SSRIs and the female brain

Abstract

One unexpected property of selective serotonin reuptake inhibitors is their ability, at doses well below those that effect 5-HT systems, to raise brain concentrations of neuroactive steroids such as the progesterone metabolite allopregnanolone. In women, rapid withdrawal from allopregnanolone when progesterone secretion drops sharply in the late luteal phase precipitates menstrual cycle-linked disorders such as premenstrual syndrome and catamenial epilepsy. Short-term, low-dose fluoxetine during the late luteal phase has the potential to prevent the development of such disorders, by raising brain allopregnanolone concentration. In female rats, withdrawal from allopregnanolone, as ovarian progesterone secretion falls rapidly in the late diestrus phase (similar to late luteal phase in women), induces upregulation of extrasynaptic GABA_A receptors on GABAergic neurons in brain regions involved in mediating anxiety-like behaviors. The functional consequence of this receptor plasticity is disinhibition of principal neurons, hyperexcitable neuronal circuitry and increased behavioral responsiveness to anxiogenic stress. These withdrawal responses were prevented by short-term treatment with fluoxetine during the late diestrus phase, which raised brain allopregnanolone concentration, so blunting the rapid physiological fall. The steroid-stimulating properties of fluoxetine offer untapped opportunities for developing new treatments for menstrual cycle-linked disorders in women, which are precipitated by abrupt falls in brain concentration of allopregnanolone.

Keywords

SSRIs fluoxetine female brain allopregnanolone neurosteroid replacement anxiety

Introduction

The selective serotonin reuptake inhibitors (SSRIs) have arguably been one of the world’s most successful classes of pharmaceutical agent. The introduction of fluoxetine for the treatment of depression in the late 1980s ushered in a new generation of antidepressant drugs. Since that time fluoxetine has become the most prescribed antidepressant drug worldwide; its use has revolutionized the treatment of depression (Wong et al., 1995). However, it is becoming clear that the name SSRI is in some sense a misnomer, since evidence is now accumulating to indicate that these compounds are far from selective exclusively for serotonin reuptake. This article shows the therapeutic potential of the neurosteroid-stimulating property of low-dose fluoxetine given during the late luteal phase to treat menstrual cycle-linked disorders in women.

One of the unexpected properties of fluoxetine and several other SSRIs is their ability to induce changes in brain concentrations of certain neuroactive steroids. More than 15 years ago, Uzunov et al. (1996) first reported that a single i.p. injection of fluoxetine in male rats was followed by an increase in brain concentration of the neuroactive metabolite of progesterone: allopregnanolone (3α,5α -tetrahydroprogesterone). The concurrent decrease that occurred in 5α -dihydroprogesterone, the immediate precursor of allopregnanolone, suggested that fluoxetine was enhancing de novo biosynthesis of allopregnanolone. The steroid-enhancing effect of fluoxetine was dose related and rapid in onset. Increases in allopregnanolone concentration in the brain could be detected within 15min after acute administration and lasted for several hours (Uzunov et al., 1996). Moreover, the effect was unchanged in adrenalectomized animals, indicating a central site of action rather than fluoxetine-stimulated release of allopregnanolone from the adrenal gland (Holzbauer et al., 1985).

Subsequent to this groundbreaking finding other laboratories confirmed the steroid-enhancing effect of fluoxetine (Marx et al., 2006; Ugale et al., 2004). It is now known that the active compound is not fluoxetine itself but its metabolite norfluoxetine (Pinna et al., 2009), and that its effect is both stereospecific (S-isomers > R-isomers) and region specific (Nelson and Pinna, 2011; Pinna et al., 2009). In contrast, inhibition of 5-HT reuptake by fluoxetine and norfluoxetine is not stereospecific. Moreover, the EC₅₀ required to inhibit 5-HT reuptake is some 10–50 times higher than for the steroid-enhancing effect (Pinna et al., 2009). Based on these properties, Pinna and co-workers (Pinna et al., 2009) suggested that the term ‘SSRI’ may be misleading in describing the pharmacological profile of fluoxetine and proposed the term ‘selective brain steroidogenic stimulant (SBSS)’ as a better descriptor.

