Social isolation stress-induced aggression in mice: a model to study the pharmacology of neurosteroidogenesis

A common genetic target for environmental and heritable influences on aggressiveness in Drosophila

Environmental and genetic factors can modulate aggressiveness, but the biological mechanisms underlying their influence are largely unknown. Social experience with conspecifics suppresses aggressiveness in both vertebrate and invertebrate species, including Drosophila. We searched for genes whose expression levels correlate with the influence of social experience on aggressiveness in Drosophila by performing microarray analysis of head tissue from socially isolated (aggressive) vs. socially experienced (nonaggressive) male flies. Among approximately 200 differentially expressed genes, only one was also present in a gene set previously identified by profiling Drosophila strains subjected to genetic selection for differences in aggressiveness [Dierick HA, Greenspan RJ (2006) Nat Genet 38:1023-1031]. This gene, Cyp6a20, encodes a cytochrome P450. Social experience increased Cyp6a20 expression and decreased aggressiveness in a reversible manner. In Cyp6a20 mutants, aggressiveness was increased in group-housed but not socially isolated flies. These data identify a common genetic target for environmental and heritable influences on aggressiveness. Cyp6a20 is expressed in a subset of nonneuronal support cells associated with pheromone-sensing olfactory sensilla, suggesting that social experience may influence aggressiveness by regulating pheromone sensitivity.

Alteration of behavioural phenotype in mice by targeted disruption of the progranulin gene

Sexual differentiation of the brain in rodents is achieved by estrogens, which are converted from androgens in the brain, during the perinatal period. We have identified the progranulin (PGRN) gene as one of the sex steroid-inducible genes that may be involved in masculinization of the rat brain. In the present study, we generated a line of mice with targeted disruption of the PGRN gene, and investigated male sexual behaviour, aggression and anxiety. PGRN-deficient mice exhibited a decrease in ejaculation incidence, while the latency and frequency of both mount and intromission were unchanged. For the aggressive behaviour test, the resident-intruder paradigm was used, and PGRN-deficient mice exhibited enhanced aggressiveness. In wild-type mice, males exhibited lower levels of anxiety than females by the open field test, while male PGRN-deficient mice exhibited an elevated level of anxiety and sex difference in anxiety was not observed. In addition, mRNA expression of the serotonergic receptor 5-HT1A, which could be related to the inhibition of aggression and anxiety, was significantly reduced in the hippocampus of PGRN-deficient mice after aggressive encounters. On the other hand, deficiency of the PGRN gene did not affect serum testosterone concentrations. These results suggest that PGRN gene plays a role in establishing sexual dimorphic behaviours at least partially by modulating the brain serotonergic system.

Neurosteroid regulation of central nervous system development

Neurosteroids are a relatively new class of neuroactive compounds brought to prominence in the past 2 decades. Despite knowing of their presence in the nervous system of various species for over 20 years and knowing of their functions as GABA(A) and N-methyl-d-aspartate (NMDA) ligands, new and unexpected functions of these compounds are continuously being identified. Absence or reduced concentrations of neurosteroids during development and in adults may be associated with neurodevelopmental, psychiatric, or behavioral disorders. Treatment with physiologic or pharmacologic concentrations of these compounds may also promote neurogenesis, neuronal survival, myelination, increased memory, and reduced neurotoxicity. This review highlights what is currently known about the neurodevelopmental functions and mechanisms of action of 4 distinct neurosteroids: pregnenolone, progesterone, allopregnanolone, and dehydroepiandrosterone (DHEA).

The relevance of neuroactive steroids in schizophrenia, depression, and anxiety disorders

