Serotonergic and dopaminergic distinctions in the behavioral pharmacology of (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and lysergic acid diethylamide (LSD)

Abstract

Rationale: After decades of social stigma, hallucinogens have reappeared in the clinical literature demonstrating unique benefits in medicine. The precise behavioral pharmacology of these compounds remains unclear, however.

Objectives: Two commonly studied hallucinogens, (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and lysergic acid diethylamide (LSD), were investigated both in vivo and in vitro to determine the pharmacology of their behavioral effects in an animal model.

Method: Rabbits were administered DOI or LSD and observed for head bob behavior after chronic drug treatment or after pretreatment with antagonist ligands. The receptor binding characteristics of DOI and LSD were studied in vitro in frontocortical homogenates from naïve rabbits or ex vivo in animals receiving an acute drug injection.

Results: Both DOI- and LSD-elicited head bobs required serotonin(2A) (5-HT(2A)) and dopamine(1) (D(1)) receptor activation. Serotonin(2B/2C) receptors were not implicated in these behaviors. In vitro studies demonstrated that LSD and the 5-HT(2A/2C) receptor antagonist, ritanserin, bound frontocortical 5-HT(2A) receptors in a pseudo-irreversible manner. In contrast, DOI and the 5-HT(2A/2C) receptor antagonist, ketanserin, bound reversibly. These binding properties were reflected in ex vivo binding studies. The two hallucinogens also differed in that LSD showed modest D(1) receptor binding affinity whereas DOI had negligible binding affinity at this receptor.

Conclusion: Although DOI and LSD differed in their receptor binding properties, activation of 5-HT(2A) and D(1) receptors was a common mechanism for eliciting head bob behavior. These findings implicate these two receptors in the mechanism of action of hallucinogens.

Fig. 1

Effects of chronic in vivo DOI administration on head bob and receptor density measures. Twenty-four hours after 8 daily injections of DOI (3µmol/kg) DOI- and LSD-elicited head bobs as well as frontocortical 5-HT_2A, 5-HT_2C, and D₁ receptor densities were measured. Data are represented as the mean fraction of control ± SEM. For each measure investigated, the head bob response or receptor density of the animal that received chronic DOI treatment is divided by the head bob response or receptor density of the paired animal that received chronic vehicle treatment (control). These calculations are made for each animal pair. *p<0.01, **p<0.005, ***p<0.001, t-test, significantly different from vehicle treated group, n=3 to 4

Fig. 2

Effect of antagonist pretreatment on DOI-elicited head bobs. Rabbits were pretreated with ritanserin (0.67µmol/kg) or SCH23390 (0.05µmol/kg) before administration of DOI (300nmol/kg). DOI-elicited head bobs are shown as the mean ± SEM. ***p<0.0001, post-hoc Dunnett test, significantly different from vehicle pretreated group, n=5 to 14

Fig. 3

Effect of antagonist pretreatment on LSD-elicited head bobs. a Rabbits were pretreated with ketanserin (3 or 10µmol/kg), ritanserin (0.67µmol/kg), SB206553 (1µmol/kg), or SCH23390 (0.05µmol/kg) before administration of LSD (30nmol/kg). LSD-elicited head bobs are shown as the mean ± SEM. b Analysis of the behavioral time course of LSD-elicited head bobs following pretreatment with vehicle (H₂O) or ketanserin (3µmol/kg). LSD-elicited head bobs are shown in 10 minute bins. The first 5 minutes is not included in the analysis due to the large stress response immediately after injection. **p<0.001, ***p<0.0001, post-hoc Dunnett test, significantly different from vehicle pretreated group, n=4 to 18.

Fig. 3

Effect of antagonist pretreatment on LSD-elicited head bobs. a Rabbits were pretreated with ketanserin (3 or 10µmol/kg), ritanserin (0.67µmol/kg), SB206553 (1µmol/kg), or SCH23390 (0.05µmol/kg) before administration of LSD (30nmol/kg). LSD-elicited head bobs are shown as the mean ± SEM. b Analysis of the behavioral time course of LSD-elicited head bobs following pretreatment with vehicle (H₂O) or ketanserin (3µmol/kg). LSD-elicited head bobs are shown in 10 minute bins. The first 5 minutes is not included in the analysis due to the large stress response immediately after injection. **p<0.001, ***p<0.0001, post-hoc Dunnett test, significantly different from vehicle pretreated group, n=4 to 18.

