Direct observations of sympathetic cholinergic vasodilatation of skeletal muscle small arteries in the cat

Central command increases muscular oxygenation of the non-exercising arm at the early period of voluntary one-armed cranking

This study aimed to examine whether central command increases oxygenation in non‐contracting arm muscles during contralateral one‐armed cranking and whether the oxygenation response caused by central command differs among skeletal muscles of the non‐exercising upper limb. In 13 male subjects, the relative changes in oxygenated‐hemoglobin concentration (Oxy‐Hb) of the non‐contracting arm muscles [the anterior deltoid, triceps brachii, biceps brachii, and extensor carpi radialis (ECR)] were measured during voluntary one‐armed cranking (intensity, 35–40% of maximal voluntary effort) and mental imagery of the one‐armed exercise for 1 min. Voluntary one‐armed cranking increased (P < 0.05) the Oxy‐Hb of the triceps, biceps, and ECR muscles to the same extent (15 ± 4% of the baseline level, 17 ± 5%, and 16 ± 4%, respectively). The greatest increase in the Oxy‐Hb was observed in the deltoid muscle. Intravenous injection of atropine (10–15 μg/kg) and/or propranolol (0.1 mg/kg) revealed that the increased Oxy‐Hb of the arm muscles consisted of the rapid atropine‐sensitive and delayed propranolol‐sensitive components. Mental imagery of the exercise increased the Oxy‐Hb of the arm muscles. Motor‐driven passive one‐armed cranking had little influence on the Oxy‐Hb of the arm muscles. It is likely that central command plays a role in the initial increase in oxygenation in the non‐contracting arm muscles via sympathetic cholinergic vasodilatation at the early period of one‐armed cranking. The centrally induced increase in oxygenation may not be different among the distal arm muscles but may augment in the deltoid muscle.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

A cholinergic contribution to the circulatory responses evoked at the onset of handgrip exercise in humans

A cholinergic (muscarinic) contribution to the initial circulatory response to exercise in humans remains controversial. Herein, we posit that this may be due to exercise mode with a cholinergic contribution being important during isometric handgrip exercise, where the hyperemic response of the muscle is relatively small compared with the onset of leg cycling, where a marked increase in muscle blood flow rapidly occurs as a consequence of multiple redundant mechanisms. We recorded blood pressure (BP; brachial artery), stroke volume (pulse contour analysis), cardiac output, and systemic vascular resistance (SVR) in young healthy males, while performing either 20 s of isometric handgrip contraction at 40% maximum voluntary contraction (protocol 1; n = 9) or 20 s of low-intensity leg cycling exercise (protocol 2; n = 8, 42 ± 8 W). Exercise trials were conducted under control (no drug) conditions and following cholinergic blockade (glycopyrrolate). Under control conditions, isometric handgrip elicited an initial increase in BP (+5 ± 2 mmHg at 3 s and +3 ± 1 mmHg at 10 s, P < 0.05), while SVR dropped after 3 s (-27 ± 6% at 20 s; P < 0.05). Cholinergic blockade abolished the isometric handgrip-induced fall in SVR and, thereby, augmented the pressor response (+13 ± 3 mmHg at 10 s; P < 0.05 vs. control). In contrast, cholinergic blockade had a nonsignificant effect on changes in BP and SVR at the onset of leg cycling exercise. These findings suggest that a cholinergic mechanism is important for the BP and SVR responses at the onset of isometric handgrip exercise in humans.

