Elisabetta Maciocco

Comparison of the effects of zaleplon, zolpidem, and triazolam at various GABAA receptor subtypes

The pyrazolopyrimidine zaleplon is a hypnotic agent that acts at the benzodiazepine recognition site of GABA(A) receptors. Zaleplon, like the hypnotic agent zolpidem but unlike classical benzodiazepines, exhibits preferential affinity for type I benzodiazepine (BZ(1)/omega(1)) receptors in binding assays. The modulatory action of zaleplon at GABA(A) receptors has now been compared with those of zolpidem and the triazolobenzodiazepine triazolam. Zaleplon potentiated GABA-evoked Cl(-) currents in Xenopus oocytes expressing human GABA(A) receptor subunits with a potency that was higher at alpha1beta2gamma2 receptors than at alpha2- or alpha3-containing receptors. Zolpidem, but not triazolam, also exhibited selectivity for alpha1-containing receptors. However, the potency of zaleplon at these various receptors was one-third to one-half that of zolpidem. Zaleplon and zolpidem also differed in their actions at receptors containing the alpha5 or gamma3 subunit. Zaleplon, zolpidem, and triazolam exhibited similar patterns of efficacy among the different receptor subtypes. The affinities of zaleplon for [(3)H]flunitrazepam or t-[(35)S]butylbicyclophosphorothionate ([(35)S]TBPS) binding sites in rat brain membranes were lower than those of zolpidem or triazolam. Furthermore, zaleplon, unlike zolpidem, exhibited virtually no affinity for the peripheral type of benzodiazepine receptor.

Clozapine, but not Haloperidol, Increases Brain Concentrations of Neuroactive Steroids in the Rat

The extrapyramidal side effects of typical antipsychotics, which are induced to a markedly reduced extent by clozapine, have been linked to a dysfunction of central γ-aminobutyric acid (GABA)-mediated neurotransmission. The effects of clozapine on the brain concentrations of 3α-hydroxy-5α-pregnan-20-one (allopregnanolone, AP) and 3α,21-dihydroxy-5α-pregnan-20-one (allotetrahydrodeoxycorticosterone, THDOC), two potent and endogenous positive allosteric modulators of GABA-mediated chloride current intensities at GABAA receptors, were compared with those of the typical antipsychotic haloperidol. A single administration of clozapine (1.25–20 mg/kg, IP), but not of haloperidol (0.1 or 0.5 mg/kg, IP), induced dose- and time-dependent increases in the concentrations of progesterone, AP, and THDOC in the cerebral cortex and striatum of rats. Clozapine (at 10 mg/kg, but not at lower doses) also increased the concentrations of these steroids as well as that of corticosterone in plasma in intact rats, but failed to increase the cortical concentrations of AP and THDOC in adrenalectomized-orchidectomized rats. An acute challenge with clozapine (10 mg/kg), administered 48 h after the termination of daily treatment with the same dose for 19 days, still increased the cortical concentrations of progesterone, AP, and THDOC. These results suggest that the clozapine-induced increases in neuroactive steroid concentrations in the brain may contribute to the atypical pharmacological profile of this antipsychotic drug.

Changes in Expression and Function of Extrasynaptic GABAA Receptors in the Rat Hippocampus during Pregnancy and after Delivery

