A Spatial Memory Deficit in Male But Not Female Rats After Neonatal Diazepam Exposure: A New Model for Developmental Sedative Neurotoxicity

BACKGROUND

Developmental anesthetic neurotoxicity is well described in animal models for GABAergic, sedating drugs. Here we investigate the role of the benzodiazepine, diazepam on spatial and recognition memory of young adult rats after neonatal exposure.

METHODS

On postnatal day 7, male (n = 30) and female (n = 30) rats were exposed to diazepam (30 mg/kg intraperitoneally) or vehicle. On postnatal day 42, animals started a series of behavioral tests including Barnes maze (spatial memory), object recognition battery (recognition memory), and open field and elevated plus maze (anxiety). In a separate cohort, blood gases were obtained from diazepam-exposed animals and compared to isoflurane-exposed animals (1 MAC for 4 hours).

RESULTS

Male animals exposed to diazepam had impaired performance in the Barnes maze and were unable to differentiate the goal quadrant from chance (1-sample t test; tdiazepam/male (14) = 1.49, P = .158). Female rats exposed to diazepam performed the same as the vehicle controls ( tdiazepam/female (12) = 3.4, P = .005, tvehicle/female (14) = 3.62, P = .003, tvehicle/male (13) = 4.76, P < .001). There were no statistical differences in either males or females in measures of recognition memory, anxiety, or locomotor activity in other behavioral tests. Physiologic measurements of arterial blood gases taken from animals under sedation with diazepam were much less aberrant than those exposed to the volatile anesthetic isoflurane by t test (pH diazepam [M = 7.56, standard deviation {SD} = 0.11] versus pH Isoflurane [M = 7.15, SD = 0.02], t (10) = 8.93, P < .001; Paco 2diazepam [M = 32.8 mm Hg, SD = 10.1] versus Paco 2Isoflurane [M = 91.8 mm Hg, SD = 5.8], t (10) = 8.93, P < .001).

CONCLUSIONS

The spatial memory results are consistent with volatile anesthetic suggesting a model in which development of the GABA system plays a critical role in determining susceptibility to behavioral deficits.

A Randomized Phase 2 Study to Evaluate Efficacy and Safety of AR36 for Prevention of Acute Mountain Sickness

INTRODUCTION

AR36 is a pharmaceutical-grade plant extract used to support cardiovascular health in traditional Chinese medicine. Studies suggest that AR36 may prevent acute mountain sickness (AMS) during gradual ascent to high altitude. This randomized, placebo-controlled Phase 2 trial aimed to evaluate dosing regimens and assess efficacy and safety of AR36 for AMS prevention during rapid ascent.

METHODS

Participants received placebo, low-dose AR36 (225 mg twice daily for 14 d prior and 5 d at altitude), or high-dose AR36 (12 d placebo, 300 mg twice daily for 2 d prior and 5 d at altitude). The primary efficacy outcome was 1993 Lake Louise Scoring System (LLSS) score on the morning after ascent. Safety was assessed through the proportion of treatment-emergent adverse events (TEAEs).

RESULTS

One hundred thirty-two participants were randomized. Mean±SD age was 31.4±8.6 (range, 19-54) y. Baseline characteristics did not differ across groups. Lake Louise Scoring System scores on Day 16 in the placebo, low-dose, and high-dose groups were 4.03 (2.88), 4.42 (3.17), and 3.5 (2.31), respectively (placebo versus low-dose, P=0.462; placebo versus high-dose, P=0.574; n=110). The incidence of AMS on Day 16 was 66.7% in the placebo, 61.1% in the low-dose, and 55.3% in the high-dose group (P=0.66). The proportion of TEAEs in the placebo, low-dose, and high-dose groups was 38.4% (81), 28.4% (60), and 33.2% (70), respectively (P=0.205; n=127). There was no statistical difference between groups in LLSS, incidence of AMS, or TEAEs.

CONCLUSIONS

AR36 did not improve LLSS or AMS incidence using the current regimens. AR36 was well tolerated.

The Effect of Route of Administration and Vehicle on the Pharmacokinetics of THC and CBD in Adult, Neonate, and Breastfed Sprague-Dawley Rats

Introduction: Basic pharmacokinetic (PK) and pharmacodynamic models of the phytocannabinoids Δ-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are critical for developing translational models of exposure and toxicity. The neonatal period is a particularly important time to study the effects of cannabinoids, yet there are few studies of cannabinoid PKs by different routes such as direct injection or breast milk ingestion. To study this question, we have developed a translationally relevant rodent model of perinatal cannabinoid administration by measuring plasma levels of THC and CBD after different routes and preparations of these drugs. Materials and Methods: Adult animals and pups were injected with THC or CBD either intraperitoneally or subcutaneously, and plasma was analyzed by liquid chromatography-tandem mass spectrometry to measure cannabinoid levels collected at specified intervals. We also tested the effect of preparation of the drug using an oil-based vehicle (sesame oil) and an aqueous vehicle (Tween). Finally, we measured the plasma levels of cannabinoids in neonatal pups that were transmitted through breast milk after intraperitoneal injection to nursing dams. Results: We observed differences in the PK profiles of cannabinoids in adults and neonatal pups that were dependent on the route of administration and type of vehicle. Cannabinoids prepared in aqueous vehicle, injected intraperitoneally, resulted in a high peak in plasma concentration, which rapidly decreased. In contrast, subcutaneous injections using sesame oil as a vehicle resulted in a slow rise and low plateau in plasma concentration. Intraperitoneal injections with sesame oil as a vehicle resulted in a slower rise compared with aqueous vehicle, but an earlier and higher peak compared with subcutaneous injection. Finally, the levels of THC and CBD that were similar to direct subcutaneous injections were measured in the plasma of pups nursing from intraperitoneally injected dams. Conclusions: The route of administration and the preparation of the drug have important and significant effects on the PK profiles of THC and CBD in rats. These results can be used to create different clinically relevant exposure paradigms in pups and adults, such as short high-dose exposure or a low-chronic exposure, each of which might have significant and varying effects on development.

