Neurogenic vasodilation in the rat hairy skin measured using a laser Doppler flowmeter

Sympathetic influence on capsaicin-evoked enhancement of dorsal root reflexes in rats

A series of experiments by our group suggest that the initiation and development of neurogenic inflammation in rats are mainly mediated by dorsal root reflexes (DRRs), which are conducted centrifugally from the spinal dorsal horn in primary afferent nocieptors. In this study, DRRs were recorded in anesthetized rats from single afferent fibers in the proximal ends of cut dorsal root filaments at the L4-L6 level and tested for responses to intradermal injection of capsaicin. Sympathectomy combined with pharmacological manipulations were employed to determine if the capsaicin-evoked enhancement of DRRs was subject to sympathetic modulation. DRRs could be recorded from both myelinated (Abeta and Adelta) and unmyelinated (C) afferent fibers. After capsaicin was injected intradermally into the plantar foot, a significant enhancement of DRRs was seen in C- and Adelta-fibers but not in Abeta-fibers. This enhancement of DRRs evoked by capsaicin injection was almost completely prevented by sympathectomy. However, if peripheral alpha1-adrenoceptors were activated by intra-arterial injection of phenylephrine, the enhancement of DRRs evoked by capsaicin could be restored, whereas no such restoration was seen following pretreatment with an alpha2-adrenoceptor agonist, UK14,304. Under sympathetically intact conditions, the enhanced DRRs following capsaicin injection could be blocked by administration of terazosin, an alpha1-adrenoceptor antagonist, but not by administration of yohimbine, an alpha2-adrenoceptor antagonist. These results provide further evidence that the DRR-mediated neurogenic inflammation depends in part on intact sympathetic efferents acting on peripheral alpha1-adrenoceptors, which augment the sensitization of primary afferent nociceptors induced by capsaicin injection, helping trigger DRRs that produce vasodilation.

Expression of the high-affinity choline transporter, CHT1, in the neuronal and non-neuronal cholinergic system of human and rat skin

Choline is an essential component in acetylcholine biosynthesis, and is involved in cell signaling. It is unable to permeate the cell membrane and requires a transporter to enter the cell. Neurons that synthesize acetylcholine take up choline by a recently cloned high-affinity choline transporter (choline transporter 1) that is Na+-dependent and can be blocked by hemicholinium-3. The aim of this study was to determine the expression and to analyze the distribution of choline transporter 1 in human and rat skin. The mRNA for choline transporter 1 was detected in rat and human skin and in the human keratinocyte cell line HaCaT. A polyclonal anti-serum was developed against the N-terminal region of the human and rat protein. In rat and human skin, choline transporter 1 immunoreactivity was present in nerve fibers. In addition, keratinocytes, HaCaT cells and cells of the internal root sheath of the hair follicle contained choline transporter 1 immunoreactivity. The labeling patterns of nonconfluent vs confluent cultured cells and the distribution of choline transporter 1 along the epidermal layer suggest an association of choline transporter 1 with keratinocyte differentiation. In conclusion, this study shows the presence of the high-affinity choline transporter choline transporter 1 in nerve fibers and epithelial cells in the human and rat skin supporting the pivotal role of this transporter in both the neuronal and non-neuronal cholinergic system of the skin.

Studies with iontophoretic administration of drugs to human dermal vessels in vivo: cholinergic vasodilatation is mediated by dilator prostanoids rather than nitric oxide

AIMS

Impaired function of the vascular endothelium has been well documented in hypertension and hypercholesterolaemia. However, the 'gold standard' method for assessing endothelial function, using intra-arterial drug infusion, is invasive and has only been applied in the forearm and coronary circulations in vivo. The aim of the present study was to establish the non-invasive technique of transdermal drug iontophoresis to assess endothelial function in human dermal vessels in vivo.

METHODS

In healthy male volunteers, we delivered acetylcholine (ACh) and sodium nitroprusside (SNP) to dermal vessels of the forearm using iontophoresis, and measured vasodilatation using laser Doppler fluximetry. Drugs were diluted in a methylcellulose gel vehicle which did not induce vasodilatation. To assess the contribution of nitric oxide and vasoactive prostanoids to cholinergic dilatation, the procedure was repeated during brachial artery infusion of the nitric oxide synthase inhibitor, L-N(G)-monomethyl-arginine (L-NMMA) and after intravenous administration of the cyclooxygenase inhibitor, aspirin. As a control for the vasoconstrictor effect of L-NMMA, which was measured by venous occlusion plethysmography, iontophoresis was repeated during brachial artery infusion of noradrenaline.

RESULTS

Flux increased in response to iontophoresis of ACh (from 45 +/- 9 to 499 +/- 80 units; P < 0.0001) and SNP (from 32 +/- 11 to 607 +/- 82 units; P < 0.0001). Brachial artery infusions of L-NMMA or noradrenaline caused reductions in forearm blood flow (by 43 +/- 2% and 44 +/- 2%, respectively) but did not inhibit vasodilatation in response to iontophoresis of ACh or SNP. In contrast, aspirin inhibited the response to iontophoresis of ACh (from 473 +/- 81 to 222 +/- 43 units; P < 0.0001) but did not affect the response to SNP (from 348 +/- 59 to 355 +/- 58).

