Altered synaptic function in Alzheimer's disease

Alzheimer's disease is the leading cause of dementia in the elderly, presenting itself clinically by progressive loss of memory and learning. Since synaptic density correlates more closely with cognitive impairment than any other pathological lesion observable in the disease pathology, an increased understanding of the mechanisms behind synaptic disconnection is of vital importance. Our lab investigated the neurotransmitter-specific status of distinct cortical presynaptic bouton populations in various transgenic mouse models of the Alzheimer's-like amyloid pathology in order to assess their involvement throughout the progression of the pathology. These studies have revealed that the amyloid pathology appears to progress in a neurotransmitter-specific manner where the cholinergic terminals appear most vulnerable, followed by the glutamatergic terminals and finally by the somewhat more resilient GABAergic terminals. This review will discuss additional studies which also provide evidence of a neurotransmitter-specific pathology as well as comment on the potential explanations for the observed vulnerabilities, touching upon metabolic demand, trophic support and receptor mediated activation.

Activity-dependent release of precursor nerve growth factor, conversion to mature nerve growth factor, and its degradation by a protease cascade

In this report, we provide direct demonstration that the neurotrophin nerve growth factor (NGF) is released in the extracellular space in an activity-dependent manner in its precursor form (proNGF) and that it is in this compartment that its maturation and degradation takes place because of the coordinated release and the action of proenzymes and enzyme regulators. This converting protease cascade and its endogenous regulators (including tissue plasminogen activator, plasminogen, neuroserpin, precursor matrix metalloproteinase 9, and tissue inhibitor metalloproteinase 1) are colocalized in neurons of the cerebral cortex and released upon neuronal stimulation. We also provide evidence that this mechanism operates in in vivo conditions, as the CNS application of inhibitors of converting and degrading enzymes lead to dramatic alterations in the tissue levels of either precursor NGF or mature NGF. Pathological alterations of this cascade in the CNS might cause or contribute to a lack of proper neuronal trophic support in conditions such as cerebral ischemia, seizure and Alzheimer's disease or, conversely, to excessive local production of neurotrophins as reported in inflammatory arthritis pain.

Imbalance towards inhibition as a substrate of aging-associated cognitive impairment

The number of synapses in the cerebral cortex decreases with aging. However, how this structural change translates into the cognitive impairment observed in aged animals remains unknown. Aged animals are not a homogenous group with respect to their cognitive performances; but instead, they can be separated into aged cognitively unimpaired ("normal") and aged cognitively impaired groups using a spatial memory task such as the Morris water maze. These two aged groups provide an unprecedented opportunity to isolate synaptic properties that relate to cognitive impairment from unrelated factors associated with normal aging. Using such classification, we conducted whole-cell patch-clamp recordings to measure basal spontaneous miniature excitatory (mEPSCs) and inhibitory synaptic currents (mIPSCs) bombarding layer V pyramidal neurons in the parietal cortex. We found that the frequencies of both mEPSC and mIPSC were lower in aged normal rats when compared with young rats. In contrast, aged cognitively impaired rats displayed a reduction in mEPSC frequency only. This results in an imbalance towards inhibition that may be an important substrate of the cognitive impairment in aged animals. We also found that pyramidal neurons in both aged normal and aged cognitively impaired rats exhibit similar structural attritions. Thus, cognitive impairment may be more related to an altered balance between different neurotransmitter systems than a mere reduction in synaptic structures.