Fig. 4

Binding reversibility of 5-HT_2A receptor ligands in rabbit frontal cortex. Frontocortical embranes were pre-incubated with LSD (4nM), ritanserin (4nM), DOI (100nM), or ketanserin (4nM). Following the pre-incubation, the membranes were washed and [³H]ketnaserin binding was measured (see Methods 2.7). Specific binding is shown as mean disintegrations per minute (dpm) ± SEM. ***p<0.0001, post-hoc Dunnett test, significantly different from vehicle pre-incubated membranes, n=4

Fig. 5

Effect of acute in vivo LSD administration on frontocortical [³H]ketanserin binding. a Seventy minutes after in vivo LSD (30nmol/kg SC) or vehicle (control) injection, the frontocortical density (Bmax) of 5-HT_2A, 5-HT_2C, and D₁ receptors were measured using [³H]ketanserin, [³H]mesulergine, and [³H]SCH23390, respectively. Data are represented as the mean fraction of control ± SEM. For each receptor, the receptor density of the animal that received acute LSD treatment is divided by the receptor density of the paired animal that received acute vehicle treatment. These calculations are made for each animal pair. b Seventy minutes after in vivo LSD injection (0, 3, 30, or 300nmol/kg IV), the density of frontocortical 5-HT_2A receptors was measured using [³H]ketanserin. Receptor density is reported as Fmol/mg tissue and shown as mean ± SEM. Frontocortical membranes were prepared with either a cold (square) or warm (triangle) pre-incubation. **p<0.001, t-test, significantly different from vehicle treated group, n=4; ##p<0.001, ###p<0.0001, post-hoc Bonferroni test, significantly different from vehicle treated group, n=4

Fig. 5

Effect of acute in vivo LSD administration on frontocortical [³H]ketanserin binding. a Seventy minutes after in vivo LSD (30nmol/kg SC) or vehicle (control) injection, the frontocortical density (Bmax) of 5-HT_2A, 5-HT_2C, and D₁ receptors were measured using [³H]ketanserin, [³H]mesulergine, and [³H]SCH23390, respectively. Data are represented as the mean fraction of control ± SEM. For each receptor, the receptor density of the animal that received acute LSD treatment is divided by the receptor density of the paired animal that received acute vehicle treatment. These calculations are made for each animal pair. b Seventy minutes after in vivo LSD injection (0, 3, 30, or 300nmol/kg IV), the density of frontocortical 5-HT_2A receptors was measured using [³H]ketanserin. Receptor density is reported as Fmol/mg tissue and shown as mean ± SEM. Frontocortical membranes were prepared with either a cold (square) or warm (triangle) pre-incubation. **p<0.001, t-test, significantly different from vehicle treated group, n=4; ##p<0.001, ###p<0.0001, post-hoc Bonferroni test, significantly different from vehicle treated group, n=4

Fig. 6

Effects of acute in vivo ketanserin or ritanserin on frontocortical [³H]ketanserin binding. a Rabbits were injected with ketanserin (3µmol/kg) or vehicle (H₂O) followed 60 minutes later by LSD (30nmol/kg) or vehicle (control). Seventy minutes later, rabbits were sacrificed and frontal cortex tissue harvested for analysis of [³H]ketanserin binding. A Scatchard plot shows binding in representative animals from Veh-Veh (filled square), Ket-Veh (open square), Veh-LSD (filled triangle), and Ket-LSD (open triangle) groups. b Other rabbits were acutely injected with ritanserin (0.67µmol/kg), sacrificed 70 minutes later, and frontal cortex tissues harvested for analysis of [³H]ketanserin binding. A Scatchard plot shows binding in a representative vehicle (square) and ritanserin (triangle) animal. All membranes were warmed for 60 minutes at 37°C (see Methods 2.4). n=3 to 5

Fig. 6

Effects of acute in vivo ketanserin or ritanserin on frontocortical [³H]ketanserin binding. a Rabbits were injected with ketanserin (3µmol/kg) or vehicle (H₂O) followed 60 minutes later by LSD (30nmol/kg) or vehicle (control). Seventy minutes later, rabbits were sacrificed and frontal cortex tissue harvested for analysis of [³H]ketanserin binding. A Scatchard plot shows binding in representative animals from Veh-Veh (filled square), Ket-Veh (open square), Veh-LSD (filled triangle), and Ket-LSD (open triangle) groups. b Other rabbits were acutely injected with ritanserin (0.67µmol/kg), sacrificed 70 minutes later, and frontal cortex tissues harvested for analysis of [³H]ketanserin binding. A Scatchard plot shows binding in a representative vehicle (square) and ritanserin (triangle) animal. All membranes were warmed for 60 minutes at 37°C (see Methods 2.4). n=3 to 5