Testosterone is Sufficient to Impart Susceptibility to Isoflurane Neurotoxicity in Female Neonatal Rats

BACKGROUND

Volatile anesthetic exposure during development leads to long-term cognitive deficits in rats which are dependent on age and sex. Female rats are protected relative to male rats for the same exposure on postnatal day 7. Here we test our hypothesis that androgens can modulate chloride cotransporter expression to alter the susceptibility to neurotoxicity from GABAergic drugs using female rats with exogenous testosterone exposure.

METHODS

Female rats were injected with testosterone (100 μg/animal) or vehicle on postnatal days 1 to 6. On postnatal day 7, the animals were randomized to either isoflurane exposure or sham. Spatial memory was assessed with the Barnes maze starting on postnatal day 41. Western blots were run from testosterone treated postnatal day 7 animals to measure levels of chloride cotransporters sodium-potassium-chloride symporter (NKCC1) and chloride-potassium symporter 5 (KCC2).

RESULTS

Exogenous testosterone modulated isoflurane anesthetic neurotoxicity in female rats based on poor performance in the probe trial of the Barnes Maze. By contrast, females with vehicle and isoflurane exposure were able to differentiate the goal position. These behavioral differences corresponded to differences in the protein levels of NKCC1 and KCC2 after exogenous testosterone exposure, with NKCC1 increasing ( P <0.001) and KCC2 decreasing ( P =0.003) relative to female controls.

CONCLUSIONS

The expression of chloride cotransporters, NKCC1 and KCC2, is altered by testosterone in female rats and corresponds to a cognitive deficit after isoflurane exposure. This confirms the role of androgens in perinatal anesthetic neurotoxicity and supports our hypothesis that the developing GABAergic system plays a critical role in the underlying mechanism.

Anesthesia-induced Recognition Deficit Is Improved in Postnatally Gonadectomized Male Rats

BACKGROUND

Preclinical investigations of the effects of general anesthesia on the young brain show differences in vulnerability of males and females to anesthetic exposure at different times during development. However, the mechanism underlying this sex difference is poorly understood. Perinatal testosterone is the primary determinant of sexual differentiation and likely plays an important role in defining the period of susceptibility to anesthetic injury. We investigated whether the removal of testosterone through gonadectomy shortly after birth would improve cognitive outcomes in male rodents after early anesthesia exposure.

METHODS

Male Sprague Dawley rats underwent gonadectomy at postnatal day 2 (P2), followed by exposure to 6 hours of isoflurane at P7. A control cohort of gonad-intact male littermates was simultaneously exposed. All rats were subjected to a series of object recognition and association tasks beginning at P42. Cell death in the thalamus and hippocampus was assessed in a separate cohort.

RESULTS

All groups performed similarly on the Novel Object Recognition task; however, the gonad-intact isoflurane group exhibited decreased performance in the more difficult tasks. This deficit was ameliorated in the gonadectomized group. Cell death was similar between both isoflurane-exposed groups, regardless of gonadectomy.

CONCLUSIONS

The absence of testosterone does not block cell death after anesthesia in specific brain regions of interest; however, does provide some neuroprotection as evidenced by the improved cognitive test performance during adulthood. These findings suggest that testosterone may be mechanistically involved in the sex-specific effects of anesthetic injury on the developing brain by extending the vulnerable period in male rats.

Standards for preclinical research and publications in developmental anaesthetic neurotoxicity: expert opinion statement from the SmartTots preclinical working group

In March 2019, SmartTots, a public-private partnership between the US Food and Drug Administration and the International Anesthesia Research Society, hosted a meeting attended by research experts, anaesthesia journal editors, and government agency representatives to discuss the continued need for rigorous preclinical research and the importance of establishing reporting standards for the field of anaesthetic perinatal neurotoxicity. This group affirmed the importance of preclinical research in the field, and welcomed novel and mechanistic approaches to answer some of the field's largest questions. The attendees concluded that summarising the benefits and disadvantages of specific model systems, and providing guidance for reporting results, would be helpful for designing new experiments and interpreting results across laboratories. This expert opinion report is a summary of these discussions, and includes a focused review of current animal models and reporting standards for the field of perinatal anaesthetic neurotoxicity. This will serve as a practical guide and road map for novel and rigorous experimental work.

