Relationship between cholinesterase inhibition and thermoregulation following exposure to diisopropyl fluorophosphate in the rat

Survey on Hypothermia and Hyperthermia in Poisoned Patients in a Unique Referral Hospital, Tehran, Iran

Background

Body temperature is a critical criterion of health. Drugs and a variety of poisons can affect body temperature in poisoned patients, causing hyperthermia and hyperpyrexia.

Objectives

Our previous study’s findings in patients poisoned with organophosphate led us to the goal of this study: obtaining the initial tympanic temperature in patients poisoned by a variety of toxins.

Materials and Methods

A cross-sectional study reviewed the records of poisoned patients who were admitted to the toxicological intensive care unit (TICU) at Loghman Hakim hospital poison center (LHHPC) from February 2014 to February 2015. The data collected included gender, age, type of poisoning, the season during which poisoning occurred, vital signs, initial tympanic temperature (first four hours), presence of seizures, white blood cell (WBC) count, creatinine phosphokinase (CPK), length of stay and patient outcome. We determined the mean (SD) for normally distributed continuous variables, the median and interquartile range for non-normally distributed continuous variables, and the absolute and relative frequency (%) for categorical variables. All were determined using SPSS version 16.

Results

Data were collected from 310 eligible patients. The mean patient age was 32.65 (with a standard deviation of 14.40). Of the patients in the study, 183 (59%) were male. Intentional poisoning in an attempted suicide was documented in 253 (81.6%) patients. The most prevalent poisoning agent was aluminum phosphate (18.70%), followed by methadone (10%) and opium (10%). Seventy percent of the patients (n = 217) were diagnosed and classified with fever or hyperthermia. A temperature ≥ 40°C was detected in just three cases. The highest mean temperature was found in patients poisoned with amphetamine, organophosphate and tramadol. Patients with alcohol and phenobarbital poisoning were included in the sample, but these patients were not diagnosed with hypothermia. WBC ≥ 10,000 cells/mL and CPK ≥ 975 IU/L were recorded in 57.7% and 13.2% of subjects, respectively.

Conclusions

Body temperature changes in human poisonings are a matter in need of special attention. A literature review did not reveal any controversy over hypothermia, but poisoning cases exhibit a variety of patterns of fever and hyperthermia. If there are no limits to the diagnosis of fever and hyperthermia, all cases with a poor prognosis which fail to respond to treatment could be categorized as drug-induced hyperthermia. Therefore, a different approach is needed for poisoning cases.

Temperature changes among organophosphate poisoned patients, Tehran- Iran

Background

Acute poisoning with organophosphorus compounds (OPs) is a major global clinical problem in the developing countries. There have been many animal studies and few human surveys on the effects of organophosphorus pesticide (OP) poisoning on thermoregulation. The aim of this prospective study was to document the pattern of tympanic temperature changes among OP poisoned patients throughout the length of their hospital stay.

Methods

60 patients with diagnose of organophosphate poisoning were included in this study. Questioner was filled out by trained nurses including demographic, clinical and paraclinical data. Tympanic temperature and Pulse rate data of the cases were collected on five- occasions after admission.

Results

There were 41 (68.3%) male and 19 (31.7%) female, with a mean age of 34.4 ±19.4 years (range 13–89 years). Forty five patients had intentional poisoning for suicidal attempt. At the time of entry, the mean tympanic temperature, pulse rate, respiratory rate and blood pressure (systolic and diastolic) of the OP poisoned patients were respectively 37.1+/−0.6°C (36.0- 39.5), 91+/−18 (55–145), 18+/−5.6 (8–44), 116+/−20 mm Hg (70–170) and 75+/−11.6 mm Hg (40–110). 41.7% of the cases had serum butyryl cholinesterase activities (BChE) ≥ 50% normal (≥1600 mU/ml). Our patients had normal temperature at the time entry (mean = 37.1). Tympanic temperature decreasing below 36°C was not detected among the patients during the study period. A rise in mean tympanic temperature was found after atropine administration.

Conclusion

Our study showed hypothermia was not considerable factor among organophosphate poisoned patients, although more studies with similar situations in tropical countries are needed.

