Effects of pentobarbital in mice selected for differential sensitivity to ethanol

Two lines of mice have been genetically selected for differential sensitivity to ethanol. These lines have been designated long sleep (LS) and short sleep (SS) on the basis of their hypnotic response to the ethanol selection dose. Earlier studies of these mice suggested that this difference was limited to alcohols and did not extend to other classes of hypnotics. The present study examined hypnotic and hypothermic responses produced by pentobarbital in recent generations of these mice. Dose-dependent differences in sleep time and in hypothermia were found, with SS mice affected to a greater degree than LS mice. Pharmacokinetic studies showed that the half-life of pentobarbital disappearance from SS blood was twice that reported for SS mice of the 18th generation. The half-life in the LS line had not changed. The volumes of distribution and waking brain concentrations were identical in LS and SS mice. An altered rate of elimination (not differential CNS sensitivity) appeared to be the major factor responsible for the differences observed between these lines.     

3H]inositol trisphosphate specific binding in two brain regions of long and short sleep mice

The present investigation determined the binding characteristics of the radiolabeled second messenger, [3H]In(1,4,5)P3, in cerebral cortical and cerebellar membrane fragments from ethanol sensitive (LS) and resistant (SS) mice. The data demonstrated that in the two brain regions examined, LS and SS mouse lines do not differ with regard to their maximal receptor binding capacities or dissociation constants for [3H]In(1,4,5)P3. Moreover, [3H]In(1,4,5)P3 specific binding in cerebellum is inhibited in the presence of ethanol to the same extent in both mouse lines. It is concluded that the differential behavioral and cerebellar sensitivities to ethanol previously observed in these mouse lines do not reflect differences in [3H]In(1,4,5)P3 receptor binding characteristics of the brain regions examined.     

Ethanol-induced changes in tyrosine hydroxylase activity in brains of mice selectively bred for differences in sensitivity to ethanol

The effects of ethanol on tyrosine hydroxylase (TH) activity in five brain areas were analyzed in two lines of mice selectively bred for their differences in sensitivity to ethanol. Following a 4.1 g/kg dose of ethanol, intraperitoneally, short sleep (SS) mice lose their righting reflex for a duration of 20 minutes and long sleep (LS) mice fail to regain their righting reflex until 120 minutes. A significant increase in TH activity occurred in the striatum, locus coeruleus and frontal cortex in both lines of mice approximately 25 minutes following ethanol administration. A decrease in TH activity occurred in the substantia nigra of SS mice at 5 minutes following ethanol administration. However, there was no significant difference in TH activity in any of these four brain regions between LS and SS mice at any time following ethanol administration. In contrast, hypothalamic TH activity was significantly increased at 25 minutes in the SS mice and at 125 minutes in the LS mice following the administration of ethanol, times which coincided with the regaining of the righting reflex. These data suggest that activation of TH in the hypothalamus of LS and SS mice in response to ethanol is associated with arousal from ethanol induced narcosis.     

Effect of hypnotics on mice genetically selected for sensitivity to ethanol

It was previously shown that the rate of disappearance of blood ethanol was identical for 2 lines of mice selectivity bred for differences in sleep-time after ethanol administration. The ED₅₀ values for the loss of righting response with ethanol were significantly different at 3.64 g per kg for the SS line and 1.65 g per kg for the LS line. In the present study the mean sleep time is 367 sec for SS mice and 9342 sec for LS mice. The ED₅₀ values remain essentially the same as previously reported. Unchanged LD₅₀ values for ethanol, however, are not different at 4.8 g per kg for the SS and 4.5 g per kg for the LS line of mice. The ED₅₀ values for loss for righting response following administration of methanol, butanol and t-butanol is approximately 2 fold greater for the SS line of mice than for the LS line. The ED₅₀ values for sodium pentobarbital or ether in the 2 lines of mice for loss of righting response are virtually identical. In addition, the sleep-time values obtained after the administration of pentobarbital, chloral hydrate, trichloroethanol and paraldehyde are not significantly different. These data indicate that while the SS and LS lines of mice differ in central nervous system sensitivity to ethanol, methanol, butanol and t-butanol it is implied that they do not differ in central nervous system sensitivity to other hypnotic agents tested. Proof of this latter suggestion awaits determination of metabolic rates, and brain levels of these other depressants.     

