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Department of Molecular Neuroimmunology, Institute of Anatomy and Cell Biology, Philipps University, Marburg, Germany; and
* Section on Molecular Neuroscience, Laboratory of Cellular and Molecular Regulation, National Institute of Mental Health, Bethesda, Maryland 20892, USA
1Correspondence: Department of Molecular Neuroimmunology, Institute of Anatomy and Cell Biology, Philipps University, Robt-Koch Strasse 6, D35033 Marburg, Germany. E-mail weihe{at}mailer.uni-marburg.de
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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Key Words: aminergic neuron • endocrine cell • CNS • amine-handling cell • autonomic • immune • inflammatory
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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. In the peripheral nervous system,
amine-containing neurons make up most or all of the neurons of the
sympathetic and parasympathetic (autonomic) nervous systems. Amines are
largely absent from the sensory nervous system, where the transmitters
are neuropeptides and glutamate.
Biogenic amine-containing neuroendocrine cells are derived both from
the neural crest (e.g., chromaffin cells) and from local stem cells
(e.g., gut enterochromaffin and skin Merkel cells; ref 2
).
Biogenic amine-handling cells of the immune/inflammatory axis are of
presumptive hematopoietic origin (3)
. These are
histamine-containing mast cells and basophils, and serotonin-containing
platelets. The proteins expressed by all of these cell populations that
make them ‘amine handling’ include enzymes for synthesizing amines,
plasma membrane transporters for scavenging and recycling amines from
the extracellular space, and intracellular transporters for amine
storage in secretory vesicles. To store amines in secretory organelles,
amine-handling cells all express one of three vesicular amine
transporters (VATs): the biogenic amine transporters VMAT1 and VMAT2,
and the acetylcholine transporter VAChT. It is from the secretory
granules or vesicles within the cell that amines are released in a
highly regulated fashion to act as informationally specific hormones,
autocrine or paracrine factors, and neurotransmitters. Thus, the
chemical neuroanatomy of amine-handling cells is determined by a
precisely regulated coexpression of enzymes imparting synthesis and
reuptake capability for a particular amine, together with one of the
three VATs.
The expression patterns of biosynthetic enzymes and VATs that give rise
to each of the amine-handling phenotypes of the nervous,
neuroendocrine, and immune/inflammatory systems are summarized in
Table 1
. In this review, we will describe current information about VAT
expression and function that illuminates the role of amine-handling
cells in each of these three major homeostatic systems.
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AMINERGIC NEURONS |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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. All cholinergic neurons of
the mammalian nervous system, both central and peripheral, therefore
express VAChT and ChAT (5
6
7
8)
. Despite their coordinated
expression and regulation in mammals (9
10
11
12)
, the relative
levels of VAChT and ChAT mRNAs can differ substantially under certain
conditions. VAChT and ChAT mRNA levels are differentially up-regulated
by NGF in rat cholinergic septal neurons (13)
. VAChT mRNA
is more abundant than ChAT mRNA in whole brain during early development
(14)
. This pattern is due to higher levels of VAChT than
ChAT transcripts in nascent cholinergic neurons, and it persists into
adulthood in peripheral cholinergic neurons. In the adult central
nervous system, ChAT and VAChT mRNA levels are similar
(15)
.
VAChT staining is a valuable adjunct to ChAT in investigating the
chemical neuroanatomy of cholinergic systems. Staining for VAChT has
been largely confirmatory of the existence of cholinergic neuronal cell
groups in both the central nervous system (CNS) and peripheral nervous
system (PNS), inferred from staining for acetylcholine esterase and
ChAT. Beyond this, VAChT has the special advantage of staining
cholinergic nerve terminals in proportion to their density of synaptic
vesicle clusters (Fig. 1
). For this reason, VAChT immunohistochemistry has allowed
identification of previously unsuspected or uncharacterized cholinergic
projection fields in several areas of brain and in the periphery. A
dense plexus of cholinergic nerve endings is now known to exist, for
example, in the hypothalamic median eminence in mouse, rat, and primate
(6
, 8
, 16
, 17)
as previously only inferred by measurement
of ChAT enzymatic activity (18)
. The primary sensory
mesencephalic nucleus has been shown to express the cholinergic
phenotype based on VAChT- and ChAT-positive staining. Cholinergic
hypothalamic nuclei, including an arcuate cell group that may be the
source of cholinergic projections to the median eminence, have been
visualized with VAChT staining and in situ hybridization
histochemistry in both mouse and rat (5
, 6
, 8
, 16
, 17
, 19)
.
