Early effects of cervical sympathetic stimulation on cerebral, ocular and cochlear blood flow

The choroid plexus: a missing link in our understanding of brain development and function

Studies of the choroid plexus lag behind those of the more widely known blood-brain barrier, despite a much longer history. This review has two overall aims. The first is to outline long-standing areas of research where there are unanswered questions, such as control of cerebrospinal fluid (CSF) secretion and blood flow. The second aim is to review research over the past 10 years where the focus has shifted to the idea that there are choroid plexuses located in each of the brain's ventricles that make specific contributions to brain development and function through molecules they generate for delivery via the CSF. These factors appear to be particularly important for aspects of normal brain growth. Most research carried out during the twentieth century dealt with the choroid plexus, a brain barrier interface making critical contributions to the composition and stability of the brain's internal environment throughout life. More recent research in the twenty-first century has shown the importance of choroid plexus-generated CSF in neurogenesis, influence of sex and other hormones on choroid plexus function, and choroid plexus involvement in circadian rhythms and sleep. The advancement of technologies to facilitate delivery of brain-specific therapies via the CSF to treat neurological disorders is a rapidly growing area of research. Conversely, understanding the basic mechanisms and implications of how maternal drug exposure during pregnancy impacts the developing brain represents another key area of research.

Increased Incidence of Tinnitus Following a Hyperthyroidism Diagnosis: A Population-Based Longitudinal Study

BACKGROUND

An association between thyroid disease and tinnitus has been described previously but further longitudinal, population-based studies are limited.

OBJECTIVE

To investigate the incidence of tinnitus in patients with hyperthyroidism in a national sample, and to identify risk level and associated factors for tinnitus in hyperthyroidism patients.

DESIGN

Retrospective cohort study. Patient data were collected from the Longitudinal Health Insurance Database (LHID 2000), which includes national claims data of patient expenditures for admissions or ambulatory care from 1996 to 2011.

SETTING

Taiwan hospitals and clinics providing healthcare nationwide.

PARTICIPANTS

Patients aged 20 years and older with newly diagnosed hyperthyroidism (ICD-9-CM code 242) between 2000-2010 were selected as the study cohort. Hyperthyroidism patient cohort were identified from the LHID2000. Those with tinnitus history (ICD-9-CM code 388.3) before the index date (first hyperthyroidism diagnosis), younger than 20 years, and with incomplete demographic data were excluded. The non-hyperthyroidism cohort included patients with no history of hyperthyroidism and no documented tinnitus.

MAIN OUTCOMES AND MEASURES

Incidence of tinnitus was the primary outcome. Baseline demographic factors and comorbidities possibly associated with tinnitus, including age, sex, and comorbidities of hearing loss, vertigo, insomnia and anxiety, were retrieved from the LHID 2000. Patients were followed until end of 2011.

RESULTS

During the study period, 780 (4.9%) hyperthyroidism patients and 2007 (3.2%) non-hyperthyroidism controls developed tinnitus. Incidence rate of tinnitus in the hyperthyroidism cohort was significantly higher in hyperthyroidism cohort (7.86 vs. 5.05 per 1000 person-years) than that in non-hyperthyroidism cohort. A higher proportion of patients with hyperthyroidism had comorbid insomnia (45.1% vs. 30.9%) and anxiety (14.0% vs. 5.73%) than those without hyperthyroidism. After adjusting for age, gender and comorbidities (vertigo, insomnia, anxiety, hearing loss), hyperthyroidism patients had 1.38-fold higher risk of tinnitus (95% CI = 1.27-1.50) than those without hyperthyroidism.

CONCLUSIONS

This large population-based study suggests patients with diagnosed hyperthyroidism was more prone to develop tinnitus. Our findings suggest evaluation for comorbid vertigo, insomnia, anxiety and/or hearing loss may identify patients who are at high risk of developing tinnitus in patients with hyperthyroidism.

