on beta-adrenergic receptors exert a very potent regulation of forms of hippocampal synaptic plasticity that last for very long-periods of time (>24 h), and occur in conjunction with novel spatial learning. Antagonism of these receptors not only prevents long-term potentiation (LTP) and long-term depression (LTD), but prevents the memory of the spatial event that, under normal circumstances, leads to the perpetuation of these plasticity forms. Spatial learning behavior that normally comes easily to rats, such as object-place learning and spatial reference learning, becomes increasingly impaired with aging. Middle-aged animals display aging-related deficits of specific, but not all, components of spatial learning, and one possibility is that this initial manifestation of decrements in learning ability that become apparent in middle-age relate to changes in motivation, attention and/or the regulation by neuromodulatory systems of these behavioral states. Here, we compared the regulation by dopaminergic D1/D5 and beta-adrenergic receptors of persistent LTP in young (2-4 month old) and middle-aged (8-14 month old) rats. We observed in young rats, that weak potentiation that typically lasts for ca. 2 h could be strengthened into persistent (>24 h) LTP by pharmacological activation of either D1/D5 or beta-adrenergic receptors. By contrast, no such facilitation occurred in middle-aged rats. This difference was not related to an ostensible learning deficit: a facilitation of weak potentiation into LTP by spatial learning was possible both in young and middle-aged rats. It was also not directly linked to deficits in LTP: strong afferent stimulation resulted in equivalent LTP in both age groups. We postulate that this change in catecholaminergic control of synaptic plasticity that emerges with aging, does not relate to a learning deficit per se, rather it derives from an increase in behavioral thresholds for novelty and motivation that emerge with increasing age that impact, in turn, on learning efficacy.

Effects of Lifelong Ethanol Consumption on Brain Monoamine Transmitters in Alcohol-Preferring Alko Alcohol (AA) Rats

The purpose of the present study was to examine the combined effects of aging and lifelong ethanol exposure on the levels of monoamine neurotransmitters in different regions of the brain. This work is part of a project addressing interactions of aging and lifelong ethanol consumption in alcohol-preferring AA (Alko Alcohol) line of rats, selected for high voluntary consumption of ethanol. Intake of ethanol on the level of 4.5–5 g/kg/day for about 20 months induced only limited changes in the neurotransmitter levels; the concentration of noradrenaline was significantly reduced in the frontal cortex. There was also a trend towards lower levels of dopamine and 5-hydroxytryptamine (5-HT) in the frontal cortex, and towards a lower noradrenaline level in the dorsal cortex. Aging was associated with a decreased concentration of dopamine in the dorsal cortex and with a declining trend in the striatum. The levels of 5-HT in the limbic forebrain were higher in the aged than in the young animals, and in the striatum, there was a trend towards higher levels in older animals. The data suggest that a continuous intake of moderate amounts of ethanol does not enhance the age-related alterations in brain monoamine neurotransmission, while the decline in the brain level of dopamine associated with aging may be a factor contributing to age-related neurological disorders.

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.

Noradrenergic control of error perseveration in medial prefrontal cortex

The medial prefrontal cortex (mPFC) plays a key role in behavioral variability, action monitoring, and inhibitory control. The functional role of mPFC may change over the lifespan due to a number of aging-related issues, including dendritic regression, increased cAMP signaling, and reductions in the efficacy of neuromodulators to influence mPFC processing. A key neurotransmitter in mPFC is norepinephrine. Previous studies have reported aging-related changes in the sensitivity of mPFC-dependent tasks to noradrenergic agonist drugs, such as guanfacine. Here, we assessed the effects of yohimbine, an alpha-2 noradrenergic antagonist, in cohorts of younger and older rats in a classic test of spatial working memory (using a T-maze). Older rats (23–29 mo.) were impaired by a lower dose of yohimbine compared to younger animals (5–10 mo.). To determine if the drug acts on alpha-2 noradrenergic receptors in mPFC and if its effects are specific to memory-guided performance, we made infusions of yohimbine into mPFC of a cohort of young rats (6 mo.) using an operant delayed response task. The task involved testing rats in blocks of trials with memory- and stimulus-guided performance. Yohimbine selectively impaired memory-guided performance and was associated with error perseveration. Infusions of muscimol (a GABA-A agonist) at the same sites also selectively impaired memory-guided performance, but did not lead to error perseveration. Based on these results, we propose several potential interpretations for the role for the noradrenergic system in the performance of delayed response tasks, including the encoding of previous response locations, task rules (i.e., using a win-stay strategy instead of a win-shift strategy), and performance monitoring (e.g., prospective encoding of outcomes).

