Pathway specific expression of neuropeptides and autonomic control of the vasculature

Characterization of trigeminal C-fiber reactivity through capsaicin-induced release of calcitonin gene-related peptide

OBJECTIVE

We hypothesized that the response of trigeminal dermal blood flow (DBF) in the trigeminal system and consecutive expansion of flare response to capsaicin would differ from the somatosensory system (arm). We also investigated whether there are differences between patients with migraine and healthy controls (HC).

BACKGROUND

Functional differences between the trigeminal and extracephalic somatosensory systems may partly explain the susceptibility for headaches in patients with migraine. Capsaicin-induced activation of nociceptive C-fibers in the skin is mainly mediated by calcitonin gene-related peptide (CGRP) and induces cutaneous vessel dilatation and flare response.

METHODS

Female patients with migraine (n = 38) and age-matched HC (n = 35) underwent DBF measurement at baseline and after topical capsaicin administration using laser speckle imaging. DBF before and after capsaicin stimulation was analyzed over ophthalmic nerve/maxillary nerve/mandibular nerve (V1/V2/V3) dermatomes and the forearm as an extracephalic control.

RESULTS

Capsaicin-induced DBF increased more in the trigeminal dermatomes than on the forearm. The V1 dermatome showed a smaller increase of DBF in patients with migraine compared to HC.

CONCLUSION

Our results suggest that the trigeminovascular system reacts differently from extracephalic areas, which may explain the trigeminal susceptibility to CGRP-mediated pain attacks. By demonstrating a different reactivity of the V1 dermatome in patients with migraine, our finding suggests that the first trigeminal branch is functionally different from the second and third branches; however, only in patients with migraine.

Detailed Characterization of Sympathetic Chain Ganglia (SChG) Neurons Supplying the Skin of the Porcine Hindlimb

It is generally known that in the skin sympathetic fibers innervate various dermal structures, including sweat glands, blood vessels, arrectores pilorum muscles and hair follicles. However, there is a lack of data about the distribution and chemical phenotyping of the sympathetic chain ganglia (SChG) neurons projecting to the skin of the pig, a model that is physiologically and anatomically very representative for humans. Thus, the present study was designed to establish the origin of the sympathetic fibers supplying the porcine skin of the hind leg, and the pattern(s) of putative co-incidence of dopamine-β-hydroxylase (DβH) with pituitary adenylate cyclase-activating polypeptide (PACAP), somatostatin (SOM), neuronal nitric oxide synthase, substance P, vasoactive intestinal peptide, neuropeptide Y (NPY), leu5-enkephalin and galanin (GAL) using combined retrograde tracing and double-labeling immunohistochemistry. The Fast Blue-positive neurons were found in the L₂-S₂ ganglia. Most of them were small-sized and contained DβH with PACAP, SOM, NPY or GAL. The findings of the present study provide a detailed description of the distribution and chemical coding of the SChG neurons projecting to the skin of the porcine hind leg. Such data may be the basis for further studies concerning the plasticity of these ganglia under experimental or pathological conditions.

Structure and composition of pulmonary arteries, capillaries, and veins

The pulmonary vasculature comprises three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. Although, in general, this vasculature is thin-walled, structure is nonetheless complex. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. This review is multifaceted, bringing together information regarding (i) classification of pulmonary vessels, (ii) branching geometry in the pulmonary vascular tree, (iii) a quantitative view of structure based on morphometry of the vascular wall, (iv) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, (v) heterogeneity within cell populations and between vascular compartments, (vi) homo- and heterotypic cell-cell junctional complexes, and (vii) the relation of the pulmonary vasculature to that of airways. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted.

