Pharmacological challenges examining the underlying mechanism of altered response inhibition and attention due to circadian disruption in adult Long-Evans rats.

Endogenous circadian rhythms govern behavior and physiology, while circadian disruption is an environmental factor that impacts cognition by altering the circadian clock at a molecular level. We modeled the effects of 2 sources of circadian disruption - activity occurring during typical rest periods and untimely light exposure - to evaluate the effects of circadian disruption on behavior and underlying neurochemistry. Firstly, adult Long-Evans rats of both sexes were maintained on a 12 h:12 h light:dark cycle and tested using a 5-choice serial reaction time task (5-CSRTT) under 3 conditions: 4 h into the dark phase with no exposure to ambient light during testing (control), 4 h into the dark phase with exposure to ambient light during testing, and 4 h into the light phase. Both models resulted in impulsive behavior and reduced attention compared to control. We established that changes in the diurnal expression pattern occur in the clock gene Period 2 (Per2) in the light phase-tested model. Choline acetyltransferase (Chat) and Dopamine receptor 1 (Drd1) showed rhythmic expression with peak expression during the dark phase regardless of light-testing condition. Next, we performed drug challenges in a new rat cohort to examine the interaction between the cholinergic and dopaminergic neurotransmitter systems in regulating the behavioral changes caused by circadian disruption. We administered the cholinergic agonist nicotine and either the dopamine-1 receptor (DR1) antagonist SCH23390 or the DR2 antagonist eticlopride under the 3 circadian conditions to identify differential drug responses between treatment groups. Rats in both models demonstrated increased sensitivity to nicotine as compared to control, while SCH23390 and eticlopride ameliorated the effect of nicotine on 5-CSRTT performance in both models. Our study is the first to identify detrimental effects of both models of circadian disruption on impulsive behavior, and that the effects of circadian disruption are mediated by an interaction between cholinergic and dopaminergic systems.

Toxicology of Avermectins and Milbemycins (Macrocyclic Lactones) and the Role of P-Glycoprotein in Dogs and Cats.

Overdoses of macrocyclic lactones in dogs and cats can result in such signs as tremors, ataxia, seizures, coma, and blindness. Dogs with the ABCB1-1Δ gene defect are predisposed to macrocyclic lactone toxicosis at lower dosages than dogs without the defect. Intravenous lipid emulsion therapy has been suggested for treatment of macrocyclic lactone toxicosis but evidence of efficacy is limited. Initial decontamination and supportive care remain the mainstays of therapy for macrocyclic lactone toxicosis.

Cholinergic and dopaminergic interactions alter attention and response inhibition in Long-Evans rats performing the 5-choice serial reaction time task.

Acetylcholine (ACh) neurotransmission is important for attention, while dopamine (DA) signaling modulates impulsive behavior. Prior studies have established an existing relationship between ACh and DA that mediates dopamine release in the prefrontal cortex of the brain in rats performing the 5-choice serial reaction time task (5-CSRTT). This study is aimed to identify cholinergic and dopaminergic interactions that govern attention and impulsive behavior, using adult Long-Evans rats of both sexes and a 5-CSRTT, with variable short and long cue delays. In Experiment 1, the effects of single cholinergic and dopaminergic drugs were evaluated on 5-CSRTT performance. Drugs like nicotinic ACh receptor (nAChR) agonist nicotine, amphetamine, and GBR12909 that increase the synaptic levels of ACh and DA respectively all increased impulsive behavior. In addition, amphetamine and GBR 12909 decreased attention while nicotine had no effect on attention. The antagonists mecamylamine, a general nAChR antagonist, flupenthixol a DA 1/2 receptor antagonist, and SCH 23390 a DA 1 receptor antagonist, all decreased impulsive behavior, with mixed effects on attention. In contrast, dihydro-ß-erythroidine hydrobromide (DHBE), an α4ß2 subunit-specific nAChR antagonist, had no significant effects on attention or impulsivity across doses administered. Eticlopride, a DA 2 receptor antagonist, decreased attention at the shortest cue delay but did not affect impulsivity. The acetylcholinesterase inhibitor donepezil decreased both attention and impulsive behavior. Subsequently in Experiment 2, effects of nicotine and amphetamine were determined after pretreatment with SCH 23390 or eticlopride. SCH 23390 attenuated the effects of nicotine and amphetamine to increase impulsivity, while eticlopride only attenuated the effect of nicotine on impulsivity. Minimal effects were seen on attention in the combination trials. This study confirms that dopamine D1 receptor plays an essential role in modulation of impulsive behavior, as measured by the 5-CSRTT. More importantly, it establishes that impulsive behavior is altered by interactions between cholinergic and dopaminergic neurotransmission.