Dysfunctional nucleus tractus solitarius: its crucial role in promoting neuropathogenetic cascade of Alzheimer's dementia--a novel hypothesis

The pathophysiological mechanism(s) underlying Alzheimer's disease (AD) still remain unclear, and no disease-modifying or prophylactic therapies are currently available. Unraveling the fundamental neuropathogenesis of AD is an important challenge. Several studies on AD have suggested lesions in a number of CNS areas including the basal forebrain, hippocampus, entorhinal cortex, amygdale/insula, and the locus coeruleus. However, plausible unifying studies on the upstream factors that involve these heterogeneous regions and herald the onset of AD pathogenesis are not available. The current article presents a novel nucleus tractus solitarius (NTS) vector hypothesis that underpins several disparate biological mechanisms and neural circuits, and identifies relevant hallmarks of major presumptive causative factor(s) linked to the NTS, in older/aging individuals. Aging, obesity, infection, sleep apnea, smoking, neuropsychological states, and hypothermia-all activate inflammatory cytokines and oxidative stress. The synergistic impact of systemic proinflammatory mediators activates microglia and promotes neuroinflammation. Acutely, the innate immune response is protective defending against pathogens/toxins; however, when chronic, it causes neuroinflammation and neuronal dysfunction, particularly in brainstem and neocortex. The NTS in the brainstem is an essential multiple signaling hub, and an extremely important central integration site of baroreceptor, chemoreceptor, and a multitude of sensory afferents from gustatory, gastrointestinal, cardiac, pulmonary, and upper airway systems. Owing to persistent neuroinflammation, the dysfunctional NTS exerts deleterious impact on nucleus ambiguus, dorsal motor nucleus of vagus, hypoglossal, parabrachial, locus coeruleus and many key nuclei in the brainstem, and the hippocampus, entorhinal cortex, prefrontal cortex, amygdala, insula, and basal forebrain in the neocortex. The neuronal and synaptic dysfunction emanating from the inflamed NTS may affect its interconnected pathways impacting almost the entire CNS--which is already primed by neuroinflammation, thus promoting cognitive and neuropsychiatric symptoms. The upstream factors discussed here may underpin the neuropathopgenesis of AD. AD pathology is multifactorial; the current perspective underscores the value of attenuating disparate upstream factors--in conjunction with anticholinesterase, anti-inflammatory, immunosuppressive, and anti-oxidant pharmacotherapy. Amelioration of the NTS pathology may be of central importance in countering the neuropathological cascade of AD. The NTS, therefore, may be a potential target of novel therapeutic strategies.

Developmental changes in pacemaker currents in mouse locus coeruleus neurons

The present study compares the electrophysiological properties and the primary pacemaker currents that flow during the interspike interval in locus coeruleus (LC) neurons from infant (P7-12 days) and young adult (8-12 weeks) mice. The magnitude of the primary pacemaker currents, which consist of an excitatory TTX-sensitive Na(+) current and an inhibitory voltage-dependent K(+) current, increased in parallel during development. We found no evidence for the involvement of hyperpolarization-activated (I(H)) or Ca(2+) currents in pacemaking in infant or adult LC neurons. The incidence of TTX-resistant spikes, observed during current clamp recordings, was greater in adult neurons. Neurons from adult animals also showed an increase in voltage fluctuations, during the interspike interval, as revealed in the presence of the K(+) channel blocker, 4-AP (1mM). In summary, our results suggest that mouse LC neurons undergo changes in basic electrophysiological properties during development that influence pacemaking and hence spontaneous firing in LC neurons.

Transcription factor Phox2 upregulates expression of norepinephrine transporter and dopamine β-hydroxylase in adult rat brains

Degeneration of the noradrenergic locus coeruleus (LC) in aging and neurodegenerative diseases is well documented. Slowing or reversing this effect may have therapeutic implications. Phox2a and Phox2b are homeodomain transcriptional factors that function as determinants of the noradrenergic phenotype during embryogenesis. In the present study, recombinant lentiviral eGFP-Phox2a and -Phox2b (vPhox2a and vPhox2b) were constructed to study the effects of Phox2a/2b over-expression on dopamine β-hydroxylase (DBH) and norepinephrine transporter (NET) levels in central noradrenergic neurons. Microinjection of vPhox2 into the LC of adult rats significantly increased Phox2 mRNA levels in the LC region. Over-expression of either Phox2a or Phox2b in the LC was paralleled by significant increases in mRNA and protein levels of DBH and NET in the LC. Similar increases in DBH and NET protein levels were observed in the hippocampus following vPhox2 microinjection. In the frontal cortex, only NET protein levels were significantly increased by vPhox2 microinjection. Over-expression of Phox2 genes resulted in a significant increase in BrdU-positive cells in the hippocampal dentate gyrus. The present study demonstrates an upregulatory effect of Phox2a and Phox2b on the expression of DBH and NET in noradrenergic neurons of rat brains, an effect not previously shown in adult animals. Phox2 genes may play an important role in maintaining the function of the noradrenergic neurons after birth, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.

