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Evaluation of cerebral blood flow in the hippocampus, thalamus, and basal ganglia and the volume of the hippocampus in dogs before and during treatment with prednisolone

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  • 1 Department of Veterinary Medicine, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo, Japan 183-8509.
  • | 2 Animal Medical Center, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo, Japan 183-8509.

Abstract

OBJECTIVE

To examine whether glucocorticoid (GC) administration alters hippocampal cerebral blood flow (CBF) or volume in dogs.

ANIMALS

6 clinically normal adult Beagles.

PROCEDURES

Each dog underwent CT and MRI to measure the CBF in the hippocampus, basal ganglia, thalamus, and cerebral cortex and the volume of the hippocampus in each hemisphere of the brain before (day 0) and during (days 7 and 21) a 21-day treatment with prednisolone (1.0 mg/kg, PO, q 24 h) and famotidine (0.5 mg/kg, PO, q 12 h). Results for hippocampal volume, anesthesia-related variables, and semiquantitative measurements of CBF (hemisphere-specific ratios of the CBF in the hippocampus, basal ganglia, and thalamus relative to the CBF in the ipsilateral cerebral cortex and the left cerebral cortex CBF-to-right cerebral cortex CBF ratio) were compared across assessment time points (days 0, 7, and 21).

RESULTS

The ratios of CBF in the right hippocampus and right thalamus to that in the right cerebral cortex on day 21 were significantly lower than those on day 0. No meaningful differences were detected in results for the hippocampal volume in either hemisphere or for the anesthesia-related variables across the 3 time points.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that GC administration reduced CBF in the hippocampus and thalamus in dogs of the present study, similar to that which occurs in humans. Research on GC-related brain alteration in dogs could potentially contribute to advancements in understanding Alzheimer disease in humans and neurodegenerative conditions in dogs.

Abstract

OBJECTIVE

To examine whether glucocorticoid (GC) administration alters hippocampal cerebral blood flow (CBF) or volume in dogs.

ANIMALS

6 clinically normal adult Beagles.

PROCEDURES

Each dog underwent CT and MRI to measure the CBF in the hippocampus, basal ganglia, thalamus, and cerebral cortex and the volume of the hippocampus in each hemisphere of the brain before (day 0) and during (days 7 and 21) a 21-day treatment with prednisolone (1.0 mg/kg, PO, q 24 h) and famotidine (0.5 mg/kg, PO, q 12 h). Results for hippocampal volume, anesthesia-related variables, and semiquantitative measurements of CBF (hemisphere-specific ratios of the CBF in the hippocampus, basal ganglia, and thalamus relative to the CBF in the ipsilateral cerebral cortex and the left cerebral cortex CBF-to-right cerebral cortex CBF ratio) were compared across assessment time points (days 0, 7, and 21).

RESULTS

The ratios of CBF in the right hippocampus and right thalamus to that in the right cerebral cortex on day 21 were significantly lower than those on day 0. No meaningful differences were detected in results for the hippocampal volume in either hemisphere or for the anesthesia-related variables across the 3 time points.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that GC administration reduced CBF in the hippocampus and thalamus in dogs of the present study, similar to that which occurs in humans. Research on GC-related brain alteration in dogs could potentially contribute to advancements in understanding Alzheimer disease in humans and neurodegenerative conditions in dogs.

Introduction

Alzheimer disease is a neurodegenerative disease that causes progressive impairment of cognitive functions. The pathological mechanism of AD is not clear; however, some studies1,2 show that the exposure of a brain to excess GCs secreted during stress is a risk factor in the advancement of AD.