Circadian disruption affects initial learning but not cognitive flexibility in an automated set-shifting task in adult Long-Evans rats.

A sizeable percent of adults are subject to circadian disturbances such as shift work, which involves misalignment of time of light exposure, activity periods, sleep, and eating. Chronic adherence to disruptive circadian schedules can negatively impact cognitive functioning. Developing preclinical models of circadian disruption allow investigation of the relationship and underlying mechanisms between circadian disruption and cognitive functioning. We placed adult Long-Evans rats of both sexes on a 12:12h light:dark schedule in which rats performed an automated operant-behavior task for 3months, with daily testing occurring either 4h after lights-on or lights-off. At the end of this period, rats were tested on an automated set-shifting task to compare the effects of the 2 testing schedules on cognitive flexibility, which is the focus of this report. Over the initial 3-month period, day-tested rats shifted to a diurnal activity schedule, with males shifting more effectively than females, while night-tested rats remained nocturnal. Upon beginning the set-shifting task, night-tested rats took longer to reach criterion performance in the initial, visual-cue detection stage as compared to day-tested rats. The groups did not differ in performance on subsequent egocentric-cue based and reversal phases. Sex-related differences in task performance unrelated to testing schedule, particularly longer latencies to lever press in females, were also detected. One possible explanation for our findings is that the night-tested rats also experienced a form of circadian disruption when they were exposed to ambient light during the daily testing sessions, and that the form they experienced was more detrimental to initial acquisition of the task than testing during the light phase. Subsequent experiments will incorporate a night-tested group that is not exposed to ambient light in order to better understand the effect seen in the night-tested rats in the current study.

Developmental PCB Exposure Increases Audiogenic Seizures and Decreases Glutamic Acid Decarboxylase in the Inferior Colliculus.

Previously, we observed that developmental polychlorinated biphenyl (PCB) exposure resulted in an increase in audiogenic seizures (AGSs) in rats. However, the rats were exposed to loud noise in adulthood, and were not tested for AGS until after 1 year of age, either of which could have interacted with early PCB exposure to increase AGS susceptibility. This study assessed susceptibility to AGS in young adult rats following developmental PCB exposure alone (without loud noise exposure) and investigated whether there was a decrease in GABA inhibitory neurotransmission in the inferior colliculus (IC) that could potentially explain this effect. Female Long-Evans rats were dosed orally with 0 or 6 mg/kg/day of an environmentally relevant PCB mixture from 28 days prior to breeding until the pups were weaned at postnatal day 21. One male-female pair from each litter was retained for the AGS study whilst another was retained for Western blot analysis of glutamic acid decarboxylase (GAD) and GABAAα1 receptor in the IC, the site in the auditory midbrain where AGS are initiated. There was a significant increase in the number and severity of AGSs in the PCB groups, with females somewhat more affected than males. GAD65 was decreased but there was no change in GAD67 or GABAAα1 in the IC indicating decreased inhibitory regulation in the PCB group. These results confirm that developmental PCB exposure alone is sufficient to increase susceptibility to AGS, and provide the first evidence for a possible mechanism of action at the level of the IC.