Factors Associated With Age-related Hearing Impairment: A Retrospective Cohort Study

Age-related hearing impairment (ARHI) is a complex degenerative disease in the elderly. As multiple factors interact during the development of ARHI, it is important to elucidate the major influencing factors to understand and prevent ARHI. We aimed to identify risk factors associated with the development of ARHI with a retrospective cohort from 2001 to 2010. The records of the adult subjects over 40 years of age who consecutively underwent a comprehensive health checkup including pure-tone audiometry at the Health Promotion Center were reviewed. During this period, 1560 subjects who underwent pure-tone audiometry more than twice, had no other otologic diseases, and were followed-up more than 2 years were included. A pure-tone average (PTA: 0.5, 1, 2, 4 kHz) was calculated. Development of ARHI was defined as a PTA at follow-up more than 10 dB greater than the baseline PTA. Times to the first development of ARHI were investigated. Overall, 12.7% of subjects developed ARHI within the first 4 years. High blood ionized calcium (hazard ratio [HR] 0.084), albumin (HR 0.239), systolic blood pressure (HR 0.577), thyroid hormone (T3) (HR 0.593), and alpha fetoprotein levels (HR 0.883) were associated with decreased hazard for the development of ARHI. In contrast, high blood high-density lipoprotein (HR 2.105), uric acid (HR 1.684), total protein (HR 1.423), and total bilirubin levels (HR 1.220) were potential risk factors for the development of ARHI. Development of ARHI is common among the aged population, and a variety of factors may interact during this process. The results of this study can be used for counseling of adults at high-risk of developing ARHI with regard to regular audiological check-up.

The acute effects of stellate ganglion block on circulation in human ocular fundus

PURPOSE

To study the acute effects of local-anesthetic stellate ganglion block (SGB) on tissue circulation in the human fundus.

METHODS

Eleven patients with Bell's palsy (age 56+/-6 y, mean+/-SD) who underwent SGB for its treatment participated in the study. Using the laser speckle method, normalized blur (NB) value, a quantitative index of tissue blood velocity, was measured every 0.125 s over an area located halfway between the macula and the optic nerve head (ONH) with no discrete visible vessels and averaged over 3 pulses when fixation was satisfactory (NB(ch-ret)). NB(ch-ret) and intraocular pressure (IOP) in both eyes, blood pressure (BP), and pulse rate (PR) were measured before, and 10, 20, 30, and 60 min after SGB. SGB was induced by injecting 1% mepivacaine hydrochloride (5 ml) into the vicinity of the seventh cervical vertebra on the paralyzed side.

RESULTS

The IOP in the blocked side significantly decreased between 20 and 60 min following SGB, compared to the baseline, while IOP in the unblocked side remained unchanged. The NB(ch-ret) was significantly increased after 10 min by about 8% in the blocked side, but its effect almost disappeared at 60 min. There was no significant change in NB(ch-ret) in the unblocked side, BP or PR throughout the experimental period.

CONCLUSION

SGB increased tissue circulation in the fundus in the blocked side, but its effect was thought to be small and transient.

Effect of superior cervical ganglionectomy on catecholamine concentration in rat cochlea

Both noradrenergic and dopaminergic nerve terminals have been described in the cochlea. The present report focused on the effect of superior cervical ganglionectomy (SCGx) on monoamine concentration in adult rat cochlea. In homogenates of whole cochleas, we measured the concentrations of norepinephrine (NE), dopamine (DA) and its main metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), by HPLC coupled to electrochemical detection. Measurements were carried out 4 h, 24 h or 6 days after unilateral SCGx. Most of the NE (approximately 82%) was lost after sympathectomy on the ipsilateral side, indicating that the principal localization of cochlear NE is in peripheral sympathetic fibers. Since about 18% of NE remained detectable 6 days after SCGx, a second origin of cochlear noradrenergic fibers may exist. Cochlear concentrations of DA or its metabolites did not change after SCGx. Therefore, DA and NE are located in two different populations of fibers within the cochlea, and are presumably related to distinct functional roles.

Clinical observations on acute low-tone sensorineural hearing loss. Survey and analysis of 137 patients

We retrospectively analyzed the clinical records of 137 patients who were treated at our clinic for acute low-tone sensorineural hearing loss of unknown cause over a period of 8 years. The analyses of the clinical records indicated the following clinical characteristics: female preponderance; peak incidence during the fourth decade of life; frequent accompanying tinnitus, sensation of ear fullness, and/ or autophony; suspicion of bilateral involvement; and association with autonomic imbalance. The hearing in most of the patients completely recovered, but some selected patients experienced fluctuating hearing loss or progression to Meniere's disease. Our results suggest that a subgroup of patients with severe initial hearing loss should be carefully followed up, although it is difficult to predict the outcome of this disorder.