1. Neuroactive steroids are steroid hormones that exert rapid, nongenomic effects at ligand-gated ion channels. There is increasing awareness of the possible role of these steroids in the pathology and manifestation of symptoms of psychiatric disorders. The aim of this paper is to review the current knowledge of neuroactive steroid functioning in the central nervous system, and to assess the role of neuroactive steroids in the pathophysiology and treatment of symptoms of schizophrenia, depression, and anxiety disorders. Particular emphasis will be placed on GABAA receptor modulation, given the extensive knowledge of the interactions between this receptor complex, neuroactive steroids, and psychiatric illness. 2. A brief description of neuroactive steroid metabolism is followed by a discussion of the interactions of neuroactive steroids with acute and chronic stress and the HPA axis. Preclinical and clinical studies related to psychiatric disorders that have been conducted on neuroactive steroids are also described. 3. Plasma concentrations of some neuroactive steroids are altered in individuals suffering from schizophrenia, depression, or anxiety disorders compared to values in healthy controls. Some drugs used to treat these disorders have been reported to alter plasma and brain concentrations in clinical and preclinical studies, respectively. 4. Further research is warranted into the role of neuroactive steroids in the pathophysiology of psychiatric illnesses and the possible role of these steroids in the successful treatment of these disorders.

Effects of progesterone and its reduced metabolites, dihydroprogesterone and tetrahydroprogesterone, on the expression and phosphorylation of glycogen synthase kinase-3 and the microtubule-associated protein tau in the rat cerebellum

Progesterone exerts a variety of actions in the brain, where it is rapidly metabolized to 5alpha-dihydroprogesterone (DHP) and 3alpha,5alpha-tetrahydroprogesterone (THP). The effect of progesterone and its metabolites on the expression and phosphorylation of the microtubule-associated protein Tau and glycogen synthase kinase 3beta (GSK3beta), a kinase involved in Tau phosphorylation, were assessed in two progesterone-sensitive brain areas: the hypothalamus and the cerebellum. Administration of progesterone, DHP, and THP to ovariectomized rats did not affect Tau and GSK3beta assessed in whole hypothalamic homogenates. In contrast, progesterone and its metabolites resulted in a significant decrease in the expression of Tau and GSK3beta in the cerebellum. Furthermore, progesterone administration resulted in an increase in the phosphorylation of two epitopes of Tau (Tau-1 and PHF-1) phosphorylated by GSK3beta, but did not affect the phosphorylation of an epitope of Tau (Ser262) that is GSK3beta insensitive. These effects were accompanied by a decrease in the phosphorylation of GSK3beta in serine, which is associated to an increase in its activity, suggesting that the effect of progesterone on Tau-1 and PHF-1 phosphorylation in the cerebellum is mediated by GSK3beta. The regulation of Tau expression and phosphorylation by progesterone may contribute to the hormonal regulation of cerebellar function by the modification of neuronal cytoskeleton.

Mechanisms of neurosteroid interactions with GABA(A) receptors

Neuroactive steroids have some of their most potent actions by augmenting the function of GABA(A) receptors. Endogenous steroid actions on GABA(A) receptors may underlie important effects on mood and behavior. Exogenous neuroactive steroids have potential as anesthetics, anticonvulsants, and neuroprotectants. We have taken multiple approaches to understand more completely the interaction of neuroactive steroids with GABA(A) receptors. We have developed many novel steroid analogues in this effort. Recent work has resulted in synthesis of new enantiomer analogue pairs, novel ligands that probe various properties of the steroid pharmacophore, fluorescent neuroactive steroid analogues, and photoaffinity labels. Using these tools, combined with receptor binding and electrophysiological assays, we have begun to untangle the complexity of steroid actions at this important class of ligand-gated ion channel.

GABA(A) receptor neurotransmission dysfunction in a mouse model of social isolation-induced stress: possible insights into a non-serotonergic mechanism of action of SSRIs in mood and anxiety disorders

