The effects of the indolehallucinogen psilocybin, a mixed 5-HT2 and 5-HT1 agonist, on regional cerebral glucose metabolism were investigated in 10 healthy volunteers with PET and [F-18]-fluorodeoxyglucose (FDG) prior to and following a 15- or 20-mg dose of psilocybin. Psychotomimetic doses of psilocybin were found to produce a global increase in cerebral metabolic rate of glucose (CMRglu) with significant and most marked increases in the frontomedial and frontolateral cortex (24.3%), anterior cingulate (24.9%), and temporomedial cortex (25.3%). Somewhat smaller increases of CMRglu were found in the basal ganglia (18.5%), and the smallest increases were found in the sensorimotor (14.7%) and occipital cortex (14.4%). The increases of CMRglu in the prefrontal cortex, anterior cingulate, temporomedial cortex, and putamen correlated positively with psychotic symptom formation, in particular with hallucinatory ego disintegration. The present data suggest that excessive 5-HT2 receptor activation results in a hyperfrontal metablic pattern that parallels comparable metabolic findings associated with acute psychotic episodes in chronic schizophrenics and contrasts with the hypofrontality in chronic schizophrenic patients.

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The modulating effects of serotonin on dopamine neurotransmission are not well understood, particularly in acute psychotic states. Positron emission tomography was used to examine the effect of psilocybin on the in vivo binding of [ 11 C]raclopride to D 2 -dopamine receptors in the striatum in healthy volunteers after placebo and a psychotomimetic dose of psilocybin (n ϭ 7). Psilocybin is a potent indoleamine hallucinogen and a mixed 5-HTPsilocybin, a potent indoleamine hallucinogen and 5-HT receptor stimulating agent, has been reported to produce a psychosis-like syndrome in man resembling the first manifestation of schizophrenia in certain respects [for review see (Fischman 1983;Gouzoulis-Mayfrank et al. 1998)]. Particularly, ego-disorders (Rüm-mele and Gnirss 1961;Vollenweider et al. 1997), affective changes (Rümmele and Gnirss 1961), loosened associations (Spitzer et al. 1996) and perceptual alterations commonly seen in psilocybin-induced psychosis are also observed in incipient acute schizophrenic stages (Bowers and Freedman 1966;Heimann, 1986;Gouzoulis et al. 1994). In support of these suggested clinical similarities, we recently found that psilocybin produced a marked prefrontal and anterior cingulate activation in normal subjects comparable to the hyperfrontal pattern observed in some (Cleghorn et al. 1989;Ebmeier et al. 1993Ebmeier et al. , 1995Catafau et al. 1994 1994;Ebmeier et al. 1995;Sabri et al. 1997) but not all (Andreasen et al. 1992;Buchsbaum et al. 1992;Liddle et al. 1992;Siegel et al. 1993) acute schizophrenic patients. A number of functional animal studies have suggested that indoleamine (psilocybin, LSD) and phenylethylamine (DOI, mescaline) hallucinogens produce their psychotomimetic effects primarily through excessive stimulation of 5-HT, and 5-HT 2A receptors in particular (McKenna et al. 1989;Pierce and Peroutka 1989;Aghajanian 1994;Sipes and Geyer 1994;Padich et al. 1996). This assumption was corroborated in a recent human study demonstrating that the psychotomimetic effects of psilocybin can be blocked dose-dependently by pretreatment with the preferential 5-HT 2A receptor antagonist ketanserin (Vollenweider et al. 1998). However, pretreatment with the D 2 -antagonist haloperidol also reduces some of the positive symptoms of psilocybin psychosis. This raises the possibility that symptoms of psilocybin are secondary responses to increased dopaminergic transmission (Vollenweider et al. 1998).Hence, a contribution of the dopamine systems to the effects of psilocybin, presumably through a serotonindopamine interaction, cannot be ruled out. Functional interactions between central serotonin (5-HT) and dopamine (DA) systems have been well documented. Electrophysiological, biochemical, and behavioral evidence suggests that the ascending serotonergic pathways from the medial and dorsal raphe modulate or control the function of the mesolimbic and mesostriatal dopamine systems (Joyce 1993;Zazpe et al. 1994;Kapur and Remington 1996). The modulating effect of serotonin on striatal dopamine release...

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Serotonin (5-HT) release is the primary pharmacological mechanism of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') action in the primate brain. Dopamine release and direct stimulation of dopamine D2 and serotonin 5-HT2A receptors also contributes to the overall action of MDMA. The role of 5-HT1A receptors in the human psychopharmacology of MDMA, however, has not yet been elucidated. In order to reveal the consequences of manipulation at the 5-HT1A receptor system on cognitive and subjective effects of MDMA, a receptor blocking study using the mixed beta-adrenoreceptor blocker/5-HT1A antagonist pindolol was performed. Using a double-blind, placebo-controlled within-subject design, 15 healthy male subjects were examined under placebo (PL), 20 mg pindolol (PIN), MDMA (1.6 mg/kg b.wt.), MDMA following pre-treatment with pindolol (PIN-MDMA). Tasks from the Cambridge Neuropsychological Test Automated Battery were used for the assessment of cognitive performance. Psychometric questionnaires were applied to measure effects of treatment on core dimensions of Altered States of Consciousness, mood and state anxiety. Compared with PL, MDMA significantly impaired sustained attention and visual-spatial memory, but did not affect executive functions. Pre-treatment with PIN did not significantly alter MDMA-induced impairment of cognitive performance and only exerted a minor modulating effect on two psychometric scales affected by MDMA treatment ('positive derealization' and 'dreaminess'). Our findings suggest that MDMA differentially affects higher cognitive functions, but does not support the hypothesis from animal studies, that some of the MDMA effects are causally mediated through action at the 5-HT1A receptor system.