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VAChT immunoreactivity in cell bodies and nerve terminals allows
greater ability to detect pathophysiological alterations in cholinergic
neurons of the CNS by providing an estimate of dynamic changes in
cholinergic terminal field density and allowing colocalization of VAChT
with other neurotransmission-relevant molecules. For example,
down-regulation of cyclooxygenase immunoreactivity in basal forebrain
cholinergic neurons and decreased cholinergic projection density in
cortex have both been observed in SIV-infected primates with
qualitative and semiquantitative VAChT immunohistochemistry
(20)
. Misawa et al. have noted the loss of VAChT-positive
homotypic synapses on ventral horn spinal motor neurons in amyotrophic
lateral sclerosis spinal cord, suggesting a role for these synapses in
the initiation or progression of cholinergic motor neuron dysfunction
in this disease (21)
. Debeir and co-workers have noted a
specific decrease in VAChT-immunoreactive synapses compared to
synaptophysin (total) immunoreactive synapses in rat cortex after local
administration of anti-NGF antibody or TrkA peptide antagonist
(22)
Biogenic amine-containing neurons of the CNS
VMAT1 and VMAT2 represent isoforms of the vesicular monoamine
transporter that are in general segregated into endocrine and neuronal
monoamine-containing cells, respectively (23
, 24)
. VMAT2
is the exclusive VMAT isoform found in serotoninergic, noradrenergic,
dopaminergic, histaminergic, and adrenergic neurons of the CNS (see
Table 1
). It has been suggested that VMAT1-positive neurons exist in
the developing rat CNS based on in situ hybridization
histochemical data (25)
. However, others have obtained no
evidence for neurons positive for VMAT1 mRNA or protein in either
developing or adult rat CNS (23
, 24
, 26)
. The lack of
catecholamine storage in the CNS of VMAT2-deficient mice during
development is also consistent with exclusive VMAT2 expression in CNS
monoaminergic neurons in the rodent (27)
.
Expression of VMAT2 in developing and adult CNS neurons defines
cell groups with an incomplete (‘orphan’) biogenic amine phenotype
As expected, VMAT2 is a general histochemical marker for biogenic
amine-handling neurons of the CNS. However, neurons have now been
identified that, despite being strongly positive for VMAT2, apparently
lack biosynthetic enzymes for any of the known biogenic amines. Other
neuronal cell groups contain a full complement of biosynthetic enzymes
but lack VMAT2 expression. For example, distinct thalamocortical and
intrathalamic neuronal cell groups have been identified that are
transiently VMAT2 positive during late gestation and early
postembryonic life in mouse and rat, but lack tyrosine hydroxylase (TH)
necessary to produce catecholamines or tryptophan hydroxylase for the
synthesis of serotonin (26
, 28
, 29)
. These neurons express
in addition to VMAT2 the serotonin plasma membrane transporter and
under certain conditions may function as transient serotoninergic
neurons by releasing exogenous serotonin scavenged from the
extracellular space (28
, 29)
.
These observations highlight the participation of VMAT2 in
multiple overlapping neurotransmitter ‘regulons’ in the brain and
raise the possibility that neurotransmitter circuits utilizing
nonclassical neurotransmitter substrates for VMAT2 remain to be
discovered. VMAT2 recognizes as transport substrates the biogenic
amines phenylethylamine, tyramine, and octopamine (30
, 31)
, which are present in trace amounts in adult and developing
rodent brain and sympathetic nervous system (32
, 33)
.
Neurons that contain aromatic amino acid decarboxylase and VMAT2 would
be candidate tyraminergic cells; those also containing dopamine
ß-hydroxylase would be candidate octopaminergic mammalian neurons.
Flies with a mutation in tyrosine decarboxylase, an aromatic amino acid
decarboxylase (AADC) that converts tyrosine to tyramine, have reduced
levels of tyramine and are resistant to the sensitizing effects of
repeated administration of cocaine (34)
. It is not yet
known whether these neurons use VMAT2 as their vesicular transporter.
Another class of neurons in the rodent CNS has been identified that
express biogenic amine biosynthetic enzymes, but not a vesicular
monoamine transporter. In the principal olfactory bulb, periglomerular
neurons clearly express both TH and AADC and presumably synthesize
dopamine (35)
, but lack VMAT2 (24)
. The
dynamic regulation of TH in these neurons suggests a role for the
synthesized catecholamine (36)
, but the functional status
of these ‘dopaminergic’ cells is presently unclear. Other neurons
that express tyrosine hydroxylase but lack VMAT2 include extensive
plexus of TH+/VMAT2- neurons in the supraoptic nucleus, area postrema,
and nucleus tractus solitarius (Fig. 2
). The absence of VMAT2 in TH-positive neuronal cell groups previously
classified as ‘catecholaminergic’ invites consideration of the
functional significance of amine synthesis and storage in neurons that
lack the capability for vesicular accumulation of transmitter. The
possibilities of nonvesicular catecholamine release or the presence of
vesicular transporters other than VMAT2 have yet to be explored.