Neuronal and nonneuronal cholinergic structures in the mouse gastrointestinal tract and spleen

Accumulating evidence demonstrates that acetylcholine can directly modulate immune function in peripheral tissues including the spleen and gastrointestinal tract. However, the anatomical relationships between the peripheral cholinergic system and immune cells located in these lymphoid tissues remain unclear due to inherent technical difficulties with currently available neuroanatomical methods. In this study, mice with specific expression of the tdTomato fluorescent protein in choline acetyltransferase (ChAT)-expressing cells were used to label preganglionic and postganglionic cholinergic neurons and their projections to lymphoid tissues. Notably, our anatomical observations revealed an abundant innervation in the intestinal lamina propria of the entire gastrointestinal tract principally originating from cholinergic enteric neurons. The aforementioned innervation frequently approached macrophages, plasma cells, and lymphocytes located in the lamina propria and, to a lesser extent, lymphocytes in the interfollicular areas of Peyer's patches. In addition to the above innervation, we observed labeled epithelial cells in the gallbladder and lower intestines, as well as Microfold cells and T-cells within Peyer's patches. In contrast, we found only a sparse innervation in the spleen consisting of neuronal fibers of spinal origin present around arterioles and in lymphocyte-containing areas of the white pulp. Lastly, a small population of ChAT-expressing lymphocytes was identified in the spleen including both T- and B-cells. In summary, this study describes the variety of cholinergic neuronal and nonneuronal cells in a position to modulate gastrointestinal and splenic immunity in the mouse.

Localization of peripheral autonomic neurons innervating the boar urinary bladder trigone and neurochemical features of the sympathetic component

The urinary bladder trigone (UBT) is a limited area through which the majority of vessels and nerve fibers penetrate into the urinary bladder and where nerve fibers and intramural neurons are more concentrated. We localized the extramural post-ganglionic autonomic neurons supplying the porcine UBT by means of retrograde tracing (Fast Blue, FB). Moreover, we investigated the phenotype of sympathetic trunk ganglia (STG) and caudal mesenteric ganglia (CMG) neurons positive to FB (FB+) by coupling retrograde tracing and double-labeling immunofluorescence methods. A mean number of 1845.1±259.3 FB+ neurons were localized bilaterally in the L1-S3 STG, which appeared as small pericarya (465.6±82.7 µm²) mainly localized along an edge of the ganglion. A large number (4287.5±1450.6) of small (476.1±103.9 µm²) FB+ neurons were localized mainly along a border of both CMG. The largest number (4793.3±1990.8) of FB+ neurons was observed in the pelvic plexus (PP), where labeled neurons were often clustered within different microganglia and had smaller soma cross-sectional area (374.9±85.4 µm²). STG and CMG FB+ neurons were immunoreactive (IR) for tyrosine hydroxylase (TH) (66±10.1% and 52.7±8.2%, respectively), dopamine beta-hydroxylase (DβH) (62±6.2% and 52±6.2%, respectively), neuropeptide Y (NPY) (59±8.2% and 65.8±7.3%, respectively), calcitonin-gene-related peptide (CGRP) (24.1±3.3% and 22.1±3.3%, respectively), substance P (SP) (21.6±2.4% and 37.7±7.5%, respectively), vasoactive intestinal polypeptide (VIP) (18.9±2.3% and 35.4±4.4%, respectively), neuronal nitric oxide synthase (nNOS) (15.3±2% and 32.9±7.7%, respectively), vesicular acetylcholine transporter (VAChT) (15±2% and 34.7±4.5%, respectively), leuenkephalin (LENK) (14.3±7.1% and 25.9±8.9%, respectively), and somatostatin (SOM) (12.4±3% and 31.8±7.3%, respectively). UBT-projecting neurons were also surrounded by VAChT-, CGRP-, LENK-, and nNOSIR fibers. The possible role of these neurons and fibers in the neural pathways of the UBT is discussed.

Long distance conduction of vasodilation: a passive or regenerative process?

The mechanism enabling coordination of the resistance of feed arteries with microcirculatory arterioles to rapidly regulate tissue blood flow in line with changes in metabolic demand has preoccupied scientists for a quarter of a century. As experiments uncovered the underlying electrical events, it was frequently questioned how vasodilation could conduct over long distances without appreciable attenuation. This perspective reviews the data pertinent to this phenomenon and provides evidence that this remarkable response could be made possible by a simple mechanism based on the steep relationship between membrane potential and calcium entry demonstrated by the voltage-dependent calcium channels which mediate the control of vascular tone in vivo.

