Regulation Of Cochlear Blood Flow Research Articles

The tonic sympathetic input to the cochlear vasculature in guinea pig

Vascular tone is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activation of the sympathetic fibers to the cochlear vasculature, other studies have failed to show evidence of sympathetic contribution to CBF regulation when the cervical sympathetic fibers were unilaterally sectioned. We hypothesized that the bilateral ‘sympathectomy of the stellate ganglia’ would remove sufficient sympathetic input to the cochlea to yield a change in CBF resting level. To test this hypothesis a new technique was used to expose the stellate ganglia (SG) bilaterally and induce a chemical sympathectomy. We observed that unilateral SG blockade with 2 μl of 4 mM lidocaine hydrochloride on either side produced a 5–10% increase in CBF, which recovered to baseline during the following 2 min. A subsequent blockade of the contralateral SG produced a rapid 25–35% increase, which then recovered partially during the following 3–4 min, remaining 5–15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Our results provide evidence for the existence of a tonic sympathetic component in the control of vascular tone in guinea pig cochlea. This neural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiological studies on the bilaterality of evoked changes in CBF due to electric stimulation of SG.

Nitric oxide mediates capsaicin-induced increase in cochlear blood flow

Capsaicin has been previously shown to increase cochlear blood flow (CBF) in a dose-dependent manner. The aim of this study was to define the role of nitric oxide (NO) in capsaicin-induced changes in CBF. This was investigated in the anesthetized guinea pig, utilizing laser Doppler flowmetry. Application of capsaicin (64.8 and 6.48 nmol in 2 μl of saline) to the round window membrane (RWM) caused increases in CBF (34 ± 2.8% of baseline (BL) and 28 ± 2.3% BL, respectively ( P < 0.001)). Application of the NO synthase inhibitor, N G-nitro-l-arginine methyl ester (l-NAME) (10 mg/kg intravenously or topically to the RWM) reduced blood flow in the cochlea, as previously reported. After pretreatment with i.v. l-NAME, the effect of capsaicin on CBF was significantly decreased. With the dose of capsaicin at 64.8 nmol, the increase in CBF fell from 34 ± 2.8% BL to 6.9 ± 1.5% BL ( P < 0.001), and at 6.48 nmol it fell from 28 ± 2.3% BL to 4.8 ± 1.6% BL ( P < 0.001). RWM l-NAME application also decreased the capsaicin vasodilatation effect. A capsaicin dose of 64.8 nmol resulted in only a 10 ± 2.5% BL increase in CBF, and with 6.48 nmol capsaicin the increase was 7.8 ± 2.2% of BL ( P < 0.001). Capsaicin-sensitive sensory neurons in other systems are generally known to release substance P (SP), which in turn elicits release of endothelium derived relaxing factor (NO). The results of this study indicate that NO is a mediator of capsaicin-sensitive sensory neuronal function in CBF regulation.

Autoregulation of cochlear blood flow in the hydropic guinea pig

Previous data suggest that regulation of cochlear blood flow (CBF) may be abnormal in the hydropic guinea pig. The purpose of this study was to employ the technique of anterior inferior cerebellar artery (AICA) occlusion to measure CBF autoregulation in experimental endolymphatic hydrops. This study also addresses the role of the cochlear sympathetic neural innervation and nitric oxide in CBF regulation with hydrops. In anesthetized guinea pigs, CBF was measured with a laser Doppler flowmeter probe while the AICA was intermittently occluded with a microvascular occluder. The CBF response was measured in normal, 6-week, and 12-week chronically hydropic animals. The gain factors (0 = no autoregulation, 1 = complete autoregulation) for 1-min occlusion were 0.95 ± 0.16 (control), 0.77 ± 0.28 (6 week, P = 0.164), and 0.67 ± 0.25 (12 week, P = 0.037). N G-nitro- l-arginine methyl ester ( l-NAME), a competitive inhibitor of nitric oxide synthase, was infused intravenously to assess basal nitric oxide (an endogenous vasodilator) production in the hydropic ear. With infusion of l-NAME, CBF was reduced by 9.16 ± 11%, 10.7 ± 10% ( P = 0.87), and 16.6 ± 18% ( P = 0.95), in the control, 6-week, and 12-week animals, respectively. In a separate group of 12-week hydropic animals, the left superior cervical ganglion (SCG) was anesthetized with lidocaine, and AICA occlusions were performed pre-and post-blockade. Prior to blocking the SCG, the gain was 0.712 ± 0.02 and afterwards 0.708 ± 0.051 ( P = 0.93). The above results show that there was a statistically significant reduction in CBF autoregulation in the 12-week hydropic animals. There was no difference in basal nitric oxide production in normal versus hydropic animals nor was there a change in autoregulation following blockade of the SCG. These data provide clear evidence for reduced CBF autoregulation in experimental endolymphatic 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 μl, 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-Pro 2, d-Trp 7,9]-SP, after which none of the capsaicin concentrations used induced a change in CBF. Moreover, after RWM application of 50 nmol/2 μl 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.