Voluntary Exercise Rescues the Spatial Memory Deficit Associated With Early Life Isoflurane Exposure in Male Rats

BACKGROUND

Early life anesthesia exposure results in long-term cognitive deficits in rats. Environmental enrichment consisting of social housing, a stimulating environment, and voluntary exercise can rescue this deficit. We hypothesized that exercise alone is sufficient to rescue the cognitive deficit associated with perinatal anesthesia.

METHODS

Postnatal day 7 male rats (P7) underwent isoflurane (Iso) or sham exposure and were subsequently weaned at P21. They were then singly housed in a cage with a running wheel or a fixed wheel. After 3 weeks of exercise, animals underwent behavioral testing for spatial and recognition memory assessments. Animals were killed at various time points to accomplish either bromodeoxyuridine (BrdU) labeling or quantitative real-time polymerase chain reaction (qRT-PCR) to quantify brain-derived neurotrophic factor (BDNF) messenger ribonucleic acid (mRNA) levels.

RESULTS

Postweaning voluntary exercise rescued the long-term spatial memory deficit associated with perinatal Iso exposure. Iso-sedentary animals did not discriminate the goal quadrant, spending no more time than chance during the Barnes maze probe trial (1-sample t test, P = .524) while all other groups did (1-sample t test, PIso-exercise = .033; Pcontrol [Con]-sedentary = .004). We did not find a deficit in recognition memory tasks after Iso exposure as we observed previously. BrdU incorporation in the adult hippocampus of Iso-sedentary animals was decreased compared to sedentary controls (Tukey P = .005). Exercise prevented this decrease, with Iso-exercise animals having more proliferation than Iso-sedentary (Tukey P < .001). There was no effect of exercise or Iso on BDNF mRNA in either the cortex or hippocampus (cortex: FExercise[1,32] = 0.236, P = .631; FIso [1,32] = 0.038, P = .847; FInteraction [1,32] = 1.543, P = .223; and hippocampus: FExercise[1,33] = 1.186, P = .284; FIso [1,33] = 1.46, P = .236; FInteraction[1,33] = 1.78, P = .191).

CONCLUSIONS

Exercise restores BrdU incorporation and rescues a spatial memory deficit after early life anesthesia exposure. This demonstrates sufficiency of exercise alone in the context of environmental enrichment to recover a behavioral phenotype after a perinatal insult.

Female rats are more vulnerable to lasting cognitive impairment after isoflurane exposure on postnatal day 4 than 7

BACKGROUND

The factors determining peak susceptibility of the developing brain to anaesthetics are unclear. It is unknown why postnatal day 7 (P7) male rats are more vulnerable to anaesthesia-induced memory deficits than littermate females. Given the precocious development of certain regions in the female brain during the neonatal critical period, we hypothesised that females are susceptible to anaesthetic brain injury at an earlier time point than previously tested.

METHODS

Female rats were exposed to isoflurane (Iso) 1 minimum alveolar concentration or sham anaesthesia at P4 or P7. Starting at P35, rats underwent a series of behavioural tasks to test their spatial and recognition memory. Cell death immediately after anaesthesia was quantified by Fluoro-Jade C staining in select brain regions, and developmental expression of the chloride transporters KCC2 and NKCC1 was analysed by immunoblotting in male and female rats at P4 and P7.

RESULTS

Female rats exposed to Iso at P4 displayed impaired spatial, object-place, -context, and social recognition memory, and increased cell death in the hippocampus and laterodorsal thalamus. Female rats exposed at P7 exhibited only decreased performance in object-context compared with control. The ratio of NKCC1/KCC2 expression in cerebral cortex was higher in P4 females than in P7 females, and similar to that in P7 males.

CONCLUSIONS

Female rats exposed to Iso at P4 are sensitive to anaesthetic injury historically observed in P7 males. This is consistent with a comparably immature developmental state in P4 females and P7 males. The window of anaesthetic vulnerability correlates with sex-specific cortical expression of chloride transporters NKCC1 and KCC2. These findings suggest that both sex and developmental age play important roles in determining the outcome after early anaesthesia exposure.

Resection of gliomas deemed inoperable by neurosurgeons based on preoperative imaging studies

OBJECTIVE

Maximal safe resection is a primary objective in the management of gliomas. Despite this objective, surgeons and referring physicians may, on the basis of radiological studies alone, assume a glioma to be unresectable. Because imaging studies, including functional MRI, may not localize brain functions (such as language) with high fidelity, this simplistic approach may exclude some patients from what could be a safe resection. Intraoperative direct electrical stimulation (DES) allows for the accurate localization of functional areas, thereby enabling maximal resection of tumors, including those that may appear inoperable based solely on radiological studies. In this paper the authors describe the extent of resection (EOR) and functional outcomes following resections of tumors deemed inoperable by referring physicians and neurosurgeons.

METHODS

The authors retrospectively examined the cases of 58 adult patients who underwent glioma resection within 6 months of undergoing a brain biopsy of the same lesion at an outside hospital. All patients exhibited unifocal supratentorial disease and preoperative Karnofsky Performance Scale scores ≥ 70. The EOR and 6-month functional outcomes for this population were characterized.

