Franklin Porter Bymaster

Olanzapine increases in vivo dopamine and norepinephrine release in rat prefrontal cortex, nucleus accumbens and striatum

Abstract 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 (SC) 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 (SC) were slightly larger than those induced by clozapine at the same doses. In contrast, haloperidol (0.5 and 2 mg/kg, SC) 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.

Neurochemical evidence for antagonism by olanzapine of dopamine, serotonin, α1-adrenergic and muscarinic receptors in vivo in rats

The ability of the atypical antipsychotic drug candidate olanzapine to antagonize dopamine, serotonin, α-adrenergic and muscarinic receptors in vivo was assessed by various neurochemical measurements in rat brain. Olanzapine increased the concentrations of the dopamine metabolites DOPAC and HVA in striatum and nucleus accumbens. Olanzapine antagonized the pergolide-induced decrease of striatal DOPA concentrations in rats treated with gammabutyrolactone and NSD1015 and increased striatal 3-methoxytyramine concentrations in nomifensine-treated rats (but not after gammabutyrolactone administration), suggesting that olanzapine blocked terminal and somatodendritic autoreceptors on dopamine neurons. Inactivation of dopamine D1 and D2 receptors by EEDQ was antagonized by olanzapine. The ex vivo binding of the 5HT2 radioligand [3H]-ketanserin was inhibited by olanzapine treatment, as was quipazine-induced increases in MHPG-SO4, evidence suggesting that olanzapine antagonized 5HT2 receptors. At higher doses, olanzapine increased the concentration of the norepinephrine metabolite, MHPG-SO4, probably by blocking α1-adrenergic receptors. Olanzapine inhibited ex vivo binding of the muscarinic antagonist radioligand [3H]-pirenzepine and lowered concentrations of striatal, but not hippocampal, acetylcholine levels. The findings provide evidence that olanzapine antagonized dopamine, serotonin, α-adrenergic and muscarinic receptors in vivo, consistent with its high affinity for these receptor sites in vitro.

Chronic effects of fluoxetine, a selective inhibitor of serotonin uptake, on neurotransmitter receptors

Fluoxetine administration to rats at a dose of 10 mg/kg i.p. daily up to 12 or 24 days failed to change the concentration-dependent binding of [3H]WB4101, [3H]clonidme and [3H]dihydroalprenolol toα 1 −,α 2 − andβ-adrenergic receptors, respectively; [3H]quinuclidinyl benzilate to muscarinic receptors; [3H]pyrilamine to histamine H1 receptors and [3H]naloxone to opiate receptors. Persistent and significant decreases in receptor number (Bmax value) without changes in the dissociation constant (KD value) of [3H]5-HT binding in cortical membranes were observed upon chronic treatment with fluoxetine administered either by intraperitoneal injection or incorporation in the diet, A detectable reduction of 5-HT1 receptor number occurred after once-daily injections of fluoxetine at 10 mg/kg i.p. within 49 hours. After pretreatment for 3 days with p-chlorophenylalanine, an inhibitor of 5-HT synthesis, followed by repeated administration of fluoxetine, 5-HT1 receptor numbers were higher than those of normal rats, suggesting a dependence on synaptic concentration of 5-HT for fluoxetine to affect a receptor down-regulation. These studies provide further evidence for the selectivity of fluoxetine as an inhibitor of 5-HT reuptake, resulting in a selective down-regulation of 5-HT1 receptors in the cerebral cortex of rat brain.

Lu AE58054, a 5-HT6 antagonist, reverses cognitive impairment induced by subchronic phencyclidine in a novel object recognition test in rats.

The in-vitro potency and selectivity, in-vivo binding affinity and effect of the 5-HT(6)R antagonist Lu AE58054 ([2-(6-fluoro-1H-indol-3-yl)-ethyl]-[3-(2,2,3,3-tetrafluoropropoxy)-benzyl]-amine) on impaired cognition were evaluated. Lu AE58054 displayed high affinity to the human 5-HT(6) receptor (5-HT(6)R) with a Ki of 0.83 nm. In a 5-HT(6) GTPgammaS efficacy assay Lu AE58054 showed no agonist activity, but demonstrated potent inhibition of 5-HT-mediated activation. Besides medium affinity to adrenergic alpha(1A)- and alpha(1B)-adrenoreceptors, Lu AE58054 demonstrated >50-fold selectivity for more than 70 targets examined. Orally administered Lu AE58054 potently inhibited striatal in-vivo binding of the 5-HT(6) antagonist radioligand [(3)H]Lu AE60157 ([(3)H]8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline), with an ED(50) of 2.7 mg/kg. Steady-state modelling of an acute pharmacokinetic/5-HT(6)R occupancy time-course experiment indicated a plasma EC(50) value of 20 ng/ml. Administration of Lu AE58054 in a dose range (5-20 mg/kg p.o.) leading to above 65% striatal 5-HT(6)R binding occupancy in vivo, reversed cognitive impairment in a rat novel object recognition task induced after subchronic treatment for 7 d with phencyclidine (PCP 2 mg/kg b.i.d., i.p. for 7 d, followed by 7 d drug free). The results indicate that Lu AE58054 is a selective antagonist of 5-HT(6)Rs with good oral bioavailability and robust efficacy in a rat model of cognitive impairment in schizophrenia. Lu AE58054 may be useful for the pharmacotherapy of cognitive dysfunction in disease states such as schizophrenia and Alzheimers disease.

