Reduction of reticulata neuronal activity by zolpidem and alpidem, two imidazopyridines with high affinity for type I benzodiazepine receptors

Zolpidem and alpidem, two imidazopyridines with high affinity for the type I benzodiazepine recognition site, have recently been proposed as preferential hypnotic (zolpidem) and anxiolytic (alpidem) drugs notable for the minor incidence of side-effects. To further characterize the molecular mechanism involved in the action of these drugs, we studied their effects in comparison with those of diazepam on the spontaneous electrical activity of substantia nigra pars reticulata (SNR) neurons. These cells have been shown to be extremely sensitive to various positive and negative modulators of GABAergic transmission. All three drugs consistently produced a dose-dependent (0.03-8.0 mg/kg i.v.) inhibition of the firing of SNR cells when administered as a single bolus. However, zolpidem was more potent and efficacious than diazepam or alpidem. The ID50s were 0.076, 0.492 and 0.821 mg/kg, respectively. When the drugs were injected in exponentially (ratio 2) increasing doses up to 8.0 mg/kg, the rank order for tachyphylaxis was zolpidem much greater than diazepam greater than alpidem. Since the effects of the drugs were abolished and prevented by a small dose (0.5 mg/kg i.v.) of flumazenil (Ro 15-1788), it is likely that the effects were mediated through activation of benzodiazepine receptors. The results indicate that the hypnotic, zolpidem, has a more potent inhibitory action on SNR cell activity than the anxiolytics, alpidem and diazepam.     

Developmental and age-related changes in D1-dopamine receptors and dopamine content in the rat striatum

The relationship between the postnatal development of dopaminergic (DAergic) nerve endings and the maturation of D1 DA receptors in the rat striatum was analyzed by measuring the content of DA and dihydroxyphenylacetic acid (DOPAC), two biochemical markers of DAergic nerve terminal proliferation, and the ontogenetic changes in [3H]SCH 23390 binding sites. DA-stimulated adenylate cyclase (AC) activity was also measured in order to characterize the coupling of [3H]SCH 23390 binding sites to the responses mediated by the activation of D1 DA receptors. Striatal levels of DA and DOPAC, as well as the density and affinity of [3H]SCH 23390 binding sites and DA-stimulated AC activity were also measured in senescent rats. The striatal content of DA increased slowly after birth, reaching adult levels by postnatal day 60 and remaining constant through adulthood and senescence (up to 20 months of age). The density of [3H]SCH 23390 binding sites increased 14-fold from birth to postnatal day 35, when a peak value was reached, whereas a significant decrease was observed in the striatum of aged rats. In contrast, the affinity of D1 DA receptors for [3H]SCH 23390 remained unchanged from birth through senescence. The stimulation of cyclic AMP formation induced by 100 microM DA increased 4-fold from birth to postnatal day 14, when the maximal responsiveness to DA was observed and then returned to adult levels. No significant alterations were observed in the Km values during development, whereas the stimulatory effect of 100 microM DA on AC activity was significantly decreased in senescent rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Involvement of D1 and D2 Dopamine Receptors in the Control of Horizontal Cell Electrical Coupling in the Turtle Retina

We studied the actions of D1 and D2 dopamine agonists and antagonists on the coupling of horizontal cell axons in the turtle retina by a combination of pharmacological and electrophysiological methods. Both D1 and D2 receptors were identified in membrane fractions by radioligand binding using [3H]-SCH 23390 and [3H]-spiperone, respectively. The KD of both receptor classes were identical (0.21 nM) but D1 receptor density exceeded that of D2 receptors by more than four-fold. D1 agonists increased the activity of adenylate cyclase in a dose-dependent manner, whereas D2 agonists were without significant effect by themselves, nor did D2 antagonists block the D1-mediated increase in adenylate cyclase activity. Intracellular recordings and Lucifer Yellow dye injections were used to characterize the modifications of the receptive field profile of horizontal cell axons (H1AT) exposed to different pharmacological agents. Dopamine or D1 agonists (0.05 - 10 microM) induced a marked constriction of the H1AT receptive field, whereas D2 agonists elicited a small expansion of the receptive field. However, in the presence of a D1 antagonist, as well as IBMX to inhibit phosphodiesterase, D2 agonists (10 - 70 microM) induced a marked increase in the receptive field profile. These results indicate that both D1 and D2 dopamine receptors play a role in shaping the receptive field profile of the horizontal cell axon terminal in the turtle retina.     