The amyloid pathology progresses in a neurotransmitter-specific manner

Past studies using transgenic models of early-staged amyloid pathology, have suggested that the amyloid pathology progresses in a neurotransmitter-specific manner where cholinergic terminals appear most vulnerable, followed by glutamatergic terminals and finally by somewhat more resistant GABAergic terminals. To determine whether this neurotransmitter-specific progression persists at later pathological stages, presynaptic bouton densities, and the areas of occupation and localization of plaque adjacent dystrophic neurites were quantified in 18-month-old APP(K670N, M671L)+PS1(M146L) doubly transgenic mice. Quantification revealed that transgenic animals had significantly lower cholinergic, glutamatergic and GABAergic presynaptic bouton densities. Cholinergic and glutamatergic dystrophic neurites appear to be heavily influenced by fibrillar Abeta as both types displayed a decreasing area of occupation with respect to increasing plaque size. Furthermore, cholinergic dystrophic neurites reside in closer proximity to plaques than glutamatergic dystrophic neurites, while GABAergic dystrophic neurites were minimal regardless of plaque size. To investigate whether similarities exist in the human AD pathology, a monoclonal antibody (McKA1) against the human vesicular glutamate transporter 1 (VGluT1) was developed. Subsequent staining in AD brain tissue revealed the novel presence of glutamatergic dystrophic neurites, to our knowledge the first evidence of a structural glutamatergic deficit in the AD pathology.

Translational control of hippocampal synaptic plasticity and memory by the eIF2alpha kinase GCN2

Studies on various forms of synaptic plasticity have shown a link between messenger RNA translation, learning and memory. Like memory, synaptic plasticity includes an early phase that depends on modification of pre-existing proteins, and a late phase that requires transcription and synthesis of new proteins. Activation of postsynaptic targets seems to trigger the transcription of plasticity-related genes. The new mRNAs are either translated in the soma or transported to synapses before translation. GCN2, a key protein kinase, regulates the initiation of translation. Here we report a unique feature of hippocampal slices from GCN2(-/-) mice: in CA1, a single 100-Hz train induces a strong and sustained long-term potentiation (late LTP or L-LTP), which is dependent on transcription and translation. In contrast, stimulation that elicits L-LTP in wild-type slices, such as four 100-Hz trains or forskolin, fails to evoke L-LTP in GCN2(-/-) slices. This aberrant synaptic plasticity is mirrored in the behaviour of GCN2(-/-) mice in the Morris water maze: after weak training, their spatial memory is enhanced, but it is impaired after more intense training. Activated GCN2 stimulates mRNA translation of ATF4, an antagonist of cyclic-AMP-response-element-binding protein (CREB). Thus, in the hippocampus of GCN2(-/-) mice, the expression of ATF4 is reduced and CREB activity is increased. Our study provides genetic, physiological, behavioural and molecular evidence that GCN2 regulates synaptic plasticity, as well as learning and memory, through modulation of the ATF4/CREB pathway.

Intracellular and extracellular Abeta, a tale of two neuropathologies

The central pathological cause of Alzheimer disease (AD) is hypothesized to be an excess of beta-amyloid (Abeta) which accumulates into toxic fibrillar deposits within extracellular areas of the brain. These deposits disrupt neural and synaptic function and ultimately lead to neuronal degeneration and dementia. In addition to the pathological roles attributed to Abeta, evidence from our laboratory would suggest that Abeta serves a physiological role in the modulation of CRE-directed gene expression. This commentary also highlights some of the pathological consequences of the accumulation of intracellular Abeta. Finally it discusses the impact of cortical Abeta burden on transmitter-specific synaptic numbers as well as the generation of dystrophic neurites. The fundamental thesis of my proposal is that the Abeta pathology seen in AD is a continuous process from an initial abnormal Abeta intracellular accumulation to the well-established extracellular Abeta aggregation, culminating in the formation of amyloid plaques and dystrophic neurites.

Early dysregulation of hippocampal proteins in transgenic rats with Alzheimer's disease-linked mutations in amyloid precursor protein and presenilin 1

The response of the hippocampal proteome to expression of mutant proteins present in familial forms of Alzheimer's disease (AD) was studied using transgenic rats. These animals carry both the amyloid precursor protein Swedish and 717 mutation (APP(SW+717)) as well as the presenilin 1 Finnish mutation (PS1(FINN)). This transgenic rat model displays intracellular amyloid beta (Abeta) in neurons of the neocortex and the hippocampus (CA2 and CA3). The hippocampus was selected as it is one of the first brain regions affected in AD and is involved in the processing of short-term memory and spatial memory. Applying a proteomic approach, we demonstrate that the expression of APP(SW+717) and PS1(FINN) transgenes causes changes in expression of hippocampal proteins, some of which have been previously linked to learning and memory formation. The protein alterations documented here occur in the absence of plaque formation and prior to the onset of cognitive deficits later observed in these transgenic rats. This indicates that molecular changes take place in the hippocampal neurons in response to expression of mutant proteins APP(SW+717) and PS1(FINN), which precede the occurrence of overt extracellular accumulation of extracellular amyloid. The implications of these findings on our understanding of the early stages of AD are discussed.