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 administration, 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.

The trajectory of the sympathetic nerve fibers to the rat cochlea as revealed by anterograde and retrograde WGA-HRP tracing

Wheat germ agglutinin-horseradish peroxidase conjugate was injected in the unilateral superior cervical ganglion (SCG), and the projection pathways of postganglionic sympathetic nerve fibers innervating the cochlea were traced in the rat. The labeled axons advanced along the internal carotid artery (ICA), and a few advanced caudally in the major petrosal nerve (MPN) and entered the facial nerve, while the majority ran rostral to the pterygopalatine ganglion at the point where they crossed the MPN in the carotid canal. The rest of the labeled fibers remained on the surface of the ICA and advanced to the cranial cavity. Most of the labeled fibers along the facial nerve joined the cochlear nerve and finally reached the osseous spiral lamina through the spiral ganglion. Some of the labeled fibers ran along the anterior inferior cerebellar artery from the basilar artery which was previously thought to have been the only pathway. We could not find any labeled fiber on the modiolar artery from anterior inferior cerebellar artery in the cochlea. These observations are consistent with our hypothesis that the sympathetic fibers innervating the neural tissues or related structures follow nerve fibers and meninges as matrices of projection pathways rather than arteries.

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 beta 1-blocker diminished the BP increase due to ESS, while the electrically-evoked reduction in CBF remained. Local application of an alpha-blocker on the round window blocked 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Student Research Award 1990. Catecholaminergic innervation of the inner ear

Cochlear blood flow has been shown to be controlled, in part, by the sympathetic nervous system. We have used immunocytochemical staining for tyrosine hydroxylase (TH) to further demonstrate the extent of catecholaminergic innervation of the cochlea. Deeply anesthetized Mongolian gerbils were systemically perfused with phosphate-buffered saline, followed by 4% paraformaldehyde. The cochleae were dissected out and post-fixed for 3 hours in 4% paraformaldehyde. They were then incubated in anti-TH antibody and subsequently processed using the avidin-biotin immunoperoxidase method. Microscopic examination of whole-mounted tissue revealed many immunoreactive fibers on the spiral modiolar artery. TH-positive fibers have also been found on both radiating arterioles and radial collecting venules, which has not been previously reported. With innervation of these small vessels, blood flow in the cochlea may be segmentally controlled. We also further describe the organization of TH-positive fibers in the osseous spiral lamina.

Chronic electrical stimulation of sympathetic nerves: effects on blood-aqueous barrier

The effect of electrical stimulation of the sympathetic nerves on the blood-aqueous barrier was investigated in rabbits. The permeability of the barrier was assessed during either acute, chronic or following chronic nerve stimulation. During acute and chronic stimulation of the sympathetic nerves, fluorescein entered the anterior chamber at a rate significantly slower than in control eyes. After chronic stimulation, both the rate of entry of fluorescein and the aqueous humor protein concentration were much greater than in control eyes indicating breakdown of the blood-aqueous barrier. Treatment with the non-steroidal anti-inflammatory drugs, indomethacin and suprofen, completely blocked the breakdown of the blood-aqueous barrier. These results indicate that sympathetic nerve stimulation can cause the local synthesis of prostaglandins and that these can affect the blood-aqueous barrier.

HPLC detection of dopamine and noradrenaline in the cochlea of adult and developing rats

The presence and postnatal development of dopamine (DA) and noradrenaline (NA) in the rat cochlea were detected by high-performance liquid chromatography coupled with electrochemical detection. Cochlear DA content rose gradually after birth. Conversely, NA concentrations rose rapidly between postnatal day 1 and 8; then, up to day 30, it increased more slowly. On day 30, both DA and NA levels were around 5 times higher than on day 1. In the adult rat cochlea, NA mean content was 234 +/- 22.2 pg/mg protein, while DA mean content was 23.6 +/- 1.1 pg/mg protein. Adrenaline was always undetectable. The present study is the first report describing directly the presence of DA in the rat cochlea. DA might serve as one of the lateral efferent neurotransmitters, whereas NA probably acts as a neurotransmitter of the sympathetic cochlear innervation. Nevertheless, their influences on the cochlear physiology, either in adulthood or during development are still a matter of discussion.

