Biomedical subjects

H A Tilson

Publications and source records attributed to H A Tilson.

At least 55 records · Page 3Linked to original sources

PubMed1994

Differential effects of polychlorinated biphenyl congeners on phosphoinositide hydrolysis and protein kinase C translocation in rat cerebellar granule cells.

Previous reports from our laboratory have suggested that the neuroactivity of some polychlorinated biphenyl (PCB) congeners is associated with perturbations in cellular Ca(2+)-homeostasis. We have characterized further the neurochemical effects of PCBs on signal transduction in primary cultures of cerebellar granule cells. The present experiments found that neither 2,2'-dichlorobiphenyl (DCBP), an ortho-substituted congener, nor 3,3',4,4',5-pentachlorobiphenyl (PCBP), a non-ortho-substituted congener, affected basal phosphoinositide (PI) hydrolysis in cerebellar granule cells. However, at concentrations up to 50 microM, DCBP potentiated carbachol-stimulated PI hydrolysis, while decreasing it at 100 microM. PCBP, on the other hand, had no effect on carbachol-stimulated PI hydrolysis in concentrations up to 100 microM. [3H]Phorbol ester ([3H]PDBu) binding was used to determine protein kinase C (PKC) translocation. DCBP increased [3H]PDBu binding in a concentration-dependent manner and a twofold increase was observed at 100 microM in cerebellar granule cells. PCBP had no effect on [3H]PDBu binding at concentrations up to 100 microM. The effect of DCBP on [3H]PDBu binding was time-dependent and was also dependent on the presence of external Ca2+ in the medium. To test the hypothesis that DCBP increases [3H]PDBu binding by acting on receptor-activated calcium channels, the effects of DCBP were compared to those of L-glutamate. The effects of DCBP (50 microM) and glutamate (20 microM) were additive.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Mechanisms underlying AlCl3 inhibition of agonist-stimulated inositol phosphate accumulation. Role of calcium, G-proteins, phospholipase C and protein kinase C.

Possible mechanisms of AlCl3-induced inhibition of agonist-stimulated inositol phosphate (IP) accumulation were investigated using rat brain cortex slices, synaptosomes or homogenates. Under conditions in which AlCl3 inhibits carbachol (CARB)-stimulated IP accumulation (Gp-mediated), AlCl3 did not affect CARB (100 microM)-induced decreases (Gi-mediated) in 30 microM forskolin-stimulated cAMP accumulation, suggesting that AlCl3 may be specific for Gp-mediated signal transduction. To determine whether AlCl3 interfered with Gp function and/or phosphatidylinositol-specific phospholipase C (PiPLC) activity, effects of AlCl3 on CARB- and Ca(2+)-stimulated IP accumulation were examined in cortical synaptosomes. AlCl3 (500 microM) decreased CARB (1 mM)- and Ca2+ (20 microM ionomycin)-stimulated IP accumulation to 77 and 75% of control, respectively, suggesting that AlCl3 may not directly affect Gp activity, but does inhibit PiPLC activity. In cortical homogenates, AlCl3 (10-500 microM) inhibited hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate (PIP2) by PiPLC in a concentration-dependent manner with an estimated IC50 of 100 microM. The effects of AlCl3 on modulation of IP accumulation by extracellular Ca2+ and PKC were also examined as potential mechanisms. Decreasing the extracellular Ca2+ concentration ([Ca2+]e) from 1.0 to 0.1 mM decreased CARB-stimulated IP accumulation in slices. AlCl3 (500 microM) decreased significantly 1 mM CARB-stimulated IP accumulation in 1.0 and 0.1 mM Ca2+ solutions; however, the effect of AlCl3 on IP accumulation did not depend on [Ca2+]e. In cortical slices, inhibition of 1 mM CARB-stimulated IP accumulation by 500 microM AlCl3 was not altered by the PKC activator phorbol 12,13-dibutyrate (PdBu, 1 microM), or the PKC inhibitor H-7 (10 microM), suggesting that AlCl3 does not interfere with IP accumulation by activation of PKC. Other studies found that AlCl3 (10-100 microM) inhibited PKC activity in a concentration-dependent manner in both cytosolic and membrane fractions of cortical homogenates with an estimated IC50 of 60 microM. These results support the hypothesis that AlCl3 inhibition of agonist-stimulated IP accumulation may be mediated by inhibition of PiPLC activity, rather than disruption of G-protein function or modulation of the IP signalling system by Ca2+ or PKC.