Endoplasmic reticulum stress in wake-active neurons progresses with aging

Fragmentation of wakefulness and sleep are expected outcomes of advanced aging. We hypothesize that wake neurons develop endoplasmic reticulum dyshomeostasis with aging, in parallel with impaired wakefulness. In this series of experiments, we sought to more fully characterize age-related changes in wakefulness and then, in relevant wake neuronal populations, explore functionality and endoplasmic reticulum homeostasis. We report that old mice show greater sleep/wake transitions in the active period with markedly shortened wake periods, shortened latencies to sleep, and less wake time in the subjective day in response to a novel social encounter. Consistent with sleep/wake instability and reduced social encounter wakefulness, orexinergic and noradrenergic wake neurons in aged mice show reduced c-fos response to wakefulness and endoplasmic reticulum dyshomeostasis with increased nuclear translocation of CHOP and GADD34. We have identified an age-related unfolded protein response injury to and dysfunction of wake neurons. It is anticipated that these changes contribute to sleep/wake fragmentation and cognitive impairment in aging.

Age-related synaptic loss of the medial olivocochlear efferent innervation

Age-related functional decline of the nervous system is consistently observed, though cellular and molecular events responsible for this decline remain largely unknown. One of the most prevalent age-related functional declines is age-related hearing loss (presbycusis), a major cause of which is the loss of outer hair cells (OHCs) and spiral ganglion neurons. Previous studies have also identified an age-related functional decline in the medial olivocochlear (MOC) efferent system prior to age-related loss of OHCs. The present study evaluated the hypothesis that this functional decline of the MOC efferent system is due to age-related synaptic loss of the efferent innervation of the OHCs. To this end, we used a recently-identified transgenic mouse line in which the expression of yellow fluorescent protein (YFP), under the control of neuron-specific elements from the thy1 gene, permits the visualization of the synaptic connections between MOC efferent fibers and OHCs. In this model, there was a dramatic synaptic loss between the MOC efferent fibers and the OHCs in older mice. However, age-related loss of efferent synapses was independent of OHC status. These data demonstrate for the first time that age-related loss of efferent synapses may contribute to the functional decline of the MOC efferent system and that this synaptic loss is not necessary for age-related loss of OHCs.

Involvement of BDNF in age-dependent alterations in the hippocampus

It is known since a long time that the hippocampus is sensitive to aging. Thus, there is a reduction in the hippocampal volume during aging. This age-related volume reduction is paralleled by behavioral and functional deficits in hippocampus-dependent learning and memory tasks. This age-related volume reduction of the hippocampus is not a consequence of an age-related loss of hippocampal neurons. The morphological changes associated with aging include reductions in the branching pattern of dendrites, as well as reductions in spine densities, reductions in the densities of fibers projecting into the hippocampus as well as declines in the rate of neurogenesis. It is very unlikely that a single factor or a single class of molecules is responsible for all these age-related morphological changes in the hippocampus. Nevertheless, it would be of advantage to identify possible neuromodulators or neuropeptides that may contribute to these age-related changes. In this context, growth factors may play an important role in the maintenance of the postnatal hippocampal architecture. In this review it is hypothesized that brain-derived neurotrophic factor (BDNF) is a factor critically involved in the regulation of age-related processes in the hippocampus. Moreover, evidences suggest that disturbances in the BDNF-system also affect hippocampal dysfunctions, as e.g. seen in major depression or in Alzheimer disease.

Reduced brain-derived neurotrophic factor is associated with a loss of serotonergic innervation in the hippocampus of aging mice

Brain-derived neurotrophic factor (BDNF) regulates monoamine neuronal growth, survival and function in development and throughout adulthood. At 18 months of age, mice with constitutive reductions in BDNF expression show decreased serotonin innervation in the hippocampus compared with age-matched wildtype mice. It is not known, however, whether age-accelerated loss of serotonergic innervation in BDNF(+/-) mice occurs in other brain regions, advances beyond 18 months or is associated with alterations in other neurotransmitter systems. In this study, immunocytochemistry was used to assess serotonergic and catecholaminergic innervation in 26-month-old BDNF(+/-) mice. Age-related loss of serotonin axons in the hippocampus was potentiated in BDNF(+/-) mice compared with wildtype mice at this late age, particularly in the CA1 subregion. By contrast, aging BDNF(+/-) mice showed increased serotonin innervation of the basomedial nucleus of the amygdala. In the noradrenergic system, BDNF(+/-) mice showed reduced numbers of cell bodies and fibers in the locus coeruleus compared with age-matched wildtype mice, whereas no changes were observed in dopaminergic innervation with respect to genotype. In vivo zero net flux microdialysis in awake mice showed a significant decrease in extracellular serotonin levels in the hippocampus in BDNF(+/-) mice at 20 months of age. Thus, reduced BDNF is associated with altered serotonergic and noradrenergic innervation in aging mice and, in particular, with accelerated loss of serotonergic innervation to the hippocampus that is manifest as a decrease in basal neurotransmission.