Changes of blood flow volume in the superior mesenteric artery and brachial artery with abdominal thermal stimulation

In traditional Chinese medicine, moxibustion is a local thermal therapy that is used for several conditions. Quantifying the effects of moxibustion therapy has been difficult because the treatment temperature depends on the physician's experience, and the temperature distribution in the target area is not uniform. This prospective observational study aims to quantify the effect of local thermal stimulation to the abdomen. We developed a heat transfer control device (HTCD) for local thermal stimulation. Twenty-four healthy subjects were enrolled and they underwent abdominal thermal stimulation to the para-umbilical region with the device for 20 min. Blood flow volume in the superior mesenteric artery (SMA) and brachial artery (BA), the heart rate and the blood pressure were measured at rest, 15 min after starting thermal stimulation and 10, 20, 30 and 40 min after completing thermal stimulation. Blood flow parameters were measured by high-resolution ultrasound. In the SMA, blood flow volume was significantly increased during thermal stimulation (P < .01), as well as at 10 min (P < .01) and 20 min (P < .05) after stimulation. In the BA, blood flow volume decreased at 40 min after stimulation (P < .01). In conclusion we could quantify the effect of local thermal stimulation with an HTCD and high-resolution ultrasound. Thermal stimulation of the para-umbilical region increased blood flow in the SMA 20 min after stimulation in healthy subjects.

Distribution pattern and chemical coding of neurons of the sympathetic chain ganglia supplying the descending colon in the pig

Sympathetic chain ganglia (SChG) neurons projecting to the descending colon of the pig were studied by means of retrograde tracing (Fast Blue, FB) and double-labelling immunofluorescence methods. FB was injected into the gut wall and after three weeks survival time the animals were transcardially perfused with paraformaldehyde and the bilateral sympathetic trunks were collected. The FB-positive neurons were localised only in the lumbar (L(1)-L(5)) ganglia of the sympathetic trunk and appeared either as small (30-50 microm in diameter) round-shaped perikarya forming clusters localised in caudal-ventral area or, rarely, as bigger (50-80 microm) and dispersed solitary irregular perikarya. Immunohistochemical staining revealed the catecholaminergic (tyrosine hydroxylase-/dopamine beta-hydroxylase-immunoreactive) character of the great majority of FB-positive neurons which preferentially co-expressed neuropeptide Y. In addition, none of the FB-positive perikarya was immunopositive to galanin, somatostatin, choline acetyltransferase, vasoactive intestinal peptide, pituitary adenylate cyclase-activating peptide, leu(5)-enkephalin, nitric oxide synthase, substance P and calcitonin-generelated peptide.

Phenotyping of sympathetic chain ganglia (SChG) neurons in porcine colitis

Colonic inflammation involves the changes in chemical coding of not only sensory but also enteric and sympathetic prevertebral neurons innervating inflamed target tissue. Since the sympathetic chain ganglia neurons (SChG) affect colonic function, the purpose of our study was to determine the precise location of the SChG neurons innervating the porcine descending colon, define their chemical coding and answer the question of whether paraformaldehyde-induced colitis in female pigs is able to change the chemical phenotype of the perikarya. Using the Fast Blue (FB) retrograde tracing, the greatest concentration of the (FB+) perikarya was revealed in L₃ ganglia. Applying double immunohistochemistry, we detected a strongly catecholaminergic character and in majority neuropeptide Y (NPY)-positive staining of the colon-projecting FB+ neurons both in the control and the inflamed animals. Furthermore, colonic inflammation caused the significant increase in density of galanin- (P<0.001), somatostatin- (P<0.005), leu5-enkephalin- (P<0.05), substance P- (P<0.005), and calcitonin gene-related peptide- (P<0.005) immunoreactive (-IR) nerve fibers surrounding the FB+ perikarya, while the density of nitric oxide synthase-IR and NPY-IR fibers remained unaffected compared to the control animals. The increase in visible immunofluorescent detection of neuropeptide-containing fibers could possibly be caused by enhanced demand and release of neuropeptides that modulate intestinal immune response and regulate the gastrointestinal activity through inflammatory mediators.

Increased innervation and ripening of the prepartum murine cervix

OBJECTIVE

Ripening of the cervix before birth is coincident with reduced collagen content and leukocyte immigration, characteristics that are analogous to a neurogenic inflammatory-like process. We sought to assess the morphologic relationship between innervation and remodeling of the peripartum cervix.

