Investigation of mechanisms of the flare and wheal reactions in human skin by band method

NGF-dependent neurons and neurobiology of emotions and feelings: Lessons from congenital insensitivity to pain with anhidrosis

NGF is a well-studied neurotrophic factor, and TrkA is a receptor tyrosine kinase for NGF. The NGF-TrkA system supports the survival and maintenance of NGF-dependent neurons during development. Congenital insensitivity to pain with anhidrosis (CIPA) is an autosomal recessive genetic disorder due to loss-of-function mutations in the NTRK1 gene encoding TrkA. Individuals with CIPA lack NGF-dependent neurons, including NGF-dependent primary afferents and sympathetic postganglionic neurons, in otherwise intact systems. Thus, the pathophysiology of CIPA can provide intriguing findings to elucidate the unique functions that NGF-dependent neurons serve in humans, which might be difficult to evaluate in animal studies. Preceding studies have shown that the NGF-TrkA system plays critical roles in pain, itching and inflammation. This review focuses on the clinical and neurobiological aspects of CIPA and explains that NGF-dependent neurons in the peripheral nervous system play pivotal roles in interoception and homeostasis of our body, as well as in the stress response. Furthermore, these NGF-dependent neurons are likely requisite for neurobiological processes of 'emotions and feelings' in our species.

Central activation by histamine-induced itch: analogies to pain processing: a correlational analysis of O-15 H2O positron emission tomography studies

The aim of this study was to identify the functional cerebral network involved in the central processing of itch and to detect analogies and differences to previously identified cerebral activation patterns triggered by painful noxious stimuli. Repeated positron emission tomography regional cerebral blood flow (rCBF) measurements using O15-labeled water were performed in six healthy right-handed male subjects (mean age 32 +/- 2 years). Each subject underwent 12 sequential rCBF measurements. In all subjects a standardized skin prick test was performed on the right forearm 2 min before each rCBF measurement. For activation, histamine was applied in nine tests in logarithmically increasing concentrations from 0.03 to 8%. Three tests were performed with isotonic saline solution serving as a control condition. Itch intensity and unpleasantness were registered with a visual analogue scale during each test. Subtraction analysis between activation and control conditions as well as correlation analysis with covariates were performed. Itch induced a significant activation in the predominantly contralateral somatosensory cortex and in the ipsilateral and contralateral motor areas (supplementary motor area (SMA), premotor cortex, primary motor cortex). Additional significant activations were found in the prefrontal cortex and the cingulate gyrus, but not in subcortical structures nor in the secondary somatosensory cortex. In correlation analyses, several cortical areas showed a graded increase in rCBF with the logarithm of the histamine concentration (bilateral sensorimotor areas and cingulate cortex; contralateral insula, superior temporal cortex and prefrontal cortex) and with itch unpleasantness (contralateral sensorimotor cortex, prefrontal cortex and posterior insula; ipsilateral SMA). Induction of itch results in the activation of a distributed cerebral network. Itch and pain seem to share common pathways (a medial and a lateral processing pathway and a strong projection to the motor system). In contrast to pain activation studies, no subcortical (i.e. thalamic) activations were detected and correlation analyses suggest differences in subjective processing of the two sensations.

