Vol. 279, Issue 1, R202-R209, July 2000
Regional regulation of choroidal blood flow by autonomic
innervation in the rat
Jena J.
Steinle,
Dora
Krizsan-Agbas, and
Peter G.
Smith
Department of Molecular and Integrative Physiology and R. L. Smith Mental Retardation Research Center, University of Kansas
Medical Center, Kansas City, Kansas 66160-7401
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ABSTRACT |
Regional influences of parasympathetic and sympathetic
innervation on choroidal blood flow were investigated in anesthetized rats. Parasympathetic pterygopalatine neurons were activated by electrically stimulating the superior salivatory nucleus, whereas sympathetic neurons were activated by cervical sympathetic trunk stimulation and uveal blood flow was measured by laser Doppler flowmetry. Parasympathetic stimulation increased flux in the anterior choroid and nasal vortex veins but not in the posterior choroid. Vasodilation was blocked completely by the neuronal nitric oxide synthase inhibitor 1-(2-trifluoromethylphenyl)imidazole but was unaffected by atropine. Sympathetic stimulation decreased flux in all
regions, and this was blocked by prazosin. Parasympathetic stimulation
did not affect vasoconstrictor responses to sympathetic stimulation in
the posterior choroid but attenuated the decrease in blood flow through
the anterior choroid and vortex veins via a nitrergic mechanism. We
conclude that sympathetic 
-noradrenergic vasoconstriction occurs throughout the choroid, whereas
parasympathetic nitrergic vasodilation plays a selective role in
modulating blood flow in anterior tissues of the eye.
parasympathetic nervous system; sympathetic nervous system; laser
Doppler flowmetry; nitric oxide
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INTRODUCTION |
OCULAR BLOOD FLOW
MUST be closely regulated to maintain normal retinal function
(8). The choroidal blood vessels, which lie between the
outer retina and sclera, provide much of the retinal perfusion in
humans and other mammals. The choroid contains dense sympathetic
innervation originating in the ipsilateral superior cervical ganglion
(24). Stimulation of the cervical sympathetic trunk (CST),
which provides preganglionic innervation to the superior cervical
ganglion, diminishes choroidal blood flow predominantly by
1-adrenoreceptor activation (21). The
choroid also receives parasympathetic innervation from the ipsilateral
pterygopalatine ganglion (30), which facial nerve
stimulation studies suggest is vasodilatory (35).
Therefore, the choroid contains sympathetic vasoconstrictor and
parasympathetic vasodilator nerves whose activity and interactions
apparently determine the level of choroidal perfusion.
Although autonomic innervation may be important in regulating choroidal
blood flow, it is not known whether sympathetic and parasympathetic
nerves determine total choroidal perfusion through interactions on
common vascular beds or whether different types of innervation
predominate in different regions. Questions also remain regarding the
extent to which parasympathetic nerves mediate the facial nerve
vasodilatory response and the identity of vasodilatory neurotransmitters. In the present study, we used laser Doppler flowmetry, which offers a relatively high degree of spatial resolution, to determine whether responses to autonomic nerve stimulation vary
within different regions of the choroid. We have combined this
methodology with discrete stimulation of parasympathetic and
sympathetic innervation and selective pharmacological blockade to
assess the regional regulation of choroidal blood flow by autonomic innervation in the rat.
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MATERIALS AND METHODS |
Studies were conducted on 25 adult female Sprague-Dawley rats
(Harlan) aged ~60-80 days and weighing 180-240 g. Rats were anesthetized with urethan (1.25 g/kg ip), rectal temperature was maintained at 36°C, and a femoral artery and vein were cannulated for
blood pressure recording (Statham; Costa Mesa, CA) and drug administration, respectively. The distal right facial nerve was exposed
and cut to eliminate movement of the facial musculature. The CST was
exposed through a ventral midline incision in the neck, and a cuff
electrode (Roboz Microprobe; Rockville, MD) was placed around the
intact nerve just caudal to the superior cervical ganglion for
stimulation. The wires were externalized, the incision was closed with
silk suture, and the rat was placed in a stereotaxic frame (Stoelting;
Wood Dale, IL). Surgical procedures and all subsequent experimental
manipulations were approved by the Institutional Animal Care and Use
Committee of the University of Kansas Medical Center.
