Effects of head down tilt on episcleral venous pressure in a rabbit model.

In humans, changing from upright to supine elicits an approximately 10 mmHg increase in cephalic venous pressure caused by the hydrostatic column effect, but episcleral venous pressure (EVP) and intraocular pressure (IOP) rise by only a few mmHg. The dissociation of the small increases in IOP and EVP compared to the larger increase in cephalic venous pressure suggests a regulatory mechanism controlling EVP. The aim of the present study was to determine if the rabbit model is suitable to study the effects of postural changes on EVP despite its short hydrostatic column. In anesthetized rabbits (n = 43), we measured arterial pressure (AP), IOP, and orbital venous pressure (OVP) by direct cannulation; carotid blood flow (BFcar) by transit time ultrasound, heart rate (HR) by digital cardiotachometer, and EVP with a servonull micropressure system. The goal of the protocol was to obtain measurement of supine EVP for ≈10 min, followed by ≈10 min of EVP measurement with the rabbit in a head down tilt. The data were analyzed by paired t-tests and the results reported as the mean ± standard error of the mean. In a separate group of animals (n = 35), aqueous flow was measured by fluorophotometry. This protocol entailed measurement of aqueous flow in the supine position for ≈60 min, followed by ≈60 min of aqueous flow measurement with the rabbit in a head down tilt. From supine to head down tilt, AP and BFcar were unchanged, IOP increased by 2.3 ± 0.4 mmHg (p < 0.001), EVP increased by 2.4 ± 0.4 mmHg (p < 0.001), OVP increased by 2.5 ± 0.2 mmHg (p < 0.001) and HR decreased by 9 ± 3 bpm (p = 0.002). Head down tilt caused no significant change in aqueous flow. Although the hydrostatic column in the rabbit is shorter than humans, the rabbit model permits sufficiently sensitive measurements of the pressures and systemic parameters likely involved in the EVP responses to posture change. The present results indicate directionally similar EVP and IOP responses to tilt as occur in humans and, as in humans, the responses are smaller than would be expected from the change in the hydrostatic column height. Also, as in humans, the model reveals no change in aqueous flow during head down tilt. We conclude the rabbit model is appropriate for studying the mechanisms responsible for the relative immunity of EVP and IOP to posture change.

A novel, microscope based, non-invasive laser Doppler flowmeter for choroidal blood flow assessment.

Impaired ocular blood flow is involved in the pathogenesis of numerous ocular diseases like glaucoma or AMD. The purpose of the present study was to introduce and validate a novel, microscope based, non-invasive Laser Doppler Flowmeter (NI-LDF) for measurement of blood flow in the choroid. The custom made NI-LDF was compared with a commercial fiber optic based laser Doppler flowmeter (Perimed PF4000). Linearity and stability of the NI-LDF were assessed in a silastic tubing model (i.d. 0.3 mm) at different flow rates (range 0.4-3 ml/h). In a rabbit model continuous choroidal blood flow measurements were performed with both instruments simultaneously. During blood flow measurements ocular perfusion pressure was changed by manipulations of intraocular pressure via intravitreal saline infusions. The NI-LDF measurement correlated linearly to intraluminal flow rates in the perfused tubing model (r = 0.99, p < 0.05) and remained stable during a 1 h measurement at a constant flow rate. Rabbit choroidal blood flow measured by the PF4000 and the NI-LDF linearly correlated with each other over the entire measurement range (r = 0.99, y = x∗1.01-12.35 P.U., p < 0.001). In conclusion, the NI-LDF provides valid, semi quantitative measurements of capillary blood flow in comparison to an established LDF instrument and is suitable for measurements at the posterior pole of the eye.

[Biological and physical aspects of intraocular pressure]. / Biologische und physikalische Aspekte des Augendrucks.

A thorough understanding of intraocular pressure homeostasis is the biological foundation for the development of new strategies to treat patients with elevated intraocular pressure or glaucoma. However, investigations on the physiology of intraocular pressure homeostasis are also important to gain more comprehensive insights into the pathogenesis of glaucoma and other diseases with associated alterations of intraocular pressure. The present review intends to give alternative insights into the biological and physical aspects of intraocular pressure regulation. The pressure-volume as well as the hydraulic model of intraocular pressure and also the relationship between ciliary blood flow and aqueous humor production, which has moved into the centre of interest because of its possible clinical relevance for glaucoma patients, will be explained. The authors Have attempted to interrelate the different aspects of intraocular pressure genesis and regulation in a comprehensive but understandable way.

