Gianna Ragagnin

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 Alzheimers 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 and mood disorders

UNLABELLED Certain women experience negative mood symptoms during the menstrual cycle and progesterone addition in estrogen treatments. In women with PMDD increased negative mood symptoms related to allopregnanolone increase during the luteal phase of ovulatory menstrual cycles. In anovulatory cycles no symptom or sex steroid increase occurs. This is unexpected as positive modulators of the GABA-A receptor are generally increasing mood. This paradoxical effect has brought forward a hypothesis that the symptoms are provoked by allopregnanolone the GABA-A receptor system. GABA-A is the major inhibitory system in the brain. Positive modulators of the GABA-A receptor include the progesterone metabolites allopregnanolone and pregnanolone, benzodiazepines, barbiturates, and alcohol. GABA-A receptor modulators are known, in low concentrations to induce adverse, anxiogenic effects whereas in higher concentrations show beneficial, calming properties. Positive GABA-A receptor modulators induce strong paradoxical effects e.g. negative mood in 3-8% of those exposed, while up to 25% have moderate symptoms thus similar as the prevalence of PMDD, 3-8% among women in fertile ages, and up to 25% have moderate symptoms of premenstrual syndrome (PMS). The mechanism behind paradoxical reaction might be similar among them who react on positive GABA-A receptor modulators and in women with PMDD. In women the severity of these mood symptoms are related to the allopregnanolone serum concentrations in an inverted U-shaped curve. Negative mood symptoms occur when the serum concentration of allopregnanolone is similar to endogenous luteal phase levels, while low and high concentrations have less effect on mood. Low to moderate progesterone/allopregnanolone concentrations in women increases the activity in the amygdala (measured with fMRI) similar to the changes seen during anxiety reactions. Higher concentrations give decreased amygdala activity similar as seen during benzodiazepine treatment with calming anxiolytic effects. Patients with PMDD show decreased sensitivity in GABA-A receptor sensitivity to diazepam and pregnanolone while increased sensitivity to allopregnanolone. This agrees with findings in animals showing a relation between changes in alpha4 and delta subunits of the GABA-A receptor and anxiogenic effects of allopregnanolone. CONCLUSION These findings suggest that negative mood symptoms in women with PMDD are caused by the paradoxical effect of allopregnanolone mediated via the GABA-A receptor.

GR3027 antagonizes GABAA receptor-potentiating neurosteroids and restores spatial learning and motor coordination in rats with chronic hyperammonemia and hepatic encephalopathy

Hepatic encephalopathy (HE) is one of the primary complications of liver cirrhosis. Current treatments for HE, mainly directed to reduction of ammonia levels, are not effective enough because they cannot completely eliminate hyperammonemia and inflammation, which induce the neurological alterations. Studies in animal models show that overactivation of GABAA receptors is involved in cognitive and motor impairment in HE and that reducing this activation restores these functions. We have developed a new compound, GR3027, that selectively antagonizes the enhanced activation of GABAA receptors by neurosteroids such as allopregnanolone and 3α,21-dihydroxy-5α-pregnan-20-one (THDOC). This work aimed to assess whether GR3027 improves motor incoordination, spatial learning, and circadian rhythms of activity in rats with HE. GR3027 was administered subcutaneously to two main models of HE: rats with chronic hyperammonemia due to ammonia feeding and rats with portacaval shunts (PCS). Motor coordination was assessed in beam walking and spatial learning and memory in the Morris water maze and the radial maze. Circadian rhythms of ambulatory and vertical activity were also assessed. In both hyperammonemic and PCS rats, GR3027 restores motor coordination, spatial memory in the Morris water maze, and spatial learning in the radial maze. GR3027 also partially restores circadian rhythms of ambulatory and vertical activity in PCS rats. GR3027 is a novel approach to treatment of HE that would normalize neurological functions altered because of enhanced GABAergic tone, affording more complete normalization of cognitive and motor function than current treatments for HE.

GABAA receptor modulating steroid antagonists (GAMSA) are functional in vivo

GABAA receptor modulating steroid antagonists (GAMSA) selectively inhibit neurosteroid-mediated enhancement of GABA-evoked currents at the GABAA receptor. 3α-hydroxy-neurosteroids, notably allopregnanolone and tetrahydrodeoxycorticosterone (THDOC), potentiate GABAA receptor-mediated currents. On the contrary, various 3β-hydroxy-steroids antagonize this positive neurosteroid-mediated modulation. Importantly, GAMSAs are specific antagonists of the positive neurosteroid-modulation of the receptor and do not inhibit GABA-evoked currents. Allopregnanolone and THDOC have both negative and positive actions. Allopregnanolone can impair encoding/consolidation and retrieval of memories. Chronic administration of a physiological allopregnanolone concentration reduces cognition in mice models of Alzheimers disease. In humans an allopregnanolone challenge impairs episodic memory and in hepatic encephalopathy cognitive deficits are accompanied by increased brain ammonia and allopregnanolone. Hippocampal slices react in vitro to ammonia by allopregnanolone synthesis in CA1 neurons, which blocks long-term potentiation (LTP). Thus, allopregnanolone may impair learning and memory by interfering with hippocampal LTP. Contrary, pharmacological treatment with allopregnanolone can promote neurogenesis and positively influence learning and memory of trace eye-blink conditioning in mice. In rat the GAMSA UC1011 inhibits an allopregnanolone-induced learning impairment and the GAMSA GR3027 restores learning and motor coordination in rats with hepatic encephalopathy. In addition, the GAMSA isoallopregnanolone antagonizes allopregnanolone-induced anesthesia in rats, and in humans it antagonizes allopregnanolone-induced sedation and reductions in saccadic eye velocity. 17PA is also an effective GAMSA in vivo, as it antagonizes allopregnanolone-induced anesthesia and spinal analgesia in rats. In vitro the allopregnanolone/THDOC-increased GABA-mediated GABAA receptor activity is antagonized by isoallopregnanolone, UC1011, GR3027 and 17PA, while the effect of GABA itself is not affected.