One of the known effects of high blood concentrations of GCs in humans is low CBF.3 In people, CBF of the medial temporal lobe, visual cortex, and cerebellum is reduced during declarative memory retrieval tasks 1 hour after GC administration, and the reduction of CBF correlates with the impairment of cognitive function.3 The medial temporal lobe includes the hippocampus, which mediates declarative memory,4 and human patients with AD have higher serum GC concentrations and lower hippocampal CBF, compared with age-matched control patients.5 Also in people, the CBF in the thalamus and basal ganglia is higher under stress.6 The thalamus is the central region for the neural response to acute stress and underlying stress-related changes in connectivity across the whole brain.7 Thalamic changes can affect episodic memory, and thalamic dysfunctions may contribute to or be responsible for some of the earliest symptoms of mild cognitive impairment and AD.8 Therefore, it is important to clarify the effects of changes in CBF induced by exposure to high blood concentrations of GCs.

Analyses of this mechanism are mainly conducted on rodents instead of people for ethical concerns. In prior research,9 rats that received GC (each implanted with a 100-mg corticosterone bead) for 3 months had neurodegeneration (characterized by shrinkage, condensation, and nuclear pyknosis) in the CA1 and CA3 cell fields and lower CBF in CA3 cell fields of the hippocampus than did control rats.9 However, investigations with rodents have 2 key disadvantages when results are extrapolated to people. First, the type of GC secreted from the adrenal cortex differs for people (cortisol) versus rodents (corticosterone). Second, cortisol has a high affinity for mineral corticoid receptors in people,10 whereas corticosterone has a low affinity for mineral corticoid receptors in rodents.11 Dogs, however, have outcomes more similar to those in people in that the predominant GC in dogs is cortisol, and the level of affinity of cortisol for mineral corticoid receptors is similar in dogs and people.12 Furthermore, behavioral changes and cognitive impairment have been reported as adverse effects of GC treatment in people,13 and behavioral changes (eg, nervousness, restlessness, and aggression) and signs of suspected cognitive dysfunction have been reported in dogs treated with GCs.14,15 Therefore, it is likely that dogs are more suitable than rodents for elucidating the potential effects in humans with low hippocampal CBF induced by exposure to high blood concentrations of GCs.

Currently, it is unknown whether high blood concentrations of GC in dogs cause changes in hippocampal CBF. Thus, in the study reported here, we aimed to examine whether GC administration alters hippocampal CBF and volume in dogs. We hypothesized that administration of prednisolone would decrease CBF in the hippocampus of treated dogs and, if so, dogs could be useful in research related to AD in people and GC effects on behavior and cognitive dysfunction in dogs.

Materials and Methods

Animals

Six university-owned, clinically normal young adult Beagles were enrolled. Prior to any experiments, the dogs underwent physical and neurologic examinations, thoracic and abdominal radiography, echocardiography, indirect blood pressure measurements, and serum biochemical analyses. Serum urea nitrogen, creatinine, and glucose concentrations and glutamic-pyruvic transaminase, glutamic-oxaloacetic transaminase, and alanine aminotransferase activities were recorded. All experiments were carried out in accordance with guidelines outlined in the US Animal Welfare Act and were approved by the Animal Experiments Committee at the Tokyo University of Agriculture and Technology.

Anesthesia

On day 0, dogs underwent general anesthesia for all imaging procedures. Food was withheld for 8 hours and water for 3 hours before the dogs underwent anesthesia for CT and MRI. For each dog, a 22-gauge IV catheter was aseptically placed in the left cephalic vein, and atropine sulfatea (50 μg/kg, SC) and butorphanol tartrateb (0.2 mg/kg, IV) were administered as preanesthetic medications. Anesthesia was induced with propofolc (6.0 mg/kg, IV, to effect), dogs were intubated, and then anesthesia was maintained with isofluraned (1% to 1.5% [vaporizer setting] during CT and 1% to 2% during MRI) in oxygen (2.0 L/min). Vital signs (ie, Spo2, Petco2, rectal temperature, HR, and MAP) were monitored during anesthesia. To help prevent dehydration and renal failure, dogs received acetated Ringer solution (5.0 mL/kg/h, IV) from 1 hour before to 1 hour after the imaging procedure.

