Antinociceptive, subjective and behavioral effects of smoked marijuana in humans
Introduction
Cannabinoid receptor agonists generally produce antinociception in animals; however, the test compound, assay, species, dose and route of administration used can modulate this effect (Harris, 1971, Dewey, 1986, Adams and Martin, 1996, Martin and Lichtman, 1998). Neuropharmacological studies, most using the tail-flick assay, have begun to characterize the mechanisms of cannabinoid antinociception. Two cannabinoid receptor subtypes have been identified; CB1 receptors (nervous system) account for most of marijuana’s effects, whereas CB2 receptors (spleen) mediate immune response (Adams and Martin, 1996). The primary brain site of tetrahydrocannabinol (THC) antinociceptive action is at CB1 receptors in the periacqueductal gray (Lichtman et al., 1996), also a principal site of opioid antinociception (Basbaum and Fields, 1984). Cannabinoid antinociception in this region is blocked by the CB1 antagonist SR-141716A (Lichtman and Martin, 1997, Welch et al., 1998) but not systemically administered opioid antagonists (Welch et al., 1995, Vivian et al., 1998), indicating a centrally mediated, opioid-independent mode of action. Recent studies also implicate brainstem circuitry that mediates THC, but not morphine, analgesia (Martin et al., 1998, Meng et al., 1999).
Early studies using opioid antagonists, at high doses that presumably blocked mu, kappa and delta receptors, demonstrated partial attenuation of THC-induced antinociception (Wilson and May, 1975, Tulunay et al., 1981, Ferri et al., 1986). These results suggested that endogenous opiates might partly mediate this effect. Subsequently, a second site of cannabinoid antinociception was identified in the spinal cord (Yaksh, 1981, Lichtman and Martin, 1991, Smith and Martin, 1992), where this effect is blocked by i.t. administration of kappa-opioid antagonists (Welch, 1993a, Welch, 1993b, Smith et al., 1994, Reche et al., 1996) but only partially attenuated by SR-141716A (Welch et al., 1995).
Effects of THC on clinical pain have been infrequently studied in humans, with mixed results (Noyes et al., 1975, Noyes et al., 1976, Raft et al., 1977). Analgesic effects of THC are usually overshadowed by side-effects, e.g. sedation. Cannabinoid effects on human pain sensitivity have been studied in double-blind, placebo-controlled laboratory situations, again with conflicting findings. Single doses of oral THC did not increase pain threshold using cold pressor (25 mg; Karniol et al., 1975) or electrocutaneous stimuli (12 mg; Hill et al., 1974), whereas i.v. THC (0.022 and 0.044 mg/kg) increased pain detection but not tolerance threshold using pressure and electrocutaneous stimulation (Raft et al., 1977). Acute marijuana (cigarettes with ≈1.0% THC) versus placebo smoking did not affect radiant heat (thermode on the forearm) sensitivity in either cannabis-experienced or non-experienced volunteers (Milstein et al., 1974). These negative results may be due to the use of a single low THC dose, smoking procedures that were less well-controlled, a response criterion that was closer to a sensory than a pain threshold, and short mean reaction times (i.e. possibly indicating a ceiling effect). In contrast, Milstein et al. (1975) found significant increases in pain tolerance following marijuana smoking, with slightly (but not significantly) greater effects for cannabis-experienced than non-experienced volunteers. Clark et al. (1981) found a hyperalgesic effect of chronic daily marijuana smoking, relative to a presmoking wash-out period, during a long-term residential study.
Contributing to this uncertain body of literature is the fact that procedures for measuring human pain sensitivity are often unreliable. Lee and Stitzer (1995) developed a procedure in humans that is comparable to the tail-flick assay. This procedure, which requires the participant to withdraw the finger from a radiant heat source under pain threshold instructions, was shown to have better reliability (within- and between-sessions) than an electrocutaneous method. This is also important because THC actually lowered pain threshold (produced hyperalgesia rather than antinociception) in healthy volunteers using electrocutaneous stimulation (Hill et al., 1974). Thermal stimulation has been the predominant laboratory model for evaluating cannabinoid antinociception in animals; we therefore used this method to probe pain sensitivity in marijuana-smoking human volunteers.
This study had three aims. The first aim was to determine whether marijuana smoking produced dose-dependent thermal antinociception in humans, similar to animals. These data were compared with biological, subjective effects, objective signs and performance measures for a complete profile. Psychomotor measures were used to evaluate whether marijuana produced motor deficits versus an inhibition of pain transmission. The second aim was to extend the method of Chait et al. (1988), in which participants smoked 0, 2, 4 and 8 cumulative active (and placebo) puffs across four smoking bouts from cigarettes with 1.4% THC. In the present study, participants smoked a wider range of doses — 0, 3, 9 and 18 cumulative active (and placebo) puffs from cigarettes with 3.55% THC across four bouts. In the Chait et al. study, smoking occurred at 20-min intervals; in the present study, bouts were scheduled every 40-min to collect a broader range of data (see first aim). The third aim was to assess whether endogenous opiates influence marijuana effects in humans, i.e. whether naltrexone pretreatment produced rightward shifts in marijuana dose-response curves. Based on evidence that kappa-opioid antagonists attenuate THC-induced spinal antinociception in animals, we tested high doses of naltrexone to block a higher fraction of kappa (and other opioid) receptors. Naltrexone was used because there are no kappa-selective antagonists presently available for human use, and it has a long duration of action (Verebey et al., 1976) to span the cumulative dosing period.
Section snippets
Participants and screening
The local Institutional Review Board approved this study. All volunteers provided informed consent prior to participation. Male and female recreational drug users aged 18–45 years old were recruited by newspaper advertisements and paid for participation. Before the study, participants underwent a complete medical examination (blood chemistry, electrocardiogram, medical history, urine samples for drug analysis and pregnancy testing, physical examination) and were interviewed about their current
Participant characteristics
A total of 13 participants (nine male, four female) aged 19–27 enrolled in the study and participated in at least the first experimental session. A total of eight (four male, four female) were terminated after the first session. All four females disliked the strength of the marijuana effect. Among the four males, one could not tolerate the blood withdrawal procedure, one coughed excessively which disrupted the controlled smoking procedure, one disliked the strength of the marijuana effect, and
Discussion
The goal of this study was to determine whether smoked marijuana produces dose-related antinociception in humans. The procedure used to test pain sensitivity (finger withdrawal from radiant heat) parallels the tail-flick assay often used in animals to demonstrate the antinociceptive effects of naturally-occurring and synthetic cannabinoid receptor ligands. A cumulative marijuana smoking procedure, extended from work by Chait et al. (1988), was used to evaluate this hypothesis and to efficiently
Acknowledgements
This research was supported by US Public Health Service Research Grant DA-05880 and Research Training Grant DA-07209 from the National Institute on Drug Abuse. The authors are grateful to David Ginn for medical screening, Rolley E. Johnson for drug preparation, John Yingling for technical assistance, Paula Pakulski for data collection, Valerie Schindler of RTI for analyses of plasma THC, and Mike DiMarino for consultation in statistical analysis.
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