The dual orexin receptor antagonist, DORA-22, lowers histamine levels in the lateral hypothalamus and prefrontal cortex without lowering hippocampal acetylcholine.

Chronic insomnia is defined as a persistent difficulty with sleep initiation maintenance or non-restorative sleep. The therapeutic standard of care for this condition is treatment with gamma-aminobutyric acid (GABA)A receptor modulators, which promote sleep but are associated with a panoply of side effects, including cognitive and memory impairment. Dual orexin receptor antagonists (DORAs) have recently emerged as an alternative therapeutic approach that acts via a distinct and more selective wake-attenuating mechanism with the potential to be associated with milder side effects. Given their distinct mechanism of action, the current work tested the hypothesis that DORAs and GABAA receptor modulators differentially regulate neurochemical pathways associated with differences in sleep architecture and cognitive performance induced by these pharmacological mechanisms. Our findings showed that DORA-22 suppresses the release of the wake neurotransmitter histamine in the lateral hypothalamus, prefrontal cortex, and hippocampus with no significant alterations in acetylcholine levels. In contrast, eszopiclone, commonly used as a GABAA modulator, inhibited acetylcholine secretion across brain regions with variable effects on histamine release depending on the extent of wakefulness induction. In normal waking rats, eszopiclone only transiently suppressed histamine secretion, whereas this suppression was more obvious under caffeine-induced wakefulness. Compared with the GABAA modulator eszopiclone, DORA-22 elicits a neurotransmitter profile consistent with wake reduction that does not impinge on neurotransmitter levels associated with cognition and rapid eye movement sleep.

Role of estrogen receptors in neuroprotection by estradiol against MPTP toxicity.

Estradiol protects against striatal dopamine terminal loss caused by the neurotoxin MPTP in mice. This effect of estradiol is thought to be mediated by an interaction with estrogen receptors (ER), of which there are two: ERalpha and ERbeta. In the present study, the role of these two ERs in MPTP toxicity and its neuroprotection by estradiol was investigated using ER knock out mice (ERKO). MPTP (7, 9, or 11 mg/kg administered four times at 2h intervals) caused a dose-dependent decrease in striatal dopamine and dopamine metabolite DOPAC concentrations in wild type (WT) mice. The degree of dopamine and DOPAC depletion after MPTP was greater in the ERKOalpha mice than WT mice, whereas the ERKObeta mice exhibited no change in MPTP sensitivity. ERKObeta mice showed a lower DA turnover than WT and ERKOalpha mice. WT, ERKOalpha and ERKObeta mice were also treated for 10 days with exogenous estradiol and on day 5 of treatment were challenged with MPTP (9 mg/kg administered four times at 2h intervals). In the WT mice, estradiol partially prevented the MPTP-induced decrease in striatal dopamine and DOPAC concentrations. However, estradiol treatment was without significant neuroprotective effects in the ERKOalpha and ERKObeta mice. These results show a greater susceptibility to MPTP toxicity of ERKOalpha mice compared to WT and ERKObeta mice and a role for both ER receptors in striatal DA neuroprotection.

Pramipexole inhibits MPTP toxicity in mice by dopamine D3 receptor dependent and independent mechanisms.

The role of dopamine D3 receptors was investigated in mediating the neuroprotective effect of the dopamine D2/D3 receptor agonist (S)-2-amino-4,5,6,7-tetrahydro-6-propylamine-benzothiazole (pramipexole) in vivo. Pramipexole retained the ability to inhibit 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopamine depletion in mice in which the dopamine D3 receptor had been deleted. However, the neuroprotective efficacy was reduced in the dopamine D3 receptor-deleted mice compared to that in littermates expressing the wildtype receptor. Furthermore, the dopamine D3 receptor selective antagonist 2-(3-[4-(2-tert-butyl-6-trifluoromethyl-4-pyrimidinyl)-1-piperazinyl]propylthio)-4-pyrimidinol (A-437203) partially inhibited the neuroprotective effect of pramipexole in dopamine D3 receptor expressing mice but not in receptor-deleted mice. These results indicate that pramipexole protects dopamine neurons from MPTP-induced toxicity by mechanisms that are both dependent and independent of an interaction with dopamine D3 receptors.

Regulation of dopamine signaling in the striatum by phosphodiesterase inhibitors: novel therapeutics to treat neurological and psychiatric disorders.

Abnormal dopamine neurotransmission has been linked to a wide array of motor, cognitive, and psychiatric disorders. Dopamine binds and regulates intracellular signals through D1-like (D1 and D5) and D2-like (D2, D3, and D4) G-protein coupled receptors. Activation of D1- like receptors stimulates cAMP/PKA signaling via Gs mediated activation of adenylyl cyclase. In contrast, activation of D2-like receptors inhibits cAMP/PKA signaling by Gi inhibition of adenylyl cyclase. In the brain, dopamine signaling is tightly regulated by cyclic nucleotide phosphodiesterases (PDEs). PDEs are a family of enzymes that selectively degrade cAMP and cGMP. There are 11 different families of PDEs that vary in their substrate specificity, kinetic properties, mode of regulation, intracellular localization, and tissue expression patterns. A number of PDE families are highly expressed in the striatum including PDE1, PDE2, PDE4, and PDE10. There is a growing amount of evidence to suggest that these enzymes play a critical role in modulating dopamine signaling and selective inhibitors of these enzymes are currently being explored as novel therapeutics to treat schizophrenia, Huntington's disease, cognitive disorders and Parkinson's disease. The aim of this review is to summarize the distinct roles of different PDEs in regulating dopamine signaling in the striatum. In addition, we will briefly review the therapeutic potential of selective PDE inhibitors to treat neurological and psychiatric disorders associated with abnormal striatal function.