Pregnancy is associated with changes in mood and anxiety level as well as with marked hormonal fluctuations. Increases in the brain concentrations of neuroactive steroids during pregnancy in rats are accompanied by changes in expression of subunits of the GABA type A receptor (GABAA-R) in the brain. Granule cells of the dentate gyrus (DGGCs) exhibit two components of inhibitory GABAergic transmission: a phasic component mediated by synaptic GABAA-Rs, and a tonic component mediated by extrasynaptic GABAA-Rs. Recordings of GABAergic currents were obtained from hippocampal slices prepared from rats in estrus, at pregnancy day 15 (P15) or P19, or at 2 d after delivery. Exogenous GABA or 3α,5α-THP induced an increase in tonic current in DGGCs that was significantly greater at P19 than in estrus. Neither tonic nor phasic currents were affected by pregnancy in CA1 pyramidal cells. Immunohistochemical analysis revealed a marked increase in the abundance of the δ subunit of the GABAA-R and a concomitant decrease in that of the γ2 subunit in the hippocampus at P19. Expression of the α4 subunit did not change during pregnancy but was increased 2 d after delivery. Treatment of rats from P12 to P18 with the 5α-reductase inhibitor finasteride prevented the changes in tonic current and in δ and γ2 subunit expression normally apparent at P19. These data suggest that the number of extrasynaptic GABAA-Rs is increased in DGGCs during late pregnancy as a consequence of the associated marked fluctuations in the brain levels of neuroactive steroids.

Increase in expression of the GABA(A) receptor alpha(4) subunit gene induced by withdrawal of, but not by long-term treatment with, benzodiazepine full or partial agonists.

The effects of long-term exposure to, and subsequent withdrawal of, diazepam or imidazenil (full and partial agonists of the benzodiazepine receptor, respectively) on the abundance of GABA(A) receptor subunit mRNAs and peptides were investigated in rat cerebellar granule cells in culture. Exposure of cells to 10 microM diazepam for 5 days significantly reduced the amounts of alpha(1) and gamma(2) subunit mRNAs, and had no effect on the amount of alpha(4) mRNA. These effects were accompanied by a decrease in the levels of alpha(1) and gamma(2) protein and by a reduction in the efficacy of diazepam with regard to potentiation of GABA-evoked Cl- current. Similar long-term treatment with 10 microM imidazenil significantly reduced the abundance of only the gamma(2)S subunit mRNA and had no effect on GABA(A) receptor function. Withdrawal of diazepam or imidazenil induced a marked increase in the amount of alpha(4) mRNA; withdrawal of imidazenil also reduced the amounts of alpha(1) and gamma(2) mRNAs. In addition, withdrawal of diazepam or imidazenil was associated with a reduced ability of diazepam to potentiate GABA action. These data give new insights into the different molecular events related to GABA(A) receptor gene expression and function produced by chronic treatment and withdrawal of benzodiazepines with full or partial agonist properties.

Physicochemical characterization and in vivo properties of Zolpidem in solid dispersions with polyethylene glycol 4000 and 6000.

Solid dispersions and physical mixtures of Zolpidem in polyethylene glycol 4000 (PEG 4000) and 6000 (PEG 6000) were prepared with the aim to increase its aqueous solubility. These PEG based formulations of the drug were characterized in solid state by FT-IR spectroscopy, X-ray powder diffraction, and differential scanning calorimetry. By these physical determinations no drug-polymer interactions were evidenced. Both solubility and dissolution rate of the drug in these formulations were increased. Each individual dissolution profile of PEG based formulation fitted Baker-Lonsdale and first order kinetic models. Finally, significant differences in ataxic induction time were observed between Zolpidem orally administered as suspension of drug alone and as solid dispersion or physical mixture. These formulations, indeed, showed almost two- to three-fold longer ataxic induction times suggesting that, in the presence of PEG, the intestinal membrane permeability is probably the rate-limiting factor of the absorption process.