Parasympathetic and sympathetic stimulation.
A scalp incision was made over the sagittal suture, and a craniotomy
was performed. A semi-microbipolar concentric electrode (100 µm
contact diameter, Rhodes Medical Instruments; Woodland Hills, CA) was
positioned stereotaxically within the superior salivatory nucleus
[SSN, 9.5 mm posterior, 9.5 mm ventral, 2.5 mm lateral to bregma
(27)], as described previously (2,
22, 32). This nucleus provides the
preganglionic innervation to the parasympathetic pterygopalatine
ganglion (34). Parasympathetic innervation was activated
by electrical stimulation of the SSN at 12-20 Hz, 3 V for
40-60 s (Grass SD9 stimulator; Quincy, MA), which we have
shown previously to elicit maximal activation of orbital
parasympathetic innervation (2); this was confirmed for
blood flow in preliminary experiments. Electrode placement within the
preganglionic parasympathetic nucleus was confirmed by porphyrin
discharge from the harderian gland during stimulation (36)
and by electrolytic lesioning at the end of the experiment followed by
examination of cresyl violet-stained frozen sections to verify
electrode position within the SSN as described previously (2).
Ocular sympathetic innervation was stimulated using the cuff electrode
placed around the preganglionic CST. Stimulations were performed at
12-20 Hz, 30 V for 40-60 s, which is supramaximal for
sympathetic activation (2, 33), as confirmed
for blood flow changes in preliminary investigations in the present study.
Blood flow measurements.
Blood flow was measured using laser Doppler flowmetry (floLAB model
with MP3 probe, Moor Instruments; Devon, UK). This technique is based
on the concept that laser light reflected from a moving object, such as
a red blood cell, undergoes a Doppler frequency shift that is
determined by the relative concentration of blood cells and their
average velocity (28). Flux, which is directly proportional to blood flow (28), was recorded (time
constant of 0.5 s) on a computer using Polyview software
(Astro-Med, Grass Instruments). Previous studies using this technique
have shown linear flow measurements for vessels with diameters similar
to those from which measurements were obtained in the present study (17).
To assess blood flow in the anterior choroid, a flow probe (1 mm tip
diameter) was positioned extraocularly ~3 mm distal to the limbus, as
described by others (18). To obtain recordings from the
posterior choroid and the vortex veins, the pupil was dilated by
topical application of a 0.01% solution of epinephrine, the cornea was
incised, the fiberoptic probe was inserted using a micromanipulator
through the anterior and posterior chambers and the vitreous body, and
mineral oil was applied to prevent corneal drying. Because of technical
constraints, it was not possible to sample all intraocular regions.
Therefore, recordings were restricted to the nasal postequatorial
(hereafter referred to as nasal) hemisphere. At the end of the
experiment, background flux was recorded following circulatory arrest
induced by anesthetic overdose.
In preliminary experiments the relationship between flux and vascular
topography was explored. Flux characteristics were recorded from
different intraocular regions. Some posterior nasal regions were
consistently found to have high parasympathetically evoked flux levels
and were readily distinguished from all adjacent regions that showed no
consistent response to SSN stimulation. These sites were assessed
anatomically by two methods. Some rats were killed by anesthetic
overdose with the probe in situ, and the sclera was excised to expose
the recording site. In others, the recording site was determined either
by using a suture tie as a landmark or by advancing the probe to induce
local trauma and erythrocyte extravasation. The vasculature was then
visualized by a peroxidase reaction. Eyes were removed following
circulatory arrest and immersion fixed in 4% buffered formaldehyde for
2 h. After fixation, the anterior chamber was removed, and the
remainder of the eye was placed for 1 h in a 24-well plate with 1 mg/1 ml diaminobenzidine in 0.005% hydrogen peroxide solution (Sigma;
St. Louis, MO). The reaction was terminated by removing the tissue to
distilled water. Endogenous erythrocyte peroxidase activity resulted in
a brown precipitate, which defined the pattern of arterial and venous vessels. Both methods confirmed that recording sites showing
high-evoked flux levels corresponded to the vortex veins, an aggregate
of vessels with diameters of about 150-250 µm. In subsequent
physiological experiments, vortex venous recording sites could be
consistently identified based on their nasal location and markedly
increased flux during SSN stimulation.