A psoriasis vulgaris protective gene maps close to the HLA-C locus on the EH18.2-extended haplotype

We determined the molecular haplotypes of the HLA-A, HLA-C and HLA-B loci and the MHC class I-B-related (MIB) microsatellite in 179 unrelated psoriatic patients (72 familial cases) and in 120 controls. The HLA-A*3002-Cw*0501-B*1801-MIB1 haplotype showed a strong negative association with psoriasis vulgaris (PV) and in particular with familial PV, revealing the presence of a PV-protective gene. Analysis of association and linkage disequilibrium of the single alleles and the various two-three-four-locus segments of this haplotype indicated the presence of a protective gene telomeric to the HLA-C locus. This finding was confirmed in 13 informative multiplex PV families, in which at least one parent carried the EH18.2 haplotype. In two families, an affected sibling presented HLA-A/C recombination on the EH18.2 haplotype. A study of 12 polymorphic microsatellites in all members of the informative families, 145 PV patients, 120 controls and 32 EH18.2 homozygous healthy individuals demonstrated that the protection conferred by the EH18.2 haplotype lies within a 170 kb interval between the C143 and C244 loci, most probably in a 60 kb segment between the C132 and C244 loci.     

The abstinence syndrome in diazepam-dependent cats is precipitated by Ro 15-1788 and Ro 15-4513 but not by the benzodiazepine receptor antagonist ZK 93426

Diazepam-dependent cats were challenged with different benzodiazepine recognition site ligands 24 h after the last dose of chronic treatment with diazepam (7 mg/kg, i.p. at 08.00 and 20.00 h, for 21 consecutive days). The benzodiazepine derivatives Ro 15-4513 (a partial inverse agonist) and Ro 15-1788 (a pure antagonist), precipitated an abstinence syndrome within minutes after i.p. administration. Abstinence signs included tremors, increased muscle tone, irritability, fear, pupillary dilation and vocalizations. On the contrary, the beta-carboline derivatives ZK 93426 (an antagonist) and FG 7142 (a partial inverse agonist) failed to precipitate abstinence signs in diazepam-dependent cats when given at doses that prevented the acute effects of diazepam. Our results demonstrate that the ability to induce withdrawal signs in diazepam-dependent cats strongly depends on the benzodiazepine structure of the challenge drug since beta-carboline antagonist and partial inverse agonists lack this property.     

Molecular and neurochemical evaluation of the effects of etizolam on GABAA receptors under normal and stress conditions

The thienobenzodiazepine derivative etizolam (CAS 40054-69-1, 6-(o-chlorophenyl)-8-ethyl-1-methyl-4H-s-triazolo-(3,4-c)thienol(1 ,4) diazepine) is a potent anxiolytic with a pharmacological profile similar to that of classical benzodiazepines. In order to rationalize the therapeutic use of etizolam, its pharmacodynamics properties on GABAA receptors were investigated by a comparative study with other ligands on human recombinant GABAA as well as rat brain native receptors. Etizolam inhibited in a concentration-dependent manner [3H]flunitrazepam (CAS 1622-62-4) binding to rat cortical membranes, with an affinity of 4.5 nmol/l greater than that of alprazolam (CAS 28981-97-7) (7.9 nmol/l). Ethizolam enhanced GABA-induced Cl- currents in oocytes expressing human cloned GABAA receptors. With alpha 1 beta 2 gamma 2S subunit combination, etizolam produced a 73% increase in GABA-induced currents with an EC50 of 92 nmol/l. At the same receptor type, alprazolam showed a higher degree of potentiation and potency (98%, EC50 56 nmol/l). At alpha 2 beta 2 gamma 2S or alpha 3 beta 2 gamma 2S subunit constructs, the effects of etizolam were similar to those of alprazolam. Flumazenil (CAS 78755-81-4) completely blocked both etizolam and alprazolam effects on GABA-induced currents. Etizolam, administered i.p., was uneffective in changing ex vivo t-[35S]butylbicyclophosphorothionate ([35S]-TBPS) binding to rat cerebral cortex, whereas alprazolam and abecarnil (CAS 111841-85-1) significantly reduced this parameter. However, etizolam similarly to abecarnil and alprazolam, antagonized isoniazid-induced increase (61%) in [35S]-TBPS binding to rat cortical membranes. Further, etizolam inhibited in a dose-dependent manner basal acetylcholine release from both hippocampus and prefrontal cortex, and reversed foot-shock-induced increase of basal acetylcholine release to a control level. Altogether, these results suggest that etizolam may have a reduced intrinsic activity, at least at specific subpopulations of GABAA receptors. This property, together with the pharmacokinetic indication of a short-acting drug, may characterize etizolam as a ligand endowed with less side-effects typical of full agonits such as diazepam (CAS 439-14-5) and alprazolam. Finally, given its marked efficacy under conditions of GABAergic deficit, etizolam may represent a possible drug of choice with reduced liability to produce tolerance and dependence after long-term treatment of anxiety and stress syndromes.     

2-Phenyl-imidazo[1,2-a]pyridine derivatives as ligands for peripheral benzodiazepine receptors: stimulation of neurosteroid synthesis and anticonflict action in rats.

