Changes in properties and neurosteroid regulation of GABAergic synapses in the supraoptic nucleus during the mammalian female reproductive cycle. (1/331)

1. GABAA receptor-mediated synaptic innervation of oxytocin neurones in the supraoptic nucleus (SON) was analysed in adult female rats going through their first reproductive cycle by recording the spontaneous inhibitory postsynaptic currents (sIPSCs) at six stages of female reproduction. 2. During pregnancy we observed a reduction in the interval between monoquantal sIPSCs. The synaptic current amplitude, current decay and neurosteroid sensitivity of postsynaptic GABAA receptors observed at this stage were not distinguishable from those measured in virgin stage SON. 3. Upon parturition an increase in monoquantal synaptic current decay occurred, whereas potentiation by the progesterone metabolite allopregnanolone (3alpha-OH-DHP) was suppressed. 4. Throughout a substantial part of the lactation period the decay of synaptic currents remained attenuated, whilst the potentiation by 3alpha-OH-DHP remained suppressed. 5. Several weeks after the end of lactation sIPSC intervals, their current decay velocity as well as the potentiation by 3alpha-OH-DHP were restored to pre-pregnancy levels, which is indicative of the cyclical nature of synaptic plasticity in the adult SON. 6. Competitive polymerase chain reaction (PCR) analysis showed that virgin animals expressed alpha1 and alpha2 GABAA receptor subunit mRNA at a relative ratio of 2 : 1 compared with beta-actin. After pregnancy both alpha1 and alpha2 subunit mRNA levels were transiently increased, although at a relative ratio of 1 : 4, in line with the hypothesis that alpha2 plays a large role in postsynaptic receptor functioning. During post-lactation both alpha subunits were downregulated. 7. We propose that synaptic remodelling in the SON during pregnancy includes changes in the putative number of GABA release sites per neurone. At parturition, and during the two consecutive weeks of lactation, a subtype of postsynaptic GABAA receptors was observed, distinct from the one being expressed before and during pregnancy. Synaptic current densities, calculated in order to compare the impact of synaptic inhibition, showed that, in particular, the differences in 3alpha-OH-DHP potentiation of these two distinct GABAA receptor subtypes produce robust shifts in the impact of synaptic inhibition of oxytocin neurones at the different stages of female reproduction.  (+info)

Neurosteroid modulation of synaptic and GABA-evoked currents in neurons from the rat medial preoptic nucleus. (2/331)

The effects of the neurosteroid 3alpha-hydroxy-5alpha-pregnane-20-one (allopregnanolone) on synaptic and GABA-evoked currents in acutely dissociated neurons from the medial preoptic nucleus of rat were investigated by perforated-patch recordings under voltage-clamp conditions. The effect of 2.0 microM allopregnanolone on GABA-evoked currents depended strongly on the GABA concentration: the currents evoked by 100 microM GABA were markedly depressed and the desensitization was faster, but the decay after GABA application was prolonged. In contrast, the currents evoked by 1.0 microM GABA were markedly potentiated, the activation was faster, a prominent desensitization was induced, and the decay after GABA application was prolonged. In the absence of externally applied GABA, 2.0 microM allopregnanolone induced a slow current that could be attributed to Cl-. Allopregnanolone did not significantly affect the amplitude of spontaneous tetrodotoxin-insensitive (miniature) synaptic currents (mIPSCs) originating from synaptic terminals releasing GABA onto the dissociated neurons. However, the mIPSC decay phase was dramatically prolonged, with half-maximal effect at approximately 50 nM allopregnanolone. A qualitatively similar effect of allopregnanolone was seen when KCl was used to evoke synchronous GABA release. The frequency of mIPSCs was also affected, on average increased 3.5-fold, by 2.0 microM allopregnanolone, suggesting a presynaptic steroid action.  (+info)

Differential modulation of the gamma-aminobutyric acid type C receptor by neuroactive steroids. (3/331)

