Adrenoceptor subtypes involved in neurally evoked sympathetic vasoconstriction in the anterior choroid of cats

Real-Time Evaluation of Regional Arterial Stiffening, Resistance, and Ocular Circulation During Systemic Administration of Adrenaline in White Rabbits

Purpose

To evaluate continuous variations of ocular microcirculation by laser speckle flowgraphy and those of regional stiffening by pulse wave velocity (PWV) and vascular resistance under systemic adrenaline administration in rabbits.

Methods

Six 16-week-old male rabbits were evaluated. The mean blur rates in the retinal vessel (MBR-RV) and choroid (MBR-CH) were measured. We assessed blood pressure (BP), femoral and carotid vascular resistance, and the heart–ankle (ha)-PWV, heart–femoral (hf)-PWV, and femoral–ankle (fa)-PWV. Adrenaline (100, 300, and 1000 ng/kg) was intravenously administered over a 10-minute period during which the parameters were measured simultaneously every 2 minutes.

Results

The MBR-RV and MBR-CH values were dose-dependently increased by the adrenaline in parallel with increased BP. At the load of 100 ng/kg adrenaline, the ΔMBR-RV and ΔMBR-CH showed positive correlations with the variation rate in mean arterial blood pressure. Also, the variation rate in carotid vascular resistance and the Δfa-PWV and Δhf-PWV were significantly positively correlated with both the ΔMBR-RV and ΔMBR-CH. At the 300-ng/kg phase, the correlations between the Δha-PWV and both ΔMBR-RV and ΔMBR-CH were canceled; instead, the Δhf-PWV showed a significant negative correlation with the ΔMBR-RV and ΔMBR-CH. At the 1000-ng/kg phase, Δha-PWV again showed significant positive correlations with the ΔMBR-RV and ΔMBR-CH.

Conclusions

These results indicate the possibility that under a systemic administration of adrenaline in rabbits, not only the BP value but also the vascular resistance and arterial function are related to the variation in ocular microcirculation.

Translational Relevance

A real-time evaluation system of systemic regional arterial function and ocular microcirculation in rabbits was developed.

Choriocapillaris: Fundamentals and advancements

The choriocapillaris is the innermost structure of the choroid that directly nourishes the retinal pigment epithelium and photoreceptors. This article provides an overview of its hemovasculogenesis development to achieve its final architecture as a lobular vasculature, and also summarizes the current histological and molecular knowledge about choriocapillaris and its dysfunction. After describing the existing state-of-the-art tools to image the choriocapillaris, we report the findings in the choriocapillaris encountered in the most frequent retinochoroidal diseases including vascular diseases, inflammatory diseases, myopia, pachychoroid disease spectrum disorders, and glaucoma. The final section focuses on the development of imaging technology to optimize visualization of the choriocapillaris as well as current treatments of retinochoroidal disorders that specifically target the choriocapillaris. We conclude the article with pertinent unanswered questions and future directions in research for the choriocapillaris.

Microvascular contributions to age-related macular degeneration (AMD): from mechanisms of choriocapillaris aging to novel interventions

Aging of the microcirculatory network plays a central role in the pathogenesis of a wide range of age-related diseases, from heart failure to Alzheimer's disease. In the eye, changes in the choroid and choroidal microcirculation (choriocapillaris) also occur with age, and these changes can play a critical role in the pathogenesis of age-related macular degeneration (AMD). In order to develop novel treatments for amelioration of choriocapillaris aging and prevention of AMD, it is essential to understand the cellular and functional changes that occur in the choroid and choriocapillaris during aging. In this review, recent advances in in vivo analysis of choroidal structure and function in AMD patients and patients at risk for AMD are discussed. The pathophysiological roles of fundamental cellular and molecular mechanisms of aging including oxidative stress, mitochondrial dysfunction, and impaired resistance to molecular stressors in the choriocapillaris are also considered in terms of their contribution to the pathogenesis of AMD. The pathogenic roles of cardiovascular risk factors that exacerbate microvascular aging processes, such as smoking, hypertension, and obesity as they relate to AMD and choroid and choriocapillaris changes in patients with these cardiovascular risk factors, are also discussed. Finally, future directions and opportunities to develop novel interventions to prevent/delay AMD by targeting fundamental cellular and molecular aging processes are presented.

Defective Choroidal Blood Flow Baroregulation and Retinal Dysfunction and Pathology Following Sympathetic Denervation of Choroid

Purpose

We sought to determine if sympathetic denervation of choroid impairs choroidal blood flow (ChBF) regulation and harms retina.

Methods

Rats received bilateral superior cervical ganglionectomy (SCGx), which depleted choroid of sympathetic but not parasympathetic innervation. The flash-evoked scotopic ERG and visual acuity were measured 2 to 3 months after SCGx, and vasoconstrictive ChBF baroregulation during high systemic arterial blood pressure (ABP) induced by LNAME was assessed by laser Doppler flowmetry (LDF). Eyes were harvested for histologic evaluation.

