D. E. McMillan

CHARACTERISTICS OF TETRAHYDROCANNABINOL TOLERANCE

Tolerance to a drug is said to have developed “when, after repeated administration, a given dose of a drug produces a decreasing effect, or conversely when increasingly larger doses must be administered to obtain the effects observed with the original dose.”’ There is evidenv that tolerance develops to the effects of marijuana and its active constituents in animaW6 and in man.7 However, many investigators have not observed tolerance to marijuana or its active constituents in animalss and in mang-lls or have suggested that the repeated administration of marijuana may result in an increased sensitivity to the subjective effects of marijuana in man.12 There are many factors that might have contributed to the conflicting findings concerning the development of tolerance to marijuana. Among the most serious of these factors has been the difficulty in controlling the dose level of the active constituents in marijuana, due to the varying purity of the preparations from the plant material.I3 Since tolerance to a drug is defined either in terms of a diminishing response to a constant dose or in terms of a constant response to graded increases in the dose, failure to control the dose adequately makes the demonstration of tolerance difficult. Other problems in demonstrating tolerance to marijuana and its active constituents may arise from differences in the methods used to assess tolerance and from species differences. We have attempted to minimize these problems by using only those constituents of marijuana that have been shown to have activity in man14-17 and are available in relatively pure form,ls and by using standardized procedures for measuring the development of tolerance in several species. Recently, schedule-controlled behavior has been used as a sensitive and stable baseline for assessing the development of tolerance to a number of d r ~ g s ~ * ~ J ~ ~ ~ in several species. The basic techniques for studying schedule-controlled behavior are well known.22 In our experiments, pigeons are trained to peck a translucent plastic response key in order to obtain a four-second access to mixed grain. Each peck breaks an electrical circuit, which permits the peck to be counted and recorded. Occasionally, depending on the schedule of food reward, pecks produce access to grain. The reward schedule for pigeons that has been used most often in tolerance s t ~ d i e s ~ ~ J ~ is the multiple fixed-ratio 30-response, fixed-interval fiveminute (mult FR 30 FI 5) schedule of food presentation. Under this schedule, when a blue light behind the translucent key is on, 30 key pecks are required to produce food (FR 30 component), and when a red light behind the key is on, the first key peck after five minutes produces food (FI 5 component). The two schedule components alternate throughout a session, with a 40-second time limit applying to both components of the schedule, so that the schedule can change automatically from one component to the other component if the pigeon is not responding at the time food is available. At these schedule parameters, pigeons usually respond at a high steady rate under the FR component, while responding under the FI 5 component is characterized by a pause early in the interval, followed by an increasing rate of responding as the availability of food appr0aches.2~

l-Dgr9-trans-Tetrahydrocannabinol in Pigeons: Tolerance to the Behavioral Effects.

Δ9-Tetrahydrocannabinol was injected daily, in increasing doses, in pigeons under a multiple schedule of food presentation. Within a week, a dose that initially abolished responding completely was without effect. This dose was gradually increased to 20 times its original value without disrupting the behavior. No withdrawal syndrome was detected when the cannabinol was discontinued.

Interaction of the discriminative stimulus properties of diazepam and ethanol in pigeons.

One group of pigeons (n = 5) was trained to discriminate between the effects induced by 5.6 mg/kg of diazepam (DZP) and the vehicle whereas other pigeons (n = 5) had to discriminate between 3.0 g/kg of ethanol (ETOH) and the vehicle, administered intragastrically (IG) 10 and 40 min prior to the training sessions respectively. Once trained, the pigeons were tested with either diazepam or ethanol alone and in combination. The birds trained to discriminate between DZP and the vehicle mostly performed non-drug associated responses when tested with ETOH (0.56 to 3.0 g/kg). Tests with other doses of DZP (0.3 to 3.0 mg/kg) in the diazepam-trained birds resulted in an ED50 value of 1.4 mg/kg. The birds trained to discriminate between ETOH and the vehicle generalized DZP to ETOH, the ED50 value for diazepam being 3.0 mg/kg. Tests with other doses of ETOH (0.56 to 2.0 g/kg) in this latter group resulted in an ED50 value of 1.3 g/kg. Tests with combinations of DZP and ETOH produced a shift of the dose-response curves to the left indicating drug additivity. The discrimination of 5.6 mg/kg of IG administered DZP but not that of ETOH (3.0 g/kg) was attenuated by injections of the analeptic bemegride (ED50 = 5.5 mg/kg), thus suggesting a difference in the cueing processes of the two drugs. When tested singly, bemegride induced non-drug responding or complete suppression of responding in the birds at the doses of 3.0 and 10.0 mg/kg respectively. In conclusion, the discriminable effects of DZP and ETOH are additive or even supra-additive, but the stimulus properties of the two drugs are not identical.

