Effects of long-term turpentine inhalation on rat brain protein metabolism

Strategies in herbivory by mammals revisited: The role of liver metabolism in a juniper specialist (Neotoma stephensi) and a generalist (Neotoma albigula)

Although herbivory is widespread among mammals, few species have adopted a strategy of dietary specialization. Feeding on a single plant species often exposes herbivores to high doses of plant secondary metabolites (PSMs), which may exceed the animal's detoxification capacities. Theory predicts that specialists will have unique detoxification mechanisms to process high levels of dietary toxins. To evaluate this hypothesis, we compared liver microsomal metabolism of a juniper specialist, Neotoma stephensi (diet >85% juniper), to a generalist, N. albigula (diet ≤30% juniper). Specifically, we quantified the concentration of a key detoxification enzyme, cytochrome P450 2B (CYP2B) in liver microsomes, and the metabolism of α-pinene, the most abundant terpene in the juniper species consumed by the specialist woodrat. In both species, a 30% juniper diet increased the total CYP2B concentration (2-3×) in microsomes and microsomal α-pinene metabolism rates (4-fold). In N. stephensi, higher levels of dietary juniper (60% and 100%) further induced CYP2B and increased metabolism rates of α-pinene. Although no species-specific differences in metabolism rates were observed at 30% dietary juniper, total microsomal CYP2B concentration was 1.7× higher in N. stephensi than in N. albigula (p < .01), suggesting N. stephensi produces one or more variant of CYP2B that is less efficient at processing α-pinene. In N. stephensi, the rates of α-pinene metabolism increased with dietary juniper and were positively correlated with CYP2B concentration. The ability of N. stephensi to elevate CYP2B concentration and rate of α-pinene metabolism with increasing levels of juniper in the diet may facilitate juniper specialization in this species.

Terpenes May Serve as Feeding Deterrents and Foraging Cues for Mammalian Herbivores

Terpenes, volatile plant secondary compounds produced by woody plants, have historically been thought to act as feeding deterrents for mammalian herbivores. However, three species of woodrats, Neotoma stephensi, N. lepida, and N. albigula, regularly consume juniper, which is high in terpenes, and N. stephensi and N. lepida are considered juniper specialists. By investigating the terpene profiles in Juniperus monosperma and J. osteosperma, which are browsed or avoided by woodrats in the field, and recording the caching and consumption of juniper foliage by woodrats in the lab, we have evidence that terpenes may serve as feeding and/or foraging cues. The obligate specialist N. stephensi chose to forage on trees higher in p-cymene and preferred to consume juniper rather than caching it in a laboratory setting. These observations provide evidence that terpenes serve as a feeding cue and that the obligate specialist's physiological mechanism for metabolizing the terpenes present in juniper may negate the need for caching. The facultative specialist N. lepida chose to forage on trees lower in four terpenes and cached more juniper than the obligate specialist N. stephensi, providing evidence that terpenes serve as a feeding deterrent for N. lepida and that this woodrat species relies on behavioral mechanisms to minimize terpene intake. The generalist N. albigula foraged on trees with higher terpenes levels but consumed the least amount of juniper in the lab and preferred to cache juniper rather than consume it, evidence that terpenes act as foraging but not feeding cues in the generalist. Our findings suggest that volatile plant secondary compounds can act as feeding and/or foraging cues and not just feeding deterrents in mammalian herbivores.

Using the Specialization Framework to Determine Degree of Dietary Specialization in a Herbivorous Woodrat