A COMPARISON OF THE PHARMACOLOGICAL PROPERTIES OF RECOMBINANT HUMAN AND RAT α1β2γ2L GABAA RECEPTORS IN XENOPUS OOCYTES

1 In the present study, we compared the pharmacology, particularly neurosteroid modulation of the GABAA receptor, between human and rat a1b2g2L GABAA receptors and between human receptors containing the long (L) and short (S) forms of the g2‐subunit. 2 We observed that maximum responses to GABA were significantly higher with the human a1b2g2L receptor compared with the rat receptor. In terms of neurosteroid modulation, increases in the EC15 response to GABA induced by 3a‐OH‐5b‐pregnan‐20‐one (3a5bP), 5a‐androstane‐3a,17b‐diol (3a5aADL) and 5a‐pregnane‐3a,20b‐diol (3a5a‐diol) were significantly greater for the rat compared with the human receptor. Responses to 30 mmol/L GABA were inhibited by 3b‐OH‐5a‐pregnan‐20‐one (UC1010) and 5b‐pregnan‐3b,20(R)‐diol (UC1020) to a greater degree for human and rat receptors, respectively. Responses to GABA + 3a5aTHDOC were inhibited by 5a‐pregnan‐3b,20(S)‐diol (UC1019) and pregnenolone sulphate to a greater degree for human and rat receptors, respectively. 3 The GABA dose–response curves for human a1b2g2S and a1b2g2L receptors were identical. However, the maximum GABA‐evoked current, the direct gating effect of pentobarbital and the allosteric potentiation of the GABA EC15 response by 3a5aTHDOC and 3a5bP were significantly higher with a1b2g2S than a1b2g2L receptors. Inhibition of the response to 30 mmol/L GABA by UC1010 and UC1020 was greater for a1b2g2L and a1b2g2S receptors, respectively. Inhibition of responses to 3a5aTHDOC + GABA by UC1019 and UC1010 was significantly higher for a1b2g2L receptors. 4 In conclusion, the site of activation by GABA and neurosteroid modulation differ between human and rat a1b2g2L receptors, as well as between human receptors containing the L and S splice variants of the g2‐subunit.

Allopregnanolone involvement in feeding regulation, overeating and obesity

Obesity is strongly associated with ill health, primarily caused by consumption of excessive calories, and promoted (inter alia) by gamma-amino-butyric-acid (GABA) stimulating food intake by activating GABAA receptors (primarily with α3 and α2 subunits) in the hypothalamic arcuate nucleus and paraventricular nucleus. Allopregnanolone is a potent positive GABAA receptor modulating steroid (GAMS). As reviewed here, elevated allopregnanolone levels are associated with increases in food intake, preferences for energy-rich food, and obesity in humans and other mammals. In women with polycystic ovarian disease, high serum allopregnanolone concentrations are linked to uncontrolled eating, and perturbed sensitivity to allopregnanolone. Increases in weight during pregnancy also correlate with increases in allopregnanolone levels. Moreover, Prader-Willis syndrome is associated with massive overeating, absence of a GABAA receptor (with compensatory >12-, >5- and >1.5-fold increases in α4, γ2, and α1, α3 subunits), and increases in the α4, βx, δ receptor subtype, which is highly sensitive to allopregnanolone. GABA and positive GABA-A receptor modulating steroids like allopregnanolone stimulates food intake and weight gain.

Neuroactive Steroids in Brain and Relevance to Mood

Depression and anxiety often affect women in relation to reproductive events like menarche, premenstrual periods, post-partum and perimenopause. A prominent example of the interaction between mood, neuroactive-steroids and the GABA system is premenstrual dysphoric disorder (PMDD). Severe premenstrual negative mood symptoms occur in 3–8% of women. Sex and stress hormones are metabolized to neuroactive steroids with effects on brain function as positive modulators of the GABA A receptor (called GABA-steroids) similar to benzodiazepines, barbiturates and alcohol. One example of a neuroactive sex steroid is allopregnanolone, and other GABA-steroids, are produced within the brain, by the adrenals at stress and from the ovary during the menstrual cycle. Animal and human studies show that benzodiazepines, barbiturates, alcohol and allopregnanolone have a bimodal effect on behavior. In high dosages or concentrations the positive GABA A receptor modulators are CNS depressants, anesthetic, and anxiolytic, whereas in certain sensitive individuals low concentrations instead of being anxiolytic cause severe anxiety, irritability, aggressiveness and depressive mood in 3–6% of individuals, and moderate symptoms in up to 30%. Low concentrations of GABA-steroids are found endogenously during the luteal phase and induce adverse emotional reactions. In women with PMDD/ PMS this paradoxical effect of neuroactive steroids seems to provoke negative mood symptoms as tension, irritability and depression. The mechanism behind the effect is called disinhibition that acts together with tolerance development by GABA A receptor active substances. Effective treatments are inhibition of ovarian steroid production or changing the CNS response to neuroactive steroids.