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VMAT2 and VAChT expression in the autonomic nervous system
Autonomic regulation of visceral functions depends on a dual
innervation from the sympathetic and parasympathetic nervous systems.
Centrally located preganglionic cholinergic cell groups are connected
to sympathetic ganglia whose cell bodies are arranged within the
paravertebral and prevertebral sympathetic chains and to
parasympathetic ganglia contained both intrinsically in target organs
such as heart, gut, iris, genitourinary tract, and lung, as well as in
cranial and sacral extrinsic ganglia. Sympathetic and parasympathetic
ganglia in turn supply dual innervation to the visceral organs,
cardiovascular system, and the skin. Sympathetic neurons are in general
noradrenergic, and parasympathetic neurons are, like the preganglionic
neurons, cholinergic. An exception to the simple rule of chemical
coding for noradrenergic neurotransmission in sympathetics and
cholinergic neurotransmission in parasympathetics is found in the
paravertebral sympathetic chain. Here, 
5% of the ganglionic neurons
are cholinergic, and project to the eccrine sweat glands, the
skeletomuscular vasculature, and the periosteum (7
, 37
, 38)
. These targets are also unique in receiving no
parasympathetic innervation (see ref 38
and references
therein).
Expression of the vesicular amine transporters in the developing as
well as mature sympathetic and parasympathetic nervous systems has
provided insight into the mechanisms of lineage determination leading
to chemical coding of neurotransmission (26
, 39
, 40)
.
Sympathetic neurons
The paravertebral sympathetic chain contains mainly TH+ neurons
that coexpress VMAT2. A second population of cells exists that is
TH-/VAChT+/VMAT2-. Depending on the rostral-caudal level, this
population varies from 1 to 5% of the total number of principal
neurons in the ganglion. The prevertebral chain contains exclusively
TH+/VMAT2+ noradrenergic neurons in the rat, but in some species
may occasionally contain rare prevertebral cholinergic (VAChT-positive)
neurons as in the paravertebral chain. Developmentally, the
noradrenergic neurons of the sympathetic chains appear to express
vesicular transporter as early as they express catecholaminergic
biosynthetic enzymes, so that the ability to store and release
catecholamines is an early emerging feature of the sympathetic nervous
system. As with VAChT, VMAT2 immunohistochemistry allows facile
visualization of terminal fields for all autonomic target organs
(Fig. 3
).
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Cholinergic differentiation in the sympathetic nervous system is
a more complex matter. VAChT expression in the developing sympathetic
chain appears to be coincident with both VMAT2 appearance and
preganglionic innervation of the chain anlage. We recently quantified
the number of VAChT-positive neurons in the developing sympathetic
chain. They represent approximately the adult proportion (5% in
stellate) as early as embryonic day 16 (Gördes and E. Weihe,
unpublished observations). Since VAChT-positive fibers are also seen
innervating sweat glands as early as postembryonic day 2, when forepaw
sweat glands are still developing, the initial cholinergic sympathetic
differentiation appears to be independent of the establishment of
synaptic relationships between these neurons and their final peripheral
targets (40)
. It has been suggested that the cholinergic
sympathetic neurons that innervate the sweat glands, which form only
after birth, achieve the cholinergic phenotype by switching from a
noradrenergic to a cholinergic expression program under the influence
of factors secreted from the sweat glands around postembryonic day 2
until about postembryonic day 10 (37
, 38
, 41
, 42)
. Several
studies suggest that the cholinergic phenotype can be elicited de
novo from nearby neurons that are purely noradrenergic when
induced to innervate a transplant consisting in normally
cholinergically innervated target tissue (e.g., sweat gland,
periosteum) (38
, 41)
. On the other hand, VAChT+ neurons
exist, at least in the stellate ganglion, as early as embryonic day 17
and comprise a fairly constant 5–7% of the total cell number of the
ganglion through postembryonic day 10 (15
, 40)
. Costaining
with VAChT and VMAT2, along with TH, indicates that at least three
populations of neurons exist in the stellate ganglion innervating the
sweat glands and other peripheral targets. These include the
majority of principal ganglion cells, which are fully and
exclusively noradrenergic (TH+/VMAT2+/VAChT-), and two populations of
VAChT+/VMAT2- cells—one positive for TH. The VAChT+/TH+/VMAT2-
neurons innervate the sweat glands (Fig. 4
). The VAChT+/TH-/VMAT2- neurons likely are non-sudomotor and may
innervate the skeletal muscle vasculature.
