The in vivo effects of olanzapine on the extracellular monoamine levels in rat prefrontal cortex (Pfc), nucleus accumbens (Acb) and striatum (Cpu) were investigated by means of microdialysis. Sequential doses of olanzapine at 0.5, 3 and 10 mg/kg (s.c.) dose-dependently increased the extracellular dopamine (DA) and norepinephrine (NE) levels in all three brain areas. The increases appeared 30 min after olanzapine administration, reached peaks around 60-90 min and lasted for at least 2 h. The highest DA increases in the Acb and Cpu were induced by olanzapine at 3 mg/kg but at 10 mg/kg in the Pfc. The peak DA increase in the Pfc (421% +/- 46 of the baseline) was significantly larger than those in the Acb (287% +/- 24) and Cpu (278% +/- 28). Similarly, the highest NE increase in the Pfc (414% +/- 40) induced by 10 mg/kg olanzapine was larger than those in the Acb (233% +/- 39) and Cpu (223% +/- 24). The DA and NE increases in the Pfc induced by olanzapine at 3 and 10 mg/kg (s.c.) were slightly larger than those induced by clozapine at the same doses. In contrast, haloperidol (0.5 and 2 mg/kg, s.c.) did not change Pfc DA and NE levels. Extracellular levels of a DA metabolite, DOPAC, and tissue concentrations of a released DA metabolite, 3-methoxytyramine, were also increased by olanzapine, consistent with enhanced DA release. However, olanzapine at the three sequential doses did not alter the extracellular levels of either 5-HT or its metabolite, 5-HIAA, in any of the three brain areas. In conclusion, the present studies demonstrate that in the case of sequential dosing olanzapine more effectively enhances DA and NE release in the Pfc than in the subcortical areas, which may have an impact on its atypical antipsychotic actions.

The atypical antipsychotic olanzapine was compared to other atypical as well as typical antipsychotic agents for in vivo occupancy of D1, D2, D3, 5HT2, and muscarinic receptors in rat brain. Blockade of D2 receptors was determined by measuring the levels of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC). To assess the interaction with phosphoinositide (PI)-coupled 5HT2A and muscarinic receptors in vivo, we used a novel radiometric technique to measure in vivo PI hydrolysis. The antagonism of olanzapine and other antipsychotic agents on 5HT2A and muscarinic receptors was determined by in vivo blockade of PI hydrolysis, stimulated by the 5HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) or the muscarinic agonist pilocarpine. Olanzapine inhibited 5HT2, D2, and D3 in vivo binding with high potency (ID50=0.15, 0.6 and 1.2 mg/kg, IP, respectively), while inhibiting D1 and muscarinic in vivo binding with much less potency (ID50 > 10 mg/kg, IP). The binding of olanzapine to D2 receptors in neostriatum was well correlated with the increase of DOPAC (ED200 = 0.8 mg/kg, IP) in vivo, indicating dopamine D2 antagonism. In vivo PI hydrolysis was increased by DOI in frontal cortex and by pilocarpine in hippocampus up to 2- and 7-fold above the basal level, respectively. The agonist-induced increases in PI hydrolysis were fully blocked by the 5HT2A antagonist MDL100907 and the muscarinic antagonist scopolamine, indicating the mediation by 5HT2A receptors in frontal cortex and PI-coupled muscarinic receptors (ml, m3, and m5) in hippocampus, respectively. Olanzapine was about 8-fold more potent in vivo in blocking DOI-induced stimulation of PI hydrolysis (ID50 = 0.1 mg/kg, IP) than pilocarpine-induced stimulation of PI hydrolysis (ID50 = 0.8 mg/kg, IP). In conclusion, olanzapine is more potent in blocking the 5HT2A receptor than D1, D2, D3 and muscarinic receptors in vivo, consistent with its favorable clinical profiles. In addition, the novel in vivo PI hydrolysis assay proved to be a useful and reliable in vivo method to assess the functional efficacy of compounds that interact with the 5HT2 and muscarinic receptors.

Olanzapine, an atypical antipsychotic, has a broad receptor binding profile, which may account for its pharmacological effects in schizophrenia. In vitro receptor binding studies showed a high affinity for dopamine D2, D3, and D4 receptors; all 5-HT2 receptor subtypes and the 5-HT6 receptor; muscarinic receptors, especially the M1 subtype; and α1-adrenergic receptors. In vivo studies showed that olanzapine had potent activity at D2 and 5 -HT2A receptors, but much less activity at D1 and muscarinic receptors, and that it inhibited dopaminergic neurons in the A10 but not the A9 tract, suggesting that this agent will not cause extrapyramidal side-effects (EPS). Microdialysis studies showed that olanzapine increased the extracellular levels of norepinephrine and dopamine, but not 5-HT, in the prefrontal cortex, and increased extracellular dopamine levels in the neostriatum and nucleus accumbens, areas ofthe brain associated with schizophrenia. Studies of gene expression showed that olanzapine 10 mg/kg also increased Fos expression in the prefrontal cortex, the dorsolateral striatum, and the nucleus accumbens. These findings are consistent with the effectiveness of olanzapine on both negative and positive symptoms and suggest that, with careful dosing, olanzapine should not cause EPS.

A large body of evidence indicates that muscarinic acetylcholine receptors (mAChRs) play critical roles in regulating the activity of many important functions of the central and peripheral nervous systems. However, identification of the physiological and pathophysiological roles of the individual mAChR subtypes (M(1)-M(5)) has proven a difficult task, primarily due to the lack of ligands endowed with a high degree of receptor subtype selectivity and the fact that most tissues and organs express multiple mAChRs. To circumvent these difficulties, we used gene targeting technology to generate mutant mouse lines containing inactivating mutations of the M(1)-M(5) mAChR genes. The different mAChR mutant mice and the corresponding wild-type control animals were subjected to a battery of physiological, pharmacological, behavioral, biochemical, and neurochemical tests. The M(1)-M(5) mAChR mutant mice were viable and reproduced normally. However, each mutant line displayed specific functional deficits, suggesting that each mAChR subtype mediates distinct physiological functions. These results should offer new perspectives for the rational development of novel muscarinic drugs.

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