Phase-sensitive neutron reflectometry measurements applied in the study of photovoltaic films.

Due to low charge carrier mobilities in polymer-based solar cells, device performance is dictated by the nanoscale morphology of the active layer components. However, their morphological details are notoriously difficult to distinguish due to the low electron contrast difference between the components. Phase-sensitive neutron reflectivity (PSNR) is uniquely suited to characterize these systems due to the large, natural scattering length density difference between two common device materials, poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Using PSNR we find a high concentration of PCBM at the substrate and near but not at the air interface. Herein we discuss the method of applying PSNR to polymer-based solar cells, the results obtained, and an evaluation of its effectiveness.

Autoregulation of choroidal blood flow in the rabbit.

Previous studies show that choroidal blood flow is not autoregulated when intraocular pressure (IOP) is increased to raise venous pressure and lower the perfusion pressure gradient. However, the autoregulatory response to changes in mean arterial pressure (MAP) is unclear. In the current study, the perfusion pressure gradient (MAP-IOP) was altered by (1) decreasing MAP while IOP was held at 5, 15, and 25 mmHg, and (2) increasing the IOP at the prevailing MAP in anesthetized rabbits (n = 8). An occluder on the thoracic vena cava was used to vary MAP; this was monitored through an ear artery catheter. Two catheters were inserted in the vitreous to monitor and control IOP. Choroidal blood flow was measured by laser Doppler flowmetry using a slender stainless-steel probe positioned next to the retinal surface. The efficacy of autoregulation depended on the IOP. When IOP was held constant at 5 mmHg, choroidal blood flow did not fall until the perfusion pressure gradient was less than 40 mmHg. The pressure-flow relationship became progressively more linear (ie, the efficacy of autoregulation decreased) when the IOP was held constant at 15 and 25 mmHg. When IOP was varied and MAP was held constant, the pressure-flow relationship was linear at IOPs greater than 20-25 mmHg. However, choroidal blood flow was pressure independent when the IOP was less than 20-25 mmHg. Simultations using a myogenic mathematic model of the choroid gave results similar to the experimental observations. It was concluded that a myogenic mechanism may be responsible for the autoregulation of choroidal blood flow in the rabbit.

Adrenergic modulation of choroidal blood flow in the rabbit.

PURPOSE: To determine whether the choroidal pressure-flow is altered by the alpha-adrenergic antagonist, phentolamine, or the beta-adrenergic antagonist, propranolol. METHODS: In two groups of pentobarbital- anesthetized rabbits, the choroidal pressure-flow relationships were determined by raising the intraocular pressure (IOP) at mean arterial pressure (MAP) of 50, 60, 70, and 80 mm Hg before and after phentolamine (0.5 mg/kg, intravenously, n=7) and propranolol (0.25 mg/kg intravenously, n=7) administration. Hydraulic occluders on the thoracic aorta and inferior vena cava were used to control MAP, which was measured in the central ear artery. The eye was cannulated with two 23-gauge needles, one to manipulate the ocular volume and the other to measure the IOP. Choroidal blood flow was measured by laser Doppler flowmetry with a probe positioned over the posterior retina. The protocol consisted of setting the MAP, then infusing saline into the eye at 30 microliters/minute until the IOP increased from baseline to 100 mm Hg. RESULTS: At the MAP of 70 mm Hg, alpha-adrenergic blockade caused an upward shift in the choroidal pressure-flow relationship; beta-blockade shifted the relationship downward. CONCLUSIONS: In the pentobarbital-anesthetized rabbit, alpha- and beta- adrenergic blockade cause choroidal vasodilation and vasoconstriction, respectively. Results indicate the presence of alpha- and beta-adrenergic receptors in the rabbit choroid and a tonic level of adrenergic vascular tone in this preparation.

Ocular perfusion pressure and choroidal blood flow in the rabbit.