Differential effects of long-chain alcohols in long- and short-sleep mice

Observations that the long-sleep (LS) and short-sleep (SS) mouse lines differ in their depressant response to barbiturates, and that the difference between lines becomes greater as lipid solubility increases, prompted this investigation of the effects of alcohols that differ in lipid solubility. Results indicate that LS and SS mice differ significantly in their sleep time responses to propanol, butanol, and 3-methyl butanol, as well as ethanol: their hypothermic responses showed a similar pattern, but only the response to ethanol differed significantly between lines. For both sleep time and hypothermia, the difference between lines decreased with increasing lipid solubility. In all cases, the LS mice were more sensitive than the SS to the depressant effects of the alcohol. Similar ratios of SS:LS waking brain ethanol and butanol levels indicated that CNS sensitivity to long-chain alcohols is similar to that for ethanol. A pharmacokinetic study revealed higher ethanol levels for LS than for SS mice at all time points in blood, fat, and brain body compartments. Blood ethanol elimination curves showed that the SS mice eliminate ethanol at a faster rate than do the LS.     

Brain neurotransmitter receptor systems in mice genetically selected for differences in sensitivity to ethanol

The density of receptors for neurotransmitters and receptor-mediated cellular responsiveness were determined in tissues from two lines of mice selectively bred for differences in sleep time after ethanol administration. The studies were undertaken to ascertain in differences in neurotransmitter receptors between the lines might explain any of the differences seen in initial sensitivity to ethanol. The results were compared with those obtained in studies using the parent heterogeneous stock (HS). The animals used in the current experiments did not receive ethanol. β-Adrenergic receptor density, as measured by the binding of (¹²⁵I)-iodohydroxybenzylpindolol, was lower in cortices of alcohol-sensitive long-sleep mice (LS) as compared to the short-sleep (SS) animals or to HS controls. The densities of β₁ and β₂-adrenergic receptors were lower by approximately the same amount. The results were tissue specific in that there was no difference in the density of β-adrenergic receptors in the cerebellum. Cyclic AMP accumulation in response to isoproterenol was not significantly different in the cortex of LS mice despite the decrease in β-adrenergic receptors. No differences were found in ³H-spiroperidol binding to dopamine receptors in the striatum or in ³H-quinuclidinylbenzilate binding to muscarinic cholinergic receptors in the cortex, striatum or hippocampus. Dopamine-stimulated adenylate cyclase activity was, how- ever, lower in striata of SS mice. The affinities of the receptors for the various ligands studied were the same in the three lines of mice.     

Stimulant and depressant properties of sedative-hypnotics in mice selectively bred for differential sensitivity to ethanol

A genetic analysis of sedative-hypnotic response in selectively bred Long-Sleep (LS) and Short-Sleep (SS) mice showed LS mice to be more depressed by acetaldehyde, ethanol (ETOH), and t-butanol, but less sensitive to pentobarbital. Intermediate inheritance was shown by the two reciprocal F₁ hybrids for the alcohols, but dominance to the LS genotype occurred for the aldehyde and the barbiturate. At severa subhypnotic doses of ETOH in two experiments, LS mice showed less locomotor stimulation and greater disruption of coordination than the SS mice. The two reciprocal F₁ hybrids did not differ form one another and had dose-response curves intermediated to the two parental lines. Study of the effects of t-butanol on locomotor activity revealed a pattern of line differences similar to that for ETOH. The genetic selection for LS and SS mice appears to have differentiated loci that pleiotropically influence a variety of behavioral responses to alcohols.     

Neonatal cerebellectomy alters ethanol-induced sleep time of short sleep but not long sleep mice

The effects of neonatal cerebellectomy on ethanol-induced sleep times in long sleep (LS) and short sleep (SS) mice were investigated. Cerebellectomy did not alter the ethanol sensitivity of LS animals for loss of righting reflex. In contrast, SS mice became more sensitive to alcohol after cerebellectomy. Even so, large differences were still observed between the alcohol-induced sleep times of cerebellectomized LS and SS mice. The data indicate that, while the cerebellum must have a prominant influence on alcohol sleep time in SS animals, this brain structure is not solely responsible for the observed differences in righting reflex sensitivity to ethanol in these two mouse lines. We postulate the existence of noncerebellar central neurons with differential sensitivities to the depressant effects of ethanol in LS and SS mice.     

