THC-Tolerant Pigeons Showed Selective Cross-Tolerance but Broader Changes in Drug Potentiation
A 1980 pigeon experiment distinguished cross-tolerance to another drug from attenuation of THC's potentiating effect. Those were different comparisons and did not produce the same pattern across pentobarbital, amphetamine and morphine.
THC could modify another drug without changing baseline rate
An acute THC dose that did not itself affect response rate potentiated the rate-decreasing effects of the three comparator drugs. Lack of an overt THC-alone rate effect therefore did not establish absence of an interaction.
Repeated exposure changed the pattern
THC-tolerant birds showed cross-tolerance to pentobarbital but not to amphetamine or morphine. Yet THC tolerance attenuated potentiation of the effects of all three drugs. A statement about one comparison cannot substitute for the result of the other.
Plasma measurements supplied a mechanistic clue
The authors interpreted pentobarbital plasma findings as suggesting a dispositional contribution to that cross-tolerance. This was not evidence that every interaction in the experiment had the same metabolic explanation. The behavior was also specific to reinforcement schedules and tested doses.
Interpreting the evidence
An interaction assessment should distinguish a drug's own effect, the second drug's effect and the combination after a defined exposure history. This bird study does not establish human medication adjustments or protection from impairment in tolerant cannabis users.
Related reading: interpreting cannabis tolerance claims.
Research reference
McMillan DE; Leander JD; Dudley KH (1980). Effects of drugs on behavior in pigeons tolerant to delta 9-tetrahydrocannabinol. The Journal of pharmacology and experimental therapeutics. PubMed research record.
This analysis draws on the supplied abstract.
