Effects of inhibitors of acetylcholine synthesis on brain acetylcholine and survival in soman-intoxicated animals

Use of C1300 neuroblastoma cells to evaluate the protective value of hexamethonium, trimethaphan, hemicholinium, and triethylcholine against diisopropyl phosphorofluoridate toxicity

Our intent was to evaluate the C1300 neuroblastoma cell as an in vitro system for studying the mode of action and efficacy of drugs used to treat or prevent organophosphate intoxication. The anticholinergic drugs hexamethonium, trimethaphan, and hemocholinium and the triethylcholine and cholinesterase/reactivator 2-pyridine aldoxime methochloride (2-PAM) have been shown to be effective in preventing intoxication by diisopropyl phosphorofluoridate (also known as diisopropyl fluorophosphate, DFP) in vivo. We determined their efficacy in preventing cell death (as measured by trypan blue exclusion) of neuroblastoma cells alone or in combination. We also determined their efficacy in reversing the cytotoxic effects of DFP on cell DNA synthesis (as measured by [3H]-thymidine incorporation), cell RNA synthesis (as measured by [3H]uridine incorporation), and on cell protein synthesis (as measured by [3H]leucine incorporation). The maximal nontoxic doses of the drugs in vitro were determined. All anticholinergic agents studied reduced the cytotoxicity of DFP using one or more parameters. 2-PAM, the cholinesterase reactivator, enhanced the cytotoxicity of DFP on cultured cells at a high concentration (1 mg/mL) and reduced it at a lower concentration (0.3 mg/mL). All four anticholinergic agents were capable of enhancing the uptake of [3H]thymidine. Only hexamethonium and hemicholinium reversed DFP inhibition of DNA synthesis. RNA synthesis was not affected by any anticholinergic agent and no agent reversed DFP inhibition of RNA synthesis. Protein synthesis was enhanced by every anticholinergic agent except hemicholinium; the inhibition of protein synthesis by DFP was reversed by trimethaphan and triethylcholine.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Carbamyl-N-allylquinuclidinium bromide. Effects on cholinergic activity and protection against soman

3-Carbamyl-N-allylquinuclidinium bromide (CAB) was synthesized and evaluated for its pharmacological effects on cholinergic activity and for protection in vivo against soman toxicity in guinea pigs. This carbamylated derivative of N-allyl-3-quinuclidinol (NAQ), a potent inhibitor of high-affinity choline uptake, demonstrated stereospecific alterations of cholinergic function as well as protection against soman. The R-isomer, but not the S-isomer, of CAB inhibited erythrocyte acetylcholinesterase (AChE) and plasma pseudocholinesterase (pChE) in a concentration-response manner (IC50 = 25 and 29 microM, respectively). The R-isomer of CAB was also a more potent inhibitor of high-affinity choline uptake (IC50 = 4.8 microM) than S-CAB (IC50 = 63 microM). When R-CAB (10 mumol/kg, i.m.) was administered to guinea pigs 30 min prior to soman in conjunction with atropine (16 mg/kg, i.m.) given 1 min post-soman, the compound significantly reduced lethality up to 5 LD50S. This represents enhanced protection when compared to NAQ (up to 100 mumol/kg); the S-isomer of CAB failed to protect against soman intoxication. The results demonstrate that reversible inhibition of AChE with suppression of acetylcholine synthesis into a single compound, CAB, enhances the protection against organophosphates.

Inhibition of brain choline acetyltransferase in vivo: (E)-1-methyl-4-(1-naphthylvinyl)-1,2,3,6-tetrahydropyridine hydrochloride (B115), a depot form of a potent inhibitor

The quaternary ammonium salt (E)-4-(1-naphthylvinyl)pyridine hydroxyethyl bromide (B111) and the tertiary amine salt (E)-1-methyl-4-(1-naphthylvinyl)-1,2,3,6-tetrahydropyridine hydrochloride (B115), both previously shown to protect against organophosphate (OP) toxicity, were examined in vivo for effects on rat brain choline acetyltransferase (CAT) activity and acetylcholine (ACh) levels. When administered iv, but not when given ip, B111 was able to inhibit brain CAT 29% and reduce brain ACh levels 25%, yet was unable to prevent soman-induced increases in ACh. B115, which may serve as a depot form of a quaternary ammonium analogue, was able to decrease CAT activity as much as 80% upon multiple ip administration. This CAT inhibitory potency was unprecedented for a tertiary amine salt of its structure. However, ACh levels were reduced by no more than 25% and B115 was ineffective in preventing soman- and sarin-induced increases in ACh. Since the degree of inhibition of CAT activity produced by B111 and B115 was not accompanied by a corresponding decrease in ACh levels, the protection afforded by these compounds against OP toxicity is most likely not related to CAT inhibition. B115 was also tested for its ability to affect cholinergic receptor binding. B115 was administered to rats ip, twice daily, at low doses throughout a 3-week period. Analysis of cortex tissue revealed a 45% increase in nicotinic receptor binding with no change in either total muscarinic receptor binding (M-1 and M-2) or high-affinity muscarinic receptor binding (M-2 alone).