CT perfusion of the head

Once anesthetized, each dog underwent CT with a 64-slice multidetector-row CT systeme to image the perfusion (CT perfusion) of the ROIs, which in each hemisphere of the brain included the hippocampus, thalamus, basal ganglia, and cerebral cortex. Each dog was positioned in dorsal recumbency with its head perpendicular to the CT table. First, plane CT of the head was performed (tube voltage, 120 kV; tube current, auto exposure control; slice thickness, 0.5 mm; rotation speed, 0.5 s/rotation). Next, the range for dynamic CT was set from the posterior clinoid process to 32 mm dorsally. Nonionic contrast mediumf (iodixanol; 320 mg of I/mL; 400 mg I/kg, IV, 3.0 mL/s) was administered, and dynamic CT was initiated (tube voltage, 100 kV; tube current, 100 mA; slice thickness, 0.5 mm; rotation speed, 0.5 s/rotation). The contrast medium was stored at 37°C until just prior to use and administered by a power injector through a 20-gauge IV catheter aseptically placed in the right cephalic vein. Dynamic CT was performed at 1.5-second intervals from 0 to 40 seconds after initiation, then at 2.0-second intervals from 40 to 60 seconds. Scan-synchronized breath holding was induced to avoid motion artifacts. Obtained images were reconstructed with the reconstruction function FC03 (soft tissue function), iterative reconstruction (dose reduction level, strong), and a slice thickness of 4.0 mm. All dynamic CT procedures were performed between 6 pm and 9 pm.

MRI of the head

During the same anesthetic event and after CT perfusion imaging, each dog underwent MRI of the head with a 0.3-T MRI system.g All MRIs were executed by fast spin echo with dogs positioned in sternal recumbency and their heads held within the quadrature detection coil. Transverse T1WIs and transverse, sagittal, and dorsal T2WIs were acquired for all dogs to measure hippocampal volume. Transverse plane image slices were oriented perpendicularly to the base of the brain. Dorsal plane image slices were gathered parallel to the hard palate. Sagittal plane image slices were obtained parallel to the longitudinal cerebral fissure. The MRI parameters were transverse T1WI (TR, 15 milliseconds; TE, 620 milliseconds; slice thickness, 2.5 mm; field of view, 150 × 150 mm; matrix size, 512 × 512; voxel, 0.23 × 0.23 mm; and slice gap, 3.0 mm), transverse T2WI (TR, 120 milliseconds; TE, 4,000 milliseconds; slice thickness, 2.5 mm; field of view, 150 × 150 mm; matrix size, 512 × 512; voxel, 0.23 × 0.23 mm; and slice gap, 3.0 mm), sagittal T2WI (TR, 120 milliseconds; TE, 4,000 milliseconds; slice thickness, 2.5 mm; field of view, 150 × 150 mm; matrix size, 512 × 512; voxel, 0.23 × 0.23 mm; and slice gap, 3.0 mm), and dorsal T2WI (TR, 120 milliseconds; TE, 4,000 milliseconds; slice thickness, 2.0 mm; field of view, 150 × 150 mm; matrix size, 512 × 512; voxel, 0.23 × 0.23 mm; and slice gap, 2.5 mm).

CBF analysis

Analysis of CBF was performed with the deconvolution method and preinstalled perfusion CT software.h The mathematical model of delay-corrected singular value decomposition was used. The matrix size for analysis setting was 512 × 512 pixels, and noise-reduction levels appeared strong within the software. To generate cerebral perfusion maps, arterial and venous time–density curves were generated. Over these perfusion maps, the tissue ROIs (ie, the hippocampus, thalamus, basal ganglia, and cerebral cortex in each hemisphere of the brain) were manually placed to measure CBF. Each ROI was determined in reference to the dorsal T2WIs and plane CT images with an ROI diameter of 1.0 mm. The CBF values obtained by CT perfusion were regarded as qualitative rather than quantitative. Accordingly, because interpatient comparisons of CBF values themselves would have been inaccurate, the ratio of the CBF in the right hippocampus to that in the right cerebral cortex and the ratio of the CBF in the left hippocampus to that in the left cerebral cortex were calculated. Additionally, the ratios of the CBF in the right and left basal ganglia and thalamus relative to the CBF in the respective ipsilateral cerebral cortex were calculated similarly. These values were regarded as semiquantitative.16

