Louis S. Harris

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.

Naloxone-precipitated jumping activity in mice following the acute administration of morphine

Abstract Jumping behavior in mice was elicited by naloxone as early as 10 min after acute administration of morphine. The jumping response was determined to be time dependent and a function of both morphine and naloxone dosage. It is suggested that jumping behavior in mice may result either from a sensitization or unmasking action by morphine to a stimulatory effect of naloxone or from an unantagonized stimulatory action of morphine.

The effect of narcotics and narcotic antagonists on the tail‐flick response in spinal mice

Irwin, Houde & others (1951) reported that the tail-flick response of rats to radiant heat has the characters of a simple reflex. The ability of spinal rats to respond to this stimulus has been confirmed and extended (Winter & Flataker, 1951 ; Bonnycastle, Cook & Ipsen, 1953). Morphine inhibited the reflex in spinal rats but a quantitative difference observed between its effect in spinal rats and intact animals indicated that a second action might exist in its capacity to increase supraspinal inhibitory mechanisms. We have used the tail-flick test in mice to elucidate the mechanism of action of morphine and the narcotic-antagonist analgesics (Harris & Pierson, 1964 ; Dewey, Harris & others, 1969 ; Harris, Dewey & others, 1969) and have found a high correlation of activity with a number of narcotic analgesics to exist between species. We have confirmed and extended the observation that cholinergic agents such as oxotremorine and physostigmine also reduced this response (Harris & others, 1969; Howes, Harris & others, 1969). In addition, we have shown that an increase in central adrenergic or 5-hydroxytryptamine tone will increase the activity of morphine in intact mice in the tail-flick test (Dewey, Harris & others, 1968). We have now attempted to increase our knowledge about this testing procedure by studying the narcotics, the narcotic antagonists, and some of the neurochemicals discussed above in spinal mice. Male albino mice of the Swiss-Webster strain (18-25 g) had transections made under ether anaesthesia. A dorsal midline incision was made and the spinal cord was exposed between the fifth and sixth thoracic vertebrae. The cord was cauterized and the wound was closed with silk sutures. Attempts to transect the cord at a higher level resulted in death from respiratory paralysis or uncontrolled bleeding. There were few deaths from the surgical procedure. Water and food were presented ad libitum. Within 3 to 4 h all mice were quite active. Simple physiological stimuli showed that the cord section was positive. Most of the mice responded to the radiant stimulus of the tail-flick apparatus within 4 s, the variability among the reaction times being less than is usually observed in normal mice. Animals not responding within 4 s were not used. The results obtained were averaged with a second reading taken 30 min later, after which the drug was given subcutaneously in the flank; readings were made 20 min later. Mice not responding within 10 s were removed from the apparatus and considered to be 100% affected. The % maximal possible inhibition was calculated using the following formula :

Effects of intravenous tetrahydrocannabinol on experimental and surgical pain; Psychological correlates of the analgesic response

Two intravenous doses of tetrahydrocannabinol (THC) (0.022 mg/kg and 0.044 mg/kg) were compared to intravenous diazepam (0.157 mg/kg) and to placebo (Ringers lactate) as premedication for dental extraction in 10 healthy volunteers. Pain detection and tolerance thresholds were measured and psychiatric interviews were supplemented by Minnesota Multiphasic Personality Inventories (MMPI), the Zung Depression Scale (ZDS), Beck Depression Inventories (BDI), and the State‐Trait Anxiety Inventory (STAI). Pain detection thresholds were altered unpredictably with high THC doses, but analgesia as indicated by pain tolerance was less than that after diazepam and placebo. In three subjects low‐dose THC (0.022 mg/kg) was a better analgesic than placebo but not diazepam. Six subjects preferred placebo to low‐dose THC as an analgesic; this group experienced increases in subjective surgical pain and were submissive, rigid, and less introspective with high State Anxiety and MMPI profiles that differed from subjects whose pain was not increased. STAI following THC presaged a poor analgesic response in this group.

The effect of 1-trans-Δ9-tetrahydrocannabinol on the hypothalamo-hypophyseal-adrenal axis of rats

Abstract 1-trans- Δ 9 -Tetrahydrocannabinol ( Δ 9 -THC) was found to be a potent stimulator of ACTH secretion, as evidenced by depletion of adrena ascorbic acid in rats. Low doses of Δ 9 -THC that were inactive in unanesthetized rats stimulated ACTH secretion in pentobarbital anesthetized rats. This observation differentiates Δ 9 -THC from other drugs reported to stimulate ACTH secretion. A nonstimulating dose of Δ 9 -THC did not block the secretion of ACTH induced by epinephrine. The concentration of adrenal ascorbic acid one hour after a fifth daily injection did not differ significantly from that observed after a single injection.

Antipyretic, analgesic and anti-inflammatory effects of Δ9-tetrahydrocannabinol in the rat

The effects on body temperature produced by graded doses of Δ9-tetrahydrocannabinol (Δ9-THC) and phenylbutazone were compared in both normal and pyretic rats. Dose related hypothermic responses were produced by the oral administration of Δ9-THC in normal animals. Moreover, Δ9-THC significantly reduced elevated temperatures in yeast-induced pyretic rats to near normal levels at doses which exhibited little hypothermic activity in normal rats. The oral antipyretic potency of Δ9-THC was approximately 2 times that of phenylbutazone. The comparative oral antinociceptive activity of Δ9-THC and selected narcotic and non-narcotic analgesics was determined by the increase in response latency to pressure applied to normal and yeast-inflamed paws. Δ9-THC administered orally was essentially inactive at dose levels below those producing pronounced central nervous system depression. The oral anti-inflammatory efficacy of Δ9-THC was compared to phenylbutazone and acetylsalicylic acid. Δ9-THC was ineffective in inhibiting carrageenin-induced edema of the rat paw following acute or chronic administration.

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.

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.