RESULTS

Intraoperative DES mapping was performed on 96.6% (56 of 58) of patients. Nearly half of the patients (46.6%, 27 of 58) underwent an awake surgical procedure with DES. Overall, the mean EOR was 87.6% ± 13.6% (range 39.0%–100%). Gross-total resection (resection of more than 99% of the preoperative tumor volume) was achieved in 29.3% (17 of 58) of patients. Subtotal resection (95%–99% resection) and partial resection (PR; < 95% resection) were achieved in 12.1% (7 of 58) and 58.6% (34 of 58) of patients, respectively. Of the cases that involved PR, the mean EOR was 79.4% ± 12.2%. Six months after surgery, no patient was found to have a new postoperative neurological deficit. The majority of patients (89.7%, 52 of 58) were free of neurological deficits both pre- and postoperatively. The remainder of patients exhibited either residual but stable deficits (5.2%, 3 of 58) or complete correction of preoperative deficits (5.2%, 3 of 58).

CONCLUSIONS

The use of DES enabled maximal safe resections of gliomas deemed inoperable by referring neurosurgeons. With rare exceptions, tumor resectability cannot be determined solely by radiological studies.

Increased Cytokines at High Altitude: Lack of Effect of Ibuprofen on Acute Mountain Sickness, Physiological Variables, or Cytokine Levels

Lundeberg, Jenny, John R. Feiner, Andrew Schober, Jeffrey W. Sall, Helge Eilers, and Philip E. Bickler. Increased cytokines at high altitude: lack of effect of ibuprofen on acute mountain sickness, physiological variables or cytokine levels. High Alt Med Biol. 19:249-258, 2018.

INTRODUCTION

There is no consensus on the role of inflammation in high-altitude acclimatization.

AIMS

To determine the effects of a nonsteroidal anti-inflammatory drug (ibuprofen 400 mg every 8 hours) on blood cytokines, acclimatization, acute mountain sickness (AMS, Lake Louise Score), and noninvasive oxygenation in brain and muscle in healthy volunteers.

MATERIALS AND METHODS

In this double-blind study, 20 volunteers were randomized to receive ibuprofen or placebo at sea level and for 48 hours at 3800 m altitude. Arterial, brain, and leg muscle saturation with near infrared spectroscopy, pulse oximetry, and heart rate were measured. Blood samples were collected for cytokine levels and cytokine gene expression.

RESULTS

All of the placebo subjects and 8 of 11 ibuprofen subjects developed AMS at altitude (p = 0.22, comparing placebo and ibuprofen). On arrival at altitude, the oxygen saturation as measured by pulse oximetry (S_pO₂) was 84.5% ± 5.4% (mean ± standard deviation). Increase in blood interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), tumor necrosis factor-α (TNF-α), and granulocyte-macrophage colony-stimulating factor (GM-CSF) levels occurred comparably in the placebo and ibuprofen groups (all not significant, univariate test by Wilcoxon rank sum). Increased IL-6 was associated with higher AMS scores (p = 0.002 by Spearman rank correlation). However, we found no difference or association in AMS score and blood or tissue oxygenation between the ibuprofen and placebo groups.

CONCLUSIONS

We found that ibuprofen, at the package-recommended adult dose, did not have a significant effect on altitude-related increases in cytokines, AMS scores, blood, or tissue oxygenation in a population of healthy subjects with a high incidence of AMS.

Is a short anesthetic exposure in children safe? Time will tell: a focused commentary of the GAS and PANDA trials

Early life exposure to general anesthesia in preclinical studies has consistently led to permanent cognitive deficits later in life. However, the extent to which this finding is translatable to humans is the subject of much debate as the results from clinical studies have been mixed. Recently two well-designed clinical trials have attempted to add clarity to our murky understanding. The General Anesthesia compared to Spinal anesthesia (GAS) trial, was an international, prospective, randomized, multicenter, equivalence trial comparing infants undergoing herniorrhaphy receiving general anesthesia vs. neuraxial anesthesia. The results released are from a pre-determined secondary outcome of a behavioral/developmental assessment of 2 years old that found equivalence between the two groups. The Pediatric Anesthesia NeuroDevelopment Assessment (PANDA) trial was an ambi-directional cohort trial, comparing patients receiving general anesthesia for hernia repair before 3 years old vs. sibling-matched controls. The neuropsychological battery performed showed no difference between siblings. Taken together, there is cautious optimism that short anesthesia exposure may not lead to significant cognitive decline in humans, but one should also consider that the GAS trial has yet to release the primary endpoint, IQ testing at age 5, and the PANDA trial may not represent the general population given the high socioeconomic status and high control IQ scores. Furthermore, as seen in preclinical studies, the cognitive deficit might not be significant until later in life, and longer exposures to anesthesia may have a more deleterious effect on cognitive function. While these new studies greatly increase our understanding in humans, there are many more questions that need to be addressed.

Summary of the Update Session on Clinical Neurotoxicity Studies

During the Fifth Pediatric Anesthesia Neurodevelopmental Assessment Symposium, experts and stakeholders met to present and discuss recent advances made in the study of neurodevelopmental outcomes after exposure to anesthetic drugs in infants and children. This article summarizes the update of 5 ongoing clinical studies: General Anesthesia compared to Spinal Anesthesia, Toxicity of Remifentanil and Dexmedetomidine, Mayo Anesthesia Safety in Kids, the University of California San Francisco human cohort study, and Columbia University Medical Center Neonatal Magnetic Resonance Imaging study. The purpose of this summary is to discuss the contributions and limitations of these studies, how they fit into the published literature, and what questions remain to be answered.

