Separation of tolerance to the behavioral effects of LSD from changes in serotonin receptor binding in cats

Cross-tolerance studies of serotonin receptors involved in behavioral effects of LSD in rats

Like hallucinogenic 5-HT2 agonists, LSD (d-lysergic acid diethylamide) produces characteristic decreases in locomotor activity and investigatory behaviors of rats tested in a novel environment. Because LSD is an agonist at both 5-HT1A and 5-HT2 receptors, however, the respective influences of these different receptors in the behavioral effects of LSD remain unclear. In particular, the paucity of selective 5-HT1A antagonists has made it difficult to assess the specific contribution of 5-HT1A receptors to the effects of LSD. An alternative approach to the delineation of receptor-specific effects is the use of cross-tolerance regimens. In the present studies, rats were pretreated with saline, 8-hydroxy-2(di-n-propylamino)-tetralin (8-OH-DPAT) (0.5 mg/kg SC), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (1.0 mg/kg SC), or LSD (60 micrograms/kg SC), every 12 h for 5 or 8 days. Thirty-six hours later, rats were tested in a behavioral pattern monitor 10 min after injection of saline, 0.5 mg/kg 8-OH-DPAT, 1.0 mg/kg DOI, or 60 micrograms/kg LSD. As expected, tolerance to the decreases in locomotor activity produced by acute administrations of 8-OH-DPAT, DOI, or LSD occurred when rats were pretreated chronically with 8-OH-DPAT, DOI, or LSD, respectively. Furthermore, pretreatment with either 8-OH-DPAT or DOI produced cross-tolerance to LSD. These results support the hypothesis that the effects of LSD in this model reflect a combination of 5-HT1A and 5-HT2 effects and support the view that there is an interaction between 5-HT1A and 5-HT2 receptors.

Hallucinogenic drug research--if so, so what? Symposium summary and commentary

The conference brought a critical focus both on older findings and new strategies in hallucinogenic drug research as well as on behavioral and electrophysiologic tools that correlate with neurobehavioral and neurochemical events. A number of unsolved problems to be investigated were noted. The sensitizing effects of both mescaline and LSD on other inputs at motor nuclei or at limbic sites, and effects on afferent processes were examples of important new directions as were drug discrimination tests that can determine the relative roles of aminergic systems in drug effect. Fixed ratio operant schedules can differentiate serotonin blocking agents, detect active indole psychotomimetics and demonstrate temporal and tolerance parameters relevant to drug induced neurochemical changes. Data and critique as well as pharmacokinetics no longer support the presynaptic-disinhibition model of LSD effects, nor is LSD induced "slowed turnover" of serotonin seen as an accurate description of neurochemical changes. The distinct nerve ending biochemical changes after LSD are reviewed. Subcellular compartmental analyses require a radical revision of the picture of the ratio in the nerve ending of cytoplasmic and vesicular amine as well as factors normally regulating accessibility of amine to intraneuronal MAO.