Mechanism of the steroid-stimulating action of fluoxetine

Allopregnanolone is synthesized from progesterone by the sequential action of two reducing enzymes: 5α-reductase (5α-R) type I, which transforms progesterone into 5α-DHP, and 3α-hydroxysteroid dehydrogenase (3α-HSD), which transforms 5α-DHP into allopregnanolone and vice versa (Figure 1). The precise mechanism whereby fluoxetine interacts with this pathway is still not clear. The fluoxetine-induced elevation of brain ALLO concentration and the concomitant fall in 5α-DHPROG implies an effect of the drug on 3α-HSD activity. However, the experimental findings to date are equivocal. In transfected cells, Griffin and Mellon (1999) described an activation (decrease in K_m) of purified recombinant rat liver type 1 (AKR1C9) and human type 3 (AKR1C2) 3α-HSD enzymes by fluoxetine, with no effect on rat type 1 5α-reductase activity. However, Trauger et al. (2002) were unable to show an effect of fluoxetine or of other SSRIs on the purified recombinant human type 3 (AKR1C2) 3α-HSD enzyme.

Other steroid-enhancing effects

In addition to allopregnanolone, fluoxetine has been reported to evoke changes in brain and serum levels of the neuroactive androgen, 3α5α-androstane-3α,17β-diol (3α,5α-Adiol), which is derived from testosterone by the action of 5α-reductase and 3α-HSD (Higashi et al., 2009). 3α,5α-Adiol shows structural similarities to allopregnanolone and is also a potent positive modulator of GABA_A receptors (Reddy, 2010). However, the concentration of 3α,5α-Adiol in the brain is orders of magnitude below that of allopregnanolone (Higashi et al., 2009) so it is not yet clear whether the effect of fluoxetine on this pathway is of biological relevance.

Clinical application of steroid-enhancing properties of SSRIs

Depressed patients and patients with post-traumatic stress disorder have lower plasma and cerebrospinal fluid (CSF) concentrations of allopregnanolone than normal individuals; an inverse relationship has also been demonstrated between allopregnanolone concentration and the severity of depressive illness (Nappi et al., 2001, Rasmusson et al., 2006; Uzunova et al., 2006). Since clinically efficacious treatment with SSRIs is associated with increases in allopregnanolone in the CSF (Romeo et al., 1998; Strohle et al., 1999; Uzunova et al., 1998), it seems likely that the therapeutic action of fluoxetine could be due in part to its steroid-enhancing properties, as well as its effect on 5-HT reuptake. Direct evidence to support this view comes from studies using animal models. The low allopregnanolone content in the brain of socially isolated mice could be normalized by treatment with fluoxetine, which also rescued the behavioral deficits associated with social isolation (Pinna, 2010).

Given the undisputed steroid-enhancing properties of fluoxetine, it is perhaps surprising that this aspect of its therapeutic potential has been realized only slowly. It is well established in the clinical literature that the antidepressant action of fluoxetine is slow to establish, even in responders. Typically around 10 days of continuous treatment with SSRIs is required before patients show a clinical response (Katz et al., 2004). Moreover, in the short term, SSRI treatment may lead to an increase in anxiety in some individuals (Amsterdam et al., 1994; Lipinski et al., 1989; Ravindran et al., 2007). The paradoxical, acute anxiogenic effect has been demonstrated in numerous rodent models using male animals (Birkett et al., 2011; Ravinder et al., 2011; Salchner and Singewald, 2002; Silva and Brandão, 2000). The anxiogenic effect is associated with increases in excitability of neuronal populations in the lateral amygdala (Ravinder et al., 2011) mediated by 5HT₂ receptors (Salchner and Singewald, 2002). However, the anxiogenic and steroid-enhancing effects of acute administration of SSRIs such as fluoxetine can readily be dissociated by appropriate dosing, since the latter occurs at doses well below those normally utilized for influencing 5-HT systems (Pinna et al., 2009). This property offers a unique opportunity to use low-dose SSRI treatment to treat short-lived, hormone-linked maladaptive states such as premenstrual syndrome (PMS) in females.

Neuroactive steroid-linked behavioral changes in females – effects of progesterone withdrawal

The aforementioned studies, and indeed almost all work associated with steroid-enhancing properties of SSRIs, has been carried out on male animal models. The female brain operates in a different chemical milieu, due to ovarian secretion of progesterone and other hormones, which can influence brain function. Due to their lipophilic nature, steroids such as progesterone pass readily into the brain from the plasma. Changes in plasma concentration of progesterone (and its neuroactive metabolite allopregnanolone) during the estrous cycle (menstrual cycle in women) are reflected by parallel changes in concentration within the brain. Indeed, peripherally secreted progesterone is the major determinant of the hormone’s concentration in the brain (Corpéchot et al., 1993).

