Development of cerebral arterial innervation: synchronous development of neuropeptide Y (NPY)- and vasoactive intestinal polypeptide (VIP)-containing fibers and some observations on growth cones

Parasympathetic innervation of vertebrobasilar arteries: is this a potential clinical target?

This review aims to summarise the contemporary evidence for the presence and function of the parasympathetic innervation of the cerebral circulation with emphasis on the vertebral and basilar arteries (the posterior cerebral circulation). We consider whether the parasympathetic innervation of blood vessels could be used as a means to increase cerebral blood flow. This may have clinical implications for pathologies associated with cerebral hypoperfusion such as stroke, dementia and hypertension. Relative to the anterior cerebral circulation little is known of the origins and neurochemical phenotypes of the parasympathetic innervation of the vertebrobasilar arteries. These vessels normally provide blood flow to the brainstem and cerebellum but can, via the Circle of Willis upon stenosis of the internal carotid arteries, supply blood to the anterior cerebral circulation too. We review the multiple types of parasympathetic fibres and their distinct transmitter mechanisms and how these vary with age, disease and species. We highlight the importance of parasympathetic fibres for mediating the vasodilatory response to sympathetic activation. Current trials are investigating the possibility of electrically stimulating the postganglionic parasympathetic ganglia to improve cerebal blood flow to reduce the penumbra following stroke. We conclude that although there are substantial gaps in our understanding of the origins of parasympathetic innervation of the vertebrobasilar arteries, activation of this system under some conditions might bring therapeutic benefits.

Positive correlation between the density of neuropeptide y positive neurons in the amygdala and parameters of self-reported anxiety and depression in mesiotemporal lobe epilepsy patients

BACKGROUND

Neuropeptide Y (NPY) has been implicated in depression, anxiety, and memory. Expression of human NPY and the number of NPY-positive neurons in the rodent amygdala correlate with anxiety and stress-related behavior. Increased NPY expression in the epileptic brain is supposed to represent an adaptive mechanism counteracting epilepsy-related hyperexcitability. We attempted to investigate whether NPY-positive neurons in the human amygdala are involved in these processes.

METHODS

In 34 adult epileptic patients undergoing temporal lobe surgery for seizure control, the density of NPY-positive neurons was assessed in the basal, lateral, and accessory-basal amygdala nuclei. Cell counts were related to self-reported depression, anxiety, quality of life, clinical parameters (onset and duration of epilepsy, seizure frequency), antiepileptic medication, and amygdala and hippocampal magnetic resonance imaging volumetric measures.

RESULTS

Densities of NPY-positive basolateral amygdala neurons showed significant positive correlations with depression and anxiety scores, and they were negatively correlated with lamotrigine dosage. In contrast, NPY cell counts showed no relation to clinical factors or amygdalar and hippocampal volumes.

CONCLUSIONS

The results point to a role of amygdalar NPY in negative emotion and might reflect state processes at least in patients with temporal lobe epilepsy. Correlations with common clinical parameters of epilepsy were not found. The question of a disease-related reduction of the density of NPY-positive amygdalar neurons in temporal lobe epilepsy requires further investigation.

On the presence of neurotrophin p75 receptor on rat sympathetic cerebrovascular nerves