PURPOSE: To compare choroidal blood pressure versus flow relationships obtained by three different methods of changing the ocular perfusion pressure. METHODS: Experiments were performed in pentobarbital-anesthetized rabbits with occluders on the aorta and inferior vena cava to control mean arterial pressure (MAP). The central ear artery was cannulated to measure MAP. Two 23-gauge needles were inserted through the pars plana into the vitreous: one connected to a saline-filled syringe to vary the ocular volume and the other to a pressure transducer to measure intraocular pressure (IOP). Choroidal perfusion was measured by laser-Doppler flowmetry with the probe in the vitreous over the posterior pole. In group 1 (n = 15), the MAP was varied while holding the IOP at 10, 15, 20, 25 and 30 mm Hg. In group 2 (n = 19), the IOP was increased while holding the MAP at 80, 70, 60, 50, 40, 30 and 20 mm Hg. In group 3 (n = 21), the MAP was varied without controlling the IOP. RESULTS: Group 1 baseline choroidal flows were similar at the five IOPs. When the flow was plotted against MAP, the curves diverged and extrapolated to intersect the pressure axis when the MAP equaled the set IOP. Group 2 baseline flows were similar at MAPs greater than 40 mm Hg. When the flow was plotted against the IOP, the curves diverged and intersected the pressure axis when the IOP equaled the MAP. In both groups, plotting the flow against the perfusion pressure (i.e., MAP minus IOP) collapsed the data points into single curves. Choroidal autoregulation occurred in all three groups; however, the low end of the autoregulatory perfusion pressure range was approximately 50 mm Hg in group 1, approximately 40 mm Hg in group 2, and approximately 30 mm Hg in group 3. CONCLUSIONS: The results show that the effective choroidal perfusion pressure gradient equals the MAP minus the IOP, and that choroidal autoregulation is most effective when the MAP varies and IOP is not controlled.

Effects of latanoprost and timolol-XE on hydrodynamics in the normal eye.

PURPOSE: To compare the effects of latanoprost and timolol-XE on ocular pressure and perfusion in healthy adults, with respect to episcleral venous pressure. METHODS: A double-masked, placebo-controlled crossover study of weeklong bedtime treatment with one drop of drug, with placebo contralaterally, followed by a 3-week washout and alternate-drug/contralateral-placebo repeat. Intraocular pressure was measured by applanation and by pneumotonometry, providing pulsatile ocular circulatory estimates. Measurements of episcleral venous pressure were obtained (Friberg method). RESULTS: Twenty subjects participated (five men, 15 women; mean age, 39 years (range, 21 to 55 years); mean baseline intraocular pressure, 13.4 mm Hg). A greater decrease in intraocular pressure was seen among these subjects the morning after initiating treatment with latanoprost (-2.0 mm Hg; P <.0001) than with timolol-XE (-0.9 mm Hg; P =.051) (latanoprost versus timolol DeltaP =.008). This ocular hypotensive effect remained significant that evening with latanoprost (-3.2 mm Hg; P <.0001) but not with timolol XE (-1.0 mm Hg; P =.2). By the morning of day 8, mean intraocular pressure remained 3.2 mm Hg below baseline with latanoprost and 2.3 mm Hg below baseline with timolol-XE (P <.0001 for both drugs). Neither drug altered episcleral venous pressure. Among a subgroup of nine subjects with comparable intraocular pressure reductions with the two drugs, latanoprost treatment was associated with a 16.7% increase in mean pulsatile ocular blood flow (P =.04) through the weeklong treatment interval, consistently higher than during timolol-XE treatment of the same subjects. CONCLUSION: Latanoprost caused an overnight decrease in intraocular pressure in normotensive normal eyes, and both drugs significantly reduced intraocular pressure within 1 week. Intraocular pressure remained higher than episcleral venous pressure throughout treatment with both drugs. Latanoprost was associated with enhanced pulsatile ocular perfusion not seen with timolol-XE treatment.

Effect of ocular perfusion pressure on retinal function in the rabbit.

The electroretinogram (ERG) vs perfusion pressure relation was studied in anesthetized rabbits during mechanically induced changes in mean arterial pressure (MAP). After 1 hr of dark adaptation, ERGs were obtained at perfusion pressures from 10 to 95 mmHg with each pressure held for 30-60 sec, and at three levels from 15 to 75 mmHg with each pressure held for 5 min. The slopes of the b-wave amplitude vs perfusion pressure data were not significantly different from zero during either the brief or longer manipulations of perfusion pressure. However, the b-wave was nearly extinguished 5 min after death. The results indicate that the rabbit ERG is maintained over a wide range of perfusion pressure.