Electrically stimulated increases in cochlear blood flow: I. Frequency and intensity effects

Charge-balanced, sinusoidal current was passed differentially between the apex and round window of the guinea pig cochlea. Cochlear blood flow was measured using a laser Doppler flow monitor. Systemic blood pressure was monitored from a cannula within the common carotid artery. Electrical stimulation increased cochlear blood flow, while systemic blood pressure was unaffected. A cochlear blood flow response parameter, normalized for transient changes in systemic blood pressure, was defined. The magnitude of the response parameter was found to be frequency selective and was also found to be an increasing function of current intensity, with maximum responses obtained with 500 Hz sinusoids. This cochlear blood flow response was not observed in dead animals; was present in preparations paralyzed with gallamine hydrochloride; and was correlated with an increase in cochlear red blood cell velocity, as directly observed by intravital microscopy. These observations imply that electrical stimulation induces a local vasodilation within the temporal bone. The fact that decreased cochlear blood flow was never observed with current injection implies that ischemia is not a likely mechanism of electrically induced tissue damage within the inner ear. The mechanism of this cochlear blood flow response is addressed in a companion report.

Cochlear blood flow: measurement techniques

The measurement of inner ear blood flow and other microvascular variables is subject to unique technical problems which are compounded by methodological limitations. As a result, the interpretation of experimental results is often difficult. This report discusses the most important methods currently available for cochlear blood circulation measurements and the technical problems associated with their use. The use of a combination of measurements to resolve problems of interpretation is stressed. An extensive review of the pertinent literature is provided in relation to each method.

Local glucose utilization of the brain and pineal gland during stimulation of the cervical sympathetic trunk

The quantitative autoradiographic 2-[14C]deoxyglucose method was employed to map the metabolic activity of the superior cervical ganglion and the entire brain during unilateral electrical stimulation of the cervical sympathetic trunk in the urethane-anesthetized rat. Stimulation of the cervical sympathetic trunk increased glucose utilization in the ipsilateral superior cervical ganglion (+95%) but did not produce side-to-side differences in glucose utilization in any of the brain structures examined in this study. Compared to the control nonstimulated animals, the rate of glucose metabolism in the pineal gland was increased 71% following stimulation of the cervical sympathetic trunk. The pineal gland was the only brain region out of 87 structures examined in which glucose utilization was increased by electrical stimulation of its sympathetic innervation.

Microsphere determination of cochlear blood flow in chickens

Chickens were injected with 9-micron-diameter radioactive microspheres. Cochleas were removed through the external auditory meatus, and the positions of all embedded microspheres were drawn under camera-lucida. Constant measurements of arterial pressures and postinjection blood-gas determinations confirmed that injections were made into normal circulatory systems. The averaged estimate of cochlear blood flow in chickens is 0.75 microliter/min. Variability in these data from chickens is similar to that reported from mammals. A potentially important but puzzling observation is an inverse relationship between blood flow to the cochlea and to the brain. The ease of cochlear extraction makes chickens ideal models for study of cochlear blood flow.

The influence of bilateral electrical preganglionic sympathetic stimulation on intra- and extracranial blood flow

The effects of bilateral electrical stimulation (SS) of the cervical sympathetic chain on intra- and extra cerebral blood flows were studied with the labelled microsphere method in the rabbit. Control blood flow was determined before the SS was started. The stimulation frequency was 7 Hz, the impulse duration 2 ms, the intensity 7 V and the stimulation time varied between 1 to 5 minutes before the second blood flow determination. Arterial blood gas values and blood pressure were unaffected by the stimulation. Due to the SS there were blood flow decrements in the extracranial tissues between 60-96%. The blood flow in the eyes, the dura, pineal gland and choroid plexa was markedly reduced during the SS. No obvious effect was elicited by the SS in the regional or total cerebral blood flow. The stimulation to control blood flow ratio ranged between 0.92 +/- 0.08 to 1.13 +/- 0.09 in different parts of the brain. The conclusions are that SS elicits vasoconstriction in several extra- and intracranial nonneuronal tissues and in the eye. Cerebral blood flow is not influenced by the SS.