Calcium influence on neuronal sensitivity to ethanol in selectively bred mouse lines

Sensitivity to ethanol, as measured by blood ethanol concentration at loss of righting reflex, was increased significantly in SS but not LS mice following intracerebroventricular (ICV) administration of calcium chloride or A23187, a calcium ionophore. Magnesium chloride or lanthanum chloride, ICV, did not alter sensitivity to ethanol in either SS or LS mice, further indicating a specificity for calcium cation. Calcium was without effect on sensitivity to halothane narcosis in LS or SS mice. Endogenous brain calcium content was similar in these mouse lines, and ethanol administration either in vivo or in vitro did not alter brain calcium concentration. These results indicate that differences in brain sensitivity to ethanol are mediated, in part, by genetic differences in calcium-related processes and support the hypothesis that ethanol-induced narcosis may be due to alterations in calcium metabolism in the CNS.     

Sedative-hypnotic anomalies related to dose of pentobarbital in long-sleep and short-sleep selectively-bred mice

Hypnotic effects following administration of three doses of pentobarbital were evaluated in mice selectively-bred for differential hypnotic sensitivity to ethanol. Although the ethanol-sensitive Long-Sleep (LS) line displays greater sedation to a wide variety of CNS depressants (alcohols, barbiturates, benzodiazepines, general anesthetics), when compared to the ethanol-insensitive Short-Sleep (SS) line, the response pattern to pentobarbital remains equivocal. Thus, to clarify the effect of pentobarbital, certain variables (dose, sex, circadian rhythmicity) believed to be important in the expression of sleep time were evaluated. For all doses examined "sex" and "time of day tested" impacted on sleep time. With these provisos, 40 mg/kg consistently induced shorter sleep time in SS mice. The 60 mg/kg dose either failed to distinguish these two lines, or induced greater sleep times in the SS mice. The 80 mg/kg dose tended to have the same effect as the 60 mg/kg dose, but to a greater degree. Overall, it appears that for each line the dose response curve for pentobarbital is sigmoidal, but that the slope of the curve for the middle range of doses is greater for the SS line. Since pentobarbital has a unique effect on these lines of mice that is dissimilar to those reported for other barbiturates, the implication is that an additional factor, that is unimportant for other barbiturates, is essential for pentobarbital-induced hypnosis. Factors that could be responsible for this effect include differential metabolism of Gabaergic receptor dynamics.     

Autoradiographic demonstration of the selectivity of two 1-N-trifluoroethyl benzodiazepines for the BZD-1 receptors in the rat brain

Receptor autoradiographic techniques have been used to demonstrate the selectivity of two trifluoroethyl-containing benzodiazepines for one of the subtypes of benzodiazepine receptor. Indirect localization of the binding sites for quazepam and halazepam was accomplished by using the ability of these compounds to displace [3H]-flunitrazepam binding. The appropriate binding parameters were selected on the basis of initial studies aimed at identifying the binding characteristics of several benzodiazepine compounds in comparison with the triazolopyridine CL218,872. Autoradiographic analysis of the benzodiazepine sites displaceable with quazepam and halazepam revealed the two benzodiazepine compounds preferentially labeled receptor sites in regions of the brain dominated by the type 1 benzodiazepine receptor subtype. Thus, quazepam and halazepam preferentially bind to benzodiazepine type-1 receptors in lamina IV of the cerebral cortex, the zona incerta, substantia nigra and the cerebellum.     

Locomotor responses to benzodiazepines,barbiturates and ethanol in diazepam-sensitive (DS) and -resistant (DR) mice

Diazepam-sensitive (DS) and -resistant (DR) mice were selectively bred for increased and reduced sensitivity to the ataxic effects of diazepam (40 mg/kg). Other response differences between DS and DR mice may reflect pleiotropic effects of the genes fixed during their selection. These mice were tested for their sensitivity to the locomtor stimulant effects of several doses of diazepam, flunitrazepam, pentobarbital, phenobarbital, and ethanol. DR mice were more sensitive than DS mice to the locomotor stimulant effects of all drugs except phenobarbital. These results largely support the hypothesis that a common biological mechanism mediates sensitivity to the stimulant effects of sedative-hypnotic drugs. Receptor mediation of the benzodiazepine effects was examined by administering the benzodiazepine receptor antagonist, RO15-1788. Locomotor depression produced by diazepam and flunitrazepam in DS mice was blocked by RO15-1788. However, while the locomotor stimulation produced by diazepam in DR mice was antagonized, the stimulant effect of flunitrazepam was not. This suggests that binding of flunitrazepam to the GABA_A-benzodiazepine receptor is not necessary for production of locomotor stimulation.     