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Thus, while copious evidence exists for increased expression of cholinergic markers at sweat gland nerve terminal during postnatal development (see above), there is also increasing evidence that cholinergic neurons, both those coexpressing noradrenergic traits and those not, already exist in the stellate ganglion, innervating the forepaw sweat glands, and that VAChT-positive terminals exist at sweat glands as early as the glands themselves exist. One possible resolution of this seeming paradox is that the dramatic increase in the density of VAChT-positive terminals in the sweat glands and periosteum that occurs postnatally is due not to up-regulation of the cholinergic gene locus at the transcriptional level, but to reorganization of vesicle trafficking and increased numbers of small synaptic vesicles at these nerve terminals as the sympathetic cholinergic innervation of these target tissues matures.
Parasympathetic neurons
Parasympathetic postganglionic neurons are organized into ganglia
extrinsic to the innervated organs in the cranial and sacral regions
and in intrinsic ganglia throughout the viscera of the head and neck,
and the thoracic, abdominal and pelvic cavities. These neurons are
purely cholinergic (VAChT+/VMAT2-) in the adult, but initially
differentiate as VAChT/VMAT2 copositive cells in the otic,
sphenopalatine, and submandibular cranial parasympathetic ganglia
(26)
. Since these neurons are TH- during the period of
expression of VMAT2, the functional significance of transient VMAT2
expression, as in the CNS, is unclear. The ciliary parasympathetic
ganglion also expresses VAChT prenatally. However, the ciliary ganglion
expresses no VMAT2 but, rather, a TH phenotype that persists into
adulthood (43
and references therein). Thus, the ability
to visualize multiple components of the noradrenergic and cholinergic
phenotypes, including the VATs, reveals a complex coexpression of
noradrenergic traits in both sympathetic and parasympathetic neurons.
This coexpression indicates that some neuronal populations pass through
a developmental stage in which neurotransmitter-associated traits are
not as stringently regulated as in the fully differentiated neuron. It
may impart properties of neurotransmission that are unique to
developing neurons.
Since VAChT and ChAT are invariably coexpressed in cholinergic neurons,
VAChT has become the marker of choice for detailed examination of
cholinergic terminal fields, particularly in the PNS (7)
.
Visualization of target organs and tissues of the parasympathetic
nervous system with VAChT antibodies has revealed a far more extensive
innervation in ‘classical’ targets of the cholinergic parasympathetic
nervous system than previously appreciated. This is true for all three
major categories of parasympathetic innervation: the secretory cells
and contractile myoepithelial investment of exocrine secretory glands
(e.g., salivary glands, lacrimal glands, exocrine pancreas); visceral
organ vasculature; and visceral organ smooth muscle walls (e.g., gut,
urinary bladder, ureter, urethra, and vas deferens). Especially
striking is the density of cholinergic innervation of ventricular
striated muscle in the heart, which is much more prominent in both
rodent and primate than reported using ChAT as a marker. Coronary
microvessels within the heart itself are also heavily innervated by
VAChT-positive terminals (7)
.
Intrinsic neurons of the gut
Biogenic amine intrinsic innervation of the gut revealed by VMAT2
costaining with biogenic amine markers
The nature of chemical coding of biogenic amine neurons in the
gastrointestinal tract, i.e., the enteric nervous system, has been
intensively studied during development and in the adult, yet several
major unanswered questions remain about amine-handling cell populations
of the enteric nervous system remain. This is due in large part to the
historical assumptions that if a neuron exhibits a particular marker
for a chemically coded phenotype, it possesses the other attributes of
that functional phenotype. Thus, TH-positive neurons without DßH are
often assumed to be dopaminergic, neurons expressing the serotonin
plasma membrane transporter (SERT) are assumed to be fully
serotonergic, and so forth. As indicated in Table 1
and
Table 2
, histochemical evidence for a full functional neurotransmitter
phenotype in the gut requires the visualization of all components
contributing to the phenotype. Even the use of labeled serotonin uptake
as an indicator of functional phenotype is suspect, since
exogenous serotonin can be accumulated by VMAT2/SERT-positive neurons
that lack tryptophan hydroxylase (26
, 28)
.