METHODS

Cervix was obtained from C3H/HeN mice on days 15 and 18 of pregnancy, 1 day postpartum, and from non-pregnant controls. Tissues were immersion-fixed, paraffin-embedded, and some sections stained with Picrosirius red to assess collagen content and complexity of organization. By image analysis of optical density, collagen content and structure were significantly decreased by the day before birth. Other sections were processed to visualize nerve fibers by immunohistochemistry with antibodies against neuron-specific epitopes, PGP9.5, peripherin, as well as brain nitric oxide synthase (bNOS), calcitonin gene-related peptide (CGRP), and other neuropeptides. Fiber density was assessed stereologically and normalized to cell density in non-pregnant cervix to correct for tissue hypertrophy due to reproductive status.

RESULTS

In groups of non-pregnant, day 15 pregnant, and postpartum mice, cervix contained nerve fibers that were immunoreactive for the pan-neural markers PGP9.5 and peripherin. Punctate and beaded varicosities were sparsely distributed in stroma, subepithelium, and in proximity to vascular structures. By day 18 of pregnancy, 1 day before birth, fiber density was increased fourfold or more compared to other groups. bNOS fibers and, to a lesser extent, CGRP accounted for most of the increased innervation of the murine cervix by the day before birth, a period when macrophage numbers are enhanced.

CONCLUSIONS

The findings suggest that increased bNOS and CGRP innervation contribute to early inflammatory-like processes that ripen the cervix before birth.

Neurochemical properties of the middle cervical ganglion in the sheep

The neurochemical properties of the ovine middle cervical ganglion (MCG) were studied using antibodies raised against tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), neuropeptide Y (NPY), substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP) and galanin (GAL). Double-labelling immunocytochemistry revealed that the vast majority (95.5 +/- 0.8%) of postganglionic sympathetic MCG neurons expressed simultaneously both catecholamine-synthesizing enzymes (neurons were TH/DbetaH-positive). A large population of noradrenergic neurons exhibited immunoreactivity (IR) to NPY (62.2 +/- 2.2%), but single NPY-positive perikarya-lacking noradrenergic markers were also observed (2.0 +/- 0.3%). None of the examined MCG neuronal somata contained SP, CGRP, GAL or VIP. A moderate number of noradrenergic nerve fibres located amongst neuronal cell bodies was also found. In small number of these terminals the presence of NPYor GAL (but not CGRP or VIP) was detected. The ovine MCG was numerously innervated with SP-immunoreactive nerve fibres which sometimes formed basket-like formations around postganglionic neurons. The MCG exhibited a sparse CGRP-immunoreactive innervation and lacked VIP-positive nerve terminals. In many aspects the chemical coding of MCG postganglionic neurons and nerve terminals resembles that found in other mammalian cervico-thoracic paravertebral ganglia, but some important species-dependent differences exist. The functional implications of these differences remain to be elucidated.

Nicotinamide adenine dinucleotide is released from sympathetic nerve terminals via a botulinum neurotoxin A-mediated mechanism in canine mesenteric artery

Using high-performance liquid chromatography techniques with fluorescence and electrochemical detection, we found that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in response to electrical field stimulation (4-16 Hz, 0.3 ms, 15 V, 120 s) along with ATP and norepinephrine (NE) in the canine isolated mesenteric arteries. The release of beta-NAD increases with number of pulses/stimulation frequencies. Immunohistochemistry analysis showed dense distribution of tyrosine hydroxylase-like immunoreactivity (TH-LI) and sparse distribution of TH-LI-negative nerve processes, suggesting that these blood vessels are primarily under sympathetic nervous system control with some contribution of other (e.g., sensory) neurons. Exogenous NE (3 micromol/l), alpha,beta-methylene ATP (1 micromol/l), neuropeptide Y (NPY, 0.1 micromol/l), CGRP (0.1 micromol/l), vasoactive intestinal peptide (VIP, 0.1 micromol/l), and substance P (SP, 0.1 micromol/l) had no effect on the basal release of beta-NAD, suggesting that the overflow of beta-NAD is evoked by neither the sympathetic neurotransmitters NE, ATP, and NPY, nor the neuropeptides CGRP, VIP, and SP. Botulinum neurotoxin A (BoNTA, 0.1 micromol/l) abolished the evoked release of NE, ATP, and beta-NAD at 4 Hz, suggesting that at low levels of neural activity, release of these neurotransmitters results from N-ethylmaleimide-sensitive factor attachment protein receptor/synaptosomal-associated protein of 25 kDa-mediated exocytosis. At 16 Hz, however, the evoked release of NE, ATP, and beta-NAD was reduced by BoNTA by approximately 90, 60, and 80%, respectively, suggesting that at higher levels of neural activity, beta-NAD is likely to be released from different populations of synaptic vesicles or different populations of nerve terminals (i.e., sympathetic and sensory terminals).