Endogenous β-amyloid peptide synthesis modulates cAMP response element-regulated gene expression in PC12 cells

Extracellular-regulated kinases play a fundamental role in several neuroplasticity processes. In order to test whether endogenous beta-amyloid peptides play a role in the activation of extracellular-regulated kinase, we investigated the Rap1-extracellular-regulated kinase pathway in PC12 cells expressing human beta-amyloid precursor protein containing familial Alzheimer's disease mutations. In PC12 cells transfected with mutant human beta-amyloid precursor proteins that lead to higher levels of endogenous beta-amyloid, we observed an up-regulation of phospho-extracellular-regulated kinase and higher levels of activity-induced cAMP response element-directed gene expression. These results suggest that moderate levels of endogenous beta-amyloid peptides stimulate cAMP response element-directed gene expression. This stimulation was via a Rap1/MEK/extracellular-regulated kinase signaling pathway, as it was blocked by inhibition of Rap1 and MEK activities, and it requires beta-amyloid precursor protein cleavage at the gamma-site as it was abolished by a gamma-secretase inhibitor. Interestingly, in agreement with the previous observations, micromolar levels of extracellular fibrillar beta-amyloid blocked the cAMP response element-regulated gene expression stimulated by potassium and forskolin. This indicates that beta-amyloid can provoke different responses on cAMP response element-directed gene expression, such that low beta-amyloid levels may play a physiological role favoring synaptic plasticity under normal conditions while it would inhibit this mechanism under pathological conditions.

Early changes in neurons of the hippocampus and neocortex in transgenic rats expressing intracellular human a-beta

Alzheimer's disease (AD) studies typically focus on the extracellular impact of the amyloid-beta (Abeta) protein, however recent findings also implicate intracellular Abeta (iAbeta) accumulation in the disease's molecular neuropathology. In a double mutant transgenic rat model (AbetaPP and PS1 mutations, UKUR25), stably expressing intracellular human Abeta fragments in an environment devoid of both amyloid plaques and neurofibrillary tangles, we investigated the impact of iAbeta burden on both the incidence and relative cross sectional areas of the Golgi apparatus, lysosomes and lipofuscin bodies. Pyramidal cells within the hippocampus and neocortex of both transgenic and non-transgenic age matched controls were compared. This comparison revealed a significant increase in both the proportional area occupied by Golgi apparatus elements as well as in the mean individual cross sectional area of Golgi compartments in the hippocampus of transgenic rats as compared to controls. Elevated lysosome and lipofuscin elements in the hippocampi of transgenic rats were observed, as was an increase in the mean individual, cross sectional area of lipofuscin bodies in the cortex of transgenic rats as compared to controls. These findings support the hypothesis that intracellular Abeta accumulation not only has an impact on subcellular compartments but also potentially contributes to the neuronal cell pathology observed in AD.

Rat transgenic models with a phenotype of intracellular Abeta accumulation in hippocampus and cortex

