Tolerance to the sedative and anxiolytic effects of diazepam is associated with different alterations of GABAA receptors in rat cerebral cortex.

The clinical use of benzodiazepines is limited by the development of tolerance to their pharmacological effects. Tolerance to each of the pharmacological actions of benzodiazepines develops at different rates. The aim of this work was to investigate the mechanism of tolerance by performing behavioral tests in combination with biochemical studies. To this end, we administered prolonged treatments of diazepam to rats for 7 or 14 days. Tolerance to the sedative effects of diazepam was detected by means of the open field test after the 7- and 14-day treatments, whereas tolerance to the anxiolytic actions of benzodiazepine manifested following only the 14-day treatment in the elevated plus maze. The cerebral cortical concentrations of diazepam did not decline after the diazepam treatments, indicating that tolerance was not due to alterations in pharmacokinetic factors. The uncoupling of GABA/benzodiazepine site interactions and an increase in the degree of phosphorylation of the GABAA receptor γ2 subunit at serine 327 in the cerebral cortex were produced by day 7 of diazepam treatment and persisted after 14 days of exposure to benzodiazepine. Thus, these alterations could be part of the mechanism of tolerance to the sedative effects of diazepam. An increase in the percentage of α1-containing GABAA receptors in the cerebral cortex was observed following the 14-day treatment with diazepam but not the 7-day treatment, suggesting that tolerance to the anxiolytic effects is associated with a change in receptor subunit composition. The understanding of the molecular bases of tolerance could be important for the development of new drugs that maintain their efficacies over long-term treatments.

GABA-induced uncoupling of GABA/benzodiazepine site interactions is mediated by increased GABAA receptor internalization and associated with a change in subunit composition.

Persistent activation of GABAA receptors triggers compensatory changes in receptor function that are relevant to physiological, pathological and pharmacological conditions. Chronic treatment of cultured neurons with GABA for 48h has been shown to produce a down-regulation of receptor number and an uncoupling of GABA/benzodiazepine site interactions with a half-time of 24-25h. Down-regulation is the result of a transcriptional repression of GABAA receptor subunit genes and depends on activation of L-type voltage-gated calcium channels. The mechanism of this uncoupling is currently unknown. We have previously demonstrated that a single brief exposure of rat primary neocortical cultures to GABA for 5-10min (t½=3min) initiates a process that results in uncoupling hours later (t½=12h) without a change in receptor number. Uncoupling is contingent upon GABAA receptor activation and independent of voltage-gated calcium influx. This process is accompanied by a selective decrease in subunit mRNA levels. Here, we report that the brief GABA exposure induces a decrease in the percentage of α3-containing receptors, a receptor subtype that exhibits a high degree of coupling between GABA and benzodiazepine binding sites. Initiation of GABA-induced uncoupling is prevented by co-incubation of GABA with high concentrations of sucrose suggesting that it is dependent on a receptor internalization step. Moreover, results from immunocytochemical and biochemical experiments indicate that GABA exposure causes an increase in GABAA receptor endocytosis. Together, these data suggest that the uncoupling mechanism involves an initial increase in receptor internalization followed by activation of a signaling cascade that leads to selective changes in receptor subunit levels. These changes might result in the assembly of receptors with altered subunit compositions that display a lower degree of coupling between GABA and benzodiazepine sites. Uncoupling might represent a homeostatic mechanism that negatively regulates GABAergic transmission under physiological conditions in which synaptic GABAA receptors are transiently activated for several minutes.

Genetic disruption of the autism spectrum disorder risk gene PLAUR induces GABAA receptor subunit changes.