Sympathetic neural activity to the cardiovascular system: integrator of systemic physiology and interindividual characteristics

The sympathetic nervous system is a ubiquitous, integrating controller of myriad physiological functions. In the present article, we review the physiology of sympathetic neural control of cardiovascular function with a focus on integrative mechanisms in humans. Direct measurement of sympathetic neural activity (SNA) in humans can be accomplished using microneurography, most commonly performed in the peroneal (fibular) nerve. In humans, muscle SNA (MSNA) is composed of vasoconstrictor fibers; its best-recognized characteristic is its participation in transient, moment-to-moment control of arterial blood pressure via the arterial baroreflex. This property of MSNA contributes to its typical "bursting" pattern which is strongly linked to the cardiac cycle. Recent evidence suggests that sympathetic neural mechanisms and the baroreflex have important roles in the long term control of blood pressure as well. One of the striking characteristics of MSNA is its large interindividual variability. However, in young, normotensive humans, higher MSNA is not linked to higher blood pressure due to balancing influences of other cardiovascular variables. In men, an inverse relationship between MSNA and cardiac output is a major factor in this balance, whereas in women, beta-adrenergic vasodilation offsets the vasoconstrictor/pressor effects of higher MSNA. As people get older (and in people with hypertension) higher MSNA is more likely to be linked to higher blood pressure. Skin SNA (SSNA) can also be measured in humans, although interpretation of SSNA signals is complicated by multiple types of neurons involved (vasoconstrictor, vasodilator, sudomotor and pilomotor). In addition to blood pressure regulation, the sympathetic nervous system contributes to cardiovascular regulation during numerous other reflexes, including those involved in exercise, thermoregulation, chemoreflex regulation, and responses to mental stress.

Differential contribution of ACh-muscarinic and β-adrenergic receptors to vasodilatation in noncontracting muscle during voluntary one-legged exercise

We have demonstrated the centrally induced cholinergic vasodilatation in skeletal muscle at the early period of voluntary one-legged exercise and during motor imagery in humans. The purpose of this study was to examine whether central command may also cause β-adrenergic vasodilatation during the exercise and motor imagery. Relative changes in oxygenated hemoglobin concentration (Oxy-Hb) of bilateral vastus lateralis (VL) muscles, as index of tissue blood flow, and femoral blood flow to nonexercising limb were measured during one-legged cycling and mental imagery of the exercise for 1 min before and after propranolol (0.1 mg/kg iv). The Oxy-Hb of noncontracting muscle increased (P < 0.05) at the early period of exercise and the increase was sustained throughout exercise, whereas the Oxy-Hb of contracting muscle increased at the early period but thereafter decreased. We subtracted the Oxy-Hb response with propranolol from the control response in individual subjects to identify the propranolol-sensitive component of the Oxy-Hb response during exercise. In both noncontracting and contracting VL muscles, the increase in Oxy-Hb at the early period of one-legged exercise did not involve a significant propranolol-sensitive component. However, as the exercise proceeded, the propranolol-sensitive component of the Oxy-Hb response was developed during the later period of exercise. Propranolol also failed to affect the initial increases in femoral blood flow and vascular conductance of nonexercising leg but significantly attenuated (P < 0.05) their later increases during exercise. Subsequent atropine (10-15 μg/kg iv) abolished the initial increases in Oxy-Hb of both VL muscles. Mental imagery of the one-legged exercise caused the bilateral increases in Oxy-Hb, which were not altered by propranolol but abolished by subsequent atropine. It is likely that the rapid cholinergic and delayed β-adrenergic vasodilator mechanisms cooperate to increase muscle blood flow during exercise.

Superimposed traumatic brain injury modulates vasomotor responses in third-order vessels after hemorrhagic shock

BACKGROUND

Traumatic brain injury (TBI) and hemorrhagic shock (HS) are the leading causes of death in trauma. Recent studies suggest that TBI may influence physiological responses to acute blood loss. This study was designed to assess to what extent superimposed TBI may modulate physiologic vasomotor responses in third-order blood vessels in the context of HS.

METHODS

We have combined two established experimental models of pressure-controlled hemorrhagic shock (HS; MAP 50 mmHg/60 min) and TBI (lateral fluid percussion (LFP)) to assess vasomotor responses and microcirculatory changes in third-order vessels by intravital microscopy in a spinotrapezius muscle preparation. 23 male Sprague-Dawley rats (260-320 g) were randomly assigned to experimental groups: i) Sham, ii) HS, iii) TBI + HS, subjected to impact or sham operation, and assessed.