In vivo protection against soman toxicity by known inhibitors of acetylcholine synthesis in vitro

Soman inhibits the enzyme acetylcholinesterase, essentially irreversibly, producing an accumulation of acetylcholine (ACh) which is responsible for many of its toxic effects. Current approaches to treatment include: (1) atropine, a muscarinic receptor blocker; (2) pyridine-2-aldoxime methylchloride (2-PAM), an enzyme reactivator; and (3) carbamate protection of the enzyme. However, no fully satisfactory regimen has been found, primarily because of the rapid aging process. In this study, compounds known to inhibit ACh synthesis in vitro were evaluated in combination with atropine and 2-PAM so as to assess their potential utility in protection against soman toxicity in rats. Acetylsecohemicholinium (100 micrograms/kg, i.c.v.t., 30 min prior to soman), an inhibitor of high affinity choline uptake (HAChU) and cholineacetyltransferase (ChAT) activity in vitro, enhanced the protective effects of atropine and 2-PAM, reducing the mortality within the first 2 hr following soman. N-Hydroxyethylnaphthylvinylpyridine (NHENVP), a quaternary ChAT inhibitor (1.7 mumol/kg, i.m.), significantly reduced the overall percent mortality due to soman from 80% to 20%. The compound was most effective when administered 2-3 min prior to soman and was effective only by the intramuscular route. N-Allyl-3-quinuclidinol, a potent HAChU inhibitor (1 mumol/kg, i.m.) was the most effective quinuclidine analog evaluated, also reducing the percent mortality for a 24-hr period. Unlike NHENVP, it was most effective when given 30-60 min prior to soman. It is suggested from the data that compounds that disrupt presynaptic ACh synthesis in vitro may prove effective in treating organophosphate poisoning. The results demonstrate interesting differences among the compounds studied and provide insight for the design of protectants against soman toxicity. These findings further underscore the need to examine the structure activity and pharmacokinetic properties of these compounds, i.e. comparison of routes of administration, dose-response relationships, and time to effect.

Changes in ACh levels in the rat brain during subacute administration of diisopropylfluorophosphate

Rats were treated with diisopropylfluorophosphate (DFP) acutely and daily for 14 days. The total, free, and bound acetylcholine (ACh) levels were monitored in striatum, hippocampus, and frontal cortex after DFP administration. Thirty minutes after daily administration of DFP, the total and free ACh levels were significantly increased and remained constant after each successive dose. The bound ACh levels in striatum and frontal cortex were also significantly increased; however, they were comparable to control levels after the 14th injection of DFP. The total ACh levels 30 min after a challenge dose of 2 mg/kg DFP in saline and DFP subacutely treated rats were significantly increased in hippocampus (34 and 76%) and frontal cortex (49 and 64%) and were not significantly different between the two groups. However, the level of total ACh in striatum was increased less in the tolerant rats (10%) than in the acutely treated rats (36%). The levels of free and bound ACh after acute administration of 2 mg/kg DFP were markedly increased in three brain regions. After subacute administration, the levels of bound ACh were significantly increased in hippocampus (84%) and frontal cortex (40%); however, that in striatum did not change. The increase in the bound ACh level in the subacute treatment group was less than that in acutely treated rats in all three brain regions; however, the duration of the elevation of the free ACh in striatum was shorter in subacutely treated rats. These results suggest that the presynaptic cholinergic storage sites for ACh might be changed during subacute administration of DFP.

Pathophysiology of soman intoxication in primates

Adult baboons were monitored during intravenous infusion of Soman (1,2,2-trimethylpropyl ester, phosphonofluoridate). Three groups of animals were studied. Two groups were anesthetized with sodium pentobarbital (initial dose, 20 mg/kg), instrumented for measurement of systemic blood pressure (BP), pulmonary artery pressure, cardiac output (CO), ECG, ventilatory flow, translaryngeal pressure (PTL), transdiaphragmatic pressure (Pdi), transpulmonary pressure (PTP), diaphragm EMG, and efferent phrenic nerve traffic (Eph). One group received no Soman and served as controls. In the other group, Soman was infused over 10 min at doses of 13.1, 8.21, 4.92, or 3.3 micrograms/kg. The onset of intoxication occurred within 7-8 min (before the end of the 10-min infusion), manifested by muscular fasciculations, stridorous breathing, copious secretions, and atrioventricular arrhythmias. Mean BP decreased to 30 mm Hg by the combination of decreased CO and decreased vascular resistance. There was a dose-related response in the onset and duration of these effects. Apnea occurred in most animals and coincided with cessation of the Eph signal. Stimulation of the diaphragm via the phrenic nerve following apnea yielded Pdi values unchanged from baseline, indicating an intact neuromuscular apparatus. All animals required ventilatory support. Some surviving animals exhibited severe behavior changes. The third group of animals was studied without anesthesia. Instrumentation was performed 3 days before using a tether system for the measurement of BP, CO, and ECG, and an arterial line for blood withdrawal. Soman was infused over 10 min at a dose of 13.1 micrograms/kg. The onset of intoxication occurred within 2-3 min, manifested by hyperactivity, severe muscle fasciculations which simulated grand-mal convulsions, stridorous respiratory sounds, copious secretions, and cardiac arrhythmias. Apnea and severe lactic acid acidosis developed in all animals and all required ventilatory assistance. None recovered spontaneous ventilation at the end of 4 hr.