Water-soluble salts of aminoacid esters of the anaesthetic agent Propofol

Abstract The glycinates 4 , 5 , acetates 6 , 7 , 10 , propionate 8 , butyrate 9 and carbonate 11 were synthesized and evaluated as potential water-soluble prodrugs of Propofol (2,6-diisopropylphenol) 1 suitable for parenteral administration. The 4–9 · HCl salts were also prepared and some of them (i.e. 4 · HCl and 6 · HCl) were found sufficiently soluble in aqueous solutions. The kinetics of hydrolysis of the esters 4–11 and 4–9 · HCl salts were studied in 0.05 M phosphate buffer pH 7.4, and a number of derivatives ( 4 , 6 , 7 , and corresponding HCl salts) were examined for their stability in human plasma and brain homogenate. Our results indicated that the salts 4 · HCl and 6 · HCl, sufficiently soluble in water, are relatively stable in physiological media. Most of the examined compounds, in particular compound 6 , were found to inhibit the binding of [ 35 S]- tert -butylbicyclophosphorothionate ([ 35 S]TBPS) demonstrating to possess affinity for the Propofol recognition site on GABA A receptors.

Effects of voluntary ethanol consumption on emotional state and stress responsiveness in socially isolated rats

Social isolation of rats immediately after weaning is thought to represent an animal model of anxiety-like disorders. This mildly stressful condition reduces the cerebrocortical and plasma concentrations of 3α-hydroxy-5α-pregnan-20-one (3α,5α-TH PROG) as well as increases the sensitivity of rats to the effects of acute ethanol administration on the concentrations of this neuroactive steroid. We further investigated the effects of voluntary consumption of ethanol at concentrations increasing from 2.5 to 10% over 4 weeks of isolation. Isolated rats showed a reduced ethanol preference compared with group-housed animals. Ethanol consumption did not affect the isolation-induced down-regulation of BDNF or Arc, but it attenuated the increase in the cerebrocortical concentration of 3α,5α-TH PROG induced by foot-shock stress in both isolated and group-housed animals as well as increased the percentage of number of entries made by socially isolated rats into the open arms in the elevated plus-maze test. Ethanol consumption did not affect expression of the α₄ subunit of the GABA(A) receptor in the hippocampus of group-housed or isolated rats, whereas it up-regulated the δ subunit throughout the hippocampus under both conditions. The results suggest that low consumption of ethanol may ameliorate some negative effects of social isolation on stress sensitivity and behavior.

Changes in expression of the δ subunit of the GABAA receptor and in receptor function induced by progesterone exposure and withdrawal

Type A receptors for GABA (GABAA receptors) that contain the δ subunit are located predominantly at extrasynaptic sites and are implicated in modulation of neuronal excitability through tonic inhibition. We have examined the effects of chronic exposure to and subsequent withdrawal of progesterone or the progesterone metabolite 3α‐hydroxy‐5α‐pregnan‐20‐one (3α,5α‐THPROG) on expression of the δ subunit of GABAA receptors and on receptor function in cultured rat hippocampal neurons. Progesterone treatment for 1 day increased the amounts of both δ subunit mRNA and protein, whereas such treatment for 6 days induced marked decreases in the abundance of both the mRNA and protein. Subsequent progesterone withdrawal up‐regulated expression of the δ subunit, which was significantly increased at 9–12 h after withdrawal. These effects of progesterone were mimicked by 3α,5α‐THPROG and blocked by the 5α‐reductase inhibitor finasteride. They were also accompanied by parallel changes in the function of GABAA receptors in hippocampal neurons. These results show that chronic exposure to and withdrawal of progesterone induce differential effects on both expression of the δ subunit of GABAA receptors and receptor function that are mediated by 3α,5α‐THPROG. They are consistent with the notion that this progesterone metabolite plays a key physiological role in modulation of GABAergic synapses.

Characterization of the electrophysiological and pharmacological effects of 4‐iodo‐2,6‐diisopropylphenol, a propofol analogue devoid of sedative‐anaesthetic properties