Ciliary blood flow and aqueous humor production.

Aqueous humor production is a metabolically active process sustained by the delivery of oxygen and nutrients and removal of metabolic waste by the ciliary circulation. This article describes our investigations into the relationship between ciliary blood flow and aqueous humor production. The results presented indicate that there is a dynamic relationship between ciliary blood flow and aqueous humor production, with production being blood flow independent above a critical level of perfusion, and blood flow dependent below it. The results also show that the plateau portion of the relationship shifts up or down depending on the level of secretory stimulation or inhibition, and that oxygen is one critical factor provided by ciliary blood flow. Also presented is a theoretical model of ocular hydrodynamics incorporating these new findings.

ROP surgery and ocular circulation.

PURPOSE: Visual results following vitreoretinal surgery for stages 4 and 5 retinopathy of prematurity are often disappointing, even when anatomic results are good. This poses the question whether the surgery or the post-operative care causes the optic atrophy. A hypothesis is proposed that ocular perfusion pressure (mean blood pressure minus intraocular pressure) during or after surgery may be too low to provide adequate ocular blood flow. METHODS: This report analyses the published results of retinopathy of prematurity surgery, the techniques used, as well as data about blood pressure and intraocular pressure in premature infants. RESULTS: Mean blood pressure in conscious premature infants is low and labile; it falls further under anaesthesia. Pre-operative intraocular pressure in retinopathy of prematurity patients is unknown, but intraocular pressure during vitrectomy is elevated, and likely elevated postoperatively. CONCLUSIONS: Conditions during and after vitreoretinal surgery for retinopathy of prematurity are conducive to low ocular perfusion pressure and consequent ischaemia of the retina and optic nerve, which can contribute to poor visual results. Improved monitoring and control of ocular perfusion pressure is warranted.

Effects of a topical alpha2 adrenergic agonist on ciliary blood flow and aqueous production in rabbits.

The relationship between ciliary perfusion and aqueous production is poorly understood. It was recently reported that aqueous production decreases when ciliary blood flow is reduced by lowering the ocular perfusion pressure, and hypothesized that drug-induced reduction of ciliary blood flow would also decrease aqueous production. In the present study, we test this hypothesis with an alpha2 adrenergic agonist (brimonidine) formulated for topical application. When used acutely, brimonidine decreases intraocular pressure (IOP) by suppressing aqueous production, although its mechanism of action is unclear. The experiments were performed in four groups of anesthetized rabbits (n=33) in which the following variables were measured: ocular mean arterial pressure (OMAP), IOP, orbital venous pressure (OVP), aqueous flow, ciliary blood flow, ciliary oxygen tension (PO2), episcleral venous pressure (EVP), carotid blood flow and heart rate. The measurements were made before and after brimonidine (0.15%, 40 microl) was applied to the cornea. Brimonidine decreased IOP (-33%, p<0.01), aqueous flow (-39%, p<0.01), ciliary blood flow (-37%, p<0.01), EVP (-42%, p<0.01) and ciliary PO2 (-32%, p<0.05). We conclude that topical brimonidine is a ciliary vasoconstrictor, and that alpha2 adrenergic agonist-induced decreases in ciliary blood flow decrease aqueous production.

Choroidal myogenic autoregulation and intraocular pressure.