Gamma-aminobutyric acid type C receptor channels (GABA(C)Rs) composed of rho subunits are pharmacologically distinct from GABA(A) receptor channels (GABA(A)Rs). This difference is illustrated by the insensitivity of homo-oligomeric rho(1) receptor channels to many known modulators of GABA(A)Rs, such as barbiturates and benzodiazepines. A number of endogenous metabolites of corticosterone and progesterone, known as neuroactive steroids, compose yet another class of compounds that can modulate GABA(A)Rs. Here, several neuroactive steroids are shown to also modulate the rho(1) receptor channel. 5alpha-Pregnane-3alpha,21-diol-20-one (allotetrahydrodeoxycorticosterone), 5alpha-pregnane-3alpha-ol-11, 20-dione (alphaxalone), and 5alpha-pregnane-3alpha-ol-20-one (allopregnanolone) potentiated the GABA-evoked currents from rho(1) receptor channels and concomitantly altered the deactivation kinetics by prolonging the decay time. In contrast, 5beta-pregnane-3alpha-ol-20-one (pregnanolone), 5beta-pregnane-3, 20-dione (5beta-dihydroprogesterone), and 5beta-pregnane-3alpha, 21-diol-20-one (tetrahydrodeoxycorticosterone), all potentiators of GABA(A)Rs, inhibited the GABA-elicited currents of the rho(1) receptor channel. In comparison to GABA(A)Rs, the modulation of rho(1) receptor channels by these neuroactive compounds occurred with relatively high concentrations of the neuroactive steroids and was more prominent in the presence of low concentrations of GABA, equivalent to fractions of the EC(50) value of the rho(1) receptor channel. Structural comparison of these six neuroactive steroids reveals that the key parameter in determining the mode of modulation for the rho(1) receptor channel is the position of the hydrogen atom bound to the fifth carbon, imposing a trans- or cis-configuration in the backbone structure. This is the first demonstration of isomeric compounds that can differentially modulate the activity of the rho(1) receptor channel.  (+info)

Anaesthetic effects of pregnanolone in combination with allopregnanolone, thiopental, hexobarbital and flurazepam: an EEG study in the rat. (4/331)

The anaesthetic interactions of the steroid, 3 alpha-hydroxy-5 beta-pregnan-20-one, in male rats were investigated in different fixed binary combinations with the steroid allopregnanolone (3 alpha-hydroxy-5 alpha-pregnan-20-one), two barbiturates (thiopental and hexobarbital) and the benzodiazepine, flurazepam. Anaesthetic effects were determined using an EEG threshold method. Interactions were assessed using an isobolographic method. The interaction between the two steroids, pregnanolone and allopregnanolone, showed an anaesthetic effect significantly less than additive (antagonistic). The interactions between pregnanolone and the two barbiturates and the benzodiazepine showed an anaesthetic effect significantly greater than additive (potentiation) in all tests performed. These results could be explained by a pharmacodynamic interaction at the hypothetical GABA-benzodiazepine-barbiturate-steroid complex in the CNS.  (+info)

Selective serotonin reuptake inhibitors directly alter activity of neurosteroidogenic enzymes. (5/331)

The neurosteroid 3alpha-hydroxysteroid-5alpha-pregnan-20-one (allopregnanolone) acts as a positive allosteric modulator of gamma-aminobutyric acid at gamma-aminobutyric acid type A receptors and hence is a powerful anxiolytic, anticonvulsant, and anesthetic agent. Allopregnanolone is synthesized from progesterone by reduction to 5alpha-dihydroprogesterone, mediated by 5alpha-reductase, and by reduction to allopregnanolone, mediated by 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD). Previous reports suggested that some selective serotonin reuptake inhibitors (SSRIs) could alter concentrations of allopregnanolone in human cerebral spinal fluid and in rat brain sections. We determined whether SSRIs directly altered the activities of either 5alpha-reductase or 3alpha-HSD, using an in vitro system containing purified recombinant proteins. Although rats appear to express a single 3alpha-HSD isoform, the human brain contains several isoforms of this enzyme, including a new isoform we cloned from human fetal brains. Our results indicate that the SSRIs fluoxetine, sertraline, and paroxetine decrease the K(m) of the conversion of 5alpha-dihydroprogesterone to allopregnanolone by human 3alpha-HSD type III 10- to 30-fold. Only sertraline inhibited the reverse oxidative reaction. SSRIs also affected conversions of androgens to 3alpha- and 3alpha, 17beta-reduced or -oxidized androgens mediated by 3alpha-HSD type II(Brain). Another antidepressant, imipramine, was without any effect on allopregnanolone or androstanediol production. The region-specific expression of 3alpha-HSD type II(Brain) and 3alpha-HSD type III mRNAs suggest that SSRIs will affect neurosteroid production in a region-specific manner. Our results may thus help explain the rapid alleviation of the anxiety and dysphoria associated with late luteal phase dysphoria disorder and major unipolar depression by these SSRIs.  (+info)

Response-rate suppression in operant paradigm as predictor of soporific potency in rats and identification of three novel sedative-hypnotic neuroactive steroids. (6/331)