Effect of a stereospecific D2-dopamine agonist on acetylcholine concentration in corpus striatum of rat brain

The enantiomers of LY141865, trans-(±)-4,4a,5,6,7,8a,9-octahydro-5-propyl-2H-pyrazolo [3,4-g]quinoline, were compared as dopamine D2 agonists by determining their abilities to elevate acetylcholine concentrations in rat corpus striatum. The levorotatory isomer, LY156258, increased striatal acetlycholine concentration at doses of 0.1–1 mg/kg i.p., whereas the dextrorotatory isomer had no effect even at doses as high as 30 mg/kg. The levorotatory isomer also decreased striatal concentrations of the dopamine metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, but did not significantly alter dopamine or 5-hydroxyindoleacetic acid concentration. The dextrorotatory isomer had no effect on any of these substances alone and did not alter the effects of the levorotatory isomer. The elevation of striatal acetylcholine levels by LY156258 was mimicked by pergolide, a dopamine agonist, and was totally prevented by pretreatment with haloperidol, a dopamine antagonist. The elevation of striatal acetylcholine concentration by LY157258 was maximal at 0.5 hour and declined thereafter, following a time course similar to that of pergolide. Neither LY141865 nor LY156258 shared with peroglide and dopamine the ability to activate striatal adenylate cyclasein vitro, an effect mediated by D1 receptors. LY141865 and LY156258 (but not the dextrorotatory isomer) inhibited the binding of tritiated apomorphine and spiperone to striatal membrane receptors, but were not as potent as pergolide, they also had less effect, or no effect, on the binding of other tritiated ligands (dopamine, WB4101, clonidine, dihydroalprenolol, pyrilamine or quinuclidinyl benzilate) to their membrane receptors. These results indicate that LY156258 stereospecifically activates dopamine D2 receptors and the studies are the first evidence of stereospecificity of dopamine receptors mediating an increase in striatal acetylcholine concentration.

Antipsychotic―induced gene regulation in multiple brain regions

J. Neurochem. (2010) 10.1111/j.1471‐4159.2010.06585.x

The tyramine pressor test may have limited sensitivity, especially in the presence of dual serotonin/norepinephrine uptake inhibition.

The Tyramine Pressor Test May Have Limited Sensitivity, Especially in the Presence of Dual Serotonin/Norepinephrine Uptake Inhibition ★

Novel alkoxy-oxazolyl-tetrahydropyridine muscarinic cholinergic receptor antagonists

The purpose of the present studies was to compare a novel series of alkoxy-oxazolyl-tetrahydropyridines (A-OXTPs) as muscarinic receptor antagonists. The affinity of these compounds for muscarinic receptors was determined by inhibition of [3H]pirenzepine to M1 receptors in hippocampus, [3H]QNB to M2 receptors in brainstem, and [3H]oxotremorine-M to high affinity muscarinic agonist binding sites in cortex. All of the compounds had higher affinity for [3H]pirenzepine than for [3H]QNB or [3H]oxotremorine-M labeled receptors, consistent with an interpretation that they are relatively selective M1 receptor antagonists, although none were as selective as pirenzepine. In addition, dose-response curves were determined for antagonism of oxotremorine-induced salivation (mediated by M3 receptors) and tremor (mediated by non-M1 receptors) in mice. In general, the A-OXTPs were equipotent and equieffective in antagonizing both salivation and tremor, although there were modest differences for some compounds. Dose-response curves also were determined on behavior maintained under a spatial-alternation schedule of food presentation in rats as a measure of effects on working memory. The A-OXTPs produced dose-related decreases in percent correct responding at doses three- to ten-fold lower than those which decreased rates of responding. However, only one compound, MB-OXTP, produced effects on percent correct responding consistent with a selective effect on memory as opposed to non-memory variables. The present results provide evidence that these alkoxy-oxazolyltetrahydropyridines are a novel series of modestly M1-selective muscarinic receptor antagonists, and that one member of the series, MB-OXTP, appears to be more selective in its effects on memory than previously studied muscarinic antagonists.

Nisoxetine prevents the 6-hydroxydopamine induced decrease in post-decapitation convulsions

Abstract Intraventricular injection of 6-hydroxydopamine (60HDA) to rats caused a marked reduction in post-decapitation convulsions (PDC), which was also observed in rats given 60HDA systemically at birth. The reduction in PDC and norepinephrine (NE) content in brain and spinal cord was completely prevented by pretreatment with the selective norepinephrine uptake inhibitor, nisoxetine, but not by fluoxetine, a specific serotonin uptake inhibitor. Presumably nisoxetine prevented the reduction in PDC and NE levels by blocking the entry of 60HDA into the neuron via the membrane uptake pump, and thus preventing subsequent NE depletion and neuron degeneration. These data imply that NE neurons are involved in the neurological mechanism of PDC, although this does not exclude a role for other neurotransmitters such as serotonin (5HT) and dopamine (DA).

Studies on 4-(p-bromophenyl)-bicyclo (2,2,2) octan-1-amine as an inhibitor of uptake into serotonin neurones

Abstract 4-( p -Bromophenyl)-bicyclo (2,2,2)octan-1-amine was a potent antagonist of p-chloroamphe-tamine-induced depletion of brain serotonin and a weaker antagonist of 6-hydroxydopamine-induced depletion of heart norepinephrine in rats and in mice. In both species, this compound was more active in antagonizing serotonin depletion and less active in antagonizing norepinephrine depletion than was the parent compound lacking the p -bromo substituent. In vitro , the bromo analogue was markedly less active than the parent compound in blocking norepinephrine uptake by rat brain synaptosomes and was slightly less active in blocking serotonin uptake. Brain levels of the p -bromo compound were higher than those of the parent compound in rats, apparently accounting for the relatively greater effectiveness of the f -bromo compound in blocking serotonin depeletion in vivo . Thus, 4-( p -bromophenyl)-bicyclo (2,2,2,)octan-1-amine appears to be a potent and relatively selective inhibitor of uptake into serotonin neurones.