Adrenergic and peptidergic innervation of cochlear blood vessels

Substance P inhibits potassium and calcium currents in inner ear spiral ganglion neurons

Substance P (SP), a member of the tachykinin family of neurotransmitters and neuromodulators, has been identified on spiral ganglion neurons (SGNs) in the inner ear; however, its high affinity receptor, neurokinin-1 (NK1), has not been identified and the physiological effects of SP on SGNs are not well understood. To address these issues, immunolabeling, RT-PCR, Western blots and whole-cell patch-clamp recordings were made from SGNs in P0-P5 mouse cochlear organotypic cultures. The NK1 receptor was detected on SGNs by immunocytochemistry, the protein was detected in cochlear tissues by Western blots, and the mRNA for the NK1 receptor was also found in cochlear tissues of postnatal mice (P2) by RT-PCR. Application of SP (1 to 25 microM) significantly increased the latency of SGN action potentials (APs) (mean increase 7.8 +/- 4 ms; 25 microM of SP), prolonged the duration of the action potential and made the resting potential (RP) more positive (mean 9.0 +/- 7 mV) relative to normal values (-54 +/- 6 mV). SP (1 to 25 microM) also suppressed voltage-activated potassium currents (IK+) and calcium currents (ICa2+). Puffing 25 microM of SP onto SGNs suppressed IK+ by 43 +/- 9% (n = 7) and ICa2+ by 40.6 +/- 5.6% (n = 7); both currents recovered when SP was washed out. A SP antagonist blocked the SP-induced suppression of IK+ and ICa2+. These results indicate that SP acting through NK1 receptors can have direct neuromodulatory effects on SGNs.

Capsaicin stimulation of the cochlea and electric stimulation of the trigeminal ganglion mediate vascular permeability in cochlear and vertebro-basilar arteries: a potential cause of inner ear dysfunction in headache

Trigeminal neurogenic inflammation is one explanation for the development of vascular migraine. The triggers for this inflammation and pain are not well understood, but are probably vasoactive components acting on the blood vessel wall. Migraine-related inner ear symptoms like phonophobia, tinnitus, fluctuation in hearing perception and increased noise sensitivity provide indirect evidence that cochlear blood vessels are also affected by basilar artery migraine. The purpose of this investigation was to determine if a functional connection exists between the cochlea and the basilar artery. Neuronally mediated permeability changes in the cochlea and basilar artery were measured by colloidal silver and Evans Blue extravasation, following orthodromic and antidromic stimulation of the trigeminal ganglion innervating the cochlea. Capsaicin and electrical stimulation induced both dose- and time-dependent plasma extravasation of colloidal silver and Evans Blue from the basilar artery and anterior inferior cerebellar artery. Both orthodromic and antidromic activation of trigeminal sensory fibers also induced cochlear vascular permeability changes and significant quantitative differences between the treated and control groups in spectrophotometric assays. These results characterize a vasoactive connection between the cochlea and vertebro-basilar system through the trigeminal sensory neurons. We propose that vertigo, tinnitus and hearing deficits associated with basilar migraine could arise by excitation of the trigeminal nerve fibers in the cochlea, resulting in local plasma extravasation. In addition, cochlear "dysfunction" may also trigger basilar and cluster headache by afferent input to the trigeminal system.

Comparison of the vascular innervation of the rat cochlea and vestibular system

In order to gain a better understanding of the neuronal and local control of inner ear blood flow, the vascular innervation to the rat cochlea and vestibular system was examined. Specimens were removed in toto beginning at the basilar artery extending to the anterior inferior cerebellar artery, labyrinthine artery, common cochlear artery, modiolar artery and anterior vestibular artery. When possible the vessels were dissected in continuity through the cribrose area. The vestibular endorgans were also removed. Specimens were examined using immunohistochemical techniques for the presence of vasoactive intestinal peptide, neuronal nitric oxide synthase, neuropeptide-Y, substance P and calcitonin gene related peptide. Results show that the vasculature to the cochlea and vestibular portion of the inner ear receive similar types of nonadrenergic innervation, that within the vestibular endorgans, only CGRP and SP were found in the neuroepithelium or in association with vessels, and that within the vestibular system, the majority of the vascular innervation appears to stop at or near the cribrose area. In the cochlea however, it extends to include the radiating arterioles. These findings suggest that cochlear blood flow is under finer control and that neuronally induced changes in blood flow may have a more global effect in the vestibular periphery.