RESULTS

HS led to a significant decrease in arteriolar diameters by 20% to baseline (p < 0.01). In TBI + HS this vasoconstriction was less pronounced (5%, non-significant). At completed and at 60 minutes of resuscitation arteriolar diameters had recovered to pre-injury baseline values. Assessment of venular diameters revealed similar results. Arteriolar and venular RBC velocity and blood flow decreased sharply to < 20% of baseline in HS and TBI + HS (p < 0.01). Immediately after and at 60 minutes of resuscitation, an overshoot in arterial RBC velocity (140% of baseline) and blood flow (134.2%) was observed in TBI + HS.

CONCLUSION

Superimposed TBI modulated arteriolar and venular responses to HS in third-order vessels in a spinotrapezius muscle preparation. Further research is necessary to precisely define the role of TBI on the microcirculation in tissues vulnerable to HS.

Evidence for centrally induced cholinergic vasodilatation in skeletal muscle during voluntary one-legged cycling and motor imagery in humans

We have recently reported that central command contributes to increased blood flow in both noncontracting and contracting vastus lateralis (VL) muscles at the early period of voluntary one-legged cycling. The purpose of this study was to examine whether sympathetic cholinergic vasodilatation mediates the increases in blood flows of both muscles during one-legged exercise. Following intravenous administration of atropine (10 μg/kg), eight subjects performed voluntary 1-min one-legged cycling (at 35% of maximal voluntary effort) and mental imagery of the exercise. The relative concentrations of oxygenated- and deoxygenated-hemoglobin (Oxy- and Deoxy-Hb) in the bilateral VL were measured as an index of muscle tissue blood flow with near-infrared spectroscopy (NIRS). The Oxy-Hb in both noncontracting and contracting VL increased at the early period of one-legged cycling, whereas the Deoxy-Hb did not alter at that period. Atropine blunted (P < 0.05) the Oxy-Hb responses of both VL muscles but did not affect the Deoxy-Hb responses. The time course and magnitude of the atropine-sensitive component in the Oxy-Hb response were quite similar between the noncontracting and contracting VL muscles. With no changes in the Deoxy-Hb and hemodynamics, imagery of one-legged cycling induced the bilateral increases in the Oxy-Hb, which were completely abolished by atropine. In contrast, imagery of a circle (with no relation to exercise) did not alter the NIRS signals, irrespective of the presence or absence of atropine. It is concluded that central command evokes cholinergic vasodilatation equally in bilateral VL muscles during voluntary one-legged cycling and motor imagery.

Have we missed that neural vasodilator mechanisms may contribute to exercise hyperemia at onset of voluntary exercise?

Whether neurally-mediated vasodilatation may contribute to exercise hyperemia has not been completely understood. Bülbring and Burn (1935) found for the first time the existence of sympathetic cholinergic nerve to skeletal muscle contributing to vasodilatation in animals. Blair et al. (1959) reported that atropine-sensitive vasodilatation in skeletal muscle appeared during arousal behavior or mental stress in humans. However, such sympathetic vasodilator mechanism for muscle vascular bed in humans is generally denied at present, because surgical sympathectomy, autonomic blockade, and local anesthesia of sympathetic nerves cause no substantial influence on vasodilatation in muscle not only during mental stress but also during exercise. On the other hand, neural mechanisms may play an important role in regulating blood flow to non-contracting muscle. Careful consideration of the neural mechanisms may lead us to an insight about a possible neural mechanism responsible for exercise hyperemia in contracting muscle. Referring to our recent study measuring muscle tissue blood flow with higher time resolution, this review has focused on whether or not central command may transmit vasodilator signal to skeletal muscle especially at the onset of voluntary exercise.