Hippocampal volume analysis

The hippocampal volume was measured with medical imaging software.l The ROI was determined by manually tracing the hippocampus regions of the left and right hemispheres on each transverse T1WI, and the area (mm2) of the hippocampus was calculated. The anatomic landmarks and borderlines of the hippocampus were determined from transverse, sagittal, and dorsal T2WIs as previously described.17 The volume (mm3) of the hippocampus in each hemisphere was obtained as the sum of the volumes calculated by multiplying the area (mm2) of the hippocampus on each image by the slice thickness (mm).

GC treatment

On day 1 (the day after CT perfusion and MRI), treatment with prednisolonej (1.0 mg/kg, PO, q 24 h, between 9 am and 10 am) and famotidinek (0.5 mg/kg, PO, q 12 h) was initiated and continued for 21 days. Famotidine was used to reduce the risk of adverse digestive effects of prednisolone.

Repeated imaging and continued monitoring

For each dog, CT perfusion and MRI were repeated on days 7 and 21 of prednisolone administration. In addition, at 24 hours, 72 hours, and 7 days after the administration of contrast medium for diagnostic imaging, a physical examination and serum creatinine concentration measurement were performed on each dog to assess for the presence of contrast-induced nephropathy or other late adverse effects.

Statistical analysis

Data distributions were assessed with the Kolmogorov-Smirnov test or Shapiro-Wilk test. Results across multiple measurement days (days 0, 7, and 21) were compiled and assessed for differences in the hippocampal volume for each hemisphere, the CBF ratios for the left versus right ROIs, and the left cerebral cortex CBF-to-right cerebral cortex CBF ratio. Data with a normal distribution were assessed with 1-way ANOVA, and data not normally distributed were assessed with the Friedman test. When significant (P < 0.05) differences were identified, a post hoc Bonferroni-Dunn test was performed. Similarly, results for anesthesia-related variables (ie, dose of propofol administered, duration from induction of anesthesia to the onset of dynamic CT, and vital signs [ie, Spo2, Petco2, rectal temperature, HR, and MAP] immediately before dynamic CT) were also assessed for differences across days 0, 7, and 21, with normally distributed data assessed with 1-way ANOVA and nonnormally distributed data assessed with the Friedman test. Available softwarel was used for statistical analysis; values of P < 0.05 were considered statistically significant.

Results

Animals

Six university-owned, clinically normal Beagles were enrolled in the study. All dogs were 6 years old, and the mean ± SD body weight was 11.17 ± 2.37 kg. There were 2 castrated males and 4 sexually intact females. Examinations revealed no neurologic, cardiac, or renal disorders among the dogs and confirmed that all were clinically normal.

Anesthesia and adverse events

All dogs underwent and recovered safely from anesthesia for each CT perfusion and subsequent MRI. Results for anesthesia-related variables (duration from induction of anesthesia to onset of dynamic CT, dose of propofol, and vital signs [Spo2, Petco2, rectal temperature, HR, and MAP]) had normal distribution and were assessed with 1-way ANOVA (Table 1). The MAP in dogs before dynamic CT ranged from 80 to 160 mm Hg for all dogs across all procedure days (0, 7, and 21). However, the MAP was ≤ 90 mm Hg before dynamic CT in 2 dogs on day 0, 1 dog on day 7, and 4 dogs on day 21. There were no significant differences in results for anesthesia-related variables across assessment time points.

Table 1

Comparisons of the mean ± SD results for anesthesia-related variables for 6 clinically normal adult Beagles that underwent CT and MRI for assessment of CBF and hippocampal volume before (day 0) and during (days 7 and 21) administration of prednisolone (1.0 mg/kg, PO, q 24 h between 9 am and 10 am) and famotidine (0.5 mg/kg, PO, q 12 h) for 21 days.