Several derivatives and analogues of the general anaesthetic 2,6‐diisopropylphenol (propofol) have been recently synthesised with the aim of exploring the structure‐activity relationships. In the present study, the effects of one such compound, 4‐iodo‐2,6‐diisopropylphenol (4‐I‐Pro), on γ‐aminobutyric acid type A (GABAA) receptors in vitro were compared with its in vivo effects in rodents. Human GABAA receptors were expressed in Xenopus oocytes, and the actions of 4‐I‐Pro on receptor function were compared with those of propofol by two‐electrode voltage‐clamp recording. Similar to propofol, 4‐I‐Pro directly activated Cl− currents in the absence of GABA at all combinations of receptor subunits tested. However, the efficacy of 4‐I‐Pro in inducing direct activation of α1β2γ2S receptors was markedly less than that of propofol. Similarly to propofol, 4‐I‐Pro potentiated in a concentration‐dependent manner GABA‐evoked Cl− currents measured at different GABAA receptor constructs. As expected, intraperitoneal injection of propofol induced sedation, ataxia, and loss of the righting reflex in rats. In contrast, administration of 4‐I‐Pro failed to produce any of these behavioural effects. Administration of 4‐I‐Pro to rats reduced in a dose‐dependent manner the incidence of tonic‐clonic seizures induced by pentylenetetrazol and induced an anticonflict effect as measured in the Vogel test. Microdialysis revealed that, like propofol, administration of 4‐I‐Pro reduced acetylcholine release in the hippocampus of freely moving rats. These results demonstrate that para‐substitution of the phenol ring of propofol with iodine yields a compound that exhibits anticonvulsant and anticonflict effects, but is devoid of sedative‐hypnotic and anaesthetic properties. Thus, 4‐I‐Pro possesses pharmacological characteristics more similar to anxiolytic and anticonvulsant drugs than to general anaesthetics.

The effect of cyclopyrrolones on GABAA receptor function is different from that of benzodiazepines.

The effects of the cyclopyrrolones zopiclone and suriclone on the function of the central γ-aminobutyric acid type A (GABAAA) receptor complex in mouse brain were evaluated both in vitro and in vivo. Added in vitro to mouse cerebral cortical membranes, these compounds potently inhibited [3H]flumazenil binding with IC50 (50% inhibitory concentration) values of 35.8 nM (zopiclone) and 1.1 nM (suriclone). Similar results were obtained with cerebellar membranes, indicating that these drugs do not discriminate between putative type I and type II benzodiazepine receptors. The interaction of cyclopyrrolones with recognition sites present at the level of the GABA receptor complex appears to be competitive, because zopiclone decreased the affinity of the receptors for [3H]flumazenil without affecting the maximal number of binding sites. Moreover, zopiclone and suriclone did not affect the rate of dissociation of [3H]flumazenil from benzodiazepine receptors. The in vitro efficacy of zopiclone appeared different from that of suriclone and the benzodiazepines diazepam and flunitrazepam. Thus, zopiclone failed to affect muscimol-stimulated 36Cl− uptake and only slightly inhibited t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding. In contrast, like diazepam and flunitrazepam, suriclone increased muscimol-stimulated 36Cl− uptake and markedly inhibited [35S]TBPS binding. On the other hand, suriclone, like zopiclone, did not modify [3H]muscimol binding to mouse cerebral cortical membranes. Moreover, zopiclone antagonized the reduction in [35S]TBPS binding elicited by the benzodiazepine receptor full agonist diazepam. Consistent with its low efficacy in vitro, oral administration of zopiclone (2.5 to 100 mg/kg, p.o.) in mice failed to modify [35S]TBPS binding subsequently measured in cerebral cortical membranes “ex vivo”. In contrast, suriclone (10 to 20 mg/kg, p.o.), like diazepam, decreased [35S]TBPS binding measured ex vivo. Moreover, both zopiclone (50 to 100 mg/kg, p.o.) and suriclone (1 to 10 mg/kg, p.o.) abolished the increase in [35S]TBPS binding induced by isoniazid (200 mg/kg, s.c.). These results suggest that suriclone may enhance GABAergic transmission with an efficacy similar to that of diazepam. In contrast, the low efficacy of zopiclone both in vitro and in vivo suggests that this drug may act as a partial agonist at benzodiazepine receptors.