Modulation of cochlear blood flow by extracellular purines

Humoral adenosine 5'-triphosphate (ATP), adenosine and uridine 5'-triphosphate (UTP) have been shown to have a role in controlling local blood flow in a variety of tissues. The presence of P1 and P2 receptors in the cochlea, and particularly the highly vascular region, the stria vascularis, implies a vasoactive role for these compounds in the inner ear. To test the effect of extracellular purines and pyrimidines on cochlear blood flow, cochleae from anaesthetised guinea-pigs were perfused with ATP (1 microM-10 mM), adenosine (1 microM-10 mM) and UTP (1 mM) in artificial perilymph while blood flow through the cochlea was measured. An acute perilymphatic perfusion technique was established via tubing placed through a hole in the bone overlying scala tympani of the first cochlear turn, with an outlet hole in scala vestibuli of the fourth turn. Blood flow was measured by placing the probe of a laser Doppler blood perfusion monitor on the bone overlying the stria vascularis in the third cochlear turn. ATP and adenosine produced a significant dose dependent increase in cochlear blood flow (28.8-229.0% and 35.8-258.1%, respectively). The effect of ATP (100 microM) on cochlear blood flow was reduced in the presence of reactive blue 2 (1 mM) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (1 mM). The blood flow response to adenosine (10 microM) was reduced in the presence of 1,3-dimethylxanthine (theophylline, 100 microM), but not with either 3,7-dimethyl-1-propargylxanthine (10 microM) or 8-cyclopentyl-1,3-dipropylxanthine (10 microM). UTP did not produce any change in the cochlear blood flow. To determine if the ATP effect was also mediated by adenosine derived from ectonucleotidase activity, the perilymphatic compartment was perfused with either ATP plus theophylline (100 microM) or with the non-metabolisable form of ATP, adenosine 5'-O-(3-thiophosphate) (ATPgammaS, 100 microM). The effect of ATP on cochlear blood flow was unaffected with the inclusion of theophylline while ATPgammaS produced an increase in cochlear blood flow similar to the one observed with ATP. These findings indicate that extracellular ATP and its metabolite adenosine have a modulatory role in cochlear blood flow possibly mediated by both P1 and P2 receptors.

Direct evidence of trigeminal innervation of the cochlear blood vessels

This paper provides the first detailed description of the trigeminal innervation of the inner ear vasculature. This system provides a newly discovered neural substrate for rapid vasodilatatory responses of the inner ear to high levels of activity and sensory input. Moreover, this discovery may provide an alternative mechanism for a set of clinical disturbances (imbalance, hearing loss, tinnitus and headache) for which a central neural basis has been speculated. Iontophoretic injections of biocytin were made via a glass microelectrode into the trigeminal ganglion in guinea-pigs. Tissue for histological sections was obtained 24 h later. Labeled fibers from the injection site were observed as bundles around the ipsilateral spiral modiolar blood vessels, as individual labeled fibers in the interscala septae, and in the ipsilateral stria vascularis. The dark cell region of the cristae ampullaris in the vestibular labyrinth was also intensively labeled. No labeled fibers were observed in the neuroepithelium of the cristae ampullaris or the semicircular canals. These results confirm and localize an earlier indirect observation of the trigeminal ganglion projection to the cochlea. This innervation may play a role in normal vascular tone and in some inner ear disturbances, e.g., sudden hearing loss may reflect an abnormal activity of trigeminal ganglion projections to the cochlear blood vessels.

Guinea pig vestibular blood flow in response to calcitonin-gene related peptide

Little is known about the physiologic regulation of the vestibular end organ blood flow. The purpose of the current study was to examine posterior semicircular canal ampulla blood flow in addition to systemic factors during intravenous infusions of calcitonin-gene related peptide (CGRP), a factor involved in the tonic regulation of blood flow. Receptors for this factor are known to be available to the vascular supply of the vestibular organs. Local blood flow using laser Doppler flowmetry and systemic parameters were monitored during infusion of CGRP. CGRP antagonists and control vehicle. The results show relatively stable vestibular blood flow (VBF), concentration-dependent decreases in systemic blood pressure, and elevations in heart rate. Pretreatment with CGRP(8-37), a specific receptor antagonist, attenuated these responses to subsequent CGRP infusions. These findings suggest a rigid regulation of VBF in the presence of a systemically active vasodilator.

