Selective Brain Steroidogenic Stimulants Research Articles

Ganaxolone improves behavioral deficits in a mouse model of post-traumatic stress disorder

Allopregnanolone and its equipotent stereoisomer, pregnanolone (together termed ALLO), are neuroactive steroids that positively and allosterically modulate the action of gamma-amino-butyric acid (GABA) at GABAA receptors. Levels of ALLO are reduced in the cerebrospinal fluid of female premenopausal patients with post-traumatic stress disorder (PTSD), a severe, neuropsychiatric condition that affects millions, yet is without a consistently effective therapy. This suggests that restoring downregulated brain ALLO levels in PTSD may be beneficial. ALLO biosynthesis is also decreased in association with the emergence of PTSD-like behaviors in socially isolated (SI) mice. Similar to PTSD patients, SI mice also exhibit changes in the frontocortical and hippocampal expression of GABAA receptor subunits, resulting in resistance to benzodiazepine-mediated sedation and anxiolysis. ALLO acts at a larger spectrum of GABAA receptor subunits than benzodiazepines, and increasing corticolimbic ALLO levels in SI mice by injecting ALLO or stimulating ALLO biosynthesis with a selective brain steroidogenic stimulant, such as S-norfluoxetine, at doses far below those that block serotonin reuptake, reduces PTSD-like behavior in these mice. This suggests that synthetic analogs of ALLO, such as ganaxolone, may also improve anxiety, aggression, and other PTSD-like behaviors in the SI mouse model. Consistent with this hypothesis, ganaxolone (3.75–30 mg/kg, s.c.) injected 60 min before testing of SI mice, induced a dose-dependent reduction in aggression toward a same-sex intruder and anxiety-like behavior in an elevated plus maze. The EC50 dose of ganaxolone used in these tests also normalized exaggerated contextual fear conditioning and, remarkably, enhanced fear extinction retention in SI mice. At these doses, ganaxolone failed to change locomotion in an open field test. Therefore, unlike benzodiazepines, ganaxolone at non-sedating concentrations appears to improve dysfunctional emotional behavior associated with deficits in ALLO in mice and may provide an alternative treatment for PTSD patients with deficits in the synthesis of ALLO. Selective serotonin reuptake inhibitors (SSRIs) are the only medications currently approved by the FDA for treatment of PTSD, although they are ineffective in a substantial proportion of PTSD patients. Hence, an ALLO analog such as ganaxolone may offer a therapeutic GABAergic alternative to SSRIs for the treatment of PTSD or other disorders in which ALLO biosynthesis may be impaired.

5α-reductase type I expression is downregulated in the prefrontal cortex/Brodmann's area 9 (BA9) of depressed patients.

The implications of the neurosteroid 3α-hydroxy-5α-pregnan-20-one [allopregnanolone (Allo)] in neuropsychiatric disorders have been highlighted in several recent clinical investigations. For instance, Allo levels are decreased in the cerebrospinal fluid (CSF) of patients with posttraumatic stress disorder (PTSD) and major unipolar depression. Neurosteroidogenic antidepressants [i.e., selective brain steroidogenic stimulants (SBSSs)], including fluoxetine and analogs, correct this decrease in a manner that correlates with improved depressive symptoms. Allo positively and allosterically modulates GABA action at postsynaptic and extrasynaptic GABAA receptors. It is synthesized in both the human and rodent brain cortices by principal glutamatergic pyramidal neurons from progesterone by the sequential action of 5α-reductase type I (5α-RI), which is the rate-limiting step enzyme in Allo biosynthesis, and 3α-hydroxysteroid dehydrogenase (3α-HSD), which converts 5α-dehydroprogesterone into Allo. We thus hypothesized that decreased CSF levels of Allo in depressed patients could reflect a brain dysfunction of 5α-RI. In a pilot study of samples from six patients per group [six depressed patients and six nonpsychiatric subjects (NPS)], we studied the expression of 5α-RI messenger RNA (mRNA) in prefrontal cortex Brodmann's area 9 (BA9) and cerebellum from depressed patients obtained from the Maryland Brain Collection at the Maryland Psychiatric Research Center (Baltimore, MD) that were age-matched with NPS. The levels of 5α-RI mRNA were decreased from 25 ± 5.8 in NPS to 9.1 ± 3.1 fmol/pmol neuronal specific enolase (NSE) (t1,10 = 2.7, P = 0.02) in depressed patients. These differences are absent in the cerebellum of the same patients. The levels of neurosteroids were determined in the prefrontal cortex BA9 of depressed patients obtained from the Stanley Foundation Brain Bank Neuropathology Consortium, Bethesda (MD). The BA9 levels of Allo in male depressed patients failed to reach statistical difference from the levels of NPS (1.63 ± 1.01 pg/mg, n = 8, in NPS and 0.82 ± 0.33 pg/mg, n = 5, in nontreated depressed patients). However, depressed patients who had received antidepressant treatment (three patients SSRI and one TCA) exhibited increased BA9 Allo levels (6.16 ± 2.5 pg/mg, n = 4, t1,9 = 2.4, P = 0.047) when compared with nontreated depressed patients. Although in a small number of patients, this finding is in-line with previous reports in the field that have observed an increase of Allo levels in CSF and plasma of depressed patients following antidepressant treatment. Hence, the molecular mechanisms underlying major depression may include a GABAergic neurotransmission deficit caused by a brain Allo biosynthesis downregulation, which can be normalized by SBSSs.