Selective activation of peripheral benzodiazepine receptors (PBRs) in adrenal cells and brain oligodendrocytes promotes steroidogenesis. Three 2-phenyl-imidazo[1,2-a]pyridine derivatives (CB 34, CB 50 and CB 54) have now been investigated with regard to their selectivity for PBRs and their ability to stimulate central and peripheral steroidogenesis in rats. The three CB compounds (10(-10)-10(-4) M) potently inhibited the binding of the PBR ligand [3H]-PK 11195 to brain and ovary membranes in vitro, without substantially affecting [3H]-flunitrazepam binding to central benzodiazepine receptors. These compounds (10(-7)-10(-4) M) also had little or no marked effects on GABA-evoked Cl- currents in voltage-clamped Xenopus oocytes expressing human alpha1beta2gamma2S GABA(A) receptors. In addition, they failed to affect ligands binding to GABA(B), D1/D2 dopamine, muscarinic acetylcholine, N-methyl-D-aspartic acid and opiate receptors. Intraperitoneal administration of CB compounds (3-50 mg kg(-1)) induced a dose-dependent increase in the concentrations of neuroactive steroids in plasma and brain. The brain concentrations of pregnenolone, progesterone, allopregnanolone and allotetrahydrodeoxycorticosterone (THDOC) showed maximal increases in 96+/-3, 126+/-14, 110+/-12 and 70+/-13% above control, respectively, 30 to 60 min after injection of CB 34 (25 mg kg(-1)). CB 34 also increased the brain concentrations of neuroactive steroids in adrenalectomized-orchiectomized rats, although to a lesser extent than in sham-operated animals, suggesting that CB compounds stimulate brain steroidogenesis independently of their effects on peripheral tissues. The increase in brain and plasma neurosteroid content induced by CB 34 was associated with a marked anticonflict effect in the Vogel test. Our results indicate that the three CB compounds tested are specific and potent agonists at peripheral benzodiazepine receptors, and that they stimulate steroidogenesis in both the brain and periphery.     

Inhibition of basal and stress-induced dopamine release in the cerebral cortex and nucleus accumbens of freely moving rats by the neurosteroid allopregnanolone

The neurosteroid allopregnanolone is a potent and efficacious modulator of γ-aminobutyric acid (GABA) type A receptors. The effects of intracerebroventricular injection of allopregnanolone (5 to 15 μg in 5 μl) on basal and stress-induced changes in the extracellular concentrations of dopamine were investigated by microdialysis in various brain areas of freely moving rats and compared with those of the benzodiazepine midazolam (1 to 10 μg in 5 μl). Allopregnanolone reduced (by a maximum of 65 to 75%) basal dopamine content in the prefrontal cortex and nucleus accumbens in a dose-dependent manner, but had no effect on dopamine output in the striatum. Allopregnanolone (10 to 15 μg) also completely prevented the increase in extracellular dopamine concentrations in the nucleus accumbens and cerebral cortex induced by foot-shock stress. Midazolam reduced basal dopamine content in all three brain regions studied as well as the stress- induced increase in dopamine content in the nucleus accumbens and cerebral cortex with a greater potency than allopregnanolone. These results suggest that endogenous neurosteroids may participate in the GABAergic modulation of dopaminergic transmission in the rat cerebral cortex and nucleus accumbens, two brain areas which are important in the regulation of emotional processes. These agents do not appear to affect striatal dopaminergic transmission which modulates motor function.     

Molecular mechanisms of tolerance to and withdrawal of GABAA receptor modulators

Here, we summarize recent data pertaining to the effects of GABA_A receptor modulators on the receptor gene expression in order to elucidate the molecular mechanisms behind tolerance and dependence induced by these drugs. Drug selectivity and intrinsic activity seems to be important to evidence at the molecular level the GABA_A receptor tolerance. On the contrary, we suggested that all drug tested are equally potentially prone to induce dependence. Our results demonstrate that long-lasting exposure of GABA_A receptors to endogenous steroids, benzodiazepines and ethanol, as well as their withdrawal, induce marked effects on receptor structure and function. These results suggest the possible synergic action between endogenous steroids and these drugs in modulating the functional activity of specific neuronal populations. We report here that endogenous steroids may play a crucial role in the action of ethanol on dopaminergic neurons.     

Pivagabine-induced increases in the abundance of CRF mRNA in the cerebral cortex and hypothalamus of rats

The effect of treatment of rats with pivagabine (4-[(2,2-dimethyl-1-oxopropyl) amino] butanoic acid) for 4 days on the abundance of corticotropin-releasing factor (CRF) mRNA in the brain was investigated. Such treatment resulted in dose-dependent (100-300 mg/kg, i.p.) increases in the amount of CRF mRNA in both the hypothalamus and cerebral cortex. The maximal increases were thus apparent with the dose of 300 mg/kg in the hypothalamus (+108%) and cerebral cortex (+49%) 30 or 60 min, respectively, after the last drug injection. Foot-shock stress administered 30 min after the final drug injection had no effect on the pivagabine-induced increases in the abundance of CRF mRNA in the hypothalamus or cerebral cortex. Such stress also had no effect on the amounts of CRF mRNA in these brain regions of vehicle-treated rats. These results demonstrate that pivagabine increases the amount of CRF mRNA in both the hypothalamus and cerebral cortex of rats, effects that might be relevant to the action of this drug in preventing the stress-induced changes in CRF hypothalamic concentration.