Pressure-sensitive and -insensitive coupling in gamma-aminobutyric acid(A) receptors

RATIONALE: Previous behavioral and biochemical studies suggest that allosteric coupling processes initiated by benzodiazepines, barbiturates and neuroactive steroids can be sub-categorized on the basis of their sensitivities to antagonism by increased atmospheric pressure. However, biochemical evidence supporting this hypothesis was limited to single concentration studies in long sleep (LS) mice. OBJECTIVE: The present paper addresses these issues by extending biochemical investigation of pressure effects on allosteric modulators across a range of concentrations that allosterically enhance gamma-aminobutyric acid (GABA)A receptor function and alter behavior using two mouse genotypes. In addition, the effects of pressure on ligand binding were explored to further investigate the mechanism of pressure antagonism of allosteric modulation. METHODS: The effects of 12 times normal atmospheric pressure (ATA) of helium-oxygen gas (heliox) on allosteric modulation of GABA(A) receptor function and [3H]flunitrazepam binding was tested in LS and C57BL mouse brain membranes (microsacs) using chloride flux and high-affinity binding assays. RESULTS: In both genotypes, exposure to 12 ATA heliox antagonized the allosteric enhancement of GABA(A) receptor function by flunitrazepam (0.1-10 microM) and pentobarbital (0.1-50 microM) but did not affect allosteric modulation by 3alpha-hydroxy-5beta-pregnan-20-one (0.1-1 microM). Pressure did not affect benzodiazepine receptor affinity (Kd) or the number of benzodiazepine receptors (Bmax). CONCLUSIONS: The results: (1) confirm that there are differences in sensitivity to pressure antagonism of allosteric coupling among GABA(A) allosteric modulators; (2) demonstrate that these differences are not concentration or genotype dependent; (3) add evidence that pressure antagonizes allosteric modulation by uncoupling the receptor and (4) support the hypothesis that allosteric modulation of receptor function can be sub-categorized on the basis of sensitivity to pressure antagonism.     

Benzodiazepine receptors in the brain as affected by different experimental stresses: The changes are small and not undirectional

Rats and mice were exposed to several different stress situations to investigate whether brain benzodiazepine receptors were sensitive to altered external or internal environmental circumstances. All stresses were applied for several days. Electrical foot shock and post-natal isolation of newborn pups resulted in small (7–25%; P<0.05–0.001) decreases in benzodiazepine receptor binding in some cerebral cortex or hippocampal areas while immobilization stress resulted in a small (9%; P<0.05) increase in frontal cortex. Other brain areas (i.e., striatum, cerebellum, pons-medulla, and occipital cortex) and other stress forms (isolation of male mice, forced swimming in cold water, or chronic amphetamine intoxication) did not change receptor binding. The effect of prolonged stress on benzodiazepine receptors is complex and not very pronounced.     

Effect of taurine on ethanol-induced sleep time in mice genetically bred for differences in ethanol sensitivity

Long Sleep (LS) and Short Sleep (SS) mice were used in this study to investigate the interaction between ethanol and taurine. Sleep time (hypnosis) was selected as an index of ethanol-induced central nervous system depression. In order to achieve a similar degree of central nervous system depression with ethanol, SS and LS mice received 5.3 and 3.0 g/kg, IP, of ethanol, respectively. When taurine (7.5, 15 and 25 mumol/kg) was administered intracerebroventricularly (ICV) to LS and SS mice immediately after regaining the righting reflex following ethanol injection, a return to sleep time was produced. This effect of taurine was immediate in onset and occurred in a dose-dependent fashion. LS mice exhibited a greater effect from taurine administration than SS mice. In another experiment LS and SS mice were given ICV TAG, a taurine antagonist (6-aminomethyl-3-methyl-4H-1,2,4-benzothiadiazine-1,1-dioxide HCl), which significantly reduced the effect of taurine to produce a return to sleep time in the presence of ethanol. TAG did not affect ethanol-induced sleep time. In control experiments, in the absence of ethanol, neither taurine (25 mumol/kg, ICV) nor TAG (1 mumol/kg, ICV) caused a significant loss of the righting reflex (sleep time). When pentobarbital (50 mg/kg, IP) was injected instead of ethanol in the sleep time experiments, taurine (7.5, 15 and 25 mumol/kg, ICV) produced a return to sleep time in LS and SS mice that resembled the effect of taurine with ethanol in SS mice. These results indicate that taurine (ICV) can enhance the central depressant action of ethanol and pentobarbital and that the greatest effect of taurine occurred with LS mice in the presence of ethanol. It is possible that taurine may have some role in the central nervous system depressant properties of ethanol.     