Aluminum Chloride↗

PubMed1994

Comparison of intracranial infusions of colchicine 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.

Animals↗

PubMed1994

Aluminum alters calcium transport in plasma membrane and endoplasmic reticulum from rat brain.

Calcium is actively transported into intracellular organelles and out of the cytoplasm by Ca2+/Mg(2+)-ATPases located in the endoplasmic reticulum and plasma membranes. We studied the effects of aluminum on calcium transport in the adult rat brain. We examined 45Ca-uptake in microsomes and Ca(2+)-ATPase activity in microsomes and synaptosomes isolated from the frontal cortex and cerebellum of adult male Long-Evans rats. ATP-dependent 45Ca-uptake was similar in microsomes from both brain regions. The addition of 50-800 microM AlCl3 resulted in a concentration-dependent inhibition of 45Ca-uptake. Mg(2+)-dependent Ca(2+)-ATPase activity was significantly lower in synaptosomes compared to microsomes in both frontal cortex and cerebellum. In contrast to the uptake studies, AlCl3 stimulated Mg(2+)-dependent Ca(2+)-ATPase activity in both microsomes and synaptosomes from both brain regions. To determine the relationship between aluminum and Mg2+, we measured ATPase activity in the presence of increasing concentrations of Mg2+ or AlCl3. Maximal ATPase activity was obtained between 3 and 6 mM Mg2+. When we substituted AlCl3 for Mg2+, ATPase activity was also stimulated in a concentration-dependent manner, but to a greater extent than with Mg2+. One interpretation of these data is that aluminum acts at multiple sites to displace both Mg2+ and Ca2+, increasing the activity of the Ca(2+)-ATPase, but disrupting transport of calcium.

Aluminum Chloride↗

PubMed1994

Fenthion produces a persistent decrease in muscarinic receptor function in the adult rat retina.

Several reports have suggested that exposure to organophosphate pesticides damages the visual system. The prolonged effects of an acute dose of fenthion (dimethyl 3-methyl-4-methylthiophenyl phosphorothionate) were studied on the cholinergic system of the rat retina. Fenthion was administered in a single dose of 0 or 100 mg/kg (sc, in corn oil) to adult, male, Long-Evans rats. The animals were killed 4, 14, or 56 days after treatment and cholinesterase (ChE) activity as well as muscarinic receptor (mChR) function measured in the retina and frontal cortex. Fenthion produced 89% inhibition of ChE activity in both tissues at 4 days, and, although there was recovery, slight (15%) inhibition of the enzyme activity was still observed at 56 days in both tissues. A long-lasting decrease in carbachol-stimulated inositolphosphate (IP) release was observed following fenthion treatment in the retina: IP release was depressed at 4 days and this depression persisted up to 56 days after dosing. The density of mChR in the retina as well as in the cortex was decreased by 14-20% at 4 days and returned to control levels by 56 days. Fenthion had no effect on the metabolism of phospholipids in the retina following intraocular injections of labeled precursors [3H]myo-inositol, [methyl-14C]choline, or [2-3H]glycerol 4 days after fenthion treatment. These prolonged effects of fenthion on mChR function (signal transduction) appear to be specific to the retina as the cortex showed no change in receptor-stimulated IP release even in the presence of significant mChR down-regulation and ChE inhibition. This dose of fenthion did not produce overt morphological changes in the retina or in the cortex, as observed with light microscopy, although an increase in glial fibrillary acidic protein immunoreactivity (GFAP IR) extending from the internal limiting membrane to the external limiting membrane of the retina was noted. This increase in GFAP IR was observed at 14 days and persisted as long as 56 days post-treatment in the retina, but was not noted in the cortex at any of the time points studied. Thus, this long-lasting perturbation in the retinal cholinergic second messenger system induced by fenthion may occur independently of depressed ChE activity and down-regulation of mChR.

Animals↗

PubMed1994

Compensatory changes in the hippocampus following intradentate infusion of colchicine.

