Rate-determining factors for ethanol metabolism in fasted and castrated male rats

The effects of castration and fasting upon the alcohol elimination rate, liver alcohol dehydrogenase (LADH) maximum activity (V_max), and hepatic concentrations of ethanol, acetaldehyde, and free NADH during ethanol oxidation were examined in male Wistar rats. Castration increased the V_max of LADH and, to a lesser extent, the alcohol elimination rate in vivo. On the other hand, fasting reduced the V_max of LADH and the alcohol elimination rate in sham-operated and castrated rats but it did not nullify the effect of castration. Castration produced small but significant changes in the hepatic concentrations of ethanol, acetyldehyde and free NADH in fed rats during ethanol oxidation. Fasting also caused significant increases in the concentration of free NADH during alcohol oxidation in both the sham-operated and castrated groups. The ratio of the steady-state velocities of LADH in situ to the maximum velocities of LADH ($\text{ν}\text{V}{}_{\mathrm{<mtext>max</mtext>}}$) under the different experimental conditions was calculated by using the steady-state rate equation for the enzyme mechanism of rat LADH and its kinetic constants. The calculated $\text{ν}\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ ratios were 50–62%, indicating that LADH activity was limited to about the same extent by its substrates and products under these conditions and that the changes in alcohol elimination rates produced by fasting and castration mainly reflected changes in the V_max of LADH. The calculated steady-state velocities in situ (ν) were 14–28% lower than the measured rates of alcohol elimination in vivo. The extent of agreement is probably acceptable in view of the assumptions needed to determine the free NADH concentration in liver and the existence of non-LADH-related processes for alcohol elimination in vivo.     

Effect of testosterone on ethanol oxidation during chronic alcoholism in castrated rats

It has been shown in non-alcoholized male rats that castration significantly and appreciably raises the level of endogenous ethanol. In chronic alcoholization of castrated and non-castrated rats, the rate of ethanol elimination (REE) is noticeably increased, with testosterone producing no essential effect on the REE. In the liver, alcohol dehydrogenase activity rises insignificantly under the effect of testosterone, whereas aldehyde dehydrogenase activity declines 30--100%.     

Effect of stress by repeated immobilization on hepatic alcohol dehydrogenase activity and ethanol metabolism.

The hepatic activity of alcohol dehydrogenase was increased after 7, 14 and 42 days of stress induced by immobilization of rats for 2.5 $\text{hr}\text{day}$. A single immobilization had no effect on the activity of alcohol dehydrogenase. Immobilization for 14 days resulted in increases in the rates of ethanol metabolism. This was not associated with changes in the activity of the microsomal ethanol-oxidizing system, microsomal catalase, cytochrome P-450, or NADPH cytochrome c reductase. A decrease in the hepatic phosphorylation potential ([ATP]/[ADP][P_i]) was found to be due to a decrease in [ATP] and an increase in [P_i]; however, there were no changes in O₂ consumption by liver slices or in hepatic (Na⁺ + K⁺)-stimulated adenosine triphosphatase activity. The increased rate of ethanol metabolism after stress remains unexplained since alcohol dehydrogenase activity is not rate-limiting in ethanol oxidation and there were no increases in ethanol oxidation by microsomes or in mitochondrial oxidative rate.     

Depression of alcohol dehydrogenase activity in rat hepatocyte culture by dihydrotestosterone

Hepatocytes harvested from castrated rats retained a higher alcohol dehydrogenase (EC 1.1.1.1) activity than hepatocytes harvested from normal rats during 7 days of culture. Dihydro-testosterone (1 μM) decreased the enzyme activity, after 2 and 5 days of culture, in hepatocytes from castrated and control animals respectively. Dihydrotestosterone decreased the enzyme activity to similar values in both groups of hepatocytes by the end of 7 days of culture. Testosterone (1 μM) had no effect on the enzyme activity in normal hepatocytes and only a transitory effect in decreasing the enzyme activity in hepatocytes from castrated animals. The increases in alcohol dehydrogenase activity after castration and their suppression by dihydrotestosterone were associated with parallel changes in the rate of ethanol elimination. Additions of substrates of the malate-aspartate shuttle or dinitrophenol did not modify ethanol elimination. These observations indicate that dihydrotestosterone has a direct suppressant effect on hepatocyte alcohol dehydrogenase and that the enzyme activity is a major determinant of the rate of ethanol elimination.     