Hypothermia and Fever after organophosphorus poisoning in humans--a prospective case series

There have been many animal studies on the effects of organophosphorus pesticide (OP) poisoning on thermoregulation with inconsistent results. There have been no prospective human studies. Our aim was to document the changes in body temperature with OP poisoning. A prospective study was conducted in a rural hospital in Polonnaruwa, Sri Lanka. We collected data on sequential patients with OP poisoning and analyzed 12 patients selected from 53 presentations who had overt signs and symptoms of OP poisoning and who had not received atropine prior to arrival. All patients subsequently received specific management with atropine and/or pralidoxime and general supportive care. Tympanic temperature, ambient temperature, heart rate, and clinical examination and interventions were recorded prospectively throughout their hospitalization. Initial hypothermia as low as 32°C was observed in untreated patients. Tympanic temperature increased over time from an early hypothermia (<35°C in 6/12 patients) to later fever (7/12 patients >38°C at some later point). While some of the late high temperatures occurred in the setting of marked tachycardia, it was also apparent that in some cases fever was not accompanied by tachycardia, making excessive atropine or severe infection an unlikely explanation for all the fevers. In humans, OP poisoning causes an initial hypothermia, and this is followed by a period of normal to high body temperature. Atropine and respiratory complications may contribute to fever but do not account for all cases.

Development of a rat pilocarpine model of seizure/status epilepticus that mimics chemical warfare nerve agent exposure

We developed a rat pilocarpine seizure/status epilepticus (SE) model, which closely resembles 1.6-2.0 x LD50 soman exposure, to analyse the molecular mechanism of neuronal damage and to screen effective neuroprotectants against cholinergic agonist and chemical warfare nerve agent (CWNA) exposure. Rats implanted with radiotelemetry probes capable of recording electroencephalogram (EEG), electrocardiogram (ECG), temperature, and physical activity were treated with lithium chloride (5 mEq/kg, im), followed 24 h later by (ip) doses of pilocarpine hydrochloride. Based on radiotelemetry analysis, a dose of 240 mg/kg (ip) pilocarpine generated seizure/SE analogous to 1.6-2.0 x LD50 of soman. The model was refined by reducing the peripheral convulsions without affecting the central nervous system (CNS) by administering methylscopolamine bromide (1 mg/kg, ip), an anti-cholinergic that does not cross the blood-brain barrier. However, when methylscopolamine bromide was administered, a higher dose of pilocarpine (320 mg/kg, ip) was required to generate the equivalent seizure/SE. Histopathology data indicated that pilocarpine induces significant damage to the hippocampal region of the brain, with similar neuropathology to that of 1.6-2.0 x LD50 soman exposure. There was a reduction in body temperature after the administration of pilocarpine, as observed in organophosphate (OP) nerve agents exposure. The heart-rate of pilocarpine-treated animals increased compared to the normal range. The pilocarpine seizure/SE model was also reproducible in the absence of lithium chloride. These results support that pilocarpine seizure/SE model is useful in studying the molecular mechanisms of neuropathology and screening neuroprotectants following cholinergic agonist and CWNA exposure.

Thermoregulatory aspects of environmental exposure to anticholinesterase agents

Anticholinesterase (antiChE) agents can be highly toxic to birds and mammals and constitute a major proportion of the pesticides used throughout the world. AntiChEs consist of the organophosphates (OP), which irreversibly inhibit the enzyme acetylcholinesterase (AChE), and the carbamates (CB), which reversibly inhibit AChE. AChE inhibition elicits cholinergic stimulation in the central nervous system and in peripheral tissues and organs, which can lead to marked dysfunction of homeostatic systems, including temperature regulation. The control of body temperature uses cholinergic pathways in the integration and central processing of thermal information, as well as in the control of thermoeffector responses. Hence, the cholinergic stimulation elicited from exposure to antiChEs has profound effects on body temperature at rest as well as during exercise. Ambient heat and cold stress can also modulate the animal's sensitivity to antiChE exposure. After exposure to most OPs, rodents and other small species undergo a marked hypothermic response lasting up to 24 hours. On the other hand, humans exposed to OP pesticides rarely become hypothermic but rather experience a fever that may last many days. Recent studies monitoring body temperature in OP-exposed, telemetered rats demonstrated that the initial hypothermic response is followed by a period of hyperthermia lasting several days. That the hyperthermia can be blocked with administration of sodium salicylate suggests that the hyperthermia is a fever. Thus, the antiChE-induced effects on body temperature and other physiological systems cannot be explained solely by the immediate consequences of AChE inhibition and stimulation of cholinergic systems. Research into the mechanisms of action of antiChE toxicity will be improved with a better understanding of their effects on temperature regulation.