Targeting neurosteroidogenesis as therapy for PTSD

Posttraumatic stress disorder (PTSD) is a severe condition resulting from exposure to traumatic events, such as combat situations, sexual assault, serious injury or the threat of death. Symptoms include disturbing recurring flashbacks, avoidance or numbing of memories of the event, and hyperarousal, which continue for more than a month after the traumatic event. Reduced cortical GABA (Kugaya et al., 2003) and cebrospinal fluid (CSF) allopregnanolone levels (Rasmusson et al., 2006) that positively and allosterically modulate GABA action at GABAA receptors (Belelli and Lambert, 2005) suggest that in PTSD patients, a perturbation of GABAergic neurotransmission plays a role in the pathogenesis of this disorder. Thus restoring downregulated brain allopregnanolone levels may be beneficial in treating PTSD. There is a general consensus that maladaptive fear responses (i.e., impaired fear extinction) are a core feature of stress-induced PTSD (Myers and Davis, 2007; Maren, 2008). Exaggerated fear responses and impaired extinction learning, or the inability to extinguish fear memories, are often treated with exposure-based therapy (EBT), which involves the exposure of the patient to the feared context without any danger (Joseph and Gray, 2008). This closely approximates the procedure used to simulate and study fear responses and fear extinction learning in PTSD mouse models (Marks, 1979). While psychological therapy has been highly effective both in treating PTSD and in preventing the progression of the event sequelae that leads to consolidation of fear memories, one challenge of PTSD therapy is the spontaneous recovery of fear that often reemerges following successful EBT. For this reason, pharmacological treatment may be advantageous alone or in combination with EBT. Selective serotonin reuptake inhibitors (SSRIs) are currently the drugs of choice in treating PTSD. They are effective in facilitating and restoring the neurobiological changes altered in PTSD patients, and they are devoid of the unwanted side effects that plague the use of benzodiazepines, more importantly, SSRIs are potent therapeutics where benzodiazepines fail to be beneficial. Following the observation that low non-serotonergic doses of fluoxetine and congeners increase allopregnanolone levels as their primary mechanism of action, we suggested that SSRIs acting as selective brain steroidogenic stimulants (SBSSs) can improve dysfunctional emotional behavior and may be of advantage in PTSD treatment. In addition to its use in PTSD, this novel steroidogenic mechanism of action of SSRIs given at low doses offers enormous therapeutic potentials for the treatment of other psychiatric disorders, including anxiety spectrum disorders, premenstrual dysphoria, and probably depression, as these disorders may be caused by a downregulation of neurosteroid biosynthesis (Uzunov et al., 1996; Westenberg, 1996; Guidotti and Costa, 1998; Romeo et al., 1998; Uzunova et al., 1998; Steiner and Pearlstein, 2000; Berton and Nestler, 2006; Pinna et al., 2006a, 2009; Pinna, 2010; Ipser and Stein, 2012; Pinna and Rasmusson, 2012; Lovick, 2013). In vitro studies show that SSRIs may activate 3α-hydroxysteroid dehydrogenase, thereby facilitating the reduction of 5α-dihydroprogesterone into allopregnanolone (Griffin and Mellon, 1999). Nonetheless, the precise neuronal mechanisms involved in the neurosteroidogenic action of SSRIs remain unclear. Drug design welcomed allopregnanolone biosynthesis as a target for novel rapidly acting anxiolytics devoid of sedation, tolerance, and withdrawal liabilities (Rupprecht et al., 2009, 2010; Schule et al., 2011), and, in addition to low doses of SSRIs, selective ligands for the (18 kDa) translocase protein (TSPO), which increase allopregnanolone levels, may be beneficial in anxiety and PTSD (Rupprecht et al., 2009).

