Distinct binding patterns of [3H]raclopride and [3H]spiperone at dopamine D2 receptors in vivo in rat brain. Implications for pet studies.

Positron emission tomography studies (PET) on dopamine (DA) D2 receptors of schizophrenics provided conflicting data, perhaps because the ligands generally used, raclopride (RAC) and spiperone (SPI), did not label the same sites. In this study, we found that the in vivo binding characteristics of [3H]RAC labeled twice as many sites in striatum and olfactory tubercle and [3H]SPI twice as many sites in pituitary. 2) The kinetic was much shorter with [3H]RAC than [3H]SPI in striatum. 3) RAC, unlike SPI, did not exhibit limbic selectivity. 4) The modulation of [3H]RAC and [3H]SPI binding by endogenous DA were diametrically opposite: D-amphetamine decreased, and reserpine + alpha-methyl-p-tyrosine increased [3H]RAC binding in striatum whereas the opposite occurred with [3H]SPI. This distinct binding pattern of [3H]RAC and [3H]SPI suggests that these two radioligands do not label the same receptor sites.

Computer-aided molecular modeling and design of DNA-inserting molecules.

Intercalators are molecules capable of sliding between base pairs without disturbing the overall stacking pattern. In addition, there may exist molecules capable of inserting into a base pair thereby disrupting the hydrogen bonds and replacing them with new hydrogen bonds. A molecule probably capable of inserting, i.e., an insertor, is the diketopiperazine cyclo-[Gly-Gly] (1). A barbiturate (2), alloxan (3), a pyrimidine derivative (4) and a hydantoin (5) were also studied as possible insertors. Furthermore, molecules such as ethyleneurea (6), succinimide (7), as well as a malonamide derivative (8) and oxamide derivatives (9-11) were studied in order to investigate the arrangement and the number of hydrogen bonds necessary for insertion. Molecules 12-14 were designed and studied for their capacity to act as bisinsertors and/or bisintercalators. These molecules feature two diketopiperazine moieties which are connected via a diphenyl(thio)ether, i.e., 12 and 13, or a bisphenol A spacer, i.e., 14. The latter molecule (14) seems a promising candidate as a bisinsertor.

Endogenous dopamine (DA) modulates [3H]spiperone binding in vivo in rat brain.

[3H]spiperone (SPI) binding in vivo, biochemical parameters and behavior were measured after modulating DA levels by various drug treatments. DA releasers and uptake inhibitors increased SPI binding in rat striatum. In other brain areas, the effects were variable, but only the pituitary remained unaffected. Surprisingly, nomifensine decreased SPI binding in frontal cortex. The effects of these drugs were monitored by measuring DA, serotonin (5-HT) and their metabolites in the same rats. The increased SPI binding in striatum was parallel to the locomotor stimulation with the following rank order: amfonelic acid greater than nomifensine greater than D-amphetamine greater than or equal to methylphenidate greater than amineptine greater than bupropion. Decreasing DA levels with reserpine or alpha-methyl-para-tyrosine reduced SPI binding by 45% in striatum only when both drugs were combined. In contrast, reserpine enhanced SPI binding in pituitary. Thus, the amount of releasable DA seems to modulate SPI binding characteristics. It is suggested that in vivo, DA receptors are submitted to dynamic regulation in response to changes in intrasynaptic concentrations of DA.

Reversal by apomorphine of the gabaculine-induced GABA accumulation in mouse cortex.

To test the assumption that in the mice cortex the rate of accumulation of gamma-aminobutyric acid (GABA) after irreversible inhibition of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19; GABA-T) represents an index of GABA turnover, we examined whether the reversal of the gabaculine-induced accumulation of GABA elicited by apomorphine was due to a decrease in GABA turnover or to a modulation of the activity of the GABA-T inhibitor. Therefore, we simultaneously measured the action of apomorphine on gabaculine-induced accumulation of GABA and on GABA-T activity. In vitro, apomorphine (3 and 30 microM) did not alter the concentration-dependent inhibition of GABA-T by gabaculine. Ex vivo, apomorphine (2 x 0.5 mg/kg s.c.) markedly decreased (69%) gabaculine-induced (150 mg/kg i.p.) accumulation of GABA. This drug had no direct effect on GABA-T activity, but significantly reduced from 83 to 71% the inhibition of GABA-T by gabaculine. The linear correlation found between GABA levels and GABA-T activity allowed the quantification of the decrease in GABA turnover elicited by apomorphine. The results showed that apomorphine decreased significantly (P less than 0.001) the rate of GABA synthesis from 7.48 to 3.36 micromol GABA/g per h, if the partial reversal of gabaculine-induced inhibition of GABA-T is considered and 2.44 micromol/g per h if not. Apomorphine effect on GABA accumulation is mainly due to a decrease of the rate of GABA synthesis and to a lesser extent to a reversal of the inhibitory activity of gabaculine. Thus, inhibition of GABA-T by gabaculine is a sensitive and reliable method for the estimation of the rate of synthesis.