Tadahiko Suzuki

Acute Effects of Pyrethroids on Serotonin Release in the Striatum of Awake Rats: An In Vivo Microdialysis Study

The present study examined the acute neurotoxic effects of three different pyrethroids, allethrin, cyhalothrin, and deltamethrin on the release of serotonin (5‐HT) and its metabolite 5‐hydroxyindoleacetic acid (5‐HIAA) in the striatum of conscious rats using microdialysis. Allethrin 10 mg/kg reduced extracellular levels of 5‐HT to 46%, whereas 20 and 60 mg/kg increased the release to 177% and 243% of baseline, respectively. Cyhalothrin increased 5‐HT release to 145–204% and deltamethrin decreased to 58–32% of baseline in a dose‐dependent manner. None of the pyrethroids tested altered extracellular levels of 5‐HIAA. Local infusion of the voltage‐gated sodium channel antagonist tetrodotoxin (TTX) into striatum completely prevented the effects of allethrin, cyhalothrin, and deltamethrin (10 and 20 mg/kg) on 5‐HT release. The effect of deltamethrin at 60 mg/kg was completely abolished by striatal infusion of nimodipine (L‐type Ca++ channel antagonist) with TTX. These findings suggest that pyrethroids disrupt the serotonergic neurotransmission in striatum in a dose‐related manner with Na+ and Ca2+ channel‐dependent mechanisms.

Correlation of Binding Sites for Diisopropyl Phosphorofluoridate with Cholinesterase and Neuropathy Target Esterase in Membrane and Cytosol Preparations from Hen

To find new putative target(s) for organophosphorus induced delayed neurotoxicity (OPIDN), we investigated the biochemical and pharmacological characteristics of [3H] diisopropyl phosphorofluoridate (DFP) binding to membrane and cytosol preparations from the brain and spinal cord of hens. Specific [3H]DFP binding was determined by subtracting non-specific binding from total binding. The binding sites of [3H]DFP, an organophosphate that induces OPIDN, were found not only on membrane but also in cytosol. Reduction of subsequent ex vivo specific [3H]DFP binding by in vivo pretreatment with unlabeled DFP was found in cytosol, not membrane. The reduced binding lasted to the onset of OPIDN, especially in spinal cord. These results suggest that the specific DFP binding sites in cytosol, rather than on membrane, are the most important with regard to the initiation of OPIDN. Inhibitors of cholinesterase (ChE) and neuropathy target esterase (NTE) other than DFP did not affect specific [3H]DFP binding to either membranes or cytosol in vivo. Additionally, inhibition of the activities of these esterases by these compounds was not consistent with either the degree of inhibition of the [3H]DFP binding or a time-dependent manner of OPIDN. These results suggest that DFP binding site(s) involved in the initiation of OPIDN may be different from the active sites of ChE and NTE.

A Comparative Study of Binding Sites for Diisopropyl Phosphorofluoridate in Membrane and Cytosol Preparations from Spinal Cord and Brain of Hens

Biochemical events in the initiation of organophosphorus induced delayed neurotoxicity (OPIDN) are not well understood. To find new putative target(s) for OPIDN, we investigated the biochemical and pharmacological characteristics of [3H] diisopropyl phosphorofluoridate (DFP) binding to membrane and cytosol preparations from the brain and spinal cord of hens in vitro. [3H]DFP binding to both preparations was determined by the specific binding obtained by subtracting non-specific binding from total binding. The specific binding sites of [3H]DFP were found not only on membrane but also in cytosol. Kd values were higher and Bmax values were lower in cytosol than in membrane. Moreover, the Kd values in both membrane and cytosol preparations from spinal cord were lower than those of brain. The Bmax values in membrane and cytosol were similar between brain and spinal cord. The specific binding to both preparations was markedly displaced by unlabeled DFP. The specific binding of DFP to the membrane was highly or partly displaced by organophosphorus compounds (OPs) or a carbamate, respectively. However, both the OPs and the carbamate had considerably weaker blocking effects on the specific binding of DFP to cytosol. None of the compounds known to interact with neuropathy target esterase (NTE) had a strong blocking effect on the specific binding of DFP to either membrane or cytosol. These results show that the specific binding of DFP to the membrane may be binding with cholinesterase (ChE). However, cytosol, especially in spinal cord, may have DFP binding sites other than ChE and NTE.

