Effects of LU-111995 in three models of disrupted prepulse inhibition in rats.
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LU-111995 is a novel antipsychotic drug in clinical development. It has a clozapine-like receptor profile and affinities for dopamine D(4) and 5-hydroxytryptamine(2A) receptors. The effects of LU-111995 were examined in three models of disrupted prepulse inhibition (PPI) in rats. The first model tested the hypothesis that LU-111995 would normalize the deficit in PPI exhibited by rats treated with the dopamine agonist apomorphine. LU-111995 significantly reduced the effect of apomorphine on PPI but also slightly increased PPI by itself. Thus, the increases in PPI were not specific to the animals treated with apomorphine but reflected an effect of LU-111995 on PPI. LU-111995 also attenuated the apomorphine-induced increase in startle reactivity. The second model tested the hypothesis that LU-111995 would normalize the deficit in PPI exhibited by rats treated with the psychotomimetic phencyclidine (PCP). LU-111995 significantly blocked the PCP-induced increase in startle reactivity but did not alter the PPI-disruptive effects of PCP. The third model tested the hypothesis that LU-111995 would normalize the deficit in PPI exhibited by isolation-reared rats tested as adults. Isolation rearing of rats produced deficits in PPI. LU-111995 reversed the isolation rearing-induced deficit in PPI without having any significant effect on PPI in socially reared rats. In summary, LU-111995 exhibits potential antipsychotic-like activity in two models of disrupted PPI. It remains to be elucidated whether its effects on PPI can be attributed to a blockade of single dopamine and 5-hydroxytryptamine receptor subtypes, especially D(4) and 5-hydroxytryptamine(2A), or a combination of both. (+info)
Multisecond oscillations in firing rate in the globus pallidus: synergistic modulation by D1 and D2 dopamine receptors.
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The firing rates of many basal ganglia neurons recorded in awake rats oscillate at seconds-to-minutes time scales, and the D1/D2 agonist apomorphine has been shown to robustly modulate these oscillations. The use of selective D1 and D2 antagonists suggested that both these receptor subfamilies are involved in apomorphine's effects. In the present study, spectral analysis revealed that baseline multisecond oscillations were significantly periodic in 71% of globus pallidus neurons. Baseline oscillations had a wide range of periods within the analyzed range, with a population mean of 32 +/- 2 s. Administration of the D1 agonist SKF 81297 (6-chloroPB) at 1.0 or 5.0 mg/kg significantly changed these oscillations, reducing means of spectral peak periods to 14 to 16 s (i.e., increasing oscillatory frequency). This effect was attenuated by D2 antagonist pretreatment. The D2 agonist quinpirole did not cause a significant population change in multisecond periodicities. The strongest effects on multisecond periodicities occurred after combined treatment with SKF 81297 and quinpirole. Low, ineffective doses of SKF 81297 and quinpirole, when combined, produced a significant increase in oscillatory frequency. Also, when quinpirole was administered after an already effective dose of SKF 81297, quinpirole shifted oscillations to an even faster range (typically to periods of <10 s). The dopaminergic control of multisecond periodicities in globus pallidus firing rate demonstrates D1/D2 receptor synergism, in that the effects of D1 agonists are potentiated by and partially dependent on D2 receptor activity. Modulation of multisecond oscillations in firing rate represents a novel means by which dopamine can influence globus pallidus physiology. (+info)
Enhancement of D1 dopamine receptor-mediated locomotor stimulation in M(4) muscarinic acetylcholine receptor knockout mice.
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Muscarinic acetylcholine receptors (M(1)-M(5)) regulate many key functions of the central and peripheral nervous system. Primarily because of the lack of receptor subtype-selective ligands, the precise physiological roles of the individual muscarinic receptor subtypes remain to be elucidated. Interestingly, the M(4) receptor subtype is expressed abundantly in the striatum and various other forebrain regions. To study its potential role in the regulation of locomotor activity and other central functions, we used gene-targeting technology to create mice that lack functional M(4) receptors. Pharmacologic analysis of M(4) receptor-deficient mice indicated that M(4) receptors are not required for muscarinic receptor-mediated analgesia, tremor, hypothermia, and salivation. Strikingly, M(4) receptor-deficient mice showed an increase in basal locomotor activity and greatly enhanced locomotor responses (as compared with their wild-type littermates) after activation of D1 dopamine receptors. These results indicate that M(4) receptors exert inhibitory control on D1 receptor-mediated locomotor stimulation, probably at the level of striatal projection neurons where the two receptors are coexpressed at high levels. Our findings offer new perspectives for the treatment of Parkinson's disease and other movement disorders that are characterized by an imbalance between muscarinic cholinergic and dopaminergic neurotransmission. (+info)
Pertussis toxin lesions of the rat substantia nigra block the inhibitory effects of the gamma-hydroxybutyrate agent, S(-)HA-966 without affecting the basal firing properties of dopamine neurons.
