Acute effects of diisopropyl fluorophosphate (DFP) on autonomic and behavioral thermoregulatory responses in the Long-Evans rat

Fenitrothion, an organophosphorous insecticide, impairs locomotory function and alters body temperatures in Sminthopsis macroura (Gould 1845) without reducing metabolic rates during running endurance and thermogenic performance tests

Endemic Australian mammal species are exposed to pesticides used for locust control as they occupy the same habitat as the target insect. The authors examined the impact of an ultra-low volume formulation of the organophosphorous insecticide fenitrothion (O,O-dimethyl-O-[3-methyl-4-nitrophenol]-phosphorothioate) on a suite of physiological measures that affect the ability of animals to survive in free-living conditions: locomotory and thermogenic functions, metabolic performance, body mass, and hematocrit and hemoglobin levels. Plasma and brain cholinesterase activity in relation to time since exposure to pesticide were also determined. An orally applied dose of 90 mg kg(-1) fenitrothion reduced running endurance in the stripe-faced dunnart, Sminthopsis macroura, by 80% the day after exposure concomitantly with a reduction of approximately 50% in plasma and 45% in brain acetylcholinesterase activity. These adverse effects disappeared by 10 d postexposure. Maximal metabolic rates reached during running were unaffected by pesticide, as were body mass and hemoglobin and hematocrit levels. Maximal cold-induced metabolic rate (measured as peak 2 min metabolic rate attained during cold exposure), time taken to reach peak metabolic rate on cold exposure, cumulative total oxygen consumed during shivering thermogenesis, and body temperature before and after cold exposure were unaffected by fenitrothion. Dunnart rectal temperatures showed a reduction of up to 5 °C after exposure to fenitrothion but returned to pre-exposure levels by 10 d postdose. Such physiological compromises in otherwise asymptomatic animals demonstrate the importance of considering performance-based measures in pesticide risk assessments.

Early differential induction of C-jun in the central nervous system of hens treated with diisopropylphosphorofluoridate (DFP)

Diisopropyl phosphorofluoridate (DFP) produces organophosphorus-ester induced delayed neurotoxicity (OPIDN) in the hen, human and other sensitive species. We studied the effect of a single dose of DFP (1.7 mg/kg/sc) on the expression of c-jun, which is one of the heterodimerizing ITFs (Inducible Transcriptional Factors) of the AP-1 family. The hens were sacrificed at different time points ie 0.25, .0.50, 1 and 2 hrs. Total RNA was extracted from the following brain regions: cerebrum, cerebellum, brainstem, midbrain and as well as spinal cord. Northern blots prepared using standard protocols were hybridized with c-jun as well as b-actin and 18S RNA cDNA (control) probes. The results indicate differential regulation of c-jun levels which may be due to the activation of both cholinergic and non-cholinergic pathways of CNS, besides changing roles of c-jun (as mediator of degeneration or regeneration) depending on heterodimerization with other ITFs. In the highly susceptible tissues like brainstem and spinal cord c-jun transcript levels increased at 15 minutes and continued to increase gradually till it reached the maximum at 2 hrs. Overall spinal cord showed the maximum levels of c-jun induction (207%) at 2 hrs time point of all the CNS tissues. The enhancement of cholinergic transmisson by the inhibition of cholinestrase may be responsible for the gradual increase mediated by neural and vascular factors. In contrast, less susceptible tissue, cerebellum showed almost immediate induction to high level of (179%) at 15 minutes and the levels stayed more or less the same until it peaked to 185% at 2 hrs. Relatively low abundance of cholinergic neurons and high number of sensitized specialized cell types like Bergman glia and Purkinje cells may be responsible for the immediate higher induction. Non-susceptible tissue cerebrum did not show any changes in the c-jun levels. In midbrain the induction pattern was very similar to that of brainstem. This differential induction pattern of c-jun encomposing the differences in the quantity and time course was directly proportionate to the degree of susceptibility and cellular heterogeneity of different regions of CNS. The significant increase in c-jun levels along with our earlier observation on the increased c-fos levels indicate that AP-1 family of genes may be one of the IEGs involved in the long term changes which eventually lead to OPIDN.