Disruption of the GABAergic system has been implicated in multiple developmental disorders, including epilepsy, autism spectrum disorder and schizophrenia. The human gene encoding uPAR (PLAUR) has been shown recently to be associated with the risk of autism. The uPAR(-/-) mouse exhibits a regionally-selective reduction in GABAergic interneurons in frontal and parietal regions of the cerebral cortex as well as in the CA1 and dentate gyrus subfields of the hippocampus. Behaviorally, these mice exhibit increased sensitivity to pharmacologically-induced seizures, heightened anxiety, and atypical social behavior. Here, we explore potential alterations in GABAergic circuitry that may occur in the context of altered interneuron development. Analysis of gene expression for 13 GABA(A) receptor subunits using quantitative real-time polymerase chain reaction (PCR) indicates seven subunit mRNAs (alpha(1), alpha(2), alpha(3), beta(2), beta(3), gamma(2S) and gamma(2L)) of interest. Semi-quantitative in situ hybridization analysis focusing on these subunit mRNAs reveals a complex pattern of potential gene regulatory adaptations. The levels of alpha(2) subunit mRNAs increase in frontal cortex, CA1 and CA3, while those of alpha3 decrease in frontal cortex and CA1. In contrast, alpha(1) subunit mRNAs are unaltered in any region examined. beta(2) subunit mRNAs are increased in frontal cortex whereas beta(3) subunit mRNAs are decreased in parietal cortex. Finally, gamma(2S) subunit mRNAs are increased in parietal cortex while gamma(2L) subunit mRNAs are increased in the dentate gyrus, potentially altering the gamma(2S):gamma(2L) ratio in these two regions. For all subunits, no changes were observed in forebrain regions where GABAergic interneuron numbers are normal. We propose that disrupted differentiation of GABAergic neurons specifically in frontal and parietal cortices leads to regionally-selective alterations in local circuitry and subsequent adaptive changes in receptor subunit composition. Future electrophysiological studies will be useful in determining how alterations in network activity in the cortex and hippocampus relate to the observed behavioral phenotype.

Role of allopregnanolone on cerebellar granule cells neurogenesis.

The role of allopregnanolone on immature cerebellar granule cells (CGC) proliferation was studied. Allopregnanolone (0.1-1 microM) increased [(3)H]thymidine incorporation and cell number determined by neuronal counting and by an MTT colorimetric assay. The effect of the neurosteroid was completely prevented by preincubation with 10 mM MgCl(2), 10 microM nifedipine, 10 microM picrotoxin or by 50 microM bicuculine. We conclude that ALLO affects cerebellar neurogenesis by increasing calcium influx through voltage-gated calcium channels and GABA(A) receptors activation.

Diazepam fails to potentiate GABA-induced chloride uptake and to produce anxiolytic-like action in aged rats.

The pharmacological response to benzodiazepines has been demonstrated to be different in aged individuals in comparison to adults. We studied the age-dependent changes in some of the in vitro and behavioral effects of diazepam in aged (24 months old) rats, comparing them to adults (3 months old). We evaluated the in vitro gamma-aminobutyric acid (GABA)-induced 36Cl- uptake and the diazepam potentiation of GABA-stimulated 36Cl- uptake in microsacs from cerebral cortex of both groups of animals. We found no differences in the GABA-stimulated 36Cl- uptake between adult and aged animals, and diazepam failed to potentiate GABA-induced 36Cl- flux in the aged cortical microsacs. We also examined the effect of 0.03-10 mg of diazepam on locomotor activity in an open-field test and the anxiolytic-like action of diazepam in doses ranging from 0.03 to 1 in a dark-light transition test. We observed no anxiolytic-like action of the drug in the dark-light transition test in the aged rats, while there was a shift to the left in the diminution of locomotor activity evaluated by the open-field test. We conclude that the pharmacodynamic changes observed in cortical GABA(A) receptors in aged rats could partially explain the lack of anxiolytic-like action but not the oversedation evidenced in this group of animals.

Modulatory action of zinc on GABA(A) receptor complex during avian CNS development.

