The acute toxicity of drugs acting at cholinoceptive sites and twenty-four hour rhythms in brain acetylcholine

Two Players in the Field: Hierarchical Model of Interaction between the Dopamine and Acetylcholine Signaling Systems in the Striatum

Tight interactions exist between dopamine and acetylcholine signaling in the striatum. Dopaminergic neurons express muscarinic and nicotinic receptors, and cholinergic interneurons express dopamine receptors. All neurons in the striatum are pacemakers. An increase in dopamine release is activated by stopping acetylcholine release. The coordinated timing or synchrony of the direct and indirect pathways is critical for refined movements. Changes in neurotransmitter ratios are considered a prominent factor in Parkinson’s disease. In general, drugs increase striatal dopamine release, and others can potentiate both dopamine and acetylcholine release. Both neurotransmitters and their receptors show diurnal variations. Recently, it was observed that reward function is modulated by the circadian system, and behavioral changes (hyperactivity and hypoactivity during the light and dark phases, respectively) are present in an animal model of Parkinson’s disease. The striatum is one of the key structures responsible for increased locomotion in the active (dark) period in mice lacking M₄ muscarinic receptors. Thus, we propose here a hierarchical model of the interaction between dopamine and acetylcholine signaling systems in the striatum. The basis of this model is their functional morphology. The next highest mode of interaction between these two neurotransmitter systems is their interaction at the neurotransmitter/receptor/signaling level. Furthermore, these interactions contribute to locomotor activity regulation and reward behavior, and the topmost level of interaction represents their biological rhythmicity.

Diurnal variation in thermoregulatory response to chlorpyrifos and carbaryl in the rat

Time of day of exposure is rarely considered in the study of insecticide toxicology. It would be expected that the circadian temperature rhythm (CTR) as well as the circadian rhythms of other physiological processes would affect the efficacy of anticholinesterase (antiChE) insecticides. The ability of antiChE insecticides to alter core temperature (T(c)) could be affected by time of exposure in relation to the CTR. To this end, we assessed time of exposure on the efficacy of the antiChE insecticides chlorpyrifos (CHP) and carbaryl (CAR) to alter T(c) in the rat. T(c) and motor activity (MA) were monitored by radiotelemetry. Rats were dosed orally with 0, 30, and 50 mg/kg CHP or 0, 25 and 75 mg/kg CAR at 09:00 and 15:00 h. Both insecticides caused an acute decrease followed by a delayed increase in T(c) by 24-48 h post-exposure. The temperature index (TI) (area under curve of DeltaT(c) with time) was significantly greater when CHP was given at 15:00 h as compared with 09:00 h. The maximum decrease in T(c) was similar for morning and afternoon CHP. The TI following CAR was similar for morning and afternoon exposure. CHP suppressed the 24 h MA equally when given in the morning and afternoon. CAR was more effective in reducing MA when given in the morning as compared with the afternoon. The T(c) increase measured 24 h after dosing was greater when CHP was given in the morning. Overall, time of day affected the thermoregulatory toxicity of CHP but not CAR. Another experiment showed that the hypothermic efficacy of oxotremorine, a muscarinic agonist, was greater when injected at 09:00 h as compared with 15:00 h. Hence, cholinergic stimulation is probably not the only mechanism to explain the effects of the chronotoxicogical effects of some antiChE insecticides.

A comparison of cholinergic effects of HI-6 and pralidoxime-2-chloride (2-PAM) in soman poisoning

The effects of HI-6 and pralidoxime chloride (2-PAM) on soman-induced lethality, time to death and several cholinergic parameters in rats were compared to understand the beneficial action of HI-6. Treatment with atropine sulfate (ATS) or HI-6 alone protected against 1.2 and 2.5 LD50s of soman respectively, whereas 2-PAM or methylated atropine (AMN) alone afforded no protection. Addition of ATS, but not AMN, to HI-6-treated rats enhanced the protection from 2.5 to 5.5 LD50s. HI-6 increased the time-to-death, while 2-PAM had no effect; a combination of HI-6 and ATS provided the most significant increase in time-to-death. Cholinesterase (ChE) activity was not altered in any tissue by ATS, HI-6 or 2-PAM treatment individually, but was markedly inhibited in all tissues by 100 micrograms/kg of soman. In soman-poisoned rats, the HI-6, but not the 2-PAM, group had significantly higher levels of ChE in blood and other peripheral tissues than did the group given soman alone. Neither HI-6 nor 2-PAM affected soman-inhibited ChE in the brain. Additional ATS treatment had no effect on ChE activity. HI-6 and 2-PAM neither modified baseline brain acetylcholine (ACh) or choline (Ch) levels nor protected against soman-induced ACh or Ch elevation. 2-PAM exhibited a 4-fold more potent in vitro inhibition of 3H-quinuclidinyl benzilate (3H-QNB) binding and sodium-dependent high-affinity Ch uptake (HACU) than did HI-6 in brain tissues. The findings that 2-PAM is a more potent in vitro inhibitor of muscarinic receptor binding and HACU than HI-6, and yet neither elevates ChE activity in the periphery nor protects rats against soman poisoning, indicate the importance of higher ChE activity in the periphery of HI-6-treated rats. Maintenance by HI-6 of a certain amount of active ChE in the periphery appears to be important for survival after soman exposure.