This study tests the hypothesis that choroidal myogenic autoregulation participates in the intraocular pressure (IOP) response to mean arterial pressure (MAP) by minimizing arterial-pressure-dependent changes in choroidal blood volume. To test this hypothesis, the IOP response to MAP was quantified before and after impairing choroidal autoregulation. In a previous study, the efficacy of choroidal myogenic autoregulation was inversely related to IOP. Therefore, in one series of experiments (n = 6), the relationship between MAP and IOP was determined at normal and elevated baseline IOP (i.e., 15 and 25 mmHg, respectively). In a second series of experiments (n = 6), the relationship between MAP and IOP was determined at the normal IOP (15 mmHg) before and after administering hydralazine, an arterial vasodilator. In both series, the MAP manipulations were kept brief to avoid the confounding effects of aqueous compensation. The experiments were performed in pentobarbital anesthetized rabbits with hydraulic occluders placed on the thoracic descending aorta and inferior vena cava to raise and lower MAP, respectively. MAP was measured via a central ear artery catheter. The right eye was cannulated with two 23 gauge needles; one cannula was used to set the IOP by varying the ocular volume and the other was used to measure the IOP. The protocol consisted of inflating the occluders to cause brief (1-1.5 min) ramp increases and decreases in MAP over a wide pressure range. Baseline IOP was set prior to each occlusion, but was otherwise uncontrolled. In the first series, the MAP range was 30 to 95 mmHg and IOP changed by 6 mmHg at the normal baseline IOP and by 18 mmHg at the elevated baseline IOP. The corresponding volume shifts were 7.7 and 14.5 microliters, respectively. In the second series, the MAP range was 25 to 95 mmHg and IOP changed by 6 mmHg during control and by 14 mmHg after hydralazine. The corresponding volume shifts were 6.7 and 13.8 microliters, respectively. In both series, the prompt return of IOP to baseline upon restoration of normal MAP indicated that the volume changes were due to changes in ocular blood volume. Additional experiments confirmed that elevating the baseline IOP and administering hydralazine impaired choroidal autoregulation but did not alter the cranial venous pressure response to varying MAP.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin modulation of choroidal blood flow in the rabbit.

Based on the previous finding that locally produced nitric oxide (NO) and endothelin (ET) exert competing effects on choroidal resistance vessels, the present study sought to further characterize the pharmacology of ET in the choroid. The specific goal was to quantify the choroidal blood flow responses to acute changes in perfusion pressure before and after administering endothelin 1 (ET1), a non-selective ET antagonist, and selective antagonists for the endothelin A (ETA) and endothelin B (ETB) receptor subtypes. Anesthetized rabbits were instrumented with an ear artery cannula to measure mean arterial pressure (MAP), occluders on the aorta and vena cava to control MAP, and a vitreous cannula to measure intraocular pressure (IOP). Choroidal blood flow was measured by laser Doppler flowmetry with a vitreous fiber optic probe. The protocol entailed changing the ocular perfusion pressure by varying MAP before and after ET1 (0.9 microg kg(-1), i.v., n = 14), non-selective ET blockade (A-182086, 3 mg kg(-1), i.v., n = 10), selective ETA blockade (FR-139317, 3 mg kg(-1), i.v., n = 12), and selective ETB blockade (A-192621, 3 mg kg(-1), i.v., n = 14). ET1 and ETB blockade shifted the choroidal pressure-flow relation downward, while the non-selective antagonist and the selective ETA antagonist had no effect. The choroid had a biphasic response to exogenous ET1 as seen in other tissues (i.e. initial brief dilation followed by prolonged constriction) that was blocked by the non-selective antagonist whereas the ETA antagonist enhanced the dilation and blocked the constriction, and the ETB antagonist blocked the dilation and enhanced the constriction. These results indicate that ETA and ETB receptors are present and mediate opposing effects on choroidal vascular resistance. The results also suggest that endogenous ET preferentially elicits ETB vasodilation, most likely by stimulating endothelial nitric oxide release.

The effect of arterial pressure on the ocular pressure-volume relationship in the rabbit.

The purpose of this study was to quantify the effect of mean arterial pressure (MAP) on the ocular pressure-volume relationship. The experiments were performed in pentobarbital anesthetized rabbits instrumented with occluders on the thoracic aorta and inferior vena cava to control MAP which was measured via a cannula in the central ear artery. To vary the ocular volume and measure the intraocular pressure (IOP), two 23 gauge needles were inserted through the pars plana into the vitreous: one needle was connected to a saline-filled syringe and the other needle was connected to a pressure transducer. In one group of animals (n = 5), pressure-volume curves were determined at MAPs of 100, 80, 60, 40 and 20 mmHg and post mortem by cumulative saline injections (2 microliters) every 1-1.5 sec. In a second group (n = 7), pressure-volume curves were obtained at MAPs of 80, 60 and 40 mmHg and post mortem by saline infusion at 0.5 microliter sec-1 until the IOP reached 100 mmHg. The infusion protocol was repeated in a third group (n = 11) where the choroidal flux and the concentration of moving blood cells (CMBC) were measured by a laser Doppler flowmeter as indices of choroidal blood flow and the blood volume, respectively. MAP had three primary effects on the ocular pressure-volume relationship: (1) the baseline IOP varied exponentially with MAP, (2) the steepness of the initial portion of the pressure-volume curves was MAP-dependent and (3) the curves exhibited a 'plateau' as the IOP approached the prevailing MAP at MAPs > or = 40 mmHg. All of the curves in the living eye intersected and became indistinguishable from the post mortem curve when the IOP exceeded the prevailing MAP. The flux and CMBC measurements indicated that the MAP-dependence of the initial portion of the curves was due to failure of choroidal autoregulation and diminished increases in choroidal blood volume at the lower MAPs, and that the plateau portion of the curves was due to expulsion of blood from the eye. It is concluded that MAP has a significant effect on the ocular pressure-volume relationship.

