Failure of Acute Ethanol Administration to Alter Cerebrocortical and Hippocampal Allopregnanolone Levels in C57BL/6J and DBA/2J Mice

Allopregnanolone, the 3α,5α-reduced metabolite of progesterone, is a neuroactive steroid that produces inhibitory neurobehavioral effects such as anxiolytic, anticonvulsant, and sedative/hypnotic actions via interaction with γ-aminobutyric acid type A (GABAA) receptors (Biggio and Purdy, 2001). Allopregnanolone concentrations are altered in several neuropsychiatric disorders, including anxiety, depression, premenstrual dysphoric disorder, schizophrenia, and drug addiction (Biggio and Purdy, 2001; Morrow et al., 2006). Allopregnanolone levels are increased in rat brain and plasma by administration of various psycho-active drugs, including ethanol (EtOH; Morrow et al., 1999), caffeine (Concas et al., 2000), nicotine (Porcu et al., 2003), tetrahydrocannabinol (Grobin et al., 2005), morphine (Concas et al., 2006; Grobin et al., 2005), and gamma-hydroxybutyric acid (Porcu et al., 2004). Systemic EtOH administration increases brain and plasma allopregnanolone levels to physiologically relevant concentrations, capable of enhancing GABAergic transmission and contribute to sensitivity to behavioral effects of EtOH in rats (Barbaccia et al., 1999; Morrow et al., 1999; VanDoren et al., 2000). In fact, allopregnanolone modulates EtOH's anticonvulsant effects (VanDoren et al., 2000), sedation (Khisti et al., 2003), impairment of spatial memory (Matthews et al., 2002), anxiolytic-like (Hirani et al., 2005), anti-depressant-like (Hirani et al., 2002), and proaggressive actions (Fish et al., 2001). Allopregnanolone also mediates EtOH's ability to inhibit the spontaneous firing of medial septal neurons (VanDoren et al., 2000) and hippocampal pyramidal neurons (Tokunaga et al., 2003) in rats. These behavioral and electrophysiological responses are inhibited by pretreatment with finasteride (a 5α-reductase inhibitor) and/or by prior adrenalectomy. In fact, the EtOH-induced increase in cerebrocortical allopregnanolone levels is mediated by the hypothalamic–pituitary–adrenal (HPA) axis, since it is absent following adrenalectomy/gonadectomy (Khisti et al., 2003; O'Dell et al., 2004; Porcu et al., 2004) or hypophysectomy (Boyd et al., 2010). However, EtOH can increase allopregnanolone content in hippocampal slices from intact and adrenalectomized/gonadectomized rats (Follesa et al., 2006). The evidence regarding a role for allopregnanolone in EtOH's actions has been mostly obtained in rats. Studies in mice have mainly focused on C57BL/6J and DBA/2J mice, two of the most widely used inbred strains in addiction research because of their different sensitivity to several drugs of abuse (Belknap et al., 1993a,b). With respect to alcohol, DBA/2J mice have more severe handling-induced convulsions after acute (Roberts et al., 1992) and chronic (Crabbe et al., 1983) EtOH withdrawal. Further, DBA/2J mice show greater acute functional tolerance (Gallaher et al., 1996), locomotor activity (Crabbe et al., 1983), and hypnosis (Linsenbardt et al., 2009). These strains also differ in EtOH preference and consumption. In the 2-bottle-choice paradigm, C57BL/6J mice drink alcohol, while DBA/2J mice avoid it (Belknap et al., 1977; Phillips et al., 1994). However, when allowed to self-administer EtOH intravenously, these 2 strains do not differ in the amount of EtOH self-administered, suggesting that EtOH's reinforcing properties are similar in both strains (Grahame and Cunningham, 1997). C57BL/6J and DBA/2J mice also differ in the behavioral response to neuroactive steroids. C57BL/6J mice are more sensitive to the anxiolytic, locomotor stimulant, and anticonvulsant effects of allopregnanolone than DBA/2J mice (Finn et al., 1997b). Endogenous allopregnanolone may play a role in sensitivity to EtOH in these strains. Allopregnanolone modulates EtOH intake (Ford et al., 2005, 2007) and reinstates EtOH seeking behavior in C57BL/6J mice (Finn et al., 2008); it is also thought to modulate EtOH withdrawal severity across mouse strains (Finn et al., 2004a; Gililland and Finn, 2007). Acute EtOH administration increases whole brain allopregnanolone levels in male DBA/2J (Gabriel et al., 2004) and C57BL/6J mice, although in this strain, the effect did not reach statistical significance (Finn et al., 2004b). Moreover, we recently reported that a single administration of EtOH (2 g/kg, i.p.) failed to increase plasma allopregnanolone levels in male DBA/2J mice and it actually decreased them in male C57BL/6J mice (Porcu et al., 2010). To further explore these discrepancies and to better understand the effects of EtOH on brain neurosteroidogenesis in mice, we examined whether acute EtOH administration alters cerebrocortical and hippocampal levels of allopregnanolone and progesterone in C57BL/6J and DBA/2J mice. We further describe the effects of acute EtOH administration to these strains on cerebrocortical and hippocampal levels of corticosterone, the main corticosteroid synthesized by the adrenals following HPA axis activation.