To be considered a dietary specialist, mammalian herbivores must consume large quantities of a plant species considered "difficult" with respect to nutrient or toxin content, and possess specialized adaptations to deal with plant defensive compounds or low nutritional content. Populations of Neotoma lepida in the Great Basin consume Juniperus osteosperma, a plant heavily defended by terpenes, but a detailed dietary analysis of this population is lacking. Therefore, we investigated the extent of dietary specialization in this species in comparison with the better-studied specialist species, N. stephensi. Microhistological analysis of feces from N. lepida revealed that greater than 90% of their diet in nature was comprised of juniper. In laboratory tolerance trials, N. lepida tolerated a diet of 80% J. osteosperma, similar to that observed for N. stephensi. There was no difference in the abilities of N. lepida and N. stephensi to metabolize hexobarbital, a proxy compound for terpene metabolism. In preference tests of native and non-native juniper species, N. lepida did not exhibit a preference for its native or co-occurring juniper, J. osteosperma, over the non-native species, J. monosperma, whereas N. stephensi preferred its native or co-occurring juniper J. monosperma over non-native J. osteosperma. Behavioral and habitat differences between these woodrat species lead to the categorization of N. stephensi as an obligate juniper specialist with a small range that overlaps that of its preferred food, J. monosperma, and N. lepida as a facultative juniper specialist with a large range, and only a portion of its distribution containing populations that feed extensively on J. osteosperma.

Biotransformation enzyme expression in the nasal epithelium of woodrats

When herbivores come in contact with volatile plant secondary compounds (PSC) that enter the nasal passages the only barrier between the nasal cavity and the brain is the nasal epithelium and the biotransformation enzymes present there. The expression of two biotransformation enzymes Cytochrome P450 2B (CYP2B) and glutathione-S-transferase (GST) was investigated in the nasal epithelia and livers of three populations of woodrats. One population of Neotoma albigula was fed juniper that contains volatile terpenes. Juniper caused upregulation of CYP2B and GST in the nasal epithelium and the expression of CYP2B and GST in the nasal epithelium was correlated to liver expression, showing that the nasal epithelia responds to PSC and the response is similar to the liver. Two populations of Neotoma bryanti were fed creosote that contains less volatile phenolics. The creosote naive animals upregulated CYP2B in their nasal epithelia while the creosote experienced animals upregulated GST. There was no correlation between CYP2B and GST expression in the nasal epithelia and livers of either population. The response of the nasal epithelium to PSC seems to be an evolved response that is PSC and experience dependent.

Differential hepatic gene expression of a dietary specialist (Neotoma stephensi) and generalist (Neotoma albigula) in response to juniper (Juniperus monosperma) ingestion

Dietary specialization is thought to be rare in mammalian herbivores because of limitations of their detoxification system in processing large doses of a single type of plant secondary compound (PSC). Therefore, in order to specialize on a single species of plant, mammalian herbivores must have a highly efficient detoxification system for the particular types of PSCs they ingest. Using microarray technology, we looked at the expression of hepatic genes of a dietary specialist, Neotoma stephensi, and a sympatric generalist, Neotoma albigula, in response to diets containing different levels of one-seeded juniper (Juniperus monosperma). We found large between species differences in gene expression, as well as large within species differences when specialists fed a low juniper diet (25% juniper) were compared to specialists fed their ecologically relevant level of juniper (70% juniper). We also tested the hypothesis that the specialist relies on less costly phase I detoxification enzymes more than phase II compared to the generalist. Although we found that the specialist had higher cumulative as well as average expression of phase I versus phase II enzymes, the generalist had a similar pattern of expression for phase I versus phase II enzymes.

Intestinal capacity of P-glycoprotein is higher in the juniper specialist, Neotoma stephensi, than the sympatric generalist, Neotoma albigula

Permeability-glycoprotein (Pgp) is a membrane-bound, ATP-dependent, transport protein that excludes many cytotoxic compounds including plant metabolites and pollutants from the barrier epithelia of many tissues including the small intestine. We hypothesized that intestinal Pgp capacity would be higher in Neotoma stephensi, a specialist on Juniperus monosperma known to be high in plant toxins, than the sympatric generalist, Neotoma albigula, which consumes juniper in the field, but is unable to tolerate a high juniper diet. We measured Pgp activity as the difference in accumulation of a known Pgp substrate, digoxin, between everted sections of small intestine exposed to ethanol vehicle control and a maximal level of a known competitive inhibitor of Pgp, cyclosporin A. We estimated intestinal capacity by averaging Pgp activity along the intestine and multiplying by total small intestine mass. These first measures of Pgp in wild mammals show a significant difference among species with the juniper specialist, N. stephensi, exhibiting a 2.4 fold higher capacity than the generalist, N. albigula. This result suggests that Pgp may play a role in the ability of N. stephensi to tolerate juniper.