CONCLUSIONS

We conclude that in healthy subjects, in contrast to the forearm circulation, dermal vasodilatation in response to iontophoresis of ACh is mediated predominately by a dilator prostanoid rather than by nitric oxide generation. Furthermore, the non-invasive technique of iontophoresis could complement existing invasive tests of endothelial function in future clinical studies.

Influence of partial nerve injury in the rat on efferent function of sympathetic and antidromically acting sensory nerve fibers

OBJECTIVE

To investigate how partial injury of a large peripheral nerve affects efferent (vasomotor) function of sympathetic and antidromically acting sensory nerve fibers.

DESIGN

Randomized animal study.

MATERIALS AND METHODS

We assessed, by laser Doppler flowmetry, skin blood flow (SBF) in the hindpaw of male Lewis rats before partial injury of the ipsilateral sciatic nerve (through loose ligation) as well as at an early stage (day 4) and at a later stage (day 21) after this procedure. This procedure has been reported to induce signs and symptoms like those observed in patients with causalgia. At the two time points after nerve injury, SBF was assessed before and after (chemical) blockade of sensory and nonsensory (sympathetic) sciatic nerve fibers. Furthermore, at day 21 we measured the density of sympathetic nerve fibers in footpad arteries.

MEASUREMENTS AND MAIN RESULTS

At day 4, compared with preligation values, we observed an increase in SBF that was reduced by blockade of sensory nerve fibers. Subsequent blockade of nonsensory nerve fibers further reduced SBF. At day 21, SBF was decreased compared with preligation values. Blockade of sensory nerve fibers further reduced SBF, and subsequent blockade of nonsensory nerve fibers did so as well. The density of sympathetic nerve fibers was lower on the ligated side than on the nonligated side.

CONCLUSIONS

Partial injury of the rat sciatic nerve causes an ipsilateral increase in SBF at an early stage, which is followed by a decrease at a later stage. At both stages, antidromically acting sensory and orthodromically acting nonsensory (sympathetic) nerve fibers are involved in the vasodilator response. At a later stage, however, neurogenic vasodilator mechanisms are overruled by a nonneurogenic vasoconstrictor mechanism. The latter may consist of supersensitivity of skin microvessels to catecholamines consequent to reduced neurogenic disposition of catecholamines.

Skin blood flow disturbances in the contralateral limb in a peripheral mononeuropathy in the rat

Electrical excitation of nociceptive afferents in an extremity has been demonstrated to increase skin blood flow in the contralateral extremity. Hence, one would expect that loose sciatic nerve ligation, which induces an experimental painful peripheral neuropathy, may also provoke a vasodilator response in the contralateral hindpaw. On the non-ligated side, such a response may involve inhibited skin vasoconstrictor activity as well as neurogenically mediated active vasodilation. We studied skin blood flow changes in the rat hindpaw consequent to contralateral loose sciatic nerve ligation. After ligation, we also investigated whether blockade of afferent input from the ligated sciatic nerve to the spinal cord, by means of lidocaine, overrules the vasodilator response in the non-ligated paw. On the non-ligated side, we assessed the vasoconstrictor response of skin microvessels to cooling of the rat abdomen as a measure of skin vasoconstrictor activity in this paw. In order to investigate the involvement of sensory and/or non-sensory nerve fibers in the non-ligated sciatic nerve on skin blood flow abnormalities in the non-ligated paw, we studied the influence of blockade of these fibers through successive capsaicin and lidocaine application. We show that loose ligation of the sciatic nerve induces a vasodilator response in the contralateral hindpaw, which is completely abolished by blockade of afferent input from the ligated sciatic nerve. From day 1 after ligation, skin vasoconstrictor activity in the non-ligated paw was reduced, as indicated by an impaired vasoconstrictor response to cooling of the rat abdomen. Besides, blockade of sensory but not of non-sensory nerve fibers on the non-ligated side attenuated the vasodilator response in this paw. The data presented here indicate that loose ligation of the rat sciatic nerve induces a vasodilator response in the contralateral hindpaw. On the non-ligated side, this vasodilator response may involve inhibition of skin vasoconstrictor activity, as well as antidromically acting sensory nerve fibers.

New insights into the local regulation of blood flow by perivascular nerves and endothelium

Blood flow, particularly in the skin, is essential for the success of plastic surgical operations. This review describes recent studies of the perivascular nerves and vascular endothelial cells which regulate blood flow. Perivascular nerves, once considered simply adrenergic or cholinergic, release many types of neurotransmitters, including peptides, purines and nitric oxide. Cotransmission (synthesis, storage and release of more than one transmitter by a single nerve) commonly takes place. Some afferent nerves have an efferent (motor) function and axon reflex control of vascular tone by these "sensory-motor" nerves is more widespread than once thought. Endothelial cells mediate both vasodilatation and vasoconstriction. The endothelial cells can store and release vasoactive substances such as acetylcholine (vasodilator) and endothelin (vasoconstrictor). The origins and functions of such vasoactive substances are discussed. Endothelial vasoactive substances may be of greater significance in the response of blood vessels to local changes while perivascular nerves may be concerned with integration of blood flow in the whole organism. The dual regulation of vascular tone by perivascular nerves and endothelial cells is altered by aging and conditions such as hypertension, as well as by trauma and surgery. Studies of vascular tone in disease and after denervation or mechanical injury suggest possible trophic interactions between perivascular nerves and endothelial cells. Such trophic interactions may be important for growth and development of the two control systems, particularly in the microvasculature where neural-endothelial separation is small.