Although the presence of neurotrophin p75 receptor on sympathetic nerves is a well-recognised feature, there is still a scarcity of details of the distribution of the receptor on cerebrovascular nerves. This study examined the distribution of p75 receptor on perivascular sympathetic nerves of the middle cerebral artery and the basilar artery of healthy young rats using immunohistochemical methods at the laser confocal microscope and transmission electron microscope levels. Immunofluorescence methods of detection of tyrosine hydroxylase (TH) in sympathetic nerves, p75 receptor associated with the nerves, and also S-100 protein in Schwann cells were applied in conjunction with confocal microscopy, while the pre-embedding single and double immunolabelling methods (ExtrAvidin and immuno-gold-silver) were applied for the electron microscopic examination. Immunofluorescence studies revealed "punctuate" distribution of the p75 receptor on sympathetic nerves including accompanying Schwann cells. Image analysis of the nerves showed that the level of co-localization of p75 receptor and TH was low. Immunolabelling applied at the electron microscope level also showed scarce co-localization of TH (which was intra-axonal) and p75. Immunoreactivity for p75 receptor was present on the cell membrane of perivascular axons and to a greater extent on the processes of accompanying Schwann cells. Some Schwann cell processes were adjacent to each other displaying strong immunoreactivity for p75 receptor; immunoreactivity was located on the extracellular sites of the adjacent cell membranes suggesting that the receptor was involved in cross talk between these. It is likely that variability of locations of p75 receptor detected in the study reflects diverse interactions of p75 receptor with axons and Schwann cells. It might also imply a diverse role for the receptor and/or the plasticity of sympathetic cerebrovascular nerves to neurotrophin signalling.

Effects of maturation on adrenergic neurotransmission in ovine cerebral arteries

The present studies examine the hypothesis that multiple adrenergic neuroeffector mechanisms are not fully developed in fetal, compared with adult, ovine middle cerebral arteries. In arteries denuded of endothelium and pretreated with 1 microM atropine to block involvement of muscarinic receptors, 10 microM capsaicin to deplete sensory peptidergic neurons, and 10 microM nitro-L-arginine methyl ester (L-NAME) to block possible influences from nitric oxidergic innervation, transmural stimulation at 16 Hz increased contractile tensions to 9.5 +/- 3.7% (n = 6) of the potassium maximum in adult arteries. Corresponding values in fetal arteries, however, were significantly less and averaged only 1.1 +/- 0.6% (n =10). However, postsynaptic sensitivity to norepinephrine (NE) was similar in the two age groups; NE pD(2) values (-log EC(50)) averaged 6.11 +/- 0.12 (n = 6) and 6.33 +/- 0.09 M (n = 9) in fetal and adult arteries, respectively. Similarly, NE content measured via HPLC was also similar in the two age groups and averaged 32.4 +/- 5.0 (n = 17) and 32.5 +/- 3.9 ng/ng wet wt (n = 13) in fetal and adult middle cerebral arteries, respectively. In contrast, stimulation-induced NE release was greater in fetal than in adult arteries, whether calculated as total mass released [883 +/- 184 (n = 17) vs. 416 +/- 106 pg NE/mg wet wt (n = 13)] or as fractional release [51.1 +/- 5.3 (n = 17) vs. 22.8 +/- 3.8 pg/pg NE content per pulse x 10(-6)]. Measured as an index of synaptic density, neuronal cocaine-sensitive NE uptake was similar in fetal and adult arteries [1.55 +/- 0.40 (n = 10) and 1.84 +/- 0.51 pmol/mg wet wt (n = 7), respectively]. Overall, age-related differences in postsynaptic sensitivity to NE, NE release, and NE uptake capacity cannot explain the corresponding age-related differences in response to stimulation. The data thus suggest that total synaptic volume and cleft width, in particular, are probably greater and/or that adrenergic corelease of vasoactive substances other than NE is altered in fetal compared with adult middle cerebral arteries.

Development of peripheral autonomic synapses: neurotransmitter receptors, neuroeffector associations and neural influences

1. The functional innervation of autonomic target tissues occurs early during development, at a time when both the nerves and post-synaptic target tissues are still differentiating. 2. Physiological responses appear soon after the arrival of the first fibres when uptake and release mechanisms within the nerves are already functional. Initial responses differ from those in the mature animal, both in the form and, frequently, in the subtypes of receptors involved. 3. Results of a number of studies suggest that the initial expression of neurotransmitter receptors during development is largely independent of neural influences. Changes recorded in neurotransmitter receptor expression during development appear to be similarly independent of neural influences. 4. While signal transduction pathways coupling adrenergic neurotransmitter receptors to effector responses appear to develop independently of the nerves, the efficient coupling of muscarinic receptors often requires the action of the neurotransmitter, acetylcholine. 5. During the period of synapse formation, the neural plexus continues to expand. While developing varicosities can release the neurotransmitter, the capacity for neurotransmitter retention appears to be restricted. Developmental changes in the neurotransmitters that produce functional responses, while well known in the sweat glands, may also be seen in more subtle forms in other target tissues. 6. Ultrastructural studies suggest that close physical associations between the membranes of the release sites of the developing nerves and the target cells may form early during development when physiological responses are still immature. These close associations could enable more specific reciprocal interactions between nerves and target cells involving known and novel growth factors, neuropeptides and cytokines important in shaping the mature synaptic characteristics.