Failure of signs of physical dependence to develop in hamsters after prolonged consumption of large doses of ethanol

Abstract Male Golden Hamsters drank large amounts of ethanol with food and water freely available, when ethanol was presented in water at concentrations of 10–40% (w/v). Although the hamsters consumed an average of 13.8 g/kg/day of ethanol for 3 months, no withdrawal signs were observed during 4 days without ethanol, nor were withdrawal signs observed during withdrawal after 4 more months of ethanol consumption. Although the Golden Hamster consumes large amounts of ethanol without the need for food or water deprivation, the Golden Hamsters may have limited usefulness as a model of physical dependence.

Ethanol and isopropanol effects on schedule-controlled responding

The effects of ethanol and isopropanol were studied on responding by pigeons under multiple fixed-ratio (FR), fixed-interval (FI) schedules of food presentation and under a fixed-interval (FI) schedule of food presentation where responding was decreased by punishment. The ethanol was rapidly absorbed into blood and decreased responding within 15 min after intubation to the opening of the proventriculus. Dose-effect determinations of the effects of ethanol showed that ethanol decreased responding in both the FR and FI components of the multiple schedules at similar doses, but there were increases in responding under an FR 100 schedule at lower doses. Isopropanol tended to decrease FR responding at doses that either increased FI responding or did not affect FI responding. Both ethanol and isopropanol (1 g/kg) produced effects on the local rates of responding within the FI which were rate-dependent in that they increased low rates while not affecting or actually decreasing the high rates of responding. Both ethanol and isopropanol increased punished responding if it was not severely suppressed by the punishment procedures.

Chronic effects of lead on schedule-controlled pigeon behavior

Abstract Pigeons were trained to peck a response key under a multiple fixed ratio 30 response (FR 30) fixed-interval 5-min (FI 5) schedule of food presentation. When rates of responding stabilized, lead acetate (6.25, 12.5, and 25 mg/kg body weight) or sodium acetate solutions were given daily by gastric intubation. Blood-lead concentrations were measured weekly by atomic absorption spectrometry, and responding under the multiple schedule was measured Monday through Friday. Control intubations of sodium acetate did not affect responding under either component of the schedule. The 25-mg/kg dose of lead decreased rates of responding after 3 to 10 days of administration and usually was lethal between 18 and 35 days. Post mortem examination of the birds showed obvious esophageal dilatation, damage to the crop, weight loss, and subarachnoid hemorrhages. Administration of the 25-mg/kg dose was discontinued in some birds after responding had ceased. Responding gradually began to recover, but rates of responding remained low and variable under both components of the schedule for many days. Daily administration of the 12.5-mg/kg dose of lead caused one death, but no obvious signs of intoxication occurred in the other birds. However, three of the four birds surviving daily 12.5-mg/kg doses of lead showed decreased rates of responding under both schedule components within 30 days of the initial intubation. The fourth bird showed a highly variable rate under the FI component of the schedule during lead administration; however, when lead administration was discontinued, dramatic increases in rates of responding above lead pretreatment levels occurred, particularly under the FI component. These rate increases persisted for 75 days thereafter. The 6.25-mg/kg dose of lead produced only small changes in rates of responding during 70 days of intubation. Blood-lead concentrations were very high (150–3470 μg/dl) during treatment with all doses of lead. Increasing doses produced increasing blood concentrations which were correlated with increasing behavioral effects, although correlations between blood-lead concentrations and behavioral changes were low within individual birds.

Distribution of radioactivity in brain of tolerant and nontolerant pigeons treated with 3H-δ9-tetrahydrocannabinol☆

Levels of radioactivity in the brain stem, cerebellum, temporal cortex and frontal cortex of birds tolerant to δ9-tetrahydrocannabinol (δ9-THC) did not differ significantly from levels in the same tissue of nontolerant birds after administration of 3H-δ9-THC. Levels of radioactivity in the lungs of both tolerant and nontolerant birds were similar to the levels of radioactivity in the brain areas after 3H-δ9-THC; however, higher levels of radioactivity were found in the livers of both groups of birds than were found in the brains. Approximately 0·1 per cent of the total dose of radioactivity was in the brain of both tolerant and nontolerant birds 2·5 hr after injection. When pigeons were injected repeatedly (seven times in 2 weeks) with 3H-δ9-THC, there was some accumulation of radioactivity in brain and lung, and an even greater accumulation in liver. These data suggest that tolerance to δ9-THC in pigeons is not due to a decreased concentration of total cannabinoids in the brain.