Electrodeposition of alginate gels for construction of vascular-like structures

In this study, tubular hydrogel structures were constructed via electrodeposition using alginate gels. Electrolysis of water in alginate solutions with calcium carbonate particles induced gel aggregation around Pt wire electrodes, forming tubular alginate gel structures. The simple method is a promising approach for construction of multi-layer tubular hydrogel structures for tissue engineering.

Central command contributes to increased blood flow in the noncontracting muscle at the start of one-legged dynamic exercise in humans

Whether neurogenic vasodilatation contributes to exercise hyperemia is still controversial. Blood flow to noncontracting muscle, however, is chiefly regulated by a neural mechanism. Although vasodilatation in the nonexercising limb was shown at the onset of exercise, it was unclear whether central command or muscle mechanoreflex is responsible for the vasodilatation. To clarify this, using voluntary one-legged cycling with the right leg in humans, we measured the relative changes in concentrations of oxygenated-hemoglobin (Oxy-Hb) of the noncontracting vastus lateralis (VL) muscle with near-infrared spectroscopy as an index of tissue blood flow and femoral blood flow to the nonexercising leg. Oxy-Hb in the noncontracting VL and femoral blood flow increased ( P < 0.05) at the start period of voluntary one-legged cycling without accompanying a rise in arterial blood pressure. In contrast, no increases in Oxy-Hb and femoral blood flow were detected at the start period of passive one-legged cycling, suggesting that muscle mechanoreflex cannot explain the initial vasodilatation of the noncontracting muscle during voluntary one-legged cycling. Motor imagery of the voluntary one-legged cycling increased Oxy-Hb of not only the right but also the left VL. Furthermore, an increase in Oxy-Hb of the contracting VL, which was observed at the start period of voluntary one-legged cycling, had the same time course and magnitude as the increase in Oxy-Hb of the noncontracting muscle. Thus it is concluded that the centrally induced vasodilator signal is equally transmitted to the bilateral VL muscles, not only during imagery of exercise but also at the start period of voluntary exercise in humans.

Electrical stimulation of the mesencephalic ventral tegmental area evokes skeletal muscle vasodilatation in the cat and rat

To test the hypothesis that the mesencephalic ventral tegmental area (VTA) plays a role in autonomic control of the cardiovascular system, we examined the cardiovascular effects of electrical stimulation of the mesencephalic ventral areas in anesthetized, paralyzed cats and rats. Electrical stimulation of the VTA for 30 s (100-μA current intensity; 40-50-Hz pulse frequency; 0.5-1-ms pulse duration) increased femoral blood flow by 130-162% in anesthetized cats and rats, whereas the identical stimulation of the substantial nigra (SN) failed to increase femoral blood flow. Electrical stimulation of the VTA also increased the arterial blood pressure and heart rate in anesthetized rats, but did not alter them in anesthetized cats. Accordingly, femoral vascular conductance was increased by 102-134% in both cats and rats. Atropine methyl nitrate (0.1 mg/kg) injected intravenously in the cats markedly attenuated the increases in femoral blood flow and vascular conductance. VTA stimulation was able to produce substantial increases in femoral blood flow and vascular conductance following a decerebration procedure performed at the premammillary and precollicular level in the cats, although their responses tended to attenuate to 55-69% of the control before the decerebration. Thus, it is likely that electrical stimulation of the VTA, but not the SN, is capable of evoking skeletal muscle vasodilatation, particularly via a sympathetically mediated cholinergic mechanism in the cat, and that the ascending projection from the VTA to the forebrain may not be responsible for the muscle vasodilatation.