Results

ChBF increased in parallel with ABP in SCGx rats over an ABP range of 90% to 140% of baseline ABP, while in sham rats ChBF remained stable and uncorrelated with ABP. ERG a- and b-wave latencies and amplitudes, and visual acuity were significantly reduced after SCGx. In SCGx retina, Müller cell GFAP immunolabeling was upregulated 2.5-fold, and Iba1+ microglia were increased 3-fold. Dopaminergic amacrine cell fibers in inner plexiform layer were reduced in SCGx rats, and photoreceptors were slightly depleted. Functional deficits and pathology were correlated with impairments in sympathetic regulation of ChBF.

Conclusions

These studies indicate that sympathetic denervation of choroid impairs ChBF baroregulation during elevated ABP, leading to choroidal overperfusion. This defect in ChBF regulation is associated with impaired retinal function and retinal pathology. As sympathetic ChBF baroregulatory defects have been observed in young individuals with complement factor H (CFH) polymorphisms associated with risk for AMD, our results suggest these defects may harm retina, perhaps contributing to AMD pathogenesis.

Neural control of choroidal blood flow

The choroid is richly innervated by parasympathetic, sympathetic and trigeminal sensory nerve fibers that regulate choroidal blood flow in birds and mammals, and presumably other vertebrate classes as well. The parasympathetic innervation has been shown to vasodilate and increase choroidal blood flow, the sympathetic input has been shown to vasoconstrict and decrease choroidal blood flow, and the sensory input has been shown to both convey pain and thermal information centrally and act locally to vasodilate and increase choroidal blood flow. As the choroid lies behind the retina and cannot respond readily to retinal metabolic signals, its innervation is important for adjustments in flow required by either retinal activity, by fluctuations in the systemic blood pressure driving choroidal perfusion, and possibly by retinal temperature. The former two appear to be mediated by the sympathetic and parasympathetic nervous systems, via central circuits responsive to retinal activity and systemic blood pressure, but adjustments for ocular perfusion pressure also appear to be influenced by local autoregulatory myogenic mechanisms. Adaptive choroidal responses to temperature may be mediated by trigeminal sensory fibers. Impairments in the neural control of choroidal blood flow occur with aging, and various ocular or systemic diseases such as glaucoma, age-related macular degeneration (AMD), hypertension, and diabetes, and may contribute to retinal pathology and dysfunction in these conditions, or in the case of AMD be a precondition. The present manuscript reviews findings in birds and mammals that contribute to the above-summarized understanding of the roles of the autonomic and sensory innervation of the choroid in controlling choroidal blood flow, and in the importance of such regulation for maintaining retinal health.

Different effect of serotonin on intracellular calcium ion dynamics in the smooth muscle cells between rat posterior ciliary artery and vorticose vein

5-hydroxytriptamine (5-HT: serotonin) is an important transmitter that causes vessel constriction, although few studies have examined the effect of 5-HT on venous smooth muscles. The intracellular Ca(2+) concentration ([Ca(2+)]i) plays an essential role in stimulus-response coupling in numerous tissue/cells including vascular smooth muscle cells. The present study was performed to examine whether differences between arteries and veins in the response to 5-HT can be detected under confocal microscope with respect to [Ca(2+)]i dynamics. In posterior ciliary arteries of rats, 5-HT induced a [Ca(2+)]i increase. The 5-HT-induced responses were caused by both Ca(2+) influx and mobilization. Agonist and antagonist experiments revealed that arterial smooth muscles possess 5-HT1a, 1b, 2 (Gprotein-coupled type) and 5-HT3 (ion channel type) receptors, and that 5-HT2 in particular plays a major role in these responses. For vorticose veins, the 5-HT-induced responses were also caused by both Ca(2+) influx and mobilization. However, the cAMP dependent pathway (5-HT4-7) was found to be significant in vasocontraction with respect to 5-HT in these vessels. Thus, Ca(2+) mobilization was induced by 5-HT2 and 5-HT4-7 in a vessel-dependent manner, whereas Ca(2+) influx universally was induced by 5-HT3. These results indicate that the posterior ciliary arteries and vorticose veins in the same tissue might differ greatly in their responses to stimulus.

The Effect of Systemic Tamsulosin Hydrochloride on Choroidal Thickness Measured by Enhanced Depth Imaging Spectral Domain Optical Coherence Tomography

BACKGROUND

To evaluate the effects of selective α1A-adrenoceptor antagonist tamsulosin hydrochloride on choroidal thickness using enhanced depth imaging spectral-domain optical coherence tomography (EDI-OCT).