Beyond anesthetic properties: the effects of isoflurane on brain cell death, neurogenesis, and long-term neurocognitive function

Anesthetic drugs cause brain cell death and long-term neurocognitive dysfunction in neonatal rats. Recently, human data also suggest that anesthesia early in life may cause cognitive impairment. The connection between cell death and neurocognitive decline is uncertain. It is conceivable that mechanisms other than brain cell death contribute to neurocognitive outcome of neonatal anesthesia. In a series of experiments, we demonstrate that isoflurane exposure causes significant hypercarbia in postnatal day 7 rats and that exposure to isoflurane or carbon dioxide for 4 h provoked brain cell death. However, 1 h of isoflurane exposure was not sufficient to cause brain cell death. Moreover, only 4 h of isoflurane exposure, but not 1 or 2 h of exposure or 4 h of carbon dioxide, led to impaired hippocampal function,questioning the association between anesthesia-induced brain cell death and neurocognitive dysfunction. Neurogenesis both in the developing and adult dentate gyrus is important for hippocampal function, specifically learning and memory. γ-Amino-butyric-acid regulates proliferation and neuronal differentiation both in the developing and the adult brain. Inhaled anesthetics are γ-amino-butyric-acid-ergic and may therefore affect neurogenesis, which could be an alternative mechanism mediating anesthesia-induced neurocognitive decline in immature rats. Understanding the mechanism will help guide clinical trials aiming to define the scope of the problem in humans and may lead to preventive and therapeutic strategies.

Effect of general anesthesia in infancy on long-term recognition memory in humans and rats

Anesthesia in infancy impairs performance in recognition memory tasks in mammalian animals, but it is unknown if this occurs in humans. Successful recognition can be based on stimulus familiarity or recollection of event details. Several brain structures involved in recollection are affected by anesthesia-induced neurodegeneration in animals. Therefore, we hypothesized that anesthesia in infancy impairs recollection later in life in humans and rats. Twenty eight children ages 6-11 who had undergone a procedure requiring general anesthesia before age 1 were compared with 28 age- and gender-matched children who had not undergone anesthesia. Recollection and familiarity were assessed in an object recognition memory test using receiver operator characteristic analysis. In addition, IQ and Child Behavior Checklist scores were assessed. In parallel, thirty three 7-day-old rats were randomized to receive anesthesia or sham anesthesia. Over 10 months, recollection and familiarity were assessed using an odor recognition test. We found that anesthetized children had significantly lower recollection scores and were impaired at recollecting associative information compared with controls. Familiarity, IQ, and Child Behavior Checklist scores were not different between groups. In rats, anesthetized subjects had significantly lower recollection scores than controls while familiarity was unaffected. Rats that had undergone tissue injury during anesthesia had similar recollection indices as rats that had been anesthetized without tissue injury. These findings suggest that general anesthesia in infancy impairs recollection later in life in humans and rats. In rats, this effect is independent of underlying disease or tissue injury.

Early exposure to volatile anesthetics impairs long-term associative learning and recognition memory

BACKGROUND

Anesthetic exposure early in life affects neural development and long-term cognitive function, but our understanding of the types of memory that are altered is incomplete. Specific cognitive tests in rodents that isolate different memory processes provide a useful approach for gaining insight into this issue.

METHODS

Postnatal day 7 (P7) rats were exposed to either desflurane or isoflurane at 1 Minimum Alveolar Concentration for 4 h. Acute neuronal death was assessed 12 h later in the thalamus, CA1-3 regions of hippocampus, and dentate gyrus. In separate behavioral experiments, beginning at P48, subjects were evaluated in a series of object recognition tests relying on associative learning, as well as social recognition.

RESULTS

Exposure to either anesthetic led to a significant increase in neuroapoptosis in each brain region. The extent of neuronal death did not differ between groups. Subjects were unaffected in simple tasks of novel object and object-location recognition. However, anesthetized animals from both groups were impaired in allocentric object-location memory and a more complex task requiring subjects to associate an object with its location and contextual setting. Isoflurane exposure led to additional impairment in object-context association and social memory.

CONCLUSION

Isoflurane and desflurane exposure during development result in deficits in tasks relying on associative learning and recognition memory. Isoflurane may potentially cause worse impairment than desflurane.

Effect of combining anesthetics in neonates on long-term cognitive function

BACKGROUND

With growing evidence that anesthesia exposure in infancy affects cognitive development, it is important to understand how distinct anesthetic agents and combinations can alter long-term memory. Investigations of neuronal death suggest that combining anesthetic agents increases the extent of neuronal injury. However, it is unclear how the use of simultaneously combined anesthetics affects cognitive outcome relative to the use of a single agent.

METHODS

Postnatal day 7 (P7) male rats were administered either sevoflurane as a single agent or the combined delivery of sevoflurane with nitrous oxide at 1 Minimum Alveolar Concentration for 4 h. Behavior was assessed in adulthood using the forced alternating T-maze, social recognition, and context-specific object recognition tasks.

RESULTS

Animals exposed to either anesthetic were unimpaired in the forced alternating T-maze test and had intact social recognition. Subjects treated with the combined anesthetic displayed a deficit, however, in the object recognition task, while those treated with sevoflurane alone were unaffected.