Short term inhalation exposure to turpentine: toxicokinetics and acute effects in men

OBJECTIVES

This study describes the toxicokinetics, pulmonary function, and subjective ratings of discomfort in volunteers experimentally exposed to turpentine vapour (a mixture of monoterpenes). The results were compared with similar exposure to single monoterpenes to look in the toxicokinetics and acute effects for signs of interactions between the monoterpenes.

METHODS

Eight male volunteers were exposed to 450 mg/m3 turpentine by inhalation (2 h, 50 W) in an exposure chamber.

RESULTS

The mean relative uptakes of alpha-pinene, beta-pinene, and 3-carene were 62%, 66%, and 68% respectively, of the amount supplied. Between 2% and 5% of the net uptake was excreted unchanged in the expired air after the end of exposure. The mean blood clearance 21 hours after exposure (CL21h) of alpha-pinene, beta-pinene and 3-carene, were 0.8, 0.5, and 0.4 l.kg-1.h-1, respectively. The mean half lives (t1/2) of the last phase of alpha-pinene, beta-pinene, and 3-carene averaged 32, 25, and 42 hours, respectively. The t1/2s agreed with previously calculated half lives from single exposures. The total blood clearance CL21h of 3-carene found in this turpentine study was lower, and CL4h of 3-carene was significantly lower than the values obtained from similar exposure to pure 3-carene. The subjects attending both exposure to turpentine and to pure alpha-pinene at 450 mg/m3 had lower CL4h during the exposure to turpentine, when they experienced more discomfort of the throat or the airways (F = 5.7, P = 0.048) than during exposure to control concentrations. After experimental exposure to turpentine an increase in airway resistance was found that differed significantly from results of exposure to 3-carene at 10 mg/m3 (P = 0.021) or 450 mg/m3 (P = 0.047).

CONCLUSIONS

Toxicokinetics and acute effects show small, if any, interactions between alpha-pinene, beta-pinene, and 3-carene. The subjects experienced discomfort in the throat and airways during exposure to turpentine and airway resistance was increased after the end of exposure.

d-limonene exposure to humans by inhalation: uptake, distribution, elimination, and effects on the pulmonary function

The toxicokinetics of d-limonene were studied in human volunteers exposed by inhalation (2 h, work load 50 W) in an exposure chamber on three different occasions. The exposure concentrations were approximately 10, 225, and 450 mg/m3 d-limonene. The relative pulmonary uptake was high, approximately 70% of the amount supplied. The blood clearance of d-limonene observed in this study, 1.1 l kg-1 h-1, indicates that d-limonene is metabolized readily. About 1% of the total uptake was eliminated unchanged in the expired air after the end of exposure, while approximately 0.003% was eliminated in the urine. A long half-time in blood was observed in the slow elimination phase, which indicates accumulation in adipose tissues. A decrease in vital capacity was observed after exposure to d-limonene at a high exposure level. The subjects did not experience any irritative symptoms or symptoms related to the central nervous system (CNS).

Human exposure to 3-carene by inhalation: toxicokinetics, effects on pulmonary function and occurrence of irritative and CNS symptoms

Terpenes, especially 3-carene, may irritate the skin and mucous membranes and prolonged exposure may result in allergic contact dermatitis or chronic lung function impairment. The toxicokinetics of 3-carene were studied in human volunteers exposed by inhalation (2 hr 50 W) in an exposure chamber on three occasions. The exposure concentrations were approximately 10, 225, and 450 mg/m3 3-carene. The relative pulmonary uptake was high, approximately 70% for the higher exposure levels. Total uptake increased linearly with increasing exposure. The blood clearance of 3-carene observed in this study, 0.9 liter.kg-1.hr-1, indicates that 3-carene is fairly readily metabolized. About 3% of the total uptake was eliminated unchanged via the lungs while less than 0.001% was eliminated in the urine after the end of exposure. A long half time in blood was observed in the terminal phase which indicates a high affinity to adipose tissues. A statistically significant divergence between ratings of irritation during the high exposure level and during the medium and control levels was observed. The difference in airway resistance after exposure to a high concentration of 3-carene compared to control level was not significant (P = 0.02).