Up‐Regulation of Neurosteroid Biosynthesis as a Pharmacological Strategy to Improve Behavioural Deficits in a Putative Mouse Model of Post‐Traumatic Stress Disorder

Benzodiazepines remain the most frequently used psychotropic drugs for the treatment of anxiety spectrum disorders; however, their use is associated with the development of tolerance and dependence. Another major hindrance is represented by their lack of efficacy in many patients, including patients with post-traumatic stress disorder (PTSD). For these nonresponders, the use of selective serotonin reuptake inhibitors (SSRIs) has been the therapy of choice. In the past decade, clinical studies have suggested that the pharmacological action of SSRIs may include the ability of these drugs to normalise decreased brain levels of neurosteroids in patients with depression and PTSD; in particular, the progesterone derivative allopregnanolone, which potently, positively and allosterically modulates the action of GABA at GABA(A) receptors. Preclinical studies using the socially-isolated mouse as an animal model of PTSD have demonstrated that fluoxetine and congeners ameliorate anxiety-like behaviour, fear responses and aggressive behaviour expressed by such mice by increasing corticolimbic levels of allopregnanolone. This is a novel and more selective mechanism than serotonin reuptake inhibition, which, for half a century, has been considered to be the main molecular mechanism for the therapeutic action of SSRIs. Importantly, this finding may shed light on the high rates of SSRI resistance among patients with PTSD and depression, comprising disorders in which there appears to be a block in allopregnanolone synthesis. There are several different mechanisms by which such a block may occur, and SSRIs may only be corrective under some conditions. Thus, the up-regulation of allopregnanolone biosynthesis in corticolimbic neurones may offer a novel nontraditional pharmacological target for a new generation of potent nonsedating, anxiolytic medications for the treatment of anxiety, depression, and PTSD: selective brain steroidogenic stimulants.

Neurosteroids reduce social isolation-induced behavioral deficits: a proposed link with neurosteroid-mediated upregulation of BDNF expression.

The pharmacological action of selective serotonin reuptake inhibitor antidepressants may include a normalization of the decreased brain levels of the brain-derived neurotrophic factor (BDNF) and of neurosteroids such as the progesterone metabolite allopregnanolone, which are decreased in patients with depression and posttraumatic stress disorders (PTSD). The allopregnanolone and BDNF level decrease in PTSD and depressed patients is associated with behavioral symptom severity. Antidepressant treatment upregulates both allopregnanolone levels and the expression of BDNF in a manner that significantly correlates with improved symptomatology, which suggests that neurosteroid biosynthesis and BDNF expression may be interrelated. Preclinical studies using the socially isolated mouse as an animal model of behavioral deficits, which resemble some of the symptoms observed in PTSD patients, have shown that fluoxetine and derivatives improve anxiety-like behavior, fear responses and aggressive behavior by elevating the corticolimbic levels of allopregnanolone and BDNF mRNA expression. These actions appeared to be independent and more selective than the action of these drugs on serotonin reuptake inhibition. Hence, this review addresses the hypothesis that in PTSD or depressed patients, brain allopregnanolone levels, and BDNF expression upregulation may be mechanisms at least partially involved in the beneficial actions of antidepressants or other selective brain steroidogenic stimulant molecules.