In this communication we report the characterization of several transgenic rat lines expressing human AbetaPP carrying the Swedish and Indiana mutations (coded UKUR28), the human presenilin 1 transgene with the 'Finn' mutation (coded UKUR19) and double transgenic rats expressing both transgenes (coded UKUR25). In these Tg rats, the AbetaPP and PS1 transgene expression was largely restricted to the hippocampus and neocortex. The PS1 transgenic rats did not produce visible changes in Abeta immunoreactivity. The AbetaPP transgenic rats (both the single Tg UKUR28, and double Tg UKUR25) generated a phenotype of intra-neuronalbeta accumulation without plaque formation and with no increased immunoreactivity for AbetaPP amino and carboxyl-terminal epitopes. This phenotype was apparent as early as 6 months of age in the transgenic rat lines carrying the human AbetaPP transgene. No senile plaques of aggregated Abeta were observed in any of the transgenic lines generated, up to 24 months of age. The hAbetaPP single homozygous Tg line (UKUR28) showed an increase in ERK2, without changes in glycogen synthase kinase 3 (GSK3) activity. A preliminary protein analysis of the hippocampus of the double transgenic rat (UKUR25) by mass spectrometry showed differences in the protein profile between this transgenic line and controls.

Long-lasting rescue of age-associated deficits in cognition and the CNS cholinergic phenotype by a partial agonist peptidomimetic ligand of TrkA

Previously, we developed a proteolytically stable small molecule peptidomimetic termed D3 as a selective ligand of the extracellular domain of the TrkA receptor for the NGF. Ex vivo D3 was defined as a selective, partial TrkA agonist. Here, the in vivo efficacy of D3 as a potential therapeutic for cholinergic neurons was tested in cognitively impaired aged rats, and we compared the consequence of partial TrkA activation (D3) versus full TrkA/p75 activation (NGF). We show that in vivo D3 binds to TrkA receptors and affords a significant and long-lived phenotypic rescue of the cholinergic phenotype both in the cortex and in the nucleus basalis. The cholinergic rescue was selective and correlates with a significant improvement of memory/learning in cognitively impaired aged rats. The effects of the synthetic ligand D3 and the natural ligand NGF were comparable. Small, proteolytically stable ligands with selective agonistic activity at a growth factor receptor may have therapeutic potential for neurodegenerative disorders.

Structural involvement of the glutamatergic presynaptic boutons in a transgenic mouse model expressing early onset amyloid pathology

While the cholinergic depletion in Alzheimer's disease (AD) has been known for some time, a definitive involvement of other neurotransmitter systems has been somewhat more elusive. Our study demonstrates a clear involvement of both glutamatergic and, to a lesser extent, GABAergic neurons in an early onset transgenic mouse model of AD-like amyloid pathology. Immunohistochemical staining and subsequent quantification has revealed a statistically significant increased density of glutamatergic and GABAergic presynaptic boutons in both the plaque free and plaque adjacent cortical neuropile areas of transgenic mice as compared to non-transgenic controls. Furthermore, amyloid plaque size was shown to have a statistically significant effect on the relative area occupied by dystrophic glutamatergic neurites in the peri-plaque neuropile. These findings support our hypothesis that the amyloid pathology progresses in a time and neurotransmitter specific manner, first in the cholinergic system which appears to be most vulnerable, followed by the glutamatergic presynaptic boutons and finally the somewhat more resilient GABAergic terminals.

Altered mitogen-activated protein kinase signaling, tau hyperphosphorylation and mild spatial learning dysfunction in transgenic rats expressing the β-amyloid peptide intracellularly in hippocampal and cortical neurons

The pathological significance of intracellular Abeta accumulation in vivo is not yet fully understood. To address this, we have studied transgenic rats expressing Alzheimer's-related transgenes that accumulate Abeta intraneuronally in the cerebral and hippocampal cortices but do not develop extracellular amyloid plaques. In these rats, the presence of intraneuronal Abeta is sufficient to provoke up-regulation of the phosphorylated form of extracellular-regulated kinase (ERK) 2 and its enzymatic activity in the hippocampus while no changes were observed in the activity or phosphorylation status of other putative tau kinases such as p38, glycogen synthase kinase 3, and cycline-dependent kinase 5. The increase in active phospho-ERK2 was accompanied by increased levels of tau phosphorylation at S396 and S404 ERK2 sites and a decrease in the phosphorylation of the CREB kinase p90RSK. In a water maze paradigm, male transgenic rats displayed a mild spatial learning deficit relative to control littermates. Our results suggest that in the absence of plaques, intraneuronal accumulation of Abeta peptide correlates with the initial steps in the tau-phosphorylation cascade, alterations in ERK2 signaling and impairment of higher CNS functions in male rats.