Direct infusion of colchicine into the dentate gyrus of the hippocampus kills granule cells and elicits behavioral, neurochemical and neuroanatomical changes. Colchicine-treated rats are less sensitive to the behavioral effects of cholinergic muscarinic receptor antagonists and more sensitive to cholinergic agonists. These behavioral changes are associated with time- and dose-dependent alterations in the cholinergic signal transduction mechanism. Carbachol-stimulated turnover of phosphoinositides is increased in the hippocampus of colchicine-treated rats; similar changes are not observed in the cortex or striatum of colchicine-treated animals. Intradentate colchicine produces a significant increase in choline- acetyltransferase activity and staining for acetylcholinesterase activity in the hippocampus, suggesting reactive synaptogenesis of cholinergic fibers. Other studies have shown that the integrity of the septohippocampal pathway is necessary for these colchicine-induced compensatory changes to occur. It is suggested that the mechanism for these neurochemical changes in colchicine-treated animals may be occurring via alterations in negative feedback control of receptor-G-protein-mediated phosphoinositide hydrolysis.

Animals↗

PubMed1993

Aluminum decreases muscarinic, adrenergic, and metabotropic receptor-stimulated phosphoinositide hydrolysis in hippocampal and cortical slices from rat brain.

Effects of aluminum chloride (AlCl3) (0.1 to 1000 microM) on inositol phosphate (IP) accumulation stimulated by carbachol (CARB), norepinephrine (NE) or quisqualate (QUIS) were examined in rat hippocampal and cortical slices. In the absence of agonist, only 1000 microM AlCl3 significantly reduced basal accumulation of IPs. For CARB-stimulated IP accumulation, 100 microM and greater AlCl3 significantly inhibited IP accumulation. In cortical slices, 1000 microM AlCl3 reduced CARB-stimulated IP accumulation by 55% and in hippocampal slices 1000 microM AlCl3 inhibited IP accumulation by 40%. Similar effects of AlCl3 were observed for NE-stimulated IP accumulation. In cortical slices, the concentration-response for AlCl3 effects on agonist-stimulated IP accumulation was significantly different from that in hippocampal slices. For QUIS-stimulated accumulation of IPs, 1000 microM AlCl3 significantly inhibited IP accumulation in hippocampal slices. However, in cortical slices a biphasic effect of AlCl3 was observed. 500 and 1000 microM AlCl3 significantly inhibited IP accumulation, whereas 10 and 50 microM AlCl3 significantly enhanced QUIS-stimulated IP accumulation. In both hippocampal and cortical slices, 500 microM AlCl3 significantly inhibited CARB-, NE- or QUIS-stimulated IP accumulation at all agonist concentrations (0.1 to 10000 microM) tested, indicating a post-receptor effect on agonist-mediated IP accumulation. Stimulation of G-proteins with NaF (5-30 mM) resulted in accumulation of IPs in hippocampal and cortical slices in the absence of added agonists. NaF (5-30 mM) plus 1 mM CARB produced increased accumulation of IPs over CARB or NaF alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum Chloride↗

PubMed1993

Characterization of disulfoton-induced behavioral and neurochemical effects following repeated exposure.

These experiments examined the relationship between behavioral alterations and neurochemical changes in rats exposed repeatedly to disulfoton, an organophosphate cholinesterase inhibitor. Male Long-Evans rats were injected ip for 30 days with 0, 0.5, 1, or 2 mg/kg of disulfoton in corn oil. Clinical signs and motor activity were measured during the course of repeated exposure. Cognitive function, as measured in the Morris water maze, and passive avoidance procedures were assessed near the end of the dosing regimen. Regional brain acetylcholinesterase (AChE) activity was measured during the course of dosing while the total number of muscarinic receptors was measured at the end of the dosing regimen. Tolerance developed rapidly to the clinical signs produced by disulfoton, but not to the disulfoton-induced decrease in motor activity. Disulfoton affected the acquisition of water maze performance, but had no effect on passive avoidance acquisition or retention. Repeated exposure to disulfoton decreased brain AChE activity and the number of [3H]quinuclidinyl benzilate binding sites. These data indicate that, in spite of muscarinic receptor down-regulation that followed repeated exposure to disulfoton, animals become tolerant to only some of the functional effects produced by this chemical.

Animals↗

PubMed1993

Effects of selected neuroactive chemicals on calcium transporting systems in rat cerebellum and on survival of cerebellar granule cells.