Effect of endolymphatic hydrops on capsaicin-evoked increase in cochlear blood flow

Capsaicin, which causes release of neuropeptides including substance P, has been shown to cause dose-related increases in cochlear blood flow (CBF). Since CBF regulation is altered in animal models with endolymphatic hydrops, this study was designed to examine the effect of hydrops on capsaicin-induced CBF changes. CBF responses to 0.01 and 0.001% capsaicin applied to the round window membrane were measured in normal and 12-week hydropic guinea pigs using laser Doppler flowmetry. With 0.01% capsaicin, CBF increased by only 13 +/- 7.8% in the hydropic ear, compared with 34% +/- 13% in the normal animal (p = 0.027). With 0.001% capsaicin, CBF increased by only 7.5 +/- 4.2% in the hydropic ear, compared with 28 +/- 16% in the normal ear (p < 0.001). These findings demonstrate a reduced responsiveness to capsaicin with endolymphatic hydrops and suggest that hydrops causes an alteration in the peptidergic sensory fibers of the inner ear or in the vascular smooth muscle or both. The results are consistent with previous reports of reduced vascular responsiveness in hydropic guinea pigs and provide further evidence for abnormal CBF regulation in hydrops.

Capsaicin-induced release of substance P increases cochlear blood flow in the guinea pig

Physiological evidence from several studies suggests that endogenous vasoactive peptides, such as substance P (SP), and their respective receptor populations may participate in the mechanisms that govern the autoregulatory capacity of the cochlear vascular system. However, these studies do not provide evidence regarding the origin or mechanism of action of SP. Capsaicin sensitivity has been used as a marker for sensory neurons, and the release of SP following capsaicin treatment suggests a sensory transmitter role for SP. The present investigation examines the relationship between the capsaicin-sensitive sensory neurons and SP in the regulation of cochlear blood flow (CBF). In 75 pigmented guinea pigs, the cochlea was surgically exposed and a laser Doppler flowmeter probe placed on the bony surface of the first turn to monitor CBF. Capsaicin solutions (2 microliters, 0.01%, 0.001% and 0.0001%) applied to the round-window membrane (RWM) resulted in a dose-related CBF increase, without change in the systemic blood pressure. This effect could be inhibited by application of a specific SP receptor antagonist, [D-Pro2,D-Trp7,9]-SP, after which none of the capsaicin concentrations used induced a change in CBF. Moreover, after RWM application of 50 nmol/2 microliters of SP there was a significant increase in CBF. No CBF change was observed with the lower concentrations of 10 nmol SP or 100 pmol SP. These results indicate a role of SP in CBF regulation and give indirect evidence that SP is released from capsaicin-sensitive primary sensory neurons.

Possible involvement of capsaicin-sensitive sensory nerves in the regulation of cochlear blood flow in the guinea pig

Capsaicin-induced microcirculatory changes in the cochlea of anaesthetized guinea pigs were examined by laser-Doppler flowmetry. Close intraarterial capsaicin infusion into the anterior inferior cerebral artery at doses of 10-50 pmol/min was followed by dose-dependent vasodilatation. Capsaicin infused in a dose of 150-200 pmol/min or above resulted in vasoconstriction in the region examined. Topical capsaicin administration into the cochlea (50-150 pmol) resulted in very moderate vasodilation with a latency of 1-2 min. Perivascular capsaicin application onto the anterior inferior cerebellar artery elicited an elevated blood flow in the cochlea, too. It is concluded that the release of vasoactive substances from capsaicin-sensitive nerve fibres in the inner ear of the guinea pig may play a role in the control of the local microcirculation. These nerves may also be involved in the neurogenic inflammatory processes in the region.

