Inhibition of efferent sympathetic nerve activity by centrally administered paraoxon in the cat

Yohimbine antagonism of the vasodepression elicited by organophosphates applied on ventral medulla oblongata

The rostral ventral surface of medulla oblongata (RVMO) has been shown to constitute a selective target for organophosphate (op) cholinesterase inhibitors. The action of soman (S) as compared with (7-nitrobenz-2-oxa-1,3 diazole)aminopentyl methylphosphonofluoridate (NBD-AP-MPF), a fluorescent organophosphate has now been examined in anesthetized cats pretreated with atropine sulphate. Blood pressure (BP), electrocardiogram (ECG) and respiration (R) were recorded. In some animals a cannula was implanted into the right lateral ventricle. Chemicals were bilaterally applied on RVMO by means of a perspex cannula and removed after 5 min. The application of 2.5 micrograms S or 60 micrograms NBD-AP-MPF elicited severe fall of BP which recovered only after 2 h in the case of the former and up to 45 min in the latter. Smaller doses produced corresponding responses of lesser magnitude. Accompanying R changes consisted in most cases of increased rate and reduced amplitude whereas in others the opposite or mixed alterations occurred. Frequently, sigh-like movements intermingled at periodic intervals with regular R deflections. The sighs were interpreted as aiming to correct blood gases balance. After application of atropine on RVMO--but not by systemic administration--BP and R were restored whereas single repeated i.v. injection of 1 microgram/kg noradrenaline produced only transient reversals without influencing the course of long lasting vasodepression. In contrast, the intraventricular administration of 250-500 micrograms yohimbine considerably reduced both the magnitude and extent of the vasodepression elicited by topically applied organophosphates. It is postulated that central alpha 2-adrenoceptors in contrast to vascular sites are likely involved in the op-induced vasodepression. The present work provides an indication that effective antagonists might be developed considering blockade of these receptors.

[Brain changes in parathion poisoning: observations in 42 cases]

A total of 42 cases were examined neuropathologically to determine possible toxic changes occurring in the brain after parathion intoxication. Sporadic anoxic alterations were observed in 7% of 41 cases in which the cause of death was acute intoxication. Nearly all cases (93%), however, showed marked hyperemia, often coupled with small reactionsless, periventricular hemorrhages which occurred in 40% of the cases. In one third of the cases (33%) there was moderate swelling of the oligodendroglia. Whereas histological evidence of edema was found in nearly one third of the cases (30%), comparison of the brain weights in these subjects with those of a large comparative collective showed definite pathologic brain weights in only five cases (12%) with 95% confidence limits. It is true that in 18 cases (42%) the brain weight was above the normal value if the confidence limit is ignored. Pronounced anoxic alterations were observed in only one case in which the individual survived the acute intoxication for 4 weeks after initial respiratory arrest. Predominantly toxic changes, however, could not be detected in any of the cases examined. This negative morphologic finding does not agree with the physiologic alterations reported by other investigators using animal models; they considered the cause of death in cases of parathion intoxication to be the result of toxic paralysis of the respiratory center. The literature was discussed.

The central effects of paraoxon on blood pressure of rabbits after intravenous administration or infusion via a vertebral artery

In the present study paraoxon (28-825 micrograms/kg, i.v.) was administered into rabbits treated with N-methylatropine (1 mg/kg). In urethane-anaesthetized rabbits pressor and depressor effects were observed. In pentobarbitone-anaesthetized rabbits, however, only depresssor effects were noticed. Dexetimide (0.25-0.5 mg/kg) prevented the pressor and depressor action of paraoxon. Furthermore, paraoxon (83-825 micrograms) was infused into a vertebral artery of pentobarbitone-anaesthetized animals. After administration via this route the depressor effect was similar to i.v. administration. 30 min after administration of paraoxon acetylcholinesterase activity was determined in the medulla oblongata, the pons and the hypothalamus. The enzyme has to be inhibited almost completely to induce an effect on blood pressure. It is concluded that the vascular effects of paraoxon seem to be mediated by a mechanism involving stimulation of central muscarinic receptors, probably by accumulated acetylcholine, which in turn induces a decrease in blood pressure.