Blood flow increase in the orofacial area of humans induced by painful stimulation

The purpose of this study was to investigate if painful stimulation produces blood flow changes in the tooth pulp and the facial skin in humans. Also, we attempted to find out if the possible blood flow changes induced by painful stimulation could be explained by central sympathetic and parasympathetic reflex mechanisms, by an antidromic activation of nociceptive axons (axon reflex), or by a change in central cardiovascular parameters. Laser Doppler flowmeter was used to assess the blood flow changes. Electrical tooth pulp stimulation at painful intensities induced a blood flow increase in the ipsilateral lip adjacent to the stimulus site, and vice versa. Nonpainful stimulation had no effects. Painful thermal stimulation of the upper lip also produced an increase in the blood flow of the ipsilateral upper incisor. The blood flow changes in the lip produced by dental stimulation were not correlated with changes in systemic blood pressure or heart rate. Painful electrical stimulation of the hand did not induce any changes in the pulpal blood flow, whereas painful dental stimulation produced a blood flow decrease in the finger but no change in the contralateral lip or cheek. In monkey experiments a regional block of the central conduction of the inferior alveolar nerve at the level of the mandibular foramen produced varying results: the blood flow increase in the lower incisor produced by noxious thermal stimulation of the ipsilateral lower lip was not abolished in two experiments but was abolished in other two experiments. It is concluded that painful stimulation can induce significant increases in the blood flow of the orofacial regions in humans. This increase is predominantly restricted to the region adjacent to the stimulus site and cannot be explained by changes in the central cardiovascular parameters. Central neuronal reflex mechanisms and an axon reflex may both underlie these blood flow increases.

Nitric oxide and sensory nerves are involved in the vasodilator response to acetylcholine but not calcitonin gene-related peptide in rat skin microvasculature

1. The contributions of sensory nerves and nitric oxide (NO) to vasodilator responses to acetylcholine (ACh) and calcitonin gene-related peptide (CGRP) were examined in rat skin microvasculature with a laser Doppler flowmeter to monitor relative blood flow. 2. Perfusion of ACh (100 microM; for 30 min) over a blister base on the rat hind footpad elicited microvascular vasodilatation and this response was not sustained. CGRP (1 microM; 10 min perfusion) also elicited vasodilatation and this response was maintained even when CGRP was no longer in contact with the blister base. 3. The vasodilator response to ACh was significantly smaller in rats pretreated as neonates with capsaicin to destroy primary sensory afferents than it was in age-matched controls. The vasodilator response to CGRP was unaffected by capsaicin pretreatment. 4. Selective inhibitors of NO synthase, NG-nitro-L-arginine (L-NOARG) and NG-monomethyl-L-arginine (L-NMMA) (both at 100 microM) attenuated the vasodilator response to ACh in control rats, but had no effect on the vasodilator response to CGRP. There was a significant L-NOARG-resistant component in control rats while in capsaicin-treated rats the vasodilator response to ACh was virtually abolished by L-NOARG. The inactive stereoisomer NG-monomethyl-D-arginine (100 microM) did not affect the vasodilator response to ACh. 5. The efficacy of L-NOARG and L-NMMA as inhibitors of endothelium-dependent responses was confirmed by use of an endothelium-dependent vasodilator, the calcium ionophore A23187 (100 microM; 10 min perfusion). Vasodilatation to A23187 was strongly attenuated by both L-NOARG and L-NMMA.6. These results suggest that sensory nerves and NO are both involved in the dilatation produced by ACh in rat skin microvasculature. A component of the vasodilator response elicited by ACh involves a direct action on the microvascular endothelium with subsequent generation of NO, while an additional component is elicited via activation of sensory nerves. The vasodilator mediator(s) released by ACh from sensory nerves acts largely independently of NO.7. The vasodilator response to CGRP is independent of a prejunctional action on sensory nerves and of NO.

Laser measurement of myocardial blood flow

A He-Ne laser flowmeter (wavelength: 632.8 nm, power: 2 mW) has already been used to measure blood flow in skin, muscle, and mucosa. However, the application of this flowmeter for the beating heart has been left undeveloped due to its difficulty. For the purpose of establishment of the laser measurement of myocardial blood flow, we redesigned a flow probe to be small and light and used a connecting paste to support the subepicardial flow probe. After resolving many obstacles, the noninvasive and real-time blood flow measurement of the beating heart was made possible. The flow volume in subendocardial myocardium was greater than the volume in subepicardial myocardium, and the average ratio of subendocardial flow to subepicardial flow was 1.62 +/- 0.21.