S-norfluoxetine microinfused into the basolateral amygdala increases allopregnanolone levels and reduces aggression in socially isolated mice

A decrease of brain allopregnanolone biosynthesis may play a role in emotion, impulsive behavior, and anxiety spectrum disorders by decreasing GABAergic neurotransmission. In male mice, four weeks of social isolation induces behavioral dysfunctions such as aggression, fear, and anxiety-like behavior associated with a decrease in allopregnanolone biosynthesis in selected corticolimbic structures comprising the basolateral amygdala (BLA), olfactory bulb, hippocampus, and medial prefrontal cortex. Importantly, no decrease in allopregnanolone biosynthesis has been found in the striatum and cerebellum. Given the importance of the amygdaloid complex in emotional behavior, we hypothesized that this brain area may play a pivotal role in decreasing social isolation-induced aggression. Thus, socially isolated mice were directly infused with S-norfluoxetine (S-NFLX) or pregnanolone (an analog of allopregnanolone) into the BLA and striatum. When S-NFLX (2.5, 3.75, and 5 nmol/0.2 μl) or pregnanolone (1.25, 2.5, 3.75, and 5.0 nmol/0.2 μl) is directly infused into the BLA, these agents dose-dependently reduced aggression (S-NFLX up to 93% and pregnanolone up to 96%) of a socially isolated mouse to a same-sex intruder. However, S-NFLX (3.75 and 5 nmol) infused directly into the striatum failed to alter aggression. Allopregnanolone content in the BLA after S-NFLX (3.75 nmol) infusion was increased by 3-fold and in the hippocampus, by 80%. Allopregnanolone levels did not change in the olfactory bulb or in the frontal cortex of the same mice. S-NFLX (3.75 nmol) infused into the striatum failed to increase the levels of allopregnanolone. These results suggest that S-NFLX, acting as a selective brain steroidogenic stimulant (SBSS), increases corticolimbic allopregnanolone levels and regulates aggression, which underscores the pivotal role of the BLA and hippocampus in the regulation of aggressiveness in socially isolated mice. This article is part of a Special Issue entitled ‘Trends in Neuropharmacology: In Memory of Erminio Costa’.

In a mouse model relevant for post-traumatic stress disorder, selective brain steroidogenic stimulants (SBSS) improve behavioral deficits by normalizing allopregnanolone biosynthesis

The pathophysiological role of the neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) in neuropsychiatric disorders has been highlighted in several recent investigations. For instance, allopregnanolone levels are decreased in the CSF of patients with post-traumatic stress disorder (PTSD) and major unipolar depression. Neurosteroidogenic antidepressants, including fluoxetine and analogs, correct this decrease in a manner that correlates with improved depressive symptoms. PTSD-like behavioral dysfunctions, including heightened aggression, exaggerated fear, and anxiety-like behavior associated with a decrease in corticolimbic allopregnanolone content are modeled in mice by protracted social isolation stress. Allopregnanolone is not only synthesized by principal glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons, but also locally, potently, positively, and allosterically modulates GABA action at postsynaptic and extrasynaptic GABAA receptors. Hence, this paper will review preclinical studies, which show that in socially isolated mice, rather than selective serotonin reuptake inhibitor mechanisms, allopregnanolone biosynthesis in glutamatergic corticolimbic neurons offers a nontraditional target for fluoxetine to decrease signs of aggression, normalize fear responses, and decrease anxiety-like behavior. At low selective serotonin reuptake inhibitor-inactive doses, fluoxetine and related congeners potently increase allopregnanolone levels by acting as potent selective brain steroidogenic stimulants (SBSS), thereby facilitating GABAA receptor neurotransmission and improving behavioral dysfunctions. Although the precise molecular mechanisms that underlie the action of these drugs are not fully understood, findings from socially isolated mice may ultimately generate insights into novel drug targets for the treatment of psychiatric disorders, such as anxiety and panic disorders, depression, and PTSD.