Assessment time pointsP values
VariableDay 0Day 7Day 21Day 0 vs day 7Day 0 vs day 21Day 7 vs day 21
DA (min)27.3 ± 6.823.3 ± 8.021.2 ± 2.70.990.42> 0.99
DP (mg/kg)5.2 ± 1.44.7 ± 1.05.2 ± 1.70.850.44> 0.99
Petco2 (mm Hg)33.8 ± 1.632.8 ± 1.535.3 ± 2.0> 0.990.540.10
MAP (mm Hg)93.0 ± 9.3116.0 ± 25.090.5 ± 11.00.14> 0.990.09
HR (beats/min)123.7 ± 33.3135.2 ± 23.7133.5 ± 24.1> 0.99> 0.99> 0.99
RT (°C)38.0 ± 0.438.3 ± 0.238.1 ± 0.20.60> 0.99> 0.99

DA = Duration of anesthesia from induction to the onset of dynamic CT. DP = Dose of propofol. RT = Rectal temperature.

No increases in serum creatinine concentration were detected for any dog on day 1, 3, or 7 after any of the 3 administrations of contrast medium. Further, no other adverse effects were seen in any dog.

ROIs

The arterial ROIs were placed on the anterior cerebral artery at reconstructed slices between 8.0 and 12.0 mm or 12.0 and 16.0 mm dorsally from the posterior clinoid process in all dogs. The venous ROI was placed over the superior sagittal sinus at reconstructed slices between 24.0 and 28.0 mm dorsally from the posterior clinoid process in 4 dogs and over the transverse sinus at reconstructed slices between 4.0 and 8.0 mm dorsally from the posterior clinoid process in 2 dogs. Because it was difficult to define the boundaries between the amygdala and hippocampus on some images, we measured the area of the hippocampus on the slices where the hippocampus was clearly seen within the lateral ventricle. All results had normal distribution; therefore, 1-way ANOVA was used in statistical analyses.

CBF analysis

Results for the ratios of CBF in each the hippocampus, thalamus, and basal ganglia relative to the CBF in the ipsilateral cerebral cortex in each hemisphere and the left cerebral cortex CBF-to-right cerebral cortex CBF ratio before prednisolone administration and on days 7 and 21 of prednisolone administration were assessed (Table 2). The mean ± SD ratios of CBF in the hippocampus and thalamus relative to the CBF in the cerebral cortex of the ipsilateral hemisphere were significantly (P = 0.03 and P = 0.02, respectively) lower for the right hemispheres of dogs on day 21 (0.58 ± 0.09 and 0.64 ± 0.13, respectively) than on day 0 (0.74 ± 0.10 and 0.87 ± 0.10, respectively). There were no other meaningful differences detected in the ratios assessed across the 3 time points.

Table 2

Comparisons of the mean ± SD CBF ratios in the dogs described in Table 1 before (day 0) and during (days 7 and 21) administration of the prednisolone and famotidine treatment.

Assessment time pointsP values
CBF variableCerebral hemisphereDay 0Day 7Day 21Day 0 vs day 7Day 0 vs day 21Day 7 vs day 21
H:CLeft0.70 ± 0.130.68 ± 0.110.55 ± 0.12> 0.990.190.33
Right0.74 ± 0.100.68 ± 0.080.58 ± 0.090.900.030.23
T:CLeft0.86 ± 0.090.78 ± 0.110.66 ± 0.19> 0.990.130.58
Right0.87 ± 0.100.79 ± 0.130.64 ± 0.130.850.020.21
B:CLeft1.02 ± 0.030.85 ± 0.100.94 ± 0.210.42> 0.99> 0.99
Right1.10 ± 0.280.84 ± 0.111.01 ± 0.110.14> 0.990.50
L:R1.01 ± 0.051.02 ± 0.031.01 ± 0.07> 0.99> 0.99> 0.99

B:C = Ratio of CBF in the basal ganglia to CBF in the ipsilateral cerebral cortex. H:C = Ratio of CBF in the hippocampus to CBF in the ipsilateral cerebral cortex. L:R = Ratio of CBF in the cerebral cortex of the left hemisphere to CBF in the cerebral cortex of the right hemisphere. T:C = Ratio of CBF in the thalamus to CBF in the ipsilateral cerebral cortex.