Sustained cerebral vasoconstriction during bilateral sympathetic stimulation in anesthetized rabbits

Temporal aspects of bilateral sympathetic nerve stimulation on cerebral blood flow (CBF) were examined in anesthetized rabbits (n = 7). CBF ranged from 32 to 50 ml/min per 100 g. Bilateral stimulation reduced blood flow by 17-31% to cerebrum, diencephalon-mesencephalon and cerebellum, and responses were constant between 2 and 6 min of stimulation. Sustained cerebral vasoconstriction is consistent with an important role for sympathetic nerves in the regulation of CBF.

Role of prostaglandins in modulating sympathetic vasoconstriction in the cerebral circulation in anesthetized rabbits

The effects of the interaction between sympathetic nerves and prostaglandins in the cerebral circulation were examined. The hypothesis tested was that inhibition of prostaglandin synthesis by indomethacin would potentiate decreases in CBF caused by sympathetic nerve stimulation. In anesthetized rabbits, following administration of either indomethacin (10 mg/kg) or vehicle, CBF was measured with 15-micron microspheres prior to stimulation and following 3-5 min of electrical stimulation (4, 8, 16 Hz) of both superior cervical ganglia. In the vehicle group, CBF was 33-42 ml/min/100 g prior to stimulation. Bilateral sympathetic stimulation reduced blood flow to the cerebrum by 12 +/- 6% (mean +/- SEM) (p less than 0.05) at 4 Hz (n = 8), by 20 +/- 4% (p less than 0.05) at 8 Hz (n = 12), and 21 +/- 6% (p less than 0.05) at 16 Hz (n = 11). In the indomethacin group, CBF was 37-48 ml/min/100 g prior to stimulation. Bilateral stimulation decreased blood flow to the cerebrum by 7 +/- 5% (NS) at 4 Hz (n = 8), by 25 +/- 3% (p less than 0.05) at 8 Hz (n = 6), and by 20 +/- 6% (NS) at 16 Hz (n = 6). Decreases in CBF during nerve stimulation were blocked by prazosin, an alpha-adrenergic antagonist. In additional experiments, cerebral vascular constrictor responses to hypocapnia were found to be similar in the vehicle and indomethacin groups. This study provides evidence that sympathetic nerves can decrease CBF substantially even at low stimulation frequencies. Further, results of this study indicate that prostaglandins do not attenuate the effects of sympathetic stimulation on the cerebral circulation.

Effects of asymmetric vertebral blood flow upon the vestibulo-ocular reflex of the rabbit

Per-rotational nystagmus was recorded in rabbits with unilaterally narrowed vertebral arteries or following unilateral cervical sympathectomies. Asymmetry of per-rotational nystagmus could not be observed when the animal's systemic arterial blood pressure was within the normal range (about 90-120 mm Hg). However, directional preponderance of nystagmus occurred when blood pressure either increased or decreased beyond this level. The reason for the occurrence of directional preponderance can be interpreted as a failure of autoregulation of cerebral and/or inner ear blood flow.

The role of sympathetic efferent activity in the regulation of brain temperature

The role of nasal heat exchange in the control of brain temperature has been studied in cats, pigs, ducks and rabbits during acute experiments under general anaesthesia. Nasal air flow at physiological rates caused hypothalamic temperature to fall at between 0.2 and 0.5 degrees C/min in cats, pigs and ducks, which all have arterial rete systems that can cool blood flowing to the brain, but not in rabbits, which lack an arterial rete. Bilateral stimulation of cervical sympathetic trunks reduced or abolished the brain cooling effect of nasal air flow in cats, pigs and ducks. After a period of airflow during which brain cooling was reduced by sympathetic stimulation, the end of stimulation was sometimes followed by marked and rapid brain cooling, indicating re-perfusion through ischaemic cooled tissues. Cervical sympathetic stimulation caused a reduction in resistance to nasal airflow in all species studied, by inducing vasoconstriction and shrinkage of the nasal mucosa. In species with well-developed arterial retia, the effect of cervical sympathetic stimulation in regulating nasal cooling of the brain is probably mediated by controlling blood flow through the nasal mucosa. Although this vascular control also occurs in rabbits, they cannot selectively cool the brain and sympathetic stimulation has no effect on rabbit brain temperature.