Dual orexin receptor antagonists show distinct effects on locomotor performance, ethanol interaction and sleep architecture relative to gamma-aminobutyric acid-A receptor modulators.

Dual orexin receptor antagonists (DORAs) are a potential treatment for insomnia that function by blocking both the orexin 1 and orexin 2 receptors. The objective of the current study was to further confirm the impact of therapeutic mechanisms targeting insomnia on locomotor coordination and ethanol interaction using DORAs and gamma-aminobutyric acid (GABA)-A receptor modulators of distinct chemical structure and pharmacological properties in the context of sleep-promoting potential. The current study compared rat motor co-ordination after administration of DORAs, DORA-12 and almorexant, and GABA-A receptor modulators, zolpidem, eszopiclone, and diazepam, alone or each in combination with ethanol. Motor performance was assessed by measuring time spent walking on a rotarod apparatus. Zolpidem, eszopiclone and diazepam [0.3-30 mg/kg administered orally (PO)] impaired rotarod performance in a dose-dependent manner. Furthermore, all three GABA-A receptor modulators potentiated ethanol- (0.25-1.5 g/kg) induced impairment on the rotarod. By contrast, neither DORA-12 (10-100 mg/kg, PO) nor almorexant (30-300 mg/kg, PO) impaired motor performance alone or in combination with ethanol. In addition, distinct differences in sleep architecture were observed between ethanol, GABA-A receptor modulators (zolpidem, eszopiclone, and diazepam) and DORA-12 in electroencephalogram studies in rats. These findings provide further evidence that orexin receptor antagonists have an improved motor side-effect profile compared with currently available sleep-promoting agents based on preclinical data and strengthen the rationale for further evaluation of these agents in clinical development.

Modulation of NMDA receptor function by inhibition of D-amino acid oxidase in rodent brain.

Observations that N-Methyl-D-Aspartate (NMDA) antagonists produce symptoms in humans that are similar to those seen in schizophrenia have led to the current hypothesis that schizophrenia might result from NMDA receptor hypofunction. Inhibition of D-amino acid oxidase (DAAO), the enzyme responsible for degradation of D-serine, should lead to increased levels of this co-agonist at the NMDA receptor, and thereby provide a therapeutic approach to schizophrenia. We have profiled some of the preclinical biochemical, electrophysiological, and behavioral consequences of administering potent and selective inhibitors of DAAO to rodents to begin to test this hypothesis. Inhibition of DAAO activity resulted in a significant dose and time dependent increase in D-serine only in the cerebellum, although a time delay was observed between peak plasma or brain drug concentration and cerebellum D-serine response. Pharmacokinetic/pharmacodynamic (PK/PD) modeling employing a mechanism-based indirect response model was used to characterize the correlation between free brain drug concentration and D-serine accumulation. DAAO inhibitors had little or no activity in rodent models considered predictive for antipsychotic activity. The inhibitors did, however, affect cortical activity in the Mescaline-Induced Scratching model, produced a modest but significant increase in NMDA receptor-mediated synaptic currents in primary neuronal cultures from rat hippocampus, and resulted in a significant increase in evoked hippocampal theta rhythm, an in vivo electrophysiological model of hippocampal activity. These findings demonstrate that although DAAO inhibition did not cause a measurable increase in D-serine in forebrain, it did affect hippocampal and cortical activity, possibly through augmentation of NMDA receptor-mediated currents.

Repeated estradiol treatment prevents MPTP-induced dopamine depletion in male mice.

Epidemiological data suggest that the steroid hormone 17beta-estradiol plays an important role in protecting the brain from neurodegenerative processes, including that causing the loss of dopamine (DA) neurons in Parkinson's disease. Determining the mechanisms of neuroprotection in experimental systems may facilitate the development of estrogenic therapies for these diseases. The present study sought to further investigate the mechanism of the neuroprotective effect of 17beta-estradiol in a murine model of Parkinson's disease, i.e. 1-methyl- 4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced striatal DA depletion. Consistent with previous findings, 17beta-estradiol was found to inhibit MPTP-induced DA depletion under a dosing regimen (repeated daily administration) that mimicked physiological levels of the steroid. However, high doses of the steroid administered repeatedly or acutely failed to inhibit toxicity, as did 17alpha-estradiol. These data suggest that the neuroprotective effect of 17beta-estradiol was mediated through an interaction with one of the nuclear estrogen receptors, and is not the result of an antioxidant action. In order to realize the therapeutic potential of the neuroprotective effect of 17beta-estradiol for Parkinson's disease, it will be necessary to identify synthetic estrogen receptor modulators that lack the activity of the steroid on peripheral tissue. In this study, raloxifene failed to mimic the neuroprotective effect of 17beta-estradiol against MPTP toxicity. Thus, exploration of new compounds with different pharmacological and/or physiochemical properties is warranted.