Age-dependent changes in dopaminergic projections from the substantia nigra pars compacta to the neostriatum

Age-dependent changes in dopaminergic (DA) innervation of the neostriatum (Str) were studied in male F344/N rats. Projections from the substantia nigra pars compacta (SNc) to the neostriatum were quantified using electrophysiological methods at age points from 6 to 24 months. The percentage of DA neurons activated antidromically by electrical stimulation (P-index) of Str increased between 18 and 24 months. Additionally, the percentage of DA neurons showing multiple antidromic latencies from striatal stimulation (M-index), which suggests axonal branching of individual DA neurons, increased significantly between 6 and 12 months and 6 and 24 months. These results suggest that DA neurons exhibit increased axonal branching in the aged brain.

Age-related changes in sympathetic neurotransmission in rat retina and choroid

While age-related night vision loss and age-related macular degeneration are well characterized, less is known about the normal aging process in the retina and choroid. The purpose of this study was to ascertain whether dopamine beta-hydroxylase (DBH), beta1- and beta2-adrenergic receptor gene and protein expression are altered in the retina and choroid with age. The retina and choroid were dissected from F344xBNF1 hybrid rats aged 8, 22, and 32 months. Real-time PCR and Western blot analysis were conducted to determine steady-state mRNA and protein expression. Immunohistochemistry (IHC) was conducted to localize DBH protein expression in the retina. DBH protein expression was substantially decreased with age in the retina, particularly in the outer nuclear layer, with no changes in DBH expression noted in the choroid. There was a significant increase in beta1-adrenergic receptor protein expression in retinal samples at 22 months, while beta2-adrenergic receptor protein expression was not affected by age. Decreased expression of DBH with age in the retina could lead to reduced production of norepinephrine, potentially resulting in an increase of beta1-adrenergic receptor expression due to denervation supersensitivity. Gene expression for DBH, beta1- and beta2-adrenergic receptors were observed to peak at 22 months and return to baseline levels by 32 months of age in the choroid. Our findings suggest that the retina may be more sensitive to age-related loss of sympathetic neurotransmission than the choroid, which may partially explain normal age-related vision loss in the elderly.

Involvement of neurotrophic factors in aging of noradrenergic innervations in hippocampus and frontal cortex

In the present study, we investigated the age-dependent changes in the axon terminals of the locus coeruleus (LC) neurons in the frontal cortex and hippocampus, in which a high degree of axonal branching in the middle-aged brain was suggested to occur in our previous electrophysiological study. We used 6-, 13- and 25-month-old male F344/N rats, and performed Western blot analysis of the norepinephrine transporter (NET), brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF). NET expression level increased in the 13-month-old hippocampus, but was not altered by aging in the frontal cortex. BDNF expression level increased in the hippocampus, but did not change with age in the frontal cortex. On the other hand, GDNF expression level was increased with age in the frontal cortex, but was not in the hippocampus. These results suggest that the LC noradrenergic innervations may be locally regulated by different neurotrophic factors that exert their trophic actions at different target sites.

Age-dependent interactive changes in serotonergic and noradrenergic cortical axon terminals in F344 rats

In the frontal cortex of aging rats, we found an increase in sprouting of the noradrenergic (NA) axons originated from the locus coeruleus (LC). The serotonergic (5-HT) axons originating from the dorsal raphe (DR) share the same cortical area and their age-dependent changes and interactions with NA axons were still unclear. To compare quantitatively the extent of axonal sprouting of DR and LC neurons in the frontal cortex, we extracellularly recorded from both DR and LC neurons in the same animals and antidromically stimulated 32 cortical sites (a pair of stimulating electrodes was moved at 100-mum intervals from 500 to 2000 microm in depth). In addition, to examine the effects of degeneration of 5-HT axons on NA axons, and vice versa, we used specific neurotoxins for 5-HT (PCA) or NA (DSP-4) axons. We also used noradrenaline uptake inhibitor (maprotiline) to verify the effects of NA on degeneration of 5-HT axons. Results suggested that 5-HT axons sprouted between 15 and 17 months of age and noradrenaline accelerated the age-dependent change of 5-HT axons.

