Pretreatment of Guinea pigs with galantamine prevents immediate and delayed effects of soman on inhibitory synaptic transmission in the hippocampus

Abstract

Galantamine has emerged as a potential antidote to prevent the acute toxicity of organophosphorus (OP) compounds. Changes in inhibitory GABAergic activity in different brain regions can contribute to both induction and maintenance of seizures in subjects exposed to the OP nerve agent soman. Here, we tested the hypothesis that galantamine can prevent immediate and delayed effects of soman on hippocampal inhibitory synaptic transmission. Spontaneous inhibitory postsynaptic currents (IPSCs) were recorded from CA1 pyramidal neurons in hippocampal slices obtained at 1 h, 24 h, or 6 to 9 days after the injection of guinea pigs with saline (0.5 ml/kg i.m.), 1xLD(50) soman (26.3 microg/kg s.c.), galantamine (8 mg/kg i.m.), or galantamine at 30 min before soman. Soman-challenged animals that were not pretreated showed mild, moderate, or severe signs of acute intoxication. At 1 h after the soman injection, the mean IPSC amplitude recorded from slices of mildly intoxicated animals and the mean IPSC frequency recorded from slices of severely intoxicated animals were larger and lower, respectively, than those recorded from slices of control animals. Regardless of the severity of the acute toxicity, at 24 h after the soman challenge the mean IPSC frequency was lower than that recorded from slices of control animals. At 6 to 9 days after the challenge, the IPSC frequency had returned to control levels, whereas the mean IPSC amplitude became larger than control. Pretreatment with galantamine prevented soman-induced changes in IPSCs. Counteracting the effects of soman on inhibitory transmission can be an important determinant of the antidotal effectiveness of galantamine.

Fig. 1.

Sample recordings of spontaneous IPSCs obtained from CA1 pyramidal neurons in hippocampal slices obtained from saline-injected guinea pigs. A, postsynaptic currents recorded from a CA1 pyramidal neuron at 0 mV before (control) and 10 min after perfusion of the slice with ACSF containing the GABA_A receptor antagonist bicuculline (10 μM). B, representative traces of IPSCs recorded from another neuron at different membrane potentials. ACSF contained the glutamate receptor antagonists 2-amino-5-phosphonopentanoic acid (50 μM) and 6-cyano-7-nitroquinoxaline-2,3-dione (10 μM).

Fig. 2.

Relationship between the acute toxicity and the mean IPSC frequency and amplitude recorded from hippocampal slices taken at different times from guinea pigs exposed to 1×LD₅₀ soman. Box plots of toxicity scores versus frequency (A) or amplitude (B) of IPSCs in slices obtained at 1 h, 24 h, or 6 to 9 days after the injection of guinea pigs with soman (26.3 μg/kg s.c.). Scores refer to mild (0–1), moderate (2–3), and severe (4–5) signs of acute intoxication, as described in Table 1. Numbers of animals per group are shown in parentheses. The IPSC amplitudes recorded from CA1 pyramidal neurons in slices taken at 1 h after soman challenge decreased as the severity of intoxication increased. *, p < 0.05 and ***, p < 0.001 according to ANOVA followed by Bonferroni post hoc test.

Fig. 3.

Changes in frequency and amplitude of IPSCs recorded from CA1 pyramidal neurons in slices taken at 1 h after soman (1×LD₅₀) challenge. A, amplitudes of IPSCs recorded from neurons in slices from mildly intoxicated guinea pigs (scores 0–1) were significantly higher than those recorded from neurons in slices of saline-injected (control) animals. B, graph shows the frequency of IPSCs recorded from pyramidal neurons from the same slices as in A. IPSC frequency in slices from severely intoxicated guinea pigs (scores 4–5) was significantly lower than control. Graph and error bars represent mean and S.E.M., respectively, obtained from the following number of neurons: 30 (saline), 3 (scores 0–1), 9 (scores 2–3), and 7 (scores 4–5). *, p < 0.05 according to ANOVA followed by Dunnett post hoc comparison with control.

Fig. 4.

Changes in IPSC frequency and amplitude recorded from CA1 pyramidal neurons at 24 h or 6 to 9 days after injection of guinea pigs with 1×LD₅₀ soman. A, mean frequency of IPSCs recorded from neurons in slices taken at 24 h after soman was significantly lower than control. **, p < 0.01 according to ANOVA followed by Dunnett post hoc comparison with control. B, cumulative probability plot of inter-event intervals recorded at 24 h after soman was displaced to the right in comparison with control. p < 0.05 according to the Mann–Whitney test. C, mean amplitudes of IPSCs recorded from neurons in slices obtained at 6 to 9 days after soman were significantly larger than control. *, p < 0.05 according to ANOVA followed by Dunnett post hoc comparison with control. D, compared with control, the histogram and cumulative probability plot of IPSC amplitudes recorded at 6 to 9 days after soman were skewed toward larger amplitudes. p < 0.05 according to the Mann–Whitney test. Data are presented as mean and S.E.M. of results obtained from 30 (saline), 21 (24 h after soman), and 22 neurons (6–9 days after soman).

Fig. 5.

Pretreatment of the guinea pigs with galantamine prevented soman-induced changes in the frequency and amplitude of IPSCs. Animals were treated with galantamine (8 mg/kg i.m.) and 30 min later they were challenged with 1×LD₅₀ soman (26.3 μg/kg s.c.). Hippocampal slices were obtained from animals at 1 h, 24 h, or 6 to 9 days after the soman challenge. Graphs show frequency (A) and amplitudes (B) of IPSCs in galantamine-treated, soman-challenged animals were comparable with control levels. Graph and error bars represent mean and S.E.M., respectively, of results obtained from the following number of neurons: 30 (saline), 12 (1 h after soman), 13 (24 h after soman), and 10 (6–9 days after soman).

Fig. 6.

Effects of the acute treatment of guinea pigs with galantamine on IPSC frequency and amplitudes. A and B, graphs of mean IPSC frequency (A) and amplitudes (B) recorded from CA1 pyramidal neurons in slices obtained at 1.5 h, 24 h, or 6 to 9 days after the injection of guinea pigs with galantamine (8 mg/kg i.m.). Treatment with galantamine caused a significant transient increase in the IPSC amplitudes. *, p < 0.05 according to ANOVA followed by Dunnett post hoc comparison with control. C, the histogram and cumulative probability plot of IPSC amplitudes recorded at 1.5 h after the injection of galantamine were displaced toward larger amplitudes in comparison with control. p < 0.05 according to the Mann–Whitney test. Data are presented as mean and S.E.M. of results obtained from the following number of neurons: 30 (saline), 17 (galantamine, 1.5 h), 12 (galantamine, 24 h), and 11 (galantamine, 6–9 days).