Age-related differences in soman toxicity and in blood and brain regional cholinesterase activity

The toxicity (lethality, acute toxic signs and body weight loss) of the irreversible ChE inhibitor soman was assessed in four groups of male rats differing in age: 30, 60, 120 and 240 days old. Plasma and brain regional ChE activity profiles were also studied in these groups. All measures of the toxicity of soman were found to increase with age. The calculated 24-hr LD50s were 110, 87, 66 and 59 micrograms/kg, IM, for 30-, 60-, 120- and 240-day-old rats, respectively. A significant and positive age-related effect on toxic sign rating scores was observed at one hr following soman injection. Furthermore, during a 14-day postsoman observation period, it was observed that young rats had less initial weight loss and more rapid, sustained recovery of growth than older animals. Survivors from the two oldest age groups did not recover to baseline body weights by the end of the 14-day observation period. Basal level of plasma ChE activity did not change significantly with age, while brain regional ChE showed two distinct age-dependent patterns: a linear decrease in the brainstem, midbrain and cerebellum and an inverted U-shaped change in the cortex, hippocampus and striatum. Our data suggest a relationship between soman toxicity and the aging process, but fails to demonstrate a definite relationship between soman toxicity and basal ChE activity in blood and brain of rats.

Effects of soman and sarin on high affinity choline uptake by rat brain synaptosomes

Synaptosomes were incubated at various time intervals following injection of 120 micrograms/kg SC of soman or sarin or with various concentrations (10(-8) to 10(-2) M) of soman or sarin in vitro. Total cholinesterase (ChE) activities in each brain region were also measured. Following soman injection, sodium-dependent, high affinity choline uptake (SDHACU) was decreased from 1 to 4 hr in the cortex and from 1 to 2 hr in the hippocampus, but increased from 2 to 24 hr in the striatum. Similarly, following sarin injection SDHACU was decreased at 0.5 hr in the cortex and from 1 to 4 hr in the hippocampus, but increased at 1 hr in the striatum. Injection of soman severely inhibited (83-99%) total ChE activity in the cortex, hippocampus and striatum from 1 to 24 hr. In contrast, sarin did not severely inhibit ChE activity in these regions and maximal inhibition (40-60%) did not occur until 24 hr after injection. With both compounds, by 168 hr ChE activity in all regions had partially recovered. Incubation of synaptosomes with soman or sarin in vitro at concentrations below 10(-4) M did not affect SDHACU in any of the brain regions. These data demonstrated that acute soman and sarin injection produced similar effects upon SDHACU in different brain regions, although the time-course of these effects was different for the two compounds. These effects were probably neither due to a direct action of these compounds on the uptake process nor dependent on ChE inhibition.

Time course effects of soman on acetylcholine and choline levels in six discrete areas of the rat brain

The time course of changes in rat brain levels of acetylcholine (ACh) and choline (Ch) was investigated following a single SC injection of soman (0.9 LD50, 120 micrograms/kg) to understand the relationship between central neurotransmitter alteration and soman toxicity. Of the animals exposed to the dose of soman, 46% died within 24 h, with maximum mortality occurring during the first 40 min following soman administration. In a second group, surviving rats were killed at various times after treatment by a beam of focused microwave radiation to the head, and ACh and Ch levels were determined by gas chromatography-mass spectrometry. Soman produced a maximal ACh elevation in the brain stem at 20 min (34.4%), in cerebellum at 40 min (51.9%), in cortex and striatum at 2 h (320.3% and 35.2%, respectively), and in hippocampus and midbrain at 3 h (94.5% and 56.8%, respectively). ACh levels remained above normal approximately 30 min in the brain stem; 2 h in the midbrain, cerebellum, and striatum; 8 h in the cortex; and 16 h in the hippocampus. Ch levels were elevated in all areas except the striatum. Ch maxima occurred at 10-40 min and returned to control levels approximately 3 h after injection. Results suggest that perturbation of ACh levels due to soman was not uniform throughout the brain and that soman toxicity may reflect ACh changes in multiple areas, rather than changes in any given area. These data further suggest a possible relationship between elevated Ch levels and soman toxicity.