METHODS

This is a prospective observational study including 29 eyes of 29 patients with newly diagnosed benign prostatic hyperplasia. Choroidal thickness and retrobulbar ocular blood flow measurements were performed at baseline and after 3 months of treatment. Results were analyzed by the masked observer.

RESULTS

The mean subfoveal choroidal thickness (275.8-291.9 µm) and thicknesses 750 µm nasal (257.9-270.4 µm) and 750 µm temporal (262.4-277.0 µm) to the fovea were significantly increased after 3 months of treatment (p < 0.001). No statistically significant change was found in retrobulbar ocular blood flow.

CONCLUSIONS

Tamsulosin causes a significant increase in EDI-OCT-based choroidal thickness measurements. This increase might be associated with choroidal vasodilation in consequence of blockade of sympathetic α1A-adrenoceptors, which is critical for the maintenance of vascular tone and resistance in the choroidal vascular architecture. This should be kept in mind when choroidal disease and its response to treatment are followed by EDI-OCT imaging.

Diurnal variation in choroidal thickness in relation to sex, axial length, and baseline choroidal thickness in healthy Korean subjects

PURPOSE

To investigate diurnal variations in choroidal thickness (CT) in relation to various factors in healthy Korean subjects using spectral-domain optical coherence tomography.

METHODS

A prospective study was conducted on 100 healthy volunteers who each underwent measurements of CT in the same subfoveal area using an enhanced depth imaging technique with spectral-domain optical coherence tomography at 8 AM, 11 AM, 2 PM, and 5 PM. Possible correlations between the diurnal variation of CT and other factors, such as sex, axial length (AL), baseline CT (8 AM), blood pressure, and intraocular pressure, were evaluated.

RESULTS

One hundred volunteers with a mean age of 30.1 years were scanned. A significant pattern of diurnal variation was observed, with a mean CT of 278.28 ± 91.78 μm at 8 AM, 271.57 ± 89.08 μm at 11 AM, 266.39 ± 86.18 μm at 2 PM, and 264.92 ± 87.10 μm at 5 PM in a right eye. Right eye and left eye had a similar trend of diurnal cycle. Based on sex, AL, and baseline CT, the pattern of diurnal variation was greater and the amplitude significantly was greater in men, AL ≤ 24 mm and baseline CT ≥ 300 μm (men vs. women: P = 0.048, AL ≤ 24 mm vs. AL > 24 mm: P = 0.036, baseline CT ≥ 300 μm vs. baseline CT < 200 μm: P = 0.002, baseline CT ≥ 300 μm vs. 200 μm ≤ baseline CT < 300 μm: P = 0.008). There were no significant correlations between the diurnal variation of CT and systolic blood pressure, diastolic blood pressure, or intraocular pressure.

CONCLUSION

In men as well as in those with a shorter AL and thick baseline CT, a greater pattern of diurnal variation with significantly greater amplitude was observed.

Intrinsic vasomotricity and adrenergic effects in a model of isolated rabbit eye

PURPOSE

We aimed to investigate the responsiveness of the ocular arteries to adrenergic drugs in a model of perfused isolated rabbit eye.

METHODS

Rabbit external ophthalmic arteries (n = 15) in a head-mounted preparation were cannulated and the retinal and uveal vasculature perfused at a constant flow with warmed tyrode. The three-way polypropylene catheter was further connected to a pressure transducer and intraluminal pressure was taken as a measure of vascular resistance. Effects of intra-arterial injections of phenylephrine (group A, n = 5), prazosin (group B, n = 5) and phentolamine (group C, n = 5) on the recorded pressure were obtained. Student's paired-t test and one-way analysis of variance were used for statistical analysis (p < 0.05).

RESULTS

Intrinsic vasomotricity was observed in all preparations prior to any drug administration. Phenylephrine produced an increase in total vascular resistance. Intrinsic vasomotricity became more evident, showing a lower frequency but higher amplitude of oscillations. Evoked vasomotor responses with phenylephrine (250 microg/ml) were inhibited by intra-arterial administration of the selective alpha(1)-adrenergic antagonist, prazosin (0.5 mg/ml), as well as the non-selective alpha-adrenergic antagonist phentolamine (6 mg/ml).

CONCLUSIONS

Rabbit external ophthalmic arteries showed spontaneous contractions under constant perfusion. Phenylephrine elicited a vasoconstrictor response that was inhibited by adrenergic antagonists. In addition, the intrinsic vasomotricity was enhanced by phenylephrine and blocked by adrenergic antagonists. These results show that under in vitro perfusion the territory presents similar responses to adrenergic drugs to those observed in in vivo models and also provides evidence of myogenic autoregulatory properties in the rabbit ophthalmic artery and/or choroid.