Benzodiazepine receptor binding response to acute and chronic stress is increased in aging animals.

Benzodiazepine receptor binding in vivo, as determined by the uptake of the high-affinity specific benzodiazepine receptor ligand [3H]Ro15-1788, was examined following acute and chronic defeat stress in male mice aged 6 weeks, 7 months and 1 year. Specific uptake in 6-week-old mice was increased from control values only in the cerebellum following acute but not chronic stress. Specific uptake in the cortex and hypothalamus was unchanged from control values following both acute and chronic stress. Seven-month-old mice demonstrated an increased specific uptake in the cortex and cerebellum when measured immediately following both acute stress and the final session of chronic stress. This enhanced binding returned to baseline levels by 24 h after stress. One-year-old mice demonstrated no change in specific uptake when measured after acute stress, while binding was enhanced in all brain regions after the final session of chronic stress. This increased binding was still evident at 24 h after the cessation of chronic stress. Changes in benzodiazepine binding differ as a response to acute and chronic stress, and this response varies markedly with age.     

Cocaine produces locomotor stimulation in SS but not LS mice: Relationship to dopaminergic function

Cocaine produces several behavioral effects, most notably locomotor stimulation. While low doses of cocaine have been shown to decrease locomotor activity, moderate to high doses in the range of 5–50 mg/kg usually produce a marked increase in locomotor activity in rodents. This study examined the effects of a range of cocaine doses, 1–75 mg/kg, on locomotor activity in LS/Ibg (LS) and SS/Ibg (SS) mice. At the lowest doses, activity was depressed in both lines, but to a greater extent in LS mice. As the dose of cocaine was increased, activity returned to baseline, and at the highest doses, increases in locomotor activity were found, but only in SS mice. In LS mice, cocaine was ineffective in increasing locomotor activity at any of the doses tested. Since striatal dopaminergic neurons influence locomotor activity, we also assessed ligand affinity and receptor density of dopamine transporters and dopaminergic D₁ and D₂ receptors in striatal tissue obtained from these two selected lines. No differences in these receptor binding parameters were found. However, because of their anomalous locomotor response to cocaine, LS mice may prove to be a valuable tool in increasing our understanding of those sites which mediate specific effects of cocaine.     

Comparison of benzodiazepine receptor binding in membranes from human or rat brain

Specific high affinity binding of [3H]flunitrazepam to membranes from human brain was stimulated by gamma-aminobutyric acid (GABA), pentobarbital, 1-ethyl-4-(isopropylidene-hydrazino)-1H-pyrazolo[3,4b]pyridine-5-carboxy lic acid ethyl ester hydrochloride (SQ 20009) and avermectin B1a and was unaffected by 2 microM 4'-chlorodiazepam (Ro 5-4864) indicating that [3H]flunitrazepam in human brain as well as in rat brain predominantly binds to benzodiazepine receptors specific to brain, which was associated with a GABA receptor and several modulatory binding sites for drugs. The potency of several selective and non-selective ligands for benzodiazepine receptors for inhibition of the binding of [3H]flunitrazepam was compared in membranes from human or rat brain cerebellum, hippocampus and cerebral cortex. It was demonstrated that all these compounds, derived from different chemical structures, had a remarkably similar potency for inhibition of the binding of [3H]flunitrazepam in the corresponding regions of the human or rat brain. However, irreversible labelling of benzodiazepine binding sites with [3H]flunitrazepam and subsequent SDS-polyacrylamide gel electrophoresis and fluorography revealed more photolabelled protein bands in human than in rat cerebellum and hippocampus. The results seem to indicate that, although the pharmacological properties of reversible binding of [3H]flunitrazepam are remarkably similar in membranes from rat or human brain, the molecular heterogeneity of benzodiazepine binding sites is even greater in human than in rat brain.     