A dual effect of sympathetic nerve stimulation on jaw muscle spindles

In anaesthetized and paralyzed rabbits, electrical stimulation of the cervical sympathetic nerve at physiological frequencies induces in jaw muscle spindle afferents a short-latency decrease or suppression of discharge. This effect is very stereotyped in pattern and is attributed to direct sympathetic innervation of spindles. It is mediated by preganglionic S1-S2 sympathetic fiber groups. A longer-latency facilitatory effect follows, probably vasomotor in origin and mediated by S3-S4 groups. Both responses are eliminated by administration of alpha-adrenergic blocking agents. The latencies, patterns, thresholds, durations and reproducibility of these responses have been studied and the mechanisms possibly involved are discussed.

Effect of nicotinic acid on cochlear blood flow

Effects of nicotinic acid on cochlear blood flow were studied in rats by the microsphere method. One measurement was performed before drug administration and the second after i.v. injection of the nicotinic acid 25 mg/kg body weight. No measurable effect on cochlear blood flow was observed in normal animals. In one group of animals the cervical sympathetic trunks were cut and the distal part on one side was electrically stimulated during the measurements. On the unstimulated side the cochlear blood flow was significantly higher than in intact animals, but uninfluenced by nicotinic acid. On the sympathetically stimulated side a significant increase of the cochlear blood flow occurred after nicotinic acid administration.

Effects of cervical sympathetic stimulation on cerebral and ocular blood flows during hemorrhagic hypotension and moderate hypoxia

The effect of cervical sympathetic stimulation upon regional blood flows was investigated in albino rabbits during graded hemorrhagic hypotension and mild to moderate hypoxic hypoxia. Regional blood flows were determined using labelled microspheres. Cerebral blood flow (CBF) decreased in response to progressive hypotension and increased considerably during hypoxia (100-200%). Unilateral sympathetic stimulation did not change the ipsilateral cerebral flow responses under either condition. There was a greater tendency to autoregulate down to lower blood pressures in deep than in superficial cerebral structures. During hypoxia cortical gray matter blood flow increased relatively more than did white matter blood flow. Blood flow in different parts of the eye decreased during hypotension and tended to increase during hypoxia. Unilateral sympathetic stimulation reduced flow rates on the stimulated side (10-50% of control side) under both conditions. The vasoconstrictory effect upon retinal blood flow tended, however, to be less during hypoxia. Dural blood flow showed a poor autoregulation and also no consistent vasodilatory response upon hypoxia. Sympathetic stimulation had a very marked effect. The results suggest that the cervical sympathetic nerves do not have any appreciable effect upon cerebral circulation during profound hypotensive and moderate hypoxic states. Dural and most ocular blood flows seem, however, to be clearly affected by sympathetic stimulation even under these extreme conditions.

Sympathetic effects on cerebral and ocular blood flow in rabbits pretreated with indomethacin

The effects of unilateral stimulation of the cervical sympathetic chain on cerebral and ocular blood flow was investigated in 8 rabbits anesthetized with pentobarbital sodium and pretreated with indomethacin in order to inhibit the formation of prostaglandins. Blood flow determinations were made with the labelled microsphere method during normotension and acute arterial hypertension. Hypertension was induced by ligation of the thoracic aorta. Evans blue was given as a tracer for protein leakage during hypertension. Sympathetic stimulation had no significant effect on the blood flow in the brain under the two conditions studied. In the uvea marked effects of sympathetic stimulation were obtained at normotension as well as at hypertension. There were no indications of breakdown of the blood-brain barrier or the blood-aqueous barrier. Thus, there was no evidence for any prostaglandin-mediated inhibition of sympathetic effects in the brain or the eye.

Effects of sympathetic stimulation on cerebral and ocular blood flow. Modification by hypertension, hypercapnia, acetazolamide, PGI2 and papaverine