Hippocampal volume analysis

The hippocampal volumes of each hemisphere before prednisolone administration (day 0) and on days 7 and 21 were assessed (Table 3). No significant differences in the mean ± SD hippocampal volume were detected in either hemisphere across the 3 time points.

Table 3

Comparisons of the mean ± SD hippocampal volumes (mm3) for the dogs described in Table 1 before (day 0) and during (days 7 and 21) administration of the prednisolone and famotidine treatment.

Assessment time pointsP values
Cerebral hemisphereDay 0Day 7Day 21Day 0 vs day 7Day 0 vs day 21Day 7 vs day 21
Left526.8 ± 31.3507.5 ± 31.3511.0 ± 30.6> 0.99> 0.99> 0.99
Right556.7 ± 30.7546.7 ± 37.3542.4 ± 30.5> 0.99> 0.99> 0.99

Discussion

Our findings indicated that during GC administration, a significant reduction occurred in the ratio of the CBF in the hippocampus to the CBF in the cerebral cortex for the right hemisphere selectively, but no significant differences were detected in the results for the left cerebral cortex CBF-to-right cerebral cortex CBF ratio across the assessment time points. These results suggested that GC administration reduced the CBF in the hippocampus of dogs in the present study, as has been reported in people.3 Moreover, the hemispheric lateralization of CBF reduction in dogs of the present study was the same as that seen in people during declarative memory retrieval tasks 1 hour after GC administration.3 These findings supported our hypothesis that administration of prednisolone would decrease CBF in the hippocampus in treated dogs. However, the present study did not measure the actual, quantitative value for CBF. Therefore, additional research with instruments capable of measuring CBF with quantitative values is needed to investigate CBF changes induced by GC administration.

In addition, our findings indicated that the ratio of the CBF in the thalamus to the CBF in the ipsilateral cerebral cortex was reduced during GC administration in the dogs of the present study, whereas no GC-induced decrease in the ratio of the CBF in the basal ganglia to CBF in the ipsilateral cerebral cortex was found. These results implied that GC administration for 21 days could reduce the CBF in the thalamus but not in the basal ganglia of dogs. The thalamus is connected to the hippocampus by the Papez circuit. In people, thalamic changes can affect episodic memory, and thalamic dysfunctions may contribute to or be responsible for some of the earliest cognitive symptoms of mild cognitive impairment and AD.8 In contrast to the findings of the present study, people have increased CBF in the thalamus and the basal ganglia during short-term stress tasks.6 This difference could be attributed to the short duration of stressor application in the human study6 versus the 21-day duration of prednisolone administration to dogs in our study. It was unclear whether short-term exposure to high blood concentrations of GC could cause long-term increases in the CBF in the thalamus and basal ganglia of dogs or whether prolonged exposure to such concentrations of GC would eventually turn off this increased effect.

In the present study, we found no significant reductions in the hippocampal volume for dogs treated with prednisolone for 21 days, whereas in people, hippocampal volume reductions were linked to the use of prednisolone for 8 years18 or hydrocortisone for just 3 days.19 In addition, right hippocampal volume was significantly lower in people with moderate to high levels of stress, compared with those who have low levels of stress.20 Studies19,21 also show that GC-induced reductions in hippocampal volume occur more readily in men than in women,19,21 and results of a previous study22 suggest that hippocampal neurodegeneration under stress conditions is observed to have a higher degree in older rats, compared with younger rats. However, our study was not designed to assess relationships between sex, age, and hippocampal changes induced by GC administration. In addition, there are technical differences in measuring hippocampal volume in dogs versus people. The brain size of dogs is smaller than that of humans,23,24 and raters of hippocampal volume in human studies23,25 were more extensively trained for the assessments than were those in our study and other dog studies.26,27 Furthermore, the MRI data in the present study were acquired with relatively thick slices and interslice gaps (3.0 mm) by use of a low–magnetic field device (0.3 T); thus, there was a possibility that these factors caused over− or underestimation of hippocampal volume because of hippocampus-amygdala boundary misjudgment.28