Secondary hyperalgesia persists in capsaicin desensitized skin

Several lines of evidence suggest that secondary hyperalgesia to punctate mechanical stimuli arises from central sensitization to the input from primary afferent nociceptors. Conventional C-fiber nociceptors respond to heat stimuli and yet heat hyperalgesia is absent in the region of secondary hyperalgesia. This evidence suggests that the central sensitization to nociceptor input does not involve heat sensitive nociceptors. To test this hypothesis, we investigated whether desensitization of heat sensitive nociceptors by topical application of capsaicin led to an alteration in the secondary hyperalgesia. Two 2x2 cm areas on the volar forearm, separated by 1 cm, were treated in 10 healthy volunteers. One of the areas was desensitized by treatment with 10% topical capsaicin (6 h/day for 2 days). The other site served as vehicle control. Hyperalgesia was produced 2 days later by an intradermal injection of capsaicin (50 microg, 10 microl) at a point midway between the two treatment areas. Secondary hyperalgesia to noxious mechanical stimuli was investigated by using a blade probe (32 and 64 g) attached to a computer-controlled mechanical stimulator. In the area of topical capsaicin treatment, there was a marked increase in heat pain threshold and decrease in heat pain ratings indicating a pronounced desensitization of heat sensitive nociceptors. However, touch threshold and pain to pinching stimuli were not significantly altered. The intradermal capsaicin injection led to the development of a similar degree of secondary hyperalgesia at both the vehicle and capsaicin treatment areas. These results indicate that capsaicin insensitive nociceptive afferents play a dominant role not only in normal mechanical pain but also in secondary hyperalgesia to noxious mechanical stimuli.

Dilatation of subcutaneous perforating blood vessels associated with capsaicin-induced cutaneous axon reflex: demonstration with subtraction thermography

The axon reflex induced by intracutaneous application of capsaicin to the forearm of human subjects and the back of anesthetized rats pretreated with intravenous injection of Evans blue was investigated using sequential subtraction thermography. In the human experiment, thermograms showed an immediate and general temperature decrease after capsaicin injection. Four min after application, several spotty areas with a temperature increase ('hot spots') appeared within and outside of the flare caused by capsaicin-induced axon reflex. A vascular murmur was observed on ultrasonic Doppler flowscopy at the hot spots. In the rat experiment, two hot spots appeared, one cranial to and one caudal to the site of injection, within different dermatomes. Hot spots appeared in rats with the pretreatment of intravenous hexamethonium and surgical removal of the bilateral lumbar paravertebral sympathetic trunks. Postmortem examination of the rats revealed that these hot spots coincided with perforating blood vessels. It was suggested that hot spots in the axon reflex identified by subtraction thermograms are induced by a passive dilatation of perforating vessels which supply blood to the flare.

Nervous control of blood flow in the orofacial region

The blood vessels of orofacial tissues are innervated by cranial parasympathetic, superior cervical sympathetic, and trigeminal nerves, a situation somewhat different from that seen in body skin. This review summarizes our current knowledge of the nervous control of blood flow in the orofacial region, and focuses on what we know of the respective roles of sympathetic, parasympathetic, and trigeminal sensory nerves in the regulation of blood flow in this region, with particular attention being paid to the mutual interaction between them.

Heat-evoked vasodilatation in human hairy skin: axon reflexes due to low-level activity of nociceptive afferents

1. Spreading vasodilatation of the axon reflex type was evoked by contact heat stimulation of the hairy skin in the human forearm (13.3 cm2 stimulus area) and was detected by laser Doppler flowmetry at 8, 19 and 30 mm distance. 2. From a base temperature of 35 degrees C, rapidly rising short heat stimuli (4 degrees C s-1, 2 s plateau) elicited vasodilatation at an average threshold of 39.4 degrees C. For slowly rising sustained heat stimuli (64 s duration) the average threshold was 39.6 degrees C (n.s.) Laser Doppler flowmetry revealed a rapid onset within about 4 s, a long duration of several minutes beyond the end of the stimulus, and a rapid spread of vasodilatation to remote skin areas. These characteristics are typical for vasodilatation by an axon reflex of nociceptive afferents. 3. Axon reflex thresholds matched the lower range of C fibre nociceptor heat thresholds. Thermal stimuli that were adjusted to elicit about half-maximal phasic responses in warm fibres (steps from 30 to 35 degrees C), but were below the range of C fibre nociceptor thresholds, did not cause any vasodilatation. 4. Pain thresholds were higher than axon reflex thresholds for both rapidly and slowly rising heat stimuli and strongly depended on the stimulus pattern (40.1 degrees C for rapidly rising stimuli and > 43 degrees C for slowly rising stimuli). This observation is consistent with recent reports that the phasic response of nociceptive afferents is essential to overcome the summation requirements at central synapses. 5. In conclusion, axon reflex vasodilatation in response to heat stimuli in the hairy skin of humans is elicited by activation of heat-sensitive nociceptors, even in the absence of a conscious perception of heat pain. The dissociation of pain and vasodilatation thresholds supports the concept of two operating ranges of primary nociceptive afferents. Warm fibres do not contribute to axon reflex vasodilatation in the hairy skin of the human forearm. Release of vasoactive peptides by nociceptive primary afferents may also contribute to local heat-evoked vasodilatation at temperatures above 40 degrees C.