Intravenous administration of clonidine reduces intraocular pressure and alters ocular blood flow

AIM

To evaluate the effect of intravenously administered clonidine on ocular blood flow in healthy volunteers.

METHODS

A randomised, double-masked, placebo-controlled, two-way crossover study was performed in 12 healthy young volunteers. Clonidine (0.2 microg/kg/min) or placebo was administered intravenously over 10 minutes. The effects of clonidine were studied at baseline and up to 150 minutes after infusion. Ocular haemodynamics were measured using laser Doppler flowmetry, laser Doppler velocimetry and a retinal vessel analyser.

RESULTS

Clonidine significantly decreased mean arterial pressure (MAP) and intraocular pressure (IOP). Calculated ocular perfusion pressure decreased significantly by -8.7+/-8.7% after infusion of clonidine (p<0.01 vs placebo). Retinal arterial diameters increased by +4.4+/-2.7% (p = 0.012 vs placebo), whereas no significant change was observed in retinal veins. Red blood cell velocity decreased by -16+/-14% (p<0.01 vs placebo) after infusion of clonidine. Hence, calculated retinal blood flow decreased by -14+/-12% (p = 0.033 vs placebo). Choroidal blood flow increased by +18+/-19% (p<0.01 vs placebo) and optic nerve head blood flow increased by +16+/-23% (p = 0.046 vs placebo) 30 minutes after administration of clonidine but both returned to baseline thereafter.

CONCLUSION

The short-time increase in choroidal and optic nerve head blood flow indicates a transient vasodilatory effect of clonidine due to an unknown mechanism. The decrease in retinal blood flow indicates clonidine-induced vasoconstriction in the retinal microvasculature.

Sensory but not parasympathetic nerves are required for ocular vascular remodeling following chronic sympathectomy in rat

Choroidal vascularity increases following chronic sympathetic denervation in rats. The mechanisms of this remodeling are unclear. Since both nitric oxide and substance P/CGRP have been suggested as angiogenic factors in other targets, we hypothesized that sensory or parasympathetic nerves may also participate in ocular vascular remodeling. To test this hypothesis, sympathetic denervation was accomplished by superior cervical ganglionectomy. Sensory denervation was induced by subcutaneous injections of capsaicin on postnatal days 2 and 9, and ocular parasympathetic innervation was ablated by pterygopalatine ganglion excision on postnatal day 60. Eyes were processed and sectioned for light microscopic histomorphometry. Sympathetic denervation for 6 weeks resulted in increased choroidal thickness, vascular luminal area, numbers of large venules and large arterioles, and capillaries in the outer nuclear layer. Capsaicin pretreatment prevented sympathectomy-induced increases in choroidal thickness, vascular luminal area and large venules and large arterioles, whereas pterygopalatine ganglionectomy was without effect. Both sensory and parasympathetic denervation attenuated increases in outer nuclear layer capillaries. These studies indicate that increased choroidal vascularity noted after chronic sympathectomy requires intact sensory innervation.

Nerve growth factor regulates human choroidal, but not retinal, endothelial cell migration and proliferation

We have previously shown that sympathetic denervation results in significant blood vessel growth of the choroid and retina. The mechanism of this growth remains unclear. Since sympathetic denervation can result in increased nerve growth factor (NGF) levels, it was the goal of this study to determine if choroidal and retinal endothelial cells in culture would respond to nerve growth factor and if nerve growth factor promote endothelial cell migration and proliferation, two components of angiogenesis. Western blotting with phospho-specific antibodies, cell migration, and cell proliferation assays were employed to determine NGF effects on both choroidal and retinal cell growth. NGF treatment produced phosphorylation of TrkA in choroidal and retinal endothelial cells. NGF stimulation resulted in activation of ERK1/2, Akt, and Src in choroidal endothelial cells, while little phosphorylation was noted following NGF treatment in retinal endothelial cells. NGF increased choroidal endothelial cell migration by 50% over control and this was inhibited by pretreatment with LY294002 (PI3K inhibitor), Akt inhibitor, and MMP2/9 inhibitor. KT5823, PD98059, and PP2 did not affect choroidal cell migration. NGF also produced a 47% increase in choroidal endothelial cell proliferation, which was blocked by PP2, LY294002, Akt inhibitor, KT5823, and PD98059. NGF stimulation did not alter retinal endothelial cell migration or proliferation. Thus, it appears that increased NGF levels that may be noted after sympathectomy are capable of producing some aspects of vascular remodeling via different signaling cascades in choroidal endothelial cells in culture.

Endocrine abnormalities in patients with central serous chorioretinopathy

PURPOSE

To investigate and to identify endocrine and metabolic abnormalities in patients with central serous chorioretinopathy (CSCR).

DESIGN

Observational case series.

PARTICIPANTS

Twenty-four patients with CSCR.