This investigation examined the relationship between alteration of Ca(2+)-transport systems and cytotoxicity in vitro for a number of neuroactive chemicals including environmental pollutants. 45Ca2+ uptake as a measure of Ca2+ sequestration was determined in mitochondria and microsomes isolated from cerebella of adult male Long-Evans hooded rats by differential centrifugation. Ca2+ extrusion, measured as Ca(2+)-ATPase activity, was determined in synaptosomes prepared by sucrose density gradient. Cytotoxicity (lactate dehydrogenase leakage) was assessed in primary cultures of cerebellar granule cells from 6- to 8-day-old Long-Evans rats. N-Methyl-D-aspartic acid (NMDA) did not alter synaptosomal Ca(2+)-ATPase activity or 45Ca2+ uptake in mitochondria and microsomes. However, chlorpromazine (CPZ), aluminum (Al), permethrin (PER), and deltamethrin (DEL) inhibited Ca2+ sequestration by mitochondria and microsomes. The IC50 values (microM) for CPZ, Al, PER, and DEL were 67.8, 671, 4.2, and 91.2 for mitochondrial 45Ca2+ uptake, and 129.9, 1384, > 50, and > 200 for microsomal 45Ca2+ uptake, respectively. CPZ, PER, and DEL also inhibited synaptosomal Ca(2+)-ATPase activity in a concentration-dependent manner with IC50 values of 62.5, > 400, and 246.9 microM, indicating an effect on the Ca(2+)-extrusion process. Al increased Ca(2+)-ATPase activity (EC50 = 833 microM). Although NMDA did not inhibit Ca(2+)-transport systems, it was cytotoxic at 250 microM and higher concentrations after 2 hr of exposure. Similarly, CPZ was cytotoxic at concentrations of 25 and 10 microM after 4 hr exposure. However, PER, DEL, and Al were not cytotoxic at any concentration up to 500 microM. Of all the chemicals tested, CPZ was the most potent in inhibiting Ca(2+)-transporting systems and was also cytotoxic.(ABSTRACT TRUNCATED AT 250 WORDS)

Aluminum↗

PubMed1993

Correlation of the anticholinesterase activity of a series of organophosphates with their ability to compete with agonist binding to muscarinic receptors.

Some compounds that inhibit acetylcholinesterase (AChE) activity compete directly with quinuclidinyl benzilate (QNB) binding, a muscarinic antagonist which binds to all subtypes equally, and with cis-methyldioxolane (CD), an agonist that binds with high affinity to the M2 subtype of muscarinic receptors. The relationship between inhibition of AChE activity and the capability to affect muscarinic receptors directly has not been systematically explored. The interaction of eight organophosphates with muscarinic receptors was compared to their ability to inhibit AChE activity in vitro in tissue homogenates from rat hippocampus and frontal cortex, two cholinergically enriched areas of the brain. Of the compounds tested only echothiophate competed for [3H]QNB binding and only at concentrations greater than 100 microM. The anticholinesterase compounds were also tested for their ability to compete with a muscarinic receptor agonist, [3H]CD, which binds with high affinity (approximate KD = 3.5 nM) to 10 and 3% of the muscarinic receptors in the frontal cortex and hippocampus, respectively. The anticholinesterase compounds inhibited high-affinity [3H]CD binding up to 80% and the effects were similar in both tissues. Echothiophate and DFP were potent inhibitors of [3H]CD binding, as were the active "oxon" forms of parathion, malathion, and disulfoton. The parent "thio" forms of these insecticides, however, were much less effective in competing for [3H]CD binding. A similar pattern of potency was observed for the inhibition of brain AChE activity. A strong correlation was found between the ability of a compound to inhibit AChE activity and the ability to compete with [3H]CD binding. These data suggest that the biological effects of cholinesterase-inhibiting compounds may be due to more than their ability to inhibit AChE.

Acetylcholinesterase↗

PubMed1993

Comparative effects of two polychlorinated biphenyl congeners on calcium homeostasis in rat cerebellar granule cells.