Reflex parasympathetic vasodilatation in facial skin

1. The present report summarizes data from recent studies dealing with parasympathetic innervation of blood vessels in the lower lips (gingiva) of cats. 2. A study using the HRP tracing technique shows that blood vessels in the lower lip are innervated by postganglionic fibres originating in the otic ganglion, but not in the pterygopalatine ganglion. 3. There is a dual innervation of the cat lower lip by two groups of parasympathetic vasodilator fibres; in one case, fibres originating from the facial nerve root are distributed to the lower lip via chorda tympani nerve and in the other, fibres emanating from the glossopharyngeal nerve root project to the lower lip via the otic ganglion. 4. Parasympathetic reflex vasodilatation can be elicited by activation of the trigeminal (somatic), vagus (visceral), chorda tympani (gustatory) and nasal (chemical and mechanical) stimulation in the lower lips of cat. 5. Parasympathetic reflex vasodilatation elicited by somatic stimulation is mediated via the otic ganglion but not via the pterygopalatine ganglion, indicating that parasympathetic neurons, particularly those running as efferents in the glossopharyngeal nerve, are involved in the vasodilatation elicited by somatic, visceral and nasal stimulation.

Capsaicin applied to rat lumbar intervertebral disc causes extravasation in the groin skin: a possible mechanism of referred pain of the intervertebral disc

Administration of 10 micrograms of capsaicin into the anterior portion of a lumbar intervertebral disc of rats pretreated with Evans blue (i.v.) caused dye extravasation in the groin skin. This phenomenon occurred even when the spinal nerve of the same segment, bilateral sympathetic trunks, and/or the anterior L2 spinal root had been cut; it did not occur in rats with a cut genitofemoral nerve. Infiltration of capsaicin solution directly into the genitofemoral nerve did not cause dye extravasation. These results suggested the presence of dichotomizing sensory C-fibers which innervate both the intervertebral discs and the groin skin in the L2 spinal nerve. If such dichotomizing fibers exist in man, they may contribute to the groin pain sometimes associated with intervertebral disc lesions.

Innervation of the cat lip by two groups of parasympathetic vasodilator fibres

1. Electrical stimulation of the peripheral cut ends of the chorda tympani nerve proper (CTNP) and the chorda-lingual nerve (CLN) elicited a blood flow increase in the ipsilateral lower lip, tongue and submandibular gland in a stimulus intensity-dependent manner in anaesthetized cats. 2. Pretreatment with hexamethonium (1.0 mg kg-1, i.v.), an autonomic ganglionic blocker, significantly reduced the CTNP-induced blood flow increases in all of the above three sites as well as the CLN-induced blood flow in the lower lip, but it had no effects on the CLN-induced blood flow increases in the tongue and submandibular gland. 3. The CTNP stimulation-induced lower lip blood flow was not influenced by sectioning the lingual nerve proper, but it was abolished by section of either the CLN or the inferior alveolar nerve (IAN) in the mandibular canal. 4. The lip blood flow increases elicited reflexly by electrical stimulation of the upper gingiva, the central cut ends of the mylohyoid nerve and CLN were not affected by cutting of the CTNP, but were markedly reduced by pretreatment with hexamethonium and abolished by the section of the inferior alveolar nerve just distal to the mylohyoid nerve. These observations imply that the parasympathetic vasodilator fibres involved in trigeminally induced reflex vasodilatation responses do not travel with the CTNP. 5. These results suggest that there is a dual innervation of the cat lower lip by two groups of parasympathetic vasodilator fibres; in one case fibres originating from the facial nerve root are distributed to the lower lip via the CTNP, CLN and IAN and in the other fibres emanating from the glossopharyngeal nerve root project to the lower lip via the mandibular nerve and the IAN.