CONCLUSION

A combined sevoflurane and nitrous oxide anesthetic led to a distinct behavioral outcome compared with sevoflurane alone, suggesting that the simultaneous use of multiple agents may uniquely influence early neural and cognitive development and potentially impacts associative memory.

Isoflurane exposure in newborn rats induces long-term cognitive dysfunction in males but not females

Volatile anesthetics are used widely for achieving a state of unconsciousness, yet these agents are incompletely understood in their mechanisms of action and effects on neural development. There is mounting evidence that children exposed to anesthetic agents sustain lasting effects on learning and memory. The explanation for these behavioral changes remains elusive, although acute neuronal death after anesthesia is commonly believed to be a principal cause. Rodent models have shown that isoflurane exposure in newborns induces acute neuroapoptosis and long-term cognitive impairment. However, the assessment of predisposing factors is lacking. We investigated the role of sex by delivering isoflurane to postnatal day (P)7 male and female Sprague Dawley rats for 4 h. Brain cell death was assessed 12 h later using FluoroJade C staining in the thalamus, CA1-3 regions of hippocampus, and dentate gyrus. Behavior was assessed separately using a series of object recognition tasks and a test of social memory beginning at P38. We found that isoflurane exposure significantly increased neuronal death in each brain region with no difference between sexes. Behavioral outcome was also equivalent in simple novel object recognition. However, only males were impaired in the recognition of objects in different locations and contexts. Males also exhibited deficient social memory while females were intact. The profound behavioral impairment in males relative to females, in spite of comparable cell death, suggests that males are more susceptible to long-term cognitive effects and this outcome may not be exclusively attributed to neuronal death.

Accuracy of the Lifebox pulse oximeter during hypoxia in healthy volunteers

Pulse oximetry is a standard of care during anaesthesia in high-income countries. However, 70% of operating environments in low- and middle-income countries have no pulse oximeter. The 'Lifebox' oximetry project set out to bridge this gap with an inexpensive oximeter meeting CE (European Conformity) and ISO (International Organization for Standardization) standards. To date, there are no performance-specific accuracy data on this instrument. The aim of this study was to establish whether the Lifebox pulse oximeter provides clinically reliable haemoglobin oxygen saturation (Sp O2 ) readings meeting USA Food and Drug Administration 510(k) standards. Using healthy volunteers, inspired oxygen fraction was adjusted to produce arterial haemoglobin oxygen saturation (Sa O2 ) readings between 71% and 100% measured with a multi-wavelength oximeter. Lifebox accuracy was expressed using bias (Sp O2 - Sa O2 ), precision (SD of the bias) and the root mean square error (Arms). Simultaneous readings of Sa O2 and Sp O2 in 57 subjects showed a mean (SD) bias of -0.41% (2.28%) and Arms 2.31%. The Lifebox pulse oximeter meets current USA Food and Drug Administration standards for accuracy, thus representing an inexpensive solution for patient monitoring without compromising standards.

Technical communication: stability of propofol in polystyrene-based tissue culture plates

Propofol has been reported to have high stability in glass and relatively high stability up to 24 hours in polyvinyl chloride-based medical plastics. Recent publications have observed the effects of propofol on cells and tissues grown in culture. Many cell culture plastics are formulated from polystyrene but we could find little information on the stability of propofol exposed to these products. We observed very little change in the concentration of propofol diluted in cell culture medium over 24 hours when exposed to glass, but substantial loss of the drug when exposed to 96-well polystyrene cell culture plates. This decrease was most rapid in the first hour but continued until 24 hours. The type of plastic used in cell and tissue culture experiments with propofol may influence the results by increasing the apparent dose required to see an effect.

Distinct long-term neurocognitive outcomes after equipotent sevoflurane or isoflurane anaesthesia in immature rats

BACKGROUND

Many anaesthetics when given to young animals cause cell death and learning deficits that persist until much later in life. Recent attempts to compare the relative safety or toxicity between different agents have not adequately controlled for the relative dose of anaesthetic given, thereby making direct comparisons difficult.

METHODS

Isoflurane or sevoflurane were given at 1 minimum alveolar concentration (MAC) for 4 h to postnatal day 7 (P7) rat pups. Beginning at P75 these animals underwent fear conditioning and at P83 Morris water maze testing to assess working memory, short-term memory and early long-term memory using delays of 1 min, 1 h, and 4 h.

RESULTS

No difference between groups was seen in fear conditioning experiments. Morris water maze learning was equivalent between groups, and no difference was seen in working memory. Sevoflurane-treated animals had a deficit in early long-term memory, and isoflurane-treated animals had a deficit in both short-term and early long-term memory.

CONCLUSIONS

Both isoflurane and sevoflurane delivered at 1 MAC for 4 h to immature rats caused a deficit in long-term memory. Isoflurane also caused a deficit in short-term memory. Isoflurane might be more detrimental than sevoflurane in very young animals.