Canine renal pathology associated with grape or raisin ingestion: 10 cases.

Ten dogs suffered acute renal failure after ingesting > or = 3 g/kg (dry matter) of grapes or raisins. All dogs had degeneration or necrosis (or both) of proximal renal tubules with basement membranes remaining intact, and epithelial regeneration was observed in 5 out of 10 cases. Mineralized tubular debris or granular to proteinaceous casts (or both) were present in all cases. A golden-brown, globular, intracellular pigment of varying amounts and sizes was observed in 6 out of 10 cases with variable reaction with Prussian blue. Multifocal fibrinous arteritis of the large colon was seen in 2 out of 5 cases with globulin insudation of vessel wall demonstrated by immunohistochemical staining for immunoglobulin (Ig)G and IgM. Mineral analysis on frozen renal tissue from 2 out of 2 cases revealed mildly elevated Ca:P ratio in both. Clinically significant observations were preservation of the integrity of basement membranes after grape-induced tubular injury and presence of early epithelial regeneration. Thus, recovery may be possible if anuria is aggressively managed. With respect to potential pathophysiologic mechanisms, further research into the roles of calcium homeostasis, vascular reactivity, and the significance of the golden-brown pigment is indicated.

Acute renal failure in dogs after the ingestion of grapes or raisins: a retrospective evaluation of 43 dogs (1992-2002).

A review of records from the AnTox database of the American Society for the Prevention of Cruelty to Animals Animal Poison Control Center identified 43 dogs that developed increased blood urea nitrogen concentration, serum creatinine concentration, or both as well as clinical signs after ingesting grapes, raisins, or both. Clinical findings, laboratory findings, histopathological findings, treatments performed, and outcome were evaluated. All dogs vomited, and lethargy, anorexia, and diarrhea were other common clinical signs. Decreased urine output, ataxia, or weakness were associated with a negative outcome. High calcium x phosphorus product (Ca x P), hyperphosphatemia, and hypercalcemia were present in 95%, 90%, and 62% of the dogs in which these variables were evaluated. Extremely high initial total calcium concentration, peak total calcium concentration, initial Ca x P, and peak Ca x P were negative prognostic indicators. Proximal renal tubular necrosis was the most consistent finding in dogs for which histopathology was evaluated. Fifty-three percent of the 43 dogs survived, with 15 of these 23 having a complete resolution of clinical signs and azotemia. Although the mechanism of renal injury from grapes and raisins remains unclear, the findings of this study contribute to an understanding of the clinical course of acute renal failure that can occur after ingestion of grapes or raisins in dogs.

Sex differences in response to amphetamine in adult Long-Evans rats performing a delay-discounting task.

The use of animal models to investigate experimental questions about impulsive behavior can provide valuable insight into problems that affect human health. The delay-discounting paradigm involves subjects choosing between smaller reinforcers delivered immediately and larger reinforcers that are delivered after a delay. This is an important experimental paradigm for examining impulsive choice in both laboratory species and humans. However, a shortcoming of previously published delay-discounting studies in animals is that typically only males were studied, reducing the applicability of these studies to human populations. In the present study, both female and male adult Long-Evans rats were trained to perform a delay-discounting task, with delays of 0, 5, 10, 20 and 40 s before delivery of the larger reinforcer. Because dopaminergic signaling is important in mediating this task, the effects of d-amphetamine and the dopamine receptor antagonist, cis-flupenthixol, on task performance were then examined. The main experimental measure was percent larger-reinforcer choice, which was defined as the percentage of experimental trials at each delay in which the delayed, larger reinforcer was chosen. There was no sex difference in percent larger-reinforcer choice during baseline performance of the task. However, d-amphetamine administration disrupted choice in females, as evidenced by <80% larger-reinforcer choice in half of the females, but none of the males, at 0.5 mg/kg. D-Amphetamine also differentially altered the latency to choose between immediate versus delayed reinforcers in females compared to males. In contrast, cis-flupenthixol did not have a sex-related effect on percent larger-reinforcer choice. These findings parallel the sex differences in response to amphetamine seen in human delay-discounting studies and underscore the importance of evaluating sex-based differences in baseline performance and in response to pharmacologic agents when utilizing animal models.

