Evidence for a capsaicin-sensitive vasomotor mechanism in the ventral medullary chemosensitive area of the cat

Capsaicin-sensitive neurogenic sensory vasodilatation in the dura mater of the rat

The neurogenic sensory vascular responses of the dura mater encephali are considered to contribute significantly to the mechanisms of meningeal nociception and headache. Although the fundamental role of capsaicin-sensitive afferent nerves in the development of the neurogenic inflammatory responses of a variety of tissues is well established, their participation in meningeal vascular reactions is unclear. In the present study, the effects of the topical application of capsaicin on the dural blood flow and on the morphology of the dural nerve fibres were examined in control and capsaicin-pretreated rats by means of laser Doppler flowmetry and electron microscopy, respectively. In the control rats, the dural application of capsaicin at concentrations of 50 and 100 nM induced significant increases in blood flow in the branches of the medial meningeal artery. This capsaicin-induced vasodilatation was abolished by capsazepine, a transient receptor potential vanilloid 1 (TRPV1) receptor antagonist, and by hCGRP8-37, a calcitonin gene-related peptide (CGRP) receptor antagonist. Administration of capsaicin at higher concentrations (1 and 10 microM) resulted in marked, dose-dependent decreases in dural blood flow. The capsaicin-induced vasodilatation was abolished, whereas vasoconstriction was augmented, by systemic pretreatment of the animals with capsaicin. Electron microscopy revealed degenerating unmyelinated axons in the dura mater after an acute exposure to capsaicin (10 microM), providing support for the existence and possible functional role of capsaicin-sensitive dural afferent nerves. The results indicate that capsaicin-induced vasodilatation in the rat dura mater is mediated by the release of CGRP from the sensory nerves, whereas the vasoconstrictor response may be attributed to a direct action of capsaicin on the vascular smooth muscle. The present study demonstrates for the first time that capsaicin-sensitive nociceptive afferent nerves contribute significantly to the dural vasodilatory responses and suggests an important role in meningeal nociception.

Thermogenesis mediated by a capsaicin-sensitive area in the ventrolateral medulla

Systemic administration of capsaicin elicits heat production, which can be observed in decerebrated preparations but is blocked by spinal transection. To identify the critical locus involved in the capsaicin-induced thermogenesis in the brainstem, we studied the effect of capsaicin on rats with bilateral electrolytic lesions in the premotor areas of sympathoadrenal preganglionic neurons. Lesions in the rostral ventrolateral medulla (RVLM), but not in other regions, largely attenuated the capsaicin-induced heat production. Unilateral microinjection of 30-100 nl capsaicin (0.5%, w/v) into the RVLM elicited a heat production response, whereas capsaicin injection in neighboring areas or vehicle injection into the RVLM did not affect heat production. These results suggest that the thermogenic effect of capsaicin is mediated, at least in part, by some capsaicin-sensitive structure in the RVLM.

Are the capsaicin-sensitive structures of ventral medulla involved in the temperature response to endotoxin in rats?

In chronic experiments on rats pretreated with bilateral microinjection of 25 nl 1% capsaicin to the caudal ventrolateral medulla under ketamine-xylazine-acepromazine anesthesia, an enhancement of the temperature response to intraperitoneal application of 3 microg/kg E. coli lipopolysaccharide as compared to animals who received vehicle to the caudal ventrolateral medulla was found. This is indicative of the involvement of the capsaicin-sensitive bulbar structures in thermoregulatory processes during endotoxemia.

Activation of chemosensitive neurons in the ventrolateral medulla by capsaicin in cats

We examined effects of centrally administered capsaicin on sympathetic nerve activity (SNA), blood pressure (BP) and heart rate (HR) in chloralose anesthetized cats (n = 18). Upon perfusion of the lower brain stem via the left vertebral artery, capsaicin (0.1-1.0 microM) caused dose-dependent increases in preganglionic SNA (recorded from the white ramus T3) that were associated with rises in BP and HR. These responses resembled closely those obtained during perfusions with CO2-enriched (40-80%) saline. Coadministration of capsaicin and CO2 resulted in additively increased responses. The effects of capsaicin, but not those of CO2, were significantly counteracted by the capsaicin antagonist capsazepine and ruthenium red. These results suggest that a specific central chemosensitivity activated by vanilloid receptor agonists may modulate hypercapnic and/or acidic sympathoexcitatory stimuli in vivo.