Recordings from the posterior choroid were obtained by positioning the
probe posterior to the vortex veins. Aside from the vortex veins, all
other recording sites in the posterior choroid appeared to be highly
uniform with regard to basal flux, and none showed any appreciable
increase with SSN stimulation.
Drugs.
The following pharmacological agents were dissolved in distilled water
and administered through the femoral venous cannula: 1-(2-trifluoromethylphenyl)imidazole (TRIM, 40-60 mg/kg iv,
Research Biochemicals International; Natick, MA), a selective inhibitor of neuronal nitric oxide synthase (NOS) (14), with the
water vehicle slightly acidified to promote solubility; atropine methyl nitrate (0.5 mg/kg iv, Sigma Chemical; St. Louis, MO), a muscarinic receptor antagonist; prazosin (1 mg/kg iv, Sigma Chemical), an 1-adrenoreceptor antagonist. Choroidal blood
flow changes after drug administration were obtained 5-40 min postinjection.
Statistical analysis.
Responses to stimulation and drugs were compared statistically by
one-way or repeated-measures analysis of variance with post hoc
comparisons by the Student-Newman-Keuls method and by Student's t-test (SigmaStat 2.03, Jandel Scientific; Corte
Madera, CA). All values are presented as means ± SE, and
P < 0.05 was considered statistically significant.
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RESULTS |
Vortex veins.
Basal blood flux recorded from the nasal vortex veins showed 0.2- to
0.3-Hz oscillations of low amplitude (Fig.
1), which were not synchronous with
respiration and may reflect spontaneous uveal vasomotor activity
reported by others (29). During SSN stimulation, peak flux
increased one- to twofold (Figs. 1 and 2,
P = 0.015), reaching a maximum within 20 s that
was maintained until stimulation was stopped after 40-60 s. Flux
values returned to baseline levels in ~20-40 s. Although the
oscillation frequency did not change during stimulation, amplitude
increased substantially (Fig. 1). SSN stimulation did not alter
systemic blood pressure (Table 1).
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Fig. 1.
Laser Doppler flux recordings from the anterior choroid,
posterior choroid, and the nasal vortex veins during stimulation of
either the superior salivatory nucleus (SSN) or the cervical
sympathetic trunk (CST). All recordings were obtained from a single rat
with the exception of the anterior choroid. Responses obtained from the
vortex veins and anterior choroid during CST stimulation were similar
to that shown for the posterior choroid. Bar indicates period of SSN
stimulation (20 Hz, 3 V) or CST stimulation (12 Hz, 30 V).
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Fig. 2.
Effect of parasympathetic SSN stimulation on vortex
venous flux under control conditions and after the administration of
atropine (0.5 mg/kg) and 1-(2-trifluoromethylphenyl)imidazole (TRIM,
40-60 mg/kg). Data are displayed as means ± SE for a group
of 5 rats. A, P < 0.05 vs. prestimulation flux; B, not
significant vs. control; C, P < 0.01 vs. control and
atropine; D, not significant vs. prestimulation flux.
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Atropine did not influence basal flux or systemic blood pressure (Table
1), and the increase in flux during SSN stimulation was not affected
(Fig. 2). The selective neuronal NOS inhibitor TRIM caused a transient
decrease in systemic arterial blood pressure and choroidal flux, both
returning to baseline within 3 min (Table 1). TRIM blocked the response
to SSN stimulation (P = 0.004 vs. stimulation before
TRIM; not significant vs. prestimulation, Fig. 2).
In a separate group of rats, CST stimulation caused vortex venous flux
to decrease significantly without obvious effects on amplitude or
frequency of oscillations. CST stimulation decreased vortex venous flux
by ~60% (Fig. 3, P = 0.002). CST stimulation did not affect systemic arterial blood pressure
(Table 1). The -adrenoceptor antagonist prazosin significantly
attenuated the CST stimulation-induced decrease (Fig. 3,
P < 0.001 vs. stimulation before prazosin), and flux
during stimulation was not significantly different from prestimulation
basal flux. However, prazosin induced a marked decrease in systemic
arterial pressure (Table 1) with an accompanying decrease in basal
flux.