Centrally evoked increase in adrenal sympathetic outflow elicits immediate secretion of adrenaline in anaesthetized rats

To examine whether feedforward control by central command activates preganglionic adrenal sympathetic nerve activity (AdSNA) and releases catecholamines from the adrenal medulla, we investigated the effects of electrical stimulation of the hypothalamic locomotor region on preganglionic AdSNA and secretion rate of adrenal catecholamines in anaesthetized rats. Pre- or postganglionic AdSNA was verified by temporary sympathetic ganglionic blockade with trimethaphan. Adrenal venous blood was collected every 30 s to determine adrenal catecholamine output and blood flow. Hypothalamic stimulation for 30 s (50 Hz, 100-200 microA) induced rapid activation of preganglionic AdSNA by 83-181% depending on current intensity, which was followed by an immediate increase of 123-233% in adrenal adrenaline output. Hypothalamic stimulation also increased postganglionic AdSNA by 42-113% and renal sympathetic nerve activity by 94-171%. Hypothalamic stimulation induced preferential secretion of adrenal adrenaline compared with noradrenaline, because the ratio of adrenaline to noradrenaline increased greatly during hypothalamic stimulation. As soon as the hypothalamic stimulation was terminated, preganglionic AdSNA returned to the prestimulation level in a few seconds, and the elevated catecholamine output decayed within 30-60 s. Adrenal blood flow and vascular resistance were not affected or slightly decreased by hypothalamic stimulation. Thus, it is likely that feedforward control of catecholamine secretion from the adrenal medulla plays a role in conducting rapid hormonal control of the cardiovascular system at the beginning of exercise.

Sympathetic cholinergic nerve contributes to increased muscle blood flow at the onset of voluntary static exercise in conscious cats

We examined whether a sympathetic cholinergic mechanism contributed to increased blood flow of the exercising muscle at the onset of voluntary static exercise in conscious cats. After six cats were operantly conditioned to perform static bar press exercise with a forelimb while maintaining a sitting posture, a Transonic or pulsed Doppler flow probe was implanted on the brachial artery of the exercising forelimb, and catheters were inserted into the left carotid artery and jugular vein. After the baseline brachial blood flow and vascular conductance decreased and became stable in progress of postoperative recovery, the static exercise experiments were started. Brachial blood flow and vascular conductance began to increase simultaneously with the onset of exercise. Their initial increases reached 52 +/- 8% and 40 +/- 6% at 3 s from the exercise onset, respectively. Both a sympathetic ganglionic blocker (hexamethonium bromide) and atropine sulfate or methyl nitrate blunted the increase in brachial vascular conductance at the onset of static exercise, whereas an inhibitor of nitric oxide synthesis (N(omega)-nitro-l-arginine methyl ester) did not alter the increase in brachial vascular resistance. Brachial blood flow and vascular conductance increased during natural grooming behavior with the forelimb in which the flow probe was implanted, whereas they decreased during grooming with the opposite forelimb and during eating behavior. Thus it is likely that the sympathetic cholinergic mechanism is capable of evoking muscle vasodilatation at the onset of voluntary static exercise in conscious cats.

Differential sympathetic outflow and vasoconstriction responses at kidney and skeletal muscles during fictive locomotion

We compared sympathetic and circulatory responses between kidney and skeletal muscles during fictive locomotion evoked by electrical stimulation of the mesencephalic locomotor region (MLR) in decerebrate and paralyzed rats ( n = 8). Stimulation of the MLR for 30 s at 40-μA current intensity significantly increased arterial pressure (+38 ± 6 mmHg), triceps surae muscle blood flow (+17 ± 3%), and both renal and lumbar sympathetic nerve activities (RSNA +113 ± 16%, LSNA +31 ± 7%). The stimulation also significantly decreased renal cortical blood flow (−18 ± 6%) and both renal cortical and triceps surae muscle vascular conductances (RCVC −38 ± 5%, TSMVC −17 ± 3%). The sympathetic and vascular conductance changes were significantly dependent on current intensity for stimulation at 20, 30, and 40 μA. The changes in LSNA and TSMVC were significantly less than those in RSNA and RCVC, respectively, at all current intensities. At the early stage of stimulation (0–10 s), decreases in RCVC and TSMVC were significantly correlated with increases in RSNA and LSNA, respectively. These data demonstrate that fictive locomotion induces less vasoconstriction in skeletal muscles than in kidney because of less sympathetic activation. This suggests that a neural mechanism mediated by central command contributes to blood flow distribution by evoking differential sympathetic outflow during exercise.