Substance P increases cochlear blood flow without changing cochlear electrophysiology in rats

Carotid artery infusions of substance P yielded reductions in systemic blood pressure and elevations in cochlear blood flow (CoBF), measured via laser Doppler flowmeter, with no alterations in cochlear action potentials or cochlear microphonics in Wistar-Kyoto rats. Additionally, direct micro-infusions of substance P into the anterior inferior cerebellar artery, which contributes to the local vascular perfusion of the cochlea, yielded elevations in CoBF with no changes in systemic blood pressure. Pretreatment with a specific substance P receptor antagonist, ([D-Pro2,D-Trp7,9]SP) via the carotid artery or the anterior inferior cerebellar artery, diminished subsequent substance P-induced vascular responses. These results suggest that endogenous substance P, like other vasoactive peptides, may interact with a substance P-specific receptor population in the cochlea and may therefore participate in the ongoing regulation of CoBF. These findings also support the premise that vasodilatory hormones, along with vasoconstrictive agents, may be involved in the autoregulation of CoBF.

Peptides and vasomotor mechanisms

The multiple and diverse roles played by neuropeptide Y, vasoactive intestinal polypeptide, substance P, calcitonin gene-related peptide and other biologically active peptides in the cardiovascular system are considered. A model of the vascular neuroeffector junction is described, which illustrates the interactions of peptidergic and nonpeptidergic transmitters that are possible at pre- and postjunctional sites. The effects of peptides on specific endothelial receptors are also described, which highlights the ability of these agents to act as dual regulators of vascular tone at both adventitial and intimal surfaces, following local release from nerves, or from endothelial cells themselves. Changes in expression of vascular neuropeptides that occur during development and aging in some disease situations and following nerve lesion are discussed.

Immunocytochemical study of catecholaminergic innervation in the guinea pig cochlea

The enzymes for synthesis of catecholamine, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyl transferase (PNMT), have been immunocytochemically localized in the guinea pig cochlea. Two groups of catecholamine-containing neurons could be distinguished. The first group, which was TH-positive/DBH, PNMT-negative, was found in the inner spiral bundle and the tunnel spiral bundle. The other group was TH, DBH-positive/PNMT-negative, and was found around the blood vessels. The immunocytochemical evidence obtained in this experiment suggests that the catecholamines should play a functional role in the guinea pig cochlea.

Drug-induced alterations in cochlear blood flow as recorded by the laser Doppler flowmeter

A number of clinical entities may result from a decrease in inner ear blood flow. Investigators have attempted to determine a relationship between decreased blood flow and such entities. In this article, we shall relate the effects of administration of drugs in animal models on cochlear blood flow, using the laser Doppler flowmeter.

Immunocytochemical detection of peptides in the guinea pig cochlea

The cochleae of juvenile guinea pigs were investigated for the presence of several neuropeptides. Glucagon, insulin, CCK and beta-endorphin immunoreactive neurons and nerve fibers as well as hair cells were demonstrated by the peroxidase antiperoxidase technique. Small amounts of substance P were also found in different sites in the inner ear. In contrast, prolactin-like material could not be found at all. These findings have significance with regard to the putative role of neuropeptides in neuromodulation.

Neuropeptides contained in peripheral cardiovascular nerves

The neuropeptides, substance P, vasoactive intestinal peptide (VIP), neuropeptide Y and enkephalin have been found in nerves associated with the heart and blood vessels of a range of mammals, including man. There is also evidence for some cardiovascular nerves with gastrin releasing peptide and neurotensin immunoreactivity. Substance P is in sensory nerves with a widespread distribution to the heart and all vascular beds. In general, large arteries have the densest innervation and the density of nerves decreases as arterial size decreases. In adult guinea-pigs, an adequate treatment with capsaicin causes the degeneration of almost all cardiovascular substance P nerves. Using capsaicin as a tool it has been shown that the substance P containing sensory nerves are not essential for baroreceptor reflexes. VIP nerves also have a widespread distribution, being particularly prominent in the cerebral arteries, uterine arteries and arteries of erectile and secretory tissues. Neuropeptide Y is located in the same cardiovascular nerves as noradrenaline. It is depleted from the nerves by reserpine or 6-hydroxydopamine. Enkephalin nerves have been reported with small arteries in only a few vascular beds.

Nerve fibres containing gastrin-releasing peptide around pial vessels

Nerve fibres containing immunoreactive gastrin-releasing peptide (GRP) were demonstrated around pial blood vessels of cat, guinea pig, rat, and mouse. A sparse supply was found around spinal cord blood vessels, whereas the choroid plexus seemed to be devoid of GRP fibres. Sympathectomy did not affect the number or distribution of the GRP fibres. The administration of neither GRP nor its closely related analogue, bombesin, contracted or dilated feline pial arteries in vitro.