S. Barone

Alterations in flash evoked potentials (FEPs) in rats produced by 3,3′-iminodipropionitrile (IDPN)☆☆☆

3,3-Iminodipropionitrile (IDPN) is a neurotoxicant that produces changes in flash evoked potentials (FEPs) 18 weeks after treatment. We examined dose- and time-related effects of IDPN on FEPs at earlier time points than previously studied (52). Adult male Long-Evans rats were given IDPN (0, 100, 200, 400 mg/kg/day x 3 days, i.p.) and FEPs were recorded 14 days later. IDPN (400 mg/kg/day) decreased the amplitudes of some of the early and middle FEP peaks (n30 and N56), and increased the latencies of some early peaks (P21 and P46). A separate group of rats was treated with IDPN (0 or 400 mg/kg/day x 3 days, i.p.) and FEPs were recorded 1, 3, 7, 14, and 35 days later. The latencies of of all portions of FEPs were increased by IDPN, with maximal changes occurring at 7 and/or 14 days. The amplitude of the middle portions of FEPs (peaks N56, P63, N70, P90) were altered as early as day 3, and some changes were observed up to day 14. In contrast, the late portion of FEPs (peak N160) was affected at later times (days 14 and 35). Corneal opacities were noted on days 3 and 7, but were largely reversible by day 14. In the time-course study, IDPN decreased colonic temperature on days 1, 3, 7, and 14. The present results suggest that IDPN alters both the early FEP peaks related to the initial afferent sensory volley, and cortical processing associated with the middle and later portions of FEPs.

Tissue Carboxylesterases and Chlorpyrifos Toxicity in the Developing Rat

Young animals are more sensitive than adults to the neurotoxic effects of some organophosphorus insecticides. Many investigators attribute this difference in sensitivity to the immaturity of the detoxification capacity of preweanling rats. Chlorpyrifos [O,O-diethylO-(3,5,6-trichloro-2-pyridyl)phosphorothionate] is an organophosphorus insecticide that demonstrates considerable age-related sensitivity. The carboxylesterases are a group of related enzymes that detoxify organophosphorus insecticides by stoichiometrically binding these molecules before they can inhibit acetylcholinesterase. This study presents in vitro and in vivo evidence demonstrating that the carboxylesterases are critical for explaining the age-related sensitivity of chlorpyrifos. The data show that the fetal rat and the postnatal day 17 (PND17) rat pup have fewer molecules of carboxylesterase (less activity), less sensitive molecules of carboxylesterase, and a larger proportion of chlorpyrifos-insensitive molecules of carboxylesterase. An in vitro mixing experiment, using adult striatum as a source of acetylcholinesterase and liver homogenates as a source of carboxylesterase, demonstrates that the adult liver carboxylesterases are superior to the PND17 liver carboxylesterases for detoxifying chlorpyrifos. In the in vivo experiments the time course profiles of carboxylesterase and cholinesterase activity following a maximum tolerated dose of chlorpyrifos also suggest that the carboxylesterases of the PND17 rat were less capable of detoxifying chlorpyrifos. Carboxylesterase activity in the preweanling rat was not as severely inhibited as in the adult, but decrements in cholinesterase activity as a result of chlorpyrifos treatment were comparable. These in vitro and in vivo findings support the previously proffered postulate that the carboxylesterases are critical for determining the age-related sensitivity of chlorpyrifos. In addition, these detailed experiments allow us to propose that the detoxification potential of these enzymes is multifaceted, and depends on the (1) amount of activity (i.e., number of molecules), (2) affinity for the insecticide or metabolite, and (3) amount of carboxylesterase activity that is refractory to inhibition by the insecticide or metabolite.

Comparison of intracranial infusions of colchirine and ibotenic acid as models of neurodegeneration in the basal forebrain

Colchicine and ibotenic acid were compared for their ability to produce neurodegeneration and cognitive deficit after bilateral infusions into the nucleus basalis magnocellularis of male Long-Evans rats. Four weeks post-lesion, there was no difference in locomotor activity following infusion of either neurotoxicant or vehicle. In a passive avoidance task, both treated groups had significantly shorter step-through latencies compared with vehicle. Five weeks post-lesion, rats were killed for neurochemistry or histochemistry. Choline acetyltransferase (ChAT) activity in both the frontal and parietal cortex was significantly decreased (25-35%) in the colchicine- and ibotenic acid-infused rats when compared to control. There was no effect of either neurotoxicant on ChAT activity in the hippocampus or striatum. Both neurotoxicants produced damage in the general area of the ventromedial pallidum, although ibotenic acid infusion consistently produced a larger area of damage as assessed in Nissl-stained sections. Analysis of the number of ChAT-immunoreactive cells in the nucleus basalis magnocellularis (NBM) showed an average 60% cell loss following colchicine infusion and a 75% cell loss after ibotenic acid infusion. Area of glutamic acid decarboxylase (GAD) staining was significantly decreased in several regions surrounding the NBM for ibotenic acid (51% average decrease), and showed non-significant decreases (28%) following colchicine infusion. Colchicine infusion decreased dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum; ibotenic acid had no effect on brain catechol of indoleamine levels. The results indicate that although similar cholinergic hypofunction and behavioral deficits were achieved, several non-cholinergic differences between the neurotoxicants were detected.

Time course of changes in cholinergic and neurotrophin-related markers after infusion of colchicine into the basal forebrain.

After bilateral infusions of colchicine or vehicle in the rat nucleus basalis magnocellularis, the time course of changes in several cholinergic and neurotrophin-related markers were assessed. Animals were sacrificed at 3, 7, 14, 28, 35 and 84 days post-lesion, and both the NBM and cortical areas were assessed. Sections were stained immunohistochemically for choline acetyltransferase (ChAT) or p140trk (trk) or histochemically for acetylcholinesterase (AChE). ChAT activity and neurotrophin protein levels were assessed regionally. The number of ChAT immunoreactive NBM neuronal profiles decreased beginning 3 days post-lesion and reach maximal loss by 28 days post-lesion, with no recovery. Examination of trk-IR around the NBM revealed a time-dependent decrease in trk-IR of magnocellular neuron and an increase in trk-IR of astrocytes at 14 and 28 days post-lesion. The density of AChE-stained cortical fibers was maximally decreased 3 days post-lesion followed by an increase in fiber staining across the remaining time points. Cortical ChAT activity showed the largest decrease at 7 days followed by recovery 84 days after colchicine infusion. There was an increase in NGF in the parietal cortex after colchicine infusion but no change in BDNF level. These patterns of changes in the cholinergic and neurotrophin-related markers suggest an association between NGF and lesion-induced compensatory responses in the basal forebrain cholinergic system.