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Fig. 3.
Effect of CST stimulation on vortex venous flux under
control conditions and after the administration of prazosin (1.0 mg/kg). Data are displayed as means ± SE for 5 rats. A,
P = 0.002 vs. prestimulation flux; B, P < 0.001 vs. control; C, not significant vs. prestimulation flux.
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To assess interactions between parasympathetic and sympathetic
innervation on vortex venous flux, another group of rats was prepared
for SSN and CST costimulation. In these rats, either the SSN or CST was
stimulated for 20 s, after which stimulation of the other site
commenced in combination with the initial stimulus. Stimulations of
both sites were maintained for a total 60 s, with measurements
obtained 40 s after costimulation commenced. Maximal changes in
vortex venous flux were essentially identical to those obtained in
previous experiments in which one site or the other was stimulated
alone (Fig. 4). Irrespective of the order
of stimulation, activation of the second site attenuated the effect of
the first. SSN stimulation reduced the CST-induced decrease in flux to
11.5 ± 2.5% of prestimulation baseline, and CST stimulation
attenuated the SSN-induced increase in flux to 7.7 ± 5% of
baseline. Flux during sympathetic and parasympathetic costimulation was
not significantly different from prestimulation basal flux [Fig. 4,
not significant by paired t-test].
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Fig. 4.
Changes in vortex venous flux in response to stimulation
of the parasympathetic SSN alone (SSN stimulation), stimulation of the
CST (CST stimulation), and simultaneous stimulation of both
parasympathetic and sympathetic innervation (costimulation).
Venous flux responses during costimulation were assessed
following administration of TRIM (40-60 mg/kg, TRIM + costimulation), atropine (0.5 mg/kg, atropine + TRIM + costimulation), and prazosin (1.0 mg/kg, prazosin + atropine + TRIM + costimulation). Data are displayed as means ± SE
for 5 rats in all groups except costimulation, which was 6, and
included experiments using both stimulation sequences. A,
P < 0.005 vs. prestimulation; B, P < 0.001 vs. SSN or CST stimulation; C, not significant vs.
prestimulation;, D, P < 0.001 vs. costimulation; E,
not significant vs. CST stimulation; F, not significant vs. TRIM + costimulation; G, P < 0.001 vs. CST stimulation and
atropine + TRIM + costimulation.
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Costimulation after TRIM administration produced a decrease in flux
that was significantly greater than that obtained in the absence of
TRIM (P < 0.001) and was not significantly different from that during CST stimulation alone (Fig. 4). Administration of
atropine following TRIM did not elicit any further change (Fig. 4).
Prazosin, when given after TRIM and atropine, prevented any significant
change in flux (Fig. 4, P < 0.001 vs. CST stimulation prior to prazosin, not significantly different from prestimulation flux).
Posterior choroid.
In another group of rats prepared for costimulation with the flow probe
positioned to record from the posterior choroidal vasculature,
activation of the SSN alone resulted in no consistent effect on
posterior choroidal blood flux (Figs. 1 and
5, P < 0.005 vs. vortex
vein response). The absence of any apparent effect of SSN stimulation
persisted in all accessible regions of the posterior choroid with the
exception of those containing the vortex veins. In contrast, CST
stimulation decreased posterior choroidal flux in a fashion similar to
that observed for the vortex veins (Figs. 1 and 5). Costimulation
yielded flux changes that were comparable to those obtained from the
posterior choroid with CST stimulation alone (Fig. 5) and were
significantly less than flux changes recorded from the vortex veins
during costimulation (P < 0.001). Because
parasympathetic stimulation exerted no effects on flux, neither TRIM
nor atropine was administered. Prazosin abolished all effects of CST
stimulation (Fig. 5).
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Fig. 5.