Progesterone, via its metabolite allopregnanolone, acts as a potent positive allosteric modulator of the actions of GABA at GABA_A receptors, where it potentiates the intensity of GABA-gated chloride currents (Belelli and Lambert, 2005; Lambert et al., 2003). Allopregnanolone has a high affinity for extrasynaptic GABA_A receptors (Wang, 2011) whose level of expression changes markedly in response to fluctuations in extracellular concentration of the steroid. In several brain areas, withdrawal of rats from chronic dosing with exogenous progesterone triggers upregulation of GABA_A receptors containing α4 and δ subunits, with consequent changes in neuronal excitability and the induction of anxiety-linked behaviors (Devall et al., 2009; Griffiths and Lovick 2005a; Hsu and Smith 2003; Smith et al., 1998). The withdrawal response is mediated not by progesterone itself but by allopregnanolone, via a mechanism that utilizes the early growth response factor-3 pathway egr3 (Gangisetty and Reddy, 2010; Smith et al., 1998).

In males, similar effects of withdrawal from exogenous progesterone have been demonstrated in some studies (Gulinello et al., 2002) but not in others (Gulinello et al., 2003), suggesting that there may be sex differences in the response to neuroactive steroids. However, it is debatable whether the withdrawal effect in males is more than an epiphenomenon, since endogenous levels of progesterone in the brain are extremely low in males (Higashi et al., 2009) and withdrawal from progesterone is an unlikely event under physiological circumstances. On the other hand, in females of reproductive age, withdrawal from progesterone is a common event. Progesterone concentrations in the plasma – and hence the brain – undergo significant fluctuations due to cyclical changes in the level of secretion of the steroid by the ovaries during the ovarian cycle.

Secretion of progesterone secretion undergoes two surges during the 45day estrous cycle in rats. During estrous there is a brief surge in progesterone lasting around 6h (Figure 2) (Butcher et al., 1974). However, GABA_A receptor expression remains unchanged (Lovick et al., 2005), probably because the change in brain concentration of the hormone is not sufficiently long-lasting to induce changes in subunit expression. In other brain regions, upregulation of α4 subunits occurred only after longer periods of exposure to or withdrawal from the hormone (Gulinello et al., 2001, 2002; Smith et al., 1998). Following the initial surge in progesterone during estrus, a second, steady increase occurs over 24h during the early diestrus phase, which is then followed by an abrupt fall during late diestrus (Figure 2). This is sufficient to trigger a withdrawal response that includes expression of new extrasynaptic receptors containing α4, β1 and δ subunits (Lovick et al., 2005). In the periaqueductal grey matter (PAG), a region associated with mediating aversive behavioral reactions, the new receptors were expressed principally by the GABAergic interneuron population (Griffiths and Lovick, 2005b). Recombinant α₄β₁δ GABA_A receptors show a low EC₅₀ for GABA that is consistent with activation by GABA at the concentration present in the extracellular fluid, unlike synaptically located GABA_A receptors (Lerma et al., 1986; Lovick et al., 2005). Receptors that contain δ subunits are likely to be located extrasynaptically and to generate tonic currents (Farrant and Nusser, 2005; Mody, 2001, 2005). An increase in the tonic current carried by GABAergic neurons would be expected to decrease their excitability. Thus when expression of α₄, β₁ and δ GABA_A receptor subunits is increased, the level of GABAergic tone on output neurons should decrease, rendering them more excitable (Brack and Lovick, 2007). This increase in intrinsic excitability of the PAG circuitry may contribute to the enhanced responsiveness to acute psychogenic stress that characterizes the late diestrus phase (Devall et al., 2009).