Some polychlorinated biphenyls (PCBs) have been reported to alter locomotor activity and decrease brain dopamine function in laboratory animals. PCBs with ortho- and/or parachlorine substitutions and varying number of chlorinations are known to decrease cell dopamine content in vitro and have been detected in brains of animals exposed to PCBs, suggesting that the neurotoxicity could be mediated by ortho-substituted congeners. Dopamine or other neurotransmitter uptake and release phenomena are dependent on the maintenance of intracellular Ca2+ homeostasis, and perturbations in Ca2+ homeostasis could lead to altered cell function and/or death. We compared the effects of two PCB congeners on Ca2+ homeostasis in cerebellar granule cells: 2,2'-dichlorobiphenyl (DCBP), a putative neurotoxic congener, and 3,3',4,4',5-pentachlorobiphenyl (PCBP), a presumed nonneurotoxic congener. In cerebellar granule cells (6-8 days in vitro), DCBP was cytotoxic as indicated by a significant increase in LDH leakage at 200 microM after 2 hr of exposure and at 100 microM after 4 hr exposure. PCBP, on the other hand, did not affect LDH leakage even at 200 microM for up to 4 hr. Although both congeners increased cerebellar granule cell [Ca2+]i, DCPB was more effective in increasing [Ca2+]i to a greater extent than PCBP. The increase in [Ca2+]i produced by both congeners was not transient, but a steady rise was observed with time. To understand cellular Ca(2+)-buffering capacity, Ca2+ sequestration and Ca2+ extrusion were studied in mitochondria, microsomes, and synaptosomes, isolated from adult rat cerebellum. DCBP was a potent inhibitor of 45Ca2+ uptake by mitochondria (IC50 = 6.17 +/- 0.53 microM) and microsomes (IC50 = 7.61 +/- 0.35 microM). PCBP inhibited Ca2+ sequestration by mitochondria (68% of control) and microsomes (72% of control), but the effects were much less than those produced by equivalent concentrations of DCBP. Synaptosomal Ca(2+)-ATPase was inhibited by DCBP, but not by PCBP. These results indicate that at concentrations where cytotoxicity in cerebellar granule cells was not observed, DCBP increased intracellular [Ca2+]i, and at the same concentrations, Ca2+ sequestration by intracellular organelles and Ca(2+)-ATPase in synaptic plasma membrane were inhibited. Although PCBP increased [Ca2+]i in cerebellar granule cells to some extent, it was not potent in affecting Ca2+ sequestration or Ca2+ extrusion in adult cerebellar components. Hence, PCBP-induced slight increase of [Ca2+]i levels in the cells might have been associated with effective Ca2+ sequestration by intracellular organelles, as seen in cerebellar preparations. The results of this study support the hypothesis that the position of chlorine substitution on the biphenyl ring and/or number of chlorine substitutions may have significant implications for predicting potential effects of PCB congeners in the nervous system, and perturbations in Ca2+ homeostasis might play a significant role in the neuroactivity of PCBs.

Animals↗

PubMed1993

Effect of repeated organophosphate administration on carbachol-stimulated phosphoinositide hydrolysis in the rat brain.

The effects of repeated exposure to two organophosphates on the turnover of phosphoinositides, the second messenger system coupled to the M1 and M3 subtypes of muscarinic receptors, were examined in the rat hippocampus. Repeated diisopropylfluorophosphate (DFP) exposure (0.2-0.8 mg/kg, SC) decreased brain acetylcholinesterase activity and muscarinic receptor density. The incorporation of [3H]myoinositol into brain slices was also decreased. Phosphoinositide turnover was measured as the accumulation of [3H]inositol phosphates (IP) in the presence of lithium. DFP did not affect basal IP accumulation, but decreased carbachol-stimulated IP accumulation in the hippocampus after 0.4 and 0.8 mg/kg. The effects of repeated disulfoton administration (2.0 mg/kg, IP) were also examined in the hippocampus. Similar to DFP, repeated disulfoton exposure decreased acetylcholinesterase activity, receptor density, and carbachol-stimulated IP accumulation. The incorporation of myoinositol, however, was increased in disulfoton-treated rats. These data indicate that repeated organophosphate exposure results in a functional decrease in muscarinic receptor activity, as well as changes in myoinositol incorporation into phospholipids.

Animals↗

PubMed1993

Neurobehavioral methods used in neurotoxicological research.