Effects of BDNF infusion on the axon terminals of locus coeruleus neurons of aging rats

Using in vivo electrophysiological techniques and continuous local infusion methods, we examined the effects of brain-derived neurotrophic factor (BDNF) and its specific antibody (anti-BDNF) on the noradrenergic axon terminals of the locus coeruleus (LC) neurons in the frontal cortex of aging rats. Recently, we observed that LC neurons with multiple-threshold antidromic responses (multi-threshold LC neurons) increased critically between 15 and 17 months of age. To examine whether the BDNF is involved in this change occurred in the aging brain, we continuously infused BDNF into the frontal cortex for 14 days. Exogenous BDNF produced a marked increase in the multi-threshold LC neurons in the 13-month-old brain, accompanied with a decrease in threshold current. However, no morphological change in the noradrenergic axons was observed in the BDNF-infused cortex. In contrast, infusion of anti-BDNF led to a dose-dependent reduction of the multi-threshold LC neurons in the 19-month-old brain, accompanied with an increase in threshold current. These findings suggest that BDNF may contribute to functional changes in the presynaptic axon terminals of LC neurons in the aging brain.

Development of the norepinephrine transporter in the rat CNS

The norepinephrine transporter (NET) plays a major role in regulating the actions of norepinephrine by removing norepinephrine from the synapse. Many studies suggest norepinephrine plays an important role in regulating development of the CNS, pointing to NET as an important factor in this process. We examined the ontogeny of NET expression in rat brain at 5, 10, 15, 20 and 25 days postnatally (PND) and in adults, using quantitative autoradiography with [3H]nisoxetine as ligand. At PND 5 and 10 most forebrain areas had low NET expression (1-2 fmol/mg tissue). By PND 15 most forebrain areas increased NET expression approximately five-fold compared with PND 10, levels generally similar to those found in the adult brain. In contrast, NET development in the brainstem exhibited elevated densities at PND 5, 10 and 20 that decreased in the adult. The locus coeruleus, in particular, had very high NET expression in the early postnatal period that decreased dramatically in the adult brain. These data illustrate a dynamic ontogenic profile for NET, characterized by developmental increases in forebrain structures and contrasting decreases in the brainstem. The early postnatal expression of NET in brainstem and the subsequent decrease or loss of NET expression with maturation suggest an important role for this transporter and for norepinephrine in the development of many brain regions. These studies also have important implications for use of drugs targeting the noradrenergic system in children and adolescents, such as antidepressants and drugs of abuse.

Cognitive impairment in aged rhesus monkeys associated with monoamine receptors in the prefrontal cortex

The "frontal aging hypothesis" has been proposed by many researchers suggesting that the earliest and most severe age-related changes in the cortex occur in the frontal lobes. Two of these changes include decreases in cognitive functions mediated by the prefrontal cortex (PFC) and significant decreases in norepinephrine (NE) and dopamine (DA). To investigate whether the changes in these neurotransmitter systems are directly related to the cognitive decline seen in aging we utilized the rhesus monkey as a model of normal human aging. Our goal was to determine if age-related changes in cognition is associated with changes in norepinephrine and dopamine receptor binding density in the PFC. Eight young monkeys between five and ten years of age (six males and two female) and eight aged monkeys between 25 and 32 years of age (five males and three females) were behaviorally characterized. Subsequently on-the-slide in vitro binding assays were used to quantify the alpha-1 adrenergic, alpha-2 adrenergic and DA1 receptors as well as the NE and DA uptake receptors. Aged animals as a group demonstrated significant cognitive impairments and aging produced a significant decrease in alpha-1 adrenergic and alpha-2 adrenergic receptor binding in the PFC but no significant change in binding for the DA1 receptor or the NE or DA uptake receptors. Further analysis revealed a significant relationship between monoamine receptor binding and cognitive performance on three tasks: delayed non-matching to sample, delayed recognition span test and the conceptual set-shifting task.

Age-associated synapse elimination in mouse parasympathetic ganglia

Little is known about the effects of aging on synapses in the mammalian nervous system. We examined the innervation of individual mouse submandibular ganglion (SMG) neurons for evidence of age-related changes in synapse efficacy and number. For approximately 85% of adult life expectancy (30 months) the efficacy of synaptic transmission, as determined by excitatory postsynaptic potential (EPSP) amplitudes, remains constant. Similarly, the number of synapses contacting individual SMG neurons is also unchanged. After 30 months of age, however, some neurons (23%) dramatically lose synaptic input exhibiting both smaller EPSP amplitude and fewer synaptic boutons. Attenuation of both the amplitude and frequency of miniature EPSPs was also observed in neurons from aged animals. Electron micrographs revealed that, although there were many vesicle-laden preganglionic axonal processes in the vicinity of the postsynaptic membrane, the number of synaptic contacts was significantly lower in old animals. These results demonstrate primary, age-associated synapse elimination with functional consequences that cannot be explained by pre- or postsynaptic cell death.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