METHODS

Serum and urinary catecholamines, glucocorticoids, mineralocorticoids, serum testosterone, and thyroid-stimulating hormone (TSH) function were evaluated prospectively.

RESULTS

Fifty percent (12 of 24) of patients with active acute CSCR showed elevated 24-hour urine cortisol or tetrahydroaldosterone levels. Serum aldosterone levels were low in 7 of 24 (29.1%) patients. Single morning plasma catecholamine levels were elevated in 7 of 24 patients, although 24-hour urine metanephrines (catecholamine breakdown products) were normal. Serum testosterone and TSH levels were normal in nearly all (23 of 24) patients.

CONCLUSION

Many patients with acute CSCR have elevated 24-hour urine corticosteroids, which may contribute to the pathogenesis of the disorder. Endogenous mineralocorticoid dysfunction is a newly described feature of CSCR.

Role of adrenergic receptors in vascular remodelling of the rat choroid

1. Choroidal blood vessels, located between the sclera and retina, constitute the principle source of blood flow to ocular structures. The choroid is innervated by vasoconstrictor sympathetic and vasodilator parasympathetic nerves. 2. We have shown previously that sympathetic denervation for 6 weeks leads to significant increases in choroidal thickness, percentage of choroid occupied by vascular lumina, and numbers of choroidal venules, large arterioles and outer retinal capillaries. Sympathetic deafferentation produces similar increases, indicating that loss of sympathetic nerve activity is responsible for increased vascularity after sympathectomy. Thus, sympathetic neurotransmission normally may be important in suppressing vascular proliferation in the adult rodent eye. 3. The aim of the present study was to determine whether sympathetic nerves act by way of adrenergic receptors to maintain normal choroidal vascular integrity. 4. The alpha-adrenoceptor antagonist, phentolamine (1 mg kg(-1) day(-1)), the beta-receptor antagonist, propranolol (1 mg kg(-1) day(-1)), or saline vehicle was infused for 3 weeks using subcutaneously implanted osmotic minipumps. 5. In phentolamine treated rats, no significant changes were noted relative to saline infused controls. However, propranolol treatment resulted in increases in choroidal thickness, vascular luminal area, and numbers of large choroidal venules and both small and large arterioles, approximating the remodelling seen after chronic sympathectomy. 6. We conclude that sympathetic nerves play a role in maintaining normal choroidal vascular architecture through actions mediated primarily by beta-adrenoceptors.

Increased ocular blood vessel numbers and sizes following chronic sympathectomy in rat

Disease states characterized by ocular vascular pathology are often associated with impaired sympathetic function. This study examined the effect of sympathetic denervation on ocular vasculature of the adult rat. Uveal perfusion and choroidal and retinal blood vessel sizes and numbers were assessed in rats with intact innervation and after short- (2 days) or long-term (6 weeks) sympathetic denervation induced by ipsilateral superior cervical ganglion excision. In rats with intact innervation and after short-term sympathectomy, blood flow in both eyes was comparable. However, after long-term sympathectomy, blood flow was four-fold greater in the denervated than in the innervated eye, but was unaltered in lacrimal gland, cerebral cortex, and masseter muscle. Choroid surface area was not affected by long-term sympathectomy, but choroidal thickness was increased and choroidal cross-sectional area occupied by vascular lumina was greater. Arteriolar number per unit cross-sectional area of choroid was not altered although arteriolar diameters were enlarged. Choroidal venules were larger and more abundant. Choroidal capillary numbers were unchanged, but retinal capillaries of the outer plexiform layer were increased. To determine if these changes result from loss of sympathetic activity, sympathetic preganglionic innervation was excised chronically. This produced significant increases in choroidal thickness and vascular luminal area, and in numbers of arterioles, small venules, and capillaries in the outer plexiform layer. These findings show that sympathetic innervation is critical in regulating choroidal and retinal vascularity, and that chronic loss of sympathetic activity may contribute to abnormal vascular proliferation in diseases such as age-related macular degeneration and diabetic retinopathy.

Differential neural activation of vascular alpha-adrenoceptors in oral tissues of cats