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VMAT2 is a pan-biogenic amine neuronal marker in the gut, providing a
histochemical ‘denominator’ by which fractional expression of markers
for dopamine, norepinephrine, and serotonin can be examined in the
enteric nervous system. In both rodent and human, the greatest
abundance of VMAT2-positive nerve fibers are found at blood vessels and
around enteric ganglia. These neurons are also positive for TH and
DßH, and represent the sympathetic postganglionic innervation of the
gut. Rare VMAT2-positive fibers in muscle and mucosal layers of gut
appear to lack TH expression and may correspond to projections of
intrinsic serotonergic neurons, i.e., the also rare submucous and
myenteric plexus cell body staining for VMAT2, which in adjacent
sections appear to lack TH and DßH expression. These neurons have
not, however, been demonstrated to express tryptophan hydroxylase or
aromatic amino acid decarboxylase. Those that express SERT may obtain
serotonin either by direct biosynthesis or via scavenging of serotonin
released from enterochromaffin cells within the lamina
propria/intestinal epithelial villi. The extensive distribution of
serotonin receptors in the intestine and the many effects of endogenous
serotonin application on enteric nervous system function make this an
important area for further study. An interesting population of biogenic
aminergic neurons of the human, but not the rodent gut, are the
VMAT2-positive cells of the myenteric and submucous plexus that are
TH+/DßH- and therefore presumably dopaminergic. The function of
these neurons and whether they represent in human a stable continuation
of the so-called transiently catecholaminergic neurons of the rodent
developing gut (44)
are open questions.
Cholinergic innervation of the gut
The use of VAChT for staining of nerve terminals with efficiency
equivalent to that for vasoactive intestinal polypeptide (VIP) and
other neuropeptides has allowed a better neuroanatomical accounting of
the NANC (nonadrenergic, noncholinergic) neuronal system in the gut
(45)
. It is now clear that most intrinsic neurons of the
gut found in myenteric or submucous ganglia are in fact cholinergic as
well as VIPergic, with only a small proportion lacking VAChT
(Fig. 5
). In the colon, the percentage of VIP+/VAChT- neurons is increased,
and these may represent NANC intrinsic sensory neurons of the gut.
Nonadrenergic, noncholinergic (e.g., VIPergic) neurons may be those
that both contain and release ACh and VIP, but at different frequencies
of stimulation. By our estimate (M. Anlauf, L. E. E., and W. E.,
unpublished results), non-VIPergic NANC may comprise up to 20% of the
total neuronal numbers of the human gastrointestinal tract, which are
neither VAChT nor VMAT2 expressing. Recent studies of the ChAT/VAChT
ratio of the peripheral vs. central nervous system also demonstrate
this ratio is rather low in the PNS compared to the CNS
(15)
. This, together with the recent finding of
alternatively spliced ‘peripheral’ forms of ChAT (46)
of
potentially lesser immunoreactivity for conventional ChAT antibodies,
may account for the inability to stain for ChAT even in large
proportions of apparently cholinergically competent neurons and explain
the previously rather high estimate of the proportion of intrinsic
neurons in gut and urogenital tract that are noncholinergic.
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The concordance of VIP and VAChT in most if not all ‘parasympathetic’ or ‘autonomic’ intrinsic neurons allows further distinctions to be made between VIP+/VAChT- putative sensory neurons and VIP+/VAChT+ enteric autonomic neurons. For example, the percentage of VIP+/VAChT- intrinsic neurons in the submucous plexus of the colon (see above) approaches 30%, whereas the numbers of these neurons in the submucous plexus of the proximal gut and in the myenteric plexus at all levels is much lower (3–10%; M. Anlauf et al., unpublished observations).
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BIOGENIC AMINE-CONTAINING ENDOCRINE CELLS AND VMAT EXPRESSION |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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. As discussed above, neuronal elements of
the autonomic nervous system that develop into noradrenergic neurons
express VMAT2 and not the VMAT1 isoform of the vesicular monoamine
transporter (see previous text). In neuroendocrine cell groups and
tissues derived from the neural crest, the dominant transporter is
VMAT1, although depending on the cell type and species examined, VMAT2
may also be expressed in some neural crest-derived neuroendocrine
cells. The small intensely fluorescent (SIF) cells of the sympathetic
ganglia, for example, comprise less than 1% of the total monoaminergic
cells of a typical sympathetic ganglion and store dopamine in large
dense core vesicles that contain VMAT1 as well as VMAT2 in the rat and
mouse (26)
. VMAT1 and VMAT2 coexpression is also seen in
SIF cells in parasympathetic and placodal (vagal and glossopharyngeal)
ganglia in the rat (E. Weihe, unpublished results).
The adrenal medulla contains norepinephrine and epinephrine-storing
chromaffin cells. In rat and mouse, VMAT1 is the dominant transporter
for both types of cells, whereas in human and rhesus monkey, VMAT1 and
VMAT2 are both expressed in apparently all of the cells of adult
adrenal medulla (23
, 24
, 30)
. Other ‘chromaffin-like’
cells of autonomic origin include the glomus cells of the carotid and
aortic bodies. These cells are dopaminergic, have short processes, and
function as chemoreceptive units signaling changes in blood gas levels
to the cardiorespiratory centers of the brain stem. Glomus cells have
as their vesicular transporter predominantly VMAT1 in the adult, but
express quite high levels of VMAT2 early in development, with a decline
into adulthood.