Exposure to chemicals in the environment and workplace can have adverse effects on the nervous system. Behavioral endpoints are being used with greater frequency in the hazard identification phase of neurotoxicology risk assessment. One reason behavioral procedures are used in animal neurotoxicology studies is that they evaluate neurobiological functions known to be affected in humans exposed to neurotoxic agents, including alterations in sensory, motor, autonomic, and cognitive function. In hazard identification, behavioral tests are used in a tiered-testing context. Tests in the first tier are designed to determine the presence of neurotoxicity. Examples of first-tier behavioral tests include functional observational batteries and motor activity. Second-tier tests are used to characterize neurotoxicant-induced effects on sensory, motor, and cognitive function. Second-tier tests are usually more complex and costly to perform. Reliance on behavioral endpoints in neurotoxicology risk assessment will likely increase in the future.

Animals↗

PubMed1993

Neurobehavioral methods used in neurotoxicology.

The use of neurobehavioral techniques in toxicology has increased dramatically over the past several years. Several national and international groups have recommended that neurobehavioral tests be included in the initial stages of hazard identification, and regulatory agencies have responded by preparing testing guidelines or requiring behavioral tests data for premarket approval of environmental and pharmaceutical chemicals. In addition, neurobehavioral data have been used to set exposure limits in the workplace. In the future, neurobehavioral data will be used more frequently in the area of risk assessment, which has been defined as the "characterization of the potential adverse effects of human exposure to environmental hazards" (National Academy of Sciences 1983). Good risk assessment depends on the ability to determine whether a particular agent is or is not causally linked to a particular health effect and on the availability of dose-response data for quantitative risk assessment. Neurobehavioral techniques used in animal behavioral toxicology measure neurobiological functions similar to those measured in humans. In addition, neurobehavioral procedures can be used in longitudinal studies where the onset and duration of effects of chemical exposure can be measured in the same animal. Neurobehavioral techniques are also amenable to the study of tolerance and compensation following repeated exposure or following recovery of function that can occur following cessation of exposure. Therefore, neurobehavioral procedures provide a valuable tool for research designed to reduce major uncertainties associated with the risk assessment process, such as animal to human extrapolation (homology of animal models) and dosing issues (i.e., high-to-low dose, acute vs. repeated dosing, and continuous vs. episodic dosing). Although the use of neurobehavioral procedures has had a significant impact on neurotoxicology, their use in the risk assessment process and in monitoring populations for possible subtle changes in neurobiological function will be limited if additional research is not done to understand the neural substrates underlying neurobehavioral endpoints. The ability to link chemically induced behavioral changes to alterations at the neurophysiological, neurochemical, and neuroanatomical levels will lead to a greater acceptance of the validity and reliability of neurobehavioral endpoints in defining adverse effects of chemicals on the nervous system.

Animals↗

PubMed1993

Developmental changes in carbachol-stimulated inositolphosphate release in pigmented rat retina.

Carbachol-stimulated release of inositolphosphates (IP) was studied in the whole retina from Long-Evans rats of different ages (day 5, 10, 15, 20, adult) following in vitro incorporation of [3H]myo-inositol. Unlike the albino rat retina, the pigmented retina was highly light-sensitive, making it necessary to dark adapt the animals and perform retinal dissections under low illumination to prevent light-induced IP release. Retinae from postnatal day 10 rats showed the highest amount of carbachol-stimulated IP released. This response to carbachol decreased with age until postnatal day 20 when it reached adult levels. The pigmented rat retina showed a sharp fall in the degree of carbachol (1 mM)-stimulated IP released at the time of eye-opening (450% above basal in retinae from 10 day old animals, as compared to 230% above basal in 15 day old retinae). Basal release of IP was not altered in the retina during development. Muscarinic cholinergic receptor density was, however, found to increase 5 fold with age, reaching adult levels by PND 20. Retinal weight and protein per retina also increased (four fold) from day 5 to adult; however, the in vitro incorporation of [3H]myo-inositol into phosphoinositides (calculated as per mg protein) did not change during development. Thus, in animals prior to eye opening, a much higher proportion of phosphoinositides appears to be hydrolyzed upon muscarinic receptor stimulation. During retinal development a change in sensitivity to the agonist-sensitive pool(s) of phosphoinositides may occur and/or there may be alterations in the efficacy of receptor coupling to the second messenger system resulting in the disassociation observed between the drastic increase in receptor number and the apparent decrease in receptor-stimulated release of IP.

Aging↗

PubMed1993

The role of the septohippocampal pathway in the mediation of colchicine-induced compensatory changes in the rat hippocampus.