The aim of this study was to determine the relative contribution of alpha(1)- and alpha(2)-adrenoceptors involved in sympathetic-evoked vasoconstrictor responses in tissues perfused by the lingual arterial circulation in pentobarbital anesthetized cats. Blood flow in the lingual artery was measured by ultrasonic flowmetry. Laser-Doppler flowmetry was utilized to measure oral tissue vasoconstrictor responses in the maxillary gingiva and from the surface of the tongue. Electrical stimulation of the preganglionic superior cervical sympathetic nerve resulted in frequency-dependent blood flow decreases at all three sites. These responses were stable over time and were uniformly antagonized by administration of phentolamine (0.3 - 3.0 mg kg(-1)). The selective alpha(1)-adrenoceptor antagonist, prazosin (10 - 300 microg kg(-1)), attenuated vasoconstriction in the lingual artery and gingiva, but was ineffective in blocking vasoconstriction in the tongue. Subsequent administration of rauwolscine (300 microg kg(-1)) antagonized remaining vasoconstrictor responses. In contrast, rauwolscine (10 - 300 microg kg(-1)), given alone, blocked evoked vasoconstriction in the tongue, and was without effect on gingival or lingual artery vasoconstrictor responses. Subsequent administration of prazosin (300 microg kg(-1)) largely antagonized remaining neurally elicited responses. These results suggest that neural vasoconstrictor responses in some regional vascular beds in the cat oral cavity are mediated by both alpha(1)- and alpha(2)-adrenoceptors. In contrast, tongue surface vasoconstrictor responses to sympathetic nerve activation appear to be mediated primarily by alpha(2)-adrenoceptors.

Effects of sympathetic nerve stimulation on long posterior ciliary artery blood flow in cats

A new technique using ultrasonic flowmetry was developed in order to directly measure blood flow in the long posterior ciliary artery (LPCA) of anesthetized cats. Basal LPCA blood flow averaged about 0.6 ml/min and was stable over the experimental period. Electrical stimulation of the cervical preganglionic cervical sympathetic nerve produced frequency-dependent anterior segment ocular vasoconstrictor responses. Ipsilateral nictitating membrane contractions were simultaneously measured as a well-established index of neural sympathetic activation. LPCA frequency-response relationships were shifted to the right in comparison with those for the nictitating membrane. When elicited at two min intervals, submaximal evoked responses of both systems were stable for more than 90 min. Ocular vasoconstrictor and nictitating membrane responses were blocked in a dose-dependent fashion by intravenous treatment with the non-selective a-adrenoceptor antagonist, phentolamine (0.3-3.0 mg/kg), as well as with the selective alpha1-adrenoceptor antagonist, prazosin (3-30 microg/kg). In contrast, neither evoked response was further antagonized by subsequent administration of the alpha2-adrenoceptor antagonist, yohimbine (500 microg/kg). These results demonstrate the usefulness of ultrasonic flowmetry to study mechanisms controlling ocular anterior segment circulation and suggest that, as previously established for the nictitating membrane and anterior choroid, adrenergic neurogenic vasoconstriction in tissues perfused by the LPCA is mediated predominantly by alpha1-adrenoceptors.

Modulation of parasympathetic neuron phenotype and function by sympathetic innervation

Selective sympathetic nerve dysfunction occurs during aging and in certain disease states. Here, we review findings concerning the effects of chronic sympathetic denervation on parasympathetic innervation to orbital target tissues in the adult rat. Long-term sympathetic denervation was induced by excising the ipsilateral superior cervical ganglion for 5-6 weeks prior to analyses. Following sympathectomy, pterygopalatine ganglion parasympathetic neurons show reduced nitric oxide synthase protein in their somata and projections to vascular targets. Laser Doppler measurements of ocular blood flow indicate that sympathectomy is also accompanied by reduced nitrergic vasodilatation. In the superior tarsal muscle of the eyelid, parasympathetic varicosities, normally, are distant to smooth muscle cells but make axo-axonal contacts with sympathetic nerves, consistent with physiological evidence showing only prejunctional inhibitory effects on sympathetically mediated smooth muscle contraction. Following sympathectomy, parasympathetic varicosities proliferate and closely appose smooth muscle cells, and this is accompanied by establishment of parasympathetic-smooth muscle excitatory neurotransmission. Many pterygopalatine parasympathetic neurons normally contain nerve growth factor (NGF) protein and express NGF mRNA. However, following chronic sympathectomy or elimination of sympathetic impulse activity, NGF mRNA and protein are markedly reduced, indicating that sympathetic neurotransmission enhances NGF expression in parasympathetic neurons. Together, these findings portray a striking dependency of parasympathetic neurons on sympathetic nerves to maintain normal phenotype and function. Sympathetic influences on parasympathetic neurons may be mediated, in part, through axo-axonal synapses. NGF synthesis and release by parasympathetic neurons may represent a molecular basis underlying the formation of these synapses, and up-regulation of NGF synthesis by sympathetic nerve activity may act to reinforce these associations.

Adrenergic supersensitivity of rabbit choroidal blood vessels after sympathetic denervation

PURPOSE

To demonstrate the phenomenon of adrenergic denervation supersensitivity in rabbit choroidal blood vessels after superior cervical ganglionectomy.