Capsaicin-sensitive area in the ventral surface of the rat medulla

In acute experiments on nembutal-urethan-anesthetized rats, structures selectively sensitive to capsaicin were found near the ventral surface of the medulla at the exit of hypoglossal nerve roots. Microinjection of 5-50 nl 0.01% capsaicin to the rostral region of the capsaicin-sensitive area mostly activated respiration, arterial pressure and heart rate (HR) while that to the caudal region inhibited arterial pressure and HR. In chronic experiments on rats, injection of 25 nl 1% capsaicin to the caudal capsaicin-sensitive area led to a decrease in arterial pressure by 35-45% and in HR by 10-15% within a week after operation. Arterial pressure and HR virtually reached the control level and the rostral and caudal ventral medulla showed asymmetric distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d)-positive cells by the end of the 2nd week. It is suggested that nitric oxide may be involved in the mechanisms of neurochemical rearrangements in the brainstem after application of capsaicin to the caudal ventrolateral medulla (CVLM).

Mechanism of cyclosporine-induced sympathetic activation and acute hypertension in rats

Although intravenous cyclosporine A (CsA) previously has been shown to cause a robust sympathetically mediated increase in blood pressure in the rat, the underlying mechanism by which CsA increases the activity of the sympathetic nervous system is unknown. To determine the relative contributions of central neural versus peripheral reflex mechanisms in causing this sympathetic activation, we recorded efferent renal sympathetic nerve activity and blood pressure during intracerebroventricular or intravenous infusion of CsA, the latter performed in intact rats and in those with sinoaortic denervation, cervical or subdiaphragmatic vagotomy, or dorsal rhizotomy (T10 through L1). In intact rats, intravenous CsA (5 mg/kg), as expected, tripled renal sympathetic nerve activity and increased mean arterial pressure by 27 +/- 4 mm Hg (P < .05). The new findings are that this sympathoexcitatory effect of intravenous CsA (1) was not duplicated by central administration (either into the cerebroventricular system or directly onto the ventrolateral surface of the medulla), (2) was unaffected by sinoaortic denervation, but (3) was greatly attenuated by either cervical or subdiaphragmatic vagotomy or by dorsal rhizotomy. In additional experiments, we found that intravenous cyclosporine increased the multiunit activity of subdiaphragmatic but not cardiopulmonary vagal afferents. From these data, we conclude that in the rat CsA-induced increases in sympathetic activity and blood pressure are caused mainly by activation of excitatory neural reflexes arising in the subdiaphragmatic region. These reflex mechanisms use at least two different afferent neural pathways: one involving the subdiaphragmatic vagi and the other involving the low thoracic dorsal spinal roots.

Effect of spantide, a substance-P antagonist, on cerebral vasospasm in primates

Experimental SAH in the squirrel monkey induces an angiographically demonstrable late spasm of about 23% at six days post subarachnoid haemorrhage (SAH). The late spasm is associated with a generalized reduction in cerebral blood flow (CBF) of about 30%. Intracisternal administration of the substance P (SP) antagonist spantide two hours and three days post SAH significantly reduces the degree of late spasm and also decreases the degree of CBF reduction. The findings suggest that SP is involved in the development of both angiographical spasm and CBF changes post SAH.

Prevention of cerebral vasospasm in the rat by depletion or inhibition of substance P in conducting vessels

Cisternal blood injection in the rat induces a biphasic angiographic vasospasm, with a maximal acute spasm at 10 minutes and a maximal late spasm at 2 days after the subarachnoid hemorrhage (SAH). Depletion of substance P-containing sensory nerves to the cerebral arteries with capsaicin prior to SAH prevents the development of both acute and late spasm. Intrathecal administration of the substance P antagonist spantide 2 hours prior to SAH also prevents the development of vasospasm, while spantide administration 1 hour before SAH only hinders the occurrence of late vasospasm. Intracisternal administration of spantide 2 hours post-SAH prevents the development of late vasospasm. This antagonist per se can induce a short-lasting dose-dependent angiographic vasoconstriction. Substance P-containing nerve fibers on the cerebral arteries could constitute the sensory link in a reflex arc system involved in the development of vasospasm in which the presence of blood in the subarachnoid space stimulates sensory substance P-containing nerve fibers on the cerebral arteries inducing a centripetal impulse to the A2-nucleus tractus solitarius setting into motion the events in the brain stem leading to acute and late vasospasm.