VMAT2 is absent from the well-studied rat pheochromocytoma-derived cell
line PC12, although VMAT2 is expressed in the rat adrenal medulla
(vide supra). This has enabled an interesting set of
experiments in which VMAT2 expression has been forced in PC12 cells and
its secretory vesicle destination followed by immune electron
microscopy and subcellular fractionation (16
, 47)
.
Contrary to expectation, VMAT2 does not target to small synaptic
vesicles that contain VAChT in these cells, but exclusively to the
large dense core VMAT1-positive secretory vesicles, implying that VMAT2
expression in small synaptic vesicles in noradrenergic neurons requires
an as yet unidentified additional competence/chaperone factor not
expressed in PC12 cells. These findings suggest an additional molecular
layer of regulation in amine handling within secretory vesicles of the
nervous and endocrine systems.
The gastroenteropancreatic neuroendocrine system
Besides adrenomedullary chromaffin cells, glomus cells and SIF
cells derived from the neural crest, vesicular monoamine transporters
are expressed on a wide variety of endocrine cells of the stomach,
intestine, and pancreas. The numerous enterochromaffin cells of the
intestine are found in the crypts and villi of the intestinal
epithelium and generated along with other gut epithelial cell types
(Paneth, enterocytic, and goblet cells) from the crypt stem cell
(2)
. Enterochromaffin cells are sensory transducers of the
gut epithelium that store and release serotonin to activate local
sensory afferent neurons that modulate gut motility (48)
.
These cells express exclusively VMAT1 and, unlike the endocrine cells
of neural crest origin, do not go through a VMAT2-expressing stage
(23
, 26)
. In the oxyntic mucosa of the stomach, a second
population of enterochromaffin-like (ECL) cells stores and releases
histamine in response to gastrin secretion, leading to paracrine
activation of acid secretion from neighboring parietal cells. These
cells express exclusively VMAT2 (23
, 49
, 50)
and, again
unlike neural crest-derived endocrine cells, do not pass through a
VMAT2/VMAT1-coexpressing developmental stage. A critical difference in
VMAT1 and VMAT2 substrate recognition may provide a clue to the purpose
of this restriction: VMAT1, unlike VMAT2, does not efficiently
transport histamine (51
, 52)
. In contrast, the primordial
VMAT of Caenorhabditis elegans does transport histamine
(31)
, suggesting that the VMAT1 isoform may function to
restrict paracrine/autocrine cells from scavenging and storing the
ubiquitous autacoid histamine. The Merkel cell of the skin expresses
histidine decarboxylase (E. Weihe, unpublished observations) yet
expresses VMAT1 rather than VMAT2. VMAT1 may be a ‘transporter of
opportunity’ in these cells, similar to the proposed role of VMAT2 in
transient thalamocortical serotonergic neurons of the CNS—in this
case, functioning to take up and store histamine only when
concentrations of the latter are very high, and serotonin or another
biogenic amine when they are not.
Recently, two laboratories have reported on the presence of VMAT1 in a
population of G cells (gastrin-containing cells) of the stomach and
duodenum of the rat (53
, 54)
. Two roles for VMAT1 in these
cells have been postulated. The first is that G cells, in addition to
the well-characterized ECL cells (see above), store and secrete
histamine in the gut (54)
. The second is that VMAT1 allows
these cells to accumulate dietary biogenic amines whose accumulation in
secretory vesicles retards the processing of progastrin to the active
gastrin peptide (53)
. Other studies have failed to
document specific VMAT1 staining in G cells however (55)
,
so that the presence and role(s) of VMAT1 in G cells remain interesting
but open questions.
Significantly, many endocrine tumors of the stomach are derived from
either VMAT1-positive EC or VMAT2-positive ECL cells (55
;
Fig. 6
). VMAT2 has been proposed as a specific marker for gastric
enterochromaffin-like cell tumors, since it is absent from all other
endocrine tumors that arise in the stomach and small intestine,
including gastrin (G cell), somatostatin, EC, and poorly differentiated
endocrine carcinomas (55
, 56)
.
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VMAT1, VMAT2, and VAChT may all be expressed in human
pheochromocytoma, based on the cloning of their cDNAs from
pheochromocytoma tissue (30)
. A systematic examination of
pheochromocytoma subtyping based on the relative expression of the
three VATs has not yet been assayed. This will be of particular
interest in view of the presence of VIP, which is coexpressed with
VAChT in most cholinergic peripheral neurons in a subset of human
chromaffin cells and pheochromocytomas (57)
.