BDNF is necessary for maintenance of noradrenergic innervations in the aged rat brain

In the axon terminals of the locus coeruleus (LC) neurons, a high level of axonal branching was occurred in the middle-aged brain, and the increased branching was maintained in the aged brain. In the present study, we hypothesized that neurotrophic support is necessary for the morphological age-related changes seen in the noradrenergic innervations from the LC to frontal cortex. Through immunohistochemical and quantitative image analyses, we examined the age-dependent effects of brain-derived neurotrophic factor (BDNF) on the noradrenergic axon terminals in the frontal cortex of F344 rats. We continuously infused BDNF into the frontal cortex of young (6-months-old), middle-aged (13-months-old), or aged (25-months-old) rats. Exogenous BDNF infusion caused a marked increase in the density of noradrenergic axons in the aged brain, but no trophic action of BDNF was observed in the young and middle-aged brain. Neutralization of endogenous BDNF with a specific function-blocking antibody to BDNF led to a reduction in noradrenergic axons in the frontal cortex of 19-month-old rats. The present results suggest that BDNF is not involved in the augmentation of noradrenergic innervations in the aging brain, but it is necessary for the maintenance of noradrenergic innervations in the aged brain.

Effects of central hypocretin-1 administration on hemodynamic responses in young-adult and middle-aged rats

The prevalence of hypertension in middle age correlates with impaired autonomic regulation and as norepinephrinergic neurons decline with increasing age, and this reduction may contribute to this impairment. Central hypocretin-activated norepinephrinergic neurons contribute to sympathetic regulation. In the present study we compared sympathoadrenal effects of intracerebroventricular (i.c.v.) hypocretin-1(5 nmol) between young-adult (12-14 weeks) and middle-aged (12-14 months) rats. Arterial blood pressure, heart rate and plasma catecholamines were assessed under pentobarbital anesthesia. In addition, we compared hypocretin-1 and K(+)-evoked norepinephrine release from the cerebrocortical slices prepared from young-adult and middle-aged rats. We also examined whether the novel hypocretin receptor-1 antagonist (SB-334867) could reverse these hypocretin-1 effects both in vivo and in vitro. I.c.v. hypocretin-1 significantly increased blood pressure by some 7%, heart rate by 9% and plasma norepinephrine concentrations by 100% in young-adult rats. In middle-aged rats these parameters did not change. Plasma epinephrine did not increase in either group. There was a significant correlation between changes in mean arterial pressure and plasma norepinephrine. Similarly, hypocretin-1 evoked norepinephrine release from cerebrocortical slices prepared from young-adult rats was significantly higher than that of middle-aged rats whilst K(+)-evoked release did not differ between the groups. SB-334867 significantly attenuated hypocretin-1-increased blood pressure and both in vivo and in vitro norepinephrine release. The present data suggest that hypocretinergic neurons may contribute to the regulation of central but not adrenal sympathetic activity. Moreover, sympathetic regulation by hypocretinergic neurones may disappear in middle-age in rats.

Age-related changes in the release and uptake activity of presynaptic axon terminals of rat locus coeruleus neurons

Age-related changes in the release and uptake activity of presynaptic axon terminals of rat locus coeruleus (LC) noradrenergic neurons were studied in the frontal cortex using an extracellular single unit recording technique in vivo. Clonidine, a selective alpha(2) adrenergic agonist, and nisoxetine, a selective noradrenaline uptake inhibitor, were infused locally into the frontal cortex to examine the effects of these drugs on release and uptake activities of the axon terminals of LC neurons. Although the infusion of clonidine produced a marked suppression of release, the effect did not change with age. Infusion of nisoxetine caused an inhibition of uptake, but the effect was attenuated in aged rats. These results suggest that the release activity mediated by the presynaptic autoreceptor did not change with age, but the uptake activity mediated by the NA transporter declined with age in the axon terminals of LC neurons.

Age-dependent changes in electroencephalographic responses to alcohol consumption in subjects with aldehyde dehydrogenase-2 genetic variations

BACKGROUND

We have recently reported that alcohol consumption resulted in a significant increase in alpha power of the EEGs in aldehyde dehydrogenase-2 (ALDH2)-normal (NN) subjects but not in ALDH2-deficient heterozygote (ND) subjects. The purpose of the present study was to investigate interactive effects of individual factors such as age and ALDH2 genotype on alcohol-induced EEG changes.