Enhanced fear responses in mice treated with anabolic androgenic steroids

Corticolimbic neurons express neurosteroid biosynthesis, which is altered during anabolic androgenic steroid (AAS) treatment. The brain circuits and neurons that underlie the behavioral deficits found after AAS treatment remain undefined. We studied the effects of testosterone propionate (testosterone) on fear conditioning responses and in primary output corticolimbic neurons on 5alpha-reductase-type-I and 3alpha-hydroxysteroid-dehydrogenase expression. Testosterone fails to change cued fear responses although it induces excessive contextual fear associated with corticolimbic 5alpha-reductase-type-I mRNA expression downregulation in the prefrontal cortex, hippocampus, and basolateral amygdala glutamatergic neurons. Increased fear responses are abolished by normalizing corticolimbic allopregnanolone levels with allopregnanolone treatment (8 micromol/kg) or selective brain steroidogenic stimulants, including S-norfluoxetine (1.8 micromol/kg). Agents that increase corticolimbic allopregnanolone levels may be beneficial in treating AAS users.

SSRIs act as selective brain steroidogenic stimulants (SBSSs) at low doses that are inactive on 5-HT reuptake

Brain principal glutamatergic neurons synthesize 3alpha-hydroxy-5alpha-pregnan-20-one (Allo), a neurosteroid that potently, positively, and allosterically modulates GABA action at GABA(A) receptors. Cerebrospinal fluid (CSF) Allo levels are decreased in patients with posttraumatic stress disorder (PTSD) and major depression. This decrease is corrected by fluoxetine in doses that improve depressive symptoms. Emotional-like behavioral dysfunctions (aggression, fear, and anxiety) associated with a decrease of cortico-limbic Allo content can be induced in mice by social isolation. In socially isolated mice, fluoxetine and analogs stereospecifically normalize the decrease of Allo biosynthesis and improve behavioral dysfunctions by a mechanism independent from 5-HT reuptake inhibition. Thus, fluoxetine and related congeners facilitate GABA(A) receptor neurotransmission and effectively ameliorate emotional and anxiety disorders and depression by acting as selective brain steroidogenic stimulants (SBSSs).

Neurosteroid Biosynthesis Regulates Sexually Dimorphic Fear and Aggressive Behavior in Mice

The neurosteroid allopregnanolone is a potent positive allosteric modulator of GABA action at GABA(A) receptors. Allopregnanolone is synthesized in the brain from progesterone by the sequential action of 5alpha-reductase type I (5alpha-RI) and 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD). 5alpha-RI and 3alpha-HSD are co-expressed in cortical, hippocampal, and olfactory bulb glutamatergic neurons and in output neurons of the amygdala, thalamus, cerebellum, and striatum. Neither 5alpha-RI nor 3alpha-HSD mRNAs is expressed in glial cells or in cortical or hippocampal GABAergic interneurons. It is likely that allopregnanolone synthesized in principal output neurons locally modulates GABA(A) receptor function by reaching GABA(A) receptor intracellular sites through lateral membrane diffusion. This review will focus on the behavioral effects of allopregnanolone on mouse models that are related to a sexually dimorphic regulation of brain allopregnanolone biosynthesis. Animal models of psychiatric disorders, including socially isolated male mice or mice that receive a long-term treatment with anabolic androgenic steroids (AAS), show abnormal behaviors such as altered fear responses and aggression. In these animal models, the cortico-limbic mRNA expression of 5alpha-RI is regulated in a sexually dimorphic manner. Hence, in selected glutamatergic pyramidal neurons of the cortex, CA3, and basolateral amygdala and in granular cells of the dentate gyrus, mRNA expression of 5alpha-RI is decreased, which results in a downregulation of allopregnanolone content. In contrast, 5alpha-RI mRNA expression fails to change in the striatum medium spiny neurons and in the reticular thalamic nucleus neurons, which are GABAergic.By manipulating allopregnanolone levels in glutamatergic cortico-limbic neurons in opposite directions to improve [using the potent selective brain steroidogenic stimulant (SBSS) S-norfluoxetine] or induce (using the potent 5alpha-RI inhibitor SKF 105,111) behavioral deficits, respectively, we have established the fundamental role of cortico-limbic allopregnanolone levels in the sexually dimorphic regulation of aggression and fear. By selectively targeting allopregnanolone downregulation in glutamatergic cortico-limbic neurons, i.e., by improving the response of GABA(A) receptors to GABA, new therapeutics would offer appropriate and safe management of psychiatric conditions, including impulsive aggression, irritability, irrational fear, anxiety, posttraumatic stress disorders, and depression.