Toxicology of avermectins and milbemycins (macrocylic lactones) and the role of P-glycoprotein in dogs and cats.

The macrocyclic lactones (MLs) are parasiticides able to kill a wide variety of arthropods and nematodes. They have a high margin of safety for labeled indications, and ivermectin has become the best-selling antiparasitic in the world. Dogs of certain breeds and mixtures of those breeds have a defect in the ABCB1 gene (formerly MDR1 gene) that results in a lack of functional P-glycoprotein, which leads to accumulation of the MLs in the central nervous system and a higher risk of adverse effects when exposed. There is no specific antidote for ML toxicosis so the most important part of treatment is good supportive care.

Discriminative stimulus effects of cocaine and amphetamine in rats following developmental exposure to polychlorinated biphenyls (PCBs).

Polychlorinated biphenyls (PCBs) are environmental neurotoxicants known to affect the brain dopaminergic (DA) system. This project investigated whether developmental exposure to PCBs would alter the discriminative stimulus effects of psychostimulant drugs known to act on the DA system. Female Long-Evans rats were orally exposed to 0, 3, or 6 mg/kg/day of an environmentally relevant PCB mixture from four weeks prior to breeding through weaning of their litters on PND 21. When they reached adulthood one male and female/litter were trained to discriminate cocaine (10.0 mg/kg, IP) from saline by repeatedly pairing cocaine injections with reinforcement on one operant response lever, and saline injections with reinforcement on the other lever. After response training, generalization tests to four lower doses of cocaine (7.5, 5.0, 2.5, and 1.25 mg/kg, IP) and to amphetamine (1.0, 0.5, 0.25, and 0.125 mg/kg, IP) were given two days/week, with additional training dose days in-between. Percent responding of the PCB-exposed rats on the cocaine-paired lever was significantly higher than that of controls for the highest generalization dose of cocaine, and lower than that of controls for the highest dose of amphetamine. Response rate and percent responding on the cocaine lever did not differ among the exposure groups on the days when the training dose of cocaine was given, suggesting that the generalization test results were not due to pre-existing differences in discrimination ability or rate of responding. These findings suggest that developmental PCB exposure can alter the interoceptive cues of psychostimulants.

Attention deficit/hyperactivity disorder: a focused overview for children's environmental health researchers.

OBJECTIVES: Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed childhood neurobehavioral disorder. Much research has been done to identify genetic, environmental, and social risk factors for ADHD; however, we are still far from fully understanding its etiology. In this review we provide an overview of diagnostic criteria for ADHD and what is known about its biological basis. We also review the neuropsychological functions that are affected in ADHD. The goal is to familiarize the reader with the behavioral deficits that are hallmarks of ADHD and to facilitate comparisons with neurobehavioral deficits associated with environmental chemical exposures. DATA SOURCES: Relevant literature on ADHD is reviewed, focusing in particular on meta-analyses conducted between 2004 and the present that evaluated associations between measures of neuropsychological function and ADHD in children. Meta-analyses were obtained through searches of the PubMed electronic database using the terms "ADHD," "meta-analysis," "attention," "executive," and "neuropsychological functions." Although meta-analyses are emphasized, nonquantitative reviews are included for particular neuropsychological functions where no meta-analyses were available. DATA SYNTHESIS: The meta-analyses indicate that vigilance (sustained attention), response inhibition, and working memory are impaired in children diagnosed with ADHD. Similar but somewhat less consistent meta-analytic findings have been reported for impairments in alertness, cognitive flexibility, and planning. Additionally, the literature suggests deficits in temporal information processing and altered responses to reinforcement in children diagnosed with ADHD. Findings from brain imagining and neurochemistry studies support the behavioral findings. CONCLUSIONS: Behavioral, neuroanatomical, and neurochemical data indicate substantial differences in attention and executive functions between children diagnosed with ADHD and non-ADHD controls. Comparisons of the neurobehavioral deficits associated with ADHD and those associated with exposures to environmental chemicals may help to identify possible environmental risk factors for ADHD and/or reveal common underlying biological mechanisms.