Protection from quinidine or physostigmine against in vitro inhibition by sarin of acetylcholinesterase activity

We have studied the relative effectiveness of quinidine and physostigmine in protecting against the inhibition of acetylcholinesterase (AChE) by sarin, an organophosphate (OP) compound. The protective effects of these compounds were studied in vitro in both synaptosomal and soluble samples obtained from various regions of sarin-administered or control isolated, perfused canine brain. Although AChE activities in the sarin-administered brain were substantially lower than in the control brain, we observed regional differences in the AChE activity in both. The AChE in the control brain and the AChE remaining in sarin-administered brain had different susceptibilities to inhibition from OP compounds in vitro and, therefore, have different properties. Quinidine partially protected AChE from the inhibitory effects of sarin in vitro possibly by altering the sarin binding sites. Addition of sarin to physostigmine-treated control brain samples allowed partial recovery of the AChE activity. The protective effects of quinidine or physostigmine were lost when samples from sarin-administered brain were treated in vitro with these compounds and then again exposed to sarin. Therefore, both quinidine and physostigmine provided partial protection against the inhibitory effects of sarin in vitro if they were added prior to sarin.

Effect of HI-6, applied into the cerebral ventricles, on the inhibition of brain acetylcholinesterase by soman in rats

When applied to rats (intraperitoneally) immediately after subcutaneous injection of soman (120 micrograms/kg) HI-6 (100 mg/kg) protected about 40% of the activity of acetylcholinesterase (AChE) in the motor end plate region of the diaphragm but did not protect AChE in the brain. However, a partial protection of AChE in brain against inhibition by soman was obtained in anaesthetized, atropinized rats by the oxime injected into the cerebral ventricle 5 min before parenteral exposure to soman. The AChE activity in brain of rats pretreated with HI-6, analyzed 60 min after the injection of soman was between 10 and 19%, while that in non-protected animals did not exceed 1% of the control. The degree of protection of AChE in brain was dose-dependent. Large doses of HI-6 (greater than or equal to 100 micrograms) were tolerated by animals because of the pentobarbital anaesthesia which counteracted the lethal action of HI-6. The rate of "aging" of AChE in brain inhibited by soman was analyzed by intracerebroventricular injection of 200 micrograms of HI-6 at different time intervals after the subcutaneous injection of soman. A statistically-significant reactivation of inhibited AChE activity in brain was demonstrated when HI-6 was applied up to 20 min after soman. The protection and reactivation by HI-6 of both AChE in brain and AChE in muscle end plates in poisoning with soman appear to be quite similar.

Anticholinergic agents do not block light-induced circadian phase shifts

Depletion of brain acetylcholine (ACh) stores by central infusions of hemicholinium-3 (HC-3) did not prevent the circadian phase-shifting effects of subsequent light pulses in rats. Animals treated with atropine likewise had normal responses to light pulses. Neither delay nor advance shifts were prevented. These data do not support the view that ACh is the primary mediator of photoentrainment in the rat. The previously reported effects of carbachol injections may be non-specific.

Blockade of acetylcholine synthesis in organophosphate poisoning

In spite of worldwide research efforts in the search for the treatment of organophosphate poisoning, the substances with practical antidotal capabilities remain to be discovered. This problem has generally been approached by attempting to reactivate the inhibited acetylcholinesterase. Our approach consisted of reducing the amount of the lethal agent acetylcholine by blocking its synthesizing enzyme cholineacetylase with methyl methane thiol sulfonate (MMTS). We have taken into consideration that we are dealing with acute toxicological problems. This applies for poisoning as well as for treatment, and therefore in the present stage we can only present minimal results. The time from sarin (2 mg/kg) injection to death in rats (controls) was 2:59 min. With a MMTS dosage of 133.5 mg/kg prior to sarin, it was prolonged to 20:55 min (p less than 0.01). With the same dosage of MMTS under identical conditions, the time from soman (2 mg/kg) injection to death was prolonged from 6:08 to 14:48 min (p less than 0.01). Although MMTS cannot be used as a therapeutic agent, our attempt has demonstrated a utility in treating organophosphate poisoning in mice and rats and points in a direction where further work might be fruitful.