Decreased corticolimbic allopregnanolone expression during social isolation enhances contextual fear: A model relevant for posttraumatic stress disorder

Mice subjected to social isolation (3-4 weeks) exhibit enhanced contextual fear responses and impaired fear extinction. These responses are time-related to a decrease of 5alpha-reductase type I (5alpha-RI) mRNA expression and allopregnanolone (Allo) levels in selected neurons of the medial prefrontal cortex, hippocampus, and basolateral amygdala. Of note, the cued fear response was not different between group housed and socially isolated mice. In socially isolated mice, S-norfluoxetine, a selective brain steroidogenic stimulant (SBSS), in doses (0.45-1.8 mumol/kg) that increase brain Allo levels but fail to inhibit serotonin reuptake, greatly attenuates enhanced contextual fear response. SKF 105,111 (a potent 5alpha-RI inhibitor) decreases corticolimbic Allo levels and enhances the contextual fear response in group housed mice, which suggests that social isolation alters emotional responses by reducing the positive allosteric modulation of Allo at GABA(A) receptors in corticolimbic circuits. Thus, these procedures model emotional hyperreactivity, including enhanced contextual fear and impaired contextual fear extinction, which also is observed in posttraumatic stress disorder (PTSD) patients. A recent clinical study reported that cerebrospinal fluid Allo levels also are down-regulated in PTSD patients and correlate negatively with PTSD symptoms and negative mood. Thus, protracted social isolation of mice combined with tests of fear conditioning may be a suitable model to study emotional behavioral components associated with neurochemical alterations relating to PTSD. Importantly, drugs like SBSSs, which rapidly increase corticolimbic Allo levels, normalize the exaggerated contextual fear responses resulting from social isolation, suggesting that selective activation of neurosteroidogenesis may be useful in PTSD therapy.

Molecule of the Month

A new, non-SSRI mechanism of action for Prozac™. Unquestionably, Prozac™ was a landmark drug that redefined the treatment for depression, anxiety disorders, premenstrual syndrome and post-traumatic stress disorder as well as ushering in a wave of new therapies targeting specific serotonin (5-HT) reuptake inhibition (SSRI) [1]. In a recent manuscript (Psychopharmacology 2006, 186, 362-372), Pinna and co-workers report that fluoxetine and its N-desmethyl congener norfluoxetine stereospecifically and selectively increase brain neurosteroid content at doses that are inactive on 5-HT reuptake [2]. Pinna further states that the term ‘SSRI’ may be misleading in describing the pharmacological profile of fluoxetine and its congeners and suggests the term ‘selective brain steroidogenic stimulants’ (SBSSs) as a better descriptor [2]. Recently, it has been shown that neurosteroids, such as allopregnanolone (Allo), play critical roles in human brain pathophysiology such as anxiety disorders, premenstrual syndrome, posttraumatic stress disorder, postpartum depression and general depression. For the treatment of major depression, recent clinical trials have indicated that the pharmacological profiles of Prozac™ and related congeners correlate with an increase of Allo in the brain and cerebrospinal fluid. Importantly, Allo is a potent (nM affinity) positive allosteric modulator of gamma-aminobutyric acid (GABA) action at GABAA receptors, potentiating the intensity of GABAgated Cl- currents [3]. Thus, Prozac™ and related analogs increase Allo levels, and subsequent activation of GABAA receptors, at currently prescribed clinical doses which may not block 5-HT reuptake, but does effectively treat depression, premenstrual dysphoria and anxiety [2]. Pinna then explored this hypothesis in animal behavioral models of aggression and anxiety [2]. Their studies found that social isolation in mice is correlated with a down-regulation of Allo content and that pretreatment with Allo abolished aggressiveness as well as correcting the altered responses of GABAA receptors to specific agonists (pentobarbital) and antagonists (picrotoxin). Therefore, drugs capable of upregulating brain Allo content normalize behavioral abnormalities expressed in mice with a downregulation of brain Allo levels. Significantly, Pinna and co-workers discovered that the actions of fluoxetine and norfluoxetine on brain Allo content are stereoselective whereas 5-HT reuptake lacks stereospecificity. Prozac™ is a racemic compound. Pinna found that the EC50s of the (S)-enantiomers of fluoxetine and norfluoxetine to normalize brain Allo levels are 10- and 50-fold lower than those required to inhibit 5-HT reuptake [2]. Will this research herald new drug discovery campaigns aimed at identifying compounds capable of selectively increasing brain Allo levels under the moniker SBSSs? REFERENCES [1] Wong, D.T.; Bymaster, F.P.; Reid, L.R.; Mayle, D.A.; Krushinski, J.H.; Robertson, D.W. Norfluoxetine enetiomers as inhibitors of serotonin uptake in rat brain. Neuropsychopharmacology 1993, 8, 337-344. [2] Pinna, G.; Costa, E.; Guidotti, A. Fluoxetine and norfluoxetine stereospecifically and selectively increase brain neurosteroid content at doses that are inactive on 5-HT reuptake. Psychopharmaology 2006, 186, 362-372. [3] Lambert, J.J.; Belelli, D.; Peden, D.R.; Vardy, A.W.; Peters, J.A. Neurosteroid modualtion of GABBA receptors. Prog. Neurobiol. 2003, 71, 67-80.

