ABSTRACT The electrophysldoglcal actions of paraoxon, an irreversible blocker of acetylcholinesterase, and their antagonism by a series of organophosphate cholinesterase reactivators, were studied in area CAI of the guinea pig hippocampus in vitro. To avoid indirect effects H i e d by excitation of CA3 neurons, the CA2/3 regions were removed routinely before the recording of extracellular field potentials in CAI. Under these c o n d i s , paraoxon (1 gM) induced regular burst activity (rate, 2-1 O/min; amplitude, 0.2-1 mV; duration, 100-500 msec). The antagonism of this burst activity by atropine (0.3-1.0 MM) and pirenzepine (1.0 pM) suggested the involvement of muscarinic cholinoceptors in the mediation of this response. The reduction in frequency of paraoxon-induced bursting by the cholinesterase reactivators was taken as an index of their efficacies. The four oxime compounds tested were all active in the low micromolar range (rank order of potencies: obidoxime > HGG 12 = HL6 7 > HI 6). In experiments without paraoxon, these oximes di d not depress either evoked population spikes in normal artificial cerebrospinal fluid or bursts induced by superfusion with Mg++-free artificial cerebrospinal fluid. Thus, an unspeafic inhibitory effect of oximes can be excluded. It is concluded that the in vitro hippocampus provides a suitable system for the quantitative electrophysidogical evaluation of cholinesterase reactivators in the central nervous system. Organophosphorus compounds, which irreversibly inhibit the enzyme AChE, induce a severe poisoning which is characterized by convulsions and paralysis, respiratory failure and, finally, death. It is not known, however, whether symptoms related to the CNS, like convulsions, are due solely to the accumulation of ACh in the brain The hippocampus has been shown to develop epileptiform activity after application of various convulsants ( Klee et al., 1982) and is known to play an important role in the generation and conduction of seizures, also with respect to cholinergic systems ( Turski et al., 1983). The presence of a neuroanatomically and histochemically defined cholinergic input to this brain structure (Lewis et al., 1967; Female guinea pigs (180-250 g) were decapitated under ether anesthesia. Both hippocampi were dhected from the removed brain and transverse slices (0.5 mm) were cut using a Vibmlice (Campden Instruments, London, UK). The slices were stored in a carbogen-gaseed (96% 02-556 C 0 2 ) chamber filled with ACSF of the following composition (millimolar): NaCl, 118; KCl, 3.0; NaHC03, 25; NaH2P04, 1.2; MgC12, 1.0; CaC12, 1.5; and glucose, 10. After at least 1 hr, slices were transferred to an experimental chamber in which they were submerged and superfused continuously with gassed ACSF (pH 7.4). The bath temperature was maintained between 29 and 31'C. The exchange time of the bath was about 2 min. In moat experiments, paraoxon was added to the ACSF to give a final concentration of 1 rM. AChE reactivators and other drugs were added to the paraoxon-containing superfusate in ABBREVIATK)NS: AChE, acetylcholinesterase; CNS, central nervous system; ACh, acetylcholine; ACSF, artifidal cerebrospinal fluid; W, bursting frequency