Effects of systemic administration of 2-(4-phenyl-piperidino)-cyclohexanol (vesamicol) and an organophosphate DDVP on the cholinergic system in brain regions of rats.

Vesamicol is known to inhibit the transport of acetylcholine (ACh) into synaptic vesicles in vitro, but much less is known about its effects in the brain in vivo. To assess the effect of vesamicol in vivo, we examined cholinergic parameters, such as the subcellular distribution of ACh, activities of enzymes, uptake of choline, and muscarinic receptor binding in the striatum, hippocampus, and cerebral cortex of rats 30 and 60 min after intraperitoneal injection of vesamicol (3 mg/kg) or of vesamicol in combination with DDVP (5 mg/kg), which was administered 10 min before vasamicol. The levels of cytosolic ACh increased in all regions of the brain after injection of vesamicol, while those of vesicular ACh decreased in all regions except for the striatum. The increase in the levels of extracellular ACh and cytosolic ACh in the striatum induced by DDVP was generally enhanced after injection of vesamicol, Vesamicol did not reduce the level of vesicular ACh when DDVP had been injected previously. Vesamicol did not induce any significant changes in the activities of enzymes, choline uptake, or binding of [6H]quinuclidinyl benzilate to the muscarinic ACh receptors in the three regions. Changes in the cholinergic parameters caused by DDVP were not reversed by the combined administration of DDVP with vesamicol. The present results indicate that vesamicol can inhibit the transport of ACh into synaptic vesicles in the brain tissue in vivo, although it cannot reverse the effects of DDVP that has been injected prior to vesamicol.

Effect of a CNS-Sensitive Anticholinesterase Methane Sulfonyl Fluoride on Hippocampal Acetylcholine Release in Freely Moving Rats

Anticholinesterases (antiChEs) are used to treat Alzheimers disease. The comparative effects of two antiChEs, methanesulfonyl fluoride (MSF) and donepezil, on the extracellular levels of ACh in the hippocampus were investigated by in vivo microdialysis in freely moving rats. MSF at 1 and 2 mg/kg produced a dose-dependent increase in ACh efflux from 10 min to at least 3 hrs after injection. At 2 mg/kg, the increase was still present at 24 hr. Donepezil at 1 mg/kg showed a similar but smaller effect, and, paradoxically, 2 mg/kg showed no consistent effect. MSF at 1 and 2 mg/kg decreased acetylcholinesterase activity in the hippocampus to 54.8 and 20.1% of control, respectively. These results suggest that MSF is a suitable candidate for the treatment of Alzheimers disease.

Neurotoxicity and Brain Regional Distribution of Manganese in Mice

Manganese (Mn) is known to cause neurotoxicity in the central nervous system similar to Parkinsonism in man, but the mechanism underlying remains to be well clarified. Catalepsy is used to observe Parkinsonism in laboratory animals. In the present study, effects of repeated injection of Mn chloride (MnCl2) on catalepsy, dopamine receptors and distribution of 54Mn in the brain were investigated. [Methods] Female ICR mice were injected with 0, 10, 30 or 50 mg/kg/day of MnCl2 for 3 days, and examined for catalepsy and the binding ability of striatum, hippocampus and cerebral cortex to [3H]haloperidol to detect and change of dopamine D2‐receptors. Whole‐body burden and disposition of 54Mn in the brain regions and liver were determined after the repeated injection of 54MnCl2. Mice were given l‐dopa at 25 mg/kg 2 hr prior to MnCl2 injection to examined if the catalepsy was abolished. [Results] Mice showed catalepsy following injection of MnCl2 at 50 mg/kg, but not with less than 30 mg/kg. The catalepsy initiated about 60, 60 and 30 min after injection of MnCl2 on the 1st, 2nd and 3rd day, respectively, and lasted for about 60 min. l‐dopa slightly reversed the catalepsy. The binding of [3H]haloperidol in the three brain regions from mice treated with MnCl2 was lower than that from control. The concentration of 54Mn in the striatum and remaining areas, including substantia nigra, was the highest in the brain regions examined. [Conclusion] Since l‐dopa slightly alleviated catalepsy by MnCl2, and binding of [3H]haloperidol was decreased in brain regions, MnCl2 might induce catalepsy by suppressing D2 receptors in the striatum‐substantia nigra.