Differential involvement of N-type calcium channels in transmitter release from vasoconstrictor and vasodilator neurons

1. The effects of calcium channel blockers on co-transmission from different populations of autonomic vasomotor neurons were studied on isolated segments of uterine artery and vena cava from guinea-pigs. 2. Sympathetic, noradrenergic contractions of the uterine artery (produced by 200 pulses at 1 or 10 Hz; 600 pulses at 20 Hz) were abolished by the N-type calcium channel blocker omega-conotoxin (CTX) GVIA at 1-10 nm. 3. Biphasic sympathetic contractions of the vena cava (600 pulses at 20 Hz) mediated by noradrenaline and neuropeptide Y were abolished by 10 nm CTX GVIA. 4. Neurogenic relaxations of the uterine artery (200 pulses at 10 Hz) mediated by neuronal nitric oxide and neuropeptides were reduced <50% by CTX GVIA 10-100 nm. 5. Capsaicin (3 microm) did not affect the CTX GVIA-sensitive or CTX GVIA-resistant neurogenic relaxations of the uterine artery. 6. The novel N-type blocker CTX CVID (100-300 nm), P/Q-type blockers agatoxin IVA (10-100 nm) or CTX CVIB (100 nm), the L-type blocker nifedipine (10 microm) or the 'R-type' blocker SNX-482 (100 nm), all failed to reduce CTX GVIA-resistant relaxations. The T-type channel blocker NiCl(2) (100-300 microm) reduced but did not abolish the remaining neurogenic dilations. 7. Release of different neurotransmitters from the same autonomic vasomotor axon depends on similar subtypes of calcium channels. N-type channels are responsible for transmitter release from vasoconstrictor neurons innervating a muscular artery and capacitance vein, but only partly mediate release of nitric oxide and neuropeptides from pelvic vasodilator neurons.

Functional organization of peripheral vasomotor pathways

AIM

In this article, we review the functional organization of the peripheral autonomic pathways regulating the vasculature.

RESULTS

The final motor neurones in vasomotor pathways tend to be smaller than neurones in other autonomic pathways. This suggests that they have relatively smaller target territories and receive fewer pre-ganglionic inputs than non-vasomotor neurones. Nevertheless, single vasomotor neurones project to large areas of the vasculature separated by up to 7 mm. Different functional pools of vasomotor neurones project to specific segments of the vasculature, allowing for the selective neural control of resistance in vessels in proximal or distal regions of the vascular bed. In many cases, each functional pool of vasomotor neurones utilizes a characteristic combination of cotransmitters. The various pools of final motor neurones in vasomotor pathways receive convergent synaptic input from different pools of pre-ganglionic neurones, many of which also contain neuropeptides which enhance the excitability of the final motor neurones. The excitability of vasomotor neurones regulating gastrointestinal and mesenteric blood flow, also can be increased by the actions of peptides such as substance P that are released from visceral nociceptors.

CONCLUSIONS

We propose that autonomic pathways regulating the vasculature are organized into 'vasomotor units'. Each vasomotor unit consists of a pre-ganglionic neurone, the final motor neurones it innervates, and the blood vessels that they regulate. The vasomotor units are likely to be grouped into functional pools that can be recruited as necessary to provide highly specific, graded control of blood flow both within and between vascular beds.