P2y purinergic receptors coupled to phosphoinositide hydrolysis in tissues of the cochlear lateral wall.

The tissues of the lateral wall of the cochlea, spiral ligament and stria vascularis, maintain homeostasis of the inner ear through the production of endolymph and regulation of cochlear blood flow. We examined the effects of purinergic agonists on the release of inositol phosphates (InsPs) in intact isolated lateral wall tissues of the Fischer-344 rat. After preincubation with myo[3H]inositol, stimulation with the P2 purinergic agonist ATP-gamma-S resulted in a concentration-dependent increase in the formation of [3H]InsPs. The P2 purinergic agonist alpha,beta-methylene ATP (AMP-CPP) was a weaker stimulant, and the P1 purinergic agonist adenosine was ineffective. These results are consistent with the presence of P2y receptors and a purinergically controlled InsP3 second messenger system in the cochlear lateral wall.

Neuronal regulation of cochlear blood flow in the guinea-pig.

1. Previous studies have shown that electrical stimulation (ES) of the guinea-pig cochlea causes a neurally mediated increase in cochlear blood flow (CBF). It is known that the centrifugal neuronal input to the cochlea comes through the perivascular sympathetic plexus from the cervical sympathetic chain and along the vestibular nerve (VN) from the periolivary area of the brainstem. Both of these neuronal systems are distributed topographically in the cochlea. 2. In order to study the neural origins of ES-evoked CBF increase, laser Doppler flowmetry was used to test the following hypotheses. (a) The response is regional, that is, limited to the area of the cochlea stimulated. To test this we performed differential ES of the cochlear turns. CBF was measured from either the third or the first turn. (b) The response is mediated via autonomic receptors within the cochlea. To study this, we applied atropine, succinylcholine and idazoxan locally to the cochlea. (c) The response is influenced by neuronal input via the sympathetic cervical chain (SC) and components of the VN. We stimulated and sectioned the SC, and sectioned the VN, to test this hypothesis. 3. We observed that the CBF response was topographically restricted to the stimulated region. Locally applied muscarinic or nicotinic antagonists (atropine and succinylcholine respectively) did not affect the response. However, local idazoxan (an alpha 2-blocker) eliminated the response. Locally applied adrenaline and SC stimulation modified the dynamic range of the response. SC sectioning enhanced the responsiveness of the cochlear vasculature to ES. The VN section caused a temporary decrease in CBF and elimination of the ES-evoked CBF response. 4. We conclude that the release of dilating agents is topographical with respect to ES current flow, the ES-evoked CBF increase is peripherally mediated via alpha 2-receptors, and the response is influenced by input via the SC. The elimination of the response by VN sectioning proximal to the brainstem indicated that fibres of the VN mediate the CBF increase during direct cochlear ES. The data suggest that these fibres may be the efferent limb of a neural loop involved with the regulation of CBF. Such a system could provide a mechanism for the rapid increase in CBF with organ stress.

Basal nitric oxide production in regulation of cochlear blood flow

Nitric oxide (NO), recently identified as endothelium-derived relaxing factor, has been shown to influence both vascular and neural function. In blood vessels, NO is produced by endothelial and smooth muscle cells and may play a role in regulation of cochlear blood flow. In the central nervous system, NO functions as a neurotransmitter involved in long term potentiation. The principle hypothesis tested in this study was that basal NO production in the cochlear blood vessels contributes to regulation of CBF. Since NO is a vasodilator, diminished NO synthesis may decrease the level of CBF. Application of a competitive inhibitor of NO synthase either intravenously or to the round window membrane caused a reduction in CBF. The application to the round window membrane did not affect compound action potential thresholds. With intravenous adminstration, the effect on CBF was dose-related and could be reversed with the physiologic substrate, l-arginine. These data indicate that NO is produced in the cochlear blood vessels and contributes to the regulation of CBF.

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.

Bilateral Inner Ear Blood Folw in Rats with Unilateral Vertebral Artery Occulusion

It is well known that some kinds of vertigo and some hearing problems are caused by impairment of the vertebro-basilar circulation. The influence of unilateral vertebral artery (VA) occlution on bilateral inner ear blood flow was studied in rats by the laser Doppler method. After tracheotomy, both cochleas were exposed through a ventral approach for the measurement of cochlear blood flow (CBF). One VA was occluded by photochemically induced thrombosis. The right femoral artery was cannulated for continuous measurement of systemic blood pressure (BP). BP and CBF showed little change after unilateral VA occlu-sion alone. However, subsequent hypotension induced by venesection not only reduced CBF, but also caused bilateral imbalance between the CBF reduction rates. CBF changed independently of BP, indicating the existence of a local CBF regulation system.