METHODS

Twenty four albino rabbits of both sexes weighing 2-3 kg were randomly separated into two groups. Twelve rabbits received bilateral superior cervical sympathectomy 2 weeks prior to the study (group s). The other 12 rabbits served as controls (group n). Four different concentrations of 0.1 ml phenylephrine, 0.05%, 0.025%, 0.013%, and 0.007% were slowly injected into the vitreous body near the retinal surface in group (n) and (s) rabbits (n = 6 in each group). The choroidal blood flow (PF), blood volume (CMBC), and velocity (V) were measured simultaneously by laser Doppler flowmetry (Perimed, PF 4001).

RESULTS

The PF showed similar decreases in group (n) and (s) rabbits after injection of 0.05%, 0.025%, and 0.013% phenylephrine. With 0.007% phenylephrine, the PF remained unchanged in group (n) rabbits, but decreased significantly in group (s) rabbits (p = 0.0007). Velocity decreased similarly in both group (n) and (s) rabbits except for the 0.007% phenylephrine, concentration in which velocity decreased significantly in group s rabbits (p = 0.0001). There was no statistical difference in CMBC between group n and s rabbits at any of the test concentrations.

CONCLUSIONS

The difference in PF decrease between group (n) and (s) rabbits with 0.007% phenylephrine demonstrated the existence of choroidal blood vessel denervation supersensitivity. The decrease in PF was achieved mainly through a decrease in blood cell velocity.

Regional regulation of choroidal blood flow by autonomic innervation in the rat

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 alpha-noradrenergic vasoconstriction occurs throughout the choroid, whereas parasympathetic nitrergic vasodilation plays a selective role in modulating blood flow in anterior tissues of the eye.

Characterization of alpha(1)-adrenoceptor subtypes in the eye

The presence of the alpha(1)-adrenoceptor subtypes in various parts of the pig and rabbit eyes was investigated using [(3)H]-prazosin radioligand binding. The characterization of the subtypes was achieved by performing competition experiments with various subtype selective drugs. In the pig retina, both alpha(1A)- and alpha(1B)-adrenoceptors were detected and the proportion of sites was 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors, respectively. In the pig iris, ciliary body and choroid, which are melanin-rich tissues, the non-specific binding of [(3)H]-prazosin was too high to detect any of the alpha(1)-adrenoceptor subtypes. However, in the albino rabbit iris, ciliary body and retina both alpha(1A)- and alpha(1B)-adrenoceptors were detected. The proportion of sites in the iris was 60 % alpha(1A)- and 40% alpha(1B)-adrenoceptors, respectively. In the ciliary body and rabbit retina the proportion of sites were 70% alpha(1A)- and 30% alpha(1B)-adrenoceptors. Only the alpha(1A)-adrenoceptor subtype was detected in the rabbit choroid.

Sympathetic vasodilation in the rat anterior choroid mediated by beta(1)-adrenoceptors

Electrical stimulation of the preganglionic superior cervical nerve produced a frequency-dependent vasoconstrictor response in the anterior choroidal blood vessels of the eye of anesthetized rats. Systemic administration of phentolamine (5 mg kg(-1)) reversed the vasoconstriction to a vasodilator response. This sympathetic-evoked vasodilation was not antagonized by inhibition of nitric oxide synthase with N(G)-nitro-L-arginine methyl ester (L-NAME) (20 mg kg(-1)) or by inhibition of cyclo-oxygenase with indomethacin (20 mg kg(-1)). Intravenous administration of propranolol (1 mg kg(-1)), as well as selective beta(1)-adrenoceptor antagonists atenolol (3 mg kg(-1)), timolol (0.3 mg kg(-1)), and betaxolol (0.1 mg kg(-1)), totally abolished the sympathetic nerve evoked ocular vasodilation. In contrast, the selective beta(2)-adrenoceptor antagonist, ICI-118, 551 ((+/-)-1-[2, 3-(Dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2- butanol) (0.3 mg kg(-1), i.v.), was without effect. These results support the conclusion that the residual sympathetic ocular vasodilation observed in the rat anterior choroid after alpha-adrenoceptor blockade is mediated exclusively by neurogenic release of norepinephrine acting on vascular beta(1)-adrenoceptors.

Sympathetic vasoconstriction in the rat anterior choroid is mediated by alpha1-adrenoceptors

These experiments were undertaken in an attempt to use laser-Doppler flowmetry to measure anterior choroidal blood flow in the anesthetized rat and to study the mechanism by which sympathetic nerve stimulation might produce vasoconstriction in this vascular bed. Electrical stimulation of the preganglionic cervical sympathetic nerve produced reproducible, frequency-related ocular vasoconstrictor responses with maximal vasoconstriction seen at about 32 Hz. Ocular vasoconstrictor responses were blocked by intravenous treatment with the nonselective alpha-adrenoceptor antagonists, phentolamine (5 mg kg(-1)) and phenoxybenzamine (2 mg kg(-1)), as well as with the selective alpha1-adrenoceptor antagonist, prazosin (0.3 mg kg(-1)). In contrast, the selective alpha2-adrenoceptor blocker, rauwolscine (0.5 mg kg(-1)), only potentiated the vasoconstriction. Neither intravenous atropine (1 mg kg(-1)) nor propranolol (1 mg kg(-1)) altered the magnitude of neurally evoked vasoconstriction. These results demonstrate the usefulness of laser-Doppler flowmetry in studies of the rat anterior choroidal circulation and suggest that adrenergic neurogenic vasoconstriction in the anterior segment of the rat eye is mediated almost exclusively by alpha1-adrenoceptor mechanisms.