Δ9-THC as a discriminative stimulus in rats and pigeons: Generalization to THC metabolites and SP-111

In a drug discrimination paradigm pigeons and rats were trained with an operant procedure to discriminate between the presence and absence of the effects of Δ9-THC (1.0 and 3.0 mg/kg, injected IM 90 min and I.P. 30 min before the start of the session). Once trained, various THC metabolites as well as a water-soluble derivative of THC (SP-111), were substituted for Δ9-THC to test for generalization to the training drug. Generalization to Δ9-THC occurred with the 11-hydroxy metabolites and the potency order was 11-OH-Δ9-THC >11-OH-Δ8-THC ⩾Δ9-THC. Among the other metabolites tested (8α-OH-Δ9-THC, 8α, 11-di-OH-Δ9-THC, 8β-OH-Δ9-THC, 8β, 11-di-OH-Δ9-THC), it was only 11-di-OH-Δ9-THC that completely substituted for Δ9-THC in pigeons, albeit at very high dose levels (rats were not tested with these metabolites). SP-111 generalized to Δ9-THC in both species. However, the onset of action of SP-111 was slower than that for Δ9-THC, especially in pigeons. These studies show the importance of obtaining complete dose-effect determinations over time when assessing structure-activity relationships with drug-discrimination procedures.

Antagonism of morphine by long acting narcotic antagonists.

The effects of three narcotic antagonists, diprenorphine, naltrexone, and naloxone were studied on the schedule-controlled behavior of pigeons. Naltrexone decreased the rate of responding under the FR and FI components of a multiple fixed-interval, fixed-ratio schedule. Naltrexone and diprenorphine were equipotent in blocking the rate-decreasing effects of morphine on schedule-controlled behavior when the antagonists were given immediately before morphine, and both were more potent morphine antagonists than naloxone. Higher doses of all 3 antagonists were required to block the effects of morphine as the time between the administration of the antagonist and morphine increased. Naltrexone provided a slightly better antagonism of morphine than diprenorphine when morphine was given 2 or 6 h after the antagonist and both antagonists had a longer duration of antagonist action than naloxone.

Blood levels of 3H-Δ9-tetrahydrocannabinol and its metabolites in tolerant and nontolerant pigeons

Abstract Pigeons were made tolerant to the behavioral effects of 1-Δ 9 -trans-tetrahydrocannabinol (Δ 9 -THC) by repeated intramuscular injections. The tolerant birds, as well as birds that had no received Δ 9 -THC previously, were injected with 3 H-Δ 9 -THC and blood samples were drawn over a period from 1 min to 2 weeks after injection. High levels of radioactivity appeared in the blood 1 min after injection and peak levels were reached in 30 min, after which there was a gradual decline with some radioactivity still present after 2 weeks. The levels of radioactivity in the petroleum ether-extractable fraction [mostly Δ 9 -THC by thin-layer chromatography (TLC)], in the diethyl ether- extractable fraction (mostly hydroxylated metabolites by TLC), and in the residue fraction of the plasma were also determined over the 2-week period. There were no differences between tolerant and nontolerant birds in levels of radioactivity in total plasma, or in any of the plasma fractions. In other experiments, seven injections of 3 H-δ 9 -THC were given to pigeons over a 2-week period during which behavioral tolerance developed. Radioactivity gradually accumulated in the plasma of these birds; however, most of the radioactivity was accounted for in the residue fraction, with levels of radioactivity in the petroleum ether-extractable fraction and the diethyl ether-extractable fractions remaining at about the same levels after seven injections as after one injection. These results suggest that tolerant birds handle an injection of δ 9 -THC in much the same manner as nontolerant birds, and that levels of δ 9 -THC and its metabolites are as high as or higher in the blood of tolerant birds than they are in the blood of nontolerant birds. Thus, tolerance to δ 9 -THC in the pigeon does not appear to be metabolic in origin.