Accuracy of carboxyhemoglobin detection by pulse CO-oximetry during hypoxemia

BACKGROUND

Carbon monoxide poisoning is a significant problem in most countries, and a reliable method of quick diagnosis would greatly improve patient care. Until the recent introduction of a multiwavelength "pulse CO-oximeter" (Masimo Rainbow SET(®) Radical-7), obtaining carboxyhemoglobin (COHb) levels in blood required blood sampling and laboratory analysis. In this study, we sought to determine whether hypoxemia, which can accompany carbon monoxide poisoning, interferes with the accurate detection of COHb.

METHODS

Twelve healthy, nonsmoking, adult volunteers were fitted with 2 standard pulse-oximeter finger probes and 2 Rainbow probes for COHb detection. A radial arterial catheter was placed for blood sampling during 3 interventions: (1) increasing hypoxemia in incremental steps with arterial oxygen saturations (SaO2) of 100% to 80%; (2) normoxia with incremental increases in %COHb to 12%; and (3) elevated COHb combined with hypoxemia with SaO2 of 100% to 80%. Pulse-oximeter (SpCO) readings were compared with simultaneous arterial blood values at the various increments of hypoxemia and carboxyhemoglobinemia (≈25 samples per subject). Pulse CO-oximeter performance was analyzed by calculating the mean bias (SpCO - %COHb), standard deviation of the bias (precision), and the root-mean-square error (A(rms)).

RESULTS

The Radical-7 accurately detected hypoxemia with both normal and elevated levels of COHb (bias mean ± SD: 0.44% ± 1.69% at %COHb <4%, and -0.29% ± 1.64% at %COHb ≥4%, P < 0.0001, and A(rms) 1.74% vs 1.67%). COHb was accurately detected during normoxia and moderate hypoxia (bias mean ± SD: -0.98 ± 2.6 at SaO2 ≥95%, and -0.7 ± 4.0 at SaO2 <95%, P = 0.60, and A(rms) 2.8% vs 4.0%), but when SaO2 decreased below approximately 85%, the pulse CO-oximeter always gave low signal quality errors and did not report SpCO values.

CONCLUSIONS

In healthy volunteers, the Radical-7 pulse CO-oximeter accurately detects hypoxemia with both low and elevated COHb levels, and accurately detects COHb, but only reads SpCO when SaO2 is more than approximately 85%.

Delayed environmental enrichment reverses sevoflurane-induced memory impairment in rats

BACKGROUND

Anesthesia given to immature rodents causes cognitive decline, raising the possibility that the same might be true for millions of children undergoing surgical procedures under general anesthesia each year. We tested the hypothesis that anesthesia-induced cognitive decline in rats is treatable. We also tested if anesthesia-induced cognitive decline is aggravated by tissue injury.

METHODS

Seven-day old rats underwent sevoflurane anesthesia (1 minimum alveolar concentration, 4 h) with or without tail clamping. At 4 weeks, rats were randomized to environmental enrichment or normal housing. At 8 weeks rats underwent neurocognitive testing, which consisted of fear conditioning, spatial reference memory, and water maze-based memory consolidation tests, and interrogated working memory, short-term memory, and early long-term memory.

RESULTS

Sevoflurane-treated rats had a greater escape latency when the delay between memory acquisition and memory retrieval was increased from 1 min to 1 h, indicating that short-term memory was impaired. Delayed environmental enrichment reversed the effects of sevoflurane on short-term memory and generally improved many tested aspects of cognitive function, both in sevoflurane-treated and control animals. The performance of tail-clamped rats did not differ from those rats receiving anesthesia alone.

CONCLUSION

Sevoflurane-induced cognitive decline in rats is treatable. Delayed environmental enrichment rescued the sevoflurane-induced impairment in short-term memory. Tissue injury did not worsen the anesthesia-induced memory impairment. These findings may have relevance to neonatal and pediatric anesthesia.

Beyond anesthetic properties: the effects of isoflurane on brain cell death, neurogenesis, and long-term neurocognitive function

Anesthetic drugs cause brain cell death and long-term neurocognitive dysfunction in neonatal rats. Recently, human data also suggest that anesthesia early in life may cause cognitive impairment. The connection between cell death and neurocognitive decline is uncertain. It is conceivable that mechanisms other than brain cell death contribute to neurocognitive outcome of neonatal anesthesia. In a series of experiments, we demonstrate that isoflurane exposure causes significant hypercarbia in postnatal day 7 rats and that exposure to isoflurane or carbon dioxide for 4 h provoked brain cell death. However, 1 h of isoflurane exposure was not sufficient to cause brain cell death. Moreover, only 4 h of isoflurane exposure, but not 1 or 2 h of exposure or 4 h of carbon dioxide, led to impaired hippocampal function,questioning the association between anesthesia-induced brain cell death and neurocognitive dysfunction. Neurogenesis both in the developing and adult dentate gyrus is important for hippocampal function, specifically learning and memory. γ-Amino-butyric-acid regulates proliferation and neuronal differentiation both in the developing and the adult brain. Inhaled anesthetics are γ-amino-butyric-acid-ergic and may therefore affect neurogenesis, which could be an alternative mechanism mediating anesthesia-induced neurocognitive decline in immature rats. Understanding the mechanism will help guide clinical trials aiming to define the scope of the problem in humans and may lead to preventive and therapeutic strategies.