Prolonged elevation in blood pressure in the unrestrained rat exposed to chlorpyrifos

Organophosphate (OP) pesticides are likely to alter the regulation of blood pressure (BP) because (i) BP control centers in the brain stem utilize cholinergic synapses and (ii) the irreversible inhibition of acetylcholinesterase activity by OP's causes cholinergic stimulation in the CNS. This study used radiotelemetric techniques to monitor systolic (S), diastolic (D), mean (M) BP, pulse pressure (systolic-diastolic), heart rate (HR), core temperature (T(c)), and motor activity in male Long-Evans rats treated with the OP pesticide chlorpyrifos (CHP) at doses of 0, 5, 10, and 25 mg/kg (p.o.) at 15:00 h 10 and 25 mg/kg CHP led to parallel elevations in S-BP, M-BP, and D-BP within 2 h after dosing. BP increased 15-20 mmHg above controls and increases persisted throughout the night and into the next day. HR decreased slightly in rats administered 25 but not 10 mg/kg CHP. T(c) was reduced by treatment with 25 mg/kg CHP and then increased above controls the next day. Motor activity was reduced by treatment with 25 but not 10 mg/kg CHP. Pulse pressure was elevated by 2-4 mmHg for 40 h after exposure to 10 and 25 mg/kg CHP. The increase in BP without an increase in HR suggests that CHP increases total peripheral resistance and may alter the baroreflex control of BP. Cholinergic stimulation of the CNS may explain the initial effects of CHP on BP; however, the persistent elevation suggests an involvement of neurohumoral pressor pathways.

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.

24-hour control of body temperature in the rat: II. Diisopropyl fluorophosphate-induced hypothermia and hyperthermia

Diisopropyl fluorophosphate (DFP) and other anticholinesterase (antiChE) agents have been found to induce marked hypothermic responses in laboratory rodents. To characterize the effects of DFP on autonomic and behavioral thermoregulation, rats of the Long-Evans strain were injected with DFP while housed in a temperature gradient. The gradient allowed for the measurement of selected ambient temperature (Ta) and motor activity (MA) over a 6- to 7-day period. Core temperature (Tc) and heart rate (HR) were also monitored simultaneously using radiotelemetry. Injection of the peanut oil vehicle led to transient elevations in Tc, HR, and MA, but no change in selected Ta. The next day animals were injected with 0.25, 1.0, or 1.5 mg/kg DFP. DFP (1.0 AND 1.5 mg/kg) led to a marked reduction in Tc. The decrease in Tc was accompanied by reductions in HR, MA, and selected Ta. During the first night after DFP, selected Ta remained elevated as Tc recovered to its preinjection level. The second 24-h period after 1.0 and 1.5 mg/kg DFP was associated with a significant elevation in the daytime Tc. In conclusion, with the option of using behavioral thermoregulatory responses, the hypothermic effects of acute DFP treatment are mediated by a selection for cooler TaS. An elevation in Tc during recovery from acute DFP corroborates the many incidents of fever in humans exposed to anti-ChE agents.

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.

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

This study examined the relationship between inhibition of cholinesterase activity (CA) and thermoregulatory response in the rat following exposure to the organophosphate (OP), diisopropyl fluorophosphate (DFP). Male Long-Evans rats were injected with DFP dissolved in peanut oil in doses ranging from 0 to 1.5 mg/kg (s.c.). Colonic (Tcol) and tail skin temperature (Ttail) were recorded at 0, 1, 2 and 3 h post-injection. At 3 h post-injection the rat was sacrificed and a blood sample was taken by cardiac puncture and analyzed for CA. There was a biphasic dose effect of DFP on Tcol with slight but significant elevation in Tcol in the dose range of 0.01-0.5 mg/kg and a significant depression in Tcol at doses of 1.0 and 1.5 mg/kg. There was a dose-dependent fall in CA with DFP administration in the erythrocyte, plasma, and whole blood fractions. Hypothermia was associated with 80-87% inhibition in CA, whereas the elevation in Tcol was associated with 20-70% inhibition in CA. DFP also elicited significant elevations in Ttail. Overall, the data fail to demonstrate any clear relationship between inhibition of blood CA and thermoregulatory response following exposure to DFP. However, the elevation in Tcol following relatively low doses of DFP may be of relevance to the frequently reported symptom of fever in humans exposed to OP agents.