Novel neuroactive steroids were evaluated for their effects on operant responding, rotorod motor performance, and electroencephalogram recording in rats. Co 134444, Co 177843, and Co 127501 were compared with the prototypical gamma-aminobutyric acid(A)-positive allosteric modulators triazolam, zolpidem, pentobarbital, pregnanolone, and CCD 3693. Each of the compounds produced a dose-related decrease in response rates under a variable-interval 2-min schedule of positive reinforcement in an operant paradigm. In addition, all compounds produced a dose-related increase in ataxia and significant increases in nonrapid eye movement sleep in this experiment or have been previously reported to do so. Co 134444, Co 177843, and Co 127501 increased nonrapid eye movement sleep at doses that had no effect on rapid eye movement sleep. All of the compounds were more potent at decreasing operant responding than they were at increasing ataxia. Furthermore, the potency of compounds to produce response-rate suppression in an operant paradigm appeared to be a better predictor of soporific potency than did potency in the rotorod assay. The screening for sedative-hypnotic activity resulted in the identification of the novel orally active neuroactive steroids Co 134444, Co 177843, and Co 127501.  (+info)

Development and role of GABA(A) receptor-mediated synaptic potentials during swimming in postembryonic Xenopus laevis tadpoles. (7/331)

We have investigated the contribution of GABA(A) receptor activation to swimming in Xenopus tadpoles during the first day of postembryonic development. Around the time of hatching stage (37/8), bicuculline (10-50 microM) causes a decrease in swim episode duration and cycle period, suggesting that GABA(A) receptor activation influences embryonic swimming. Twenty-four hours later, at stage 42, GABA(A) receptor activation plays a more pronounced role in modulating larval swimming activity. Bicuculline causes short, intense swim episodes with increased burst durations and decreased cycle periods and rostrocaudal delays. Conversely, the allosteric agonist, 5beta-pregnan-3alpha-ol-20-one (1-10 microM) or the uptake inhibitor, nipecotic acid (200 microM) cause slow swimming with reduced burst durations and increased cycle periods. These effects appear to be mainly the result of GABA release from the spinal terminals of midhindbrain reticulospinal neurons but may also involve spinal GABAergic neurons. Intracellular recordings were made using KCl electrodes to reverse the sign and enhance the amplitude of chloride-dependent inhibitory postsynaptic potentials (IPSPs). Recordings from larval motoneurons in the presence of strychnine (1-5 microM), to block glycinergic IPSPs, provided no evidence for any GABAergic component to midcycle inhibition. GABA potentials were observed during episodes, but they were not phase-locked to the swimming rhythm. Bicuculline (10-50 microM) abolished these sporadic potentials and caused an apparent decrease in the level of tonic depolarization during swimming activity and an increase in spike height. Finally, in most larval preparations, GABA potentials were observed at the termination of swimming. In combination with the other evidence, our data suggest that midhindbrain reticulospinal neurons become involved in an intrinsic pathway that can prematurely terminate swim episodes. Thus during the first day of larval development, endogenous activation of GABA(A) receptors plays an increasingly important role in modulating locomotion, and GABAergic neurons become involved in an intrinsic descending pathway for terminating swim episodes.  (+info)

Differential change in neuroactive steroid sensitivity during ethanol withdrawal. (8/331)

The progesterone metabolite 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-P or allopregnanolone) is a potent positive modulator of gamma-aminobutyric acid(A) (GABA(A)) receptors. Although it is well documented that chronic ethanol (EtOH) administration produces cross-tolerance to the positive modulatory effect of benzodiazepines and GABA at GABA(A) receptors, recent findings suggest that sensitivity to 3alpha,5alpha-P is enhanced during EtOH withdrawal. In addition, EtOH-naive inbred strains of mice, which differ in EtOH withdrawal severity (DBA/2 >> C57BL/6), had marked differences in behavioral sensitivity to 3alpha,5alpha-P. Therefore, the present study was conducted to determine whether C57BL/6 (B6) and DBA/2 (D2) mice would be differentially sensitive to several of the pharmacological effects of 3alpha,5alpha-P during EtOH withdrawal. Male mice were exposed to EtOH vapor or air for 72 h. During withdrawal from EtOH, animals were injected with 3alpha,5alpha-P (0, 3.2, 10, or 17 mg/kg i.p.) and tested for activity and anxiolysis on the elevated plus maze, muscle relaxation, ataxia, and seizure protection following pentylenetetrazol. Sensitivity to the anticonvulsant effect of 3alpha,5alpha-P was enhanced during EtOH withdrawal in B6, but not D2 mice. In contrast, sensitivity to the muscle relaxant effects of 3alpha,5alpha-P was reduced in EtOH-withdrawing B6 and D2 mice, with a suggestion of decreased sensitivity to the anxiolytic effect of 3alpha,5alpha-P during EtOH withdrawal in B6. These results suggest that sensitization to the anticonvulsant effect of 3alpha,5alpha-P during EtOH withdrawal does not generalize across all genotypes nor does it generalize to all of the pharmacological effects of 3alpha,5alpha-P.  (+info)