Increasing the duration of isoflurane anesthesia decreases the minimum alveolar anesthetic concentration in 7-day-old but not in 60-day-old rats

BACKGROUND

While studying neurotoxicity in rats, we observed that the anesthetic minimum alveolar anesthetic concentration (MAC) of isoflurane decreases with increasing duration of anesthesia in 7-day-old but not in 60-day-old rats. After 15 min of anesthesia in 7-day-old rats, MAC was 3.5% compared with 1.3% at 4 h. We investigated whether kinetic or dynamic factors mediated this decrease.

METHODS

In 7-day-old rats, we measured inspired and cerebral partial pressures of isoflurane at MAC as a function of duration of anesthesia. In 60-day-old rats, we measured inspired partial pressures of isoflurane at MAC as a function of duration of anesthesia. Finally, we determined the effect of administering 1 mg/kg naloxone and of delaying the initiation of the MAC determination (pinching the tail) on MAC in 7-day-old rats.

RESULTS

In 7-day-old rats, both inspired and cerebral measures of MAC decreased from 1 to 4 h. The inspired MAC decreased 56%, whereas the cerebral MAC decreased 33%. At 4 h, the inspired MAC approximated the cerebral MAC (i.e., the partial pressures did not differ appreciably). Neither administration of 1 mg/kg naloxone nor delaying tail clamping until 3 h reversed the decrease in MAC. In 60-day-old rats, inspired MAC of isoflurane was stable from 1 to 4 h of anesthesia.

CONCLUSIONS

MAC of isoflurane decreases over 1-4 h of anesthesia in 7-day-old but not in 60-day-old rats. Both pharmacodynamic and a pharmacokinetic components contribute to the decrease in MAC in 7-day-old rats. Neither endorphins nor sensory desensitization mediate the pharmacodynamic component.

Somatostatin inhibits IGF-1 mediated induction of VEGF in human retinal pigment epithelial cells

Neovascularization stimulated by IGF-1 mediated induction of vascular endothelial growth factor (VEGF) is one of the leading causes of blindness in humans. It plays a central role in the pathogenesis of proliferative diabetic retinopathy (DR), neovascular glaucoma, exudative age-related macular degeneration (AMD) and retinopathy of prematurity. Neovascularization is a multi-step process that involves complex interactions of a variety of mitogenic factors such as VEGF and IGF-I which are produced locally in the human eye by a variety of cells including retinal pigment epithelial (RPE) cells, retinal capillary pericytes, endothelial cells, Mueller cells and ganglion cells. We hypothesized that somatostatin would inhibit the IGF-1 signal transduction pathway in RPE cells, resulting in decreased VEGF production. We have observed expression of somatostatin receptor protein in retinal pigment epithelial (RPE) cells of the human eye using immunohistochemistry and have confirmed expression of somatostatin receptors in cultured human RPE cells using reverse transcriptase-PCR. IGF-1 induced a dose dependent increase in IGF-1R phosphorylation and in VEGF mRNA levels in cultured human RPE cells. Somatostatin and octreotide, a somatostatin analogue, inhibited IGF-1 receptor (IGF-1R) phosphorylation and decreased VEGF production. Both IGF-1R phosphorylation and accumulation of VEGF mRNA were inhibited by physiological levels of somatostatin and octreotide (1 nM). These results demonstrate somatostatin and octreotide mediated attenuation of both IGF-1R signal transduction and VEGF mRNA accumulation via somatostatin receptor type 2 (sst2). Furthermore, these data suggest a rationale for the use of octreotide as a prophylactic and therapeutic option in disease states that cause ocular neovascularization.

Somatostatin receptor gene expression in human ocular tissues: RT-PCR and immunohistochemical study

PURPOSE

Somatostatin (SST) analogues have been used to treat proliferative diabetic retinopathy, pseudotumor cerebri, thyroid orbitopathy, and cystoid macular edema. There is a paucity of published data in regards to cell-specific distribution of SST receptors (SSTR) in normal human eye tissues. Gene expression for all five known SSTRs in normal human ciliary body/iris, retina, choroid, and cultured retinal pigment epithelial (RPE) cells were studied.

METHODS

mRNA was isolated from human ocular tissues (iris/ciliary body, retina, and choroid) dissected from eight pairs of normal eyes (9-62 years) and from RPE cells grown in culture. RT-PCR was done for all five SSTRs in all analyzed tissues. Immunohistochemistry for SSTR1 and SSTR2 was performed on eight pairs of normal human eyes (28-74 years) imbedded in paraffin.

RESULTS

SSTR1 to 5 genes are expressed in retina, SSTR1 and SSTR2 genes in cultured RPE cells, and SSTR1, 2, and 4 in ciliary body and choroid. SSTR1 and SSTR2 immunoreactivity (-ir) was observed on a variety of cells within all analyzed tissues including cornea, iris, trabecular meshwork, Schlemm's canal, ciliary processes, ciliary muscle, retina, choroid, cultured RPE cells, and optic nerve.

CONCLUSIONS

SSTR genes are widely expressed in normal human eye tissues, with genes for SSTR1 and SSTR2 being the most widely expressed. Genes for all SSTRs are expressed in retina. SSTR1-ir and SSTR2-ir were observed in all analyzed ocular tissues. Detailed knowledge of SSTRs distribution and function in the human eye will result in a better understanding of their role in health and disease.