The impact of Aβ-plaques on cortical cholinergic and non-cholinergic presynaptic boutons in alzheimer's disease-like transgenic mice

A previous study in our laboratory, involving early stage, amyloid pathology in 8-month-old transgenic mice, demonstrated a selective loss of cholinergic terminals in the cerebral and hippocampal cortices of doubly transgenic (APP(K670N,M671L)+PSl(M146L)) mice, an up-regulation in the single mutant APP(K670N,M671L) mice and no detectable change in the PSl(M146L) transgenics [J Neurosci 19 (1999) 2706]. The present study investigates the impact of amyloid plaques on synaptophysin and vesicular acetylcholine transporter (VAChT) immunoreactive bouton numbers in the frontal cortex of the three transgenic mouse models previously described. When compared as a whole, the frontal cortices of transgenic and control mice show no observable differences in the densities of synaptophysin-immunoreactive boutons. An individual comparison of layer V of the frontal cortex, however, shows a significant increase in density in transgenic models. Analysis of the cholinergic system alone shows significant alterations in the VAChT-immunoreactive bouton densities as evidenced by an increased density in the single (APP(K670N,M671L)) transgenics and a decreased density in the doubly transgenics (APP(K670N,M671L)+PSl(M146L)). In investigating the impact of plaque proximity on bouton density at early stages of the amyloid pathology in our doubly (APP(K670N,M671L)+PSl(M146L)) transgenic mouse line, we observed that plaque proximity reduced cholinergic pre-synaptic bouton density by 40%, and yet increased synaptophysin-immunoreactive pre-synaptic bouton density by 9.5%. Distance from plaques (up to 60 microm) seemed to have no effect on bouton density; however a significant inverse relationship was visible between plaque size and cholinergic pre-synaptic bouton density. Finally, the number of cholinergic dystrophic neurites surrounding the truly amyloid, Thioflavin-S(+) plaque core, was disproportionately large with respect to the incidence of cholinergic boutons within the total pre-synaptic bouton population. Confocal and electron microscopic observations confirmed the preferential infiltration of dystrophic cholinergic boutons into fibrillar amyloid aggregates. We therefore hypothesize that extracellular Abeta aggregation preferentially affects cholinergic terminations prior to progression onto other neurotransmitter systems. This is supported by the observable presence of non-cholinergic sprouting, which may be representative of impending neuritic degeneration.

Parasympathetic nerve fibers invade the upper dermis following sensory denervation of the rat lower lip skin

The sympathetic division of the autonomic nervous system is known to play a role in the genesis of neuropathic pain. In the skin of the rat lower lip (hairy skin), sympathetic and parasympathetic fibers normally innervate the same blood vessels in the lower dermis but do not occur in the upper dermis. However, we have shown that sympathetic fiber migration into the upper dermis occurs following mental nerve lesions (Ruocco et al. [2000] J. Comp. Neurol. 422:287-296). As sensory denervation has a dramatic effect on sympathetic fiber innervation patterns in the rat lower lip skin, we decided to investigate the possible changes in the other autonomic fiber type in the skin-the parasympathetic fiber. Sensory denervation of the rat lower lip was achieved by bilateral transection of the mental nerve, and animals were allowed to recover for 1-8 weeks. Lower lip tissue was processed for double-labeling light microscopic immunocytochemistry (ICC), using antibodies against substance P (SP), which labels a subpopulation of peptidergic sensory fibers, and against the vesicular acetycholine transporter (VAChT), as a marker for parasympathetic fibers. In sham-operated rats, SP-immunoreactive (IR) sensory fibers were found in the epidermis and upper and lower dermal regions, whereas VAChT-IR fibers were confined to the lower dermis. Mental nerve lesions induced the gradual disappearance of SP-IR fibers from all skin layers accompanied by the progressive migration of VAChT-IR fibers into the upper dermis. Cholinergic fiber migration was evident by the second week post surgery, and the ectopic innervation of the upper dermis by these fibers persisted even at the last time point studied (8 weeks) when SP-IR fibers have completely regrown. VAChT-IR fibers were observed in the upper dermis, well above the opening of the sebaceous glands into the hair follicles. These results show that considerable changes occur in the innervation patterns of parasympathetic fibers following mental nerve lesions.