The kinetics of the declining progesterone and hence allopregnanolone concentration in the brain appear to be a major determinant of the intensity of the withdrawal effect. Using an exogenous dosing regimen, Doornbos et al. (2009) showed that withdrawal from long-term dosing with progesterone was much more effective in precipitating GABA_A receptor upregulation and the concomitant increase in anxiety behavior if plasma levels of progesterone fell abruptly, rather than following a gradual decline. In spontaneously cycling rats, in which progesterone concentration declines rapidly during the late diestrus phase of the cycle, we found that short-term administration of fluoxetine (two injections 15h apart) during the late diestrus phase, using a dose sufficient to raise brain concentration of allopregnanolone (Devall et al., 2011) but subthreshold for an effect on 5-HT systems, prevented development of the increased responsiveness to anxiogenic stress that normally characterizes the late diestrus phase (Devall et al., 2009; Santos et al., 2012). When viewed in a clinical context, these results are important because they provide insight into the way that the steroid-stimulating property of fluoxetine could be harnessed to provide a novel treatment for PMS in women.

A recent Cochrane Data Review (Brown et al., 2009) concluded that SSRIs are the drug of choice for PMS. However, prescribing is based on doses found to be active as antidepressants; yet typically PMS patients are not depressed. Moreover, in depressed patients who respond to SSRIs, on average 13days of continuous dosing is required to produce a clinically significant improvement (Katz et al., 2004). On the other hand, When used for PMS SSRIs are effective in alleviating symptoms within a few days, and intermittent dosing with antidepressant doses of fluoxetine during the luteal phase or at the onset of symptoms has been reported to be as effective as continuous treatment for preventing symptoms on PMS (Ravindran et al., 2007; Steiner 1997, 2000; Steiner et al., 2008). Within such a short time frame, antidepressant effects on 5-HT systems would be unlikely.

The above findings clearly imply that the mechanism/s that underlie the effectiveness of SSRIs in preventing the development PMS is different to their antidepressant action. Indeed, PMS is not a feature of anxiety or depressive states but has its own nosological identity. The results of studies in animal models suggest that the effectiveness of SSRIs such as fluoxetine, especially when taken during the luteal phase of the cycle, may be due to their steroid-enhancing property, which offsets the sharp fall in allopregnanolone concentration in the brain to produce a more gradual decrease during the late diestrus phase (Figure 3). Effectively, fluoxetine removes the trigger for withdrawal effects. This action of fluoxetine can be achieved at doses far below the threshold for interaction with 5-HT systems, which are the target in antidepressant therapy. When translated into the clinical context, these findings suggest that current prescribing practice on SSRIs for PMS in women may not be optimal.

In women, the premenstrual phase of the ovarian cycle is associated with other disorders such as catamenial epilepsy; symptoms of several other disorders, for example panic disorder and irritable bowel syndrome, may also worsen at this time (Steiner 1997, 2000). It has been hypothesized that withdrawal of progesterone-derived neurosteroids leads to enhanced neuronal excitability predisposing to seizures (Reddy, 2009). In support of this view, rapid withdrawal from progesterone in rats has been shown to increase seizure susceptibility, and this is associated with increased expression of GABA_A receptors containing the α4 subunit (Gangisetty and Reddy, 2010; Moran and Smith, 1998). In a recent clinical trial luteal phase progesterone therapy, which would be expected to blunt the abrupt fall in plasma and brain concentration of progesterone that occurs naturally at this time, reduced seizures in women with perimenstrual catamenial epilepsy (Herzog et al., 2012; Reddy 2013). Intermittent treatment with SSRIs could provide another strategy to prevent the development of pro-convulsant withdrawal effects and prove beneficial to patients with catamenial seizures, whilst avoiding the hormonal side effects of progesterone therapy, which may interfere with the ovarian cycle and compromise fertility.

In women with PMS, neurosteroid replacement therapy during the late luteal phase has the potential to provide a safe, fast-acting, acceptable and efficacious means for preventing the development of periodic menstrual cycle-linked symptoms. By acting downstream from progesterone, steroid-stimulating agents can enhance brain allopregnanolone concentration without interfering with normal ovarian function. Thus reproductive function should not be compromised. Administration of the native steroid would provide a ready means of raising the concentration of allopregnanolone in the brain. However, allopregnanolone is not available in a formulation for human use. In contrast, fluoxetine and other SSRIs that function as steroid-stimulating agents provide a cheap, readily available indirect source of allopregnanolone. Moreover, since the doses of SSRIs needed to enhance brain allopregnanolone concentration are much lower than those used to block serotonin reuptake, and with the requirement for only intermittent dosing, the likelihood of adverse side effects which accompany use of antidepressant medication should be minimized.

Footnotes

Conflict of interest

The authors declare no conflict of interest.

Funding

The author’s work described in this paper was supported by Medical Research Council project grant MRC project grant GG0700379.

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