Reflex vasodilatation in the cat lip evoked by stimulation of vagal afferents

In 36 cats under nembutal anaesthesia, stimulation of the central end of the cut vagus nerve caused blood flow to increase in only the ipsilateral side in six cats (17%) and in the bilateral sides in 30 cats (83%) in the lower lips. Pretreatment with hexamethonium to block nicotinic synapses in autonomic ganglia resulted in a time-dependent reduction of the reflex vasodilator response, while phentolamine, propranolol (alpha-, beta-adrenoreceptor antagonists) and tripelennamine (histamine receptor antagonist) had no effect. Pretreatment with atropine (muscarinic receptor antagonist) showed a slight, but not statistically insignificant attenuation of the reflex vasodilatation. Ipsilateral section of either the glossopharyngeal nerve root or the inferior alveolar nerve completely abolished the reflex vasodilator response elicited by central vagal stimulation. The reflex vasodilator response induced by stimulation of the central end of the cut vagus nerve was abolished by topical capsaicin application on the central cut ends of the vagus nerve but not by capsaicin on the inferior alveolar nerve. These results suggest that there is a cutaneous reflex vasodilator system that can be activated via capsaicin-sensitive afferent fibres in the vagus nerve. Parasympathetic vasodilator fibres of this system emerge from the brain stem with the glossopharyngeal nerve and reach the blood vessels in the cat mandibular lip via the inferior alveolar nerve.

Somatosensory stimulation causes autonomic vasodilatation in cat lip

1. Electrical stimulation of the infra-orbital nerve and the maxillary buccal gingiva caused an increase in ipsilateral lip blood flow in a stimulus intensity-dependent manner in anaesthetized cats. 2. The reflex vasodilator response was resistant to blockade by antimuscarinic (atropine), antiadrenergic (phentolamine and propranolol) and antihistaminergic (tripelennamine) but sensitive to ganglionic blocking agent (hexamethonium). 3. The reflex vasodilator response was unaffected by section of the ipsilateral cervical sympathetic trunk and facial nerve root, but was completely abolished by ipsilateral section of the glossopharyngeal nerve root. 4. The vasodilator response elicited by stimulation of the facial and glosso-pharyngeal nerves was never affected by lesion of the ipsilateral pterygopalatine ganglion. 5. Local anaesthesia or section of the inferior alveolar nerve abolished the vasodilator effects of stimulation of the facial and the glossopharyngeal nerves. 6. These results suggest that there is a somato-autonomic reflex vasodilator system mediated via final neurons that are not cholinergic, and that the parasympathetic vasodilator fibres emerge from the brain stem with the glossopharyngeal nerve and reach the blood vessels via the otic ganglion and the inferior alveolar nerve in the cat mandibular lip.

Selective excitation of parasympathetic nerve fibers to elicit the vasodilatation in cat lip

Electrical stimulation of the tongue and the proximal cut end of the lingual nerve caused a blood flow to increase in a stimulus-intensity dependent manner in the ipsilateral lower lip of the cats. Pretreatment with hexamethonium (an autonomic ganglionic blocker, 1.0 mg/kg) abolished the vasodilator response, while atropine, phentolamine, propranolol and tripelennamine had no effect on these vasoresponses. Ipsilateral sections of either the glossopharyngeal nerve root, inferior alveolar nerve or mental nerve at the main mental foramen, but not at the posterior mental foramen, abolished the vasodilator response caused by electrical stimulation of the tongue and the lingual nerve. Electrical stimulation of the distal cut ends of the glossopharyngeal nerve root and inferior alveolar nerve caused the vasodilator and vasoconstrictor responses, whereas stimulation of the tongue and the proximal cut ends of the lingual nerve did not elicit the vasoconstrictor response. These results suggest that reflex vasodilatation in the cat mandibular division is exclusively mediated via activation of the parasympathetic nerve fibers, and that selective excitation of the parasympathetic nerve fibers in the oral area is possible.