Intracellular A-beta amyloid, a sign for worse things to come?

In this review the authors discuss the possible neuropathological role of intracellular amyloid-beta accumulation in Alzheimer's disease (AD) pathology. There is abundant evidence that at early stages of the disease, prior to A-beta amyloid plaque formation, A-beta peptides accumulate intraneuronally in the cerebral cortex and the hippocampus. The experimental evidence would indicate that intracellular amyloid-beta could originate both by intracellular biosynthesis and also from the uptake of amyloidogenic peptides from the extracellular milieu. Herein the aspects of the possible impact of intracellular amyloid-beta in human AD pathology are discussed, as well as recent observations from a rat transgenic model with a phenotype of intracellular accumulation of A-beta fragments in neurons of the hippocampus and cortex, without plaque formation. In this model, the intracellular amyloid-beta phenotype is accompanied by increased MAPK/ERK activity and tau hyperphosphorylation. Finally, the authors discuss the hypothesis that, prior to plaque formation, intracellular A-beta accumulation induces biochemical and pathological changes in the brain at the cellular level priming neurons to further cytotoxic attack of extracellular amyloidogenic peptides.

Skin blood vessels are simultaneously innervated by sensory, sympathetic, and parasympathetic fibers

Despite the known major role of skin blood vessel innervation in blood flow control, particularly in disease, little information on the co-innervation of blood vessels by sensory and autonomic fibers and the relationships of these fibers to one another is available. To fill this gap, we performed a light and electron microscopic analysis of the innervation of skin vessels by sensory and autonomic fibers by using the rat and monkey lower lips as a model. In rats, double-labeling immunocytochemistry revealed that combinations of fibers immunoreactive for substance P (SP) and dopamine-beta-hydroxylase (DbetaH), SP and vesicular acetylcholine transporter (VAChT), as well as DbetaH and VAChT occurred only around blood vessels in the lower dermis. All fiber types travelled in parallel and in close proximity to one another. In the upper dermis, blood vessels were innervated by SP-containing fibers only. Although nerve terminals displayed synaptic vesicles, synaptic specializations were never observed, suggesting that, in this territory, these fibers do not establish synaptic contacts. Quantification of the distance between the various immunoreactive terminals and their presumptive targets (smooth muscle cells and endothelial cells) revealed that both sympathetic and parasympathetic fibers were significantly closer to the endothelial cell layer and smooth muscle cells compared with sensory fibers. In monkeys, double-labeling immunocytochemistry was performed for SP-DbetaH and SP-VAChT only. The results obtained are similar to those found in rats; however, the fiber density was greater in monkeys. Our findings suggest that the regulation of skin microcirculation might be the result of the coordinated functions of sensory and autonomic fibers.

Tau function and dysfunction in neurons: its role in neurodegenerative disorders

Alzheimer's disease (AD) is the most usual neurodegenerative disorder leading to dementia in the aged human population. It is characterized by the presence of two main brain pathological hallmarks: senile plaques and neurofibrillary tangles (NFTs). NFTs are composed of fibrillar polymers of the abnormally phosphorylated cytoskeletal protein tau.

