Quetiapine and its metabolite norquetiapine: translation from in vitro pharmacology to in vivo efficacy in rodent models.

BACKGROUND AND PURPOSE: Quetiapine has a range of clinical activity distinct from other atypical antipsychotic drugs, demonstrating efficacy as monotherapy in bipolar depression, major depressive disorder and generalized anxiety disorder. The neuropharmacological mechanisms underlying this clinical profile are not completely understood; however, the major active metabolite, norquetiapine, has been shown to have a distinct in vitro pharmacological profile consistent with a broad therapeutic range and may contribute to the clinical profile of quetiapine. EXPERIMENTAL APPROACH: We evaluated quetiapine and norquetiapine, using in vitro binding and functional assays of targets known to be associated with antidepressant and anxiolytic drug actions and compared these activities with a representative range of established antipsychotics and antidepressants. To determine how the in vitro pharmacological properties translate into in vivo activity, we used preclinical animal models with translational relevance to established antidepressant-like and anxiolytic-like drug action. KEY RESULTS: Norquetiapine had equivalent activity to established antidepressants at the noradrenaline transporter (NET), while quetiapine was inactive. Norquetiapine was active in the mouse forced swimming and rat learned helplessness tests. In in vivo receptor occupancy studies, norquetiapine had significant occupancy at NET at behaviourally relevant doses. Both quetiapine and norquetiapine were agonists at 5-HT1A receptors, and the anxiolytic-like activity of norquetiapine in rat punished responding was blocked by the 5-HT1A antagonist, WAY100635. CONCLUSIONS AND IMPLICATIONS: Quetiapine and norquetiapine have multiple in vitro pharmacological actions, and results from preclinical studies suggest that activity at NET and 5-HT1A receptors contributes to the antidepressant and anxiolytic effects in patients treated with quetiapine.

Low affinity use-dependent NMDA receptor antagonists show promise for clinical development.

The success of the low affinity use-dependent NMDA receptor antagonists to reach clinical trials can be readily attributed to their wider margins of safety and lack of neurotoxicity at higher doses. Several mechanistic differences distinguish the low affinity from the high affinity use-dependent antagonists: 1) Differential regional affinities for the various NMDA receptor subtypes; 2) The static receptor blockade due to the faster on/off rate receptor kinetics which limit, but do not totally prevent the amount of Ca+2 entry into the cell during glutamate-induced depolarization; and 3) Rapid egress of the compounds from the ion channel during recovery resulting in less membrane trapping between transmission pulses. Advanced clinical trials are in progress for the following indications: epilepsy, stroke, head trauma, tardive dyskinesia, pain plus Parkinson's, Huntington's and Alzheimer's diseases.

Long-term functional end points following middle cerebral artery occlusion in the rat.

The purpose of the present study was to assess the magnitude and stability of a number of functional deficits in rats subjected to occlusion of the middle cerebral artery (MCAO). Three groups of rats, treated with 90-min, 120-min, or sham occlusion were used in functional studies for 22 weeks following surgery. The following tests were used: methamphetamine-induced rotation, the staircase test, acquisition of operant responding, running-wheel behavior, and performance of operant differential reinforcement of a low-rate responding (DRL) schedule of reinforcement. Histology performed at 23 weeks following infarct showed on average modest damage of a 19% reduction in hemispheric volume. Of the behavioral tests conducted, rotation, the staircase test, and the operant DRL were sensitive to ischemic damage, and were under some circumstances related to lesion size. These data show that long-term functional deficits following MCAO are demonstrable, and hence, assessment of long-term neuroprotection is feasible.

Time course of postnatal lead-induced changes in dopamine receptors and their relationship to changes in dopamine sensitivity.

Although alterations in dopaminergic function represent a potential neurochemical basis of Pb-induced behavioral deficits, the impact of postnatal Pb exposure on DA systems has not been adequately delineated. This study examined the effects of postnatal Pb exposure, across a broad range of concentrations, on the ontogeny of both D1 and D2 DA (dopamine) receptors in striatum and nucleus accumbens. Rat pups were exposed to Pb from 0-21 days of age via lactating dams consuming solutions of 0, 100, 350, 1000 or 2000 ppm Pb acetate. Pups were sacrificed for homogenate receptor binding assays at 7, 14, 21 or 60 days of age. Postnatal Pb exposure generally facilitated DA receptor number (Bmax) development over the first 21 days of age, in both striatum and nucleus accumbens, without any apparent effects on receptor affinity (Kd values). Residual changes in Bmax were found for both D1 and D2 receptors at 60 days of age, with Bmax changes occurring in opposite directions in the two brain regions. D1 Bmax values were increased in striatum, and decreased in nucleus accumbens at PbB (blood lead levels) of > 50 ug/dl, whereas, at PbBs of 10-20 ug/dl, but not higher, D2 Bmax values were decreased in striatum but increased in nucleus accumbens. These findings suggest a preferential vulnerability of D2 receptors to lower Pb exposure concentrations and underscore the importance of Pb exposure level and brain region to resulting receptor changes. A linear relationship was observed between changes in nucleus accumbens D2 receptor Bmax values and Pb-induced changes in D2 sensitivity as derived from a drug discrimination study using littermates of offspring from the current study (Cory-Slechta et al., 1992), suggesting nucleus accumbens as a preferential site of Pb-induced D2-mediated effects.