In the present work, we studied the effect of zinc on GABA(A) receptor complex at three developmental stages of chick optic lobe (embryonic day 14, post-hatching day 1, and adulthood), in order to investigate the role of this cation in central nervous system (CNS) functional maturation. It was demonstrated that zinc exerts an inhibitory modulation of both GABA binding and GABA-gated chloride flux in a concentration-dependent manner with maximal effects at 100 microM zinc concentration. Maximal inhibition was higher at the embryonic stage and declined thereafter, disclosing minimal values at the adult stage. The effect of zinc on saturation GABA binding experiments performed at embryonic day 14 demonstrated that the cation decreased the maximal number of binding sites (B(max)) from 7. 53 +/- 1.06 pmol/mg protein to 4.63 +/- 0.53 pmol/mg protein, in the absence and presence of 100 microM zinc, respectively, while the dissociation constant (K(d)) remained unchanged. Analysis of the GABA concentration-effect curve at the embryonic stage revealed that the addition of 100 microM zinc decreased E(max) values for GABA stimulation of chloride uptake from 26.46 +/- 2.64% to 16.40 +/- 1. 96%, while EC(50) values were unaffected. In conclusion, our results suggest that zinc acts as a non-competitive inhibitor of both GABA binding and GABA responses during avian CNS development, with its effect inversely related to age.

Presence of an endogenous factor in avian CNS with agonistic action on benzodiazepine receptors.

In the present paper we describe the presence in avian CNS of an endogenous inhibitor of [3H]flunitrazepam binding. This compound was extracted from a synaptic membrane fraction isolated from chick optic lobe and brain using an exhaustive aqueous washing procedure, then purified by means of solid-phase extraction with C18 cartridges and several HPLC steps until an homogeneous peak was obtained. Its chemical structure was studied by size-exclusion chromatography of the purified material which indicated that it possesses a molecular weight below 1350. Although its inhibitory activity was lost by HCl treatment, its peptidic nature was ruled out by an amino acid and N-terminal sequence analyses. Ultraviolet absorption spectrum showed two main peaks at 230 and 280 nm. The endogenous compound was found to inhibit competitively [3H]flunitrazepam binding to its recognition site without affecting [3H]GABA binding to the same receptor complex. The behavior of the endogenous factor in an "in vitro" GABA "shift" test and GABA-dependent chloride flux experiments were similar to that of benzodiazepine receptor agonists. In conclusion, these results demonstrate the existence in avian CNS of a competitive endogenous inhibitor of benzodiazepine binding with agonistic action on benzodiazepine receptors.

GABA-stimulated chloride uptake during avian CNS development: modulation by neurosteroids.

In the present report we studied the GABA-stimulated 36Cl- uptake during chick optic lobe development in order to establish the ontogenetic profile of the functional GABAA receptor complex. A concentration-dependent stimulation of 36Cl- influx by GABA was demonstrated, starting at developmental stages as early as 10 days of incubation. The maximal GABA-induced 36Cl- uptake changed significantly during ontogeny with highest values near hatching. However, GABA potency to stimulate ion influx remained unchanged. We also examined the effect of two neurosteroids, allopregnanolone and epipregnanolone, on GABA-stimulated 36Cl- influx at three developmental stages (embryonic day 14, post-hatching day 1 and adult stage). Both steroids enhanced ion uptake in a concentration-dependent manner, exerting greater stimulatory effects at early developmental stages. Allopregnanolone displayed EC50 values lower than epipregnanolone at all three time points and was also more potent at post-hatching stages. Analysis of the GABA concentration-effect curve disclosed that both steroid decreased EC50 values for GABA stimulation while Emax levels were unaffected. In conclusion, our results showed an early appearance of the GABA-associated chloride channel together with the ability of neurosteroids to modulate GABA-gating of such channel.

Neurosteroid modulation of the benzodiazepine receptor in developing chick optic lobe.

