Psychopharmacology (2005) 180: 427–435 DOI 10.1007/s00213-005-2183-9 ORIGINAL INVESTIGATION Danuta Marona-Lewicka . Ronald A. Thisted . David E. Nichols Distinct temporal phases in the behavioral pharmacology of LSD: dopamine D2 receptor-mediated effects in the rat and implications for psychosis Received: 13 July 2004 / Accepted: 6 January 2005 / Published online: 19 February 2005 # Springer-Verlag 2005 Abstract Rationale: The effect of LSD in humans has second temporal phase that involves dopaminergic path- been described as occurring in two temporal phases. The ways would be consistent with the widespread belief that behavioral effects in rats also occur in two temporal phases: excessive dopaminergic activity may be an underlying an initial suppression of exploration followed by increased cause of paranoid psychosis. locomotor activity. Objectives: We decided to investigate . this phenomenon from the perspective that the pharmacol- Keywords LSD Drug discrimination 5-HT2A . ogy might have relevance to the neurochemical mecha- Serotonin Dopamine D2 Schizophrenia Rat nisms underlying psychosis. Methods: Twenty-five male Sprague–Dawley rats were trained to discriminate LSD (186 nmol/kg, 0.08 mg/kg, i.p.) with a 30-min preinjection Introduction time (LSD-30, N=12) and LSD (372 nmol/kg, 0.16 mg/kg, i.p.) with a 90-min preinjection time (LSD-90, N=13) from LSD is the most potent known hallucinogenic substance, saline, using a two-lever, food-reinforced operant condi- with high affinity for a number of brain receptors (see tioning task. Results: LSD (186 or 372 nmol/kg, 0.08 or Nichols 2004 for a review). Although virtually all research 0.16 mg/kg) given 30 min prior to training produced a cue on hallucinogens, both clinical and preclinical, has focused that was completely antagonized by 5-HT2A antagonists on a unitary pharmacology for these substances, Freedman and lasted no longer than 1 h. LSD (372 nmol/kg, 0.16 mg/ (1984) has described the effects of LSD in humans as kg) injected 90 min before training produced a cue that was occurring in two temporal phases: a “psychedelic experi- not fully blocked by 5-HT2A antagonists, but instead was ence” in the early phase, with “meaningfulness and por- significantly inhibited by haloperidol. In these rats, sub- tentousness” as the prime characteristics, and a second stitution no longer occurred with the 5-HT2 agonists DOI phase that is “clearly a paranoid state.” This latter phase or LSD (30 min preinjection), but full substitution was develops about 4–6 h after LSD administration, and at obtained with the D2 agonists apomorphine, N-propyldi- times out to 10 h postdrug, subjects given LSD “...regularly hydrexidine, and quinelorane. Conclusion: The discrimi- report... they had been at the least self-centered, and usually native stimulus effect of LSD in rats occurs in two phases, suspicious, with ideas of reference or even paranoid con- and these studies provide evidence that the later temporal victions” (Freedman 1984). Freedman noted on several phase is mediated by D2 dopamine receptor stimulation. A occasions that this effect continued to be unnoticed and unstudied in research with hallucinogens. Further, he drew parallels between this later paranoid phase and amphet- amine psychosis in man and emphasized the possibility that D. Marona-Lewicka . D. E. Nichols (*) Department of Medicinal Chemistry and Molecular clues to understanding psychosis might be found in this Pharmacology, Purdue University, “paranoid” phase of LSD intoxication. Heine Pharmacy Building, 575 Stadium Mall Dr., We decided to investigate this phenomenon from the West Lafayette, IN, 47907-2091, USA perspective that the pharmacology might have relevance to e-mail: [email protected] Tel.: +1-765-4941461 the neurochemical mechanisms underlying psychosis. Fur- Fax: +1-765-4946790 thermore, we reasoned that Freedman’s observation might have relevance to the use of 5-HT2A antagonists in the R. A. Thisted treatment of some forms of psychosis because LSD and Department of Health Studies, Statistics, and Anesthesia/ Critical Care, The University of Chicago, other hallucinogens are thought to act principally as 5- 5841 S Maryland Avenue, HT2A agonists (Nichols 2004). It also seemed possible that Chicago, IL, 60637, USA the incidence of “bad trips” with LSD could be related to 428 the nature of the underlying pharmacology in the later LSD (2 h) prior to amphetamine potentiated the amphet- temporal phase of LSD intoxication. amine interoceptive cue in a drug discrimination paradigm In contrast to this observation about LSD, there is no in rats by shifting the amphetamine dose–response curve indication in the literature of a parallel in the psychophar- to the left (Marona-Lewicka and Nichols 1997). Those re- macology of hallucinogenic phenethylamines or simple sults suggested that the enhanced behavioral response to tryptamines. That is, reports of intoxication with these other amphetamine might be due either