Protracted social isolation in laboratory animals causes stress, which induces a variety of behavioral abnormalities including increased aggressiveness, anxiety-related behaviors, cognitive deficits and hyper locomotion. Many of these disorders are similar to the symptoms found in psychiatric disorders, such as depression, anxiety, premenstrual dysphoria and posttraumatic stress disorders (PTSD). Recent studies have demonstrated that male mice that have been socially isolated for more than 4 weeks show: (a) reduced responsiveness of GABA(A) receptors (GABA(A)-R) to the administrations of GABA mimetic drugs at GABA(A)-R; (b) downregulated biosynthesis of 3alpha,5alpha-tetrahydroprogesterone (3alpha,5alpha-THP) (allopregnanolone: ALLO), a neurosteroid with a potent positive allosteric modulatory effect on the action of GABA on GABA(A)-R; and (c) alterations in the expression of GABA(A)-R subunits (i.e. a decrease of alpha1/alpha2 and gamma2 subunits and an increase of alpha4 and alpha5 subunits). The selective serotonin reuptake inhibitor (SSRI) fluoxetine (FLX) and its congener norfluoxetine (Nor-FLX), when administered systemically at nmol/kg doses, normalize the reduced content of brain ALLO and the reduced responsiveness of GABA(A)-R to GABA mimetic drugs (i.e. pentobarbital) and also attenuate aggressive behavior in socially isolated mice in a stereospecific manner. Although these compounds inhibit ex vivo serotonin reuptake into brain tissue, their SSRI activities require high micromol/kg dose ranges and are not stereospecific. These studies suggest that in socially isolated mice, abnormalities of GABA(A)-R signal transduction are attributable to the downregulation of ALLO production and to a switch in heteropentameric GABA(A)-R subunit assembly composition. Hence, the normalization of ALLO biosynthesis may be a new target for the development of drugs effective for psychiatric disorders related to neurosteroid biosynthesis downregulation.

Hypothalamic-pituitary-adrenal axis modulation of GABAergic neuroactive steroids influences ethanol sensitivity and drinking behavior

Activation of the hypothalamic-pituitary-adrenal (HPA) axis leads to elevations in gamma-aminobutyric acid (GABA)-ergic neuroactive steroids that enhance GABA neurotransmission and restore homeostasis following stress. This regulation of the HPA axis maintains healthy brain function and protects against neuropsychiatric disease. Ethanol sensitivity is influenced by elevations in neuroactive steroids that enhance the GABAergic effects of ethanol, and may prevent excessive drinking in rodents and humans. Low ethanol sensitivity is associated with greater alcohol consumption and increased risk of alcoholism. Indeed, ethanol-dependent rats show blunted neurosteroid responses to ethanol administration that may contribute to ethanol tolerance and the propensity to drink greater amounts of ethanol. The review presents evidence to support the hypothesis that neurosteroids contribute to ethanol actions and prevent excessive drinking, while the lack of neurosteroid responses to ethanol may underlie innate or chronic tolerance and increased risk of excessive drinking. Neurosteroids may have therapeutic use in alcohol withdrawal or for relapse prevention.

Reduced metabolites mediate neuroprotective effects of progesterone in the adult rat hippocampus. The synthetic progestin medroxyprogesterone acetate (Provera) is not neuroprotective

The ovarian hormone progesterone is neuroprotective in different experimental models of neurodegeneration. In the nervous system, progesterone is metabolized to 5alpha-dihydroprogesterone (DHP) by the enzyme 5alpha-reductase. DHP is subsequently reduced to 3alpha,5alpha-tetrahydroprogesterone (THP) by a reversible reaction catalyzed by the enzyme 3alpha-hydroxysteroid dehydrogenase. In this study we have analyzed whether progesterone metabolism is involved in the neuroprotective effect of the hormone in the hilus of the hippocampus of ovariectomized rats injected with kainic acid, an experimental model of excitotoxic cell death. Progesterone increased the levels of DHP and THP in plasma and hippocampus and prevented kainic-acid-induced neuronal loss. In contrast to progesterone, the synthetic progestin medroxyprogesterone acetate (MPA, Provera) did not increase DHP and THP levels and did not prevent kainic-acid-induced neuronal loss. The administration of the 5alpha-reductase inhibitor finasteride prevented the increase in the levels of DHP and THP in plasma and hippocampus as a result of progesterone administration and abolished the neuroprotective effect of progesterone. Both DHP and THP were neuroprotective against kainic acid. However, the administration of indomethacin, a 3alpha-hydroxysteroid dehydrogenase inhibitor, blocked the neuroprotective effect of both DHP and THP, suggesting that both metabolites are necessary for the neuroprotective effect of progesterone. In conclusion, our findings indicate that progesterone is neuroprotective against kainic acid excitotoxicity in vivo while the synthetic progestin MPA is not and suggest that progesterone metabolism to its reduced derivatives DHP and THP is necessary for the neuroprotective effect of the hormone.

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.