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

Abstract

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.

Figure 1

Allopregnanolone biosynthesis from progesterone in corticolimbic neurons. The 5α-reductase type I specific competitive inhibitor SKF 105,111 or social isolation decrease corticolimbic allopregnanolone levels, which triggers a reduced GABA_A receptor neurotransmission. The mechanism whereby the potent selective brain steroidogenic stimulant (SBSS) S-norfluoxetine upregulates allopregnanolone levels is not clearly understood. A direct 3α-HSD activation by S-norfluoxetine has been previously suggested (Griffin and Mellon, 1999). 5α-DHP: 5α-dihydroprogesterone; 3α-HSD: 3α-hydroxy steroid dehydrogenase.

Figure 2

Physiological levels of allopregnanolone maintain the function of GABAergic neurotransmission in the brain (Pinna et al., 2000). This diagrammatic representation depicts the local action of allopregnanolone on GABA_A receptors located on synaptic membranes of pyramidal neurons. GABA released from GABAergic interneurons activates a family of postsynaptic and extrasynaptic GABA_A receptors. Allopregnanolone may facilitate the synaptic inhibitory action of GABA at postsynaptic and extrasynaptic GABA_A receptors by an autocrine mechanism (arrow 1) or may access GABA_A receptors by acting at the intracellular sites (arrow 2) of the GABA_A receptors.

Figure 3

Increase of allopregnanolone levels in the olfactory bulb of socially-isolated (SI) mice after treatment with fluoxetine (FLX) and norfluoxetine (NFLX) stereoisomers. R- and S-fluoxetine (A) and R- and S-norfluoxetine (B) were given 30 min before allopregnanolone determination. P values are from the comparison of R- and S-fluoxetine-treated and R- and S-norfluoxetine-treated socially-isolated mice with vehicle (VH)-treated socially-isolated mice. *, P < 0.05; **, P < 0.01. Each value is the mean ± SEM of six mice. GH= Group housed mice. For details see Pinna et al., (2004).

Figure 4

Ex vivo inhibition of serotonin reuptake in cortical slices from socially-isolated mice treated with stereoisomers of fluoxetine (A) and norfluoxetine (B). Drugs were administered 30 min before ex vivo [14C] 5-HT uptake measurements. Each value represents the mean ± SEM of four mice. For details see Pinna et al., (2004).

Figure 5

Fluoxetine (FLX) and norfluoxetine (NFLX) stereospecifically normalize the duration of pentobarbital-induced sedation in socially-isolated (SI) mice. R- and S-FLX (A) and R- and S-NFLX (B) were given 30 min before pentobarbital (50 mg/kg, i.p.). P values are from the comparison of FLX- or NFLX-treated SI mice with vehicle (VH)-treated SI mice. *, P 0.05; **, P 0.01. Each value is the mean ± SEM of six to eight mice. For details see Pinna et al., (2004).

Figure 6

Fluoxetine (FLX) and norfluoxetine (NFLX) dose-dependently suppress social isolation-induced aggressive behavior (% of vehicle-treated mice) in a stereospecific manner. Each point is the mean ± S.E.M. of 8–12 mice. Drugs were given 30 min before test. *, P < 0.05; **, P < 0.01, when FLX-or NFLX-treated mice were compared with vehicle-treated mice. For details see Pinna et al. (2003a).