Group-selective reagent modification of the benzodiazepine-gamma-aminobutyric acid receptor-ionophore complex reveals that low-affinity gamma-aminobutyric acid receptors stimulate benzodiazepine binding

The modification of membrane proteins with diethylpyrocarbonate (DEP) and diazotized sulfanilate was investigated on the binding of three benzodiazepine radioligands in three brain regions. Both of these reagents produced a dose-dependent inactivation of [3H] diazepam, [3H]flunitrazepam, and [3H]propyl beta-carboline-3-carboxylate binding to cortex, cerebellum, and hippocampus. Both DEP and diazotized sulfanilate decrease the Bmax of the benzodiazepine binding sites without altering the KD. The ability of muscimol and pentobarbital to enhance [3H]diazepam binding was not altered by DEP pretreatment in any of the three regions. Scatchard analysis indicated that, following the inactivation of 40-50% of [3H]diazepam binding by 1 mM DEP, pentobarbital and muscimol were still able to increase the affinity of [3H]diazepam binding in cortex, cerebellum, and hippocampus. In contrast, diazotized sulfanilate pretreatment abolishes the ability of muscimol and pentobarbital to enhance [3H]diazepam binding in these three regions. The effects of these reagents on [3H] gamma-aminobutyric acid (GABA) binding revealed that sulfanilate but not DEP eliminates the low-affinity GABA receptor sites in cortex and cerebellum. Thus, while both DEP and sulfanilate inactivate benzodiazepine binding sites, only sulfanilate abolishes the low-affinity GABA binding sites and the ability of the GABA agonists to enhance [3H]diazepam binding. These results suggest that the stimulation of benzodiazepine binding appears to be mediated by the low-affinity GABA receptors.     

Selective mouse breeding for short ethanol sleep time has led to high levels of hepatic aromatic hydrocarbon (Ah) receptor

Following a selective breeding program of heterogeneous mice for more than 30 generations, SS ("short sleep") and LS ("long sleep") lines have been developed on the basis of their sleep times when challenged with a single intraperitoneal dose of ethanol. The aromatic hydrocarbon responsiveness (Ah) locus encodes the Ah receptor, which regulates the induction of certain drug-metabolizing enzymes by polycyclic aromatic compounds such as 3-methylcholanthrene and tetrachlorodibenzo-p-dioxin. The C57BL/6 inbred mouse strain (B6; Ahb/Ahb) has a high-affinity Ah receptor, while the DBA/2 inbred mouse strain (D2; Ahd/Ahd) has a low-affinity Ah receptor. We show here that the SS inbred mouse line exhibits markedly elevated hepatic levels of the high-affinity Ah receptor, while the LS outbred mouse line contains the low-affinity Ah receptor. Among progeny of (B6D2)F1 X D2 backcross, the b/d heterozygote (having the high-affinity Ah receptor) was found to be several times more resistant than the d/d homozygote to a single dose of intraperitoneal ethanol. The D2.B6-Ahb congenic line is also several times more resistant to intraperitoneal ethanol than the B6.D2-Ahb congenic line is also several times more resistant to intraperitoneal ethanol than B6.D2-Ahd congenic line. We found that the waking blood ethanol levels are the same in b/d and d/d mice, suggesting that the relative ethanol resistance in b/d mice cannot be explained on the basis of a difference in central nervous system sensitivity. There are no differences between SS and LS mice or between b/d and d/d mice with regard to (i) blood acetaldehyde levels after a single intraperitoneal dose of ethanol, or (ii) hepatic alcohol dehydrogenase activities. There is a difference in the rate of ethanol elimination: SS more rapid than LS; b/d more rapid than d/d. Although SS mice have lower hepatic aldehyde dehydrogenase activities (cytosolic, mitochondrial low-Km: and mitochondrial high-Km forms) than LS mice, b/d and d/d do not show this difference. These data suggest that a selected mouse breeding program, based on resistance to a single intraperitoneal dose of ethanol, selects concurrently for the hepatic high-affinity Ah receptor. This selective advantage cannot be explained on the basis of changes in alcohol dehydrogenase or aldehyde dehydrogenase activities and might provide insight into the nature of the endogenous ligand for the Ah receptor.