Traditionally, analyses of relationships between AD and high blood concentrations of GCs prompted by stress have mainly been conducted on rodent subjects. However, the types of GCs secreted from the adrenal cortex as a result of stress differ between people and rodents.29 Therefore, extrapolating the results from rodents to people is difficult. On the other hand, the type of GC secreted from the adrenal cortex in dogs is the same as in people, and the living environments of pet dogs are more similar to those of humans.30 Furthermore, dogs show similar symptoms and amyloid deposition in aging-related cognitive dysfunction as in people with AD.31,32 On the basis of these issues and our findings that GC administration decreased the CBF in the hippocampus of dogs, as similarly seen in humans,3 we suggest that dogs are more suitable than rodents for use in AD research related to GC effects. However, when considering extrapolating such data for dogs to AD data for humans, it is important to recognize that pet dogs are often administered prednisolone for the treatment of inflammatory diseases, such as dermatitis. Therefore, pet dogs treated with prednisolone should be excluded from some AD research, such as research related to GC effects on the hippocampus and thalamus. This is why we selected prednisolone instead of hydrocortisone as the treatment GC in the present study.

In veterinary medicine, previous studies14,15 show that dogs receiving GC drugs, such as prednisolone, have behavioral changes (eg, signs of nervousness, restlessness, and aggression) that are suspected to stem from cognitive dysfunction. It is unclear whether CBF reductions caused by GC administration are associated with these cognitive and behavioral changes. Nonetheless, veterinarians should pay attention to these cognitive and behavioral changes, especially in dogs receiving GC drugs.

Importantly, several considerations must be kept in mind when evaluating CBF in dogs. In people, assessments of CBF may be performed on awake individuals, whereas dogs must undergo general anesthesia for assessment of CBF, and anesthesia may artificially alter CBF results. For example, propofol reduces CBF dose dependently.33 In addition, high concentrations of isoflurane increase the CBF. A study34 shows that the CBF in dogs is significantly higher in those for which anesthesia is maintained with 2.8% isoflurane, compared with 1.4% isoflurane.34 In the dogs of the present study, there was no significant difference in the propofol dose administered for the anesthetic events on days 0, 7, and 21, and the concentration of isoflurane in oxygen ranged between 1% and 1.5% during all CT procedures. Thus, there were no differences in the CBF attributed to the administration of propofol or isoflurane in the dogs of the present study. Additionally, changes in blood pressure may change the CBF. A study35 of dogs shows that CBF remains stable when the MAP ranges between 90 and 180 mm Hg; however, when the MAP is ≤ 90 mm Hg, CBF decreases in correlation with decreases in the MAP.35 In dogs of the present study, the MAP before dynamic CT ranged between 80 and 160 mm Hg for all dogs across all procedure days (0, 7, and 21). However, the MAP was ≤ 90 mm Hg before dynamic CT in 2 dogs on day 0, 1 dog on day 7, and 4 dogs on day 21. With the CBF alterations evaluated by semiquantitative values, it was unknown whether blood pressure variation affected semiquantification of the CBF because the whole brain undergoes blood pressure influences, and our results for CBF might have been artificially reduced by blood pressure variations. Such blood pressure changes need to be considered when estimating CBF in anesthetized dogs.

