Characterization of the discriminative stimulus properties of centrally administered (-)-DOM and LSD.

Despite the plausible assumption that the effects of hallucinogens predominantly arise in the central nervous system, most studies of these drugs in intact subjects have been conducted following systemic administration. The objective of the present investigation was to characterize the stimulus effects of (-)2,5-dimethoxy-4-methylamphetamine ((-)-DOM) following intracerebroventricular administration. Chronic indwelling cannulae were implanted into the lateral ventricle of male Fischer 344 rats trained to discriminate systemically administered (-)-DOM or lysergic acid diethylamide (LSD) from saline. Time-course and dose-response relationships for (-)-DOM and LSD administered intracerebroventricularly were established. For both LSD and (-)-DOM, central administration did not change the pretreatment times required for the maximal stimulus effects to occur. However, the onset of the stimulus effect was more rapid following intracerebroventricular administration. Following pretreatment periods that maximize drug-appropriate responding, central administration of (-)-DOM and LSD was approximately 2.4- and 1.5-times more potent, respectively, than systemic administration. The results of this study are consistent with the assumption that the stimulus effects of (-)-DOM and LSD are centrally mediated.

The effect of chronic treatment with clozapine and haloperidol on stimulus control by DOM.

The present study investigated the effects of chronic treatment with the atypical antipsychotic, clozapine, or the typical antipsychotic, haloperidol, on the stimulus properties of 2,5-dimethoxy-4-methylamphetamine ([-]-DOM) in rats trained to discriminate [-]-DOM (0.3 mg/kg; 75 min pre-treatment time) from vehicle. As compared with control values, treatment with clozapine (25 mg/kg.d) for 7 d caused a statistically significant 57% reduction in [-]-DOM-appropriate responding. Unlike clozapine, treatment with haloperidol (1 mg/kg.d) for 7 d did not affect the stimulus properties of [-]-DOM. These findings demonstrate that a functionally significant decrease in 5-HT2A receptor-mediated activity is a unique component of the in-vivo response to chronic treatment with clozapine but not haloperidol and, therefore, might account for some of the clinical differences associated with atypical antipsychotics.

5-HT2A receptor-stimulated phosphoinositide hydrolysis in the stimulus effects of hallucinogens.

The role of 5-HT2A-mediated stimulation of phosphoinositide hydrolysis in the discriminative effects of hallucinogens was investigated in PC12 cells stably expressing the rat 5-HT2A receptor (PC12-5-HT2A cells). The hallucinogenic compounds, D-lysergic acid diethylamide (LSD), (-)2,5-dimethoxy-4-methylamphetamine (DOM), psilocybin, N,N-dimethyltryptamine (DMT), 5-methoxy-N,N-dimethyltryptamine (MDMT) and N,N-diethyltryptamine (DET), all caused a concentration-dependent increase in the generation of [3H]inositol phosphates. The nonhallucinogenic compounds, 6-fluoro-N,N-diethyltryptamine (6-F-DET), lisuride and quipazine, also displayed significant efficacy in stimulating phosphoinositide hydrolysis, while 2-bromo-lysergic acid diethylamide (BOL), which is not a hallucinogen, did not alter inositol phosphate generation. The beta-carbolines, harmaline and harmane, also did not alter phosphoinositide hydrolysis. Comparison of these results with previous drug discrimination studies indicated the apparent lack of correlation between the degree of substitution in LSD- and DOM-trained animals and efficacy in stimulating phosphoinositide hydrolysis. The present study indicates that 5-HT2A-mediated stimulation of phosphoinositide hydrolysis does not appear to be the sole critical signaling mechanism involved in the discriminative effects of hallucinogens.

Role of serotonergic 5-HT2A receptors in the psychotomimetic actions of phencyclidine.

The psychotomimetic phencyclidine (PCP) alters various behavioural responses involving the serotonergic system including potentiating the discriminative stimulus effects of the phenethylamine hallucinogen, 2,5-dimethoxy-4-methylamphetamine (DOM). The present study was undertaken to test the hypothesis that PCP directly interacts with the 5-HT2A receptor. PC12 cells, a neuronal cell line, were stably transfected with the cDNA encoding the rat 5-HT2A receptor (PC12-5-HT2A). In these cells PCP and the related compounds, ketamine and dizocilpine, did not increase [3H]inositol phosphate generation nor did they alter 5-HT-stimulated phosphoinositide hydrolysis. These compounds also did not display appreciable affinity for the 5-HT2A receptor labelled with [3H]ketanserin. The present study indicates that the behavioural responses to PCP, ketamine and dizocilpine do not involve a direct interaction of these compounds with the 5-HT2A receptor.