Adrenal sympathetic efferent nerve and catecholamine secretion excitation caused by capsaicin in rats

Capsaicin enhances adrenal medullary catecholamine secretion. The participation of the central nervous system on this enhancement by capsaicin was investigated in alpha-chloralose-urethan- or halothane-anesthetized rats. Intravenous administration of capsaicin caused a rapid and marked increase in adrenal sympathetic nerve activity. The nerve activity began to show an increase with the administration of capsaicin at a dosage of 20 micrograms/kg and significantly increased with a dosage of 200 micrograms/kg, i.e., capsaicin was found to cause a dose-dependent increase in adrenal nerve activity. Cholinergic blocking with hexamethonium bromide and atropine sulfate (1 and 5 mg/kg iv, respectively) attenuated the adrenal epinephrine secretion caused by capsaicin. The direct action of capsaicin on adrenal catecholamine secretion was examined using a retrograde perfusion system of left adrenal gland. Up to 8.2 X 10(-5) M capsaicin did not enhance catecholamine secretion from the adrenal gland. These results suggest that the enhancement of physiological catecholamine secretion by capsaicin is mainly through activation of the central nervous system.

Cardiovascular effects of substance P and capsaicin microinjected into the nucleus tractus solitarii of the rat

This report deals with the effect of substance P (SP) and capsaicin on blood pressure and heart rate after administration into different sites of the nucleus tractus solitarii (NTS) of urethane-anesthetized rats. Microinjection of SP at 6 different coordinates throughout the NTS showed 3 sites where SP administration evoked changes in blood pressure and heart rate. The most sensitive sites where application of SP into the NTS evoked dose-dependent hypotension and bradycardia were at the level of the posterior tip of the area postrema (zero level) and at the level of the obex. Capsaicin evoked dose-dependent hypotension and bradycardia at the same sites. These results further support the possibility that SP may be a neurotransmitter or neuromodulator of baroreceptor afferents in the NTS.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

Frequent stimulation of the guinea-pig myocardium raises the inotropic efficacy of tissue-bound ouabain

3H-Ouabain binding to frequently (1 Hz) stimulated papillary muscles from reserpine-pretreated guinea pigs was evaluated at ouabain concentrations of 18.5 and 200 nmol/l. Myocardial activity increased the amount of 3H-ouabain bound to the tissue in comparison with quiescent preparations. Since the shape of the time course of ouabain binding changed with frequent stimulation, a greater number of ouabain-accessible binding sites of the Na pump as induced by the rise in intracellular Na with frequent stimulation cannot be the sole mechanism of the frequency dependence. In view of their stimulatory properties on the Na pump the effects of intracellular Na and extracellular K could be equivalent. By contrast, both interventions were differently effective. The K antagonism on 3H-ouabain binding was independent from stimulation frequency. Furthermore, the shape of the time course of binding was not altered by [K]o. As evidenced by the dependence of half-times to steady-state effect on muscle diameter, the apparent rate of diffusion of ouabain was accelerated with the frequency of contractions. This acceleration could have interfered with the time course of binding at frequent stimulation. After correlating the time courses of positive inotropic effect and ouabain binding (concentration of ouabain in the medium 200 nmol/l), frequent stimulation was found to raise the inotropic efficacy of tissue-bound ouabain. The relation of excitation-dependent Na influx to the saturable, ouabain-inhibited, Na pump explained the frequency dependence of the intropic efficacy of ouabain; that is, the observed change of efficacy was consistent with Na-pump saturation in dependence on intracellular Na.

Bronchial, cardiovascular and secretory responses after central administration of capsaicin in the guinea-pig

Capsaicin was injected intracisternally (i.c.), intrathecally (i.th.) or intravenously (i.v.) into guinea-pigs anaesthetized with urethane and ventilated artificially. The effects of 0.2-100 micrograms capsaicin on insufflation pressure, heart rate, arterial blood pressure and salivation were recorded. Low i.c. doses of 0.2 and 2 micrograms capsaicin induced bradycardia, hypertension and salivation but no change in insufflation pressure. An insufflation pressure increase, i.e. bronchoconstriction, was observed with 20 or 100 micrograms capsaicin i.c. and this was associated with tachycardia and hypertension. Bronchoconstriction after 20 micrograms capsaicin i.c. was augmented by propranolol (1 mg/kg i.v.). It was, however, unaffected by bilateral cervical vagotomy and could also be induced by i.th. capsaicin injections in the lumbar region. Capsaicin (3 micrograms/kg) injected i.v. induced bronchoconstriction and tachycardia. Propranolol enhanced bronchoconstriction but did not reduce the tachycardia indicating that capsaicin led to activation of sympathetic bronchial but not cardiac fibers. These results also indicate that i.c. capsaicin caused reflex responses consisting of salivation, bronchodilatation bradycardia and hypertension. High doses injected i.c. or i.th. also caused tachycardia and bronchoconstriction. This latter effect, however, was neither a vagal reflex nor did it seem to result from activation of central terminals of afferent fibers with subsequent release of mediators from the peripheral endings due to antidromic spread of nerve impulses. Instead, capsaicin seemed to be readily resorbed into the systemic circulation and thus acting at peripheral endings to cause bronchoconstriction and tachycardia.