Decreased in vivo acetaldehyde oxidation and hepatic aldehyde dehydrogenase inhibition in C57BL and DBA mice treated with carbon tetrachloride

Male $\text{C57BL}\text{6J}$ (C57) and $\text{DBA}\text{2J}$ (DBA) mice were dosed with 500 μl/kg ig CCl₄, 24 hr before a 3.25 g/kg ip dose of ethanol. The relative bioavailability of CCl₄ was similar in both mouse strains. CCl₄ pretreatment produced genotypically related decreases in blood ethanol elimination rates with DBA mice showing the greater decrease. Blood acetaldehyde concentrations were significantly elevated in the CCl₄-pretreated animals of both strains. DBA CCl₄-pretreated mice exhibited significantly higher blood acetaldehyde concentrations than C57 mice 1, 2, 3, and 4 hr after ethanol administration. CCl₄ pretreatment resulted in acetaldehyde elevations of approximately threefold in C57 mice four- to fivefold in DBA mice. Four hours after the ethanol dose, animals were sacrificed for enzymatic analyses of hepatic aldehyde dehydrogenase (ALDH) and alcohol dehydrogenase (ADH) activity. The specific activities and activity per gram of cytosolic and mitochondrial ALDH were decreased in both mouse strains. ADH activity was not significantly altered by CCl₄ pretreatment. Liver necrosis, as measured by serum alanine aminotransferase activity, was similar in both strains (approx fivefold increase). These data indicate that CCl₄ increased blood acetaldehyde concentrations by inhibiting hepatic cytosolic and mitochondrial ALDH and that the degree of blood acetaldehyde elevation is different in the two inbred mouse strains C57 and DBA.     

The fate of the ethanol analogue 1,3-butanediol in the dog. An in vivo-in vitro comparison

In view of the current interest in 1,3-butanediol as food additive or potential drug its pharmacokinetics have been investigated in the dog and compared to its oxidation in vitro by alcohol dehydrogenase. Plasma disappearance of i.v. doses of 5.5 mmol/kg were zero order followed by first order. Assuming Michaelis-Menten kinetics a V_max of 1.23 ± S.D. 0.27 μmol/min/g of liver and a K_m of 1.15 ± 0.85 mM could be calculated. The corresponding values for 1,3-butanediol metabolism by alcohol dehydrogenase in vitro were 1.62 ± 0.34 μmol/min/g of liver and 5.11 ± 1.45 mM. Hepatic vein catheterizations were used to measure hepatic blood flow (18.1 ± 2.8 ml/min/kg) and the fraction of butanediol disappearing in the liver, which was only 34.2 ± 6.6 per cent. Compared to ethanol, V_max of 1,3-butanediol was 15 per cent smaller in vitro, 45 per cent smaller in vivo, K_m was 3 times larger in vitro and 60 per cent smaller in vivo. The splanchnic elimination fraction of 1,3-butanediol was about $\text{1}\text{2}$ the one of ethanol. These data are consistent with the concept, that oxidation by alcohol dehydrogenase is the major route of butanediol elimination. The differences between 1,3-butanediol and ethanol metabolism, however, render different pharmacological and toxicological effects likely.     

Liver aldehyde and alcohol dehydrogenase activities in rat strains genetically selected for their ethanol preference.

Rat strains raised by genetical selection for either high (AA strain) or low (ANA strain) voluntary ethanol consumption were compared with respect to their hepatic alcohol and aldehyde dehydrogenase activities. Liver alcohol dehydrogenase activity was lower in both sexes in the AA strain compared with the ANA strain. The NAD-dependent aldehyde dehydrogenase activity was higher in the mitochondrial and microsomal fractions and lower in the soluble fraction in the AA strain than in the ANA strain. These differences were more pronounced in females than in males. The NAD-dependent utilization of acetaldehyde in liver homogenates was higher in the AA strain in both sexes, when the initial acetaldehydehyde level was 0·40 mM, but there was no difference at $\text{0¢13}\text{mM}$ acetaldehyde. It is concluded that the higher activities in the AA strain are due mainly to those aldehyde dehydrogenases of mitochondrial and microsomal fractions, which have K_m-values for aldehydes in the millimolar range. The higher alcohol dehydrogenase and lower aldehyde dehydrogenase activity in livers of rats of the ethanol-avoiding ANA strain may contribute to the previously found higher acetaldehyde levels in blood and liver of rats of this strain after ethanol administration.     