Changes in posterior choroidal flux in response to SSN
stimulation, CST stimulation, and costimulation. Posterior flux
responses during costimulation were obtained after the administration
of prazosin (1.0 mg/kg, prazosin + costimulation). Data are
displayed as means ± SE, with SSN stimulation obtained from 6 rats, CST stimulation from 5 rats, and costimulation and prazosin from
3 rats. A, P < 0.001 vs. prestimulation; B,
P < 0.001 vs. SSN stimulation; C, P < 0.001 vs. costimulation and CST stimulation and not significant vs.
baseline flux.
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Anterior choroid.
In transscleral recordings of the anterior choroid, SSN stimulation
caused an increase in flux that was similar to that recorded from the
vortex veins and was significantly greater than posterior choroid flux
(Figs. 1 and 6, P = 0.010). CST stimulation elicited a decrease in flux that was comparable
in magnitude to those observed in both the vortex veins and the
posterior choroid (Fig. 6). However, flux values during costimulation
were significantly greater in the anterior choroid than in the
posterior choroid (P < 0.001, Fig. 6). Administration
of TRIM before costimulation resulted in a decrease in flux that was
comparable to that obtained from the vortex veins and was significantly
different from flux changes in response to costimulation alone
(P < 0.001). Atropine did not affect the response to
costimulation. Prazosin effectively blocked the remaining change in
flux (Fig. 6).
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Fig. 6.
Changes in anterior choroid flux in response to SSN
stimulation, CST stimulation, and costimulation. Anterior choroid flux
responses were measured after the administration of TRIM (40-60
mg/kg, TRIM + costimulation), atropine (0.5 mg/kg, atropine + TRIM + costimulation), and prazosin (1.0 mg/kg, prazosin + atropine + TRIM + costimulation). Data are displayed as
means ± SE for 4-5 rats in each treatment. A,
P < 0.005 vs. prestimulation; B, P < 0.001 vs. SSN or CST stimulation; C, not significant vs.
prestimulation; D, P < 0.001 vs. costimulation, E, not
significant vs. CST stimulation; F, not significant vs. TRIM + costimulation; G, P < 0.001 vs. CST stimulation and
atropine + TRIM + costimulation.
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DISCUSSION |
Autonomic regulation of choroidal perfusion.
Stimulation of parasympathetic preganglionic neurons of the SSN elicits
a pronounced increase in flux within the vortex veins. Because these
vessels represent the primary route of choroidal blood efflux, this
finding is consistent with increased overall blood flow throughout the
choroid and associated tissues. Furthermore, because this was not
accompanied by changes in systemic blood pressure, it is most
consistent with a decrease in vascular resistance due to vasodilation.
These findings are in agreement with previous studies showing increased
choroidal blood flow and microsphere accumulation during facial nerve
stimulation (35). Moreover, because increased flow was
elicited by selectively stimulating preganglionic parasympathetic
neurons, our findings provide strong evidence that vasodilation
accompanying facial nerve stimulation is mediated by parasympathetic
axons and not by other fiber populations [e.g., sensory, sympathetic
(23), or somatic motor] that also travel in this nerve.
We therefore conclude that a parasympathetic nerve pathway originating
in the SSN and with postganglionic fibers traveling in the facial nerve
is a potent modulator of uveal blood flow.
Although previous microsphere studies provide evidence of
parasympathetically mediated choroidal vasodilation, regional
influences were not examined. Unlike microsphere analyses, laser
Doppler flowmetry can readily measure flow in relatively discrete
volumes of tissue (~1.5 mm3) (1) and
therefore is well suited for assessing regional differences in changes
to perfusion. This technique provides clear evidence of major regional
differences in parasympathetic modulation of choroidal flow. Thus,
whereas the nasal vortex veins showed substantial flux increases during
parasympathetic stimulation, all other sampled regions of posterior
choroid (even as close as 1 mm to the vortex veins) showed little to no
change in perfusion during stimulation. In contrast, the anterior
choroid showed large increases in flux. We conclude that increased
vortex venous flux during parasympathetic stimulation is primarily due
to increased flow in anterior vasculature without detectable changes in
posterior choroidal flow. These findings therefore may imply that
different neural control mechanisms exist within different ocular regions.