The effect of unilateral, electrical stimulation of the cervical sympathetic chain in rabbits anesthetized with pentobarbital sodium and vasodilated by hypercapnia, acetazolamide, papaverine or PGI2 was investigated to determine to what extent the sympathetic nerves to the brain and the eye cause vasoconstriction and prevent overperfusion in previously vasodilated animals. Evans blue was given as a tracer for protein leakage. Blood flow determinations were made with the labelled microsphere method during normotension and acute arterial hypertension. Hypertension was induced by ligation of the thoracic aorta and in some animals metaraminol or angiotensin was also used. Acetazolamide caused a two to threefold increase in cerebral blood flow (CBF) and hypercapnia resulted in a fivefold increase. CBF was not markedly affected by papaverine or PGI2. In the choroid plexus, the ciliary body and choroid, papaverine and hypercapnia caused significant blood flow increases on the control side. Sympathetic stimulation induced a 12% blood flow reduction in the brain in normotensive, hypercapnic animals. Marked effects of sympathetic stimulation at normotension were obtained under all conditions in the eye. In the hypertensive state the CBF reduction during sympathetic stimulation was moderate, but highly significant in hypercapnic or papaverine-treated animals as well as in controls. Leakage of Evans blue was more frequently seen on the nonstimulated side of the brain. In the eye there was leakage only on the control side except in PGI2-treated animals where 2 rabbits had bilateral leakage. The effect of sympathetic stimulation on the blood flow in the cerebrum and cerebellum in vasodilated animals seems to be small or absent if the blood pressure is normal. In the eye pronounced vasoconstriction occurs under these conditions. In acute arterial hypertension sympathetic stimulation protects both the cerebral and ocular barriers even under conditions of marked vasodilation.

Cerebral and ocular blood flow during alpha 2-blockade: evidence for a modulated sympathetic response

The effect upon regional blood flows of alpha 2-blockade by yohimbine was investigated in albino rabbits before and during unilateral cervical sympathetic stimulation. The combined effects of i.v. angiotensin, alpha 2-blockade and sympathetic stimulation were also investigated. Regional blood flows were determined using the labelled microsphere method. Yohimbine produced high flow rates in the eyes, the dura, the choroid plexus and in the peripheral organs. Sympathetic stimulation during alpha 2-blockade caused significant blood flow reductions in the brain, the eye and in all other tissues investigated. In the brain the flow reduction was 20-25% in several structures. Angiotensin caused marked reductions in blood flow in the dura and in most peripheral tissues. The effects of sympathetic stimulation were not appreciably changed. The results suggest that vascular alpha 2-receptors affect the vascular tone in some organs and that presynaptic alpha 2-receptors modulate the response to sympathetic stimulation in the brain, the optic nerve and in the choroid plexus.

Effects of facial nerve section and stimulation on cerebral and ocular blood flow in hemorrhagic hypotension

Albino rabbits were anesthetized and artificially ventilated. In two groups of animals the facial nerve on one side was sectioned and/or electrically stimulated at the internal acoustic pore. Hemorrhagic hypotension was induced to reveal a possible parasympathetic vasodilator mechanism, normally masked. The labelled microsphere method was used for flow determination. Section of the facial nerve did not produce any difference between sectioned and intact side, concerning cerebral, ocular and mandibular gland blood flows at normal or low blood pressures. Stimulation of the facial nerve at arterial hypotension produced significant ipsilateral increases in the choroidal and the mandibular gland blood flows. Regional and total cerebral blood flow remained unaffected. The results indicate no or only minimal contribution of the facial nerve to the cerebral vascular tone under conditions of general anesthesia. Further evidence is given in this study for a vasodilator pathway to the eye via the facial nerve, but the resting vasodilator tone under general anesthesia seems to be very low at normotensive as well as hypotensive states.

Cerebral circulation in acute arterial hypertension--protective effects of sympathetic nervous activity

The cervical sympathetic chain was stimulated electrically at 6 or 3 Hz on one side in anesthetized cats. Acute arterial hypertension was induced by ligation of the aorta. Evans blue was given as tracer for protein leakage. The regional blood flow in the brain was determined by using labelled microspheres. At high blood pressures there was a multifocal breakdown of the blood-brain barrier. The regions with breakdown had 10-20 times the normal flow rates. With a maintained hypertension regions which were overperfused at 5 min were still overperfused at 10 min, but there was little addition of new overperfused areas. Normalization of the pressure resulted in almost twice the normal flow rates in previously overperfused regions. The breakdown of the blood-brain barrier was restricted to the non-stimulated side, or more marked on that side. The protective effect of the sympathetic stimulation lasted more than 10 min. The results indicate that acute arterial hypertension tends to cause forced and long-lasting vasodilation in some areas in the brain but regions which are resistant to the acute rise have an increase in the vascular tone. Sympathetic activity helps in developing this tone. Normalization of the blood pressure results in partial recovery of the vascular tone in previously overperfused regions and normalization in other areas.