Effect of castration on the turnover of rat liver alcohol dehydrogenase

Castration increased liver alcohol dehydrogenase activity and enzyme protein in male rats. The turnover of alcohol dehydrogenase determined from the decline in radioactivity present in immunoprecipitated enzyme after injection of NaH¹⁴CO₃ was decreased after castration. The fractional rate of degradation (K_d) for the enzyme was 0.11 · day⁻¹ in the castrated as compared with 0.13 · day⁻¹ in the control animals (P < 0.05). The fractional rate of synthesis (K_s) of the enzyme was not affected by castration, while the absolute rate of synthesis was increased slightly. This study shows that a decrease in the rate of degradation is the principal cause for the increase in liver alcohol dehydrogenase following castration.     

Brain metabolite concentrations and redox states in rats fed diets containing 1,3-butanediol and ethanol

Both ethanol and its dimer 1,3-butanediol (BD) are substrates for alcohol dehydrogenase in liver and thus have many similar effects upon metabolite concentrations and redox states in that organ. Although the mechanism by which ethanol exerts its effects on the brain is not known, it must differ from that in liver since brain contains insignificant amounts of alcohol dehydrogenase. The effects of BD, which does not produce intoxication, and ethanol upon brain were compared in an attempt to determine which changes in metabolite content are related to depression of the CNS. Diets containing 47% of calories as ethanol, BD, or glucose were pair-fed to rats for 62 days. The brains were then removed and frozen with a new apparatus which prevents postmortem changes, and concentrations of metabolites were determined. Substitution of BD for glucose in the diet caused a decrease in the brain content of glucose, lactate and α-oxoglutarate and an increase in glutamate and citrate. Substitution of ethanol for glucose caused only a decrease in the brain content of glucose and an increase in citrate. The ethanol diet, as compared with the BD diet, caused an elevation of the brain content of glucose and a decrease in glutamate. Both BD and ethanol caused a decrease in the free cytoplasmic $\text{[}\text{NADP}{}^{\mathrm{+}}\text{]}\text{[}\text{NADPH}\text{]}$ ratio in brain without changing the $\text{[}\text{NAD}{}^{\mathrm{+}}\text{]}\text{[}\text{NADH}\text{]}$ of cytoplasm or mitochondria.Except for the differences in effect upon glucose and glutamate, BD and ethanol appear to act similarly on the metabolite content of rat brain. It is therefore concluded that the brain is resistant to changes of the measured metabolites during the process of chronic ethanol ingestion. It is further suggested that the small changes observed in the cytoplasmic $\text{[}\text{NADP}{}^{\mathrm{+}}\text{]}\text{[}\text{NADPH}\text{]}$ ratio after ethanol feeding are not related to the depression of CNS activity associated with ethanol ingestion since they also occur after feeding BD.     

Studies on ethanol and oral contraceptives: feasibility of a hepatic-gonadal link

Adult male and female rats were subjected to gonadectomy by means of surgical removal of the gonads. In the male, castration resulted in a significant decrease in both body and liver weights compared to intact controls, which persisted for at least 3 weeks. Conversely, ovariectomy was associated with a significant enhancement in both growth rate and liver weight from intact controls. Castration of male rats resulted in induction of hepatic L-ADH (cytosolic alcohol dehydrogenase) and L-ALDH (cytosolic aldehyde dehydrogenase) as contrasted with inhibition of mitochondrial ALDH which was evident in the enzyme with the apparent high Km. Kinetic studies indicate that there was an increase in apparent Km of L-ADH, and hence reduced affinity to hepatic metabolism of ethanol as a consequence of castration in the male rat. This is compared with few changes occurring in the apparent Km value of L-ALDH. Ovariectomy did not alter endogenous L-ADH or L-ALDH. Short-term administration of a synthetic estrogenic steroid ethinyl estradiol, inhibited liver mitochondrial ALDH in the intact female rat but not in the ovariectomized female. Short-term administration of the same dose of an androgen, testosterone, did not alter specific activities of the liver enzymes measured in the intact or in the castrated male rat. Administration of both components of OCs (oral contraceptives) combined or the estrogen alone in behavioral experiments profoundly reduced ethanol drinking by voluntary intake of diluted ethanol solution by the intact female rat. These results suggest a hepatic-gonadal link may exist and that a toxic interaction between the OCs and alcohol drinking is definitely possible.     