Modulation of choroidal autoregulation in the rabbit.

Previous studies show that the choroid can maintain its blood flow despite changes in perfusion pressure, behaviour possibly mediated by an autoregulatory mechanism. However, the choroid's rich autonomic innervation suggests possible neural involvement in the response. To evaluate the potential neural contribution, choroidal blood flow was measured by laser Doppler flowmetry over a wide range of perfusion pressure before and after ganglionic blockade in anaesthetized rabbits. Although an upward shift in the pressure-flow (P-F) curve was anticipated due to loss of adrenergic tone, ganglionic blockade shifted the P-F curve downward, prompting a search for a neural vasodilator to explain the response. Cholinergic blockade with atropine failed to alter the P-F curve suggesting little parasympathetic involvement. By contrast, inhibition of nitric oxide (NO) synthase with nitro-L-arginine methyl ester (L-NAME) caused a dramatic downward shift in the P-F curve, suggesting the nitridergic nerves as the source of vasodilatory tone. However, the downward shift in the P-F curve with L-NAME was greater than seen with ganglionic blockade, indicating that endothelial NO rather than neural dilator tone predominates. Moreover, calcium channel blockade after L-NAME reversed the downward shift in the P-F curve suggesting that choroidal vascular tone is modulated by an interaction between NO and an unknown vasoconstrictor. Neither hexamethonium, losartan or a vasopressin antagonist given after L-NAME reversed the downward shift in the P-F curve, ruling out a neural vasoconstrictor, angiotensin II and vasopressin as the source of constrictor tone. Phentolamine given after L-NAME caused a small but significant reversal of the downward shift in the P-F curve, suggesting a minor adrenergic contribution. By contrast, the non-selective endothelin antagonist, A-182086, given after L-NAME significantly attenuated the choroidal constriction. These results indicate that an unknown neural dilator and locally produced NO and endothelin exert competing influences on the inherent reactivity of the choroidal resistance vessels as they respond to changes in perfusion pressure, and that this local regulation is likely modulated by extrinsic neurohumoral factors that are relatively quiet in the anesthetized rabbit.

Effect of fasting and refeeding on mesenteric autoregulation in conscious rabbits.

To determine whether feeding improves the efficacy of mesenteric autoregulation in conscious animals, rabbits were instrumented with pulsed-Doppler flow probes on the superior mesenteric artery and distal abdominal aorta to record mesenteric and hindquarters blood flow velocity. Hydraulic occluders were placed on the abdominal aorta (just below the celiac artery) and the thoracic vena cava to vary mesenteric and hindquarters arterial pressure (MAP), which was monitored via a catheter positioned in the distal abdominal aorta. Heart rate (HR) and lumbar sympathetic nerve activity (LSNA) were monitored as indexes of sympathetic nervous system activity. Pressure-velocity curves were obtained by aortic and caval occlusions on two consecutive days; first after a 24-h fast and after approximately 24 h of ad libitum refeeding. In the fed state, mesenteric velocity was significantly increased and MAP was decreased slightly; the slight decrease in MAP was counteracted by significant increases in HR and LSNA, whereas hindquarters perfusion was unchanged. The mesenteric and hindquarters pressure-velocity curves were all highly linear (r greater than or equal to 0.9) and showed no evidence of autoregulation in the fasted or fed state when pressure was changed by either the aortic or the caval occlusions. We conclude that autoregulation plays a minor role in the short-term regulation of mesenteric blood flow in the conscious rabbit.