Homogeneity of regional brain lead concentrations.

It has been proposed that the neurobiological basis of many of the behavioral manifestations arising in response to Pb exposure may be due to selective vulnerability of particular brain regions, such as hippocampus, a region in which preferential accumulation has been reported in some studies. However, these findings have not been invariant and, in fact, have been found to vary with dosing parameters. This study examined potential regional accumulation of Pb in brain following postnatal exposure of rats to Pb via nursing dams consuming Pb acetate solutions of 0, 100, 350, 1000, or 2000 ppm. Offspring were sacrificed at 7, 14, 21, 40 or 60 days of age and brains dissected into 12 regions, including striatum, nucleus accumbens, hypothalamus, thalamus, ventral (tegmentum) and dorsal (tectum) midbrain, hippocampus, frontal, parietal/temporal, and occipital cortices, cerebellum and brain stem for regional Pb analyses. All brain regions generally accumulated Pb in a similar pattern. Regional elimination half-lives, determined for the 350, 1000 and 2000 ppm exposure groups, averaged about 20 days and did not vary between regions. Regional brain Pb levels were not different whether brains were perfused or not prior to brain Pb determinations. However, regional differences in brain Pb levels could be introduced by using dry, rather than wet weights in the calculation of Pb concentrations, and by procedures including soaking in formaldehyde prior to drying, probably as a result of the differential fluid volume of different brain regions. Taken together, these data do not support previous findings of selective regional accumulation of Pb in any brain region, including the hippocampus, at least under conditions of postnatal Pb exposure and wet weight measures. While there may indeed be differences among brain regions in sensitivity to Pb, any differences in response among brain regions should probably be ascribed to differences in the interactions of Pb with biochemical or cellular targets unique or enriched in that region rather than to differences in accumulation of Pb.

Apparent mediation of the stimulus properties of a low dose of quinpirole by dopaminergic autoreceptors.

This study examined the hypothesis that, if the stimulus properties of the D2 agonist quinpirole (QUIN) were mediated by autoreceptors, then pharmacological treatments engendering a decline in dopamine (DA) release and consequent decrease in postsynaptic DA receptor stimulation should result in QUIN-appropriate responding; those that activate postsynaptic receptors should result in saline-appropriate responding. In rats trained to discriminate 0.05 mg/kg of QUIN from saline using standard operant drug discrimination procedures, QUIN (up to the training dose), two other putative D2 autoreceptor agonists (low-dose apomorphine and N-propylnorapomorphine), the DA depeleter alpha-methyl-p-tyrosine (AMPT) and the D1 antagonist SCH 23390 all produced primarily QUIN-lever responding. Moreover, coadministration of alpha-methyl-p-tyrosine with QUIN potentiated QUIN-stimulus properties. Higher doses of apomorphine, known to stimulate postsynaptic D1 and D2 receptors and pretreatment with the D2 antagonist haloperidol decreased QUIN-lever responding. Neither the D1 agonist SKF38393, the indirect D1/D2 agonist d-amphetamine, the D2 antagonist haloperidol or coadministration of SCH 23390 and d-amphetamine substituted for QUIN. Coadministration of either SKF 38393 or d-amphetamine with QUIN decreased levels of QUIN responding. Taken together, the data are consistent with the contention that the stimulus properties of 0.05 mg/kg of QUIN are primarily mediated by D2-type presynaptic autoreceptors and that these stimulus properties involve a decline in DA release and consequent decreased postsynaptic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional alterations in dopamine systems assessed using drug discrimination procedures.