Uptake,distribution, and effects of carbon tetrachloride in rainbow trout (Salmo gairdneri)

Uptake, distribution, and effects of CCl₄ were studied in rainbow trout. Carbon tetrachloride (1 ml/kg, ip) produced 5- to 10-fold increases in serum GOT, GPT, and ICD activities, whereas exposure of trout to CCl₄ in the tank water (1–80 mg/liter) produced neither mortality nor significant changes in enzyme activities. CCl₄ residue(s) appeared highest in concentration in the adipose tissue, followed by liver, brain, and spleen, and was lowest in gill regardless of the administration route. The elimination rates of ¹⁴C residue(s) from the tissue samples were most rapid in muscle ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{β = 1.7}\text{hr}$) and relatively prolonged for liver ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{β = 38.9}\text{hr}$). Maximum liver concentrations of ¹⁴C residue(s) were reached at 2 hr by either ip (1 ml/kg) or water exposure (80 mg/liter) and were 4.8 μmol/g and 0.75 μmol/g, respectively. No increase in liver triglyceride (TG) concentrations were noted at liver CCl₄ concentrations that have been associated with increased TG levels in the rat. Histological examination of tissues revealed varying degrees of liver and splenic necrosis 6 hr after administration of CCl₄.     

Placental transfer and tissue localization of digoxin in the pregnant rat

Serum and tissue digoxin concentrations were estimated by radioimmunoassay in maternal and fetal animals at various times following the injection of pregnant rats, on Days 19 or 20 of gestation, with digoxin (0.1 mg/kg iv). Maternal serum digoxin concentrations were 3.5 to 5 times greater than corresponding fetal concentrations during the early period of sampling (2–10 min) and ranged from 1.8- to 2.7-fold higher for the remainder of each experiment. The elimination of digoxin from maternal serum followed a biexponential pattern ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{α = 0.36}\text{hr}\text{; t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{β = 3.52}\text{hr}\text{; K}{}_{\mathrm{<mtext>el</mtext>}}\text{= 0.39}\text{hr}{}^{\mathrm{?1}}$) and was similar to that observed in placental tissue and the fetal circulation. Maternal tissue concentrations of digoxin were highest in the liver followed in descending order by the skeletal muscle, heart, and kidney. The distribution pattern in fetal tissues differed significantly as indicated: heart > kidney > liver. These data have demonstrated that digoxin is rapidly transferred across the placenta of the pregnant rat. Differences in the fetal/maternal tissue distribution patterns of the drug may reflect developmental influences upon tissue binding and/or regional perfusion.     

Modification of some of the toxic effects of daunomycin (NSC-82,151) by pretreatment with the antineoplastic agent ICRF 159 (NSC-129,943)

Daunomycin (50 mg/kg) was lethal to Syrian golden hamsters ($\text{10}\text{10}$) within 2–3 days, however, when this same dose was given 30 min after 100 mg/kg of 2,6-piperazinedione, 4,4′-propylenedi-, [±]-(ICRF 159) half of the animals ($\text{5}\text{10}$) survived over 21 days. Between 40 and 51% of the total dose of daunomycin (50 mg/kg) could be recovered unchanged from hamster serum, urine, heart, lung, liver, kidney and spleen. The amount of unchanged daunomycin recovered from these tissues was higher (44–69%) in animals pretreated with ICRF 159. The amount of daunomycinone, a metabolite of daunomycin, recovered from the various tissues was less at all intervals in animals pretreated with ICRF 159 (0–3% of the total daunomycin dose) when compared with 4–9% recovered from the saline-pretreated hamsters. The iv administration of daunomycin (50 mg/kg) in the rhesus monkey produced hyperglycemia (140%) within 15 min, followed by a secondary hypoglycemia after 1–3 hr. Pretreatment with ICRF 159 (100 mg/kg) blocked the initial increase, but not the secondary decrease, in blood sugar over a 3-hr period. Daunomycin increased creatine phosphokinase and glutamic-oxaloacetic transaminase serum enzyme activities 13 and 4.5 times, respectively. Smaller increases were detected in serum enzyme activities of glutamic-pyruvic transaminase (3.5 times) and lactic acid dehydrogenase (2.5 times). The increases in serum enzyme activities were attenuated in animals pretreated with ICRF 159. In the present experiments, a decrease in production of daunomycinone may be an important factor in the reduction of daunomycin toxicity.     