Effects of timolol and betaxolol on choroidal blood flow in the rabbit

This study evaluated the effects of the topical beta1-adrenergic antagonist betaxolol and the non-selective beta-adrenergic antagonist timolol on the choroidal pressure-flow relationship. Pentobarbital-anesthetized rabbits were instrumented with hydraulic occluders on the aorta and inferior vena cava to control MAP, an ear artery cannula to measure mean arterial pressure (MAP), and two vitreous cannulas to control and measure intraocular pressure (IOP). Choroidal blood flow was measured by laser Doppler flowmetry with the fiber-optic probe tip positioned over the posterior pole. Choroidal pressure-flow curves were obtained before and 30 min after topical application of 0.1 ml of betaxolol (Betoptic, 0.5%, n=10), timolol (Timoptic, 0.5%, n=10) or saline (n=8) by varying the MAP without controlling the IOP and by raising IOP while holding the MAP constant at 70 mmHg. The IOP was significantly reduced by betaxolol and timolol but not by saline. MAP was also slightly, but significantly, reduced after betaxolol but not after timolol or saline. However, the systemic hypotensive response to isoproterenol (8 microgram/kg, i.v.) was blunted after betaxolol and timolol indicating appreciable systemic absorption of both drugs. Timolol, but not betaxolol or saline, caused a significant, small increase in baseline choroidal vascular resistance. Timolol also attenuated the IOP response to MAP; however, none of the treatments had a significant effect on the choroidal pressure-flow relationship. We conclude that both drugs reach the systemic circulation after topical application, but neither betaxolol nor timolol alter the choroidal response to acute changes in perfusion pressure.

Role of nitric oxide in post-ischemic cerebral hyperemia in anesthetized rats

This study was undertaken to determine the extent to which nitric oxide (NO) mechanisms are involved in cerebral hyperemia following global brain ischemia. The vertebral arteries were cauterized through the first alar foramina in anesthetized male Sprague-Dawley rats and followed by 20-min occlusion of the common carotid arteries. Blood flow from the parietal cerebral cortex was measured using laser-Doppler flowmetry. In saline-treated animals, carotid occlusion reduced cerebral blood flow by approximately 95% with a maximal hyperemia of about 400% observed after 15 min of reperfusion. Pre-treatment with the nonspecific NO synthase inhibitor, L-NAME (NG-nitro-L-arginine methyl ester; 2, 10 and 50 mg kg(-1)), produced dose-related depression of post-ischemic hyperemia, whereas D-NAME (10 mg kg(-1)) was inactive. Pre-treatment with L-arginine (300 mg kg(-1), i.v.) prevented L-NAME attenuation of cerebral hyperemia. The selective neuronal NO synthase inhibitor, 7-nitroindazole (30 mg kg(-1)), was without significant depressant effect. These results suggest that NO (largely from vascular endothelium) is instrumental in development of post-ischemic cerebral hyperemia.

Central serous chorioretinopathy associated with inhaled or intranasal corticosteroids

OBJECTIVE

The purpose of the study is to investigate the relationship between inhaled or intranasal adrenergic agonists and corticosteroids and the development of central serous chorioretinopathy (CSC).

DESIGN

The medical records of three patients with CSC who were found to use inhaled adrenergic agents or corticosteroids or both were identified prospectively. A survey of members of the Retina, Macula, and Vitreous societies and the National Registry of Drug-Induced Ocular Side Effects identified three additional cases.

RESULTS

Six patients with CSC were found to be chronic users of corticosteroid (four patients) or both beta adrenergic agonist and corticosteroid (two patients) metered dose inhalers or nasal sprays. In three cases, there was a close temporal correlation between the use of a corticosteroid nasal spray and the development of CSC.

CONCLUSIONS

These findings suggest that, in patients who are susceptible, the periocular or systemic absorption of inhaled corticosteroids may be sufficient to produce CSC in humans, supporting previous hypotheses regarding the pathogenesis of the disorder. Further studies are needed to confirm this association and to determine whether inhaled adrenergic agents also contribute to the development of this disorder. Patients in whom CSC develops while using corticosteroid inhalers or nasal sprays should be alerted to the possible relationship between CSC and these agents.