Interaction of reversible and irreversible cholinesterase inhibitors on the monosynaptic reflex in neonatal rats

Diisopropylfluorophosphate-induced depression of segmental monosynaptic transmission in neonatal rat spinal cord is also mediated by increased axonal activity

Involvement of dorsal and ventral root activity for the depressant action of diisopropylfluorophosphate (DFP) on synaptic transmission was examined using in vitro spinal cord/root preparations. Superfusion of DFP produced a dose-dependent depression of monosynaptic reflex (MSR) and maximal depression of about 80% occurred at 1000 microM. The concentration to produce 50% of the maximal inhibition was about 100 microM of DFP. The DFP (100 microM)-induced depression of MSR was reversed by atropine (0.5 microM) but not by mecamylamine (0.5 microM). Contrary to the action on MSR, DFP potentiated the ventral root potential and 1st peak of dorsal root potential. The maximal potentiation was about 25% of control in both the root potentials at 100 microM of DFP. However, the second peak of dorsal root potential was slightly depressed (10-20% of control) by DFP (1-1000 microM). Further, the cords treated with DFP (100 microM) showed significant decrease in the cholinesterase (ChE) activity (27% of control). Results suggest that the DFP-induced depression was mediated at least by two different mechanisms, one through the inhibition of ChE activity and the other through the activation axonal activity having inhibitory inputs to the segmental synaptic transmission. These inputs mediate their action through muscarinic receptors.

Biphasic action of sarin on monosynaptic reflex in the neonatal rat spinal cord in vitro

The action of sarin, an organophosphorus (OP) compound, was examined in vitro for its effects on the spinal monosynaptic reflex (MSR) in neonatal rats. The effects of sarin were biphasic, i.e. facilitation at lower concentrations (2-20 nM) followed by depression of the MSR at concentrations above 30 nM. Facilitation of MSR was maximal (150% of control) at 20 nM sarin. The depression of MSR was maximal (70% of control) at 200 nM sarin, with half maximal inhibition occurring at 90 nM sarin. Atropine (200-500 nM) effectively reversed the depression caused by sarin, while pretreatment with low concentrations of atropine (10 nM) completely blocked the depression otherwise observed with sarin. Benactyzine was also effective in preventing sarin-induced depression, while pirenzepine was less effective. The nicotinic blocking agents tubocurarine and mecamylamine were, however, ineffective in preventing or reversing sarin-induced depression. The facilitation of MSR seen with lower concentrations (2-20 nM) correlated well with the blockade of late phase inhibition (between 30 and 50 ms conditioning-test interval) elicited in spinal cord by stimulating the adjacent dorsal root at various condition-test intervals, which has been shown elsewhere to be sensitive to bicuculline (Deshpande and Warnick 1988). Thus it is speculated that sarin at lower concentrations blocks GABA transmission, producing facilitation, and at higher concentrations activates the muscarinic receptors producing depression of MSR. The beneficial action of pretreatment with antimuscarinic agents may be attributed to the protection of the muscarinic receptors.