VMAT2 is expressed in ß cells of the pancreas, as well as in
pancreatic tumors (E. Weihe, unpublished observations; Fig. 7
). Whether VMAT2 is expressed in large dense core vesicles or small
vesicles will be of interest to determine, particularly in light of the
observations that other markers generally associated with the synaptic
vesicle in neurons are present in pancreatic islet neuroendocrine cells
(58)
.
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THE INFLAMMATORY SYSTEM |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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Megakaryocytes/platelets
The major role of platelet secretory granule activation in most
species appears to be the local release of serotonin. That histamine is
a major component of the platelet granule in some species may be due to
the fact that VMAT2 is the platelet vesicular transporter (E. Weihe,
unpublished results; Fig. 8
) and has an affinity for histamine that approaches that for serotonin
in some species. The species-specific presence of HDC in platelets has
not yet been examined.
|
Basophils
The original cloning of VMAT2 from a rat basophilic leukemia cell
expression library focused interest on VMAT2 expression and function in
the basophil lineage (59)
. Plasma basophils (E. W. and L.
E. E., unpublished observations) and tissue mast cells (see below) both
express VMAT2, consistent with the apparent generation of both
basophils and mast cells from CD34+ pluripotent bone marrow progenitor
cells and the storage of histamine, a preferred substrate for VMAT2
compared to VMAT1, in granules of both cell types (60
, 61)
.
Tissue mast cells
VMAT2 is found in histamine-storing and secreting tissue mast
cells of brain, tonsils, and skin (Fig. 9
). Visualization of VMAT2 in mucosal mast cells of the stomach has been
difficult compared to other tissues (23)
. For example, gut
tissue mast cells in adventitia and muscle layers are clearly VMAT2
positive as are tonsillar mast cells (Fig. 9)
. We have previously
speculated that VMAT2 down-regulation in stomach mast cells may
function to limit mast cell action to histamine release there and to
prevent chronic reaccumulation and release of histamine by these cells.
|
Dendritic cells
As in the basophil, the dendritic cell function of VMAT2 remains
unclear, as do the domains of the VMAT2 gene that determine its
expression in the hematopoietic cell lineage. It is of interest that
pre-B cells express the VMAT2 gene, which is up-regulated along with
the histidine decarboxylase gene by increased intracellular calcium
(62)
.
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VISUALIZATION OF OTHER VESICULAR NEUROTRANSMITTER TRANSPORTERS IN THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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. The functional
identity of this protein in the worm was based on three important
pieces of circumstantial information: 1) its occurrence in
identified GABAergic neurons, 2) its mislocalization to cell
bodies in mutants deficient in axonal transport of synaptic vesicles to
nerve terminals, and 3) its homology to a family of amino
acid permeases in bacteria (64)
. In the same report, the
functional characterization of a mammalian homologue to unc-47 as a
GABA transporter was described. Independently, Gasnier and co-workers
identified a mammalian homologue of the likely unc-47 cDNA in silico
and subsequently characterized the corresponding protein as a mammalian
GABA and glycine transporter (65)
. This protein, perhaps
more appropriately called VIAAT (for vesicular inhibitory amino acid
transporter), is present in both glycinergic and GABAergic nerve
terminals in mammals (66
, 67)
; it is speculated that the
higher affinity of VIAAT for GABA compared to glycine explains the
preferential accumulation of GABA in neurons that express glutamic acid
decarboxylase and therefore synthesize GABA (66)
. Evidence
exists for GABA/glycine cotransmission in the mammalian spinal cord,
consistent with VIAAT’s dual transporter specificity (67
, 68)
.
The availability of antibodies against each of the major known
vesicular neurotransmitter transporters provides the unique opportunity
to compare directly the density of each type of chemically defined
synapse in a given brain region as well as the synaptic patency for a
given chemically coded neuronal projection system in health or disease.