Neurosteroids regulate mouse aggression induced by anabolic androgenic steroids

Anabolic androgenic steroid abuse triggers impulsive aggression, anxiety, and depression, which suggests a dysfunction of GABAergic neurotransmission. Socially isolated female mice that have received testosterone propionate (1.45 micromol/kg) treatment for 3 weeks during social isolation express aggression, neurosteroid downregulation, and changes in the cortical mRNA expression of several gamma-aminobutyric acid type A receptor subunits (alpha1, alpha2, gamma2 are decreased by 30-40%, and alpha4 and alpha5 are increased by 50%). Administration of allopregnanolone or the potent selective brain steroidogenic stimulant S-norfluoxetine, in doses (1.8-3.6 micromol/kg) that fail to inhibit 5-hydroxytryptamine reuptake, normalizes olfactory bulb neurosteroid level downregulation and abolishes aggression. This work underscores the role of neurosteroids in the regulation of aggression elicited by testosterone propionate in socially isolated female mice.

Fluoxetine and norfluoxetine stereospecifically and selectively increase brain neurosteroid content at doses that are inactive on 5-HT reuptake

It has recently become more clearly understood that in human brain pathophysiology, neurosteroids play a role in anxiety disorders, premenstrual syndrome, postpartum depression, posttraumatic stress disorder, and depression. In the treatment of major depression, recent clinical studies indicate that the pharmacological profiles of fluoxetine and fluvoxamine are correlated with the ability of these drugs to increase the brain and cerebrospinal fluid content of allopregnanolone (Allo), a potent positive allosteric modulator of gamma-aminobutyric acid (GABA) action at GABAA receptors. Thus, the neurosteroid-induced positive allosteric modulation of GABA action at GABAA receptors is facilitated by fluoxetine or its congeners (i.e., paroxetine, fluvoxamine, sertraline), which may not block 5-HT reuptake at the doses currently prescribed in the clinic. However, these doses are effective in the treatment of premenstrual dysphoria, anxiety, and depression. In socially isolated mice, we tested the hypothesis that fluoxetine, norfluoxetine, and other specific serotonin reuptake inhibitor (SSRI) congeners stereoselectively upregulate neurosteroid content at doses insufficient to inhibit 5-HT reuptake; although they potentiate pentobarbital-induced sedation and exert antiaggressive action. Very importantly, the inhibition of 5-HT reuptake lacks stereospecificity and requires fluoxetine and norfluoxetine doses that are 50-fold greater than those required to increase brain Allo content, potentiate the action of pentobarbital, or antagonize isolation-induced aggression. Based on these findings, it could be inferred that the increase of brain Allo content elicited by fluoxetine and norfluoxetine, rather than the inhibition selective of 5-HT reuptake, may be operative in the fluoxetine-induced remission of the behavioral abnormalities associated with mood disorders. Therefore, the term "SSRI" may be misleading in defining the pharmacological profile of fluoxetine and its congeners. To this extent, the term "selective brain steroidogenic stimulants" (SBSSs) could be proposed.