These striking regional differences in parasympathetic regulation may
reflect differences in origin and function of the choroidal vasculature. Anterior and posterior ocular structures derive their primary blood supplies from different sources. In the rat, the posterior choroid is perfused by the short posterior ciliary and central retinal arteries, whereas the anterior uvea is perfused by the
long posterior and anterior ciliary arteries (6,
9). It is therefore probable that the arterioles supplied
by the long posterior and anterior ciliary arteries are strongly
influenced by pterygopalatine parasympathetic innervation, whereas
those of the short ciliary and central retinal artery vessels are
appreciably less affected. The absence of detectable parasympathetic
influences on posterior choroidal flow is somewhat surprising in light
of immunohistochemical studies showing NOS-immunoreactive nerves in the
posterior choroid of the rat (39) and pigeon
(4). Nonetheless, these fibers may simply be axons of
passage en route to more anterior targets, or the posterior choroidal
vascular smooth muscle may be relatively insensitive to released
parasympathetic neurotransmitters.
Functionally, these findings may provide insight as to the role of
parasympathetic innervation. The anterior tissue from which we recorded
likely contains some anteriorly projecting blood vessels from posterior
choroid but is probably dominated by branches from the major arterial
circle and pars plana venules that drain the more anterior tissues
(supplied by the long posterior and anterior ciliary arteries),
particularly the ciliary body and processes (10). These
structures are integrally involved in formation of aqueous humor.
Because aqueous humor formation is determined in part by the extent of
perfusion of the anterior uveal structures, increased blood flow via a
parasympathetic projection may provide a selective mechanism for
modulating perfusion of these tissues. Indeed, it is well established
that stimulation of the facial nerve increases intraocular pressure
(11), as does stimulation of diencephalic regions
(12) that project to the SSN [e.g., lateral hypothalamus
(34)]. These findings therefore are accordant with the
view that a parasympathetic pathway from the SSN via the
pterygopalatine ganglion plays an important role in regulating anterior
uveal perfusion.
Neuronal NOS mediates parasympathetic choroidal vasodilation.
Previous studies have shown that facial nerve vasodilation is atropine
resistant and therefore not likely to be cholinergic (3),
a finding confirmed in the present experiments. In light of the potent
vasodilatory effects of NO in other systems (13), this
neurotransmitter would be a logical candidate. Indeed, nonselective NOS
inhibitors decrease basal choroidal blood flow in pigs, dogs, and rats
(7, 15, 20) and attenuate the
increased flow during facial nerve stimulation in the rabbit
(26). However, because nonselective NOS inhibitors elevate
basal systemic blood pressure, thereby increasing choroidal perfusion,
it has been unclear whether other transmitters (e.g., vasoactive
intestinal polypeptide) may also participate in parasympathetically
mediated vasodilation (26). In the present study,
the highly selective neuronal NOS inhibitor TRIM (14) did
not affect systemic blood pressure or basal uveal flow, a finding
similar to that reported for another neuronal NOS antagonist, 7-nitro
indazole (7-NI) (20). However, TRIM completely
abolished effects of parasympathetic stimulation on anterior choroidal
and vortex venous blood flow, even at the relatively high stimulation
frequencies used in these studies. Although this is the first study to
document the involvement of NO in neurally evoked uveal vasodilation of
the rat, similar findings were obtained in the pigeon, where
vasodilation mediated by oculomotor nerve efferents was eliminated by
7-NI (40). However, the selectivity of this NOS inhibitor
has been questioned recently (5). In contrast to previous
studies using nonselective NOS inhibitors (19), TRIM in
the present study did not affect basal flux at doses that completely
blocked vasodilation during parasympathetic nerve stimulation. These
findings support the idea that parasympathetic nitrergic mechanisms do
not play an appreciable role in determining basal choroidal blood flow
but are responsible for choroidal vasodilation that accompanies
parasympathetic stimulation in the anesthetized rat.
Interactions between parasympathetic and sympathetic innervation in
the regulation of choroidal perfusion.
Together with previous studies, our findings show that sympathetic
innervation exerts potent 1-adrenoceptor-mediated
vasoconstriction in the anterior (18) and posterior
choroid, whereas parasympathetic vasodilation influences primarily
anterior uveal tissues. Because both sympathetic and parasympathetic
nerves regulate anterior perfusion, blood flow in this region should
reflect the summation of the interactions between these populations.
