Antagonism of presynaptic dopamine receptors by phenothiazine drug metabolites

Diminished catalepsy and dopamine metabolism distinguish aripiprazole from haloperidol or risperidone

Catalepsy and changes in striatal and limbic dopamine metabolism were investigated in mice after oral administration of aripiprazole, haloperidol, and risperidone. Catalepsy duration decreased with chronic (21 day) aripiprazole compared with acute (single dose) treatment across a wide dose range, whereas catalepsy duration persisted with chronic haloperidol treatment. At the time of maximal catalepsy, acute aripiprazole did not alter neostriatal dopamine metabolite/dopamine ratios or homovanillic acid (HVA) levels, and produced small increases in dihydroxyphenylacetic acid (DOPAC). Effects were similar in the olfactory tubercle. Dopamine metabolism was essentially unchanged in both regions after chronic aripiprazole. Acute treatments with haloperidol or risperidone elevated DOPAC, HVA, and metabolite/dopamine ratios in both brain areas and these remained elevated with chronic treatment. The subtle effects of aripiprazole on striatal and limbic dopamine metabolism, and the decrease in catalepsy with chronic administration, illustrate fundamental differences in dopamine neurochemical actions and behavioral sequelae of aripiprazole compared to haloperidol or risperidone.

8-OH DPAT can restore the locomotor stimulant effects of cocaine blocked by haloperidol

In the first experiment, separate groups of rats (n = 7) were treated with either saline, cocaine (10 mg/kg), haloperidol (0.1 mg/kg), or cocaine (10 mg/kg) plus haloperidol (0.1 mg/kg). Locomotor behavior was measured in an open-field environment, and cocaine induced a reliable locomotor stimulant effect compared to saline-treated animals. Haloperidol produced a progressive decline in locomotion over the 5 test days. Haloperidol also blocked cocaine stimulant effects compared to cocaine-treated animals. In the second experiment, five groups (n = 7) of animals were treated either with saline, cocaine (10 mg/kg), 8-OH DPAT (0.2 mg/kg), 8-OH DPAT (0.2 mg/kg) plus haloperidol (0.1 mg/kg), or 8-OH DPAT (0.2 mg/kg) plus haloperidol 0.1 mg/kg plus cocaine (10 mg/kg). Over the course of 5 days of treatment, cocaine induced a locomotor stimulant effect. Saline and 8-OH DPAT animals did not differ in terms of locomotion. The 0.1 mg/kg haloperidol plus 0.2 mg/kg 8-OH DPAT treatment decreased locomotion compared to the saline group, but the group given 0.2 mg/kg 8-OH DPAT plus 0.1 mg/kg haloperidol plus cocaine (10 mg/kg) exhibited a locomotor stimulant effect equivalent to the cocaine group. In a third experiment, it was found that the 0.2 mg/kg 8-OH DPAT treatment did not enhance the locomotor stimulant effect of cocaine. Thus, the 8-OH DPAT treatment was able to restore a cocaine locomotor stimulant effect in animals treated with haloperidol without directly enhancing the locomotor stimulant effects of cocaine. In Experiments 2 and 3, entries into the central zone of the open field were measured. Cocaine reliably increased central zone entries. The 8-OH DPAT treatment, however, selectively blocked this behavioral effect of cocaine suggesting a qualitative influence of 5-HT(1A) receptors upon cocaine, independent of locomotion activation by cocaine. Ex vivo measurements of dopamine and 5-hydroxytryptamine metabolism in limbic tissue were consistent with the established effects of cocaine, haloperidol, and 8-OH DPAT upon dopamine and 5-hydroxytryptamine neurotransmission. In addition, measurement of cocaine brain concentration indicated that neither haloperidol or 8-OH DPAT affected cocaine concentration in brain.

Different effects of chronic administration of haloperidol and pimozide on dopamine metabolism in the rat brain

We investigated the differences between the action of haloperidol and pimozide on dopamine metabolism and on catalepsy in periods up to 6 weeks after cessation of chronic administration of the neuroleptics to male Wistar rats. Dopamine and its metabolites (dihydroxyphenylacetic and homovanillic acids) were measured, using high-performance liquid chromatography (HPLC), in the frontal cortex, nucleus accumbens, and striatum. Both neuroleptics produced similar effects after a single dose: catalepsy and an increase of dopamine metabolism in the brain structures. However, haloperidol and pimozide differed after chronic treatment. In haloperidol-treated rats hypersensitivity of the dopaminergic system developed at the end of 2 weeks' administration, as evidenced by depression of dopamine metabolism. The biochemical changes were accompanied by behavioral hyperactivity that lasted up to 3 weeks. Dopamine metabolism in rats treated with pimozide was normal from 24 h after the end of the treatment, while catalepsy was maintained at the high level for up to 8 days and was observable up to 3 weeks after the last dose. Our results suggest that in contrast to haloperidol, pimozide is not able to produce adaptive changes leading to supersensitivity of the dopaminergic system. This may be the consequence of its potent Ca2+ channel blocking action.

Metabolism of levomepromazine in man

The phenothiazine drug, levomepromazine (LM), is used in the treatment of psychiatric disorders and as an analgesic. A single 50 or 100 mg dose of LM was given to healthy male volunteers, and urine samples were collected for 24 h. The urine was treated with beta-glucuronidase, purified by solid-phase extraction, and analyzed on a GC-MS system for identification of LM metabolites. Mass spectra suggesting 14 different LM metabolites were obtained from the samples. Our of these, 13 spectra could be ascribed to specific metabolites, 5 of which have not previously been identified. All these 5 metabolites were hydroxylated at the phenothiazine nucleus. Although the applied method did not determine the positions of hydroxyl groups on phenothiazine nuclei. 3 of the 5 metabolites were identified as O-desmethyl 3-hydroxy LM, O-desmethyl 7-hydroxy LM, and N,O-didesmethyl 7-hydroxy LM, based on their chromatographic properties. In addition two metabolites, one being hydroxylated on the phenothiazine nucleus, and one being O-demethylated and hydroxylated on the nucleus, were found. It is suggested that these were 8-hydroxy LM and O-desmethyl 8-hydroxy LM. The concentrations of 3-hydroxy LM (free+conjugated) appeared to be much higher than the concentrations of any other metabolite in the samples.