The modulation of [3H]flunitrazepam binding by 3 alpha-hydroxy-5 alpha-dihydroprogesterone (3 alpha-OH-DHP) and 3 alpha, 5 alpha-tetrahydrodeoxicorticosterone (3 alpha-THDOC) was investigated in synaptic membranes isolated from chick optic lobe at three developmental stages, in order to evaluate the role of neurosteroids in central nervous system functional maturation. It was demonstrated that both steroids modulate [3H]flunitrazepam binding in a concentration-dependent manner at embryonic day 14, hatching and adult stage, producing maximal [3H]flunitrazepam binding enhancement at early stages of development and declining thereafter. EC50 values for 3 alpha-OH-DHP were lower than for 3 alpha-THDOC at all stages examined. On the other hand, Emax values were higher with 3 alpha-OH-DHP than with 3 alpha-THDOC. Scatchard analysis performed at embryonic day 14, hatching and adult stage showed that dissociation constants (Kd) were of 22.23 +/- 0.15, 18.38 +/- 1.15 and 19.86 +/- 0.62 nM, and maximal number of binding sites (Bmax) were 1.95 +/- 0.15, 3.13 +/- 0.21 and 2.25 +/- 0.13 pmol/mg protein, respectively. By adding 4 microM of either 3 alpha-OH-DHP or 3 alpha-THDOC, Kd values decreased significantly to 10.65 +/- 0.62, 9.71 +/- 0.85 and 13.25 +/- 0.74 nM or 9.54 +/- 0.65, 11.20 +/- 1.27 and 12.96 +/- 1.38 nM, at the above mentioned stages, respectively. Thus, either drug at the given concentration increased the affinity of benzodiazepine receptor sites for [3H]flunitrazepam, while the density of receptor sites remained unchanged. Our results suggest that these steroids display a positive allosteric modulation of benzodiazepine receptor sites which is inversely related to age.

Ontogeny of two different benzodiazepine binding sites in the chick optic lobe.

The developmental time-course of type I and type II benzodiazepine receptors in the chick optic lobe was determined using a triazolopyridazine, CL 218872. At embryonic day 13 most of the binding sites corresponded to type II (98.23%), while type I represented only a minor proportion (1.77%). During development there was an increase in type I binding sites which reached 62.88% in adulthood, while type II binding sites decreased to 37.12%. These results demonstrate a differential ontogeny of two benzodiazepine receptor subtypes. Changes in the benzodiazepine binding population may account for the variability in the GABA-benzodiazepine receptor interaction during chick optic lobe development.

Presence of endogenous inhibitors of benzodiazepine binding in developing chick optic lobe.

An endogenous inhibitor of benzodiazepine receptor binding was removed from synaptic membranes of developing chick optic lobe by an exhaustive buffer washing procedure. This treatment increased [3.H]flunitrazepam binding at all stages of development, although this effect was greater at early stages (embryonic day 14-16). Scatchard analysis performed on exhaustively washed membranes at embryonic day 16 and postnatal day 15 revealed the presence of a single population of flunitrazepam binding sites with apparent dissociation constants (Kd) of 1.99 +/- 0.11 and 3.28 +/- 0.10 nM and a maximal number of binding sites (Bmax) of 1.07 +/- 0.08 and 1.22 +/- 0.11 pmol mg-1 protein, respectively. At both stages of development, the addition of inhibitory material, present in the supernatants obtained after the washing procedure, reduced the affinity of recognition sites for [3H]flunitrazepam with Kd of 4.92 +/- 0.13 and 5.62 +/- 0.12 nM, respectively while their Bmax values remained unchanged. These studies demonstrate the presence of an endogenous material in developing chick optic lobe which competitively inhibits [3H]flunitrazepam binding to its receptor site.

Methods for removing endogenous factors from CNS membrane preparations: differences in [3H]GABA binding parameters and developmental-related effects.