Aging causes a preferential loss of cholinergic innervation of characterized neocortical pyramidal neurons

Aging is known to markedly affect the number and structural characteristics of both pre- and post-synaptic sites in the cerebral cortex. There is evidence that lamina V pyramidal neurons, and their basilar dendrites in particular, are affected by age-related decline. Furthermore, layer V is the area where the greatest overall age- related losses in the total population of synaptic boutons and of cholinergic boutons are observed. Since both pyramidal neurons and cortical cholinergic input are characteristically compromised in aging, we investigated whether aging altered the pattern of cholinergic boutons in apposition to the soma, proximal and distal basal dendrites of intracellularly labeled lamina V large pyramidal neurons in the parietal cortex of young and aged rats. We observed a significant age-related decrease in the population of both total and cholinergic boutons apposed to proximal and distal dendrites of layer V large pyramidal neurons. However, the age-related decreases of cholinergic presynaptic boutons were higher than those in the total bouton population apposed to the pyramidal neurons. The average decrease in cholinergic boutons in aged rats was 3.7-fold more pronounced than the diminution in the overall number of presynaptic boutons. Our results add important new evidence in support of the concept that the age-related learning and memory deficits are attributable, at least partially, to a decline in the functional integrity of the forebrain cholinergic systems.

Sympathectomies lead to transient substance P-immunoreactive sensory fibre plasticity in the rat skin

Research using animal models of neuropathic pain has revealed sympathetic sprouting onto dorsal root ganglion cells. More recently, sensory fibre sprouting onto dorsal root ganglion cells has also been observed. Previous work in our laboratory demonstrated persistent sympathetic fibre sprouting in the skin of the rat lower lip following sensory denervation of this region. Therefore, we applied immunocytochemistry to determine the effects of sympathectomies on the terminal fields of sensory fibres. The superior cervical ganglia were removed bilaterally and the effects on the innervation of the skin of the rat lower lip were observed 1, 2, 3, 4, 6 and 8 weeks post-surgery. Substance P and dopamine-beta-hydroxylase immunoreactivities were used to identify a subset of sensory and sympathetic fibres, respectively. We also assessed neurokinin-1 receptor immunoreactivity. Quantitative data was obtained with the aid of an image analysis system. In controls, the epidermis and upper dermis were innervated by substance P-immunoreactive fibres only and upper dermal blood vessels possessed the highest density of neurokinin-1 receptor immunoreactivity. Blood vessels in the lower dermis were innervated by both substance P- and dopamine-beta-hydroxylase-immunoreactive fibres. Following sympathectomies, substance P-immunoreactive fibres in the epidermis and upper dermis were more intensely labelled only 1 and 2 weeks post-surgery when compared to sham controls. The length of substance P-immunoreactive fibres in this region was also increased only on the second week. Neurokinin-1 receptor immunoreactivity in the upper dermis was slightly decreased 1 and 2 weeks post-surgery. In the lower dermis, substance P-immunoreactive fibres associated with blood vessels were more intensely labelled only 1 and 2 weeks post-surgery, and at all post-surgical time points studied, blood vessels in this region were devoid of dopamine-beta-hydroxylase-immunoreactive fibres. The length of substance P-immunoreactive fibres was increased from the first to the third week post-surgery in the lower dermis. These results indicate that sympathectomies lead to transient changes in substance P-immunoreactive fibre innervation and neurokinin-1 receptor expression in rat lower lip skin. The effects are most prominent in the lower dermis probably due to a greater local concentration of nerve growth factor in this region. The plasticity of the interactions between sensory and sympathetic fibres may prove important in the regulation of skin microcirculation and in the generation of painful sensations under normal conditions or following peripheral nerve injuries.

Cholinergic nerve terminals establish classical synapses in the rat cerebral cortex: synaptic pattern and age-related atrophy

This study addresses the issue of whether cholinergic varicosities in the cerebral cortex establish 'classical synapses' or whether they communicate with their targets non-synaptically by 'volume transmission'. Most recent studies in the neocortex have suggested that acetylcholine acts non-synaptically, however in the present study we provide ultrastructural evidence that suggests synaptic mechanisms prevail. This conclusion is based upon our ultrastructural observations that cholinergic boutons--as revealed by immunoreactivity for the specific cholinergic market, vesicular acetylcholine transporter--establish a high percentage of classical synapses in layer V of the rat parietal cortex. Furthermore, the combination of this approach with the intracellular labeling of large pyramidal neurons on slice preparations revealed significant incidences of cholinergic contacts abutting preferentially on dendritic shafts. Finally, we have gathered information suggesting that cholinergic boutons undergo atrophy with aging which could be related to the well-known cholinergic and cognitive decline. These results illustrate that the cholinergic terminations in the neocortex establish proper synaptic connections and that they experience important age-dependent atrophy.