To study the involvement of the septohippocampal pathway in colchicine-induced changes in the hippocampus, colchicine was used to lesion the septum and/or hippocampus of male, Fischer-344 rats. Rats were killed 12 weeks post-lesion and histochemical and biochemical measurements were performed. [3H]-QNB binding, choline acetyltransferase (ChAT) activity and agonist-stimulated release of inositol phosphates (IPs) were measured in hippocampal slices. AChE histochemistry was also performed to visualize AChE positive fibers in the hippocampus. Increases in ChAT activity, AChE staining and carbachol-stimulated IP release observed in hippocampal-lesioned animals were attenuated in animals receiving both septal and hippocampal lesions. However, the decrease observed in [3H]-QNB binding sites after intradentate colchicine was not affected by septal lesions. Subsequent studies also found enhanced sensitivity to excitatory amino acid (EAA)-stimulated IP release in hippocampal-lesioned animals. Similar to the changes observed in carbachol-stimulated PI hydrolysis, this increase was also long-lasting. However, the hyperstimulation of EAA-induced IP release was not attenuated by the septal lesion. Thus, it appears that the neurochemical and morphological changes observed in the hippocampus following intradentate colchicine are dependent upon more than one afferent projection to the hippocampus.

Animals↗

PubMed1992

The neurobiological effects of colchicine: modulation by nerve growth factor.

To study the effects of exogenously applied nerve growth factor (NGF) on colchicine-induced neurodegeneration in the dentate gyrus of the rat hippocampal formation, colchicine (COLCH) or artificial cerebrospinal fluid (ACSF) was infused into the dorsal hippocampus (HPC) followed by unilateral infusion of either purified beta-NGF (in ACSF) or cytochrome C. One week later, animals were tested in activity chambers when NGF treatment was found to reduce the COLCH-induced hyperactivity. Animals were sacrificed 3 or 12 weeks postlesion for neurochemical or morphological analysis. Carbachol-stimulated phosphatidyl inositol (PI) turnover performed in hippocampal slices was not affected by any treatment at 3 weeks. However, 12 weeks after the lesion, CARB stimulation of PI hydrolysis was increased in the COLCH/ACSF group. NGF treatment significantly reduced the hyperstimulation in COLCH-treated rats. Morphological analysis showed that COLCH treatment increased AChE staining in the hippocampus, whereas NGF treatment had no effect on AChE staining. There was no difference in the number of septal ChAT immunoreactive cell bodies of controls or colchicine-treated rats at either time point examined. However, NGF treatment resulted in a significant increase in the number of ChAT immunoreactive cell bodies 3 weeks postlesion. Results from this study indicate that NGF can modify colchicine-induced compensatory changes in hippocampal signal transduction and has transitory influences on cholinergic cells in the medial septum.

Acetylcholinesterase↗

PubMed1992

Comparison of screening approaches.

Neurobehavioral techniques have been used extensively in animal toxicology studies because, in many cases, such procedures are designed to evaluate neurobiological functions thought to be affected in chemical-exposed humans, e.g., changes in sensorimotor function. Procedures used to identify or screen for the presence of neurotoxicity are usually designed to test large numbers of animals and are not considered to be as sensitive to subtle effects as more specialized tests for neurobiological dysfunction. For purposes of screening, the use of a functional observational battery (FOB) is now generally accepted. In general, FOB evaluations in animals are similar to clinical neurological examinations in humans in that they rate the presence and, in some cases, the severity of behavioral and neurological signs. A number of batteries containing different observations and measurements have been developed in several laboratories for rodents, dogs, and non-human primates. Frequently, the FOB is used in conjunction with other measures of neurotoxicity, i.e., neuropathology or sensory evoked potentials. FOB used in screening typically assess several neurobiological domains including neuromuscular (i.e., weakness, incoordination, abnormal movements, gait, motor seizures, myoclonia, rigidity and tremor), sensory (i.e., auditory, visual and somatosensory) and autonomic (i.e., pupil response, salivation) functions. Most FOB used for screening do not assess cognitive function (i.e., learning and memory). FOB evaluations can yield important information concerning dose-response characteristics and data on the onset, duration and persistence of an effect. FOB should be able to differentiate neurotoxicants from non-neurotoxicants and neurotoxicants having different mechanism(s) or site(s) of action.

Animals↗