Forelimb vasodilatation induced by hypothalamic stimulation is greatly mediated with nitric oxide in anesthetized cats

The aim of this study was to examine whether or not stimulation of the hypothalamic defense area is capable of inducing sympathetic vasodilatation of the forelimb vascular bed in anesthetized cats. When the hypothalamic defense area was electrically stimulated, brachial blood flow velocity (brachial BFV) and vascular conductance were increased as well as femoral BFV and vascular conductance. Brachial BFV and vascular conductance increased by 110-139% during hypothalamic stimulation. These increases were blunted to approximately one-fifth of the control responses following i.v. injection of a synthesis inhibitor of nitric oxide, N(omega)-nitro-L-arginine methyl ester (L-NAME). The attenuating effect of L-NAME on forelimb vasodilatation evoked by hypothalamic stimulation was greater than that on hindlimb vasodilatation. The combined administration of L-NAME and atropine sulfate eliminated nearly all of the increases in brachial BFV and vascular conductance during hypothalamic stimulation. From the present results, we conclude that stimulation of the hypothalamic defense area is able to induce neurogenic vasodilatation of the cat forelimb vascular bed, which is greatly mediated with a nitric oxide mechanism. The contribution of nitric oxide to neurogenic vasodilatation seems to be greater in the forelimbs than hindlimbs.

Differences in the density of sympathetic nerve fibers in the arteriolar walls of the rat extensor digitorum longus muscle

In this study, we electron-microscopically investigated a number of sympathetic axons in the arteriolar walls of the extensor digitorum longus muscles of the rat rear leg. Arterioles in the muscle were divided into two groups: (i) one group consisted of arterioles with accompanying muscle spindles, and (ii) the other consisted of arterioles without accompanying muscle spindles. The number of sympathetic axons present in the arteriolar walls and the ratios to the total number of sympathetic and non-sympathetic axons were compared between the groups. For electron-microscopic identification of sympathetic axons, 5-hydroxydopamine, a pseudotransmitter agent, was used. The number and ratio of sympathetic axons were significantly higher in arterioles with accompanying muscle spindles than arterioles possibly unrelated to muscle spindles. Additionally, amine- and immunohistochemistry were used to confirm the above observation.

Role of perivascular sympathetic nerves and regional differences in the features of sympathetic innervation of the vascular system

Maintenance of blood pressure is mostly dependent on sympathetic "tone", and the sympathetic nerve innervates the entire vascular bed, excepting the capillaries. Although norepinephrine (NE) is the principal neurotransmitter released upon sympathetic nerve stimulation, neuropeptide Y and ATP are cotransmitters in various vascular tissues. In addition, dopamine and epinephrine, as well as acetylcholine, have been shown to be sympathetic neurotransmitters in specific vasculatures. Transmitter NE release is modified by a number of endogenous substances including the transmitter itself. Chronic denervation of the preganglionic fiber induces an increase in NE release per pulse, indicating postganglionic neuronal supersensitivity. So far, three main adrenoceptor types have been shown, alpha1, alpha2 and beta, each of which is further divided into at least three subtypes, as well as the alpha1L-adrenoceptor, a phenotype of the cloned alpha1a-adrenoceptor, in the blood vessel. Thus, the response of vessels with different receptor types to a transmitter varies quantitatively and even qualitatively from one vessel to another. The remarkable diversity in the sympathetic innervation mechanism in the vascular system may play an important role in regional variations in the regulation of blood flow. The sympathetic nerve also exerts long-term trophic action on the blood vessel. In conclusion, the sympathetic nervous system plays an important role not only in the regulation of cardiovascular dynamics but in the maintenance of the vessel structure, as well.