Stellate ganglion drives sympathetic regulation of cochlear blood flow

The functional properties of the sympathetic fibers innervating the cochlea are not well understood. Adrenergic fibers supplying lateral wall structures of the cochlea have been observed terminating on radiating arterioles and collecting venules. Adrenergic fibers also terminate as ‘free’ endings in the spiral osseous lamina. Stimulation or transection of sympathetic fibers originating from superior cervical chain and supplying the cochlea have yielded mixed results concerning many aspects of cochlea physiology. In order to clarify the origin of sympathetic fibers and their role in control of cochlear blood flow (CBF), we examined the effect of electrical stimulation of the stellate ganglion (ESS) and transection of postganglionic fibers from the stellate on CBF measured by laser Doppler flowmetry and on systemic blood pressure (BP) in the guinea pig. ESS produced a 20–35% increase in BP and 10–15% decrease in CBF. The decrease in CBF presumably reflects the net result of increased perfusion pressure, local autoregulatory mechanisms, and a direct sympathetic-induced vasoconstriction. Section of the immediate postganglionic sympathetic trunk had little or no effect on the ESS-related change in BP; however, it eliminated the CBF reduction. Intravenously infused β1-blocker diminished the BP increase due to ESS, while the electrically-evoked reduction in CBF remained. Local application of an α-blocker on the round window blocker ESS evoked CBF reductions without altering the BP increase. These data confirm the functional role of sympathetic projections from the stellate ganglion in CBF regulation in guinea pig. Taken together with previous studies showing that the superior cervical ganglion mediates a sympathetic input into inner ear vasculature in guinea pig, our results indicate the action of two or more autonomic sympathetic fiber systems in the control of CBF.

Glyoxylic acid in the study of autonomic innervation in the gerbil cochlea.

The autonomic innervation of the inner ear has been investigated earlier, yet questions concerning the origin, function, extent, and distribution of sympathetic nerves in the cochlea still remain unanswered. This study investigates the extent and distribution of adrenergic nerves in the cochlea. Our procedure combines the glyoxylic acid method of catecholamine fluorophore identification with rapid dissection, decalcification, and whole-mount slide preparation techniques to topographically trace the adrenergic innervation of the cochlear infrastructure. We have demonstrated that perivascular adrenergic innervation extends beyond the immediate branches of the modiolar artery and reaches into radiating arterioles. These findings also suggest the possibility of segmental regulation of cochlear blood flow.

Probational Treatment of Sudden Deafness with Prostacyclin: A Pilot Study

Sudden idiopathic hearing loss has occasionally been supposed to be caused by a disturbed microcirculation in the inner ear of unknown origin. Little is known about the regulation of cochlear blood flow and the effectiveness of drugs in cochlear microcirculation. Because animal experiments gave evidence that prostacyclin (PGI2) might be one biochemical substratum of local regulators in the flow of blood in the stria vascularis, 11 patients with sudden idiopathic hearing loss were treated once for 6 h with prostacyclin (10 ng/kg body weight/min) in a first open clinical trial. In most cases prostacyclin increased hearing level (mean value: 7.4 dB/frequency/day) more than a standard therapy with pentoxifylline. The substitution of PGI2 could be another indication of a rheologic disorder--whether per se or within a larger context of inflammation-like interaction--in the inner ear of patients with sudden hearing impairment.

Neural basis for regulation of cochlear blood flow: Peptidergic and adrenergic innervation of the spiral modiolar artery of the guinea pig

The spiral modiolar artery is the terminal artery in the cochlea, and as such is expected to play a major role in the control of cochlear blood flow. In this study, we examined the distribution of adrenergic and peptidergic nerve fibres on the spiral modiolar artery of the guinea pig using histofluorescence and immunofluorescence techniques. The spiral modiolar artery was dissected from the modiolus so that the entire length of the vessel and its branches, could be observed. Noradrenaline was identified using the glyoxylic acid histofluorescence technique. The presence of the vasoactive peptides substance P, calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP), was investigated using antibodies against these peptides. Each putative transmitter tested yielded labelled nerve fibres throughout the length of the spiral modiolar artery and its branches. Double-labelling experiments confirmed that CGRP and substance P are contained in the same fibres but that VIP and substance P appear to be contained in different populations of fibres. These results establish that nerve fibres containing vasoactive peptides and noradrenaline supply the spiral modiolar artery and suggest that they are involved in the regulation of cochlear blood flow.