Long-term social isolation of laboratory animals is a model to study the behavioral and neurochemical consequences of the absence of social interaction in rodents. Many of the symptoms induced by isolation resemble depression and anxiety disorder symptomatology. Our studies have revealed that male mice socially isolated for more than 4 weeks, exhibit increased aggressiveness, a reduced responsiveness to GABA_A receptor acting drugs, and a downregulation of brain levels of 3α,5α-tetrahydroprogesterone (allopregnanolone: 3α,5α-THP), a neurosteroid endowed with potent positive allosteric modulatory activity of the action of GABA at various GABA_A receptor subtypes. This downregulation of 3α,5α-THP appeared to be associated with the reduction of brain type I 5α-reductase mRNA and protein expression. Systemic administration of the selective serotonin reuptake inhibitor fluoxetine and its metabolite norfluoxetine normalized brain 3α,5α-THP content and reduced responsiveness to GABA_A mimetic drugs in a stereospecific manner. These drugs in nanomolar doses also reduced social isolation-induced aggressiveness with the same stereospecificity as detected in their action on 3α,5α-THP brain content, while their ex vivo inhibition of serotonin reuptake occurred at high μmolar doses and lacked stereospecificity. From these results we infer that the brain 3α,5α-THP content physiologically upregulates GABA_A receptor responsiveness to GABA and that social isolation induces a reduction of brain 3α,5α-THP content that is probably causally related to the onset of aggression.

In rat cerebellar slices, we compared whole-cell recordings of spontaneous inhibitory postsynaptic currents (sIPSCs) with Cl- currents resulting from pulses of GABA (1 mM, < 2 ms) to outside-out patches from Purkinje and granule neurons. sIPSCs in Purkinje cells decayed with a single fast exponential, as previously reported, whereas in granule cells sIPSC decay was best described by the sum of a fast and a slow exponential curve, with a variable contribution of the slow component to the peak current. GABA pulses to nucleated patches from granule cells elicited Cl- currents with decays similar to sIPSC decays, whereas in patches from Purkinje neurons GABA pulses produced Cl- currents decaying largely with a fast component, but often followed by a slower exponential. GABA concentration steps produced rapidly desensitizing currents in patches from both cerebellar neurons. In distinct cerebellar neurons, specific functional properties of GABAA receptors may relate to the presence of distinct receptor subtypes.

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 3α,5α-tetrahydroprogesterone (3α,5α-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 α1/α2 and γ2 subunits and an increase of α4 and α5 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 μmol/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.