Circadian rhythm in rat brain opiate receptor

To investigate diurnal variations in opiate receptor binding, the amount of specifically bound [3H]naloxone was measured at 4-h intervals across a 24-h period in the forebrains of rats that had been housed under a controlled light--dark cycle (lights on from 07.00 to 19.00 h) for 3 weeks. A significant rhythm with a peak at 22.00 h was found, the amplitude was 46--78%. In the absence of time cues, this circadian rhythm persisted with a peak at 02.00--06.00 h and an amplitude of 88%. Scatchard analysis indicated that the differences in binding throughout the day were due not to changes in affinity, but to changes in the number of binding sites.

Effects of chronic lead exposure on levels of acetylcholine and choline and on acetylcholine turnover rate in rat brain areas in vivo

Rats were exposed to lead acetate from birth, and were killed at the age of 44--51 days for analysis of levels and turnover rates of acetylcholine (ACh). Steady-state levels of ACh were not altered in midbrain, cortex, hippocampus, or striatum of lead-exposed rats. Similarly, no changes in choline (Ch) concentrations were found in cortex, hippocampus, or striatum. In the midbrain, however, a 30% reduction in Ch levels was observed. Changes in specific activity of Ch and ACh were measured as a function of time in selected brain areas of rats infused with a radio-labeled precursor of Ch. Specific activities of ACh were not altered. Ch specific activities were, however, significantly elevated in all brain areas examined, as compared with age-matched control rats. The in vivo ACh turnover rate in cortex, hippocampus, and striatum was diminished by 35%, 54%, 51%, and 33%, respectively. These findings provide direct evidence for an inhibitory effect of lead exposure from birth on central cholinergic function in vivo. Since a significant reduction of body weight was found in those animals treated with lead acetate, the alteration of central cholinergic function may partially be attributed to malnutrition observed in the lead-exposed animals.

Chronopharmacology of strychnine and allylglycine in the mouse

1. The response of individually caged mice to the lethal actions of allylglycine and strychnine was evaluated in animals previously conditioned on an LD 12:12 (12 h light-12 h darkness) schedule in a controlled environment. 2. These convulsant agents were most toxic at 18.00 hours (during the light phase), and least toxic in the dark phase of the programmed lighting schedule. The relationship is considered of the circadian fluctuations in levels of inhibitory transmitter substances to the time-linked action of convulsant agents.

Relation of rat brain acetylcholine levels to duration of self-stimulation and escape behavior

Total brain acetylcholine (ACh) was assayed in groups of animals after various periods of operant responding maintained by electrical stimulation of the lateral posterior hypothalamus or of escape behavior induced by electrical stimulation of the midbrain tegmentum. Different groups of trained rats were placed in identical Skinner boxes for periods of 1 to 24 hr. The following groups were studied: controls, self-stimulators receiving electrical stimulation, escapers from brain stimulation or peripherally applied aversive stimulation, self-stimulators not receiving electrical stimulation prior to decapitation, tubocurarine-paralyzed respired rats with electrodes in the posterior-lateral hypothalamus not receiving stimulation, and a group of tubocurarine-paralyzed, respired rats receiving electrical stimulation automatically. It was found that brain stimulation decreased total brain ACh, regardless of whether the stimulation was positive, as during self-stimulation behavior, or negative, as during escape behavior. Animals that receivied positive stimulation while being paralyzed showed similar decreases in total brain ACh, but the change in ACh was smaller. No changes occurred in animals that were paralyzed that recieved no electrical stimulation. It is concluded that brain usage produced by electrical stimulation of discrete functional pathways causes a reduction of total ACh, but this is unrelated to the specific motivational properties of the electrical stimuli.

Multiphasic retention deficits at periodic intervals after passive-avoidance learning

Aftetr one-trial passive-avoidance training, indepelenet groups of rats tested promptly after training or at successive 6-hour intervals displayed a repetitive pattern of high then low retention scores. These results suggest that some physiological rhythm may interact with retention performance.