Questions that require further research stemmed from our findings. We found that prednisolone administration for 21 days reduced the CBF in the hippocampus and thalamus but did not reduce the CBF in the basal ganglia. This result was inconsistent with findings from a study6 of human medicine that shows more CBF in the thalamus and basal ganglia under short-term stress tasks. This difference may lie in the duration of the exposure to high blood concentrations of GCs (short-term [eg, 12 minutes] stress exposure in humans6 vs relatively long-term [ie, from 7 and 21 days] prednisolone treatment in dogs of the present study). However, we did not examine the potential shorter-term effects of prednisolone on the CBF in the thalamus and basal ganglia, and there was a possibility that an initial increase in the CBF could have been turned off with longer-term prednisolone influence. Another question that stemmed from our findings in the present study was why there was no significant reduction in the hippocampal volume with prednisolone administration up to 21 days, whereas people have reductions of the hippocampal volume when treated with hydrocortisone for 3 days19 or have long-term moderate to high levels of stress.20 This discrepancy also involves questions regarding differences in types and sources of GCs, such as endogenous versus exogenous.

Our study had limitations. Only 6 dogs were included, and there were more females than males. Moreover, all dogs were Beagles; therefore, the potential effects of dog breed on hippocampal changes induced by GC administration remain unclear. Furthermore, the dogs in our study were relatively young. It has been reported36 that the CBF in the fronto− and temporocortical regions and in the subcortical region in aged dogs is lower than that in young dogs and that the CBF in the frontocortical region negatively correlates with age. Therefore, the impact of GCs on CBF could be more prominent or different in aged dogs, compared with the adult dogs in the present study. Another limitation was that the dogs in our study received famotidine in addition to prednisolone. We did not investigate whether famotidine affects CBF, and we did not perform a placebo-controlled aspect of the study. In addition, we manually placed ROIs on CT images, all measurements were performed by the same investigator, and the investigator was not blinded to information about the treatment.

Overall, our findings indicated that GC administration reduced the CBF in the hippocampus and thalamus in dogs and that the reduction of the CBF in the hippocampus was similar to that reported in people.3 Therefore, we suggest that dogs are a suitable species for AD models involving GC effects. Additionally, the types of GCs administered to dogs need to be considered when researching the effects of high blood concentrations of GC on the hippocampus in dogs. Furthermore, our findings could contribute to research efforts on cognitive dysfunction in dogs and humans.

Acknowledgments

No third-party funding or support was received in connection with the present study or the writing or publication of the manuscript. The authors declare that there were no conflicts of interest.

Abbreviations

AD

Alzheimer disease

CBF

Cerebral blood flow

GC

Glucocorticoid

HR

Heart rate

MAP

Mean arterial blood pressure

Petco2

End-tidal partial pressure of carbon dioxide

ROI

Region of interest

Spo2

Oxygen saturation of hemoglobin as measured by pulse oximetry

T1WI

T1-weighted image

T2WI

T2-weighted image

TE

Echo time

TR

Repetition time

Footnotes

a.

Atropine sulfate injection, Mitsubishi Tanabe Pharmaceutical Co Ltd, Osaka, Japan.

b.

Vetorphale, Meiji Seika Pharma Co Ltd, Tokyo, Japan.

c.

Fresenius Kabi Japan, Tokyo, Japan.

d.

Isoflu, Sumitomo Dainippon Pharma Co Ltd, Osaka, Japan.

e.

Aquilion CXL, Canon Medical Systems Corp, Tochigi, Japan.

f.

Visipaque 320, GE Healthcare, Chicago, Ill.

g.

AIRIS-II-1A Comfort, Hitachi Healthcare Manufacturing Ltd, Chiba, Japan.

h.

Body Perfusion, Canon Medical Systems Corp, Tochigi, Japan.

i.

OsiriX N-20.14, Newton Graphics Inc, Hokkaido, Japan.

j.

Prednisolone tablets, Mylan Seiyaku Ltd, Osaka, Japan.

k.

Gaster tablets, LTL Pharma Co Ltd, Tokyo, Japan.

l.

SPSS Statistics, version 22.0, IBM Corp, Armonk, NY.

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Contributor Notes

Address correspondence to Dr. Kishimoto (miori@cc.tuat.ac.jp).