Lead and PCBs as risk factors for attention deficit/hyperactivity disorder.

OBJECTIVES: Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed neurobehavioral disorder of childhood, yet its etiology is not well understood. In this review we present evidence that environmental chemicals, particularly polychlorinated biphenyls (PCBs) and lead, are associated with deficits in many neurobehavioral functions that are also impaired in ADHD. DATA SOURCES: Human and animal studies of developmental PCB or lead exposures that assessed specific functional domains shown to be impaired in ADHD children were identified via searches of PubMed using "lead" or "PCB exposure" in combination with key words, including "attention," "working memory," "response inhibition," "executive function," "cognitive function," "behavior," and "ADHD." DATA SYNTHESIS: Children and laboratory animals exposed to lead or PCBs show deficits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance, and alertness. Studies conducted to date suggest that lead may reduce both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low-level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted. CONCLUSIONS: We speculate that exposures to environmental contaminants, including lead and PCBs, may increase the prevalence of ADHD.

Developmental exposure to PCBs and/or MeHg: effects on a differential reinforcement of low rates (DRL) operant task before and after amphetamine drug challenge.

The current study assessed the effects of developmental PCB and/or MeHg exposure on an operant task of timing and inhibitory control and determined if amphetamine (AMPH) drug challenges differentially affected performance. Long-Evans rats were exposed to corn oil (control), PCBs alone (1 or 3 mg/kg), MeHg alone (1.5 or 4.5 ppm), the low combination (1 mg/kg PCBs+1.5 ppm MeHg), or the high combination (3 mg/kg PCBs+4.5 ppm MeHg) throughout gestation and lactation. An environmentally relevant, formulated PCB mixture was used. Male and female offspring were trained to asymptotic performance on a differential reinforcement of low rates (DRL) operant task as adults. PCB-exposed groups had a lower ratio of reinforced to non-reinforced responses than controls. Groups exposed to MeHg alone were not impaired and the deficits observed in PCB-exposed groups were not seen when PCBs were co-administered with MeHg. AMPH was less disruptive to responding in males receiving PCBs alone, MeHg alone, and 1.0 mg/kg PCB+1.5 ppm MeHg. Paradoxically, the disruption in responding by AMPH in males given 3.0 mg/kg PCB+4.5 ppm MeHg did not differ from controls. Exposed females from all treatment groups did not differ from controls in their AMPH response. Overall, the findings suggest that developmental exposure to PCBs can decrease DRL performance. Co-exposure to MeHg seemed to mitigate the detrimental effects of PCBs on performance. The finding that the disruptive effects of AMPH on DRL performance were lessened in some groups of exposed males suggests that alterations in dopaminergic functioning may have a role in behavioral changes seen after perinatal PCB and MeHg exposure.

Gastric outflow obstruction after ingestion of wood glue in a dog.

A 2-year-old, male, mixed-breed dog presented with a 12-day history of vomiting, depression, and weight loss after ingestion of industrial-strength wood glue containing diphenylmethane diisocyanate as its active ingredient. A diagnosis of gastric foreign body was made from survey abdominal radiographs. A large aggregate of solidified wood glue was surgically removed, and the dog recovered uneventfully. Fourteen other cases have been reported to the Animal Poison Control Center at the American Society for the Prevention of Cruelty to Animals (ASPCA). Eight of those 14 cases required surgical intervention. All cases recovered completely.