Strain as a determinant factor in the differential responsiveness of rats to chemicals

The beneficial effects derived from the use of chemicals in agriculture, energy production, transportation, pharmaceuticals, and other products that improve the quality of life are clearly established. However, continued exposure to these chemicals is only advantageous in conditions where the benefit far outweighs toxic manifestations. By law, determination of risk of toxicity necessitates the use of laboratory animals to establish whether chemical exposure is safe for humans. To simulate the human condition, it is incumbent upon investigators to choose a species in which pharmacokinetic and toxicokinetic principles are established and resemble those of humans. Some of the advantages to the use of rat in chemical toxicity testing include (a) similarities in metabolism, anatomy, and physiological parameters to humans; (b) the short life span, especially for carcinogenesis study; (c) the availability, ease of breeding, and maintenance at a relatively low cost; and (d) the existence of a large database to enable comparison of present to reported literature findings. However, the choice of rat can be complicated by several factors such as sex, age, and nutrition, but especially strain, where currently there are over 200 different strains of rat known to exist. The aim of this review is to demonstrate that there are differences in the responsiveness of rat strains to chemicals and that the susceptibility observed is dependent on the tissue examined. It is evident that the genotype differs among strains, and this may be responsible for differences in sensitivities to chemicals. Awareness of strain as a factor in susceptibility to toxicant action needs to be taken into account in interpretation of relevance of risk of toxicity for humans.

Autonomic and behavioral responses of selectively bred hypercholinergic rats to oxotremorine and diisopropyl fluorophosphate

The hypercholinergic Flinders Sensitive Line (FSL) rat was significantly more sensitive than the Flinders Resistant Line (FRL) rat to the biotelemetrically recorded hypothermic effects of oxotremorine, a direct-acting muscarinic agonist, and diisopropyl fluorophosphate (DFP), an anticholinesterase agent. The effects of these agents on heart rate and motor activity, also recorded biotelemetrically, indicate either small differences (DFP) or no significant effect (oxotremorine) between the lines. These findings confirm the dramatic differences in temperature responses to cholinergic compounds between FSL and FRL rats, for which they were selectively bred, but suggest that a general increase in the sensitivity of the FSL rats to all muscarinic-mediated responses may not occur.

Thermoregulatory effects of chlorpyrifos in the rat: long-term changes in cholinergic and noradrenergic sensitivity

Subcutaneous injection of a sublethal dose of chlorpyrifos (CHLP), an organophosphate (OP) pesticide, causes long-term inhibition in cholinesterase activity (ChE) of brain, blood, and other tissues. Such prolonged inhibition in ChE should lead to marked behavioral and autonomic thermoregulatory patterns, especially in terms of altered noradrenergic and cholinergic sensitivity. To evaluate the behavioral and autonomic effects of long-term ChE inhibition, Long-Evans rats were implanted with radiotelemetry transmitters that continuously monitored core temperature (Tc), heart rate (HR), and motor activity (MA). These parameters were monitored for 7 days following a single injection of peanut oil (vehicle control) or 280 mg/kg CHLP. CHLP led to a significant reduction in Tc during the first night after treatment but had no other effects on Tc. CHLP also resulted in a significant elevation in HR which lasted for approximately 72 h. Motor activity was unaffected by CHLP. Cholinergic and noradrenergic drug sensitivity was assessed between 7 and 25 days after CHLP. CHLP-treated rats were more sensitive to norepinephrine as based on a greater hyperthermic response. MA of CHLP-treated rats was more sensitive to scopolamine. On the other hand, the hypothermic effects of oxotremorine (0.4 mg/kg) were nearly abolished by CHLP treatment, indicating tolerance to cholinergic stimulation. The tachycardic effects of methyscopolamine were also greater in the CHLP group. Overall, the acute effects of CHLP are unusual compared to other OP's in that there is no hypothermic response, an attenuated nocturnal elevation in Tc and a prolonged elevation in HR.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and delayed effects of diisopropyl fluorophosphate on body temperature, heart rate, and motor activity in the awake, unrestrained rat