Numerous studies have suggested that Pb-induced perturbations of dopamine (DA) systems and DA functions could underlie the behavioral impairments attributed to Pb exposure. However, the precise nature of the effects of Pb on DA systems, either at the receptor/biochemical level or at the behavioral level have never been precisely delineated, much less interrelated. Substantial advances in the understanding of DA neuropharmacology provide new opportunities to more precisely elaborate Pb-induced changes in DA systems and DA function. Recently completed studies from our laboratories using drug discrimination procedures indicate that low-level postweaning Pb exposure produces a functional DA supersensitivity manifest at both the D1 and D2 receptor subtypes. Postnatal Pb exposure likewise induces functional DA supersensitivity, although this effect appeared to be restricted to the D2 receptor subtype. In conjunction with the drug discrimination studies, standard homogenate receptor binding assays of D1 and D2 receptors carried out in 5 different brain regions (striatum, nucleus accumbens, frontal cortex, midbrain and cerebellum) suggested a correspondence between D2 DA behavioral supersensitivity and Bmax changes found in nucleus accumbens, suggesting it as a possible site for Pb-induced supersensitivity to DA agonist stimulus properties. Collectively, the nature of the changes in DA sensitivity in the drug discrimination studies and the changes in D1 and D2 receptor number raise the possibility that Pb could, in part, produce a net functional autoreceptor agonism.

Postnatal lead exposure induces supersensitivity to the stimulus properties of a D2-D3 agonist.

To examine the impact of lead (Pb) exposure during the ontogeny of dopaminergic (DA) systems on resultant DA function, rats were exposed postnatally (0-21 days of age) via the lactating dam to 0, 100 or 350 ppm Pb acetate in drinking water. At 2 months of age, the postnatally Pb-exposed rats were trained to discriminate the stimulus properties of either the D1 receptor agonist SKF38393 (6.0 mg/kg) or the D2-D3 receptor family subtype agonist quinpirole (0.05 mg/kg) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. In each training group, dose-effect curves describing drug lever responding to lower doses of the training drug and to preadministration of selective DA antagonists were obtained to examine Pb-induced changes in DA sensitivity, and doses of non-DA compounds were substituted to determine the specificity of any changes in DA sensitivity. In the D1/saline training condition, Pb exposure was not associated with any specific or consistent changes in DA sensitivity. In contrast, exposure to Pb was associated with D2-D3 receptor subtype supersensitivity as was indicated by significantly elevated levels of drug lever responding in the presence of quinpirole and haloperidol and to at least one dose of apomorphine. No differences in the dose-effect curves for either (+)-amphetamine or NMDA were observed in the D2-D3-trained control and Pb-exposed groups, but an increase in drug lever responding in the presence of pentobarbital was noted in the Pb-exposed group relative to control. Taken together, these findings are consistent with a Pb-induced functional D2-D3 supersensitivity possibly mediated via autoreceptors. Moreover, this functional D2-D3 supersensitivity necessarily represents a permanent effect of postnatal Pb exposure since both blood and brain Pb levels were negligible at the time drug discrimination training began.

Lead retards development of Drosophila melanogaster.

1. Lead (Pb) is a ubiquitous environmental toxicant which has been reported to have growth-retarding effects. That premise was examined in the current study of the effects of developmental exposure to Pb on the maturation of the fruitfly Drosophila melanogaster. 2. Flies were raised from egg to adulthood in media to which 0, 100, 250, or 500 ppm Pb acetate added. 3. There was a dose-dependent delay of maturation but no apparent effect on survival to adulthood. There were no significant differences in this effect between male and female flies. 4. Weights of fly offspring and their fecundity were not related to increasing exposure levels, suggesting that the delays were not due to gross nutritional deficits. 5. Analyses of Pb content indicated exposure-dependent body burdens of Pb in flies. Timed analyses of the Pb content of media itself indicated a heterogeneous distribution of Pb in the media, suggesting some precipitation of Pb at the highest exposure, occurring primarily during the first hour. 6. The mechanistic bases of the Pb-induced retardation of D. melanogaster development remain unknown, but it is concluded that because of the extensive body of knowledge on D. melanogaster genetics, molecular biology, and developmental biology, this procedure could serve as a model system for further study of the developmental consequences of exposure to Pb or other toxicants. 7. Environmental Pb exposure resulting in retarded development could have deleterious repercussions for insect populations exposed chronically to high levels of Pb.

Low level lead exposure increases sensitivity to the stimulus properties of dopamine D1 and D2 agonists.