Immunohistochemical study of rabbit choroidal innervation

Immunocytochemical methods with antibodies to the light (68 kDa), medium (160 kDa), and heavy (200 kDa) chain subunits of the neurofilament triplet have been used to visualize neuronal structures in rabbit choroids. Choroidal nerve fibers were present in the suprachoroid and vascular laminae and absent in the choriocapillary layer. These fibers may be classified as perivascular and intervascular. Perivascular fibers surround all arterial and venous blood vessels and form a network; these fibers were labeled with the three NF antibodies, although they were more easily visualized with anti NF-160 and anti NF-200 than anti NF-68. Intervascular fibers formed two groups. The first group consisted of fibers situated between the blood vessels and parallel to the blood vessel wall surface (paravascular fibers); these fibers were better observed using anti NF-160 and NF-200 than anti NF-68. The second group consisted of fibers which travel the entire length of the choroid until they reach the nerve plexus of the ciliary body (long tract fibers). The plexus was observed with anti NF-68, anti NF-160 and anti NF-200; however, the long tract fibers were more clearly visualized with anti NF-160 and anti NF-200 than with anti NF-68. Two types of choroidal cell were also labeled: ganglion cells and melanocytes. Ganglion cells are small, scarce neurons situated in the peripheral choroid; they were labeled with anti NF-160 and anti NF-200. The melanocytes were only labeled with anti NF-200 and they were the only non neuronal structure visualized using antibodies against neurofilaments.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Areal differences of NPY mRNA-expressing neurons are established in the late postnatal rat visual cortex in vivo, but not in organotypic cultures

In order to learn about the factors regulating the postnatal development of neocortical peptidergic neuron populations, we have analysed neurons expressing neuropeptide Y (NPY) by immunohistochemistry and in situ hybridization in developing and adult rat visual cortical areas 17 and 18a in vivo, and in organotypic slice cultures of rat visual cortex. For quantitative analysis, the percentage of NPY mRNA-expressing neurons was determined in supragranular layers I-IV, in infragranular layers V and VI and in the white matter. In vivo, this percentage increased in visual areas 17 and 18a until postnatal day 21 in supra- and infragranular layers. Initially, in both areas the neurons were about equally distributed in supra- and infragranular layers (a ratio of 1:1). During the second postnatal month, the percentage of NPY mRNA-expressing neurons in area 18a declined by approximately 50% in both supra- and infragranular layers, so that the ratio of 1:1 remained constant. In contrast, in area 17 the percentage of neurons in supragranular layers remained fairly constant, but it declined to 50% in infragranular layers, so that by postnatal day 70 the ratio was gradually shifted to 2:1. Throughout development, area 18a contained significantly more NPY mRNA-expressing neurons than area 17. In organotypic slice cultures, a high density of NPY mRNA-expressing neurons had appeared by 10 days in vitro. A much higher percentage of neurons expressed NPY mRNA. The ratio of labelled neurons in supra- versus infragranular layers was 1:1. Both ratio and percentage remained constant from 10-85 days in vitro. The decline in vivo was not caused by an elimination of transient cell types. All cell types persisted into adulthood. Four NPY peptide-immunoreactive neuronal types were classified by axonal morphology in organotypic slice cultures and in vivo; they include (i) cells in layer VI/white matter with horizontal axons and ascending collaterals, (ii) cells in layers V/VI with descending axon and horizontal collaterals, (iii) Martinotti cells in layers V/VI with ascending axons, and (iv) cells in layers III-V with columnar axons. Two further types, bipolar cells with axons descending from dendrites and small basket cells with short horizontal axons, both found in vivo in layers II/III, could not be unequivocally identified in organotypic slice cultures. The NPY-immunoreactive neuron types had already formed a dense innervation of the cultures by 10 days in vitro, which remained stable for up to 85 days in vitro, and resembled the innervation observed in vivo. NPY peptide-immunoreactive neurons in organotypic slice cultures and in vivo were distributed in cortical layers II/III, V and VI and the white matter, but rarely in layers I and IV, which corresponded to the distribution of NPY mRNA-expressing neurons. However, with in situ hybridization more neurons were detectable, especially in layers II/III. A majority of NPY mRNA-expressing neurons co-localized NPY peptide, somatostatin and calbindin. We conclude that intrinsic cues were sufficient to drive the molecular expression of the NPY phenotype, the morphological differentiation and the stabilization of an organotypic NPY innervation in organotypic slice cultures. However, the area- and lamina-specific changes observed in vivo were not observed under monoculture conditions.