METHODS

We examined EEG power spectral changes induced by 0.4 ml/kg of alcohol ingestion in 53 NN and 21 ND subjects of two different age groups: younger and older groups. Blood ethanol and acetaldehyde levels were also determined in 17 NN and 13 ND subjects in separate studies.

RESULTS

Alcohol consumption markedly increased EEG power in the NN subjects of the older group, especially in theta and slow alpha power, whereas only slight increases were noted in fast alpha and beta power in the NN subjects of the younger group. However, no such differences between the two age groups were observed in the ND subjects. It should be noted that there were no differences in blood ethanol and acetaldehyde level at 30 min after alcohol ingestion between the different age groups in both genotypes. However, there was a significant increase in frequency of alcohol intake in the older group of both genotype groups. The multiple regression analysis indicated that both alcohol use habits and genotype, as well as aging, significantly modulated EEG changes after alcohol ingestion.

CONCLUSIONS

The results suggest that both ALDH2 genotype and age as well as alcohol use habits modify alcohol sensitivity in the central nervous system, resulting in greater increases in EEG energy in response to alcohol intake in the older group of the NN subjects.

Presynaptic regulation of neurotransmitter release in the cortex of aged rats with differential memory impairments

Cluster analysis of water-maze reference-memory performances of 25-27-month-old (compared to 3-5-month-old) rats distinguished subpopulations of young adult rats (YOUNG), aged rats with no significant impairment (AU), aged rats with moderate impairment (AMI), and aged rats with severe impairment (ASI). In the frontoparietal cortex, we subsequently assessed the electrically evoked release of tritium in slices preloaded with [3H]choline, [3H]noradrenaline (NA), or [3H]serotonin (5-HT) and the effects of an agonist (oxotremorine, UK 14,304, and CP 93,129) of the respective autoreceptors. Cholinergic and monoaminergic markers were measured in homogenates. Overall, aged rats exhibited reduced accumulation of [3H]choline (-25%) and weaker evoked transmitter release (in % of accumulated tritium: -44%, -20%, and -34%, for [3H]acetylcholine, [3H]NA, and [3H]5-HT, respectively). In all rats, the inhibitory effects of the autoreceptor agonists on the evoked release of [3H] were comparable. Acetylcholinesterase (AChE), not choline acetyltransferase (ChAT), activity was reduced. The results suggest age-related modifications in the cholinergic, noradrenergic, and serotonergic innervation of the frontoparietal cortex, alterations of evoked transmitter release, but no interference with presynaptic autoinhibition of the release. Neither of these alterations seemed to account for the cognitive impairment assessed.

Age-related decline in the tyrosine hydroxylase-immunoreactive innervation of the amygdala and dentate gyrus in mice

Numbers of catecholaminergic neurons are known to decline with aging. Whether projections of these neurons to the forebrain are similarly affected is not known. High densities of tyrosine hydroxylase-immunoreactive (TH-ir) fibers are found in the hippocampal formation (CA1-3, dentate gyrus) and in the amygdala of normal adult mice. We report here that densities of TH-ir fibers in the amygdala and hippocampus in aged mice (21-26 months) decrease dramatically and in a subregion-specific fashion. There is a reduction of 35% in the dentate gyrus, while hippocampal regions CA1 through CA3 are almost entirely spared. In the amygdala the lateral, basolateral, basomedial, and central nucleus were affected, with fiber reduction ranging from 19% to 34%. These results indicate that the age-related decline of TH-ir catecholaminergic cell bodies in the substantia nigra and the ventral tegmental area induces substantial losses of TH-ir fibers in the amygdala and dentate gyrus, but not in other areas of the hippocampal formation. This suggests that region-specific factors may be implicated in the regulation of maintenance vs. degeneration of TH-ir fibers during aging.

Ageing and the nigrostriatal dopaminergic system

BACKGROUND

Brain dopamine has been the focus of numerous studies owing to its crucial role in motor function and in neurological and psychiatric disease processes. Whilst early work relied on postmortem data, functional imaging has allowed a more sophisticated approach to the quantification of receptor density, affinity and functional capacity. This review aims to summarise changes in the nigrostriatal dopaminergic system which accompany normal ageing.

METHODS

A literature search focussed on postmortem and neuroimaging studies of normal ageing within the nigrostriatal dopaminergic tract. The functional significance of age-related effects was also considered.

RESULTS

There are significant reductions in pre- and post-synaptic markers of brain dopamine activity during normal ageing: However the rate of decline (linear or exponential), the effects of gender and heterogeneity and the mechanisms by which these changes occur remain undetermined. Limited data suggest there is a significant association between postsynaptic receptor density and specific aspects of motor and cognitive function.