to an increased sensitivity agents appear to describe a unitary psychopharmacology of dopaminergic neurons or to an enhanced release of with typical dose–response curves (Shulgin and Shulgin dopamine by amphetamine (Marona-Lewicka and Nichols 1991, 1997). We therefore speculated that the two temporal 1997). intoxication “phases” of LSD were not related simply to 5- Nevertheless, the acute (15–30 min prior to testing) HT2 agonist activity. These observations led us to hypo- discriminative cue of LSD is clearly mediated by activation thesize that a “two-phase” temporal action was unique to of the 5-HT2A receptor (Colpaert et al. 1982; Colpaert and LSD and was related to some pharmacological property of Janssen 1983; Glennon et al. 1983, 1984; Glennon 1999; LSD that distinguished it from other chemical classes of Ismaiel et al. 1993; Leysen et al. 1982; Schreiber et al. 1994; hallucinogens. Winter 1994; Winter and Rabin 1988; Winter et al. 1999). A significant pharmacological difference between LSD Furthermore, time–response curves in rats given LSD and other types of hallucinogens is the direct dopaminergic 30 min prior to training showed a maximum of the 5-HT2A- effect of LSD, which is not a component of the pharma- mediated discriminative cue at about 30 min, with a loss cology of phenethylamine- and tryptamine-type hallucino- of the cue by about 1 h. We reasoned, therefore, that if gens. Dopaminergic effects of LSD have been recognized rats discriminated an LSD cue at longer pretreatment for many years (see Watts et al. 1995 and references there- times, it would likely be mediated by other pharmacolo- in), but the relevance of these actions to the psychophar- gical processes. In particular, if LSD sensitized the do- macology of LSD has never been fully appreciated. In view pamine system, as we had shown in our amphetamine of the recognized role of dopamine in psychosis (Seeman studies (Marona-Lewicka and Nichols 1997), rats might 1987; Bennett 1998; Carlsson 2001), we felt that a closer also be more sensitive to the dopaminergic effects of LSD focus in this area might be fruitful. Early drug discrimi- at later time periods. nation studies did note that LSD substituted partially (70– In the present study, a drug discrimination procedure in 80%) for the nonselective dopamine agonist apomorphine, rats was used to evaluate the discriminative stimulus effect and that apomorphine was able to generate 50% substitu- of LSD administered 90 min prior to testing, at a time when tion in LSD-trained rats (Holohean et al. 1982). These the 5-HT2A receptor-mediated cue was no longer signif- authors concluded that although serotonin plays a promi- icant. These rats were compared with rats trained using our nent role in the discriminative stimulus effects of LSD, standard protocol where LSD is administered 30 min prior dopamine receptor activation is a secondary factor in the to training. LSD cue. Holohean et al. (1982) further suggested that the dopaminergic effect of LSD might become evident in drug discrimination only when rats were trained to “attend” to Material and methods dopamine receptor activation. There has never been any evidence from drug discrimination experiments, however, Animals that the nature of the LSD discriminative cue might be time-dependent. Twenty-five male Sprague–Dawley rats (Harlan Labora- Evidence from animal studies of time-dependent behav- tories, Indianapolis, IN) weighing 180–200 g at the begin- ioral pharmacology for LSD comes from the studies of ning of the study were used as subjects. Rats were divided Mittman and Geyer (1991), who showed that LSD-induced into two groups and trained to discriminate LSD (186 nmol/ behavioral effects in rats occurred in two temporal phases: kg, 0.08 mg/kg, i.p.) with a 30-min preinjection time (LSD- an initial suppression of behavioral responding, followed by 30, N=12) and LSD (372 nmol/kg, 0.16 mg/kg, i.p.) with a a subsequent increase in locomotor activity that was not 90-min preinjection time (LSD-90, N=13) from saline, observed with other serotonergic agonists. Using pretreat- using a two-lever, food-reinforced operant conditioning ment with ritanserin and/or propanolol, these workers con- task. None of the rats had previously received drugs or cluded that the two temporal phases of LSD effects in rats behavioral training. Water was freely available in the in- were mediated by different serotonergic or β-adrenergic dividual home cages, and a rationed amount of supplemen- receptors. Due to the limited number of pretreatments that tal feed (LabDiet-5001, PMI, Nutrition International, LLC, were employed in that study, and the possibility of dopa- Brentwood, MO) was made available after experimental mine–serotonin functional interactions, it is not possible to sessions so as to maintain ∼80% of free-feeding weight. deduce clearly a mechanistic basis for the effects of LSD Lights were on from 0700 to 1900 h. The laboratory and observed in that study.