Patterns of peptide-containing perivascular nerves in the circle of Willis: their absence in intracranial arteriovenous malformations

Using standard immunohistochemical techniques and an improved procedure for whole-mount vascular preparations, the authors describe the pattern and density of innervation of calcitonin gene-related peptide (CGRP)-like, neuropeptide Y (NPY)-like and vasoactive intestinal polypeptide (VIP)-like immunoreactivity in major arteries of postmortem adult human circles of Willis. Calcitonin gene-related peptide-, NPY-, and VIP-LI exhibited a variety of varicose and nonvaricose single axons, and small and large perivascular nerve bundles. Although the density of innervation within each vascular segment was highly variable, the pattern of innervation for each neuropeptide observed was consistent throughout the circle of Willis. With the use of human and rat circles of Willis as positive control preparations, the lack of CGRP-LI, NPY-LI, and VIP-LI in vessel segments taken from five cases of intracranial arteriovenous malformations (AVMs) is also reported. It is concluded that adult human circles of Willis exhibit CGRP-LI, NPY-LI, and VIP-LI perivascular nerves. In addition, intracranial AVMs do not possess these peptide-containing nerves that, in animals, normally mediate neurogenic control in the cerebrovasculature. It is hypothesized that this lack of innervation, and hence neurotrophic influence, may contribute to the development of AVMs.

Neuropeptides and a neuronal marker in cutaneous innervation during human foetal development

There is evidence that foetal body movements first occur at 6 weeks gestation, and that the reflex arc is functional at 8 weeks. This correlates with the detection of the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP) in spinal cord at 10 weeks gestation. However, the development of cutaneous neuropeptide-containing nerves is not well documented in humans. We have investigated the early appearance and distribution pattern of CGRP, SP, vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), as well as those of the general neuronal marker protein gene product 9.5 (PGP) in various areas of foetal skin at different gestational ages. PGP-immunoreactive nerves were first seen in the subepidermal plexus at 6 weeks gestational age. Initially, the immunoreactive nerves are thick, club-shaped and distributed in the superficial dermis. Beaded adult-like fibres become more numerous only at later ages (10-12 weeks), and extend from this plexus to penetrate the epidermis. Histologically, the skin of the hand develops faster than that of other body areas and at 9 weeks, more PGP-immunoreactive nerves were seen in the palm than in the dorsum. Primitive sweat glands were first noted in axillary skin at 17 weeks, accompanied by a few PGP-immunoreactive nerves. Occasional, small CGRP-immunoreactive fibres were first noticed in the dermis at 7 weeks, but it was at 17 weeks that the presence of this neuropeptide was unequivocal in the subepidermal plexus. Sparse VIP-, SP- and NPY-immunoreactive fibres were not found until 16-17 weeks gestation, when they were seen in the dermis and around small blood vessels.(ABSTRACT TRUNCATED AT 250 WORDS)