Figure 10
demonstrates cholinergic cell bodies and homotypic synapses in ventral
horn motoneurons, preganglionic sympathetic cell bodies in the intermediolateral column, and cholinergic terminal and fibers in the
dorsal horn; calcitonin gene-related peptide (CGRP) -positive
motoneurons in ventral horn and sensory input from dorsal root
ganglion in ventral horn; and GABA/glycinergic synapses in the same
area visualized with an antibody raised against a carboxyl-terminal
peptide containing sequences common to rat and mouse VGAT
(64
, 65)
. The differences in density of cholinergic,
peptidergic, and inhibitory (GABA+glycinergic) innervation in the
spinal cord is striking: the amount of inhibitory innervation far
exceeds that contributed by either the sensory or motor systems in the
spinal cord. Thus, comparative staining density and intensity for
vesicular transporters is likely in general to be highly indicative
of the relative contributions of discrete types of chemically coded
neurotransmission at specific neuroanatomical locations. Likewise,
changes in the intensity and density of staining for a given
vesicular neurotransmitter transporter indicate directly the loss of
synaptic patency that can occur early in the course of some
neurodegenerative diseases (20
, 21
, 69
70
71)
. The
application of antibodies directed against the vesicular GABA/glycine
and glutamate transporters (72
, 73)
is likely to
contribute to the understanding of excitatory and inhibitory
neurotransmission in the failing brain, with the high degree of
anatomical specificity required to unravel mechanisms of
neurodegeneration operative in stroke and other human diseases.
|
|
CHEMICAL CODING OF AMINE-HANDLING CELLS: FUNCTIONAL AND PHARMACOLOGICAL IMPLICATIONS AND FUTURE PROSPECTS |
|---|
|
TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
|---|
Developmentally, it is clear that the VATs are regulated in many cases
independently of the biosynthetic enzymes for any of the major
classical neurotransmitters. Insofar as neurons expressing these
biosynthetic enzymes do not express the appropriate VAT, the phenotype
of neurons thought to be catecholaminergic due to the expression of TH
cannot be considered fully functional unless VMAT2 is concomitantly
expressed. Neurons expressing VATs and containing no known classical
neurotransmitter have orphan status. Potential neurotransmitter
candidates for orphan neuronal systems, such as the transient
thalamocortical VMAT2+ neurons of the developing brain, include
serotonin as a transmitter of opportunity and possibly so-called trace
biogenic amines. For example, tyramine can be generated from tyrosine
and stored in neurons that contain only aromatic amino acid
decarboxylase and VMAT2; octopamine can be generated in such neurons if
tyramine ß-hydroxylase is expressed. The latter enzyme has been
identified in Drosophila but not in mammals
(74)
. A classical neurotransmitter may even exist in some
neuronal cell groups but remain uncharacterized: for example, a
potential histaminergic phenotype for VMAT2+/TH- neurons of the
developing parasympathetic nervous system would be conferred by the
presence of histidine decarboxylase, a possibility not yet examined in
these neurons. Special emphasis should be placed on the prominent
‘nonclassical’ amine-synthesizing neuronal cell groups now known to
exist in the CNS. These include the nucleus tractus solitarius
TH+/VMAT2- neuronal cell group, for which the mode of secretion of
dopamine (assuming these neurons contain AADC) remains unknown. These
neurons are concentrated in a small region of the brain known to be
critical for the central regulation of blood pressure, making the
identification of their transmitter status of particular importance.
The presence of VMAT2 in unique and previously unsuspected locations,
including tonsils and developing brain, provides a potential for
imaging with reagents that bind VMAT2 in the clinical contexts of
neuroneonatology and inflammatory/immune disease, such as for diagnosis
of neurodegeneration of central cholinergic neuronal systems with
vesamicol-like reagents (see ref 75
).
Finally, the superior visualization of aminergic systems with
antibodies directed toward the VATs will enable cotransmitters to be
identified, particularly subtypes of amine-handling cells in the brain,
endocrine system, and inflammatory axis. Examples already cited include
coexpression of the prostaglandin biosynthetic enzyme cyclooxygenase
along with VAChT in primate basal forebrain, but not spinal motoneuron
projection systems (20)
. This finding implies
cotransmitter specificity between subdivisions of the cholinergic
nervous system with functional implications for the treatment of
neurodegenerative diseases, such as Alzheimer’s disease, that involve
cholinergic function. Measurements of synaptic patency in the CNS have
also been assayed for the cholinergic system using VAChT staining
corrected for synaptophysin as a general synaptic marker
(22)
. A transient population of VMAT2-positive
thalamocortical projection neurons has been identified in mammals
(26
, 28
, 29)
. Cholinergic and monoaminergic innervation of
immunocytes in the gut has been documented with VAT
immunohistochemistry. These studies mark a new beginning in the
in situ exploration of plasticity, regulation, and
degeneration of specific sets of amine-handling neurons in the brain
and periphery, and the function of amine-handling inflammatory and
immune cells.
|
ACKNOWLEDGMENTS |
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|
REFERENCES |
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TOPABSTRACTINTRODUCTIONAMINERGIC NEURONSBIOGENIC AMINE-CONTAINING...THE INFLAMMATORY SYSTEMVISUALIZATION OF OTHER VESICULAR...CHEMICAL CODING OF AMINE...REFERENCES |
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