To examine the impact of Pb exposure on dopaminergic (DA) function, weanling rats were chronically exposed to 0, 50 or 250 ppm Pb acetate in drinking water. At 3 months of age, the rats were trained to discriminate the stimulus properties of either the D1 agonist SKF38393 (3.0 mg/kg i.p.; D1/sal) or the D2 agonist quinpirole (0.05 mg/kg i.p., D2/sal) from saline using a standard two-lever operant food-reinforced drug discrimination paradigm. Lead-exposed rats learned the discriminations faster than respective controls. Moreover, they exhibited greater levels of drug lever responding to lower doses of the training drugs (D1/sal and D2/sal), and to selected doses of other direct and indirect DA agonists (D2/sal only), including apomorphine, cocaine and (+)-amphetamine, and less blockade of drug lever responding by haloperidol (D2/sal). Taken together, these findings are consistent with a generalized DA supersensitivity. There were no differential Pb effects when non-DA compounds including morphine, pentobarbital and MK-801 were substituted for the training drugs, indicating the selectivity of the DA effects in the context of these experiments, and the improbability of a non-specific behavioral causation. Pb-exposed rats in the D2/sal group also showed a pronounced enhancement of drug lever responding when NMDA was substituted for quinpirole, suggesting the possibility of a Pb-induced NMDA supersensitivity as well.

Behavioral manifestations of prolonged lead exposure initiated at different stages of the life cycle: II. Delayed spatial alternation.

Since both Pb exposure and aging have been associated with alterations in memory functions, this study compared the effects of Pb exposure initiated at early, middle and later stages of the life cycle on delayed spatial alternation performance. Young (21 day old), adult (8 mon old) and old (16 mon old) rats were exposed for a total duration of 8.5 mon to 0, 2 or 10 mg Pb acetate/kg/day (young rats) or 0, 1.9 or 9.3 mg Pb acetate/kg/day (adult and old rats) in drinking water. These doses were designed to produce equivalent brain Pb concentrations across the three age groups. Behavioral testing began after 4 mon of exposure utilizing a standard delayed alternation paradigm with delay values of 0, 3, 6, 9 and 12 sec presented randomly during each experimental session for a total of 40 sessions. Aging, as expected, was associated with impairments of accuracy, but the disruption of accuracy at the 0 sec delay suggested that these were performance rather than memory deficits. Pb exposure actually improved delayed spatial alternation performance in both young and old animals: accuracy values were increased, while error frequencies declined in the two age groups. The increases in accuracy occurred primarily at the longest delay value in young rats, but were more prominent at short delay values in old rats. This difference may have been the result of a ceiling effect, since accuracy for young rats at short delay values was already quite high. Delayed spatial alternation performance of adult rats was generally unaffected by Pb exposure. A possible explanation of the improved performance of young and old Pb-exposed rats is one based on response perseveration, i.e., a facilitation of rote alternation behavior engendered by the cued alternation training program. The results of this study suggest that both early and late stages of the life cycle must be considered periods of enhanced vulnerability to Pb-induced behavioral changes.

Behavioral differentiation of the stimulus properties of a dopaminergic D1 agonist from a D2 agonist.

This study examined the hypothesis that the stimulus properties of a dopaminergic D1 agonist should be functionally discriminable from those of a D2 agonist. Rats were trained in a two-lever food-reinforced drug discrimination paradigm to discriminate either the D1 agonist SKF 38393 from apomorphine (SKF/APO), SKF from saline (SKF/SAL) or apomorphine from saline (APO/SAL), at an APO dose previously established to have D2-mediated stimulus properties. Results showed the SKF/APO discrimination to be readily acquired. The SKF cue showed similar stimulus properties under both the SKF/APO and SKF/SAL training conditions which were mediated by the D1 receptor: the SKF response was blocked by the D1 antagonist SCH 23390, but not by the D2 antagonist haloperidol. Run length data suggested a graded rather than quantal effect of SKF dose on responding during SKF generalization tests. In APO/SAL-trained animals, the APO cue was D2 mediated; responding was blocked by haloperidol, but not by SCH 23390 and APO responding generalized to the selective D2 agonist LY 171555 (quinpirole), but not to SKF. In the SKF/APO condition, APO responding did not decrease with APO dose, nor were antagonists effective in blocking responding, reflecting the lack of an alternative no-drug response option. Higher doses of APO appear to have D1 properties; doses above the training dose engendered SKF responding in both the SKF/SAL and SKF/APO groups, a response which could be blocked by the D1 antagonist SCH 23390, but not by the D2 antagonist, haloperidol. In all three training conditions, generalization test outcomes could be influenced by the conditions of the previous test session.(ABSTRACT TRUNCATED AT 250 WORDS)