Neuropeptide Y effects on vasorelaxation and intestinal contraction in the Atlantic cod Gadus morhua

Neuropeptide Y (NPY) has prominent cardiovascular effects in mammals and sharks, but no such effect has previously been demonstrated in any teleost fish. In the Atlantic cod, we found that cod NPY (10(-10)-10(-6) M) relaxed celiac arteries precontracted with epinephrine, and weak contractions were elicited in intestinal ring preparations. A few NPY-immunoreactive nerve fibers were present along small gut arteries. The results suggest that cod NPY produces vasorelaxation both by a direct action on smooth muscle and by release of prostaglandins, but with no involvement of nitric oxide, leukotrienes, or endothelium-derived relaxing factors. An additional indirect effect involving another neurotransmitter may occur. Cod NPY (10(-7) M) and human NPY (10(-7) M) had identical effects on the vessels. Small differences only in the effects of porcine [Leu(31),Pro(34)]NPY, NPY-(13-36), and cod NPY suggest the presence of a Y(1) subfamily receptor, similar to the zebrafish Ya receptor. A physiological role for NPY in teleost vasculature is concluded, but surprisingly the effect, a vasodilation, is opposite to that in mammals and is mediated by prostaglandins.

Differential inhibition by botulinum neurotoxin A of cotransmitters released from autonomic vasodilator neurons

The role of the soluble NSF attachment protein receptor (SNARE) protein complex in release of multiple cotransmitters from autonomic vasodilator neurons was examined in isolated segments of guinea pig uterine arteries treated with botulinum neurotoxin A (BoNTA; 50 nM). Western blotting of protein extracts from uterine arteries demonstrated partial cleavage of synaptosomal-associated protein of 25 kDa (SNAP-25) to a NH2-terminal fragment of approximately 24 kDa by BoNTA. BoNTA reduced the amplitude (by 70-80%) of isometric contractions of arteries in response to repeated electrical stimulation of sympathetic axons at 1 or 10 Hz. The amplitude of neurogenic relaxations mediated by neuronal nitric oxide (NO) was not affected by BoNTA, whereas the duration of peptide-mediated neurogenic relaxations to stimulation at 10 Hz was reduced (67% reduction in integrated responses). In contrast, presynaptic cholinergic inhibition of neurogenic relaxations was abolished by BoNTA. These results demonstrate that the SNARE complex has differential involvement in release of cotransmitters from the same autonomic neurons: NO release is not dependent on synaptic vesicle exocytosis, acetylcholine release from small vesicles is highly dependent on the SNARE complex, and neuropeptide release from large vesicles involves SNARE proteins that may interact differently with regulatory factors such as calcium.

Neuropeptide Y immunoreactivity in cutaneous sympathetic and sensory neurons during development of the guinea pig

Different levels of the cutaneous vasculature are innervated selectively by subpopulations of sympathetic neurons distinguished by the presence or absence of immunoreactivity (-IR) for neuropeptide Y (NPY). This study used multiple-labelling immunohistochemistry to examine the appearance of NPY-IR in neurons innervating cutaneous vessels in the ear pinna of embryonic, fetal, and neonatal guinea pigs. NPY-immunoreactive axons were detected in the ear bud at embryonic day 25. However, these axons lacked IR for tyrosine hydroxylase (TH) and often ran in bundles with substance P (SP)-immunoreactive axons close to the epidermis. Many neuronal somata in the cervical dorsal root ganglia (DRG) at late embryonic stages contained NPY-IR with or without SP-IR, but no NPY-IR was detected in DRG or subepidermal axons by late fetal stages. IR for calcitonin gene-related peptide increased in DRG neurons from midfetal to late fetal stages, after the decrease in NPY-IR. Populations of TH-IR neurons with or without NPY-IR were present in the superior cervical ganglion (SCG) from midembryonic stages. TH-immunoreactive axons were not detected in the ear pinna until midfetal stages, when axons with TH-IR and NPY-IR innervated proximal arteries and TH-immunoreactive axons without NPY-IR innervated distal vessels. Vasoactive intestinal peptide-IR was detected transiently in most fetal SCG neurons with TH-IR and NPY-IR but was not detected in cutaneous axons. These results demonstrate that selective expression of NPY by subpopulations of sympathetic neurons occurs prior to innervation of their targets. This suggests that target contact is not required to establish appropriate patterns of expression of peptide neurotransmitters by cutaneous sympathetic neurons.