Effect of carbidine on parameters of ethanol oxidation in experimental alcoholic intoxication

The effect of carbidine on enzymes of ethanol and acetaldehyde oxidation, the rate of ethanol elimination and the parameters of ethanol consumption were studied during long-term alcoholic intoxication. Carbidine administration was shown to increase the activity of alcohol dehydrogenase of the liver tissue, to decrease the activity of aldehyde dehydrogenase with a low Km to acetaldehyde. Also, the rate of ethanol elimination and a relative amount of consumed ethanol increase at the expense of an increase of the volume of consumed liquid.     

Inhibition of water intake by ouabain administration in sheep

Intracarotid infusion of ouabain (1280 ng/min) over $\text{4}\text{1}\text{2}\text{hr}$ virtually abolished water intake of sheep in response to intracarotid infusion of either angiotensin II (800 ng/min) or 4 M NaCl (1.6 ml/min for 20 min). Ouabain treatment did not affect mean arterial pressure either before or during infusion of angiotensin. Neither ouabain nor angiotensin administration affected plasma [Na] or [K] or CSF [K]. During ouabain, but not during control infusion, angiotensin administration significantly decreased CSF [Na]. Ouabain administration also decreased water intake after $\text{23}\text{1}\text{2}\text{or}\text{48}\text{hr}$ water deprivation. In the $\text{23}\text{1}\text{2}\text{hr}$ deprivation experiments, food was made available immediately prior to water presentation and the ingestion of food appeared to ameliorate the reduction in water intake. Food intake itself, was decreased in some animals, during ouabain treatment. Ouabain infused at 960 ng/min resulted in significant, but smaller, reductions in water intake induced by angiotensin, 4 M NaCl, and 48 hr water deprivation. It was concluded that ouabain treatment affected water intake by influence on Na transport either in the thirst receptors or at some other level in the neural system between receptor and effector.     

Effects of solvent exposure on testosterone levels and butyrylcholinesterase activity in mice

In female and male mice the effect of exposure to trichloroethylene (TCE) seen at the lowest concentration is an increase in liver weight. The activity of plasma butyrylcholinesterase (BuChE) increases even more than the liver weight at corresponding concentrations, but only in the males. Depletion of testosterone through castration or destruction of the pituitary gland or hypothalamus, are the only other ways to experimentally induce corresponding increases in BuChE. Plasma BuChE activity increase was found to be a common reaction after exposure to TCE, perchloroethylene, chloroform, methylene chloride and carbon tetrachloride and also after exposure to ethanol. Other solvents such as toluene, xylene, benzene and 1,1,1-trichloroethane had little or no effect on BuChE activity. Normal and castrated male mice were continuously exposed for one month to 150 p.p.m. TCE. The increase in BuChE activity after the exposure was of the same magnitude as the increase seen after castration. BuChE activity in castrated males was not further increased by TCE exposure. Administration of testosterone with osmotic minipumps for 13 days almost restored the normal testosterone and BuChE levels in castrates. The effect of TCE exposure on BuChE activity in these animals was the same as on normal males. Testosterone levels were not influenced by the TCE exposure in normal males or in castrates given testosterone. No sex hormone binding globulins (SHBG) could be detected in the mice. BuChE activity changes induced through solvent exposure are therefore neither directly nor indirectly (through SHBG) due to effects on testosterone. The results from these animal experiments do not support the epidemiological findings of decreased testosterone levels in humans exposed to solvents.     