Acute exposure to diisopropyl fluorophosphate (DFP) causes irreversible inhibition of acetylcholinesterase activity, leading to various behavioral and autonomic sequelae including hypothermia, reduced motor activity, and other neurological dysfunctions. To characterize the acute response and recovery of autonomic and behavioral processes to DFP exposure, rats of the Long-Evans strain were implanted with radiotransmitters that allowed the monitoring of core temperature, heart rate, and motor activity in unrestrained animals 24 h/d. These parameters were monitored for 96 h following subcutaneous injection of DFP at a dose of 0, 0.1, or 1.0 mg/kg. Rats given 0 and 0.1 mg/kg DFP displayed an increase in core temperature and motor activity during the first 24 h postinjection. The 1.0 mg/kg group showed a typical hypothermic response for the first 24 h following DFP administration. Core temperature decreased a maximum of 1.9 degrees C by 5 h after DFP and then started to recover, reaching control levels by 17 h after DFP treatment. Motor activity was also depressed during the first 24-h period in the 1.0 mg/kg group. Heart rate was initially elevated above basal levels in all treatment groups for several hours after treatment, but the 1.0 mg/kg group showed a decrease in heart rate at the time when core temperature began its recovery from hypothermia. Core temperature was the only parameter significantly affected by DFP during the 24-96 h recovery phase. The 0.1 and 1.0 mg/kg groups showed a significant elevation in core temperature for the 3 d after DFP administration. The elevation in core temperature during the recovery from DFP treatment may represent an important facet of the acute cholinergic neurotoxicity of organophosphate compounds.

Strain comparisons of DFP neurotoxicity in rats

The purpose of this study was to assess intraspecies differences in behavioral and autonomic function in three strains of rat following administration of diisopropyl fluorophosphate (DFP), an irreversible inhibitor of acetylcholinesterase activity. Male rats of the Long-Evans (LE), Fischer 344 (F344), and Sprague-Dawley (SD) strains wer administered DFP at doses of 0-1.5 mg/kg (sc). The animals were placed 60 min later into one of two motor activity chambers and tested for 30 min. Motor activity was measured using either a Doppler-based system or a commercial photocell device. Following measurement of motor activity in the Doppler system, body temperature (Tb) was measured and blood was then withdrawn by cardiac puncture and analyzed for serum cholinesterase activity (ChE). The remaining rats were retested 1 d after DFP administration in the photocell device. The results showed a significant influence of strain on the effects of DFP. Motor activity of LE rats was reduced by DFP at doses of 1.0 and 1.5 mg/kg, whereas the activity of F344 rats was reduced only at 1.5 mg/kg. The relative sensitivity of SD rats depended on the device used to measure motor activity. The SD rats resembled F344 rats in their response to DFP when motor activity was measured in the photocell device, and LE rats when motor activity was measured in the Doppler system. The Tb of F344 rats was unaffected by DFP, while the LE and SD rats became hypothermic at 1.5 mg/kg. The DFP-induced inhibition of serum ChE activity was significantly less in F344 rats. All three strains retested the day after DFP still showed significant decreases in motor activity. Overall, it appears that the F344 strain is relatively resistant to the behavioral and autonomic effects of DFP. This intraspecies variability should be considered in selecting appropriate experimental models for assessing the neurotoxicological hazards of cholinesterase-inhibiting pesticides.

Relationship between serum cholinesterase activity and the change in body temperature and motor activity in the rat: a dose-response study of diisopropyl fluorophosphate

Risk assessment of the neurotoxicology of organophosphate (OP) pesticides calls for a thorough understanding of the relationship between tissue cholinesterase (ChE) activity and changes in behavioral and autonomic responses to OP treatment. To address this issue, motor activity, core and skin temperature, and serum ChE activity were measured 2 h after rats of the Long-Evans strain were treated with the OP, diisopropyl fluorophosphate (DFP) at a dose of 0, 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, and 1.5 mg/kg (SC). DFP doses > or = 0.25 mg/kg led to significant decreases in serum ChE activity, whereas doses of > or = 0.5 mg/kg caused reductions in motor activity and body temperature. The highest dose of DFP caused an increase in tail skin temperature, indicating an elevation in skin blood flow. A hockey stick regression analysis was used to determine threshold inhibition in ChE activity associated with depressions in motor activity and colonic temperature. The threshold serum ChE activity, relative to controls for inhibition of motor activity and reduction in body temperature was 46%. A wide range in individual motor activity and colonic temperature responses was noted when the inhibition in ChE activity exceeded threshold levels. This may be indicative of marked genetic variability to ChE inhibition. That is, rats appear to be either responsive or unresponsive when subjected to extreme inhibition in ChE activity. This pattern has been reported in other rodents and may represent a fundamental aspect of ChE toxicity.