Sympathetic cholinergic vasodilation of skeletal muscle small arteries

Recently we have studied the direct vasomotor response of the hindlimb extramuscular large arteries (internal diameter, 500-1400 microm) and intramuscular small arteries (internal diameter, 50-500 microm) of in vivo thick skeletal muscle during activation of sympathetic cholinergic nerve in anesthetized cats. The hypothalamic defense area was electrically stimulated so as to induce a profound increase in femoral blood flow mediated by sympathetic cholinergic fibers. To visualize the vascular arrangement from the extramuscular large feeding arteries to small arteries in the triceps surae muscle, we developed a new X-ray TV system. The internal diameter, flow velocity, and volume flow of arterial blood vessels were directly measured before and during stimulation of the hypothalamic defense area. The major new finding is that the hypothalamic stimulation causes an intense increase in the internal diameter of small arteries in skeletal muscle, which is abolished either by cholinergic blockade or by the section of the sciatic nerve, but not by combined alpha- and beta-adrenergic blockade. In contrast, the internal diameter of the extramuscular larger arteries does not change during the hypothalamic stimulation, but their flow velocity and volume flow increase. These findings indicate that sympathetic cholinergic vasodilation occurs at intramuscular small arteries with internal diameter of 50-500 microm, which in turn increases flow velocity and volume flow of upstream blood vessels.

Absence of cholinergic sympathetic innervation from limb muscle vasculature in rats and mice

Although the existence of cholinergic sympathetic vasodilatory innervation in limb muscle vasculature is well established for some species, previous pharmacological studies have failed to reveal the presence of such innervation in rats. Recently, Schafer and colleagues [Schafer, M.K., Eiden, L.E., Weihe, E., 1998. Cholinergic neurons and terminal fields revealed by immunohistochemistry for the vesicular acetylcholine transporter. II. The peripheral nervous system. Neuroscience 84(2), 361-376] reported that vesicular acetylcholine transporter immunoreactivity (VAChT-IR), a marker for cholinergic terminals, is present in the innervation of the microvasculature of rat hindlimb skeletal muscle and concluded that rats possess cholinergic sympathetic innervation of limb muscle vasculature. Because of our interest in identifying targets of cholinergic sympathetic neurons, we have analyzed the transmitter properties of the innervation of muscle vessels in rat and mouse limbs. We found that the innervation of vasculature in muscle is noradrenergic, exhibiting robust catecholamine histofluorescence and immunoreactivity for tyrosine hydroxylase (TH) and the peptide transmitters, neuropeptide Y (NPY) and occasionally vasoactive intestinal peptide (VIP). In contrast, cholinergic phenotypic markers,VAChT-IR and acetylcholinesterase (AChE) activity, are absent. Neuron cell bodies in sympathetic ganglia, retrogradely labeled with injections of tracer into limb muscles, also lacked VAChT but contained TH-IR. The innervation of large extramuscular feed arteries in hindlimbs was also devoid of cholinergic markers, as were the cell bodies of sympathetic neurons innervating extramuscular femoral arteries. These results, like those of previous physiological studies, provide no evidence for the presence of cholinergic sympathetic innervation of muscle vasculature in rats or mice.

Cellular and molecular determinants of sympathetic neuron development

The development of the sympathetic nervous system can be divided into three overlapping stages. First, the precursors of sympathetic neurons arise from undifferentiated neural crest cells that migrate ventrally, aggregate adjacent to the dorsal aorta, and ultimately differentiate into catecholaminergic neurons. Second, cell number is refined during a period of cell death when neurotrophic factors determine the number of neuronal precursors and neurons that survive. The final stage of sympathetic development is the establishment and maturation of synaptic connections, which for sympathetic neurons can include alterations in neurotransmitter phenotype. Considerable progress has been made recently in elucidating the cellular and molecular mechanisms that direct each of these developmental decisions. We review the current understanding of each of these, focusing primarily on events in the peripheral nervous system of rodents.