CONCLUSION

The identification of strategies to improve dopaminergic transmission may delay the onset of motor and cognitive deficits associated with normal ageing. In order to develop effective preventative strategies, the causative mechanisms underlying age-related changes and the interaction between synaptic structure and function need to be more clearly elucidated.

Effects of age and GDNF on noradrenergic innervation of the hippocampal formation: studies from intraocular grafts

Recent studies have suggested that factors in the target tissue influence the degree of plasticity and regeneration following aging and/or specific insults. We have investigated whether young or aged targets differ in their noradrenergic innervation from fetal locus coeruleus (LC) neurons, and also if a specific growth factor, glial cell line-derived neurotrophic factor (GDNF) can affect this innervation pattern. Tissue pieces of fetal brainstem and young (3 months) or old (18 months) iris tissue were transplanted simultaneously into the anterior chamber of the eye of adult hosts. We found that aged iris transplants became innervated to a significantly lesser degree by the cografted LC neurons than young iris transplants. Fetal hippocampal tissue was then grafted to adult hosts, and a fetal brainstem graft containing LC neurons was placed adjacent to the first graft, either at 3 or 21 months post-grafting. Thus, old/young chimeras of the noradrenergic coeruleo-hippocampal pathway were created. Aged hippocampal grafts received a much less dense innervation from co-grafted LC neurons than young hippocampal grafts. Tyrosine hydroxylase-positive-immunoreactive innervation was only found in the outskirts of aged grafts, while the young hippocampal grafts contained an even innervation pattern. The innervation density of hippocampal grafts was significantly enhanced by GDNF treatment. These findings demonstrate that target-derived factors may regulate neuronal plasticity, and that the age of the target is more important for innervation properties than the age of the neuron innervating a particular target.

Changes in cortical noradrenergic axon terminals of locus coeruleus neurons in aged F344 rats

The noradrenergic innervations and noradrenaline contents of the frontal cortex in two age groups (9 and 25 months) of male F344 rats have been quantified by electrophysiological and biochemical methods. In the electrophysiological study, the percentage of locus coeruleus (LC) neurons activated antidromically from the frontal cortex decreased with age. In contrast, the percentage of LC neurons showing multiple antidromic latencies, which suggests axonal branching of individual LC neurons, increased markedly between 9 and 25 months in the frontal cortex. In the biochemical study, we found no significant difference in noradrenaline levels in the cortical terminal fields of LC neurons during aging. These results suggest that LC neurons give rise to axonal branches to retain noradrenaline levels in their target fields in the aged brain. Our findings show that LC neurons preserve a strong capability for remodeling their axon terminals even in the aged brain.

Age-dependent changes in noradrenergic innervations of the frontal cortex in F344 rats

Noradrenergic innervations of the frontal cortex with advancing age (9, 13 and 25 months) in male F344 rats were quantified by immunohistochemistry for dopamine-beta-hydroxylase (DBH), which is a marker enzyme for noradrenergic axons. The density of DBH-positive axons, varicosities (swellings along an axon from which noradrenaline is released), and the number of varicosities per unit length of axon were measured in the frontal cortex. We found that the density of axons and varicosities significantly decreased at an earlier stage of aging (9-13 months), but not at a later stage (13-25 months). On the other hand, the number of varicosities per unit length of axon did not change with age. The result shows that the density of varicosities, which represent the synapses of noradrenergic neurons, decrease in the frontal cortex in the early aging process.

Age-dependent changes in axonal branching of single locus coeruleus neurons projecting to two different terminal fields

Age-dependent changes in the axonal branching patterns of single locus coeruleus neurons, which innervate both the frontal cortex and hippocampus dentate gyrus, have been studied in male F344 rats. We used an electrophysiological approach involving antidromic activation to differentiate single from multi-threshold locus coeruleus neurons in each terminal field with age (7-27 mo of age). Most of these neurons have a single threshold in the young rats, whereas in the older brains, the neurons have multi-threshold responses. This implies an increased amount of axonal branching in the older brains. The time course of the increase differs in the two terminal fields, suggesting that the degree of plasticity or age-dependent increase in branching can differ across terminal fields.

Changes in electrophysiological properties of axon terminals of locus coeruleus neurons with age in F344 rat

Age-dependent changes in electrophysiological properties of locus coeruleus (LC) neurons were studied in urethane anesthetized male F344 rats aged 8, 17 and 22 months. These properties, such as spontaneous firing rate, conduction time, and threshold currents were measured for individual LC neurons antidromically activated from terminals in the frontal cortex. We found no change in the electrophysiological properties with age except for an increased number of low-threshold LC neurons in 22-month-old rats compared to that of 8-month-old rats. These results suggested that the electrophysiological properties of cortical axon terminals of LC neurons changed with age.