Neurosteroid modulation of recombinant rat alpha5beta2gamma2L and alpha1beta2gamma2L GABA(A) receptors in Xenopus oocyte.

GABA(A) receptors containing alpha(5)-subunit have an important role in cognitive function. As the agonistic effect of 3alpha-hydroxy ring-A reduced steroids depends on subunit combinations of the GABA(A) receptor, the antagonistic effect of pregnenolone sulfate and 3beta-hydroxypregnane steroids may vary between alpha(5)-subunit and alpha(1)-subunit containing receptors. We investigated the effect of agonist and antagonist steroids in the recombinant rat alpha(1)beta(2)gamma(2L) and alpha(5)beta(2)gamma(2L) receptors expressed in Xenopus oocytes using a two electrodes voltage-clamp technique. We did not find any significant difference in potency and efficacy of GABA response between alpha(1)beta(2)gamma(2L) and alpha(5)beta(2)gamma(2L) receptors. Compared to the alpha(1)beta(2)gamma(2L) receptor, a significantly lower degree of desensitization was observed in the alpha(5)beta(2)gamma(2L) receptor. In addition, the potencies of 3alpha-OH-5alpha-pregnan-20-one (3alpha5alphaP), 5alpha-pregnan-3alpha,21-diol-20-one (3alpha5alphaTHDOC) and 5alpha-androstane-3alpha,17beta-diol (3alpha5alphaADL) to enhance GABA response were significantly higher in the alpha(5)beta(2)gamma(2L) receptor, whereas their efficacies remained unchanged between two receptors. In either receptor, the efficacy of 3alpha5alphaTHDOC was significantly higher than 3alpha5alphaP and 3alpha5alphaADL. The efficacies of 5beta-pregnan-3beta,21-diol-20-one(UC1015) and 5alpha-pregnan-3beta,20alpha-diol(UC1019) to inhibit 30 microM GABA response, and the efficacies of 3beta-OH-5beta-pregnan-20-one (UC1014) and 5beta-pregnan-3beta, 20beta-diol (UC1020) to inhibit 3 microM 3alpha5alphaTHDOC+3 microM GABA response were higher in the alpha(5)beta(2)gamma(2L) receptor compared to the alpha(1)beta(2)gamma(2L) receptor. The potencies of pregnenolone sulfate and 3beta-hydroxypregnane steroids to inhibit the GABA response and the 3alpha5alphaTHDOC+GABA response did not vary between two receptors. Interestingly, the potencies and efficacies of pregnenolone sulfate and 3beta-hydroxypregnane steroids to inhibit the GABA response were positively correlated to their potencies and efficacies to inhibit the 3alpha5alphaTHDOC+GABA response. Results from the current study revealed a different modulation pattern by neurosteroids between the alpha(1)beta(2)gamma(2L) and alpha(5)beta(2)gamma(2L) receptor.

GABA(A) receptor changes in acute allopregnanolone tolerance.

To study acute tolerance, rats were anesthetized with interrupted i.v. allopregnanolone infusions where the "silent second" in the electroencephalogram (EEG) was the target. Animals were killed either directly at the first silent second or at the silent second level after 30 or 90 min of anaesthesia. Acute tolerance was demonstrated at 90 min of anaesthesia as earlier shown. In situ hybridization showed a decreased expression of the gamma-aminobutyric acid(A) (GABA(A)) receptor subunit alpha4mRNA amount in the thalamus ventral-posteriomedial nucleus of the tolerant rats. A parallel change in the abundance of the alpha4 subunit was detected with immunohistochemistry. The increase in maintenance dose rate (MDR) was significantly negatively correlated with the alpha4mRNA in the thalamus ventral-posteriomedial nucleus, and positively correlated with alpha2mRNA in different hippocampal subregions. There was also a positive relationship between the alpha1mRNA amounts in the different hippocampal subregions, with significant differences between groups. These changes in GABA(A) receptor subunits mRNA expression and protein (alpha4) might be of importance for the development of acute tolerance to allopregnanolone.

Neuroactive steroid effects on cognitive functions with a focus on the serotonin and GABA systems.

This article will review neuroactive steroid effects on serotonin and GABA systems, along with the subsequent effects on cognitive functions. Neurosteroids (such as estrogen, progesterone, and allopregnanolone) are synthesized in the central and peripheral nervous system, in addition to other tissues. They are involved in the regulation of mood and memory, in premenstrual syndrome, and mood changes related to hormone replacement therapy, as well as postnatal and major depression, anxiety disorders, and Alzheimer's disease. Estrogen and progesterone have their respective hormone receptors, whereas allopregnanolone acts via the GABA(A) receptor. The action of estrogen and progesterone can be direct genomic, indirect genomic, or non-genomic, also influencing several neurotransmitter systems, such as the serotonin and GABA systems. Estrogen alone, or in combination with antidepressant drugs affecting the serotonin system, has been related to improved mood and well being. In contrast, progesterone can have negative effects on mood and memory. Estrogen alone, or in combination with progesterone, affects the brain serotonin system differently in different parts of the brain, which can at least partly explain the opposite effects on mood of those hormones. Many of the progesterone effects in the brain are mediated by its metabolite allopregnanolone. Allopregnanolone, by changing GABA(A) receptor expression or sensitivity, is involved in premenstrual mood changes; and it also induces cognitive deficits, such as spatial-learning impairment. We have shown that the 3beta-hydroxypregnane steroid UC1011 can inhibit allopregnanolone-induced learning impairment and chloride uptake potentiation in vitro and in vivo. It would be important to find a substance that antagonizes allopregnanolone-induced adverse effects.

Allopregnanolone inhibits learning in the Morris water maze.

The progesterone metabolite allopregnanolone (3alpha-OH-5alpha-pregnane-20-one) inhibits neural functions, enhancing the GABA induced GABA(A) receptor activation. This effect is benzodiazepine like and benzodiazepines are known to impair memory. Acute effects of allopregnanolone on the hippocampus dependent spatial learning in the Morris water maze have not been studied. Adult male Wistar rats where injected (i.v.) with allopregnanolone (2 mg/kg), or vehicle, daily for 11 days. At 8 or 20 min after each injection, studies of place navigation were performed in the Morris water maze. Allopregnanolone concentrations in plasma and in nine different brain areas where analyzed by radioimmunoassay. The latency to find the platform was increased 8 min after the allopregnanolone injection, while normal learning was seen after 20 min. Swim speed did not differ between groups. A higher number of rats were swimming close to the pool wall (thigmotaxis) in the 8 min allopregnanolone group compared to the other groups. Allopregnanolone concentrations in the brain tissue at 8 min were 1.5 to 2.5 times higher then at 20 min after the allopregnanolone injections. After vehicle injections the brain concentrations of allopregnanolone were at control levels. Plasma concentrations of allopregnanolone followed the same pattern as in the brain, with the exception of an increase 8 min after vehicle injections. The natural progesterone metabolite allopregnanolone can inhibit learning in the Morris water maze, an effect not caused by motor impairment. The learning impairment might be due to a combination of changed swimming behavior and difficulties in navigation.