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S(-)3-amino-1-hydroxypyrrolidone-2 (S(-)HA-966), a potent gamma-hydroxybutyrate-like drug, inhibits spontaneous firing and induces a pacemaker-like discharge pattern in nigral dopamine (DA)-containing neurons. Recent evidence has suggested that these effects could be mediated by GABAB receptors and, thus, is likely to involve G protein intermediaries. To test this hypothesis, extracellular single-unit recording techniques were used to assess the effects of S(-)HA-966 in animals that had received an intranigral injection of pertussis toxin (PT). Failure to respond to the inhibitory effects of apomorphine was taken as presumptive evidence that PT-sensitive G protein-coupled receptors had been inactivated. No significant differences were observed in the basal firing properties of DA cells recorded in control and PT-lesioned animals. However, in marked contrast to the inhibitory effects observed in uninjected and sham-lesioned animals, S(-)HA-966 significantly increased the firing rate of apomorphine-insensitive DA neurons in PT-lesioned rats. The excitatory effects of S(-)HA-966 were accompanied by a significant reduction in bursting activity and an increase in the regularity of firing. These data indicate that the inhibitory effects of S(-)HA-966 are mediated locally within the substantia nigra by a PT-sensitive substrate, presumably a G protein-coupled receptor. (+info)
Long-term doxycycline-controlled expression of human tyrosine hydroxylase after direct adenovirus-mediated gene transfer to a rat model of Parkinson's disease.
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Developments of technologies for delivery of foreign genes to the central nervous system are opening the field to promising treatments for human neurodegenerative diseases. Gene delivery vectors need to fulfill several criteria of efficacy and safety before being applied to humans. The ability to drive expression of a therapeutic gene in an adequate number of cells, to maintain long-term expression, and to allow exogenous control over the transgene product are essential requirements for clinical application. We describe the use of an adenovirus vector encoding human tyrosine hydroxylase (TH) 1 under the negative control of the tetracycline-sensitive gene regulatory system for direct injection into the dopamine-depleted striatum of a rat model of Parkinson's disease. This vector mediated synthesis of TH in numerous striatal cells and transgene expression was observed in a large proportion of them for at least 17 weeks. Furthermore, doxycyline, a tetracycline analog, allowed efficient and reversible control of transgene expression. Thus, the insertion of a tetracycline-sensitive regulatory cassette into a single adenovirus vector provides a promising system for the development of successful and safe therapies for human neurological diseases. Our results also confirm that future effective gene replacement approaches to Parkinson's disease will have to consider the concomitant transfer of TH and GTP-cyclohydrolase transgenes because the synthesis of the TH cofactor tetrahydrobiopterin may be crucial for restoration of the dopaminergic deficit. (+info)
Dopamine agonists both stimulate and inhibit prolactin release in GH4ZR7 cells.
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Prolactin secretion from the anterior pituitary gland is regulated by multiple factors including prolactin-release inhibiting factors (PIFs) and prolactin releasing factors. PIFs, however, usually dominate to exert a tonic inhibition in the biological system, and the physiological PIF is believed to be dopamine. However, there is accumulating evidence that dopamine can not only inhibit but also stimulate prolactin release. Many investigators believe that this is achieved by activating inhibitory and stimulatory subtypes of dopamine receptors. We tried to demonstrate that one subtype of dopamine receptors is capable of both inhibiting or stimulating prolactin release using GH(4)ZR(7) cells. GH(4)ZR(7) cells express only a short form of dopamine D(2) receptors (D(2s)). Low concentrations of three well-established D(2) receptor agonists (dopamine, apomorphine and bromocriptine) stimulated prolactin release from GH(4)ZR(7) cells while high concentrations inhibited the release. Haloperidol, a D(2) receptor antagonist, blocked the inhibitory action, but was unable to block the dopamine-induced stimulatory action. Pretreatment of cells with phenoxybenzamine, a receptor alkylating agent, abolished both the dopamine-induced stimulatory and inhibitory actions. Our results support the thesis that the stimulation of prolactin release induced by dopamine is mediated through dopamine D(2s) receptors since the GH(4)ZR(7) cells have only D(2s) receptors among dopamine receptors. We have concluded that the D(2s) receptor is capable of both stimulating and inhibiting prolactin release, probably via the activation of a G(s) protein by low concentrations and a G(i) protein by high concentrations of dopaminergic agents. (+info)
Evaluation of apomorphine HCl effects on reproductive endpoints: studies in male rats and dogs.