Recordings of flux from the vortex veins show that under normal
conditions, flow during sympathetic and parasympathetic costimulation is similar to or slightly below prestimulation levels. Because the
posterior choroid is markedly vasoconstricted during costimulation, vortex venous outflow may reflect a relative predominance of
parasympathetic vasodilation in the anterior choroid, and indeed flux
during costimulation is significantly greater in the anterior choroid
than in the posterior choroid. Therefore, whereas sympathetic
vasoconstriction is not detectably affected by parasympathetic
activation in the posterior choroid, it appears to be negated by
parasympathetic nerves in the anterior region.
Parasympathetic nerves can nullify sympathetic actions through two
potential mechanisms. Target cell activity may reflect equal or
preferential responsivity to parasympathetic transmitters or these
neuronal populations can interact prejunctionally, with parasympathetic
nerves inhibiting sympathetic transmitter release. This latter
mechanism is particularly important in many target systems.
Parasympathetic nerves inhibit sympathetic neurotransmitter release via
prejunctional muscarinic receptors in a variety of organ systems,
including the heart, vasculature, vas deferens, and periorbital smooth
muscle (2, 25, 37,
38). If this mechanism is important in regulating
choroidal perfusion, then parasympathetic stimulation during
sympathetic activation in the neuronal NOS-inhibited preparation should
still attenuate sympathetic vasoconstriction and this should be
countered by the administration of atropine. However, our findings from
anterior choroidal and vortex venous recording sites show that NOS
inhibition completely blocks the parasympathetic effects and that
atropine has no significant actions. Therefore, prejunctional
muscarinic inhibition of sympathetic neurotransmission by
parasympathetic nerves apparently is not an important mechanism in
regulating choroidal blood flow.
It is unclear why this mechanism, which is widespread in the
peripheral nervous system, is not operative in the choroid. One possibility is that sympathetic neurons that project to the choroid lack prejunctional muscarinic receptors. Although a high proportion (65-85%) of superior cervical ganglion neurons express
high-affinity muscarinic binding sites (16), we cannot
rule out the possibility that choroidal sympathetic nerves lack
muscarinic receptors. Alternatively, even though both parasympathetic
and sympathetic axons travel together within Schwann cells in some
parts of the choroid (31), their terminals may be
segregated, such that acetylcholine released by parasympathetic nerves
is unable to activate prejunctional muscarinic receptors. In any event,
it appears that autonomic projections to the choroid display a high
degree of functional segregation as revealed by their selective
regional distribution and absence of prejunctional muscarinic interactions.
Perspectives
This study provides evidence that regional differences exist in
the autonomic regulation of blood flow in the eye. Specifically, the
posterior choroid is dominated by sympathetic vasoconstrictive input,
whereas parasympathetic nitrergic vasodilatory nerves preferentially regulate flow in anterior tissues. Because many ocular disease states
are associated with regional disturbances in ocular perfusion, it is
interesting to speculate as to possible roles of autonomic innervation
in the etiology of these diseases. Moreover, manipulation of either
sympathetic or parasympathetic innervation may selectively impact
different tissues, depending on their autonomic profile. Indeed, it may
be predicted from these studies that vascular parasympathetic innervation plays an important role in regulating perfusion of the
anterior tissues, including those associated with aqueous humor
formation. It is particularly interesting therefore that NOS inhibitors
are proving effective in clinical trials at combating elevated
intraocular pressure associated with glaucoma. Whether other
therapeutic strategies can also take advantage of regional selectivity
of ocular autonomic effects remains to be determined.
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ACKNOWLEDGEMENTS |
This work was supported by the National Institute of Child Health
and Human Development Grant HD-33025, with core support from center
Grant HD-02528.
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FOOTNOTES |
Address for reprint requests and other correspondence:
P. G. Smith, Dept. of Molecular and Integrative Physiology, Univ.
of Kansas Medical Center, Kansas City, KS 66160-7401 (E-mail:
psmith{at}kumc.edu).
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Received 15 September 1999; accepted in final form 3 February 2000.
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