The present report describes a systematic study comparing and combining methods currently used for the removal of endogenous factors known to affect the interaction of GABA with its receptor. The effects of these methods were analyzed by performing [3H]GABA binding studies, and by measuring the amount of residual GABA left in the different membrane preparations. The effectiveness of these methods were also applied to different developmental stages. The results show that: 1) an exhaustive buffer washing procedure is necessary to accurately measure the maximal binding capacity (Bmax) of the low-affinity GABA binding site, and 2) the use of more drastic methods, including freeze-thawing and Triton treatment allows a clear demonstration of receptor heterogeneity and a precise measurement of the Bmax of the high-affinity GABA binding site as well as increases the affinity of the low-affinity site. The analysis of the Bmax values obtained with these different procedures in relation to the values of GABA removal, strongly indicates that the exhaustive washing procedure removes some unknown endogenous substances required for Triton treatment to exhibit its maximal effectiveness. Finally, a detailed analysis of Kd and Bmax values obtained with these three methods in the developing nervous tissue shows the existence of significant differences with regard to their effectiveness in removing endogenous substances when applied in different developmental stages.

The postnatal development of benzodiazepine receptor sites in the chick optic lobe is modulated by environmental lighting.

The present paper describes the ability of benzodiazepine receptor sites to undergo light mediated-plastic changes during the early postnatal development of the chick optic lobe. The postnatal development pattern of these receptors was studied under different levels of light stimulation, i.e. normal-, light-and dark-rearing. At hatching the specific binding of [3H]Flunitrazepam was 0.23 +/- 0.01 pmol/mg protein. The developmental profile shows a sharp and transient peak of receptor overexpression between the 1st and the 2nd postnatal day in three experimental groups. Between the 2nd and the 6th day significant differences were found between the three groups, being this difference maximal during the peak of overexpression. In fact, on the 2nd day the specific [3H]Flunitrazepam binding showed an increase of 17% (P < 0.0005) and a decrease of 34% (P < 0.0005) for light- and dark-reared animals as compared with normally-reared ones. The changes in receptor density were transient since from the 6th day onward they gradually disappeared, being almost identical in the three groups by the day 15. At this moment the number of benzodiazepine receptor sites stabilized at the adult level. Scatchard analysis at the 2nd postnatal day revealed that the differences observed in the high affinity benzodiazepine binding sites between the three groups were due to modifications in the total number of binding sites while the affinity remained unchanged. The maximal number of binding sites were: 2.76 +/- 0.03, 3.40 +/- 0.01 and 1.46 +/- 0.11 pmol/mg protein in normally-, light- and dark-reared chicks, respectively; while the apparent dissociation constants were unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepine receptor sites in the chick optic lobe: development and pharmacological characterization.

To investigate the interaction between gamma-aminobutyric acid (GABA) and benzodiazepine (BZD) receptor sites during development, the time-course of appearance of flunitrazepam (FNZ) binding sites and their pharmacological characterization were studied in developing chick optic lobe. At the earliest stage examined, embryonic day (Ed) 12, the receptor density was 30.9% (0.05 +/- 0.01 pmol/mg protein) of that found in the chick optic lobes of adult chicks. The adult value was achieved on Ed 16 (0.16 +/- 0.01 pmol/mg protein). After this stage there was a sharp and transient increase in specific [3H]FNZ binding of about two-fold reaching a maximal value between hatching and the postnatal day (pnd) 2 (0.33 +/- 0.01 pmol/mg protein). Scatchard analysis at different stages of development revealed the presence of a single population of specific FNZ binding sites. The increase in [3H]FNZ binding during development was due to a large number of binding sites while their affinity remained unchanged. Competition experiments in the chick optic lobe revealed that the order of potency for displacement of specific [3H]FNZ binding paralleled the pharmacological potency of the BZDs tested. The IC50s for clonazepam, flunitrazepam, Ro 15-1788 and chlordiazepoxide were 3.02, 4.30, 0.32, and 4778.64 nM respectively. Ro 5-4864, a potent inhibitor of BZD binding to peripheral tissues, had no effect on specific [3H]FNZ binding indicating that only central BZD binding sites are present in the chick optic lobe. The peak of maximal expression of BZD receptor sites precedes in 5-6 days the peak of GABA receptor sites indicating a precocious development of BZD receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)