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Apomorphine HCl is currently in phase III clinical trials for the treatment of erectile dysfunction. The potential for reproductive toxicity in males was assessed based on a 13-week rat study--a fertility study in male rats--and a 6-month study in dogs. The subcutaneous (s.c.) route was selected to simulate the sublingual route in humans. Dosages of apomorphine were 0.0 (vehicle), 0.8, 2, and 8 mg/kg/day in the 13-week study in rats (20/group), with 8 mg/kg/day used for only 9 weeks. In the fertility study, 24 males/group were cohabited with females, and doses were 0.0, 0.2, 0.8, and 2 mg/kg/day. Males were treated for 4 weeks prior to cohabitation and for 5 weeks throughout cohabitation. Organ weights, including testis and left epididymis, sperm count and morphology in the epididymis, and sperm motility in the vas deferens were evaluated. Male fertility index, and in females, the numbers of fetuses, implantation sites, and corpora lutea were counted. Male dogs (five/group) were dosed with 0.04, 0.1, or 0.4 mg/kg/day for 6 months. Epididymal and prostate weight, and testicular and epididymal histology were evaluated. Daily morbidity/mortality, weekly clinical observations, body weight, and food consumption were evaluated in all studies. No adverse effect was observed in any of the reproductive parameters in the studies. The NOEL for reproductive toxicity was approximately equal to 0.4 mg/kg/day in dogs and > or = 2 mg/kg/day in rats. These doses in rats and dogs correlated with plasma levels of approximately 240 and 130 ng/mL, and AUCs of 200 and 100 ng.h/mL, respectively. These levels suggest a safety margin for the evaluated male reproductive endpoints of at least 104 times based on the Cmax, and 44 times based on AUC of the clinical dose. (+info)
Contrasting EEG profiles elicited by antipsychotic agents in the prefrontal cortex of the conscious rat: antagonism of the effects of clozapine by modafinil.
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1. Power spectra (0-30 Hz) were recorded from transcortical electrodes implanted in prefrontal and sensorimotor cortex in conscious rats. For each animal, the spectra in the presence of a drug were divided by the spectra in the presence of vehicle to give a drug-related differential display of the power spectra, the profile of EEG effects. 2. The profiles of a range of antipsychotic agents of different classes were compared. Haloperidol (0.5 mg kg-1 and 1 mg kg-1 s.c., peak 8 - 12 Hz), chlorpromazine (0.5 mg kg-1, i. p., peak 8 Hz), levomepromazine (1 mg kg-1, i.p., peak 8 Hz), quetiapine (2.5 mg kg-1, s.c., peak 9 - 12 Hz), sertindole (2.5 mg kg-1, s.c., peak 6 - 14 Hz), risperidone (0.5 and 1 mg kg-1 i.p., peak 9 Hz), clozapine (0.1, 0.2, 0.3 and 5 mg kg-1, s.c., peak 8 Hz) and MDL100907 (0.01 mg kg-1 s.c. peak 2 Hz) synchronized the EEG, increasing the power spectra between 2 and 30 Hz, although there were marked differences between the individual profile of EEG effects for each drug. 3. In contrast, the benzamides, sulpiride (7.5 and 15 mg kg-1 i.p.), and amisulpiride (1 and 15 mg kg-1 i.p.) caused marked asynchronous changes in the EEG. Raclopride (2.5 mg kg-1 i.p.), caused an initial peak at 9 Hz, but the effects of raclopride desynchronized over a 3 h time period. 4 Modafinil and apomorphine, administered alone, decreased the power spectra at frequencies higher than 4. Hz. Modafinil (62.4 mg kg-1, i.p.) selectively antagonized the effects of clozapine, but did not antagonize the effects of raclopride. 5. Different pharmacological classes of antipsychotic show synchronization or desynchronization of the EEG in the prefrontal cortex, with the benzamides showing a distinctive spectrum. There appears to be a specific interaction between modafinil and clozapine. Thus, modulation of prefrontal cortical function, perhaps by thalamic gating, may be important for antipsychotic activity. (+info)