Liquid/air partition coefficients of four terpenes

The liquid/air partition coefficients of four common terpenes, alpha-pinene, beta-pinene, 3-carene, and limonene, have been determined in vitro using head space technique. The liquids used were water, human blood, and olive oil. alpha-Pinene, beta-pinene, and 3-carene were practically insoluble in water and limonene was slightly soluble; all were readily dissolved in olive oil. The oil/air partition coefficients ranged from 2900 to 5700 in the order alpha-pinene, beta-pinene, 3-carene, and limonene. The blood/air partition coefficients ranged from 15 to 42 in the same order as for oil/air.

Cerebral cortex and body growth development of progeny of rats exposed to thinner and turpentine inhalation

1. Turpentine and thinner exposed rats, during the last stage of pregnancy, delivered 59% and 20% of dead newborn, respectively. 2. Turpentine and thinner delayed foetal growth and caused no effects on neonates body size. 3. Cerebral cortex of newborn rats, prenatally exposed to organic solvents showed to histological alterations.

Burden and dose-related neurochemical effects of intermittent cyclohexane vapour inhalation in rats

Intermittent daily inhalation exposure to 300, 1000 or 2000 ppm of cyclohexane vapour resulted in a dose-dependent solvent concentration in the perirenal fat in rats. The linear relationship changed between the first and second week of exposure as the body solvent burden decreased, despite the continued exposure: this was especially clear in the brain cyclohexane analyses. The salient feature in the brain was the reduction in the activity of azoreductase, while no change could be found in the RNA or glutathione content or in glutathione peroxidase activity. The azoreductase activity was somewhat below the control range after a 2-week withdrawal period, while no solvent could be found and other biochemical variables were within the control ranges.

Neurochemical effects on rats of n-heptane inhalation exposure

Inhalation exposure of male rats at three dose levels (4.2, 21, 62 mol/L) to n-heptane vapor caused a dose-dependent brain and body solvent burden, which increased during two weeks of exposure. Initial neurochemical effects included reduced RNA concentration and increased NADPH-diaphorase in brain at the lowest dose. Increased proteolysis was detected in the cerebral samples in the second week at all doses, and the NADPH-diaphorase returned to the control range. Brain RNA content tended to be larger than in the controls. All biochemical effects were abolished after two weeks of withdrawal from the two-week exposure, with the exception of reduced glutathione at the lowest dose. None of the rats presented clinical signs of neuropathy, which indicates that heptane and its metabolites do not share the specific neurotoxicity of hexane and its metabolites.

Enhancement of hepatic drug biotransformation by a short-term intermittent turpentine exposure in the rat

An inhalation exposure of male rats to 300 p.p.m. of a commercial turpentine 6 hrs daily 5 days a week for 8 weeks enhanced the activities of drug biotransformation enzymes of liver microsomes considerably. The activities of NADPH cytochrome c reductase and 7-ethoxycoumarin deethylase, and microsomal content of cytochrome P-450 were increased 35-60% during the first weeks of the experiment, but had a tendency to return towards the control values later on. A similar enhancement of activities was also found in liver microsomal epoxide hydratase and UDP glucuronosyltransferase, but these enzyme activities tended to adapt less during the experiment. The turpentine treatment increased the affinity of liver microsomal cytochrome P-450 to alpha-pinene (the main component of the turpentine). The present data suggests that exposure to turpentine is able to modify considerably the biotransformation of drugs.