Light and electron microscopic study of the distribution of substance P-immunoreactive fibers and neurokinin-1 receptors in the skin of the rat lower lip

Cutaneous antidromic vasodilatation and plasma extravasation, two phenomena that occur in neurogenic inflammation, are partially blocked by substance P (SP) receptor antagonists and are known to be mediated in part by mast cell-released substances, such as histamine, serotonin, and nitric oxide. In an attempt to provide a morphological substrate for the above phenomena, we applied light and electron microscopic immunocytochemistry to investigate the pattern of SP innervation of blood vessels and its relationship to mast cells in the skin of the rat lower lip. Furthermore, we examined the distribution of SP (neurokinin-1) receptors and their relationship to SP-immunoreactive (IR) fibers. Our results confirmed that SP-IR fibers are found in cutaneous nerves and that terminal branches are observed around blood vessels and penetrating the epidermis. SP-IR fibers also innervated hair follicles and sebaceous glands. At the ultrastructural level, SP-IR varicosities were observed adjacent to arterioles, capillaries, venules, and mast cells. The varicosities possessed both dense core vesicles and agranular synaptic vesicles. We quantified the distance between SP-IR varicosities and blood vessel endothelial cells. SP-IR terminals were located within 0.23-5.99 microm from the endothelial cell layer in 82.7% of arterioles, in 90.2% of capillaries, and in 86.9% of venules. Although there was a trend for SP-IR fibers to be located closer to the endothelium of venules, this difference was not significant. Neurokinin-1 receptor (NK-1r) immunoreactivity was most abundant in the upper dermis and was associated with the wall of blood vessels. NK-1r were located in equal amounts on the walls of arterioles, capillaries, and venules that were innervated by SP-IR fibers. The present results favor the concept of a participation of SP in cutaneous neurogenic vasodilatation and plasma extravasation both by an action on blood vessels after binding to the NK-1r and by causing the release of substances from mast cells after diffusion through the connective tissue.

Loss of presynaptic and postsynaptic structures is accompanied by compensatory increase in action potential-dependent synaptic input to layer V neocortical pyramidal neurons in aged rats

Reduction in both presynaptic and postsynaptic structures in the aging neocortex may significantly affect functional synaptic properties in this area. To directly address this issue, we combined whole-cell patch-clamp recording of spontaneously occurring postsynaptic currents (PSCs) with morphological analysis of layer V pyramidal neurons in the parietal cortex of young adult (1- to 2-month-old) and aged (28- to 37-month-old) BN x F344 F(1) hybrid rats. Analysis of spontaneous PSCs was used to contrast functional properties of basal synaptic input with structural alterations in the dendritic tree of pyramidal neurons and density of terminals in contact with these cells. We observed significant changes in a number of morphological parameters of pyramidal neurons in aged rats. These include smaller cell body size and fewer basal dendritic branches (but not of oblique dendrites and dendritic tufts) and spines. Ultrastructural analysis also revealed a lower density of presynaptic terminals per unit length of postsynaptic membrane of labeled pyramidal neurons in the aged brain. This reduction in both presynaptic and postsynaptic elements was paralleled by a significant decrease in frequency of tetrodotoxin-insensitive miniature (action potential-independent) PSCs (mPSCs). The frequency of excitatory and inhibitory mPSCs was reduced to the same extent. In contrast, no significant change was observed in the frequency of spontaneous PSCs recorded in absence of tetrodotoxin (sPSCs), indicating an increase in action potential-dependent (frequency(sPSCs) - frequency(mPSCs)) input to pyramidal neurons in the aged group. This functional compensation may explain the lack of drastic loss of spontaneous neuronal activity in normal aging.