Vasodilator responses following intracranial stimulation of the trigeminal, facial and glossopharyngeal nerves in the cat gingiva

The effects of electrical stimulation of the trigeminal, facial and glossopharyngeal nerves on gingival blood flow in the cat were studied. The intracranial part of these nerves was stimulated electrically, and gingival blood flow was measured by the laser Doppler technique. Electrical stimulation of the trigeminal, facial and glossopharyngeal nerves caused blood flow to increase in the ipsilateral gingiva both with the cranial nerve intact and after cutting it to the medulla. Stimulation of the distal cut ends of the facial and glossopharyngeal nerves elicited an increase in blood flow but no increase in systemic blood pressure. Pretreatment with hexamethonium reduced the increase in blood flow elicited by electrical stimulation of the facial and glossopharyngeal nerves, but had no effect on that elicited by stimulation of the trigeminal nerve. In contrast, pretreatment with tripelennamine attenuated the trigeminal nerve-stimulated blood flow increase, but not that elicited by stimulation of the facial and glossopharyngeal nerves. Atropine, propranolol and phentolamine had no effect on these responses. These results suggest that the autonomic nervous system, particularly the parasympathetic nervous system, is responsible for the blood flow increase elicited by facial and glossopharyngeal nerve stimulation, and that the trigeminal nerve-stimulated blood flow increase is induced by antidromic vasodilatation of the trigeminal sensory nerve.

The effects of capsaicin applied topically to inferior alveolar nerve on antidromic vasodilatation in cat gingiva

Electrical stimulation of the cut inferior alveolar nerve caused 3 different patterns of vasoresponses in the cat gingiva: vasodilatation, vasoconstriction, and biphasic response consisting of vasoconstriction and vasodilatation. Topical capsaicin application onto the inferior alveolar nerve produced a vasodilatation in all of cats tested. After the repeated application of capsaicin, the vasodilator response was no more elicited by electrical stimulation of the inferior alveolar nerve, while the vasoconstrictor response was observed in every preparation. The vasoconstrictor response caused by electrical stimulation of the inferior alveolar nerve was not affected by the capsaicin application, but was completely inhibited by phentolamine, sympathetic alpha-adrenergic receptor antagonist. The present results suggest that vasodilatation induced by electrical stimulation of the inferior alveolar nerve occurs via the sensory nerve, and vasoconstriction via the sympathetic nerve.

The nervous control of gingival blood flow in cats

The purpose of the present study was to investigate the nervous control of gingival blood flow in cats. Gingival blood flow was measured by laser Doppler flowmeter in 75 cats during electrical stimulation and cutting or ligation of the inferior alveolar nerve and cervical sympathetic nerve without sympathectomy or pretreatment with adrenoceptor blocking agents. Three different patterns of responses in gingival blood flow were observed following electrical stimulation of the inferior alveolar nerve in cats. In 45 cats there was an increase in blood flow, in 4 cats a decrease in blood flow, and in 7 cats a biphasic change consisting of an initial decrease and a successive increase in blood flow. The vasodilator effect was significantly reduced by pretreatment with (D-Pro2, D-Trp7.9)-substance P. tripelennamine, and methysergide. Pretreatment with cimetidine, atropine, hexamethonium, phentolamine, or propranolol had no effect on vasodilatation. The vasoconstrictor response was completely inhibited by pretreatment with phentolamine; in this case the vasodilator response appeared after stimulation of the inferior alveolar nerve. Ligation or cutting of the inferior alveolar nerve always elicited an increase in gingival blood flow. Cutting the cervical sympathetic nerve had no effect on gingival blood flow in 8 of 10 cats and caused an increase in gingival blood flow in 2 cats; however, electrical stimulation of the cervical sympathetic nerve always caused a decrease in gingival blood flow in the cats investigated. The present results suggest that cat gingival blood flow is controlled by sympathetic alpha-adrenergic fibers for vasoconstriction and by sensory fibers and mast cells for vasodilatation.