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Marona-Lewicka D, Thisted RA, Nichols DE. 
“Distinct temporal phases in the behavioral pharmacology of LSD: dopamine D2 receptor-mediated effects in the rat and implications for psychosis”. 
Psychopharmacology (Berl). 2005 Jul;180(3):427-35.

RATIONALE: The effect of LSD in humans has been described as occurring in two temporal phases. The behavioral effects in rats also occur in two temporal phases: an initial suppression of exploration followed by increased locomotor activity.
OBJECTIVE: We decided to investigate this phenomenon from the perspective that the pharmacology might have relevance to the neurochemical mechanisms 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 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 saline, using a two-lever, food-reinforced operant conditioning task.

RESULTS: LSD (186 or 372 nmol/kg, 0.08 or 0.16 mg/kg) given 30 min prior to training produced a cue that was completely antagonized by 5-HT(2A) antagonists and lasted no longer than 1 h. LSD (372 nmol/kg, 0.16 mg/kg) injected 90 min before training produced a cue that was not fully blocked by 5-HT(2A) antagonists, but instead was significantly inhibited by haloperidol. In these rats, substitution no longer occurred with the 5-HT(2) agonists DOI or LSD (30 min preinjection), but full substitution was obtained with the D(2) agonists apomorphine, N-propyldihydrexidine, and quinelorane. CONCLUSION: The discriminative stimulus effect of LSD in rats occurs in two phases, and these studies provide evidence that the later temporal phase is mediated by D(2) dopamine receptor stimulation. A second temporal phase that involves dopaminergic pathways would be consistent with the widespread belief that excessive dopaminergic activity may be an underlying cause of paranoid psychosis.
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