Determinants of plasma free acetaldehyde levels during the oxidation of ethanol: Effects of chronic ethanol feeding

Plasma free acetaldehyde levels were measured by an improved method in baboons fed ethanol chronically and in pair-fed controls during an intravenous infusion of ethanol. After an appropriate loading dose, ethanol was infused at the rate of its elimination to achieve a steady state at one of three different blood ethanol levels (50 ± 10 mM, 10 ± 2 mM, or 5 ± 1 mM). The rate of production of acetaldehyde was calculated from the rate of ethanol elimination by subtracting losses into urine and expired air. Liver mitochondrial aldehyde dehydrogenase (AlDH) activity was measured in surgical biopsy samples with 50 μM acetaldehyde as substrate. Chronic ethanol administration resulted in both higher plasma free acetaldehyde levels and faster acetaldehyde production at each level of blood ethanol. When the blood level of ethanol was increased from 5 to 50 mM, the level of plasma free acetaldehyde also rose in both groups of animals. The rate of acetaldehyde production, however, increased only in alcohol-fed baboons. Plasma free acetaldehyde had a significant positive correlation with production rate of acetaldehyde (r = 0.69) and a significant negative correlation with liver mitochondrial AIDH specific activity (r = ?0.59). When these two parameters were combined (acetaldehyde production rate/AIDH activity), a correlation coefficient of 0.84 resulted, suggesting that, in addition to increased production, decreased catabolism may contribute to the higher acetaldehyde levels seen after chronic consumption.     

Metabolism of ethanol and sorbitol in clofibrate-treated rats

The rates of ethanol and sorbitol removal and the cytoplasmic and mitochondrial redox states of the liver were determined in female rats pretreated with clofibrate (ethyl-α-p-chlorophenoxyiso-butyrate) for 2–15 days. The drug significantly increased the rate of elimination of ethanol even within the first two days. A significant increase in liver mass took place within a week but cannot explain the initial increase in ethanol removal. Although the liver mass was increased by clofibrate, the rate of sorbitol removal was significantly decreased. A decrease in liver sorbitol dehydrogenase activity was also observed. The sum of the removal of ethanol and sorbitol, when they were simultaneously metabolized, was significantly decreased in clofibrate-treated rats as compared with control ones. Sorbitol inhibition of ethanol elimination was increased but ethanol inhibition of sorbitol elimination was abolished by clofibrate administration. Ethanol and sorbitol caused similar changes in cytoplasmic (lactate/pyruvate) and mitochondrial (β-OH-butyrate/acetoacetate) redox states of clofibrate-treated rat liver, as has been earlier observed in livers of control animals.     

Quantitative correlation of ethanol elimination rates in vivo with liver alcohol dehydrogenase activities in fed, fasted and food-restricted rats.

The effects of nutritional states upon liver alcohol dehydrogenase (ADH) activity and ethanol elimination rate in vivo have been examined in the rat. Male Sprague-Dawley rats, 250–280 g, were studied in the fed state, after fasting for 24, 48 and 72 hr, and after 9 days of food restriction (5g food/day). Total ADH activity per liver or per animal (2.20 m-moles/hr in fed rats) decreased after a 24-hr fast and was 1.32 and 0.94 m-moles/hr after a 48-hr fast and food restriction respectively. Cytosolic protein and liver wet weight decreased in parallel with total ADH activity, but DNA content exhibited only a 10% decrease with fasting and a 20% decrease with food restriction. Ethanol elimination rate in vivo per animal after intraperitoneal injection of 2g ethanol/kg was 1.92, 1.14 and 0.84 m-moles/hr in the fed, 48 hr-fasted and food-restricted rats, respectively. These data indicate that the decrease in the ethanol elimination rate with fasting and food restriction may be caused by decreasing ADH activity, since the cytosolic free NAD⁺/ NADH in liver after acute administration of alcohol in vivo has been reported to be nearly identical in the fed and 48 hr-fasted rats. The close agreement between liver ADH activity and ethanol elimination rate in vivo suggests that the total enzymatic activity of liver ADH is an important rate-limiting factor in ethanol metabolism under the nutritional conditions examined.     

Effects of apomorphine on self-stimulation

In rats, self-stimulation (SS) from posterior lateral hypothalamus and ventromedial tegmentum was suppressed by the ip administration of alpha-methyl-para-tyrosine methyl ester (α-MPT, 100 mg/kg). Apomorphine (0.25 or 0.5 mg/kg) was injected ip $\text{3}\text{1}\text{2}\text{hr}$ after α-MPT treatment. Self-stimulation was reinstated to a significant degree, after 2 hr for the 0.25 mg/kg group, and 3 hr for the 0.5 mg/kg group. Apomorphine given after saline control, elicited an immediate suppression of SS for approximately $\text{1}\text{2}\text{hr}$ in the case of 0.25 mg/kg and $\text{1}\text{1}\text{2}\text{hr}$ for 0.5 mg/kg.